Electrolytic pretreatment of coal for enhanced liquefaction

Fuel Processing Technology, 31 (1992) 221-232

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Elsevier Science Publishers B.V., Amsterdam

Electrolytic pretreatment of coal for enhanced liquefaction S.B. Lalvani a, P. Rajagopal", J.A. Koropchak b, C. Chavez b, B. Akash a and C.B. Muchmorea aDepartment of Mechanical Engineering and Energy Processes, Southern Illinois University, Carbondale, IL 62901 (USA) bDepartment of Chemistry, Southern Illinois University, Carbondale, IL 62901 (USA) (Received March 9th, 1992; accepted May 26th, 1992 )

Abstract

Mild oxidation of coal can result in enhanced coal liquefaction yields, presumably due to the formation and decomposition of carbonyl or hyperoxide groups from the benzylic groups of alkyl bridges in the coal structure. Due to the ability to closely control the oxidizing or reducing conditions at the electrode, electrochemical routes for coal pretreatment have attracted considerable interest. Electrolysis of an Illinois coal in NaOH and H2SO4 electrolytes at potentials of between 1.0 and 1.3 V vs. saturated calomel electrode (SCE) resulted in hydrogen production at current efficiencies of about 100% while a significant amount of sulfur was removed from the coal. Liquefaction of the coal was carried out in tetralin at 375 °C with an initial hydrogen pressure of 140 psig (10.5 bar). The data obtained clearly indicate that electrochemical oxidation of coal slurries results in significant enhancement (up to 27% ) in coal liquefication yields. The products (liquid) obtained from the liquefication of the electrolyzed coal contained substantially larger amounts of the more desirable pentane soluble fraction, while the undesirable benzene insoluble fraction content diminished. The data also show that the molecular weight of the liquid products obtained from pre-electrolyzed coal samples is very much lower than the corresponding molecular weight of the liquid products obtained from an untreated coal under the same liquefaction conditions.

INTRODUCTION

Coal modification by chemical treatment can lead to enhancement in coal extractability and liquefaction yields presumably due to selective bond cleavage. For example, Graft et al. [1,2] and other researchers [3,4] have shown that hot water treatment of coal enhanced its reactivity towards pyrolysis accomplished by steam (at 50 atm and 320-360° C). In principle, any successful chemical oxidation should have its electrochemical counterpart, and anodic Correspondence to: Dr. S.B. Lalvani, Department of Mechanical Engineering and Energy Processes, Southern Illinois University, Carbondale, IL 62901, USA. Tel.: (618) 536-2396, Fax: (618) 453-7455.

0378-3820/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

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oxidation of coal has been the subject of considerable interest since the 1920s. Due to the ability to closely control the potential, electrochemical methods offer certain advantages over the corresponding chemical techniques. Lalvani and Coughlin [5] electrolyzed coal slurries in acidic as well as basic electrolytes at different oxidizing potentials. The partially reacted coal residues were subjected to Soxhlet extraction with benzene and ethanol. Coal electrolyzed in acidic electrolytes was more extractable. Electrolysis at 3.1 V vs. SCE (saturated calomel electrode) of a North Dakota lignite enhanced the solubility of coal in the benzene-ethanol mixture by 33 %. They speculated that in basic electrolytes, coal depolymerization could take place due to nucleophilic attack of O H - . In another study, Pomfret et al. [6 ] electrolyzed coal and coal derived humic acids at a cell voltage of 1.7 V. Because of the complicating presence of inorganic materials in coals, humic acids extracted from lignites were used as model substrates. Considerable reductions in molecular mass, as revealed by size exclusion chromatography, were achieved by anodic oxidation of humic acids slurried in acid, and more especially for solutions in alkali. NMR spectroscopy of methylated humic acids before and after oxidation suggests cleavage of ether linkages between aromatic clusters, with generation of carboxyl groups. Pomfret et al. [6] further state that the incorporation of electrochemical oxidation into schemes for the liquefaction of coal could, therefore, be achieved by anodic oxidation to produce carboxylic acids at voltages below that required for CO2 generation. In a preliminary study, Nand et al. [ 7 ] have shown that although electrolysis of Illinois No. 6 bituminous coal at 1.4 V vs. SCE in an acidic electrolyte did not significantly change the total volatile matter content of the solid, its evolution was shifted to much lower temperatures. An increase of about 400% devolatilization yield for an electrolyzed coal residue over the parent coal below 400°C was observed. Similar but less profound effects were caused by electrolysis in alkaline aqueous electrolytes in which a significant portion of the smaller oxidized fragments could dissolve. Conversion of the acidic electrolysis residue in tetralin at 375 °C and about 1500 psi (100 bar) H2 pressure to filterable liquid was about 13% greater than that of the parent coal. In another study, Hoo et al. [8] describe the influence of electrolysis on coal reactivity. The electrolyzed coal samples were pyrolyzed in an atmosphere of 800 °C and subsequently gasified with carbon dioxide at 800 °C in a fixed bed reactor. The reactivity of coal electrolyzed in 2 M NaOH is found to be several orders of magnitude higher than that of the original coal. Both electrochemical oxidation and reduction reactions of coal have been studied. Whereas oxidation has been studied in acidic and basic electrolytes, reduction has been performed in non-aqueous solvents. A review article [9] compares the chemical reactions (oxidation and reduction) of coal with the corresponding electrochemical pathways. The objectives of this study were to

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liquefy the electrolyzed coal under mild (i.e. low pressure) reaction conditions in a hydrogen donor solvent in the absence of a catalyst, and conduct preliminary characterization of the reaction products. EXPERIMENTAL

A bituminous coal (Illinois Basin coal sample no. 105) obtained from the Argonne Premium coal sample program was used in this study. The coal sample ground to - 2 0 0 mesh ( < 75 ~tm) was slurried in 375 ml of electrolyte and charged to a stirred glass reactor at 60 ° C. The working electrode (anode for oxidation) is separated from the counter electrode by a porous glass frit. A saturated calomel electrode (SCE) serves as the reference electrode. All reactions were carried out at constant potential applied for 2 h at the working electrode. Experimental details about electrolysis can be found elsewhere [ 10 ]. The electrolyzed coal slurry was filtered and the solids obtained were then washed with distilled water. Four grams of the dried electrolyzed coal was liquefied in 120 ml of tetralin in a 300-ml glass-lined autoclave for 1 h. The reaction time reported refers to the time after the reactor has obtained the desired temperature. A constant temperature was maintained for the reaction time after which the heat supply to the reactor was shut off. During the course of the reaction, the gas pressure was observed to increase. Figure I is a typical plot of the temperature and pressure of the reactor. For a reaction time of 1 h, it was observed that the maximum pressure in the reactor rose to about 400 psig (28 bar). The unreacted coal was filtered, dried, and weighed. Several of the liquefaction products were subjected to spectroscopic, chromatographic, and elemental analyses. The samples were first vacuum distilled to remove tetralin. The concentrated products were subsequently separated 500

400

~400 ~l~ o

3001

¢.0 300 "~" ~7 L~ 200 100

0

60

120

180

Time, minute Fig. 1. Pressure a n d temperature vs. time. Four grams of coal pretreated with 1 M N a O H at 60 ° C were liquefied in 120 ml tetralin under initial hydrogen gas pressure of 140 psig (10.5 bar).

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into benzene-soluble and benzene-insoluble fractions; the benzene-soluble fraction was subsequently extracted with pentane to generate a pentane-soluble fraction [11]. To prepare soluble forms of the benzene-insoluble fractions, these samples were first silylated by refluxing with hexamethyl disilazane and trimethylchlorosilane, using standard procedures [12]. NMR spectroscopic data were collected using a Varian VXR-300, 300-MHz instrument. Proton experiments were done in a conventional manner [13]. R E S U L T S AND D I S C U S S I O N

Figure 2 is a current vs. time plot of coal (10 g) electrolyzed in 375 ml of 1 M NaOH at 1.0 V vs. SCE at 60 ° C for 2 h. At the beginning of electrolysis, the current observed is high and it gradually declined and reached a steady state value after 40 min of electrolysis. The amount of hydrogen gas produced at the cathode as a function of the time of electrolysis is also shown in Fig. 2. The hydrogen gas produced is a linear function of time. The coulometric efficiency for these experiments was found to be about 100%. These results are consistent with the findings of other investigators [5,10,14,15]. Preliminary results of the influence of coal electrolysis on liquefaction yields are presented in Table 1. Electrolysis was conducted in NaOH and H2SO4 electrolytes. In some experiments, coal was slurried in these electrolytes under open-circuit conditions. The amount of coal conversion is based upon the mass of solids reacted (dry basis). The data presented clearly indicate that the electrolytic pretreatment of coal slurries under mild reaction conditions resulted in significant enhancement (up to 27% ) in coal conversion under the liquefaction conditions described in this paper. However, the enhancement in coal conversion due to electrolytic pretreatment is found to be different when the

E 0 90

"

hl 0,010 0,70

0 . 5 0 ~ 0

/

f io.oo,

.... , 30

-

, 60

" 90

¢~ ,~

" 120

+0.000

Time, minute Fig. 2. Current and hydrogen evolution vs. time. Ten grams of coal were electrolyzed in 375 ml of 1 M N a O H at 1.0 V vs. SCE at 60°C. The anode was a P t - m e s h (6.5 cm2).

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TABLE 1

Coal conversion yields: Preliminary results. Ten grams of coal were either slurried (open-circuit) or electrolyzed in N a O H / H 2 S 0 4 for 2 h. The pretreated coal (4 g) was liquefied in 120 ml tetralin in at 375°C for 60 rain a under initial hydrogen pressure of 140 psig Sample

Conversion a'b

EnhancemenV 'b

(%)

(%)

Enhancement a'b over slurried coal

(%) 1. Coal (untreated) 2. Coal electrolyzed in 1 M N a O H at 1.3 V vs. S C E and 60 ° C 3. Coal electrolyzed in 1 M H2S04 at 1.0 V vs. S C E and 60 ° C 4. Coal slurried in 1 M N a O H at 60°C 5. Coal slurried in 1 M H2SO4 at 6 0 ° C

43.38/42.25/44.5 (52.88) 54.98 (62.50)

-

-

26.8 (18.2)

35.8 (31.1)

53.86 (60.26)

21.1 (14.0)

12.8 (9.7)

40.50 (47.69) 47.75 (54.92)

-6.6 (-9.8) 10.07 (3.9)

-

aTime measured since the reactor attained the desired reaction temperature. bQuantities in parentheses refer to ash- and moisture-free basis. Conversion is defined as: [ ( m a s s coal )in - ( m a s s coal ) out] / (mass coal) in" CEnhancement over corresponding conversion for untreated coal. dEnhancement over corresponding conversion for coal that was slurried in N a O H or H2S04 under open-circuit conditions.

data are presented on ash- and moisture-free basis (Table 1 ). Nonetheless, the data clearly show that electrolytic pretreatment of coal resulted in an enhancement in its conversion during the liquefaction step. The results also show that the pretreatment technique consisting of slurrying the coal in the electrolytes used under open-circuit conditions resulted in insignificant (4% ash-free basis, H2SO4 electrolyte) to no ( - 10% ash-free basis, NaOH electrolyte) enhancement in coal conversion under subsequent liquefaction step. Enhancement in electrochemically pretreated coal conversion over corresponding conversion for coal that was slurried in NaOH or H2S04 under open-circuit conditions is also reported in Table 1. Significant enhancement in coal conversion is observed for coal electrolyzed in NaOH solution. Solubility of coal in benzene was used as a measure of the extent of coal hydrogenation. Solubility in benzene and pentane is a measure of the "oil" produced. The pentane-insoluble, but benzene-soluble fraction is called asphaltenes. Asphaltenes are high molecular weight intermediate products in coal liquefaction. Table 2 lists the mass distribution of three liquefied products fractioned into pentane solubles, asphaltenes, and benzene insolubles. The

226

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TABLE 2 Fractionation of liquefied products obtained at 375 ° C. Experimental conditions as in Table 1 Sample

Pentane soluble

Asphaltenes (%)

Benzene insolubles

36.10

50.84

13.06

50.30

42.90

6.90

57.00

35.50

7.60

(%)

1. Coal liquids obtained at 375°C from untreated coal (UC) 2. Liquids obtained from liquefaction of coal electrolyzed in 1 M NaOH at 1.3 V vs. SCE and 60°C (BC) 3. Liquids obtained from liquefication of coal electrolyzed in 1 M H2SO4 at 1.0 V vs. SCE and 60°C (AC)

(%)

products represent three liquefied coals: (i) untreated (UC), (ii) pre-electrolyzed in NaOH base catalyzed (BC), and (iii) pre-electrolyzed in H2SO4 acid catalyzed (AC). The primary observation from this data is that the pretreated coal produced a liquefied product containing substantially larger mounts of the more desirable pentane soluble fraction, while the undesirable benzene insoluble fraction content decreased significantly. Size exclusion chromatography data for the various fractions was also determined (Table 3 ). The number average molecular weight (Mn) and the weight average molecular weight (Mw) are reported in Table 3. The dispersivity, D, is defined as: MwM~ '. D is an index for the homogeneity of the sample. IfD = 1.0, then all molecules of the sample have the same size. A notable observation is the significantly lower molecular weights of the liquids (BC and AC ) obtained by liquefication of the electrolyzed coal samples in NaOH and H2SO4 solutions, as compared to the molecular weights of the liquids (UC) obtained by liquefication of an untreated coal. The dispersivity data indicate that as compared to UC liquid fractions, the AC and BC liquid fractions contain a wide variety of different size molecules. Proton NMR spectra were obtained for various fractions and classified by using the system described in Table 4. The results of this classification are listed in Table 5. The notable difference between the liquid samples of the pentane-soluble BC and UC fractions are as follows. The former shows very significant enhancement in H F and HA+OH levels (22% and 23%, respectively ), while there is a significant decrease in Hy and Hz levels (32 % and 27 %, respectively). It appears that the base-electrolyzed coal sample upon liquefaction yields a liquid product which is more aromatic. Similar, although less pronounced, trends are observed when the liquid samples of the pentane-soluble AC fractions are compared. When the corresponding asphaltene fractions of BC, AC and UC samples are compared, the following observations can be made.

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TABLE 3 Size exclusion chromatography data for liquid fractions. The data for molecular weights is corrected for symmetrical band broadening and skew. Experimental conditions as in Table 2 Samples Pentane solubles UC BC AC Asphaltenes UC BC AC Benzene insolubles (silylated) UC BC AC Overall molecular weight data UC BC AC

Mn

Mw

Dispersivity

224 83 93

576 383 616

2.6 4.6 6.6

648 162 290

892 68O 784

1.4 4.2 2.7

371 1012 757

2691 2286 2200

7.3 2.3 2.9

459 180 213

1013 641 793

2.2 3.6 3.7

UC: Coal liquids obtained at 375°C from untreated coal. BC: Liquids obtained from liquefaction of coal electrolyzed in 1 M NaOH at 1.3 V vs. SCE and 60°C. AC: Liquids obtained from liquefaction of coal electrolyzed in 1 M H2S04 at 1.0 V vs. SCE and 60°C. TABLE4 NMR Shift classification Hydrogen type

Symbol

Chemical shift (ppm)

Aromatic Phenolic Ring-joining methylene CH3, CH2 and CH (c~ to an aromatic ring) CH3, CH2 and CH (fl or farther from aromatic ring + paraffinic CH2 + CH ) CH3 (y or farther from aromatic ring+paraffinic CH3

HA Ho~ HF Ha Hp

6.0-9.0 5.0-9.0 3.4-5.0 1.9-3.4 1.0-1.9

H~,

0.5-1.0

P r e t r e a t m e n t l e a d s t o l o w e r l e v e l s o f HA+OH. T h e H a , H p a n d H~ l e v e l s a r e enhanced for the AC sample. This may indicate enhanced hydrocracking. The d a t a f o r a l i p h a t i c - t o - a r o m a t i c H r a t i o s a r e a l s o p r e s e n t e d ( T a b l e 5 ). T h e p e n -

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TABLE 5 Proton NMR distribution~ (%) of products obtained at 375°C. Experimental conditions as in Table 3 Type of proton

Coal liquid UC

BC

AC

6.0 22.9 29.1 7.7 34.2

7.2 28.7 32.4 2.4 29.3

4.9 23.3 31.9 9.1 30.7

6.6 27.3 34.4 6.0 25.8

Pentane solubles

H~, Hz Ha HH HA+OH

8.9 31.2 29.7 2.4 27.8 Asphaltenes

H~ Hp Ha HH HA+OH

5.7 21.1 32.1 9.4 31.7

Product

Aliphatic-to-aromatic ratios uc

Pentane solubles Asphaltenes

2.59 2.39

BC 2.14 2.60

AC 2.62 3.25

aObtained using Varian VXR-300 MHz NMR spectrometerwith CDC13as solvent. tane-soluble BC sample has a lower aliphatic-to-aromatic H ratio than the corresponding ratio of UC and AC samples. However, the asphaltene fractions of the pretreated coals (BC and AC) have significantly higher aliphatic-toaromatic ratios t h a n the corresponding ratio of the untreated coal sample. The elemental analysis of various coal samples was carried out and data on atomic C / H ratios and sulfur content are shown in Tables 6 and 7. It is interesting to note t h a t electrolysis in N a O H (Table 6) resulted in a significant sulfur removal from coal (33%). A considerable amount of sulfur removal (27%) is also observed when the coal is contacted with NaOH under open circuit conditions. These results are in agreement with previous findings [10]. The C / H (atomic) ratios of the base-treated and electrolyzed coals are lower t h a n the corresponding ratio of the untreated (raw) coal. However, upon liquefaction, the C / H ratio of the untreated coal decreased, while t h a t of the treated coals increased very significantly and was found to be greater t h a n the corresponding ratio of the solid residue of the liquefied coal (untreated). The data permit the following speculation. Electrolysis results in coal depolymerization due to the nucleophilic attack of OH - on Ar- ( C=O ) - R moieties leading

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TABLE 6 Elemental analysis of coal subjected to caustic (NaOH) treatment. Experimental conditions as in Table 1 Coal sample

Sulfur (% )

C/H (atomic ratio )

Reduction in sulfur content a

(%) A 1. Raw (untreated) 2. Liquefied (untreated) B 1. Electrolyzed 2. Liquefied (electrolyzed) B 1. Stirred (open-circuit) 2. Liquefied (stirred)

5,27 4.40 3,52 2.82 3,85 3.06

1.08 1.05 0.99 1.47 1.00 1.22

16.5 33.2 46.5 27.0 41.9

a( ~0S . . . . . . 1--% Scoal)/(~0 S . . . . . . 1).

TABLE 7 Elemental analysis of coal subjected to acidic (H2S04) treatment. Experimental conditions as in Table 1 Coal sample

Sulfur (% )

C/H (atomic ratio )

Reduction in sulfur content a

(%) A 1. Raw (untreated) 2. Liquefied (untreated) B 1. Electrolyzed 2. Liquefied (electrolyzed) C 1. Stirred (open-circuit) 2. Liquefied (stirred)

5.27 4.40 4.40 3.11 4.14 2.97

1.08 1.05 0.98 1.09 1.06 1.17

16.5 16.5 41.0 21.4 43.6

a(• S . . . . . . 1--% Scoal)/(~o S . . . . . . 1).

to the formation of carboxylic acids. However, carboxylic acids are alkali-soluble, thus the electrolyzed coal residue is richer in hydrogen than the parent coal. The depolymerized coal residue is more amenable to liquefaction in the presence of a hydrogen donor solvent. It is also possible that mild electroxidation of coal can result in enhanced coal liquefaction yields, presumably due to the formation and decomposition of carbonyl or hyperoxide groups from the benzylic groups of alkyl bridges in the coal structure. Therefore, the resultant residue obtained after liquefaction is poorer in hydrogen as indicated by the higher C/H ratio. The C/H ratios of the pretreated coals and the solids obtained upon liquefaction for experiments conducted in HeSO4 follow the same trends (Table 7) as explained earlier for the coal samples subjected to caustic pretreatment.

27.75 40.00 41.75 43.8/42.25/44.5 43.3/41.0/44.0/45.0 55.73/57.8/52.00/57.39

21.25 32.50 43.25 54.98/47.25/48.75 51.86/47.25 49.50

--23.4 - 18.8 3.6 16.0 14.4 - 11.2 62.50 64.5/68.5 62.34 65.09

-

(%)

70.0/78.7/71.25 81.5/78.0 71.75/69.75 71.20

Pretreated coal conversion, (%)

U n t r e a t e d (raw) coal conversion,

Enhancement, (%)

Untreated (raw) coal conversion, (%)

Pretreated coal conversion, (%)

Liquefaction at 400 ° C ~

Liquefaction at 375 ° C b

aTime measured since the reactor attained the desired reaction temperature. bConversion is defined as: [ (mass c o a l ) i n - (mass coal)out ] / ( m a s s coal)i..

0 15 30 60 85 120

Liquefaction time ( min ) ~

17.3 19.9 13.5 9.4

-

Enhancement, (%)

Coal conversion yields: Kinetic studies. Ten grams of coal were electrolyzed ( 1.0 V vs. SCE ) in 375 ml of i M N a O H for 2 h at 60 ° C. T h e pretreated coal (4 g) was liquefied in 120 ml of tetralin under initial hydrogen pressure of 140 psig

TABLE 8

i

5~

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A number of experiments reported below were carried out to further study the influence of electrolytic pretreatment on coal liquefaction. W h e n the electrolyzed coal was liquefied at temperatures below 375 ° C, no enhancement in coal conversion was observed. The influence of the time and temperature of liquefaction on coal conversion is shown in Table 8. At short reaction times, for experiments conducted at 375 ° C, there was no enhancement in conversion of the electrolytically treated coal. The lower initial conversions could be attributed to the absence of pyrite, which is removed during the pretreatment step. It is known that pyrite and mineral matter in coal are good catalysts for liquefaction. However, with an increase in the reaction time, enhancement in coal conversion was observed. It appears that enhancement in coal conversion reaches a maximum, and then declines with further increase in reaction time. Similar trends were observed when the electrolyzed coal was liquefied at 400 ° C. A maximum enhancement in coal liquefaction yield of about 20 % was observed for a reaction carried out for 60 min at 400 ° C. The temperature dependence of the enhancement in coal conversion due to electrolytic pretreatment can be attributed to the relatively high temperature needed for the softening of the coal. For short reaction times, the observed decrease in conversion of the electrolytically treated coals could be ascribed to the relative absence of oxygen functionalities (ketones, particularly) and other labile moieties that may be responsible for the higher observed conversion of the untreated coal. The electrochemically pretreated coal, unlike the raw coal, has a great portion of its mineral matter removed. Mineral matter, as mentioned before, is known to act as a catalyst for coal liquefaction. In light of this fact, lower conversion yields for the pretreated coal are not surprising for short reaction time. Coal electrolyzed in H2S04 was also liquefied at 400°C for various reaction periods. Enhancement in coal conversion of the order of 18% was observed for the reaction time corresponding to 30 min. However, for longer reaction times, there was no enhancement in coal conversion that resulted from the electrolytic (H2SO4) treatment. CONCLUSIONS

Electrochemical pretreatment of coal results in a significant amount of sulfur removal from coal. Liquefaction of the electrolyzed coal in tetralin results in enhanced conversion with respect to that obtained for an untreated coal. Preliminary data obtained by fractionation of liquefaction products and size exclusion chromatography clearly indicate that electrolytic pretreatment prior to coal liquefaction results in enhanced product quality.

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