Distribution of sulphur in the tertiary coals of Meghalaya, India

International Journal of Coal Geology, 3 (1983) 63--75 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

63

D I S T R I B U T I O N OF S U L P H U R IN THE T E R T I A R Y COALS OF MEGHALAYA, INDIA

D. CHANDRA, K. MAZUMDAR and S. BASUMALLICK Department of Applied Geology, Indian School of Mines, Dhanbad 826004 (India) (Received April 2, 1982; revised and accepted January 27, 1983)

ABSTRACT Chandra, D., Mazumdar, K. and Basumallick, S., 1983. Distribution of sulphur in the Tertiary coals of Meghalaya, India. Int. J. Coal Geol., 3: 63--75. The distribution pattern of the different forms of sulphur in the coal seams of Meghalaya has been worked out. In general, the total sulphur content is found to increase from the base to the top of a seam. There is also a general trend of upward increase of sulphur content in the seam-wise sequence, i.e. the upper seam is found to contain more sulphur than the seam lying below. Pyritic sulphur shows an upward increasing tendency both intraseam- and interseam-wise. Variation in organic sulphur shows an apparent parallelism with that of total sulphur. A gradual decrease in total sulphur content is also seen in the coals of Meghalaya from east to west within the basin of deposition. This regional lateral variation in sulphur content was due to the more marine nature of the peat-forming swamps of the Khasi and Jaintia Hills (eastern Meghalaya) as compared to that of the Garo Hills (western Meghalaya).

INTRODUCTION T h e r e are extensive deposits o f E o c e n e coal in the states o f Assam and Meghalaya. These coals are o f g o o d quality, particularly due to their characteristically l o w ash c o n t e n t and m e d i u m c o k i n g p r o p e r t y . These coals c o u l d n o t be c o m m e r c i a l l y utilized, however, due to their r e p o r t e d l y high sulphur content. The p r o b l e m o f the utilization o f these coals has b e c o m e m o r e a c u t e n o t only because o f the high total sulphur c o n t e n t b u t m a i n l y due to the presence o f relatively high p r o p o r t i o n s o f organic sulphur, the removal of w h i c h b y c o n v e n t i o n a l m e t h o d s is e x t r e m e l y difficult. Coals o f identical n a t u r e with high organic sulphur c o n t e n t are k n o w n t o o c c u r in various c o u n t r i e s o f the world, such as Australia, Holland, P o l a n d , B o r n e o , Pakistan and Yugoslavia. H o w e v e r , there are few r e p o r t s o n the d i s t r i b u t i o n o f d i f f e r e n t f o r m s o f s u l p h u r in the coal seams. T h e c o n c e p t t h a t all the T e r t i a r y coals o f Assam and Meghalaya are high in t o t a l sulphur c o n t e n t is m a i n l y based on sporadic samples. C o n t r a r y t o this belief, C h a n d r a et al. ( 1 9 8 0 a ) have s h o w n t h a t in the Tertiary coals 0166-5162/83/$03.00

© 1983 Elsevier Science Publishers B.V.

64 of A s s a m a n d A r u n a c h a l Pradesh t h e r e are s e a m s / s e c t i o n s with c o m p a r a tively low s u l p h u r c o n t e n t s . T h e objective o f the p r e s e n t investigation was to find o u t the d i s t r i b u t i o n p a t t e r n of the d i f f e r e n t f o r m s o f s u l p h u r in the coal seams o f M e g h a l a y a and to delineate t h e areas, if t h e r e s h o u l d be a n y , o f (a) low s u l p h u r c o n t e n t or (b) high s u l p h u r c o n t e n t with a low p r o p o r t i o n o f organic s u l p h u r in a coal seam. While c a r r y i n g o u t this w o r k the o p p o r t u n i t y was t a k e n to r e c o n s t r u c t the e n v i r o n m e n t o f d e p o s i t i o n of s u l p h u r in the coals o f Meghalaya. STRATIGRAPHIC FRAMEWORK T h e coal deposits of M e g h a l a y a o c c u r within the T e r t i a r y S y s t e m , which f o r m s a n a r r o w belt e x t e n d i n g f r o m the G a r o Hills in the west, t h r o u g h t h e Khasi Hills in the m i d d l e to the J a i n t i a Hills in t h e east, ranging in age TABLE I Tertiary stratigraphic succession of Meghalaya Age

Series

Upper Eocene

Khasi and Jaintia Hills

Garo Hills

Stage

Lithology

Stage

Lithology

Kopili

Shale, sandstone (alterate bands of calcareous sandstone and shale)

Rewak

Alterations of shale, limestone and sandstone, lower part highly fossiliferous

Prang limestone Narpuh sandstone Umlatdoh limestone

Siju

Grey to buff coloured limestone, marl and shale parting with organic remains

Tura

Coarse, crossbedded sandstone, clay and shale, coal with microscopic plant remains

Middle Eocene Sylhet

i Lakadong sandstone/ eoal Lower Eocene

<

Lakadong limestone

z

Upper TheriaHard sandstone/coal Palaeocene

Theria

Upper Cretaceous Langpar

Permian

Lower TheriaLimestone and calcareous shale Calcareous shale, limestone and sandstone Lower Gondwana sandstone

65 between Palaeocene and Eocene. Though all the rock types are grouped under the Jaintia Series, their development in the Khasi and Jaintia Hills districts has been markedly different from that of the Garo Hills district. Table I gives the generalized stratigraphic succession of Meghalaya. The Theria Sandstone and Sylhet Limestone stages axe the principal coal-bearing horizons in the Khasi and Jaintia Hills, while in the Garo Hills the Tura Sandstone stage is the only coal-bearing horizon. Development of coalfields in this region is given in Table II. TABLE II Coalfields of Meghalaya Area

Colliery

Coalfield

Stage

Jaintia H i l l s

Bapung Lumshnong

Bapung Lumshnong

Theria Sylhet

I Borsora Rongshokham Jathan Mawlong Laitryngew Siju Nangalbibra

Langrin Mawsynram

Sylhet Sylhet

Ishamati Laitryngew Siju West Daranggiri

Sylhet Sylhet Tura (basal) Tura (basal)

Khasi Hills

f

Garo Hills

METHOD OF INVESTIGATION Samples were collected from different collieries of West Daranggiri, Siju, Langrin, Mawsynram, Ishamati, Laitryngew, Bapung and Lumshnong, coalfields covering entire Tertiary coal deposits of Meghalaya (Fig. 1). While collecting the samples the lithology of the seams, roof and floor strata, intervening dirt band and partings were also studied (Fig. 2). Seams were subdivided into sections on the basis of lithology. Channel samples were obtained from each section. The numbers of samples analysed in the different coalfields have been shown in Fig. 1. For the determination of sulphur, chemical analyses of the samples were carried out according to the standard procedure (Indian Standards, 1969). RESULTS AND DISCUSSIONS The sampled coals represent almost all the exposed coal seams of Meghalaya. Correlation of coal seams is difficult because of their sporadic distribution. Coal seams are limited in continuity and coal deposits are isolated. Moreover, the physical nature of the seams does not permit correlation (variable thickness, variable nature of dirt band etc.). However, the faunal associations of the overlying and underlying formations suggest that the

--~o-Fi ~ -

I

F~

HI

~qo"

I~.~, ~ I

9,'~0"

I I / ~

HI

R ./"h

~'l o"

HILLS

u

9zIo"

j

p

4> +

OF MEGHALAYA SHOWING COAL DEPOSITS Scale Zl° ol ZOl 401 601 Km

Fig. 1. Map of Meghalaya showing coal deposits.

?-5 O'

O'

26'

so.Jo'

MAP

,

I

~

~o.

9o.

7o 9o. 'i"', o ,ooi

~J o"

~ - ~ ORGANIC SULPHUR -~THICKNESS OF TOP SEAM -~THICKNESS OF BOTTOM SEAM i~N~NUMBEP, OF SAMPLES ' ' ANALYSED COAL I)EPOS~TtFIELD'~1

SULPHATE SULPHUR

PYRLT{C SULPHUR ~

TOTAL SULPHUR

~I

I N D E X

~

93"I0"

I

N

D E X

~SANOSTONE

i

•

,..

-....

•

BALCHUA NALA "~ • SECTION DAMUK NALA -'_

~,JU AREA

ADITZB2S

i

SANDSTONE

SHALE

SEQUENCE

LANGRIN ~

AREA

SCALE

....

.::: i

'.'.'.'1 .... i

I[

TURA,

~

MAWSYNRAM

AREA

LAKADANG

AND

THERIA

COAL

'- "-_':

measures o f Meghalaya.

J

_

~M

LAITRYNGEW ISHAMATI LUMSHNONG BAPUNG AREA AREA AREA AREA • , ~ ~

i [

RONGSHOKHAM JATHANG A RONGSHOKHAM B

I

:!!i

• .','1

~

l i

'

m

ADIT2

L O

,4

OF

MEGHALAYA

POINTS OF

SAMPLING MEASURES

BORSORA COLLIEARY ADIT2425 ADITIS ADIT6

AT

Fig. 2. Lithological sequence at sampling points of Tura, Lakadang and Theria coal

WAIT NALA SECTION

lEST DARANGGIBI

- ~ CLAY/ LITHO~TLIHESTONE ~-~- MERGIC CLAY : ~ CARBONACEOUS "~SANDSTONE

CARBONACEOUS I ~ P Y R I T I F E R O U S 3SHALE ---~SHALE ~PYRITIFEROUS

~

mCOAL

ITHOLOG[CAL

r~ 4

68 coals are broadly of the same age (established to be Eocene; Fox and Ghosh, 1940; Ghosh, 1954). Chemical analyses revealed that the carbon and hydrogen plots of these coals on Seyler's Chart fall within and above the normal "Coal Band" and can be classified in general as lignitous coal with normal and above normal hydrogen content (perhydrous). Chemical analyses of the coals of Meghalaya have been summarized in Table III. The total sulphur content of the Meghalaya coals ranges from 1.61 to 8.92%, the average being 3.5%. The sectional variation of total sulphur reveals that in general the total sulphur content increases from b o t t o m to the top of a seam (Figs. 3b--f), with one exception, viz. in the top seam of Nangalbibra colliery (Fig. 3a). Besides the above-mentioned increase of sulphur content from base to the top of a seam, there is also a general trend of upward increase of sulphur content in the seam-wise sequence, i.e. the upper seam is found to contain in general more sulphur than the seam lying below. For instance, in West Daranggiri coalfield, Langrin coalfield and Bapung coalfield, where there are more than one seam, this trend of seam-wise increase in sulphur c o n t e n t is obvious. But there is an exception in Laitryngew coalfield (Fig. 3g), where the oldest three seams follow the above-mentioned trend, whereas the uppermost seam contains less sulphur than the underlying seam. It has been revealed by the sectional distribution of different forms of sulphur that in majority of the cases pyritic sulphur increases upward both inter- and intra-seam-wise, though an occasional tendency to concentration of pyritic sulphur towards both top and b o t t o m of a seam is also recorded. Moreover, the pyritic sulphur content is found to increase from the western to the eastern part of Meghalaya, which is mainly due to the prevalent marine conditions at the time of deposition in the eastern part of the basin (Ghosh, 1976). Coals of Meghalaya are characteristically high in sulphate-sulphur content (0.02--0.98%). The high sulphate sulphur content in the coals of Meghalaya is especially conspicuous whenever there is a thin overburden. The comparatively higher content of sulphate sulphur may be due to the weathered nature of these coals. The organic sulphur content is found to be fairly consistent over the entire basin of deposition in Meghalaya. Since part of the total sulphur is organic sulphur (56.7--97.5%), there is an apparent parallelism between organic sulphur and the total sulphur (Figs. 3a--c}. Sectional analysis has shown that all the coals of Meghalaya are not necessarily of high organic sulphur content, since there are also sections of comparatively low organic sulphur content in the seams (Figs. 3b, 3c). The overall sulphur content of the coals of Meghalaya also reflects a gradual trend of variation from west to east within the basin of deposition. If we conceive a single basin for deposition of coals of Meghalaya, then it may be seen that in the western part of the basin, coals contain low total sulphur content (Nangalbibra colliery, West Daranggiri coalfield:

S e a m No.

II I

Wait Nala S e c t i o n

II I

Rongshokham Section

IV III II I

--

II I

--

Coalfield

Nangalbibra

Siju

Langrin

Mawsynram

Laitryngew

Ishamati

Bapung

Lumshnong

Chemical analyses of coals o f M e g h a l a y a

T A B L E III

1.30

2.50 2.50

1.80

2.30 3.50 3.00 1.60

4.00

3.00 2.35

3.56

3.80 8.50

10.94

7.25 12.50

9.15

11.30 6.90 14.80 25.60

12.53

6.80 5.30

5.80

4.50 8.20

42.40

45.20 48.20

50.10

45.00 38.20 40.60 43.30

52.50

47.90 40.90

42.20

51.00 49.30

Volatile m a t t e r (%)

Moisture (%)

Ash (%)

D m f basis

Air dried basis

85.00

77.20 79.50

78.80

81.10

78.03

78.20 81.00

79.00

77.50 76.40

Carbon (%)

6.00

5.60 5.80

6.20

5.80

5.60

5.30 5.10

6.20

5.40 6.50

Hydrogen (%)

D m f basis

1.20

0.80 1.00

1.20

1.10

1.00

1.00 0.80

0.90

1.40 1.00

Nitrogen (%)

6.30

10.60 10.30

9.90

8.70

9.50

14.00 12.10

13.90

12.70 13.40

Oxygen (%)

¢,D

70

z

0

Fw

:E

W ~D

2.5 2.4

TOP

SEAM

/

VII

2.0-

LEGE.ND

1-5--

1-Oq II!

0-5-II

I

I

O0

I

I

2-0 3.0 Percentage of SuLphur

1.0

L

I

4.0

5.0

__

,J

BOTTOM SEAM 0.6 0.5

i

0.4.

)

0-3-

/

0.2o.i-

0.0 (a)

I

I

2.0 Percentage of Sulphur

1.0

3.0

Fig. 3a. Relationship between total, pyritic, sulphate and organic sulphur in coal seams of Nangalbibra colliery, West Daranggiri Coalfield, Garo Hills.

71

1.0

TOP SEAM

I

~1 ~ o.s_~

I

I

I 1-0

0.0

=

• A

LEGEND Total Sutphur Organic Sutphur Pyritic Sutphur

D

Sulphate Sutphur

o

I I 2.0 3.0 Percentage of Sulphur

1 4.0

T-

,'T,, ,,o, BOTTOM SEAM 9

/ > I o.o

1.o

I 2-0 3.0 Percentage of Sutphur

I 4.0

(b) Fig. 3b. Relationship between total, pyritic, sulphate and organic sulphur in coal seams of Borsora Colliery, Langrin Coalfield, Khasi Hills.

72 BAPUNG

COLLIERY

L)

.Q

j

LEGEND o Total Sulphur

L)

f

l

J

LU~

El•

• Organic Sulphur A Pyritic Sulphur

2!o

3!o

[] Sulphate Sulphur

4!o

61o

5!o

7to

Percentage of Sutphur

e!o

9'o

B!o

9!o

iolo

>

m

I

~o

3to

41o

go

6'.o

~o

Percentage of Sulphur

(c)

Fig. 3c. Relationship between total, pyritic, sulphate and organic sulphur in coal seams of Bapung Colliery, Bapung Coalfield, Jaintia Hills. ,=,, P-

o

1.5 1.4--, IV

11111~6

12--

I[I

E_N_O / /

/ /

i I /i//~

0.8-:

0.6--1 I 0.4-- I

0 Total Sulphur • Organic Sulphur A Pyritic Sulphur

/ f

[3 Sulphate Sulphur

o.z-I [

0.'0

I

I

I

I

1.0

2'.0

3-0

4.0

PERCENTAGE OF SULPHUR (d)

Fig. 3d. Relationship between total pyritic, organic and sulphate sulphur in coal seams of Mawlong Colliery, Ishamati Coalfield, Jaintia Hills.

73

uJ

5 LD

~.?_

RK = COAL OF RONGSHKHAM COLLIERY J - COAL OF JATHANG COLLIERY

)

K

RK

RK LEGEND

! '

Total

Sulphur

Or~jan ic Sulphur Pyritic Sulphur Sulphate Sulphur

5 o.o -

I 1,o 2.0 ~ Percentage of Sutphur

I

3-0

(e)

z C,

1-0 IV

IIl 0.5-__ ll! I

--I I = :

('f)

0.0

/

~

1

L

1.0

2.0 ~

-

I,

I

3.0 Percentage

4.0 If

J

5.0

~_1 5.5

l 6-0

Sulphur

Fig. 3e. Relationship b e t w e e n total, pyritic, sulphate and organic sulphur in coal seams of Mawsynram Coalfield, Khasi Hills. Fig. 3f. Relationship b e t w e e n total, pyritic, sulphate and organic sulphur in coal seams of L u m s h n o n g Colliery, L u m s h n o n g Coalfield, Jaintia Hills.

74 L AITP, Y NGEW

COL LIE P,Y

E~

LEGEND

/

?~.-_ ', /

/

~.~ UJ~

(g)

/ •

310 4'0 ,'o alo Percentageof Su{phur---~

/

o Total SuLphur • Organic SuLphur A Pyrltlc Sulphur [] SuLphate Sulphur

i

j 2'0 310 4!0 I'0 Percentage of SuLphur-~

Fig. 3g. Relationship between total, pyritic, sulphate and organic sulphur in coal seams of Laitryngew Colliery, Laitryngew Coalfield, Khasi Hills.

1.61--4.29%, average 2.55; Siju colliery, Siju coalfield: 1.73--2.32%, average 1.99), whereas in the eastern part of the basin the coals contain high sulphur (Mawlong colliery, Ishamati coalfield: 2.8--3.9%, average 3.62; Bapung colliery, Bapung coalfield: 3.00--8.92%, average 6.83; Lumshnong colliery, Lumshnong coalfield: 3.35--5.40%, average 5.23) (Fig. 1). To summarize, the following three types of variation in total sulphur content have been noted in the coals of Meghalaya: (a) intra-seam vertical variation; (b) inter-seam vertical variation; and (c) regional inter-field lateral variation. An attempt will now be made to reconstruct the depositional environment of the coals of Meghalaya and the causes of both local vertical variation and regional lateral variation of sulphur. The intra- and inter-seam vertical variation of sulphur, specially in the eastern part of the basin, has been ascribed to two major factors: (1) pH values decreased with depth (i.e. alkalinity increased from b o t t o m to the surface), as a result, sulphur content increased upward in the sequence, since the alkaline condition is conducive to the deposition of sulphur; (2) the calcium-rich solution from the reef complex which was at the southern side of the basin (Ghosh, 1976) generated CO2 which migrated upwards and increased the reducing condition from b o t t o m to the surface. The carbonic acid (HCO3) that was formed remained at the b o t t o m , thereby resulting in pH differences from the b o t t o m to the surface. The regional lateral variation in sulphur content was strictly palaeoenvironmental. In other words, the increase in sulphur content from west to east was due to the more marine nature of the peat-forming swamps of the Khasi and Jaintia Hills as compared to ~hat of the Garo Hills. The coals of the eastern part of Meghalaya contain a high amount of structureless collinite as well as phlobaphinite, indicative of marine origin of the eastern coal seams, as compared to the relatively higher content of sclerotinite and

75 d e g r a d o f u s i n i t e in the western coal seams o f s u p p o s e d l y deltaic origin (Mazumdar, 1980). T h o u g h the coals o f the Garo Hills were d e p o s i t e d in general in a deltaic e n v i r o n m e n t , there are indications o f gradual marine transgression leading t o m o r e t o x i c i t y , with c o n c o m i t a n t increase in sulphur c o n t e n t . This cond i t i o n p r o b a b l y prevailed up to the d e p o s i t i o n o f the middle section o f the Seam No. II o f Nangalbibra colliery. T h e transgression a p p e a r e d t o be t e m p o r a r i l y arrested, leading to a partial d r o p in sulphur c o n t e n t in the u p p e r part, b e f o r e giving way to a p r o m i n e n t transgressive phase in the Middle E o c e n e (Siju L i m e s t o n e ) . CONCLUSION T h e m o s t i m p o r t a n t finding o f the investigation appears to be the recognit i o n o f b o t h inter-seam and intra-seam vertical variations o f sulphur c o n t e n t o f the coals; this opens up the possibility of selective e x p l o r a t i o n as well as e x p l o i t a t i o n o f relatively s u l p h u r - p o o r coal h o r i z o n s amidst otherwise high-sulphur coals. T h e present investigation has also e m p h a s i z e d t h a t the s t u d y o f d i f f e r e n t forms and distribution of sulphur w o u l d provide a guideline in choosing t h e p r o p e r m e t h o d f o r removing sulphur f r o m coal. F o r e x a m p l e , if the sulphur in a coal is m o s t l y in pyritic f o r m , t h e n it is a m e n a b l e to inorganic removal or b y microbial removal using the bacteria, Thiobacillus ferrooxidans ( C h a n d r a et al., 1980b). On the c o n t r a r y , if t h e coal contains high a m o u n t s o f organic sulphur t h e n it is n o t readily a m e n a b l e to inorganic removal, b u t it can be r e m o v e d by bacterial means by isolating m i c r o b e s f r o m d i b e n z o t h i o p h e n e {Chandra et al., 1979).

REFERENCES Chandra, D., Roy, P., Mishra, A.K., Chakraborti, J.N. and Sengupta, B., 1979. Microbial removal of organic sulphur from coal. Fuel, 58: 549--550. Chandra, D., Choudhuri, S.G. and Ghose, S., 1980a. Distribution of sulphur in coals seams with special reference to the Tertiary coals of northeastern India. Fuel, 59: 357--359. Chandra, D., Roy, P., Mishra, A.K., Chakraborti, J.N., Prasad, N. and Choudhuri, S.C., 1980b. Removal of Sulphur from coal by Thiobacillus ferrooxidans and by mixed acidophilic bacteria present in coal. Fuel, 59: 249--252. Fox, C.S. and Ghosh, A.M.N., 1940. The stratigraphic position of Cherra Sandstone, Assam. Rec. Geol. Soc. India, LCCV, Proc. Paper No. 4. Ghosh, A.M.N., 1954. The Tura Sandstone stage of the Garo Hills, its possible stratigraphic position. Rec. Geol. Soc. India, LXXXVII, part 2. Ghosh, D., 1976. Palaeocene reef rock complex of Meghalaya and its oil potentialities: Sci & Cult., V.43. Indian Standard, 1969. Methods of test for coal and coke, Pt. III, Determination of Sulphur. Indian Standard 1350 (Part III), 1969, 5. Mazumdar, K., 1980. Geology and Distribution of Sulphur in the Eocene Coals of Meghalaya. Ph.D. Thesis, India School of Mines (unpubl.).