Properties and reactivities of solid residue from coal electrolysis

Fuel Processing Technology, 21 (1989) 125-134 Elsevier Science Publishers B.V., AmsterdAm-- Printed in The Netherlands

125

Properties and Reactivities of Solid Residue from Coal Electrolysis SACHCHIDA NAND 1, HAMMAM EL-ABD 2, ROBERT W. COUGHLIN 3 and SHASHI B. LALVANI Department of Mechanical Engineering and Energy Processes, Southern Illinois University, Carbondale, Illinois 62901 (U.S.A.)

ABSTRACT

Although electrolysis of Illinois No. 6 bituminous coal at 1.4 V vs. SCE for 4 hours in an acidic electrolyte did not substantially 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 electrolyte 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 H2 pressure to filterable liquid was about 13% greater than for the parent coal.

INTRODUCTION Electrolysis of coal previously reported [ 1-3 ] to reduce t h e cell voltage necessary to liberate h y d r o g e n from water has more recently been f o u n d [4,5] to remove significant a m o u n t s of sulfur from t h e coal as well. Moreover, this t r e a t m e n t can produce dilute acid a n d remove other non-organic m a t t e r from t h e coal, especially w h e n acidic electrolytes are used. In addition to hydrogen gas a n d an electrolysis residue of reduced sulfur content, t h e electrolytic treatm e n t also produces a significant q u a n t i t y of low molecular weight material [47 ] which can be solvent extracted from the electrolyte a n d from the solid residue. We report here f u r t h e r investigation of coal electrolysis residues by therm o g r a v i m e t r y a n d liquefication experiments. In addition to providing more f u n d a m e n t a l i n f o r m a t i o n a b o u t the effects of electrolysis, these results also indicate t h a t a wide range of chemical products can be produced by electrolytic t r e a t m e n t of coal [ 7,8 ]. 1The Fertilizer Association of India, New Delhi 110, 067, India. 2Dept. of Chemical Eng., King Abdul Aziz Univ., Jeddah, P.O. Box 9027, Saudi Arabia. 3Dept. of Chemical Eng., Univ. of Connecticut, Storrs, CT, U.S.A.

0378-3820/89/$03.50

© 1989 Elsevier Science Publishers B.V.

126

A number of chemical methods [9] for cleaning and pretreating coal are known. Only in few instances [ 10,11] have thorough chemical investigations been made to study the effects of such chemical treatment on the coal residue. EXPERIMENTAL

Coal was electrolyzed in a Pyrex cell described elsewhere [4 ] and also shown schematically in Fig. 1. The cell is divided into two sections: (a) an anode compartment (350 ml) where powdered coal is kept in suspension in the electrolyte by a magnetic stirrer; a Luggin capillary filled with saturated KC1 extends into this compartment from a saturated calomel reference electrode (SCE). A thermometer is also inserted in the electrolyte. (b) a cathode compartment separated from the anode compartment by a porous glass frit that confines the coal particles to the anode compartment. Both electrodes are platinum mesh. Samples of Illinois No. 6 coal in pulverized form were obtained from the U.S. Department of Energy, Pittsburgh Energy Center and were ground further to the mesh size desired. Ultimate analysis of the coal is given in Table 1. Samples (15-20 mg) of coal or coal residue after electrolysis were pyrolyzed in a stream of nitrogen (60 ml/min. ) using a Du Pont 990 thermogravimetric balance. The heating rate was 50 ° C/min. and the final temperature 800 ° C. Similar samples were liquefied in a 1000 ml stainless steel, magnetically stirred autoclave using 200 ml of tetralin and 25 g of coal. After leak testing

I

i =

F

- - 4

5

ii

--6

7

Fig. 1. Coal electrolysis cell. 1 Calomel reference electrode, 2 Pt working electrode, 3 condenser, 4 cathode compartment, 5 thermometer, 6 Pt counter electrode, 7 glass frit.

127 TABLE 1 Elemental analysis of Illinois No. 6 coal (dry basis) Element

Weight %

Carbon Hydrogen Nitrogen Oxygen (diff.) Sulfur Ash

63.2 4.8 1.2 13.4 3.8 13.6

and purging with nitrogen, the autoclave was pressurized with hydrogen and heated to 375 °C over a period of 40 minutes during which the pressure rose from 1000 psi to around 2000 psi (70-140 bar). Pressure and temperature were then held constant for an hour while the reaction mixture was stirred at 1500 rpm. Thereafter, the mixture was cooled by circulating water through a coil immersed therein. The contents of the reactor were filtered through W h a t m a n No. 541 paper to separate the insoluble residue which was then dried and weighed. Liquefication conversion was computed as Conversion = ( W°~-o - We) × 1 0 0 where W ° is the dry weight of solid at the onset of the experiment, and Wc is the weight of the filtered and dried solids after liquefaction. All masses and conversions are reported on an ash-free basis. RESULTS AND DISCUSSION

Table 2 summarizes the electrolysis experiments and the results of pyrolysis experiments are shown in Figs. 2-8 as rate of weight loss against temperature. For unreacted coal Fig. 2 shows a rapid evolution of volatile products which begins around 400 ° C, peaks sharply at 550 °C and drops off very rapidly above 500 ° C. For most coals the most important pyrolytic reactions occur between 400 ° C and 600 ° C, the range of so-called active thermal decomposition of coal [12]. Rate of weight loss becomes constant at 700°C and remains approximately at the same level up to 800 ° C, the final temperature of pyrolysis. Pyrolysis between 700-800 °C is accompanied by secondary reactions of char and released gases [ 12 ].

Experiments in acidic electrolytes Figure 2 also shows the weight loss curve for the coal which has been contacted with a mixture of HC1 and H2SO4, but without any electrolysis. It is

128 TABLE 2 Electrolysis of Illinois No. 6 coal" Experiment No.

Initial coal mass, g

Electrolyte

Anode potential, V vs. SCE

Current mA

15%HCl + 5% H2S04 15% HCl+5% H2S04 9% HC1 15% HCl+5% H2S04 15% HCl+5% H2S04 15% HCl+5% H2S04 15% HCl+5% H2S0a

open circuit b 1.0 1.4 1.4 2.0 2.0¢ 3.0

-100 1,000 1,300 3,000 550 3,100

2 M NaOH 2 M NaOH 2 M NaOH

open circuit 1.4 3.0

-300 3,000

Electrolysis in acidic electrolytes 1 2 3 4 5 6 7

6.05 12.69 11.15 13.76 12.95 6.18 12.71

Electrolysis in basic electrolyte 8 9 10

13.02 12.61 13.03

aCoal slurries (0.036 g/ml) of Illinois No. 6 (79-105/~m) were electrolyzed in a stirred reactor at 75°C and Pt-mesh anode (4 cm2) for a period of 4 hours. bCoal was stirred in electrolyte for 8 hours with no external applied voltage. cCoal was electrolyzed at 30 oC. 9

4 2

0 100

20o

3oo

40o

50o

66o

760

soo

Temperature I°C

Fig. 2. Rate of pyrolysis weight loss vs. temperature; o untreated Illinois No. 6 coal, A coal washed with 15% HCl+5% H2SO4for 8 h at 75°C. e v i d e n t t h a t this t r e a t m e n t causes no a p p r e c i a b l e c h a n g e in p y r o l y s i s b e h a v i o r c o m p a r e d to t h e u n t r e a t e d coal. I n c o n t r a s t , t h e residues f r o m acidic electrolysis display a r a p i d w e i g h t loss at t e m p e r a t u r e s as low as 2 5 0 ° C (Figs. 3 a n d 4). I n fact, t h e r e are two w e i g h t loss m a x i m a in t h e rate curve, one at 3 0 0 ° C a n d t h e o t h e r at 550 ° C. T h e m a x i m u m at 550 ° C, also o b s e r v e d in t h e u n r e a c t e d coal, b e c o m e s less p r o n o u n c e d a f t e r electrolysis. T h e m a x i m u m ob-

129

R

121-% ~o]

/ ] /

t

, i b, {

~m

¥ 12.

/!

~1o

§8

'i

"6 4

IO0

4

2 ~- ....

260 ~60 ~60 ~00 660 750 860

Temperature, °C

.o.- -~

26o 35o 46o 500 600 700 soo Temperature,

°C

Fig. 3. Rate of pyrolysis weight loss vs. temperature; o u n t r e a t e d Illinois No. 6 coal; ~ coal electrolysed (1.4 V vs. S C E ) in 9% HCI for 4 h a t 75°C.

Fig. 4. Rate of pyrolysis weight loss vs. temperature; o untreated Illinois No. 6 coal, ~ coal electrolysed (1.4 V vs. SCE) in 15% HCl+9% H2S04 for 4 h at 75°C, ~ coal electrolysed (2.0 V vs. SCE) in 15% HCl+9% H2S04 for 4 h at 75°C.

served at lower temperatures can be ascribed to devolatilization of the lighter fragments formed during electrolytic oxidation of the coal, and to decomposition of carboxylic groups formed by the oxidation. This maximum is more pronounced for electrolysis in a mixture of acids (HCI and H2SO4) than for electrolysis in HC1 alone. Higher reaction rates are thought to occur in the acid mixtures thereby forming larger quantities of lighter products. The total weight losses are respectively 36.9, 44 and 38.8% for unreacted coal, residue from electrolysis in the acid mixture and that from electrolysis in HC1. Although there is no appreciable difference in the total weight loss at lower potentials, the thermograms do show that more of the weight loss occurs at lower temperatures for the electrolyzed residue. When the anodic potential and hence the rate of electrochemical reaction is increased, the pyrolysis weight loss maximum at 300°C becomes more pronounced as shown in Fig. 4. The pyrolysis weight loss of the electrolysis residue is plotted vs. applied potential in Fig. 5 where it is seen that while the total weight loss initially decreases with the potential, it increases as the potential is raised from 1.0 to 2.0 V vs. SCE and then remains constant from about 2.0 to 3.0 V vs. SCE. Beyond a potential of about 2.0 V vs. SCE, there was no further increase in the electrolytic current (Table 2, experiment nos. 5 and 7) and this suggests that the process might have been transport-limited at higher potentials. We believe that the observed decrease in weight loss at lower potentials could be due to the dissolution of lighter fragments which could have contributed to the volatile content of the coal had they been not dissolved

130

50_

40rm

u~ ~ 30-

.~

20-

. j

10-

0.0 1,0 2.0 AppLied potentla[,

3.0

V vs. SCE

Fig. 5. Pyrolysis weight loss vs. potential. Illinois No. 6 coal was electrolysed in 15% HCl+9% H2S04 for 4 h at 75°C; o weight loss between 200-400°C, [] weight loss between 400-700°C, ~ total weight loss. o~ E 10-

% 8-

_o

6

3 4 O 2

0 IO0

250

360

l

4()0 5()0 6()0 700 800 Temperature, °C

Fig. 6. Rate of pyrolysis weight loss vs. temperature; a coal electrolysed (2.0 V vs. SCE) in 15% HCl+9% H2SO4 at 30°C for 4 h, [] coal residue obtained by Soxhlet extraction (equivolumetric benzene + ethanol) of the same electrolysed coal.

away. The increase in weight loss at higher potentials may be attributed to the various oxygen functionalities (such as carboxylic compounds) which evolve readily during thermal treatment. Various oxygen functionalities have been identified on electrolyzed coal surface [13 ]. There is a sharp increase in the pyrolysis weight loss between 200-400ac and a correspondingly smaller decrease in weight loss between 400-700 °C as the potential is raised from 1.0 to 2.0 V vs. SCE. After an electrolysis coal residue was Soxhlet extracted with an equivolumetric mixture of benzene and ethanol, the pyrolysis rate maximum at 300 ° C was suppressed as shown in Fig. 6. It can be seen also that the solvent extraction process removes a significant amount of volatile material from the electrolysis residue. G C - M S analysis of solvent extracts of electrolyzed coal residue detected the presence of anthracene, naphthalene, phenanthrene etc.; data

131

regarding product analysis and the enhancement of solubility of electrolysis residue in the solvent used is provided elsewhere [5]. It is likely that these compounds were formed during electrolysis and are, at least, in part responsible for the weight loss observed in the temperature range of 200-400 ° C. Untreated coal and electrolysis residue were also compared regarding their susceptibility to liquefication in tetralin at 375 °C and about 1500-2000 psi hydrogen pressure. The extents of liquefication observed were 56.5, 62.8 and 63.6% respectively for untreated coal and for the residues from electrolysis at 1.4 and 2.0 V vs. SCE in acidic electrolytes. The observed 13% increase in coal liquefication due to electrolysis is explained as below. Researchers have shown that coal electrolysis results in coal depolymerization. These results were obtained by extracting the coal with organic solvents [ 5 ] and it was shown that the electrolyzed coal is more soluble than the raw parent coal. Thus, the electrolyzed coal residue is more reactive than the raw coal. Our data on devolatilization of an electro[:~ed coal and a Soxhlet-extracted electrolyzed coal residue (Fig. 6) also support this hypothesis. We believe that electrolysis somehow results in the cleavages of C-C, CO-C, and C-S-C scille bonds, thus making the resulting structure more amenable to liquefication.

Experiments in basic electrolytes Residues from coals electrolyzed in aqueous NaOH also show some increase in the pyrolysis weight loss in the temperature range of 150-400 ° C. As is evident from Fig. 7, however, this increase is small and there is no pronounced maximum in this temperature range. This behavior suggests that the tom-

12-

E

/

_ 10

/

,"'i I

'

g~ 2-

1oo 260 300 460 ~So 660 760 8So Temperature,

°C

Fig. 7. Rate of pyrolysis weight loss vs. temperature; o u n t r e a t e d Illinois No. 6 coal, ~ coal electrolysed (1.4 V vs. S C E ) in 2 M N a O H at 75°C for 4 h, o coal electrolysed (3.0 V vs. S C E ) in 2 M N a O H at 75 ° C for 4 h.

132

pounds formed as a result of electrochemical oxidation are soluble in the alkaline electrolyte. The sharp pyrolysis rate maximum at 550 °C observed for the untreated coal is greatly reduced by merely washing the coal as shown in Fig. 8. It can be seen from Fig. 7 that the effect of electrolysis in NaOH is similar but more pronounced regarding the 550 °C maximum. Untreated coal shows a weight loss of 29.5% in the temperature range of 400-700°C and the residue of the coal electrolyzed in base at 1.4 V vs. SCE looses only 18.5% of its weight in the same temperature range. The total amounts of volatile matter (weight loss up to 800°C) are 36.8, 34.0, 30.0 and 40.5% respectively for untreated coal, coal washed with NaOH, coal residue from electrolysis in NaOH at 1.4 V vs. SCE and from electrolysis in NaOH at 3.0 V vs. SCE. It appears that NaOH dissolves a small part of volatile matter. Mild electrolysis produces additional compounds soluble in NaOH, thus leaving a coal with lower volatile content. Electrolysis in basic electrolyte produces a solid residue which shows lower pyrolysis weight loss than the residue from acidic electrolysis under comparable reaction conditions. This may be attributed generally to the greater solubility of oxidized carboxyl-rich molecular fragments in alkaline solutions [ 7 ]. It is interesting to compare the results of products obtained from pyrolysis of raw coal, coal electrolyzed in HC1 at 1.4 V vs. SCE and coal electrolyzed in NaOH at 1.4 V vs. SCE (Table 3). The content of low-molecular weight fragments obtained, such as hydrogen, carbon monoxide, carbon dioxide is higher for the electrolyzed coal samples. Electrolysis results in a coal which has low sulfur content [ 5 ], this fact is borne out by the reduced levels of sulfur dioxide, hydrogen sulfide and thiophene in the gases observed by the pyrolysis of coal (Table 3). The influence of potential on pyrolysis weight loss of the electrolysis residue

-

'c

/ q

12-

/ /

E

/

~- 100 t I J

x

4

.0"

~00

250

36o 450

s60

Temperaturej

650

75o

86o

°C

Fig. 8. Rate of pyrolysisweightlossvs. temperature;o untreatedIllinoisNo. 6 coal, ~ coalwashed with 2 M NaOH at 75°C for 4 h.

133 TABLE 3 Pyrolysis products of coal and electrolyzed coal* Product

Untreated Illinois No. 6 coal

Coal electrolyzed at 1.4 V vs. SCE in 9% HC1 {Expt. No. 3)

Coal electrolyzed at 1.4 V vs. SCE in 2 M NaOH (Expt. No. 9)

Hydrogen Carbon monoxide Methane Carbon dioxide Ethylene Acetylene Ethane Hydrogen sulfide Carbonyl sulfide Propylene Propane Water Sulfur dioxide Benzene Toluene Xylenes Indene Napthalene Phenol Cresols Dimethylphenols Thiophene Methylthiophenes Dimethylthiophenes Benzothiophene Methylbenzothiophenes

15.8 74.4 32.8 27.0 10.7 1.0 3.9 2.4 0.7 3.4 0.8 21.7 5.5 2.26 2.12 1.18 0.37 0.34 1.17 2.58 1.33 0.15 0.31 0.19 0.18 0.21

16.4 94.3 24.1 30.6 6.3 0.2 2.1 2.0 0.8 2.3 0.5 15.8 3.4 2.68 2.32 1.29 0.35 0.43 1.30 2.27 0.99 0.12 0.30 0.16 0.16 0.17

25.1 94.0 21.6 71.6 7.1 -2.7 1.2 0.4 2.3 0.7 29.3 0.2 1.89 2.02 1.22 0.31 0.39 1.58 1.71 0.64 0.10 0.27 0.15 0.15 0.15

*Products obtained by pyrolyzing coal at 850 °C for 10 s are reported on the basis of mg product per g of coal sample. Experimental conditions are as in Table 2.

is m o r e c o m p l e x , b u t t h e b e h a v i o r s e e m s to be p a r a l l e l for b a s i c a n d acid elect r o l y t e s , viz. a d e c r e a s e in w e i g h t loss a t lower p o t e n t i a l s a n d a n i n c r e a s e in w e i g h t loss a t h i g h e r p o t e n t i a l s , c o m p a r e d to t h e p a r e n t coal. T h e d e c r e a s e a t lower p o t e n t i a l s m a y be a t t r i b u t e d to d i s s o l u t i o n o f low m o l e c u l a r w e i g h t fragm e n t s w h i c h w o u l d h a v e c o n t r i b u t e d to t h e volatile c o n t e n t if t h e y h a d rem a i n e d p a r t o f t h e i n s o l u b l e residue. T h e i n c r e a s e in p y r o l y s i s w e i g h t loss obs e r v e d for residues of e l e c t r o l y s i s a t h i g h e r p o t e n t i a l s c a n be a s c r i b e d to t h e f o r m a t i o n o f large p o p u l a t i o n s of c a r b o x y l i c g r o u p s o n t h e large residue molecules.

134 CONCLUSIONS 1. During pyrolysis, the residue from coal electrolysis in acidic medium was generally observed to have a higher concentration of volatile matter t h a n the parent raw coal at higher electrode potentials. Electrolysis fosters an increase in volatile matter liberation in the temperature range of 200-400 ° C. 2. Reaction of the original coal and the residue obtained by electrolysis in tetralin at 375 ° C and 1500 psi of hydrogen caused up to 13% more liquefication of the electrolysis residue as compared to the untreated coal. 3. As compared to the raw coal, electrolysis renders coal more soluble in a mixture of benzene and ethanol. ACKNOWLEDGMENTS We t h a n k the Koppers Corporation for financial support. One of us (SBL) acknowledges support from the U.S. D e p a r t m e n t of Energy and the American Chemical Society - - Petroleum Research Fund.

REFERENCES 1 2 3 4 5 6 7 8 9 10

11 12 13

Coughlin,R.W. and Farooque, M., 1979. Fuel, 48: 705. Coughlin,R.W. and Farooque, M., 1979. Nature (London), 279: 301. Coughlin,R.W. and Farooque, M., 1980. Ind. Eng. Chem. Process Des. Dev., 19: 211. Lalvani,S.B., Pata, M. and Coughlin,R.W., 1983. Fuel, 62: 427. Lalvani,S.B., Nand, S. and Coughlin,R.W., 1985. Fuel ProcessingTechnology,11: 25-36. Lalvani,S.B. and Coughlin,R.W., 1985. Fuel ProcessingTechnology,11: 37-46. Lalvani,S.B. and Coughlin,R.W., 1986. Fuel, 65: 122-128. Murphy,O.J., Bockris,J.O'M. and Later, D.W., 1985. Int. J. HydrogenEnergy, 10: 453-474. Huettenhain, M., Ruby, J.D. and Yim, Y.J., 1980. Evaluation of Chemical Coal Cleaning Processes.Bechtel Report submitted to the U S DOE, July. Hsu, G.C., Kalvinskas,J.J.,Gangli, P.S. and Gavlas, G.R., 1977. In: Wheelock, T.D. (Ed.), Coal Desulfurization:Chemical and Physical Methods, ACS Syrup. Series,Vol. 64, pp. 206217. Kalvinskas,J.J.and Rohtagi, N., 1981. Coal Processing Technology, a C E P Technical Manual Published by AICHE, pp. 141-143. Probstein,R.F. and Wicks, R.E., 1982. Synthetic Fuels, McGraw-Hill, New York, NY, pp. 95. Lalvani, S.B., 1987. Investigationsof Anodically Oxidized Coal, Div. of Fuel Chemistry, American Chemical Society,preprints of papers presented at Denver, Colorado, April,pp. 107-17.