@chirimiri Coals, Koriya District, Chhatisgarh, India
@chirimiri Coals, Koriya District, Chhatisgarh, India
@chirimiri Coals, Koriya District, Chhatisgarh, India
Abstract
Representative coal samples were collected from different coal seams of the Chirimiri coalfield which cov-
ered the entire stratigraphic sequence. These samples were tested for Chemical analysis, Crossing Point
Temperature (CPT), Petrography, Infrared studies (IR) and Differential Thermal Analysis (DTA). All the test
results vindicated that the aforesaid parameters had a definite relationship with the stratigraphic disposition
or the ranks of coal. The low rank coals found as younger seams in the stratigraphic sequence were more
prone to spontaneous combustion whereas the higher rank coals found at the bottom of stratigraphic se-
quence were less prone to spontaneous combustion. Through combustibility characterisation by different
tests, it was found that the upper Duman and Kaperti seams placed as younger seams in the stratigraphic se-
quence are highly prone to spontaneous combustion whereas the lower Karakoh and Sonawani seams seem
to be least prone to spontaneous combustion.
Keywords: Chirimiri Coalfield, Crossing Point Temperature (CPT), Infrared (IR) Studies, Differential
Thermal Analysis (DTA), Spontaneous Combustion
is a part of Son-valley basin. It falls within 23˚08'N and and eastern margins. Due to unevenness of the Precam-
23˚15'N latitudes and 82˚17'E and 82˚25'E longitudes brian basement, varying thickness of the Talchir sedi-
and covers an area of 130 sq. km. This coalfield has a ments is preserved at different places. The Talchirs are
unique physiographic setting. Unlike other Gondwana composed of olive green shale and lemon yellow fine
basins, this coalfield is marked by high hills with steep grained sand stone. The sand stone is usually compact
scarp faces and deep gorges along the course of stream with unaltered feldspars. Towards the top of the Talchirs,
flows. The mean altitude is about 650 m above mean sea a transition zone is well defined. This zone is character-
level (MSL) which is unique as compared with other ised by grey shales interbanded with green shales. The
Gondwana coalfields in India. This coalfield forms a grey shales on weathering develop distinctive greenish
plateau amidst the surrounding plains formed by Talchir shale and break into splintery fragments. The transitional
sediments. The geological map of Chirimiri coalfield is zone contains thick units of light grey, fine to medium
shown in Figure 1. grained sandstones which at places are not distinguish-
able from the Barakar strata.
2.1. Stratigraphic Formations 2.1.3. Barakars
The Talchirs grade upwards into the Barakar Formation
2.1.1. Precambrians which crops out on the highlands and occupies the cen-
The Precambrian rocks do not crop out in the vicinity of tral part of the basin. It is composed of light grey, coarse
the coalfield. These are found to the northwest side of the grained sandstones and the cement is normally kaolin-
area and comprise granites, gneisses and few outcrops of ized feldspar. Lenticular bands of pebbly sandstone are
quartzite. also common. The proportion of fine grained sandstone
is less compared to coarse grained sandstone. Ripple drift
2.1.2. Talchirs laminated shales are also found. Coal seams, however,
Talchir Formation covers a large tract of the low lying show prominent horizons within the sand stone domi-
plains surrounding the coalfield on the western, southern nated cycles.
Figure 1. (a) Outline map of India with location of Chirimiri coalfield. (b) Geological map of Chirimiri coalfield (modified
after Raja Rao [5]).
2.2. Basic Flows, Dykes and Sills rank of all seams. Similar results have been proved in
Raniganj coalfield [6], Talchir coalfield [7] and Ib-valley
Basic flows overlie the Barakar sequence and occur on coalfield [8]. Gradual decrease of moisture and volatile
the tops of the hills giving rise to steep escarpments. A matter down the stratigraphic sequence is observed.
prominent sill of dolerite defines the northern boundary
of the Chirimiri coalfield which continues further north- Table 1. Stratigraphy.
ward into the Sonhat coalfield. The maximum thickness
Age Formation Lithology
of the sill is reported to be 100 m. A few dykes are also
Basic flows, dykes
reported. Upper Cretaceous to
Deccan Trap & sills (60 m. to
The stratigraphy of the area is shown in Table 1. Eocene
100 m)
Sandstones with
2.2.1. Geology of Coal Seams subordinate shales
Lower Permian Barakar
and coal seams
Chirimiri coalfield is one of the best and extensively de- (230 m to 435 m)
veloped coalfields in Chhatisgarh. There are seven work- Predominantly
ing collieries in this field, namely Kurasia, Chirimiri, Upper Carboniferous olive green shales
Talchir
New Chirimiri Pondi Hill (NCPH), West Chirimiri, Du- to Lower Permian and fine grained
sand stones (+9 m)
man Hill, North Chirimiri and Koriya colliery. --------------- --------------
The most important seam in Chirimiri basin is the Ka- Unconformity
----- -----
rakoh seam which is exposed in all the blocks. It is lo- Precambrians
Granite, gneisses
cally named as Bijora seam in Koriya colliery and Ghor- and quartzites
ghella seam in Duman Hill and North Chirimiri collieries.
This is a marker seam and co relatable in all collieries. Table 2. Generalised sequence of coal horizons in Chirimiri
The other co relatable seam is the Sonawani seam which coalfield.
is the lower most seam in Chirimiri coalfield. It is also Seam Thickness (m)
referred as Kotmi seam in Duman Hill and North Chiri- Duman 0.2 to 2.3
miri collieries. The sequence of coal seams is reflected in Parting 6 to 50
Table 2. Kaperti 0.2 to 8
Parting 12 to 44
2.2.2. Chemical Analysis Karakoh - Bijora - Ghorghella 1.5 to 19.8
The chemical analysis of the Chirimiri coals (Table 3) Parting 7.0 to 60
reveals that the top most seam known as Duman seam is Sonawani - Kotmi 0.1 to 7.8
the lowest in rank whereas the Sonawani seam found at Parting 90 to 130
the bottom of the stratigraphic sequence shows highest Talchir Formation
Kk/6 137
Kk/5 140
Kk/4 144
Karakoh 143.1
Kk/3 142
Kk/2 147
Kk/1 148.5
So/3 151
Figure 4. Evolution paths of macerals of Chirimiri coals Sonawani So/2 148 151.3
(After VAN KREVELEN [11]). So/1 155
Table 5. Maceral composition (volume%) and reflectance (Rm% and Rmax%) of vitrinite of the coals of the Chirimiri coal-
field.
bottom seam. Pyrite and other mineral matter vary from increases with the increase of vitrinite and exinite whereas
5.2% to 18.2% in which pyrite contributes to 0.2% to it decreases with increase of inertinite.
3%. The mean reflectance (Rm%) of vitrinite varies be-
Correlations have been drawn between vitrinite and tween 0.52% and 0.61% which suggests the Chirimiri
CPT (Figure 5), exinite and CPT (Figure 6) and iner- coals to be of low rank. The rank increases from the top
tinite and CPT (Figure 7). It is seen that CPT decreases Duman seam to bottom Sonawani seam in harmony with
with increase of vitrinite and exinite. On the other hand, increasing reflectance value. The maximum reflectance
CPT increases with increase of intertinite. Thus suscepti- (Rmax%) of vitrinite of the Chirimiri coals is correlated
bility to spontaneous combustion of the Chirimiri coals to volatile matter (Figure 8) and the elemental carbon
Figure 9. Relation between elemental carbon and reflectance (Rmax%) of vitrinite of Chirimiri coals.
Two sharp but small peaks appear in the region be- the rate of auto-oxidation; hence lower the crossing point
tween 3000 cm–1 and 2800 cm–1. Most of the coal sam- temperature. Thus a higher content of aromatic C = O
ples show characteristic absorption at 2920 ± 10 cm–1 could be responsible for susceptibility to spontaneous
and 2850 ± 10 cm–1. These spectral bands indicate the combustion of Duman and the Kaperti seams.
presence of aliphatic CH, CH2 and CH3 groups [14]. The peaks at 1020 ± 10 cm–1 occur due to presence of
These aliphatic absorption bands are stronger in Duman kaolinite or clay minerals containing large amount of
and Kaperti seams and less intense in Karakoh and kaolinite. Sharp peaks at 600 cm–1, 530 cm–1, 460 cm–1
Sonawani seams. This type of aliphatic chains also oc- and 340 cm–1 are also indicative of the presence of min-
curs at 1450 ± 10 cm–1. The intensities of these bands in eral matter.
different seams vary in the same fashion as mentioned
above. 2.6. Differential Thermal Analysis
The 1600 ± 10 cm–1 absorption band corresponding to
the double bond stretching variation of aromatic C = O, The Differential Thermal Analysis (DTA) technique has
is the most spectacular among all the spectra. The coals been proved to be useful to assess the proneness of coal
of Duman and Kaperti seams show stronger absorbance to spontaneous combustion. It is the rate of rise of the
than the coals of other seams. Choudhury et al. [15] es- heat evolution of coal during aerial oxidation which other-
tablished that higher the aromatic content, the faster is wise controls the proneness to spontaneous combustion.
Chandra et al. [12] used this technique to assess the the peak maxima are found between 400˚C and 415˚C,
degree of proneness to spontaneous combustion in Rani- whereas for the samples of the Kaperti and the Duman,
ganj coals. Stott & Baker [16] observed that in DTA, the peak maxima are in between 390˚C and 410˚C. The
initial stage of spontaneous heating of coal is due to exothermic peaks due to the combustion of fixed carbon
evaporation which leads to cooling effect, but the exo- are found in the temperature range of 455˚C to 490˚C in
thermic reaction due to oxidation soon gains ground. the samples of Sonawani seam and the Karakoh seam
Banerjee & Chakraborty [17] studied DTA on coal and whereas for the samples of Kaperti and the Duman seams,
found that at stage-I, the reaction is predominantly en- the peaks range from 455˚C to 495˚C.
dothermic due to the release of moisture, but after some- A comparison of DTA peak temperatures and the cor-
time the reaction becomes exothermic at stage-II due to responding crossing point temperatures is also shown in
oxidation. The rate of rise of heat evolution in stage-II Table 6. A graphical relation between the CPT and the
process is much lower if the coal is poorly combustible. first DTA endothermic peak temperature is shown in
Banerjee [18] further observed that in DTA the degree of Figure 12.
cooling in stage-I is directly proportional to inherent
moisture content, but subsequent exothermic reaction 2.7. Correlation Co-Efficient for DTA and CPT
follows due to oxidative heating. Banerjee et al. [19]
further concluded that oxidation kinetics in DTA studies The correlation co-efficient between two parameters
facilitates air entry due to opening up of active centres in such as DTA and CPT was calculated and the best fit line
the surface of coal. Chandra et al. [12] and Behera & was drawn (Figure 12). The r value of these two pa-
Chandra [20] applied DTA technique to evaluate the rameters was found to be 0.9192 and the‘t’ value was
degree of proneness to spontaneous heating of coal in
Raniganj and Ib-valley coalfields respectively.
n 2
calculated by using the formula t = r where t
The experimental results of the DTA of the Chirimiri 1 r 2
Table 6. Comparison of DTA vis-a-vis CPT of the coals of the Chirimiri coalfield.
Figure 12. Correlation between DTA endothermic peak temperature and crossing point temperature of Chirimiri coals.
of coal,” Fuel, Vol. 32, 1953, p. 415. “Rate of Studies of Aerial Oxidation of Coal at Low
[17] S. C. Banerjee and R. N. Chakravorty, “Use of D.T.A. in Temperature (30˚C - 170˚C),” Fuel, Vol. 49, No. 3, 1972,
the Study of Spontaneous Combustion of Coal,” Journal pp. 324-332. doi:10.1016/0016-2361(70)90024-4
of Mines, Metals, Fuels, Vol. 15, No. 1, 1967, pp. 1-5. [20] P. Behera and D. Chandra, “Spontaneous combustion of
[18] S. C. Banerjee, “Spontaneous Combustion of Coal,” Ph.D. the Ib-valley coals of Orissa (India)—A DTA Study,”
Thesis, Calcutta University, Calcutta, 1969. Minetech, Vol. 16 No. 1-2, 1995, pp. 52-57.
[19] S. C. Banerjee, B. D. Banerjee and R. N. Chakravorty,