FORMATION OF INORGANIC SUBMICRON PARTICLES
UNDER SIMULATED PULVERIZED COAL COMBUSTION CONDITIONS
by
Matthew Neville
B.S.,
University of New Hampshire
(1977)
SUBMITTED IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE
DEGREE OF
DOCTOR OF SCIENCE
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
August, 1982
Signature of Author
Department of Chemical Engineering
Certified by
Adel F. Sarofimn, Thesis Advisor
Accepted by
ulenn C. Williams, Chairman
Departmental Committee on Graduate Theses
ArchTve
WFTTS INSTITUTE
OFC
DEC
19OLOGY
31982
LIBRARIES
FORMATION OF INORGANIC SUBMICRON PARTICLES
UNDER SIMULATED PULVERIZED COAL COMBUSTION CONDITIONS
by
Matthew Neville
Department of Chemical Engineering
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
August, 1982
Professor Jack P. Ruina
Secretary of the Faculty
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
Dear Professor Ruina:
I
In accordance with the regulations of the Faculty,
herewith submit a thesis, entitled, "Formation of Inorganic
Submicron Particles Under Simulated Pulverized Coal Combustion Conditions", in partial fulfillment of the requirements
for the Degree of Doctor in Science in Chemical Engineering
at Massachusetts Intitute of Technology.
Respectfully submitted,
Matthew Neville
iii
FORMATION OF INORGANIC SUBMICRON PARTICLES
UNDER SIMULATED PULVERIZED COAL COMBUSTION CONDITIONS
by
Matthew Neville
Submitted to the Department of Chemical Engineering in August, 1982 in
partial fulfillment of the requirements for the Degree of Doctor of Science
from the Massachusetts Institute of Technology
ABSRTACT
The factors governing the physico-chemical characteristics of the
inorganic submicron particles produced by the combustion of coal have been
examined by burning size-graded Montana lignite particles in a laminar flow
drop-tube furnace at 1700 K. The particle combustion temperature achieved
vary from 1800 K to 2800 K as the oxygen concentration in which the coal
particles burn is increased from 5 percent to 100 percent.
Size fractionation of the ash yields a bimodel size distribution.
The average size of the primary particles in the submicron mode varies from
less than 50 angstroms to greater than 400 angstroms, depending on the
combustion conditions. The submicron particles are produced by vaporization
of the mineral matter species during combustion and their subsequent
recondensation. The amount of fume produced as metal oxide increases from
0.1 percent at 1800 K to 20 percent at 2800 K. The growth of the submicron
particles is well predicted by classical coagulation theory where theory
provides a good representation of the observed primary particle size as a
In the early
function of residence time and degree of ash vaporization.
stages of coagulation the particles coalesce, but as the temperature in the
vicinity of the particles decreases subsequent to the completion of combustion of the char and as the primary particle diameter increases, the
coalescence rate decreases and the agglomerate produced appears as aggregates of spheres.
The chemical composition of the Montana lignite fume is dominated
by MgO and CaO at all but the lowest particle combustion temperature (<
1900 K), apparently because the vaporization of these refractory oxides is
augmented by their chemical reduction to the more volatile metal vapor in
the locally reducing atmosphere within the burning char particle. The
inorganic submicron particles are found to consist of a core of MgO, CaO,
and FeO with an inner coating of silica and an outer coating of sodium,
arsenic, and other trace metals. Furthermore, silicon and the volatile and
trace species show an increase in concentration with decreasing particle
size in the submicron range (< 400 angstroms).
From a steady state one dimensional model the reoxidation of the
vaporized reduced-state species of Mg, Ca, and Fe away from the char
surface results in the supersaturation of these refractory oxides in the
boundary layer of the burning char particle which is predicted to provide
the necessary driving for the formation of a new condensed phase by homogeneous nucleation. Silicon deposition subsequently occurs as a probable
consequence of the low rate of oxidation of the SiO vapors released by the
burning coal particle. The volatile and trace species condense on the outer
surface as the combustion products are cooled. Most of the ash surface is
provided by submicron particles even though they constitute only a small
fraction of the total ash. Both the residual and submicron particles are
coated with a surface deposit of volatile (Na) and trace (As,Sb) elements.
The distribution of the volatile elements between the two size modes of the
ash is influenced by the amount of submicron particles produced during
combustion and the rate at which the combustion products are cooled.
However, in the case of the trace species, where an insufficient driving
force for diffusion-controlled condensation exists, the surface deposition
of these species is believed to be chemically controlled.
Consequently,
the distribution of the trace species between the two size modes is influenced by both the relative reactivity and surface area between the two
size modes.
Thesis Advisor: Adel F. Sarofim
Professor of Chemical Engineering
ACKNOWLEDGEMENTS
The author wishes to express his sincere graditude to
his advisor,
Professor Adel F.
continued
his
for
Sarofim,
encouragement, guidence, and enthusiasm throughout the course
tional goal would not have been possible.
this
of
without which the attaining
of this thesis,
educa-
The principles
in
which Professor Sarofim has instilled in the author have made
a lasting impression.
Special graditude is
Quann,
Modestino,
Anthony J.
also
extented
and
Charles A. Mims,
and
Haynes for their support, advise,
Richard
to
J.
Brian
S.
with
the
assistance
experimental and conceptual aspects of this study. The Author
is also greatful to John Martin for his assistance with Auger
Spectroscopy
and
ESCA,
Sande
Vander
Professor
Garret-Ried for their assistance with
Scanning
Tony
and
Transmission
Electron Microscope, and Morteza Janghorbani for the
Neutron
Activation Analysis Service.
This work was supported by
Electric
Power
Research
Institute and N.I.E.H.S.
To all
express
enough
members
of
appreciation
my
family,
for
their
the
author
cannot
encouragement
and
support.
Finally, I wish to thank the members of my office and
vi
the members of the combustion group with special graditude to
Myongsook Lee, Lyle Timothy, Harvey Stanger, and Kim Ritrievi
for their continued support
coarse of this thesis work.
and
encouragement
through
the
iiv
TABLE OF CONTENTS
Page
.
TITLE PAGE..
.
.
.
AUTHORIZATION PAGE. .
.
.
.
.
. . . .
.
.
.
.
. . . .
iii
.
.
.
.
.
.
.
. . .
ACKNOWLEDGEMENTS .
.
.
.
.
.
.
.
.
.
.
TABLE OF CONTENTS
.
.
.
.
.
.
.
.
.
.
LIST OF FIGURES .
.
.
.
.
.
.
.
.
.
.
ABSTRACT..
.
.
.
V
.
.
.
.
.
.
.
.
.
SUMMARY .
.
.
.
.
.
.
.
.
1.1 Introduction . .
.
.
.
.
.
.
1.2 Experimental Apparatus .
.
.
LIST OF TABLES
CHAPTER ONE.
.
.
.
1.3 Results. .
.
.6
..........
.
.
2
tion on
1.3.1 Dependence of Concen
.
6
1.3.2 Surface Enrichment.
7
Particle Size .
1.4 Discussion. .
.
.
.
.
15
.
. . .
1.4.1 Particle Nucleation
15
1.4.2 Particle Growth .
21
.
ion.
1.4.3 Heterogeneous Conden
25
37
1.5 Conclusions. .
.
.
.
. . .
BACKGROUND
.
.
.
.
2.1 Introduction. .
.
.
. . .
.
39
.
.
.
. . .
.
46
CHAPTER TWO.
2.2 Objectives. .
CHAPTER THREE.
.
.
39
.
.
COMBUSTION AND SAMPLING SYSTEM
3.1 Combustion Furnace.
3.2 Feeder System
.
.
.
.
.
.
.
.
.
.
.
.
.
48
49
.
.
.
51
iiiv
3.3
Collection Probe .
.
.
.
.
.
.
.
.
.
.
.
54
3.4
Cascade Impactor
.
.
.
.
.
.
.
.
.
.
.
56
ANALYTICAL TECHNIQUES
.
.
.
.
.
.
.
.
62
CHAPTER FOUR.
.
4.1 Transmission Electron Microscope. .
. ....
62
4.2 Automated Image Analyzer. .
.
.
.
. . .
4.3 Mobility Classifier .
.
.
.
.
.
.
.
.
.
.
66
4.4 Bulk Analysis .
.
.
.
.
.
.
.
.
.
.
67
4.5 Scanning Transmission Electron Microscope
69
.
.
.
4.6 Auger Spectroscopy .
.
4.7 ESCA .
.
CHAPTER FIVE.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
66
.
................
103
FACTORS INFLUENCING THE BULK
CHARACTERISTICS OF THE SUBMICRON
PARTICLES..
. . . . . . . . . . .
.
110
5.1 Fly Ash Mass Distribution. .
.
. . . .
.
110
5.2 Bulk Chemical Composition. .
.
.
.
120
. . .
5.3 Combustion Temperature
Time Histrory .
.
126
5.4 Vaporization . .
.
.
134
.
.
.
.
.
.
.
.
.
.
CHAPTER SIX. CHEMICAL CHARACTERIZATION OF THE
SUBMICRON PARTICLES . . . . . . . . . . . .
6.1 Bulk Composition
.
.
.
.
.
.
.
.
147
6.2 Dependence of Concentration on Particle Size .
149
......
6.3 Surface Enrichment
6.4 Chemical
State Analysis .
CHAPTER SEVEN. DISCUSSION. .
7.1 Nucleation
7.2 Coagulation.
.
.
.
.
. . . .
.
.
.
.
.
.
.
.
.
. . .
.
.
.
.
.
.
.
.
.
.
.
.
.
..
.
.
156
..
..
165
. .
.
178
179
........
.
.
.
.
.
...
.
.
.
.
.
7.3 Heterogeneous Condensation .
7.3.1 Refractory Oxides.
.
. .
.
.
.
147
...
.
.
.
.
.
.
193
209
.
212
__
__li_
_
__1_
_II
__
:jl_
~I
_1
~__
ix
.
.
217
.
.
240
.. .
.
241
. . . . . . . . . . . . .
.
244
. . . . . . . . . . . . . . . . . . . . . .
246
.
..
CHAPTER EIGHT. CONCLUSIONS AND RECOMMENDATIONS .
.
7.3.2 Volatile and Trace Species .
8.1 Conclusions .
..............
8.2 Recommendations .
REFERENCES .
.
~----- :. -I-~ -;--..-.i~-~.~---------
---
-----
-
--
-
_--
LIST OF FIGURES
Page
Figure
1.1
Elemental Concentration for Primary Particles
by STEM Verses Inverse Diameter . .........
1.2
1.3
1.4
1.5
1.6
Concentration of Various Elements in the size
Classified Fume Verses Inverse Diameter ......
Model Prediction for the Supersaturation of
. . . . . . . . . . .
Magnesium Oxide . . ..
1.8
2.1
2.2
9
.
.
.
20
Experimental and Predicted Time Dependence for
the Mean Diameter of the Primary Submicron
.
Particles ...................
.
.
22
Predicted Coalescence Time for Crystaline
Magnesium Oxide Particles . ............
24
Tendency for Critical Supersaturation of Sodium
Sulfate as Combustion Products are Cooled in the
.
.
.
31
Concentration of Sodium in the Residual Fly Ash
Verses Inverse Diameter Squared . . . . . . . .
.
.
34
Fraction of Sodium Collected With the Fume for
Various Degrees of Vaporization of the Total Ash. .
36
Schematic Diagram of Fly Ash Formation During
. . . . . . . . . .
Pulverized Coal Combustion .
43
Presence of Residual and Submicron Particles.
1.7
8
.
Fly Ash Particle Size Distribution Obtained
From a Utility Boiler . ..............
45
3.1
Schematic Diagram of Laminar Flow Furnace ....
.
50
3.2
Gas Temperature Profiles in Laminar Flow Furnace. .
52
3.3
Schematic Diagram of Coal Feeder.
3.4
Schematic Diagram of Water Cooled Collection
.
..
...........
Probe ........
3.5
4.1
4.2
..
...
.
53
....
55
..
Schematic Diagram Showing Two Stages of a Cascade
..
. . .
Impactor. . . . . . . . . . . . . . ...
.
57
Calibration of the Mobility Classifier With
Respect to Primary Partcile Diameter. . ......
.
68
Decay in Beam Current and Specimen Count Rate
4.3
4.4
With Time . . . . . . . . . . . . . . . . . . . . .
74
STEM X-Ray Spectra Before (A) and After (B)
Modifying the Aperature Configuration . ......
79
Adsorption Correction for Various Particle
Diameters . .................
.
.
.
.
82
4.5
Beam Broadening
for Various PArticle Diameters.
.
.
84
4.6
STEM Sensitivity Correction Factors Referenced
to Sulfur . . . .. . . . . . . . . . . . . . . .
.
.
89
4.7
Auger Peak Amplitude Verses Beam Current. . ....
95
4.8
Auger Peak Amplitude Verses Sensitivity Settings. .
97
4.9
Auger Peak Amplitude Verses Modulation Amplitude
. . . . . .
Setting . ................
98
4.10
Auger Spectra of Mg and MgO . ... .
4.11
ESCA High Resolution Scan of Carbon C is Line
..
...
for Fume Sample ..........
5.1
.
5.3
106
.. .
Mass Distribution of Particle Combustion Products
on Cascade Impactor Stages and Final Filter .
5.2
101
.......
.
..
111
Distribution of Particulate Combustion Products
for Various Combustion Conditions . . . . . . . .
. 114
Fume Particles Produced From the Combustion of
Montana Lignite in 40 (A) 20 (B) and 10 (C)
Percent Oxygen in Nitrogen. ............
. 115
5.4
Volume-Mean Diameter of Submicron Primary Particles
118
for Various Oxygen Partial Pressure . .......
5.5
Particle Size Distribution of Particulate
...............
Combustion Products . . . . ..
119
5.6
Variation of Composition of the Fume for Various
Combustion Conditions . . . . . . . . . . . . . . . 121
5.7
Sulfur Retention in
5.8
Comparison of S to Na and K in
5.9
Average Particle Temperature of Montana Lignite
...
(90/105 um) by Two Color Pyrometry. . ....
5.10
Buring Time
the Fume. .
for Montana Lignite
.
.
..
the Fume
...........
122
......
127
(90/105 um)
.
.
. 129
.
.
131
xii
5.11
Reciprocal of Burning Time Verses Ln(l + Xo2)
5.12
Dependence on Oxygen Partial Pressure of
Elemental and Ash Fraction in Fume. ........
.
.
.
133
135
5.13
Variation of Elemental Mass Fraction with Particle
. 137
Size for the Residual Fly Ash ...........
5.14
Concentration of As in Size Classified Residual
Fly Ash Particles.
...
. .
.........
.
139
Temperature Dependence of the Elemental
Vaporization Rates .
. . . . . . . . . . . . . .
. 142
5.15
5.16
5.17
Time Resolved Profile of Maximum Char Yield of
Char and Submicron Particle Constituents. . . .
143
Time Resolve Measurement of the Mean Primary
Particle Radius for the Submicron Particles .
6.1
..
.
.
.
145
Elemantal Concentration for Size Classified Samples
. ..........
by INAA . . . . . . . . . . . ...
151
6.2
Elemental Concentration for Size Classified Samples
152
...
by INAA ...................
6.3
Elemental Concentration for Primary Particle
by STEM . . . ................
.....
.
. .. .
6.4
6.5
153
Sectioned Particle Showing the Contribution from
the Outer Suface (A) and the Shaded Regions from
the Ion Sputtering Gun.
..........
.
. .
. .
. 160
Escape Depth of Auger Electron Verses Electron
........
Energy. ........
. . . ..
. 162
6.6
ESCA High Energy Scan of Carbon C is Line for the
. ..
. . . 166
.........
........
Fume. . . . . . .
6.7
ESCA High Resolution Scan of Magnesium Mg 2 p line
for the Fume .
...
. ..
. . ........
. .
. . . 168
6.8
X-Ray Diffraction Lines for Submicron Particles
6.9
ESCA High Resolution Scan for Iron Fe 2 p Line
. .....
for the Fume. . ...........
6.10
6.11
.
.
.
ESCA High Resolution Scan of Silicon Si 2 p line
for the Fume. ....
.......... . .
. .
....
. . .
ESCA High Resolution Scan of Sulfur S 2 p line
..
........
for the Fume. . ........
169
170
. 172
.
173
xiii
6.12
6.13
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
7.10
7.11
7.12
7.13
7.14
7.15
7.16
ESCA High Resolution
for the Fume. . .........
line
Scan of Sodium Na Is
. . .......
.
175
.
176
Equilibrium Vapor Pressure for Magnesium for
Various Fuel Equivalence Ratio. . .........
.
181
Schematic Diagram of Magnesium Vaporization and
Recondensation During Char Combustion . .....
.
183
Model Prediction for the Supersaturation and Cond.
. ............
densation of Magnesium.
191
for the Fume
Size Distribustion
Primary Particle
with the Self-Preserving Size Distribution. . ...
198
Experimental and Predicted Growth of the Primary
Submicron Particles . . . . . . . . . . . . . . . .
200
ESCA High Resolution Scan of Calcium Ca
for the Fume. . ...............
Predicted Coalescence Time for Solid
. ............
Magnesium Oxide Particles
2
p
Line
...
Crystaline
204
Variation of Mean Diameter of Primary Submicron
with Degree of Vaporization of the Ash .
Particle
206
Experimental and Predicted Growth of the Primary
. . . . . . . . . . . . . . . .
Submicron Particles
207
Profile
Char .
of Silicon at Various Distances from
. . . . . . . . . . . . . . . . . . . . .
Elemental Concentration for Primary Particle
by STEM Verses Inverse Diameter . ........
Tendency for Critical Supersaturation of Sodium
Sulfate as Combustion Products are Cooled in the
Presence of Residual and Submicron Particles .
.
.
214
.
216
.
223
Concentration of Various Elements in the Size
Fume Verses Inverse Diameter . .....
Classified
Concentration of Na and K in the residual
Verses Inverse Daimeter Squared . .......
Concentration of As and
Verses Inverse Diameter
225
Fly Ash
.
.
227
Sb in the Residual Fly Ash
. . . . . .
. .......
229
Area of Fume with Predictions
Superficial
. ....
Self-Preserving Size Distribution
from
. .
Fraction of Sodium Collected With the Fume for
.
. 232
xiv
7.17
7.18
Various Degrees of Vaporization of the Total Ash.
.
234
Concentration of Na in the Residual Fly Ash
Verses Inverse Diameter Squared . .......
.
.
235
Fraction of As and Sb Collected With the Fume for
Various Degrees of Vaporization of the Total Ash.
.
239
xv
LIST OF TABLES
Page
Table
1.1
1.2
1.3
Analytical Instruments Used for the PhysicoChemical Characterization of the Submicron
Particles . . . . . . . . . . . . . . . . .. .
Auger Spectroscopy of the Fume Before and
After Sputtering. . ............
for 100 Coals.
.
3.1
Calibration of the Ambient Andersen
3.2
Elemental Material Balance on Furnace and
Collection System . . . . . ..........
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
.
.
.
.
5
.
11
13
.
41
.
.
59
.
.
61
.
.
63
.........
Mean Composition
4.2
.
Ratio of High and Low Energy Auger Electron
Emission from the Submicron Particles and Homogeneous Bulk Standard . ..............
1.4
4.1
.
.
.
Impactor.
Analytical Instrumentation Used for PhysicoChemical Characterization of the Submicron
Particles . . . . . . . . . . . . . . . . . .
.
Calibration of the Magnification Settings for
........
. .. .
the Philips 200 TEM . ....
INAA Analysis of Opel Glass, NBS Standard
Number 91 . . . . . . . . . . . . . . . . .
65
.
.
.
70
Sources of Error and Correction Implimented in
Order to Obtain Quantitative X-Ray Analysis of
the Submicron Particles with the STEM . . . . .
.
.
72
.
Repetitive Analysis of Submiron Particles for
. ..
the Elements Silicon and Iron . .......
77
Theoretical and Experimentally Determined
Sensitivity Correction Factors for 100 Kv .
88
....
Comparison of the Chemical Analysis by the STEM
and Neutron Activation (INAA) . . . . . . . . . .
Elemental Retention During Analysis with the
High Intensity Electron Beam of the STEM. ....
Relative Sensitivity Factors Obtained
Experimentally (Ep = 5.0 Kev) . .......
Calibration of ESCA Spectrometer with Au,
.
Ag,
.
90
.
92
.
. 102
xvi
and Cu Metal Standards.
.
.....
..
. 107
.......
4.11
Comparison of the Peak Position from Standards
with the Reference Binding Energy Compiled by
PHI . . . . . . . . . . . . . . . . . . . . . . . . 108
5.1
Coal Composition for Major, Minor, and Trace
Elements. . . . . . . . . . . . . . . . . . . .
. . 124
5.2
Submicron Fume Composition .
.
. 125
6.1
Chemical Analysis of the Submicron Particles
Produced from the Combustion of Montana Lignite
(45/53 um) at Furnace Temperature of 1700 K in
.
...
Twenty Percent Oxygen in Nitrogen . ...
.
149
. . . . . . . . .
6.2
STEM Microprobe Analysis of Individual Submicron
Particles Produced from the Combustion of Montana
Lignite (150 um) at Furnace Temperature of 1700 K
. 155
in Seventy Five Percent Oxygen. . .........
6.3
Auger Spectroscopy of the Fume Produced from the
Combustion of Montana Lignite (45/53 um) at a
Furnace Temperature of 1700 K in Twenty Percent
Oxygen in Nitrogen. . ...............
. 157
Auger Spectroscopy of the Fume Before and After
.
Sputtering. . . . . . . . . . . . . . . . . ...
. 159
Ratio of High and Low Energy Auger Electron
Emission from the Submicron Particles and
....
Homogeneous Bulk Standard . ......
. 163
6.4
6.5
.
CHAPTER ONE -
SUMMARY
1.1 INTRODUCTION
Interest in the control of fine particles emitted
considerations
coal combustion is motivated by environmental
a
relatively
high
trace
toxic
of
concentration
and
lungs
the
since fine particles can penetrate deep into
contain
by
emitted
by
utility boilers (Markowski et. al., 1980) shows that most
of
species. Studies of size distribution of the ash
the ash is concentrated in a size range of 1
to
microns
20
total ash by weight, is present in the submicron
which shows up as a
distinct
spike
the
in
the
of
but that a small amount, of the order of one percent
range
size
size
particle
distribution. The mechanisms for the formation of the ash
the two size ranges are different.
coal (Padia, 1976).
are
The larger particles
matter
produced by the fusion and coalescence of mineral
The finer particles,
in
in
in
submicron
the
size range, are produced by the vaporization and condensation
composition of larger ash particles
have
of
Studies
of the mineral constituents (Flagan, 1979).
shown
have concentrations of volatile trace species
that
that
the
these
increase
1974)
and
that the surface layers of the particles are enriched in
the
with decreasing particle size (Davison
et.
al.,
coal combustion with the rate of deposition being
on
during
produced
particles
ash
the surface of the residual
condense
vapors
tent with a hypothesis that trace metal
consis-
is
This
same trace species (Linton et. al., 1977).
controlled
by mass transfer (Flagan and Friedlander, 1978).
In general, the toxic effects of the fine
tes (i.e.,
ing
submicron) are unknown because field data pertaintheir
to
particula-
are
characteristics
physico-chemical
fragmentary, and relevent toxicological experiments have
yet been conducted. The objectives
of
are
thesis
this
determine the physico-chemical characteristics of the
cron particles under
simulated
coal
pulverized
not
to
submi-
combustion
about
the
evolution of these particles with particular emphasis on
the
better
conditions and to obtain a
understanding
sequence of condensation for the vaporized inorganic elements
distribution
and the
the
of
and
volatile
species
trace
between the two size modes.
1.2 EXPERIMENTAL APPARATUS
A dilute cloud of coal particles is burned under well
controlled,
furnace.
conditions
reproducible
in
a
Small homogeneous samples of well
coal particles are fed through a narrow water
tube
and
are
injected
oxygen/nitrogen mixture.
axially
into
laminar
size
classified
cooled
a
flow
feeder
preheated
The coal particles heat up rapidly,
ignite and burn in a narrow stream within 6.2 x 10-3
ignite, and burn in a narrow stream within 6.2 x i0
meters
meters
of the center axis,
where radial dispersion of the
particles
is minimized by the stable laminar flow field.
All the combustion products, both gaseous and
solid,
are collected at a specified distance from the point of
injection in the
furnace
by
a
water
cooled
coal
probe
whose
position may be adjusted in order to vary residence time.
the entrance
to
the
probe,
the
rapidly quenched and diluted.
combustion
products
At
are
Particle deposition within the
probe is avoided by transpiring gas through a porous liner in
the probe.
The particles and gas from the
collection
probe
are passed through an Andersen Impactor for on-line aerodynamic size classification of the particles greater than 0.3 pm.
The submicron particles, because of their
through the impactor.
Several
utilized for these submicron
information sought.
small
collection
particles
size,
pass
procedures
were
depending
Collection on 0.2 pm
upon
fluoropore
the
filter
was utilized for weight and chemical analysis, deposition
on
transmission electron microscope grids placed in an electrostatic precipitator for microscopy studies,
on-line
size
classification using a mobility classifier (Knutson et.
al.,
1975) for measurement of the variation
in
or
composition
with
particle size.
Insight on the fundamental
formation of the
submicron
aerosol
processes
can
be
detailed physico-chemical characterization of
particles.
governing
obtained
the
the
from
submicron
A list of the analytical techniques employed
for
Table 1.1.
Particle
for
distibutions
size
the
submicron
an
automated
particles are obtained from TEM micrographs by
image
activation
analysis
and
(AA),
adsorption
atomic
(INAA),
neutron
instrumental
by
determined
submicron particles is
the
of
composition
elemental
bulk
The
analyzer.
in
presented
the characterization of submicron particles is
The elemental
liquid chromatography (Mizisir et. al., 1978).
with
composition of individual primary particles is obtained
a 100kv field emission scanning transmission electron microscope (STEM) fitted with an energy dispersive x-ray spectrome-
adsorption
characteristics
of
the
the
sensitivity
detector,
and
Al,
correction factors for the light elements (Mg,
had to be determined.
in
uncertainty
of
Because
ter (Vander Sande, 1979).
system
The unique feature of this
Si)
is
its twenty angstrom spatial resolution for elemental analysis
of single
particles.
high
the
under
However,
intensity
electron beam the volatile species such as sulfur and
Consequently,
sium were preferentially lost during analysis.
analysis by this method is
limited to
stable
the
electron beam.
elements:
signals
silicon, iron, calcium, and magnesium, the x-ray
which remain constant during exposure to the
Auger spectroscopy is used to obtain informasubmicron
angstroms.
Auger
composition
electron
and
is
of
which
the
is
a
order
mean
the
The depth of analysis is determined by
free path of the
elemental
of
intensity
high
tion about the elemental stratification within the
particles.
potas-
function
of
of
twenty
The instrument used was a PHI 590A scanning Auger
Table 1.1 - Analytical instrumentation used for
physicochemical characterization of
the submicron particles
APPLICATION
INSTRUMENT
Physical Characterization
TEM
Particle
morphology
Automated image analyzer
PSD
Mobility classifier
Discrete sized Samples
within the submicron
mode
Chemical Characterization
INAA, AA, and
liquid chromatography
Bulk analysis
STEM
Individual particle
analysis
AUGER spectroscopy
Surface analysis
ESCA, mossbauer, and
x-ray diffraction
Chemical state
microprobe with submicron spatial resolution of two tenths of
both
The 590A utilizes
a micron for analysis.
single
electron optics for image analysis and an argon ion
ing gun for depth profiling.
sputterfactors
sensitivity
Relative
found
compounds
had to be redetermined for the specific
pass
in
the fume since the peak to peak amplitude of the differential
In order to minimize
some of the elements.
state
chemical
signal was found to be sensitive to the
for
charging
severe
under the incident electron beam in Auger analysis, a conductive silver membrane filter was employed for
the
collection
of the submicron particles.
1.3 RESULTS
For carefully controlled combustion
conditions,
the
important variables which govern the physico-chemical characteristics of the submicron particles (i.e., size, amount, and
bulk
composition)
were
examined.
The
details
have
been
et.
al.,
presented elsewhere (Neville et. al., 1980; Haynes
1981; Quann, 1982).
detail on the
The results presented here will focus in
chemical
characteristics
of
the
submicron particles produced from the combustion
individual
Montana
of
lignite 45/53 pm in twenty percent oxygen in nitrogen.
1.3.1 Dependence of concentration on particle size
Data were obtained by two methods. A STEM was used to
analyze individual particles and INAA
was
used
size segregated particles collected and sized by
to
a
analyze
mobility
classifier.
and
Particles with diameters ranging between 80
400
angstroms were analyzed at random using the STEM. The elemental concentration of individual particles
diameter is plotted in Figure 1.1.
Ca, and Fe show no major trend
silicon exhibits a
marked
reciprocal
versus
The concentration of
with
concentration
in
increase
whereas
size
particle
with
Mg,
decreasing particle size.
Four discrete cuts of
tained from
the
mobility
the
with
classifier
The relative
of
concentration
particles
mean
diameters of 100, 200, 300, and 400 angstroms
by INAA.
analyzed
were
Na,
Ca,
Fe,
Mg,
These results
As,and Sb are reported in Figure 1.2.
ob-
particles
submicron
show
a
systematic increase in concentration with decreasing particle
The concen-
size for all elements other than Mg, Ca, and Fe.
trations of iron, calcium, and magnesium did
particle size in
either
the
STEM
analysis
not
vary
of
with
individual
particles or the INAA analysis of size classified samples.
1.3.2 Surface Enrichment
elements
The enrichment of the particle's surface in
selected
the
surface
was
determined
by:
(1)
comparing
concentration, measured by Auger spectroscopy, with the
concentration
measured
by
AA,
INAA,
chromatography; (2) determining the variation
tion measured by Auger spectroscopy
as
the
liquid
and
in
bulk
concentra-
surface
layers
100
STEM
80
A
- Mg x 0.2
[
- si x 10
0
- Fe x 0.5
O - ca
60
CD
z
40
El
z
-)El
0.000
0.003
0.006
0.009
INVERSE DIAMETER
0.012
(Ao)-1
Figure 1.1 - Elemental concentration for primary
particles by STEM versus inverse diameter
100
MOBILITY CLASSIFIER
A
- Na x 1.OE 1
+- - As x 4.8E 3
O
- Sb x 2.BE 4
[] - Ca
75
-
- Mg x 2.8E-1
- Fe
A
CD
F-4
50
L--
LU
CD
L3
25 H
0
0. 000
1
i
0.003
0.006
INVERSE DIAMETER
0.009
0.012
(A0 ) -i
Figure 1.2 - Concentration of various elements in the
size classified fume versus inverse diameter
comparing
(3)
were sputtered away; and
the
of
ratio
the
emitted
intensity of the high and low energy Auger electrons
from the sample to the ratio for a homogeneous bulk standard.
electron
The surface concentration measured by Auger
the
with
spectroscopy (column one) is compared in Table 1.2
bulk particle concentration by INAA (Na2 0, CaO, MgO, FeO), AA
(K20 , SiO2 ),
four).
(column
and liquid chromatography (SO3 )
A
All elements are reported as their oxides for convenience.
that
shows
comparison of the two analyses
sulfur,
sodium,
potassium, and silicon are highly concentrated in the surface
layers, while magnesium
and
are
iron
comparison of the two analysis,
calcium
is
the
From
depleted.
highly
neither
enriched nor depleted in the surface layers.
In order to obtain the concentration as a function of
depth from the surface,
the fume sample was
at
sputtered
a
milling rate that corresponds to a surface regression rate of
about twenty five angstroms per minute for a Te20 5 sputtering
analyses
Auger
standard (Martin, 1981).
original
the
of
surface and at several sputtered times are presented in Table
1.2.
The concentration of species in which
enriched,
such as that
decrease as the surface
of
layers
are
sputtered
species in which the surface is depleted, such
and iron, increase.
The relative
concentration
with successive sputtering remains constant
is
potassium,
and
sulfur,
sodium,
surface
the
while
away,
magnesium
as
of
calcium
suggesting
that
calcium is present near the surface as well as in the core of
Table 1.2
species
Na20
surface conc.
(wt%)
11.8
5.6
16.4
so03
Auger spectroscopy of the fume
before and after sputtering.
3 minute
(wt%)
bulk cone.
(INAA)*
(wt%)
6.0
4.6
5.3
3.7
1.8
7.9
6.4
3.5
1.6
sputter
1 minute
(wt%)
sputter
14.9
13.3
13.8
3.3
CaO
12.3
11.3
11.7
9.7
MgO
29.3
49.1
53.1
SiO
FeO
2
3.9
6.5
7.8
58.6
11.8
* supplemented with AA and
liquid chromatography
It should be noted that the sputtering rate is
the particle.
uniform over a horizontal cross-section and
that
Auger
the
emission for a sectioned particle will always include contributions from surface layers.
Another
measure
of
the
elemental
stratification
within the submicron particle is provided by a comparison
intensities from the KLL and LMM Auger emission for
elements in the fume
with
those
from
standard (Kane, 1974; King, 1979).
a
specific
homogeneous
bulk
The extent of attenuation
of the Auger electron moving through a solid will
dependent on its own mean free path.
The
be
escape
mean free path X ) of the low energy LMM Auger
100 ev)
highly
1000 ev).
the intensity of the high energy
electrons
the
to
low
obtained from a homogeneous bulk standard,
the distance of the
Therefore,
elements
a comparison
of
from
the
the
the
to
surface
intensity
ratio
increase
will
of
Auger
energy
relative
(-
energy
Consequently, the ratio
element,
emission for a subsurface
(or
depth
is considerably smaller than that of the high
KLL Auger electrons (
of
as
increases.
ratios
for
the
elements in the submicron particles with those from homogeneous bulk standard provides a measure of the
distribution
sulfur, magnesium, and silicon as a function
Table 1.3).
greater than,
of
the ratio from
the
homogeneous
the
slightly
sample,
bulk
which suggests that sulfur is in the outer surface
the submicron particle. For magnesium,
(see
depth
For sulfur, the ratio is close to, but
of
layer
intensity
of
ratio
13
Table 1.3
element
Ratio of high and low energy Auger
electron emission from the submicron
particles and homogeneous bulk standards
electron
energy
A AO
fume ratio
standard ratio
120
1.5
1834
76
10.4
1606
Mg
35
Mg
1190
20
14
greatly exceeds that
ratio
suggesting that magnesium is
The extent of attenuation of
silicon indicates that this element lies
surface layer enriched in elements such
the
between
as
for
emission
energy
low
the
particle.
the
of
core
in the
standard
homogeneous
the
from
outer
the
and
sulfur
particle's core enriched in elements such as magnesium.
of
The above data are only semi-quantitative because
complexities introduced by the different degrees of
attenua-
different
elements
by
tion for the Auger electrons emitted
and because of the averaging of the emissions from
chords in a spherical
results
The
particle.
complicated by the attenuation of the Auger
are
in
by
The
an
layer
intermediate
mean
layer. This apparent anomaly is explained by the larger
free path
of
silicon
KLL
compared to that of sulfur
outer
the
sample will attenuate the sulfur signal to a
angstroms)
The
angstroms).
(4.3
carbon contamination present on
(25
electron
auger
a
results
surface
the
whereas those in Table 1.3 suggest it is in
further
electrons
carbon film that forms on the sample's surface.
in Table 1.2 suggest that silicon is
different
surface
greater
adsorbed
of
the
extent
than the attenuation of the silicon signal by surface layers.
Therefore, Auger analysis yields an apparently
surface
concentration.
However,
by
the
use
high
silicon
of
several
measurement techniques such as those employed in this
a coherent story can be developed.
study,
1.4 DISCUSSION
The elemental
record of the condensation
particle provides a
the various inorganic vapors.
The
believed to be responsible
condensed
by
phase
for
in
the
Fe)
are
and
Ca,
Mg,
sequence
present
elements
core of the submicron particles (ie.,
initial
the
for the formation of
homogeneous
submicron
the
within
stratification
those
while
nucleation,
the
elements such as Na and K which form the outer layers of
at
submicron particles are believed to condense
temperatures.
lower
much
formation
The phenomena involving the initial
of the submicron particles should be evident by examining the
which
vaporization and condensation of magnesium,
resides in
the core of
the
submicron
particles,
not
only
but
also
accounts for over sixty percent of the submicron particle
by
weight.
1.4.1 Particle Nucleation
compounds
From the rate of vaporization of different
it
is possible to infer their partial pressure at the surface
magnesium,
the
vapor
was
found
to exceed the equilibrium vapor pressure for magnesium
oxide
of the
burning
char
particle.
For
pressure calculated from the rate of vaporization,
by several orders of magnitude (Quann,
of vaporization is
explained by
1982).
chemical
reduction
oxide to the more volatile metal vapor in the
cing atmosphere within a burning char
The high
of
rate
the
locally
redu-
The
metal
particle.
thus providing the neccessary driving force
homogeneous
for
the
from
away
temperature
in
decrease
The
nucleation.
supersaturated,
be
will
Upon reoxidation, the oxide vapors
environment.
oxidizing
surrounding
the
char particle into
burning
the
vapors will reoxidize as they diffuse away from
particle can only accentuate this tendency.
A steady
phenomena
examine in detail the
nucleation of
oxide.
magnesium
involving
to
homogeneous
the
the
estimates
model
The
order
in
developed
was
model
state
temperature, oxygen partial pressure, magnesium metal partial
pressure, and
partial
oxide
magnesium
from the char particle.
distance
of
function
governing differential equations as a
the
from
pressure
The temperature at various distances
from the char particle was derived from Fourier's law of heat
conduction
1
r2
(
r
2
k
rT
= 0
Eq. 1.1
dr
dr
where the particle temperature was determined
pyrometry
(Timothy
et.
al.,
1980).
from
pressure at various distances
the
The
char
by
color
two
oxygen
partial
particle
was
derived from Fick's first law,
1 (
2 dr
r2 D
dn2) = 0
Eq. 1.2
AB dr
where the oxygen partial pressure at the surface of the
particle was calculated from char oxidation kinetics
char
(Field,
1967).
distan-
The partial pressure of magnesium at various
differential
the
ces from the cnar particle was governed by
equation,
(d
r2 dr
r
2
D
dnMg
Eq. 1.3
-Kngn
Mg 02
AB dr
char particle was calculated from
data (Quann, 1982).
The reaction
The modeling
of
the
homogeneous
-KnMgno2
by
vapor
for the consumption of magnesium metal
gas
accounts
,
oxidation.
oxidation
phase
coefficient
magnesium will entail establishing the rate
the
vaporization
experimental
term,
of
surface
where the magnesium partial pressure at the
of
for
the governing bimolecular reaction
MgO(v) + 0
Mg(v) + 02 ---
as proposed by
Markstein
(1963).
The
Eq. 1.4
pressure,
low
low
temperature studies concluded that the bimolecular
homogene-
ous reaction will account for initial formation of
magnesium
homogeneously
oxide vapors which, upon supersaturation, will
nucleate to form the initial particles.
high temperature environment
of
For the
combustion,
atmospheric
estimated
the
rate coefficient from collision theory was used to model
bimolecular oxidation
of
magnesium
metal
vapors
the
(Laider,
1965).
The partial pressure of magnesium
oxide
at
various
distances from the char surface was governed by the differen-
18
tial
Equation,
1 (d
r2 dr
r 2 DAB dnMg)
= Rrxn + Rnucl + Rhet
Eq. 1.6
AB dr/
= K nM
Rrxn
no
Rnucl = I
3
av
= F'i(d) -a
Rhet
where the reaction terms on the right hand side
1.6 account for the continual production of
equation
of
oxide
magnesium
consump-
the
vapors by oxidation of the metal vapors, Rrxn,
tion of magnesium oxide by homogeneous nucleation, Rnucl, and
vapors
the consumption of magnesium oxide
condensation on the stable nuclei of
heterogeneous
by
magnesium
oxide.
rate of homogeneous nucleation of a spherical liquid
for single component
droplet
from
calculated
was
I,
vapors,
The
the
classical theory of nucleation of Volmer and Weber and Becker
and Doering (Hirth et. al., 1963) using
the
predicted
mag-
nesium oxide partial pressure and equilibrium vapor pressure.
The size of the stable nuclei forming by homogeneous
nuclea-
tion is specified by the critical cluster size, r*, which
a cluster of size such that the addition of one monomer
lead to the formation of
heterogeneous
stable
a
of
condensation
the
cluster.
The
condensable
the kinetic theory
>> 10)
of
which is derived
gases
(Friedlander,
from
1977)
will
rate
of
vapors
is
determined from the rate of deposition per particle for
molecular regime (Kn
is
the
and
free
from
the
number density, N , for the stable nuclei formed by homogene-
monodisperse
particle
size distribution, a simple solution to the kinetic
equation
ous nucleation.
In assuming a nearly
rate
for brownian coagulation can be used to approximate the
of
of change in the number density and average particle size
the stable nuclei as a result of coagulation.
model's
The
magnesium
for
predictions
corresponding
combustion
the
to
Montana lignite in twenty
percent
presented in Figure 1.3.
The
of
condensation
the
condition
for
nitrogen
are
in
oxygen
supersaturation
magnesim
of
oxide is predicted to occur within the thermal boundary layer
the
of the char particle due to the relatively high rate for
homogeneous oxidation of magnesium. The model predicts a very
sharp
rise
increases.
in
This
the
nucleation
the
uncertainties in the assumed values for
and
pressures at the char particle's surface.
rate
oxidation
1965),
constant of Mg(v) and MgO surface tension (Smithells,
and the estimate of the magnesium oxide
to
insensitive
relatively
is
position
supersaturation
as
rate
partial
oxygen
Since the onset of
the
in
changes
nucleation is very sensitive to temperature,
values assumed for the above parameters will result in only a
nuclea-
slight displacement of the position for the onset of
tion because of the steepness
near the particle surface.
the
of
temperature
The duration
of
wave is attributed to the continual addition
homogeneous gas phase oxidation
of
the
of
magnesium
Since the transient time for establishing
the
gradient
nucleation
monomer
metal
steady
by
vapor.
state
20
18
-3
En
-,-
E
-I
4-J
-4
16
-5
14
-6
12W
U-)
CL
OC
Er
a.r
EL
JI
z
I-
-7
10 <
-8
8
LJ
zo
ID
-9
6
1
5
13
9
R/Rp
Figure 1.3 - Model prediction for the supersaturation
of magnesium oxide
17
gas and temperature profiles arround
extremely short and the fact that
the
char
particle
times
characteristic
the
is
for oxidation and nucleation are over two orders of magnitude
shorter than the time for
char
burnout,
magnesium
assumed to homogeneously nucleate very early
on
can
during
be
the
combustion process, where the temperature is relatively high.
1.4.2 PARTICLE GROWTH
The classical nucleation theory predicts
the
forma-
tion of a large number of particles (a nucleation rate of 1.0
x 1017 nuclei/cc sec) of extremely
cles of SiO
2
size.
density, the
initially high particle number
grow rapidly.
small
1976),
1973), and soot (Wagner, 1979) have been
governed by coagulation theory.
In
inoganic
to
shown
present
the
partiHomer,
be
study
well
the
parti-
submicron
theory was shown to apply to the inorganic
an
will
and
(Graham
Pb
to
particles
The studies of the growth of
(Ulrich et. al.,
Due
cles produced from simulated pulverized coal combustion where
the rate of coagulation was governed by
quency of the particles due to
random
that particle coalescence was assumed
the
particle
motion
Brownian
to
contact.
at
diameter
the predictions from coagulation theory in Figure
with the self preserving distribution function
and
good
The
various
distances from the point of coal injection are compared
particle size distribution is found to be in
fre-
instantane-
occur
ously to form single spherical particles upon
experimentally measured mean
collision
1.4.
with
The
agreement
(Friedlander,
300
- PRIMARY PARTICLE
n
AGGLOMERATED PARTICLES
PROJECTED AREA DIAMETER
-
COAGULATION THEORY
0
S
200
CHAR
COMBUSTION
rr
-ZONE
I
<JI
7
O
II
100
II
I
0
0
5
10
15
20
25
DISTANCE FROM COAL INJECTION (cm)
Figure 1.4 - Experimental and predicted time dependence
for the mean diameter of the primary
submicron particles
For
1977).
early
periods
growth
theory
coagulation
the
provides an excellent representation of the change in primary
particle size with time.
curve,
it
From extrapolation
is apparent that the
formation
of
particles is occurring at the initation of
The continuous
addition
of
new
of
the
the
char
material
growth
submicron
combustion.
due
to
further
vaporization during char combustion is accomidated by adjustment in the volume fraction.
in Figure 1.4 the
primary
In examining the
particle
growth
growth
ceases
curve
shortly
after char burnout as the gas temperature drops back down
the furnace gas temperature of 1700 K
diameter
approaches
160
and
angstroms.
as
This
the
particle
apparent
between theory and experimental observation can be
by a decrease in coalescence rate.
The time for
governed by solid-state diffusion for
two
to
break
explained
coalescence,
particles
having
the same diameter may be estimated from the relationship
-__-
t
5/2
AL
Lo
( 2 K T) r6/
(20 a D*)
where AL/Lo is the fractional
Eq. 1.7
5
shrinkage
of
the
coalescing
particles, a3 is atomic volume of diffusing vacancy,
was selected to derive
Figure 1.5.
estimates
The decrease in
of
solid
magnesium
oxide
coalescence
temperature
D*
A
is the self diffusion coefficient (Kingery, 1965).
phase diffusion coefficient for crystalline
and
on
times
completion
in
of
combustion and the increase in particle diameter both contribute to the decrease of the coalescence
rate.
The
primary
0.0
oC)
G1)
IF)
-1.0
LLJ
M:
H-
LUJ
LLJ
C-)
w
Co
L3
-2.0
CD
G
L).0
CD
-J
-3.0
-4.0
1400
1600
1800
TEMPERATURE
2000
K
Figure 1.5 - Predicted coalecsence time for crystaline
magnesium oxide particles
25
particle diameter is
agglomerated
of
formation
particles
particles where the discrete size of the colliding
is
The
conserved.
projected
agglomerated
the
of
areas
particles at a sampling position of 20 cm from the
injection of the coal is
in
agreement
good
very
particles ceases at relatively high temperatures
point
of
with
the
of
these
(i.e.
1700
The fact that the coalescence
coagulation theory.
point,
Beyond this
when both these factors become important.
further coagulation leads to the
angstroms
160
to
120
in the range of
K) is consistent with the hypothesis that the species Na,
are
products
being
cooled
since
combustion
the
and K condense at much lower temperatures as
would
impurities
these
S,
undoubtedly increase the coalescence rate at 1700 K (Kingery,
1965).
1.4.3 CONDENSATION
In a manner analogous to magnesium,
vaporization
the
the
chemical
more
volatile
of calcium, iron, and, silicon is augmented by
reduction of the
refractory
oxides
to
the
suboxide vapor (Sio) and metal vapor (Ca and Fe)
the
the locally reducing environment within
As
these
vapors
diffuse
surrounding the char
into
particle,
the
they
char
oxidizing
will
sequence of condensation for the various
to the formation of stable nuclei, one
environment
refractory
pressures.
might
of
particle.
reoxidize.
will be highly dependent on the rate coefficients
oxidation and their relative partial
because
for
The
species
their
Subsequent
anticipate
an
26
oxidation
increase in the condensation rate by heterogeneous
oxygen
of the metal or suboxide vapors by collision with the
The contribution
of
this
is
formation
MgO
to
mechanism
particles.
growing
the
atom layer formed on the surface of
estimated to be two orders of magnitude smaller than that due
to
homogeneous
even
oxidation,
Silicon
calcium,
condenses
is
slow.
magnesium,
to
relative
and iron because the silicon
and its rate of oxidation
1959).
1969; Glassman,
late
low
is
concentration
monoxide
silicon
The
at
surface
collision efficiency of magnesium atoms with the
high temperatures (Markstein,
low
the
neglecting
when
vapor pressure at the surface is reduced to a level below the
equilibrium value because of a diffusional resistance. Such a
diffusional limitation is not
and
magnesium
for
important
calcium because magnesium and calcium are finely dispersed
an elemental form in
to silicon which is
(Quann,
1982).
SiO to SiO 2 is
present in
discrete
contrast
in
the original coal particle
inclusions
mineral
The rate of oxidation of a monoxide
expected to be much slower than
oxidation of the metal to the
monoxide;
this
based on an analogy with aluminum for which it
the oxidation of aluminum by 02
to
magnitude faster than that of AlO to
A1O
A10 2
that
two
is
as
of
the
orders
iron
surface of the burning char particle as determined
is
that
found
(Fontijn,
The proximity of the saturation of calcium and
such
statement
is
in
of
1976).
to
the
from
the
steady state model suggests that these species like magnesium
condense
However,
after
shortly
onset
the
char
of
combustion.
temperature
drops
back
upon
temperature
furnace
the
to
gas
local
silicon is shown to saturate only as the
completion of the combustion of the char particles.
mean free path of the gas molecules,
the rate of
the
than
smaller
Since the submicron particles are
heterogene-
ous condensation of the vapor molecules on the particles will
the
be governed by
with
frequency
which
the
vapors collide with the surface of
the
condensable
the
In
particles.
free molecular regime, the rate of heterogeneous condensation
per particle, F i
(d), as calculated from the
of gases (Friedlander,
is given by
1977)
F.(d) =
d 2
(P
-
mk
(2
Peq, i
T)1 / 2
)
Eq. 1.8
Consequently, the concentration dependence of
cron particles is predicted to be inversely
From
STEM
the
analysis
species
those
which condense heterogeneously on the surface of
particle diameter.
theory
kinetic
submi-
the
proportional
to
individual
of
submicron particles, the concentration dependence for silicon
is best correlated with inverse diameter,
the concentration of magnesium,
dependence on particle size.
calcium,
Figure
and
while
1.1,
iron
show
no
The interparticle concentration
of those species which condense relatively early
during
growth process should be relatively uniform since the
the
evolu-
tion of the submicron particles will entail the coalesence of
28
a large number (i.e.,
However,
size
in the case of silicon, the predicted particle
the
of
ously after the coalescence
The late condensation of
other refractory oxides is
heterogene-
condenses
dependence is preserved since silicon
ceased.
particles.
1.0 x 104) of various sized
primary
particles
has
to
the
relative
silicon
spec-
Auger
further supported by
troscopy which has shown silicon to be present near the outer
calcium,
surface of the submicron particles while magnesium,
up
and iron are shown to make
the
submicron
the
of
core
particles.
of calcium, magnesium,
iron,
much
species
dependent on the area of the residual and
be
will
homogeneously
highly
submicron
primary
condensation
particles availible for heterogeneous
vapor
higher
Whether these
pressures than the refractory oxides.
or
they
and silicon either because
are present in dilute quantities or have
condense heterogeneously
oxides
refractory
dense at much lower temperatures than the
con-
to
assumed
The volatile and trace species are
and
the
rate at which the combustion products are cooled.
The method employed for determining whether homogeneous nucleation of the volatile and trace species can occur in
the presence of the pre-existing particles will be determined
for
by establishing whether or not the criteria
nucleation has been exceeded (i.e.,
(Hirth et. al.,
1963).
I
=
1
homogeneous
nuclei/cc
sec)
As the combustion products are cooled
the volatile and trace species will eventually
saturate.
As
the partial pressure of species
the gases are cooled further,
i will be determined by calculating the amount of the condenon
the
pre-existing particles over the short time increment at
each
sable
temperature.
heterogeneously
condenses
that
vapor
The amount of condensable vapor species remain-
next
starting amount for the
condensation,
increment
time
of
the amount
condensing
determined from the flux integral /n(v)
used
the
at
the
as
next
heterogeneous
diffusion-limited
For
temperture.
be
will
ing in the gas phase after each step
volatiles
will
be
F i dv
For
the
.
residual ash particle mode which obeys continuum behavior (Kn
function,
distribution
size
<< 1), the particle
approximated numerically from the experimental data
n(v),
is
and
the
diffusion flux, Fi(d), which is given by
Fi(d)
dp D (Pi
k T
= 2
Kn >> 10),
by
represented
n(v) is
function,
* (7),
Peq,i
)
the
size
Eq. 1.9
free molecular
mode which exhibits
For the submicron particle
behavior (i.e.,
-
self-preserving
size
distribution
and the molecular diffusion flux,
obtained from equation 1.8.
condensable vapors
to
the
Kelvin effect (Friedlander,
In
establishing
submicron
1977)
function
distribution
F'i(d),
the
particles,
and the
reduction
area of the primary particles due to agglomeration
flux
is
of
both
the
in
the
(Medalin,
1967) will be taken into account.
The model's predictions for the extent of supersatur-
30
the presence of the residual and submicron
ation of sodium in
1.6.
For
sulfate is
sodium
lignite,
chlorine
low
a
as
Montana
thermodynamically
favored
such
coal
content
the
Figure
in
particles for various cooling rates are presented
sodium
vapor species over the temperature range at which the
condense
to
is anticipated
state
Furthermore, chemical
sodium
is
particles.
present
as
(McNellan
analysis
the
in
sulfate
sodium
that
shows
ESCA
by
1981).
al.,
et.
submicron
The equilibrium vapor pressure for sodium sulfate
is calculated assuming a pure condensed phase.
The
particle
size distribution for the residual and submicron particles is
the
based on the fly ash distribution obtained from
combus-
tion of Montana lignite 45/53 um at a furnace temperature
of
rate
at
The
nitrogen.
1700 K in twenty percent oxygen in
which the combustion products are cooled as they approach the
water cooled collection probe was estimated from the temperature profiles in the furnace to range between 2,000 K/sec
7,000
Assuming
K/sec.
all
sodium
the
about 1400 K.
during
vaporizes
combustion, the sodium vapors are predicted
saturate
to
heterogeneously.
all
the
sodium
to
predicted
is
Only at much higher cooling
condense
rates,
K/sec, is the partial pressure of sodium predicted to
the crtical supersaturation partial
the onset of homogeneous
pressure
nucleation.
cooling rate of 40,000 K/sec,
still
at
As the gases are cooled below 1400 K at a rate
of 5,000 K/sec,
sodium is
to
over
exceed
for
necessary
However,
ninety
40,000
at
even
percent
of
a
the
predicted to condensed heterogeneously before
I
1 U')
4-,
(0
-4
-
I
I
Cooling rate
5,000 K/sec
Na2 SO4 ,
2 - 20, 000 K/sec
3 - 40, 000 K/sec
LLJ
Lf)
c-f)
PNa ,SO,
LL
C
-J
CL0
-6
-8 F-
CD
.
cI
~_4
I
1000
I
1200
I
I
1600
1400
TEMPERATURE
K
Figure 1.6 - Tendency for critical super saturation of Na2SO4 as combustion
products are cooled in the presence of residual and submicron particles
32
the criteria for homogeneous nucleation is
assumed
sodium, the other volatile and trace species are
as
heterogeneously
condense
the
Like
satisfied.
are
products
combustion
to
cooled.
concentration
For diffusion-limited condensation the
dependence of those species which condense heterogeneously on
the submicron particles
have
proportional to particle size.
Sb, Fe,
Ca,
been
The concentration of Na,
As,
of
size
and Mg obtained from the INAA
classified samples of the
inversely
be
to
shown
submicron
discrete
versus reciprocal diameter in Figure
observation
The
1.2.
plotted
are
particles
is
that the concentration dependence of As, Sb, and Na
predic-
model's
correlated by d-1 relationship supports the
best
tion that the volatile and trace species condense heterogeneously as the gases are being cooled.
determination
The
submi-
the stratification of the various elements within the
form
species
outer
the
most
of
surface
volatile
the
that
cron particle by Auger spectroscopy show
of
the
submicron
particle.
the
Under the combustion conditions examined,
tile species had been previously assumed to totally
during combustion due to
their
high
the
discrepency
between
the
amount
of
two
size
volatile
appearing in the submicron particles with that in the
coal (i.e.,
-
forty
percent)
cannot
be
vaporize
However,
volatility.
based on the respective surface area of the
vola-
explained
modes
species
parent
by
the
33
amount of material condensing on the residual fly ash
parti-
cles since the submicron particles account
ninety
of
percent
free
the
for
available
area
for
heterogeneous
the
in
The concentration of sodium
condensation.
over
residual
fly ash from the combustion of Montana lignite in 45/53 um at
oxygen
a furnace temperature of 1700 K in twenty percent
shown in Figure 1.7 to be inversely proportional to
sodium in the residual fly ash is
present
of
the
a
result
of
fact
that
the
as
the
However,
condensation.
heterogeneous
diameter
substantial fraction
which suggests that a
squared,
is
2
intercept for the sodium concentration versus 1/d
posi-
is
tive also suggests that not all the sodium volatilized during
combustion.
From
the
about
trend,
concentration
In order
percent of the sodium is estimated to vaporize.
explain the distribution of the vaporized sodium,
primary
tition between the
residual fly ash particles
sodium
was
vapors
vaporization,
varied,
4.
of
spheres
a
as
examined
for
sink
by
various
diffusion-limited
(i.e.,
condensation,
the
of
fume
identity).
distribution
of
volatile species between the two modes can be estimated
the flux integral for the residual fly ash particles and
primary spheres of the submicron
stated,
particles.
the Kelvin effect and the reduction in
As
the
ash
is
remain
will
the
of
the
condensing
vaporized
ash
self-preserving
and
degrees
As the fraction of total ash
the total area of the residual fly
00.6
the
to
compe-
the
fume
for
relatively constant while the total area
vary
the
seventy
will
For
the
from
the
previously
area
of
8
0
z
CD
rl
H-
LU
:
2
0
2
0
4
INVERSE DIAMETER SQUARE
Figure 1.7
6
(um)-2
- Concentration of sodium in the residual fly ash
versus inverse diameter squared
8
35
into
taken
are
agglomeration
the primary particles due to
account in the flux integral for the submicron particles.
experimental
and
A comparison between the predicted
degrees
fraction of sodium appearing in the fume for various
of ash vaporization is
of sodium vaporized
trends for each condition.
sodium
The fraction of vaporized
The increase in
estimated
rate
which is the lower bound for the cooling
the fraction of sodium
twenty
,
is
percent
can be attributed to the increase in the area of the fume
40.6.
in
appear-
ing in the fume as the amount of total ash vaporized,
increased from less than one percent to over
to
K/sec
2,000
of
rate
be fairly well predicted by a cooling
shown
is
fume)
appearing in the submicron particles (ie.,
the furnace.
concentration
the
from
estimated
was
amount
the
shown in Figure 1.8 where
The variation in the prediction of sodium appearing
by
in
the fume with cooling rate can be attributed to the fact that
at lower cooling rates the concentration of condensable vapor
is shown to remain
fairly
pressure (see Figure 1.7).
rates,
Consequently,
the
that
the
potential
particles. However,
Kelvin
for
as the
effect
condensation
cooling
supersaturation of the condesable
rate
vapor
vapor
saturation
low
for
(Pi - Peq,i
the potential for condensation,
small enough
reduce
the
to
close
will
)
cooling
, will
preferentially
submicron
to
the
is
increased
the
and
the
increases
Kelvin effect becomes less pronounced due to the increase
the potential for condensation.
be
in
1.0
LLJ
LL
0.8 -
-N
Co
Z
C2
UJ
NJ
0.6
--/i
LL.
0.4
Li
F-
CD)
CD
Cooling rate
0.2
-/ 2
1 - 7,000 K/sec
U-
2 - 2,000 K/sec
0.0 1
0. )0
I
0.05
I
0.10
I
I
0.15
0.20
FRACTION OF TOTAL ASH VAPORIZED
Figure 1.8 - Fraction of sodium collected with the fume for various
degrees of vaporization of the total ash
1.5 CONCLUSION
The submicron
inorganic
coal combustion have a stratified
the
core of the particles in
Montana
a
on
study
inner
The
concentration.
present
during
produced
particles
lignite consists mainly of the oxides of magnesium,
iron, and
calcium produced by the vaporization of the metals
generated
in
the
particle,
locally
reducing
The nuclei coagulate at
of
the
which
is
In
the
surface
nucleation of the resulting oxides near the
burning particle.
homogeneous
the
oxidation of the metal vapors and
char
burning
a
within
regime
a
rate
adequately predicted by classical coagulation theory.
early stages of coagulation the particles coalesce,
the
the temperature in the vicinity of
particles
but,
as
decreases
subsequent to the completion of combustion of the char and as
the primary particle diameter increases,
the coalescence rate
aggregates
decreases and the agglomerates produced appear as
of spheres. Silicon monoxide generated in the
oxidized
to
condenses
on
and magnesium oxides.
As
cing atmosphere of the burning char particle is
silica at a relatively slow rate and therefore
the core produced by iron, calcium,
the combustion products
are
cooled
volatile
compounds
metals such as Na, As, and Sb condense to produce
layer of the inorganic particles.
redu-
locally
The
volatile species between the two size modes is
outer
the
distribution
of
sensitive
the rate at which the combustion products are cooled and
fraction of the total ash vaporized.
of
the
to
the
The hypotheses relating
38
to the condensation sequence are supported by measurements on
the stratification of the elements within the particle and by
the observation that the concentration of the
in the outer layer is proportional to the reciprocal
particle diameter.
found
species
of
the
39
CHAPTER TWO - BACKGROUND
2.1 INTRODUCTION
Coal
is
atmospheric
of
source
major
the
still
operation
particulates, inspite of improvements in combustor
and the implementation
electrostatic
of
precipitators
and
Interest in
the
scrubbers for particulate emission control.
control of fine particles emitted by pulverized coal
combus-
tion is motivated by environmental considerations
since
fine particles can penitrate deep into the lungs,
have
atmospheric residence times,
relatively
high
are
and
concentrations
of
believed
to
ash
Studies of the size distribution of the fly
contain
by
emitted
al.,
et.
McCain
utility boilers (Markowski, et. al., 1980;
long
species.
trace
toxic
the
1975) show that most of the fly ash is concentrated in a size
range of 1 to 20 microns but that
a
small
order of one to two percent of the total
present in the submicron size
range
ash
which
the
of
amount,
is
weight
by
up
shows
as
distinct spike in the particle size distribution. Studies
a
of
shown
that
these have concentrations of the volatile trace species
that
et.
al.,
the composition of the larger ash particles have
increase with decreasing
1974) and that
the
particle
surface
layers
size
of
(Davison
the
particles
are
40
enriched in the same trace species (Linton et. al., 1977).
physical
Fly ash is formed from the high temperature
been
extraneous.
original
plant
material
or
association
the coal formation period.
Adventitious
during
water
ion-exchange processes with circulating ground
of
result
a
as
the
to
bonded
chemically
carbonaceous matter of the coal because of their
the
or
those
encompasses
matter
The inherent mineral
coal
of
adventitious,
inherent,
as
categorized
inorganic elements intimately or
with
present
matter
The ash forming constituents
with the parent coal.
have
mineral
the
and chemical transformation of
matter
mineral
is
present as finely distributed micron sized mineral crystallyBecause of the association
tes in the carbonaceous material.
of
mineral
adventitious
material, the mineral inclusions can also be
inherent.
Extraneous
mineral
matter
is
catigorized
associaand
minor,
The major,
trace element content in coals are listed in Table 2.1 as
average of 100
US
coals.
major
The
earth minerals, the most abundent
calcite, and pyrite.
being
in
occur
common
kaolinite,
quarts,
and
With respect to their vaporization
can
condensation characteristics, the inorganic elements
catigorized as refractory
or
volatile
depending
volatility and/or abundence in the parent coal.
an
which
consitituents,
generally
include; Si, Al, Fe, Ca, and Mg
as
from
dirt
free
appreciable
adjacent sedimentary bands without any
tion with the carbonaceous material.
carbonaceous
the
with
matter
The
on
be
their
refrac-
Table 2.1 - Mean composition for 100 coals
CONSTITUENTS
MEAN
STANDARD
DEVIATION
MIN
MAX
Refractory
Al
1.29
0.45
0.43
3.04
Ca
0.77
0.55
0.05
2.67
Fe
1.92
0.79
0.34
4.32
Mg
0.05
0.04
0.01
0.25
Si
2.49
0.80
0.58
6.09
0.50
93.00
Volatile
17.7
As
14.02 PPM
Cd
2.52 PPM
7.60
0.10
65.00
Cr
13.75 PPM
7.26
4.00
54.00
Hg
0.20 PPM
0.20
0.02
1.60
Ni
21.07 PPM
12.35
3.00
80.00
Pb
34.78 PPM
43.69
4.00
218.00
Sb
1.26 PPM
1.32
0.20
8.90
Zn
27.29 PPM
694.23
6.00
5350.00
55.23
80.14
Ultimate
Analysis
C
70.28 %
3.87
H
4.95 %
0.31
4.033
5.79
N
1.30 %
0.22
0.78
5.79
O
8.68 %
2.44
4.15
16.03
11.44 %
2.89
2.20
25.80
Ash
42
while
oxides
major
tory species encompass the non-volatile
the volatile species encompass the volatile and trace species
carcinogens,
(i.e.,
toxic
potentially
are
which
some of
mutagens, and heavy metals).
At high combustion temperatures, mineral matter
will
transformations.
The
undergo both
and
physical
scenario depicting
the
chemical
mechanisms
formation
ash
fly
for
during pulverized coal combustion is presented in Figure 2.1.
sions averaging around one
to
two
small
inclu-
(Padia,
1976).
as
The inherent mineral matter is represented
microns
Initially, as the coal particle is heated, volatile hydrocarpyroly-
bons originally present in the coal or formed during
sis are vaporized. A residual char particle
is
proceeds to burn by heterogeneous oxidation of
surface.
which
left,
external
the
For non-swelling coals, the char surface receeds as
combustion proceeds, exposing more and more inclusions on the
surface. Because ash has a high surface tension, it does
inclu-
wet the exposed carbon surface, instead these surface
sions will coalesce to form spherical ash droplets
1969). Near the tail end of
will fracture,
each
combustion,
leading
coalesced ash particle (Padia,
to
the
1976).
the
(Ramsden,
particle
char
of
single
mineral
matter
formation
The
not
residue of the burned-out suspended coal particles
typically
have mass mean diameters of 10 to 20 microns and are commonly
refered to as residual fly ash.
released in this way.
However, not all the ash
is
During combustion, the char combustion
COAGULATION
NUCLEATION
0
o o.
Inorganic
Vapors
mineral
Inclusion
0 0
00 0
VAPORIZATION
ash
particle
/
HETEROGENEOUS.
CONDENSATION
Inorganic Submicron
Particle
0
Coal
Particle
Char
Particle
CHAR
BURNOUT
0 0
Residual Fly Ash
I - 20 Ia
-
DECREASING TEMPERATURE -
Figure 2.1 - Schematic diagram of fly ash formation during pulverized coal combustion
temperatures (1700 - 2200 K) are high enough that some of the
As the
mineral matter will volatilize.
products
combustion
are cooled, the inorganic vapors must condense either heteroparticles
geneously on the free surface of residual fly
ash
or by homogeneous nucleation if the inorganic
vapors
supersaturated thereby leading to
phase
condensed
extremely
of
formation
the
nucleation, the large number of stable nuclei should
1979).
Upon
rapidly
formation
grow by coagulation, eventually leading to the
Desrosier, et. al.,
The presence of submicron aerosol
where the size distribution
of
(1975),
fly
ash
to
found
was
It should be noted that
be
1980)
Results from a more recent study (Marcowski,
are presented in Figure 2.2.
partiin
the
lumped
and
of
ten
Studies of the composition of the residual fly
ash
cles greater than ten microns which
preseperator of the cascade impactor
were
have
collected
been
subsequently classified as having particle diameters
microns.
of
1979;
(Flagan,
um
was first substantiated in the field study of McCain
bimodal.
new
a
of
particles.
small
particles with mass mean diameter of 0.1
become
particles
have
shown
that
these
have
concentrations
of
volatile trace species that increase with decreasing particle
size (Davison, et. al.,
1974) and that the surface layers
(Linton,
et.
This is consistent with the hypothesis that
the
the particles are enriched in the same species
al., 1977).
of
trace metal vapors condense on the surface
of
the
residual
ash particles produced during pulverized coal combustion with
the rate of deposition
being
controlled
by
mass
transfer
45
2.8
I
I
I III
I I
I
2.4
2.0
,,
1.6
1.2
0.8
e--FIT TO EASA DATA
-
CASCADEIMPACTOR
0.4
0.01
0.1
1
GEOMETRIC AEROOL DIAMETER (micron IS)
Figure 2.2 - Fly ash particle size distribution obtained
from a utility boiler (Marcowski, 1980)
(Flagan and Friedlander, 1978).
2.2 OBJECTIVES
In general, the toxic effect of
the
fine
(i.e., submicron) are unknown because field
to their physico-chemical
characteristics
and relavent toxicalogical
conducted.
data
experiments
are
have
particles
pertaining
fragmentary,
not
yet
been
Furthermore, reevaluation of particulate emission
standards will most probably require
fine particle emission.
the
reduction
In obtaining a better
of the evolution of the submicron particles,
in
the
understanding
insitu
altera-
tion of the submicron particles through combustion
modifica-
tions may provide
both
the
determine
the
a
viable
means
for
reducing
emission and toxic effect of those particles.
The objectives of this thesis are
physico-chemical characteristics of the
produced under a wide range of
laboratory laminar
flow
to
submicron
combustion
furnace
and
understanding about the evolution of
to
these
particles
conditions
obtain
in
a
better
The
particles.
analytical capabilities for the specific characterization
the submicron particles will be developed inorder
mine the bulk composition, chemical
individual
submicron
particle
surface,
within
the
submicron
mode,
chemical
state
of
and
profile), chemical characteristics of discrete size
of
constituents, particle morphology, and particle size
of
deter-
to
characteristics
(bulk,
a
the
depth
sampling
inorganic
distri-
47
bution (PSD).
of
Since the phenomena
vaporization
ash
of this thesis will pertain to those
vaporization
with
specific
emphasis
process
on
the
focus
the
being examined in detail by Richard Quann (1982),
is
subsequent
to
condensation
sequence of the vaporized inorganics, particle growth process
by coagulation and agglomeration,
and
vaporized volatile and trace species
modes.
distribution
between
the
of
two
the
size
CHAPTER THREE
COMBUSTION AND SAMPLING SYSTEM
The central features of the laboratory scale
continuous
tion system are a laminar flow-drop tube furnace,
collection
coal feeding system, and water cooled
combus-
probe
for
products
continuous recovery and quenching of the combustion
followed by an Andersen impactor for on line size classification of the residual fly ash
particles.
The
advantages
an
apparatus
conducting combustion experiments on such
of
are
that: (1) the combustion conditions can be precisely defined,
controlled, and monitored to permit detailed investigation of
a number of
effecting
parameters
of
small homogeneous samples
particle
well
be
(2)
coal
are
characterized
employed for the experiments; (3) all
leaving the furnace can
formation;
particulate
completely
collected
productes
and
size
classified for subsequent analysis which eliminates questions
inevitably present
in
field
studies
to
the
extent
collected samples are representative; and (4) the
laboratory
system can simulate environments encountered in both
tional pulverized coal fired boilers and
extreme
that
conven-
conditions
such as those proposed for advanced energy conversion systems
(e.g., MHD).
In this chapter, the combustion
furnace,
continuous
coal
feeding
capabilities,
system,
and
collection
operating
system,
parameters
monitoring
described
are
in
detail.
3.1 COMBUSTION FURNACE
flow-drop
tube
furnace (Astro Model 1000a) is presented in Figure 3.1.
The
elements,
the
A schematic diagram of
has
furnace
the
graphite
heated
electrically
laminar
current
temperature of which are regulated with an automated
controller.
In order to protect the
ments
the
from
isolated from the
muffle tube.
oxidizing
central
graphite
zone
by
flows through an alumina honeycomb at
zone, an isothermal region
of
uniform
laminar
oxygen-nitrogen gas mixture is regulated by
controllers.
through a
Well size classified
narrow
water
cooled
coal
feeder
axially into the main gas stream just
dual
mass
are
particles
tube
below
hot
The
temperature
composition
The
velocity.
it
preheater,
and
delivering the main gas at the specified furnace
a
enters
1979).
(House,
the
1800
the
of
top
honeycomb serves as both a flow straightener
with
is
where
furnace
the
m
0.1
by
gas,
The main gas, a pre-mixed oxygen inert
are
alumina
an
temperature
at 1.0 x 10- 4 m 3 /s through the top of the
ele-
elements
imposed
Due to the thermal limitation
alumina, the maximum operating furnace
Kelvin.
the
environment,
combustion
heating
flow
fed
injected
and
the
of
honeycomb.
The coal particles are rapidly heated and combustion
begins.
Radial dispersion of the particles is minimized by the stable
50
Coal Particles
and Carrier Gas
,Water Cooled
Particle Injector
Flow
Figure 3.1 - Schematic diagram of laminar flow furnace
laminar flow field.
Visual observation shows that inside the
- 2 m,
the
furnace, which has an internal diameter of 5.0 x 10
-3
m of the axis.
burning particles remain within 6.2 x 10
Axial temperature profiles of the gas in the hot zone
measurements.
of the furnace were determined by thermocouple
These measurements were preformed under
flow
conditions
liters/min main gas) identical to those
used
in
(6
combustion
experiments. Coal was not fed, however, for the gas
tempera-
ture measurements. The measured axial gas temperature
profiset
les were obtained with the water cooled collection probe
at 0.15 m from the honeycomb flow
the probe, Figure 3.2.
and
straightener
without
At a furnace wall temperature of 1750
K, the average gas temperature in the hot zone of the furnace
the
is 1700 K without the water cooled probe and 1650 K with
water cooled probe.
ture was cooled at
Following the hot zone the gas
rate
a
of
2,000
K/s
and
temperaK/s,
7,000
respectively.
3.2 FEEDER SYSTEM
Because
of
experimental
the
apparatus was designed
to
feed
continuously and reproducibly.
small
scale,
quantities
diagram
A schematic
feeding system is presented in Figure 3.3.
cles are entrained in 1.0 x 10- 6 m 3 /s of
which flows over the surface of the
the
The
inert
agitated
into the stationary fine gauge tubing. The
gas
coal
feeding
coal
of
of
the
parti-
carrier
gas
bed
and
coal
velocity
in
1700
Is
1400
LU
H-
1100
LU
F-
800
500
5
10
15
25
20
DISTANCE FROM POINT OF COAL INJECTION
(cm)
Figure 3.2 - Gas temperature profiles in laminar flow furnace
To Furnace
er Gas
e Gauge Tubing
O-Ring
i Vial
Bed
Figure 3.3 - Schematic diagram of coal feeder
54
the fine gauge tubing is sufficient to keep the particles
suspension.
The rate of entrainment is
rate at which the
coal
feed
vial
established
is
driven
by
the
towards
the
stationary fine gauge tubing by the syring pump. A
feeding rates from 1.7 x 10- 4 grams/s to 1.7 x
in
range
10- 3
of
grams/s
is obtainable by changing the speed of the syringe pump.
For
a given syringe setting, a fixed clearance between the top of
the coal bed and the fine
gauge
after an intial transient.
tube
will
be
One to three grams
established
of
coal
are
normally fed per experiment.
3.3 COLLECTION PROBE
All the combustion products, both gaseous
are collected at a given height in the
cooled probe which is inserted
furnace.
resolved studies.
A
schematic
diagram
probe is presented in Figure 3.4.
water cooled collection probe
is
The
fitted
a
water
bottem
the
adjustable
The probe position is
solid
by
furnace
through
and
to
of
collection
core
with
the
time
enable
the
inner
of
a
of
the
stainless
The
steel porous tubing through which gas is transpired.
-2
1.27 x 10
m ID porous tubing is constructed from fused five
micron stainless steel spheres.
In the
top
x
2.5
10- 2
m
combustion products are rapidly
-4
quenched at a rate of 1.0 x 10-4 degrees per second by 3.0 x
section of the probe, the
10- 4 m 3 /s of nitrogen.
6.6 x 10- 5 m 3 /s
of
A minimal inward radial gas
nitrogen
velocity of 3.5 x 10- 3
which
corresponds
m/s is maitained through
flow
to
the
a
of
gas
subse-
55
COMBUSTION
QUENCH ZONE
O-RING
FURNACE
MUFFLE TUBE
POROUS TUBE
QUENCH GAS -
COOLING WATER
WALL GAS -
TO COLLECTION AND
ANALYSIS
Figure 3.4 - Schematic diagram of water cooled
collection probe
56
quent section of the porous tubing
to
counter
phoretic velocity of the particles (e.g., 1.8
the
thermo-
10- 4
x
m/s),
thereby, preventing particle deposition on the inner wall
of
the probe.
3.4 CASCADE IMPACTOR
probe,
From the collection
enter directly into an Andersen 20-800
Ambient
Sampler
is
a
multistage
gas
Sampler
for
The Andersen
aerodynamic classification of the particulates.
Ambient
and
particles
the
impactor
cascade
which
contains eight stages followed by a 0.2 um Fluoropore filter.
Each stage
of
the
impactor
contains
multicircular
jets,
The
designed to permit highly efficient size classification.
principle involved in this technique is illustrated in Figure
3.5.
velocity determined by the
size
of
the
The
orifice.
stream is deflected by a collection plate.
at
jet)
The aerosol flow through an orifice (i.e.,
a
gas
Smaller particles
are able to modify their direction of flow and follow the gas
streamlines, while the larger particles
possessing
a
large
diame-
kinetic energy impact on the plate. Since the orifice
of
ter decreases with succeeding stages, the size
particles will decrease with succeeding stages.
gas flow rate for this impactor is 4.72
will accomodate
the
high
gas
flow
x
The
10- 4
rates
impacting
optimal
m 3 /s
which
neccessary
for
quenching the combustion products.
Several
laboratory and field studies
(Ondov,
1975;
57
o
0.
p
0
•
Colltion Plate
Air
oFlow
.I
2
CIMm.n
Collction Pl
Figure 3.5 - Schematic diagram showing two stages of
a cascade impactor
(Friedlander, 1977)
58
McCain, 1978; and Rao, 1978) have noted that the
of
impactors
can
be
severly
hampered
reentrainment, and particle bounce.
by
wall
The problems
with reentrainment and particle bounce can be
implementing the use of
greased
aluminum
apezon grease type H) and minimizing
avoid particle overloading on each
the
the
furnace.
fly
ash
particles
associated
eliminated
time
incorporating
was
recalibrated
in
the
laboratory
stage
obtained from scanning electron micrographs by
an
image analyser, where between two hundred
five
and
described in greater detail in section 4.1.2.
hundred
will
particle size distribution are presented in Table
theoretical volume mean diameter for each stage
for
the
3.1.
were
The
calcu-
lated for spherical particles with specific density of
three
Due to the close aggreement with theory
each
and the observed narrow distribution on
evident that problems associated
with
reentrainment have been eliminated.
Consequently, the inlet
transpired
stage,
and
Significant
wall
loss
wall
nozzle
was
through
the
is
bounce
of
the
inlet
redesigned
its
graphite wall to eliminate particle deposition.
of selected inorganic elements on
it
particle
has been observed to occur on the inner
be
be
For each stage
the volume mean diameter and the standard deviation
that gases could
was
automated
particles were sized. The particle sizing technique
nozzle.
to
By
The particle size distribution for each
(Friedlander, 1977).
by
(i.e.,
sampling
stage.
generated
loss,
substrate
these modifications, the Andersen impactor
for
performance
impactor
inner
so
porous
The recovery
stages
and
59
Table 3.1 -
Calibration of the ambient Andersen impactor
stage number
experimental
Theory*
D
D
P
p
12.2
5.65
1.8
5.52
3.46
0.89
3.63
2.54
0.46
2.47
1.58
0.37
1.65
0.9
0.20
0.95
0.55
0.23
0.54
* particle density= 3.0 g/cc
60
final filter should verify whether significant wall
of
of
selected
after
combus-
inorganic particles is occuring. The percentage
elements in the coal feed ulimately recovered
loss
Generally, more than ninety
tion are presented in Table 3.2.
percent of an element in the coal feed is
recovered
in
particulate product. This indicates that no significant
of particles or inorganic vapors
are
occurring
within
the
loss
the
furnace, collection probe, or Andersen Impactor.
The submicron particulates, because
size pass
through
the
impactor.
presently employed
for
the
particles for the
Three
of
sampling
of
preparation
their
the
various
analytical
techniques
collection on a filter for
subsequent
weight
analysis;
grids
(2) deposited on transmission
in
the
electrostatic
studies; and (3)
classifier.
on-line
size
and
for
classification
methods
submicron
are;
(1)
chemical
microscope
electron
precipitator
small
microscopy
by
mobility
These various option will be descused in greater
detail in chapter four.
61
Table 3.2 - Elemental material balance on furnace and
collection system. Gas temperature 1750 K
oxygen partial pressure 0.5 atmospheres
Element
percent recovered
Al
77.3
As
99.6
Ca
96.0
Cr
100.5
Fe
91.7
La
100.4
Mg
80.7
Mn
90.0
Na
89.0
Sb
100.0
62
CHAPTER FOUR - ANALYTICAL TECHNIQUES
processes
Insight on the fundamental
nature of the submicron particles
detailed physicochemical
particles.
can
be
charaterization
governing
obtained
of
the
from
this
section
The focus of this
will be to evaluate the analytical capability of
a
submicron
A list of the analytical techniques used in
study are presented in Table 4.1.
the
tech-
each
nique for quantitative analysis.
4.1 TRANSMISSION ELECTRON MICROSCOPE
A Philips 200 transmission electron microscope (TEM),
with a working magnification
of
125,000
morphological characterization of
The high resolution of the TEM
of several angstroms can
produced
by
thermionic
be
achieved.
emission
"hairpin" filament held at -100 kV.
are accelerated and partially
from
The
collimated
wehnelt electrode surrounding the
a
resolution
electrons
heated
emitted
by
filament.
highly
a
point
The
a
for
particles.
with
obtained
collimated electron beam, where a point to
used
was
submicron
the
is
x,
are
tungsten
electrons
controlling
With
magnetic
lenses, the electron beam is first condensed and than focused
onto the electron thin specimen held in the
specimen
stage.
63
Table 4.1 - Analytical instrumentation used
for physico-chemical characterization
of the submicron particles
INSTRUMENT
APPLICATION
Physical Characterization
TEM
particle
morphology
Automated image analyzer
PSD
Mobility classifier
discrete sized Samples
within the submicron
mode
Chemical Characterization
INAA, AA, and
liquid chromatography
bulk analysis
STEM
individual particle
analysis
Auger spectroscopy
surface analysis
ESCA, mossbauer, and
x-ray diffraction
chemical state
The transmitted electrons are then magnified to form a planar
image of the specimen on the viewing screen at the bottem
ob-
are
images
the
Micrographs of
the microscope column.
of
tained with a plate camera which is located below the viewing
screen.
Samples of the submicron particles
prepared
are
by
depositing the particles on transmission electron
microscope
grid placed in an electrostatic percipitator down
stream
the cascade impactor.
of
The TEM grid is a 200 mesh copper grid
with a vacuum deposited carbon support film.
By allowing the
electrostatic precipitator to fill uniformally with particles
before each pulse of
presentative
collection
the
samples
the
of
plate
submicron
re-
electrode,
can
particles
be
obtained.
the
Quantitative calibration of
set-
magnification
tings for the Philips 200 was obtained with a carbon diffraction grading replica standard, containing
lines per millimeter.
At
low
magnification the distance between
used.
The
calibration
of
the
the
magnification
between the large number of lines
is
two
diffraction
2,160
used,
while
discrete
magnification
compaired with the manufacture's for the Philips
4.2.
distance
points
setting
200,
Due to slight drift in the magnification settings
time, recalibrations were periodically conducted.
high
at
is
are
Table
with
Table 4.2 - Calibration of the magnification
the Philips 200 TEM
settings for
magnification
setting
manufacture
calibration
12
9,900
8,700
17,000
18,100
30,500
32,100
55,000
58,200
94,000
97,100
4.2 AUTOMATED IMAGE ANALYZER
micrographs
and agglomerated particles are obtained from TEM
of the submicron particles by
with
the
software
computerized
a
establishing
for
calabilities
equivalent spherical diameter of both primary and
ted particles.
Since the growth process
upon
instantaneously frozen
by
with
sampling
analyzer
image
automated
The automated image analyzer is
(Magiscan).
system
an
particles
primary
of
The particle size distibution
agglomera-
coagulation
the
is
collection
coagula-
probe, time resolved studies can be used to examine
tion kinetics where the relative rate of coalescence leads to
agglo-
the formation of either single spherical particles or
merated particles.
4.3 MOBILITY CLASIFIER
The mobility classifier (Knutson and Whitby, 1975) is
an on line instrument developed for particle size classification of submicron particles where the classification is based
on the electrical mobility of unicharged particles.
feature of the mobility
classifier
is
its
An added
capability
for
of
the
extracting discrete sized samples.
In order to obtain
discrete
primary particles for bulk chemical
sized
samples
analysis,
the
mobility
classifier had to be calibrated with respect to the inorganic
submicron
particles
since
slight
particles will occur before sampling.
agglomeration
The
primary
of
these
particle
size distributions for the discrete sized cuts obtained
from
the mobility classifier are broader than that predicted
from
theory.
However,
the
of
breadth
the
particle
size
distributions, which is a consequence of agglomeration,
not pose a significant problem in obtaining several
sized cuts of the
particle mode.
primary
particles
will
discrete
within
the
submircon
The calibration was conducted
for
submicron
particles produced from the
(53/63 um) at a furnace
combustion
temperature
percent oxygen in nitrogen.
of
of
Montana
1700
The new voltage
mean diameters for the primary
particles
K
Lignite
in
twenty
settings
are
verse
presented
in
Figure 4.1.
4.4 Bulk Analysis
The bulk elemental composition for the major,
and trace inorganic elements is
determined
by
neutron activation analysis (INAA) supplemented
absorption (AA) for silicon, potassium, and
liquid chromatography (Mizisir et.
instrumental
with
atomic
phosphorus,
for
1978)
al.,
minor,
and
sulfur.
INAA, a multielemental analysis technique, is a reliable
sensitive method for the determination of trace
The
amounts of one nanogram or less.
elements
employs
technique
and
in
a
high neutron flux source for the production of
radionuclides
The
radionuclides
(i.e., isotopes) from the target element.
decay, leading to the emission
of
gamma
rays
of
specific
energy which are measured with a gamma spectrometer, a Ge(Li)
68
250
200
r1-
0
150
CD
FCD
cr
LJ
U
100
z
50 -O
0
100
200
MEAN
DIAMETER
300
(Ao)
Figure 4.1 - Calibration of the mobility classifier with
respect to primary particle diameter
69
solid state detector coupled to a multichannel analyzer.
The
precise and accurate amount of each element in the sample
calculated by ratioing the measured activity
with the activity of a standard of known
NBS coal).
The accuracy of INAA
several Opal Glass samples "NBS
was
of
the
sample
composition
(i.e.,
checked
standard
by
analyzing
number
91".
The
composition of the oxides of Ca, Al, and Na as determined
INAA are within one percent of
the
NBS
is
composition,
by
Table
4.3.
The submicron particles are collected on
high purity filter (i.e., Fluoropore) placed
stage of the cascade impactor.
polytetrafluoroethylene
in
0.2
micron
the
filter
The flouropore filters are
substrate
bonded
to
high
a
density
polyethylene net for improved ease of handling.
4.5 SCANNING TRANSMISSION ELECTRON MICROSCOPE
The
elemental
composition
of
individual
primary
particles are obtained with a 100kv field emmission
scanning
transmission electron microscope (STEM) fitted with an energy
dispersive x-ray spectrometer. In order to
obtain
quantitative results a computerized data acquisition
accurate,
system,
known as quantex-ray, is available for x-ray evaluation.
The
STEM provides a wide range of image and analysis capabilities
not previously combined in a single instrument,
a major advance in instrumental microanalysis
and Hall, 1979).
The unique
feature
of
the
representing
(Vander
STEM
is
Sande
its
70
Table 4.3 -
element
INAA analysis of Opal Glass, NBS standard
number 91
INAA
NBS analysis
Na 2
34.5%
34.1%
Al203
24.8%
24.1%
CaO
40.7%
41.8%
for
elemental
analysis. This permits microprobe analysis of the
individual
resolution
spatial
of
angstroms
twenty
parti-
primary paticles where the contribution from adjacent
cles can be neglected.
Submicron particle samples for the STEM are
prepared
elec-
transmission
by depositing the submicron particles on
tron microscope grid placed in the electrostatic precipitator
down stream of the cascade impactor.
samples
in
the
uncertainty
in
the
When X-ray spectra of standards and
STEM are
aquired,
there
exists
some
analysis, culminating from both systematic and random errors.
The approach taken in minimizing the
systematic
errors
are
outlined in Table 4.4.
4.5.1 Reproducability
A. Beam Current Decay
The decay in beam current results from
contamination
build up on the filament of the field emission gun, the
of which depends on
1979).
the
chamber
gun
decay
The significance of beam current
the fact that the intensity of induced
the
vacuum
specimen
is
directly
the
(Garret-Ried,
arises
from
emission
from
beam
current
After cleaning the
filament
proportional
(Goldstien and Williams, 1977).
with a high current pulse,
x-ray
rate
objective
to
aperture
current
(indirect monitor of beam current) and count rate for inconel
Table 4.4 - Sources of error and corrections implemented in
order to obtain quantitative x-ray analysis of
the submicron particles with the STEM
Phenomena
I.
II.
Correction
Reproducibility
A. Decaying probe current
monitor for each analysis
B. Specimen contamination
bake samples before analysis
C. Drift
mechanically realign sample
D. Counting statistics
determine the uncertainty in
x-ray counting and propagate
through calculation
Maximize signal/bremsstrahlung
A. Bremsstrahlung
establish spurious sources and
eliminate contribution
B. Hole count
use virtial objective aperture
in back focal plane
III. Quantitative analysis
A. Thin film criteria
1. Absorption
2. Fluorescence
3. Beam spreading
establish significance of
absorption, fluorescence, and
beam spreading making the
necessary adjustment to
raw intensities
B. Standards
implement use of experimental
sensitivity factors Kab
C. Specimen stability
establish stable species
73
600 standard (i.e.,
Iron) were monitored
seconds,
1500
for
X-ray analysis obtained at different
Figure 4.2 (Was, 1980).
time intervals during the decay cycle will hamper comparative
analysis of the raw intensity.
intensities
x-ray
The variation in
decaying beam current could be eliminated by
acquisition
analysis.
and
time
cleaning
resulting
from
decreasing
the
each
before
filament
the
of
life
However, this would sharply decrease the
time.
the filament and subsequently increase instrument down
the
normalize
Instead, the corrective procedure used was to
x-ray intensities with respect to beam current.
B. Specimen Contamination
Contamination build up on the specimen
will
surface
the
attenuate both the primary electrons and x-ray signal of
specimen.
The
effect
of
analysis is illustrated in Figure 4.2,
for Fe drops off more sharply
current with time.
build
contamination
than
where the
the
during
up
rate
count
objective
aperture
The effect of contamination build
up
more serious for the light elements (e.g., Na and Mg),
is
since
their low energy x-rays will be attenuated more severely.
During analysis,
surface
adsorbed
hydrocarbon
carbon oxides (i.e., contamination) will coke under the
and
high
energy electron beam leaving a residual film of carbon (Ennos
1953; Heide, 1963).
In the high vacuum sample
rate of residual vapor deposition
will
be
chamber,
extremely
the
slow.
74
1.0
FC - specimen count rate (Fe)
O
- objective aperature current
0.8
0.6
0.4
0.2
I
I
0.0
0
500
TIME
1000
1500
(sec)
Figure 4.2- Decay in beam current and specimen
count rate with time
75
Therefore, the observed contamination build up during
sis has been attributed to the migration of surface
hydrocarbons and carbon
oxides
into
the
Proper sample preparation by removing
will eliminate
analysis.
substantial
contamination
adsorbed
analysis
adsorbed
region.
contaminates
build
This is achieved by baking the TEM
analy-
up
grids
during
at
200
centigrade in a vacuum of 10- 6 torr for an hour.
C. Drift
For extended counting periods
(i.e.,
>20
seconds),
the drift of the specimen can occur such that the particle of
interest is no longer being analyzed.
from expansion of
the
carbon
Specimen drift results
support
film
with
heating,
and/or movement in the specimen holder or retaining ring
to vibrations.
In
analysis, continual
order
to
compensate
realinement
of
the
for
during
drift
specimen
due
will
be
required.
D. Counting Statistics
Since x-ray photon emission is a random process, some
variance (i.e., counting error) in the
will exist for
a
series
measurement of acquired
of
measured
repetitive
counts,
N,
the
intensities
For
analyses.
standard
a
counting
error is represented by the relationship (Kevex, 1980)
a =
(N)1 / 2
Eq.
4.1
76
particle,
For a series of repetitive analyses of a submicron
the standard deviation in the normalized counts for Si and Fe
with
was determined and is compared
deviation in Table 4.5.
the
standard
counting
that
indicates
The close agreement
the contribution from all sources of error other
coun-
than
ting error are relatively small.
is
The precision of each analysis which
by
defined
the relationship
S1
Precision
N
(N)
can be greatly enhanced by increasing
Eq. 4.2
1/2
the
time
acquisition
(i.e., number of counts) while maintaining a high
signal
to
background ratio.
4.5.2 Maximize Signal/Bremstrahlung
A. Bremsstrahlung
Background is the result of the detection of unwanted
x-rays
or
electrons.
The
primary
source
of
background
electrons
originates from inelastic collision of the primary
with the specimen, resulting in the production of a continuum
of x-rays.
This
continuum
bremsstrahlung or braking
is
sometimes
radiation.
The
to
referred
high
as
background
level detracts from the statistical precision of the
results
by increasing the difficulty in interpreting the signal
from
77
Table 4.5 - Repetitive analysis of submicron particles
for the elements silicon and iron.
number
average
standard
deviation
standard
counting
deviation
total count
Si
Fe
258
187
269
226
311
213
268
199
276
206
background, Figure 4.3 (a).
bremsstrahlung,
extraneous
the
background, minimization of
high
and
rates
counting
low
the
of
Because
the source of which can be traced by the presence of spurious
reduction.
From
examination, the largest source of bremsstrahlung was
attri-
x-rays,
will
be
data
for
essential
buted to the copper grid matrix whose x-ray signal dwarfs the
unusually high production
of
through
prepaired thin foil standard.
For
a
specially
a
in
hole
a
convergent
perfectly
standard
beam, no interactions between the electron beam and
should occur.
extremely
However,
high
objective
the
hole count was eliminated by shifting the objective
sharp
in the spurious x-ray and bremsstruhlung emanating
copper grid matrix was
achieved.
Figure 4.3 by examining the
after (b) modifying
the
x-ray
objective
This
increased
current
convergence of the electron beam.
reduction
from
(a)
before
spectra
aperture
density
aperture
illustrated
is
The increase in x-ray counts from the specimen
result of the
high
The
tron beam which accounts for the high hole count.
Consequently, a
elec-
the
aperture to the specimen causes the scattering of
into the back focal plane.
were
counts
hole
The close proximity of
detected (Was, 1980).
the
Here,
matrix) was resolved by examining the hole count.
electron beam is directed
grid
(i.e.,
x-rays
spurious
the
for
explanation
The
specimen signal, Figure 4.3 (a).
due
the
in
and
configuration.
is
to
a
direct
improved
2
4
6
X-RAY ENERGY
8
10
8
10
(KeV)
(A)
0
2
4
6
X-RAY ENERGY
(KeV)
(B)
Figure 4.3 - STEM x-ray spectra before (A) and after (B)
modifying the aperture configuration
80
4.5.3 Quantitative Analysis
A.
Thin Film Criterion
The thin film criterion states that no adsorption
or
specimen
is
fluorescence corrections need be applied if the
infinitely thin. Many people interpret this criterion to mean
that if the film is transparent to electrons (i.e.,
thin) at the operating potential
film criterion is satisfied.
being
employed,
electron
thin
the
However, absorption effects can
be appreciable for electron thin films (Tixier, 1979), if the
combination of mass absorption coefficient and film thickness
are large enough. For this reason, the theoretical extent
of
absorption and fluorescence were examined for a range of film
thicknesses.
The extent of absorption can be calculated
approximation, if one considers that
the
average
half
the
thickness
x-ray excitation occurs at one
film.
If the path length
represented as
(tcsc a )/2,
for
the
in
absorption
generalized
first
to
depth
the
form
of
film
of
of
the
is
the
equation reduces to
Kab = K
abT.F.
e-(Xb-a)
u
AX
b-a
spec
b
(
p t/2)
Eq. 4.3
u
c c sc
p
spec
a
81
film,
where Kab)T.F.
)T.F is the Kab ratio for an infinately thin
and u/pspea
u/Pspec
(spec), respectively,
a
1977).
al.,
magni-
The
will
tude of the binary absorption coefficient
as
increase
elements
the difference between the atomic number of the two
Al
and
The calculated absorption factors for Mg
increases.
the
is
is the take off angle, and t
electron path length (Goldstien et.
film
thin
the
in
characteristic x-rays of element a and b
for
coefficient
absorption
mass
are
relative to Fe are presented in Figure 4.4, where the absorp-
et. al.,
absorption can be
for
neglected
x-ray
of
effect
The
1980; Henke et. al., 1956).
(Bender
literature
tion coefficients were obtained from the
less
than
750
calculated
from
the
particles
angstroms in diameter.
is
The extent of fluorescence
relationship
I
r -1
a
f
Ia
where
C
b
a
ra
A
u
a
pb
Ab
Pb
spec
a
I a,
Wb
is
the
element b, ra is the absorption jump
b
a
2
ca
Ecb
is the fluorescence intensity which is added to the
primary intensity
u/
E
u
is the mass
absorption
radiation in element a,
ratio
coefficient
element
Aa and Ab are the atomic
element a and b, and Eca and Ecb are the critical
energies for the characteristic lines of
a,
element
for
of
for
yield
fluorescence
elements
b's
weights
of
excitation
a
and
b
1.08
a
b
Mg
Fe
I - Al
Fe
O -
1.06
1.04
1.02
1.00
0
250
500
PARTICLE DIAMETER
750
(Ao)
Figure 4.4 - Adsorption correction for various
particle diameters
1000
83
cence of the detected elements
the
in
submicron
particles
for
particles
percent
one
were calculated to be less than
fluores-
the
4.4,
(Philibert et. al., 1975). From equation
less than 1000 angstroms in diameter.
be
The spacial resolution for analysis is assumed to
of
on the order
(i.e.,
angstroms
twenty
through the specimen.
pass
they
as
scattered
However, electrons are elastically
diameter).
beam
Consequently, the analysis volume from
beam diameter depending on
extent
the
electron
primary
the
exceed
which x-rays are produced can
beam
of
spreading.
Goldstien et. al. (1977) has estimated the effective electron
beam spreading or broadening,
b = 625
Eq. 4.5
t 3/2
p 1/2
A
Z
Eo
where b is the extent of broadening for a zero diameter beam,
Z is the atomic number, A is the atomic weight ,
accelerating voltage in KeV, p is the density
and t is
thickness.
specimen
a
of
In
particles
summary
beam
greater
Consequently,
broadening
than
250
will
of
beam
function
of
film
be
angstroms
Figure
apreciable
in
for
diameter.
the effect of beam broadening will have
considered before comparing the raw intensities from
sized particles.
film,
the
thicknesses for pure element targets are presented in
4.5.
the
is
values
Calculated
broadening for a zero diameter beam as
Eo
to
be
various
84
125
100
0
75
-- I
Z
Lu
Co
50
J
Wi
£0
25
0
250
500
PARTICLE DIAMETER
750
1000
(Ao)
Figure 4.5 - Beam broadening for various particle
diameters
B. Standards
Quantitative
microanalysis
x-ray
of
particle is simplified, since the effect
used
as the Cliff and Lorimer method was
in
analysis
a
submicron
to
binary
For
x-ray
the
from
determined
mixtures, the weight percent is
known
direct
obtain
percent.
weight
and
adsorption
A simple ratio technique
fluorescence can be ignored.
quantitative
of
intensities by the relationship,
Ca
Cb
Ia
b
K
Cb
ab
where I is the measured x-ray
Eq. 4.6
Ib
intensity,
C
weight
the
is
fraction of the species, and K is the sensitivity
correction
The factor, K, accounts for the relative
efficiency
factor.
of x-ray production by the analyzed element and the detection
efficiency of the x-rays at
Analysis
of
multicomponent
characteristic
their
mixtures
can
be
binary
developing a series of K factors
from
referencing all K factors to the
response
of
energies.
achieved
mixtures
and
element.
one
The weight fraction of species i can be determined
by
divi-
by
ding the adjusted intensity for species i by the sum
of
the
adjusted intensity of all species detected.
Ki I i
C
Where
=
1
K
I
WC
Eq. 4.7
IJ
i
= 1
quantitative
for
The sensitivity correction factors
x-ray analysis are determined from bulk standards, thin
foil
standards, or calculated from theory (Goldstien, 1979).
For
the
of
application
thin
film
sensitivity
the
analysis,
correction factors determined from bulk standards
have
will
to be adjusted to account for the effects of x-ray adsorption
and fluorescence (i.e.,
section.
previous
the
ZAF corrections has been outlined in
for
The procedure
ZAF corrections).
Since thin foil standards, which are of known mass thickness,
meet the thin foil criterion, ZAF corrections are not needed.
lated from theory
by
ionization cross section, the fluorescence yield,
charac-
Because of the uncertainty in
teristics of the EDS detector.
EDS
the absorption characteristics of the
in
(Kevex,
1980),
manufacturing
the
of
theoretically
calculated
(Cliff
and
Lorimer,
sensitivity
the
extent
sensitivity
1975)
with
theoretically
estimation of the K factors for the light elements
due
to
of
factors
correction
sensitivity correction factors (Goldstien, 1977).
stien (i.e., Na and Mg) arises
of
light elements) by
This is evident from the comparison
determined
to
window
beryllium
the
absorption of the low energy x-rays (i.e.,
experimentally
due
detector
correction factors may not properly estimate
the detector window.
frac-
the
tion of total intensity measured, and the adsorption
nonuniformity
the
element;
each
for
establishing
calcu-
directly
be
Sensitivity correction factors can also
calculated
The
under
by
Gold-
uncertainty
in
the
87
thickness of the detector's beryllium window, Table 4.6.
The approach used
in
establishing
the
sensitivity
correction factors for the STEM was derived from the combination of the three techniques.
adsorption characteristics
thickness), sensitivity
Because of the uncertainty
of
the
correction
(i.e.,
detector
factors
for
elements had to be determined experimentally.
in
window
the
light
a
Mg-Fe
From
thin film standard, the sensitivity correction factor for
Mg
relative to Fe was found to be 7.5 (Hall, 1978). From a Mg-Si
bulk standard,
the
sensitivity
correction
relative to Si after ZAF corrections was
For the
remaining
elements
of
factor
found
interest,
to
the
correction factors were calculated from theory.
for
Mg
6.7.
be
sensitivity
The
derived
sensitivity correction factors which have been referenced
sulfur are presented in Figure 4.6.
The
to
high
unusually
K
values for the light elements (i.e., Mg and Si) results
from
the high adsorption of their low energy x-rays
thin
by
the
mylar film which has been placed on the outer surface of
the
beryllium window in order to absorb the low energy electrons.
By
comparing
the
average
primary particles determined from
concentration
for
the
with
the
bulk
the
STEM
composition of submicron particles by INAA, the
derived
sensitivity
correction
factors
analysis of the submicron particles can be
4.7.
Ca,
for
use
of
the
quantitative
justified,
The variation in the relative concentration of Mg,
Table
Si,
and Fe by the STEM with that by INAA can be attributed to
88
Table 4.6 - Theoretical and experimentally determined
sensitivity correction factors Kx/Si for
100 kV.
Z
Kexperimental
(Cliff, 75)
Ktheoretical
(Goldstien, 77)
11 Na
5.77
1.66
12 Mg
2.07
1.25
13 Al
1.47
1.12
14 Si
1.00
1.00
20 Ca
1.00
1.02
26 Fe
1.27
1.33
10
8
CE
CD
I-
Cf)
N.I.
12
16
20
24
28
ATOMIC NUMBER
Figure 4.6 - STEM
Sensitivity correction factors referenced to sulfur
90
Table 4.7 - Comparison of the chemical analysis by
the STEM and neutron activation (INAA)
element
Mg
conc. (wt%)
conc. (wt%)
STEM
INAA
64.4
6.1
Si
4.1 + 0.8
P
1.2
S
K
+
60.3
2.8
0.2
3.3
0.7
0.1
5.4
0.5
0.1
2.3
Ca
13.3
Fe
15.5
1.3
11.9
1.5
14.1
+
91
the inherent counting error for STEM analysis.
However,
apparent breakdown in analysis for S, K, and P
can
only
explained by examining the stability of those elements
the high intensity electron beam.
the
be
under
This will be discussed
in
greater detail in the following section.
C. Sample Stability
Morphological changes in the
the particle under
observed.
the
high
internal
intensity
structure
electron
beam
The observed mobility within the particle
explained in terms of localized heating on an
The stability of
each
element
was
in
analysis.
In
comparing
volatile species S,
particles
by
INAA,
P, and K were found to
the
Table
be
level.
to
elements
average
concentration of the primary particle with the bulk
tion of the submicron
be
order
determined whether preferential loss of some of the
was occurring during
was
may
atomic
examined
of
composi4.7,
the
substantially
depleted. The loss phosphorous has been noted in the analysis
of biological specimen (Sudenfield, 1979).
the elements Si, Ca, Fe, Mg, and
S
were
The stability
examined
detail where the total counts after each of four
analyses were compiled.
4.8.
S,
in
of
more
consecutive
The results are presented
in
Table
The behavior of the volatile species, as represented by
clearly demonstrates
intensity electron beam.
for Si,
Mg,
their
instability
The variation in the
and Fe are attributed to
error for x-ray analysis.
under
the
in
high
counts
total
inherent
The gradual decrease
the
counting
Ca
with
Table 4.8 - Elemental retention during analysis with
the high intensity electron beam of the
STEM
Intensities*
Element
Standard
Counting
Deviation
(time sec)
150
250
350
Mg
494
485
509
475
22.5
Si
187
226
213
199
17.1
Ca
512
531
434
409
24.6
Fe
258
269
311
268
18.1
72
33
9.6
* Normalized to beam current
93
be
successive analysis can not
however,
error alone,
Ca
by
justified
will
the
counting
analysis, since its retention for the first
the
into
incorporated
be
seconds
200
is
bounded within the uncertainty for x-ray analysis.
only
Quantitative x-ray microanalysis will
con-
be
ducted for the elements Si, Ca, Fe, and Mg where the error in
analysis will be specified by the counting error.
4.6 AUGER SPECTROSCOPY
The surface sensitive technique of Auger Spectroscopy
(Powell and Spicer, 1978; Kane and Larraby, 1974) was used to
extract information about the elemental stratification within
the submicron particle.
Scanning
Electronic 590A
The
system
Auger
a
was
used
Physical
submicron
with
Microprobe
The 590A utilizes
spatial resolution of 0.2 um for analysis.
and
both single pass electron optics for image analysis,
Auger
Ar ion sputtering gun for depth profiling.
copy is a
highly
number of escaping Auger electrons
exponentially with
distance
into
atomic layers.
the
the
sample
decays
matrix,
the
first
few
the
electron
the Auger signal is more readily detected by
energy
the
Because the Auger peaks are superimposed on a
continuous background from the secondary
the
sample
from
analysis should represent the composition of
SpectrosSince
technique.
sensitive
surface
an
distribution
function
(e.g.,
emission,
differentiating
dN(E)/dE).
The
submicron particles are collected on a silver membrane filter
94
charging
sample
severe
(0.2 um pore size) which eliminates
during analysis.
Ep,
cross section
transition.
of
the
ele-
of
range
The optimum Auger yield for the
Auger
the
in
involved
level
core
ionization
impact
is largely determined by the electron
energy,
beam
primary
The Auger electron yield with
ments present in the fume will be obtained for a primary beam
energy of 5 KeV.
The other important instrumental parameters
the sensitivity (sens),
for
and,
responce
peak
to
peak
the
of
ensure
to
order
In
spectrum, the modulation energy (Em).
quantitative analysis, the
dN(E)/dE
conventional
the
(Ip),
current
for peak enhancement are the primary electron
be
examined
increases
linearly
to
amplitude with instrumental parameters need
for both the low and high energy Auger signals.
The Auger peak to peak amplitude
and Mg KLL Auger signal (1174 eV) in
accelerating
for excessively high beam current densities and
voltages, this linear relationship will
beam damage of the specimen.
break
Consequently,
the beam current will be maintained below
Under
accelerating voltage of 3 and 5 kV.
conditions no significant loss of
evident.
the
However,
4.7.
Figure
eV)
(32
signal
with beam current as shown for Mg LMM Auger
due
down
analysis
during
one
for
microamp
these
volatile
The very slight decay in the signal from
to
operating
species
is
homogene-
ous standards with time is attributed to contamination
up on the specimen surface, since the signal returns
build
to
its
95
I
I
EO - Mg (32 eV)
O
- Mg (1174 eV)
41
LU
Jz
-
2 -
Oz
Z1
O
0.0
I
I
0.5
1.0
BEAM CURRENT
(microamp)
Figure 4.7 - Auger peak amplitude verse beam current
1.5
96
original amplitude upon cleaning with the ion gun.
For S LMM Auger signal (152 eV),
the
peak
to
peak
amplitude increases linearly over the entire range of
sensi-
tivity settings, Figure 4.8.
The Auger peak to peak ampiltude
signal (32 eV) and
S
KLL
Auger
verses
eV)
(2117
signal
Auger
LMM
Mg
for
to
The Auger peak
modulation energy is shown in Figure 4.9.
peak amplitude is proportional to the modulation energy
when
Auger
peak
width. However, as the modulation energy
becomes
will
of Mg LMM Auger signal
(32
for
narrow peak width, becomes nonlinear
greater than 2 eV.
Since
its
with
eV),
peak
the
electron energy, the peak to peak
Consequently,
amplitude
peak
correspondingly
modulation
energy
with
increases
width
amplitude
the
as
nonlinear
The peak to
modulation energy is increased.
the
increased,
is
broadened.
Auger peak width is instrumentally
the peak to peak amplitude
the
to
the modulation energy is small relative
of
the
S
KLL
Auger signal (2114 eV) increase linearly through a modulation
energy of 8 eV.
Therefore, in
using
enhance the peak to peak amplitude,
ity for the various elements must be
modulation
the extent of
incorporated
energy
to
nonlinearinto
the
the
mass
peak to peak normalization.
Theoretically,
quantitative
analysis
of
concentration for the surface layers can be
determined
the peak to peak amplitudes with the use of
relative
from
sensi-
97
20 -
O - S (152
eV)
L
5-
O
--I
S
10
L1
D I0
)-j
CD
0
50
100
150
200
SENSITIVITY SETTING
Figure 4.8 - Auger peak amplitude verse sensitivity
setting
20
O
- Mg (32 eV)
[-1 - S (2117 eV)
LU
15
F--j
ul
10
LU
N
._J
2:
5F
I
I
I
I
10
15
20
MODULATION AMPLITUDE SETTING
Figure 4.9 - Auger peak amplitude verse modulation
amplitude setting
25
99
tivity factors.
The effects
roughness) is believed
decrease
to
the analysis will only be
understood
relative
The
semi-quantitative.
ionization
sensitivity factors accommodate for variations in
electrons
Auger
cross sections and escape depths of various
for homogeneous samples.
relative
generating
The method of
normal-
sensitivity factors from homogeneous standards is to
ize the intensities (i.e., peak to peak
element to one specific
each
amplitude)
of
silver)
using
(i.e.,
element
until
However,
better
the full effects of surface roughness are
by
signals
Auger
all
1974).
nearly the same percentage (Fadley,
(e.g.,
topography
surface
of
the
relationship
x/Ag
a
Eq.
x
a +b
4.8
IAg
where Ix and IAg are the peak to peak amplitude for element x
and pure silver standard, respectively (Davis et. al.,
A and b are the chemical formula indices
of
1976).
compound
XaYb
-
The determination of relative sensitivity factors for quantihighly
tative Auger spectroscopy will be
standards chosen.
The relative
dependent
sensitivity
factors
Whether the impurity
the standard, thereby reducing the
atomic
sorbed on the surface of the standard (i.e.,
thereby leading to the preferential
is
the
obtain
lead
to
inherent
in
from standards with small amounts of impurities can
substantially errors.
on
density,
or
ad-
contamination),
adsorption
of
the
low
energy Auger electrons, the relative sensitivity factors will
100
be inaccurate.
The Auger spectra varies dramatically between
peak
In the case of magnesium, the
the metal and the oxide.
the
of
location and shape (i.e., number of satellite peaks)
Auger signal varies between the metal and the oxide, but more
importantly, the peak to peak amplitude of
signal (i.e.,
to
appears
dN(E)/dE)
al,
et
were
1976)
the
to
sensitive
be
oxidation state of magnesium, Figure 4.10.
for Mg and MgO (Davis
differential
the
The Auger spectra
for
obtained
a
Auger
peak,
the sensitivity factor for the oxide (1174 ev) is over
three
primary beam energy, Ep, of 3 KeV.
times
greater
addition,
than
that
For Mg
the ratio of the relative
sensitivity
The
In
factors
of
is
the Mg KLL auger peak to the Mg LMM auger peak
the metal and 1.09 for the oxide.
ev).
(1186
metal
the
for
KLL
for
0.23
factors
sensitivity
for the specific chemical state of the detectable elements in
the fume are presented in Table 4.9 for a primary beam energy
of 5 KeV.
The chemical state analysis by ESCA for
generated from the combustion of Montana lignite
fume
the
um)
(45/53
in twenty percent oxygen in nitrogen at a furnace temperature
of 1750 K is presented in chapter six.
Despite the fact that sputtering is a widely accepted
technique for obtaining elemental depth profiles,
the
sput-
artifacts,
where
be used in the interpretation of the results from
tering depth
profile
due
to
sputtering
certain species can be preferential sputtered
Hathorne,
1972; Sigmund,
1969).
care should
(Andersen
and
>
I
MgO
x 2.5
x 2.5
dN
x4 2.
x 2.5
dN
dE
dE
x 2.5
1174
S=
0.416
S= 0.283
S=0.311
45
0
100
"
1100
1200
Electron Energy (eV)
1300
0
100
/
1100
1200 '
Electron Energy (eV)
Figure 4.10 - Auger spectra of Mg and MgO
(Davis et. al., 1976)
1300
102
Table 4.9 -
element
Relative sensitivity factors obtained
experimentally (Ep = 5.0 KeV)
electron energy
(eV)
LMM
152
KLL
2117
Sx/Ag
0.724
0.0138
LMM
76
0.084
KLL
1606
0.367
Mg
IMM
32
0.164
Mg
KLL
1174
0.246
KLL
990
0.25
KLL
252
0.90
KLL
291
0.478
KLL
703
0.21
Fe
103
4.7 ESCA
was used to determine the chemical
species.
inorganic
the
of
state
(ESCA)
analysis
chemical
Electron spectroscopy for
The system used was a PHI 548 ESCA/Auger unit
with
double
pass
a
a precision electron analyzer. The analyzer,
cylindrical
mirror
the
measures
kinetic
energy
of
the
input,
grid
retarding
with
analyzer
9
below 10
chamber
is
analysis
during
400
of
cromatic Mg k alfa x-rays at 1253.6 eV with a source
The sample
mono-
produces
excitation source is a magnesium anode which
watts.
The
photoelectron.
maintained
Torr.
monochromatic
with
In ESCA, the specimen is excited
x-ray beam. In returning to their ground state,
atoms
these
eject photoelectrons, whose measure kinetic energy is defined
by
Eq. 4.9
KE = hv - BE - Os
where hv is the energy of the
binding energy of the electron
electron.
KE
Using
is
this
the
BE
the
orbital
state
from
spectrometer
work
in
kinetic
relation,
calculated once the energy of the
the
of
energy
x-rays
exciting
chemical
the
energies
binding
emitted electron's kinetic energy are known.
the binding energies are known,
the
photon,
which the electron originated, Os is
function, and
is
incident
emitted
can
and
be
the
In theory, when
state
information
104
can be deduced,
(how
since the binding energy
are held to their
chemical state.
In general,
(>
1.0
eV)
the various compounds is
limited,
the
oxidation
differences
measurable
the
various
differentiation
between
for
exist
oxidation states. On the other hand,
they
to
sensitive
identification of the
state can be easily determined since
in the binding energy
is
atoms)
respective
tightly
since the difference in the
resulting shift maybe small (< 1.0 eV).
chemical
The identification of
state
depends
pri-
marily on the accurate determination of the binding energies.
narrow
To determine binding energies accurately, a peak with
sweep range
must
be
recorded
with
good
statistics,
and
accurate correction must be made for insulating specimen when
static charging is present.
During analysis, insulating samples tend to acquire a
steady state charge.
between electron
loss
This steady state charge is
from
the
balance
emission
by
surface
a
and
electron gain by conduction or by acquisition of slow thermal
electrons from the vacuum space.
problem
A serious
exact determination of the extent of
shifting.
charge
steady state charge, usually positive, adds to
tion of the photoelectron and tends to make
higher binding energy. Charging can shift
is
the
peak
peak
the
The
retardaappear
at
location
by
more than 2 eV as well as affecting the shape of the peak.
The method for valid charge correction for insulating
105
samples is to reference the carbon Cls line (284.6 eV) of the
on
adventitious hydrocarbon present
Any shift in the Cls peak from the
is
charge correction procedure
eV
284.6
taken as a measure of the static charge.
surface.
sample's
the
of
An example
presented
the
4.11.
Figure
in
be
can
value
Verification of the measured extent of static charge shifting
can be obtained by establishing the
species
least one of the
by
an
chemical
state
for
(ie.,
method
independent
at
mossbauer, x-ray diffraction, and Auger spectroscopy).
Instrumental calibration is essential
tive analysis.
conductive
calibration.
for
quantita-
To eliminate problems arising from
charging,
samples
The
first
for
employed
were
analysis
spectrometer
the
a
established
linearity
problem with the electronics, where the deviation in apparent
binding
binding energies (ie., measured) from the referenced
energies for three conductive standards (eg., Cu, Ag, and Pt)
increased with increasing binding energy.
After
realinement
tenths
of the spectrometer, the deviation was within two
of
an eV, Table 4.10.
for
After correcting
charge
static
shifting,
chemical state of inorganic species in the fume
mined by comparing the apparent binding
ference binding energies compiled by
PHI
deter-
were
energy
to
(Wagner
the
the
et.
real.,
identification
was
obtained by checking the reference binding energies with
our
79).
Conformation
for
chemical
own standards, Table 4.11.
state
The error in
the
peak
location
1
Charge Shift of 2.7 eV
S#
A - Measured carbon
peak (287.3 eV)
%.,
B - Reference C is
line (284.6 eV)
LL
I'A
'A
n
293
289
m
I
285
BINDING ENERGY
I
281
(eV)
Figure 4.11 - ESCA high resolution scan of carbon C is line for fume sample
107
Table 4.10 - Calibration of ESCA spectrometer with
Au, Ag, and Cu metal standards
apparent
binding energy
reference
binding energy
83.8
84.0
Ag
367.9
367.9
Cu
932.4
932.4
standard
Au
peak
4f
7
/
2
108
Table 4.11 -
element
Mg (2p)
Comparison of the peak position from
standards with the reference binding
energy compiled by PHI (Wagner, 79)
compound
Standard's
binding energy
(ev)
51.6 *
49.4
MgO
711.4
Fe (2P 3 / )
2
Si (2p)
SiO2
S
Na 2 SO
(2p)
reference
binding energy
(eV)
4
Na (is)
Na2SO4
Ca (is)
CaO
103.1
103.4
168.8
168.6
1071.2
1071.3
347.4
346.3 *
* Castleton (81) also
disputes PHI B.E.
109
for Mg and Ca for the spectra
compiled by
PHI (Wagner et. al.,
1979) has been confirmed from work conducted at the DOE research
facility, METC (Castleton,
1981).
110
CHAPTER FIVE
FACTORS INFLUENCING THE BULK CHARACTERISTICS
OF THE SUBMICRON PARTICLES
combust a dilute stream of pulverized
furnace.
drop
the
well controlled conditions in
coal
characteristics of the
Specifically,
the
conditions,
residence time,
of
physico-chemical
particles
submicron
influence
the
govern
which
examined.
were
condition,
combustion
the
flow
laminar
For the carefully controlled combustion
the important variables
under
particles
tube
size
and coal type on mass loading, particle
distribution (PSD),
particle morphology,
to
was
The experimental basis of this investigation
chemical
bulk
and
composition of the submicron particles were examined.
5.1 FLY ASH MASS DISTRIBUTION
particulate
The mass distribution of the
products (ie.,
final filter
(FF)
from
(45/53 um) at a furnace
the
combustion
temperature
stages
impactor
fly ash) on the cascade
of
of
combustion
Montana
1700
K
lignite
in
twenty
The
percent oxygen in nitrogen are presented in Figure 5.1.
mass distribution of the
fly
ash
was
obtained
by
and
weight
analysis of the various impactor stages and final filter. The
weights are based on one gram of coal burned.
Clearly,
bulk of the mass is collected in the preseperator
(stage
the
0)
111
.01
C
CE
cD
.0001
F-
F-
FF
.00001
-1.0
7
6
5
I
0.0
4
3
2
1
0
_
1.0
LOGIo(AERODYNAMIC DIAMETER
um)
Figure 5.1 - Mass distribution of particulate combustion products
on cascade impactor stages and final filter
112
greater
where the aerodynamic diameters of the particles are
than 11.5 microns.
mass
The
the
stages are refered to as
are
which
ash
fly
residual
impactor
various
the
a
(stages
percent of the mass is collected on the final stages
The particles collected on
of
tenths
decreasing particle size where less than three
6 and 7).
with
successively
decreases
inclusions
produced by fusion and coalescence of the mineral
The variation in ash
in each coal particle during combustion.
particle size, which is considerably greater than that of the
parent coal particle,
is
attributed
content of individual coal
particles,
by the ash (Ramsden,
1977;
1969; Sarofim et. al.,
Williams, 1965; and Raask,
1966).
increase in the amount of material
filter
fragmentation
to
a
is
There
on
collected
of
formation
and to cenosphere
coal particles during combustion,
ash
in
differences
to
and
Raask
sigificant
final
the
relative to stages 6 and 7 which reflects the
bimodal
field
studies
in
observed
nature of the fly ash previouly
(Marcowski et. al.,1980; McCain et. al., 1975).
the inorganic vapors, which pass through the
filter
submi-
condensation
cron mode is believed to be formed from the
impactor and collect on the final
The
cascade
entire
because
of
their
of
less
than
0.5
obtained
from
the
cascade impactor for various combustion conditions where
the
small size with
aeorodynamic
diameters
of
microns.
The fly ash
mass
distributions
main gas oxygen partial pressure was varied from ten
percent
113
to sixty six percent for the combustion
of
Montana
(45/53 um) at a furnace temperature of 1700 K, are
in Figure 5.2.
The relative
mass
stages depends
only
on
(i.e.,
weakly
loading
the
on
lignite
presented
the
combustion
various
conditions
oxygen partial pressure) where a slight shift in
loading to the lower impactor stages with
partial pressure is observed.
loading on the final filter
increasing
On the other
is
extremely
hand,
collecting on the final filter increases
over
magnitude as the oxygen partial pressure
is
oxygen
the
sensitive
specific combustion conditions where the amount
mass
to
of
an
mass
the
material
order
increased
of
from
ten percent to sixty six percent.
Transmission electron
collecting on the final filter
micrographs
of
are presented
the
in
material
Figure
5.3
in
10,
for the combustion of Montana lignite (45/53 microns)
20, and 40 percent oxygen
in
nitrogen.
The
samples
were
prepared for the TEM by depositing the submicron particles on
TEM grids
placed
in
the
electrostatic
stream from the cascade impactor.
the final filter
are as
extremely
small,
less
Under these conditions
the
which
are
existence of clustered and chained
agglomerates,
composed on the average of three to four
particles, is observed.
down
The particles collecting on
anticipated
than 1000 angstroms in diameter.
precipitator
The average
discrete
primary
spherical
particle
increases with increasing oxygen partial pressure.
oxygen partial pressures where the mass loading is
size
For
low
small,
the
114
0.3
U'
0.2
A
- stage 0 & I (x 0.1)
D
- stage 2 & 3 (x 0.25)
0
- stage 4 & 5
+
- stage 6 & 7
X
- final filter
CD
H-
7
CD
CD
F-
0.0
0.0
0.2
0.4
OXYGEN PARTIAL PRESSURE
0.6
(atms)
Figure 5.2 - Distribution of particulate combustion
products for various combustion conditions
CT
2~I-
0
.
,.
w
01i
::
~ i~~
W
k
Figure 5.3 - Fume particles produced from the combustion of Montana lignite
in 40 (A),
20 (B),
and 10 (C) percent oxygen in nitrogen.
I
116
many
of
presence
observed,
transmission by their faint haloes is
(C).
Figure
5.3
their
mor-
by
identified
The faint haloes have been
in
visible
made
particles
transparent
phology as sulfuric acid mist (Frank and
Lodge,
1967).
As
the oxygen partial pressure is increased
(i.e.,
increase
in
mass loading), the observed presence of
sulfuric
acid
observed.
mist
acid
sulfuric
the
of
presence
percent the
fifteen
of
pressure
partial
diminishes where above oxygen
mist
not
is
For low oxygen partial pressures, the simultaneous
collection of sulfuric acid mist with the inorganic submicron
integrity
environment.
The collection
this
in
particles
inorganic
the
of
reactive
filtration
mist has been shown to be highly dependent on the
temperature and
the
flow
gas
(Cheney et. al., 1979).
filtration
104
(ie.,
temperature
results from the dilution of the
collected
under
those
concern about anomalies
for
in
the
in
dew
order
analysis
The
point)
Consequently,
analysis
chemical
conditions
m3 /sec.
products (e.g.,
combustion
SO 3 and H2 0) by the quench gas upon sampling.
all samples being prepared
temperature
filtration
above 323 K for gas flow rates above 1.7 x
low
time)
The collection of sulfuric acid mist
can be eliminated by maintaining the
relatively
contact
(i.e.,
rate
acid
sulfuric
for
efficiency
the
about
concern
raises
filter
final
the
particles on
due
to
to
will
be
alleviate
sampling
artifacts.
The volume-mean diameter
of
the
primary
submicron
117
where
particles produced under various combustion conditions
ten
the main gas oxygen partial pressure was varied from
seventy five percent for the combustion
was
from
obtained
TEM
micrographs
by
the
cles were sized for each condition. As previously noted,
submicron
the
with increasing oxygen partial pressure
increases
particles
(i.e.,
an
parti-
automated image analyzer where between 200 and a 1000
volume-mean diameter of
in
submi-
primary
The volume-mean diameter of the
cron particles
shown
is
K
(45/53 um) at a furnace temperature of 1700
Figure 5.4.
lignite
Montana
of
to
increase
in
mass loading of fume).
cron range (i.e.,
filter)
those
particles
and on various impactor stages for the combustion
fifteen
in
Montana lignite (45/53
um)
oxygen in nitrogen are
shown
5.5.
Figure
in
were
used.
The
determination
actual
The
through
actual
the
of
of
percent
forty
and
volume-mean diameters of the particles at stages one
seven,
final
the
on
collected
submi-
the
The volume distributions of particles in
volume-mean diameters for each of the stages is discussed
in
section 3.4.
The actual volume distribution of the particles
collected in
the
preseparator
is
presented
order
in
to
illustrate the distribution of the residual fly ash particles
greater than 5.65 microns.
The distinct
separation
between
the residual fly ash mode and the submicrom particulate
clearly demonstrates the bimodal nature
from the direct
combustion
of
of
pulverized
fly
coal
ash
mode
formed
particles.
118
500
400
o
<
cc
LU
300
FH
LU
r2:
I
LJ
200
CD
-
100
0 .
0.0
0.2
0.4
0.6
0.8
1.0
OXYGEN PARTIAL PRESSURE (atms)
Figure 5.4 - Volume-mean diameter of submicron primary
particles for various oxygen partial pressure
I
-6H
I
I
I
Montana lignite fly ash PSD
o
- 15% 021N2
O - 40% 0 2/N2
C.
o
.-
CD
CD
-8H
_0
0o
CD
CD
I
-10 -
-12
-3
-2
-1
LOG1 o DIAMETER
pm
Figure 5.5 - Particle size distribution of particulate
combustion products
120
Comparison
of
the
distributions
for
the
two
conditions clearly illustrated the sensitivity of
mass loading and volume mean diameter of the
combustion
the
total
submicron
mode
to the combustion condition.
5.2 BULK CHEMICAL COMPOSITION
The composition of
the
major
oxides
in
the
fume
produced under various combustion conditions where
the
main
gas oxygen partial pressure was varied from
fifty percent for the combustion of
shown
is
The concentration of the major oxides
by INAA, AA,
and
liquid
MgO
partial pressures the refractory oxides,
counts for most of the mass,
while
were
At
chromatography.
at
low
the concentration
of
sulfur
in
increasing oxygen partial pressure,
the
to
(45/53
in
Figure
determined
higher
and
oxygen
ac-
CaO,
oxygen
pressures the volatile species Na20 accounts for
fraction of the mass of the particles.
percent
lignite
Montana
um) at a furnace temperature of 1700 K
5.6.
ten
partial
significant
Despite the fact that
fume
decreases
with
the relative retention of
sulfur in the fume increases from one to six percent,
Figure
5.7.
Since the rate of oxidation of SO 2 to SO 3 is
extremely
slow,
the increase
stable
inorganic
in
the
formation
of
sulfate with increasing amount of fume appears to account for
the observed decrease in the amount of sulfuric acid mist
at
ambiant temperatures.
The concentration of
the
major,
minor,
and
trace
121
100
I
I
I
I
MgO
CaO
4-
Si02
10
S0
3
Na2 0
.0
0.1
0.2
0.3
0.4
0.5
OXYGEN PARTIAL PRESSURE (atms)
Figure 5.6 - Variation of composition of the fume
for various combustion conditions
0.6
122
1
1
I
0.06
LL
ICD
L
F-
0.04 -
2:
LL
-j
HF-
0.02 H
0.00
12
WEIGHT OF FUME
16
(mg/gcoal
Figure 5.7 - Sulfur retention in the fume
20
123
inorganic elements are given in
Montana lignite, North
Pittsburgh No 8.
Table
Dakota
lignite,
tion of the mineral matter
[SiO2 ],
for
discrete
kaolinite
the
metal
From detailed
1982),
the
mineral
phases
such
pyrite
[FeS],
4
],
[CaCO 3 ], and dolomite [MgCa(CO 3 )2 ]. The
occur as either impurities in
content
coals
as
were
quarts
calcite
elements
trace
major
the
and
characteriza-
(Quann,
[Al 2 SiO5 (OH)
coals:
Rosa,
Alabama
The major, minor, and trace
of the different coals do vary.
found to contain
5.1
forms
mineral
can
as
discrete mineral phases in extremely small quantities
or
as
chelated compounds in the organic matter of the coal.
In
the
occur
as
the
high rank bituminious coals,
phases
mineral
micron sized mineral inclusions embedded in the
matrix. By contrast,
rank lignites are not
carbonacious
the alkaline earth
metals
in
the
any
extent
as
discrete
present
to
low
mineral phases but rather as an intimate part of the chemical
structure of the coal. More
precisely,
they
in
occur
ion
exchanged form on carboxyl and phenolic function groups.
The amount and composition of the fumes produced from
temperature
the combustion of each of the coals at a furnace
of 1700 K in twenty percent oxygen in nitrogen are
Table 5.2.
The amount and concentration
much more dramaticaly than
anticipated
amoung the original coals.
For
amount of fume produced is
the
of
from
the
the
bituminous
less than that for
coals, particularly the high sodium containing
the
given
fume
in
vary
variation
coals,
the
low
rank
North
Dakato
124
TABLE 5.1 - Coal composition for major, minor, and trace
elements
element
MS(L)
ND(L)
weight
AL(B)
PT#8(B)
percent
Mg
0.61
0.31
Si
0.75
0.55
1.65
.36
Al
0.71
0.41
0.66
.88
Ca
1.72
1.13
Fe
0.20
0.49
0.36
.89
S
0.55
0.55
0.77
.83
.073
.59
parts p er million
Na
250.0
7360.0
330.0
650.0
K
300.0
300.0
1700.0
810.0
3.4
21.0
As
4.0
Zn
11.0
62.0
Cr
7.5
16.0
Sc
0.83
1.1
2.0
7.5
6.9
7.1
ASTM
ash
<10.0
2.0
7.4
* weight percent
125
Table 5.2 - Submicron fume composition
element
MS(L)
ND(L)
Al(B)
weight percent
59.9
13.3
0.6
3.0
SiO2
3.6
1.4
22.6
46.9
A1 2 0 3
0.4
0.1
1.3
1.0
CaO
8.9
2.7
1.07
1.5
FeO
12.4
14.3
54.5
20.9
Na 2 0
4.3
38.4
9.4
13.9
SO 3
6.6
29.8
5.6
MgO
Pt#8(B)
parts per million
As
375.0
200.0
6490.0
984.0
Zn
1140.0
650.0
3900.0
4160.0
Cr
360.0
154.0
962.0
3630.0
Sc
0.8
0.3
42.0
4.3
2.86
8.85
Percent of
ASTM ash
1.63
1.56
^~l~-i-lll^-~-~I~I~YPVL
~
L -126
lignite.
The fume from the bituminous coals are dominated by
silicon and iron while in contrast the
fumes
from
the
low
rank coals have significant and dominant amounts of magnesium
and calcium in addition to iron. The
fume
lignite is mostly composed of sodium.
The
for the high sodium fume of
North
of
North
sulfur
Dakota
retention
lignite
thirty percent while for the relatively low
Dakota
is
sodium
fume
Alabama rosa, the sulfur retention is below one percent.
sulfur content in the various fumes
correlates
with the sodium and potassium content in each
examined,
over
of
The
fairly
well
the
fumes
of
Figure 5.8.
The combustion conditions have been shown to
influence the nature of the particulate
where the amount,
composition,
and
particles are extremely sensitive
combustion
size
to
of
the
combustion
greatly
products
submicron
condition.
the combustion
In order to facilitate a better understanding,
process will be reviewed.
5.3 COMBUSTION TEMPERATURE TIME HISTORY
Extensive theoretical and experimental
been devoted to the subject
(Field et.
al.,
of
1967;Smith,
conditions being investigated,
pulverized
1978).
Under
where the coal
efforts
have
coal
combustion
the
combustion
particles
are
less than 150 microns in diameter and the particle combustion
temperatures are above 1600 K, the rate of combustion will be
externally diffusion
limited.
Therefore,
the
temperature
127
4
E
- Beulah N.D.
0
S-
Ill # 6
Montana lignite
E
A - Alabama Rosa
Co
3
DOC
D
DI
I
1
2
3
LOGio (Na AND K IN FUME ug/gcoal)
Figure 5.8 - Comparison of S to Na and K in the fume
4
128
the
in
pressure
dependent on the oxygen partial
highly
be
will
particle
char
time history of the burning
gas
bulk
phase.
lignite particles will be determined
color optical pyrometry.
greater
(Timothy,
82;
discussed
else-
Alterichter,
81).
employed where
Briefly, a slightly modified feeding system is
coal
the measurement of the temperature of single
presented was
obtained
(1980),
(1979), Altrichter
particle burning
times
were
invesigations
the
and
(1981)
Timothy
obtained
particles
be
to
data
The temperature-time
from
allow
will
This
single particles can be fed individually.
from ignition to burnout.
of
application
The priniciple and
detail
two
by
experimentally
this technique to the combustion furnace is
where in
Montana
for
The combustion temperature-time history
Dictor
by
the
where
with
simultaneously
particle temperature measurements for single coal particles.
Temperature measurements by two color optical pyrometry were obtained for individual Montana lignite particles in
oxygen partial pressures greater than fifteen percent, Figure
5.9.
parti-
At each condition, fifteen to twenty individual
cles were examined.
At lower oxygen
light emitted from the burning particle was
of
the
insufficient
Consequently,
detection.
intensity for photomultiplier
pressures
partial
at
extremely low oxygen partial pressure the particle combustion
temperatures
temperatures.
were
assumed
As
evident,
to
approach
increasing
the
the
bulk
oxygen
gas
partial
129
2800
2400
LU
2000
UJ
02
LU
I-
1600
1200
0 .0
0.2
0.4
0.6
0.8
1.0
OXYGEN PARTIAL PRESSURE (atms)
Figure 5.9 - Average particle temperature of Montana lignite
(90/105 pm) by two color pyrometry.
130
pressure results in an increase in the combustion temperature
The particle
of the coal particles.
exceeds the bulk gas temperature due
heat
released
oxidation
reaction
the
to
from the highly exothermic heterogeneous
occurring at the surface of the
temperature
combustion
oxygen
low
For
particle.
partial pressure in argon the particle combustion temperature
is
However,
pressure.
increase in
the
at
oxygen
high
particle
partial
combustion
gradual with increasing oxygen
partial
oxygen
in
variation
to
sensitive
extremely
the
pressures
temperature
is
pressure.
This
has
of
the
partial
been attributed to the high temperature dissociation
more
heat
gases in the particle boundary layer which enhances the
transfer away from the burning char particle (Sarofim,
1979).
For the two furnace temperatures examined (e.g., 1700
K
1250 K),
the characteristic trends in
the particle combustion
temperatures with oxygen partial pressure are
the magnitude of the particle
temperatures
combustion
dependent
is
shown
to
be
and
while
similar
during
achieved
on
furnace
the
temperature.
The average time elapsed between ignition and burnout
of 90/105 micron Montana lignite particles for oxygen partial
pressures from ten to one hundred precent are shown in Figure
5.10.
The various studies
qualitatively
display
response: as the oxygen partial pressure decreases the
burning time of the particle increases.
fusion limited
combustion
of
For
non-swelling
same
the
total
external
coals
such
difas
131
150
I
I
I
I
A - Dictor 80 (75/90 pm)
125 k
O - Altrichter 81
0
- Timothy 81
100
G)
(i)
E
LLJ
F-
75 k
O
50
A
CD
Z
FD
Q3
El
A
A
25
0 I
0.0
I
0.2
I
0.4
I
I
0.6
0.8
OXYGEN PARTIAL PRESSURE (atms)
Figure 5.10 - Burning time for Montana lignite
(90/105 pm)
1.0
132
governed
Montana lignite the burning time should be
by
the
Eq.
5.1
relations
b
t
24 cw D
2
where tb is
d°
Ln(1 + X0
02
the total burning time,
with respect to carbon content, DO
cient for 02,
and XO222 is
)
p is the particle density
of
fraction
the mole
coeffi-
is the diffusion
is
XO2)
limited
obtained, Figure 5.11, indicating external diffusion
combustion.
As
oxygen.
evident, a linear correlation of do2/tb with In(l +
The burning times for Montana lignite at a given
oxygen partial pressure will proportional to
of
square
the
The main gas temperature does
the intial particle diameters.
not appear to have significant effect on burning time as seen
from the comparison of the burning times between Timothy with
a main gas temperature of 1250 K and Alterichter at 1640 K.
particle
The temperature/time history for the single
are
feed experiments by two color optical pyrometry
to apply to the combustion of particles in
experiment.
From those studies,
dilute
the
cloud
the temperature/time history
shown
for the combustion of Montana lignite has been
the
main
gas
results
in
corresponding increase in combustion rate and temperature
burning coal particle.
examine the mechanism of
The focus of the
ash
next
vaporization,
be
to
Increasing
highly dependent on the oxygen partial pressure.
the oxygen partial pressure in
assumed
section
the
extent
a
of
will
of
133
0.010 -
0.008 C
U)
E
CU-
0.004 -
0.002
0.000
0.0
0.4
0.2
Ln (
0.6
+ X 2)
Figure 5.11 - Reciprocal of burning time verse
In (1 + Xo0)
0.8
134
which
will
be
highly
sensitive
to
the
temperature/time
history of the mineral constituents associated with the
particle. Since the submicron particles are formed
volatilized inorganics,
the
vaporization
establish the influence that the
from
mechanisms
combustion
coal type have on the ultimate composition,
char
should
conditions
amount,
the
and
and
size
of the submicron particles.
5.4 VAPORIZATION
It
has been proposed that the
elements in the fume,
final filter,
determination
of
the
that material which is collected on the
should prove to be a sensitive measure
degree of ash vaporization,
of
the
since the submicron particles are
formed from the condensation of
the
vaporized
ash
(Quann,
1982).
The fraction of the total
ash
elements appearing on the final filter
and
the
for various combustion
conditions where the oxygen partial pressure is
six to fifty percent oxygen for
lignite (45/53 um)
Figure 5.12.
Ca,
Si,
of
from
Montana
shown in
The fraction of refractory constituents Mg, Al,
order of magnitude
as
the
In contrast,
the
oxygen
particle combustion temperature)
Na,
combustion
varied
at furnace temperature 1700 K, is
Fe and Sc appearing in
fifty percent.
the
constituent
the
is
fume
increase
increased
from
six
constituents
volatile
on
an
pressure (i.e.,
partial
and Sb show only a slight dependence
over
oxygen
to
As,
partial
135
1.0
o
o-
0A
N
Mg
Mn
0.I
Co
ASH
0.01
AI
MONTANA LIGNITE
GAS TEMPERATURE 17500 K
0.001
0.I
0.2
0.4
0.3
P [02])
0.5
0.6
ATM
Figure 5.12 - Dependence on oxygen partial pressure of
elemental and ash fraction in fume
136
pressure where the fraction of these
the fume is relatively high (>
forty
combustion conditions examined,
the
should
have
pressure.
totally
vaporized
elements
appearing
percent).
volatile
due
to
Under
trace
their
in
the
species
high
vapor
The discrepency between the amount of the volatile
species appearing on the final filter
and that in the
parent
coal may be attributed to the fact that some of
these
tile species will heterogeneous condensation on
the
of the larger ash particles.
Evidence for the
condensation of the volatile species on the
dependence of their concentration
classified samples of the
on
residual
heterogeneous
of
the
examining
the
particle
fly
surface
surface
residual fly ash particles can be obtained by
ash
vola-
size.
Size
particles
for
chemical analysis by INAA were obtained by using the
cascade
impactor.
species
The concentration of some of the
on the various stages
of
the
cascade
typical
impactor
and
final
The concentration of
those
species in the overall ash are shown on the right axis.
The
concentration
and
filter
are shown in
of
Figure 5.13.
magnesium,
calcium,
iron,
alumina,
scandium show no systematic variation with particle size.
the other hand,
antimony,
the volatile trace species such
and sodium all show pronounced
as
increasing
On
arsenic,
concen-
tration trends with decreasing particle size. Relative to the
parent ash,
sodium in
the
concentration
of
arsenic,
antimony,
and
the larger particles are depleted while the smaller
ones are highly enriched.
tration for sodium,
The observed increase
arsenic,
and
antimony
with
in
concen-
decreasing
137
IMPACTOR STAGE
Al
Al
Mg
I4
10
No
No-
B
Mn
As
As-
10
I
-%
0.2
I
I
I
I
II
0.5
t
10
dp (,um)
Figure 5.13 - Variation of elemental mass fraction with
particle size for the residual fly ash
138
particle size suggest that these species condense heterogene-
1979).
al.,
et.
Smith
al., 1974; Ondov et. al., 1979; and
et.
(Davison
ously on the surface of the residual fly ash.
in
The concentration trends for the volatile element arsenic
the residual fly ash produced from the combustion of
lignite (45/53 um)
oxygen
are
The observed concentration trends
for
in 5,
shown in Figure 5.14.
Montana
10, 20,
and 50
percent
with
the different combustion conditions are best correlated
inverse
diameter.
increased,
As
the
the slope of the
particle diameter is
amount
concentration
arsenic
with
coencides
with
of
This
less accentuated.
fume,
the increase in the amount of arsenic collected in the
Figure 5.12.
The
parallel
and
paths
competing
chapter seven.
residual and submicron) will
for
the
of
the
modes
deposition of the volatile species in the two
fly ash (e.g.,
is
produced
fume
of
be
examined
in
vaporiza-
The determination of the degree of
tion and hence the rate of vaporization will be restricted to
the refractory species.
the
The details of vaporization of
mineral
various
constituents have been investigated in a complimentary
(1)
have
The finding from his investigation
by R. Quann (1982).
established;
study
the
internal and external
vaporization
vapor
rate
transport
is
contolled
by
(2)
the
processes;
volatilization of the refractory oxides is enhanced by
chemical reduction to their more volatile suboxide
metal (Mg) vapors due to the extremely
reducing
their
(SiO)
or
environment
139
1600
1200
rzn
0m
0m
800
H
C-
CI
U
o
400
0
1
INVERSE DIAMETER
2
(Ao)
Figure 5.14 - Concentration of As in sized classified
residual fly ash particles
140
of vaporization
particle size,
(extraneous,
coal
original
the
on
dependent
be
will
rate
the overall
(3)
found within the burning char particle;
forms
the degree of dispersion of the mineral
inherent micron size
inclusion,
and
dispersed within carbon matrix),
present in the specific coal (e.g.,
or
the
on
atomically
forms
mineral
quartzs vs Kaolinite
for
silicon).
external
The rate expression for
diffusion
limited
vaporization can be derived from Fick's first law:
dni
Rvap
2
_
dt
7
Di
Ps (T )
s p
d (t)
p
2 R
RT
Eq. 5.2
where P (T ) is the vapor pressure at the char surface, T is
P
s p
the particle temperature, n i is the number of moles vaporized
from the single particle, Di
d (t)
of
diffusivity
is the
is the char particle diameter at time t.
burning char particle is modeled as a shrinking
and
nonswel-
For
lignite
ling coals which is the case for Montana
i,
coal,
the
sphere
with
the time dependence
d (t) = d
1
t)1/ 2
Eq. 5.3
where d0 is the intial diameter and tb is the burning time of
the
char
particle.
In
substituting
relationship for char particle
size,
the
equation
integrated over the total burning time. The
ized is
time
dependence
5.2
fraction
can
be
vapor-
141
4 (tb)
8 Di
Ps(T)
tb
Eq. 5.4
Eq. 5.4
2
Co R T do
where Co is now the initial volume concentration of species i
in the particle. Assuming a Clausius - Clapeyron
tempertaure
dependence of the vapor pressure, the fraction vaporized will
have the following tempertaure dependence.
dln b
1
dHvap
R
Eq. 5.5
tb
where Hvap is
the
heat
of
diffusion limited vaporization, the log of
time)
rate (extent vaporized per unit
proportional to particle temperature.
5.15,
a linear
externally
For
vaporization.
the
should
vaporization
inversely
be
As evident
in
relationship is obtained between the
the rate of vaporization verses particle combustion
ture for Mg, Al, Si, Ca, Fe, and Sc where
the
Figure
log
of
tempera-
burning
time
and average particle temperature were obtained from two color
optical
pyrometry
studies.
At
conventional
combustion
temperatures (~2000 K) the rate of vaporization increases
the sequence Al, Ca,
Fe,
in
and Mg where the relative rates span
over three orders of magnitude.
Time-resolved measurements of the
Quann (1982) are shown in Figure 5.16.
ash
vaporized
by
In these experiments,
Montana lignite (nominal 140 micron diameter) was
burned
in
142
K
100
3500 3000 2750 2500
2250
2000
10
No
As
Zn
I
Mg
Fe
0.1
.01
I
I
I
3.0
4.0
5.0
I x 104
Tp
K-1
Figure 5.15 - Temperature dependence of the elemental
vaporization rates
50
TIME (msec)
100
150
200
250
300
2800
2600
2400 -u
P:1
2200
C
;r
2000 m
1800
2.5
5.0
7.5
10.0
12.5
DISTANCE FROM INJECTION (cm)
15.0
Figure 5.16 - Time-resolved profiles of maximum yield of char and submicron
particle constituents
144
thirty percent oxygen in nitrogen at a furnace temperature of
1750 K.
The position of the collection probe was adjusted to
quench the char particle
stages of burnout.
determine the
combustion
Weight
extent
loss
of
process
at
measurements
burnout
at
successive
were
each
used
position.
fraction of ash vaporized was determined from the
to
The
amount
of
fume collected on the final filter of the cascade impactor at
each residence time.
During the first 100
coal is
the
heated
initial period,
to
furnace
milliseconds,
temperature.
the
After
this
the char particles ignite and burn to comple-
tion at about 12.5 cm.
The
data
demonstrate
vaporizes only during the char combustion
expected because it
is
during
this
that
process.
period
that
experiences the locally reducing atmosphere within
the
ash
This
is
the
ash
the
char
and the tempertaure overshoot caused by the
exothermic
char
It
post
combustion reation.
is
of note that in the
tion period there is no measurable
change
in
combusmass
the
of
submicron material.
For the time resolve vaporization study,
the
varia-
tion in primary particle size was examined, Figure 5.17.
submicron particles are shown to form at the
combustion,
where their
size
increases
very
The
of
char
rapidly
with
onset
position.
The
vaporization
charateristics
elements which are dependent on both
the
tions and coal type ultimately determine
of
the
inorganic
combustion
the
amount,
condisize,
145
200
150
o
C)
LU
CI
100
F-
L
__J
50
4
8
12
DISTANCE FROM COAL INJECTION (cm)
Figure 5.17 - Time resolve measurements of the mean primary
particle radius for the submicron particles
16
146
and composition of the submicron
fume produced
depends
on
the
particles.
combustion
history and the characteristics of the
The
amount
of
temperature/time
mineral
since
forms
on
the extent of vaporization has been shown to be dependent
these two parameters.
The large contribution of
the
refac-
tory oxides which account for the bulk of the fume is
attrioxides
refractory
buted to the enhanced vaporization of the
by their chemical reduction to the more volatile suboxide
the
metal due to the reducing environment found within
particle
during
combustion.
The
char
from
composition
fume
or
different coals vary to a greater degree than in the
overall
ash as a result of the influence of the
in
differences
degree of dispersion of the inorganics and
forms present.
type
of
the
mineral
147
CHAPTER SIX
CHEMICAL CHARACTERIZATION OF
THE SUBMICRON PARTICLES
The following investigation examined the chemistry of
combustion
the inorganic submicron particles formed from the
of Montana lignite (45/53 pm) at furnace temperature of
With the development
K in twenty percent oxygen in nitrogen.
of the analytical capabilities of the
electron microscope (STEM),
ESCA,
transmission
scanning
Auger
size,
elemental
the
the
individual primary particles,
and
spectroscopy,
concentra-
INAA detailed information about the dependence of
tion on particle
1700
composition
elemental
the
of
stratification
the
within the submicron particle, and the chemical state of
major elements was determined.
6.1 BULK COMPOSITION
(K2 0 and SiO2),
and
liquid
chromatography
AA
INAA,
The average bulk particle concentration by
(SO3 )
of
four
The concen-
separate fume samples is presented in Table 6.1.
tration of those species in the overall ash, calculated
the coal composition with an ash content of
also presented in Table 6.1.
All the elements are
in their highest oxidation state.
of the submicron
particles
percent
7.1
are
The dominant
the
refractory
from
is
presented
constituents
oxides
of
148
Table 6.1
Chemical analysis of the submicron
particles produced from the combustion
of Montana lignite (45/53 um) at
furnace temperature of 1750 K in
twenty percent oxygen in nitrogen.
element
weight percent
MgO
59.9
3.6
SiO2
Al 2 0
0.4
3
CaO
8.9
FeO
12.4
Na20
4.3
SO 3
6.6
parts per million
375.0
Zn
1140.0
Cr
360.0
Sc
0.8
Percent of
ASTM ash
2.86
149
The
percent).
trace
a
particles, having
of
concentration
submicron
over
two
order
(i.e., oxide weight), approximately fifteen
the fume with its extremely high surface
makes
the
of
adsorption
Repetitive
(Rothenbergand Cheng, 1980).
showed that the dry weight
of
filter
was
analysis
of
percent
the
The hydroscopic nature of
weight could not be accounted for.
m2/g),
the
chemical
by
determined
of
When
ash.
final
the
on
weight of the material collected
amount
the
overall
magnitude greater than that in the
compared with the
in
enriched
antimony, and sodium are highly
arsenic,
as
such
species
volatile
(8.0
iron
magnesium (56 percent), calcium (10 percent), and
the
133.0
(i.e.,
area
unavoidable
water
experiments
drying
filter coincided with the weight of the oxides.
final
the
on
material
When the dry
filter was exposed to the room environment, the filter sample
The
regained its original weight within ten minutes.
experiments suggested the presence
thus it can be assumed that
the
of
total
water,
adsorbed
amount
drying
of
and
material
collected on the filter is now accounted for.
6.2 DEPENDENCE OF CONCENTRATION ON PARTICLE SIZE
Size classified samples of
the
submicron
particles
for chemical analysis by INAA were obtained with the mobility
classifier.
The
chemical
composition
of
the
individual
submicron particles were obtained from microprobe analysis by
the STEM.
150
400
ang-
classifier.
The
and
with mean particle diameter of 100, 200, 300,
stroms were
obtained
with
mobility
the
particles
submicron
the
Four discrete size cuts of
in
As
relative concentration of Mg, Ca, Fe, Al, Na, Sb, and
each of the four size cuts are shown in Figure 6.1 and Figure
6.2.
Sodium, arsenic, chromium, antimony, and
all
aluminum
show increasing concentration trends with decreasing particle
size.
Iron, and calcium
show
systematic
no
slight decrease in
concentration
shows
magnesium
while
concentration with particle size,
a
particle
decreasing
with
in
variation
size.
The STEM's capability for microprobe analysis
provi-
des for the first time detailed information about the
compo-
sition of
individual
As
particles.
submicron
mentioned in section 4.5, only those
previously
(i.e.,
species
stable
over
Si, Mg, Fe, and Ca) whose x-ray signals remain constant
time
under
examined.
the
high
Microprobe
electron
intensity
the
of
analysis
particles, with diameters ranging
primary
between
hundred angstroms were choosen at random.
the
elemental
particles.
concentration
the
for
There is no major trend in the
magnesium, calcium, and iron with
beam,
eighty
Figure
various
size.
magnesium and iron do show a slight variation
is relatively constant.
submicron
four
and
6.3
shows
primary
concentration
particle
tion from particle to particle, while
be
will
calcium
in
of
However,
concentra-
concentration
Silicon, on the other hand, exhibits
151
100
80
60
H
7-
Cr-
40
CU
£3
20
0
100
200
DIAMETER
300
(Ao )
Figure 6.1 - Elemental concentration for size
classified samples by INAA
400
152
10
A - Na
0E
- Al x 1.4E 1
0- - Sb x 2.8E 3
8
+
- As x 5.1E 2
0-
CD
)---
UJ
4
L1
CD
2
0
0
I
I
I
I
100
200
300
400
DIAMETER
(AO)
Figure 6.2 - Elemental concentration for size
classified samples by INAA
153
100
A - Mg x 0.2
OL - si x 10
80
0 - ca
+ - Fe x 0.5
60
CH-
z..
40
20 20
0
0
I
I
I
I
100
200
300
400
DIAMETER
(AO)
Figure 6.3 - Elemental concentration for primary
particles by STEM
154
decreasing
particle
Results showing the concentration trends for
calcium,
a marked increase in concentration with
size.
analysis
STEM
iron, and magnesium with particle size by
individual particles and by INAA of sized classified
of
samples
of a large number of particles are in agreement.
decreas-
The observed increase in concentration with
is
ing particle size for Na, As, Sb, Al,and Si
the
composed
particles
pre-existing
elements
these
to result from heterogeneous condensation of
on the surface of
hypothesized
of
magnesium, iron, and calcium. The concentration trends result
from the fact that the surface to volume ratio increases with
decreasing particle size.
The microprobe analysis of submicron particles formed
from the
combustion
of
(120/150
lignite
Montana
um)
in
seventy five percent oxygen shows a dramatic variation in the
Table
composition irrespective of particle size,
calcium,
relative concentration of silicon,
The
6.2.
and
magnesium,
iron in the bulk fume sample as determined by INAA and AA are
also shown in Table 6.2.
A
large
the
of
number
primary
particles are highly enriched in either silicon (98 percent),
percent),
calcium (86 percent), or magnesium (76
remaining particles have
percent).
unusually
The analysis suggests
high
iron
these
that
selectively condense, forming relatively
pure
particle
is
due
the
content
(43
species
might
particles
of
presence
of
silicon, magnesium, calcium, and iron where the
other constituents in each
while
to
subsequent
155
Table 6.2
particle
diameter Ao
STEM microprobe analysis of individual
submicron particles produced from the
combustion of Montana lignite (150 um) at
furnace temperature of 1700 K in seventy
five percent oxygen.
Sio
2
(wt%)
CaO
MgO
FeO
(wt%)
(wt%)
(wt%)
80
8
66
15
140
1
51
32
3
2
3
1
1
43
1
0
1
23
60
10
250
92
300
350
425
98
450
14
75
6
480
86
7
3
52
35
3
bulk fume
conc. *
* by INAA and AA
156
heterogeneous condensation and coagulation between relatively
pure particles.
6.3 SURFACE ENRICHMENT
The elemental stratification in
ticle
was
determined
concentration, measured
particle concentration
by:
by
measured
by
INAA,
bulk
with
spectroscopy,
Auger
surface
the
comparing
(1)
par-
submicron
the
liquid
and
AA,
concentra-
chromatrography; (2) determining the variation in
tion measured by Auger spectroscopy as the surface layers are
sputtered away; and (3) comparing the ratio of the
intensity
of the high and low energy Auger electrons emitted
from
the
sample to the ratio for a homogeneous bulk standard.
The surface concentration by
Auger
is
spectroscopy
compared in Table 6.3 with the bulk particle concentration by
INAA (Na2 0, MgO, CaO, and
liquid chromatography (SO3).
FeO),
All
AA
(K2 0,
elements
and
are
and
SiO 2 ),
reported
as
two
the
A
comparison
of
analyses shows that sodium, sulfur,
potassium,
and
silicon
are highly concentrated in the surface layers, while
conver-
their oxides
for
convenience.
sely magnesium and iron are depleted.
From the comparison of
nor
the two analyses, calcium is neither highly concentrated
depleted in the surface layers.
In order to obtain the concentration as a function of
depth from the surface,
the fume sample was
sputtered
at
a
milling rate that corresponds to a surface regression rate of
157
Table 6.3
species
Auger spectroscopy of the fume produced
from the combustion of Montana lignite
(45/53 um) at furnace temperature of
1700 K in twenty precent oxygen in nitrogen.
surface conc.
(Auger)
(wt%)
bulk conc.
(INAA) *
(wt%)
Na 2 0
11.8
5.3
SO 3
16.4
7.9
5.6
K20
1.6
14.9
3.5
CaO
12.3
9.7
MgO
29.3
SiO
FeO
2
3.9
58.6
11.8
* supplemented with AA and
liquid chromatography
158
about twenty five angstroms per minute for a Te205 sputtering
standard (Martin, 1980).
analyses
Auger
the
of
original
surface and at several sputtered times are presented in Table
6.4.
the
The concentration of species in which
enriched, such as sodium,
and
sulfur,
surface
potassium
decreases
surface
with extent of sputtering, while species inwhich the
is
depleted,
such
magnesium
iron,
and
The relative concentration of
proportionally.
sucessive
as
sputtering
remains
increase
with
calcium
that
suggesting
constant,
is
calcium is present near the surface as well as in the core of
the particle.
The asymptotic concentration shows a
signifiThis is
cant contribution from the surface enriched species.
will
unavoidable since the emission for a sectioned particle
6.4
always include contributions from surface layers, Figure
(A).
This is
compounded
by
the
of
nature
the
sample's
surface topography, since the ion sputtering source is at low
grazing angle to the sample's surface, while the detector and
the primary electron source are perpendicular to the sample's
surface, Figure 6.4 (B).
Consequently, shaded regions
are highly enriched with Na, S, K, and Si will
which
persist
with
sputtering.
Another
measure
of
the
elemental
stratification
comparing
the
within the submicron particle is provided
by
intensities from the low energy LMM Auger
electron
emission
Auger
electron
emission
(~100 ev) and the high energy
KLL
(~1000 ev) for each element (Kane and Larraby,,
1974;
King,
159
Table 6.4
Auger spectroscopy of the fume
before and after
species
surface conc.
(wt%)
Na20
11.8
5.6
16.4
SO 3
sputtering.
sputter
1 minute
(wt%)
sputter
3 minute
(wt%)
6.0
4.6
3.7
1.8
6.4
3.5
14.9
13.3
13.8
CaO
12.3
11.3
11.7
MgO
29.3
49.1
53.1
6.5
7.8
SiO
FeO
2
3.9
160
(A)
(B)
FIGURE 6.4 - Sectioned particle showing the contibution
from the outer surface (A) and the shaded
regions from the ion sputtering gun
which are in view of the detector (B).
161
1979).
The extent of
electrons
Auger
the
of
attenuation
moving through a solid will be highly dependent on their mean
free
path
(or
escape
depth
X )
the
in
shown
is
as
relationship;
I(x) = I(o) e - X/A
where I(x) is the intensity
of
Eq. 6.1
Auger
traversed through a solid of thickness x,
sity of the original signal,
the Auger electron.
and
after
having
I(o) is the
inten-
signal
the mean free path
A is
of
For electron energies above 100 eV,
the
to
the
escape depth of an Auger
is
electron
energy of that electron, Figure 6.5.
proportional
Consequently, the ratio
emission
of the intensity of high energy to low energy Auger
for a subsurface element,
relative to the ratio obtained from
a homogeneous bulk standard, will increase as the distance of
the element from the surface increases.
Therefore, a compar-
ison of the intensity ratios for the elements in
cron particles with those
from
homogeneous
the
bulk
For
6.5.
the ratio is close to, but slightly greater than,
from the homogeneous bulk sample.
The observed
identified surface-adsorbed carbon (as CO 2 and
standards
magnesium,
provides a measure of the distribution of sulfur,
and silicon as a function of depth, Table
submi-
the
sulfur
ratio
presence
of
hydrocarbon),
less than a monolayer in thickness on the fume surface,
will
energy
Auger
lead to slight
emission.
attenuation
of
sulfur's
low
Consequently, the results suggest that
in the outer surface layer of the
submicron
sulfur
particle.
is
For
---
O
0
Tarng & Wohnor 1972 Phys. Elect. Conf.
Eastman 1972 P. E. C.Albuquerque
EJ Ridgeway & Hanemsn Surface Spienc- 24
0
451 26
83 (1971)
Palmberg & Rhodin J. Appl. Phys. 39 2425 (1968)
Jacobi Surface Science 26 54 (1971)
A
Baer et al S. S. . 8, 1479 (1970)
Steinhardt et at ICES Asilomar (1971)
+
Seah Surface Science 32 703 (1972)
Au
AL
Ag
C
Au
F.
I
-
I
I
I
•
•I
100
I
!
200
I
.
.r
300
i
1000
I
1500
ELECTRON ENERGY (eV)
Figure 6.5 - Escape depth of Auger electron verses electron energy
N)
163
Table 6.5
element
Ratio of high and low energy Auger
electron emission from the submicron
particles and homogeneous bulk standards
electron
energy
~ AAO
fume ratio
standard ratio
120
1.5
1834
28
76
4
10.4
Si
1606
Mg
35
Mg
1190
20
164
magnesium, the ratio greatly
exceeds
that
ratio
The magnitude of the
homogeneous standard.
sity ratio, a result of extreme
of
gests that magnesium is in the core
the
magnesium's
layer,
sug-
particle.
The
surface
of
the
inten-
sample's
attenuation
low energy Auger emission by the outer
from
extent of attentuation of the low energy emission for silicon
indicates that this element lies between
the
outer
layer enriched in elements such as sulfur and the
surface
particle's
core enriched in elements such as magnesium.
part
The above data is only semi-quantitative due in
to the complexities derived from
the
attenuation by the Auger
electrons
elements and also due to
the
differing
emitted
from
of
averaging
The
is
highly
results
surface layers, whereas the
in
from
results
enriched
Table
of
different
emissions
different chords in the sphercal particles.
Table 6.3 suggest that silicon
degrees
6.5
in
in
the
suggest
silicon is in an intermediate layer. The apparent anomaly can
be explained by comparing the mean free path of
Auger electron (~25 angstroms)
to
the
mean
silicon
free
KLL
path
of
sulfur KLL Auger electron (~4 angstroms). The adsorbed carbon
contamination present on the outer surface of the sample will
probably attenuate the sulfur signal by a greater extent than
the attenuation of the silicon signal by
the
outer
surface
of
the
outer
surface
layers.
yields
Therefore,
an
Auger
apparently
analysis
high
spectroscopy for analysis
of
silicon
the
concentration.
elemental
Auger
stratification
165
the
within individual particles can be justified even though
area,
analysis
the
than
particles are much smaller
since
by
STEM
ESCA
was
determined for magnesium, iron, silicon, sulfur, sodium,
and
inter particle
homogeneity
established
been
has
analysis.
6.4 CHEMICAL STATE ANALYSIS
Accurate chemical state identification
calcium.
In general, identification of the
by
state
oxidation
can be easily determined since measurable differences in
binding energy (> 1.0 eV) exist
states.
On
the
hand,
other
for
differentiation
various compounds is limited, since
the
oxidation
various
the
between
the
in
the
difference
However, a
resulting chemical shift may be small (< 1.0 eV).
because
eliminated
large number of compounds can be readily
of the detailed knowledge about the chemistry of
the
the
submi-
cron particle.
The extent of static charging for the fume sample was
determined by
comparing
carbon Cls line (i.e.,
energy (Wagner et. al.,
line (i.e.,
284.6 eV),
observed binding energy
the
observed
287.3 eV) with the referenced
1979)
for
Figure 6.6.
for
adventitious
static
binding
Cls
in
the
carbon
charging
from
its
of
the
Therefore, in order to correct
shifting, all observed binding energies will be
for
carbon
The 2.7 eV shift
adventitious
reference binding energy is due to
insulating fume sample.
energy
binding
adjusted
this
by
I
I
I
I
I
I
Charge Shift of 2.7 eV
I
/
A - Measured carbon
peak (287.3 eV)
"\
B - Reference C is
line (284.6 eV)
LLuj
zl
I
A
293
289
..
285
BINDING ENERGY
281
(eV)
Figure 6.6 - ESCA high resolution scan of carbon C Is line for fume sample
167
2.7 eV.
Figure 6.7 shows the high resolution
nesium Mg2p line.
A
comparison
energy for magnesium (49.1 eV)
of
with
the
the
scan
for
observed
binding
reference
binding
energy (Castleton, 1981) for magnesium metal
(47.2
eV)
and
in
the
magnesium oxide (49.4 ev) suggest that the magnesium
fume is
present
in
its
highest
diffraction analysis of the fume
oxidation
confirms
crystaline magnesium oxide, Figure 6.8.
the diffraction lines
is
magnesium oxide crystals
a
result
(Padro,
state.
the
X-ray
presence
of
The slight shift
in
of
impurities
1981).
Since
in
the
cles is composed of magnesium, calcium, and iron,
in the diffraction lines is probably due
After
correcting
for
to
the
Auger
analysis has indicated that the core of the submicron
and calcium.
mag-
parti-
the
shift
dissolved
static
iron
charging,
the
observed binding energy for magnesium, which has
been
pendently shown to be present as magnesium oxide,
is
within
magnesium
oxide.
0.2 eV of the reference binding energy for
This excellent agreement
the
verifies
use
of
inde-
carbon
the
referencing technique to accurately determine the
extent
of
static charge shifting for the insulating fume sample.
The high resolution scan for
present in Figure 6.9.
1979)
Fe2p3/ 2
A comparison of the observed
energy for iron (711.1 eV) to the
(Wagner .et al.,
iron
reference
for iron (706.9 eV),
line
binding
binding
FeO
is
(709.5
energy
eV),
Fe 2 0 3 (711.0 eV), and Fe30 4 (711.4 eV) suggests that the iron
I
I
49.3
I
54
I
I
I
I
I
I
47.2
I
I
46
50
BINDING ENERGY
I
(eV)
Figure 6.7 - ESCA high resolution scan of magnesium Mg2p line for fume sample
O
FII
LLJ
F-
39
44
54
49
59
64
20
Figure 6.8 - X-Ray diffraction lines for submicron particles
LJ
23
725
715
BINDING ENERGY
4
705
(eV)
Figure 6.9 - ESCA high resolution scan of iron Fe2p line for fume sample
171
oxidation
state
resolution
scan
highest
its
in the fume is present in
as
either Fe20 3 or Fe3 0 .
4
Figure
silicon Si2p line.
high
the
shows
6.10
binding
observed
the
of
comparison
A
for
energy for silicon (101.7 eV) to the reference binding energy
ESCA of the
However,
silicate
specific
eV)
in
its
by
Identification
silicate.
a
highest oxidation state as
(101.78
present
is
fume
the
suggests that the silicon in
silicate
and
eV), silicon dioxide (103.4 eV),
(99.0
metal
silicon
for
(Wagner, Gale, and Raymond, 1979)
possible.
not
is
compound
the chemical analysis also suggest that both alumina
of
the
The concentration of aluminum in
the
sublayer
and some of the calcium are present in the
particle with silicon.
submicron particles displays the same
This
dence as silicon, Figure 6.2.
suggests
heterogeneously condenses on the surface
thereby placing aluminum
submicron particle.
at
near
or
that
surface
the
alumina
particles,
the
of
depen-
size
particle
the
of
Auger analysis suggests that some of the
calcium is near the surface of the particle.
The high resolution
presented in Figure
6.11.
scan
A
for
is
sulfur
S2p
line
of
the
observed
comparison
binding energy for sulfur (168.8 eV) to the reference binding
=4
energy (Wagner et. al., 1979) for suflate ion (SO = 4
eV), sulfite ion (SO= 3 ~ 166.3 eV), elemental
163.9 eV) and sulfide ion (S
~ 161.4
eV)
~
sulfur
establishes
168.6
(SO
~
that
suflur is present in its highest oxidation state as a sulfate
I
I
I
I
I
I
LLI
I-
106
Si0 2
silicate
Si
103.4
101.8
99.0
I
,
I
102
BINDING ENERGY
I
I
I
98
(eV)
Figure 6.10 - ESCA high resolution scan of silicon Si2p line for fume sample
LU
-,4
168.6
I
I
174
so
S0=3
16
166 3
I
170
166
BINDING ENERGY
(eV)
=
16.4
I
I
I
162
Figure 6.10 - ESCA high resolution scan of sulfur S 2p line for fume sample
174
ion. Auger analysis shows that sulfur is highly
enriched
charac-
chemical
the outer suface of the particle. From the
in
terization of the submicron particle the bulk of
the
sulfur
will be present either as sodium sulfate
eV)
and/or
calcium sulfate (169.3 eV).
sulfur S2p line,
The
(168.6
shift
chemical
slight
sodium,
due to the cation of calcium verses
between
differentiation
is not significant enough to permit
in
the two compounds.
Figure 6.12 shows the high resolution scan for sodium
Nals line.
The photoelectron chemical shift is
than 2 ev, for the
less
However,
sodium.
of
states
oxidation
small,
differentiation between the binding energy for some plausible
obtained with good statistics (i.e.,
can
be
width).
A
resolution
sodium compounds is possible when a high
narrow peak
(1071.4
comparison of the observed binding energy for sodium
eV) to the reference binding energy (Wagner et. al., 1979) of
Na2 0 (1072.5 ev), sodium chloride
sulfate (1071.3 eV)
(1071.5
suggests that the sodium in
present as either the chloride or
ratio of sulfur to sodium in
the fume is
1:2,
ratio
insufficient chlorine in
of
sodium
to
is
molar
The
while the molar
ratio of chlorine to sodium is less than 0.1:1.
stoichiometric molar
fume
the
sulfate.
the
sodium
and
eV),
The
sulfur
the fume tends to suggest
proper
and
the
that
the
scan
for
bulk of the sodium is present as sodium sulfate.
Figure 6.13 presents the
high
calcium Ca2P3/ 2 line. A comparison of
resolution
the
observed
binding
LL
w
1078
1066
1072
BINDING ENERGY
(eV)
Figure 6.10 - ESCA high resolution scan of sodium Na is line for fume sample
II
II
Ii
II
I
I
LLJ
CaS04
348.7
I
354
I
I
CaO
487.4
I
350
BINDING ENERGY
346
(eV)
Figure 6.13 - ESCA high resolution scan of calcium Ca2p3/2 line for the fume
177
oxide
1981) for calcium
energy (Castleton,
referenced
the
to
eV)
(347.1
energy for calcium
binding
and
eV)
(347.4
present
calcium sulfate (348.7 eV), suggests that calcium is
as calcium oxide.
The multiple analytical tools used in this investiga-
the elemental stratification of the submicron
core of the particle is composed of magnesium,
presence of
dissolved
crystals. The outer
constituents whose
dependence with
is
surface
concentration
the
analysis shows that two of
composed
of
display
a
par-
on
the
suggests
oxide
volatile
the
size
state
Chemical
elements,
enriched
surface
the
and
particle
species.
trace
other
calcium,
magnesium
the
in
impurities
The
analysis
x-ray
the
ticle size. Furthermore,
particle.
dependence
iron where their concentrations show no
about
story
tion have lead to the development of a coherent
sodium and sulfur, are present as sodium sulfate. The subsurface layer between these two strata
silicon whose concentration also
dependence.
Chemical
state
is
which
has
been
analysis to be present near the surface as
core of the particle, coexist in the
silicon.
and/or
a
well
subsurface
size
Auger
by
as
size
calcium.
particle
shown
of
that
demonstrates
analysis
Both alumina, whose concentration displays
calcium,
particle
a
displays
silicon is present as a silicate of alumina
dependence, and
mostly
composed
in
layer
the
with
178
CHAPTER SEVEN DISCUSSION
The examination of the bulk characterizatics
submicron particles in chapter five has provided
information
about the amount, size, and bulk composition of these
cles which have been
shown
to
be
combustion environment and the
of the mineral matter.
dependent
vaporization
The detailed
the
of
parti-
both
on
the
characteristics
chemical
characteriza-
tion of the individual submicron particles in chapter six has
provided specific information about the
size
dependence
composition for particles in the submicron range and
elemental stratification within these particles.
mation about
the
physico-chemical
on
the
The
evolution
five
of
and
the
of
the
six
has
submicron
particles subsequent to their initial formation.
The
of this section will be to examine the hypotheses
about
formation and subsequent growth of
that can be inferred from the
the
submicron
physico-chemical
tics of the submicron particles.
the
infor-
characteristics
submicron particles presented in chapter
provided some insight
on
of
focus
the
particles
characteris-
The discussion will pertain
most closely to the conditions corresponding to
tion of Montana lignite (45/53 um) at a
furnace
of 1700 K in twenty percent oxygen in nitrogen.
the
combus-
temperature
179
7.1 NUCLEATION
more
In the absence of a condensed phase, one or
the
vaporized
should
elements
inorganic
supersaturate,
onset
thereby, providing the necessary driving force for the
available
made
The free surface
of homogeneous nucleation.
(i.e.,
with the formation of the stable clusters
precursors
the
of the submicron particles) can be expected to alleviate
supersaturation of the other inorganic vapors as a result
heterogeneous
The
Martinez-Sanchez et. al., 1978).
stratifica-
elemental
provided
tion of the submicron particles is believed to have
condensation
a chronological signature of the
The elements
the various inorganic vapors.
sequence
present
core of the submicron particles are believed to be
ble for the formation
of
the
initial
condensed
homogeneous nucleation, while those elements which
in
initial
much
later.
formation
of
The
the
factors
submicron
responsible
particles
for
the
responsiphase
by
form
the
outer layers of the submicron particles are believed to
condensed
of
1981;
al.,
et.
(McNellan
condensation
of
for
should
have
the
be
evident by examining the phenomena involving the vaporization
and subsequent condensation
presides in the core of
the
of
which
not
only
particle,
but
also
magnesium,
submicron
accounts for over sixty percent of the submicron particle
by
weight.
The vaporization
of
the
mineral
matter
has
been
180
(1982).
Quann
Richard
examined in a complimentary study by
In the case of Montana lignite, the vaporization of magnesium
was
be
to
determined
externally
diffusion-limited.
The
for
mag-
vaporization
experimentally observed high rate of
vaporization
nesium could not be explained by the direct
the fact that
partial
the
magnesium
of
pressure
from
evident
was
This
magnesium oxide into the gas phase.
of
the
at
surface of the char particle calculated from the experimental
vapor
equilibrium
vaporization data was found to exceed the
magnitude
of
pressure for magnesium oxide by several orders
combustion
conditions
examined in this study (i.e., particle combustion
temperatu-
the rate of char oxidation will be
externally
(Quann,
res > 2000 K),
the
For
1982).
diffusion-limited
(Smith,
range
1978;
of
1967).
al.,
et.
Field
Consequently, a locally reducing atmosphere will exist within
the burning char particle, since oxygen penetration into
porous structure of the char particle will be minimal due
the high oxidation rate at the char surface.
stable
magnesium
presence of H, C, S, N, and 02 over
equivalence ratios.
equilibrium
compounds
wide
in
of
range
The results are presented in Figure
where for simplicity only CO2 ,
shown.
a
02,
MgO(v),
The equilibrium vapor pressure
for
and
Mg(v)
of
magnesium
oxide
fairly constant up to a fuel equivalence ratio of
the
fuel
7.1
are
increases
Mg(v)
sharply as the conditions are changed from fuel lean to
rich, while the vapor pressure
to
The NASA sp-273
equilibrium program can be used to calculate the
vapor pressure for the
the
fuel
remains
2.5.
For
181
1
TEMP = 2080 K
-1
Mg
F-
LLJ
cn
cn
n~
-4-
2:
CO2
-5
-1
or
I
CD
0
-7
CD
I_
02
-90.5
0.5
1.0
1.5
2.0
2.5
3.0
EQUIVALENCE RATIO
Figure 7.1 - Equilibrium vapor pressure for Magnesium for
various fuel equivalence ratios
182
char
particle
(i.e., fuel equivalence ratio > 1) the metal is the
dominate
burning
the condition anticipated within the
is
vapor species for magnesium which
vapor pressure of the oxide by several orders
magnesium can be
explained
by
of
rate
The experimentally observed high
the
the
exceed
to
shown
of
magnitude.
for
vaporization
chemical
magnesium oxide in the condensed phase to the
reduction
of
more
volatile
as
magnesium
metal vapor.
The dramatic change in the environment
metal vapors diffuses from the locally
within the burning
char
particle
out
environment
reducing
into
atmosphere surrounding the char particle should
the supersaturation of magnesium vapor.
vaporization and
subsequent
presented in Figure 7.2.
oxidizing
the
account
for
for
the
A scenerio
condensation
of
magnesium
atmosphere
In the locally reducing
oxide
within the char particle the vaporization of magnesium
will occur by chemical reduction to form
metal vapor.
The metal vapor
surface of the char particle.
will
be
the
is
volatile
more
to
transported
the
metal
Once at the surface the
vapors will diffuse away from the burning char particle
into
the surrounding oxidizing environment, where the metal vapors
will reoxide.
Upon reoxidization, the magnesium oxide vapors
will be supersaturated, thus providing the necessary
force for homogeneous nucleation.
The decrease
ture away from the burning char particle can only
driving
tempera-
in
accentuate
the tendency for the supersaturation of the magnesium
vapors
diffusion of Mg
vapors from char
oxidation
MgO(v) + 0
Mg (v) + 02 -
-----------
\
MgO(s)
+ CO -
Mg(v)
+ CO2
formation of condensed
phase
'V'1
I
char particle
surface
REDUCING ATMOSPHERE
Peq. Mg
P eq. MgO
OXIDIZING ATMOSPHERE
Pmagnesium >> P eq. MgO0
Figure 7.2 - Schematic diagram of magnesium vaporization and recondensation
during char combustion
184
supersatura-
The point at which magnesium
upon reoxidation.
tes will be dependent on the rate of oxidation and the oxygen
potential,
subsequent
the
magnesium
of
pressure
partial
oxide, and the gas temperature at various distances from
the
char particle.
A steady state one dimenstional model
in order to examine in detail
homogeneous nucleation of magnesium oxide
particle.
pressure,
partial
magnesium
oxygen
pressure,
partial
metal
the
char
the
arround
temperature,
the
estimates
model
The
involving
phenomana
the
developed
was
and
magnesium oxide partial pressure from the governing differential equation
particle.
as
function
a
of
distance
The major assumption about the
from
present
the
model
that a barrier exist for both the oxidation of the metal
char
is
and
the initial formation of the condense phase despite the large
potentials that may exist.
From two color optical pyrometry the particle combustion temperature has been shown to overshoots
1982).
temperature (Timothy,
Consequently,
the
the bulk gas phase.
The
at
temperature
surface
various
from the char particle was derived for spherical
gas
temperature
a
gradient will exist between the char particle's
bulk
and
distances
coordinates
from Fourier's first law of heat conduction,
1
d (r2 k dt)
r
dr
dr
-
0
Eq. 7.1
185
color
where the particle temperature was determined from two
The magnesium partial pressure is low enough that
pyrometry.
the heat released upon oxidation and condensation should
perturb the
layer.
tempertaure
in
profile
boundary
thermal
the
the
The heat effect from the oxidation of CO in
gas
oxidation
of
since the rate
stream has not been considered,
not
extremely slow.
of CO is
For
combustion,
diffusion-limited
externally
an
oxygen gradient will exist between the bulk gas phase and the
various distances from the
partial
oxygen
The
char particle's surface.
r
r2 DAB
derived
2) = 0
Eq. 7.2
where the oxygen partial pressure at the surface of the
from
The
(Field et. al., 1967).
boundary layer by the
for
dr
dr
particle was calculated
at
law,
spherical coordinates from Fick's first
12 d
was
particle
char
pressure
the
oxidation
char
of
consumption
oxidation
at
the
kinetics
in
the
vapors
and
oxygen
metal
char
carbon monoxide have not been considered.
The partial pressure of magnesium at various distance
from the
char
surface
was
governed
equation for spherical coordinates
by
the
differential
186
1
r
2 d
r 22 DAB dnMg
=
-K n
Eq.
n
7.3
2
dr
dr
surface
where the partial pressure of magnesium metal at the
of the char particle was calculated from experimental vapori-
for
hand side of equation 7.3 accounts
magnesium metal by oxidation.
right
the
on
The reaction term
1982).
zation data (Quann,
consumption
the
of
homogene-
The modeling of the
ous gas phase oxidation of magnesium will entail establishing
the rate coefficient for the governing bimolecular reaction
as proposed by
Markstien
Eq. 7.4
MgO(v) + O
Mg(v) + 02 --
The
(1963).
low
pressure,
low
temperature studies concluded that the bimolecular
homogene-
ous reaction will account for initial formation of
magnesium
homogeneously
oxide vapors which, upon supersaturation, will
nucleate to form the initial particles. For
high temperature environment
of
combustion,
the
atmospheric
the
estimated
rate coefficient from collision theory was used to model
bimolecular oxidation
of
magnesium
metal
vapors
the
(Laider,
1965)
K
=
A Tn e - E/RT
Eq.
7.5
187
A = 4.44 x 1018 m 3 /mole sec
n
= 1/2
E = 1.28 x 105
joules/mole
various
at
oxide
The partial pressure of magnesium
distance from the char surface was governed by the
differen-
tial equation in spherical coordinates,
1
d
r1dr
r
dr
2
dnMgO
-AB Rrxn + Rnucl + Rhet
dn
dr
Eq.
7.6
Rrxn =K nM n
I Mg
Rnucl
Rhet ~ F'(d)
where the reaction terms on the right hand side
7.6 accounts for; the continual addition of
magnesium
of
and the consumption
magnesium
nucleation,
vapors
by
oxide
consumption
vapor by oxidation of the metal vapor, Rrxn; the
of magnesium oxide vapors by homogeneous
equation
of
Rnucl;
heterogeneous
condensation on the stable nuclei of magnesium oxide.
The rate of homogeneous
nucleation
of
a
liquid droplet from unary vapors can be calculated
spherical
from
the
classical theory of nucleation of volmer and weber and Becker
and Doering (Hirth and Pound, 1963).
I (nuclie/cc s),
I=
The rate of nucleation,
is given by
P. 2
2a
(2cm
/ 2
1/2V
(kT) 2 m
1m)
exp -
3
6
3 k T AGv 2216
Eq.
7.7
188
\GV
0Gv =In
m
where ac is the condensation
P eq
eq,i
eq,i
(
species i,
a is the surface tension of liquid i, Vm
molecular volume,
constant,
and
m is
the mass of molecule i,
droplet formation per unit volume.
the
the
energy
for
of
the
development
The
classical nucleation theory is based on
condsiderations
stability) and kinetics
imposed by both thermodynamics (i.e.,
Inother words, the rate
(i.e., probability).
is
boltzman
k is
free
the change in Gibbs'
AGv is
vapor
the
are the partial and equilibrium pressure of
of
and
Pi
stick),
and
embryo
fraction
(the
coefficient
molecules that collide with an
i
ln
k T
of
nucleation
is governed by the net number of clusters per unit time which
grow larger than the critical
size.
cluster
The
critical
cluster is a cluster of size such that its rate of growth
is
equal to its rate of decay. Any cluster which fluctuates to a
size larger than the critical
macroscopic size while clusters
size will most
likely
shrink
size
will
smaller
back
to
probably
that
the
the
grow
to
critical
monomer.
The
critical cluster size is determined from the relation,
r*
=
2 aV
m
k T In (Pi/Peq,i)
Eq.
7.8
where the addition of one monimor leads to the formation of a
stable cluster.
189
The rate of heterogeneous condensation of the condensable vapor molecules on the newly formed stable
cluster
of
magnesium oxide whose size is much smaller than the mean free
frequency
path of the gas molecule will be controlled by the
with which the vapor molecules collide
with
the
The rate of deposition per particle of
size
dp
from the kinetic theory of gases (Friedlander,
F'
(d)
=
particles.
dirived
is
1977)
*)
(P. - P
d 2 a
eq, i
1
c
p
(2 r m k T)
where ac is the condensation coefficient and
have a value of be unity.
Eq.
is
assumed
nuclei of magnesium oxide.
the
number
rate
of
stable
of
density
to
conden-
The rate of heterogeneous
sation for the condensable vapors obtained from the
deposition per particle and
7.9
The overall rate of heterogeneous
condensation will be highly dependent on the
and size distribution of the particles.
of
The details
growth process of the particles by coagulation will
with in detail in section 7.2.
density
number
However, in
delt
be
assuming
the
nearly
monodispersed particle size, a simple solution to the kinetic
equation for brownian coagulation can be used to
the number density and average particle size
nuclei of magnesium oxide.
of
approximate
the
stable
The decay in the total number
of
particles with time is approximated from the rate expressions
dN
dr
1 K N2
2
Eq. 7.10
190
where the collision frequency factor, K, is given by
K =
Eq. 7.11
The average particle size can be approximated from the volume
fraction and the total number of particles.
model's
The
magnesium are presented in
7.3
Figure
condensation
the
for
prediction
conditions
the
for
twenty
corresponding to the combustion of Montana lignite in
percent oxygen in nitrogen.
is
The supersaturation of magnesium
boundary
predicted to occur within the thermal
the char particle due to the relatively
This
increases.
nucleation
position
rate
surface
estimate of the partial
tension
for
oxygen at the char particle's surface.
layer
of
for
the
predicts
a
supersaturation
as
insensitive
oxidation
the
magnesium
Since
to
rate
and
the
oxide
and
1965),
(Smithells,
pressure
rate
model
relatively
is
uncertainties in the assumed values for
constant and
high
The
homogeneous oxidation of magnesium.
very sharp rise in the
of
onset
the
nucleation is very sensitive to temperature, changes
in
of
the
values assumed for the above parameters will result in only a
slight displacement of the position of nucleation because
the steep
surface.
temperature
gradient
near
the
The duration of the nucleation is
char
of
particle's
attributed to the
20
ov
-3
18
-4
16
P Mg
D
co
cn
14
-5 N
0LU
0
LN
Mg
-6
12
c
CD
F-4
0 <
-7.eq,
o
MgO
8
--8
z
rCD
CD
9I
1
5
I
I
I
9
13
17
6
R/Rp
Figure 7.3 - Model prediction for the supersaturation and condensation of magnesium
192
continual
addition
of
by
monomer
phase
gas
homogeneous
time
oxidation of magnesium metal vapor. Since the transient
profi-
for establishing the steady state gas and temperature
les arround the char particle is extremely short and the fact
are over two orders of magnitude shorter than
char burnout, magnesium
can
nucleate very early on during the
combustion
homogeneously
combustion
process,
observa-
as
in
shown
early
very
tions indicate that the submicron particles form
on during the
where
process
Experimental
high.
the temperature is relatively
for
time
the
to
inferred
be
nucleation
and
oxidation
that the characteristic times for
time
the
resolve study in Figure 5.17.
cles form very early on during the combustion
is supported by
which
process
This
observation.
experimental
parti-
submicron
The present model predict that the
been
has
vapori-
shown to be a direct consequence of the mechanism of
zation where the refractory oxides are chemically reduced
to
form the more volatile metal vapor, and the sharp temperature
gradient surrounding the char particle.
diffuse into the
oxidizing
As the metal
vapors
metal
vapors
environment,
reoxides forming supersaturated oxide
for the early formation of the
the
vapors
submicron
examining the sequence of condensation for
ganic species relative to magnesium, the
account
which
particles.
other
the
growth
the
inor-
process
the submicron particles will be examined more closely,
the presence of these particles may influence
Before
of
since
condensa-
193
tion process for the other inorganic vapors.
7.2 COAGULATION
particles
submicron
inorganic
The formation of the
of
have been shown to result from the homogeneous nucleation
boundary
the
in
the supersatured vapors of magnesium oxide
The classical nucleation
layer of the burning char particle.
theory predicts the formation of an extremely large number of
stable nuclei with critical radii of six angstroms. The
high
particles
will
coalesce
to
"coagulation"
form
stick
which
particles
spherical
single
growth
the
describes
collisions.
particle-particle
result in a high frequency of
The colliding
small
extremely
particle mobility of these
upon
aerosol
for
process
in
the
number
total
of
term
The
particles.
account
systems where the random particle-particle collision
for the reduction
should
contact
particles
coalescence accounts for the increase in the average size
the primary particles.
and
of
From the physical characterization of
the submicron particles in chapter
primary particles was found
to
be
five,
of
the
both
the
size
the
on
dependent
residence time and the total mass in the submicron mode.
By convention, the rate of coagulation is
designated
by the collision frequency of the particles where the
quent coalescence of the colliding particles
occur instantaneously.
is
The kinetics governing the
subse-
assumed
to
collision
frequency of the particles as a result of brownian motion
in
194
determine the decay in
the total
can
1917)
a uniform flow field (Smoluchowski,
number
of
used
be
particles
to
with
time
dN
=-
2
(v,u,t)
n(v,t)
dt
n(u,t)
dv du
Eq. 7.12
where n(u,t) is the particle volume distribution function and
B(v,u) is the collision frequency between particles of volume
v and u (Lai et. al.,
For particles much smaller than
1972).
free molecular
the mean free path of the gas molecules (i.e.,
regime),
B(v,u),
the collision frequency factor,
is
derived
collisions
amoung
molecules that behave as rigid elastic spheres (i.e.,
nonin-
kinetic
from the
theory
of
gases
for
teracting spheres)
B(v,u) =
where k is boltzman constant, T is
P is the particle density, and
/3+
/2
+
1/6/2
a
the absolute
a
the
is
For
1970).
the
case
of
In
examining
kinetics for molten lead
the
aerosols,
quency of the particles (i.e.,
G=2)
the
and
spheres,
intermolecular
are
coagulation
forces
were
collision
fre-
dispersion
found to account for the enhancement in
coeffi-
(Hidy
unity
molecular
free
Eq. 7.13
temperature,
noninteracting
interaction forces such as electronic or
neglected.
2
sticking
of
cient which is assumed to have a value
Brock,
u/3
(Graham and Homer,
1972).
The origin of the dispersion forces resulted from a theoreti-
195
cal treatment of london van der waals forces (Hamaker, 1937).
The total number of particles per unit volume, N. ,
expressed
be
volume fraction of the dispersed phase, V, can
the
and
in the form:
on(V,t) dv
Eq. 7.14
v n(v,t) dv
Eq. 7.15
N,(t) =
V
For
pure
=
absence
the
in
coagulation
condensation,
of
evaporization, or particle loss by settling or diffusion, the
volume fraction, V, will be invariant with respect to time.
The partial integro-differential
governing
equation
increase
calculations become extremely combersome due to the
in particle inventory with time.
However,
the
however
particle coagulation can be solved numerically,
with the discovery
that the evolving particle size distribution
aerosol
an
of
asymo-
whose growth is governed by coagulation approaches an
class
of
analytical solutions have been developed based on the use
of
totic form with time
(i.e.,
a similarity transformation for the particle
tion (Swift and
1966).
Friedlander,
The unique feature of
1964;
the
size distribution function is that
distribution conforms to
the
a
self-preserving),
size
Friedlander
self-preserving
once
the
self-preserving
distribuand
Wang,
particle
particle
size
distribution,
the normalized distribution will be invariant with
time
and
196
independent of the
The
particle size distribution.
of
peculiarity
individual
is based
n(v,t),
initial
transformation
similarity
for the particle size distribution,
the
the
on
assumption that the fraction of the particles in a given size
the mean particle volume,
=
n(v,t)
V,
N
2
Eq.
# (7)
V (t)
7
where
=
v
N-(t)
V
7.16
v
is a
B(u,v),
Since the collision frequency factor,
ous function,
normalized by
v,
range is a function only of particle volume,
the similarity transformation of
the
size distribution will reduce the kinetic equation
the decay of the total number of particles,
homogeneparticle
governing
to
equation 7.12,
an ordinary integro-differential equation
3
dN
dt
dt
where 8 is
the
VI
)
-
/ 6
N
N4
2
dimensionless
11/6
1/6
6kT
collision
0 (7),
and is
by numerical analysis (Lai et. al.,
1972).
self-preserving
distribution,
/ 6
Eq. 7.17
6
integral
for
found to be
The
the
6.67
analytical
solution to equation 7.17 yields a simple expression for
time dependence of the number of particles in the system
the
197
=
N(t)
(Kth
theory
6/5
1
Vl
6t
K
where
Ktheory
(Ktheory
Therefore,
V1/
6
the
be
will
t)
12 47
on
times
For coagulation
2
3 1/6(6kT )1/
5
where
Eq. 7.18
1 5/6)
N
+
order
much
G
P
of
millisecond,
one
(1/N)5/
than
larger
6
equation 7.18 can be further simplified
N(t) = (Ktheory V/6 t)
- 6/ 5
Eq. 7.19
be
will
where the predicted number density, N(t),
dent of the initial particle number density for
several
times greater than
particle volume idenity,
v = V/N , the
particle
the volume average
diameter
the
dependence
time
of
coagulation
From
milliseconds.
indepen-
the
mean
of
coagulating
particles can be obtained:
d
p
=()
Eq. 7.20
1/3 (Ktheory V t)2/5
In order to examine the volume fraction and time
for the growth of the
that
7.20),
predicted
by
inorganic
coagulation
submicron
kinetics
particles
(i.e.,
the distribution of the submicron particles
self-preserving.
In Figure 7.4,
dependence
with
equation
must
be
the particle size distribu-
.
198
2
[] - 15% 02
I -
OA
._
- 40% 0
0
0
CL
-
20% 02
-3
Oo
CD
I
-4
SELF PRESERVING
SIZE DISTRIBUTION
-5
I
-6I
-1.0
-0.6
-0.2
0.2
0.6
LOGio (dp/dpv)
Figure 7.4 - Primary particle size distribution for the
fume with the self-preserving distribution
199
combustion of Montana lignite 45/53 um in
percent
fairly
are
oxygen
the
by
represented
well
40
and
20,
15,
the
from
produced
tion of the primary submicron particles
The calculated transiant
self-perserving size distribution.
for
time for the transformation of the initial distribution
order
the
the submicron particles to self-preserving is on
of several milliseconds (Hyde and Lilly, 1965).
In order to examine
growth of the
primary
the
particles,
submicron
measurements were conducted where samples of
resolve
time
the
the
for
dependence
time
submicron
particles were collected at various distances from the point
The time
of coal injection.
resolve
measurements
of
the
diameters
for
the
experimentally determined mean particle
primary submicron particles produced from the combustion
of
Montana lignite 45/53 um at a furnace temperature of 1700
K
in twenty percent oxygen in nitrogen are compared with those
predicted from coagulation theory (i.e.,
Figure 7.5.
The
predicted
curve
for
equation
the
7.20)
mean
in
particle
diameters were generated using the appropriate value for the
volume fraction and elapsed time where the formation of
the
at
the
initial submicron particles was
initiation of
char
combustion.
assumed
to
The
volume
occur
fraction
is
obtained from the measured value of the fractional vaporization for the metal oxides, t, assuming that the fume
parti-
cles remain within the core along the furnace axis in
the coal particles are visually observed to burn (i.e.,
which
core
200
O
300
C1 - PRIMARY PARTICLE
AGGLOMERATED PARTICLES
PROJECTED AREA DIAMETER
-
COAGULATION THEORY
0
S5
200
CHAR
COMBUSTION
ZONE
rC
LIJ
I
L.
L
I
I
Z
I
I
LL
5
100
I I
I
I
I I
I
I
I I
0
0
I
I
I
I
5
10
15
20
DISTANCE FROM COAL INJECTION (cm)
Figure 7.5 - Experimental and predicted growth of the
primary submicron particles
25
201
radius of 0.62 centimeters).
the time scale of the
This is reasonable because
on
transport
by
particle
experiments,
diffusion can be neglected.
diameter
The experimental mean
of the primary particle is determined by the automated image
analyser from electron micrographs as previously described.
coagu-
In examining the growth curve in Figure 7.5,
the
of
representation
lation theory provides an excellent
changes in the primary particle size with time for the early
coalesce.
and
brings about the decrease in particle
2.76 x 10
large
the
particle size is observed to increase rapidly as
number of small nuclie collide
primary
The
growth period between 3.81 to 6.35 centimeter.
Coagulation
from
density
number
particles/cc at 3.81 centimeters to 6.24 x
1010
volume
mean
6.35
particles/cc at
centimeters
the
while
diameters of the particle increases from 34 angstroms to 139
new
combustion,
char
angstroms,
respectively.
material is
continiously added to the vapor phase by further
vaporization.
Originally,
During
had
it
assumed
been
because
the
of the inorganic vapor continually released
no
forma-
initial
subsequent nucleation would occur after the
tion of the submicron particles,
that
concentration
during
combus-
tion was anticipated to remain close to the saturation vapor
pressure as a consequence of heterogeneous
condensation
on
the surface of the particles produced by the initial nucleation wave (McNellan et. al.,
1978).
However,
1981; Martinez-Sanchez et. al.,
the thermophoretic
forces
resulting
from
202
char
the steep temperature gradient surrounding the burning
the
from
away
particles
the
particle will rapidly drive
dimen-
state
one
will
continously
nucleate close to the char particle as long as
vaporization
char's surface. Consequently, the steady
tional
model
persists.
that
suggests
magnesium
Since magnesium is
the
the contribution from
makes up the submicron particles,
condensing
other inorganic vapors
the
combustion
char
during
of
addition
However, the continuous
will be minimal.
which
species
dominate
new
particles should be rapidly accomodated, since a much higher
rate of coagulation will
exist
particles
older
particles with their high mobility and the
formed
newly
the
between
1977).
with their larger cross sectional area (Friedlander,
The success of the
coagulation
growth of the submicron particles
model
in
predicting
for
the
zone further supports the assumption that
combustion
char
the
the
addition
of
new particles will be rapidly accomodated.
In examining the growth curve
growth of the primary particles
The
apparent
brake
Figure
shortly
ceases
sampling point at 8.9 centimeters from
injection.
in
point
the
between
theory
observed experimentally may be explained by a
7.5,
the
after
the
of
coal
and
that
decreased
the coalescence rate after char burnout as the gas
tempera-
ture drops back down to the furnace gas temperature of
K and as the particle
diameter
approaches
160
in
1700
angstroms.
Since the particles during the early growth period have been
203
and
and calcium
iron,
proposed to be composed of magnesium,
the fact that these particles should be solid below 1800
the coalescence rate for these particles at the furnace
will
be
governed
difference
in
free
K
temperature of 1700
The
diffusion.
chemical
or
energy
potential between the neck area and the surface of
material
two
between the
diffusion
(i.e.,
centers
governed
The time for coalescence,
coalescence).
the
for
particles'
the
between
distance
decrease in the
boundary
grain
responsible
be
will
particles
the
along
two
the
particles provide a driving force for solid state
where the transfer of
gas
state
solid
by
K,
by
solid state diffusion for two solid particles with identical
diameters may be estimated from the relationship
where
AL/L
5/ 2
( 2 a t T)
D*) r6/5
(20
t ALL
t
7.21
Eq.
the
is the fractional shrinkage of
coalescing
particles, a 3 is atomic volume of diffusing vacancy, and
is
the self diffusion
inspection the coalescence time
particle size,
(Kingery,
coefficient
be
crystaline
From
1965).
by
governed
and temperature.
composition,
diffusion coefficient for
will
A solid
magnesium
D*
the
phase
oxide
was
selected to derive estimates of the coalescence time for the
primary submicron
particles
during
growth where only magnesium, calcium,
proposed to
condensed.
Figure
7.6
the
early
and
shows
Iron
the
stages
have
of
been
estimated
204
0
COMBUSTIO
TEMPERATU
FURNACE
TEMPERATURE
0
I
I
I
I
LLU
DP
CD
=
80 AO
LD
CD
-3
-4
= 40 AO
=p
I
I
I
1400
1600
1800
-4
TEMPERATURE
2000
K
Figure 7.6 - Predicted coalescence time for solid
crystaline magnesium oxide particles
205
coalescence
time as
The local gas
particle size.
temperature
and
temperature
surrounding
the
furnace
gas
main
the
exceed
will
burning char particle
both
of
function
a
temperature due to the liberation of heat during combustion.
Consequently,
both the decrease in temperature on completion
of combustion and the increase
in
contribute to the decrease of
the
primary particle diameter is
particle
diameter
both
rate.
The
coalescence
to
120
of
range
in the
Beyond
angstroms when both these factors become important.
this point further coagulation leads
agglomerated particlces
of
size
discrete
the
where
formation
the
to
160
of
the
The projected area of the
colliding particles is conserved.
agglomerated particle at sampling position of 20 centimeters
with coagulation theory.
agreement
good
from the point of coal injection is in very
The fact that the coalescences
of
these particles cease at relatively high temperatures (i.e.,
Na, S,
and,
K
particles have solidified
only
and
the
upon
cooling
of
the
would
tend
impurities
combustion products since these
increase the coalescence rate
after
well
condense
heterogeneously
(Kingery,
species
the
1700 K) is consistant with the hypothesis that
1965).
average
Coagulation theory predicts that the volume
particles diameter for equivalent
vary as V2 / 5 , or as
time
coagulation
2/5 since the volume
fraction,
proportional to the fractional vaporization,
ticle size dependence for the
primary
to
4.
submicron
The
should
V,
is
par-
particles
206
are shown to be well governed in Figure
tional vaporization to the
2 /5th
power.
7.7
by
frac-
the
4
The value of
0.002 to 0.15 were generated from the combustion of
from
Montana
lignite (38/45 um) in oxygen concentration varying from
ten
percent to seventy five precent in nitrogen at 1700 K.
The
mean
major assumption in presenting the
verses the fractional vaporization of the
the two fifths power is
diameter
particle
oxides
metal
time
that the coagulation
primary particles for the range of conditions
to
for
the
examined
are
the
primary
submicron particles produced from the combustion of
Montana
nitrogen,
Figure
In examining the growth curve for
equivalent.
lignite in seventy five percent oxygen in
7.8, the duration for
produced
particles
percent oxygen is
ference
in
anticipated,
the
in
twenty
relatively
time
burning
coalescence
percent
equivalent
for
the
coagulation theory is
representation of the change in
growth of the
primary
submicron
and
seventy
five
the
dif-
despite
two
conditions.
the
period.
particle
However,
sizes (i.e.,
400 angstroms)
good
the
shortly
ceases
the
The coalescence of much larger
As
particle
primary
after the sampling point of 8.9 centimeters from
of coal injection.
a
shown to provide
the
size with time for the early growth
submicron
the
for
point
particle
tends to suggest that the
local
gas temperature is much higher than the main gas temperature
of 1700 K during and immediately following char
combustion.
The relationship between mean particle radius and fractional
vaporization for similar growth times is
further
conforma-
400
O
MONT. LIGNITE 38/45
GAS TEMP. 1700 K
C
300
uJ
200
z
L
100
0
0.0
0.1
0.3
0.2
0.4
02/5
Figure 7.7 - Variation of mean diameter of primary submicron particle with
degree of vaporization of the ash
0.5
208
800
700
600
P
500
CT
S400
z
-
300
El
LL
200
100
0
0
5
10
15
20
DISTANCE FROM COAL INJECTION (cm)
Figure 7.8 - Experimental and predicted growth of the
primary submicron particles
25
209
governed
tion that particle growth is well
coagulation
by
kinetics.
7.3 HETEROGENEOUS CONDENSATION
particles
submicron
The formation of the inorganic
have been shown to result from the homogeneous nucleation of
the supersaturated vapors of magnesium oxide in the boundary
the
If
layer of the burning char particle.
to
the
driving
force
for
homogeneous
heterogene-
condense
ously on the surface of these particles,
of
layer
those species which saturate outside the boundary
the char particle are anticipated
high,
sufficiently
the stable nuclei of magnesium oxide is
of
area
total
thereby,
decreasing
those
of
nucleation
condensable vapors.
The mechanism of mass transfer between particles and
the gas phase will depend on the particle size
the mean free path of the gas molecules.
This
relationship
between particle size and mean free path of the gas
les is defined by the knudsen number,
to
relative
Kn (i.e.,
molecu-
Kn = 2 X/dp).
For particles much larger than the mean free path
of
the gas molecules (i.e., continuum regime Kn << 1), a concentration gradient of
condensable
arround each particle.
gas
will
molecules
For continuum particle
dynamics
<< 1), the rate of diffusion-limited heterogeneous
tion is given by
exist
(Kn
condensa-
210
2 7rd
F.(d)
p
D (P.
kT
where Fi(d) is
-
Peq, i)
eq,i
1
Eq.
the flow of condensable gas molecules per unit
The number of molecules
1977).
particles of size dp per unit mass of
the
diameter
dp
condensing
on
can
be
of
time to the surface of a spherical particle
(Friedlander,
7.22
particle
expressed as
F.(d)
a
1
( P p(
Thus,
/6)
d
3 )
for the continuum regime the
1
Eq.
-
7.23
d 2
concentration
dependence
of a species which condense heterogeneously will be inversely
proportional to the particle diameter squared.
For particles much smaller than the mean free path of
Kn >> 1),
the
rate of deposition of the condensing inorganic vapors on
the
the gas molecules (i.e.,
free molecular regime,
which
the
Therefore,
vapor
molecules
collide
the rate of deposition of
the
with
the
with
frequency
particle's surface will be controlled by the
particles.
condensable
inor-
ganic vapors on the surface of those particles will occur
gradient.
a random fashion in the absence of a concentration
The rate of deposition per particle of
size
from the kinetic theory of gases (Friedlander,
dp
by
is
1977)
derived
211
F'
dp(P
c
=
(d)
i
-
P eq,i
(2 = m k T)1
where
c
Eq.
/ 2
is
is the condensation coefficient and
have a value of unity
assumed
7.24
to
The number of molecules condensing on
a particle of size dp
per
unit
mass
of
can
particle
be
expresed as
1
F'l (d)
1
( P (
p
Thus,
Eq. 7.25
dp
d 3)
p
/6)
depen-
for the free molecular regime the concentration
dence of a condensing species will be inversely
proportional
to the particle diameter.
the
In the transition regime between
free molecular regime (i.e.,
10
>
Kn
>
1),
deposition of the condensable inorganic vapors
continuum
the
rate
and
of
is governed by
the relationship proposed by Fuchs and Sutugin (1971).
F.i1 (d)
2 rd
=
p
D (P.-P
k T
1
eg,i
.)
(1 + Kn)
2
(1 + 1.71Kn + 1.33Kn )
Eq. 7.26
Kn
In the limit of the continuum particle dynamics (i.e.,
1),
equation 7.26 reduces to
equation
7.22,
limit of the free molecular particle dynamics
will approach the form of
equation
7.24
if
while
in
equation
the
<<
the
7.26
diffusion
212
velocity
coefficient is given by D = 1/3 C where the average
of the vapor molecules,
C,
is expressed as (8kT/rm)1/2
7.3.1 REFRACTORY OXIDES
presented
The steady state one dimentional model
pathway
section 7.1 will be used to examine the condensation
(i.e., homogeneous verses heterogeneous ) for the
iron, and silicon in
calcium,
oxides
the
of
presence
the
in
of
stable
nuclei of magnesium oxide.
In a manor analogous to magnesium,
of calcium, iron, and silicon is augmented
the
chemical
more
volatile
by
reduction of these refractory oxides to their
suboxide (SiO) or metal (Ca and Fe)
vaporization
the
locally
vapor due to the
1982).
reducing environment within the char particle (Quann,
As
those
vapors
diffuse
into
the
surrounding the burning char particle,
reduced species will reoxidize.
oxidizing
the
environment
The sequence of condensation
for the refractory oxides of calcium,
tion and their relative partial pressure.
in
will
iron, and silicon
oxida-
be highly dependent on the rate coefficient for their
formation of stable nuclei,
those
of
vapors
the
Subsequent to
increase
one might anticipate an
the oxidation rate by heterogeneous oxidation of the metal
or suboxide vapors by collision with the
oxygen
atom
layer
formed on the surface of the growing particles. The contribution of this mechanism to Mgo formation is
estimated
two orders of magnitude smaller than than that due
to
to
be
homo-
213
calcium,
late
condenses
and its rate of oxidation
slow.
is
1959).
to
relative
and iron because the silicon
high
at
surface
the
1969; Glassman,
temperatures (Markstien,
Silicon
with
atoms
magnesium
efficiency of
collision
low
the
neglecting
even when
geneous oxidation,
magnesium,
monoxide
silicon
The
low
is
concentration
vapor pressure at the surface of the char particle is reduced
to a level below the equilibrium value because of a diffusiodiffusional
Such a
nal resistence within the char particle.
limitation is not important for magnesium and calcium because
ginal coal particle
1982).
present
is
in contrast to silicon which
in discrete mineral inclusions (Quann,
ori-
the
in
magnesium and calcium are atomically dispersed
rate
The
of
oxidation of a monoxide such as SiO to SiO 2 is expected to be
much slower than that for the oxidation of
a
the
to
metal
monoxide; this statement is based on an analogy with aluminum
for which it
to AlO is
was found that the oxidation of aluminum
by
02
two orders of magnitude faster than that of A10
to
saturation
of
A10 2 (Fontijn, 1976).
The proximity
the
of
calcium and iron to the surface of the buring
as determined from steady state
model
char combustion.
However,
condensation of silicon in
magnesium oxide,
the
model's
the
suggest that
onset
of
for
the
prediction
the presence of stable
Figure 7.9,
the
silicon will condense heterogeneously only upon
these
that
suggests
species like magnesium condense shortly after
particle
char
nuclei
bulk
the
of
of
the
comple-
E
4-J
LU
cr
cn-5
Psio
-6
C)
C)
L
r
Peq, Si0,
-7-7
0
iPsi0
CD
-8
9
1
5
13
9
17
R/Rp
Figure 7.9 - Profile of SiO and SiO
particle's surface
2
at various distances from char
215
tion of char combustion where the local gas temperature drops
back to the furnace temperature of 1700 K.
to the surface of the
that
suggest
particle
char
burning
iron
and
The proximity of the saturation of calcium
these species like magnesium condense shortly after the onset
of char combustion,
as the local
drops
temperature
gas
char
the
of
combustion
temperature upon completion of the
furnace
the
to
back
only
saturate
shown to
while silicon is
particles.
free molecular behavior (i.e., Kn ~ 100).
concentration dependence
the
Consequently,
condense
which
species
those
of
exhibit
to
The submicron particles are small enough
heterogeneously on the surface of the submicron particles
to
predicted to be inverse proportional
equation 7.25.
diameter,
particle
only
Of the major refractory oxides examined
silicon displays a particle
dependence,
size
6.3.
section
From the STEM analysis of individual submicron particles,
magnesium,
size.
Figure
calcium,
7.10
while
the
concentration
of
particle
on
and iron show no dependence
the
with
concentration dependence for silicon is best correlated
inverse diameter,
is
From the growth process of the submicron particles the
relative uniform concentration for
iron with
particle
size
cain
be
magnesium,
calcium,
and
During
and
explained.
shortly after the completion of the combustion of
char,
the
the growth of the submicron particles have been shown
to
well predicted by the classical coagulation theory where
be
the
216
1
1
r
100
A - Mg x 0.2
80
O - Si x 10
0 - ca
<-
4
Fe x 0.5
J
60
z3D
F0C
U
I-UZ)
40
z
20 P
A
n I
0.000
I
0.003
80
0.006
0.009
INVERSE DIAMETER
0.012
(Ao )
Figure 7.10 - Elemental concentration for primary particle
by STEM verses inverse diameter
217
colliding particles rapidly coalesce upon contact.
as the local
gas
temperature
back
drops
However,
furnace
the
to
temperature upon completion of the combustion of the char and
rate
as the primary particle size increases, the coalescence
leads
is shown to decrease sharply where further coagulation
to
condensation
of
calcium and iron are predicted to account for
the
formation
the
magnesium,
of
aggregates.
The
initial formation of the submicron particles where the growth
of the particles
from
the
angstroms to the final primary particle size of
six
of
size
cluster
critical
hundred
one
fifty angstroms will entail the coalescence of a large number
(i.e.,
the
~1.0 x 104) of various sized particles.
interparticle
concentration
species
those
of
Consequently,
which
condense relatively early during the growth process should be
relatively uniform as is observed for the case of
calcium,
and iron.
However,
the
in
case
of
magnesium,
silicon,
the
predicted particle size dependence is preserved since silicon
has been shown to condense heterogeneously after the
coales-
The late
conden-
cence of the primary particles has ceased.
sation of silicon relative to the other refractory oxides
further supported by Auger spectroscopy which has
bulk of the silicon to be present near the outer
the submicron particles while magnesium,
are shown to make up the core
of
section 6.3.
7.3.2 VOLATILE AND TRACE SPECIES
the
calcium,
submicron
is
shown
the
surface
of
and
iron
particles,
218
The volatile and trace species are
magnesium,
conoxides
refractory
dense at much lower temperatures than the
of calcium,
to
assumed
they
iron, and silicon either because
they
vapor pressures than the refractory
oxides.
or
species condense heterogeneously
these
Whether
will
homogeneously
the
of
highly dependent on the area
higher
much
have
are present in dilute quantities or
and
residual
be
primary
condensation
submicron particles availible for heterogeneous
and the rate at which the combustion products are cooled
The method employed for determining whether homogeneous nucleation of the volatile and trace species can occur in
the presence of the pre-existing particles will be determined
for
the criteria
Classically,
nucleation has been exceeded.
homogeneous
for
by establishing whether or not the criteria
homogeneous nucleation has been defined by that condition
which the
nucleation
rate
one
exceeds
nuclei
cal nucleation theory,
section 7.1,
species will eventually saturate.
the
As
of
homogeneous
can be calculated.
I = 1 nuclei/cc sec)
the combustion products are cooled
classi-
the critical supersatura-
tion partial pressure required for the onset
nucleation (i.e.,
cubic
per
From
centimeter per second (Hirth and Pound, 1963).
at
the
volatile
gases
and
are
As
trace
cooled
further, the partial pressure of species i will be determined
by calculating the
amount
of
the
condenses heterogeneously on the the
over the short
time
increment
at
condensable
pre-existing
each
vapor
that
particles
temperature.
The
219
amount of condensable vapor
in
remaining
species
the
gas
phase after each step will be used as the starting amount for
temperature.
next
the
the next time increment at
partial pressure of species i exceeds its
turation
partial
pressure
supersa-
during
cooling,
assumed
to
presummed
to
homogeneous nucleation in the classical sense is
occur.
The trace
volatile
and
are
species
condense only as the gases are being cooled
ticle size distribution for the residual
ash particles are well established.
the
critical
point
any
at
If
where
and
Therefore,
tion from both the residual and submicron fly
the
submicron
parfly
the contribuash
particles
will be considered in the present model.
For diffusion-limited heterogeneous condensation,
the
from
the
amount of condensing volatiles will be
determined
flux integral:
Eq. 7.27
Sn(v) Fi dv
For the residual ash particle mode,
bution function, n(v),
experimental data.
the particle size distri-
is approximated numerically
Thus,
from
the
the flux integral becomes
7
Snk
Fi(dk)
Eq. 7.28
stage
k=O
where nk is
the number density of particles in the size range
220
corresponding
and is given by nk =
to stage k,
4
Here,
kV/vk.
and
k is the fraction of the total ash appearing on stage k
vk is the average particle volume for that stage
3.1) while V is
7.2).
In
Table
section
(see
fraction
the total ash volume
(see
order to account for the non continuum behavior
7),
the smaller residual fly ash particles (stage 6 and
molecular diffusion flux, Fi(dk),
is substituded into
required
The
7.28.
equation
the
regime
transition
for the
of
sum
is
there fore
12VD (P
- P
1 RT e,
)
f2FS (d k
1d
)
Eq. 7.29
stage
k=0
The
behavior
distribution
t ('7),
function,
7 =
when
size
self-preserving
f
v/
vf
and
(Friedlander,
volume
average submicron particle
molecular
free
the
and obey
kn > 10)
(i.e.,
exhibit
particles
submicron
transforming the size distribution
the
1977).
In
of
the
n(v),
function,
is
submicron particles for self-preserving
(see section 7.2) and
substituting the
flux,
molecular
diffusion
F'i(dp),
equation 7.24 to account for the free molecular
1
of
the flux integral becomes
the submicron particles,
12VD (P.
behavior
from
-
RT
P
eq,i
)
0.65 X d
G
Eq.
7.30
221
G
where
The 2/3-moment of
72 /
=
3
#(7)
d7
0.90.
represented by G has the value
#(q)
Depending on the magnitude of the potential for condensation,
(P
1
the
- Peq, i),
eq,i
which
effect,
kelvin
accounts
the
for
elevated vapor pressure over extremely small particles due to
reduces
will preferentially
their curvature,
for condensation to the smaller particles (i.e.,
1000 angstroms).
An expression for the vapor
potential
the
<
diameter
pressure,
Pd'
over a drop of diameter dp has been derived from thermodynam1977)
ics (Friedlander,
Eq.
Km
exp
P
Pd
7.31
where Peq is designated as the equilibrium vapor pressure for
From equation 7.31,
a flat surface.
a drop
is
decreases.
shown
to
increases
the vapor pressure
as
the
particle
Since the volatile and trace species are
over
diameter
assumed
to condense well after coalescence of the submicron particles
has ceased, the area of the primary particles
condensation
agglomeration.
will
be
dependent
on
available
the
extent
The projected area of such agglomerates
been shown to be Agg = n 0
87
A
where
A
is
the
agglomerate
(Medalin,
establishing the flux of condensable vapors to the
of
have
crossecof
such
1967).
In
tional area of a primary sphere and n is the number
primary particles in the
for
submicron
222
particles,
primary submicron particles will be accounted for.
the
For
the
to
assumed
are
agglomerates
present calculations, the
of
area
both the kelvin effect and the true
be
composed of five primary particles (see section 7.2)
The model's predictions for the extent of supersaturation of sodium in
the presence of the residual and submicron
7.11.
For a
low
Montana
as
such
coal
content
chlorine
Figure
in
particles for various cooling rates are presented
vapor
lignite, sodium sulfate is the thermodynamicly favored
anticipated
to
Furthermore, chemical
state
analysis
section 6.4.
sodium is present as sodium sulfate in the fume,
Therefore,
based
the present calculations for sodium are
the assumption that sodium presides as sodium
sulfate.
equilibrium vapor pressure for sodium sulfate
is
assuming a pure condensed
thermochemistry involved,
Because
phase.
accounting
for
of
the
considered.
The
particle
size
residual and submicron particles is
temperature
percent oxygen in nitrogen.
The
complex
the
formation
of
will
not
distribution
for
the
the
fly
ash
based
on
distribution obtained from the combustion of Montana
(45/53 um) at a furnace
on
calculated
sodium silicate and other sodium bearing solutions
be
that
shows
ESCA
by
is
1981).
al.,
et.
(McNallan
condense
sodium
the
which
species over the temperature range at
of
1700
K
in
lignite
twenty
The rate at which the combustion
products are cooled as they approach the water cooled collection probe was estimated from the temperature profiles in the
I
I
I
P
N82S0,
Cooling rate
U,
1 - 5,000 K/sec
2 - 20, 000 K/sec
3 - 40, 000 K/sec
E
t
crit
-4
Na SO4 , eq
LLJ
:l
U)
LLJ
0n-
I
I
I
Na SO4
-6
3
Er-
-8
-2
O_J..
-n
1000
1200
1600
1400
TEMPERATURE
K
Figure 7 .11 - Tendency for critical supersaturation of Na2 SO4 as combustion products
are cooled in the presence of residual and submicron particles
224
7,000
Assuming
K/sec.
combustion,
the
sodium
during
vaporizes
the sodium vapors are predicted
about 1400 K.
saturate
to
heterogeneously.
all
the
sodium
condense
to
predicted
is
rates,
Only at much higher cooling
40,000
exceed
is the partial pressure of sodium predicted to
the onset of homogeneous
nucleation.
cooling rate of 40,000 K/sec,
still
over
condense
of
percent
a
the
predicted to condensed heterogeneously before
the criteria for homogeneous nucleation is
sodium,
at
even
However,
ninety
for
necessary
pressure
the crtical supersaturation partial
sodium is
at
As the gases are cooled below 1400 K at a rate
of 5,000 K/sec,
K/sec,
all
to
K/sec
2,000
between
range
furnace (see Figure 3.2) to
satisfied.
as
the
combustion
to
assumed
the other volatile and trace species are
heterogeneously
Like
are
products
cooled.
For diffusion-limited condensation the
concentration
dependence of those species which condense heterogeneously on
the submicron particles
have
proportional to particle size.
Sb, Fe,
Ca,
been
to
inversely
be
As,
The concentration of Na,
and Mg obtained from the
classified samples of the
shown
submicron
INAA
of
particles
verses reciprecal diameter in Figure 7.12.
that the concentration dependence of As,
The
are
plotted
observation
Sb, and Na
correlated by d-1 relationship supports the
size
discete
model's
is
best
predic-
tion that the volatile and trace species condense heterogeneously as the gases are being cooled.
The
determination
of
225
100
MOBILITY CLASSIFIER
75
A
- Na x i.OE I
+
- As x 4.8E 3
0
- Sb x 2.8E 4
[
- Ca
X - Mg x 2.8E-1
- Fe
-
CDI
502:
LL
H
U
OC
U-
25 F
0 L
0.000
0.003
0.006
INVERSE DIAMETER
0.009
0.012
(Ao)-I
Figure 7.12 - Concentration of various elements in the size
classified fume verses inverse diameter
226
submi-
the stratification of the various elements within the
trace species form
the
the
of
layer
surface
most
outer
volatile
the
that
cron particle by Auger spectroscopy show
submicron particles.
Under the combustion conditions examined,
have
totally
(Quann,
1982).
volatile
species
to
assumed
tile and trace species have
been
vaporized due to their high
vapor
pressure
The discrepency between the amount of
the
vola-
the
appearing with the submicron particles and that in the parent
heterogeneously
volatile and trace species will condense
the surface of residual fly ash particles.
condensation of the volatile and trace
on
the
the
re-
on
species
For
size.
particle
concentra-
diffusion-limited
dependence
heterogeneous condensation, the concentration
the
the volatile and trace species in
residual
predicted to be inversely proportional to
square (i.e.,
continuum regime Kn > 1).
that a significant fraction of sodium and
residual fly ash is present
as
condensation on these particles.
tration trend for both sodium and
a
of
is
diameter
shows
7.13
potassium
in
the
heterogeneous
of
from
potassium
that some of the sodium and potassium may
vaporized during combustion.
Figure
ash
fly
particle
result
However,
on
Evidence for
sidual fly ash can be obtained by examining their
tion dependence
the
of
coal were initially attributed to the fact that some
it
not
Despite their high
the
concen-
is
evident
have
totally
volatility,
the concentration of sodium and potassium for the very
large
10
8
CD
FH
LLJ
C-)
12:
C-)
2
4
INVERSE DIAMETER SQUARE
Figure 7.13
6
(um)-2
- Concentration of Na and K in the residual fly ash verses inverse
diameter squared
228
residual fly ash particles do not tend toward
zero,
instead
their concentration appears to reach an asymotote.
There
is
some question as to whether a condensation driving
force
is
the experimental system for the volatile
actually achieved in
trace species.
the
In
case
of
arsenic,
the
example,
for
partial pressure of arsenic in the sampling system at 500 K
-9
less than the
considerably
is 1.3 x 10 9 atms. This is
equilibrium vapor pressure
of
coolest point in the system ( 60 C in
As40 6 )
(as
arsenic
since
impactor),
the
the
at
the boiling point of arsenic (as As 4 0 6 ) is 730.2 K (Weast et.
al.,
eighty to ninety percent of the arsenic
Yet,
1974).
the coal is recovered with the ash in the
The fact that the arsenic
implies a low arsenic
at
deposits
activity
the
in
impactor.
cascade
all
system
the
in
phase.
solid
The
high-temperature gas-phase chemistry of As has been shown
<H3 AsO 4
be dominated by the forms HAsO 2 < AsO
Srivastava, 1975).
The very low concentration of arsenic
tion of As 2 species during cooling,
forms of
arsenic
temperatures.
In
can
be
this case,
expected
Consequently,
to
dominate
the condensed phase.
species
the
at
acid
lower
the mechanism of deposition could
involve the reaction of the acid with bases such as
CaO in
in
forma-
the
even though such
may become thermodynamically favored.
to
and
(Farber
our sampling system precludes on kinetic grounds
in
MgO
and
Similar considerations may apply
to other volatile trace species as well (i.e.,
Sb).
The rate
of adsorption of gaseous species reacting at the surface of a
spherical particle in the continuum regime is
229
F.(d) =
1
P.
7a D
2.60 A
where a is the probability of
1
d 2
p
Eq.7.32
on
reaction
The
collision.
of
number of molecules depositing on particle
size
dp
per
unit mass of the particle can be expressed as
Fi(d)
( P(
Thus,
1d
p
=/6) d 3)
the concentration dependence of
which is adsorption controlled in
be
to
expected
diameter.
the
continuum
particle
correlated by the d - 1 relationship than the d -
2
that
the
adsorption-limited.
of
deposition
Since,
ash particles,
are
these species can be assumed to
is
species
these
particles
better
relationship,
As
the concentration of
tend toward zero for the very large
some
of
Figure 7.14 shows that the concentration
Sb
is
regime
the
to
of the volatile trace species such as As and
confirming
species
depositing
a
proportional
inversely
Eq. 7.33
dp
and
fly
residual
have
Sb
totally
vaporized.
The concentration dependence of Na, K, As, and Sb
the residual fly ash
volatile and trace species in
result
of
that
confirms
condensation
the
and/or
the
presences
residual
adsorption.
fly
of
ash
However,
in
the
is
a
the
230
100
Q
K
- As x 1.0E 5
- Sb x 5.OE 6
75
0
4-)
CD
H
F--
50
CD
LLU
25-
0
0
1
INVERSE DIAMETER
Figure 7.14
3
2
(um)1
- Concentration of As and Sb in the residual
fly ash verses inverse diameter
231
sidual fly ash is not obvious,
re-
the
in
species
explanation for the magnitude of those
particles
since the submicron
should dominate as the sink for the condensing vapors because
particles
of the high specific surface area of the submicron
relative to the residual fly ash particles.
The
been
has
function
distribution
residual-mode
shown to be more or less independent of the overall degree of
ash vaporization,
flux
to
this
conditions.
f,
mode
However,
the self preserving size distribution,
area
the area of
dependence
fume
as
0.6.
for
fume
vapors can be fully described in terms of the overall
volatile
the distribution of the
species
modes can be estimated from the flux
sidual fly ash particles,
spheres of the fume,
equation
equation 7.30.
As
degree
condensation,
between
the
two
for
the
re-
integral
7.29,
and
condensing
the
. For diffusion-limited
the
7.15.
Figure
The competition between the primary spheres of the
the residual fly ash particles as a sink for
the
0.6
to
submicron particles is clearly demonstrated in
of ash vaporization,
obeys
submicron-mode
which determines the flux to this mode should vary
The conformation of the
funcoverall
the
to
sensitive
Since the
degree of ash vaporization.
combustion
distribution
the submicron-mode
tion has been shown to be highly
the
of
independent
is
specific
the
Therefore,
(section 5.1).
the
primary
previously
stated,
and
of
the
primary particles due to agglomeration will be accounted
for
the kelvin effect and
the
reduction
in
the
area
232
OXYGEN PARTIAL PRESSURE { ATM)
0.1 0.15 0.2
0.4
0
U
0.3
L
O
W
0.2
LL
0
0
Li
a:
0.1
0.05
0.15
0.25
, 3/5
Figure 7.15 - Superficial area of fume with predictions
for self-preserving size distribution
233
in the flux integral for the submicron particles.
A comparison between the predicted
fraction of
vaporized
appearing
sodium
various cooling rates is
experimental
and
in
the
fume
The amount of
shown in Figure 7.16.
trends
sodium vaporized was estimated from the concentration
for each condition, Figure 7.
17.
The
fraction
apperaing in the submicron particles (i.e.,
fume)
of
rate
be fairly well predicted by a cooling
of
is shown to
K/sec
in
estimated
the fraction of sodium
The increase in
sodium
2,000
which is the lower bound for the cooling rates
the furnace.
for
appear-
ing in the fume as the amount of the total ash vaporized,
is
increased from
less
one
than
to
percent
,
twenty
over
percent can be attributed to the increase in the area of the
0.6
the prediction of sodium
. The variation in
fume by 0
to
appearing in the fume with cooling rate can be attributed
condensable vapor is
shown to
Peq,i
)
to
that
the
preferentially reduce the potential for
However,
creased the supersaturation
of
as the
the
the
Consequently,
the potential for condensation,
will be small enough
submicron particles.
close
7.11).
Figure
of
concentration
fairly
remain
saturation vapor pressure (see
for low cooling rates,
the
rates
the fact that at lower cooling
(Pi
effect
kelvin
-
will
condenstion
to
the
rate
is
in-
vapor
in-
cooling
condensable
creases and the kelvin effect becomes less pronounced due
the increase in the potiential for condensation.
to
1.0
0
0.8 H-
2:
LLJ
O
ru
0. 6 -
CL
0.4
LL
CD
C
Cooling rate
I - 7,000 K/sec
2 - 2,000 K/sec
0.2
2
LL
LL
0.0 ~L
0.00
0.05
0.10
0.15
0.20
FRACTION OF TOTAL ASH VAPORIZED
Figure 7.16 - Fraction of sodium collected with the fume for various degrees
of vaporization of the total ash
235
2
0-50%
02
A
00
CD
H-
r-r
C---
LLU
1
0
0.0
I
I
I
I
0.1
0.2
0.3
0.4
INVERSE DIAMETER SQUARE
Figure 7.17
0.5
(um)2
- Concentration of Na in the residual fly ash
verses inverse diameter squared
236
For
adsorption-limited
deposition,
amount
the
of
trace volatiles will be determined from the area integral.
the particle size distri-
For the residual ash particle mode,
n(v),
bution function,
Eq. 7.34
d 2 dv
n(v)
is approximated numerically
fly ash indicate that there
is
neglible
internal
Consequently only the superfical area of the
surface.
particles
need
Thus the area integral becomes
7
7
=
nk dk2
Eq. 7.35
k/dK
6V
stage
k=O
stage
k=O
In transforming the size distribution function,
of the submicron
the
The BET area measurements of the residual
experimental data.
to be considered.
from
for
particles
self-preserving,
n(v),
area
the
integral for the submicron particles becomes
6V
df
G =
where
The 2/3-moment of #(77)
G
Eq. 7.36
j
2/3 I(7)
d7
represented by G has the
The reduction in the area of the
primary
0.90.
value
particles
due
agglomeration will be accounted for in the area integral
the submicron particles.
to
for
237
As
of
fraction
the
fume
for
shown in Figure
7.18.
The
in
appearing
Sb
and
adsorption-limited deposition is
experimental
and
A comparison between the predicted
elements As and Sb unlike sodium and potassium appear to have
ash
for the very large residual fly
Therfore,
7.14).
depositing
fraction of these elements in the vapor phase
particles and residual
If
ash
equal,
submi-
the fraction of As and Sb collected with the
However,
cron particles is less than that predicted.
residual
reactivity of the submicron particles and
particles
differ
significantly,
depositing material can be expected
Chemical characterization of
the
the
distribution
submicron
the
fly
ash
of
the
particles
mainly
shown the outer layers of the particles are
if
accordingly.
change
to
the
sink.
dominate
the
submicron particles are predicted to be
Clearly,
are
particles
on
submicron
the reactivity of the
fly
the
of
elements recovered with the fume is a direct measure
the submicron particles.
these
of
mass
input
the fraction of the
Figure
(see
particles
zero
to
completely vaporized since their concentration tend
have
composed
of SiO 2, Na 2 SO 4 ' and K2 SO 4 , while the surface of the residual
particles will be partially composed of MgO and CaO since the
deposition of the volatile species will only
the
outer
surface
Consequently,
of arsenic,
should be
of
the
residual
partially
ash
fly
for the hypothesis proposed for the
coat
particles.
deposition
the reactivity of the residual fly ash
particles
the
submicron
greater
than
the
reactivity
of
I
1.0
I
_
_
LU
0.8
C=
HI
A
ADSORPTION CONTROL
)-
I
0.6
LL
-
l
3
0.4 -
0.2 H
0.0
0. 00
A- As
C= e
y- Sb
0.05
0.10
0.15
0.20
FRACTION OF TOTAL ASH VAPORIZED
Figure 7.18 - Fraction of As and Sb collected with the fume for various degrees
of vaporization of the total ash
239
particles.
As shown in Figure 7.18,
reactivity of the fume,
residual fly ash,
C,
is
eight
surface
the relative
if
the
than
less
times
the predicted distribution curve looks very
Because
Sb.
much like that found experimentally for As and
of the complex thermochemistry involved the exact
determinais
tion of the relative reactivity of the two particle modes
not possible.
However,
results
the
do
that
indicate
relative reactivity of the two particle modes will
the
influence
the
the fraction of the volatile trace species depositing on
submicron particles.
Magnesium,
Iron,
the
and Calcium are
submicron particles where their
condense to form the initiial
to
concentration are relatively homogeneous due
quent coagulation of
which
species
various
sized
subse-
the
the
While
particles.
concentration of those species which condense heterogeneously
after coalescence has ceased will
dependence.
display
a
size
particle
Futhermore the elemental stratification
of
the
submicron particle by auger spectroscopy has shown that those
species which condense heterogeneously form the outer surface
layers of the particles.
The
amount
of
volatile
condensing on the submicron particles has been
shown
species
to
be
highly sensitive to the rate at which the combustion products
are cooled and the degree of total ash vaporization.
In
the
case of the volatile trace species As and Sb whose concentration display the same particle size dependence as
the submicron particles can be assumed to be highly
sodium
in
enriched
240
on the outer
surface
of
these
particles.
amount
The
volatile trace species depositing on the submicron
particles
whose deposition has been shown to be adsorption-limited
be highly sensitive to the relative
particle mode.
reactivity
of
of
the
will
two
241
CHAPTER EIGHT
CONCLUSIONS AND RECOMMENDATIONS
8.1 CONCLUSIONS
The factors governing the physico-chemical characteristics
of the inorganic
by
submicron particles produced
combustion of coal have been examined by burning
the
size-graded
Montana lignite particles in a laminar flow drop-tube furnace
vary
at 1700 K. The particle combustion temperature achieved
from 1800 K to 2800 K as the oxygen
the coal particles burn is
Seperation of
percent.
concentration
increased from 5
submicron
the
percent
particles
residual fly ash particles which are
deposited
cascade impactor and the utilization
of
schemes for these
necessary
for
particles
accurate
has
out
several
provided
detailed
in
the
which
to
100
from
the
in
the
collection
versatility
characterization
of
the
submicron particles.
Size fractionation of the ash yields a
distribution. The average size of the
the submicron mode varies from
greater than
400
angstroms,
less
primary
than
depending
50
on
bimodel
size
particles
in
angstroms
to
the
combustion
conditions. The submicron particles are produced by vaporization of the mineral
matter
species
during
combustion
and
242
The amount of fume produced
their subsequent recondensation.
to
20
percent at 2800 K.
The growth of the submicron particles
is
a
good
observed
the
of
representation
particles coalesce,
the particles
In
the early
stages
of
completion
diameter
particle
combustion of the char and as the primary
the
the vicinity of
the
to
subsequent
of
coagulation
of
but as the temperature in
decreases
primary
degree
particle size as a function of residence time and
ash vaporization.
theory
where
well predicted by classical coagulation theory
provides
K
1800
as metal oxide increases from 0.1 percent at
increases, the coalescence rate decreases and the agglomerate
produced appears as aggregates of spheres.
The chemical composition of the Montana lignite
is dominated by MgO and CaO at all but
combustion temperature
(< 1900
K),
particle
lowest
the
apparently
fume
the
because
vaporization of these refractory oxides is augmented by their
chemical reduction to the more volatile metal
the
in
vapor
locally reducing atmosphere within the burning char particle.
The inorganic submicron particles are found to consist
of
a
core of MgO, CaO, and FeO with an inner coating of silica and
an outer coating of sodium, arsenic,
Furthermore,
the concentration of silicon
and trace species which
are
found
metals.
and other trace
to
and
make
the
up
volatile
the
outer
surface of the submicron particles show a sharp increase with
decreasing particle size.
243
reoxi-
From a steady state one dimensional model the
and
dation of the vaporized reduced-state species of Mg, Ca,
supersaturation
Fe away from the char surface results in the
burning char particle
is
which
predicted
of
the
provide
the
layer
boundary
the
in
of these refractory oxides
to
phase
necessary driving for the formation of a new condensed
subsequently
deposition
Silicon
by homogeneous nucleation.
occurs as a probable consequence of the low rate of oxidation
The
of the SiO vapors released by the burning coal particle.
surface
volatile and trace species condense on the outer
as
the combustion products are cooled. Most of the
ash
surface
is provided by submicron particles even though
they
consti-
total
ash.
residual and submicron particles are coated
with
deposit of volatile (Na)
the
of
fraction
tute only a small
(As,Sb)
trace
and
distribution of the volatile elements between
amount
the
modes of the ash is influenced by
particles produced during combustion and the
the combustion products are cooled.
the trace species,
However,
where an insufficient
diffusion-controlled condensation exists,
tion
of
these
controlled.
species
Consequently,
is
believed
the
surface
a
The
elements.
the
of
rate
size
two
submicron
at
which
in the case
driving
force
of
for
the surface deposito
distribution
be
of
chemically
the
species between the two size modes is
influenced by both
relative reactivity and surface area
between
modes.
the
Both
the
two
trace
the
size
244
8.2 RECOMMENDATION
(A)
the
A more detailed investigation of
facilitate
growth studies should be conducted in order to
to
coalescence
of
one
from
particles
submicron
a
of
the coagulation
better understanding of the transition in
the
resolved
time
The approach would be to examine the growth of
agglomeration.
the
where
coals
the fume produced from other
contribution
rich
Focusing on those fumes
combustion of Montana lignite.
the
from
produced
fume
the
in
less pronounced than that
much
is
from the refractory oxides of calcium and magnesium
temperatures
in iron, coalescence should occur at much lower
and between much larger particles.
(B)
A more detailed investigation of the vaporization
of
to
be
the volatile
examined,
constituents,
with specific
various mineral forms.
such
as
sodium,
emphasis
on
its
needs
vaporization
char despite the increase
decreasing
times.
burning
the
rational is that at higher temperatures
sions will be molten and consequently,
the sodium will be greatly enhanced.
of
the
temperatures
combustion
char
in
for
time
sodium appears to be sensitive to the burnout
with
the
For the case of Montana lignite where
sodium is atomicly dispersed, the extent of
associated
in
activity
present
The
mineral
inclu-
their interaction with
In obtaining
a
better
understanding of the thermochemistry and examining the extent
of vaporization
of
sodium
in
different
association of sodium is varied, a
more
coals
where
fundamental
the
under-
245
standing of the vaporization of sodium can be obtained.
The
(C)
between
competition
the
of
condensation
the
vaporized volatile species on the submicron particles
verses
the residual fly ash particles
highly
is
predicted
dependent on the rate at which the
cooled. In
the
to
combustion
be
products
obtaining better control of the cooling
combustion
products,
competition between the two
a
better
size
rate
for
of
the
for
the
understanding
modes
as
are
sinks
volatile species can be obtained.
(D)
The competition between the deposition of
species between the two size modes has been
the
trace
proposed
to
dependent on the relative reactivity of the surfaces for
two size
modes.
The
distribution
between the two size modes for
the
of
the
trace
fly
ash
produced
be
the
species
from
different coals suggests that the relative reactivity between
the two size modes is dependent on coal type.
zing the surfaces of the fly ash produced
from
In characterithe
various
coals of interest and obtaining a better understanding of the
thermochemistry may facilitate a better understanding of
distribution for the
modes.
trace
species
between
the
two
the
size
246
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