Journal of Analytical and Applied Pyrolysis
30 (1994) 91-100
ELSEVIER
JOURNALOI
ANALYTICALati
APPLIED PYROLYSIS
Behavior of sulfur during coal pyrolysis
Dakang
a Center
Shao a, Erik J. Hutchinson a, Jenny Heidbrink
Wei-Ping Pan a,*, Chen-Lin Chou b
a,
for Coal Science, Deptartment
of Chemistry, Western Kentucky University, Bowling Green,
KY 42101, USA
b Illinois State Geological Survey, 615 East Peabody Dr., Champaign, IL 61820, USA
Received 15 September 1993; accepted 8 April 1994
Abstract
The behavior of sulfur in Illinois coals during pyrolysis was evaluated by thermogravimetry/
Fourier transform-infrared
spectroscopy
(TG/FT-IR)
techniques. SO,, COS, and H,S were
major gaseous sulfur-containing
products observed during coal pyrolysis. The release rates of
the gaseous sulfur species showed several peaks within the temperature ranges, which were due
to the emission of different forms of sulfur in coal.
Keywords:
Coal; Sulfur; Pyrolysis
1. Introduction
It is generally accepted that three forms of sulfur are present in coal: organic
sulfur (an integral part of the coal structure);
pyritic (generally discrete pyrite and
marcasite particles or ‘lumps’); sulfate (as salts of metals such as calcium or iron).
A minor amount of elemental sulfur also occurs in coal [l-3].
During the pyrolysis of coals of various types, the mechanisms
of the release of
sulfur species are influenced by numerous factors such as the forms of sulfur present
in the coals, coal rank, and experimental
conditions.
Monitoring
the time- and temperature-dependent
evolution of sulfur-containing
gases during coal pyrolysis provides insight into the sulfur chemistry affecting the
* Corresponding
01652370/94/$07.00
author.
0
1994 - Elsevier
SSDZ 0165-2370(94)00807-D
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92
D. Shao et al. / J. Anal. Appl. Pyrolysis
30 (1994) 91-100
evolution profiles. Programmed-temperature
studies of sulfur gas evolution have
often been limited to H,S [4,5]. In some cases, all the sulfur gases are studied
collectively by reducing or oxidizing them to H,S or SO1 [6,7].
In 1987, Carangelo et al. [8] investigated hydrocarbon structure and the kinetics
of gas evolution by using a TG/FT-IR technique to study the pyrolysis of coal,
char, tar and oil shale. They identified the individual gaseous species such as CO,
CO*, H,O, CH,, C2H6, CzH4, C,H,, C,Hs, benzene, heavy paraffins, heavy olefins,
HCN, HCl, NH,, SOZ, CS,, COS, CH,OH, CH,COOH and CH,COCH,.
In 1988, Oh et al. [9] investigated the evolution of sulfur gases during the
pyrolysis of eight Argonne premium coal samples in an inert atmosphere (argon),
in the temperature range 25-900°C with a slow heating rate (4S”C/min). They
monitored the real-time evolution of sulfur gases during slow-heating pyrolysis
using a triple quadrupole mass spectrometer (TQMS). Their studies demonstrated
that the sulfur gaseous species were SO,, COS, CS,, H,S, and some organic sulfur
gases such as C,H$,
CH,SH, C,H,SH, C3H,SH, CH,C,H&
C,H,SH, and
CH,C$H$H evolved during the coal pyrolysis.
In 1989, Khan [lo] investigated the distribution of coal sulfur in the gaseous,
liquid, and solid products that occur during devolatilization (pyrolysis) in an inert
atmosphere, at a relatively low temperature (about 500°C) of bituminous coal
containing various amounts of total, pyritic, and organic sulfur. 32 coal samples,
mostly of the high-volatile bituminous type, were investigated. His study demonstrated that 31% (sum of H,S and COS) of the coal sulfur appeared in the gaseous
products of the coal pyrolysis, while about 61% of the coal sulfur appeared in char.
Hydrogen sulfide and COS were the only gaseous sulfur species analyzed in the
pyrolysis gaseous products. Khan did not establish and evaluate the release profile
of the gaseous sulfur species (e.g. H,S and COS) during the coal pyrolysis.
In 1990, Solomon et al. [ 1l] analyzed the Argonne premium coal samples using
TG/FT-IR techniques to determine volatile composition, volatile release kinetics,
and relative char reactivity. The results reported that one of the SO, peaks appears
to coincide with one of the COS peaks. The paper does not report on the profile of
H,S. In addition, Whelan [ 121suggested that some of the SO, from kerogens comes
from organic matter.
In 1991, Coburn et al. [ 131 investigated a mechanism for H,S release during the
pyrolysis of Illinois #6 coal in an argon atmosphere. They used a temperature
programmed, gas evolution technique with an argon sweep of 20 ml/min to study
the evolution of H,S detected by a quadrupole MS at different temperatures. At a
10 K/min heating rate, the rate of H,S release from both organic sulfur and pyrite
by the H-donor mechanism reaches a maximum at 700 K and then decreases. The
H,S release by the unimolecular decomposition of coal pyrite to FeS and S2, also
by the H-donor mechanism, then occurs sharply at 830 K (557°C).
The evolution of H,S during the pyrolysis of Argonne premium coal has been
studied with a mass spectrometer in temperature programmed decomposition
(TPD) experiments [ 141. This work indicated that the decomposition reaction of
organic sulfides to form H,S proceeds prior to most of the conversion of hydrocarbons to volatile species during pyrolysis [ 141.
D. Shao et al. 1 J. Anal. Appl. Pyrqlysis
30 (1994) 91-100
93
The purpose of this work was to determine gaseous sulfur species such as H,S,
S02, and COS evolved during the pyrolysis of Illinois coals, and to evaluate their
release behavior using TG/FT-IR experiments.
2. Experimental
Four coal samples ( -60 mesh, air dried) from the Illinois Basin Coal Sample
Program were used in the experiments of coal pyrolysis. The coal samples (called
IBC coals for short) contain 1.13-4.54% sulfur on a dry basis. The compositions of
the coal samples are shown in Table 1.
By using the combined TG/FT-IR instrumental system, the volatile species
produced on a DuPont 951 TG during pyrolysis of a coal sample are analyzed using
a Perkin Elmer 1650 FT-IR spectrometer (resolution, 4 cm-‘; detector, DTGS).
The system is able to continuously measure the mass change of the coal sample with
increasing temperature at a heating rate of lOC/min (by TG) from 20 to 950°C as
well as to identify qualitatively and/or determine quantitatively the individual
gaseous species such as HCl, NH3, COS, SOZ, H$, H20, CH,, C2H4, C3H6, etc. (by
FT-IR) emitted during coal pyrolysis. The sample chamber in the TG furnace is
connected to the 100 x 24 mm gas cell in the FT-IR spectrometer via a 1 mm
diameter Teflon tube. Both the Teflon tube and the gas cell are heated by heating
coils up to 150°C and the temperature is controlled continuously by a temperature
controller. The time lag between the sample chamber and the gas cell is one minute
with an oxygen-free nitrogen flow at a rate of 50 ml/min during pyrolysis. The
corrected temperature is reported for the FT-IR results in this study. The FT-IR
spectrometer could be used to automatically scan the pyrolysis gas mixture, and the
spectra could be taken automatically every minute or every 10°C during heating, by
using Galactic Industries Corporation’s LabCalc software. Calibration of the
FT-IR spectra was made by scanning individual pure gases (SO*, H,S, CHJ or
prepared gas mixtures (SO,, H,S and CH, in nitrogen). Details of the experimental
setup and procedures are reported in other papers [ 15,161. Temperature calibration
Table 1
Compositions of Illinois Basin coal samples
Composition
IBC-103
IBC-105
IBC-106
IBC-109
%
%
%
%
%
%
%
%
%
5.70
8.70
36.10
74.48
5.04
1.73
2.30
7.56
0.18
9.40
18.60
36.80
63.60
4.58
1.22
4.55
7.39
0.10
10.40
9.00
39.70
71.86
4.93
1.67
3.77
8.76
0.02
9.20
8.20
35.00
75.05
4.89
1.74
1.13
8.53
0.42
Moisture
Ash
Vol Matter
Carbon
Hydrogen
Nitrogen
Sulfur
Oxygen
Chlorine
D. Shao et al. 1 J. Anal. Appl. Pyrolysis
94
for the difference between the thermocouple
system was not carried out.
30 (1994) 91-100
and the sample temperature in the TG
3. Results and discussion
3.1. TG results
As shown in Fig. 1, the TG curves obtained by heating approximately 100 mg of
the Illinois coal samples illustrate the decomposition behavior of the samples during
pyrolysis from ambient temperature to 950°C. The second significant weight loss for
the four Illinois coals occurs in the sample temperature range of approximately
350-950°C
with the quantities in the order IBC-109 (27.45%) < IBC-103
(30.35%) < IBC-105 (30.49%) < IBC-106 (34.60%). However, IBC-109 and IBC106 have very close maximum rates of weight loss (1.79 and 1.69%/min, respectively) and similar T,, (433 and 425°C respectively). The two coals, IBC-103 and
IBC-105, have weight loss rates of 2.42 and 2.37%/min at 426 and 411”C, respectively. The weight losses in the 350-950°C temperature range correspond basically
to the loss of the volatile species resulting from the decomposition of the coals, and
they are comparable with the values of the volatile matter content in the coal. The
decomposition of some minerals (calcite, pyrite) also contributes to weight losses in
this temperature range.
105
55 !
-200
0
200
Fig. 1. TG curves
400
600
Tempereture ('Cl
for Illinois
Basin coal samples
800
I
1000
1200
Overlay Vl.OD TA Inst.2000
during
pyrolysis.
D. Shao et al.
/ J. Anal. Appl. Pyrolysis 30 (1994) 91-100
95
Table 2
Sulfur content and forms of sulfur present in Illinois coals as analyzed by the ASTM D2492 method
Coal
IBC-103
IBC-105
IBC-106
IBC-109
3.2. FT-IR
Sulfur
Sulfur
Sulfur
Sulfur
Total
Pyritic
Sulfate
Organic
2.30%
4.55%
3.11%
1.13%
1.11%
2.52%
1.86%
0.50%
0.02%
0.00%
0.01%
0.00%
1.16%
2.03%
1.90%
0.63%
results
Table 2 outlines the results of the total sulfur and forms of sulfur as determined
by ASTM D2492 method. Since there are several forms of sulfur present in Illinois
coals, several gaseous sulfur species can be produced by the decomposition of the
coals under the pyrolysis conditions. In this study, three gaseous sulfur species were
observed: H,S, SOz, and COS.
Hydrogen sulfide, H&j, has the most difficult absorption peak to resolve in the
FT-IR spectra. For instance, its two largest absorbance bands overlap greatly with
other major gaseous species (such as CO,, H,O, and CH,) known to form during the
pyrolysis of coal. However, this can be resolved in another manner. Specifically, H,S
absorbs at 1268 cm-‘, but so does CH,. Therefore, a combined H,S/CH, release
profile can be created. In the CH, standard FT-IR spectrum, the absorbance intensities
of both the 1304 cm- ’ and 1268 cm-’ absorption peaks can be identified. Thus, it is
possible to calculate the ratio of the absorbance intensities of the 1268 cm- ’ peak to
the 1304 cm-’ peak for CH,, which is 0.16. Multiplying the CH4- 1304 cm-’ peak
absorbance intensities by this ratio at varying temperatures will give the net CH41268 cm-’ peak absorbance intensities for a particular temperature range. Finally, by
subtracting the net CH4 gas release profile from the H,S/CH, release profile, a net H,S
release profile can be created. To illustrate this, Fig. 2 shows the plotted H,S/CH,
absorbance from 200 to 750°C for IBC-106 and IBC-109. There are three peak maxima
at 440,510, and 570°C. However, by applying the method noted above, the 5,lO”Cpeak
maximum was found to be mainly due to the evolution of CH4 (Fig. 3).
Thus, the low-temperature decomposition of both the organic sulfur species and
pyrite may generate H$ during the pyrolysis of Illinois coals under our experimental conditions [ 10,131, and results in the lower temperature peak of 440°C on the net
H,S release profiles. The organic sulfur species, which undergoes pyrolysis decomposition at a relatively low temperature, may include aliphatic sulfides, aromatic
disulfides, aromatic-aliphatic disulfides and some aromatic-aliphatic monosulfides
[lo]. As controlled by the following reactions:
FeS, s
H2S + FeS
FeS 2z$FeS+$,-
H-donor H2S + FeS
96
Ll. Shao et a& / J. Anal. Appl. Pyrolysis 30 (1994) 91-100
-
106/H2scH4/1268cm-1+
Fig. 2. pyrolysis combined H,S-CH4
109/H2sCH4/12mcm1
release profiles for IBC-106 and IBC-109 by TG/FT-IR.
6ALmrbance
IXE-3
.
5432l-
Temperature
rcl
-
1O6/ktZS/126Ecmlkal
+
1O9/l-W12E8cml/cal
Fig. 3. Calculated pyrolysis net H,S release profiles for IBC-106 and IBC-109 by TG/FT-IR.
the decomposition of pyrite could also take place to liberate H2S at the relatively
low temperature of 440°C under the pyrolysis conditions [ 131. As a consequence,
the combination of H2S formulated from both the organic and pyritic sulfur species
decomposition contributes to the large 440°C peaks on the net H2S release profiles.
The decomposition of the aromatic monosulfides and some of the aromaticaliphatic sulfur species might occur at a relatively high temperature (around 490°C)
to evolve H$, leading to the small 490°C peaks on the net H2S release profiles. The
small 590°C peak on the net H2S release profiles is mainly due to the emission of
H,S by hydrolysis of a small part of the pyritic sulfur species, which takes place at
a relatively high temperature (around 590°C) and is controlled by the following
reactions [ 131:
3FeS2 + 4H,O + 4H,S + Fe,04 + Sz
3FeS + 4H,O --t 3H,S + Fe,O, + H,
D. Shao et al. / J. Anal. Appl. Pyrolysis
200250300350400450wo
Temperalure
ITP
-
1Ec-106/m/2073cm1
+
30 (1994) 91-100
91
600660700750
IBc-m9/m/m7Xml
Fig. 4. Pyrolysis COS release profiles for IBC-109 and IBC-106 by TG/FT-IR.
Carbonyl sulfide, COS, can be monitored much more easily than H,S. COS
absorbs at 2073 cm- ‘, and Fig. 4 shows the pyrolysis COS gas release profiles
versus temperature. In Fig. 4, three peak maxima are present at 430, 560, and
610°C. The reactions to form COS could be both gas-solid and gas phase
secondary reactions, as has been suggested by Attar and Dupuis [6] and Oh et al.
PI.
1. Formation
of COS from gas phase reactions:
H,S+CO+COS+H2
(1)
H,S + CO, + COS + Hz0
(2)
2. Formation
of COS from gas-solid
FeSz + CO + COS + FeS
reaction:
(3)
3. Elimination of COS:
H,S + COS + CS* + H,O
(4)
2cos
(5)
+ cs, + co,
Our experiments show that the COS release profiles OEraw Illinois coals have two
relatively large peaks around 430 and 560°C and a small broad peak around
610°C. According to Oh et al. [9], the larger 430°C peaks are probably due to the
combined release of COS from two different sources of sulfur species - the organic
sulfur species and the pyritic sulfur. The release of COS from the aromatic sulfur
species is controlled by the gas-phase reaction of CO and/or CO, with HIS
(reactions 1 and 2), and the H,S is produced from the decomposition of different
types of the organic sulfur species, as discussed above. The release of COS from one
type of the pyritic sulfur is also controlled by the reaction of CO with H,S (reaction
l), but the H2S is produced from the decomposition of the pyritic sulfur species.
The large 560°C and small broad 610°C peaks are probably due to the release of
COS by the gas-solid reactions of CO with two different types of the pyritic sulfur
98
D. Shao et al. / J. Anal. Appl. Pyrolysis
Fig. 5. FT-IR
30 (1994) 91-100
106/HS/1268cm!/cal
+ 106/CCS/2703cm1
comparisons
of Pyrolysis
COS and net H,S release profiles
for IBC-106.
species (isometric pyrite and orthorhombic marcasite), and reaction 3 controls the
COS release.
According to Ferm [ 171, reaction 4 occurs at a temperature between 340 and
900°C and reaction 5 is slow but its rate reaches a maximum at 600°C. Thus, these
two reactions compete with the formation of COS between 350 and 900°C and they
probably dominate around 550 and 600°C. This probably leads to the smaller 560
and 610°C peaks on the COS release profiles of Illinois coals.
As discussed above, the formation of COS should be related closely to the
formation of H,S under the pyrolysis conditions (reactions l-3). By comparing the
COS and net H,S release profiles of IBC-106 shown in Fig. 5, the COS profile is
similar to the H,S profile in the temperature range of 300-7OO”C, except for the
absence of the 560°C peak on the H,S profile. This is evidence that reactions l-3
do control the formation of COS and the elimination of H,S during the pyrolysis.
SO2 absorbs at 1374 cm-‘, and Fig. 6 shows the pyrolysis SO2 release profiles of
IBC-109 and IBC-106. The pyrolysis SO, release profiles are similar for both coals,
the difference being in their absorbance intensities; they both have three broad
Tempearture TCI
Fig. 6. Pyrolysis
106/So2/1374cm-I
SO, release profiles
+
109/SoL/1374crnl
for IBC-106
and IBC-109
by TG/FT-IR.
D. Shao et al. 1 J. Anal. Appl. Pyrolysis 30 (1994) 91-100
99
peaks around 320, 440, and 550°C. The small 320°C peak is due to the decomposition of aliphatic and/or elemental sulfur species which takes place readily at low
temperatures [ 181. The large 440 and 550°C overlapping peaks are probably due to
the decomposition of both the aromatic and pyritic sulfur species in the coals. The
second peak is due mostly to pyritic sulfur. This assignment is confirmed by the
reduced peak areas of clean coals [ 191. Most of the pyritic sulfur is removed by the
deep-cleaning process. IBC-103 and IBC-105 have similar pyrolysis SO2 release
profiles.
4. Conclusions
The following conclusions can be drawn from this study.
(1) Hydrogen sulfide, COS and SO, are the major gaseous sulfur-containing
compounds created during the pyrolysis of Illinois coals.
(2) Evolution of COS follows the formation of H,S under the pyrolysis conditions.
(3) *Three peak maxima are present in the evolution of SO,. These peaks were
probably due to the emission of aliphatic, pyritic, and aromatic sulfur.
Acknowledgments
This work was prepared with support in part by grants made possible by the
Illinois Department of Energy and Natural Resources through its Coal Development Board and Illinois Clean Coal Institute, and by the U.S. Department of
Energy (Grant Number DE-FG22-91334). However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors, and do not
necessarily reflect the views of IDENR, ICCI, and the DOE.
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