Upload
doque
View
225
Download
0
Embed Size (px)
Citation preview
18
CHAPTER 3
Petrography of Coal: A tool for evaluation
of CBM potential
3.1 Introduction
Petrography of coal is concerned with the macroscopic and microscopic description of the
coal materials, i.e., macerals or maceral groups (an indicator of coal type), mineral matter,
elemental composition and the vitrinite reflectance is usually considered as rank indicator.
The percentage of vitrinite reflectance, fixed carbon, volatile matter, moisture content and
ash determine the rank of coal. Appreciable volume of methane including other
hydrocarbons is generated during coalification processes which transform woody materials
into coal (Fig. 3.1). Maximum volume of methane obtained from coalbeds are formed by
thermogenic process, where interns are controlled by burial history, maceral composition
and basin hydrodynamics [Berbesi et al., 2009, Scott and Kaiser, 1996]. A part of the
generated gas is retained into the coal in adsorbed condition and remaining escapes from
coalbeds. The petrographic composition, coal rank and the type of organic matter are some
of the main factors controlling the quality and quantity of gaseous hydrocarbons retained by
coal [Gurdal and Yacin, 2000; Quick and Brill, 2002; Mastalerz et al., 2008]. Coal rank
indicates the level of maturation of coal, potential of methane generation, the storage
capacity and development of cleat network in coal/coaly bed. Out of all four ranks of coal
(peat, lignite, sub-bituminous, bituminous and anthracite), bituminous shows the maximum
capacity to adsorb coalbed methane as shown in Fig. 3.2. The methane retention capacity
becomes economical in high volatile bituminous coals and peaks at medium volatile
bituminous coals. Gas holding capacity of coal is related to carbon percentage, moisture
content, ash percentage and volatile matter into the coal. In most of the cases, moisture
content decreases with increasing depth and increasing rank of coal [Sivek et al., 2010].
Theoretically, lower moisture and ash content in coal can provide more sites for methane
adsorption [Bustine and Clarkson, 1998; Crosdale et al., 2008; Joubert et al., 1974; Ozdemir
CHAPTER-3 Petrography of coal
19
and Schroeder, 2009], hence gas holding capacity in high rank of coal is more than the
lower one. However, again gas adsorption capacity starts to decrease in low volatile
bituminous to Anthracite because of reduction in porosity. In lignite coal although it has
abundant porosity, the gas retention capacity is less due to presence of higher moisture
content. Mineral matter plays an important role for methane retention capacity into the coal.
Mineral matter in coal acts as an inert component and reduces the number of sorption sites.
So with increasing mineral matter in coal the sorption capacity decreases [Brown et al.,
1996]. Petrological aspect is important for evaluation of rank of coal and it depends upon
the volume of maceral group, i.e., vitrinite, semi-vitrinite, liptinite, inertinite, mineral
matter and the mean vitrinite reflectance into the coalbed. The vitrinite maceral is related to
CBM potentiality because of its methane storage capacity, presence of higher proportion of
micropores which results in more surface area and high adsorption capacity.
Fig. 3.1: Generation of Coal
(Source http://www.uow.edu.au/eng/pillar/images/about/coal_rank.jpg)
CHAPTER-3 Petrography of coal
20
Commercial CBM generating coals have reflectance values between 0.7-2.0% [Chandra,
1997]. Reflectance of vitrinite is the main component for development of natural fracture
and cleats into the coalbeds which are responsible for storage and flow of gases through
coalbeds. The cleats are generated during coalification process. The development of cleat
systems within the coal is controlled by exogenic and endogenic processes [Acosta et al.,
2007]. Ammosov and Eremin [1963] and Pashin et al., [1999] observed that most of the
cleat were generated during the bituminous rank stage while their abundance is decreased at
high rank due to an increase in the thermal maturation with the progress of coalification.
Developments of microcleats are more near the medium volatile bituminous rank of coal
and then start to decrease. Initial increase in the development of cleats takes place upto
vitrinite reflectance Ro~1.3 (medium volatile bituminous rank), thereafter, followed by a
reduction in cleat formation [Laubach et al., 1998, and Su et al., 2001]. Depending upon the
cleat density and opening of cleats, permeability of coal varies which increases with
increasing cleat density and opening. So, studies of coal quality parameters which include
grade, chemical composition, maceral composition, rank and physico-mechanical properties
are important for characterizing CBM reservoir and gas flow potential of coal [Kim, 1977;
Levine, 1993; Lamberson and Bustin, 1993; Bustin and Clarkson, 1998, Paul and
Chatterjee, 2011a, b].
Fig. 3.2: Relation between rank and gas content of coal
CHAPTER-3 Petrography of coal
21
CBM reservoir exploration and its characterization may be complex due to heterogeneity
and spatial variation in maceral composition [Chalmers and Bustin, 2007].
The objectives of this chapter are (i) to evaluate coal chemical properties from proximate
and elemental analysis of coal samples, and (ii) determination of maceral groups present in
coal samples under study, (iii) Measurement of vitrinite reflectance, (iv) SEM study and
FTIR study for identification of functional groups present in the samples. Based on
chemical and petrographic characteristics, an attempt has been made to estimate the rank of
coal and capacity for production of CBM.
3.2 Proximate analysis
Proximate analysis is the study of physico-chemical parameters of coal and can be used to
establish the grade and rank of coals as mentioned earlier. For the present study, Bureau of
Indian Standard procedures [BIS Standard: 1350, Part-1, 2003] were followed to determine
moisture, ash and volatile matter content of coals. Coal proximate analysis was carried out
in the Coalbed Methane Laboratory, Department of Petroleum Engineering, Indian School
of Mines, Dhanbad.
3.2.1 Determination of Moisture
One gm of the powdered air-dried sample was weighed and spread uniformly in a petri-dish
which was pre-heated at 108˚C, cooled and weighed. The uncovered petri-dish containing
the sample was weighed and heated in a drying oven at 108˚C±2˚C for 1.5 hours until there
is no further loss in mass. The petri dish was covered, cooled in a desiccator and weighed
again as soon as the samples have reached room temperature. The loss in mass during
drying is reported as the percentage of moisture content in the sample. Calculate the
moisture percentage in the analysis sample as follows:
Moisture in analysis sample, 2 3% 100
2 1
W W
W W
Where, W1 is the weight of empty glass petri dish with lid, (gm), W2 is the mass of the glass
petri dish with lid and coal sample before heating, (gm) and W3 is the mass of the glass
petri dish with lid and sample after heating.
CHAPTER-3 Petrography of coal
22
3.2.2 Determination of Ash content
Total 0.50 gm of the powdered air dried sample was taken in a clean platinum crucible of
known weight. The uncovered vessel containing the sample was weighed and inserted into
a muffle furnace and heated in air to 500˚C in 30 minutes. The temperature was raised from
500˚C to 815˚C±10˚C for a further 30-60 minutes and maintained at this temperature for
another 60 minutes until there is no further loss in mass. The crucible was covered with a
lid, removed from the muffle furnace and cooled in a desiccator. The cooled vessel was
weighed. The mass of remaining of burnt coal is expressed as a percentage of the total mass
of sample and is reported as the percentage of ash yield in the sample.
Ash in analysis sample, 3 1% 100
2 1
W W
W W
Where, W1 is the weight of empty platinum crucible with lid, (gm), W2 is the mass of the
platinum crucible with lid and coal sample before heating, (gm) and W3 is the mass of the
platinum crucible with lid and sample after heating.
3.2.3 Determination of Volatile matter
Total 0.5 gm of the powdered air dried sample was taken in a clean platinum crucible of
known weight. The covered crucible containing the sample was weighed, inserted into a
muffle furnace and heated out of contact with air at 900˚C ±10˚C for 7 minutes in absence
of air. The covered crucible was then removed from the furnace, cooled in a desiccator and
weighed. The percentage of loss in mass with respect to the total is expressed as percentage
of volatile matter.
Volatile matter in analysis sample, 2 3% 100 %
2 1
W WM
W W
Where, ‘W1’ is the weight of empty platinum crucible with lid, (gm), ‘W2’ is the mass of
the platinum crucible with lid and coal sample before heating, (gm), ‘W3’ is the mass of the
platinum crucible with lid and sample after heating and ‘M’ is the percentage of moisture in
the sample on air dried basis.
CHAPTER-3 Petrography of coal
23
3.2.4 Determination of Fixed Carbon
Percentage of fixed carbon is obtained by difference of the moisture percentage, ash
percentage, and volatile matter percentage from 100.
FC= 100-(% M +% A+% VM)
Where, ‘FC’ is the fixed carbon percentage, ‘%M’ is the moisture percentage, ‘%A’ is the
ash percentage and ‘%VM’ is the percentage of volatile matter.
3.2.5 Dry ash-free basis (daf) calculations
For comparison of the results of proximate analysis of different coals, the results obtained
on air dried coal is recalculated on dry ash free basis or generally known as pure coal basis.
Therefore, for Dry ash free (daf) basis calculation the formulae is as follows:
V.M. (daf), % = {V.M. (air dried) ×100}/ {100-(M+A)}
Where, ‘M’ is the percentage of moisture on air dried basis, ‘A’ is the ash percentage on air
dried coal and ‘VM’ is the volatile matter in air dried basis.
Classification of India coal: The general classification of Indian coals and lignites is given
in Table 3.1.
3.3 Elemental analysis
The elemental analysis (C, H, N, S and O) of coal samples collected from different wells of
Jharia coalfield and Singareni coalfield has been analyzed by elemental analyzer
(ElementarVario EL III- CHNS analyzer) in the Department of Fuel and Mineral
Engineering, Indian School of Mines, Dhanbad. The experimental set-up and schematic
diagram of the setup are shown in Fig 3.3 and Fig. 3.4.
Some features of the instrument are:
I. Small pellets with a pinch of titanium oxide along with the sample are prepared and
used for analysis.
CHAPTER-3 Petrography of coal
24
II. Long time stability of the multipoint calibration of the analyzer allows maintenance
free operation over night.
III. The solid samples vary in weight from micro (< 1mg) to macro (1 g) depending on
sample composition whereas bulky samples of up to 1 ml volume can also be
analyzed.
IV. During quantitative high temperature decomposition, the temperature up to 1200°C
(1800°C during combustion) for analytical accuracy was applied.
V. Gas separation with "purge and trap" technique serves for large concentration range,
detection sensitivity.
VI. Helium is used as the carrier gas.
VII. Jet injection of oxygen supplies high concentration at the place of combustion with
very low total usage.
VIII. Carbon is converted to carbon dioxide, nitrogen to nitric oxide, sulphur to sulphur
dioxide and hydrogen to water vapour. Each of these components is collected in
separate chambers.
IX. Analysis time is automatically optimized.
X. All analytical results are automatically calculated and complete operating conditions
for each sample are stored.
XI. Easy operation and maintenance, low consumable costs and no expensive special
catalysts.
XII. Compact design, easy installation and handling, cost effective operation 230/110 V,
50/60 Hz, approx. 1800 VA.
Fig. 3.3: ElementarVario EL III- CHNS analyzer
CHAPTER-3 Petrography of coal
25
Fig. 3.4: Schematic diagram of CHNS analyzer
3.3.1 Importance of elemental analysis
One of the major parameters for determination of coal ranks are elemental compositions
mainly carbon content of coal. Rank of coal increases with increasing carbon content. The
particular organic matters present in coalbeds are classified based on H/C and O/C atomic
ratios [van Krevelen, 1993]. Three main types of kerogen (Type I, Type II and Type III)
occur which are associated in specific geological settings [Tissot and Welte, 1984]. The
elemental analyses data are used for classification of kerogen type in a van Krevelen
diagram (Fig. 3.5) and this graph commonly demonstrates the path of chemical changes of
kerogen and coal macerals which can associate with coalification process and hydrocarbon
generation. The H/C and O/C atomic ratios decrease with increased thermal maturation of
organic matter along the paths of van Krevelen diagram which indicates the corresponding
kerogen and maceral types.
Type I Kerogen: Type I kerogen formed from proteins and lipids and Alginate is the main
organic constitute. The O/C atomic ratio is less than 0.15 and H/C atomic ratio is greater
than 1.25 for type I kerogen.
CHAPTER-3 Petrography of coal
26
Type II Kerogen: Type II kerogen is formed from lipid deposits under reducing conditions
and sapropelic organic matter (Liptinite) is the main constitutent. In Type II kerogen the
O/C ratio ranges from 0.03 to 0.18 and H/C ratio is less than 1.25.
Type III Kerogen: Vitrinite (humic organic matter) is the main organic component in type
III kerogen. For type III kerogen the H/C ratio is less than 1 and O/C ratio varies from 0.03
to 0.30. Type III kerogen is the product of plant material enriched in vitrinite maceral and
recognized as highly gas prone compared to type I and II kerogen.
Fig. 3.5: A van Krevelen diagram showing different kerogen types and positions of coal
and maceral groups relative to hydrogen-carbon and oxygen-carbon atomic ratios [modified
from Cornelius, 1978]
CHAPTER-3 Petrography of coal
28
3.4 Petrographic analysis of coal in laboratory
Coal petrographic analysis for collected coal samples from different coalfields and different
wells were carried out at the Central Institute for Mining and Fuel Research, Dhanbad.
Identification of various maceral groups and measurements of vitrinite reflectance (VRo) in
oil medium was carried out on polished coal surface in reflectance with white light and
fluorescence illumination using Leica DMRXP-HC advance research polarizing
microscope. Analysis was carried out as per Bureau of Indian Standards [BIS No 9127
part 2 for preparation, grinding and polishing of pellet, part 3 for maceral and mineral
identification and part 5 for VRo measurement].
Preparation of coal samples and its studies (maceral analysis and VRo) are described below:
3.4.1 Preparation of representative coal sample
For the preparation of coal samples following steps were followed:
I. The coal samples were broken at 1/2 to 1/4 inches then 1/8 to 1/16 inches and
reduce the volume by quarter and coning method to the required level, i.e., 50 gm to
100 gm in accordance with ISO 1988.
II. The sample was crushed to 16 mesh (1119 micron or 1.19 mm) by controlled
crushing and was sieved through 16 mesh/18 mesh sieve. The filtered samples were
used for pellet preparation which was retained by 18 mesh sieve.
III. The filtered samples (18+ mesh) were spread on the flat surface/ paper in a circular
fashion and then a representative coal sample (5-10 gm) was prepared for pellet
preparation by lifting the spread sample from all corners and middle by
spoon/spatula.
3.4.2 Pellet preparation from representative coal sample
Pellets used for experimental purposes were prepared as per Indian standard [IS. 9127, Pt. 2,
2002/ISO 7404-2:1985]. The volume of the pellet was 25 mm3
and the size of coal sample
was within 840 micron to 1 mm. The representative sample was very carefully crushed so
that it did not produce too much fines. About 3 gm to 6 gm of the sample was mixed with a
CHAPTER-3 Petrography of coal
29
synthetic resin in a mould of the above size. The pellet when hardened was ground and
polished as per the method laid down in the Indian standard [IS No. 9127, Pt. 2, 2002].
3.4.3 Instrument
Leica DMRX polarised light microscope fitted with a 50X oil immersion objective and 10X
paired oculars and a swift point counting was used for petrographic analysis. One of the
oculars was equipped with an adjustable eye piece in which a micrometer graticule was
placed for maceral analysis and reflectance measurements. The slide of an automatic point
counter capable of holding and moving the polished surface was fixed to the stage of the
microscope. The number of push buttons corresponds to the number of macerals and the
mineral matter was classified.
3.4.4 Analytical procedure
3.4.4a Maceral analysis
The maceral analysis has been carried out as per the method described in IS 9127(Part 3):
2002. The polished pellet was fixed on the holder of the slide in such a position that
measurement began in one corner of the specimen. The initial readings of the point counter
were read off and recorded. The point-to-point and line-to-line distances were chosen
according to the diameter of the largest grain, i.e., for a grain size below 1 mm, both the
maximum point-to-point and line-to-line distances were 0.5 mm. However, the normal
distances for maceral analyses lie between 0.3 mm and 0.5 mm. With 0.5 mm intervals, a
polished surface of 2.5 x 2.5 cm represents 800 to 1000 points so that the required
minimum of 500 points on coal was recorded with certainty.
In each case the maceral coincident with the intersection of the cross hairs was taken into
consideration. The total area of the pellet was examined, because macerals are distributed
irregularly. At the end of the measurement the values indicated in the counter were read off
and recorded. The mineral substances either associated with the macerals or free were
recorded as a total, without subdivision into individual minerals that were, however, made as
and when necessary.
CHAPTER-3 Petrography of coal
30
After completion of the analyses the number of grains counted for each individual maceral,
maceral group or mineral was expressed as a percentage of the total of points recorded.
Thus, the values obtained were looked upon as volume percentages.
3.4.4b Reflectance Measurement
Reflectance measurements were carried out using the same microscope following the Indian
and International standard methods [IS 9127, Part. 5: 2002]. Microscope is attached with
photomultiplier and following the analyses by MSP software and ‘J’ and ‘M’ make control
unit. In all the cases random reflectance (under oil) was measured on vitrinite grains for
rank determination.
3.5 Calculation of Vitrinite Reflectance from empirical equation
Vitrinite reflectance of coal was determined by using empirical equation given by Rice
[1993] and from laboratory studies. The value of vitrinite reflectance (VRo %) gives an idea
about the coal rank. The vitrinite reflectance value of coals is calculated by using the
following formula:
% 2.712 log( ) 5.092oR VM 3.1
Where, ‘Ro’ is reflectance value of vitrinite macerals and ‘VM’ is the dry, ash free volatile
matter of coal. This equation was developed for USA coal samples. Empirical correlation
for the Indian coal has been developed in the present study also and described in this
chapter.
3.6 Results and Discussion
The coal samples have been collected from three different wells (Well J1, Well J2 and Well
J3) of Jharia coalfield, Jharkhand, two different wells (Well R1and Well R2) of Raniganj
coalfield, West Bengal and two different boreholes from Singareni coalfield, Andhra
Pradesh. The rank and CBM potential of collected coal samples from different wells was
evaluated on the basis of chemical analysis and petrographic results.
CHAPTER-3 Petrography of coal
31
3.6.1 Chemical characteristics
3.6.1.1 Jharia coalfield
Total 40 numbers of samples were collected from 3 different wells (well J1, well J2 and
well J3) of Jharia coalfield and characterized in laboratory.
3.6.1.1a Proximate analysis
Ash content: Ash yields, which is an indicative measure of mineral matter, vary from 11.5
wt% to 26.6 wt% for well J1, from 10.26 wt% to 24.17 wt% for well J2 and from 14.50
wt% to 40.06 wt% for well J3. Ash content with respect to depth of seam burial for well J1
is shown in Fig. 3.6. Ash content is relatively higher in the coal samples collected from
deeper depths of well J1.
Fig. 3.6: Variation of ash content with depth (m)
Moisture content: Moisture content of samples varies from 0.50 wt% to 0.73 wt% for well
J1, from 0.17 wt% to 1.80 wt% for well J2 wt% and from 0.20 wt% to 0.32 wt% for well
J3. A cross plot between moisture content and depth exhibits a negative correlation and is
shown in Fig.3.7 and Fig. 3.8 for well J1 and well J2 respectively.
CHAPTER-3 Petrography of coal
32
Fig. 3.7: Variation of moisture content with depth (m) for well J1
Fig. 3.8: Variation of moisture content with depth (m) for well J2
CHAPTER-3 Petrography of coal
33
Volatile matter: Volatile matter on dry-ash free basis ranges from 28.4 wt%. to 16.5 wt%
for well J1, from 9.19 wt% to 24.15 wt% for well J2 and from 15.60 wt% to 26.12 wt% for
well J3. As expected dry, ash free volatile matter values, in general, are found to decrease
with depth. Volatile matter exhibits decreasing trend with sample burial depth as shown in
Fig.3.9, Fig. 3.10 and Fig. 3.11 for well J1, well J2 and well J3 respectively.
Fixed carbon: Fixed carbon of the samples collected from well J1, well J2 and well J3
varies from 71.65 wt%. to 83.53 wt%, 76.24 wt% to 91.05 wt%. and 74.15 wt%. to 84.70
wt%. respectively. Fig. 3.12 Fig. 3.13 and Fig. 3.14 displaying the plots between fixed
carbon percentage and depth of coal samples are found to increase with increasing depth of
the burial.
The fixed carbon percentages are plotted against depth for all the samples of the study area
(well J1, well J2 and well J3) of Jharia coalfield (Fig. 3.15). The overall trend of the fixed
carbon percentage increases linearly with depth, i.e., maturity of coal increases with depth
of coal seams.
Based on the values of volatile matter (daf) and fixed carbon percentage, the rank of most
of the coals recovered from Well J1, well J2 and well J3 are inferred to be medium-volatile
bituminous (mvb) to low-volatile-bituminous (lvb) rank as per classification of Indian coals
and Lignites, their properties and utilization [IS: 770-1977, clause 7.1], (Table 3.1).
Medium volatile bituminous to low volatile bituminous coals are important for prospect of
CBM than anthracite or lower rank coals because of amount of methane retention and
development of cleat network as mentioned earlier. Moisture and volatile matters are driven
off during coalification, subsidence, compaction and indurations of plant materials.
Proximate analysis results show that the volatile matter ranges from low to medium and
moisture content of the samples are < 2%, so due to presence of lower values of moisture
content and volatile percentage the study area may be zone of CBM potential.
Proximate analysis results for well J1, well J2 and well J3 are shown in Table 3.2, Table 3.3
and Table 3.4 respectively.
CHAPTER-3 Petrography of coal
34
Fig. 3.9: Variation of Volatile matter with depth (m) for well J1
Fig. 3.10: Variation of Volatile matter with depth (m) for well J2
CHAPTER-3 Petrography of coal
35
Fig. 3.11: Variation of Volatile matter with depth (m) for well J3
Fig. 3.12: Variation of fixed carbon percentage with depth for well J1
CHAPTER-3 Petrography of coal
36
Fig. 3.13: Variation of fixed carbon percentage with depth for well J2
Fig. 3.14: Variation of fixed carbon percentage with depth for well J3
CHAPTER-3 Petrography of coal
37
Fig. 3.15: Relation of fixed carbon percentage against depth for 3 wells in Jharia coalfield
Table 3.2: Proximate analysis result of samples from Well J1 of Jharia coalfield
Sample Interval Mean
Intv.
(m)
Moist
(%) Ash
(%) VM
(%)
VM
(daf)
(%)
FC
(%)
FC
(daf)
(%) from (m) to (m)
680.35 680.66 680.505 0.64 11.54 24.90 28.35 62.92 71.65
682.33 682.7 682.515 0.72 13.00 21.03 24.37 65.25 75.63
684.35 684.68 684.515 0.73 15.25 21.11 25.12 62.91 74.88
769.37 769.69 769.53 0.71 12.00 19.52 22.36 67.77 77.64
771.03 771.33 771.18 0.68 16.21 17.64 21.22 65.47 78.78
908.34 908.94 908.64 0.67 12.42 17.33 19.94 69.58 80.06
973.20 973.50 973.35 0.65 16.26 14.91 17.94 68.18 82.06
1060.17 1060.47 1060.32 0.51 24.85 14.03 18.80 60.61 81.20
1061.81 1062.11 1061.96 0.53 18.81 14.86 18.42 65.8 81.58
1064.07 1064.37 1064.22 0.55 18.92 15.52 19.27 65.01 80.73
1067.03 1067.33 1067.18 0.52 17.51 13.50 16.46 68.47 83.53
1068.94 1069.24 1069.09 0.52 26.63 13.26 18.20 59.59 81.80
1071.53 1071.83 1071.68 0.50 21.02 14.83 18.90 63.65 81.10
CHAPTER-3 Petrography of coal
38
Table 3.3: Proximate analysis result of samples from Well J2 of Jharia coalfield
Sample interval Mean
depth
(m)
Moist
(%)
Ash
(%)
VM
(%)
VM
(daf),
%
F.C.
(%)
F.C
(daf),
%
from (m) to (m)
1042.62 1042.94 1042.78 0.35 19.52 16.28 16.69 67.08 83.71
1116.81 1117.13 1116.97 1.00 20.75 8.94 9.19 71.50 91.05
1117.97 1118.29 1118.13 0.79 18.56 10.13 12.56 70.52 87.44
1130.31 1130.63 1130.47 0.42 13.99 20.34 23.76 65.25 76.24
1131.16 1131.48 1131.32 1.55 16.18 16.4 19.93 65.87 80.07
1183.76 1184.03 1183.895 0.49 24.17 12.77 16.94 62.57 83.05
1184.96 1185.26 1185.11 0.17 16.36 14.5 17.37 68.97 82.63
1409.83 1410.15 1409.99 1.36 16.86 10.57 12.92 71.21 87.08
1412.58 1412.90 1412.74 0.34 14.15 10.92 12.77 74.59 87.23
1417.03 1417.35 1417.19 0.24 13.91 10.36 12.06 75.49 87.93
1418.33 1418.64 1418.485 0.71 12.24 10.81 12.41 76.24 87.58
1420.68 1420.99 1420.835 0.57 16.38 11.21 13.50 71.84 86.50
1422.75 1423.07 1422.91 0.43 24.08 10.24 13.56 65.25 86.44
1459.15 1459.47 1459.31 0.34 10.26 9.88 11.05 79.52 88.95
1462.02 1462.33 1462.175 0.31 10.89 10.09 11.36 78.71 88.64
CHAPTER-3 Petrography of coal
39
Table 3.4: Proximate analysis result of samples from Well J3 of Jharia coalfield
Sample Interval Mean
depth
(m)
Moist
(%)
Ash
(%)
VM
(%)
VM
(daf)
(%)
FC
(%)
FC
(daf)
(%) from (m) to (m)
1073.36 1073.86 1073.61 0.35 30.11 16.64 23.92 52.9 76.07
1074.25 1074.75 1074.5 0.30 15.02 18.20 21.50 66.48 78.51
1127.31 1127.61 1127.46 0.26 14.50 17.54 20.57 67.7 79.42
1127.95 1128.26 1128.10 0.25 23.16 19.80 25.85 56.79 74.15
1128.82 1129.12 1128.97 0.35 16.68 16.80 20.24 66.17 79.75
1186.72 1187.02 1186.87 0.22 21.96 15.83 20.34 61.99 79.66
1187.49 1187.79 1187.64 0.25 40.06 14.55 24.37 45.14 75.62
1132.90 1233.20 1233.10 0.30 16.46 16.33 19.13 68.87 82.74
1233.39 1233.69 1233.54 0.25 23.27 15.63 20.43 60.85 79.56
1234.10 1234.40 1234.25 0.22 27.84 14.28 19.84 57.66 80.15
1451.00 1456.25 1453.62 0.20 16.15 12.80 15.30 70.85 84.70
3.6.1.1b Elemental analysis
Total 26 number of coal samples from 2 different wells namely well J1 and well J2 of
Jharia coalfield have been studied for elemental analysis in the laboratory. The main
chemical constituents for coals are generally carbon, hydrogen, nitrogen, oxygen, sulfur and
a small amount of nitrogen varying from 1 to 1.5% [Chandra et al., 1992]. Elemental
compositions of the samples under study are shown in Table 3.5 and Table 3.6.
Carbon content: Elemental compositions of coal samples vary that the carbon content
varies from 79.64% to 90.23% for well J1 and 84.34% to 88.09% for well J2.
CHAPTER-3 Petrography of coal
40
Hydrogen content: Hydrogen content of the sample from well J1 and well J2 varies from
3.84% to 5.30% and 4.25% to 5.26% respectively.
Nitrogen content: Nitrogen content varies from 0.60% to 2.11% for well J1 and 0.58% to
1.99% for well J2.
Sulfur content: Sulfur content of the samples collected from well J1 and well J2 varies
from 0.49% to 2.10% and 0.56% to 1.72% respectively.
Oxygen content: Oxygen content for samples from well J1 varies from 3.26% to 10.98%
and for well J2 varies from 4.69% to 8.94%.
All analysis data presented here are based on dry ash free basis. The enrichment of
hydrogen in the coal samples is possibly due to the prevalence of anoxic conditions of
bacterial decay. The different researchers like Newman and Newman, 1982; Gentzia and
Goodarzi, 1994; Gurba and Ward, 1998; Petersen and Rosenberg, 1998; and Singh et al.,
2.10a, 2012 have documented that enrichment of hydrogen occurs due to organic matter
deposition in a sedimentary basin where depositional condition was anaerobic conditions.
The figures (Fig. 3.16, Fig. 3.17) show a continuous increase in carbon content with
decreasing trend of oxygen content. The carbon content is a better measure of rank of coal
[Chandra, 1992]. Range of carbon contents in samples again confirms that the samples
under study belong to medium-volatile bituminous (mvb) to low-volatile-bituminous (lvb)
rank.
Table 3.5 and Table 3.6 list the experimental results of elemental composition of coal
samples. The plots (Fig. 3.18 and Fig. 3.19) between H/C and O/C ratio in the van Krevelen
diagram fall in the zone of type III kerogen and rank of coal samples indicate the
bituminous rank which indicates a gas prone zone. The diagram (Fig. 3.20) shows the
overall plot of H/C and O/C atomic ratio in the van Krevelen diagram for study area (well
J1 and well J2) located at Jharia coalfield and fall in the zone of type III kerogen.
CHAPTER-3 Petrography of coal
41
Fig. 3.16: Relationship between carbon and oxygen content of coal samples from well J1 of
Jharia coalfield
Fig. 3.17: Relationship between carbon and oxygen content of coal samples from well J2
CHAPTER-3 Petrography of coal
42
Table 3.5: Elemental analysis data of samples from Well J1 of Jharia coalfield
mean depth
(m)
Elemental data (%), daf basis Atomic ratios
C H N S O H/C O/C
680.66 79.64 4.16 2.16 0.80 13.24 0.63 0.03
682.7 82.52 4.06 1.74 0.70 10.99 0.59 0.03
684.68 85.46 4.28 1.55 0.76 7.95 0.60 0.03
769.69 84.58 3.85 1.58 0.62 9.37 0.55 0.02
771.33 82.54 5.17 0.60 2.17 9.52 0.75 0.04
908.94 82.99 4.60 0.58 1.96 9.87 0.67 0.02
973.50 82.56 5.30 0.60 1.44 10.10 0.77 0.03
1060.47 88.29 4.56 1.74 0.80 4.61 0.62 0.02
1062.11 91.52 4.60 1.52 0.50 1.86 0.60 0.02
1064.37 90.03 4.35 1.74 0.62 3.27 0.58 0.02
1067.33 81.37 5.25 1.83 0.73 10.82 0.77 0.02
1069.24 87.71 5.07 2.03 0.52 4.68 0.69 0.02
1071.83 87.16 5.30 2.12 0.59 4.84 0.73 0.04
1074.20 90.24 4.66 1.65 0.69 2.77 0.62 0.02
CHAPTER-3 Petrography of coal
43
Table 3.6: Elemental analysis data of samples from Well J2 of Jharia coalfield
mean depth
(m)
Elemental data (%), daf basis Atomic Ratios
C H N S O H/C O/C
1116.97 85.30 5.23 1.96 0.88 6.63 0.74 0.03
1118.13 87.04 4.34 1.67 0.87 6.08 0.60 0.02
1130.47 87.04 4.91 1.75 0.58 5.71 0.68 0.03
1183.895 86.81 4.25 1.73 0.80 6.42 0.59 0.02
1185.11 86.86 5.09 1.51 0.77 5.77 0.70 0.04
1409.99 85.74 4.89 1.65 0.56 7.15 0.68 0.04
1412.74 87.12 5.26 1.80 0.58 5.23 0.72 0.03
1417.19 86.90 5.01 0.58 1.63 5.88 0.69 0.03
1418.485 84.50 4.82 0.75 1.72 8.20 0.69 0.03
1420.835 87.72 4.70 0.60 1.57 5.42 0.64 0.03
1422.91 88.09 4.57 1.99 0.66 4.69 0.62 0.01
1459.31 84.34 4.50 1.45 0.77 8.94 0.64 0.03
Fig. 3.18: Variation of Kerogen types & position of coal relative to H/C and O/C atomic
ratios in van Krevelen diagram for well J1
CHAPTER-3 Petrography of coal
44
Fig. 3.19: Variation of Kerogen types & position of coal relative to H/C and O/C atomic
ratios in van Krevelen diagram for well J2
3.6.1.2 Samples from Raniganj coalfield
The proximate analysis results of well R1 of Raniganj field indicate that the percentage of
moisture is low whereas ash and volatile matter is quite high. The moisture content varies
from 0.77 wt% to 0.96 wt% (average 0.87 wt%) and ash percentage ranges from 12.07 wt%
to 24.92 wt% (average 17.65 wt%) respectively. The coal samples are generally medium
volatile which varies from 32.4 wt% to 38.89 wt% having an average value of 35.48 wt%
CHAPTER-3 Petrography of coal
45
(from 41.87 wt% to 46.45 wt% daf basis, average 43.55 wt%) and the fixed carbon
percentage ranges from 41.91 wt% to 50.57 wt%, average 45.99 wt% (from 53.54 wt% to
58.12 wt% as daf basis, average 56.43 wt%).
Fig. 3.20: Variation of Kerogen types & position of coal relative to H/C and O/C atomic
ratios in van Krevelen diagram for well J1 and well J2.
CHAPTER-3 Petrography of coal
46
Ash percentage of these samples from well R2 of this field is low and varies from 7.6 wt%
to 8.3 wt%. Coal of this field are generally low to medium volatile (from 19.8wt % to 20.7
wt%) coals, moisture percentage are ~ 2% and fixed carbon percentage varies from 69 wt%
to 70.7 wt% (from 76.92 wt% to 78.12 wt% as daf basis). The proximate analysis results
indicate that the coal samples belong to medium volatile bituminous rank. The proximate
analysis result has been summarized in Table 3.7
Table 3.7: Proximate analysis result of samples from 2 different wells of Raniganj coalfield
Sample Interval Mean
Intv.
(m)
Moist
(%)
Ash
(%)
VM
(%)
VM
(daf)
(%)
FC
(%)
FC
(daf)
(%)
from (m) to (m)
WELL R1
662.33 662.63 662.48 0.93 17.23 35.22 43.03 46.62 56.96
662.58 663.88 663.23 0.96 18.55 34.47 42.82 46.02 57.17
708.40 708.70 708.55 0.77 24.92 32.4 43.60 41.91 56.39
711.88 712.18 712.03 0.93 12.07 36.43 41.87 50.57 58.12
1002.03 1002.34 1002.20 0.78 15.5 38.89 46.45 44.83 53.54
WELL R2
972.33 972.83 972.58 2.00 8.30 20.70 23.07 69.00 76.92
973.43 973.90 973.66 1.90 7.60 19.80 21.87 70.70 78.12
CHAPTER-3 Petrography of coal
47
3.6.1.3 Samples from Singareni coalfield
3.6.1.3a Proximate analysis
Total 12 numbers of coal samples were collected from Dorli-Bellampalli coal belt of
Singareni coalfield, Andhra Pradesh. From the proximate analysis results, it was observed
that the ash content varies from 10.52 wt% to 26.59 wt%. Proximate analysis of the
investigated coal samples revealed that the moisture content (M %) varies from 2.46% to
3.82%, whereas volatile matter ranges from 23.30 wt% to 40.26 wt% and fixed carbon
content varies from 26.01 wt% to 53.21 wt%. It is recognized that the fixed carbon of coal
increases with increase in coal depth which is directly proportional to the coal maturity and
coal rank. The similar trend is observed in the present study as shown in Fig. 3.21.
The proximate analysis results and calculated vitrinite reflectance values are summarized in
Table 3.8.
Fig. 3.21: Variation of fixed carbon percentage with depth
CHAPTER-3 Petrography of coal
48
3.6.1.3b Elemental analysis results
From the elemental results of coal samples obtained from 2 different bore holes of
Singareni coalfield was found that carbon content varies from 59.93% to 83.84% (daf
basis), hydrogen content varies from 5.11% to 5.87% (daf basis), nitrogen content varies
from 1.92% to 2.87% (daf basis), sulfur content varies from 0.58% to 0.97% (daf basis) and
oxygen content varies from 8.21% to 15.97% (daf basis). The elemental analysis results are
represented in Table 3.9. Carbon content of the samples are plotted against oxygen content
(Fig. 3.22) and it is inferred that carbon content increases with the decrease of oxygen
percentage. Atomic ratio of H/C are plotted against O/C atomic ratio in the van Krevelen
diagram (Fig. 3.23) and observed that the coal samples from this coalfield belongs to
kerogen type III organic matter with a maturity of Lignite and sub-bituminous stage, hence
gas content capacity is low.
Table 3.8: Proximate analysis result of samples from Singareni coalfield
Sample Interval Mean
depth
(m)
Moist
(%)
Ash
(%)
VM
(%)
VM
(daf)
(%)
FC
(%)
FC
(daf)
(%) from (m) to (m)
Bore hole No. BPA210
426.95 428.95 427.95 3.76 26.00 32.50 46.27 37.74 53.73
429.43 432.00 430.71 3.01 24.94 32.75 45.45 39.3 54.55
498.23 498.35 498.29 3.04 26.59 26.30 37.37 44.07 62.63
499.61 499.86 499.73 3.38 22.82 31.62 42.85 42.18 57.15
501.72 504.00 502.86 3.12 17.03 30.46 38.15 49.39 61.85
540.17 541.77 540.97 3.53 22.65 23.30 31.56 50.52 68.44
Bore hole No. R1255
368.59 369.19 368.89 2.95 23.00 28.96 39.11 45.09 60.89
370.60 372.40 371.5 2.46 45.99 25.45 49.37 26.1 50.63
434.67 436.57 435.62 3.43 25.17 33.61 47.07 37.79 52.93
438.03 439.57 438.8 3.72 15.39 27.68 34.22 53.21 65.78
442.72 444.02 443.37 3.15 10.52 40.26 46.64 46.07 53.36
455.08 458.12 456.6 3.82 11.15 31.11 36.59 53.92 63.41
CHAPTER-3 Petrography of coal
49
Table 3.9: Elemental analysis data of samples from 2 wells of Singareni coalfield
mean depth (m) Elemental data (%), daf basis Atomic Ratios
C H N S O H/C O/C
Bore hole No. BPA210
426.95 77.82 5.82 2.51 0.97 12.88 0.90 0.12
429.43 79.79 5.72 2.26 0.92 11.31 0.86 0.11
498.23 81.67 5.39 2.40 0.84 9.71 0.79 0.09
499.61 80.51 5.72 2.34 0.75 10.68 0.85 0.10
501.72 82.87 5.44 1.97 0.71 9.02 0.79 0.08
540.17 83.84 5.82 2.51 0.97 12.88 0.90 0.12
Bore hole No. R1255
368.89 83.84 5.11 2.15 0.58 8.32 0.73 0.15
435.62 74.57 5.65 2.87 0.95 15.97 0.91 0.16
438.8 78.94 5.77 2.34 0.71 12.24 0.88 0.12
443.37 83.67 5.33 2.11 0.68 8.21 0.76 0.07
456.6 81.36 5.87 1.92 0.73 10.11 0.87 0.09
Fig. 3.22: Relationship between carbon and oxygen content of coal samples from 2 wells of
Singareni coalfield
CHAPTER-3 Petrography of coal
50
Fig. 3.23: Variation of Kerogen types & position of coal relative to H/C and O/C atomic
ratios in van Krevelen diagram for 2 blocks of Singareni coalfield.
3.6.2 Petrographic study
The petrography of coal samples was determined from laboratory studies and from
empirical equation.
3.6.2.1 Sample from Jharia coalfield
Vitrinite: From petrographic study, it is found that Vitrinite is the dominant maceral group
followed by the Inertinite and Liptinite. The volume of vitrinite ranges from 32.04% to
73.35% for well J1, 41.20% to 74.90% for well J2 and 43.70% to 59.00% for well J3.
CHAPTER-3 Petrography of coal
51
Semi-vitrinite: Semi-vitrinite ranges from 0.60% to 4.10% for well J1, from 1.80% to
2.60% for well J2 and from 0.60% to 2.00% for well J3.
Inertinite: Inertinite ranges from 15.02% to 51.03% for well J1, from 18.20% to 35.00%
for well J2 and from 20.00% to 45.10% for well J3.
Liptinite: Liptinite value for well J1, well J2 and well J3 ranges from 0.02% to 0.98%,
0.20% and 0.20% to 0.40% respectively.
Visible Mineral matter: Mineral matter ranges from 5.73% to 23.00% for well J1, from
4.60% to 5.80% for well J2 and from 6.20% to 35.10% for well J3.
Mean vitrinite reflectance (VRO): Mean vitrinite reflectance of the samples from well J1,
well J2 and well J3 ranges from 1.23% to 1.65%, 1.58% to 2.03% and 1.42% to 1.79%
respectively.
Empirical correlation
For studied coal samples, Rice equation was modified for the Indian coal and also used to
calculate the value of vitrinite reflectance (Ro%) and compared with the results determined
experimentally in laboratory.
% 2.056 log( ) 4.023oR VM 3.2
Vitrinite reflectance of coal is also determined by using Rice empirical equation (1993) and
modified equation for Jharia coalfield. The plots (Fig. 3.24, Fig. 3.25 and Fig. 3.26) show
the variation of vitrinite reflectance with increasing depth of burial for well J1, well J2 and
well J3 of Jharia coalfield. Mean vitrinite reflectance also depends upon the volatile matter
and with increasing volatile matter percentage of vitrinite decreases (Fig.3.27, Fig. 3.28 and
Fig. 3.29). With increase in depth, the volatile matter decreases as mentioned earlier and the
volume of vitrinite increases in coal. Presence of increased volume of vitrinite reflects more
rays incident on its surface.
Variations in the experimental and calculated data were observed for almost every sample
because Rice equation was developed mainly for USA coal which is different from that of
Indian coal. However, similar trends were observed for both the cases.
CHAPTER-3 Petrography of coal
52
Fig. 3.24: Variation of vitrinite reflectance with depth for well J1
Fig. 3.25: Variation of vitrinite reflectance (analysis) with depth for well J2
CHAPTER-3 Petrography of coal
53
Fig. 3.26: Variation of vitrinite reflectance with depth for well J3
Fig. 3.27: Variation of vitrinite reflectance with volatile matter for well J1
CHAPTER-3 Petrography of coal
54
Fig. 3.28: Variation of vitrinite reflectance with volatile matter for well J2
Fig. 3.29: Variation of vitrinite reflectance with volatile matter for well J3
CHAPTER-3 Petrography of coal
55
The vitrinite maceral is more favorable for CBM potentiality because of development of
higher proportion of micropores which can enhance the gas storage capacity and adsorption
capacity of coal. At a given rank of coal, vitrinite contains more micropores compared to
inertinite and liptinite [Unsworth et al., 1989; Lamberson and Bustin, 1993; Beamish and
Crosdale, 1995]. Surface area of coal is guided by the pore size distribution of a sample
whereby the surface area progressively increases with decline in pore size (microporosity)
for a given pore volume. Table 3.10 shows the variation of pore volume with rank and
depth of coal seam and also reported the gas content and methane adsorption capacity with
pore volume. Some investigators [Gan et al., 1972; Clarkson and Bustin, 1996; Prinz et al.,
2004; Printz and Littke, 2005] reported that micro porosity increases with higher rank of
coal and this is the reason why methane capacity increases with rank. Table 3.11, Table
3.12 and Table 3.13 lists the variation of Vitrinite reflectance and maceral group obtained
from laboratory analysis and established correlation.
Table 3.10: Variation of pore volume, surface area and gas content with depth and rank of
samples from Well J1 of Jharia coalfield
Parameters Well J1 Well J1 Well J1
Depth (m) 682.50 1130.57 1418.33
Vitrinite (%) 73.35 60.20 51.50
Total gas content (cc/g) 12.28 10.14 8.73
CH4 volume 21.90 20.20 17.10
Micropore volume(cc/g) 0.100 0.024 0.046
Micropore area (m²/g) 189.650 31.747 86.476
External surface area (m²/g) 46.111 37.770 21.468
Surface Area (m²/g ) 235.762 83.944 107.944
Total pore volume (cc/g) 1.900e-01 1.269e-01 1.172e-01
Average pore Radius (Å) 1.61186e+01 3.02376e+01 2.17186e+01
CHAPTER-3 Petrography of coal
56
Table 3.11: Petrographic analysis result of samples from Well J1 of Jharia coalfield
Mean
Intv. (m)
Maceral (Vol.)%
Mean
Ro%
Ro%
From Rice’s
equation
Vitrinite Semi-
vitrinite
Liptinite Inertinite Visible
Mineral
matter
680.50 73.35 0.70 0.20 20.02 5.73 1.23 1.15
682.51 59.02 0.60 0.98 29.50 9.90 1.34 1.33
684.51 68.20 0.80 0.20 23.80 7.00 1.33 1.29
769.53 70.98 0.85 0.89 15.02 12.26 1.42 1.43
771.18 48.51 1.10 0.71 37.44 12.24 1.40 1.49
908.64 56.45 1.39 0.48 32.05 9.62 1.49 1.57
973.35 32.04 1.40 0.50 43.06 23.00 1.55 1.69
1060.32 32.45 3.98 0.20 51.03 12.34 1.57 1.64
1061.96 36.80 4.10 0.20 42.03 16.87 1.61 1.66
1064.22 45.29 3.30 0.09 42.33 8.99 1.57 1.61
1067.18 36.46 2.20 0.50 39.86 20.98 1.61 1.79
1069.09 63.01 1.82 0.37 27.42 7.38 1.65 1.67
1071.68 55.28 3.60 0.02 34.25 6.85 1.62 1.63
1074.05 38.45 2.20 0.20 50.13 9.02 1.62 1.12
Table 3.12: Petrographic analysis result of samples from Well J2 of Jharia coalfield
Mean
depth
(m)
Maceral (Vol.)% Mean
Ro%
Ro%
From Rice’s
equation Vitrinite Semi-
vitrinite
Liptinite Inertinite Visible
Mineral
matter
1116.81 58.2 1.8 0.2 35.00 4.8 1.58 2.81
1130.57 60.2 2.5 0.2 32.1 5.2 1.63 1.54
1185.11 41.2 1.4 0.0 51.4 5.8 1.72 1.94
1418.33 51.5 2.6 0.0 40.1 5.8 2.02 2.39
1459.31 74.9 2.3 0.0 18.2 4.6 2.03 2.29
CHAPTER-3 Petrography of coal
57
Table 3.13: Petrographic analysis result of samples from Well J3 of Jharia coalfield
Mean
depth
(m)
Maceral (Vol.)% Mean
Ro%
Ro%
From Rice’s
equation Vitrinite Semi-
vitrinite
Liptinite Inertinite Visible
Mineral
matter
1073.61 43.7 0.8 0.4 20.0 35.1 1.42 1.35
1127.46 46.7 0.7 0.2 45.1 7.3 1.51 1.53
1128.97 48.2 0.6 0.2 44.2 6.8 1.52 1.55
1233.54 59.0 1.5 0.0 33.3 6.2 1.56 1.54
1451.63 48.0 2.0 0.0 39.6 10.4 1.79 1.88
The vitrinite reflectance ranges between 1.23% to 2.03% and hence it may be concluded
that the study areas are good for CBM potential. Based on chemical analysis and
petrographic analysis results, the coals are inferred to range between medium volatile
bituminous (mvb) to low volatile bituminous (lvb) rank.
Relation of Vitrinite reflectance with depth of burial for Jharia coalfield
The plot between mean vitrinite reflectance and depth of coal samples indicates that with
increasing depth of burial of coal seam vitrinite reflectance increases (Fig. 3.30).
Fig. 3.30: Variation of mean vitrinite reflectance with depth of 3 wells of Jharia coalfield
CHAPTER-3 Petrography of coal
58
The plot (Fig. 3.31) illustrates that with increase of hydrogen content of coal, vitrinite
reflectance increases. Rajpardi lignite deposit of Gujarat and Kalimantan coals of Indonesia
also show the similar trend [Singh et al., 2010a].
Fig. 3.31: Relationship between hydrogen content (daf) and vitrinite reflectance of coal
samples from 2 wells (well J1 and well J2) of Jharia coalfiled
3.6.2.2 Raniganj coalfield
From the proximate analysis data, calculated vitrinite reflectance value ranges from 0.57%
to 0.69% for well R1and from 1.40% to 1.45% for well R2. Hence the coal samples from
well R1 under study are inferred to be high volatile-C bituminous in rank and for well R2
coal rank are medium volatile bituminous which are good for CBM. Table 3.14 displays the
results of vitrinite reflectance derived from empirical equation.
Table 3.14: Results of Vitrinite reflectance for Raniganj coalfield from empirical equation
CHAPTER-3 Petrography of coal
59
Sample interval Mean Intv. (m)
VRo %
From Rice’s equation
from
(m) to (m)
WELL R1
662.33 662.63 662.48 0.66
662.58 663.88 663.23 0.66
708.40 708.70 708.55 0.64
711.88 712.18 712.03 0.69
1002.03 1002.34 1002.20 0.57
WELL R2
972.33 972.83 972.58 1.40
973.43 973.90 973.66 1.45
3.6.2.3 Singareni coalfield
The Vitrinite reflectances of samples are determined and value ranges from 0.50% to 1.02%
(Table 3.15).
3.7 Scanning Electron Microscope (SEM) analysis
Scanning electron microscopy is widely used for morphological/surface of the surface
analytical techniques. For SEM analysis, the samples were coated with gold to prevent
charging of the sample [Deydier et al., 2005]. The SEM micrographs at different
magnifications of coal samples from Jharia coalfield are shown in Fig. 3.32, Fig.3.33 and
Fig. 3.34 respectively. The figures indicate the presence of different particle sizes and
granular shapes with irregular hexagonal features.
CHAPTER-3 Petrography of coal
60
Table 3.15: Results of Vitrinite reflectance for Singareni coalfield from empirical equation
Mean Intv. (m)
VRo %
From Rice’s equation
Bore hole No. BPA210
426.95 0.57
429.43 0.60
498.23 0.82
499.61 0.66
501.72 0.80
540.17 1.02
Bore hole No. R1255
368.89 0.77
435.62 0.50
438.8 0.55
443.37 0.93
456.6 0.56
3.8 FTIR spectroscopic analysis of the samples
The FT-IR spectra obtained from coal samples of different depth intervals (680.50 m,
682.70 m and 908.64 m) from Jharia coalfield are shown in Fig. 3.35, Fig. 3.36 and Fig.
3.37 respectively. The intensity of broad band at 3431 cm-1
and 3436 cm-1
is due to
stretching frequency of OH group. Intensity of the bands at 2900 cm-1
and 2924 cm-1
is
corresponding to the stretching vibration of aromatic and aliphatic C-H bonds. Coal
consists of polycyclic materials. The bands observed at 1603 and 1607 cm-1
can be assigned
CHAPTER-3 Petrography of coal
61
to skeletal C-C stretching modes. The bands at 1401 cm-1
, 1403 cm-1
and 1439 cm-1
are
assigned to –CH2 scissoring and –OH bending vibration respectively. The bands 1232 cm-1
and 1262 cm-1
are due to C-C stretching vibration. The intensity bands at 1031 cm-1
and
1031 cm-1
are due to C-O-C stretching vibration. Signals related to mineral matter
contained in coal at 800-400 cm-1
was observed in both the samples.
Fig. 3.32: SEM of sample from Jharia coalfield of 680.50 m depth
Fig. 3.33: SEM of sample from Jharia coalfield of 682.70 m depth
CHAPTER-3 Petrography of coal
62
Fig. 3.34: SEM of sample from Jharia coalfield of 908.64 m depth
Fig. 3.35: FT-IR spectra of sample from Jharia coalfield of 680.50 m depth
CHAPTER-3 Petrography of coal
63
Fig. 3.36: FT-IR spectra of sample from Jharia coalfield of 682.70 m depth
Fig. 3.37: FT-IR spectra of sample from Jharia coalfield of 908.64 m depth
CHAPTER-3 Petrography of coal
64
3.9 Conclusions
Based on the laboratory analysis results, the following conclusions are drawn:
1. Coal samples from different wells of Jharia coalfield have been tested for their
petrological properties to determine their CBM generating potential. It has been
observed that samples from all three wells of Jharia coalfield can be considered
viable as potential for CBM gas resources.
2. Fixed carbon percentage shows an increasing trend with depth, whereas moisture
and volatile matter content decreases with increasing depth which is normal for
maximum coal.
3. Vitrinite reflectance values vary from 1.23% to 2.03% as analysed and increases
with increase of depth. On the basis of proximate and petrographic analysis results,
the rank of coal from three wells of Jharia coalfield can be inferred to be medium-
volatile-bituminous (mvb) to low-volatile-bituminous (lvb) rank. Vitrinite
reflectance values are within the range of 0.7% to 2.0% as demarcated by Chandra
(1997) and these 3 wells (Well J1, Well J2 and Well J3) of Jharia coalfield are
highly prospective for CBM exploration. Petrographically, the coals are dominated
by vitrinite macerals followed by inertinite and liptinite in low concentration and
cleat density can be high due to presence of high values of vitrinite macerals.
4. From the proximate analysis results of the samples collected from well R1 of
Raniganj coalfield, the coal samples are inferred to be high volatile-C bituminous in
rank. High volatile-C bituminous rank of coal is less effective for generation of
methane and development of cleat system. While samples from well R2 of this field
are designated as medium volatile bituminous coal and have comparatively higher
methane generation capacity than the lower rank.
5. SEM, IR and surface area analysis gives the insights of particle size and shapes,
functional groups and pore size distribution of the samples.