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Energy & Fuels 2002, 16, 48-53
Role of Water in Hydrogenation of Coal without Catalyst Addition Yoshiharu Yoneyama,* Makoto Okamura, Kanako Morinaga, and Noritatsu Tsubaki Department of Material Science System Engineering and Life Science, Faculty of Engineering, Toyama University, 3190 Gofuku, Toyama, 930-8555, Japan Received July 3, 2001. Revised Manuscript Received October 16, 2001
Several coals, including Argonne Premium coal, were noncatalytically hydrogenated with or without water addition at 673 K to investigate the effect of added water on coal conversion. For comparison, similar experiments in nitrogen or undecane (n-C11) were also carried out, as critical temperature of n-C11 is similar to that of water. In nitrogen or hydrogen atmosphere, added water promoted coal conversions. But adding n-C11 did not change or decreased the conversions. Especially added n-C11 inhibited coal conversions in nitrogen for higher-rank coal. The conversion of coals, using nitrogen and water, increased with increasing carbon content of coals. On the other hand, while hydrogen and water were used, there existed no clear relationship between the coal conversion and carbon content of coals. Under pressurized hydrogen, the coals containing larger amount of pyrite gave significantly large conversions. There existed synergistic effect between hydrogen and water on the conversions of coals, and the effect was more obvious for the coals containing larger amount of pyrite. These results suggested that pyrite in coals acted as the catalyst and played an important role in synergistic effect between hydrogen and water on the conversion of coal.
Introduction Low-cost hydrogen sources are required for converting coals to liquid or gaseous fuels and chemicals. To make clean fuels or chemicals, large amount of hydrogen is needed to remove heteroatoms in coal or coal-derived liquid. Water is a promising candidate among hydrogen sources. Many researchers used water as hydrogen source through water-gas shift reaction (WGSR) or as an effective pretreatment method for hydrogenation and pyrolysis of coals.1-5 Ross et al. conducted hydrogenation of Illinois #6 coal using carbon monoxide and water, to utilize hydrogen from water through WGSR, and obtained 100% pyridine soluble product.1 Furthermore, these solvent soluble products were analyzed, and their structural features were found to be similar to those obtained from the hydrogenation using tetralin.4 Synthesis gas was also used for the reaction instead of carbon monoxide.6 Water at supercritical state or steam state has been used as extraction or pretreatment vehicles of coals as well.7-16 Graff et al. reported that steam improved the reactivity of Illinois #6 coal for * Corresponding author. Tel/Fax: (81) 76-445-6847. E-mail:
[email protected]. (1) Ross, D. S.; Blessing, G. G. Fuel 1978, 57, 379. (2) Ross, D. S. Coal Sci. 3, 301. (3) Ross, D. S.; Blessing, J. E.; Nguyen, Q. C.; Hum, G. P. Fuel 1984, 63, 1206. (4) Ross, D. S.; Laine, R. M.; Green, T. K.; Hirschon, A. S.; Hum, G. P. Fuel 1985, 64, 1323. (5) Ross, D. S.; Hum, G. P.; Miin, T.-C., Green, T. K.; Mansani, R. Fuel Process. Technol. 1986, 12, 277. (6) Sondreal, E. A.; Wilson, W. G.; Stenberg, V. I. Fuel 1982, 61, 925. (7) Tse, D. S.; Hirschon, A. S.; Malhotra, R.; McMillen D. S.; Ross, D. S. Prepr.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36, 23.
pyrolysis, and coal was depolymerized by the steam treatment.11-13 After the steam treatment, the yield of pyridine soluble product of Illinois #6 coal increased from 17% to 30%.11 It was considered that some ether linkages in coal were cleaved by the steam treatment because the amount of hydroxyl group was doubled after steam treatment. It was also reported that the steam treatment was only effective within the temperature between 593 and 633 K.12,13 However, Khan et al. and Mapstone independently reported that there did not exist the promoting effect of steam on conversion of all kinds of coal.14,15 Mapstone carried out steam treatment of coals over 573 K and reported that hydrothermal treatment of Illinois #6 coal significantly increased the tetrahydrofuran (THF) soluble material but that of Pittsburgh coal did not increase. Artok et al. also found the promoting effect of water on coal conversion and estimated from 13C solid-state NMR and FT-IR spectra that water treatments promoted the cleavage of ether linkages and the decarboxylation of coals.16 Decarboxy(8) Vasilakos, N. P.; Dobbs, J. M.; Parisi, A. S. Prepr.sAm. Chem. Soc., Div. Fuel Chem. 1983, 28, 212. (9) Desphpande, G. V.; Holder, G. D.; Bishop, A. A.; Gopal, J.; Wender, I. Fuel 1984, 63, 956. (10) Kershaw, J. R. Fuel Process. Technol. 1986, 13, 111. (11) Graff, R. A.; Brandes, S. D. Energy Fuels 1987, 1, 84. (12) Brandes, S. D.; Graff, R. A.; Gorbaty, M. L.; Siskin, M. Energy Fuels 1989, 3, 494. (13) Ivanenko, O.; Graff, R. A.; Balogh-Nair, V.; Brathwaite, C. Energy Fuels 1997, 11, 206. (14) Khan, M. R.; Chen, W.-Y.; Suuberg, E. Energy Fuels 1989, 3, 223. (15) Mapstone, J. O. Energy Fuels 1991, 5, 695. (16) Artok, L.; Schobert, H. H.; Nomura, M.; Erbatur, O.; Kidena, K. Energy Fuels 1998, 12, 1200.
10.1021/ef010147r CCC: $22.00 © 2002 American Chemical Society Published on Web 11/30/2001
Role of Water in Hydrogenation of Coal
lation of coal weakened noncovalent bonds such as hydrogen bonds in coals and, thus, increased the coal conversions.16 Since increasing coal rank means increasing carbon content of coal, in general, amounts of labile oxygencontaining functional groups such as aliphatic ethers and carboxylic groups decrease with the increasing of coal rank. Solum et al. reported from 13C solid-state NMR study of Argonne coals that increasing coal rank decreased the amounts of oxygen-containing functional groups, and only Beulah-Zap, Wyodak, and Blind Canyon coals contained carbonyl carbons.17 As described above, the water treatment of coal promoted the cleavage of ether linkage and decarboxylation of coals.16 Therefore, it is considered that the water treatment would be more effective for lower-rank coals. In the catalytic hydrogenation of coals, it is wellknown that water or steam deactivates hydrotreating catalyst. Several groups reported the negative effect of water on catalytic hydroliquefaction.18-20 It was considered that complete removal of water is necessary for better conversion of coal. Recently, it was found that water addition effectively promoted the hydrogenation of Wyodak coal, using ammonium tetrathiomolybdate (ATTM) as a precursor to form the dispersed MoS2 catalyst in situ.21,22 At first, it seemed that hydrogen of water promoted the coal conversion. However, it was found later, from the results of hydrogenation of model compounds such as 2,2′-dinaphthyl ether and naphthylmethyl bibenzyl, that the addition of water was effective for the generation of highly active MoS2 catalyst from ATTM.23,24 Water itself inhibited the conversion of model compounds in noncatalytic hydrogenation.23 Even if it seems that water exhibited negative effect on coal conversion in hydrogenation with or without catalysts, addition of water increased the CO2 formation and the contents of phenolic compounds in the obtained oils.9,10,16,25,26 It was also reported that phenolic compounds formed in the extraction of supercritical water or thermal treatment of coal with water.18,21,27-29 These results strongly suggest that water interacts with coals, irrespective of the presence or the absence of catalysts, and acts as a chemical reagent. In this study, to clarify the role of water in the coal hydrogenation, Wandoan and Argonne premium coal were noncatalytically hydrogenated at 673 K with water (17) Solum, N. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (18) Kamiya, Y.; Nobusawa, T.; Futamura S. Fuel Process. Technol. 1988, 18, 1. (19) Bockrath, B. C.; Finseth, D. H.; Illig, E. G. Fuel Process. Technol. 1986, 12, 175. (20) Ruether, J. A.; Mima, J. A.; Kornosky, R. M.; Ha, B. C. Energy Fuels 1987, 1, 198. (21) Song, C.; Saini, A. K. Energy Fuels 1995, 9, 188. (22) Song, C.; Saini, A. K.; Yoneyama, Y. Fuels 2000, 79, 249. (23) Yoneyama, Y.; Song, C. Prepr.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 52. (24) Yoneyama, Y.; Song, C. Catal. Today 1999, 50, 19. (25) Adschiri T.; Nagashima, S.; Shibuichi, H.; Shishido, M.; Arai, K. J. Jpn. Inst. Energy 1996, 75, 742. (26) Hu, H.; Guo, S.; Hedden, K. Fuel Process. Technol. 1998, 53, 269. (27) Bianco A. D.; Grirardi E.; Stroppa F. Fuel 1990, 69, 240. (28) Ross, D. S.; Loo, B. H.; Tse, D. S.; Hirschon, A. S. Fuel 1991, 70, 289. (29) Hughes, C. P.; Sridhar, T.; Chuan, L. S.; Redlich, P. J.; Jackson, W. R.; Larkins, F. P. Fuel 1993, 72, 205.
Energy & Fuels, Vol. 16, No. 1, 2002 49 Table 1. Elemental Analysis of Coals (%, daf) coal
C
H
N
O + Sa
Pyriteb
Beulah-Zap Wandoan Wyodak Illinois #6 Blind Canyon Lewiston-Stockton Pittsburgh #8 Upper Freeport
72.9 73.4 75.0 77.7 80.7 82.6 83.2 85.5
4.8 6.2 5.4 5.0 5.8 5.3 5.3 4.7
1.1 1.1 1.1 1.4 1.6 2.6 1.6 1.6
21.2 19.4 18.5 16.0 12.0 9.6 9.8 8.3
0.2 0.1 5.0 0.4 0.3 2.5 3.5
a
By difference
b
Wt %, dry base.
addition. If catalysts exist in the coal hydrogenation, the effect of catalysts would be too strong to observe the effect of water addition on coal hydrogenation. For comparison, experiments under nitrogen and with addition of n-undecane (n-C11) were also carried out. n-C11 is regarded as an essentially nonreducing organic medium with a critical temperature similar to that of water (estimated to be 636 K) and is used as an unreactive hydrocarbon solvent, to differentiate aqueous from purely thermal reactions in the hydrothermal reactions of coal and coal model compounds.28,30 Thus, we used n-C11 as a reference. Experimental Section Basic properties of coals are listed in Table 1. For coals different ranks with structural information,17,31 Argonne coals except for Pocahontas #3 coal were used. The coals were dried at 333 K under vacuum for over 48 h to constant weight before use. Undecane (n-C11) was used without further purification. Hydrogen or nitrogen of 99.999% was used to minimize oxidation of coal during the reaction. Hydrogenation reactions were carried out using coal of 1.5 g with or without adding water of 0.8 g or adding n-C11 of 0.8 g in a 25 mL horizontal microautoclave reactor at 673 K for 60 min under an initial hydrogen pressure of 6.9 MPa. For comparison, similar experiments under nitrogen were also conducted. Agitation was provided by vertical shaking at about 240 stroke/min. After the reaction, the reactor was inserted into a cold water bath. The gaseous product was collected for analysis by gas chromatograph with thermal conductivity detector. In the gaseous product, CO, CO2, methane (C1), ethane (C2) and trace amount of propane were contained, but other higher hydrocarbons such as butane were not detected. Only when water was added, some of solid products agglomerated in the reactor wall. Therefore, all the solid products were recovered completely by using a spatula. The recovered products were placed into a thimble filter and separated to tetrahydrofuran (THF) soluble product by Soxhlet extraction with THF, followed by rotary-evaporation and drying in a vacuum oven at 333 K. The THF insoluble residue was washed with acetone and hexane and subsequently dried at 333 K for over 24 h in a vacuum oven before weighing. The conversion was defined as the sum of the yields of gases and THF soluble products. When n-C11 was used, the weight of n-C11 was subtracted from the weight of THF soluble products. The experimental error range of the conversion and products yields was estimated to be within 3 wt %, from duplicated experiments. (30) Siskin, M.; Katritzky, A. Z. J. Anal. Appl. Pyrolysis 2000, 54, 193. (31) Fletcher, T. H.; Bai, S.: Pugmire, R. J.; Solum, M. S.; Wood, D.; Grant, D. M. Energy Fuels 1993, 7, 734. (32) Ramayya, S.; Brittain, A.; DeAlmeida, C.; Mok, W.; Antal, M. J., Jr. Fuel 1987, 66, 1364. (33) Trewhella, M., Jr.; Grint, A. Fuel 1987, 66, 1315. (34) Baldwin, R. M.; Vinciguerra, S. Fuel 1983, 62, 498. (35) Wasaka, S. J. Jpn. Inst. Energy 1999, 78, 807.
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Figure 1. Results of noncatalytic coal conversions using H2 or N2 with or without adding water or undecane (n-C11). Temperature, 673 K; time, 60 min.
Results and Discussion Effect of Added Water on Coal Conversion. Figure 1 shows the yields of gaseous and THF soluble products, as well as coal conversions of different coals using hydrogen or nitrogen with adding water or n-C11. Under nitrogen, adding water promoted the conversions of coals, but the conversions of Illinois #6, Pittsburgh #8, and Upper Freeport coals did not increase. However, adding n-C11 decreased the conversions of higher-rank coals, such as Upper Freeport and Pittsburgh #8. Retrogressive reactions of coals might occur predominantly in the combination of nitrogen and n-C11. Since it is considered that n-C11 was inert and did not swell coals, the coal fragments generated at 673 K were trapped in the coal matrix and therefore tended to be reincorporated.28 On the other hand, adding water seemed to promote the cleavage of ether linkage in coal structure.16,30 The reaction conditions using nitrogen and adding water were similar to those used by Graff et al., although the reaction temperature here was higher than theirs.11-13 Graff et al. found promotional
pretreatment effect of water steam on pyrolysis and solvent extraction of Illinois #6 coal and estimated that these effects were caused by the cleavage of ether linkages.11 Although Khan’s and Mapstone’s research could not verify this promoting effect of water on conversion of all kinds of coal,14,15 under the conditions we used, added water increased THF soluble product yields for many coals. Under hydrogen, the conversions of coals were larger and adding water was more effective for increasing the conversion. The effect of n-C11 addition was similar to those in the case of nitrogen atmosphere except for Upper Freeport coal by considering experimental error. The conversion of Upper Freeport coal did not change with adding n-C11 in hydrogen, although it decreased under nitrogen. For Beulah-Zap lignite, adding water increased gas formation and decreased THF soluble products. These results indicate that depolymerization of coal occurred significantly in the combination of water and hydrogen. The effectiveness of the combination of water and hydrogen will be discussed later. The behav-
Role of Water in Hydrogenation of Coal
Energy & Fuels, Vol. 16, No. 1, 2002 51
Figure 3. Changes in yields of preasp, asp, and oil from Illinois #6 coal. Temperature, 673 K; time, 60 min; preasp, tetrahydrofuran soluble and toluene insoluble; asp, toluene soluble and hexane insoluble; oil, hexane soluble.
Figure 2. Yield of gases from the reaction with or without adding water or undecane (n-C11). Temperature, 673 K; time, 60 min; C1, methane; C2, ethane.
ior of adding n-C11 was similar under hydrogen or nitrogen. Because the features of coal conversions changes with adding n-C11 were similar under hydrogen or under nitrogen, it seems that the coal conversion using n-C11 depended on coal structure itself. Therefore, as in the case of nitrogen, it seems that since n-C11 did not swell coals, the coal fragments generated at 673 K were trapped in the coal matrix and tended to be reincorporated even under hydrogen.29 In the case of higher-rank coals, a little amount of gases was formed. However, for lower-rank coals, a significant amount of gases was produced, and adding water enhanced the gas yields. It was reported in coal conversion using water that CO2 formation was accelerated.9,10,16,25,26 Figure 2 shows the yields of gases from lower-rank coals. CO2 was predominant. CO, methane (C1), and ethane (C2) were also formed. Addition of water increased the amount of CO2. CO2 might derive from CO and water through WGSR. But only a little amount of CO formed in the reactions without water, and its amount was not enough for CO2 formation through WGSR. The CO yield did not change significantly, when n-C11 was replaced by water, under either hydrogen or nitrogen. These results indicate that CO2 was formed by chemical reaction between coal and water. CO2 might form from decarboxylation of coal, because only lower-rank coals formed much CO2. Artok et al. estimated that CO2 was generated from the decarboxylation of coals.16 It is referred that lower-rank coals contains more carboxyl groups than higher-rank
ones.17 As described above, Solum et al. reported that increasing rank of Argonne coals decreased the amount of oxygen-containing functional groups. And only Beulah-Zap, Wyodak and Blind Canyon coals contained carbonyl carbon, while other coals did not.17 Therefore it seems that, since there existed little amount of carboxylic groups in the higher-rank coals, a little amount of CO2 was formed in the hydrogenation of higher-rank coals. Using CO and water for coal liquefaction was reported.1-5 However, since only very little CO was present in the gases, the WGSR might not contribute to the promoting coal conversion in this study. This assumption was also supported by the fact that the conversion of Illinois #6 coal was the largest, with very small amount of gases formed. The fractions from Illinois #6 coal treated at 673 K for 60 min were shown in Figure 3. The hydrogenated products were separated into gas, oil (hexane soluble), asphaltene (toluene soluble and hexane insoluble), and preasphaltene (THF soluble and toluene insoluble) portions. For comparison, runs with adding n-C11, with adding water under nitrogen, or with n-C11 were also carried out. Under nitrogen, the promoting effect of adding water on oil yield was small, but under hydrogen, the addition of water significantly promoted the oil yield. Adding water increased oil yield from 8.4 to 33.2 wt %, indicating that water played a significant role in depolymerization of coal. These results clearly indicate that synergism existed between hydrogen and water on depolymerization of coal. On the mechanism of water treatment of coal, it is proposed that proton attacked the ipso position of ether linkages connecting aromatic moieties, and then ether linkages broke.16 This mechanism was also supported by model compounds studies.30 It is known that supercritical water acts as acid because ion product of water (pKw) under supercritical conditions is about 11 at 673 K under 69 MPa.32 This means that water becomes a strong acid under supercritical conditions. However, the pKw of steam is about 26 at 673 K under 13.8 MPa.32 This means that water steam is a very weak acid. Under our reaction conditions, water existed as steam, and it seems that proton did not attack coal structure effectively. Conversions of coal were mainly enhanced by the presence of water under hydrogen or under nitrogen. As described below, it seems that pyrite in coal played a significant role in promoting coal conversion under
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Figure 5. Influence of reaction time on coal conversion of Illinois #6 coal. Temperature, 673 K.
Figure 4. Relationship between carbon % of original coals and conversion under nitrogen or hydrogen with adding water or undecane (n-C11). Temperature, 673 K; time, 60 min.
hydrogen. Considering the coal conversion under nitrogen, it seems that steam also disrupted and depolymerized the coal structure by breaking covalent bonds, similar to those estimated by Graff et al.11 and Ivanenko et al.13 Relationship between Carbon Content of Original Coal and the Conversion. In Figure 4, the relationship is shown between the carbon contents of original coals and the conversions of coals using hydrogen or nitrogen, where water or n-C11 was added. In the combination of nitrogen and water, with increased carbon content of original coals except for Upper Freeport coal, the conversions of coals were enhanced, although it was estimated that water addition was effective for lower-rank coal. The conversions using nitrogen and n-C11 were similar to, or smaller than, those using nitrogen only. It seems that since n-C11 did not swell coals, the coal fragments generated at 673 K were trapped in the coal matrix and easy to be reincorporated.29 On the other hand, in the combination of hydrogen and water or hydrogen and n-C11, there existed no clear relationship between the carbon content of original coals and the coal conversions. As mentioned above, the effectiveness of n-C11 was different for different types of coals, but adding water promoted coal conversions. It is found that the conversions of both Illinois #6 and Pittsburgh #8 coals were irregularly large. The conversion of Illinois #6 coal increased about 20% with added water, even though added water in nitrogen did not promote the coal conversion significantly. To clarify the presence of synergism between water and hydrogen, effect of reaction time on conversion of Illinois #6 was investigated. In Figure 5, the conversions of Illinois #6 coal with or without water addition for 30 and 60 min reaction time are exhibited. Under nitrogen, the addition of water did not increase the conversion, even though the reaction time became longer. On the other hand, under hydrogen, increasing reaction time from 30 to 60 min increased the conversion form 54.1 to 65.6% in hydrogen only, and from 62.7 to 82.6% in hydrogen and water. These results clearly indicate that
Figure 6. Relationship between pyrite content in original coal and the conversion. Temperature, 673 K; time, 60 min.
synergism exists between hydrogen and water for coal conversion. Effect of Pyrite Content on Coal Conversion. Since Illinois #6 and Pittsburgh #8 coals contain more pyrite than other coals as compared in Table 1, it is speculated that pyrite played an important role in the promoting effect of water on coal conversions using hydrogen. It is well-known that pyrite in coals acts as catalysts in coal hydrogenation.33 Figure 6 exhibits the relationship between the pyrite contents of original coals and the conversions of coal under hydrogen. The conversions of coals containing more pyrite were larger except for Upper Freeport coal. As Illinois #6 and Pittsburgh #8 coals contain large amount of pyrite, the synergistic effect between hydrogen and water seems to be large. The conversion of Upper Freeport coal was low, even if its content of pyrite was large. As the strength of connecting linkages between aromatic moieties in coal becomes strong with increasing coal rank,17 it seems that the conversion of Upper Freeport coal was lower under the same condition that what observed using Upper Freeport coal, the highest-rank coal in Table 1. It is referred that pyrite plays an important role in the synergistic effect between water and hydrogen on coal conversion. It is reported that a synergism existed between pyrite and hydrogen sulfide for coal conversion over 653-673 K.33,34 It is well-known that pyrite in coals decomposes to pyrrhotite under hydrogenation conditions and the pyrrhotite is an active catalyst for coal hydrogenation.33 The synergism between pyrite and hydrogen sulfide on coal hydrogenation was reported to be caused by pyrrhotite and hydrogen sulfide.33 The synergism appears to be due to the ability of hydrogen sulfide to maintain pyrrhotite in a sulfur-rich (i.e., irondeficient) form.33 In addition, the hydrogenation activity of pyrrhotite has been known to be greater than that of pyrite, and hydrogen sulfide also promotes the hydro-
Role of Water in Hydrogenation of Coal Scheme 1
genation of coal. Since the gases from the hydrogenation of coals in this study had an odor of hydrogen sulfide, the synergistic effect between hydrogen and water in this hydrogenation might be caused by the pyrite and hydrogen sulfide. NEDOL process is developed in Japan to liquefy bituminous coals.35 Scheme 1 shows outline of the NEDOL process. In the process, coals are hydrogenated using synthetic pyrite and vehicle oil. After hydrogenation, heavy oil is separated from the product oil and hydrogenated by an active Ni-Mo catalyst to be reused as vehicle oil for hydrogenation of coal. Synthetic pyrite is used as catalyst in the hydrogenation of coals and much water is produced in the process. From this point, it is suggested from our results that synergistic effect between pyrite and water also promotes coal conversion in the process. Conclusions To investigate the role of water on coal conversion under hydrogenation conditions, several coals were
Energy & Fuels, Vol. 16, No. 1, 2002 53
hydrogenated with or without adding water. For comparison, effect of adding n-C11 was also investigated. Adding n-C11 inhibited coal conversions in nitrogen for higher-rank coal, but adding water promoted coal conversions. The conversion of coals using combination of nitrogen and water increased with increasing carbon content of coals. For the reactions using hydrogen and water, adding water promoted the coal conversions, and synergism between water and hydrogen for conversion of coals was found. In the hydrogenation of Illinois #6 coal, degradation of coal was significantly promoted by adding water, since oil yield increased from 8.4 to 33.2 wt % with adding water. In the combination of hydrogen and water, there existed no clear relationship between the coal conversions and carbon content of coals. Under hydrogenation conditions, the coals containing larger amount of pyrite exhibited irregularly large conversion. The synergistic effect between hydrogen and water on the conversions of coals was larger for the coals containing larger amount of pyrite. These results suggested that pyrite in coals acted as a catalyst and played an important role in the synergistic effect between hydrogen and water on the conversion of coal. Because pyrite decomposed to form pyrrhotite and hydrogen sulfide, these species might be responsible for the synergistic effect between hydrogen and water. Acknowledgment. This work was supported by the Research for the Future Program of the Japan Society for the Promotion of Science (JSPS) through the 148th Committee on Coal Utilization Technology of JSPS. EF010147R
