Effect of Atmosphere on Evolution of Sulfur-Containing Gases during

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Energy & Fuels 2005, 19, 892-897

Effect of Atmosphere on Evolution of Sulfur-Containing Gases during Coal Pyrolysis Qiang Zhou, Haoquan Hu,* Quanren Liu, Shengwei Zhu, and Rui Zhao Institute of Coal Chemical Engineering, Dalian University of Technology, Dalian 116012, People’s Republic of China Received September 7, 2004. Revised Manuscript Received February 1, 2005

The evolution rates of sulfur-containing gases were measured during temperature-programmed pyrolysis of coals in an atmospheric pressure, vertical micro-fixed-bed reactor. The results indicated the following, compared with the products evolved in N2: carbon monoxide (CO) promotes the formation of carbonyl sulfide (COS); carbon dioxide (CO2) inhibits the evolution of sulfur-containing gases at temperatures below 600 °C and exhibits effects similar to those of CO above this temperature; methane (CH4) also inhibits the evolution of sulfur-containing gases at temperatures below 600 °C but promotes the formation of H2S at temperatures above 800 °C; and hydrogen gas (H2) improves the formation of hydrogen sulfide (H2S) and inhibits the formation of other sulfur-containing gases.

Introduction Coal is the main energy resource and form of energy consumed in China. It is estimated that the mined coal reserve in China is ∼114.5 billion tons and presently constitutes ∼70% of the primary energy sources. Meanwhile, it is also the main pollution contributor: ∼90% of SO2, 67% of NOx, 82% of acid rain, and 60% of particulates in the atmosphere originate from the direct combustion of coal in China. Therefore, it is very urgent to make rational and clean use of coal in China. Sulfur is a major factor in regard to inhibiting the extensive utilization of coal. Therefore, the desulfurization of coal is very necessary to improve coal quality and protect the environment. Although many methods have been developed for coal desulfurization, an effective and economic process remains to be found. Pyrolysis is not only an important intermediate stage in coal gasification, combustion and liquefaction, etc, but it is also a simple and effective method for clean conversion of coal and has been received much attention as a processing technology. In pyrolysis, both inorganic sulfur and organic sulfur can be removed, and most sulfur goes into the gas phase in the form of H2S, which is easily recovered as sulfur. Numerous factors, including coal properties and processing conditions, affect sulfur removal during the pyrolysis of coal. Among coal properties, the form of sulfur in the coal has an important role in its transformation during pyrolysis.1,2 In the pyrolysis, pyrite will decompose to the sulfide and sulfur. The nascent sulfur is very active and can capture hydrogen from coal to form H2S that is converted to gas and/or is captured by * Author to whom correspondence should be addressed. Telephone: +86-411-88993966. E-mail address: [email protected]. (1) Van Heek, K. H.; Hodek, W. Structure and Pyrolysis Behaviour of Different Coals and Relevant Model Substances. Fuel 1994, 73, 886896. (2) Cypres, R.; Furfari, S. Fixed-Bed Pyrolysis of Coal under Hydrogen Pressure at Low Heating Rates. Fuel 1981, 60, 768-778.

an organic matrix to form organic sulfur that remains in char or tar3-8 and/or is fixed by mineral matter in coal (mainly calcium, sodium, or iron compounds) to form sulfides that remain in the char.7-14 The behavior of organic sulfur in the coal is mainly dependent on the functional group of sulfur in coal.15-19 The lower the rank of coal, the larger the proportion of nonthiophenic (3) Cleyle, P. J.; Caley, W. F.; Stewart, I.; Whiteway, S. G. Decomposition of Pyrite and Trapping of Sulfur in a Coal Matrix during Pyrolysis of Coal. Fuel 1984, 60, 1579-1582. (4) Chen, H. K.; Li, B. Q.; Zhang, B. J. Decomposition of Pyrite and the Interaction of Pyrite with Coal Organic Matrix in Pyrolysis and Hydropyrolysis. Fuel 2000, 79, 1627-1631. (5) Ibarra, J. V.; Palacios, J. M.; Moliner, R.; Bonet, A. J. Evidence of Reciprocal Organic Matter-Pyrite Interactions Affecting Sulfur Removal during Coal Pyrolysis. Fuel 1994, 73, 1046-1050. (6) Ibarra, J. V.; Bonet, A. J.; Moliner, R. Release of Volatile Sulfur Compounds during Low-Temperature Pyrolysis of Coal. Fuel 1994, 73, 933-939. (7) Patrick, J. W. Sulphur Release from Pyrites in Relation to Coal Pyrolysis. Fuel 1993, 72, 281-285. (8) Liu, Q.; Hu, H.; Zhou, Q.; Zhu, S.; Chen, G. Effect of Mineral on Sulfur Behavior during Pressurized Coal Pyrolysis. Fuel Process. Technol. 2004, 85, 863-871. (9) Zhang, D. K.; Telfer, M. Sulfur Transformation in a South Australian Low-Rank Coal during Pyrolysis. Symp. (Int.) Combust., [Proc.] 1998, 2, 1703-1709. (10) Guan, R. Q.; Li, W.; Li, B. Q. Effects of Ca-Based Additives on Desulfurization during Coal Pyrolysis. Fuel 2003, 82, 1961-1966. (11) Mondragon, F.; Jaramillo, A.; Saldarriaga, F.; Quintero, G.; Fernandez, J. Effects of Morphological Changes and Mineral Matter on H2S Evolution during Coal Pyrolysis. Fuel 1999, 78, 1841-1846. (12) Telfer, M.; Zhang, D. K. The Influence of Water-Soluble and Acid-Soluble Inorganic Matter on Sulphur Transformations during Pyrolysis of Low-Rank Coals. Fuel 2001, 80, 2085-2098. (13) Sciazko, M.; Kubica, K. The Effect of Dolomite Addition on Sulphur, Chlorine and Hydrocarbons Distribution in a Fluid-Bed Mild Gasification of Coal. Fuel Process. Technol. 2002, 77-78, 95-102. (14) Gryglewicz, G. Effectiveness of High-Temperature Pyrolysis in Sulfur Removal from Coal. Fuel Process. Technol. 1996, 46, 217-226. (15) Sugawara, K.; Tozuka, Y.; Kamoshita, T.; Sholes, M. A. Dynamic Behaviour of Sulfur Forms in Rapid Pyrolysis of DensitySeparated Coals. Fuel 1994, 73, 1224-1228. (16) Telfer, M. A.; Heidenreich, C. A.; Zhang, D. K. Effect of Particle Size and Heating Rate on the Transformation of Sulphur during Pyrolysis of a South Australian Low-Rank Coal. Dev. Chem. Eng. Miner. Process. 1999, 7, 409-426. (17) Miura, K.; Mae, K.; Shimada, M.; Minami, H. Analysis of Formation Rates of Sulphur-Containing Gases during the Pyrolysis of Various Coals. Energy Fuels 2001, 15, 629-636.

10.1021/ef049773p CCC: $30.25 © 2005 American Chemical Society Published on Web 03/03/2005

Evolution of Sulfur Gases during Coal Pyrolysis

Energy & Fuels, Vol. 19, No. 3, 2005 893

Table 1. Proximate, Ultimate, and Sulfur Form Analyses of Coals Proximate Analysis (wt %)

Sulfur Forma (wt %, db)

Ultimate Analysis (wt %, daf)

coal sample

Mad

Ad

Vdaf

C

H

N

Ob

YZ DT YM HLH

2.27 2.88 6.54 16.32

10.74 13.69 21.27 24.01

44.82 32.67 42.55 50.81

81.37 76.94 73.56 71.99

5.67 4.08 4.83 5.22

1.29 0.54 1.09 1.33

9.65 18.08 20.01 21.13

a

St

Sp

Ss

Sob

3.63 1.60 2.35 0.44

1.72 1.20 1.86 0.15

0.11 0.09 0.09 0.04

1.80 0.31 0.40 0.25

St, total sulfur; Sp, pyritic sulfur; Ss, sulfatic sulfur; and So, organic sulfur. b Determined by difference.

organic sulfur in coal and the greater the extent of organic sulfur removal. The significant factor among pyrolysis conditions is the pyrolysis temperature; however, other factors (such as heating rate, time, pressure, velocity of carried gas, type of reactor, etc.) also have important roles, in regard to influencing sulfur behavior during coal pyrolysis.20,21 A better understanding of sulfur transformation during coal pyrolysis is essential for finding an effective and economical sulfur removal method. Although gases derived from coal pyrolysis (such as N2, H2, CH4, CO, and CO2) have important roles in the formation of sulfur-containing gases, little effort has been expended to understand their detailed effects. The aim of this study was to investigate the effect of those gas components on the evolution of sulfur-containing gases during coal pyrolysis. Experimental Section Coal Samples. Four Chinese coals (including Yanzhou (YZ), Datong (DT), Yima (YM), and Huolinhe (HLH) coals, which belong to different ranks and contain different sulfur contents) were used in the study. Air-dried samples were ground to -100 mesh before use. The proximate, ultimate, and sulfur-form analyses of the coals are listed in Table 1. Pyrolysis. Pyrolysis was performed in an ambient-pressure, vertical quartz micro-fixed-bed reactor with an inner diameter of 5 mm; ∼50 mg of sample, placed in the reactor, was heated from 25 °C to 1000 °C at a constant heating rate of 5 K/min under different pyrolysis atmospheres (90% N2 mixed with 10% other selected gases, i.e., H2, CO, CO2, and CH4). Air was removed from the system by purging with pyrolysis atmospheres before each run. Gaseous products were all fed into a gas chromatography (GC) system equipped with a 3-m-long glass column that was packed with 25% 1,2,3-tris(2-cyanoethoxy) propane and a flame photometric detector to analyze sulfur-containing gases. To avoid the condensation of tar in the heating zone in the reactor, a high gas flow rate (200 cm3/min) was used. Tar was trapped by quartz wool at the outlet of the reactor. To avoid adsorption of sulfur-containing gases by the system, a glass tube (with an inner diameter of 1 mm) was used as a conduit to connect the reactor with the GC equipment. The total sulfur content in the coal and char was determined by the Coulomb method (Testing Standard of China GB214-77), and the content of sulfate, pyritic sulfur, ferrous sulfide, and organic sulfur (by difference) was determined using the Gladfelter method.22 The evolution rate of sulfur(18) Cai, H. Y.; Guell, A. J.; Dugwell, D. R.; Kandiyoti, R. Heteroatom Distribution in Pyrolysis Products as a Function of Heating Rate and Pressure. Fuel 1993, 72, 321-327. (19) Sugawara, S.; Tozuka, Y.; Sugawara, T.; Nishiyama, Y. Effect of Heating Rate and Temperature on Pyrolysis Desulfurization of a Bituminous Coal. Fuel Process. Technol. 1994, 37, 73-85. (20) Miura, K.; Kazuhiro, M.; Kiyoyasu, S.; Kenji, H. Flash Pyrolysis of Coal Following Thermal Pretreatment at Low Temperature. Energy Fuels 1992, 6, 16-21. (21) Hu, H.; Zhou, Q.; Zhu, S.; Krzack, S.; Meyer, B.; Chen, G. Product Distribution and Sulfur Behavior in Coal Pyrolysis. Fuel Process. Technol. 2004, 85, 849-861. (22) Calkins, W. H., Bonifaz, C. Coal Flash Pyrolysis. 5. Pyrolysis in an Atmosphere of Methane. Fuel 1984, 63, 1716-1719.

containing gas (ERS) was defined by the following formula:

ERS(mg g-1 s-1) )

dWS WC × dt

where dWS is the weight of released sulfur-containing gases calculated in form of sulfur (given in milligrams) during a time interval dt (given in seconds), and WC is the weight of dry coal samples (given in grams).

Results and Discussion Evolution of H2S. Figure 1 shows H2S evolution during coal pyrolysis, as a function of temperature in different atmospheres. It can be seen that the evolution of H2S is affected by the coal properties and processing conditions. For DT and YM coals, only one peak (at ∼525 °C) on the H2S evolution curve with two shoulders can be observed when pyrolyzed in N2. According to the literature,6 the peak is attributed to the decomposition of pyrite. The shoulder at lower temperatures (600 °C) is related to the decomposition of aromatic sulfides. Only one main signal of H2S evolution resulted when the HLH coal was pyrolyzed in N2; however, the appearance temperature of the peak (∼425 °C) is lower than that of the DT and YM coal and may correspond to the decomposition of aliphatic sulfur. From the analytical data of raw coals (see Table 1), the percentage of organic sulfur in the HLH coal is the highest, whereas the rank of the HLH coal is the lowest in all tested coal samples; therefore, the largest proportion of nonthiophenic organic sulfur in the HLH coal and the greatest extent of organic sulfur is removed as H2S. When the YZ coal was pyrolyzed in N2, two peaks on the H2S evolution curvesat ∼425 °C and ∼550 °Cscan be observed. As stated previously, the first peak is related to the decomposition of aliphatic sulfur, and the second peak is attributed to the transformation of pyrite to pyrrhotite. Above 650 °C, because of the fixation of mineral matter,7-14 almost no H2S is released for all tested coals. Compared to H2S evolution in N2, a higher H2S evolution rate and more H2S peaks are observed for coal pyrolysis in H2. For the DT coal, four H2S peakssat ∼300, 525, 650, and 750 °C, which represents the decomposition of aliphatic sulfur, pyrite, pyrrhotite, and organic sulfur (such as thiophenes6), respectivelyscan be observed. The H2S evolution of the YZ coal is similar to that of the DT coal, except the first peak shifts to higher temperature, which indicates the organic sulfur in thermally labile structures is mainly in the form of disulfides in the YZ coal. In addition, the effect of H2 on the pyrite decomposition peak is more pronounced, which may be caused by the good swelling property of

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peratures about 400 °C can be observed. For the HLH coal, the peaks caused by the decomposition of organic sulfur in thermally labile structures and pyrite are observed at 400 °C and 550 °C, respectively, and a pronounced shoulder is present at temperatures above 600 °C, which is caused by the decomposition of pyrrhotite and thiophenes. Compared with the effect of N2 on H2S evolution, CO inhibits H2S evolution for the DT and YM coal and has little effect on the YZ and HLH coals at temperatures below 650 °C. Above 650 °C, CO promotes H2S evolution for all coals. This may be caused by the following reactions during coal pyrolysis:23,24

CO + FeS f COS + Fe COS + H2 f CO + H2S

Figure 1. Effect of atmosphere on the evolution rate of hydrogen sulfide (H2S) during the pyrolysis of coals.

the YZ coal. In N2, coal with a good swelling property may trap more sulfur from pyrite decomposition and transform it to organic sulfur, and thus less sulfur is released as H2S, whereas in the presence of H2, the nascent sulfur from pyrite decomposition can react with H2 quickly and form H2S. H2S derived from the decomposition of pyrrhotite and thiophenes results in a broad peak at ∼725 °C. For the YM coal, two H2S peakssat ∼525 and 650 °Csand a pronounced shoulder at tem-

In CO2, less H2S is released below 500 °C for all coals than that in N2. At temperature regions of 500-600 °C, more H2S evolves for the DT and YZ coals; almost same amount of H2S is formed for the YM coal and less for the HLH coal than that in N2. At high temperature, because CO2 reacts with carbon (coal) to form CO, the evolution of H2S in CO2 is similar to that in CO. In CH4, less H2S evolves than that in N2 at temperatures below 650 °C; above this temperature, more H2S evolves and the rate increases as the temperature increases. This may result from the decomposition of CH4 to form H2,22 which promotes the decomposition of pyrrhotite and/or thiophenes to form H2S. Evolution of Carbonyl Sulfide (COS). It is known that carbonyl sulfide (COS) is derived from pyrite and organic sulfur and/or secondary reactions.6,23 The profiles of COS evolution during pyrolysis of selected coals, as a function of temperature, in different atmospheres are presented in Figure 2. This figure shows that, when coal was pyrolyzed in N2, only a small amount of COS is evolved and mainly occurs in the temperature range of 450-600 °C. Compared with the COS evolution in N2, CH4 and H2 inhibits the formation of COS. H2 can react with oxygen that is contained in coal during coal pyrolysis to form water and, consequently, causes less CO formation. CO promotes the formation of COS, which will be discussed in detail later, whereas both H2 and water can react with COS to form H2S, all of which lead to less COS formation. CH4 may decompose to H2 at relative low temperature, in the presence of coal; it then has an effect similar to that of H2. Compared with the effect of N2 on COS evolution, CO promotes the formation of COS and a COS evolution peak is observed at ∼500 °C (i.e., the pyrite decomposition temperature) for all coals except for the HLH coal. This shows that CO competes with H2 to react with sulfur derived from the decomposition of pyrite. For the HLH coal, which is a brown coal with only 0.44 wt % total sulfur (because more H2 is derived during pyrolysis than other coals, as stated previously, which resulted in less COS formation), no COS peak is observed at the pyrite decomposition temperature. Above 650 °C, the rate of COS evolution increases as the temperature (23) Furimsky, E.; Palamer, A. D.; Cheng, M. Distribution of Volatile Sulfur Containing Products during Fixed Bed Pyrolysis and Gasification. Can. J. Chem. Eng. 1991, 69, 869-875. (24) Trewhella, M. J.; Grint, A. The Role of Sulfur in Coal Hydroliquefaction. Fuel 1987, 66 (10), 1315-1320.

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Figure 3. Effect of atmosphere on the evolution rate of methyl mercaptan (CH3SH) during the pyrolysis of coals.

Figure 2. Effect of atmosphere on the evolution rate of carbonyl sulfide (COS) during the pyrolysis of coals.

increases for the DT, YZ, and YM coal, and a COS peak is present at ∼900 °C. CO2 also promotes the formation of COS; however, at temperatures below 575 °C, the extent is not notable; above this temperature, more COS is formed than that in CO. This result indicates that the formation of COS is related to CO in the system and the effect of CO2 is partly due to the formation of CO by the reaction of CO2 with carbon (coal) at high temperature.

Evolution of Methyl Mercaptan (CH3SH). Methyl mercaptan (CH3SH) is derived from the decomposition of organic sulfur in coal;17 thus, the evolution of CH3SH also is reflected the form of organic sulfur in coal. Figure 3 shows the CH3SH evolution rate profiles of the select coals in different pyrolysis atmospheres. The evolution of CH3SH mainly occurs in temperature range of 200-650 °C, which indicates that the CH3SH is mainly derived from the decomposition of organic sulfurs in the form as thiols, polysulfides, disulfides, dialkyl sulfides and alkyl-aryl sulfides, etc. As coal was pyrolyzed in N2, two peakssat ∼275 °C and ∼475 °Cscan be observed for the DT coal, one peaksat ∼475 °Cscan be observed for the YZ coal, one peaksat ∼500 °Cscan be observed for the YM coal, and no peak can be observed for the HLH coal. From the temperature of the CH3SH evolution peak, it can be concluded that there should be some thiols, polysulfides, and disulfides in the DT coal, some disulfides in the YZ coal, and some dialkyl sulfides in the YM coal, respectively. The reason for no CH3SH evolution during the pyrolysis of the HLH coal may be that more hydrogen formed for the HLH coal than for other coals during

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Figure 4. Effect of atmosphere on the sulfur distribution in DT coal pyrolyis products.

pyrolysis, and H2 can selectively destroy the C-S bond to form hydrocarbon and H2S. Compared to the evolution of CH3SH in N2, less CH3SH evolves in H2, which is caused by the promotion of H2 on the decomposition of organic sulfur. CO also inhibits the formation of CH3SH and the other two atmospheres make the CH3SH peak shift to high temperature. Evolution of Other Sulfur-Containing Gases. The formation of carbon disulfide (CS2) during coal pyrolysis was also observed; however, the effect of atmosphere on the formation of CS2 is somewhat irregular. Some researchers assumed that CS2 was derived from the reaction between pyrite and CH4.25,26 There is no

evidence that CH4 is essential for the formation of CS2 in this study; however, the evolution of CS2 occurs in the temperature region of pyrite decomposition, which indicates that the formation of CS2 may be attributed to the decomposition of pyrite. A small amount of SO2 was observed during the pyrolysis of all coals in five atmospheres. Sulfur Distribution in Coal Pyrolyis Products. By integration of the evolution rate of sulfur-containing gases at different temperatures, the total amount of sulfur that evolved to the gaseous product during coal pyrolysis can be calculated. To estimate the percentage of total sulfur that can be removed from the coal for each of the gases used, the sulfur content of the char (solid

(25) Khan, M. R. Prediction of Sulphur Distribution in Products during Low-Temperature Coal Pyrolysis and Gasification. Fuel 1989, 68, 1439-1449.

(26) Oh, M. S.; Burnham, A. K.; Crawford, R. W. Evolution of Sulfur Gases during Coal Pyrolysis. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 1988, 33, 274-282.

Evolution of Sulfur Gases during Coal Pyrolysis

product) obtained at different temperatures was determined using the Coulomb method. Figure 4 shows the sulfur distribution of coal pyrolysis products, using the DT coal as an example, in which the sulfur content in the tar (liquid product) was calculated by difference. Compared with those results in N2, H2 promotes the sulfur in coal transferring to a gaseous product, whereas the other three gases promote the sulfur transforming to a liquid product. When coal pyrolysis is performed in N2, the amount of sulfur that is evolving to a gaseous product is almost constant (20 wt %) at temperatures above 550 °C. The sulfur that is evolving to a gaseous product increases slightly as the temperature increases beyond 550 °C, from 11.6 wt % to 13.8 wt % in N2 with the addition of 10% CO and from 12.0 wt % to 13.9 wt % in N2 with the addition of 10% CH4. In this case, the further increase in total sulfur removal from coal is mainly due to the sulfur transferring to a liquid product. In N2 with the addition of 10% CO2, the sulfur that is evolving to a gaseous product increases with temperature but is lower than that transferring to a liquid product. In N2 with the addition of 10% H2, the sulfur that is evolving to a gaseous product increases almost linearly with temperature, whereas the amount of sulfur transferrred to a liquid product is almost constant. The difference in the sulfur distribution of the pyrolysis products reflects the difference in desulfurization mechanisms during coal pyrolysis in different atmospheres. Conclusions Compared to the evolution of sulfur-containing gases in N2, carbon monoxide (CO) promotes the formation of

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carbonyl sulfide (COS). Carbon dioxide (CO2) inhibits the evolution of H2S at temperatures below 500 °C and shows an effect on sulfur-containing gases that is similar to that of CO at temperatures above 500 °C. Methane (CH4) inhibits the evolution of hydrogen sulfide (H2S) more intensely than CO2 at temperatures below 650 °C; however, at higher temperature, more H2S is formed. H2 promotes the formation of H2S and inhibits the formation of other sulfur-containing gases. Compared to the sulfur distribution of the coal pyrolysis products in N2, H2 promotes the sulfur in coal transferring to a gaseous product, whereas the other three gases promote the sulfur transforming to a liquid product. Although coal pyrolysis in N2 with the addition of CO, CO2, or CH4 can promote the sulfur removal from coal, the most effective atmosphere for the desulfurization of coal through pyrolysis is H2, especially at high pyrolysis temperatures. The high cost of hydrogen will be the main hindrance for a large-scale use of hydropyrolysis. The alternatives will use hydrogen-rich gases or pretreated hydrogen-containing gas (such as CH4 partial oxidation) for coal pyrolysis. Acknowledgment. This work was performed with support from the Key Program in Major Research Plan for the West of China, the National Natural Science Foundation of China (90410018), and the National Basic Research Program (973 Program) of China (G1999022101). EF049773P