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Sulphur removal from Spanish low rank coals by acid-catalyzed depolymerization
Fuel Processing Technology, 21 (1989) 223-230
223
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Sulphur Removal from Spanish Low Rank Coals by Acid-catalyzed Depolymerization B. RUBIO and A.M. MASTRAL
Instituto de Carboquimica. CSIC. Aptdo. 589 Pl. Parafso, 1, 50004-Zaragoza (Spain)
ABSTRACT Reduction in the sulphur content of three Spanish high sulphur low rank coals by acid depolymerization has been studied. The results indicate that the depolymerization removes part of the pyritic sulphur as well as organic sulphur. Generally, between 50 and 75% of THF solubles were obtained with this process. The sulphur content decreased near 80% in some cases. Under applied reaction conditions, the total decrease of sulphur attained similar values for all the treated coals, in spite of their different ash content and sulphur forms. The degree of removal of the different sulphur forms, mainly pyritic and organic, depends on the coal. Sulphates are in most cases removed above 80%, whereas pyrite removal varies between 25 and 70%. A pre-extraction of coals has not a clearly defined role in the removal of the various sulphur forms. Depending on the coal and on the solvent, the results of the various may be opposite. In general, the pre-extraction increases the yields in extracts after the depolymerization, and decreases the total sulphur content of the coals on average with respect the sulphur content of the obtained products.
INTRODUCTION
Sulphur appears to be a problem in most processes involved in coal utilization [ 1 ]. Except for hydrogenation, in which pyrite seems to play a favourable role as catalyst [2,3], a low sulphur level is desired. In some cases a pretreatment of coal to decrease its sulphur content makes the coal more reactive or more uniform to process [4]. Flue gas desulphurization, despite its demonstrated efectiveness, presents in some cases important problems with regard to economical costs and waste sludge. Chemical cleaning of coal is under research with the aim of removing especially organic sulphur, which is unaffected by other desulphurization methods. The heterogeneity in the distribution of sulphur and the lack of data on organic sulphur forms in most coals [5,6] is a troublesome point in evaluating the behaviour of a certain sulphur form under several desulphurization treatments. Because of the complex nature of the problem, coal cleaning is also
0378-3820/89/$03.50
© 1989 Elsevier Science Publishers B.V.
224
highly site specific, so a clear understanding of the nature and evolution of sulphur in a particular coal is needed to find the most adequate desulphurization method. Total sulphur contents of Spanish low rank coals vary in the range 3-11 w/ w%, the distribution into its several forms (pyrite, sulphate, organic) being widely variable and depending on the basin. This paper describes three low rank coals from Utrillas (MI12, MI18) and Mequinenza (L-7627) with a total sulphur content (dry) ranging from 3.8 to 9.6 w/w% and widely variable in their ash content and sulphur forms, which were treated by acid catalyzed depolymerization, with and without previous solvent extraction. The aim of this work was to determine the degree of desulphurization by a mild chemical process such as acid depolymerization, and how the treatment affected the distribution of total and organic sulphur in the products. The effect of a pre-extraction using non-specific solvents was considered as well. EXPERIMENTAL
The ultimate and elemental analyses of the used coals are given in Table 1. The coals were ground to 60 mesh (0.25 mm) and stored until use in small containers under nitrogen atmosphere. The extractions were carried out in a Sohxlet apparatus for 48 hours using chloroform, isopropanol and THF. The solvents were stripped off by vacuum distillation to recover the extracts, and the residues were dried at 60 ° C. The depolymerization of coals and residues was performed using boron trifluoride as catalyst and phenol in excess as solvent, as described elsewhere [7]. In this case, bitumen extracted from each depolymerized coal and residue was not further separated into asphaltenes and oils, but was considered as one coal-derived liquid for further analysis. C,H and N contents were determined in a LECO CHN-600 analyzer and the total sulphur content in a LECO S-32 apparatus. Pyritic and sulphate sulphur were determined according to ASTM method D 2492/84. This method is only applicable to residue as the extracts present a difficulty in that they are attacked by the acid solution. Organic sulphur was determined by difference and also according to the method of Block et al. [8] which consists basically in a treatment of the sample with a nitric acid solution (25% v/v) for 24 h. The sample was filtered, washed and dried and analyzed for its sulphur content. This sulphur was considered as organic sulphur. The differences found using both methods are discussed. RESULTS AND DISCUSSION
The differences between the three investigated coals not only lie in their total sulphur and ash contents (Table 1 ), but also in the distribution of their
225 TABLE 1
Elemental analyses (% dry basis) of the used coals Constituent
L-7627
M I 18
M I 13
C H N S,,,, Sso~ Spyritic S,,r,
54.9 4.4 0.9 9.6 0.1 1.1 8.4 19.9
38.6 2.7 0.4 5.4 0.6 2.9 1.9 39.0
64.6 4.1 0.9 3.8 0.6 1.4 1.8 19.0
Ash
TABLE 2
Yield, elemental analyses and sulphur forms content of extracts and residues from solvent extracted coals Coal
Solvent
% Extract*
Extracts (%) C
L-7627
chloroform isopropanol THF
MI18
chloroform isopropanol THF
MI13
chloroform isopropanol THF
4.5 4.4 6.0 6.8 3.8 12.0 3.9 2.5 5.2
H
Residues (%)
N
S .... C
H
N
St,,t Spvr~t~, Sso~
S,*r*~ S,*,*~*
0.2 0.2 0.2
8.8 8.3 8.5
52.0 52.4 52.5
4.6 4.8 4.9
0.4 0.4 0.3
9.6 9.8 9.7
0.85 0.86 0.78
0.15 0.16 0.20
8.60 8.15 8.78 7.92 8.72 8.16
7.6 0.2 7.2 0.1 6.4 0.2
4.4 4.9 5.2
37.8 3.3 37.8 3.4 3 6 . 1 3.2
0.2 0.1 0.1
5.1 5.3 5.3
2.31 2.87 2.72
0.90 0.78 0.80
1.89 1.53 1.65 1.65 1.78 1.49
69.1 7.7 0.2 73.0 7.3 0.2 69.3 6.5 0.4
3.4 3.1 3.8
59.3 4.7 60.5 4.8 65.7 4.6
0.3 4.0 0.3 3.9 0.7 3.8
1.73 1.43 1.32
1.36 1.25 1.40
1.01 1.10 1.22 1.08 1.08 1.10
71.2 8.6 69.4 8.6 67.6 8.1 70.5 62.6 64.4
*d.a.f.
**Calculated difference. ***Determined according Block et al. [8].
sulphur forms. (pyritic, sulphates and organic). The coal L-7627 from Mequinenza basin has a very high organic sulphur content, whereas M118 coal (Utrillas basin) contains mainly pyritic sulphur as well as a large amount of ash. MI13 in turn contains a moderate amount of both sulphur and ash. These coals were chosen because of their significant differences. The results obtained from extraction of the coals are given in Table 2. THF appears to be the most effective solvent with respect to extract yield. The distribution of the sulphur forms in the extracted coals do not vary significantly from one solvent to another. A great difference in the values of organic sulphur [9] obtained by difference and by Block's method was found particularly in the case of MI13 coal. The values of sulphate sulphur obtained following the
226
A S T M m e t h o d are o v e r e s t i m a t e d . In s o m e cases a c o - p r e c i p i t a t i o n of o t h e r s u b s t a n c e s , p r o b a b l y organic m a t t e r , t o g e t h e r w i t h t h e B a S O 4 w a s observed. A d e c r e a s e in p y r i t i c s u l p h u r c o n t e n t c a n be seen, w h i c h c a n be a t t r i b u t e d to i n s u f f i c i e n t a t t a c k of t h e nitric acid s o l u t i o n d u r i n g p y r i t i c s u l p h u r d e t e r m i n a t i o n r a t h e r t h a n e x t r a c t i o n of p y r i t e s b y t h e s o l v e n t s used. Since t h e values of organic s u l p h u r o b t a i n e d b y B l o c k ' s m e t h o d are lower t h a n t h e v a l u e s o b t a i n e d b y d i f f e r e n c e in a l m o s t all cases, a m o r e c o m p l e t e d i s s o l u t i o n o f p y r i t e s a n d s u l p h a t e s b y nitric acid, p e r f o r m e d at higher c o n c e n t r a t i o n a n d longer d u r a t i o n b y B l o c k ' s m e t h o d , c a n be s u p p o s e d as well as t h e a c c u m u l a t i o n of e r r o r s f r o m p y r i t i c a n d s u l p h a t e d e t e r m i n a t i o n s in t h e calc u l a t i o n of d i f f e r e n c e m e t h o d . T h e r e s u l t s o b t a i n e d in t h e d e p o l y m e r i z a t i o n of coals a n d residues are s h o w n in T a b l e 3. T h e lowest yield in T H F solubles w a s f o u n d for M I 1 8 coal, as a r e s u l t of its e l e v a t e d a s h c o n t e n t . T h e d e c r e a s e in t o t a l s u l p h u r in e x t r a c t s is v e r y s i g n i f i c a n t for all t h r e e coals. T h e p e r c e n t of r e m o v e d s u l p h u r in e x t r a c t s is given in Fig. 1. In t h e case of U t r i l l a s b a s i n coals t h e values are n e a r 80%; for L-7627 s i g n i f i c a n t l y lower, w h i c h is p r o b a b l y due to t h e a b u n d a n c e of organic s u l p h u r in t h i s coal, t h e r e m o v a l o f w h i c h is n o t so effective. A p a r t i a l d e a s h i n g of t h e t h r e e coals occurs d u r i n g acid d e p o l y m e r i z a t i o n TABLE 3 Yields and sulphur forms contents in extracts and residues of depolymerized raw and chloroform (R1), isopropanol (R2) and THF (R3) extracted coals Sample
Extracts (%)
Residues (%)
Weight*
Stot
Weight*
Sto t
Spyriti c
Sso ~
S(*)rg
So*rg*
L-7627 L-7627 R1 L-7627 R2 L-7627 R3
71.3 77.4 77.3 79.5
3.60 4.24 5.25 4.58
28.7 22.6 22.7 20.5
8.35 8.07 8.42 9.23
2.50 2.55 3.11 4.00
0.12 0.09 0.20 0.09
5.73 5.43 5.11 5.14
6.10 5.56 5.16 4.81
MI18 MI18 R1 MI18 R2 MI18 R3
45.3 50.9 54.8 54.4
1.22 1.42 1.04 0.95
54.7 49.1 45.2 45.6
5.96 4.86 4.94 6.38
4.14 4.25 3.82 4.77
0.15 0.19 0.11 0.10
1.67 0.42 0.99 1.51
1.65 1.52 1.55 1.89
MI13 MI13 R1 MI13 R2 MI13 R3
75.6 77.5 74.8 73.2
0.75 0.63 0.83 0.63
24.4 22.5 25.2 26.8
3.34 3.55 3.98 3.86
1.96 1.83 2.10 2.38
0.41 0.23 0.17 0.14
0.92 1.52 1.71 1.34
0.97 1.04 1.46 1.40
*Respective to the weight of depolymerized coal or residue. **Calculated by difference. ***According to Block et al. [8].
227 L.762
I
2
EXTRACTS
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13
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Fig. 1. Sulphur removed (%) from extracts and residues of a) depolymerized, b) chloroformextracted-depolymerized, c) isopropanol extracted-depolymerizedand d) THF extracted-depolymerized coals. As the ash content could not be determined for all depolymerized samples, the atomic S / C ratio was calculated for extracts and residues (Table 4), but the results do not disagree with the trend shown by the values of total sulphur - calculated on a dry basis. The residues obtained in the depolymerization of THF extracted coals have the highest sulphur content in all cases. It seems that by a previous extraction with THF a part of the mobile components of coal facilitates the reagents to reach the labile bonds of the coal matrix [10]. The extraction of coal prior to depolymerization also improves the yield of soluble material, as in the case of hydroliquefied low rank coal [ 11,12 ]. Pyritic and sulphate are removed in amounts widely depending on the type of coal. About 60-70% of pyritic sulphur and almost 90% of sulphate sulphur of MI13 are removed (Table 5). The lowering in pyritic S of MI18 was not so high, about 30-40%, but it is possible that the reaction time (3 h) was not enough to result in a more intensive desulphurization for this coal, taken into account its elevated content of pyrites.
228 TABLE 4 Elemental analyses and atomic S/C ratio of depolymerized raw and chloroform (R1), isopropanol (R2) and T H F (R3) extracted coals Sample
Extracts
Residues
C%
H (%)
N (%)
S/C
C (%)
H (%)
N(%)
S/C
L-7627 L-7627 R1 L-7627 R2 L-7627 R3
67.9 66.4 65.7 69.0
7.1 6.4 6.2 6.5
0.1 0.4 0.5 0.4
0.019 0.023 0.029 0.024
63.0 54.2 48.7 39.3
4.7 3.8 3.4 3.5
0.9 0.8 0.7 0.6
0.049 0.055 0.064 0.088
MI18 MI18 R1 MI18 R2 MI18 R3
70.7 69.8 72.6 73.5
6.7 6.4 6.4 7.3
0.1 0.2 0.1 0.1
0.006 0.007 0.005 0.004
54.9 38.5 56.9 56.3
3.3 2.6 3.3 4.0
0.6 0.4 0.5 0.5
0.040 0.047 0.032 0.042
MI13 MI13 R1 MI13 R2 MI13 R3
74.0 72.1 70.8 72.7
6.3 6.0 5.8 6.2
0.3 0.3 0.3 0.3
0.003 0.003 0.004 0.003
72.6 76.6 66.9 62.6
4.6 4.9 4.2 4.7
0.7 0.7 0.7 0.6
0.017 0.017 0.038 0.022
TABLE 5 Remaining and removed sulphur in depolymerized raw and extracted coals (R1 chloroform, R2 isopropanol and R3 T H F ) Sample
% S*
% removed S
Spyritic
8SO42-
Sorg
Stot
Spyritic
Sso 2-
Sorg
Stot
L-7627 L-7627 R1 L-7627 R2 L-7627 R3
0.7 0.6 0.7 0.8
0.03 0.02 0.04 0.02
4.2 4.5 5.2 4.7
4.9 5.0 5.9 5.5
36 45 36 27
70 80 60 80
50 46 38 44
49 48 38 43
MI18 MI18 R1 MI18 R2 MI18 R3
2.2 2.1 1.7 2.2
0.05 0.1 0.04 0.04
1.5 0.9 1.0 1.2
3.7 3.1 2.7 3.4
24 28 41 24
91 83 93 93
21 53 47 37
31 43 50 59
MI13 MI13 R1 MI13 R2 MI13 R3
0.46 0.40 0.52 0.65
0.09 0.04 0.02 0.03
0.8 0.8 1.0 0.8
1.81 1.6 1.5 1.5
67 71 63 54
85 93 97 95
56 56 44 56
52 58 60 60
*Calculated [% S in ext. X ext. yield + % S in res. × ( 1 0 0 - ext. yield) ]/100.
229
Also the organic sulphur decreases after depolymerization, in most cases about 50%, probably as H2S. While the organic sulphur in low rank coal is present mainly as sulfides and thiols [ 13 ], it can be expected that these groups can react under acid depolymerization conditions. Again the differences between the values of organic sulphur obtained by both methods are significant. In this case, the value of organic sulphur determined by difference is generally lower. The great dispersion for MI18 values would indicate an overestimation of the pyritic sulphur, so the percentage of removed pyrites could be higher than the calculated. Considering total removed sulphur, the desulphurization of the coals follows the order MI13> L-7627> MI18, inversely to their ash content. Further work is in progress to determine the dependence of low rank coal desulphurization on reaction time and acid concentration during depolymerization. CONCLUSIONS
Acid catalyzed depolymerization of high sulphur rank coals carried out under atmospheric pressure at 200 °C results in conversion in T H F soluble material between 45 and 75%, as well as in a partial desulphurization of the obtained products. The T H F soluble material from depolymerized coal shows a sulphur removal up to 80% depending on the coal, type as compared with the T H F solubles of the raw coal. The lowering in sulphur content of residues varies widely with the coal properties, but can be located near 10%. A previous extraction of coals with solvents leads to slightly higher conversion yields in the depolymerization as well as a higher sulphur removal for the highest ash containing coal. There is not a clear relationship between the preextraction of the coals and the removal of a certain sulphur form preferred over the others. A total balance of sulphur in the investigated coals shows that after the depolymerization most of the remaining sulphur is concentrated in the residues. Especially for the T H F pre-extracted coals, the removal of the several sulphur forms in the residues varies from one coal to another. ACKNOWLEDGEMENT
B.R. is grateful to the Ministerio de Educacidn y Ciencia (MEC) for a postdoc research grant.
230 REFERENCES 1 Engdahl, R.B., 1985. In: Attia, Y.A. (Ed.), Processing and Utilization of High Sulfur Coals. Elsevier, Amsterdam Ch. 2, p. 113. 2 Tarrer, A.R., Guin, J.A., Pitts, NS, Henley, J.P., Prather, J.W. and Styles, G.A., ACS Div. Fuel Chem. Prep., 21 (5): 59. 3 Gaugwer, T.E. and Prassad, H., 1979. Fuel 58: 577. 4 Wheelock, T.D. and Markuszewsky, R., 1984. In: Cooper, B.R. and Ellingson, W.A. (Eds.), The Science and Technology of Coal and Coal Utilization. Plenum Press, New York, NY. 5 Attar, A. and Dupuite, F., 1979. ACS Div of Fuel Chem. Prep., 24 (1): 166. 6 Lacount, R.B., Anderson, R.R., Friedman, S. and Blaustein, D.B., 1987. Fuel, 66: 909. 7 Mastral, A.M. and Rubio, B., 1985. Fuel, 64: 1127. 8 Block, S., Sharp, J.B. and Darlage, J.L., 1975. Fuel, 54, 113. 9 Chakrabarti, J.N., 1978. In: Karr, C. (Ed.), Analytical Methods for Coal and Coal Products, Vol. I. Academic Press, New York, NY, Ch. 9, p. 279. 10 Derbyshire, F.J., Stansberry, P.G., Terrer, M.T. and Mastral, A.M., 1986. Report US Department of Energy, PC-60811-9,10. 11 Mastral, A.M., Rubio, B. and Membrado, L., 1988. Fuel, 67: 263. 12 Mastral, A.M., Membrado, L., Rubio, Nicol~is, C. and Royo, J., 1987.Ingenierfa Qufmica (Energfa), 13 (5): 153. 13 Attar, A. and Dupuis, F., 1978. ACS Div. Fuel Chem., Prepr., 23 (2): 4.
223
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Sulphur Removal from Spanish Low Rank Coals by Acid-catalyzed Depolymerization B. RUBIO and A.M. MASTRAL
Instituto de Carboquimica. CSIC. Aptdo. 589 Pl. Parafso, 1, 50004-Zaragoza (Spain)
ABSTRACT Reduction in the sulphur content of three Spanish high sulphur low rank coals by acid depolymerization has been studied. The results indicate that the depolymerization removes part of the pyritic sulphur as well as organic sulphur. Generally, between 50 and 75% of THF solubles were obtained with this process. The sulphur content decreased near 80% in some cases. Under applied reaction conditions, the total decrease of sulphur attained similar values for all the treated coals, in spite of their different ash content and sulphur forms. The degree of removal of the different sulphur forms, mainly pyritic and organic, depends on the coal. Sulphates are in most cases removed above 80%, whereas pyrite removal varies between 25 and 70%. A pre-extraction of coals has not a clearly defined role in the removal of the various sulphur forms. Depending on the coal and on the solvent, the results of the various may be opposite. In general, the pre-extraction increases the yields in extracts after the depolymerization, and decreases the total sulphur content of the coals on average with respect the sulphur content of the obtained products.
INTRODUCTION
Sulphur appears to be a problem in most processes involved in coal utilization [ 1 ]. Except for hydrogenation, in which pyrite seems to play a favourable role as catalyst [2,3], a low sulphur level is desired. In some cases a pretreatment of coal to decrease its sulphur content makes the coal more reactive or more uniform to process [4]. Flue gas desulphurization, despite its demonstrated efectiveness, presents in some cases important problems with regard to economical costs and waste sludge. Chemical cleaning of coal is under research with the aim of removing especially organic sulphur, which is unaffected by other desulphurization methods. The heterogeneity in the distribution of sulphur and the lack of data on organic sulphur forms in most coals [5,6] is a troublesome point in evaluating the behaviour of a certain sulphur form under several desulphurization treatments. Because of the complex nature of the problem, coal cleaning is also
0378-3820/89/$03.50
© 1989 Elsevier Science Publishers B.V.
224
highly site specific, so a clear understanding of the nature and evolution of sulphur in a particular coal is needed to find the most adequate desulphurization method. Total sulphur contents of Spanish low rank coals vary in the range 3-11 w/ w%, the distribution into its several forms (pyrite, sulphate, organic) being widely variable and depending on the basin. This paper describes three low rank coals from Utrillas (MI12, MI18) and Mequinenza (L-7627) with a total sulphur content (dry) ranging from 3.8 to 9.6 w/w% and widely variable in their ash content and sulphur forms, which were treated by acid catalyzed depolymerization, with and without previous solvent extraction. The aim of this work was to determine the degree of desulphurization by a mild chemical process such as acid depolymerization, and how the treatment affected the distribution of total and organic sulphur in the products. The effect of a pre-extraction using non-specific solvents was considered as well. EXPERIMENTAL
The ultimate and elemental analyses of the used coals are given in Table 1. The coals were ground to 60 mesh (0.25 mm) and stored until use in small containers under nitrogen atmosphere. The extractions were carried out in a Sohxlet apparatus for 48 hours using chloroform, isopropanol and THF. The solvents were stripped off by vacuum distillation to recover the extracts, and the residues were dried at 60 ° C. The depolymerization of coals and residues was performed using boron trifluoride as catalyst and phenol in excess as solvent, as described elsewhere [7]. In this case, bitumen extracted from each depolymerized coal and residue was not further separated into asphaltenes and oils, but was considered as one coal-derived liquid for further analysis. C,H and N contents were determined in a LECO CHN-600 analyzer and the total sulphur content in a LECO S-32 apparatus. Pyritic and sulphate sulphur were determined according to ASTM method D 2492/84. This method is only applicable to residue as the extracts present a difficulty in that they are attacked by the acid solution. Organic sulphur was determined by difference and also according to the method of Block et al. [8] which consists basically in a treatment of the sample with a nitric acid solution (25% v/v) for 24 h. The sample was filtered, washed and dried and analyzed for its sulphur content. This sulphur was considered as organic sulphur. The differences found using both methods are discussed. RESULTS AND DISCUSSION
The differences between the three investigated coals not only lie in their total sulphur and ash contents (Table 1 ), but also in the distribution of their
225 TABLE 1
Elemental analyses (% dry basis) of the used coals Constituent
L-7627
M I 18
M I 13
C H N S,,,, Sso~ Spyritic S,,r,
54.9 4.4 0.9 9.6 0.1 1.1 8.4 19.9
38.6 2.7 0.4 5.4 0.6 2.9 1.9 39.0
64.6 4.1 0.9 3.8 0.6 1.4 1.8 19.0
Ash
TABLE 2
Yield, elemental analyses and sulphur forms content of extracts and residues from solvent extracted coals Coal
Solvent
% Extract*
Extracts (%) C
L-7627
chloroform isopropanol THF
MI18
chloroform isopropanol THF
MI13
chloroform isopropanol THF
4.5 4.4 6.0 6.8 3.8 12.0 3.9 2.5 5.2
H
Residues (%)
N
S .... C
H
N
St,,t Spvr~t~, Sso~
S,*r*~ S,*,*~*
0.2 0.2 0.2
8.8 8.3 8.5
52.0 52.4 52.5
4.6 4.8 4.9
0.4 0.4 0.3
9.6 9.8 9.7
0.85 0.86 0.78
0.15 0.16 0.20
8.60 8.15 8.78 7.92 8.72 8.16
7.6 0.2 7.2 0.1 6.4 0.2
4.4 4.9 5.2
37.8 3.3 37.8 3.4 3 6 . 1 3.2
0.2 0.1 0.1
5.1 5.3 5.3
2.31 2.87 2.72
0.90 0.78 0.80
1.89 1.53 1.65 1.65 1.78 1.49
69.1 7.7 0.2 73.0 7.3 0.2 69.3 6.5 0.4
3.4 3.1 3.8
59.3 4.7 60.5 4.8 65.7 4.6
0.3 4.0 0.3 3.9 0.7 3.8
1.73 1.43 1.32
1.36 1.25 1.40
1.01 1.10 1.22 1.08 1.08 1.10
71.2 8.6 69.4 8.6 67.6 8.1 70.5 62.6 64.4
*d.a.f.
**Calculated difference. ***Determined according Block et al. [8].
sulphur forms. (pyritic, sulphates and organic). The coal L-7627 from Mequinenza basin has a very high organic sulphur content, whereas M118 coal (Utrillas basin) contains mainly pyritic sulphur as well as a large amount of ash. MI13 in turn contains a moderate amount of both sulphur and ash. These coals were chosen because of their significant differences. The results obtained from extraction of the coals are given in Table 2. THF appears to be the most effective solvent with respect to extract yield. The distribution of the sulphur forms in the extracted coals do not vary significantly from one solvent to another. A great difference in the values of organic sulphur [9] obtained by difference and by Block's method was found particularly in the case of MI13 coal. The values of sulphate sulphur obtained following the
226
A S T M m e t h o d are o v e r e s t i m a t e d . In s o m e cases a c o - p r e c i p i t a t i o n of o t h e r s u b s t a n c e s , p r o b a b l y organic m a t t e r , t o g e t h e r w i t h t h e B a S O 4 w a s observed. A d e c r e a s e in p y r i t i c s u l p h u r c o n t e n t c a n be seen, w h i c h c a n be a t t r i b u t e d to i n s u f f i c i e n t a t t a c k of t h e nitric acid s o l u t i o n d u r i n g p y r i t i c s u l p h u r d e t e r m i n a t i o n r a t h e r t h a n e x t r a c t i o n of p y r i t e s b y t h e s o l v e n t s used. Since t h e values of organic s u l p h u r o b t a i n e d b y B l o c k ' s m e t h o d are lower t h a n t h e v a l u e s o b t a i n e d b y d i f f e r e n c e in a l m o s t all cases, a m o r e c o m p l e t e d i s s o l u t i o n o f p y r i t e s a n d s u l p h a t e s b y nitric acid, p e r f o r m e d at higher c o n c e n t r a t i o n a n d longer d u r a t i o n b y B l o c k ' s m e t h o d , c a n be s u p p o s e d as well as t h e a c c u m u l a t i o n of e r r o r s f r o m p y r i t i c a n d s u l p h a t e d e t e r m i n a t i o n s in t h e calc u l a t i o n of d i f f e r e n c e m e t h o d . T h e r e s u l t s o b t a i n e d in t h e d e p o l y m e r i z a t i o n of coals a n d residues are s h o w n in T a b l e 3. T h e lowest yield in T H F solubles w a s f o u n d for M I 1 8 coal, as a r e s u l t of its e l e v a t e d a s h c o n t e n t . T h e d e c r e a s e in t o t a l s u l p h u r in e x t r a c t s is v e r y s i g n i f i c a n t for all t h r e e coals. T h e p e r c e n t of r e m o v e d s u l p h u r in e x t r a c t s is given in Fig. 1. In t h e case of U t r i l l a s b a s i n coals t h e values are n e a r 80%; for L-7627 s i g n i f i c a n t l y lower, w h i c h is p r o b a b l y due to t h e a b u n d a n c e of organic s u l p h u r in t h i s coal, t h e r e m o v a l o f w h i c h is n o t so effective. A p a r t i a l d e a s h i n g of t h e t h r e e coals occurs d u r i n g acid d e p o l y m e r i z a t i o n TABLE 3 Yields and sulphur forms contents in extracts and residues of depolymerized raw and chloroform (R1), isopropanol (R2) and THF (R3) extracted coals Sample
Extracts (%)
Residues (%)
Weight*
Stot
Weight*
Sto t
Spyriti c
Sso ~
S(*)rg
So*rg*
L-7627 L-7627 R1 L-7627 R2 L-7627 R3
71.3 77.4 77.3 79.5
3.60 4.24 5.25 4.58
28.7 22.6 22.7 20.5
8.35 8.07 8.42 9.23
2.50 2.55 3.11 4.00
0.12 0.09 0.20 0.09
5.73 5.43 5.11 5.14
6.10 5.56 5.16 4.81
MI18 MI18 R1 MI18 R2 MI18 R3
45.3 50.9 54.8 54.4
1.22 1.42 1.04 0.95
54.7 49.1 45.2 45.6
5.96 4.86 4.94 6.38
4.14 4.25 3.82 4.77
0.15 0.19 0.11 0.10
1.67 0.42 0.99 1.51
1.65 1.52 1.55 1.89
MI13 MI13 R1 MI13 R2 MI13 R3
75.6 77.5 74.8 73.2
0.75 0.63 0.83 0.63
24.4 22.5 25.2 26.8
3.34 3.55 3.98 3.86
1.96 1.83 2.10 2.38
0.41 0.23 0.17 0.14
0.92 1.52 1.71 1.34
0.97 1.04 1.46 1.40
*Respective to the weight of depolymerized coal or residue. **Calculated by difference. ***According to Block et al. [8].
227 L.762
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Fig. 1. Sulphur removed (%) from extracts and residues of a) depolymerized, b) chloroformextracted-depolymerized, c) isopropanol extracted-depolymerizedand d) THF extracted-depolymerized coals. As the ash content could not be determined for all depolymerized samples, the atomic S / C ratio was calculated for extracts and residues (Table 4), but the results do not disagree with the trend shown by the values of total sulphur - calculated on a dry basis. The residues obtained in the depolymerization of THF extracted coals have the highest sulphur content in all cases. It seems that by a previous extraction with THF a part of the mobile components of coal facilitates the reagents to reach the labile bonds of the coal matrix [10]. The extraction of coal prior to depolymerization also improves the yield of soluble material, as in the case of hydroliquefied low rank coal [ 11,12 ]. Pyritic and sulphate are removed in amounts widely depending on the type of coal. About 60-70% of pyritic sulphur and almost 90% of sulphate sulphur of MI13 are removed (Table 5). The lowering in pyritic S of MI18 was not so high, about 30-40%, but it is possible that the reaction time (3 h) was not enough to result in a more intensive desulphurization for this coal, taken into account its elevated content of pyrites.
228 TABLE 4 Elemental analyses and atomic S/C ratio of depolymerized raw and chloroform (R1), isopropanol (R2) and T H F (R3) extracted coals Sample
Extracts
Residues
C%
H (%)
N (%)
S/C
C (%)
H (%)
N(%)
S/C
L-7627 L-7627 R1 L-7627 R2 L-7627 R3
67.9 66.4 65.7 69.0
7.1 6.4 6.2 6.5
0.1 0.4 0.5 0.4
0.019 0.023 0.029 0.024
63.0 54.2 48.7 39.3
4.7 3.8 3.4 3.5
0.9 0.8 0.7 0.6
0.049 0.055 0.064 0.088
MI18 MI18 R1 MI18 R2 MI18 R3
70.7 69.8 72.6 73.5
6.7 6.4 6.4 7.3
0.1 0.2 0.1 0.1
0.006 0.007 0.005 0.004
54.9 38.5 56.9 56.3
3.3 2.6 3.3 4.0
0.6 0.4 0.5 0.5
0.040 0.047 0.032 0.042
MI13 MI13 R1 MI13 R2 MI13 R3
74.0 72.1 70.8 72.7
6.3 6.0 5.8 6.2
0.3 0.3 0.3 0.3
0.003 0.003 0.004 0.003
72.6 76.6 66.9 62.6
4.6 4.9 4.2 4.7
0.7 0.7 0.7 0.6
0.017 0.017 0.038 0.022
TABLE 5 Remaining and removed sulphur in depolymerized raw and extracted coals (R1 chloroform, R2 isopropanol and R3 T H F ) Sample
% S*
% removed S
Spyritic
8SO42-
Sorg
Stot
Spyritic
Sso 2-
Sorg
Stot
L-7627 L-7627 R1 L-7627 R2 L-7627 R3
0.7 0.6 0.7 0.8
0.03 0.02 0.04 0.02
4.2 4.5 5.2 4.7
4.9 5.0 5.9 5.5
36 45 36 27
70 80 60 80
50 46 38 44
49 48 38 43
MI18 MI18 R1 MI18 R2 MI18 R3
2.2 2.1 1.7 2.2
0.05 0.1 0.04 0.04
1.5 0.9 1.0 1.2
3.7 3.1 2.7 3.4
24 28 41 24
91 83 93 93
21 53 47 37
31 43 50 59
MI13 MI13 R1 MI13 R2 MI13 R3
0.46 0.40 0.52 0.65
0.09 0.04 0.02 0.03
0.8 0.8 1.0 0.8
1.81 1.6 1.5 1.5
67 71 63 54
85 93 97 95
56 56 44 56
52 58 60 60
*Calculated [% S in ext. X ext. yield + % S in res. × ( 1 0 0 - ext. yield) ]/100.
229
Also the organic sulphur decreases after depolymerization, in most cases about 50%, probably as H2S. While the organic sulphur in low rank coal is present mainly as sulfides and thiols [ 13 ], it can be expected that these groups can react under acid depolymerization conditions. Again the differences between the values of organic sulphur obtained by both methods are significant. In this case, the value of organic sulphur determined by difference is generally lower. The great dispersion for MI18 values would indicate an overestimation of the pyritic sulphur, so the percentage of removed pyrites could be higher than the calculated. Considering total removed sulphur, the desulphurization of the coals follows the order MI13> L-7627> MI18, inversely to their ash content. Further work is in progress to determine the dependence of low rank coal desulphurization on reaction time and acid concentration during depolymerization. CONCLUSIONS
Acid catalyzed depolymerization of high sulphur rank coals carried out under atmospheric pressure at 200 °C results in conversion in T H F soluble material between 45 and 75%, as well as in a partial desulphurization of the obtained products. The T H F soluble material from depolymerized coal shows a sulphur removal up to 80% depending on the coal, type as compared with the T H F solubles of the raw coal. The lowering in sulphur content of residues varies widely with the coal properties, but can be located near 10%. A previous extraction of coals with solvents leads to slightly higher conversion yields in the depolymerization as well as a higher sulphur removal for the highest ash containing coal. There is not a clear relationship between the preextraction of the coals and the removal of a certain sulphur form preferred over the others. A total balance of sulphur in the investigated coals shows that after the depolymerization most of the remaining sulphur is concentrated in the residues. Especially for the T H F pre-extracted coals, the removal of the several sulphur forms in the residues varies from one coal to another. ACKNOWLEDGEMENT
B.R. is grateful to the Ministerio de Educacidn y Ciencia (MEC) for a postdoc research grant.
230 REFERENCES 1 Engdahl, R.B., 1985. In: Attia, Y.A. (Ed.), Processing and Utilization of High Sulfur Coals. Elsevier, Amsterdam Ch. 2, p. 113. 2 Tarrer, A.R., Guin, J.A., Pitts, NS, Henley, J.P., Prather, J.W. and Styles, G.A., ACS Div. Fuel Chem. Prep., 21 (5): 59. 3 Gaugwer, T.E. and Prassad, H., 1979. Fuel 58: 577. 4 Wheelock, T.D. and Markuszewsky, R., 1984. In: Cooper, B.R. and Ellingson, W.A. (Eds.), The Science and Technology of Coal and Coal Utilization. Plenum Press, New York, NY. 5 Attar, A. and Dupuite, F., 1979. ACS Div of Fuel Chem. Prep., 24 (1): 166. 6 Lacount, R.B., Anderson, R.R., Friedman, S. and Blaustein, D.B., 1987. Fuel, 66: 909. 7 Mastral, A.M. and Rubio, B., 1985. Fuel, 64: 1127. 8 Block, S., Sharp, J.B. and Darlage, J.L., 1975. Fuel, 54, 113. 9 Chakrabarti, J.N., 1978. In: Karr, C. (Ed.), Analytical Methods for Coal and Coal Products, Vol. I. Academic Press, New York, NY, Ch. 9, p. 279. 10 Derbyshire, F.J., Stansberry, P.G., Terrer, M.T. and Mastral, A.M., 1986. Report US Department of Energy, PC-60811-9,10. 11 Mastral, A.M., Rubio, B. and Membrado, L., 1988. Fuel, 67: 263. 12 Mastral, A.M., Membrado, L., Rubio, Nicol~is, C. and Royo, J., 1987.Ingenierfa Qufmica (Energfa), 13 (5): 153. 13 Attar, A. and Dupuis, F., 1978. ACS Div. Fuel Chem., Prepr., 23 (2): 4.