Role of Maceral Composition on the Formulation of Concentrated Coal-Water Slurry Using a Natural Surfactant

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ScienceDirect Materials Today: Proceedings 9 (2019) 542–550

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GMSP&NS’18

Role of Maceral Composition on the Formulation of Concentrated Coal-Water Slurry Using a Natural Surfactant Jibardhan Mehera, Debadutta Dasb, Akshaya K. Samalc and Pramila K. Misraa* a

Centre of Studies in Surface Science and Technology, School of Chemistry, Sambalpur University, Jyoti Vihar-768 019, Odisha, India. b Department of Chemistry, Sukanti Degree College, Subarnapur-767017, Odisha, India c Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Bengaluru, 562112, Karnataka, India

Abstract The stabilization of coal-water slurry(CWS) depends on both inherent properties of coal and the external factors like solution pH, presence of additives, temperature, modulation of coal surface by ultrasonic radiation or microwave treatment etc. The rank and chemical composition i.e. mineral and maceral composition of coal are very much important in the formulation of CWSand some other applications of coal. The present investigation involves the formulation and stabilization of CWS using four varieties of non-coking Indian coalsstabilized by a natural surfactant saponin as dispersant. Proximate and ultimate analyses of coal were investigated to get first hand information on the compositions of the coals. The reflectance and X-ray diffraction studies indicated the amount of macerals and minerals present in the coals under investigation. The rheological parameters of CWS were analyzed by coal-loading, dispersant percentage, solution pH and temperature. The CWS formed from all coals followed the Bingham plastic model and behaved as a Non-Newtonian fluid in the presence of saponin. The static stability of CWS prepared from Talcher coals was however, found to be superior to that prepared fromIb river valley coals. The extent of the maceral and mineral percentage was assigned to be the primary reason for the variation in the slurry behaviour. © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Green Methods for Separation, Purification and Nanomaterial Synthesis, GMSP&NS’18, 24–25th April 2018, Centre for Nano and Material Sciences, Jain University, Bangalore 562112, Karnataka, India.

Keywords : maceral content;

mineral content; Bingham plastic model; vitrinite, inertinite; coal loading; apparent viscosity.

___________ *Corresponding author: [email protected] (Pramila K. Misra) 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on Green Methods for Separation, Purification and Nanomaterial Synthesis, GMSP&NS’18, 24–25th April 2018, Centre for Nano and Material Sciences, Jain University, Bangalore 562112, Karnataka, India.

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1. Introduction The alternate energy sources are receiving considerable attention due to acute shortage of fossil fuels because of climatic changes and increasing demand from all sectors of society for several domestic and industrial applications. The coal water slurry (CWS), a concentrated dispersion of coal in water,has now come into forefront as an alternative substitute for gas and oil because of it is combustible, low-priced and easyto handle [1]. It can also be transported from mines to plant sites with reduced environmental pollution and delivered without exhibiting much damage to the furnace during feeding. On combustion, it burns almost completely leaving without much of ash content with concomitant generation of huge amount of energy. Adding to the fact, the presence of water makes CWS as fire and explosion-proof and therefore, CWS is receiving worldwide consideration. Diesel engines, power stations, power boilers, gas turbines are some of the commodities where CWS is extensively used[2]. Desirable condition of an effective CWS is that the volume fraction of coal should be as highas possible with admissible apparent viscosity of slurry(< 1000 Pa.s) and considerable stability[3][4].But on increasing coal concentration in the slurry the apparent viscosity increases due to increase of the coal particle-particle interaction leading to settling of the CWS which in turn causes in detrimental effect to both storage and transportation of the slurry[3][5].Hence various methods are continuously developed to reduce the apparent viscosity for obtaining stable slurry. Addition of dispersants such as surfactants and polymers;exposure to microwave irradiation;treatment with ultrasonic waves and desulfurizationetc. are some of the established techniques[3][4][6][7][8][9] for the formulation of stable slurry. During the course of formulation factors like size distribution of coal particle;temperature and pH of CWS; shear rate;amount of additive; percentage of coal load;inherent moisture and ash content of coal; composition and type of coal; oxygen containing functional groups in coal;reflectance and porosity of coal; content of carbon and O/C ratio[3][4][10][11][12][13][14][15] etc.The coal contentsare determined by the reflectance spectra[16] are also found to play a vital role on the flow characteristics of CWS [17][18][19][20].The organic fraction(macerals) and inorganic fraction (minerals)of coal are the two basic componentsof coal[21]. The maceralsincludefusinite, inertinite, liptinite, and vitrinite whose aromaticity increases in the order: liptinite< vitrinite
2. Experimental methods 2.1. Isolation of dispersant The natural dispersant, saponin was isolated from the drupes of Sapindus laurifolia tree following the procedure described by us elsewhere[3]. 2.2. Procurement and analyses of coals The coal samples from both Talcher Coal Field and the Ib river valley coal of Odisha (the south-eastern part of India ) were collected for the purpose. The coals were subjected to crushing and dashing to get cleans. Cleans were crushed first in a jaw crusher and then in double roll crusher to obtain samples with particle size below 100 µm. The proximate, chemical and ultimate analyses data were measured by air-dried basis[3].

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The chemical analyses of coals throughX-ray diffraction(XRD) data were obtained by a diffractometer using PHILLIPS X’ PERT X-Ray Diffractometer,Model PW 1830 (Almelo, Netherlands) with CuK radiation (35 kV and 30 mA) at the scan rate of 30/min and was analyzed by standard software provided with the instrument. The coals were crushed to −1 mm size and coal pellets were prepared with cannabau wax to identify the maceral composition. These samples were exposed under reflected light using Leitz MPV2 microscope having oil immersion lenses and fluorescent attachment following standard procedures (ICCP 1971, 1998, 2001)[29][30]. The macerals were identified following ICCP classification of macerals (ICCP 1963, 1971, 1975, 1998, 2001)[31][32][33]. 2.3. Rheological measurement Distilled water was used throughout for the preparation of CWS. The rheological studies of the CWS were carried out using a HAAKE Rotational viscometer (Model RV 30), consisting of measuring drive unit, temperature vessel with circulator, sensor system and a data logger[3]. The best-fit model was fitted to the shear stress-shear rate data to obtain the nature of coal-water slurry. Various rheological parameters such as yield stress, apparent viscosity etc. were investigated with respect to different parameters against shear rate, additive concentration, percentage of coal load, pH, temperature of slurry, etc. 3.Result and Discussion 3.1. Analyses of coal composition The coals under investigations were of sub-bituminous type. The proximate and ultimate analyses of coals are shown in Table 1 and 2. Basing on the ash content four types of coal samples designated as coal A(8.02%), coal B(18.14%), coal C(9.18%) and coal D(20.05%)were selected for slurrypreparation. The C/O ratio of coal A of Talcher coal field was highest(7.83) and that of coal D of Ib valley was lowest(6.87). Usually, the coal with higher C/O ratio are more preferred for the preparation of CWS[3][4]. Table 1. Proximate and chemical analyses of coal samples analysis of different rank of Talcher and Ib river valley coal samples. Talcher Ib Parameteres Coal-A Coal-B Coal-C Coal-D Moisture 13.15 13.65 12.99 12.98 Ash 8.02 18.14 9.18 20.05 Volatile Matter 33.44 28.06 32.64 27.70 Fixed Carbon 45.39 39.66 45.02 39.05 Calorific Value 6438 5745 6259 5532 21.5 23.7 (Quartz)SiO2 0.08 0.12 (Anatase) TiO2 2.52 2.85 Al2O3 0.76 0.85 (Hematite) Fe2O3 CaO 2.55 2.92 MgO 0.65 0.98 Table 2. Ultimate analysis of different rank of Talcher and Ib river valley coal samples Talcher Ib Elements Coal-A Coal-B Coal-C Coal-D Carbon % 78.70 77.89 77.65 77.34 Hydrogen % 5.83 5.91 5.72 6.02 Sulphur % 2.86 3.70 2.91 2.85 Nitrogen % 1.85 1.89 1.75 1.86 Oxygen % 10.05 10.62 10.89 11.25 C/O 7.83 7.333 7.125 6.870

Coal is characterized by two basic components: macerals(organic constituents)and minerals(inorganic constituents).The macerals are optically homogenous aggregates of organic substances, possessing distinctive physical and chemical properties, and occurring naturally in the sedimentary, metamorphic, and igneous materials of the earth[34].Frequently, the organic macerals of coal are divided into vitrinite, inertinite, and liptinite[25]. Analyses

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through sophisticated technologies like FTIR, XPS, XRD, NMR, DSC and chromatography reveal that these macerals exihibit a descending order in volatile yield and the content of hydrogen, olefins, and alkanes: liptinite > vitrinite >inertinite [25][35] whereas the content of aromatic carbon decreases in the order of inertinite > vitrinite > liptinite[24][36]. Vitrinite contains numerous long aliphatic chains and bridges and a greater number of substituents on aromatic rings. On the other hand inertinite has more oxygen functional groups, containing relatively higher aromaticity[24[37]. Thus, the percentage and type of maceral present in the coal is highly essential. All coals, contain virinite,inertinite and liptinite groups. The petrographic analyses of Talcher coals collected from different seams envisages the vitrinite content ranges from 30.11 to 61.57 % with a average of 44.22%. (43.91 to 69.47%, mean 57.64% mineral matter free basis), the inertinite content ranges from 13.31 to 56.35%, with a mean of 27.29% (24.35 to 73.70%, mean 39.65% mineral matter free basin). In case of Ib-river coal, vitrinite percentage ranges from 11.58 to 28.65, mean 22.89% (18.18 to 43.21%, mean 29.73% mineral matter free basis). The mean vitrinite reflectance of Ib coal ranges from 0.38 to 0.46 with a mean of 0.49.The concentration of mineral matter in Talcher coal ranges from 7.67 to 52.22% mean 31%. The mean vitrinite reflectance of Talcher coal ranges from 0.41 to 0.58 with a mean of 0.55.The X-ray diffraction studies (Fig. 1(a)-(b)) of the coals of under study revealed the mineral types available in the coals. The major minerals present in the coals are kaolinite, micasaite, anatase, quartz, basanite and pyrite.The mineral contents in Ib river valley coal is more than that of Talcher coal(Table3, Figs. 1(a)(b)).The petrography analyses of the coal under investigation unravelled the presence of the macerals with different amounts as provided in Table-3. On analyses it is obvious that coal A contains highest amount of vitrinite whereas coal D contains highest amount of inertinite among these four varieties of coals. Table 3. Percentage of Macerals in coals under investigation. Talcher Ib Coal-A Coal-B Coal-C Coal-D Vitrinite% 44.22 29.25 30.23 25.20 Inertinite% 27.90 41.24 42.51 51.79 Liptinite% 28.69 29.51 27.26 23.01 Maceral %

Fig. 1 X-ray diffraction pattern of (a) Talcher coal A and (b) Ib coal C

3.2. Rheological behaviour of CWS In order to investigate the effect of macerals on the CWS stabilization, the rheological behaviour of the CWS formed from both categories of coals were investigated by varying the shear rate, percentage of coal load, experimental temperature and pH keeping the concentration of the dispersant as 0.8%. In our previous study we reported from fluorescence studies that the natural dispersant, saponin used in present case is a natural surfactant having critical micellar concentration equals to 0.8%[3]. Since the isolated monomer get adsorbed at coal-water interface and provides steric hindrance for stabilizing the coal-water slurry, we judiciously maintained the saponin

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concentration 0.8% as beyond CMC, the number of isolated monomers remained constant[38-40]. The apparent viscosity of CWS goes on decreasing with increase of saponin percentage in CWS and remained constant beyond 0.8%. Hence 0.8% of saponin was taken for investigating the rheological behaviour of CWS[3]. Fig. 2(a) and Table 4 Show the effect shear rate on apparent viscosity for four types of Indian coals. There is an exponential decrease in viscosity with increased shear rate for all types of coal. The viscosity of coal-water slurries decreased rapidly with increase in shear rates of upto 50 s-1but beyond 50 s-1the extent of decrease is less. However, in general Talcher coals responded to shear rate greater than Ib coals. The apparent viscosity at 100 s-1 remains well below the admissible range[3] and the shear-thinning behavior is observed in all cases. Table 4.Apparent viscosity (η) measured with different shear rate(γ) of CWS. Talcher Ib Coal-A(in Pa.s) Coal-B(in Pa.s) Coal-C(in Pa.s) Coal-D(in Pa.s) 3.425 3.777 3.952 4.333 1.776 1.998 2.019 2.442 1.215 1.355 1.455 1.629 0.955 1.123 1.103 1.355 0.823 O.988 0.993 1.112 0.733 0.869 0.935 1.025 0.665 0.733 0.882 0.955 0.559 0.623 0.753 0.893 0.425 0.510 0.622 0.750 0.335 0.429 0.551 0.629 0.313 0.362 0.429 0.584

Shear Rate( in s-1) 0 10 20 30 40 50 60 70 80 90 100

The influence of coal load on CWS viscosity was examined keeping saponin concentration at 0.8% for all types of coal and the result is illustrated in Fig. 2(b) and Table 5. The apparent viscosity of CWS viscosity increases with increasing of coal concentrationcharacteristically due to increase of the particle-particle interactions. A maximum of 64% coal load has been achieved for all kinds of coal, above which slurry viscosity increases abruptly with a nonacceptable range of apparent viscosity. The sharp increase in viscosity may be due to the agglomeration of small particles at high concentration of coal load to form larger particles within which the water was trapped resulting in large particles. Furthermore, comparing all the curves for different types of coal, it has been observed that the apparent viscosity of Talcher coals is always lower than that of Ib river valley coals at a constant shear rate and coal loading. The rheological behaviour of all CWSs were investigated by looking to the shear stress-shear rate relationship at the different experimental conditions. Fig. 3(a) and Table 6 show the relationship between shear stress with applied shear rate of CWS (64% weight fraction) in presence of 0.8% of saponin. In all cases a linear relationship between shear stress and, shearrate is observed. However, in each case the CWS can uphold the shear upto certain limit beyond which the shear stress changes with applied shear rate similar to water. Thus, these CWS exhibit nonNewtonian character with some initial shear stress known as yield stress. These fluids data are well-fitted to the relationship of Bingham plastics fluids(equation-1). τ = τ + γ (1) where τ and γ denote shear stress and applied shear rate respectively. τ0 is the yield stress and  is defined as coefficient of rigidity. The yield stress and apparent viscosity values measured at shear rate of 77 s-1 and coal load 50-64% for all cases. On analyzing the rheological measurement data of coal water slurry it is found that all types of coal shows non-Newtonian characteristics in the coal load range of 50-64%. The yield stresses for all CWS are provided in Table7.

Coal Conc. (in %) 50 55 60 62 64

Table 5.Effect of coal concentration on apparent viscosity(η) of CWS. Talcher Ib Coal-A(in Pa.s) Coal-B(in Pa.s) Coal-C(in Pa.s) Coal-D(in Pa.s) 0.382 0.401 0.425 0.468 0.420 0.461 0.499 0.563 0.500 0.540 0.564 0.645 0.601 0.721 0.665 0.773 0.820 0.972 0.913 0.997

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Shear Rate( in s-1) 0 10 20 30 40 50 60 70 80 90 100

Coal Conc. (in %) 50 55 60 62 64

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Table 6. Applied shear rate(γ) and corresponding shear stress(τ). Talcher Ib Coal-A( in Pa) Coal-B( in Pa) Coal-C( in Pa) Coal-D( in Pa) 35 42 40 50 38 45 44 54 40 48 48 56 44 54 50 59 46 58 52 62 46 60 59 65 48 64 54 70 49 66 55 73 50 68 57 76 54 69 60 78 55 70 62 80

Table 7. Apparent viscosity(η)and yield stress(τ0) obtained for different coal concentration in CWS Yield Stress (τ0)for Ib Yield Stress(τ0) for Talcher Apparent Viscosity(η) for Ib Apparent Viscosity(η) Coals( in Pa) Coals( in Pa) Coals(in Pa.s) For Talcher Coals(in Pa.s) Coal-A Coal-B Coal-A Coal-B Coal-C Coal-D Coal-C Coal-D 0.332 0.410 30 36 0.394 0.499 30 39 0.410 0.450 34 41 0.469 0.520 35 44 0.521 0.552 48 49 0.594 0.601 52 51 0.613 0.742 58 60 0.710 0.811 62 63 0.723 0.910 60 67 0.792 0.999 65 61

Since during pipeline transport, the pH and temperature of the slurry may change which in turn may affect the slurryability and stability of CWS, the investigation of these two parameters on the CWS is necessary. On examination on the effect of pH the apparent viscosity of CWS (Fig. 3(b) and Table 8), an inverse relationship is observed between pH and apparent viscosity of the slurry. As pH increases positively charged surface species gradually diminishes and surface hydroxylation takes place which increases the negative charge density on the surface of the coal particle. Coal gradually acquires hydrophilicity above pH 6 and complete hydrophilicity above pH 10 due to the ionization of surface functional groups or the formation of hydroxy complexes of multivalent metal ions present on coal surfaces[41].

pH 1 2 3 4 5 6 7 8 9 10

Table 8. Measurement of apparent viscosity(η) with variation of pH of CWS. Talcher Ib Coal-A(in Pa.s) Coal-B(in Pa.s) Coal-C(in Pa.s) Coal-D(in Pa.s) 0.810 0.998 0.923 1.154 0.709 0.908 0.844 1.012 0.644 0.806 0.725 0.903 0.520 0.644 0.640 0.805 0.489 0.511 0.498 0.764 0.444 0.492 0.482 0.687 0.402 0.423 0.439 0.566 0.392 0.403 0.412 0.532 0.390 0.499 0.405 0.522 0.390 0.499 0.405 0.522 Table 9. Measurement of apparent viscosity(η) with variation of temperature. Talcher Ib Temperature( in K) Coal-A(in Pa.s) Coal-B(in Pa.s) Coal-C (in Pa.s) Coal-D (in Pa.s) 298 1.005 1.125 1.164 1.345 303 0.825 0.898 0.955 1.121 308 0.649 0.688 0.710 0.925 313 0.505 0.563 0.596 0.743

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Fig. 2. (a) Apparent viscosity(η) verses shear rate of CWS and (b) Percentage of coal load verses apparent viscosity(η) of CWS

Fig. 3. (a) Shear rate verses shear Stress(τ) of CWS and (b) Apparent viscosity(η) verses pH of CWS Furthermore, in the case of demineralized coal (low ash) the apparent viscosity is lower in comparison to higher ash percentage coal. With increase in ash content, the isoelectric point shifts to alkaline side. The negative charge on the coal surface increases with increase in ash pH and reaches maximum at pH 8-9. Plot apparent viscosity as a function of temperature is demonstrated in Fig. 3(b) and Table 9. The apparent viscosity of coal-water slurry is found to decrease exponentially with increase of temperature due to the decrease in inter-particle attraction. The increase in temperature increases in kinetic energy of the coal particles thereby decreasing the interparticle interactions. Because of the increased velocities of individual molecules with temperature, the interaction time among the particle also decreases. On increasing temperature, the cohesive force between the particles is reducedwhich ultimately reduces the viscosity of the liquids.The relation between viscosity and temperature was found to fit Guzman Andrade equation(equation-2, [42])satisfactorily. Equation-2 on rearrangement gives equation3. A linear relationship with correlation coefficient=0.99 was obtained in all cases. A

= A e T ln  = ln

+

(2) (3)

3.3. Static stability test and Mechanism of stabilization The stability of CWS for each coal was tested by rod penetration method[43] and the data are presented in Table 10. It is clearly evident that Talcher coals show better rheological properties under same dispersant dosage and keeping other parameters constant i.e. lower apparent viscosity and higher coal load were achieved using Talcher coal than that of Ib-river coal. Coal A is stable for a month at coal load 64 volume fraction which is quite significant. Since the

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inherent moisture, oxygen and ash content are relatively less in Talcher coal in comparison to that of Ib-river coal, the hydrophilicity of former is comparatively less. The hydrophilic coal, is likely to hold the greater amount of water, and hence less volume of water would be available in slurry and therefore, it would not promote greater fluidity of slurry. Furthermore, from the petrography study, it is apparent that inertinitedominates in Ib river coal. Literature study tells that inertinite has a charcoal-like appearance with cell texture and the cells may be either empty or filled with mineral matter[44]. Because of high porosity, inertinite becomes quite hydrophilic in water and show poor performance towards slurry ability.The vitrinite reflectance data supports the presence of higher amount of vitrinite in coal A than coal C(Table 11). The comparison of apparent viscosity between coal A and C(Table11) also depicts that the viscosity of CWS formed from Coal A is substantially lower than that of coal C. Table 10. Measurement of static stability of different types of coal with their different coal concentration in CWS. Talcher Ib Coal Conc. (in %) Coal-A (in days) Coal-B (in days) Coal-C (in days) Coal-D (in days) 50 14 14 9 8 58 18 17 12 12 60 21 19 16 14 62 23 22 20 20 64 30 27 22 19 Table 11. Comparison ofTalcher coal and Ib Coal with respect to vitrinite percentage, inertinite percentage and vitrinite reflectance. Apparent viscosity(in Pa.s) at Types of Coals Vitrinite (%) Inertinite (%) Vitrinite reflectance a coal concentration of 50% Talcher Coal ‘A’ 44.22 27.9 0.55 0.382 Ib Coal ‘C’ 22.89 14.5 0.49 0.425

The greater hydrophobicity of vitrinite than inertinite[44], is also supported from the higher value of contact angle vitrinite in comparison to that of inertinite, i.e. contact angle for vitrinite is 60-700 , whereas for inertinite the contact angle ranges between 250 to 400 which therefore, be more beneficial for coal to form slurries with higher vitrinite. Shin et al[45] have also reported that because of hydrophobic nature of vitrinite coal, these are more beneficial to form slurry.Bright surfaces(higher reflectance) are more hydrophobic than dull surfaces and hence the coal having greater percentage of vitrinite can also float better[46]. Slurryability and static stability of CWS increase with increase in carbon content and grindability index of coal. Therefore, the trend in slurryability and stability in the descending order, coal A> coal B>coal C> coal D may be attributed to the decreasing order of C/O ratio. The coal having relatively lower moisture and oxygen-containing groups would make the more highly loaded CWS because of its lower hydrophilicity[24].Among Talcher coals, Coal A is more effective than coal B and among Ib coals, Coal C is more effective than coal D. The apparent viscosity of Ib coal is always higher than that of Talcher coal at same shear rate and coal loading. This may be due to that the dissolved cations, particularly the high-valent cations, such as Ca2+, Mg2+, Al3+, Fe3+ cations, etc. built bridges among the coal particles and a huge cross net structure formed by the bridge bonding among the coal particles resulting increase in viscosity. Further, minerals influence the viscosity of CWS through hydrogen bonding [47]. 4. Conclusion In view of increasing demand for energy sources, the quest for an efficient and economic alternative engery source is highly indispensable. In the present study the formation of coal-water slurry from low rank coals with varied amount of maceral and mineral compositions was explored. Four varieties of coal with different amount of ash content, coal A, coal B, coal C and coal D were employed for the investigation. Talcher coal with high percentage of vitrinite and C/O was found to form stable and low viscous slurry with high coal concentration. Rheological characteristics of coal B and Coal C are approximately equal because two competitive parameters i.e.C/O ratio and percentage of ash content have opposite effect towards the change in viscosity.Static stability of CWS was decided by the percentage of mineral and maceral contentpredominantly.Summarizing, the petrography of coals plays a vital role in formulating coal-water slurry which therefore, needs to be properly analyzed before employing coal of different origins.

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Acknowledgments The authors thank the DST, New Delhi (Project No. DST sanction letter no. SR/S1/PC-64/2009, dated 20.04.2010) and UGC-BSR( Office Order No. 533 PGCO/RC dated 30.03.2013)for providing research fellowship to JM. Support of the UGC (DRS) and the FIST (DST) to the department are also gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47]

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