FUEL

ELSEVIER Fuel Processing Technology 46 (1996) 143-155

PROCESSING TECHNOLOGY

Supercritical desulfurization of high rank coal with alcohol/water and alcohol/KOH

Wen Li*, Shucai Guo Institute of coal chemical engineering, Dalian University of Technology, Dalian, 116012, PR China

Received 30 March 1994; accepted 22 August 1995

Abstract

A high rank coal with total sulfur of 4.90% was extracted employing alcohol/KOH and alcohol/water under supercritical conditions both in a semi-continuous reactor and in a batch reactor. In the semi-continuous reactor it was found that supercritical desulphurization is mainly taking place within about one hour at 400 “C. Pretreatment of coal with KOH up to 5% concentration was favourable for sulfur removal, but greater KOH concentration and longer soaking time brought about the opposite results. When the ethanol concentration was 95 ~01% the organosulfur removal achieved the maximum. Ethanol/KOH solution as supercritical solvent enhanced the desulfurization process, in which the inorganic sulfur was removed preferentially. In the batch reactor it was found that there was reincorporation of both organic sulfur and inorganic sulfur. KOH addition can improve the sulphur removal greatly. When KOH/coal ratio was greater than 0.5, the tendency for sulfur removal was gradually slow.

Keywords: High rank coal; Supercritical desulfurization; Alcohol/water; Alcohol/KOH

1. Introduction

China is rich in coal resources. It is the dominant fuel for industry, rail transport, and household use. For total energy consumption in 1992, more than 80% of coal was used for direct combustion [l]. Using coal as a major source of energy has been a long-term policy in China. It is reported that more than 20% yields of Chinese coals are high sulfur coal containing > 2% sulfur [a]. The emission of SO, is 18 million tons every year, of which about 90% is from coal [3]. Acid rain resulting from SO2 has a harmful effect on agriculture and destroys the ecological balance.

Concern over environmental effects attributed to acid rain has resulted in a variety of legislative proposals to address the problem. The existing technology that serves as

* Corresponding author. Present address: Institute of coal chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, 030001, Shanxi, PR China.

0378-3820/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0378-3820(95)00057-7

144 W. Li, S. GuolFuel Processing Technology 46 (1996) 143-155

a yardstick for economic comparison of any proposed approach to contend with the high sulfur coal problem is the post combustion approach-flue gas desulfurization. Although the effectiveness of this approach has been demonstrated, several inherent problems exit with this technique: high capital and operating cost, reduced availability of the power generating plant, and production of significant volumes of waste sludge that, depending on plant location, can present a formidable disposal problem. Thus, interest has grown in recent years in development of pre-combustion desulfurization processes.

Supercritical fluid extraction of coals has previously been reported as a medium for selective desulfurization of coal [4]. Supercritical fluids have unique capability to greatly enhance the solubility of organic compounds. Alcohols are expected to exhibit greater solubility for polar organic molecules due to hydrogen bonding and dipole attractive forces, also providing the opportunity for chemical reactions during the extraction due to the nuecleophilicity of the alcohol oxygen and the tendency to act as a hydrogen donor. This suggests that the application of a supercritical desulfurization process following a physical treatment process removal of pyritic sulfur could produce a solid fuel meeting environmental requirements.

Most work [5-81 on supercritical desulfurization of coal in the literature has as a primary objective the maximum percentage of total sulfur removal. The coals used are mostly low rank and the reactors used have been microautoclaves. Using a semi- continuous reactor, Lee et al. [9] studied the effect of the composition of meth- anol-water mixtures on the removal of sulfur. The removal of organic sulfur and the reaction mechanism through a model compound (dimethyl disulfide) were investig- ated by some workers [l&12]. The over all objective of this study was to gain a better understanding of a high rank coal desulfurization process with supercritical alcohol. The desulfurization of nitric acid treated coal containing only organic sulfur was specifically studied. Using a semi-continuous system, desulfurization of organic sulfur, effect of reaction time, and KOH treatment on sulfur removal were examined. Using a batch reactor, the effect of reaction time and KOH/coal ratio on sulfur removal were studied in detail.

2. Materials used

The coal used in this study was a high rank Fenxi coal obtained in Shanxi province, North China. Typical properties of this coal are listed in Table 1. A - 24 mesh particle size was used in the semi-continuous reactor and - 80 mesh in the batch reactor.

For some experiments, the coal was treated with KOH. In these experiments, 8Og of coal particles were soaked in 240g of KOH in ethanol solution with a given concentration for 10 min, filtered, washed twice with 240g of ethanol and filtered again before its use. Samples without inorganic sulfur were prepared in light of ASTM D-2492 [13]. The XPS (X-ray Photoelectron Spectroscopy) was used to examine the presence of elements in the treated coal. No iron was found (see Fig. 1). It can be concluded that after this treatment, that the inorganic sulfur was completely removed. Proximate and ultimate analyses of the treated coal are listed in Table 2. As

W. Li, S. GuoJFuel Processing Technology 46 (1996) 143-155 145

Table 1 Proximate and ultimate analysis of the Fenxi coking coal

Proximate analysis (% dry basis) Ultimate analysis (% daf basis)

Ash 12.9 Volatile Matter 18.9 Fixed carbon 68.2

Sulfate sulfur (% dry) 0.02 Pyrite sulfur (% dry) 1.96 Organic sulfur (% dry) 2.92 Total sulfur (X dry) 4.90

Carbon Hydrogen Sulfur Nitrogen Oxygen by diff.

Calorific value (bomb, dry) 21832 kJ/kg

81.92 4.73 5.63 1.32 0.40

40000 I I L

Cl+ 30000-

_ C(KLL) 01s

O(KVVI m t,

c

3

IUOO 500 BINDING ENERGY CeV3

Fig. 1. XPS analysis of element in coal treated by 1 : 7 HN03.

Table 2 Proximate and ultimate analysis of HNOa treated coal

Proximate analysis (dry basis), % Ultimate analysis (daf basis), %

Ash 11.6 Carbon 85.97 Volatile matter 17.2 Hydrogen 4.81 Fixed carbon 71.2 Sulfur 3.30

Nitrogen 1.35 Sulfur (dry) 2.92 Oxygen by diff. 3.57

146 W. Li, S. GuofFuel Processing Technology 46 (1996) 143-155

anticipated, oxygen content was increased compared to the original coal due to the oxidation effects of HN03.

3. Experimental

3.1. Semi-continuous reactor studies

The flow chart for the semi-continuous reactor has been previously reported in elsewhere in detail [14]. The experiments were initiated by charging a pre-weighed sample of dry coal (about SOg) into the extractor. The solvent was delivered to the extractor at the rate of 1 l/h with the exit from the reactor closed to permit build-up of the desired supercritical reaction pressure and temperature. The extraction pressure was controlled by an expansion value. After the required extraction time, the extractor presssure relieved entraining the solvent.

3.2. Batch reactor studies

Batch experiments utilized a 100 ml autoclave reactor. A flow chart of the reactor is given in Fig. 2. The reactor system was attached to an automatic shaker supported above a platform stand. The shaker allowed the autoclave to be agitated during reaction to ensure uniformity of reaction temperature. The heating oven was temper- ature controlled, using a thermocouple and an on-off temperature controller to ensure temperature stability.

1 Reector

2 Oven

3 Extract receptor

4 Gas receptor

5 Level vessel

6 Vent

7 Supporting equipment and shaker

Fig. 2. Schematic diagram of batch reactor system used in the supercritical desulfurization system.

W. Li, S. GuolFuel Processing Technology 46 (1996) 143-155 147

The reactor was charged with coal (5 g) and alcohol (20 g). Two pressured flushings with nitrogen were carried out to ensure the removal of oxygen after the autoclave had been sealed. Following reaction, the furnace was withdrawn and the reactor was cooled in a water bath to the room temperature. The gas product was collected by slowly opening the metering valve, Gases were analyzed using gas chromatography. The solution in the autoclave was separated from the residue by filtration.

The extracted coals were analyzed for total sulfur. The forms of sulfur and elemental composition were measured on selected products. The proximate and ultimate ana- lyses were performed using Leco MAC-400 proximate analyzer and 1106 ultimate analyzer, respectively. The sulfur forms were determined following the standard ASTM D-2492.

4. Results and discussion

4. I. Semi-continuous reactor results.

4. I. 1. EfSect of reaction time The extraction temperature was kept at 400°C. Extraction time was up to 6 h. The

results are presented in Fig. 3. The conversion indicated at time zero on the plot represent extraction and/or reaction that occurs during the pre-heating time to reaction temperature.

The sulfur removal versus time curve can be studied in two portions: a sharply increasing part and an almost constant part. In the first part the total sulfur re- moval increased sharply from 9.8% to 33.1% within 1 h. The sulfur in the organic compounds in the coal reacts with ethyl alcohol to form new compounds which were

Extraction time, h

Fig. 3. Effect of extraction time on sulfur removal.

148 W. Li, S. Guo/Fuel Processing Technology 46 (1996) 143-155

Table 3 The effect of KOH pre-treatment on supercritical desulfurization of coal with ethyl alcohol (400 “C, 1 h, and 12 MPa)

KOH concentration (X) Sulfur removal (%, cont. basis)

0 29.6 2 30.8 3 31.8 4 32.1 5 34.5

Table 4 The effect of KOH pre-treament on sulfur removal with increasing KOH concentration and soaking time

KOH concentration (%) 5 10 15 20

Soaking time (h) 0.5 2 4 0.5 4 0.5 Overnight 0.5 2 Overnight sulfur removal (%) 30.2 29.2 28.8 31.8 31.6 29.6 29.6 26.9 26.1 24.1 Vd, % in residue 14.7 14.7 15.0 14.5 14.6 14.9 15.7 15.4 15.5 15.7

removed from the coal with the ethanol during the supercritical extraction. It is possible that bonds in the coal were cleaved both by solvent attack and thermal reaction with lower activation energies due to solvation [S]. In the second part, longer reaction time did not change the sulfur removal significantly, therefore it seemed that the reaction reached a steady state within about 1 h at 400°C.

4.1.2. EfSect of KOH treatment Two kinds of treatment procedures were used in this study to evaluate whether

or not a pre-treatment of coal with KOH will enhance the desulfurization efficiency. (1) Effect of KOH soaking. The data in Table 3 indicates that an improvement of

desulfurization potential was realized with KOH pre-treatment. This may have resulted through depolymerization of the coal structure, enabling greater penetration of the supercritical alcohol.

In order to further understand the effect of KOH, a measure of increasing KOH concentration and prolonging the soaking time was adopted. The results are shown in Table 4.

It came as a surprise that when the KOH concentration was increased above lo%, the total sulfur removal was decreased. In addition, the sulfur removal was decreased with increasing soaking time at a given KOH concentration with little increase in volatile matter of the solid product. The reason for this phenomenon is not very clear at present. It may be deduced from the increase of volatile matter in the solid product that excessive KOH concentration and soaking time might bring about crossqinking between KOH and some organic bonds in coal, so the structure of coal is changed. Another possible reason is that gaseous sulfur can be more easily reincor- porated into the coal during supercritical extraction through this kind of treatment.

W. Li, S. Guo/Fuel Processing Technology 46 (1996) 143-155 149

Table 5 The effect of ethanol/KOH solution on supercritical desulfurization (400 “C, 1 h, and 12 MPa)

Amount of KOH addition (g) Sulfur removal (%, cont. basis)

0 29.6 2.5 44.3 5 41.3

15 53.3 60 59.6

Table 6 Sulfur forms distribution of residues resulted from supercritical desulfurization with ethanol/KOH solution

Amount of KOH addition (g) St. d (%) Sino. d (%) So. d (%)

0 3.27 1.36 1.91 5 2.58 0.97 1.61

60 1.98 0.52 1.46

(2) Supercritical extraction with ethanol/KOH solution. The effect of ethanol/KOH solution as a supercritical solvent was investigated. The ethanol/KOH solution was made up by dissolving different weights of KOH into 1500 ml as ethanol solution. The experimental data are listed in Table 5. It was found that the sulfur removal increased with increasing amounts of KOH. Small amounts of KOH can have notable effect on desulfurization. The sulfur removal with 60g KOH addition doubled the amount removed without KOH. The thermal decomposition and cleavage were more rapid and severe under the attack of ethanol/KOH solution. The production of hydrogen made the radical fragments stable, which were removed from the extractor with the solvent.

In order to know which sulfur forms were more easily removed by the extraction of ethanol/KOH solution, selected solid products were analyzed for sulfur forms distri- bution. The data are presented in Table 6. It is seen that the removal of both organic and inorganic sulfur was increased with KOH addition. When increased amounts of KOH were used, the pyritic sulfur was removed preferentially. For example, a com- parision between 60 g KOH addition and without KOH shows that the concentration of organic sulfur was decreased by 23.6%, however, that of inorganic sulfur was decreased by 61.8%. This suggests that intensive depolymerization of coal occurs when more KOH was used so the pyritic sulfur embedded into fine particles was exposed more directly to the solvent.

4.1.3. Super-critical extraction of coal containing organic sulfur Fig. 4 shows the sulfur removal and weight loss of coal versus reaction temperature,

where the extraction time was 1 h. Both the sulfur removal and weight loss of coal were increased with increasing temperature. The desulfurization selectivity ra- tio (percent of sulfur removal/percent of weight loss) ranged between 6.48 and 11.45.

150 W. Li, S. Guo/Fuel Processing Technology 46 (1996) 143

275 300 350 400

Reaction temperature, %

Fig. 4. Effect of reaction temperature on organic sulfur removal and weight loss of coal.

Table I Supercritical desulfurization result at different ethanol concentrations

Ethanol concentration Sulfur removal Desulfurization Residue Character % (mass basis) selectivity ratio” Vdaf (%) H/C ratio

92.1 56.22 6.48 12.87 95.0 57.80 11.12 12.78 99.5 52.62 11.46 13.01

a Desulfurization selectivity ratio = percent of sulfur removal/percent of coal extracted.

0.578 0.595 0.573

Three kinds of ethanol-water mixtures were used as supercritical solvents under the reaction conditions: 400°C 1 h and 12 MPa to study the organic sulfur removal. The data are listed in Table 7. It is seen that adding water to ethanol was favorable to supercritical desulfurization. The sulfur removal and H/C ratio reached maximum when the concentration of ethanol was 95 ~01%. It is also noticed that the volatile matter in the residues was a function of reaction temperature, and had no relation to the concentration of ethanol. Through the desulfurization of a model compound, Tao et al. [12] claimed out that the sulfur was removed from the organic coal structure by direct reaction with alcohol or its degradation product. The initial reaction product is likely mercaptan, which is then converted to sulfide. In our experiments, because both ethanol and water are polar solvents, the attractive force between them may be favorable to the reaction between sulfur containing groups and solvents.

W. Li, S. Guo/Fuel Processing Technology 46 (1996) 143-155 151

Extraction time, min.

Fig. 5. Effect of extraction time on sulfur removal at different temperatures in batch reactor: (0) 300 “C (A) 350°C (0) 400 “C, EtOH : coal = 4.

4.2. Batch reactor results

4.2. I. Efect of extraction time The reaction temperatures used were 300°C 350°C and 400°C in extraction time

runs. The extraction time was upto 120 min. Fig. 5 shows the total sulfur removal versus time at different reaction temperatures.

As can be seen, at 300°C the sulfur removal did not vary significantly with longer times, but only increased slightly from 30.0% to 32.2%. This indicates that under reaction conditions the desulfurization reaction was mainly performed during the initial 10 min. Since the coal structure was not notably changed at 3Oo”C, the materials which can be extracted were little and the effect of prolonging extraction time on sulfur removal was very limited.

At 400°C the sulfur removal increased during the first 30 min. However, a marked decrease in sulfur removal (34.5%) occurred when the extraction time was 50 min. The sulfur removal rose again (45.7%) in 60 min and appeared steady at longer times. The initial increase in sulfur removal was probably caused by the conversion of pyrite to pyrrhotite and the extraction of some gaseous organic compounds resulted from thermal decomposition of coal. With increasing extraction time, softening of the coal particles took place, during which gaseous sulfur could be reincorporated into the coal by capping of free radicals [15]. This fluid stage occurred due to cleavage of

152 W. Li, S. GuolFuel Processing Technology 46 (1996) 143-155

Table 8 Sulfur forms analyses of desulfurized coals in diferent extraction times at 400°C

Time (min) St. d (%) So. d (%) Sino. d (%)

10 2.92 2.02 0.90 30 2.19 2.01 0.78 50 3.21 2.18 1.03 60 2.66 1.98 0.68

120 2.53 1.94 0.69

cross-linkages (H, S, 0, etc.) during the dehydrogenation/polymerization process. During this stage, the characteristic dip in sulfur removal was observed. With continuously increasing extraction time, the organic molecules began to condense and subsequently develop to anisotropic coke [6,7]. As the disappearance of fluid phase, the coal began to solidify due to polymerization and the pressure increased since the supercritical gas mixture became less soluble. This resulted in the return of the liquid reactants to the vapor phase, therefore sulfur removal increased again.

In order to further understand the phenomenon of reincorporation of sulfur, sulfur forms were determined on the residues obtained at 400°C. Results are shown in Table 8.

It is seen from the data that (1) The extent of inorganic sulfur removal was greater than that of organic sulfur

removal when the total sulfur removal increased with increasing extraction time. (2) The reincorporation of inorganic sulfur involved both organic and inorganic

sulfur. The reincorporation of sulfur might result from the sudden tie-up of solvent by the organic matrix which shifts the equilibrium constant toward a high sulfur level for the iron phase [7].

(3) supercritical desulfurization reaction showed a stable state with longer extrac- tion time.

4.2.2. EfSect of KOH addition Weighed KOH was dissolved in 25 ml of ethanol and the solution was charged to

the reactor with the coal particles. The results are shown in Fig. 6. It is seen that an improvement of desulfurization potential was realized with KOH addition. The presence of KOH brings about the rapid pyrolysis and subsequent depolymerization of coal, which results in a large amount of materials extracted and thus the increase in sulfur removal. When the amount of KOH was more than 0.5 g, the increase of sulfur removal was gradual.

The reaction between ethanol and KOH takes place as follows:

CH3CH20H + KOH + CH$H,OK + HzO,

CH3CH20K + Hz0 + CH,COOK + 2Hz,

(1)

(2)

W. Li, S. GuolFuel Processing Technology 46 (1996) 143-155 153

0 0.1 0. 5 1. 0

The amount of KOH addition., g/g coal

Fig. 6. Effect of KOH addition on supercritical desulfurization with ethanol in batch reactor.

partly

CH3CH20K + CzH4 + KOH. (3)

With increasing KOH concentration a large amount of hydrogen was produced which was absorbed by the coal. The effect of hydrogenation made the radical fragments more stable. In addition, ether linkages (0, S) were split during hydrogen- ation This made the molecules smaller and more soluble in the supercritical fluid. When further KOH addition took place, reaction (2) was moderate, so the sulfur removal did not increase significantly. Makabe et al. [16] claimed that hydrogen involving from the interaction of ethyl alcohol and NaOH play role in reduction and cleavage reactions. Under this reductive condition the weak C-S bonds present in the coal are probably cleaved with ease. Alcohols can transfer hydrogen more easily in the presence of bases. In their ethanol-NaOH system study, when NaOH/coal ratio was greater than 0.5, the ethanol extraction yield remained nearly constant about 30%. Kera and Ceylan [17] discussed the mechanism of the cleavage of C-S bonds of model compounds and came to the conclusion that C-S bonds in lignites might be cleaved by the attack of base, so that some organic sulfur of lignites could be converted to sulfides and/or sulfites at 315-4Oo”C by NaOH treatment. Muchmore et al. [lo] pointed out that after the desulfurization reaction in MEOH/KOH at 350°C aromaticity increases further, and carboxyl and nitrates disappear. It is thought from our results that KOH was more favorable to the cleavage of bonds and the breaking of the side chains away from the coal matrix in the presence of solvent.

In order to know the contribution of KOH addition to removal of sulfur forms, selected solid products were analyzed for sulfur forms, as shown in Table 9. It is seen that KOH addition can reduce the content of both organic and inorganic sulfur. When continuously increasing the amount of KOH, the sulfur removal of organic

154 W. Li, S. Guo/Fuel Processing Technology 46 (1996) 143-155

Table 9 Sulfur forms analyses of residues resulted from ethanol/KOH extraction in batch reactor

Amount of KOH addition (g/g coal)

St. d (%) So. d(%) Sino. d (%)

0 2.66 1.98 0.68 0.25 2.28 1.70 0.58 1 2.02 1.76 0.26

sulfur did not increase, however, the content of inorganic sulfur was lowered signifi- cantly. This indicates that higher concentrations of KOH were favorable to the removal of inorganic sulfur, but had a limited effect on organic sulfur removal. This suggests that very strong bonds exist in coking coal, where even strong bases could not break such bonds. For the removal of pyritic sulfur, Masciantonio et al. [18] claimed a rapid reaction occurs between pyrite and caustic at temperatures near 250°C. It appears that pyritic sulfur is converted to sulfides, which are soluble in the molten caustic. In our experiments KOH reacted with the pyrite isolated from the coal particles, a further reaction with the pyrite located at the edge of the coal particles and the pyrite finely disseminated into organic structures might occur with increasing amount of KOH addition, so the pyritic sulfur was continuously removed.

It is noticeable that the content of organic sulfur in the residue increased slightly when the amount of KOH varied from 0.25 to 1 g. This might be due to the experimental conditions (inorganic sulfur of the pyrite could have diffused into the coal matrix and then become fixed in the form of organic sulfur). This phenomenon was also observed by Ge et al. [ 191, who studied the thermal behavior of sulfur in coal using SEM and TEM. The presence of KOH might be helpful to this conversion process, which needs to be further studied to obtain more direct evidence.

5. Conclusions

The experimental information obtained have led to the following conclusions (1) Supercritical desulfurization in our semi-continuous reactor took place within

1 h. (2) The effect of KOH pretreatment was found to be positive when KOH concentra-

tion was less than 5%; however, greater concentrations and longer soaking times gave the opposite results.

(3) Ethanol/KOH solutions can improve sulfur removal significantly of which the inorganic sulfur is removed preferentially.

(4) The sulfur removal of coal containing organic sulfur reaches maximum when ethanol concentration is 95 ~01% in ethanol-water mixtures.

(5) Sulfur reincorporation occurs at 4OO”C, 50 min. in batch reactor. This process involves the reincorporation of both inorganic and organic sulfur.

W. Li, S. GuoJFuel Processing Technology 46 (1996) 143-155 155

(6) In the batch reactor system KOH addition was favorable to the desulfurization process. When KOH/coal ratio was greater than 0.5, the increasing tendency for sulfur removal was reduced.

References

[l] Coal Industry Mining, China Coal Industry Yearbook 1992,1993, Economic Information & Agency, Hongkong.

[2] Chen Minghe and Li Shilun, Sym. on Coal Utli. Int’L Beijing, 1989, 9 (in Chinese). [3] Dai Hewu and Chen Wenmin, Sym. on Coal Utli. Int’L Beijing, 1989, 196 (in Chinese). [4] Li Wen and Guo Shucai, Coal Conversion, 15(2) (1992) 28 (in Chinese). [S] C.B. Munchmore, J.W. Chen, A.C. Kent et al., ACS Div. Fuel Chem., 30(2) (1985) 24. [6] N. Murdie, E.J. Hippon, W. Tao et al., Fuel Process. Technol., 18 (1988) 119. [7] E.J. Hippon. N. Murdie, J.W. Chen et al., Fuel Process. Technol., 17 (1987) 85. [S] Yuda Yurum and Ayse Tuglhan, Fuel Sci. Technol. Int., 8(3) (1990) 321. [9] S. Lee, SK. Kesavan, A. Ghosh and K.L. Fullerton, Fuel, 68(9) (1989) 1210.

[lo] C.B. Muchmore, J.W. Chen, A.C. Kent and M. Liszka, in: Coal Science and Technology II ~ 1987 International Conference on Coal Science, Maastricht, The Netherlands, 439.

[ll] C.B. Muchmore, J.W. Chen, A.C. Kent and M. Liszka, in: Proceedings of 1989 International Conference on Coal Science, pp. 1933196, Tokyo, Japan, October 1989.

[12] W.L. Tao, E.J. Hippo and C.B. Muchmore, in: Proceedings of 1991 International Conference on Coal Science, pp. 1013-1016 University of Newcastle upon Tyne, UK, September 1620, 1991.

[13] Annual Book of ASTM Standards, ASTM D-2492-80, Philadephia, PA 19103, 1983, 347 [14] Haoquan Hu and Shucai Guo, Fuel Process. Technol., 31(12) (1992) 79. [15] I.C. Lewis and L.S. Singer, in: L. Philip, Walker, Jr. and A.T. Peter, (Eds.), Chemistry and Physics of

Carbon, Vol. 17, pp. l-88, Marcel Dekker, New York. [16] M. Makable, Y. Hirano and K. Ouchi, Fuel, 57 (1978) 289. [17] H. Kara and R. Ceylan, Fuel, 67 (1988) 170. [lS] P.X. Masciantonio, Fuel, 44 (1965) 269. [19] Ge Yongpei et al., J. Fuel Chem. Technol., 20(l) (1992) 90 (in Chinese).