Analysis of Formation Rates of Sulfur-Containing Gases during the

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Analysis of Formation Rates of Sulfur-Containing Gases during the Pyrolysis of Various Coals Kouichi Miura,* Kazuhiro Mae, Makoto Shimada, and Hiroyuki Minami Department of Chemical Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Received August 15, 2000. Revised Manuscript Received March 8, 2001

To clarify the relation between organic sulfur distribution in coal and the emission of sulfurcontaining gases, six Japanese standard coals, three Argonne premium coals, and one Chinese coal were pyrolyzed using a thermogravimetric analyzer-mass spectrometer (TG-MS) and a Curiepoint pyrolyzer. The changes in sulfur distributions in coal and chars were estimated by a modified controlled-atmosphere-programmed-temperature-oxidation (CAPTO) method. It was clarified that the aliphatic sulfur decomposed below 500 °C and that the aromatic sulfur decomposed at 400700 °C, irrespective of coal type. The decomposition of thiophenic sulfur was strongly affected by coal type. Finally, the H2S formation rate and the decomposition rate of organic sulfur were analyzed using a new distributed activation energy model (DAEM).

Introduction The emission of sulfur compounds in conversion process is one of big problems to be solved for utilizing one of abundant fossil resources, coal. In conventional coal combustion power plants, the flue gas desulfurization method at low temperature has been widely adopted to remove SO2 produced during the combustion. When we utilize a large amount of coal in new efficient conversion processes such as pyrolysis, gasification, and liquefaction, it is a key factor to find effective desulfurization methods which realize high thermal efficiency. To do so, it is necessary to clarify the sulfur distribution in coal and its change during the conversion process. In general, the sulfur contents of coals are within 1 to 5%. The sulfur in coal is grouped into inorganic and organic sulfurs. The inorganic sulfur consists of pyrite (FeS2) and sulfonates, but most of it is said to be pyrite. The organic sulfur, accounting for 30 to 50% of total sulfur, is believed to consist of sulfides, disulfides, thiols, and thiophenes, but it is generally grouped into aliphatic sulfurs, aromatic sulfurs, and thiophenes for convenience. Several methods have been proposed to quantify the sulfur in coal. The ASTM method quantifies the total sulfur, the sulfonates, and FeS2 directly, but quantifies the organic sulfur from the sulfur balance. This method, therefore, cannot be applied to quantify the distribution of organic sulfurs. Attar and Dupuis1 presented a reductive heating method, in which coal was heated to 350 °C in the presence of reducing agent in liquid phase. The formation profiles of H2S, they called them kinetograms, were related to several forms of sulfurs. This method is said to overestimate thiols, and cannot * Corresponding author. Tel. +81-75-753-5578. Fax. +81-75-7535909. E-mail: [email protected]. (1) Attar, A.; Dupes, F. Coal Structure; Advances in Chemistry Series 192; American Chemical Society: Washington, DC, 1992; pp 236-256.

quantify thiophenes directly. Calkins et al.2,3 presented a pyrolysis method. The method is simple, but secondary gas-phase reactions cannot be neglected over 750 °C. This made the quantification of organic sulfurs complicated and less realiable. Lacount et al.4,5 presented a controlled-atmosphere-programmed-temperature-oxidation (CAPTO) method. This method quantifies the sulfur from the SO2 formation profile during the oxidation of coal in an oxygen atmosphere. Two organic sulfur forms were successfully quantified by the method. Nondestructive methods using XPS6 and XANES7 were also developed. George et al.7 determined the sulfur distribution in the Argonne Premium coals by XANES and clarified the validity of the method by comparing their analysis results with the other measurements. However, this method cannot be utilized for routine analyses. Of several methods presented the CAPTO method was simple and reliable, and it is judged to be suitable for practical quantification of the sulfur in coal. On the other hand, many studies have been performed to examine the mechanism of the decomposition of sulfur in coal by analyzing the sulfur-containing gases formed during the pyrolysis under various conditions as summarized in review articles.8,9 However, a few kinetic studies have been done on the formation reaction of the sulfur-containing gas in connection with the change in each sulfur form in coal.10,11 (2) Calkins, W. H. Energy Fuels 1987, 1, 59-64. (3) Calkins, W. H.; Torres-Ordonez, R. J.; Jung, B.; Gorbaty, M. L.; George, G. N.; Keleman, S. R. Energy Fuels 1987, 1, 59-64. (4) Lacount, R. B.; Anderson, R. R.; Friedman, S.; Blaustein, B. D. Fuel 1987, 66, 909-913. (5) Lacount, R. B.; Kern, D. G.; King, W. P.; Lacount, R. B., Jr.; Miltz, D. J., Jr.; Stewart, A. L.; Trulli, T. K.; Walker, D. K.; Wicker, R. K. Fuel 1993, 72, 1203-1208. (6) Keleman, S. R.; George, G. N.; Gorbaty, M. L. Fuel Process. Technol. 1990, 24, 425-429. (7) George, G. N.; Gorbaty, M. L.; Keleman, S. R.; Sansone, M. Energy Fuels 1991, 5, 93-97. (8) Davidson, R. M. Fuel 1994, 73, 988-1005. (9) Attar, A. Fuel 1978, 57, 201-212.

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Table 1. Ultimate Analyses and Total Sulfur Contents of Coals

SS001 (Newlands, NL) SS002 (Ebenezer, Eb) SS003 (Blair Athol, BS) SS004 (Daitong, DT) SS005 (Taiheiyo, TC) SS006 (Tiger Head, TH) Illinois No. 6 (IL) Pittsburgh (PITT) Upper Freeport (UF) Enshuntohson (ES) a ,bCited

ash (wt%, dry)

C

15.4 14.8 8.8 10.0 12.5 12.2 15.5 9.1 13.0 11.5

85.9 81.2 82.9 82.7 78.7 82.3 77.7 83.2 85.5 82.8

ultimate analysis [wt%, daf] H N org Sc 4.9 6.1 4.7 4.7 6.2 5.6 5.0 5.3 4.7 5.6

1.7 1.6 1.8 1.1 1.2 1.7 1.4 1.6 1.6 1.5

0.5 0.6 0.3 0.6 0.1 0.5 2.4 0.9 0.7 3.6

O

total sulfura (wt%, daf)

pyritic Sb (wt%, daf)

7.0 10.5 10.3 10.9 13.8 9.7 13.5 9.0 7.5 6.5

0.65 0.61 0.33 0.83 0.32 0.69 5.71 2.41 2.67 4.26

0.14 0.01 0.03 0.20 0.20 0.03 3.32 1.51 2.00 0.61

from refs 14 and 15. c Organic sulfur ) total sulfur - pyritic sulfur - sulfatic sulfur.

In this study we extended the CAPTO method to identify three forms of organic sulfurs of six Japanese standard coals, three Argonne premium coals, one Chinese coal, and the chars prepared from the coals at different temperatures. Then the change in the amount of each form of organic sulfur was examined in relation to the formation of sulfur-containing gases during the pyrolysis. Furthermore, we analyzed the formation reaction of H2S in relation to the decomposition reaction of aliphatic and aromatic sulfurs using a new distributed activation energy model (DAEM) which was developed by Miura et al.12-14 to analyze the pyrolysis reaction of coal. Experimental Section Samples. Six Japanese standard coals distributed from the Japan Coal Energy Center (JCOAL), three Argonne premium coals, and one Chinese coal were used. The ultimate analyses and the amounts of total sulfur and pyritic sulfur are listed in Table 1.15,16 The amounts of total sulfur are the sum of organic sulfur, pyritic sulfur, and sulfatic sulfur. The Japanese standard coals were kept in airtight bottles purged by N2, and the Argonne premium Coals were kept in amples. To minimize the oxidation of the coal by air, coal preparations, including handling and griding, were performed in a glovebox in which the oxygen concentration was maintained less than 1%. Pyrolysis of Coal. Pyrolysis of the coal was performed by two methods. One is the temperature-programmed pyrolysis in a stream of helium gas: about 3 mg of coal particles less than 74 µm in diameter were heated from 25 to 900 °C at the rate of 20 K/min using a thermogravimetric analyzer (Shimadzu, TG-50H) that was directly connected to a mass spectrometer (Shimadzu, GC-MS2000A). The resolution of the mass spectrometer was 1000. The formation rates of sulfurcontaining gases (H2S, COS, CS2, SO2, CH3SH, C2H5SH) could be measured continuously by using the experimental setup. The other is the flash pyrolysis in an inert atmosphere using a Curie-point pyrolyzer (Japan Analytical Ind., JHP-2S). About 2 mg of coal particles wrapped up tightly by a ferromagnetic foil were placed in a small quartz reactor (4.0 mm i.d.), and they were heated rapidly at the rate of 3000 K/s to the temperature of 280, 485, 590, 764, or 920 °C by an induction heating coil. The tar produced was completely trapped by the (10) Bassilakis, R.; Zhao, Y.; Solomon, P. R.; Serio, M. A. Energy Fuels 1993, 7, 710-720. (11) Yargey, A. L.; Lampe, F. W.; Vestal, M. L.; Day, A. G.; Fergusson, G. J.; Johnston, W. H.; Snyderman, J. S.; Essenhigh, R. H.; Hudson, J. E. Ind. Eng. Chem. Process Des. Dev. 1974, 13, 233240. (12) Miura, K. Energy Fuels 1995, 9, 302-307. (13) Miura, K.; Maki, T. J. Chem. Eng. Jpn. 1998, 31, 228-235. (14) Miura, K.; Maki, T. Energy Fuels 1998, 12, 864-869. (15) Vorres, K. S. In User’s Handbook for the Argonne Premium Coal sample program; Argonne National Laboratory: Argonne, IL, 1993. (16) NEDO Brain-C Report, NEDO-C-9839 (March, 1999).

quartz wool placed just after the foil. Gaseous products were all led to a gas chromatograph which was equipped with a 3.1 m long glass column packed with 25% 1,2,3-tris(2-cyanoethoxy)propane on Shimalite (Shimadzu) and a flame photometric detector (FPD) to analyze sulfur-containing gases (H2S, COS, CS2, SO2, CH3SH, C2H5SH). The yields of char and tar were measured from the weight changes of the foil and the reactor. Controlled-Atmosphere-Programmed-TemperatureOxidation (CAPTO). This method was originally developed by Lacount et al.4,5 in order to determine the distribution of sulfur in coal. In their work about 1 mg samples of coal particles were heated in an oxygen stream diluted by He (10% O2) at the rate of 5 K/min up to 900 °C, then the formation rates of SO2 and COS were measured continuously by a mass spectrometer. The SO2 formation profile was then deconvoluted into three peaks, and they were assigned to nonthiophenic sulfurs, pyritic sulfur, and thiophenic sulfurs, respectively. We applied the same method to the analysis of sulfur distribution in coal and char, but we deconvoluted the SO2 profile into four peaks which were assigned to four types of sulfurs as stated later. In this sense our method was different from Lacount’s method, then we called our method a modified CAPTO method in this paper.

Results and Discussion Quantitative Analysis of Sulfur in Coal by the Modified CAPTO Method. First, we checked if the modified CAPTO method is valid to quantify the sulfur in coal and char. Figure 1 shows the profiles of SO2 formation rates measured by the modified CAPTO method for four coals. In the original CAPTO method4,5 the formation profile of SO2 was deconvoluted into 3 peaks as stated in Experimental Section. However, the profiles of SO2 formation rates obtained here had four distinct peaks at around 320, 400, 450, and 500 °C as typically shown for IL, PT, and UF coals. For ES coal, however, the fifth peak appeared at 472 °C. So, we separated the SO2 formation profile into four or five peaks by a curve-fitting method using the Gaussian function. The fifth peak deconvoluted at 472 °C for ES coal was regarded as a satellite peak of the last peak judging from its peak temperature. The peak area of the fifth peak was much smaller than the peak area of the fourth peak. Consequently, only four main peaks were used for the analysis, and they were, respectively, assigned to the oxidation products of aliphatic sulfurs (Aliph-S) such as alkyl sulfides and thiols, aromatic sulfurs (Aroma-S) such as phenyl disulfides and dibenzylsulfides, an inorganic sulfur (Inorg-S) associated with FeS2, and thiophenic sulfurs (Thio-S) such as benzothiophenes and dibenzothiophenes as shown in

Formation of S-Containing Gases during Coal Pyrolysis

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Figure 2. Comparison of the sulfur distributions in coal estimated by the three analysis methods.

Figure 3. The sulfur distributions in coals estimated by the modified CAPTO method. Figure 1. Formation profiles of SO2 during the CAPTO analysis, and the four or five SO2 formation profiles deconvoluted using the Gaussian function for Argonne premium coals and Enshuntohson (ES) coal. Solid lines: formation rates of SO2; dotted lines: deconvoluted lines of SO2 formation profiles.

Figure 1. Each formation rate was integrated by temperature to calculate the amount of each sulfur type. On the other hand, a small amount of COS was detected for all the coals used in this study. The formation of COS was not taken into account for the analysis, because the amount of COS formed was much smaller than the amount of SO2 formed. Lacount et al. did not take into account the COS formation either.4,5 The CAPTO experiments were performed for IL coal by changing the heating rate (5, 10, and 20 K/min) and the oxygen concentration (2, 5, 10, and 20%), but the operating conditions did not affect the estimated sulfur distributions. The distributions of organic sulfurs estimated by the modified CAPTO method for three Argonne premium coals were compared with the distributions determined by XPS6 and XANES7 methods in Figure 2. The distributions of organic sulfurs estimated by the modified CAPTO method were very close to those determined by other methods. These examinations clarified that the modified CAPTO method is valid to estimate the amount of each form of sulfur in coal. The XPS method, however, could not separate the thiophenic sulfur from the aromatic sulfur. The Amount of Each Form of Sulfur in Japanese Standard Coal. Following the above analysis method,

the sulfur distributions in all the coals were estimated as shown in Figure 3. The amounts of total sulfur and pyritic sulfur determined by the modified CAPTO were almost the same as the data in Table 1. The amounts of total sulfur in IL, for example, were 5.71 wt % by ASTM and 5.6 wt % by the modified CAPTO method. The amounts of pyritic sulfur were 3.32 wt % by ASTM and 3.2 wt % by the modified CAPTO method. Thus, it was also clarified that the modified CAPTO method is valid to estimate the distribution of sulfur in coal from the comparison with ASTM. The distributions were significantly different among the coals. The amount of inorganic sulfur, pyritic sulfur, ranged from several mmol/kg-coal for SS002 to 1.03 mol/kg-coal for IL. For SS001, SS004, IL, and ES coals, the amounts of thiophenic sulfurs were very large and they accounted for 50 to 60% of total sulfur. The ratios of three organic sulfurs were rather close for SS002 and SS006 coals. Since the decomposition rate of different forms of sulfur is expected to be different, the results obtained here suggest that the formation rates of sulfur-containing gases during the coal conversion should be examined in relation to the sulfur distribution in coal to clarify the emission behavior of sulfur from coal. Formation of Sulfur-Containing Gases during Pyrolysis of Coal. Figure 4 shows the formation rates of H2S, COS, and SO2 for eight coals during the temperature-programmed pyrolysis at the heating rate of 20 K/min. Formations of other sulfur-containing gases

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Figure 4. The formation rates of sulfur-containing gases during the pyrolysis of coals.

such as CS2, CH3SH, and C2H5SH were not observed. It should be noted that the scales of ordinate for IL, ES, and SS004 coals are different from other coals. For all the coals, the formation profiles of H2S had a sharp and large peak at 590 °C. Since the magnitude of this peak corresponded to the amount of inorganic sulfur shown in Figure 3, the formation peak is judged to come from the decomposition of pyrite. For IL, ES, SS001, and SS002 coals that contained a large amount of aliphatic sulfur, formation of SO2 was observed below 400 °C, and a large peak of H2S formation appeared at around 500 °C. The formation of H2S above 600 °C was associated with the decomposition of thiophenic sulfurs. These result show that the formation of sulfur-containing gases during the coal pyrolysis was closely related to the sulfur distribution in coal. Effect of Heating Rate on the Formation of Sulfur-Containing Gases during Pyrolysis. Since the yield of pyrolysis product is known to be significantly affected by the heating rate,17 the effect of the heating rate on the formation of sulfur-containing gases was examined. Figure 5 compares the yield of sulfurcontaining gases formed below 764 °C between the temperature-programmed pyrolysis (slow pyrolysis) and the flash pyrolysis. The flash pyrolysis was conducted by use of ferromagnetic foils whose Curie-point temperatures were fixed as stated in Experimental Section. Then the yields were compared at 764 °C, the Curiepoint temperature of the foil used. The total amounts of sulfur formed during the flash pyrolysis were equal or slightly larger than the amounts formed during the slow pyrolysis. The kinds of gases formed were quite different between the two pyrolysis methods. SO2 was (17) Xu, W. C.; Tomita, A. Fuel 1987, 66, 632-636.

Miura et al.

Figure 5. Comparison of the distribution of sulfur-containing gases formed between the slow pyrolysis and the flash pyrolysis.

not formed at all, but a fairly large amount of CH3SH and C2H5SH were formed during the flash pyrolysis for all the coals. Since only the primary reaction occurs when using the Curie-point pyrolyzer, the product distribution reflects the primary decomposition reactions. Therefore, CH3SH and C2H5SH were judged to be the primary decomposition products. The thiols would be easily decomposed to form H2S, and would be oxidized to form SO2 by H2O or CO2 produced during the slow pyrolysis. However, the amount of SO2 produced under the slow pyrolysis was larger than that of thiols produced under the flash pyrolysis for ES, SS001, SS002, SS004, and SS006. This suggests that SO2 also comes from components other than thiols. Pyrite is known to be oxidized even at room temperature. From this fact, it was judged that a part of pyrite was oxidized to form SO2 by H2O and CO2 produced at the low temperature under the slow pyrolysis. Change in the Sulfur Allotment during Pyrolysis. The formation behaviors of sulfur-containing gases were found to be closely related to the sulfur distributions in coal as shown above. Next, the change in sulfur allotment during the pyrolysis was examined to clarify the decomposition mechanism of each type of sulfur, where the sulfur allotment means how the sulfur in coal was distributed into the products: char, gas, and tar. Distribution of the sulfur to the gaseous product could be estimated from the gas formation rates measured. To estimate the distribution of the sulfur into the char, the modified CAPTO analyses were performed for the chars prepared at different temperatures. Distribution of the sulfur to the tar could not be measured because we could not collect all the tar formed, and hence it was estimated from the sulfur balance. Zoller et al. reported



H2S in the gas phase for both pyrolysis methods. The aromatic sulfurs in the coal decreased above 400 °C and diminished at around 750 °C for the slow pyrolysis, and they appeared in tar and as H2S, COS, and SO2 in the gas phase. For the flash pyrolysis, the aromatic sulfurs (Ar-S-R) started to decrease from 386 °C and diminished at 764 °C. The aromatic sulfurs were reported to be decomposed into RSH via formation of RS‚ and Ar‚ radicals.9 The formed RSH was judged to appear either in tar or in gas phase as CH3SH for the flash pyrolysis. No CH3SH was detected for the slow pyrolysis, indicating that CH3SH was further decomposed into H2S, COS, and SO2 via gas-phase secondary reactions. The thiophenic sulfurs in the coal started to decrease over 500 °C, but they did not diminish even at 900 °C for both pyrolysis methods. The thiophenic sulfurs were known to be decomposed into C2H5SH by the scission of C-S and C-C bonds attached to a benzene ring.9 The formation of C2H5SH was actually observed for the flash pyolysis, whereas C2H5SH was further decomposed via gas-phase reactions and appeared in tar and in thegas phase as H2S and SO2 for slow pyrolysis. Pyrite accounting for the most of inorganic sulfur, was judged to be decomposed at around 590 °C for both pyrolysis methods, accompanying the formation of H2S by the following reaction:9

nFeS2 f nFeS + Sn Sn + organic-H f nH2S + organic

Figure 6. Change in the sulfur allotments between the slow pyrolysis and the flash pyrolysis for Illinois No. 6 coal.

that a small amount of S2 was produced at 560 °C from pyrite for IL coal.18 We could not measure the amount of S2, but it was counted as one of organic sulfurs in the char in our method. (a) Effect of the Heating Rate. Figure 6 compares the change in the sulfur allotments between the slow pyrolysis and the flash pyrolysis for Illinois No. 6 coal. The chars prepared at 300, 400, 500, 750, and 900 °C were served to the modified CAPTO analyses for the slow pyrolysis. On the other hand, the chars prepared at 386, 485, 590, 764, and 920 °C, which were the Curiepoint temperatures of the foils used, were served to the analyses for the flash pyrolysis. Since the amount of sulfur in the tar was calculated from the sulfur balance as stated above, the sulfur form in the tar could not be identified. At first sight the changes in the sulfur distributions in the chars are rather close between the slow pyrolysis and the flash pyrolysis. Significant difference appeared only in the sulfur distributions of the gaseous product between the two pyrolysis methods. Sulfurs of all forms in the coal are judged to decrease monotonically with the increase of temperature, which means that the transformation reactions of the sulfurs in solid phase are negligible. The aliphatic sulfurs in the coal completely diminished below 500 °C, and they appeared in tar and as (18) Zoller, D. L.; Johnson, M. V.; Tomic, J.; Wang, X.; Calkins, H. Energy Fuels 1999, 13, 1097-1104.

(1)

In summary, only the distributions of sulfur in the gas phase were different between the slow pyrolysis and the flash pyrolysis. During the flash pyrolysis, a large amount of CH3SH and C2H5SH were formed at above 485 °C, and they reached more than 40% in the total sulfur-containing gas evolved below 920 °C. On the other hand, the gas produced during the slow pyrolysis consisted of H2S, SO2, and COS at above 500 °C. The difference of the sulfur distributions in the product gas was judged to be caused by the difference in the contributions of secondary gas-phase reactions between the two pyrolysis methods. In other words, the secondary gas-phase reactions controlled the sulfur distribution in gas phase. (b) Effect of Coal Type. Next, we compared the sulfur allotments during the slow pyrolysis of four coals in Figure 7a-d. The four coals, ES, SS002, SS004, and SS006, were selected as typical ones for examining the effect of the organic sulfur distributions on the pyrolysis of the sulfur compounds. The formation behavior of H2S was dependent on the distribution of sulfur form in coal. For SS002 and SS006 which contained lots of aromatic sulfur, a large amount of H2S formed at between 400 and 700 °C were judged to come from the decomposition of the aromatic sulfurs. On the other hand, the decomposition behavior of thiophenic sulfur was seen to be dependent on coal type. Below 900 °C the thiophenic sulfurs in ES decreased little; on the contrary, the sulfurs in SS002, SS006, and SS004 decreased by 70 to 80%. Strictly speaking, these results suggest the existence of several forms of thiophenic sulfurs in the coals. However, most of results shown above support the validity of our analysis method which took into account four forms of sulfurs as the sulfurs in coal, and the

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results given in Figure 7a-d show that every form of sulfur is decomposed in its particular temperature region for all the coals. Kinetics of the H2S Formation Rate during the Pyrolysis of Coal. We have developed a new distributed activation energy model (DAEM) to analyze the pyrolysis reaction of coal.12-14 The model assumed that the pyrolysis reaction consists of many irreversible firstorder parallel reactions having different rate parameters. Then the amount of volatiles evolved by time t, V, is represented by eq 2:

(V* - V)/V* )

∫0∞exp[-k0 ∫0texp(-E/RT) dt]f(E)dE (2)

where V* is the ultimate amount of volatiles, f(E) is a normalized distribution function of activation energy, E, and k0 is the frequency factor that is different from reaction to reaction. When the temperature is raised linearly at the rate of a, eq 2 is rewritten as

1 - V/V* )

∫0∞Φ(E,T)f(E) dE

(3)

where

(

Φ(E,T) = exp -

k0 a

∫0Te-E/RT dT

)

(4)

This cannot be integrated analytically, but is well approximated by

(

k0RT2 -E/RT Φ(E,T) = exp e aE

)

(5)

At a selected temperature T, Φ(E,T) is the function of E, but can be approximated by a step function at E ) Es when we assume that only one reaction having the activation energy Es is occurring at the temperature T. Then eq 3 is simplified to eq 6:

V/V* ) 1 -

∫E∞ f(E) dE ) ∫0E f(E) dE s

s

(6)

The activation energy, Es, was chosen to satisfy Φ(Es,T) = 0.58 by our method, and hence the following relationship is obtained by using eq 5:

(

exp -

)

k0RT2 -ES/RT e ) 0.58 aEs

(7)

This equation relates k0, Es, and T at a specified heating rate, a. Exactly the same equation holds for the same k0 and E values but different sets of a and T at a same V/V* value as eq 6 shows. This means that Es and the corresponding k0 values can be obtained by applying eq 7 to the V/V* vs T relationship obtained at different heating rates. Practically E and k0 values at different V/V* levels can be obtained as follows. Since eq 7 is rearranged as

( )

k0R a E1 + 0.6075 ) ln 2 E R T T

(8)

Figure 7. (a,b) Change in the sulfur allotments during the slow pyrolysis of SS002 and ES coals. (c,d) Change in the sulfur allotments during the slow pyrolysis of SS004 and SS006 coals.

the Arrhenius plots of a/T2 at the selected V/V* values for different a values give the relationships between

V/V* and E and between k0 and E. Differentiating the V/V* vs E relationship by E gives f(E) as eq 6 indicates.

ln



Figure 9. The distribution curves of the activation energy for the H2S formation reaction during the pyrolysis of coals.

Figure 8. The f(E) curves and k0 vs E relationship for the pyrolysis of coals estimated by a new DAEM model.

We measured V/V* vs T relationships at three different heating rates for all the coals, then the relationships were analyzed by the new DAEM described briefly above. Figure 8 shows the f(E) curves and k0 vs E relationships estimated for 6 coals. The E values at the peak of f(E) curves ranged from 230 to 260 kJ/ mol depending on coal types. The f(E) curves were sharp for IL, SS002, and SS006 coals; on the contrary, they were rather broad for SS003, SS004, and SS005 coals. The upper figure shows that the k0 vs E relationships are little dependent on coal types and that linear relationship hold between ln k0 and E. This means that the kinds of pyrolysis reactions are almost independent of coal types. The linear ln k0 and E relationship is wellknown as a compensation effect. As described above, the model assumed that only one reaction occurs at a specified temperature, and consequently E is uniquely related to T at a specified heating rate. Once we know the E vs T relationship at a heating rate, the profiles of gas formation rates measured at the same heating rate are instantly converted to the distributions of the activation energies for the formation reactions of the gases. Practically E vs T relationship at any heating rate can be obtained by use of eq 7 when the k0 vs E relationship is given. Then we constructed the E vs T relationships at a ) 20 K/min for all the coals by use of the k0 vs E relationships shown in Figure 8. By utilizing the E vs T relationships, the relationships between the formation rate of H2S and T were converted to the distribution curves of H2S formation reactions for several coals in Figure 9. It was found that the distributions are rather broad and are quite different among the coals, but the sharp peaks coming from the decomposition reaction of pyrite appeared at around E ) 430 kJ/mol for all the coals. This means that H2S is produced by many reactions from the organic sulfurs and that their relative magnitude is highly dependent on coal types. Thus, it was found that the distributions of activation energies for the H2S formation reactions from the organic sulfurs were rather broad.

Figure 10. The distribution curves of the activation energy for the decomposition reactions of alipahtic and aromatic sulfurs for five coals.

To examine the relation between the H2S formation reaction and the form of organic sulfurs in coal, the DAEM analysis was applied to the decomposition rate of each sulfur form. This can be performed by the following procedure. The relationships between the fraction of each sulfur decomposed and temperature can be constructed from Figures 6a and 7a-d. The relationships can be converted to the distributions of activation energies for the decomposition reaction of each sulfur by the procedure described above. Figure 10 shows the distribution curves of E determined for four coals. The distributions of E for the decomposition reaction of aliphatic sulfurs ranged from 150 to 300 kJ/mol. The E values at the peaks of distributions were between 180 and 220 kJ/mol. The distributions of E for the decomposition reaction of the aromatic sulfurs ranged from 150 to 400 kJ/mol, and the E values at the peaks of distributions were ca. 230 kJ/mol for all the coals. The peak E values almost coincided with those for the H2S formation reactions (Figure 9). This well supports the experimental fact that H2S was produced mainly from the aromatic sulfurs as shown in Figures 6a to 7d. The value of 230 kJ/mol was close to the bond dissociation energy of C-S bond in phenyl sulfide. Thus, the H2S formation reaction and the decomposition reaction of

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each sulfur form were successfully analyzed by using the new DAEM. Conclusion Six Japanese standard coals, three Argonne premium coals, and one Chinese coal were pyrolyzed under a slow heating and a flash heating modes, and the formation rates of sulfur-containing gases were measured. The amounts of pyrite and three forms of organic sulfurs in the raw coals and the chars were determined using the modified CAPTO method. Combining these data, the relationship between the decomposition behavior of each form of sulfur and the formation of sulfur-containing gases was examined. The aliphatic sulfur was decomposed below 500 °C and the aromatic sulfur was decomposed at 400-700 °C accompanying the formation of H2S. The decomposition of thiophenic sulfur was strongly dependent on coal type. Furthermore, the H2S formation reaction and the decomposition reactions of

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aliphatic and aromatic sulfurs were analyzed by a new DAEM method to determine the distributions of activation energies of the reactions. The E values at the peaks of distributions were between 180 and 220 kJ/mol for the decomposition reaction of aliphatic sulfur and 230 kJ/mol for the decomposition reaction of aromatic sulfur. The peak E value for the aromatic sulfur decomposition reaction was close to that for the H2S formation reaction. It was also close to the bond dissociation energy of the C-S bond in phenyl sulfides. Thus, the kinetic parameters of sulfur decomposition reactions during the pyrolysis of coals were successfully estimated. Acknowledgment. This work was financially supported by New Energy and Industrial Technology Development Organization (NEDO) through the Basic Research (BRAIN-C). EF000185V

Analysis of Formation Rates of Sulfur-Containing Gases during the

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