phosphoric acid mixtures

FUEL PROCESSING TECHNOLOGY ELSEVIER

Fuel Processing Technology 41 (1995) 135-146

Desulfurization of bituminous coals" fluidized bed gasification of coal/phosphoric acid mixtures J.M. Stencel*, J. Y a n g , J.K. N e a t h e r y Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, K Y 40511, USA Received 16 November 1993; accepted in revised form 26 April 1994

Abstract

The processing of coal:phosphoric acid mixtures using mild temperature (932°F fluidized bed conditions is presented. Phosphoric acid promoted the desulfurization of the bituminous coals, producing chars that contained less than 20% of sulfur originally in the coals. Gaseous H2S was the primary sulfur-containing gaseous product. Pyritic sulfur was eliminated from the coal at a temperature as low as 410°F, whereas about 70% of the organic sulfur was removed from the coal at 932°F. In comparison with literature data, organic sulfur remaining in the char is suggested to contain thiophenic speciation. Phosphoric acid also decreased greatly the swelling character of the coals, and altered dramatically the yields of gas, condensate and char relative to that obtained during the processing of coal without acid.

I. Introduction Product yields and characteristics from the mild temperature gasification or pyrolysis of coal have been evaluated extensively in fundamental and process development efforts. K h a n [1, 2] discussed some of these data and showed that the sulfur distribution in products resulting from the pyrolysis at 932°F of US coals containing 0.5-6.0% sulfur could be represented by Total sulfur in gas = 0.31 x Sco~ Total sulfur in tar -- 0.06 x Scoal Total sulfur in solid = 0.61 x S~o~ This type of distribution is indicative of that obtained during thermal processing of bituminous coals, although factors such as coal type, sulfur content and form, particle

* Corresponding author. 0378-3820/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 3 7 8 - 3 8 2 0 ( 9 4 ) 0 0 0 8 1 - 4

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size, heating rates, temperature and process design also affect product distribution and characteristics, and the amount of sulfur reporting to the various product streams. For example, increasing the pyrolysis residence time usually decreases the sulfur content in the char and increases the tar and gas yields and their sulfur content. The devolatilization and pyrolysis of Illinois, Montana, and Ohio coals under slow and rapid heat-up conditions have shown that product yields varied between 6-13% gas, 3-25% tar and 91-52% char [3]. The pyrolysis of coal for the production of low-sulfur fuel when using fixed-bed and fluidized-bed reactors has also been described [4]. These tests, performed at temperatures in the range of 930-1200°F on Illinois No. 6 bituminous and Wyoming subbituminous coals, showed that there was a nonselective distribution of sulfur to the gas, condensate and char products and that a maximum desulfurization expected would be 45% for the subbituminous coal and 68% for the bituminous coal. At temperatures near 1500°F, and using slightly oxidizing atmospheres, upwards of 80% sulfur removal was obtained for Illinois basin coal [5]. Using a statistical analysis, the amount of sulfur remaining in the char was determined to be dependent on the amount of sulfur in the coal, with the amount varying between 27-97% of the sulfur in the feed coal. At higher temperatures and pressures, and for subbituminous coal, the sulfur distribution in the products has been reported to be somewhat different than presented above [6, 7]. One of the primary coal dependent factors affecting the extent of desulfurization under mild temperature conditions is the chemical form of the sulfur. Recent analytical efforts have shown that high-sulfur, US bituminous coals may contain between 30 and 70% of their organic sulfur as a refractory thiophenic species [8, 9]. Thiophenic sulfur is not decomposed at 932°F whereas other more labile sulfur species decompose at significantly lower temperatures [1, 10, 11]. Pyrite will decompose at temperatures near 842°F and, in the process, transform into pyrrhotite specie which are stable beyond 1250°F. In the current study, mild temperature pyrolysis of Illinois basin coals mixed with phosphoric acid under fluidized bed conditions is reported. The extent to which sulfur was removed from bituminous coals using a pyrolysis temperature of 932°F is reported. Product yields are compared for fixed bed and fluidized bed processing of coal:phosphoric acid mixtures and coal-only samples. Swelling characteristics of the coals are also discussed relative to operation of the fluidized bed and related to effects of acids on thermoplasticity.

2. Experimental The two coal samples were obtained from the Illinois Basin Coal Sample Program. They were both from the Springfield seam and designated as Indiana V, but were sampled at different locations in the seam and at different times. Their composition is presented in Table 1; they are labelled as Samples A and B. Approximately one-half of each of the samples was subjected to physical cleaning by column flotation subsequent to wet grinding to - 200 mesh. This cleaning was

J.M. Stencel et al. ~Fuel Processing Technology 41 (1995) 135-146

137

Table 1 Composition of parent and cleaned coals Parent A

Coals B

Cleaned A

Coals B

6.16 7.14 38.37 48.30 12 400

5.48 8.65 40.09 45.80

0.37 4.63 41.69 53.30 13 421

2.13 5.90 37.74 54.20 13 150

69.29 5.49 1.47 3.45

67.87 5.46 1.42 3.95

75.71 5.14 1.61 3.01

73.85 5.22 1.53 3.47

1.50 1.95 0.01

1.40 2.51 0.04

0.60 2.21 0.20

0.80 2.48 0.19

Proximate Moisture (%) Ash (%) Volatile mat. (%) Fixed carbon (%) Heating value (Btu/lb)

Ultimate % C H N S

Sulfi~r forms (%) Pyritic Organic Sulfatic

performed under conditions which enabled a combustible recovery of about 75%. The other one-half of the samples was used in as-received or parent form after grinding and sieving. The coals were admixed with 50% strength, reagent grade phosphoric acid to attain coal : acid weight ratios of 1.0 : 0.65 and 1.0: 0.96 or with water to produce a coal-only sample having a coal:acid ratio of 1.0:0. These mixtures were dried at 200°C in a nitrogen purged furnace then vacuum dried at the same temperature. The dried samples were stored in sealed containers subsequent to purging with argon until they were treated in a bench-scale reactor. A schematic of the fluidized bed reactor system is presented in Fig. I. The reactor had a three inch inner diameter and was 39 in. in height. Dried coals were loaded into a pressurized hopper located on a precision screw feeder. Coal was dropped from the screw outlet of the feeder into a eductor line which led to the bottom of the gasifier. Gasification tests were as long as four hours in duration with average coal feeds of 0.24-0.64 lb/h. The fluidizing gas velocity varied between 0.5-1.0 f/s. Nitrogen, nitrogen/air or nitrogen/steam were used as the fluidizing gas during pyrolysis. These gases were pre-heated in a furnace upstream from the fluid bed. Heat traced lines were used to maintain constant temperatures throughout the reactor and its associated output streams. Chars were collected in the down-leg of a cyclone and in an underbed collection flask. Underbed char was removed continuously from the bed of the reactor through a standpipe which entered the reactor from its bottom. Typically, about 70% of the char reported to the cyclone, 15% reported to the underbed zone, and 15% of the total amount of char remained in the bed at the end of a pyrolysis experiment.

138

J.M. Stencel et al./Fuel Processing Technology 41 (1995) 135-146

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J.M. Stencel et al. / Fuel Processing Technology" 41 (1995) 135-146

139

A three-stage condensate recovery section was used to trap condensate. The first stage was maintained at 284°F, the second stage was maintained at 59°F, and the third stage at ice bath temperature. Subsequent to pyrolysis and before analysis, the chars were hot water filtered to remove excess phosphorus. For these experiments, a stainless steel, 1.41 pressure filter (Model KST, Lars Lande Manufacturing, Inc.) was insulated and connected to an in-line water heater, a flow meter and a pump. At the outlet of the filter was placed an in-line pH electrode which was connected to a digital pH transmitter. A record of the pH of the effluent water from the filter was obtained by connecting a strip chart recorder to the pH transmitter. A continuous flow of distilled and deionized water at 190-210°F was pumped through the filter in which char was placed. Over 95% of the phosphorus which was removed from the chars occurred during the initial 20 min of washing. For the samples discussed herein, a minimum of 60 min washing was used at a water/coal flow rate of 0.18 lb-s-1 H20/lb coal. All tar and char products were subjected to proximate, ultimate, forms of sulfur and heating value analyses. The phosphorus content of the chars was also measured.

3. Results

Table 1 presents data on the composition of the two coals before and after physical cleaning. The parent coals were similar in composition, except that Sample B had a higher sulfur content, primarily reflected in an increased level of organic sulfur. Physical cleaning decreased by 30% the ash content and the pyritic sulfur content by approximately 50%, whereas the sulfatic sulfur contents in both of the cleaned coals increased as a consequence of cleaning. This increase was probably a result of oxidation during drying of the samples. Char yield data from the fluidized bed testing are displayed in Fig. 2(a) and tar yield data in Fig. 2(b). These yields were calculated from (weight f e e d - weight product)/weight feed and are corrected for phosphorus, ash and moisture (maf basis). The duration of each of the coal-only tests was slightly greater than one hour. For the other tests in which coal:phosphoric acid mixtures were treated, the durations were as great as four hours. This difference in time was solely a consequence of experiencing significant problems using coal-only feeds that were not experienced using coal:acid mixtures. In general, such operational problems were either associated with the agglomeration of coal within the bubbling bed-which caused defluidization and bed blockage, or the accumulation of tar products at the air-side outlet of the cyclone-which caused pressure increases within the gasification zone and, eventually, bed blockage. Significantly, no such operational problems were encountered during gasification of the coal : acid mixtures. The char yields were greater for the 1.0:0.65 coal:acid mixtures than for the 1.0: 0.96 mixtures. In addition, the char yields were greater for the cleaned coals than for the parent coals using equal phosphoric acid concentrations. This behavior may be related to the ability of phosphoric acid to initiate cross linking reactions [12] in association with its interaction with mineral matter and catalytic activity [13, 14].

140

J.M. Stencel et al./FuelProcessing Technology 41 (1995) 135-146 100-

80-

60-

o

40-

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20-

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1.0:0.0

parent parent 1.0:0.96 1.0:0.65

cleaned 1.0:0.96

cleaned 1.0:0.65

Coal: Acid Ratio

>-

0 Fig. 2. Product yields from fluidized bed pyrolysis at 932°F under nitrogen fluidization: (A) Char yield (mar), (B) tar yield (maf).

Opposite to the char yields, the tar yields were decreased significantly by the presence of phosphoric acid. A ten fold decrease in the tar yield occurred when using the 1.0:0.96 coal:acid ratios in comparison to the coal-only samples. Table 2 presents compositional data for chars produced from the two coals. As expected, the composition of the chars from the two coal samples were very similar. For comparison purposes, char composition data from treating a coal-only sample are also presented in Table 2.

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Table 2 Composition of chars and physically cleaned coals after pyrolysis at 500"C 1.0 : 0.96 coal : acid

Proximate Moisture (%) Ash (%) Volatile mat. (%) Fixed carbon (%) Heating value (Btu/lb)

Coal only

Parent A

Parent B

Cleaned A

Cleaned B

Parent B

1.88 23.61 22.45 52.01 10670

4.88 21.25 21.54 52.22 10600

2.13 15.31 23.32 59.22 11 755

5.28 14.49 18.22 62.02 11 486

3.84 10.54 20.93 64.70 12 ! 30

66.77 1.86 1.61 0.81

66.78 2.52 1.36 0.95

74.11 1.91 1.58 0.71

74.02 1.76 1.51 0.84

74.28 3.10

O.13 0.66 0.02

O.18 0.70 0.04

0.03 0.65 0.03

0.03 0.79 0.02

Ultimate % C H N S

Sulfur forms (%) Pyritic Organic Sulfatic

1.70 2.64

The amount of sulfur remaining in the chars produced from the coal : acid mixtures was weakly dependent on whether the coals were cleaned before pyrolysis. For example, the char pyritic sulfur contents from cleaned coals were less than the char pyritic sulfur contents from parent coals; nevertheless, the concentration of pyritic sulfur is small and less than 0.18%. Greater than 80% of the sulfur in chars from parent coals was organic in nature, whereas greater than 90% of the sulfur in chars from cleaned coals was organic in nature. This nearly equivalent level of organic sulfur suggests there is a common sulfur functionality or chemical form which is not eliminated during the pyrolysis treatment. The organic sulfur remaining in the chars was approximately 30% of the total organic sulfur which was in the parent coals. Approximately 20% of the sulfur originally in the coals remained in the chars after the 932°F fluidized bed testing of coal : acid mixtures, whereas nearly 70% of the sulfur remained in the char produced from the coal-only samples. The sulfur in the tars accounted for less than 1%0 of the total amount of sulfur originally in the coals, whereas over 70% of the coal sulfur reported to the gas phase as H2S. These calculations were based on a comparison of the amount of sulfur in the products with the amount in the feed coals. In general, sulfur mass balances were equal or better than 90%. Fig. 3 presents heating value data relative to the phosphorus content in the chars. At phosphorus concentrations greater than about 6% the heating values of the chars dropped precipitously to values below 11 500 Btu/lb. This drop is, in part, related to the overall phosphorus and ash content of the chars. For example, the data in Fig. 4 show that the ash content of the chars increased at a rate greater than the phosphorus

142

J.M. Stencel et al./Fuel Processing Technology 41 (1995) 135-146

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1 0 . 5 ..............................................................................................................................

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Coal A 91 , -°1................................................................................. J--/ ........................................................................................... 71 /.__../_..._ ....................................................................................... ............................................................................

=iIIIIIIZIIIIIIIZZIZIIIIiI.IIII-I~.--.-----.--XLLIII.Y]IIIIIIZ][IIII

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16

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Ash Content

Fig. 4. Plot of phosphorus content for chars from sample A of Indiana V relative to their ash content.

content (the lines drawn in Fig. 4 indicate equal increases in ash and phosphorus). The same type of plot can be presented for both coals. It was noted that a dark, glassy material was formed in the sample boats used for proximate analyses if the char phosphorus contents were greater than approximately 5%; these analyses provide the weight of 'ash' in a sample subsequent to oxygen exposure at 750°C. Since phosphorus contents were quantitatively standardized and

J.M. Stencel et al. ~Fuel Processing Technology 41 (1995) 135-146

143

determined independently of proximate analyses, it is possible that carbon in the char was not completely evolved during proximate determinations, thereby causing ash contents to be higher than expected for known amounts of phosphorus in the samples.

4. Discussion

Char yields obtained in the current study using coal-only feeds, rapid heat-up and fluidized conditions were similar to those obtained in experiments using Wyoming, Illinois and West Virginia coals [15]. For a range of temperatures between 950-1400°F I16], the char yield decreased and the tar yield increased with increasing temperature. For the gasification of Indiana III coal [,16], the yields of char were sensitive to temperature and the gasification atmosphere but were close to the value for the coal-only feeds displayed in Fig. 2. In addition, char yield data reported previously for high volatile bituminous coal using fixed bed reactors [3] were similar to that in Table 1. Hence, heat-up rate was not a primary influence on char yields. In general, addition of phosphoric acid enhanced the char yield from the two coals. This behavior has been previously discussed relative to cross-linking reactions catalyzed by phosphoric acid [17, 18]. It is well known that the phosphoric acidcontaining catalysts are highly active for the polymerization of olefins and for the alkylation of aromatic compounds [19, 20]. Such reactions can be promoted by solid or liquid phosphoric acid forms, and the state of hydration of the acid is important [21]. As dehydration occurs orthophosphoric (HaPO4) transforms into the less reactive pyrophosphoric (H2PEOT) form, which transforms and polymerizes into metaphosphates and larger polymeric forms. The initiating reaction between coal and the phosphoric acid may be associated with oxygen heteroatom sites within the coal structure [18]. However, the rapid rate of sulfur removal and the nearly equal oxygen and nitrogen concentrations in coals and chars when using different coal : acid ratio feeds suggest that the interaction of the acid with coal oxygen functionality is not the predominant initial reaction. Rather, proton transfer, akylation and polymerization may control the overall acid-coal interactions. In addition, the acid will interact with the mineral phases in the coal to produce, in some cases, insoluble Si-phosphate and Al-phosphate compounds [-22]. Char yields were greater for the coal:acid ratio of 1.0:0.65 than for the 1.0:0.96 ratio, whereas sulfur removal was increased with increasing acid concentration. This behavior may be a result of competition between alkylating and polymerization functions of phosphoric acid, in combination with the influence of creating char porosity during gasification. For example, the BET NE surface area of the char from Coal A was 445 mZ/g when using the 1.0 : 0.96 coal : acid ratio whereas it was 177 m2/g when using the 1.0 : 0.65 ratio. The extent to which sulfur can be removed from coal during hydropyrolysis has been attributed, in part to the development of porosity [23]. In addition, Lu and Do [24] presented data which showed that an increased rate of heating during gasification or pyrolysis influenced the softening, swelling and shrinking properties of the coals and changed significantly the porosity of the char. It has also been shown that

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the thermoplastic properties of coal were significantly decreased by the addition of acid (HC1) or oxidation treatments [24-26]. Phosphoric acid treatment did not, however, increase the concentration of oxygen in the chars relative to that in the coals. Interestingly, the free swelling index (FSI) of the parent Indiana VA was 3.4 whereas it was less than one after the addition of acid at the coal : acid ratio of 1.0: 0.96. This decrease in the FSI explains the absence of operational problems in the bench-scale, bubbling bed reactor for all coal:acid mixtures. The amount of sulfur removed from coal:acid mixtures was consistently and significantly greater than the amount of sulfur removed from coal-only feeds [ 1 4 , 15, 16]. The extent to which sulfur was removed is similar to that observed in fixed-bed testing using coal: phosphoric acid mixtures [17, 18], although in the current case the mean residence time of the coal within the reactor was on the order of minutes rather than hours. The SO2 emission level of the char from Coal B, using the parent and cleaned coal feed at a coal:acid ratio of 1.0:0.96, would be 1.8 and 1.5 lbs SO2/MMBtu, respectively. In contrast, the emission level of the char from a coal-only feed would be 4.4 lbs SO2/MMBtu. Even greater sulfur removal could be expected with an optimization of the processing. Practically total elimination of the pyritic sulfur occurred for all coal : acid mixture feeds. It was also observed that this elimination could be facilitated at temperatures as low as 410°F - i.e. during the initial drying/evacuation before fluidized bed treatment at 932°F. Hence, phosphoric acid decreased the temperature at which pyrite decomposition was initiated. Mild temperature gasification technology has been extensively investigated for its ability to produce a low sulfur char from relatively high sulfur coals. It can be concluded that, at a gasification temperature of 932°F, sulfur is nonselectively distributed in all products [ 1 4 , 15, 16]. As a consequence, no outstanding benefit is achieved relative to sulfur concentrations in the products. In comparison, sulfur mass balances show that the use of phosphoric acid at a coal:acid ratio of 1.0:0.96 causes the partitioning of coal sulfur to the gas with the following sulfur product distribution: Total sulfur in gas = 0.75 S¢oa~ Total sulfur in tar = 0.01 Scoa~ Total sulfur in char = 0.20 Scoa~ This distribution shows that it is possible to partition coal sulfur to the gas phase under mild temperature conditions without the use of reductive and consumable gases like H2. Such partitioning increases the viability of mild temperature processing of coal by decreasing the severity to which each of the product streams would need to be treated. The chemical form of sulfur species in Indiana V coal has been investigated by XANES spectroscopy [27]. A comparison of the distribution of sulfur in Indiana V as determined by XANES and in the current coals and chars is presented in Table 3. As noted by Huffman et al. [27], their sample was oxidized and, as such, there were oxidized sulfur forms which would originate from either pyrite or organic sulfur species. Our coals and the chars produced from them were not oxidized. However,

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145

Table 3 Comparison of sulfur distribution wt% in Indiana V coal and char produced from the pyrolysis of coal : acid mixtures Sulfur form

Indiana V a

Coal A and B b

Char A and B b

Pyrite Organic Elemental S Sulfide Thiophene Sulfone Sulfate

34 48 14 16 16 2 18

39 60

4 18

1

0.8

a Ref. [27]. b Current results, from parent coals.

irrespective of the extent of oxidation, the relative amount of refractory thiophenic species identified in Indiana V by XANES is very close to the relative amount of organic sulfur which remained in the char subsequent to pyrolysis of coal : phosphoric acid mixtures. Previously, fixed bed test results at 932°F of deeply physically cleaned Indiana V that had been mixed with phosphoric acid also showed that 20% of the organic sulfur could not be evolved from the char [28]. Hence, it is concluded that, for Indiana V and using coal:phosphoric acid processing at 932°F, the only organic sulfur species not completely eliminated from the coal is thiophenic in nature. A coal desulfurization process using phosphoric acid would benefit greatly from being able to maximize the amount of acid recycled to a pyrolysis stage. In fact, the high cost of phosphoric acid would suggest that nearly 90% acid recycle would be needed, whereas only 80% recycle has been achieved. Current work is concentrated on understanding how to increase this phosphorus removal from the char and to increase the overall efficiency of the processing.

5. Conclusions The pyrolysis of coal/orthophosphoric acid mixtures at temperatures near 932°F promotes the removal of coal sulfur as H2S. However, for the two coals investigated, the data indicate that thiophenic sulfur specie are not decomposed completely with this treatment. Further work is necessary to understand mechanisms of the observed desulfurization and the manner by which the phosphorus content in the chars could be decreased to values which optimize the use of the chars for energy production.

Acknowledgements The authors gratefully acknowledge the assistance of Dr. Jack Groppo, Chris Yates and Allen Howard. This work was supported by the Center for Applied Energy Research and the Illinois Clean Coal Institute.

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J.M. Stencel et al./ Fuel Processing Technology 41 (1995) 135-146

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Khan, M.R., 1988. ACS reprint, Div. Fuel Chem., 33(1): 253-264. Khan, M.R., 1989. Fuel, 68: 1439. Khan, M.R., 1989. Fuel, 68: 1522. Alvin, M.A., Archer, D.H. and Ahmed, M.M., EPRI AP-5005 (Project 2051-2), final report January 1987. [5] Stephenson, M.D., Williams, A.D., Ellis, S.L., Sprague, J.A. and Clauson, G.L., 1987. second International Conference on Processing and Util. of High Sulfur Coal, Elsevier, Amsterdam, pp.192-201. 16] Badi, M.F., Scaroni, A.W. and Jenkins, R.G., 1988. ACS reprints, Div. Fuel Chem., 33(1), 265-273. [7] Oh, M.S., Burnham, A.K. and Crawfor, R.W., 1988. ACS reprints, Div. Fuel Chem., 33(1): 274-282. 1-8] Gorbaty, M.L., George, G.N. and Keleman, S.R., 1990. Fuel, 69: 945. [-9] Keleman, S.R., George, G.N. and Gorbaty, M.L., 1990. Fuel, 64: 939. [10] Calkins, W.H., 1985. Determination of organic sulfur-containing structures in coal by flash pyrolysis experiments, ACS reprints, Div. Fuel Chem., 30: xxx. [11] LaCount, R.B., Anderson, R.R., Friedman, S. and Blaustein, B.D., 1986. Am. Chem. Soc., Div. Fuel Chem., 31: 70, preprint. 1-12] Derbyshire, F.J., Jagtoyen, M., McEnaney, B., Sethuraman, A.R., Stencel, J.M., Taulbee, D. and Thwaites, M.W., 1991. Am. Chem. Soc., Fuel Div. Prep., 36: 1072. [13] March, J., 1992. Advanced Organic Chemistry, 4th ed. Wiley, New York, p. 1011. [14] Roberts, J.D. and Caserio, M.C., 1981. Basic Principles of Organic Chemistry, 2nd ed. Benjamin, Menlo Park, CA, p. 1048. 1-15] Loganbach, J.R., 1991. Proceedings of the eighth Annual International Pittsburgh Coal Conference, p. 155. [16] Ness, R.O., 1989. Final Technical Report, DE-AC21-87MC24267. [17] Derbyshire, F.J., Jagtoyen, M., McEnaney, B., Rimmer, S.M., Stencel, J.M. and Thwaites, M.W., Proceedings of ICCS Conference, 9/16-20/91, Newcastle, England, Butterworth, London, pp. 480-483. [18] Jagtoyen, M., 1993. Groppo, J. and Derbyshire, F., Fuel Processing Technol., 34: 85. [19] Imai, T., Barger, P.T. and Hammershaimb, H.U., 1991. US Patent 5,043,509. [20] Yound, D.A., 1981. US Patent 4,270,017. [21] Lamartine, R., Lamsouber, F. and Perrin, R., 1988. Mol. Cryst. Liq. Cryst. Inc., Nonlin. Opt., 161: 163. [22] Pollack, S., 1992. Personal communications. [23] Sugaware, T. and Sugawara, K., 1992. ACS, Div. Fuel Chem., 37: 1701, Preprint. [24] Lu, G.Q. and Do, D.D., 1991. Fuel Processing Technol. 28: 247. [25] Walker, P.L., 1986. Carbon, 24: 379. [26] Lee, C.W. and Jenkins, R.G., 1989. Energy Fuels, 3: 703. [27] Huffman, G.P., Vaidya, S.A., Shah, N. and Huggins, F.E., 1992. Sulfur Speciation of Desulfurized Coals by XANES Spectroscopy, ACS Div. Fuel Chem., 37: 1094, Preprints. [28] Derbyshire, F.J., Jagtoyen, M., Stencel, J.M. and McEnaney, B., Synthesis of Adsorbent Carbons from Illinois Coals, CRSC, Technical Report, September 1-November 30, 1991.