Extraction of coals through alkaline degradation at plastic state under ambient pressure conditions

ELSEVIER Fuel Processing Technology 43 (1995) 147-156

FUEL

PROCESSING TECHNOLOGY

Extraction of coals through alkaline degradation at plastic state under ambient pressure conditions

D.K. Sharma”, S.K. Singh

Fuels and Biofiels Engineering, Laborato y, Centre for Enera Studies, Indian Institute of Technology, New Delhi I 10016. India

Received 19 July 1993; accepted in revised form 20 January 1995

Abstract

A new technique to degrade bituminous coals and lignites using alkali in liquid paraffin at the plastic state of coals has been developed. More than 60% of coal can be rendered extractable.

The technique is very simple and does not involve the use of pressure. The technique could be of interest for structural studies on coal. The technique is also utilized for obtaining ash-free, high calorific value (a clean fuel) from coals and lignites.

1. Introduction

The importance of alkaline degradation in the study of the organic chemical structure of natural products and natural polymers is well known. Some workers have emphasized the use of this reaction in the development of convenient processes for the conversion of coal to value-added fuels and chemicals [l]. However, most of the research work on alkaline degradation of coal has been reported [2-41 under high pressures. The use of high pressure is not convenient and cost-effective for industrial processing. The major aim of the research work on coal conversion in the authors’

laboratory is to carry out conversion of coal at atmospheric pressure conditions.

Research work on alkaline degradation of coal has been reviewed in an earlier

paper

CU.

2. Alkaline degradation of coal in the plastic state

Alkaline degradation of cellulose, lignins and lignites has been reported to be almost complete at elevated temperatures. These elevated temperatures were achieved

* Corresponding author. Fax: +91-l l-686203.

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148 D.K. Sharma, S.K. Singh/Fuel Processing Technology 43 (1995) 147-156

by using high pressure. Haber and Bruner [2] have reported that wood charcoal, coke, graphite, and pure carbon react with sodium hydroxide at 350°C producing Na2C03 and Ha. The use of high pressure in these reactions resulted in the hydrogenation of the product which was solubilized in the reaction medium. Makabe and Ouchi [3] and Chow [4] have also reported the use of high pressure in alkaline degradation of coal. However, the use of hydrogen and high pressure is not desirable for the development of a convenient process for alkaline degradation of coal for enhanced solubilization in common organic solvents. From earlier studies on the alkaine degradation of coal, it was clear that the action of heat and a degrading chemical agent like sodium hydroxide would be required to degrade coal effectively to smaller molecules having enhanced extractability in com- mon organic solvents [S]. Not much work seems to have been reported on the alkaline degradation of coal under atmospheric pressure conditions [S-S]. The object of the present work was to study the alkaline degradation of coal at elevated temperatures without using high pressure. In fact, Assam coal and most of the other coals attain plastic state in the temperature range 320-410°C. At plastic state, coal degrades to free radicals and ions and is in the depolymerized state. Most of the reactions such as hydrogenation and hydropyrolysis of coal are carried out at the plastic state of coal (by applying high hydrogen pressures). During the primary stage of liquefaction of coal, most of the strong bonds such as CC, C-O-C, C-N-C, C-S-C, -C=C-, etc., are cleaved at a temperature of 350°C. The boiling range of liquid paraffin (LP) is 33&35O”C, therefore, if Assam coal is treated with liquid paraffin under reflux conditions, it will attain plastic state even at atmospheric pressure. The melting point of NaOH is 318°C and that of KOH is 338°C;

therefore, the use of these alkalis in boiling LP would allow for a reaction between coal and fused and molten (fluid) alkali at the plastic state of coal. In a preliminary study from this laboratory, the use of alkali in LP has been reported [9, lo] to result in the depolymerization of high molecular weight coal substances which followed the alkylation of coal by liquid paraffin. Modest increases in the extractability of coal in pyridine and in ethylendiamine were achieved even after using a promoter such as potassium tertiary butoxide (PTB). Since the extraction enhancement of Assam coal was only 20-30%, it was evident that the degradation of coal had not taken place to a considerable extent, even though a coal-alkali in the high ratio of 1: 1 was used. In the present work, it was proposed to study the extraction enhancement of coal through successive stepwise alkaline degradation using LP by employing a lower coal-alkali ratio of 4 : 1. Reduction in the amount of alkali used in the reaction would make this reaction interesting for the process development to obtain value-added fuels and chemicals from coal under mostly atmospheric conditions. Preliminary results of this work were presented at the Interna- tional Rolduc Symposium [ 111.

3. Experimental

3.1. Alkaline degradation of coal in the plastic state

Assam coal (9 g), NaOH (9 g), potassium tertiary butoxide (PTB) (0.16 g) and liquid paraffin LP (150 ml) were placed in the two necked round bottom reaction flask fitted

D.K. Sharma, S.K. Singh/Fuel Processing Technology 43 (1995) 1471.56 149

with a reflux condenser and mercury sealed stirrer in central neck for continuous stirring. The reaction mixture was stirred at the boiling temperature of liquid paraffin (330-360°C) under reflux conditions for 6 h.

After the reaction about 200 ml of distilled water was added to dissolve the unused alkali and 50 ml of benzene was added to facilitate the filtration. The coal residue left after the filtration was washed with dilute acid (2% aq. HCl) to remove traces of alkali which may have been sticking to coal and to acidify any 0-Na or COO-Na linkages that might have been formed in coal. Finally, the residue was washed with distilled water up to neutral pH and Soxhlet-extracted with hexane and methanol. The residue was then dried in oven at 110°C. The residual coal obtained was Soxhlet-extracted with quinoline (Qn) for 24 h.

The residual coal, thus obtained after the alkali treatment in liquid paraffin in the first step followed by its Qn extraction was taken as a substrate for the second step of alkali treatment in liquid paraffin under similar conditions as before. The treated coal thus obtained was extracted with Qn as in the first step.

3.2. Extraction yields

The extraction yields were determined by recording the loss in weight of the coal.

This loss in weight was then calculated on the dried mineral matter free basis to determine the extractability of coal on the DMMF basis.

The total extractability of coal after all the alkali treatment steps followed by the Qn extraction steps was calculated from the loss in weight of the starting coal (original coal) after all the treatment and extraction steps. This was then calculated on the DMMF basis.

4. Results and discussion

Table 1 shows the ultimate and proximate analyses of Assam and Talcher coals and Neyveli lignite.

When Assam coal was treated with NaOH in LP using coal-NaOH in 1: 1 ratio at 330-360°C for 24 h, about 26% of the coal was rendered extractable within the reaction medium. The residual coal was found to show 8% extraction in quinoline.

Thus, in all about 35% of the coal was rendered extractable. These extraction yields were almost the same as those obtained in the blank experiment for extraction of Assam coal with LP, followed by extraction of the residual coal in Qn. These results are shown in Table 2. It was decided to use PTB as a promoter in the reaction using coal : NaOH : PTB : LP in the 4 : 1: 0.26 : 68 ratio. The reaction was performed for 24 h and about 16% of Assam coal was rendered extractable within the reaction medium.

The residual coal obtained was found to be extractable in Qn up to 35%. Thus, about 49% of the coal was rendered extractable in all. These extraction results were higher than those obtained without using any promoter under similar conditions (Table 2).

The residual coal obtained after the Qn extraction of the alkali-treated coal in the first step was subjected to alkali treatment in the second step under similar conditions

150 D.K. Sharma, S.K. SinghlFuel Processing Technology 43 (1995) 147-156 Table 1

Analysis of coals and lignites

Coal Moisture Mineral

matter

Volatile matter

Fixed carbon Proximate analysis (on % air dried basis)

Assam 2.8

Talcher 2.1

Neyveh lignite 8.7

8.3 42.0 46.9

16.9 38.8 42.6

6.1 46.6 38.6

Coal Carbon Hydrogen Sulfur Nitrogen Oxygen Atomic

Ultimate analysis (% on dry mineral matter free basis)

Assam 77.0 5.7 3.8

Talcher” 74.8 5.3 1.0

Neyveli lignite 62.0 5.3 1.4

a Could be weathered coal.

Table 2

Alkali treatment of Assam coal in plastic stage (using liquid paraffin) Run

No.

Sample Reactants Solvent Extraction on loss in Total Extraction original coal weight basis on DMMF basis

W) W)

OAC Residue OAC Above residue OAC Above residue Above residue Above residue

LP Qn

NaOH/O LP

Qn

NaOH/PTB LP

Qn

NaOH/PTB LP

- Qn

25 21

I 35

24 26

8 35

16 17

35 49

4 52

25 60

Note: Coal-NaOH-PTB, 12: 3 : 1.

Coal-Solvent, 1: 17.

Reaction time, 24 h.

Qn extraction through Soxhlet extracter.

as employed in the first step. The results obtained are presented in Table 2. About 4%

of the (residual) coal was extracted within the reaction medium. The residual coal obtained was found to show 14% extraction in Qn. Therefore, about 11% was .rendered extractable in the second step and overall, about 60% of the coal was extracted in two steps. Thus, these studies show that it is possible to extract 60% of the coal through alkali degradation of coal at its plastic state employing coal-alkali in 4 : 1 ratio in the presence of comparatively small amounts of the promotor.

D.K. Sharma. S.K. SinghJFuel Processing Technology 43 (199.5) 147-156 151

4.1. IR spectral and elemental analysis of the treated coal obtained using alkali in liquid para& in the presence of PTB

Tables 3 and 4 show IR spectral results of treated coals obtained after using alkali in LP in the presence of PTB. It was observed that the intensity of absorption at 3400-3200 cm-’ (due to OH groups) and 2940-2860 cm- ’ (due to CHJ and CH2 stretching) in the treated coals increased after the first alkali treatment step and these absorptions were found to have been decreased in the treated coals obtained after the second step of alkali treatment of coal. This observation was similar to the one observed earlier in the stepwise alkali treatment in phenol [S]. The reasons for this could be that there are more sufficiently active sites available for hydrolysis and alkaline degradation reactions in the first step than those available in the coal in second step. The aromatic character of coal was found to increase during the alkali treatment of coal. The degree of substitution of aromatic rings in treated coal, as revealed by increase in the intensity of absorption at 750 and 800 cm- ’ was found to have decreased after successive stepwise alkali treatment followed by Qn extraction.

These observations were supported by the IR spectral results obtained by considering the absorptions due to different functional groups in alkali-treated Assam coal with reference to the intensity of absorption of the standard peak of alkali-treated coals around 1600 cm- i.

The atomic H/C ratio of the alkali-treated coals was observed to decrease after the successive stepwise alkali treatment. This showed that the H-content in the coal is diminishing after the stepwise alkali treatment. Although the IR spectral studies had shown an increase in aliphatic character in the coal obtained after the first alkali treatment step, overall, there was a decrease in the H-content. This may be due to the fact that, overall, there is a dehydrogenation of coal (Table 5) as there was also an increase in the aromatic character of coal after successive stepwise alkali treatment of coal in LP.

4.2. Comparison between the alkali treatment of coal in phenol with that in LP The results of the extraction of coal obtained through the two-step alkali treatment in LP were compared with those obtained in the two-step alkali treatment using phenol at its boiling point (18OC) (and using coal-NaOH in’ a 3 : 1 ratio) [8] under similar conditions. It can be seen that the extraction of Assam coal by two-step alkali treatment in LP using coal-NaOH ratio of 4: 1 (in the presence of PTB) was higher (60%) than that obtained through alkali treatment using two-step phenol-NaOH in the 3 : 1 ratio (50%), i.e. employing even larger amounts of alkali) and that obtained through alternate phenol-NaOH and tetralin-NaOH treatments on Assam coal (56%) [S]. Thus, these studies showed that alkaline degradation of Assam coal at the plastic state using PTB as a promoter in LP results in greater degradation of coal. The reasons for this greater depolymerization could be as follows. Coal attains its plastic state in the boiling range of LP. Moreover, coal has been known to undergo extractive disintegration in the boiling range of LP. Even NaOH is melted at 318°C and is in a fluid state and thus gets ionized to react with coal which is in the process of

Table 3 IR spectral results (ca-‘) of the alkali treated coal residue obtained at plastic stage (using liquid paraffin as a solvent and PTB as a promoter) Coal samples Percentage proportion of the intensity of absorption 3600-3100 2940-2860 1890-1710 1600 1460 1370 1030 900 850 750 OAC 15.85 10.36 6.76 40.24 12.80 1.83 0.60 2.27 1.99 OAC+NaOH+PTB+LP+R 29.86 11.46 1.39 40.97 9.38 1.04 1.39 1.22 1.56 1.39 Above residue 2 R 15.18 8.38 5.20 45.03 7.85 2.62 3.66 - - Table 4 IR spectral absorption (cm- ‘) due to different functional of groups in treated Assam coal obtained by using NaOH in liquid paraffin in the presence of potassium tertiary butoxide with reference to the standard absorption peak of alkali-treated coal, i.e. around 1600 cm-’ Coal samples Area of different absorption peaks with respect to that of the absorption peak at 1600 cm- 1 3600~3100 2940-2860 1890-1710 1600 1460 1379 1030 900 850 750 OAC 0.394 0.258 0.167 0.318 0.045 0.015 - 0.061 0.053 OAC+NaOH+PTB+LP-+R 0.559 0.217 0.014 0.140 0.028 0.028 0.063 0.084 0.021 Above residue 2 R 0.342 0.125 0.033 0.183 0.042 0.033 0.033 0.075 0.042 OAC+NaOH+PTB+LP+R 17.904 6.550 1.747 52.402 9.607 2.183 1.747 1.747 3.930 2.180 Above residue + 25.974 10.065 0.649 46.429 1.299 1.299 1.922 2.922 3.896 0.974

D.K. Sharma, S.K. SinghlFuel Processing Technology 43 (1995) 147-156 153 Table 5

Elemental analysis of residual coals obtained after alkali treatment at plastic stage and followed by Qn extraction at each step

Run No.

Sample Reactants Solvent Elemental analysis Atomic

H/C c (%) H (%) N (%) ratio

1 OAC NaOH/PTB LP 15.5 4.8 3.1 0.77

2 Above residue - Qn 74.8 4.5 6.3 0.73

3 Above residue NaOH/PTB LP 78.5 4.9 5.9 0.75

4 Above residue - Qn 11.2 4.4 6.2 0.69

Note: Coal-NaOH-PTB ratio, 12: 3 : 1.

Coal-Solvent ratio, 1 : 17.

Qn extraction through Soxhlet extractor.

disintegration. The action of alkali on coal at this temperature would be similar to that on lignins. Moreover, PTB would create strongly basic conditions in the reaction medium, thus, resulting in alkaline degradation of coal macromolecules. At this state, the action of alkali would result in the cleavage of various C-C, C-O-C, C-N-C, -C-S-C- and -C=C-C=O linkages in the coal structure. Most of the hydrolytic reactions on coal would take place during washing of coal with water to remove unreacted alkali from the coal. Since the coal attains plastic state, alkali gets melted, liquid paraffin starts cracking and PTB increases the basicity of the reaction medium.

Therefore, there could be multimoiety reaction and it would be difficult to understand the dynamics of these complex reactions. The following tentative mechanism can be suggested for these alkaline degradation reactions on coal at its plastic state:

Coal + NaOH + PTB + LP + Extractive disintegrated coal + PTB + NaOH + LP

Alkaline degradation and dehydrogenation

Degraded and dehydrogenated coal.

Talcher coal was also subjected to alkaline degradation for 24 h using coal-NaOH-PTB-LP in 4 : 1: 0.26 : 68 ratio. About 21% of Talcher coal was extrac- ted within the reaction medium (Table 6). The residual coal was found to show 28%

extraction in Qn. Therefore, in all, about 49% of Talcher coal was rendered extract- able. This extraction yield was almost the same as that obtained from the Assam coal

. . . .

(49%) under similar conditions.

In another study, Talcher coal was also subjected to two-step alkali treatment in LP using PTB under similar conditions as used earlier for the alkali treatment of Assam coal. About 65% of Talcher coal was rendered extractable in total (Table 6) after two

154 D.K. Sharma, S.K. Singh/Fuel Processing Technology 43 (1995) 147- I56

alkali treatment steps (in LP in the presence of PTB) followed by Qn extraction at each step. This extraction value (65%) was more than that obtained from Assam coal (60%) under similar conditions. This showed that Talcher coal was more reactive than Assam coal towards the action of alkali in the presence of PTB in LP.

Extractability of Assam coal through alkali treatment in LP (in the presence of PTB) was more in EDA (32%) than in Qn (28%) (Table 7). This showed that EDA was a better solvent for the alkali-degraded coal in LP. However, in the present studies the use of Qn was preferred to EDA, as Qn is generally used as a standard solvent for determining the extractability of coals through chemical reactions.

4.3. Alkaline treatment of Neyveli lignite in LP in the presence of PTB

Neyveli lignite, which had larger amounts of volatile matter, atomic H/C ratio and was more reactive than Talcher and Assam coal (Table 6), was found to result in

Table 6

Alkali treatment of Talcher coal (OTC) and Neyveli lignite (ONL) at plastic stage using liquid paraffin as a solvent and PTB as a promotor

Run Sample Reactants Solvent Extraction on loss

in weight basis (W

Total extraction on DMMF basis W)

1 OTC Qn 25 27

2 OTC NaOH/PTB LP 21 23

3 Above residue - Qn 28 49

4 ONL Qn 16 17

5 ONL NaOH/PTB LP 38 41

6 Above residue - Qn 35 65

Note: Coal-NaOH-PTB ratio, 12: 3 : 1.

Coal-Solvent ratio, 1 : 17.

Qn extraction through Soxhlet extractor

Table 7

Alkali treatment of Assam coal at plastic stage using liquid paraffin as a solvent and PTB as a promoter Run

No.

Sample Reactants Solvent Extraction on loss

in weight basis W)

Total extraction on DMMF basis (%)

OAC NaOH/PTB LP 22 24

Above residue - Qn 28 39

from 1

Above residue - EDA 32 51

from 1

Note: Coal-NaOH-PTB ratio, 18 : 18 : 1.

Coal-solvent ratio, 1: 17.

Qn extraction through Soxhlet extractor.

D.K. Sharma, S.K. Singh/Fuel Processing Technology 43 (1995) 147-156 155

higher extraction yields through alkali treatment in LP in the presence of PTB under similar reaction conditions. These results are shown in Table 6. About 65% of the Neyveli lignite was found to have been extracted through alkali treatment in LP followed by Qn extraction (Table 6). Thus, these studies showed the general applica- bility of the alkaline degradation of coal or lignite in LP in coal technology.

4.4. Alkaline treatment of Assam coal using larger amount of alkali

In another study the coal-alkali ratio for the alkaline degradation of Assam coal using PTB was increased. The reaction was performed for 24 h employing Coal-NaOH-PTB-LP in the ratio 1: 1: 0.26 : 68. The results are shown in Table 7.

About 22% of the coal was extracted within the reaction medium and the residual coal showed 28% extraction in Qn. Thus, overall, about 39% of the coal was rendered extractable. This was only 3% less than that obtained using coal-NaOH in 4 : 1 ratio under similar reaction conditions. This showed that an increase in the alkali concen- tration did not have an appreciable extent of beneficial effect on increasing the extraction of coal.

4.5. Effect of reaction time

In yet another study, the reaction time was reduced to 6 h from 24 h as used in the earlier studies. Assam coal was subjected to alkaline degradation using coal-NaOH-PTB-LP in the ratio 4 : 1: 0.26 : 68. The reduction in the overall extrac- tion through alkali treatment (for 6 h) was only 15% over that obtained by performing the reaction for 24 h. Thus, these studies showed that it was possible to reduce the reaction time to 6 h by sacrificing only a little amount of extraction yields and this reaction could be interesting for process development. Solvent refined coal obtained through alkali treatment can be used as a demineralised clean fuel.

5. Conclusions

The following conclusions may be drawn on the basis of the work described in this paper.

(1) Alkaline treatment of coal or lignite in liquid paraffin in the presence of potassium tertiary butoxide offers an interesting technique to render more than 60%

coal extractable in organic solvents. This technique may be used for the study of the organic chemical structure of coal.

(2) About 60% of Assam coal and 65% of Talcher coal was rendered extractable through two alkali treatment steps in liquid paraffin (in the presence of PTB as a promoter), followed at each step by quinoline extraction.

(3) Since about 65% Neyveli lignite was rendered extractable through a single alkali treatment step in liquid paraffin in the presence of PTB promoter, this showed that lignite was more reactive towards alkali in liquid paraffin.

156 D.K. Sharma, S.K. SinghlFuel Processing Technology 43 (1995) 147- I56

(4) There is a scope to reduce (a) the amount of alkali used in the reaction and (b) the reaction time. This technique may be interesting to develop a clean coal techno- logy to obtain a solvent refined coal (an ash-free high calorific value fuel).

(5) The residual coals obtained [8] after the extraction of coal through present alkali treatment were found to contain almost the same volatilizable components as present in the starting coal. This matter can be thermally recovered by flash heating at 900°C and thus may be added to the total extraction yield through alkaline degrada- tion.

References

[l] Sharma, D.K., 1988. Solvent extraction of coal through chemical reactions at atmospheric pressure.

In: N.P. Vasilakos (Ed.), Advances in Coal Chemistry. Theophrastus Publications, Athens, Greece.

[2] Haber, F. and Bruner, L.Z., 1904. Electrochem, 10: 697.

[3] Makabe, M. and Ouchi, K., 1979. Fuel Processing Technol., 2: 131.

[4] Chow, C.K., 1983. Fuel, 62: 317.

[S] Mirza, Z.B., Sarkar, M.K. and Sharma, D.K., 1984. Fuel Processing Technol., 9: 149.

[6] Sharma, D.K. and Singh, S.K., 1988. Fuel Processing Technol., 19: 73.

[7] Withrow, J.R. and Pew, J.C., 1931. Fuel, 66: 44.

[S] Sharma, D.K. and Singh, S.K., Fuel Processing Technol., in press.

[9] Sharma, D.K., unpublished results.

IlO] Kalra, R.L., Choudhury, R. and Sarkar, M.K., 1982. Fuel, 61: 1786.

_l l] Sharma, D.K. and Singh, SK., Proceedings of the International Rolduc Symposium, May, 1993, The Netherlands.