FUNDAMENTAL ASPECTS OF
COAL-WATER FUEL DROPLET COMBUSTION AND
SECONDARY ATOMIZATION OF COAL-WATER MIXTURES
FINAL REPORT, Volume II
by
T.U. Yu, S.W. Kang, J.M. Bedr
J.D. Teare and A.F. Sarofim
MIT EL 87-003
February 1987
DOE/PC/70268-F2
FUNDAMENTAL ASPECTS OF
COAL-WATER FUEL DROPLET COMBUSTION
AND
SECONDARY ATOMIZATION OF COAL-WATER MIXTURES
FINAL REPORT
Professor J.M. Beer
Professor A.F. Sarofim
Principal Investigators
Volume II
The Energy Laboratory
and
Department of Chemical Engineering
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
Date Prepared - November 1986
Prepared for
United Stated Department of Energy
Pittsburgh Energy Technology Center
Fossil Energy Program
Under Grant Number:
DE-FG22-84PC70268
i
DISCLAIMERS
This report was prepared as an account of work sponsored by an agency
of the United States Government.
thereof, nor
any
of
their
implied, or assumes
any
party's
results
use
of
the
Neither the United States nor any agency
employees,
makes
legal liability
of
such use
or
any
warranty,
expressed
responsibility for
of any
information,
or
any third
apparatus,
product or process disclosed in this report, or represents that its use by
such third party would not infringe privately owned rights.
ii
FOREWORD
Grant
Number
Massachusetts
DE-FG22-84PC70268
Institute
September 1986.
of
covered
Technology
research
during
the
carried
period
out
at
October
the
1984-
All of this research related to combustion of coal-water
fuels, but the workscope covered two entirely separate tasks which involved
experimental work in three different facilities at MIT.
The Task 1 investigations dealt with Fundamental Aspects of Coal-Water
Fuel Droplet Combustion, and the
experiments were
carried out in Laminar
Flow
Monitor
DOE
Reactors.
The
Technical
at
the
Pittsburgh
Energy
Technology Center for this portion of the work was Dr. James D. Hickerson.
The
Task
2
research
was
on
Secondary
Atomization
of
Coal-Water
Mixtures, and this involved measurements on a Spray Test Rig to characterize
spray
atomization
quality
of
fuels
with various
treatments,
combustion tests in a 1-3 MWth Combustion Research Facility.
followed by
The Spray Test
Rig was also used to characterize the nozzle used in the Task 1 work.
The
Task 2 Technical Monitor was Mr. Charles McCann, who was also the Project
Manager for the Grant.
Because of the
disparate nature of these separate-but-related Tasks,
this Final Report is issued in two volumes; Volume I covers the Fundamental
Aspects and Volume II deals with Secondary Atomization.
iii
RESEARCH TEAK
Principal Investigators
Professor J.M. Be6r
Professor A.F. Sarofim
Research Group
Tasks
Prof. J.M. Be6r
1, 2
Dr. D. Froelich
1
Mr. S.9. Kang
1, 2
Mr. S.G. Kang
1
Prof. A.F. Sarofim
1, 2
Dr. S. Srinivasachar
1
Dr. J.D. Teare
1, 2
Mr. L.D. Timothy
1
Dr. M.A. Toqan
2
Dr. P.M. Walsh
2
Dr. T.U. Yu
2
Reports Prepared by:
Volume I
Volume II
S.W. Kang
A.F. Sarofim
J.D. Teare
T.U. Yu
S.W. Kang
J.M. Be6r
J.D. Teare
A.F. Sarofim
J.M. Be6r
ACKNOWLEDGEMENTS
The contributions of the members of MIT-CRF group to the experimental
program, and of Ms. Bonnie Caputo in the report preparation, are gratefully
acknowledged.
ABSTRACT
This Final Report is issued in two volumes, covering research into the
combustion of Coal Water Fuels (CWF).
discussed;
Volume I contains results
Two separate but related tasks are
obtained under Task 1 - Fundamental
aspects of Coal-Water Fuel Droplet Combustion in which the experiments were
carried out in Laminar Flow Reactors.
The present report, Volume II, covers
experiments under Task 2 - "Secondary Atomization of Coal-Water Mixtures".
Three
of
methods
improving
spray fineness
by
fuel
treatment were
investigated - 1) the heating of the CWF under pressure to produce steam as
the pressure drops during passage of the CWF through the atomizer nozzle 2)
the absorption of CO02 gas in the CWF to produce a similar effect, and 3) the
addition of a chemical additive which will cause microexplosions
droplets upon heating.
in the
These treatments are expected to produce disruptive
atomization, i.e., the disintegration of slurry droplets subsequent to their
leaving
the atomizing nozzle,
and therefore
to yield better burnout and
finer fly ash particle size distribution upon combustion.
The effects of
disruptive atomization upon CWF spray size distribution were studied using a
spray test chamber equipped with a laser diffraction particle size analyzer;
the
data
were
function.
fitted
The
to
the
combustion
Rosin-Rammler
characteristics
particle
of
the
size
treated
distribution
were
CWFs
investigated in the MIT Combustion Research Facility.
The spray chamber tests established that thermally-assisted atomization
produced reductions both in the mean droplet size and in the mass fraction
of large particles in the spray.
For fuel delivery temperatures up to 100*C
this effect is attributable to lowered fuel viscosity, while further heat of
the CWF (to 1500C in these experiments) produces disruptive atomization
In-flame
measurements
and
high
speed
cine
pictures
made
during
combustion tests provided detailed information for comparisons of treated
and untreated CWF.
Thermally-assisted atomization was the most effective of
the methods studied for improving carbon conversion efficiency and reducing
fly ash particle
size.
CO02 and picric acid addition techniques
substantial improvements but they were less effective.
showed
v
Table of Contents
Page
Disclaimers
Foreword
i
ii
Research Team
iii
Acknowledgements
iii
Abstract
iv
Table of Contents
v
1.
Introduction
1
2.
Experimental Apparatus and Conditions
3
2.1
2.2
3
6
2.3
3.
11
Experimental Results and Discussion
11
3.1
11
11
20
24
3.2
4.
Spray Test Rig and Optical Spray Measurement
Experimental Equipment, Measurement Methods and
Experimental Variables
Fuel Treatments
Spray Tests
3.1.1 Thermally-Assisted Atomization
3.1.2 C02 -Assisted Atomization
Combustion Tests
Summary and Conclusions
36
References
38
Appendix A
Appendix B
Appendix C
A-I
B-1
C-l
1.
Introduction
is a tendency for the coal
During coal water slurry combustion there
particles
to
the
evaporating;
decomposition
within
agglomerate
are
agglomerates
in
the
flame.
droplets
individual
then
Hence
dried
the
out
while
and
resulting
the
undergo
coal
water
is
thermal
size
particle
distribution (p.s.d.) will be determined more by the size distribution of
the
atomized fuel
spray than by the
initial particle
size
of the
coal.
increased burnout times
Large particles formed through agglomeration have
and produce large fly ash particles which accelerate convective tube bank
erosion.
Such
erosion
be
can
reduced
if
the
fly
ash
particles
are
sufficiently small that they follow the gas streamlines around tubes rather
than impact on them.
In pulverized coal combustion it is known that finer grinding of the
coal
will
yield
reduced
fly
ash
p.s.d.
The
relationship between coal
particle size and fly ash size, however, is less straight-forward for CWF.
If finer coal p.s.d. in the slurry could permit use of smaller atomizer
nozzle
orifices,
this
might
lead
to
finer
fly
ash p.s.d. via
improved
atomization, with the fineness of atomization being related to the orifice
dimensions of the atomizer.
increased slurry viscosity
Unfortunately,
for
finer solids p.s.d. leads to
a given CWF solids
loading, and this,
in
turn, may lead to coarser atomization unless the viscosity is reduced by
means of an additive or by diluting the slurry with water.
An alternative route to finer p.s.d. of the spray droplets and of the
fly ash is the use of fuel treatment to produce disruptive atomization; the
atomizer
would deliver
as
fine
a spray
as readily
achievable, but fuel
treatment or additives would cause further disintegration of the atomized
droplets, yielded finer droplet p.s.d. for combustion.
Three treatments to produce microexplosions in atomized droplets have
been studied at MIT:
i)
CWF
heating,
which
was
first
studied
in
successfully tested (2) by Babcock & Wilcox;
Germany
(1)
and
if a pressurized fuel
is heated to a value approaching its saturation temperature,
the
water content of the slurry 'flash vaporizes' as its pressure
rapidly reduced in the atomizing nozzle.
is
The resulting volumetric
change from steam release causes the droplets initially formed by
the nozzle to shatter, yielding a substantial decrease in p.s.d.
ii)
The absorption of CO2
proposed at MIT (3);
into
the
line
in the slurry under pressure, as
initially
CO2 is dissolved into the fuel by injection
between
the
pump
and
fuel
nozzle.
During
the
pressure release in the atomizing nozzle, the dissolved CO 2 evolves
in gaseous form and disrupts CWF droplets.
iii)
The use of picric acid as an additive as suggested by Dr. Kenneth
Olen (4);
upon the injection of the CWF spray into the flame the
water-soluble
and
microexplosions
on the
environment,
resulting
thermally
surface
in
unstable
of the
separations
chemical
coal particles
of partially
produces
in
the hot
agglomerated
coal particles.
Some quantitative characteristics of these three
treatment techniques
are presented in Appendix A.
In the following sections are reported the results of experimental and
analytical studies carried out at MIT to provide information on the effects
of
disruptive
particles;
atomization
in
reducing
p.s.d.
of
droplet
and
fly
ash
the influence of the three types of fuel treatment is evaluated
in terms of the spray droplet p.s.d.,
the overall flame behavior including
flame stability and carbon burnout, the p.s.d. of particulates within the
flame
and
at
Experimental
exit
from
results
the
showed
combustor,
that
the
and
fuel
the
fly
treatments
ash
were
deposition.
effective
in
reducing p.s.d. of droplet and fly ash particles (5, 6, 7).
2.
Experimental Apparatus and Conditions
2.1 Spray Test Rig and Optical Spray Measurement
A
schematic diagram of a spray test rig is shown in Figure 1.
CWF
droplet p.s.d. for a spray discharged into a 1.25 x 0.45 x 1.00 m chamber
was measured by means of a laser diffraction spray analyzer (8).
Two sides
of the chamber have plexiglas walls for optical observation and measurement;
the other sides are fitted with honeycomb sections through which outside air
to be
entrained by the
necessary
to
chamber.
A
suppress
spray can pass.
any
recirculation
This
of
supply of outside air
small
particles
description of the spray test rig and the
inside
is
the
laser diffraction
analyzer is given in Appendix B.
The measurement technique of the analyzer is based on the Fraunhofer
diffraction
droplets
in
pattern
the
superimposed
path
of
a
on
the
monochromatic
manufactured by Malvern Instruments
Inc.,
geometrical
light
comprises
image
beam.
produced
The
by
analyzer,
a laser light source
that passes light through the two plexiglas plates perpendicular to the fuel
spray flow, a 31-element photodetector that receives the light signal on the
other side of the chamber, and a minicomputer and a control terminal that
process
output signals from the photodetector
distribution parameters.
and print out droplet size
In the processing of the signal a functional form
for the size distribution is fitted to the data; for example, the software
yields
best-fit
characteristics,
Rosin-Rammler
(R-R)
parameters
including mass median diameter
from
which
other
spray
(MMD), can be calculated.
FLOW
METER
EXHAUST
FAN
EXHAUST
TO
OUTSIDE AIR
ATOMIZING
AIR SUPPLY
ROOM AIR
I I I I
WATER
STEAM OUT
SPRAY
CHAMBER
AIR FLOW
DAMPER
FILTER
CO
2
STEAM IN
INJECTION
P
: PRESSURE GAUGE
T
: TEMPERATURE GAUGE
STIRRER
V : VALVE
R
: REGULATOR
TO STORAGE
TANK
V
FUEL
MIXING
TANK
PUMP
FILTER
Figure i.
Schematic diagram of spray test facility
FUEL PUMP
WATER
5
An assessment was made of the relative error due to our choice of this R-R
size
distribution by comparison with fits based on a "model-independent"
size distribution.
The index used to show the quality of the least-square
fit of the data was defined as
31
Z Log [light intensity calculated - light intensity measured] 2
1
had a value
always
less
than 4.3,
and
its value did not
improve
(i.e.,
decrease) when the model-independent size distribution was used instead of
the R-R functional form.
The
laser
downstream
from
beam
the
was
aimed
nozzle
perpendicular
tip.
The
to
the
transmissivity
spray
of
axis,
the
30
cm
spray was
monitored, and multiple scattering effects were determined according to the
empirical calibration technique developed by Dodge
due to multiple scattering was always less than 5%.
(9).
The error on MMD
A 300 mm focal length
lens was used for the laser-diffraction particle size measurements.
This
gave an observable size range between 5 and 560 Mm.
During
spray
experiments
the
atomization techniques were tested.
163
kg/h,
while
the
diameter were varied.
atomizing
thermally-assisted
and
C0 2 -assisted
The fuel flow rate was maintained at
air/fuel mass
flow ratio
and
the
nozzle
The atomizer nozzle used for spray tests was of the
OR-KVB dual fluid type developed by Occidental Research Corp. and KVB, Inc;
it is of internal mixing and has a single exit orifice.
Both the spray tests and the combustion studies were carried out with
Coal-Water
Fuels
Prepared
by Atlantic
Research
Corporation
to
specification (coal particle size, viscosity, and fuel additive).
in the
CWF used for
prepared for the
the spray tests was
the
same
The coal
"Montcoal", and that in the CWF
combustion experiments was
"Splashdam".
The origins
of
these coals were different, but in each case the coal type was bituminous;
There
the two coals were of the same rank and were similar in composition.
is
no
to
reason
that
believe
differences
in
type
coal
used
in
the
formulation of the CWF caused any differences to exist in the qualitative
behavior of the CWF sprays used in the two types of experiments.
Indeed,
experiments were subsequently carried out at MIT with a range of coal types
in CWFs, and these later spray tests have confirmed that the behavior of a
CWF under disruptive atomization is insensitive to the coal type (10).
2.2
Equipment,
Experimental
Variables
Measurement
Methods
and
Experimental
The MIT Combustion Research Facility (CRF) is a 1.2 m x 1.2 m crosssection, 10 m long combustion tunnel equipped with a single burner capable
The combustion tunnel, shown in Figure 2, is
of up to 3 MW firing rate.
comprised of a number of 0.3 m wide, watercooled, refractory lined or bare
interchangeable sections.
metal
heat extraction
along the
The sections
length of the
can be arranged
flame, and thus
to
to control
simulate the
thermal environment of a wide range of industrial and utility flames.
facility
is
extensively
instrumented to permit accurate
The
control of flame
conditions and detailed characterization of internal flame structures.
The
experimental burner consists of an infinitely variable swirl generator
(of
IFRF moveable block design).
Combustion air from the swirl generator passes
through a 0.176 m diameter nozzle coaxial with a 0.06 m diameter fuel gun.
The
fuel
gun
and
atomizer
permitting variation
nozzle throat.
in
are
the
moveable
position of
along
fuel
the
axis
injection
of
the
burner,
relative
to
the
Detailed descriptions of the burner and combustion chamber
have been given by Be6r et al. (11) and Farmayan et al. (12).
In-flame
could be
measurements
inserted
were
obtained
with water-cooled
into the combustion chamber at various
probes
which
axial stations.
Solids sampling was carried out with probes in which the sampling lines were
Burner
Figure 2.
Experimental
Chamber
Cylindrical
Afterburner
Furnace assembly and air supply
Cold-Wall
Chamber
Exhaust
Section
either water-cooled or steam-heated.
injected
collected
into
the line
sample,
and
at the
in
the
In the former case a water spray was
sampling nozzle
latter
case
to quench reaction in the
the
probe
was
connected to
a
cascade impactor where particulates could be classified aerodynamically into
several size ranges.
The various techniques used for in-flame measurements
of temperature, gas composition and solids concentration have been reported
elsewhere (11, 12).
Fine
and
regular-grind
combustion tests.
CWFs
supplied
by
ARC
were
used
for
the
Experiments were carried out to determine the effect of
addition of CO 2 , picric acid, and the preheating of the fuel to 110*C prior
to
atomization.
Over
this
range
of preheat,
plugging in the pipeline or nozzle.
neither
slurry caused any
For each slurry a baseline study was
carried out using untreated fuel.
Specifications of the two ARC CWFs (Fine and regular-grind; Coal type:
- Splashdam)
are
included
in
Table
1.
The
experimental
conditions
maintained constant during the combustion tests are given in Table 2.
It
should be noted that the excess air in the flames studied was maintained at
a
level
unusually
improvements
due to
low
for
the fuel
coal
combustion
treatments.
(8-9%)
to
However, the
accentuate
two
the
types of CWF
(fine and regular-grind) were tested under different thermal inputs (1.0 and
1.3
MW, respectively).
Thermal
input was
increased to
raise
the
flame
temperatures in the regular-grind CWF experiments, but this would result in
improved combustion efficiency at
a given axial location relative
corresponding
Thus
fine-grind
case.
quantitative
comparisons
equivalent fine and regular-grind cases are not available.
to the
between
Table 1
Specifications on the ARC Coal-Water Fuels*
A.
Montcoal (Used for Spray Tests)
- Solids p.s.d. in slurry
Size (pm)
% Passing
- Slurry Solids:
300
100
1
2
150
97
75
81
800 ±
50 cp in interval 2000 - 8000 sec-
45
72
10
29
5
15
68.1%
-I
- App. Viscosity (Haake):
B.
Splashdam (Used for Combustion Tests)
o ARC Regular (Runs 324 to 326)
- Solids p.s.d. in slurry
Size (pm)
% Passing
- Slurry solids:
850
100
70
80
20
50
7.6
30
70.3%
-1
- App. Viscosity (Haake): 616 cp at 70*F and 102 sec
o ARC Fine (Runs 320 to 323)
- Solids p.s.d. in slurry
Size (pm)
% Passing
- Slurry solids:
600
100
75
96.9
30
80
9.9
50
4.6
30
69.6%
-1
- App. Viscosity (Haake): 416 cp at 70*F and 102 sec
o Characteristics of the Parent Coals
- Proximate Analysis: As received
1.07
% Moisture
% Ash
5.50
% Volatile
30.44
% Fixed Carbon
62.99
Btu/lb
14561
Dry Basis
5.56
30.77
63.67
14718
- Ultimate Analysis (Dry)
%
%
%
%
%
%
%
Carbon
Hydrogen
Nitrogen
Chlorine
Sulfur
Ash
Oxygen (diff.)
82.91
5.06
1.50
0.11
0.61
5.56
4.25
*Analyses of experimental slurries provided by Atlantic Research Corp.
Table 2
Experimental Conditions of Combustion Tests (Runs 320 to 326)
Fixed Conditions
o
o
o
o
o
o
Atomizer: Solid Cone 50* OR-KVB Nozzle, 3.175 mm Orifice Diameter
Burner Type: 25* Half Angle Divergence, Refractory
S = 2.8
Combustion Air Swirl:
Burner Nozzle Diameter: 0.176 m
Atomizer Position: At the entrance of the divergent quarl
Combustion Chamber Configuration (from burner to outlet):
7 Water-cooled refractory lined sections
2 Water-cooled bare metal sections
5 Water-cooled refractory lined sections
ARC Fine (Runs 320 to 323)
o
o
o
o
o
o
o
o
o
CWF Type: ARC Fine, Splashdam, 67.5%, Coal Loading
Fuel Flow Rate: - 188 kg/h (-1.0 MW Firing Rate)
Fuel Pressure at Nozzle: -1.65 MPa (-1.20 MPa with Heating)
Fuel Temperature: -26 0 C (-1100 C with Heating)
Atomizing Air Flow Rate: -35.9 kg/h
Atomizing Air Pressure: -1.20 Mpa
Combustion Air Flow Rate: - 1119 kg/h
Combustion Air Preheat: -290*C
Excess 02: -2%
ARC Regular (Runs 324 to 326)
o
o
o
o
o
o
o
o
o
CWF Type: ARC Regular, Splashdam, 69.5% Coal Loading
Fuel Flow Rate: - 232 kg/h (-1.3 MW Firing Rate)
Fuel Pressure at Nozzle: -1.72 MPa (-1.40 MPa with Heating)
Fuel Temperature: -270 C (-110 0 C with Heating)
Atomizing Air Flow Rate: -42.9 kg/h
-1.34 Mpa
Atomizing Air Pressure:
- 1570 kg/h
Combustion Air Flow Rate:
Combustion Air Preheat: -310*C
Excess 02: -2%
2.3
Fuel Treatments
For
the
thermally-assisted
atomization
studies,
steam-heated
lines
connecting the fuel pump to the Spray Test Rig and to the CRF burner are
capable of raising the CWF temperature to about 150*C.
Each line is about
12 m long and equipped with gauges for monitoring pressures and temperatures
of both steam and CWF.
For the
study of fuel treatment by CO2 absorption, a 15
cm section
containing a CO2 injection assembly can be readily inserted into either main
fuel line
at the high pressure side of the fuel pump.
schematic diagram of the injection system.
Figure
3 shows a
The maximum CO2 flow rate which
could be injected into the line without producing pulsating sprays or flames
was
approximately
4
g/kg
CWF.
This
is
about
50%
of
the
theoretical
saturation limit.
The chemical treatment of the CWF was effected by premixing picric acid
with
CWF, with special
care being
taken to
ensure uniform mixing.
The
"nominal" picric acid concentration was chosen to be 0.35 g/kg CWF, which is
within a factor of two of that required to cover the CWF coal particles with
a monolayer of picric acid molecules.
excess
of the
nominal were
Several picric acid concentrations in
tested, but visual observation of the
showed no discernable improvement in atomization quality.
flames
Thus the in-flame
measurements were carried out with the nominal picric acid concentration.
3.
Experimental Results and Discussion
3.1
Spray Tests
3.1.1
Thermally-Assisted Atomization
The mass median diameter
(MMD) of the
spray was calculated from the
laser diffraction measurement data and plotted in Figure 4 as a function of
P,T
ROTOMETER
FUEL LINE SECTION
ELECTRICAL
IIEATING
ELEMENT
CO2 INJECTION ASSEMBLY
CO 2 CYLINDER
Figure 3.
Schematic diagram of CO 2 injection system
ARC CWF (68/32, standard)
OR-KVB NOZZLE
Fuel Flow Rate 2.72 kg/min
80
E
ZL
60
O-
x
0
x
X
x
A
I-
xx *
xxx
XX
40
40
0]
U)
U)
2
Air/ Fuel
Mass Ratio
0.319
0.273
0.261
0.133
0. 133
Run
I
20 2
3
4
5
I
I
I
I
I
o2
0
Nozzle Orifice
Diameter (mm)
3.302
3.175
3.175
3.175
2.794
I
I
I
I
Coal Particles
MMD = 26.3km
I
100
50
o
B
o
I
I
I
I
I
~
I
~
I
I
150
OC
CWF TEMPERATURE
Figure 4.
Effect of CWF temperature on mass median diameter of CWF spray
(CWF - ARC Montcoal)
For both high (0.261 - 0.319) and low (-0.133) A/F ratios,
CWF temperature.
decreases
the MMD
100*C
the
effective viscosity
decrease
in
reduction
temperatures
of the
size
droplet
spray
is
increasing
CWF with
Up
to
decrease
of
temperature.
to
due
the
At
temperature.
higher
the continuing reduction in MMD may be partly due to further
in viscosity, but
vaporization
increasing CWF
monotonically with
of water
in the
to
attributable
also
is
it
It
droplets.
not
does
disruptive
flash
appear feasible
to
quantify the relative roles of these two mechanisms without first obtaining
CWF viscosity data measured at high shear rate and high temperature (in the
range 100-150*C).
For
the
ratio
high A/F
be
may
it
that
observed
the
measured MMD
approaches the MMD of the parent coal particles in the CWF, indicating the
potential
quality.
of
atomization
thermally-assisted
for
improvement
of
spray
At the low A/F ratio of 0.133 there is a similar decrease in MMD
in the range 100*C to 150*C, but the smallest droplet MMD measured is still
much larger than that of the parent coal particles.
For Run 3 (0.261 A/F ratio and 3.175 mm orifice dia.), the experimental
data
were
further
analyzed
in
the
form
of
particle
mass
distribution.
Figure 5 shows the relative mass distribution of sprays for three different
temperatures (21*C, 100*C and 148*C) and of the parent coal particles used
in the formulation of the ARC CWF.
The curves represent differential forms
of the cumulative Rosin-Rammler mass distribution.
The beneficial effect of heating CWF from room temperature to 100*C and
then to 148*C can be seen in Figure 5.
does
the
mean
diameter
becomes more uniform;
spray diminishes.
At
decrease,
i.e.,
but
As temperature increases not only
also
the
spray
size
distribution
the number of large droplets existing in the
room temperature
(-21*C),
for example,
13%
of the
2.0
0
O
O
I)
O
1.0
0
100
200
PARTICLE DIAMETER (/.Lm)
Figure 5.
Size distribution of coal particles and CWF droplets
at various CWF temperature (CWF - ARC Montcoal)
spray mass is contained in droplets greater than 100 microns, whereas at a
temperature of 148*C the corresponding percentage of the spray mass is 1.3.
Figure 5 also shows that the mass distribution of the spray at 148*C in the
large size
range is close
of
forcing
the
data
to
Due to
to that of the parent coal particles.
the
functional
Rosin-Rammler
form,
the
size
distribution of 148*C CWF shows fewer large particles above 80 pm than the
This error, however, is small ( < 1%) and does not change the
parent coal.
above conclusion.
The extent of disruptive atomization can be estimated using the area
enclosed by two mass distribution lines of 100*C and 148*C.
These two lines
intersect each other at a particle diameter of approximately 43 microns.
The enclosed area to the right of this abscissa represents the total amount
of large particle mass lost due to disruptive atomization, and it is equal
to the area to the left of 43 microns, which is the total amount of fine
particle mass gained.
The area is calculated to be 0.2,
i.e.,
it can be
said that as much as 20% of the total mass of spray droplets is converted
into finer droplets.
The
strength
of flash vaporization from a
sudden expansion
of high
temperature pressurized water into atmospheric pressure can be related to a
"volume change".
vaporize
to
The volume change occurs because some of the water will
steam
as
it
undergoes
a
saturation pressure, typically to 1 atm.
sudden
pressure
drop
below
its
The bulk liquid must cool down to
the saturation temperature corresponding to the reduced pressure, and the
heat released is available for the production of steam, which results in a
greater total volume.
K
and
500
K,
The values of enthalpy of saturated water between 373
obtained
from
Steam
Tables
(13),
can be
represented by a linear function of temperature given by
reasonably well
h -
4.32 x 103 Ts - 1.193 x 106, (J/kg) for 373 K < Ts < 500 K,
where Ts is the saturated water temperature in K.
Therefore, the change of
enthalpy of the water from its value at 373 K is
and
Ah - 4.32 x 103 Ts
- 1.612 x 106,
this
for
is
available
(J/kg),
steam production.
(1)
If it
is assumed that
the
saturated water at high temperature can be passed through a rapid pressure
drop to atmospheric pressure without any transient process of vaporization,
the volume change compared to its original volume could be represented by
Ah
AV
vfhfg
v
V-
3.20 Ts - 1.194 x 103
1
3
1 ((2)
1
1.043 +
vfv
+
=
2.255 x 106 J/kg (heat of vaporization of water,
1 atm & 373 K).
with h
vg
1
=
1- (
)Y
1.672 m3 /kg (specific volume of steam, 1 atm & 373 K)
vf-
1.043 x 10-3 m3/kg, (specific volume of water, 373 K)
Pc
1.3 x 103 kg/m 3 (density of coal),
and where 7 is the mass fraction of water in the CWF.
In addition, the mass fraction of vaporized steam is
Am
m
Ah
h
7
=
(1.916 x 10
-3
Ts - 0.715) 7
(3)
fg
These
relationships
are
approximately
linear
with
temperature
and
are
displayed in Figure 6 as a function of water mass fraction of the CWF.
For
example, if the water heated to 150*C in a pressurized line undergoes sudden
expansion to atmospheric pressure through the atomizing nozzle, the water
temperature should decrease to -100*C,
liberating an "excess" enthalpy of
2.15 x
to vaporize
105
J/kg.
This
is
sufficient
produce a 150-fold increase in specific volume.
10%
of the water
and
If the water forms part of
a 70/30 coal-water slurry, the instantaneous flash vaporization produces
Absolute Pressure (kpa)
100
500
300 r--
1500 2000
1000
0.15
El
200
E
0.1
>,
0-
0
0
°-
o
I00
0.05
100
I
I
I
I
400
150
°C
I
I
200
,
1
I
I
I
I
450
Saturation Temperature
Figure 6.
Volume and mass fraction of steam generated from CWF as a function
of saturation temperature. y: Water mass fraction in CWF
- 0.03 kg of steam per kilogram of CWF, representing a net 55-fold increase
of specific volume.
For the present case of the CWF atomization shown in Figure 4 the fuel
flow rate at 2.72 kg/min and at 150*C (corresponding to a fuel volume flow
rate of 4 x 10-5 m3/s) will result in a vaporized steam flow rate of -2.3 x
10-3 m 3 /s.
This in turn corresponds to 36% of the atomizing air volume flow
rate (6.4x10-3 m 3 /s) at the A/F ratio of 0.133.
steam
flow
changes
acts
like
from 0.133
increase
due
to
atomizing
to
0.03
air flow,
kg
of steam per
the
specific
larger
the
atomizing
that
of
the
effective
A/F
mass
(0.133 + 0.03)/(1.0-0.03) - 0.168, which
conservative, because
than
Assuming that the vaporized
CWF kilogram.
volume
of
air,
the
thus
ratio
is a 26%
The assumption
steam
calling
produced is
for
higher
is
58%
flow
velocities to maintain a given volumetric flow rate.
One question which arises is whether the heating of CWF from 100*C to
150*C provides better atomization quality than would be obtained by directly
increasing the atomizing air mass flow by 26% at 100*C.
there
have
been
no
direct
experimental
comparisons
to
As of this time,
clarify
such
a
question.
However, the data in Figure 4 show the extent of improvement achievable
with CWF at 100*C when
the A/F ratio
is doubled
Since this A/F increase actually quadruples
(from 0.133
to 0.261).
the momentum of the atomizing
air, it is reasonable to postulate that a 41% increase in A/F ratio, or a
doubling of momentum, would yield approximately half as much
improvement
relative
improvement
to
the
data
at A/F
-
0.133;
i.e.,
would
yield an
roughly equivalent to that obtained by increasing fuel temperature to 150*C
while maintaining A/F ratio unchanged at 0.133.
From
an
operational
it
viewpoint
can be
argued
that
the
cost
of
achieving a given improvement in atomization quality should be lower when
the
heating
fuel
technique
used
is
when
than
the
same
obtained by increase in flowrate of the atomizing medium.
improvement
is
The compression
work for pressurized air is usually provided by electrically-driven motors,
while use of steam as the atomizing medium will significantly increase the
requirement for demineralized make-up water supplied to the boilers;
these
factors make use of high atomizing-to-fuel mass flow ratios prohibitively
expensive.
The
experimental
tests
on
atomization
thermal
have
provided
new
information on the effectiveness of flash vaporization on CWF atomization.
However,
better
vaporization,
understanding
both
upstream
of
and
the
phenomena
downstream
of
associated
the
nozzle
with
flash
orifice,
is
required to provide an improved way of controlling and optimizing thermallyassisted atomization.
3.1.2
CO0
2 -Assisted Atomization
The role
of dissolved CO2
in spray atomization has been studied by
measuring spray droplet size distributions obtained using CWF with a range
The experimental measurements of MMD as a function
of CO 2 concentrations.
of CO2
concentration
indicate
that
CO2 by itself produces
a more modest
improvement in atomization quality (Figure 7).
One
possible
comparison
of
explanation
volume
assisted atomization.
AV
where
P
is
=
expansion
these
between
results
CO 2
can
be
atomization
seen
and
from
the
thermally-
Since the solubility of CO2 in water at 31*C is
6.38 x 10
the
for
-3
(4)
P scc/g H2 0,
nozzle fuel pressure
in kPa, the
maximum soluble CO02 to the CWF volume becomes
ratio of the volume
of
120
OR CWS (standard,
OR - KVB NOZZLE
I 10 -
NOZZLE ORIFICE DIA. 3.175mm
Fuel Flow R ate = 2.27 kg/min
A/F Moss Ratio = 0.151
Fuel Nozzle Pressure 720 KPa
E
00 --
C
70/30)
a)
aL 90rE
In
0
80
0
a
a
"3
C
o
U)
U)
Run
2
2
60 1-
501
)
I
I
I
0.1
C0 2 /CWS
Figure 7.
X
o
I
I
I
0.2
0.3
Mass Ratio (%)
I
0.4
Effect of CO2 concentration on mass median diameter
of CWF spray
6.38 x 10
AV
V
-
+
10
-33
Py
(5)
(l-y)
-
PC
where - is the mass fraction of water in CWF, and Pc is the coal density
which is approximately 1.3 x 103 kg/m 3 .
The mass fraction of CO2 is
-5
Am
-m - 1.15 x 105 PY
m
Equations
the
(5)
change
volume
CWF
are graphically displayed in
achieved
is only
2, which
achievable by
expansion
an
from
to
Figure 8.
CO2
of
when
is
6.4.
dissolved
temperature of
saturation
the
is
in water
CO 2
the
At 1000 kPa,
in
times smaller than the volume
about 25
is
heating
expansion
change
corresponding volume
the
However,
(70/30)
and (6)
(6)
150*C.
This relatively weak volume change is probably the primary reason why the
C02 -assisted atomization
did
significant
show
not
improvement
in
spray
quality.
during the
One problem noted
injected mass
amount,
the
pulsating.
of
CO2
CO 2
experiments was
exceeded approximately
resulting
sprays
from
the
50%
nozzle
of
that whenever
its
became
maximum
the
soluble
non-uniform
and
This phenomenon is probably due to CO2 gases left undissolved in
the fuel line.
Since in our case the residence time for dissolution of CO2
is finite, it is to be expected that the calculated value of CO2 solubility
in H 2 0 would be larger than the actual value.
In order to reach a final conclusion on the effectiveness of this CO2 assisted
atomization
technique,
further
studies
are
needed.
How
the
absorbed CO2 escapes from the water during a sudden expansion and/or after
primary atomization remains unanswered.
22
EE
I0
O
I;
>1>>
1.5
u
0
U0
)
o
E
I0.5
03
0
5
0.5
0,
1000
2000
Absolute Pressure (kpa)
Figure 8.
Volume and mass fraction of CO2 dissolvable into CWF as a
function of absolute pressure
-y: Water mass fraction in CWF
Combustion Tests
3.2
Experimental data of gas composition, solids concentration and carbon
conversion efficiency for the baseline case and for three fuel treatments
are
compared in Tables
The carbon conversion efficiency improvements
respectively.
treatments
at
the
3 and 4 for the ARC fine and regular-grind CWFs,
to fuel
due
are accompanied by corresponding reductions in 02 concentration
The lower 02
exit.
furnace
concentrations are
concomitant with
increased CO2 concentrations and lower final concentration of CO.
The data
show that the thermally-assisted atomization is the most effective method in
improving the carbon conversion efficiency.
CO2 injection is slightly more
effective than picric acid addition.
Photographs of the flames of ARC fine CWF taken during the four scoping
trials
(Runs 322
-0 to -3) are shown in Figure 9.
It can be seen from a
close observation of the still photos that the different treatments yield
improvements
varying
in
flame
stability and fuel-air mixing.
A
fuller
flame length was especially evident when the CWF was heated, consistent with
the measurements of more complete carbon burnout.
of
effect
The
size
particle
fuel
treatments
(p.s.d.)
distribution
cascade impactor.
Particles
was
of
studied further
flame
solids,
as
in terms
of
the
determined by a
larger than 20 pm were captured by a cyclone
upstream of the impactor and sieved manually.
Particle size distributions
of particulates taken in flames of fine and regular-grind CWFs at a distance
of
X/D
-
17.1
(D -
0.176
m
plotted in Figures 10 and 11.
carbon as
is
the
combustion
In Figure
air
nozzle
diameter)
are
11 mass percentage of unburned
a function of particle size is also plotted for the
thermally-
assisted and baseline cases, indicating substantial reduction in the amount
of unburned carbon in the larger particles for the former case.
With the
Table 3
Summary of Experimental Data from Scoping Tests with
Various Secondary Atomization Treatments for ARC Fine CWF (Run 322)
Run No.
322-0
322-1
Treatment
Base
CO2
27°C
3.9 g/kg CWF
Axial Position
X/D, D - 0.176 m
Temperature K
1080C
17.1
3.3
17.1
-
1352
-
1353
-
1354
-
0.0202 -
63.9
Ash (%)
5.7
Carbon Conversion*
Efficiency (%)
3.7
CWF Type:
0.35 g/kg CWF
3.3
CO %
Solid Concentration*
(g/m3 , NTP)
Heating
17.1
3.52
20.1
58.8
17.1
1353
-
3.30
0.71
0.0067
-
0.0076
0.0035
8.7
7.9
-
3.3
2.70
15.57
14.57
322-3
Picric Acid
3.3
02 %
CO2 %
322-2
32.1
14.98
57.0
7.4
26.3
8.8
-
17.21
22.7
2.6
11.0
52.9
ARC Fine Splashdam
*
Water-quench solids sampling probe (X/D - 3.3)
Steam-heated solids sampling probe (X/D - 17.1)
t
Carbon conversion efficiency at combustion exit (X/D - 28) for all cases was >
99%.
Table 4
Summary of Experimental Data from Scoping Tests with
Various Secondary Atomization Treatments for ARC Regular CWF (Runs 325)
Run No.
325-1
325-1
Treatment
Base
CO2
270C
Axial Position
3.3
325-2
325-3
Picric Acid
Heating
3.9 g/kg CWF
0.35 g/kg CWF
3.9
17.1
3.3
17.1
3.3
17.1
1459
-
1486
-
1490
1771
1496
17.1
1100
X/D, D = 0.176 m
Temperature K
1727
02%
0.8
2.56
-
2.46
-
2.22
CO %
0.8
0.0071
-
0.0071
-
0.0072
CO2 %
1 6.0
16.5
-
Solid Concentration*
(g/m3 , NTP)
99.0
2.3
79.01
5.8
39.0
7.4
47.6
Carbon Conversiont
Efficiency (%)
4.4
90.8
26.3
93.5
Ash (%)
CWF Type:
16.6
1.9
-
80.78
16.8
0.8
1.92
0.0057
15.9
18.2
2.2
52.2
6.7
44.0
8.0
71.5
18.0
92.5
32.3
97.7
1.5
ARC Regular Splashdam
*
Water-quench solids sampling probe (X/D - 3.3)
Steam-heated solids sampling probe (X/D - 17.1)
t
Carbon conversion efficiency at combustion exit (X/D = 28) for all cases was >
99%.
BASE (Run 322-0)
CO2 (Run 322-1)
PICRIC ACID (Run 322-2)
HEATING (Run 322-3)
Figure 9.
Photographs of CWF flames with various treatments
(Run 322, CWF - ARC Fine Splashdam)
4.5 -
o
CO 2 ASSISTED
1.5
-
PICRIC ACID
a
BASELINE
x
ARC-FINE
z
o
H
1.0
H
4
MI
0.5
0
100
200
300
PARTICLE DIAMETER ( / M )
Figure 10.
Fly ash and residual char (unburned carbon) particle size
distribution for the effect of different fuel treatments
(CWF
- ARC Fine Splashdam; Flame thermal input - 1.0 MW)
2,5
100
2.0
0
z
E o
0
50
0
50
w
Z
zm
:3:
Q
z
m
B
0,5
100
PARTICLE DIAMETER
Figure 11.
200
d(/Am)
Fly ash and residual char (unburned carbon) particle size
distribution for the effect of different fuel treatments
(CWF: ARC Regular Splashdam; Flame thermal input - 1.3 MW)
300
fuel treatments, improvement in size distribution by reduction of the large
particle mass fraction and corresponding increase in the small particle mass
CO 2 and
fraction can be seen for both the fine and the regular-grind CWFs.
picric acid addition resulted in appreciable improvement in p.s.d.
However,
the technique of CWF heating produced the best p.s.d. observed.
Trimodal
distributions
of particle
size
seen on
Figures
10
and
11
indicate that several mechanisms may contribute to the production of fly ash
and
residual
char
particles
during
CWF
combustion.
As
small
and
intermediate size particles are formed through the partial disintegration of
coal agglomerates, these may continue to burn, with each producing a single
fly ash particle;
this
is roughly
equivalent to
a yield of one
fly ash
particle per original coal particle in the CWF, and it creates the middle
Alternatively several very small fly ash particles can
peak of the p.s.d.
be produced through fragmentation of a single parent coal particle, which
corresponds
particles
to
the
formed
left-most
initially
peak
of
p.s.d.,
while
some
of
the
large
through agglomerations may still exist at
axial station at which sampling was carried out (X/D -
17.1).
the
Disruptive
atomization causes effects of coal particle size on char burn-out and fly
ash particle size to become more important.
This is evidenced by the number
of large particles surviving to the axial location of X/D - 17.1 (see Figure
10 and 11 - the right-most peaks);
the
regular-grind
the number is
CWF than for those
higher for the flames of
of the fine-grind CWF.
The middle
peaks of these trimodal distributions are centered at 45 pm and 20 Mm for
the
fine
and regular-grind cases,
respectively.
In the
latter case
the
unburned carbon content of the 20 pm particles was very small, but carbon
content was not measured at X/D - 17.1 for the fine-grind flames.
comparison of the
No direct
relative extents of burnout for these two fuels can be
made, because
(as indicated earlier) the regular-grind CWF was burned in
flames with higher thermal input than the fine-grind.
Detailed measurements (see Appendix C) on the centerline of the flames
of both fine and regular-grind CWFs were made for the case of thermallyassisted
atomization for
comparison with
the baseline
data;
traverses were also carried out for one flame (Table C-2).
some
radial
The centerline
distributions of flame velocity and temperature are plotted in Figure 12.
Figure
flame
13
shows
are
that
lower
solids
than
concentrations
those
of
the
from each
corresponding
thermally-assisted
baseline
case
and,
furthermore, the carbon burnout is better.
High speed cine pictures and photographs of the flames show a wider
spray
angle
baseline,
for
and
the
thermally-assisted
corresponding
improved
case
flame
relative
stability
to
is
that
of
the
manifested
by
reduced ignition distance and absence of low frequency fluctuations in flame
front position.
The
illustrated by SEMs
particles
baseline
captured
flames
improvement
(Scanning
from
the
in combustion conditions
Electron Micrographs)
centerlines
at X/D - 17.1.
of
the
can be further
taken for
the
solid
thermally-assisted
Comparison of the SEMs
of 250
and
- 355 pm
particles in Figure 14 shows that the state of oxidation is more advanced in
the flame with thermally-assisted atomization.
For the baseline and thermally-assisted cases further comparisons were
made by determining deposition rates of fly ash on ceramic tubes inserted
perpendicular to the flame axis and thermally equilibrated with the flame
gases.
The transverse distribution of deposition rate was determined from
the total amount of fly ash deposited per unit length of deposition probe
during a period of 20 minutes.
Effects of CWF heating on the deposition
rates for tube diameters of 25.4 and 6.35 mm are shown in Figure 15.
The
40
4
2000
V%,%
I
I
I
I
i
o BASELINE
o BASELINE
•* THERMAL
ATOMIZATION
STHERMAL
ATOMIZATION
1500
ARC-REGULAR
1000 I
I
I
ARC-FINE
L
20
Figure 12.
I
30
1000
0
I
I
10
20
Comparison of gas velocity and temperature at the centerline
of CWF flames for baseline and thermal atomization cases
Left:
Right:
CWF - ARC Regular Splashdam
Flame thermal input - 1.3 MW
CWF - ARC Fine Splashdam
Flame thermal input - 1.0 MW
3
I00
I
SIO
TIIERMAL
AIOMIZATION
BASELINE
I-
'10
E
E
z
z
I-g
I-
0
z
aj
U
z
o
10
0
a
C3
CJ
0
,)
(n
°
20
10
0
X/D,
I0 0
I
I
I
20
X/D, DISTANCE FROM BURNER
DISTANCE FROM BURNER
Figure 13.
I
Solids concentrations and unburned carbon percentage
on the centerline of CWF flames for baseline and thermal
atomization cases
Left:
Right:
CWF - ARC Regular Splashdam
Flame thermal input - 1 3 MW
CWF - ARC Fine Splashdam
Flame thermal input - 1.0 MW
I
-
(A) BASELINE
(B) THERMALLY ASSISTED ATOMIZATION
Figure 14.
SEM photographs of particles collected from the
centerline
of the flame at X/D = 17.1, 250 - 355 pm
size range
(CWF - ARC Regular Splashdam)
101
X/D= 17.1I
BASELINE THERMAL
ATOMIZATION
_
O
U0
C
TUBE DIA. 6,35 mm
TUBE DIA. 25.40 mm
E
100
t
O
W
0
10
0
20
DISTANCE
Figure 15.
40
FROM FLAME AXIS (cm)
Transverse distributions of fly ash deposition rate per unit
area of ceramic tubes at X/D - 17.1, for thermally-assisted
and baseline cases (CWF - ARC Regular Splashdam)
60
rate with
deposition
CWF heating
for a
25.4 mm tube
is
than that
less
withoug heating (baseline) case at all transverse locations, by a factor of
about 0.5 - 0.6.
As would be expected, the smaller tube has higher capture efficiency by
impaction
particle
which
results
in
higher
normalized
deposition
rates.
However, for this tube the CWF heating produces increased deposition rates
relative to the baseline case in the region close to the flame axis.
reason for
this
could be
related to
the
reduction of
the mass
The
fraction
larger particles which are capable of eroding the deposited material upon
their impaction.
but
the
heat
Thus by their removal the risk of tube erosion is reduced
exchange
surface
might
collect more
of
the
fine
particle
deposits.
4.
Summary and Conclusions
Three methods of inducing disruptive atomization to improve the quality
of spray droplet p.s.d.,
studied
using
the
MIT
and thereby yield finer fly ash p.s.d., have been
Spray
Rig
and
CRF.
They
include
1)
assisted, 2) C02 -assisted and 3) chemically-assisted atomization.
thermallyThe spray
rig studies involved measurements of droplet p.s.d. and mass median diameter
of sprays using a laser diffraction analyzer.
In-flame measurements
made
during combustion experiments in the CRF served to determine the influence
of these three methods of disruptive atomization on flame stability, carbon
burnout, and resultant fly ash p.s.d.
During the
found
to
improve
spray tests the thermally-assisted atomization of CWFs was
spray quality.
This
improvement was probably due
to a
decrease in viscosity at temperatures
lower than 100*C, and to disruptive
atomization
than
at
temperatures
higher
100*C.
Compared
to
normal
atomization the CO02-assisted method had a more modest effect upon reduction
of CWF spray particle size.
This can be explained by the differences in the
volume expansion due to flash vaporization and the evolution of gaseous
CO 2
in the CWF droplet.
During combustion experiments the characteristics of the three modes of
disruptive atomization studied to identify the effectiveness of each method
in reducing the fly ash p.s.d.
The most effective method was the thermally-
assisted atomization, judged by reduction of solids concentration and p.s.d.
determined
along the
length of
the
flames.
While
not
as
effective
as
thermal atomization, CO2 and picric acid additions to the slurry have also
given beneficial results.
The improvement in atomization quality due to CO
2
absorption was slightly greater in the flame than in sprays introduced into
a cold environment.
The chemically-assisted atomization was
place in ranking behind thermal and CO02 -assisted atomization.
in the third
38
References
1.
Merten, M. and Homer, M., Section in Final Report on "Combustion of
Coal/Water Suspension Power Plants," Steinkohlen bergbauvereim (Lignite
Mining Association), Essen, Germany, January, 1972.
2.
Daley, R.D., Farthing, G.A., Jr. and Vecci, S.J. Coal Water Slurry
Evaluation, Vol. 2, Final Report CS-3413, Research Project 1895-3 EPRI
Palo Alto, CA 1984.
3.
Reid, R.C., Sarofim, A.F., and Be6r, J.M., MIT, Cambridge, MA., private
communication (1983).
4.
Olen, K.R., "Chemically Enhanced Combustion
U.S. Patent No. 4,445,150, June 19, 1984.
5.
Yu, T.U., Kang, S.W., Toqan, M.A., Walsh, P.M., Be6r, J.M., and
Sarofim, A.F., "Secondary Atomization of Coal-Water Slurry Fuels,"
Combustion and
Slurry
on Coal
Symposium
International
Seventh
Technology, New Orleans, Louisiana, May 22-24, 1985.
6.
Yu, T.U., Kang, S.W., Toqan, M.A., Walsh, P.M., Teare, J.D., Beer, J.M.
and Sarofim, A.F., "Disruptive Atomization and Combustion of CWF," 8th
International Symposium on Coal Slurry Preparation and Utilization,
Orlando, Florida, May 27-30, 1986.
7.
Yu, T.U., Kang, S.W., Toqan, M.A., Walsh, P.M., Teare, J.D., Be6r, J.M.
"Effect of Fuel Treatment on Coal-Water Fuel
and Sarofim, A.F.,
(International) on Combustion, West
21st
Symposium
Combustion,"
Germany, August, 1986.
8.
Swithenbank, J., Bear, J.M., Taylor ,S.S., Abbot, D. and McCreath,
G.C., "A Laser Diagnostic Technique for the Measurement of Droplet and
Particle Size Distribution," AIAA 14th Aerospace Sciences Meeting, AIAA
paper No. 76-69, 1976.
9.
"Change of Calibration of Diffraction-Based Particle
Dodge, L.G.,
Sizers in Dense Sprays," Optical Engineering, Vol. 23, No. 6, 1984.
10.
Beer, J.M., Toqan, M.A., Teare, J.D., "Reduction of Fly Ash Particle
Size in Coal-Water Fuel Flames" Phase IV Final Report September, 1986.
11.
Be6r, J.M., Jacques, M.T., Farmayan, W.F., and Teare, J.D., "Design
Strategy for the Combustion of Coal-Derived Liquid Fuels," EPRI Interim
Report, (RP1412-6), Palo Alto, California, 1982.
12.
Farmayan, W.F., Srinivasachar, S., Monroe, L., Ditaranto, F., Teare,
J.D. and Beer, J.M., "NOx and Carbon Emission Control in Coal-Water
Slurry Combustion," Proceedings Sixth International Symposium on Coal
Slurry Combustion and Technology, p. 165, U.S. DOE, Pittsburgh Energy
Technology Center, 1984.
13.
Keehan, J.H., Keyes, F.C., Mill, P.G. and Moor, J.G.,
John Wiley & Sons, Inc., 1969.
of Water-Slurry
Fuels,"
"Steam Tables,"
A-i
Appendix A - Characteristics of Fuel Treatment Techniques
Al.
Thermal Atomization
If water is heated to 200*C (saturated vapor pressure then being 1550
kPa, or - 15
atm) its enthalpy in the liquid state
is 0.853 MJ/kg.
On
sudden expansion to atmospheric pressure the water temperature must decrease
to -100 0 C, which represents a drop on enthalpy of -0.43 MJ/kg H20.
This is
sufficient to vaporize -0.2 kg/kg H20, since the heat of vaporization of
water at 100*C is 2.26 MJ/kg.
Thus the net result of a sudden expansion of
water at 200*C is the conversion of -20%
of the water into steam, with a
300-fold increase in specific volume.
If the water forms part of 70/30 coal/water slurry, the instantaneous
flash vaporization produces -0.06 kg of steam per kg of CWF, representing a
net specific volume change of the mixture of -100-fold increase.
of
the
heat
stored
in
the
coal
particles
(initially
at
Transfer
-200 0 C) to
the
remaining water would add to the extent of flash vaporization and enhance
the specific volume increase.
A2.
Carbon Dioxide Injection
At 310 C the solubility of CO2 in water is -0.65 P cc (NTP) per gram of
H20,
with
P
in
atmospheres.
Thus
at
a
nozzle
pressure
of
-7atm
the
saturation level would be -4.5 cc/g H20, or -0.009 g/g H20, corresponding to
-0.0027
slurry).
g/g
It
CWF
was
(for a
70/30
expected
slurry),
or
-0.0045
g/g CWF
that pronounced secondary
(for a
50/50
atomization effects
would be observed with lesser amounts of dissolved C0 2 , at
levels on the
order of 0.1% by weight of the CWF.
Provision must be made to prevent freezing of the CWF as a result of
temperature drop in the CO2 as it is expanded from storage pressure.
This
A-2
was accomplished by making the injection section a part of the heated fuel
line discussed in Al.
A3.
Picric Acid
An upper limit estimate of the 'desirable' amount of picric acid to be
added can be made by assuming a monomolecular coating for the coal particles
in the slurry.
For instance, for spherical coal particles of diameter d,
the picric acid mass per particle would be
kg/mole, A = 20 x 10- 2 0 m2
The particle
rd2 WM/ANA kg, where WM -
0.229
(molecular 'area'), and NA = 6.02 x 1023/mole.
mass would be
rpd 3 /6 kg, with p -
1200 kg/m 3 , so
that
the
picric acid requirement would be 6WM/pdANA kg/kg of coal, or (for a 70/30
slurry) 4.2 WM/pdANA
-
3.3 x 10 .4 kg/kg CWF for d - 20 pm.
This corresponds
to 1.1 x 10 -3 kg/kg H2 0 in the slurry, which is in turn approximately 10% of
the room temperature saturated solution concentration.
Picric
acid
dissolves
saturation is too low (- 1%)
rapidly
in
water,
but
its
concentration
at
to permit addition to the CWF by metering a
saturated solution, since this would cause excessive dilution of the CWF.
For
the
CRF
experiments
the
most
practicable
additive to the fuel was by premixing.
method
of
introducing
the
I
I
~~-iL-i--~--=rr=2=~f;-re=~-~-L-----L---
-
~-~-mm~-
B-I
Appendix B. - Spray Test Rig and Optical Analyzer
B-1.
Spray Test Rig
A schematic of the spray test facility is shown in Figure 1.
Figure B-
The atomizing air supply and fuel
1 is a photograph of the spray test rig.
pump systems are shared with the 3 MW combustion facility.
The atomizing
It passes through a 1/2 inch
air is supplied at pressures up to 2200 psig.
diameter line equipped with a pressure regulator, flow meter and a flexible
steel
stainless
temperature
hose,
to
are measured
a
SPC[y nozzle.
at the- entrance
Atomizing
of the
fuel
air
gun.
and
pressure
The CWF
delivered by a Moyno pump which can provide pressures up to 550 psig.
is
On
its path to the spray nozzle the fuel passes through a flow meter (Micro
Motion Model C 25).
Fuel pressure and temperature are measured at the gun
entrance.
A spray gun transporting the CWF and the atomizing air can be adjusted
vertically and horizontally to permit the traversing of different segments
of the conical spray by the laser beam of the optical spray analyzer.
The
spray
rig test chamber
is
49"
x
18"
x
40".
Two sides of the
chamber have plexiglas walls for optical observation and measurement.
About
half of the area of the other sides of the chamber is comprised of honeycomb
sections
through which
air to be
entrained by the
spray can pass.
The
supply of outside air is necessary to suppress the recirculation of small
particles into the path of the laser beam.
Air at room temperature enters
through the honeycomb sections when the exhaust fan is switched on.
At exit
from the spray chamber this entrained air and the atomizing air stream are
separated from the CWF, and then flow through a filter and a flexible hose
en route to the exhaust system of the CRF.
storage tank through a pump.
The used CWF is collected in a
B-2
SPRAY
CHAMBER
JEL LINE
I
f"
1-
ILASER LIGHT
SOURCE
ATOMIZING
AIR LINE
I:r;
*~u
f~I
T~I2
HEATED FUEL
SUPPLY LINE
,
_
=
.~a~i
EXHAUST
HOSE
S
(
Figure B-1.
Photograph of spray test facility
B-3
Optical Analyzer
B-2.
Figure B-2 shows a schematic diagram of a Laser Diffraction Spray Analyzer.
This analyzer (Malvern Instrument) consists of a He-Ne laser light source
that passes light through the two plexiglas plates perpendicular to the fuel
spray flow,
signal
of a
31 multi-element photodetector that receives
from the other
side of the
the
light
chamber, and of a minicomputer and a
control terminal that process output signals from the photodetector.
The operational principle of the laser diffraction spray analyzer is
based on the Fraunhofer diffraction pattern superimposed on the geometrical
image, produced by the droplets in the path of the monochromatic coherent
light beam.
The diffraction pattern is large compared with the image.
The
resulting light energy distribution is collected through a lens by a multielement
detector
consisting
of
31
semi-circular
rings.
The
lens
acts
effectively as a Fourier transform lens by bringing all the scattered light
from droplets at various locations in the beam into the focal plane of the
lens.
For monosize particles
the light distribution pattern at the focal
plane would consist of alternate bright and dark fringes, the position of
which would depend upon the size of the droplets.
different
sizes
are
present
an
aggregate
light
When droplets of many
energy
distribution
obtained from which the drop size distribution can be calculated.
is
The light
energy falling on one ring of the photo-detector located between radii si
and sj can be expressed (8) according to
E.i
Ii
-
C
M
2
M Nkk
k Z1
[(J
2
0
+ J
2)
1
si
-
(J 0
0
2
+ J1
1
2)
s
]
B-
where C is a constant, N is the number of droplets of size X, J 0 and J 1 are
Bessel functions, and M is the number of drop size ranges.
The total light
energy
of
distribution
is
also
the
sum
of
the
product
the
energy
B-4
A TD
LIQUID
NOZZLE
He - Ne
LASER
I
COLLIMATED
BEAM
,,
DIFFRACTED
BEAM
MULTIELEMENT
LIGHT
DETECTOR
I
DROPL.ET SPRAY
,
I.
MINICOMPUTER
CONTROL
TERMINAL
SIZE
DISTRIBUTION
PRINT OUT
Figure B-2. Schematic diagram of laser diffraction analyzer
B-5
distribution for each size range and the weight or volume fraction in that
range.
This can be expressed in the form of a matrix equation
E -
TW
B-2
where W is the weight fraction and T contains the coefficients which define
the light energy distribution curves for each droplet.
Rewriting the above
equation as W = T-1E, then with the knowledge of the inverse matrix T-1 the
weight distribution can be calculated from the measured light energy E.
approach to
the
solution of Equation B-2
An
is to assume a form for W and
adjust the parameters by iterative means until the sum of the squared errors
Z(E-TW)2 is a minimum.
various
weight
The Malvern Instrument software is capable of using
distributions
for
W,
distribution, or "model independent".
the 31
semi-annular detectors
are
including
Rosin-Rammler,
normal
To determine the diffraction pattern
scanned
sequentially by a solid state
switch controlled by a microprocessor, both with and without the droplets
present in the beam.
If, for example, the Rosin-Rammler distribution is postulated, then in
the
processing
of
the
signal
the
microprocessor
assumes
that
the
size
distribution is a good approximation to:
R = 1 - v - exp
[-(X/X)n]
B-3
where R is the weight fraction contained in particles of diameters greater
than X, X is the Rosin-Rammler mean diameter (for which R - 36.8%), and the
exponent n indicates the spread of diameters
about the mean.
For a fuel
spray typical values of n will be in the range 1.1 to 3, but n can increase
to 15-20 for near monosize droplets.
B-6
The microprocessor selects initial values of X and n and the light
energy
distribution
corresponding
calculated through Equation B-2.
to
the
Rosin-Rammler
distribution
is
A least squares error criterion is used to
determine the quality of fit between calculated and measured light energy
distribution.
The parameters X and n are then iteratively adjusted to give
the best fit with minimum error.
The Rosin-Rammler distribution in 15 size
ranges together with the calculated and measured light energy distribution
is printed by the microprocessor using the appropriate values of X and n.
Using X
and n, the mass median diameter (MMD), which is the droplet
diameter below or above which lies 50 percent of the mass of the droplets
(i.e., R - 0.5), can be calculated by
MMD -
X [In 2]
1/n
B-4
The Sauter mean diameter, SMD, also can be related by
SMD - X/ r(l-1/n)
where r
is the gamma function.
B-5
The SMD is the diameter of a droplet having
the same volume/surface ratio as the entire spray.
The mass
distribution of a spray as the weight fraction in any size
increment is given by the derivative of Equation B-3, i.e.,
dv
v
dx
n
n
X
iXX
n
exp[-(X/X)n]
B-6
C-1
Appendix C - Experimental Data
The following four tables summarize the experimental data obtained by
in-flame
measurements
in
the
Combustion
Research
Facility.
Centerline
distributions of flame temperature, velocity, gaseous species concentrations
and particle concentrations are tabulated.
Some radial distributions are
included in Table C-2.
Table C-1
Experimental Data, Run 320 (ARC Fine, Base)*
Distance from
Gas
Gas
Air Nozzle
Temp.
Velocity
X(m)
T(K)
uz (m/s)
X/D
Mole Fractions
(as measured, dry basis)
X2
(%)
0.17
1.0
103.0
0.27
1.5
23.1
0.42
2.4
1515
0.57
3.3
1638
7.5
Particle
XCO2
XCO
XNO
(%)
(%)
(ppm)
XSO 2
Concentration
3
(g/m , NTP)
(ppm)
16.0
0.4
16.2
2.300
497
875
63.67
0.74
4.2
1681
4.0
0.88
5.0
1620
2.3
0.3
17.6
1.000
457
750
500
625
36.02
1.19
6.8
1580
1.9
1.7
16.7
0.111
1.49
8.5
1538
2.4
2.5
16.1
0.651
600
500
1.80
10.3
1450
2.6
2.3
16.3
0.028
2.09
11.9
1390
2.9
2.0
16.7
0.019
3.00
17.1
1309
2.5
2.0
16.6
0.006
492
3.61
20.6
1301
2.5
2.1
16.6
0.003
495
4.22
24.1
1297
5.4
4.52
25.8
1289
6.7
*Centerline Measurement
5.61
3.03
0
3.20
3.21
3.23
2.7
15.9
0.002
489
Table C-2
Experimental Data, Run 323 (ARC Fine, Heating)
Radial
Distance
R(m)
Distance from
Air Nozzle
X(m)
X/D
Gas
Gas
Temp.
Velocity
T(K)
u (nVs)
0
0
0
0.17
0.27
0.42
1.0
1.5
2.4
1645
1719
106.0
38.2
33.0
0
0.1
0.57
3.3
1697
1641
1581
1570
1555
1600
1602
1619
1618
1593
1583
1566
1584
1589
1541
1500
1377
1356
1326
14.3
-4.1
14.5
11.8
5.0
0.0
-3.9
-2.9
2.0
7.1
-3.4
-3.0
2.1
2.8
-1.8
1.5
2.1
2.1
2.8
CN0.2
0.3
0.4
0
0.1
0.2
0.3
0.4
0
0
0.2
0.4
0
0
0
0
0
0.88
5.0
1.19
1.49
6.2
8.5
1.80
2.09
3.00
3.61
4.22
10.3
11.9
17.1
20.6
24.1
Particle
Mole Fractions
Concentration
(as measured, dry basis)
X
X
XO
(%)
(%)
(%)
-
-
1.6
6.4
7.1
6.1
6.0
1.8
2.9
3.7
4.4
5.0
3.0
3.3
3.2
3.6
3.4
3.2
2.9
2.6
2.6
14.9
12.5
11.9
12.9
13.2
16.1
15.5
14.8
14.4
13.7
15.5
15.3
15.3
14.9
15.1
15.3
15.7
16.3
15.9
-
1.150
0.056
0.025
0.012
0.010
0.320
0.073
0.017
0.014
0.015
0.040
0.040
0.011
0.008
0.015
0.016
0.007
0.007
0.008
2
(ppm)
(g/m 3 , NTP)
(ppm)
-
-
700
580
535
550
560
590
610
600
610
600
590
570
590
600
505
-
525
513
525
538
713
650
625
625
563
550
650
620
638
650
625
625
663
575
675
26.21
9.47
6.90
4.36
7.24
5.03
4.78
4.78
2.89
2.53
3.55
2.60
3.33
C-3
Table C-3
Experimental Data, Run 324 (ARC Regular, Base)
Distance from
Air Nozzle
X(m)
X/D
G as
Ternp.
T( K)
Gas
Velocity
uz(m/s)
Mole Fractions
(as measured, dry basis)
X0
XCO
XCO
XNO
XSO
(%)
(%)
(%)
(ppm)
Particle
Concentration
(g/m 3 , NTP)
(ppm)
0.17
1.0
0.27
1.5
1393
44.6
0.42
2.4
1620
12.4
0.7
15.7
3.300
740
1125
0.57
3.3
1771
-6.9
0.8
17.0
1.600
750
775
0.74
4.2
1760
-4.3
0.5
17.7
0.800
690
763
0.88
5.0
1757
-4.2
0.4
18.1
0.900
630
788
28.66
1.19
6.8
1706
-3.7
1.8
17.2
0.093
710
600
6.92
1.49
8.5
1667
-3.7
3.6
15.5
0.027
740
525
4.30
1.80
10.3
1623
-2.6
3.1
16.1
0.016
740
538
3.04
2.09
11.9
1576
-0.9
2.8
16.3
0.011
720
575
3.00
17.1
1459
2.4
16.9
0.007
700
575
3.61
20.6
1450
2.5
16.7
0.006
710
588
4.22
24.1
1439
2.2
17.1
0.005
680
600
55.6
*Centerline Measurements
423.73
99.00
2.36
2.34
C-4
Table C-4
Experimental Data, Run 326 (ARC Regular, Heating)*
Distance from
Gas
Gas
Air Nozzle
Temp.
Velocity
(m)
T(K)
uz (m/s)
X/D
Mole Fractions
Particle
(as measured, dry basis)
X
O2
(%)
Concentration
(g/m3 , NTP)
XCO2
XCO
XNO
SO2
(%)
(%)
(ppm)
(ppm)
0.17
1.0
0.27
1.5
1543
14.0
0.34
1.9
1664
10.4
0.42
2.4
1688
-7.3
1.1
15.2
4.000
700
700
0.57
3.3
1727
-8.3
0.8
15.9
3.500
670
1150
0.63
3.6
1736
-7.8
0.74
4.2
1748
-7.0
0.9
16.9
1.300
700
525
0.88
5.0
1775
-6.4
0.9
17.3
0.900
690
575
0.94
5.4
1739
-5.7
0.7
17.8
0.700
670
575
1.19
6.8
1708
-5.5
2.1
17.4
0.175
710
575
5.84
1.49
8.5
1681
-4.0
2.9
15.9
0.037
710
600
2.52
1.80
10.3
1655
-2.1
2.5
16.8
0.025
710
625
1.44
2.09
11.9
1610
0
2.4
16.9
0.013
720
588
3.00
17.1
1490
2.2
2.2
16.8
0.006
710
638
2.00
4.22
24.1
1460
1.8
17.4
0.007
670
638
1.50
43.6
*Centerline Measurement
166.48
52.25
19.29