Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.10, pp.905-911, 2011
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905
Analytical Study of Two Differently Ranked Coals
Using UV-VIS-NIR Spectroscopy
B. Manoj
1,2
* and A.G.Kunjomana
1
1
Department of Physics, Christ University, Hosur Road, Bangalore, Karnataka, India 560029
2
Research and development Cell, Bharathiar University, Coimbatore, Tamil Nadu, India
*Corresponding Author: [e-mail]
ABSTRACT
The characterization of Indian bituminous and subbituminous coal was performed by UV-
Visible–NIR spectroscopy. Chemical leaching with varying concentration of hydrofluoric acid
was conducted on both the samples. Electronic absorption at this region was higher for higher
ranked coals. Chemical leaching increased electronic transitions in subbituminous coal with
maximum transitions for HF (10%) leached samples. The absorption maximum of benzene-
oxygen system was found between 235-270 nm and was showing a red shift with leaching. The
characteristic naphthalene ring systems (220 & 280 nm) were masked by the absorption regions
of monoaromatic rings; indicating the content of napthalenoid hydrocarbon was very low. The
bands observed in the visible region (450nm) were attributed to SO
2
in the sample and was
showing a red shift. The weak band at the 680 nm was attributed to the П-П* electronic
transitions of the polynuclear aromatic hydrocarbons which also showed red shift with leaching.
It was found that the ash content is reduced by 87.5% & 76.2% in bituminous and subbituminous
coal respectively with HF (30%) leaching.
Key words: UV-VIS-NIR spectroscopy, Coal characterization, chemical leaching, proximate
analysis.
1. INTRODUCTION
Worldwide demand for energy will continue to increase in the future primarily due to the
industrialization of the developing nations. Fossil fuels especially coal will account for most of
the energy demand. The widely distributed and abundant reserves, the relatively low price and

906 B. Manoj and A.G.Kunjomana Vol.10, No.10
scarcity in petroleum products are the most recurrent quoted advantages for the continuing use of
coal. There is, however, a growing environmental concern over the deleterious effects caused by
the use of fossil fuels such as green house effect, acid rain, and the release of trace elements etc.
In addition to this, minerals and ashes in fossil fuels can create erosion and fouling in gas turbine
and furnace. In this scenario, clean coal and its characterization is of prime important.
Vibrational spectroscopy in particular had been widely used for the study of clay mineral
structure for a very long time [1-2]. Over the past 15 years, diffuse reflectance spectrometry had
been applied to the mid-IR spectra of coals and carbons; this method gives very sharp and
reliable IR bands. Osamu Ito et al [3] reported that the broad absorption bands observed in the
region between 4000-6000 cm
-1
by this technique was attributable to electronic transitions and
not to the effect of scattering. For the UV-Visible and near-IR regions above 6000 cm
-1
,
however, diffuse reflectance spectrometry had been applied less for the characterization of coal
samples, even though this method has widely applied to solid organic and inorganic compounds.
In the present paper, it is reported that the absorption spectra in the UV-Visible and near-IR
regions, as measured by diffuse reflectance, change with the coal rank and chemical treatment.
The high heating value (HHV) of the original and leached sample was also estimated.
2. EXPERIMENTAL METHODS
Two differently ranked coal samples were taken randomly from two deposits in India. One was a
sub-bituminous coal from Godavari basin and the other high volatile bituminous coal from
Korba coal fields. The samples were powdered and dried in a dessicator to remove absorbed
water and were used without further purification. Samples were leached with hydrofluoric acid
(HF) for 24 hrs of concentration 10%, 20% and 30%. After the specified time of leaching the
samples were washed with enormous amount of distilled water and dried in an oven of about
70
o
C. The dried samples were powdered and used for the analysis.
2.1. Proximate Analysis
Proximate analyses of the samples are conducted by the standard procedure using a muffle
furnace. The highest heat value and ash content of the virgin and chemical leached samples are
determined.
2.2. UV-Vis-NIR Absorption Spectroscopy
The Praying Mantis DRA was installed into the Cary 500 spectrophotometer and aligned. The
wavelength range was set to scan from 800 to 200 nm. The ‘spectral band width’, ‘beam mode’,
‘average time’ and ‘slit height’ were set to 5 double, 0.3sec, and reduced respectively.
‘Zero/baseline’ correction was selected for baseline correction, using powdered KBr as a

Vol.10, No.10 Analytical Study of Two Differently Ranked Coals 907
reference. Diffuse reflectance spectra of the powdered coal samples were obtained using the
standard sampling cup supplied with the Praying Mantis accessory. The spectra were recorded
for leached samples also as per the described procedure and imported into origin software for
analysis.
3. RESULT & DISCUSSION
3.1. Proximate Analysis
Proximate analysis of the virgin and chemical leached samples are conducted and recorded in
table.1. Figure.1 shows the effect of HF leaching on demineralization of subbituminous and
bituminous Indian coal. It is found that the ash content in the bituminous coal is reduced from
10.20 wt% to 1.27 wt% with HF (30%) leaching. A significant reduction of 87.5% is achieved.
The fixed carbon content reported an increase of 16%. For the subbituminous coal, the ash
content varies from 12.87 % to 3.06%, a reduction of 76.22% is observed with HF (30%)
leaching. This result is better than the previous reported result of single stage leaching with HF
where 74% reduction in ash content was reported [4-5]. The fixed carbon content is increased
about 50% from initial value with HF (30%) leaching in the case of subbituminous coal. The
higher heating value (HHV) of the sample was determined and is recorded in table.1. It is found
that the HHV is increasing systematically with increasing HF concentration. The maximum
increase was for the sample leached with HF (30%) for samples, 22% and 17% for
subbituminous and bituminous coal respectively.
Table.1. Proximate analysis of virgin and leached samples
Sample FC V.M. Ash Moisture HHV(kJ/kg)
G 45.66 36.90 12.87 4.57 23024
G (10%) 66.29 28.23 04.68 0.80 27938
G (20%) 66.67 26.83 04.40 3.10 27825
G (30%) 68.89 24.26 03.06 3.79 28099
K 52.31 30.40 08.20 9.09 24024
K(10%) 60.35 30.31 04.37 4.97 26458
K (20%) 60.78 30.36 02.24 6.62 26613
K (30%) 62.05 31.18 01.27 5.50 27127

908 B. Manoj and A.G.Kunjomana Vol.10, No.10
Figure 1. Effect of chemical leaching on ash content
3.2 UV-Vis-NIR Spectrum
The electronic absorption spectrum of various coal powders had been measured by diffuse
reflectance spectrometry in the UV-Visible and NIR regions and was shown in Figure.2 and
Figure.3. The electronic absorption spectra of sub-bituminous and bituminous coal were
measured by the diffuse reflectance spectrometry in the UV-Visible and near IR regions (200-
800 nm). The molecular moieties likely to absorb light in the 200 to 800 nm region were Pi-
electron functions and hetro atoms having non-bonding valence shell electron pairs. Such light
absorbing groups are referred to as chromophores. Some chromophores like in C=O, n →П*
transitions gave rise to λ
max
at 290 nm and N=O (n → П*) transition at 275 nm. The presence of
chromophores in a molecule was best documented by UV-Visible spectroscopy. In the measured
spectra (Figure 2 and Figure 3) the σ band (260-270 nm) had defined structure. This absorption
increases with leaching with strong absorption for HF leached samples. The general shape of the
spectrum was characteristic for hydrocarbons with a single benzene ring. The presence of
napthalenoid hydrocarbons would show up in the spectrum and could be judged qualitatively
only in the 320 nm regions, where benzoid hydrocarbon give practically no absorption of UV
radiation. The two principal bands that were the characteristic for the naphthalene system (220 &
280 nm) were masked by the absorption regions of monoaromatic rings; this indicated that the
content of napthalenoid hydrocarbon was very low [1-7].The absorption maximum of benzene-
oxygen (1/1) system was found between 240-270 nm. This was due to the benzene –oxygen
charge transfer band and was showing a red shift with leaching. The two common transitions of
isolated carbonyl groups are n→p* transition was lower in energy ( λ
max
=290 nm) than the p-p*
transition (λ
max
= 180 nm), but the excitation of former is thousand times smaller than the latter.
From the spatial distribution of these orbitals it was clear that, the n-orbitals did not overlap at all
well with the p* orbital, so the probability of excitation was small.

Vol.10, No.10 Analytical Study of Two Differently Ranked Coals 909
The p-p* transitions, on the other hand, involved orbitals that had significant overlap, and the
probability is near 1. Many other kinds of conjugated Pi-electron system acted as chromophores
and absorbed light in the 200- 800 nm region. This included unsaturated aldehydes, ketones and
aromatic ring compounds. Unsaturated ketones gave strong П-П* absorption at 242 nm and
weak n-П* absorption at 300 nm. Benzene exhibited very strong absorption at 180 nm and group
of much weaker bands at 254 nm. Only the last groups were displayed in the spectrum because
of 200 nm cut-off characteristic of the spectrometer.
Olajire et al [3] reported prominent peaks at 400 nm (Soret band) and at wavelength ranges of
535-550 nm (β band) and 565-600 nm (α band) for the Nigerian coal minerals. In present study,
sample showed peaks at 400 nm which increase with leaching and was maximum for sub-
bituminous coal sample (H 10%). Frederick J et al [4] reported two maxima at 620 and 577 nm
in New Mexican coals. They designated the most intense band at 620 nm to the color of vitrinite
in transmission. The absorption of the particular maxima increased with HF leaching in sub-
bituminous coal where as in bituminous coal it decreases. The sample was almost transparent in
the region 475 nm to 650 nm and near IR region. There were small distinct absorption peaks in
the UV region and 680 nm. The intensity of the broad absorption in the visible (680 nm) and
near IR region (759 nm) were attributable to the П-П* electronic transitions of the poly nuclear
aromatic hydrocarbons, increases with the rank of coal [3, 7]. The intensity of this band
increased with chemical leaching as an indication of electronic transitions with leaching.
200 300 400 500 600 700 800
0.78
0.80
0.82
0.84
0.86
0.88
0.90
0.92
0.94
0.96
0.98
1.00
1.02
1.04
1.06
1.08
KX
HF 30%
H F 40 %
H F 20 %
absrbance
w a ve le n gth
Figure 2. UV-Vis-NIR spectrum of bituminous coal

910 B. Manoj and A.G.Kunjomana Vol.10, No.10
200 300 400 500600 700 800
0 .8 2
0 .8 4
0 .8 6
0 .8 8
0 .9 0
0 .9 2
0 .9 4
0 .9 6
0 .9 8
1 .0 0
1 .0 2
1 .0 4
1 .0 6
1 .0 8
HF 1 0 %
HF 4 0 %
HF 3 0 %
G X
absorb ance
wav ele n g th
Figure 3. UV-Vis-NIR spectrum of sub-bituminous coal sample
In the absorption spectra, small spikes were observed at between 350-400 nm and 450 nm. This
was assigned to sulphur dioxide (SO
2
) in the sample [9]. The absorption of this spikes increases
with HF leaching and was generally showing a shift towards higher wavelength in the case of
sub-bituminous coal compared to bituminous coal. This red shift in the wavelength was an
indication of decrease of absorption with leaching. Maximum absorption was shown by sample
leached with HF 10% in the case of subbituminous coal (Figure.3) and in the case of bituminous
coal HF 40% is showing maximum increase. For HF 30% and 20% absorbance was less.
Osamu Ito [3,10] reported that the absorption spectra in the UV-Visible and near-IR regions, as
measured by diffuse reflectance, change with the rank of coal. With HF leaching absorption in
both samples were showing a shift to higher wavelength. This might be due to the change in
chemical environment of the organic molecule. The electron-releasing groups increased both the
wavelength and the intensity of the secondary absorption band.
4. CONCLUSIONS
In the measured spectra, the σ band (260- 270 nm) had defined structure. This part of the
spectrum was characteristic for hydrocarbons with single benzene ring. The characteristic bands
of naphthalene ring system (220 and 280 nm) were masked by the absorption regions of the
monoaromatic rings; indicating the content of napthalenoid was very low. The band in the region
680 nm was attributed to the П-П* electronic transitions of the polynuclear aromatic
hydrocarbons. Intensity of this band increases with chemical leaching was an indication of
increase of electronic transitions with leaching. The weak bands observed in the visible region
(450 nm) were due to the presence of SO
2
in the sample and was showing a red shift with HF
leaching. The electron releasing groups increases both the wavelength and intensity of the
secondary absorption band. It was found that the ash content is reduced by 87.5% & 76.2% in
bituminous and subbituminous coal respectively with HF (30%) leaching

Vol.10, No.10 Analytical Study of Two Differently Ranked Coals 911
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