Fuel Processing Technology 59 Ž1999. 35–50
TGrFTIRrMS study of organic compounds
evolved during the co-firing of coal and
refuse-derived fuels
Richard Lu, Shobha Purushothama, Xiaodong Yang, John Hyatt,
Wei-Ping Pan, John T. Riley ) , William G. Lloyd
Department of Chemistry and Materials Characterization Center, Western Kentucky UniÕersity, Bowling
Green, KY 42101, USA
Received 15 September 1998; received in revised form 21 January 1999; accepted 1 February 1999
Abstract
Complementary thermal analytical techniques were used to analyze gaseous products evolved
during the co-firing of coal with refuse derived fuels. The combined thermogravimetric
ŽTG.rFourier transform infrared ŽFTIR.rmass spectrometry ŽMS. techniques were employed to
study the reaction pathways for the formation of gaseous products during combustion, as well as
identify molecular chlorine, HCl, CO, CO 2 , H 2 O and various organic compounds. The discovery
of molecular chlorine has led us to look for the possible formation of chlorinated organic
compounds in the combustion process. Chlorine and hydrocarbon species are released in the same
temperature range and in higher concentrations during fast heating rates compared to relatively
slow ones. These results indicate that there may be more opportunities to form chlorinated organic
compounds during the co-firing of coals with refuse-derived fuels ŽRDF. at the faster heating rates
that may occur in a fluidized-bed combustor ŽFBC. system. q 1999 Elsevier Science B.V. All
rights reserved.
Keywords: Co-firing; Coal; Refuse-derived fuel; TG; FTIR; MS
1. Introduction
Waste-to-energy conversion of municipal solid waste ŽMSW. has been an important
practice to relieve the pressure from increasing waste generation and as a source of
cheaper energy. An ongoing project at Western Kentucky University has demonstrated
)
Corresponding author. Tel.: q1-502-745-6244; fax: q1-502-745-6293
0378-3820r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 3 8 2 0 Ž 9 9 . 0 0 0 1 1 - 9
36
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
that co-firing high heat content coal with low heat content MSW or refuse-derived fuels
ŽRDF. in a fluidized-bed combustor ŽFBC. can obtain high combustion efficiency.
However, some technical problems need to be solved to reduce the possible release of
toxic chlorinated organic compounds during co-firing coal with MSW. Additional
research is needed to optimize the co-firing conditions and parameters.
Considerable work w1–3x has been done on the application of thermogravimetric ŽTG.
analysis interfaced with a Fourier transform infrared ŽFTIR. spectrometer to the on-line
analysis and characterization of the gaseous products evolved from the pyrolysis and
combustion of various materials. However, diatomic molecules that do not possess a
permanent dipole moment are not infrared active, and therefore cannot be detected using
infrared absorption. Also, overlapping peaks, such as the O–H stretching vibration in
water and carboxylic acids, presents a problem in the analysis of mixtures as a result of
interpreting the infrared absorption data. Only a few reports w4–6x have been published
about the on-line analysis of volatile products using TG interfaced with mass spectrometry ŽMS.. The mass spectrometer is very sensitive and can identify the mass range of the
species in a sample. Also, it has a faster response time than the FTIR systems. The
detectability of the MS system is usually less than 1 ppm, which is more sensitive then
an FTIR spectrometer having a general detectability of approximately 10 ppm. However,
an MS has a relatively high cost and since it measures only mass-to-charge ratios, it
does not have the capability to determine isomers. Also, oxidation can reduce the
lifetime of the MS instrument by destroying the filament. Once the TGrFTIRrMS
techniques are combined they are a powerful method for the characterization of the
gaseous products, analysis of the decomposition kinetics, and investigation of the
combustion conditions and parameters during the combustion of a variety of fuels. A
recent special issue of Thermochimica Acta provided a survey of the present state of
instrumentation and the main application areas of coupled thermal-analysis and gas-analysis methods w7x.
In previous work w8x, the study of individual materials has provided a reasonable
understanding of the characteristics and thermal behavior of these materials, their
relative thermal stability and temperature relationships, their decomposition products and
the evolution profiles of different gaseous products. The formation of molecular chlorine
during the combustion process could be a key step for the formation of chlorinated
organic compounds. The production of phenol and furan during the combustion of
newspaper and cellulose could provide the important precursors for the formation of
polychlorinated dibenzodioxin ŽPCDD. and polychlorinated dibenzofuran ŽPCDF.. These
results can help trace the sources of thermal decomposition products of blends, as well
as evaluate their combustion kinetics and interaction mechanisms. The purpose of this
study is to investigate conditions leading to the possible formation conditions of PCDDs
and PCDFs during co-firing coal with RDF. A possible mechanism that has been
suggested for the formation of PCDDs during co-firing coal with RDF starts with the
Deacon reaction w9x:
4HCl q O 2 | 2Cl 2 q 2H 2 O
Substitution of molecular chlorine on phenols leads to polychlorinated phenols:
Cl 2 q phenol™ chlorinated phenols
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
37
Condensation of chlorinated phenols leads to polychlorinated dibenzodioxins:
chlorinated phenols™ PCDDs
Following the same experimental conditions as those used with the individual
materials, a series of experiments on the blends containing different amounts of coal,
PVC, newspaper, and cellulose was conducted at two heating rates. Studying the
combustion processes at a low TG heating rate allows one to determine the kinetics and
dynamic performances of different gaseous products. The studies at a fast heating rate
allows one to develop a more realistic picture for combustion performance during the
combustion of coals with refuse-derived fuels. The fast heating rate conditions more
closely approximate those in an FBC.
2. Experimental
The detailed TG combustion profiles of the blend materials, including two coals,
polyvinyl chloride ŽPVC., newspaper, and cellulose, were investigated. The coals were a
high-chlorine, low-sulfur Illinois a6 coal Ž90003. and a low-chlorine, high-sulfur
Kentucky a9 coal Ž92073.. The PVC was a homopolymer resin Oxy225 from Occidental Chemical, the cellulose was from Whatman, and the newspaper was from a local
news agency. The analytical values for the materials used are given in Table 1.
The major source of chlorine in RDF materials is PVC. About 7% of RDF is plastic,
and PVC accounts for about 1% of MSW w10x. A greater amount of PVC was used with
the fuel in this study to provide conditions more favorable for the formation of
chlorinated hydrocarbons. The blends of fuels used in this study are given in Table 2.
The TGrFTIR system consisted of a DuPont Instruments Model 951 Thermogravimetric Analyzer interfaced to a Perkin–Elmer 1650 Fourier Transform Infrared Spectrometer. The Model 951 system has a horizontal quartz tube furnace with a ball joint
Žsize 12r5.. The socket joint Žsize 12r5, length 5 cm. was clamped by a joint clamp.
The socket was connected with a permanent heated Teflon Ž1r8 in. OD. transfer line
Table 1
Analytical values of coals and raw materials used in the study
Parameter a
Coal
90003
Coal
92073
PVC
Newspaper
Cellulose
Moisture Ž%.
Volatile matter Ž%.
Ash Ž%.
Carbon Ž%.
Hydrogen Ž%.
Sulfur Ž%.
Chlorine Ž%.
Heating value ŽBTUrlb.
8.76
34.25
8.12
75.07
5.21
1.26
0.38
13,248
9.88
32.56
26.34
59.53
3.50
4.44
0.03
10,334
0.0
100
0.36
38.71
4.20
0.22
56.45
8,556
4.21
81.21
4.12
45.67
6.67
0.05
0.03
7,741
3.12
96.88
0.0
43.44
6.80
0.02
0.00
6,723
a
All parameters except moisture are given on a dry basis.
38
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
Table 2
Composition by weight of the fuel blends used
Blend no.
Component 1
Component 2
1—03PVC
2—03New
3—03Cell
4—03PN
5—35122
6—73PVC
7—73New
8—73Cell
9—73PN
10—735122
50% coal 90003
50% coal 90003
50% coal 90003
50% coal 90003
50% coal 90003
50% coal 92073
50% coal 92073
50% coal 92073
50% coal 92073
50% coal 92073
50% PVC
50% newspaper
50% cellulose
25% PVC
10% PVC
50% PVC
50% newspaper
50% cellulose
25% PVC
10% PVC
Component 3
Component 4
25% newspaper
20% newspaper
20% cellulose
25% newspaper
20% newspaper
20% cellulose
using a custom-made Teflon reducing connector in which the Teflon heated transfer line
was controlled by a Powerstat at 30 V. The ball, socket joints and Teflon connector were
wrapped with a heating coil which was controlled by a variable autotransformer with a
thermocouple. The heated gas cell was covered with a heated jacket which was
controlled by a Barnant temperature controller. Silica tubing formed the connection
between the teflon heated transfer line and the outlet Ž1r4 in. glass tubing. for the gas
cell. This entire portion was then wrapped by a heating coil controlled by a variable
autotransformer. The heated gas cell was 10 cm by 24 mm with a KBr window.
In our TGrFTIR experiments, all samples Ž; 100 mg. were heated in air Ž50
mlrmin.. Heating rates used were a slow heating rate of 108Crmin to 7008C, a fast
heating rate of 1008Crmin to 9008C, and this temperature held for 5 min. Using the
FTIR system, the spectra and profiles of gaseous species evolving from the TG system
were recorded and analyzed within the temperature range from 100–7008C for the
108Crmin heating rate and from room temperature to 9008C for the 1008Crmin heating
rate.
The TGrMS system consisted of a DuPont Instruments Model 951 Thermogravimetric Analyzer interfaced to a Fisons Instruments model VG Thermolab Mass Spectrometer. In the TGrMS system, the mass detection range is 1–300 atomic mass unit Žamu.
and uses an electron impact ionization energy of 70 eV. A capillary transfer line heated
to 1708C was used to connect the TG to the MS, while the inlet port on the mass
spectrometer is heated to 1508C. These temperatures are necessary to minimize the
condensation of high boiling point gases in the transfer line. The capillary inlet Ž1.8 m.
was constructed from fused silicon encased within a stainless-steel sheath. It was then
covered with PTFE and then with a red fiberglass sheathing. Such a construction allows
for good mechanical strength and a means to resistively heat the long capillary line.
Another major advantage of this interface is that it produces a partial pressure in the MS
ion source proportional to that in the atmospheric pressure gas sample at the inlet probe.
A sample size of ; 20 mg was used in all runs. The same heating and air flow rates
used in the TGrFTIR experiments were followed with the TGrMS system. The MS
system can automatically sample the gases evolved from the TG system, analyze them
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
39
with its quadrupole analyzer, and save the mass spectra. The detailed experimental
conditions are discussed elsewhere w11x.
3. Results and discussion
3.1. ThermograÕimetricr differential thermograÕimetric (TG r DTG) results
Fig. 1 is a comparison of TG curves obtained for blend 035122 and its components
Žcoal 90003, PVC, cellulose, and newspaper. at the slow heating rate. It is obvious that
the blend has a much lower decomposition temperature than the coal because of mixing
with the more volatile and more easily decomposed components—newspaper, cellulose,
and PVC. This indicates that there are some mutual chemical interactions among the
components during the combustion process. For the blend curve, the first weight loss is
from loss of the moisture. The second weight loss mainly comes from the decomposition
of PVC, newspaper and cellulose. The third weight loss is from the combustion of coal
and carbon residue from PVC. It is likely that the combustion of newspaper, cellulose
and PVC induces a shift in the coal combustion curve to a much lower temperature. The
temperature of 50% weight loss ŽT1r2 . for coal is about 5008C, whereas the corresponding temperature is 3708C for the blend.
Fig. 2 is a comparison of TG curves obtained for blend 035122 and its components at
the fast heating rate. The same trend is observed in that the blend decomposes at much
Fig. 1. Comparison of TGA curves for blend 035122 and its components at the low heating rate.
40
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
Fig. 2. Comparison of TGA curves for blend H035122 and its components at the faster heating rate.
lower temperatures than the coal. The T1r2 for coal is about 7508C, and that for the
blend is about 4508C. Upon comparison with the blend curve in Fig. 1, one can see that
the combustion profile of the blend becomes more continuous, so that the second and
third weight losses noted clearly in Fig. 1 can not be separated in Fig. 2. This indicates
that the combustion behavior is different at different heating rates. Also, the T1r2 values
for the blend in the two figures suggest that the combustion curves shift to a much
higher temperature Žin the case of the above example, from 3708C to 4508C. as the
heating rate increases. The same results obtained for blend 035122 are observed in blend
735122.
Table 3 is a summary of the TGrDTG results at both heating rates for all the blends.
The maximum rates of major weight loss Ž R max . at the fast heating rate are much higher
than those at the slow heating rate for all the blends. In the case of 035122, for example,
the second weight loss rate increased from 10%rmin for the slow rate to 62%rmin for
the higher heating rate. This indicates that the decomposition reactions of fuels occur at
a faster reaction rate as the heating rate increases. The heating rates in an FBC system
are much faster than those in TG experiments. Thus, one can expect decomposition
reactions to occur at a greater R max in FBC systems.
Upon comparison of the data for the blends with that from the single materials, there
is an important difference. For most blends, the combustion profiles are compressed into
a continuous stage of weight loss at the fast heating rate. This indicates that the different
decomposition stages for most blends can only be separated from each other at a slow
heating rate. It is difficult to distinguish each decomposition stage in the combustion
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
41
Table 3
Summary of TGrDTG results for duel blends
Samplea
03PVC
H03PVC
03NEW
H03NEW
03CELL
H03CELL
03PN
H03PN
035122
H035122
73PVC
H73PVC
73NEW
H73NEW
73CELL
H73CELL
73PN
H73PN
735122
H735122
DW1 Ž%. b
c
Tmax
R dmax
3.2
3.4
3.8
2.9
3.2
1.8
3.0
4.3
70
106
60
112
60
102
61
93
1
2.7
1
2.9
1
1.3
0.5
3.8
2.5
4.3
4.3
4.3
4.3
4.0
2.5
5.7
5.2
113
70
103
70
109
64
100
60
93
1.9
1
4.0
1
3.3
0.6
2.3
1
4.8
DW2 Ž%.
Tmax
R max
DW3 Ž%.
Tmax
R max
30
42
30
93
43
92
27
94
49
51
37
82
51
73
53
82
52
85
40
49
307
354
340
349
341
361
302
345
300
325
306
347
337
374
346
383
336
344
300
331
6
71
19
41
17
67
7
81
10
62
7
57
33
99
23
99
31
98
6
70
64
55
59
457
435
463
6
32
8
48
442
18
64
419
13
44
39
51
444
595
437
7
13
5
24
444
14
30
423
19
29
432
12
42
43
418
538
8
12
a
‘H’ indicates the TGA higher heating rate was used.
D is weight loss.
c
Tma x is the temperature at R max Ž8C..
d
R ma x is maximum rate of weight loss, percent per minute.
b
profiles at the fast heating rate, except for the moisture loss stage. Unlike those at the
slow heating rate, the profiles of decomposition reactions for most blends display only
one continuous weight loss at the faster heating rate. Also, the Tmax values at the fast
heating rate are in between the two Tmax values at the slow heating rate. For example,
the Tmax for H73PN Žfast heating rate. is 3448C, but the Tmax values for 73PN Žslow
heating rate. are 3368C and 4328C.
3.2. FTIR results
All the TGrFTIR results obtained show that there is not any significant difference
between the combustion products for the two coals. These coals essentially differ only in
the chlorine and sulfur contents, which are not rank dependent. Therefore, one needs
only to analyze the data for one coal in detail. Table 4 is a summary of the tentative
identification of FTIR absorption bands for the evolved gases from the coal 92073
blends during combustion at both heating rates.
Fig. 3 is a 3D FTIR plot for blend 10, 735122, obtained at the slow heating rate. The
combustion begins around 2308C Ž z coordinate ; 800 s., speeds up after 2608C, and
reaches a maximum around 3008C Ž z ; 1200.. The combustion process can be illus-
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
42
Table 4
Tentative identification of FTIR peaks for coal 92073 blends Žwavenumber, cmy1 . a
Name
73PVC H73PV 73New H73Ne 73Cell H73Cel 73P2N2 H73PN 735122 H73122
methane
hydrogen chloride
carbon dioxide
carbon monoxide
carbonyl sulfide
carbonyl group
Žmultiplet.
sulfur dioxide
acetic acid
formic acid
methanol
ethylene
1,3-butadiene
p-xylene
furan
chlorobenzene
hydrogen cyanide
3018
2798
2358
2178
2074
1800;
1700
1374
1175
1107
–
950
908
793
–
741
712
a
3016
2798
2358
2178
2074
1800;
1700
1374
–
–
1034
950
–
–
–
741
–
3016
2798
2358
2178
2074
1800;
1700
1374
1175
1107
1034
–
–
–
745
741
712
3016
2798
2358
2178
2074
1800;
1700
1374
1175
1107
1033
950
908
793
745
741
–
3018
2798
2358
2178
2074
1800;
1700
1374
1175
1107
–
950
908
–
745
741
712
3016
2798
2358
2178
2074
1800;
1700
1374
–
1107
1033
950
–
–
745
741
–
3016
2798
2358
2178
2074
1800;
1700
1374
1175
1107
–
950
908
–
745
741
712
3016
2798
2358
2178
2074
1800;
1700
1374
1176
1107
1033
950
908
793
745
741
–
3016
2798
2358
2178
2074
1800;
1700
1374
1176
1107
–
950
906
–
745
741
712
3016
2798
2360
2178
2074
1800;
1700
1374
–
1106
–
950
–
793
745
741
712
Reference: Ref. w12x.
trated by following the emission of CO 2 Ž2230 and 670 cmy1 ., and H 2 O Ž3851 and
1652 cmy1 .. This corresponds to the second weight loss Ž DW2 . in its TG curve Žas
shown in Table 3.. In the decomposition stage, a lot of organic and inorganic species
Fig. 3. 3D FTIR spectra of gases from the combustion of blend 10 at a heating rate of 108Crmin.
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
43
such as HCl Ž2600–3000 cmy1 ., water, organic acids and alcohols are released and can
be identified in the spectra. From the analysis of single materials, it is known that the
evolution of HCl is due primarily to the decomposition of PVC. The organic acids and
alcohols are mainly from the combustion of newspaper and cellulose. The sulfur dioxide
peaks appear around 2808C Ž z ; 1100. and reach their maxima at about 3408C Ž z ;
1400.. This indicates that the coal starts to decompose rapidly at this temperature, and
reaches another combustion maximum at about 4208C Ž z ; 2000., which is the third
weight loss in Table 1. This is supported by the presence of peaks for carbon mono- and
dioxide and methane in the FTIR spectra. Similar results can be found in blend 5
Ž035122. except for the evolution of sulfur dioxide due to low content of sulfur in coal
90003.
Fig. 4 shows a 3D FTIR plot for blend 10 heated at 1008Crmin. The decomposition
reactions of the fuels occur at about 3008C and reach a maximum at 3308C, which
corresponds to the second weight loss in Table 3. As the heating rate increases, the
spectra show a trend similar to that with the raw materials, in which the decomposition
of fuels is compressed into a much narrower time range Žshorter z axis.. Compared with
those at the slow heating rate ŽFig. 3., the spectra from the combustion at the fast
heating rate display two important differences. Compounds such as hydrocarbons, HCl,
CO, COS, SO 2 , organic acids, alcohols and ethylene are produced at the same range of
time Ž Z s 200–400.. After this time, the major gaseous products are carbon dioxide and
water, which are from the complete combustion of carbon residue in the fuels. This is
different from the results at the slow heating rate in which methane and carbon
monoxide are released along with carbon dioxide, rather than with HCl. The second
Fig. 4. 3D FTIR spectra of gases from the combustion of blend 10 at a heating rate of 1008Crmin.
44
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
change is that the decomposition products at the fast heating rate have much higher
concentration Žhigher absorbance., as indicated by absorbances of methane and HCl.
The assignments for the FTIR absorption bands for some gaseous products of the
combustion of blend 10 at the fast heating rate are given in Fig. 5. The spectrum
corresponds to ; 3208C. At this temperature, furan and chlorobenzene can also be
identified at 745 and 741 cmy1 , respectively.
In summary, the concentrated hydrocarbon and chlorine species formed during the
combustion of blends are released in the same time span at the fast heating rate whereas
they evolve at different times for the slow heating rate. This supports the conclusions
from the combustion of the individual materials. This also may be a starting point for
understanding the formation of chlorinated organics. It suggests that there are more
opportunities to form chlorinated organic compounds when the combustion occurs at a
faster heating rate, which can happen in an FBC system.
3.3. Mass spectrometry results
Table 5 summarizes the identification of some peaks in the MS spectra of evolved
gases from the blends of coal 92073 at both heating rates. The compounds in bold type
were detected by using the combination of a gas trap and GCrMS techniques.
Fig. 5. Identification of some gases from the combustion of blend 10 at a heating rate of 1008Crmin.
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
45
Table 5
Tentative parent structures of MS peaks for coal 92073 blendsa
Name
dimethylnaphthalene
vinylnaphthalene
acenaphthene
biphenyl
methylbenzothiophene
phthalic anhydride
dichlorobenzene
indan-1,3-dione
methyltetrahydronaphthalene
methylnaphthalene
chlorobenzaldehyde
chlorobenzaldehyde-H
benzodihydrofuran
naphthalene
trimethylcyclohexane
benzodihydrofuran
propylbenzene
acetophenone
methylbenzaldehyde
trimethylbenzene
chlorobenzene
xylene
benzaldehyde
ethylbenzene
xylene-H
benzaldehyde-H
3-methyl-2-furanone
methylthiophene
furfural
toluene
toluene-H
hexene
thiophene
benzene
furan
sulfur dioxide
a
73PVC H73PV 73New H73Ne 73Cell H73Cell 73P2N2 H73PN 735122 H73122
156
154
154
154
148
148
146
146
146
148
148
146
146
146
148
148
146
146
146
148
148
146
146
146
142
140
142
140
139
134
128
126
120
120
120
120
120
112
106
106
106
105
105
98
98
96
92
91
84
84
78
68
64
142
140
139
134
128
126
120
120
120
120
120
112
106
106
106
105
105
98
98
96
92
91
84
84
78
68
64
142
140
139
134
128
126
120
120
120
120
120
112
106
106
106
105
105
98
98
96
92
91
84
84
78
68
64
134
128
126
120
120
120
120
120
112
106
106
106
105
105
98
98
96
92
91
84
84
78
68
64
139
134
128
126
120
120
120
120
120
112
106
106
106
105
105
98
98
96
92
91
84
84
78
68
64
134
128
120
120
120
120
120
112
106
106
106
105
105
98
98
96
92
91
84
84
78
68
64
112
106
106
106
105
105
98
98
96
92
91
84
84
78
68
64
106
106
106
105
105
98
98
96
92
91
84
84
78
68
64
112
106
106
106
105
105
98
98
96
92
91
84
84
78
68
64
120
120
120
120
120
112
106
106
106
105
105
98
98
96
92
91
84
84
78
68
64
The numbers represent m r z values for peaks in the mass spectra of products.
It is difficult to interpret the complicated spectra obtained from the combustion of all
the blends. However, one can use FTIR data as a second set of clues and concentrate our
attention on the sulfur and chlorine containing species in the evolved gases. Because of
the similarity of the different coals, we need only to discuss the data from coal 92073 as
presented in Section 3.2.
Fig. 6 shows some mass profiles from the combustion at the slow heating rate for
blend 9 Ž73PN.. There are two major decomposition stages in this figure. The first stage
46
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
Fig. 6. Profiles of some masses from blend 9 at the slow heat rate.
occurs around 3008C. In this stage, HCl Ž mrz s 36 and 38. and chlorine Ž70, 72 and 74.
from PVC, as well as furan Ž68. and furfural Ž96. from newspaper are major products.
The second stage is around 3408C. In this stage, larger aromatic molecules such as
toluene Ž92., methylthiophene Ž98., xylene Ž106., chlorobenzene Ž112., ethylbenzene
Ž120. and naphthalene Ž128. are released, which indicates the coal matrix has begun to
decompose. These two stages consist of the second weight loss in its TG curve.
There are more complicated results for blends 5 Žfile 035122. and 10 Žfile 735122..
In Fig. 7 Žblend 10., only moisture Ž mrz s 18. evolves from the fuels around 1008C.
When the temperature approaches 3008C, the fuels start decomposing and releasing as
Fig. 7. Profiles of some masses from blend 10 at the slow heat rate.
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
47
combustion products small organic and inorganic molecules such as HCl, benzene and
toluene Ž mrz s 92. from PVC, acetic acid andror carbonyl sulfide Ž mrz s 60., furan
Ž mrz s 68., phenol Ž mrz s 94. and furfural Ž mrz s 96. from newspaper and cellulose,
while carbonyl sulfide Ž mrz s 60. and sulfur dioxide Ž mrz s 64. are released from
coal. The decomposition reactions reach their maximum rate at 3208C and finish at
about 3708C. This corresponds to the second weight loss in the TG curve. After then,
only a few peaks such as mrz s 18, 36, 44, 64 and 70 appear in the mass spectra. The
absorption bands for carbon dioxide and sulfur dioxide display their maxima at about
4208C, which is consistent with the third weight loss at around 4208C in the TG curve.
As further evidence, water, HCl, carbon dioxide, methane, and sulfur dioxide all appear
as predominant peaks after 3708C Ž z ; 1500. in the 3D FTIR spectra of blend 10. In the
profiles of HCl Žmass 36. and molecular chlorine Žmass 70., three maxima appear
around 3008, 3908, and 4408C, respectively. The first maximum is due mainly to the
decomposition of PVC. The second and third ones should be attributed the decomposition of coal and newspaper. In the mass profile of newspaper, HCl Ž mrz s 36. and
molecular chlorine Ž mrz s 70. are evolved around 3808C.
An important finding in this work is that molecular chlorine, phenol and furan are
released in the same temperature range. This indicates there is the possibility of
producing chlorophenol at this temperature. However, we cannot identify chlorophenol
from the MS results due to two reasons. First, the amount of phenol Ž mrz s 94. present
in the gas phase is very small. This is evident from the profile of mrz s 94. Secondly,
chlorophenol has the same mass Ž128. as naphthalene. These two species cannot be
distinguished from each other in the TGrMS system.
The ambiguity over the mrz s 128 problem was resolved using a separation system
and identification system. In tube furnace studies conducted in our laboratory, larger
samples were loaded to produce more phenol. Also a GCrMS system was used to
separate the components to identify each compound in the gaseous mixture. The results
from reacting 10 g of blend sample showed that there was mainly naphthalene in the
Fig. 8. Profiles of some masses from blend 5 heated at the slow rate.
48
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
Fig. 9. Profiles of some masses from blend 5 at the fast heat rate.
gaseous products, and only a small amount of chlorophenol. The results from reacting
100 mg phenol with a purge gas containing 0.5% chlorine in nitrogen at the heating rates
used above showed that mono-, di-, and trichlorophenols can be formed. This indicates
that during co-firing coal with MSW in the TGrMS system, the amount of phenol
produced is too small to form chlorophenol, or the amount of chlorophenol produced is
too small to be detected by the TGrMS system w13x.
There are similar results illustrated in Fig. 8 Žblend 5, file 035122.. The peaks for
mrz s 60 Žacetic acid and carbonyl sulfide. and 64 Žsulfur dioxide. display the same
Fig. 10. Profles of some masses from blend 10 at the fast heat rate.
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
49
changes observed in the results from coal 92073. That is, sulfur dioxide Žpeak 64. is
produced during three decomposition phases and the peak for mrz s 60 shows two. The
first mrz s 60 is predominantly from acetic acid and the second one from carbonyl
sulfide. This was confirmed by the FTIR results. In the profiles for HCl and molecular
chlorine there are some differences. Both molecules show two peaks around 3008C and
3408C. The first peak may be attributed to the decomposition of PVC. The second peak
is due to the decomposition of coal 90003 Žhigh chlorine, low sulfur. and newspaper.
At the fast heating rate, the combustion of blends 5 Žfile H035122. and 10 Žfile
H735122. becomes much more complicated. The oxidation reactions of fuels at the high
temperature could rapidly produce many segments having different masses. This complicated scheme could allow two or more products having the same mass. Therefore, it
may be expected that the analysis of the MS results is very difficult. For example, in
Figs. 9 and 10, the profiles of sulfur dioxide from blend 5 Žfile H035122. and 10 Žfile
H735122. have only one decomposition stage due to the compression effect at the fast
heating rate. In addition, the expected isotope ratio of peaks 36 ŽHCl., 70 ŽCl 2 ., and 112
Žchlorobenzene. can not be observed due to the complexity of the reactions at high
temperatures and the contribution from other segments with the same masses as the
chlorine species.
4. Conclusions
. The combination of the complementary TGrFTIR and TGrMS techniques
provides a versatile analytical system for the detection and identification of gaseous
combustion products.
. The TGrFTIRrMS system makes it feasible to study the reaction pathways for
formation of gaseous products during combustion.
. The TGrFTIRrMS system was used to identify molecular chlorine, along with
HCl, CO, CO 2 , H 2 O, and other hydrocarbons in the gaseous products of combustion of
PVC resin with other fuels in air. This finding is significant in that it led to further
experiments to examine this combustion step further to look for the formation of
chlorinated organic compounds.
. Chlorine and hydrocarbon species are released in the same temperature range and
in higher concentrations at the fast heating rate. This finding indicates that there are
more opportunities to form chlorinated organic compounds during co-firing coals with
RDF at the faster heating rate that may occur in an FBC system. Under the operating
conditions of a real FBC, it is possible to produce the amount of phenol needed to react
with molecular chlorine to form polychlorophenols, and react further to form PCDDs.
Further experiments will be done at Western Kentucky University to confirm this
mechanism.
Acknowledgements
The authors gratefully acknowledge the financial support from the following agencies: the U.S. Department of Energy through the University Coal Research Program
50
R. Lu et al.r Fuel Processing Technology 59 (1999) 35–50
ŽDE-FFG-22-94 PC 94211., the National Science Foundation through the Research at
Undergraduate Institutions Program ŽCHE-9320182., Huntsman Film for a Thermal
Analysis Fellowship, and the U.S. Department of Energy University Coal Research
Internship Program.
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