Dynamic behaviour of sulfur forms in rapid pyrolysis of coals with alkali treatment

FuelVol. 74 No. 12, pp. 1823 1829, 1995

0016-2361(95)00179-4

Copyright ~) 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/95/$9.50 + 0.00

Dynamic behaviour of sulfur forms in rapid pyrolysis of coals w i t h alkali t r e a t m e n t Katsuyasu Sugawara, Keiko Abe, Takuo Sugawara, Yoshiyuki Nishiyama* and Mark A. Sholest Division of Materials Process Engineering, Mining College, Akita University, 1- 1 Tegata Gakuen-cho, Akita City, 010 Japan *Institute for Chemical Reaction Science, Tohoku University, 2- 1- 1 Katahira, Aoba, Sendal 980 Japan tDepartment of Geological Engineering, Montana Tech of the University of Montana, Butte, MT 59701, USA (Received 13 October 1994; revised 24 February 1995)

Three kinds of subbituminous and bituminous coals with added potassium hydroxide were heated at 523 K in a nitrogen stream to transform thermally stable organic sulfur to reactive species. Extents of total sulfur removal were 27-52% during the course of alkali treatment, while weight loss was 8 13%. The extent of total sulfur removal was linearly proportional to the internal surface area of the parent coal. The parent coals and alkali-treated samples were pyrolysed rapidly in a free-fall reactor in a nitrogen stream at 1233 K. Under these conditions the alkali-treated samples lost more organic sulfur than did the parent coals. The observed changes in the content of sulfur forms were successfully simulated kinetically. The combined process of rapid pyrolysis with alkali leaching was effective for reduction of organic sulfur, except for a highrank coal of small internal surface area. (Keywords: coal desulfurization; alkali treatment; rapid pyrolysis)

Coal is an abundant and low-cost energy resource in many areas and coal consumption is increasing rapidly in developing countries. However, air pollution and acid rain from the emission of SOx and NOx damage the environment, and flue gas desulfurization and denitrification processes cannot be applied in small coal-fired units used in m a n y developing countries. Efficient precleaning technology must be developed for the utilization of high-sulfur and low-grade coals. Conventional desulfurization processes generally fall into two groups: physical methods for removal of pyritic sulfur by flotation or magnetic separation, and chemical methods for removal of organic sulfur and small pyrite grains embedded in the organic matrix I. Biological treatment can be used for pyritic sulfur removal, and bacteria that effectively remove organic sulfur have been recently found 2. Molten caustic leaching 3 6, wet oxidation by l~otassium permanganate 7 and microwave irradiation ° have also been reported as chemical cleaning methods. O f the m a n y chemical desulfurization methods reported, only the T R W Gravimelt and C S I R O molten caustic leaching processes have been conducted on a pilot plant scale. In these processes sodium hydroxide or potassium hydroxide and coal are mixed and heated to 523-688 K 9. Potassium hydroxide has been reported to be a more effective leachant than sodium hydroxide for high-rank coals 4.

A series of studies on chemical coal cleaning processes has demonstrated the dynamic behaviour of sulfur forms during hydropyrolysis at 0 . 1 - 3 M P a , temperatures up to 7 7 3 K and heating rates of 1 0 - 1 0 0 K m i n l using fixed beds l° 12 and at 0.1MPa, temperatures up to 1233 K and rapid heating (6000 K s - l ) using a freefall pyrolyser ~3. These studies showed that rapid pyrolysis effectively removed organic sulfur from the solid phase in some types of coal. The release rate as well as the extent of organic sulfur removal increased with the contents of exinite and vitrinite 14. Part of the organic sulfur exists in thermally stable form which is difficult to remove from the solid phase by pyrolysis 2H2. Thermally stable organic sulfur remaining in the solid should be changed to decomposable forms to achieve efficient desulfurization in the pretreatment process. In the present study, three kinds of subbituminous and bituminous coals were treated with potassium hydroxide solution under milder conditions than those of Gravimelt and C S I R O processes, to transform the thermally stable form of organic sulfur to reactive species before pyrolysis. The alkali-treated samples were then pyrolysed rapidly in a nitrogen atmosphere in a free-fall reactor. Changes in sulfur-form distribution and extent of devolatilization were measured sequentially and simulated kinetically.

Fuel 1995 Volume 74 Number 12

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Dynamic behaviour of sulfur forms in rapid pyrolysis of coals with alkali treatment." K. Sugawara et al. Table 1 Analyses of coals used (wt% db) Sulfur forms C

H

N

S

Diff.

VM

Ash

Pyritic

Sulfate

Organic

Muswell Brook (Australia) PSOC1493 (USA)

71.1 70.1

5.5 5.4

1.9 1.1

0.7 3.1

20.8 20.3

41.1 40.3

12.5 14.9

0.07 0.39

0.02 0.46

0.57 2.20

Nan Tong (China)

73.6

4.0

0.5

4.1

17.8

17.0

17.3

1.40

0.04

2.70

Parent Coal ]

I

Immersion in K O H aq. soln.] Room Temp., 24 h ,)

Filtration Alkali immersedcoal I Heat treatment ] 523K, 4 h, N2 stream) Washing Drying

Table 2 basis)

Analyses of alkali-treated coal (wt%, alkali-treated coal Sulfur forms C

H

S

Muswell Brook 72.8 5.5 0.4 PSOC1493 68.3 5.1 1.7 Nan Tong 75.5 4.3 3.3

VM

Ash Pyritic

34.5 13.8 0.02 31.3 17.4 0.14 18.4 18.3 0.89

Sulfate Organic 0.05 0.06 0.09

0.37 1.47 2.32

applying the Dubinin-Polanyi equation to CO2 adsorption isotherms at 273 K. A scanning electron microscope was used to observe the physical structure of the coals and chars.

[Alkali treated coal [ RESULTS AND DISCUSSION

I

Rapid pyrolysis] 1233K, N2 stream J

[ Pyrolysischar ] Figure 1 Flowsheet of experimental procedure

EXPERIMENTAL Table 1 shows proximate, ultimate and sulfur-form analyses of the coals used. The particle size of the samples was 0.3-0.4 mm. Figure 1 shows a flowsheet of the experimental procedure. The coal samples were immersed in a saturated aqueous solution of potassium hydroxide for 24 h with agitation at room temperature. The wet alkaliimmersed samples were then heated at 523 K for 4 h in a nitrogen atmosphere after filtration from potassium hydroxide solution. The alkali-treated samples were washed repeatedly with distilled water to reduce the basicity and then dried at 380 K. The parent coals and their alkali-treated products were pyrolysed rapidly in a nitrogen atmosphere in a free-fall reactor, which was a fused silica tube (i.d. 36 mm), the temperature of which was controlled by an electric furnace composed of five sections, each 30 cm long and controlled independently. This apparatus enables coal particles in nitrogen gas (at 2.0 dm3min -1) to be heated at rates from 103 to 104 K s- 1. Particle residence time was varied by changing the length of the heated section. The temperature was 1233 K in the isothermal section of the reactor. Details of the free-fall pyrolyser have been described elsewhere ~3. Some improvements to the ASTM D2492 method and the Gladfelter and Dickerhoof method were used in the analysis of the forms of sulfur, as reported previouslyl°. Internal surface areas of parent and alkali-treated coals were obtained by

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Fuel 1995 Volume 74 Number 12

Pretreatment by alkali Table 2 shows ultimate, proximate and forms of sulfur

analyses for the alkali-treated samples. A weight loss of 8-13 % was observed after leaching. The carbon content increased but the hydrogen content was almost constant for each sample. Volatile matter decreased for Muswell Brook and PSOC1493. The conventional molten-caustic leaching process showed effective reduction of minerals in addition to a decrease in volatile matter and hydrogen ~5 content at a leaching temperature of ~673K . Ashforming minerals were not removed, owing to the low treatment temperature and alkali concentration. Chriswell et al. 4 reported that the sulfur concentration generally decreased with increasing leaching temperature. Extents of total sulfur removal were 42, 52 and 27%, respectively, for Muswell Brook, PSOC1493 and Nan Tong coals in the alkali treatment, the percentage sulfur removal being defined as 100 (sulfur in parent coal-sulfur in alkali treated sample)/(sulfur in parent coal). The results show a greater decrease in inorganic sulfur than in organic sulfur, the percentage removal of inorganic and organic sulfur forms being, respectively, 75 and 44 for Muswell Brook, 70 and 41 for PSOC1493, and 41 and 21 for Nan Tong. Previous molten-caustic leaching studies also showed greater removal of inorganic sulfur than of organic sulfur 15. According to XANES analysis of sulfur forms in char obtained by the molten-caustic leaching process, a considerable amount of sulfur was changed from thiophene in a parent coal to sulfate and sulfoxide in the char 16 . The poorer removal of organic sulfur for Nan Tong coal may be due to predominence of the chemically and thermally stable thiophenic sulfur in this high-rank coal. Figure 2 shows a strong correlation between the amount of total sulfur removed and the internal surface area of the coals. The desulfurization extent increased linearly with increasing internal surface area. The large internal porosity would provide easy access for

Dynamic behaviour of sulfur forms in rapid pyrolysis of coals with alkali treatment." K. Sugawara et al. Table 3

Kinetic parameters determined for volatile matter release

M uswell Brook

PSOC 1493

Nan Tong

Parent

Alkali-treated

Parent

Alkali-treated

Parent

Alkali-treated

1500 42

700 42

2000 46

3000 46

15000 59

4200 59

kv0 (s- l ) E (kJ mol-1)

60

i

i

3

,

PSOC 1493 ©

50

O

E

40

©

40

"W-----'-----F~

"7,

Muswell Brook

30 O Nan Tong

20

>

20 ok

10 0

I

0

I

I

I

40

I

80

120

Internal surface area [m2/gl Figure 2 Relation of total sulfur removed in alkali treatment to internal surface area of coal

0

0.2

0.4

0.6

0.8

t R [S]

Figure 4 Experimentalresults and simulatedcurves for volatilematter release of alkali-treated coals as a function of residence time in rapid pyrolysis

50 equations of m o m e n t u m and heat balance for a particle and volatile matter release rate. Assuming the overall process is a first-order decomposition with Arrheniustype dependence of rate constant on temperature, the volatile matter release rate is expressed as

40 "7

30 >

20 [~ff] Muswell Brook PSOC 1493 Nan Tong

10 0

0

0.2

0.4

I

I

0.6

0.8

tR [S]

Figure 3 Experimentalresults and simulated curves for volatilematter release of parent coals as a function of residence time in rapid pyrolysis potassium hydroxide into the coal particle during the alkali leaching step. The p o o r removal of both organic and inorganic sulfur for N a n Tong coal may be due to the low internal porosity as well as the existence of a stable organic sulfur form. The alkali leaching increased the internal surface area (m 2 g - l ) from 59 to t24 for Muswell Brook, from 101 to 111 for PSOC1493, and from 12 to 37 for N a n Tong.

Rapid pyrolysis Volatile matter release. Figure 3 shows the volatile yield, defined by weight loss based on the parent coal, against particle residence time in the free-fall pyrolyser. The m a x i m u m volatile yields range from 35 to 5 0 w t % for the three coals and are inversely proportional to the carbon content of the parent coal. Solid lines in the figure are simulated values obtained by solving simultaneous

d V / d t = kvo e x p ( - E / R T ) ( V * - V) where V is the cumulative amount of volatiles produced up to time t, and V* is the effective amount of volatiles for the coal. The details of the simulation have been described previously 17. The pre-exponential factors and activation energies determined are listed in Table 3. Figure 4 shows the change in volatile yield for alkalitreated samples against the residence time in the reactor. The solid lines in Figure 4 are simulated values. The residence time of alkali-treated coal particles was so long for PSOC1493 that only two data points are shown in Figure 4. The long residence time for alkali-treated samples was due to the decrease in average particle diameter from 0.34mm for the parent coal to 0.22mm after alkali treatment. Although the proximate analysis of PSOC1493 shows smaller amounts of volatile matter for the alkali-treated sample than for the parent coal, the ultimate volatile yield of the alkali-treated sample was almost the same as that of the parent coal shown in Figure 3. The same ultimate volatile yields were obtained for parent and alkali-treated samples of N a n Tong coal. The ratio of ultimate volatile yield V in the rapid pyrolysis of parent coal to volatile matter obtained by proximate analysis Vp is 1.2 for Muswell Brook, 1.2 for PSOC1493 and 1.9 for N a n Tong. For the alkali-treated samples, the increase in the ratio V/Vp, 1.3 for Muswell Brook, 1.6 for PSOC1493 and 2.0 for N a n Tong, shows the greater effect of heating rate on volatile yield for

Fuel 1995 Volume 74 Number 12

1825

Dynamic behaviour of sulfur forms in rapid pyrolysis of coals with alkafi treatment: K. Sugawara et al.

(a)

[]

(b)

m

lOOt~ m Figure 5 SEM photographs of parent coal, alkali-treated sample and rapid pyrolysischars for PSOC1493: a, parent coal; b, rapid pyrolysischar of parent coal; c, alkali-treatedsample; d, rapid pyrolysischar of alkali-treated sample

alkali-treated samples than for the parent coals. It has been reported that the molten-caustic leaching process yields less volatile matter and causes the volatile generation peak to shift to higher temperature than that for the parent coal 15. The present study shows a higher release rate for alkali-treated PSOC1493 coal but a lower release rate for alkali-treated Muswell Brook and Nan Tong coals. Figure 5 illustrates SEM photographs of the parent coal, an alkali-treated sample and rapid pyrolysis chars for PSOC1493. Many fissures were observed in the alkali-treated material, as shown in Figure 5c. When the parent coal was pyrolysed rapidly, swelling was observed, with 40% increase in particle size, development of large pores and appearance of material covering the particle. In contrast, the alkali-treated sample shows no swelling behaviour, indicating a reduction of the precursor of fluid material before the rapid pyrolysis. It was observed that the alkali-treated sample was fragmented during the initial heating stage. The average particle diameter decreased from 0.34 to 0.22mm at a

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Fuel 1995 Volume 74 Number 12

residence time of 0.27 s. There was no appreciable change in particle diameter during pyrolysis of either parent coal or alkali-treated samples of Muswell Brook coal. Nan Tong coal showed significant swelling, as the particle size doubled during the pyrolysis of both parent and alkalitreated coals. The alkali leaching was ineffective in reducing the swelling capacity, as in the desulfurization, for Nan Tong coal.

Sulfur-form distribution Figure 6 shows sequential changes in the distribution of the forms of sulfur for Muswell Brook and its alkalitreated product versus the distance X from the coal hopper or the residence time t R of the particle in the reactor. The sulfur contents of the tar and gaseous products are given for specified treatment times t R. For the parent coal shown in Figure 6a, the total sulfur content in the solid phase (indicated by circles) decreased rapidly from 0.66 to 0.29 wt% within 0.33 s and attained a constant value corresponding to the release of volatile matter. The ultimate extent of organic sulfur removal

Dynamic behaviour of sulfur forms in rapid pyrolysis of coals with alkali treatment. K. Sugawara et al. was 60% in rapid pyrolysis of the parent coal. The alkalitreated sample showed a rapid decrease of sulfur and 94% ultimate organic sulfur removal. Hence the combined process of rapid pyrolysis and alkali treatment was very successful in reducing organic sulfur in Muswell Brook coal. Thiophenic sulfur is a major component of the organic sulfur in coal, its proportion increasing from 55 to >90% with increasing carbon content from 75 to 92wt% daf for Argonne Premium coals 18. Calkins 19 reported that thiophenic compounds showed a low conversion of <3% at 1223K whereas aliphatic and aromatic sulfides, mercaptans and disulfides decomposed with high conversion at <1173K. The alkali treatment in the present study transformed the organic sulfur to readily decomposable species. The solid lines in Figure 6 show simulated values obtained by solving simultaneous equations of momentum and heat balance for a free-fall particle with a material balance of the sulfur forms based on the desulfurization scheme 13 in Figure 7. The pre-exponential factors determined for each reaction step are listed in Table 4 with activation energies from Yergey et al 2°. By comparing the frequency factors for the parent coal with those for the alkali-treated samples, it can be seen that the alkali treatment reduced the rate of step 1 (conversion of organic sulfur in the original coal into refractory organic sulfur in the char), and increased the rate of step 3 (liberation of organic sulfur to gas). Figure 8 shows sequential changes in the distribution of the forms of sulfur for (a) the parent coal and (b) the alkali-treated sample of PSOC1493. Organic sulfur was reduced by 86% relative to the parent coal, with a large portion going into the tar (Figure 8a). For the alkalitreated sample Figure 8b shows that the desulfurization reaction was almost complete within tR = 0.27 s and that a large part of the organic sulfur was released to the tar. The ultimate extent of organic sulfur removal was 86% for the alkali-treated sample, the same as for the parent coal, even though the sulfur content of the alkali-treated sample was half that of the parent coal. The preexponential factors (k20 q- k30) of steps 2 and 3, liberation of organic sulfur from solid to gas phase for the alkalitreated sample, were greater than those for the parent coal by a factor of 2. Sequential changes in the distribution of the forms of sulfur for parent coal and alkali-treated sample of Nan Tong coal are shown in Figure 9. Rapid pyrolysis of the raw coal results in rapid release of organic sulfur (within 0.23s), followed by no appreciable change with continued heat treatment (Figure 9a). Nan Tong coal shows the lowest ultimate extent of desulfurization (38%) for organic sulfur. The simulation indicates that the sulfur in the gaseous products orginated from reduction of pyrite rather than liberation of organic sulfur. The preexponential factor k30 for step 3 is zero. All the volatile organic sulfur moved to tar during rapid pyrolysis. Figure 9b shows sequential changes of sulfur forms for the alkali-treated sample. The organic sulfur was gradually removed with increasing residence time. The ultimate extent of organic sulfur removal was 43%, less than for Muswell Brook or PSOC1493. This suggests that thermally stable organic sulfur existing in the parent coal was not effectively transformed, because of insufficient permeation of potassium hydroxide into the coal matrix with small internal surface area.

(a)

tR [s] 0.25 0.33

8I

i Pyritic

1

0.57

0.68

0.86

i

i

i

Sulfur

6

,~ 4

e0

~2 0 (b) 0.27

0.38

0.56

0.76

0.89

4

~2

8O

100

120

140

160

180

200

X [cm] Figure 6 Experimental results and simulated curves for change in sulfur-form distribution with residence time t R or distance X from coal hopper for Muswell Brook: a, parent coal; b, alkali-treated coal

2

(So~)co~ 1 / ~

(Sorg)~r (Sorg)ch~

"-k

° H zS

FeS2 FeS Figure 7

Desulfurization scheme

Figure 10 compares the organic sulfur contents of the parent coal, the alkali-treated sample and the char obtained by rapid pyrolysis after alkali treatment. The combined process of rapid pyrolysis with alkali treatment was effective in reducing organic sulfur contents of 0.57 and 2.2wt% in Muswell Brook and PSOC1493 parent coals to 0.04 and 0.44wt% in the respective chars. For Nan Tong coal, however, desulfurization by the combined process was not so effective: 2.7 wt% of organic sulfur in the parent coal was reduced only to 2.3 wt% in the char. It would be necessary to permeate the coal particles with the alkali solution thoroughly and to raise the treatment temperature above 523K to promote conversion of the stable forms of organic sulfur in this instance.

Fuel 1995 Volume 74 Number 12

1827

Dynamic behaviour of sulfur forms in rapid pyrolysis of coals with alkali treatment." K. Sugawara et al. Table 4 Pre-exponential factors determined for desulfurization a Muswell Brook

N a n Tong

PSOC1493

Step i

Parent

Alkali-treated

Alkali-treated

Parent

Alkali-treated

Parent

1

1.5 × 105

4.0 × 104

5.0 × 104

1.0 × 105

5.0 × 105

1.0 × 104

2

1.5 × 105

1.5 × 105

2.0 × 105

4.0 x 105

3.0 x 105

2.0 × 104

3

7.0

104

1.0 × 105

2.0 × 104

6.0

0

0

4

8.0 x 108

8.0 × 108

8.0 × 108

1.0 × 108

8.0 × 108

8.0 X 108

aklo - k 3 0

x

(s-l), k40 [(mgS/g)°5 s-I],

(a)

Ei (i =

x

104

1-3) = 9 2 k J m o l 1, E4 = 176kJmo1-1

t~ [s] 0.26

0.33

0.41

0.48

tR [S]

(a)

0.51

40

0.25

0.33

0.57

0.27

0.38

0.68

0.86

50 30

40' 3o

~ 20 *

E ~

lO

20

d~ Q

0 .~,

"

0 0 (b)

0.27

~

0.66

1.41

2.22

(b)

2.63

20

4O

u 15

-~ 30

0.56

0.76

0.89

8

20 i r~

~, 5 0 80

100

120

140

160

180

200

X [cm]

80

100

120

140

160

180

200

X [cm]

Figure 8 Experimental results and simulated curves for change in sulfur-form distribution with residence time tR or distance X from coal hopper for PSOC1493: a, parent coal; b, alkali-treated coal

Figure 9 Experimental results and simulated curves for change in sulfur-form distribution with residence time tg or distance X from coal hopper for N a n Tong: a, parent coal; b, alkali-treated coal

Figure 11 represents the effects of time and tempera-

and the subsequent heat treatment at 523 K transformed thermally stable organic sulfur to reactive forms. The extent of total sulfur removal during the treatment showed a tendency to increase with the internal surface areas of the parent coal. Sequential changes in volatile yield and distribution of the forms of sulfur were determined quantitatively and simulated kinetically in rapid pyrolysis in a nitrogen atmosphere at temperatures up to 1233 K. Compared with the parent coals, the alkali-treated samples showed greater extent of organic sulfur removal and higher ultimate volatile yields than in proximate analysis. The organic sulfur content was reduced from 0.57, 2.2 and 2.7wt% for the parent coals to 0.04, 0.44 and 2.3wt% for the chars of Muswell Brook, PSOC1493 and Nan Tong coals, respectively. Transformation of the thermally stable organic sulfur to more labile species increased with time in the alkali treatment step.

ture of the alkali treatment on the organic sulfur content in alkali-treated coal and char. Although a treatment time of >2 h (compare C and D) was not effective in reducing organic sulfur in alkali-treated coal, the organic sulfur content of the rapid pyrolysis char decreased remarkably with increased treatment time, probably owing to conversion of thermally stable organic sulfur to other forms which are easily decomposable in rapid pyrolysis. However, as shown at E in Figure 11, treatment at the lower temperature of 473K for 4 h with alkali did not bring about a decrease in the organic sulfur in the pyrolysis char. CONCLUSIONS Alkali treatment of coals of different rank by immersion in a saturated aqueous solution of potassium hydroxide

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Fuel 1995 Volume 74 Number 12

Dynamic behaviour of sulfur forms in rapid pyrolysis of coals with alkali treatment." K. Sugawara et al. 0.6

ACKNOWLEDGEMENTS The authors thank Y. Tozuka, E. Moriyama, K. Nagai, T. Gunji and K. Fukase for their assistance. The Taro Yamashita foundation is thanked for financial support.

0.4

0.2 8

0 3

REFERENCES ""

2 1

1

2 3

o

4

8 5 • -~

1

6 o

Parent coal

A

B

C

Figure 10 Change in organic sulfur content: A, pyrolysis char; B, alkali-treated coal; C, pyrolysis char with alkali treatment

~

r"'n Parent coal Pyrolysis char Alkali treated coal Pyrolysis char with alkali treatment

0.6

7 8 9 10 11 12

0.5 13 0.4 o

14 0.3 15 0.2

"~ 0.1 ©

16

0 A

B

C

D

E

Figure 11 Effect of alkali treatment conditions on organic sulfur content of Muswell Brook coal: A, no treatment, B, 523 K, 1 h; C, 523K, 2h; D, 523K, 4h; E, 473K, 4h. Ordinate indicates contents based on dry coal, alkali-treated coal and pyrolysis char

17 18 19 20

Khoury, D. I. (Ed.). 'Coal Cleaning Technology', Noyes Data Corporation, Park Ridge, NJ, 1981, p. 3 Kilbane, J. J. In 'Bioprocessing and Biotreatment of Coals' (Ed. D. L. Wise), Dekker, New York, 1990, p. 487 Gala, H. B., Srivastava, R. D., Rhee, K. H. and Hucko, R. E. Coal Prep. 1989, 7, 1 Chriswell, C. D., Markuszewski, R. and Norton, G. A. In 'Processing and Utilization of High Sulfur Coals IV' (Eds P. R. Dugan et al.), Elsevier, Amsterdam, 1991, p. 385 Nowak, M. A., Fauth, D. J. and Knoer, J. P. In 'Processing and Utilization of High Sulfur Coals IV' (Eds P. R. Dugan et al.), Elsevier, Amsterdam, 1991, p. 399 Anastasi, J. L., Barrish, E. M., Coleman, W. B., Hart, W. D., Jones, J. F., Ledgerwood, L., McClanathan, L. C., Meyers, R. A., Shih, C. C. and Turner, W. B. In 'Processing and Utilization of High Sulfur Coals III' (Eds R. M. Markuszewski and T. D. Wheelock), Elsevier, Amsterdam, 1990, p. 371 Attia, Y. A. and Lei, W. In "Processing and Utilization of High Sulfur Coals II', (Eds Y. P. Chugh and R. D. Caudle), Elsevier, Amsterdam, 1987, p. 202 Boron, D. J. and Kollrank, R. Min. Eng. 1986, 38(2), 120 Gordon, C. R. "Advanced Coal Cleaning Technology', IEACR/ 44, lEA Coal Research, London, 1991, p. 67 Sugawara, T., Sugawara, K. and Ohashi, H. Fuel 1988, 67, 1263 Sugawara, T., Sugawara, K. and Ohashi, H. Fuel 1989, 68, 1005 Sugawara, K., Tozuka, Y., Sugawara, T. and Nishiyama, Y. Fuel Process. Technol. 1994, 37, 73 Sugawara, T., Sugawara, K., Nishiyama, Y. and Sholes, M. A. Fuel 1991, 70, 1091 Sugawara, K., Tozuka, Y., Kamoshita, T., Sugawara, T. and Sholes, M. A. Fuel 1994, 73, 1224 Mcllvried, T. S., Smouse, S. M. and Ekmann, J. M. In 'Processing and Utilization of High Sulfur Coals III', (Eds R. M. Markuszewski and T. D. Wheelock) Elsevier, Amsterdam, 1990, p. 379 Huffman, G. P., Vaidya, S. A., Shah, N. and Huggins, F. E. Am. Chem. Soc. Div. Fuel Chem. Preprints 1992, 37, 1094 Sugawara, T., Sugawara, K., Sato, S., Chambers, A. K., Kovacik, G. and Ungarian, D. Fuel 1990, 69, 1177 George, G. N., Gorbaty, M. L., Kelemen, S. R. and Sansone, M. Energy Fuels 1991, 5, 93 Calkins, W. H. Energy FueLs' 1987, 1, 59 Yergey, A. L., Lampe, F. W., Vestal, M. L. and Synderman, J. S. Ind. Eng. Chem. Process Des. Dev. 1974, 13, 233

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