Structural Parameters of Perhydrous Indian Coals

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Structural Parameters of Perhydrous Indian CoalsPuja Khare a;B. P. Baruah a

a Coal Chemistry Division, North-East Institute of Science and Technology (CSIR), Jorhat, Assam,India

Online publication date: 18 May 2010

To cite this Article Khare, Puja andBaruah, B. P.(2010) 'Structural Parameters of Perhydrous Indian Coals', InternationalJournal of Coal Preparation and Utilization, 30: 1, 44 — 67To link to this Article: DOI: 10.1080/19392691003781616URL: http://dx.doi.org/10.1080/19392691003781616

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International Journal of Coal Preparation and Utilization, 30: 44–67, 2010

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ISSN: 1939-2699 print/1939-2702 online

DOI: 10.1080/19392691003781616

STRUCTURAL PARAMETERS OF PERHYDROUSINDIAN COALS

PUJA KHARE AND B. P. BARUAH

Coal Chemistry Division, North-East Institute of

Science and Technology (CSIR), Jorhat, Assam, India

Higher hydrogen content of perhydrous coals exhibits a differentcomposition and physicochemical properties in comparison withnormal coals. In the present investigation, a structural study ofperhydrous coals and coke was done using FTIR and HPLC data.These coals have high volatile matter with high-calorific valuesand low-moisture content. The structural study suggests that themajor structural units of these coals are simple phenols withpara-alkyl substituted derivatives. They have high alkyl substitutiongroups and low aromatic compounds. The structural studies revealthat these coals contain high amounts of low-molecular weightPAH compounds with 1-2 ring structures. These 1-2 ring struc-tures have high H/C ratios as compared to large ring polyaro-matic hydrocarbons (PAHs). It may also be one of the reasonsfor high H/C ratios in these coals. The alkyl groups contributesignificantly to their high volatile matter (VM) contents. Thepresence of alcoholic groups found in pyrolytic products may alsobe due to the conversion of catechol-like structures to those ofcresols. Coal properties, such as moisture, VM, H/C ratio, and CV,

Received 6 November 2009; accepted 16 March 2010.The authors are thankful to the Director, North-East Institute of Science &

Technology (CSIR), Jorhat, Assam for constant support and encouragement. Authorsare also thankful to Dr. K. Maharaj Kumari, D.E.I., Agra and Dr. Karuna Shanker,Scientist, CIMAP, Lucknow for providing support in analysis of PAHs. The financialsupport by Council of Scientific & Industrial Research, New Delhi is gratefullyacknowledged.

Address correspondence to Puja Khare, Coal Chemistry Division, North-EastInstitute of Science and Technology (CSIR), Jorhat, Assam 785006, India. E-mail:[email protected]

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STRUCTURAL PARAMETERS OF PERHYDROUS INDIAN COALS 45

do not correlate with the rank as normally classified. A definiterelationship has been found between the characteristics of thesecoals, char/cokes, and aromatic characters (fa, Har).

Keywords: Aromaticity; Coal; Coke; Polyaromatic hydrocarbons;Pyrolysis

INTRODUCTION

The coals with abnormally high-hydrogen content in comparison totheir carbon content are generally referred to as perhydrous coals. Theenriched hydrogen content is responsible for structural modificationof perhydrous coals [1–4]. Perhydrous vitrinites have the potential togenerate hydrocarbon liquids in the course of natural coalification [5].The thermal stability of perhydrous coals is low and during their thermaltreatment they mainly generate oils/tars, even though they are humiccoals, which are almost totally or exclusively composed of vitrinite[6]. Perhydrous coals also possess high-calorific values and, therefore,they are good candidates for use as a fuel source. However, there islittle information about their structural changes during pyrolysis. Duringpyrolysis, structure of coal undergoes significant changes. Pyrolyticreactions are known to be the primary source of polyaromatic hydro-carbons [7]. The formation of PAHs enhances the aromatic character-istics of pyrolytic products. Fourier Transform Infrared Spectroscopy(FTIR) techniques have been widely used for the assessment of thestructural changes of normal coals [7] and lignites [8]. The review ofliterature shows that most of the studies address the qualitative struc-tural modification (analysis of functional groups) of pyrolytic product ofnormal coals [1, 2, 7–10].

The northeastern region of India has coal deposits of tertiary originaged from Paleocene to Oligocene. The coals from the Eocene ageare vitinite rich (>70 vol%, mineral matter free (mmf)), with moderateamounts of liptinite (>8%, mmf) and inertinite (>5 vol%, mmf) with0�37%–0�67% vitrinite reflectance [11]. These coals can be classified assub-bituminous to high volatile bituminous coals according to ASTMstandards. These coals have high-hydrogen content and are suitable forcombustion and conversion (i.e., liquefaction) processes. Some of thesecoals have caking properties used for coke making. Physicochemicalproperties and petrography of these coals have been studied [11–13];however, data on structural studies is limited [14]. Studies on quantitative

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46 P. KHARE AND B. P. BARUAH

determination of structural modification of these perhydrous coals andtheir abnormal characteristics have not been adequately reported so far.

This work focuses on the study of a set of perhydrous coals ofEocene age in order to establish the interrelations between hydrogencontent and structure and explain their anomalous properties. This isaccomplished by direct characterization of coal, char/coke, combiningwith FTIR and polyaromatic hydrocarbons (PAH) analyses.

EXPERIMENTAL

The freshly mined coal samples from Mondiati (MM), Sutanga (MS),Khlieriat (MK), and Bapung (MB) of Jaintia Hills, Meghalaya (91◦

58′–92◦50′ E longitudes and 25◦02′–25◦45′ N latitudes), India, were usedin this study. The air-dried samples were ground to 0.211mm prior topyrolysis experiments.

Pyrolysis experiments were carried out in a quartz fixed-bed reactor(19mm internal diameter (i.d.)) with a temperature programming systemin an inert atmosphere (Figure 1). In each run, about 20g of air-driedcoal sample was taken into the retort, and the free space of the retortwas filled with quartz wool and ceramic granules. Pyrolysis of perhy-drous coals were performed using a standard method [15]. The retortwith coal samples was inserted into the furnace heated up to the desired

Figure 1. Diagram of pyrolysis experiment.

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STRUCTURAL PARAMETERS OF PERHYDROUS INDIAN COALS 47

temperatures. The final temperatures of 450, 600, 850, and 1000◦C wereattained in 15, 25, 35, and 60 min, respectively, and were kept constantfor 1 hour for complete evolution of gas. The retort was cooled, andthe char/coke was weighed. The pyrolysis temperatures selected were450, 600, 850, and 1000◦C corresponding to primary coal carbonization,semi-coke formation, secondary coal carbonization, and coke formation,respectively [16]. Char/coke was grounded to 0.211mm before analysis.The carbon, hydrogen in coal, and char/coke were analyzed by usinga True Spec CHN analyzer (Leco). TGA 701 proximate analyzer(Leco) and AC-350 Automatic bomb calorimeter (Leco) were used forProximate and calorific values, respectively (Table 1).

FTIR spectra of the coals and pyrolyzed samples were recorded ina Perkin-Elmer system 2000, model 640B (wavelength 400–4000 cm−1,with an accuracy of 0.3) using a KBr pellet with same weight ofdried coal samples. The spectra for 124 scans were recorded at aresolution of 2 cm−1. Software facilities were used for baseline correc-tions of the spectra that were scaled down to 1mg sample cm−2.For quantitative measurements of spectra, duplicate pellets were used.The aliphatic hydrogen (Hal) and aromatic hydrogen (Har) contentswere calculated from the integrated absorbance areas of the bands at3000–2700 cm−1 and 900–700 cm−1, respectively. For sub-bituminouscoals, it is reported that the extinction coefficients used for convertingintegrated absorbance areas to concentration units were 541 and 710abs cm−1 mg cm−2 for the aromatic and aliphatic bands respectively [7].The apparent aromaticity, fa, of the samples was calculated by using themethod of Brown and Ladner [17] for the coals under study:

fa = 1 − Cal/C, (1)

Cal/C = (Hal/H.H/C)(Hal/Cal), (2)

Table 1. Proximate, ultimate, H/C and CV values of coal samples

Ultimate (% DMF basis)Proximate (% as-received basis)

Coalsamples C H N S H/C M Ash VM

CVKcal/kg

MK 64.08 5.72 1.4 2.9 1.07 3.1 11.5 34.60 7280MB 64.32 6.07 1.1 4.46 1.13 1.5 11.5 35.60 7690MM 70.08 6.10 1.3 3.98 1.04 2.7 11.5 41.60 7070MS 74.64 7.50 1.2 2.9 1.21 2.9 20 40.50 6545

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48 P. KHARE AND B. P. BARUAH

where Cal/C is the aliphatic carbon fraction, H/C is the hydrogen-to-carbon atomic ratio calculated from elemental analysis, Hal/H is thefraction of total hydrogen present as aliphatic hydrogen and Hal/Cal ishydrogen-to-carbon atomic ratio for aliphatic groups, which is generallytaken to be 1.8 for coals [9].

Selected zones of the FTIR spectra were studied by curve-fittingstatistical analysis. Selected regions were baseline-linearized using aninteractive procedure of the program by connecting the left and rightpoints of the interval with a straight line. After baseline adjustments, thespectra have intensities tended towards zero at both ends of the regioneliminating a possible source of artifacts in the deconvoluted spectra.Position and shape of the bands were similar to those reported by Ibarraet al. [9]. In order to understand the structural parameters, selectedzones of spectral band areas belonging to CH3 (2951–2973 cm−1), CH2

(2827–2857 cm−1), Car (1518–1617 cm−1), C=O (1613–1702 cm−1) andC–O (1062–1131 cm−1) groups have been calculated by multiplying thepeak widths (cm−1) with peak heights. Baseline adjustments yieldedspectra whose intensities tended towards zero at both the sides [10].

PAH extraction, clean up, and analysis were done using highperformance liquid chromatography (HPLC) grade solvents. The driedcoal/char samples (1.0g) sample was placed in an extraction thimblemade of filter paper Whatman # 1 in a Soxhlet extractor. Samples wereextracted in methylene chloride at the rate of four cycles per hour for 16hours under protection from light. The extract was concentrated up to1.0mL in rotary evaporator under reduced pressure. Analysis was donein triplicate.

The concentrated extract was redissolved in cyclohexane prior tocleanup using elution under gravity over a silica gel column. The slurryof 10g activated silica gel in methylene chloride was prepared andloaded into a chromatographic column (i.d. 10mm). The methylenechloride was used for the washing of column. The silica column waswashed with methylene chloride followed by 25mL of n-pentane atan elution rate of about 2mL/min. Prior to being eluted dry, 1.0mLof sample extracted in cyclohexane was loaded on the column. Anadditional 4.0mL cyclohexane was used to complete the transfer. Thecolumn was washed with 25mL of n-pentane without disturbing theupper layer and the washings were discarded. The PAHs on the columnwere eluted with 25mL of a methylene chloride/n-pentane mixture (2:3v/v) in a conical flask [18]. The eluate was evaporated to 1.0mL and

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STRUCTURAL PARAMETERS OF PERHYDROUS INDIAN COALS 49

solvent exchanged with 1.0mL acetonitrile. The extracted and cleanedsamples were protected from light and stored at 5�0◦C until the analysisby HPLC. The weight of methylene extracted and the cleaned sampleswere considered as total PAHs [18].

PAHs were characterized and quantified in each sample of coal andchar using a fully automated HPLC (Shimadzu, Kyoto, Japan) equippedwith a pump (LC-20AD), PDA detector (SPD-M 20A), column oven(CTO-20A), degasser (DGU-20A5) and Lab solution version 3.21software. The reverse phase chromatography was performed using aC18 column, Symmetry ShieldTM WATERS (4�6 × 250mm, 5�m). ThePAH standards and samples were eluted isocratically at a flow rateof 1.5mL/min using acetonitrile and water (70:30, v/v). Twenty �Lof standards as well as sample were injected and the column effluentwere recorded online from 200–600nm and stored in the LC-solutionsoftware of the HPLC system. The quantification of the targeted PAHswas performed at 254nm. The specificity of each PAH sample wasconfirmed by UV-Vis spectra matching with the standard PAHs. Theequipment was calibrated for the retention time values using PAHstandards (610-N, SUPELCO, USA). All the solvents used were ofHPLC grade (Merck, India). All the solvents were filtered through a0�45-�m Millipore membrane (Millipore, Billerica, USA). A minimumdetection limit of individual PAHs ranged from 0.5 to 5.0ng/mL (partsper billion, ppb) and their percent recovery was 82%–96%.

The statistical analysis was performed using Microsoft exceland SPSS version 11.5 program. A two-tailed ANOVA test andMann–Whitney U-test were applied for statistical analysis. For corre-lation and curve fitting, linear model was adopted.

RESULTS AND DISCUSSION

Characteristics of Coal

Physicochemical properties of the coals under study are shown inTable 1. These coals have high volatile matter and low-ash content withhigh-calorific values. H/C atomic ratios of these coals ranged from 1.1to 1.4, which were higher than reported for normal coals (0.5–0.77) [19].All the coal samples have high-sulfur contents (2.9%–4.6%).

The rank of a coal can also be determined by the moisturecontent (as-received). The relationship between moisture (as-received)and rank for Turkish coals [20] and those of the present study are

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50 P. KHARE AND B. P. BARUAH

Figure 2. Relationship between moisture content (as-received) and CV of Turkishcoals [20] and perhydrous Indian coals.

shown in Figure 2. All the coals with moisture content in excess of27.6%, 21.5%–27.6%, 15%–21.5% are classified as lignites, lignitesor sub-bituminous, and sub-bituminous coals, respectively. The coalshaving 10%–15% and less than 10% are also classified as either sub-bituminous or bituminous and bituminous, respectively [20]. All theperhydrous coals studied have a moisture content less than 5% (1.5%to 3.1%) (Table 1) corresponding to bituminous coals (Figure 1). Thecalorific values (CVs) of these coals ranging from 6545 to 7690Kcal/kgshow that these coals fall in the lignite to bituminous category and,hence, may be classified as lingo-bituminous [21].

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STRUCTURAL PARAMETERS OF PERHYDROUS INDIAN COALS 51

Figure 3. Variation in the hydrogen content of coal with its rank for normal [19] andperhydrous Indian coals.

Atomic H/C ratio can also be used to estimate rank of coal [22].A variation of the H/C ratio with its rank in normal path of coalificationalong with present data is given in Figure 3. All the coals have high H/Cratio as compared to their rank.

The abnormal behavior of these coals (low rank with high VM, lowmoisture and high CV) may be due to a modification in coal structuresby the presence of a high H/C ratio.

Characterization of Coal by FTIR

A comparison of the FTIR spectra in the region 400–4000 cm−1 forcoal and char/coke at four different temperatures is presented inFigure 4. The FTIR spectra of all the coals show the strongest inten-sities of aliphatic CH2 stretching vibration at 3000–2800 cm−1, defor-mation at 1460–1401 cm−1, aromatic C=C ring stretching at 1506–1665and 1100–1000 cm−1 due to aliphatic ethers and alcohols varying inintensity. The peak at 900–700 cm−1 is due to CH out-of-plane vibrationin poly-aromatic ring structures. CO peaks due to aromatic aldehydic,ketonic, and carboxylic groups are obtained at 1650–1720 cm−1 whilethe phenolic –OH group gives a broad peak at 3600 cm−1.

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52 P. KHARE AND B. P. BARUAH

Figure 4. FTIR spectra of perhydrous Indian coals.

There are several distinct peaks in the range from 900–1000 cm−1

and 720–730 cm−1 in the IR spectra of coals. The peak at 900–1000 cm−1

might be due to CH2 deformation in olefins, where specific peaklocations in this range are at 970 and 950 cm−1 due to CH2 defor-mation in vinyl and trans distribution, and at 910 cm−1 due to CH2

deformation in normal olefin. The peak at 720–730 cm−1 might bedue to CH2 rocking in long-chain alkanes that has shown four ormore methylene groups in the molecular structures. The bands obtainedbetween 700–900 cm−1 represent the organic moieties exclusively ratherthan mineral matters [10]. It is reported that the major structural unitsof perhydrous raw vitrains are simple phenols with a predominance ofpara-alkyl substituted derivatives [2]. The presence of such a structure isreflected through the clear absorption at 1500 cm−1 and predominanceof the 815 cm−1 peak in the 900–700 cm−1. The latter feature has alsobeen associated with a low degree of substitution/condensation of thearomatic units in low-rank coals [9].

In aliphatic stretching region (2800–3000 cm−1), there are distinctpeaks found at 2914 and 2845 cm−1, attributed to symmetric andasymmetric CH2 stretching, respectively. The methyl-to-methylene ratiocan be considered as an estimate of aliphatic chain length of thecoals and a branching index [10, 23]. The CH3/CH2 ratio varied from0.65 to 1.07 (Table 2). These ratios are higher than the normal sub-bituminous coals (0.48) [10]. It indicates the presence of more methylgroups and more branching of aliphatic chains in these perhydrous coals

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STRUCTURAL PARAMETERS OF PERHYDROUS INDIAN COALS 53

Table 2. Structural parameters of coals

Coalsamples Hal Har Hal/Har Car/C = O Cal/C CH3/CH2 Car/(C=O + Car) fa

MK 1.96 0.05 39.2 0.45 0.66 0.70 0.48 0.33MB 1.76 0.42 4.19 0.31 0.50 1.06 0.38 0.50MM 2.02 0.30 6.73 0.52 0.62 0.65 0.66 0.38MS 2.39 0.28 8.53 0.58 0.69 0.95 0.69 0.31

supporting a high-H/C ratio. The release of high amounts of C1–C4hydrocarbons during pyrolysis is also consistent with this fact. Thezone of oxygen-containing functional groups (1660–1720 cm−1), charac-terized by a very intense peak, is attributed to C=O groups. The C–Ogroups in 1067–1131 cm−1 and 1031–1041 cm−1 region are also verydistinct from the spectra. The Car/(C=O + Car) ratio is an index forassessing the degree of maturation of coal organic matter [24, 25].The Car/(C=O + Car) ratio (Table 2), surprisingly, is higher than theratio reported for lignite and sub-bituminous coals by Ibarra et al. [10].The possible reason for the high ratios is the high-hydrogen and sulfurcontent of these coals. Replacement of oxygen with sulfur [26] in thesecoal molecules may increase the Car/(C=O + Car) ratio.

Generally, high-VM content is attributed to high concentrationof methyl groups and lower hydrocarbon and oxygenated compounds.However, these coals have abnormally high CH3/CH2 and Car/(C=O +Car) ratios. Methyl groups may also contribute significantly to volatilematter as compared to oxygenated groups. Hence, values of CV of thesecoals were found to be high.

The calculated Hal and Har values are shown in Table 2. The highHal/Har ratio is consistent with the subsequent stage of evolution. Theintensity of the aliphatic peaks relative to the aromatic ones in thesecoals is characteristic of immature coal. These spectral features arecharacteristics of perhydrous coals [27].

It is also reported that the bands centered at 870 cm−1, 815 cm−1,and 780 cm−1 may give information of ring sizes of ployaromaticcompounds [28, 29]. These bands are assigned to aromatic structureswith isolated aromatic hydrogen (870 cm−1), two adjacent hydrogensper ring (815 cm−1) and four adjacent aromatic hydrogens (750 cm−1),respectively. The number of adjacent hydrogens per ring provides anestimate of the degree of aromatic substitution, and isolated aromatic

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54 P. KHARE AND B. P. BARUAH

hydrogens provides the estimate of condensation with increasingtemperature.

The peaks at 850, 803, and 750 cm−1 are attributed to isolatedaromatic hydrogen, two adjacent hydrogens per ring and four adjacentaromatic hydrogens, respectively (Table 3). Significant differencesamong the intensities of these peaks in coal samples were found dueto the variation of the relative amount of aromatic rings. In the coalsamples, peaks at 803 cm−1 and 750 cm−1 were intense, while weakat 850 cm−1 suggesting that these bands could be attributed to highlysubstituted aromatic rings. However, presence of the isolated hydro-carbon (peak at 835 cm−1) in the spectra indicates highly substitutedaromatic rings besides larger size rings in the coal structures. It isreported that the aromatic structure of non-perhydrous coals are mainlypenta-substituted (870 cm−1); whereas for the perhydrous coals studiedhere, the band at 815 cm−1 is the most prominent of the aromatic C–Hout-of-plane bending modes, indicating the presence of an aromaticstructure containing 1-2 rings with a very small contribution of large-size aromatic rings. The x-ray scattering pattern of coals of similar origin

Table 3. Peak heights of the aromatic C–H out-of-plane deformation bands of the coalsand their char/coke samples at different temperatures

Char/CokeWavenumber(cm−1)

Coalsamples 450◦C 600◦C 850◦C 1000◦C

MK850 0.0018 0.0043 0.0067 0.0084 0.014803 0.0049 0.0057 0.004 0.003 0.0015750 0.0095 0.0098 0.004 0.0025 0.0018

MB850 0.014 0.018 0.019 0.046 0.051803 0.006 0.0071 0.0047 0.002 0.0017750 0.0055 0.0079 0.0054 0.0034 0.0025

MM850 0.0037 0.0036 0.0047 0.0084 0.012803 0.0049 0.0053 0.006 0.0052 0.0051750 0.003 0.0056 0.0049 0.0046 0.0022

MS850 0.001 0.0027 0.0036 0.0049 0.0053803 0.0049 0.0058 0.002 0.0027 0.001750 0.0036 0.0052 0.004 0.002 0.0015

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STRUCTURAL PARAMETERS OF PERHYDROUS INDIAN COALS 55

also showed the presence of 8-9 C-atoms within an aromatic layer, whichis also revealed by the present study [14].

Characteristics of Pyrolyzed Products

Physicochemical parameters and yields of product are given in Table 4.Volatile matter content in the pyrolytic products is showing abnormalbehavior. Volatile matters of the cokes at 850 and 1000◦C are relativelyhigher than the values for the cokes from normal coals [6]. The varia-tions in the VM and H/C ratios show that high-hydrogen content incoals modified the macromolecular structure of perhydrous coals atthose temperatures. It is interesting to note that high volatile matter wasobserved in coke produced at 1000◦C. Generally, cokes at this temper-ature have low volatile matter due to removal of aliphatic hydrocarbons.These coals have a high concentration of substituted aliphatic hydro-carbons, which contribute significantly to the volatile matter content.

Table 4. Carbon (C), hydrogen (H), H/C, VM, and yield of pyrolized products (%, asreceived basis) obtained at different temperatures

Temperature (◦C) C H H/C VM Yield (%)

MK450 71.9 2.52 0.42 25.95 75600 77.4 2.56 0.40 23.53 68850 80.2 2.31 0.35 23.11 66.8

1000 88.9 2.53 0.34 18.23 46.9MB

450 69.4 4.11 0.71 28.17 73.5600 69.6 2.63 0.45 24.88 69.9850 70.9 2.32 0.39 23.28 65.4

1000 83.9 2.23 0.32 18.52 40.8MM

450 79.9 3.57 0.54 30.53 73.4600 82.8 3.56 0.52 30.20 72.4850 88.9 1.54 0.21 29.91 71.9

1000 89.4 1.49 0.20 19.64 47.2MS

450 78 3.12 0.48 33.74 83.3600 74.5 2.99 0.48 29.85 73.7850 76.0 3.29 0.52 28.39 70.1

1000 81.4 3.02 0.45 24.38 60.2

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56 P. KHARE AND B. P. BARUAH

It may be possible that these groups may remain in the coal matrix andincrease the volatile matter content. Presence of aliphatic hydrogen incoke at 1000◦C is also consistent with the fact.

Effect of temperatures on the intensities of characteristic bandsfor C=O, C–O, CH3, CH2, C=C and Har are clearly seen from theFigure 5. Intensities of characteristic bands for C=O, C–O, CH3, andCH2 decrease, while for C=C and Har, they increase with an increasein the pyrolytic temperatures. The intensities of characteristic bandsof C=O and C–O decrease due to the loss of aliphatic hydrogen andoxygen during devolatilization. The band at 3400 cm−1 representing OHgroups still remains in char/coke samples. This is consistent with theperhydrous nature of these coals. It is reported that the transformationof catechol like structures into phenolic-cresol-like structures occurs

Figure 5. FTIR spectra of char/coke samples obtained from the coal samples at differenttemperatures.

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STRUCTURAL PARAMETERS OF PERHYDROUS INDIAN COALS 57

during pyrolysis of such coals that is confirmed by the presence ofphenolic groups in the FTIR (peak at 3400 cm−1) spectra.

The intensities of the characteristic bands of CH3 and CH2 decreasedue to removal of aliphatic groups from the coal structure duringpyrolysis. It can be seen from Figure 5 that decrease in intensities forCH3 and CH2 bands are prominent up to 600◦C and eliminated at highertemperatures, which is attributed to the removal of aliphatic compoundsas a result of thermal cracking. The variations in characteristic bandsof C=C, Har, and apparent aromaticity (fa) are discussed in detail in thefollowing sections.

Aromaticity in Coal and Char/Coke

Apparent aromaticity (fa) and related parameters deduced from FTIRmeasurements of char/coke are depicted in Table 5. Analysis ofVariance (ANOVA) is applied to the parameters deduced from the FTIR

Table 5. Structural parameters of char/cokes obtained from the coal samples at differenttemperatures

Temperature (◦C) Hal Har Hal/Har Cal/C fa

MK450 0.46 0.06 7.6 0.10 0.86600 0.38 0.14 2.7 0.13 0.89850 0.33 0.20 1.7 0.10 0.91

1000 0.21 0.42 0.5 0.02 0.98MB

450 1.25 0.44 2.8 0.39 0.61600 0.43 0.40 1.1 0.13 0.87850 0.44 0.41 1.1 0.13 0.87

1000 0.45 0.55 0.8 0.11 0.89MM

450 0.49 0.45 1.1 0.13 0.87600 0.45 0.67 0.7 0.12 0.88850 0.38 0.85 0.4 0.09 0.91

1000 0.36 0.98 0.4 0.09 0.91MS

450 0.85 0.27 3.1 0.24 0.76600 0.46 0.42 1.1 0.13 0.87850 0.27 0.43 0.6 0.08 0.92

1000 0.22 0.84 0.3 0.06 0.94

Har: Aromatic hydrogen; Hal: Aliphatic hydrogen, Cal: Aliphatic carbon.

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58 P. KHARE AND B. P. BARUAH

at different temperatures (Table 6). ANOVA is a powerful and verysuggestive method, which separates the contribution to the net variationin a set of experimental data and tests their significance [30]. Thecontributors of variation are indicated by sum of squares (SS), that isa sum of number of squared terms representing the variation underobservation, a number of degrees of freedom (df), and a mean square(MS), represented by the former divided by the latter. MS is deployedto calculate the significance of variation by means of F-test. The MSand number of df for overall variations are respective sums of MSsand dfs of several contributing sources of variation. Such an additiveproperty simplifies the calculations giving more scientific and error-freedata [31]. The ANOVA analysis revealed variation in most of the param-eters deduced at different temperatures is significant at a level of � =0�001 (Table 6).

The variations in C=C, Hal, Har, and aromaticity fractions atdifferent temperatures are plotted in Figure 6. Aromaticity of char/coke

Table 6. One-way ANOVA analysis of structural parameters deduced from the FTIR ofcoal and char/cokes

Structuralparameters

Sum ofsquares (SS)

Degree offreedom

df

Meansquare(MS) F

Signifi-cance(�)

Hal Between groups 8�28 4 2�07 40�57 0.001Within groups 0�77 15 0�05Total 9�05 19

Har Between groups 0�55 4 0�14 2�49 0.009Within Groups 0�83 15 0�06Total 1�38 19

Cat Between groups 3�78 4 0�95 4�93 0.001Within groups 2�88 15 0�19Total 6�66 19

H Between groups 44�54 4 11�13 25�21 0.001Within groups 6�63 15 0�44Total 51�16 19

H/C Between groups 1�43 4 0�36 31�67 0.001Within groups 0�17 15 0�01Total 1�60 19

fa Between groups 0�81 4 0�20 38�77 0.001Within groups 0�08 15 0�01Total 0�89 19

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STRUCTURAL PARAMETERS OF PERHYDROUS INDIAN COALS 59

increases with an increase in temperature. Increase in the C=C, Har,and fa is more prominent during the conversion of coal to char at450◦C, which is due to high weight loss and accompanied by anincrease in aromatic fraction. A regression analysis between VM andaromaticity of all samples was performed (Figure 7). A good negativecorrelation (r2 = 0�82) between the apparent aromaticity deduced fromFTIR spectra with VM (Figure 7) suggests progressive loss of volatilematters (hydrocarbons, carbon dioxides, hydrogen, etc.) accompaniedwith a conversion of substituted aromatic structures to higher aromaticstructures. However, the residual curve fit data (dispersion of data pointsfrom a regression line) may be attributed to different physicochemicalcharacteristics of coal samples.

Figure 6. Variation of aromatic hydrogen (Har), aliphatic hydrogen (Hal) and apparentaromaticity (fa), aromatic carbons (Car) of the coals and char/coke obtained at differentpyrolytic temperatures.

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60 P. KHARE AND B. P. BARUAH

Figure 7. Relationship between volatile matters of pyrolyzed products obtained atdifferent temperatures with fa (apparent aromaticity).

Calculated values of C=C, Har, and apparent aromaticity increasesimultaneously with the increase of pyrolytic temperatures (Figure 8).The fa shows linear relation with atomic carbon (r = 0�73) (Figure 8a)and stretching band C=C (r = 0�84) (Figure 8b) of coal, char, and coke.H/C ratio is a good indicator of aromatic characteristics. Aromaticcompounds have lower H/C ratios than aliphatic ones. Formation ofaromatic compounds occurs with a decrease in H/C ratio. The fa andH/C ratios are plotted in Figure 8c where a high correlation coefficientof 0.97 is obtained over a large range of H/C atomic ratios. Using aregression equation, a fa value was calculated. A good linear relationof fa was found between the calculated values and those deduced fromFTIR spectra (Figure 8d).

In char/coke, the increase of intensity for 850 cm−1 over 803 cm−1

was found. These facts should be consistent with a large size ofthe condensed aromatic nuclei formed during pyrolysis. An importantcontribution of the bands at 870 cm−1 over the 815 and 770 cm−1

zone was observed (Table 3), which indicates the decrease of two andfour adjacent hydrogen atoms with simultaneous increase in isolatedhydrogen. The variation in the intensities of characteristic aromatichydrogen at 850 cm−1, 803 cm−1, and 750 cm−1 of all four coal samplesand their respective pyrolytic products has been depicted in Figure 6.In general, the intensity of four adjacent atomic H (750 cm−1) decreasedwith the increase of temperature. In the present study, the decrease in

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STRUCTURAL PARAMETERS OF PERHYDROUS INDIAN COALS 61

Figure 8. Regression analysis of (a) apparent aromaticity (fa) vs. Atomic Carbon;(b) apparent aromaticity (fa) vs intensity of C=C; (c) apparent aromaticity (fa) vs. H/Cratio; (d) apparent aromaticity (fa) measured vs. calculated for coal and their pyrolyzedproducts.

intensities is slow at 450◦C due to a negligible condensation ratio oflow molecular weight aromatic compounds. At lower temperatures (upto 600◦C), peak intensities of 850 cm−1 shows a gradual increase, whileat higher temperatures (above 600◦C) intensity of the same peak showsa sharp increase (Figure 9). During pyrolysis, chemical transformationand rearrangement take place to give more condensed polyaromaticstructures for the coal samples subjected to pyrolysis. Smaller polyaro-matic hydrocarbons (2-3 ring structures) are formed at lower tempera-tures and formation of larger PAHs (4-5 ring structures) take place athigher temperatures. It is also reported that the average aromatic ringsize can be inferred from the hydrogen-to-carbon ratio [31]. A valueof 0.40 is consistent with average ring sizes of four to six. In cokesamples (at 1000◦C), H/C ratios ranging from 0.20 to 0.45 indicatesthe formation of larger rings. This is similar to the reported values for

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62 P. KHARE AND B. P. BARUAH

Figure 9. Variations in characteristic FTIR peaks of isolated aromatic hydrogen(850cm−1), two adjacent hydrogens per ring (803cm−1) and four adjacent aromatichydrogens (750cm−1) coals and their pyrolyzed products at different temperatures.

low volatile bituminous and semi-anthracite [10] coals. Similar to naturalmaturation, aromaticity in char/coke increases by a release of aliphaticcompounds from the coal matrix. A value of 0.40 is indicative for anaverage polyaromatic ring size of two with four substituents per ring,and or with an average polyaromatic ring size of four to six.

PAHs in Coal and Char/Coke

The elution characteristics of 10 PAHs detected have been summarizedin Table 7. The specificity of each PAH in the coal sample as well asits pyrolytic products were confirmed by their corresponding retentiontimes (Rt) and matched with the reference compound of an individual

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STRUCTURAL PARAMETERS OF PERHYDROUS INDIAN COALS 63

Table 7. Concentration (mgg−1) of total PAHs in perhydrous Indian coal (MS) andchar/coke samples obtained at different temperatures

Char/Coke

Coal (MS) 400◦C 600◦C 850◦C 1000◦C

Total PAHs 10.2 12.8 15 15.6 16.2Naphthalene 6.880 2.943 0.262 0.188 NDAcenapthylene 1.721 1.128 0.425 ND NDFluorene 0.064 0.037 0.017 ND NDAcenaphthene 0.080 0.368 2.390 ND NDPhenanthrene ND 0.035 0.040 0.016 NDAnthracene 0.068 0.018 0.018 ND NDFluoranthene ND 0.020 0.065 0.419 2.016Pyrene ND 0.016 0.035 0.040 0.050Chrysene ND 1.113 1.214 1.315 1.48DBA ND ND 5.055 5.564 7.564

PAH. LC-MS (Shimadzu 2010EV) with electro spray ionization-atomicpressure chemical ionization (ESI-APCI) dual probe was also usedfor PAH identification. The results based on HPLC-UV are summa-rized in Table 7. Out of 16, 10 polyaormatic hydrocarbons—namelyNaphthalene, Acenapthylene, Flourene, Acenaphthene, Phenanthren,Anthracene, Flouranthene, Pyrene, Chrysene, and 1,2,3,4-DBA incoals and pyrolytic products—have been identified. On applying theMann–Whitney U-test, it was found that levels of these PAHs signifi-cantly differ in coals and its pyrolytic products at various temperaturesat a level of 0.01.

The 10 identified PAHs in coal and its char/cokes belong to threestructural classes: Naphthalene benzologues (naphthalene, acenaph-thylene), Fluorene benzologues (flourene, acenaphthene, fluoranthene),and Anthracene benzologues (phenanthrene, anthracene, pyrene,chrysene, 1,2,3,4 DBA (dibenzo(ah)anthracene)). These coal sampleshave higher concentrations of Napthalene benzologues. Generally,normal coals with similar carbon contents are dominated by 4-ringpolyaromatic hydrocarbon structures. The natural transformations ofthe oxygenated functionalities in the lignin precursors are responsiblefor the polyaromatic system [2] in normal coals. The high-hydrogencontent in coals may also cause more ring substitution than aromati-zation, which is true for the coals under study. As a result, perhydrouscoals contain mainly aromatic structures with 1-2 rings and a very small

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64 P. KHARE AND B. P. BARUAH

concentration of aromatic rings of large size. In pyrolytic products at1000◦C, as a consequence, the concentration of these benzologues wasnot detected.

Benzologues of the Fluorene group showed different behavior inthe pyrolytic process adopted. Concentration of Fluorene was notdetected in pyrolytic products formed at 1000◦C. Acenaphthene concen-tration was low in coal samples and increased in the products upto 600◦C and found to decrease at 850◦C and 1000◦C. Fluorantheneconcentration in coal samples was found below detection limit butshowed an increase with temperature. For anthracene benzologues, thelevels of anthracene were found below detection limit in the productsformed at 850◦C and 1000◦C. Other three benzologues of the groupshowed maximum concentration in the products formed at 1000◦C. Thisstudy clearly indicates the presence of lower molecular weight polyaro-matic hydrocarbons in coal. During the pyrolysis processes, these lowermolecular weight compounds condensed into higher molecular weightcompounds, which is consistent with the FTIR results. The presenceof high molecular weight PAHs at higher temperature is shown inTable 7. The concentration of Fluoranthene, Pyrene, Chrysene, and1,2,3,4-DBA was found to be higher in the high-temperature pyrolyticproduct (1000◦C). This is consistent with the results obtained by FTIR.

CONCLUSION

The tertiary coals selected for the present study are found to have perhy-drous characteristics. They are classified as lingo-bituminous coals;however they have different characteristics like high H/C atomic ratio,volatile matter content, and calorific values with low-moisture and ashcontents. The results of the FTIR and PAH analysis of coals and thepyrolyzed products showed that they contain more substituted alkylgroups with 1-2 aromatic ring structures with very small concentrationof large rings. The presence of a high amount of lower ring PAHs inthese coals also supports these findings. Modification of the structure ofthese coals such as high CH3/CH2 ratio, low aromaticity, and presenceof phenolic groups in pyrolytic products may occur due to high H/Ccontent. Hence, their physicochemical properties do not correlate withtheir rank. Their char/coke formed from these coal exhibits high-volatilematter than those obtained from normal coals. Due to more alkyl substi-tution in their structure, these coals are found suitable for conversion

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STRUCTURAL PARAMETERS OF PERHYDROUS INDIAN COALS 65

processes. Abnormalities in the behaviors of these coals are mainly dueto the high hydrogen content and its influence on the coal structureduring evolution processes.

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