Energy & Fuels 1994,8, 234-238
234
Desulfurization of Illinois No. 6 Bituminous Coal via Reductive Carboxylation in Absolute Ethanol Robert C. Duty**+and Jeffrey M. Penrod Chemistry Dept., Illinois State University, Normal, Illinois 61 761 Received May 28,1993. Revised Manuscript Received October 29, 1993'
.
This study detailed the sulfur removal from Illinois No. 6 bituminous coal through reductive carboxylation in absolute ethanol. Maximum desulfurization (68%)occurred at 300 "C using 10 g of sodium hydroxide, 500 mL of ethanol, and 10 g of 250/325 Tyler mesh coal. Sulfur removal increased with an increase in reaction temperature, lower base concentration (10 g vs 20 g of NaOH), and the use of carbon dioxide. The improved desulfurization of reactions which used carbon dioxide was the result of selective carboxylation of organosulfur compounds. The mass recovery of original coal samples ranged from 77 % to 91 96, with the lower values occurring at higher temperatures. Conversely, heat content values increased as the temperature was increased. Elemental and infrared analyses revealed that hydrogenation and loss of C-0 functional groups, as opposed to solvent incorporation, were the main causes. Elemental analyses of selected residues (acid-washed, moisture-free: AWMF) and acids recovered from basic solutions indicated that hydrogenation of coal had occurred. The degree of hydrogenation was found to increase with reaction temperature. The O/C ratios of black acids were higher than those of AWMF residues, which supported the theory that carboxylic acids were removed from the products. Most of the sulfur that was removed from the coal samples remained soluble in water and was not recovered. Solubilization occurred through a displacement reaction with sodium and through demineralization in 10% hydrochloric acid. The remaining sulfur was either lost during product separation or recovered in the acid fraction. The acid fractions were formed from carboxylation of organic precursors, some of which contained sulfur.
Introduction Coal usage has been increasing steadily since the beginning of the industrial age. Unfortunately, combustion of this abundant fuel creates environmental problems, one of which is sulfur which is converted to sulfur dioxide and sulfur tri0xide.l Sulfur is found in coal within the approximate range of 0.3-5% by weight.2 In order for the pretreatment process of sulfur removal from coal to be effective, one must remove inorganic (pyritic) and organic (thiolic, sulfidic, and thiophenic) sulfur. Although pyritic and some organic forms of sulfur are easy to remove, thiophenic sulfur has proven quite difficult.314 This study concentrated on removing the organicsulfur forms of coal and is based on several previous research efforts. Ouchi and Makabe5 have shown that hydrogen radicals which are produced from an absolute ethanol-sodium hydroxide mixture cause hydrogenation of a Taiheiyo coal. Sternberg et al. have shown that thiophenic ring opening occurs at C-S bonds as a result of reduction of the structure with sodium metal? and the Kolbe reaction involves the carboxylation of phenoxide anions using carbon dioxide.' We have shown in this t Present address: Chemistry Dept., Baylor University, Waco, TX 76798. 0 Abstract published in Aduance ACS Abstracts, December 15,1993. (1) Manahan, S.E.Environmental Chemistry;Willard Grant Press: Boston, 1970, pp 358-361. (2) Meyers, R. A. Coal Desulfurization; Marcel Dekker: New York, 1977; p 3. (3) Attar, A. 73rdAnnualMeetingoftheAmericanInstituteofChemical Engineers, 1980; p 16. (4) Attar, A.; Dupuis, F. Adu. Chem. Ser. 1981, 192, 239. (5) Makabe, M.; Ouchi, K. Fuel 1981, 60, 327. (6) Sternberg, H.W.; Delle Donne, C. L.; Markby, R. E.; Friedman, S. Ind. Eng. Chem. h o c . Des. Deu. 1974, 13(4), 433. ( 7 ) Lindsey, A. S.; Jeskey, H . C h e m . Reu. 1957, 5 7 , 583.
0887-062419412508-0234$04.50/0
laboratory that carboxylation reactions with coal do occur in aprotic solvents with alkali metals and carbon dioxide and that acids were generated in both tetrahydrofuran8 and t ~ l u e n e . ~ With this study we determined to what extent sulfur could be removed from an Illinois bituminous coal using the carboxylation reaction in a protic solvent (ethanol) with sodium hydroxide and an external source of carbon dioxide. Sulfur can be classified into four general groups in coal. These include pyritic sulfur, organic sulfur, sulfate salts, and elemental sulfur. By far the largest amount of sulfur in coal falls into either the pyritic or organic groups. Sulfate salts and elemental sulfur account for very little, usually no more than 0.1-2 % for the former and 0.2 % for the latter.'@ There are presently four main classifications for organic sulfur in coal. These are thiols, thioethers, bisthioethers (disulfides), and heterocycles (thiophenes).ll Generally sulfides account for a greater proportion of the organic sulfur in lignite along with the presence of thiols. Thiols, however, are negligible in high rank c0als.5 There have been numerous attempts to desulfurize coal including oxidation,12-22displacement reaction~,~3-~5 microwave heating with caustic,26 and hydrodesulfurization.27s28The oxidation processes were the most successful, ~~
~~~~
(8)Duty, R. C.; Hussman, G. P.; Justin, J. A. Fuel 1981, 60, 83. (9) Duty, R. C.;Chiri, I. C. Reductive Carboxylation Reactionsof Illinois No. 6 Bituminous Coal. Aduances in Coal Chemistry; Theophrastus Pub. S.A.: Athens, Greece, 1988; pp 141-157. (10) Yurovskii, A. 2.Sulfur in Coal. National Technical Information Service, Virginia, 1974. (11) Markuszewski,R.; Fan, C. W.; Greer, R. T.;Wheelock, T. D. New Approaches in Coal Chemistry. Amer. Chem. SOC.,Pittsburgh, PA, 1980; p 257.
0 1994 American Chemical Society
Coal Desulfurization via Reductive Carboxylation but one disadvantage of this method is the reduction of the heating value of the coal product. Oxidation reactions have had limited success in removing organic sulfur.29 Chlorine in chloroform reacting with lignite was reported by Camaloglu et aL30 to remove 75% of the sulfur (inorganic and organic). A 6% potassium permanganate solution at ambient temperatures removes 87 % of organic sulfur from coal,3land M ~ G o w a reported n~~ that boiling concentrated perchloric acid (203 "C) converted organic sulfur in coal to sulfate and did remove a large percentage of organic sulfur from coal. have been successful in removing Muchmore et sulfur by oxidation and subsequent treatment with base. He reacted with coal with nitric acid and subsequently treated it with methanol containing 5 w t % potassium hydroxide at 350 "C for 45 min. The sulfur content was decreased by 90%, and without nitric acid treatment the sulfur content was decreased by only 50 % . Supercritical methanol and ethanol have been used by a number of investigators.34-36 to remove sulfur from coal. When the coal is pretreated with nitric acid and reacted with supercritical ethanol and methanol, 99.9% of the sulfur was removed.3' Thiophenes are the predominant form of organic sulfur in many bituminous coals.4 Unfortunately, thiophenic sulfur is highly resistant to desulfurization methods. This
Energy & Fuels, Vol. 8, No. 1, 1994 235 study focused on the removal of thiophenes from Illinois No. 6 bituminous coal. A series of reductive carboxylation reactions were run on coal, and sulfur removal efficiency was determined. Several reaction variations were employed, including elimination of the carboxylation step, changes in the coal particle size, reaction temperature, and the amount of base used. The major objective of the reductive carboxylation reaction was removal of the majority of thiophenic sulfur from the test coal. Other objectives included (1) the efficient removal of non-thiophenic sulfur compounds from coal, (2) a substantial reduction of the ash content, and (3) recovery of at least 90% of the heating value from the original coal. Experimental Section
Coal. A grab sample of Illinois No. 6 bituminous coal was obtained from the Illinois Geological Survey. Samples were ground with a mortar and pestle and sieved through progressively finer screen to obtain particle sizes of 63-45 pm (250/325 Tyler mesh) and 45-38 pm (325/400 Tyler mesh). Sieving was accomplished by shaking the ground coal in a Cenco-Meringer shaker for 15-30 min. The sized coal was rendered moisture free (MF) by drying in a heated (100 "C) vacuum desiccator (calcium chloride and phosphorus pentoxide, desiccants) for 24 h. Elemental analyses for the MF coal and AWMF coal are given in Table 6. Absolute Ethanol. US. Industrial Chemicals, U.S.P. No (12) Warzinski, R. P.; Ruether, J. A.; Friedman, S.; Steffgen, F. W. further purification was needed. Proceedings of the Symposium on Coal Cleaning t o Achieve Energy and Sodium Hydroxide. Fisher, A.C.S., pelletized sodium hyEnvironmental Goals;Environmental Protection Agency: Washington, droxide. D.C. 1979; pp 1016-1037. (13) Joshi, J. B.; Shah, Y. T. Fuel 1981,60,612. Autoclave. Desulfurization reactions were carried out in a (14) Chuang,K. C.;Markuszewski,R.;Wheelock,T.D.FuelProcessing 1-L, stainless steel autoclave. The unit was supplied by Autoclave Tech. 1983, 7, 43. Engineers, Inc. (Erie, PA). It had a cylindricalinterior measuring (15) Chuang, K. C.; Chen, M. C.; Greer, R. T.; Markuszewski,R.; Sun, 9 in. in height and 3 in. in diameter. The pressure vessel had Y.; Wheelock, T.D.; Chem. Eng. Common. 1980, 7 (1-3), 79. (16) Agarwal, J. C.; Giberti, R. A.; Irmingar, P. R.; Petrovic, L. F.; walls measuring 11/16 in. thick which gave it a working pressure Sareen, S. S.Mining Congress Journal 1975,61 (3), 40. of 5000 psi at 343 "C. The agitator consisted of a belt-driven, (17) Sareen, S.S.CoalDesulfurization; Wheelock,L. D.,Ed.;American magnetic drive assembly (Autoclave Engineers Magnedrive 11, Chemical Society: Washington, D.C., 1977; pp 173-181. Model 75-2). (18) Morrison, G. F. Chemical Desulfurization of Coal; IEA Coal Research London, England, 1981, pp 30-34. Autoclave Reactions. The autoclave was charged with 10 g (19) Meyers, R. A.; Van Nice, L. J.; Santy, M. J. Combustion 1979,51 of moisture-free coal, 10or 20 g of NaOH, and 500 mL of absolute (e),18. ethanol. The autoclave was flushed with nitrogen for 60 s. The (20) Meyers, R. A. Hydrocarbon Processing 1975,54 (6), 93. impeller speed was lo00 rpm. Heat was applied, and reactions (21) Hsu, G. C.; Kalvinskas, J. J.; Ganguli, P. S.; Gavalas, G. R. Coal Desulfurization; Wheelbook, T. D., Ed.; American Chemical Society: were run for 24 f 4 h. With reactions with carbon dioxide, the Washington, D.C., 1977, pp 206-217. nitrogen was vented and then carbon dioxide was introduced (22) Kalvinskas, J. J.; Grohmann, K.; Rohatgi, N.; Ernest, J.; Feller, prior to the heating of the autoclave. Temperatures of the D. FinalReport. Coal Desulfurizationby Low TemperatureChlorinolysis, autoclave reactions were 100,200, and 300 "C. It took approxPhase 11. Jet ProDulsion Laboratory: Pasadena, CA, 1980. (23) Guth, E. D: Proceedings of the Symposium on Coal Cleaning to imately 45 min to reach the 300 "C temperature and slightly less Achieve Energy and Environmental Goals; US. Environmental Profor 200 and 100 OC reactions. tection Agency: Washington, D.C., 1979; pp 1141-1164. Product Separation. The reaction products from the au(24) Stambaugh, E. P. Coal Desulfurization; Wheelock, T. D., Ed.; toclave were rotary evaporated to dryness. The residue was then American Chemical Society: Washington, D.C. 1977, pp 198-205. (25) Morrison, G. F. Chemical Desulfurization of Coal; IEA Coal dried in a heated vacuum desciccator at 100 "C for approximately Research London, England, 1981, pp 43-45. 24 h. In order to solubilizean acid fraction, the dried residue was (26) Morrison, G. F. Chemical Desulfurization of Coal; IEA Coal refluxed in 500 mL of 5% sodium hydroxide for 24 h. After Research London, England, 1981, pp 45-46. cooling, the supernatant was drawn off. The remaining residue (27) Lesch, D. A.; Richardson, J. W.; Jacobson, R. A.; Angelici, R. J. J . Am. Chem. Soc. 1984,106, 2901. was washed and centrifuged 6 times, with washings added to the (28) Morrison, G. F. Chemical Desulfurization o f Coal; IEA Coal supernatant. The acids were recovered from the supernatant by Research: London, England, 1981, pp 47-48. acidifying with concentrated hydrochloric acid to a pH of 3.0 or (29) Saika, P. C.; Sain, B.; Baruah, B. P.; Bordoloi, C. S.; Mazumber, less. The acids that precipitated were washed and centrifuged B. J . Mines, Met. Fuekr 1988, 36 (5), 216. (30) Cemaloglu,M.;Yilmas,V.;Kin,B.;Emir,B.D.;Filiz,M.;Erturans, 4 times. The acids were dried over a steam bath and then placed S.;Dincers, S.Doga: Turk Kim.Derg. 1988, 12 (2), 137. in vacuum desiccator at 100 "C and weighed. (31) Attia, Y. A.; Lei, W. Process. Util. High Sulfur Coals Proc. Int. The washed and dried residues from the supernatant from the Conf. 2nd 1987, 202. acids were treated with 200 mL of 10% hydrochloric acid. The (32) McGowan, C. W.; Markuszewski,R. Fuel Process. Technoll987, 17 (l), 29. slurry was magnetically stirred for 24 h. The residues were then (33) Muchmore, C. B.; Chen, J. W.; Kent, A. C.; Liszka, M. Coal Sci. washed and centrifuged 6 or more times until no positive chloride Technol. 1987, 11, 439. test was obtained in the washings. Residues were again placed (34) Lee, S.;Fullerton, K. L.; Lee, B. G. Proc.-Ann. Int. Pittsburgh in a vacuum desiccator at 100 OC and dried overnight. Coal Conf. 1989, 6 t h (l),425. (35) Garcia, R.; Moinelo, S. R.; Snape, C. E.; Bernard, P. Fuel Process. Elemental Analyses. The elemental analyses on the acids Technol. 1990, 24, 187. and dried residues were done by Canadian Microanalytical (36) Yurum, Y.; Tugluhan, A. Fuel Sci. Technol. Int. 1990,8 (3), 221. Service, Ltd. (Vancouver,BC). Percent oxygen was determined (37) Chen, J. W.; Muchmore, C. B.; Kent, A. C. Energy Res. Abstr. by subtracting the percent C, H, N, S, and ash from 100%. 1987, 12 (5), Abstr. No.8817.
Duty and Penrod
236 Energy &Fuels, Vol. 8, No. 1, 1994
Table 2. Reactions (without COz) Using 10 g of 250/325 Tyler Mesh Coal. and 325/400 Tyler Mesh Coal. reaction AWMF MF AWMF organics temp ("C) residue wt ( 9 ) acid w t (g) residue (% S) removed (7%) 100 100 200 200 300 300
t H+
100 100 200 200 300 300
1
(3.76) 3.19 (8.736) 8.447 (0.484) 0.559 (3.65) 2.98 (9.009) 8.628 (0.305) 0.529 (7.513) 8.279 (1.299) 1.520 (2.66) 2.28 (2.57) 2.39 (7.508) 9.042 (1.338) 0.781 (8.023) 7.842 (0.662) 8.020 (1.78) 1.64 (7.306) 8.051 (0.811) 0.792 (1.95) 1.76 With 20 g of Sodium Hydroxide (8.694) 9.105 (1.459) 0.701 (3.00) 2.86 (8.786) 8.975 (0.642) 1.263 (3.12) 2.97 (8.384) 7.335 (0.962) 1.556 (2.43) 2.17 (8.069) 8.567 (1.463) 2.162 (2.57) 2.39 (7.341) 7.946 (0.535) 0.575 (1.99) 1.79 (7.502) 8.270 (0.695) 0.784 (2.07) 1.73
----(19.5) 25.8 (11.8)23.1
--- 1.9
-(10) 19.1 (7.4) 21.8
Q!f===J
a Solvent: 500 mL of absolute ethanol. Values in parentheses are for 250/325 Tyler mesh coal.
SH
C02H
Figure 1. Mechanism for reductive carboxylation of dibenzothiophene.
-Na+
1 Na
Table 3. Total Sulfur Reduction of Reductive Reaction S removed S removed mesh size temp ("C) (with C02)(%) (without COz) (%) 10 g of Sodium Hydroxide 100 40 100 38 200 51 200 49 300 68 300 62 20 g of Sodium Hydroxide 100 31 100 28 200 49 200 45 300 60 300 55
2501325 3251400 2501325 3251400 2501325 3251400
Q)yJ-@s- -
Figure 2. Mechanism for C-S bond cleavage in dibenzothiophene with sodium. Table 1. Reductive Carboxylation Reactions Using 250/325 Tyler Mesh Coal. and 324/400 Tyler Mesh Coal reaction AWMF MF AWMF organics temp ("C) residue wt ( 9 ) acid wt (9) residue (% S) removed (%) 100 100 200 200 300 300 100 100 200 200 300 300
(8.305) 7.926 (8.792) 8.902 (8.285) 7.941 (3.412) 8.168 (5.315) 8.983 (6.676) 8.496 (9.142) 8.993 (9.320) 8.774 (8.378) 8.037 (8.368) 8.065 (7.986) 7.332 (8.042) 8.474
(0.200) 1.836 (0.400) 1.842 (2.017) 1.112 (1.880) 0.884 (2.090) 1.105 (2.684) 0.566 (0.402) 2.343 (0.352) 1.525 (4.675) 0.524 (3.170) 0.585 (0.390) 3.395 (0.412) 3.028
(2.70) 2.41 (2.62) 3.04 (2.30) 2.27 (2.07) 2.26 (1.41) 1.66 (1.45) 1.66 (3.02) 3.10 (3.12) 3.25 (2.17) 2.35 (2.30) 2.50 (1.60) 1.94 (1.91) 2.05
--
-(1)(6.4) (36.2) 24.9 (34.4) 24.9
--(1.8) -(27.6) 12.2 (14.1) 7.2
0 Solvent: 500 mL of absolute ethanol. Values in parentheses are for 250/325 Tyler mesh coal.
Mineral Matter Determination (Ash Content). For each determination, a cleaned crucible and lid were heated in a muffle furnace (950 "C) for 24 h and then cooled in a desiccator and weighed. Approximately 0.5 g of sample was placed in the dried and weighed crucible and reheated in the muffle furnace for 24 h. Duplicate determinations were run on each sample, and if they differed by more than 2.5% a third determination was run. Infrared Analyses. For each spectrum, a potassium bromide pellet was made from approximately 0.015 g of sample and 0.5 g of KBr (Fischer, A.R.). Each sample was shaken in a wiggle bug for 4 min, dried in a dryingpistol with phosphorus pentoxide, and formed into pellets using a press (20 000 psi for 15 min at 1.2 Torr). The pellets were analyzed on a Perkin-Elmer Model 621 infrared spectrophotometer. Infrared spectra were made of each sample from 4000 to 600 cm-1. The carbonyl stretching of the carboxyl groups was measured a t 1695 cm-l with 1850 cm-l as the referencebackgroundlevel. With this background reading, measurements were recorded as cm/mg of acids. Gas Chromatography. In order to verify the presence of hydrogen, samples were taken during the process of heating the coal in ethanol with sodium hydroxide and carbon dioxide. A
2501325 325/400 2501325 3251400 2501325 3251400
raw AWMF coal
16 30 41 47
58 62 31 34 43 48 54 60 3.42% S
2.5-mL portion of the gas sample was injected into a Varian Aerograph Model 90-P gas chromatograph with thermal conductivity detector. Helium was the carrier gas which flowed (60 mL/min) through a 9.5 f t X 0.25 in. column filled with 40/60 molecularsieveB stationary phase (Matheson,Coleman and Bell, MX 1583-2-9325). Column temperature was maintained at 100 "C. No other gases were detected from this analysis including the hydrogen sulfide gas. Heating Value Determination. The heat of combustion of AWMF residue samples was determined using a Parr Model 1242 adiabatic calorimeter. Approximately 0.5-g samples were first pelletized under high pressure and weighed to 0.0001 g. A Parr Model 1108 bomb was loaded with sample and pressurized to 30 atm with oxygen. The bomb was submerged in 2000g of distilled water, and the sample was ignited. Heat from the calorimeter was measured to 0.001 "C. The gross heat of combustion (H,) was determined using the following equation:
Hg=
tW - e, - e2- e3 m
t = temperature increase of distilled water
W = specific heat of water e, = correction in calories for heat of formation of nitric acid (determined by titration) e2 = correction in calories for heat of formation of sulfuric acid (calculated from the percent sulfur in the sample) e3 = correction in calories for the heat of combustion of ignition wire m = mass of sample
Coal Desulfurization via Reductive Carboxylation
Energy & Fuels, Vol. 8, No. 1, 1994 237
Table 4. Carbonyl Group Absorption Intensities per Unit Weight of AWMF Residues and Their Corresponding MF Acids. AWMF residues MF acids reaction wt Codstretch CO stretch wt CO stretch CO stretch temp ("0 (mg) at 1695 cm-1 (cm) (cmlmg) (mg) at 1695 cm-1 (cm) (cm/mg) l00b 1.19 8.3 7.0 1.02 22.3 21.9 2006 1.17 7.0 6.0 1.03 23.4 22.7 3006 1.23 6.9 5.6 0.47 10.2 21.7 1W 1.20 8.3 6.9 1.25 11.9 9.5 2w 1.22 8.7 7.1 1.21 11.3 9.3 3 w 1.22 7.6 6.2 1.16 9.3 8.0 unreacted coal (mo 1-10 5.7 5.2 Solvent = 500 mL of absolute ethanol, base = 10 g of sodium hydroxide, coal mesh (Tyler) = 2501325. b Reaction using carbon dioxide. Reaction without carbon dioxide. d CO = carbonyl.
Results and Discussions The proposed reaction mechanism for the reductive carboxylation of thiophenes (using dibenzothiophene as a model) is represented in Figure 1. Ouchi and Makabe were the first to report the hydrogenation of a Taiheiyo coal with ethanol and sodium hydroxide.6 Their coal was more soluble in pyridine because of the hydrogenation reaction. The hydrogen radicals were proposed to occur from the following reaction: C,H,OH
+ NaOH
-
CH3C0,Na
+ 2H'
Chromatographic analyses of gas samples obtained from blank (no coal) reductive carboxylation reactions gave further evidence that hydrogen was produced which could be the result of hydrogen radical production. The samples were taken during the reduction phase (under nitrogen) of the reactions. Although hydrogen gas was absent in the 100"C reaction, it was present in fairly large quantities in the 200 and 300 "C reactions. Chromatographic analyses revealed no evidence for the formation of hydrogen sulfide. Consequently, no reduction of thiol groups is occurring in the proposed thiol carboxylic acid shown in Figure 1. Sternberg et ala6have studied the reduction of dibenzothiophene using sodium metal. Their proposed mechanism for the rearrangement and subsequent cleavage of a C-S bond is illustrated in Figure 2. Theory suggests that the proposed mechanism for reductive carboxylation may be inhibited by the protic solvent used in this study. However, carboxylation may compete with protonation of these carbanions, as this study will document. Tables 1and 2 reveal the amount of sulfur found in the AWMF residues. Table 1 is the reactions that were run under reductive carboxylation conditions while Table 2 was run under reductive conditions with no carbon dioxide added. In almost every case using 10 g of sodium hydroxide, the reductive carboxylation reactions reduced the total sulfur in the residues more so than without the addition of carbon dioxide. In the case of the 100 "C reactions with 10 g of sodium hydroxide, the AWMF residues come out higher in sulfur content than the AWMF coal starting material. This can be explained by assuming the coal that was solubilized by the sodium hydroxide and carbon dioxide reaction had very little sulfur in its contents. No organic sulfur was lost in any of the 100 "C reactions; however, a considerable amount of organic sulfur was lost in the 300 "C reductive carboxylation reactions with a high value of 36.2% loss of organic sulfur using the 250/ 325 mesh coal. The average value at 300 "C for reductive carboxylation was 22.7 7% reduction in organic sulfur. The reduction reactions without adding carbon dioxide also lost organic sulfur, but the average value for the 300 "C
Table 5. Heating Values of AWMF Residues mesh size temp ( O C ) heating value (Btullb) 10 g of Sodium Hydroxide 2501325 100 11&40f86 100 12 336 f 288 3251400 2501325 200 13 112 f 200 3251400 200 12 900 i 370 2501325 300 14 638 i 43 3251400 300 14 669 i 120 20 g of Sodium Hydroxide 2501325 100 12 295 f 158 3251400 100 12 208 f 34 2501325 200 13 449 f 60 3251400 200 13 383 f 108 2501325 300 14 466 f 313 300 14 548 f 58 3251400 unreacted coal 9 981 i 19 (AWMF) ~
reactions was only 17.3?6 reduction of organic sulfur, Very little organic sulfur was lost at 200 "C regardless of whether or not carbon dioxide was added. The amount of organic sulfur in Illinois No. 6 bituminous coal is reported as 2.21 5% organic sulfur,12 and this was the basis for determining the reduction of organic sulfur in our residues. In all of the AWMF residues, they were extracted with 10% hydrochloric acid for 24 h before drying. After extraction was complete, the residue was washed with distilled water until no test for chloride was detectable by AgN03 test. With this treatment, we assumed that all inorganic sulfur components would be removed. The acids formed in these reactions were formed in maximum amount at the 200 "C reaction temperature. As one would generally expect, more acids would be formed from the reductive carboxylation reactions than with the reductive reactions with no carbon dioxide added. Table 3 summarizes the total sulfur reduction from the starting coal (AWMF) to the AWMF residues recovered from each reaction. As the temperature is increased the total sulfur is reduced and reaches a maximum of 68 % at 300 "C with 10 g of sodium hydroxide and the 2501325 mesh coal. In every case with carbon dioxide, the 2501325 mesh coal sample had more sulfur removed than the 3251 400 mesh coal. This was a surprising result because one would assume the smaller particle size would render the sulfur moieties more susceptible to attack by hydrogen and/or base, which would reduce the sulfur content. The only reason for higher sulfur removal with larger particle size may result from the fact that they are surrounded by less solvent and more exposed to radical reduction by the hydrogen radicals. In one compares the results with and without carbon dioxide in Table 3, in every case using the 2501325 mesh coal the reductive carboxylation was more effective than the reduction without carbon dioxide with one exception
Duty and Penrod
238 Energy &Fuels, Vol. 8, No. 1, 1994
reaction no. 2 4 6 2 26 26
Table 6. Elemental Analysis of AWMF Residues and MF Acids elemental analysis (wt reaction temp (OC) COZused? sample C H N Oa 100 yes AWMFresidue 65.61 4.79 1.29 19.27 17.23 200 yes AWMFresidue 70.68 5.37 1.34 11.92 yes AWMFresidue 75.51 6.80 1.23 300 9.8 2.05 0.20 yes MF acids 9.67 100 17.40 100 no AWMFresidue 67.61 5.00 1.30 64.51 4.95 1.076 21.67 100 no MF acids
raw mf coal raw AWMF coal a
69.20 64.13
4.72 4.56
1.29 1.29
7.98 16.87
%)
molar ratio H/C O/C
S
ash
3.48 2.38 1.64 0.31 3.70 3.31
5.56 3.00 2.90 78.0 4.99 4.49
0.87 0.91 1.07 2.53 0.88 0.91
0.22 0.18 0.12 0.76 0.19 0.25
4.41 3.42
12.40 9.73
0.81 0.85
0.09 0.20
Calculated by difference.
(where they were equal at 100 "C). We ran reactions with and without carbon dioxide to show that carbon dioxide cannot only form sodium carbonate with sodium hydroxide but can also react with carbanions that are found in the nitrogen atmosphere in the presence of base and ethanol. The 3251400 mesh coal had a slightly higher decrease in sulfur using 20 g of sodium hydroxide vs 10 g of sodium hydroxide without carbon dioxidethan with carbon dioxide except for the 300 OC reaction. These results showing a decrease in sulfur removal with larger amounts of sodium hydroxide may result from carbon dioxide reacting with sodium hydroxide before it has a chance to react with a carbanion. Using dibenzothiophene as a model compound for our study, we reacted dibenzenethiophene under the same experimental conditions as the coal; i.e., we used a temperature of 300 O C with 20 g of sodium hydroxide and 500 mL of ethanol with carbon dioxide. The weight of dibenzothiophene in each of these reactions was 2 g, and the temperature was maintained for 24 h. Analyses of recovered dibenzothiophene were 68 % and 62 % which indicated that more than 30% had reacted. Infrared analyses were made on all acid samples and on all of the AWMF residues from the reactions. Infrared measurements were made from KBr pellets with milligram quantities of a coal sample. Quantitative results were measured from the carbonyl stretch at 1695 cm-l versus a background reading at 1850 cm-l. Table 4 lists the measurement in terms of carbonyl stretch in cm/mg for the AWMF residues and their corresponding acids for the reactions run with 10 g of NaOH and 2501325 mesh coal. Results are illustrated for reaction run with and without carbon dioxide. The striking difference is found in the recovered acids. Acids recovered from the reactions with carbon dioxide are over 100% stronger than acids recovered from reactions without addition of carbon dioxide. This is a strong proof to document that carboxylation reactions did occur in the presence of the protic solvent, ethanol, and the carbon dioxide competed very well with the acidic hydrogen from ethanol for the carbanion sites generated in the reduction reactions. There is little difference in the carbonyl stretch measurements between the reactions with and without carbon
dioxide for the AWMF residues. This can reasonably be explained by the fact that when carboxylated this portion of the residues was solubilized in the basic reflux. However, the carboxyl stretching in all cases for the AWMF residues was higher than the MF coal that was the starting material. This again documents that carboxylation did occur in the presence of the protic solvent, ethanol. Another goal of this research was to obtain residues of 90% or better of heating value than the starting material. The results of our bomb calorimeter studies are shown in Table 5. These figures reveal that in every reaction the heating value of the AWMF residues was greater than that of the starting material. In order to determine why increased heating values occurred, infrared and elemental analyses were made (see Tables 4 and 6). An examination of the HIC molar ratio of raw coal compared to that of AWMF residues revealed that hydrogenation had occurred in every reaction. Consequently, this explains why the heating value would increase. The infrared analysis of the C-0 bonds (10851050cm-) revealed that no increase occurred in this region of the spectrum. Consequently, solvent incorporation to increase the heating value can be eliminated. The ash content of the AWMF residues in every reaction was lower than the AWMF starting coal sample. The ash content of the AWMF starting coal was 9.7394, and the reduction of the ash content in the AWMF residues varied from a high of 72 % at 200 "C with 20 g of NaOH and the 2501325 mesh coal to a low of 34% at 100 "C with 10 g of coal and a mesh size of 2501325. The average reduction at 100 "C was 45%, at 200 "C was 65%,and at 300 "C was 58%. As can be seen by these results, the reductive carboxylation decreases the ash content, evidently by removing the ash particles as soluble carbonates. In conclusion, reductive carboxylation of Illinois bituminous coal did occur in the protic solvent ethanol as revealed in the acids generated and the increase in carboxyl stretching frequency in AWMF residues over the AWMF starting coal. In addition, reductive carboxylation did decrease significantlythe total sulfur content of the original coal and also accomplished the removal of organic sulfur. The total heating value of the AWMF residues increased over that of the starting coal as the result of the reduction reactions with ethanol.
