Aerial oxidation of coal-analytical methods, instrumental techniques and test methods: A survey

Indian Journal of Chemical Technology Vol. 16, March 2009, pp. 103-135

Aerial oxidation of coal-analytical methods, instrumental techniques and test methods: A survey

Raja Sen, Sunil K Srivastava* & Madan Mohan Singha

Central Institute of Mining & Fuel Research, Digwadih Campus, P.O.- F.R.I., Dhanbad 828108, India aDepartment of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi 221005, India

Email: [email protected]

Received 21 January 2008; revised 10 November 2008

Aerial oxidation of coal particularly atmospheric oxidation of coal is mainly responsible for self heating of coal and deterioration of most of its useful technological properties like calorific value, floatability and caking properties. A lot of work has been done over the years to understand the exact mechanism of the processes responsible for this phenomenon. This literature survey attempts to list and describe various analytical techniques, both chemical and instrumental, standard tests and novel experimental procedures applied by various authors to study the different aspects of this phenomenon.

Keywords: Coal oxidation, Weathering, CPT, FTIR, PyMs, NMR, XPS

Since the establishment of coal mining and steel making industry the negative effect of coal oxidation vis a vis deterioration of coking properties and the problem of self heating and spontaneous combustion have been well recognized. Over the last 150 years, particularly since the 50’s, voluminous literature has been generated in the area of aerial oxidation of coal which involve use of analytical and other investigative techniques where sophisticated instruments, chemical methods and standard tests have been extensively used. Although this has generated quantum developments in the understanding of this phenomenon the complete understanding of it still eludes the scientific community and lot of grey areas continue to exist. This is mainly due to inter alia

(i) On the molecular level coal is an extremely

complex substance and in spite of extensive research in this area the exact structure of coal has yet to be determined. Different authors have proposed various coal models from time to time. Broadly it is agreed that coal consists of heterogeneous polyaromatic clusters in a complex array resulting in a highly crosslinked macromolecular gel structure. Another opinion that coal having highly associated structure also exists.

(ii) On the macro level too coal is extremely heterogeneous consisting of various minerals and maceral components whose composition varies from coal to coal which increases the complexities in the study on aerial oxidation of coal.

(iii) Oxidation in itself is a complex process which

consists of parallel but competing/interacting reaction processes. Although at least 4 such processes are believed to exist yet the exact number, nature and kinetics of such process is not very clearly understood. Although aerial oxidation of coal is essentially a chemical reaction process, it is influenced by apart from its original chemical composition, other factors like temperature, moisture and catalytic effects of water and components in the mineral matter. Furthermore, the effects of the physical and surface properties play a role which is not properly understood.

Consequently on the practical side, progress in the science of self-heating leading to useful preventive methodologies with wide and extensive applicability on industrial level is yet to be developed. Although the negative effects of aerial oxidation on the technologically useful properties e.g. coking

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properties, lowering of tar yield, combustion behaviour, and spontaneous combustion of coal is well documented in literature, again the exact mechanisms of these phenomenon are yet to be firmly established and considerable difference of opinion still exists. It is therefore imperative that for a more clear understanding of the phenomenon more refined, improved and novel experimental techniques are needed to be developed to address the outstanding problems. This survey is an attempt to provide comprehensive information on the different experimental and analytical techniques applied by various authors in this area and the scientific information generated by them. Proximate analysis Proximate analysis of coal essentially involves the determination of ash, moisture, volatile matter (VM) and fixed carbon by difference using standard specified procedures1-7 and provides a first hand idea of coal properties and behaviour. Authors working on coal oxidation following the general trend of coal research characterize coals both fresh and those treated by various process using proximate and ultimate analysis as a preliminary step. Pisupati et al.

16 have not observed major changes in coal ash content due to oxidation good enough to follow a general consistent tend. However, Teo et al.

22

observed small increases in ash content on comparing outcrop coals with their unweathered counterpart. Results reported by various authors regarding change in volatile matter of coal with oxidation/weathering do not follow a regular pattern and vary. While some authors14-16,24-26 have reported increase in VM of oxidised/weathered coal compared to those of fresh, others8,18-20 have reported decrease. While most authors13,16,24,27 have reported increase in moisture upon oxidation, some authors10,12 have reported very little change. These variations are in agreement with suggestion of Fryer and Szladow26 that changes in volatile matter can not be easily predicted which is due to the extreme sensitivity of aliphatic and alicyclic moieties to oxidation and also depends not only upon the quantity but also the modes in which they combine which may differ from coal to coal. Similarly, Pisupati et al.

14 as well as Beier17 explained this variation by attributing the variation to the dependence of VM on the rank of the coal being examined and the form in which oxygen is incorporated in the coal structure. According to

general agreement amongst most authors the two factors that had the most statistical significance in determining the propensity for self-heating were the volatile content and the inherent moisture. According to Harper et al.

21 the progressive change in air-dried moisture content, together with the relative ease of measurement of this parameter makes moisture determination the quickest and easiest method of judging the level of oxidation of a sample of coal. The present authors also have the same opinion. Calorific value

Calorific value of coal is determined by bomb calorimeter using standard procedures28,29. Commonly used standard procedures include ASTM, ISO and BIS. Most workers13,22-24,30-32 have observed steady decrease in calorific value with progressive duration of oxidation. The steady decrease in calorific value of medium volatile bituminous coal, as observed by Iglesias et al.

13, which finally decreased by nearly 28% percentage of its original value in 14 days during oxidation at 200°C is an example of this phenomenon. Ghiroux et al.

30 observed that though there was steady decrease in CV during oxidation at 40°C, calorific content of coal did not vary for samples stored in a freezer at -15°C and thus opined that degree of reduction in CV of coal is interalia dependent on the oxidation temperature. Fryer and Szladow26 made a general estimation of loss of calorific value of both caking and noncaking coals for every 1% increase in oxygen content. Since slurry pH also decreases with duration of oxidation, considering slurry as an index of oxiduration, Yun and Meuzelaar32 plotted calorific value against slurry pH measurements and opined after studying the plots for 60 and 150°C that mechanisms of coal oxidation at both temperatures were different. Ultimate analysis

Ultimate analysis of coal involves the determination of carbon, hydrogen, nitrogen sulphur and oxygen (by difference). Various standard methods used for these determinations include BIS, ASTM and ISO methods for carbon and hydrogen33,38,45, using modified Libegs method, nitrogen using Kheldhal method34,43, total sulphur using Escha method35,40 and high temperature tube furnace combustion method41,42, forms of sulphur36,37 (sulphate, pyritic and organic), determination of carbon, hydrogen and nitrogen using instrumental techniques39. Aerial

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oxidation principally involves the incorporation of oxygen into the coal matrix. In the study of aerial oxidation of coal it is therefore important to measure the oxygen content of both fresh and oxidised coals. The organic oxygen is usually calculated by subtracting from 100 the values of C, H, N, S on a dry ash or dry mineral free basis. According to Ode46, the main disadvantage of this method is that all the errors of the other determination get incorporated into the oxygen value. According to Baltisberger et al.

47, oxygen by difference calculated on dry ash free basis may be substantially erroneous. However, Given and co-workers49-51 have suggested methods for minimizing these errors which include use of dry mineral matter free basis instead of dry ash free basis, use of organic sulphur value instead of total sulphur value in the calculation, and application of corrections for eliminating contribution of C and H from mineral matter. According to Given and Yarzab51 oxygen by difference estimated on dry mineral matter free (dmmf) basis incorporating the above corrections gives a good estimate of organic oxygen content in coal. However, oxygen by difference method continues to be the standard method recommended by most of the standard organizations including BIS33 and ASTM38. The British Standard44 incorporates most of the corrections mentioned above and is therefore is preferred. In the past, several methods for the direct determination of oxygen in coal have been developed. These methods employ oxidative52, reductive53,54, and pyrolytic which include Unterzauchers55 and modified Unterzauchers method56, pyrolysis followed by measurement of CO and CO2 at (900°C)57 and at high temperature (1950°C) using Radio frequency (RF) heating58 which gave better results than pyrolytic (900°C). The ISO method59 involves the pyrolysis of coal followed by passing the products over carbon or preferably platinised carbon at 1100°C. Oxygen in the coal in all the pyrolytic methods is thus converted to CO which can be determined by various methods60,61. Ehmann et al.

62 used a modification of the Unterzauchers method followed by measuring the generated CO2 coulometrically. All the methods mentioned above are tedious and time and energy consuming and are not used extensively. Among the sophisticated techniques namely electron microprobe analysis63, charged particle activation analysis64,65, and fast neutron activation analysis (FNNA) have been proposed of which FNNA has been widely if not extensively used

for direct determination of oxygen in coal. The earliest works on the application of Neutron activation analysis in this area was by Veal and Cook66 who first applied it to coal and also by Martin et al.

67. The FNNA method essentially involves the irradiation of samples with 14 MeV fast neutrons produced from accelerator type generators (e.g. Cockroft-Walon type or later developed Kaman type sealed tube neutron generators) and compared to slow (thermal neutrons) lead to a much simpler gamma ray spectrum. The process involves nuclear reaction 16O(np)16N. Gamma rays of 6.1 and 7.1 MeV are emitted when 16N decays. Various groups68-76 have examined this method in detail for the determination of total and organic oxygen. The problem with these methods is that they determine the total oxygen content since they do not differentiate between the oxygen in the organic and inorganic part (including moisture) in coals. The oxygen content of pure coals (Oorg) is determined by accounting for/correcting the oxygen contribution from mineral matter and moisture. Correction for moisture is usually done by62,74,78 carefully drying the coal in inert atmosphere and filling the dry coal into the FNNA rabbits in the confines of a dry box in an inert atmosphere. The correction for mineral matter is done by two methods, either by removing the mineral matter from the coal by acid demineralization62,74,78 or by subtracting from the total oxygen content determined by FNNA, the oxygen content of mineral matter which is determined by measuring the oxygen content by FNNA of either of high temperature ash (HTA)71-73,75,76 or low temperature ash (LTA) isolated by ash preparation in low temperature oxygen plasma68-71,74. However, all the three approaches have limitations. Since preparation of HTA leads to substantial alteration of mineral matter originally present in coal, it is obvious that determination of oxygen in mineral matter by FNNA using HTA will be substantially erroneous and therefore Volborth et al.

76 recommended use of LTA for this purpose. But considering the difficulties in preparing LTA they proposed71-73,75,76 a series of approximations/ corrections (while using HTA for the above purpose) which gave better oxygen values than when these corrections were not applied. However, use of LTA for the above purpose in place of HTA is also fraught with possible errors due to fixation of organic sulphur in coal as sulphate during the LTA ashing process. Organic oxygen determination by FNNA on acid demineralised coal sounds a very good idea but it has


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been reported62,68,78 to have serious limitations due to the fact that residual fluorine trapped in coal as a result of demineralization with HCl-HF can lead to serious errors as fluorine seriously interferes with FNNA determination of oxygen due to 19F(n,2n)18F reaction and also due to the fact that acid demineralization normally does not remove the mineral matter completely. Various detailed corrections have been applied to account for all these sources for errors62,78 which has brought some agreement between Oorg values from FNNA by demineralised coal method and by LTA method78 and FNNA-DMC62 and Given–BD74. However, these authors concede that Oorg determined by FNNA after incorporating various corrections remains essentially a by difference method. Although oxygen by difference method continues to be the most widely used and recommended by various standard procedures including ASTM33 and BIS38. Several authors31,80,84-86 in their studies on coal oxidation have used FNAA and charged particle activation analysis81 to determine Oorg content in coals and oxidised coals. Most authors13,15,75 who have studied the effect of aerial oxidation have in general reported decrease in carbon, and hydrogen content and increase in oxygen content. Cimadevilla et al.

12 observed little change in ultimate analysis inspite of considerable change in plastic properties for coals weathered for up to 567 days. One important application of C, H, O data from ultimate analysis of coal is the use of the Van Krevelen diagram prepared by plotting the H/C versus O/C or better (O+N)/C. The ultimate analysis data plotted on a van Krevelen diagram reveals the pathway i.e. various distinct stages during the oxidation and also the various changes that take place at every stage. Azik et al.

84-86 revealed that oxidation of Beypazari lignite at 150°C involved three distinct stages. Similar use of van Krevelen diagrams have been made by various other authors e.g. Kister et al.

88, Dereppe et al.

89, Lopez et al.90 and Iglesias et al.

13 to track various stages during coal oxidation. Another method87of utilizing ultimate analysis data involves superimposition of oxidation tracks on the Seylers coal band. Retracing an oxidation track across the Seyler’s coal band allows estimation of probable composition of original (unoxidised) coal wherever only the oxidised coal is available. Excellent reviews on oxygen in coals have been presented by Ruberto and Cronauer82 and Zhou et al.

83. According to Yun and Meuzelaar32 the total oxygen content in oxidised

coal may not actually reflect the true oxygen uptake as the uptake of oxygen is offset by parallel processes of loss in the form of water and oxides of carbon which is not insignificant. Though the present authors agree with Yun and Meuzelaar32 nevertheless increase in oxygen content of coal (by difference on DMF basis) gives significant information particularly when used in van Krevelen diagram. Quantification of oxygenated functional groups in

coal The most significant change in coal structure due to aerial oxidation is the increase in concentration of some and also generation of some oxygenated groups which has been recorded/measured by both chemical and instrumental methods such as FTIR, XPS, NMR, etc (Discussed in detail later in various respective sections). Also the increase in these functional groups is considered to lead to deterioration of many of the technical properties by way of formation of ether and ester crosslinks. Quantitative estimation of these oxygenated groups is therefore extremely important in the study of aerial oxidation of coal. Therefore, since inception of modern coal science researchers91-99 have devoted lot of efforts in the development of methods for quantitative evaluation of oxygenated functional groups distribution in coal. Functional group analysis

Various reviews petaining to these methods have been presented by various authors. Some very comprehensive and important reviews have been published by Ihnatowitcz100, Blom et al.

101, van Krevelen102, Kroger et al.

103,104 and Ignasiak et al.105,

Given and Yarzab50, and Wender et al.106 .

Total acidity (phenolic plus carboxylic)

The two most commonly used methods for estimating the total acidity (phenolic plus carboxylic) in coals are ion exchange method and method of non-aqueous titration. Over the years both methods have been compared and critically re-evaluated and improvements, corrections and even substantial modifications have been incorporated. The non-aqueous titration method by Brooks and Maher et al.

107,108 essentially involves treatment of the coal with ethylenediamine followed by potientiometric titration with sodium aminoethoxide. The ion exchange method101 involves exchange of the acidic groups with barium hydroxide and the amount of acid


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groups (total acidity) is estimated from decrease in concentration, after exchange, of barium hydroxide solution using titration. In a reappraisal of this method, Schafer109 has opined that the exchange of acidic groups by barium hydroxide alone gave too high results which has been attributed to more barium hydroxide being consumed than is required to exchange the acidic groups. This may be due to physical adsorption of the barium hydroxide reagent by coal. Schafer proposed a modified method of exchange, using barium hydroxide-barium chloride reagent which is followed by washing with a wash solution containing sodium hydroxide and barium chloride which removes the excess and adsorbed barium hydroxide without causing either hydrolysis or loss of the exchanged barium in coal. Maher and Schafer110 compared both non aqueous potentiometric titration and the ion exchange methods and observed that the values for both carboxyl and total acid group contents obtained by non aqueous titration were always higher than that obtained by ion exchange method. However, the phenolic content (obtained by difference) determined by both methods were in agreement. Although the authors proposed a number of explanations for this phenomenon they opined that further work in this area is needed. Other methods include thermometric titration with alcoholic alkali by Vaughen and Swithenbank111 and using D2O exchange by Yokoyama et al.

112. Although the method of non-aqueous titration107,108,110 is most commonly used recently doubts have been raised about its accuracy113. Allardice et al.

114 undertook reevaluation of Schafers methods for determination of total acidity and carboxylic content. Suitable modifications were suggested to make the method quicker, cheaper and safer without losing on accuracy. The modifications included addition of methanol as wetting and swelling agent during the barium exchange process which facilitates the exchange process for more hydrophobic high rank coal and heat treated coals and the use of 0.2 M HCl rather than the more dangerous perchloric acid used in Schafers original method109. Determination of hydroxyl groups In their classic work, Brown and Wyss115 measured the uptake of acetyl groups by hydroxyl groups in coal. Various authors have used acetylation for hydroxyl group determination using variations in method. Wyss116 and Blom et al.

117 used acetylation

with acetic anhydride in pyridine and Mazumdar et al.

118 and Mukherjee et al.119 both used acetylation

with sodium acetate in acetic anhydride. In a rather less time taking variation of this method coal was acetylated using 14C labeled acetic anhydride and the -OH content was estimated by measuring acetyl uptake radiochemically120 or alternatively the acetylated coal is combusted and the combustion gases collected and subjected to scintillation counting121. Some earlier authors122,123 have used titration with barium hydroxide and calcium acetate at room temperature in aqueous medium. Friedman et al.

124 introduced conversion of phenolic groups in coal to trimethylsilyl ethers by reaction with hexamethyldisilazane in boiling pyridine followed by determination of the added silicon or by IR spectroscopy. Blom125 after comparing various methods for determining OH groups in coal, prior to 1960, suggested a procedure involving acetylation of coal with acetic anhydride in pyridine medium. Pyridine is presumed to aid entry of the reagent by swelling the coal. Presently this method continues to be the most widely used routine method for estimation of -OH in coal. Method of non-aqueous titration attempted by Dutta and Holland123 was found to give lower values of phenolic –OH compare to acetylation method which they attributed to proximal phenolic groups being partially titrable. In 1996, Aida

et al.126 developed a new method for estimation of

-OH group distribution (-OH as phenolic, as alcoholic and as carboxylic) by quantifying the hydrogen evolved by reaction of hydroxyl groups in coal with borohydride in pyridine. Phenolic -OH can be determined117 seperately by esterifying the coal with diazomethane in diethyl ether, saponifying the product and finally determining the resultant -OCH3 by modified Zeisl method. Alcoholic -OH can then be determined by -OHtotal minus -OHphenolic. Lenz and Koster127 developed a method for quantitative determination of hydroxyl groups in lignites by use of activated triethylborane and measuring the ethane evolved. Determination of carboxylic groups

Carboxyl groups are generally determined by an ion exchange reaction with alkali or alkaline earth acetates followed by filtration and washing of the coal. The carboxyl content is then determined by estimating content of acetic acid (generated by the exchange process) in the filtrate by titration with


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alkali. Alternatively the treated coal is washed free of excess acetate and (-COOH) is measured by determining barium content gravimetrically. Alternatively the cations in the washed coal are acid extracted and the carboxyl content is measured by estimating the amount of acid needed to extract the cations. The original method has been attributed to Fuchs128 which involved ion exchange with calcium acetate. Various modifications of the basic experimental procedure101,129-132 are found in literature. Later method of Fuchs133 involved exchange with calcium acetate followed by potentiometric titration of the acetic acid with alkali to pH 7.9, in the presence of coal. In the methods mentioned above in this section it is assumed that phenolic groups do not get involved in the exchange process while complete exchange of carboxylic groups take place. In all cases it is extremely important to keep the hydroxyl groups out of the exchange process. This can be done by using methylation134 to block the hydroxyl groups or by conducting the exchange reaction at pH of around 8.3. In order to avoid exchange by phenolic groups Fischer and Kadner135 applied the original method of Mehlich136,137 for soils. In this method the sample is exchanged with a solution of barium chloride buffered with triethanolamine and hydrochloric acid to a pH of 8.2 approximately. This method is simple, rapid and the buffer keeps the pH at 8.2. In a detailed study of the methods for estimation of carboxylic groups in coals, Schafer138 opined that exchange process of carboxylic groups must essentially be conducted at pH of around 8.3 which ensures complete exchange while keeping phenolic groups out of the exchange process and also that the procedure using single reflux with barium acetate (pH 8.25) followed by titration in presence of the acid of coal released was the most reliable method of carboxylic determination. He further concluded139 that the method involving exchange with barium chloride with triethanolamine and hydrochloric acid gave much higher results which he attributed to physical adsorption of triethanolamine by coal. However, after further reevaluation of this method Schafer and Wornat in a later paper140 concluded that it gave reliable results provided that, prior to exchange, the sample is treated with hydrochloric acid to convert carboxyl groups, if any, in cation form to the acid form and that the calculation was done on the basis of the amount of acid used to remove the cations from the exchanged

coal and not on the amount of barium extracted. On the basis of study for the determination of carboxylic groups in Yallourn brown coal, Schafer and Wornat140 mentioned that treatment of the coal sample with hydrochloric acid prior to exchange was essential to obtain accurate results. Allardice et al.

114 reexamined the Schafer’s method138,139 in detail and proposed some modifications in the post exchange process which has been mentioned above in the total acidity section.

Determination of carbonyl group Estimation of carbonyl (=CO) functional group is usually done by oxime formation procedure117 or by reaction with phenylhydrazene101,102.

Determination of oxygenated groups using

spectroscopic methods Osawa and Shih141 using dispersive IR plotted the normalised absorbance (which they termed as specific extension coefficient) of the 3400 cm-1 band against the OH content determined by acetylation methods. Solomon142 using FTIR applied the method and co-relation of Osawa and Shih to determine OH group content during structural studies in coal. Later Solomon and Carangelo143 developed their own correlation by using the band intensity at 3400 cm-1 and determined OH contents of coals using the radiochemical method of Abdel Baset et al.

121. However, in all cases the water absorbed by coal KBr pellet is the most significant difficulty. To avoid this problem Kuehn et al.

144 and Rhoads et al.145 combined

FTIR and acetylation methods to determine OH group concentrations in coals. Using curve resolution they improved upon the original method of Durie and Sternhell146. Painter et al.

147 compared the -OH values as determined by FTIR with those determined by 13C NMR148 and tried to explain the discrepancy between the two even after the NMR data was subjected to correction. Dyrkacz and Bloomquist149

attempted quantitative estimation of -OH groups using DRIFT. Starsinic et al.

150 used a combination of Shafer's ion exchange technique and FTIR spectroscopy with curve fitting to estimate carboxyl groups in coal. Murata et al.

151 determined distribution of oxygen functional groups (phenolic, alcoholic, carboxylic, carbonyl, ether) using a combination of chemical techniques and Single Pulse Excitation/ Magic Angle Spinning (SPE/MAS) 13CNMR followed by curve fitting. However, in spite of their limitations most authors continue to use the


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classical methods for quantification of oxygenated functional groups. Calorimetry

Isothermal calorimetry

Isothermal calorimetry measures the rate of production of thermal energy when coal is heated at particular constant temperature. This rate has been used151,153 as a measure of self-heating propensity of coal. Some authors154,155 have used static isothermal method (also referred to as oxygen adsorption method) where the propensity of coal to self-heat is indicated by either the rate of oxygen consumption154

or the rate of heat generation155 at a constant starting temperature. Using an isothermal-flow reactor Wang et al.

156 studied the decomposition behaviour of solid oxygenated complexes formed by low-temperature oxidation of coal under pure nitrogen, at temperatures between 60 and 110°C. The production of CO2 and CO during thermal decomposition of the complexes was quantified by an on-line dual-column micro GC. They showed that the production rates of CO2 and CO depend on temperature, but not on particle size of the samples, indicating that this thermal decomposition process is dominated by chemical kinetics rather than diffusion. Adiabatic calorimetry

Adiabatic calorimetry157 consists of measuring the temperature rise in a coal sample reacting with oxygen under adiabatic conditions. Normally this is done by placing the sample in an oven the temperature of which is increased to match the temperature of the sample recorded using a thermometer placed within the sample. The main parameters157,158 recorded are Maximum Temperature Recorded / Total Temperature Rise (TTR) and Initial Rate of Heating (IRH) which are used to rank coals according to their self-heating propensity. For the purpose of studying spontaneous combustion of coal, Davis and Byrne159 way back in 1924 first proposed application of adiabatic oxidation technique. However, due to practical difficulties it was only in around 1970 that interest in this technique was regenerated. Combining adiabatic oxidation data (obtained under identical conditions for all coals tested) and historical spontaneous combustion records, researchers of the University of Nottingham160 developed a spontaneous combustion classification system. Moxon and Richardson161-163

using adiabatic technique (later developed as a commercial instrument) have drawn correlations between the maximum temperature rise under controlled oxidation conditions and chemical and petrographic properties. Publications by some authors164,165 have described various designs of such adiabatic apparatus. Ren et al.

157 used both IRH and TTR values to rank coal according to spontaneous combustion risk but also considered the temperature rise versus time curve. They also observed inter alia that smaller particle size had positive effect on TTR but there exists a critical size above which size effect is insignificant. Vance et al.

166 while studying spontaneous combustion of coal observed that at temperatures below 80°C maximum oxidation occurs at medium moisture content (around 7%). Smith and Lazzara167 also opined that highest self-heating risks due to low temperature oxidation were observed using humid air and not dry air. Adiabatic method for determining R70

This adiabatic test method involves the measurement of average self-heating rate of the coal between 40 and 70°C. This self-heating rate, which is known as R70, is considered as a measure of the coal's propensity to self-heat. This method although first developed by Humphreys169,170 was developed into a full standard method for testing coal for self heating propensity by Beamish et al.

168. A coal with an R70 value below 0.58 C/h is considered to be low risk, 0.5±0.88 C/h medium risk, while coals with an R70 value higher than 0.88 C/h are considered to be highly prone to self-heating171. This method has been adopted by the Australian Coal Industry Research Laboratories (ACIRL) as a service to industry on a commercial fee-paying basis. In an adiabatic oxidation study on New Zealand coals, Beamish et al.

172 observed very high R70 values for the New Zealand low-rank coals indicating that they are extremely prone to spontaneous combustion. They further observed that resin bodies present in sub-bituminous coals from the Rotowaro Coalfield show no propensity to spontaneous combustion and are therefore not a contributing factor to the high propensity to spontaneous combustion of these coals. In an attempt to assess the effect of rank and R70 self-heating index of New Zealand and Australian coals, Beamish et al.

172 and Beamish173 observed a non linear relationship between R70 and Suggate rank parameter174,175 and revised the later (Suggate).


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Significant moderating (reducing) effect of both moisture and mineral matter on the R70 coal self-heating rate have been demonstrated by Beamish and Blazak176 and Beamish and Hamilton177 respectively. Consequently although New Zealand sub-bituminous coal is 20 times more reactive to oxygen compared to high rank coal, it effectively comes down to 2 to 3 times when the effect of moisture and mineral matter is accounted for. Beamish and Arisoy178 further opined that the physico chemical effects also contribute towards the negative relationship between R70 self heating rate and mineral matter in coal apart from the heat sink effect of mineral matter which is the most common explanation. Empirical tests related to carbonization behaviour Caking coals, during carbonization, pass through a fluid stage around 350 to 550°C, which is extremely sensitive to the presence of certain components/structures generated during oxidation of parent coal. These changes in the thermoplastic and rheological properties, which are sensitive to various degrees to the effects of coal oxidation, form the basis of various empirical tests such Gieselar plastometry, Dilatometry, Free swelling index. Gieselar plastometry

Gieselar plastometry involves heating from an initial temperature of 300°C at 3°C/min of coal sample tightly packed around a rod with rabble arms, which is subjected to a constant torque of 40 g/in. Important data measured include maximum fluidity, maximum fluidity at Dial Divisions Per Minute (DDPM), initial softening point and resolidification temperature. Standard methods used include ASTM179 and BIS182 standard procedures. Earlier literature which include work of Schmidt et al.

185,186, Davis et al.

187 and Russel and Pearch188 indicate in general that the effect of coal oxidation is decreased in maximum fluidity which is accompanied by concomitant change in other Gieselar parameters such as narrowing of plastic range, increase in softening point and decrease in resolidification temperature, which also are extremely hesitative to low temperature oxidation/weathering of coal. Extensive literature generated subsequently10,12,87,189-199, while agreeing with the earlier works, have come to a general conclusion that the loss of maximum fluidity as determined by Gieselar plastometer is most sensitive parameter and responsive to low temperature

oxidation with the other parameters mentioned above to be somewhat less but also very sensitive. These parameters particularly, maximum fluidity is extremely useful since significant changes in these parameters are observed much before any significant changes in proximate and ultimate analysis, calorific value and changes in DRIFT spectra are observed. This sensitivity is true in case of both laboratory oxidation and natural weathering with reduction of Gieseler fluidity to nearly one tenth of its initial value as observed by Huffman et al.189, after just six hours of thermal oxidation at 105°C, is an example of its sensitivity. Seki et al.

195 have observed that although Gieselar maximum fluidity decreased for all coals they examined, the pattern and rate of decrease differed among coals which he observed for FSI also. Many authors10,12,188-190 have observed that the most dramatic reduction in maximum fluidity occurs in the early stages of oxidation after which the sensitivity to oxidation is markedly decreased and finally reduces to zero. Thus, for monitoring of long-term or deep oxidation Gieselar plastometry is not a useful technique since changes during long term/deep oxidation can not be monitored. Such oxidations are better monitored by FSI, which takes a much longer time to reach minimum constant value (discussed later in this section). Due to its extreme sensitivity to aerial oxidation, Gieselar fluidity has often been used21,200-203 to monitor the detoriation characteristics (due to oxidation) on storage so as to examine the efficacy of various long-term storage techniques. The surprising observation of Huggins et al.

200 that two coals stored at room temperature undergo a loss of 20-30% of its fluidity in 455 days indicated that for safe storage of coal samples more stringent techniques have to be used. An empirical test, Gieseler fluidity measurement has been used by some authors to investigate the loss of fluidity due to oxidation/weathering of coal. Casal et al.

204 while monitoring of weathering of coking coal attributed loss of Gieseler maximum fluidity to concomitant decrease in methylene groups as determined by FTIR. Clemens et al.

216 observed inter alia that the lowering

of Gieseler fluidity due to oxidation was restored to a large extent by addition of a free radical stabilizer suggesting that for fluidity development quenching of free radicals generated in the coal structure during heating was more important than cross link development. Ignasiak et al.

205 observed that the loss in caking properties, by aerial oxidation, of a high


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volatile bituminous coal as measured by Gieseler fluidity and dilation was restored to considerable degree by treatment with barium hydroxide and tentatively concluded that certain hydroxyl groups formed during oxidation were responsible for the loss in caking properties which are subsequently restored after blocking the process by barium hydroxide exchange. However, in spite of its sensitivity Gieselar plastometry has certain disadvantages, which include its usefulness only in case of caking coals and the fact that even for the same coal any small changes in maceral distribution in samples drawn can lead to different fluidity values. Another disadvantage is that unlike mossbauer spectroscopy or stain test the original values of the unoxidised coals are required. Furthermore, some authors200,206 have opined that any process other than oxidation e.g. volatilization of hydrocarbon moieties under vacuum and methane degassing that decreases alkyl groups may also be responsible for loss in Gieseler fluidity. Malony et al.

207 observed that exposure of an hvA bituminous coal to moist air at ambient temperature has more effect on subsequent swelling and softening properties than exposure to dry air and that the plastic properties of an oxidised coal may be partially restored by grinding to expose new surface.

Dilatation test

Dilatation is commonly studied applying Audibert-Arnu Dilatometer. ASTM Standard180 is commonly used. The method involves heating of, in a vertical electric furnace, a coal sample in the form of tapered cylindrical pencil of standard specified weight and dimension placed in a container with a 7.8 cm rod placed on the sample. The movement of the rod against a calibrated scale indicates volume changes (dilatation) during pyrolytic heating (3°C/min). Various studies indicate that dilatation decreases on oxidation. Alongside fluidity, dilatation is also found to be a very sensitive parameter to early stages of coal oxidation at low temperatures197-199,205. According to Ghiroux et al.

30, as observed with Gieseler fluidity, dilatation was found to decrease more rapidly with higher storage temperature. They observed inter alia that while dilatation of a coal stored at 40°C dropped to zero within 20 weeks, dilatation and Gieseler parameters remained virtually unchanged even after 60 weeks when same coal was stored at -15°C. Thomas et al.

208observed that although dilation of oxidised coal was found to be either same or less than

that of fresh coal depending on the oxidation conditions, shapes of dilation versus the pressure curves changed dramatically with oxidation. Free swelling index (FSI)

Measurement of free swelling index of coal essentially involves heating of 1 g of sample in a covered crucible at 820°C for 2½ min and grading the coke button produced from lowest size 1 to highest 9. Standard methods commonly used include ASTM181 and BIS182 methods. Most authors30,197-199 agree that FSI is much less sensitive (much slower) indicator of early stage low temperature coal oxidation compared to Gieseler maximum fluidity or dilatation. Seki et al.

195 observed FSI of two coals actually increased initially with oxidation time before decreasing. Kreulen209 also had similar observations. Larsen et al.

210 proposed that the loss of donatable hydrogens is one of the key factors responsible for the loss in FSI due to aerial oxidation which is restored by reduction with lithium aluminium hydride. Iglesias et al.

13 observed reduction of FSI from 8 to 0 after 6 h oxidation which they correlated to corresponding significant reduction in alkyl groups in the FTIR spectrum. Sole heated oven test

Sole heated oven test for coke contraction measurements, applicable for coals and coal blends used for coke making, is a large-scale laboratory test for studying the expansion or contraction of coalor coal blends during carbonization under specified conditions. ASTM standard method183 is commonly used. Small changes in the coke contraction level for coals subjected to oxidation on long-term storage in ambient were observed by some authors196,212-213. However, Khan et al.

214 failed to detect any reduction in contraction even after storage for four years although it had led to reduction in dilation from 82 to 23% Coke reactivity and strength

ASTM test method184 is the commonly used standard procedure. Giroux et al.

29 observed changes in coke reactivity and hot coke strength, as measured by CRI and CSR. While CRI increased after 6 months and then leveled out around that value at 12 months, CSR decreased from 65 to 58% during the first 6 months storage. Other investigators196,211,212,213,216 have reported decreases in hot coke strength with storage.


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Proton magnetic resonance thermal analyser (PMRTA)

The measuring instrument was originally developed in the laboratories of CSIRO, Australia. The method involves recording proton magnetic resonance signals from coal samples being heated at rate of 4°C/min up to 550°C under nitrogen atmosphere. These signals are converted to F (Fusion) values, which are a measure of mobility of the sample. Clemens et al.

216 using PMRTA pyrograms observed corresponding decrease in maximum mobility (indicated by increase in M2T10 values) with Gieselar maximum fluidity with progressive oxidation of coal. This loss of mobility and fluidity was correlated with corresponding loss of C-H bonds, which though small translated into big loss in mobility. Loss in C-H bonds results in loss of donatable hydrogens required to cap the free radical species to generate low molecular weight species required for fluidity development. Smith et al.

217 observed changes in maximum mobility as measured from F (Fusion) values (maximum F values reached during pyrolysis at around 450°C) obtained from PMRTA and used it to characterise coking coal subjected to low temperature aerial oxidation at various temperatures. They observed that fluidity remained unchanged during oxidation at 36°C for 28 days but decreased in case of same duration at 80°C, the change at 80°C being more pronounced compared to 60°C. Heat based measurements

Crossing point temperature (CPT)

This method basically involves heating powdered coal samples in a flow of air/oxygen and constantly monitoring the temperature of the both the coal bed and the heating bath during heating of the coal at a precise rate of temperature increase. The point at which the temperature of the coal bed exceeds the temperature of the heating bath is called the crossing point temperature (CPT) and signifies the onset of self-propellant self-heating. It is generally believed that Nubing and Wanner218, as early as 1915, first measured the crossing point temperature of coal using an oil bath to gradually heat coal in oxygen flow. Wheeler219 carried out similar experiment with air but preferred to call the temperature relative ignition temperature. Parr and Coons220, used Wheeler’s method but with oxygen and named this temperature as autogenous temperature. Over the years, keeping the basic principle the same, various authors221-228,287287 have incorporated various

modifications in the above experimental method and apparatus for the determination of crossing point temperature. Feng et al.

231 applied further variation in the method by taking into account further information generated during the CPT experiment such as heating rate. In India the most widely used method is that of Ganguli and Banerjee232 which was standardised and optimised by Bagchi233,234 who suggested heating rate of 0.5°C per min in a glycerin bath and with a flow rate of 80 mL/min of humid air for a bed of 20 g of coal. Bagchi234 also studied the factors affecting the determination of crossing point temperature. Interpretation of results of CPT poses quite a number of difficulties since results vary widely with experimental conditions and apparatus design. Even with identical apparatus and experimental conditions repeatability is no better than 4-5°C. Furthermore, the experimental conditions used are quite different from the conditions encountered in stockpiles. Banerjee et al.

235 pointed out certain limitations in the crossing point method, principally encountered for high moisture coals which usually generate misleading information. Nandi et al.

236 while studying the application of CPT for determination/categorisation of spontaneous combustion of coal opined that for better categorization both the CPT value and the slope of the time temperature curve (which signifies rate of temperature rise) should be considered. A coal exhibiting a sudden sharp rise in the slope of the time-temperature curve may be considered to be highly vulnerable. Similar opinion has been given by Banerjee et al.

235. Nevertheless, crossing point temperature continues to be widely used237,238 as a common method for categorisation of propensity of coal to self-ignition mainly due to the simplicity of the apparatus and absence of sophisticated instrumentation. Lower is the CPT, higher is the propensity. Nandi et al.

236,239 observed that CPT usually decreased with the increase in moisture content, volatile matter and oxygen content and with decrease in rank (carbon content). They also observed that beyond 4-6% moisture content (as received basis), CPT again exhibits an increasing trend, which probably explains the high CPT values in Lignites in spite of their low rank. Mahidin et al.

240 observed some what similar results. They further correlated the increase in oxygen and volatile matter contents in coal with concomitant increase in various functional groups in coal, determined from FTIR spectra and


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further correlated them with CPT data. They observed good correlation between FTIR and CPT data. Chen and Chong241 examined some important factors regarding CPT method for studying self-ignition kinetics of combustible solid. Nugroho et al.

242,243, to study the propensity of coals and coal blends to self-heat applied the crossing point temperature and measured it against factors such as critical ambient temperature and activation energy. Kadioglu and Varamaz244 investigated the effect of moisture content and air-drying on spontaneous combustion of coal using CPT method and concluded that CPT values increased as moisture content and particle size of the coal sample increases. In spite of its limitations the crossing point method continues to be the most common and widely used method in the Indian coal industry for assessing propensity of coal to spontaneous combustion mainly due to its ease and low cost. Basket heating test

Bowes first developed basket-heating methods for assessment of coal oxidation and Cameron245,246 based on Frank Kamentskii theoretical model247 for thermal ignition of solids in a packed state. The basic experimental technique, details of which are described by Bowes and Cameron245, involves placing a basket of particular size and shape filled with the prepared coal in an oven. The temperature of the oven is then allowed to rise to a preset level. If the coal fails to ignite the oven temperature is preset at higher level and experiment repeated till coal ignites. The total set of experiments is further repeated with identical shapes but different sizes. The critical temperature is then determined as a function of basket size. The experimental data is interpreted and extrapolated to stockpiles of significant size using energy equations of Kamentskii. The experimental data generated is used for designing safe coal stack250,251, for studying chemical kinetics of coal oxidation using Frank Kamentskii model245 and for studying coal shipping safety248. Jones has studied basket-heating method in detail249,252. Olpinski index

This method235 is popular in Poland for categorisation of propensity of coal to self-heat. In this method a pellet of powdered coal is heated in quinoline vapour bath with air flowing past the coal pellet at a constant rate. The time versus temperature curve is continuously noted till the temperature is

235°C (boiling point of quinoline). Sza is obtained by drawing a tangent at point of 235°C. Szb is then obtained by applying correction for ash. It is claimed that coal with Szb above 120 has high susceptibility to spontaneous heating. However, doubts have been raised about its accuracy since coal swells in quinoline vapour and this is rather different from true self-heating situation.

Chens method

This method of assessment of coal oxidation susceptibility, particularly studying apparent chemical kinetics of coal oxidation is of relatively recent origin and was developed by Chen and Chong241. This method is based on the fact that coal sample while being externally heated during oxidation, at a particular point of time, the heat flux disappears at a point near the centre of the sample. By measuring the temperature history at short distance from the centre line of the sample, the rate of heat release can be directly calculated and coal oxidation kinetics can be determined according to a simple kinetic model.

Heat release measurements

Jones et al.252-254 measured the rate of heat release

at oven temperature from temperature-time curve to study the kinetic parameters of air oxidation of coal. The experimental setup is the same as for basket heating test.

Titration (quantification of peroxy complex)

It is now generally accepted81,98 that aerial oxidation commences with chemisorption of oxygen by coal leading to the formation of an unstable coal oxygen complex (peroxy complex) which reaches a maximum with time and depending on the conditions finally decomposes via free radical chain reaction mechanism to generate oxygenated groups accompanied by liberation of CO, CO2 and H2O. It is generally believed that peroxy complex in coal is of hydro peroxide type. The quantitative estimation of per oxygen content has been used in studies on aerial oxidation of coal. As early as 1949 Yohe and Harman255 and Jones and Townend256,257 separately published methods for actually measuring the per oxygen content in coals. Yohe and Harman measured this per oxygen content by adding excess TiCl3 in coal and titrating back excess TiCl3 potentiometrically with FeCl3. Jones and Townend measured the per oxygen content by titration with ferrous thiocynate solution and the resultant ferric salt was measured colorimetrically. Chalishazar and Spooner258 modified


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method of Jones and Townend, which requires special apparatus to develop a simple and rapid method to determine peroxide groups in coal. They treated coal with ferrous ammonium sulphate and estimated the ferric ion produced by titration with mercurous nitrate. Banerjee and Chakravorty259 used this method for peroxy estimation to assess spontaneous heating susceptibly of coals. Though the method is simple it is only useful for broad based classifications. Chakravarty260,261 used the method of Chalishazar and Spooner258 to determine the activation energies of reaction involved in low temperature aerial oxidation of coal.

Slurry pH measurements One of the early studies on slurry pH was the work done in BCURA laboratory in the mid forties by Kramers and McKee (mentioned in Jones and Townend256) in which the pH of a slurry of coal in methanol with 5% water showed progressive decrease with increase in the oxidation of coal. Considering the acidic character of weathered/oxidised coal Gray et al.

262, in an attempt to consider a simple yet effective and practical method to assess degree of oxidation in coals suggested slurry pH as one of the candidate techniques. They observed slurry pH changes from 7.1 to 5.5 in case of a severely weathered low volatile bituminous coal. Hill et al.

263

while applying the same technique for sub-bituminous coal added 0.01% of surfactant to enhance wetting of the coal. They observed slurry pH changes from 7.2 to 5.0 and made attempt to correlate the results with concomitant closs in calorific values. The use of slurry pH has been mentioned by Iskra and Laskowski264 as one of the important methods for detecting of oxidation in weathered coal and for assessing degree of oxidation. According to Mikula et al.

265 slurry pH measurement is a sensitive test for detection of oxidation in coal. Yun et al.

32,266 for determining the degree of air oxidation of a coal sample of known origin but uncertain weathering status developed a reliable slurry pH titration method using oxidised coal rehydrated at 150°C in Parr digestion bombs. Rehydration of the oxidised coal converts the anhydrides and other condensation products back to acidic moieties. However, although for low rank coals e.g. sub-bituminous coals the initial pH value can suffice as an efficient oxidation index34,266 but higher rank coals due to their low susceptibility towards oxidation necessarily requires rehydration to enhance the slurry pH differences.

However, in spite of its ease and simplicity, and that it involves simple and low cost equipment, very few studies have been undertaken on Indian coals. More and extensive studies need to be done in this area involving Indian coals.

Optical microscopy

Microscopic detection of oxidised coal using stain test

Gray et al.262 first developed this method for

rendering oxidised coal more amenable to quantitative microscopic examination by alkali etching of petrographic sample followed by staining with a red stain (saffranin O in alcohol), which turns the red areas green. The intensity and depth of the stain depend upon the degree of oxidation. According to Marchioni22 the stain test is the most sensitive test for detection of oxidised coal and is sometimes more sensitive than Giesler plastometry with the advantage that it can detect weathering without prior knowledge of the fresh coal characteristics and furthermore that this test is not influenced by petrographic variations. Lowenhaupt and Gray268 suggested an 80% cutoff point for cationic staining of caking/coking coals to be used routinely to assess oxidation or weathering state.

Petrological studies by microscope

Standard methods include BIS269 and ASTM270 methods. Aerial oxidation of coal, whether natural or induced, causes change in the microstructure, which varies from maceral to maceral. Microscopic features of oxidised coal which can be used to detect oxidised coal and assess the extent of oxidation have been studied by Gray et al.

262, Crelling et al.271,

Lowenhaupt and Gray268, and Marchioni22. The petrological sample of coal is prepared and examined under a microscope with an oil immersion objective. In a study on aerial oxidation Ferrari272 showed that vitrinite was most affected by aerial oxidation in comparison with other macerals. Formation of oxidation rims is a commonly observed feature of oxidised coal. Ferrari272, Goodarzi and Murchinson273,274, Nandi et al.

275 all reported high reflectance oxidation rims in case of laboratory induced high temperature aerial oxidation, while Gray et al.

262, Marchioni22, Crelling et al.271 and

Nagayanki276 have reported low reflectance oxidation rims due to natural weathering. Alpern and Maume277 concluded that while low reflectance rims were associated with low temperature oxidation such as weathering, high reflectance rims are associated with


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high temperature oxidation and that low rank coals were unlikely to produce rims on oxidation. Unlike Chandra26,278 who concluded that variations observed in Rō (max) on coals oxidised in air/oxygen and on weathered (outcrop) coals were insignificant, Kojima and Ogoshe279 and Alpern and Maume277 reported significant alterations in vitrinite reflectance. However, naturally oxidised (weathered) coal had lower vitrinite reflectance than their fresh equivalents. Pearson and Kwong280, Bustin281 and Itay et al.

282 on examination of a nearly pure vitrain (petrologically examined) after oxidation suggested that a wide range of oxidation reactivities exist even among vitrinite particles of a single coal. Gentzis et al.

283 observed that vitrinite reflectance of the oxidised coals increases with temperature for almost all ranks. However, for sub-bituminous coal, the major increase in reflectance is between 100 and 200°C, which is in agreement with the earlier results of Goodarzi and Murchison273. Pisupati and Scaroni15 after studying the reflectance values of fresh and crop (weathered) coal observed no significant differences in the vitrinite reflectance values indicating that reflectance is not affected by weathering even when structural and compositional properties are altered and therefore is in agreement with Marchion22 and Kruszewska and du Cann285 that Rō (max) is an insensitive parameter for studying weathering. In a study on optical properties of oxidised coal, Calemma et al.

284 concluded that the increase of reflectance is associated with relatively small changes in coal composition, and there exists a threshold value of oxidation degree beyond which further increase in reflectance is not observed and that the increase of reflectance is a result of a rearrangement of coal structure, during oxidation. Recently Wagner286 has used petrographic technique based Abnormal Condition Analysis (previously referred to as the Weathering Index Analysis) to study the degree of secondary weathering of a suit of discard coals. This method considers features not typically characterised during routine petrographic analyses e.g. margin effects, leach holes and microcrack and fissure patterns, but which may have an impact on the technological properties of coals.

Optical anisotropy index (AI) of cokes

Optical anisotropy index (AI) is an optical texture index which is calculated by method of Moreland et al.

287. It is generally estimated using a point

counting technique using polarized light microscope. This index gives an estimation of degree of anisotropic component developed on coke surface on carbonization. A number of authors10,204,208,288-291 have observed that optical anisotropy index of cokes as measured by microscope is reduced in case of oxidised coal signifying that for cokes of both fresh and oxidised coal obtained under identical carbonization anisotropic carbon content in the latter is reduced.

Fluorescence microscopy & fluorescence spectroscopy

Fluorescence in coal has been attributed to the activity of mobile π electrons which may either be outer shell electrons shared by adjacent atoms or which occur in conjugate double bonds292,293. Fluorescence microscopy294-297 involves measurement of vitrinite and huminite fluorescence produced by blue light (459-490 nm) at fixed wavelength (650 nm)295. With oxidation/weathering of coal fluorescence intensities are found to decrease. This decrease in fluorescence intensity (fluorescence quenching) is used to detect and, with necessary calibration, can be a potential technique for quantification of oxidation in coal. It is a sensitive technique for the detection and evaluation of oxidation and is considered to be comparable in sensitivity to Gieseler plastometry. However since Gieseler plastometry is only applicable to caking coals this technique is potentially useful for non caking (low rank) coals294-297 also. McHugh et al.

296 are of the opinion that although this phenomenon is observed in coals of all ranks it was more pronounced in high volatile bituminous coals. Quick et al.

295 applied fluorescence and reflectance in tandem to develop a quotient for expressing degree of oxidation. This was further investigated by Bend et al.

297, who proposed an oxidation quotient O/Q that is calculated as

O/Q=Fluor. Int./% Rō (max) where O/Q= Oxidation Quotient, Fluor. Int.= Mean maximum fluorescence intensity and %Rō (max) = Mean maximum Reflectance. Bend et al.

297also observed that in case of a lignite and coal, heating at 70°C lead to decrease in the quotient up to 10 points. However the main disadvantages with this technique is that macerals vary widely in fluorescence intensities, with


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desmocollinite showing greatest intensity296. Furthermore, fluorescence properties of coals, particularly their response to oxidation, depends upon factors like depositional history and are very prone to regional differences296. It is because of this that fluorescence microscopy in spite of its sensitivity has yet to be standardized and made widely applicable for detection and quantification of oxidation of coal. Kister et al.

88 applied Synchronous UV Fluorescence spectroscopy in conjunction with FTIR in the studies in coal oxidation. They noted differences in the PAH distribution between chloroform and pyridine extracts of fresh and those of oxidised coal. Long-wave fluorescence intensity measurement namely Fluorescence Relative Intensity (FRI) was used by Kruszewska and du Cann285 to detect incipient oxidation in coals subjected to simulate weathering conditions. Indentation microhardness

Indentation micro-hardness either the Knoop or Vickers microhardness index is a sensitive indicator to oxidation. Nandi et al.

298 observed that accordingly the kind of indentation materials can be classified as plastic (clear indentation with smooth edges) or elastic (shallow or indistinct indentation). They observed that for coals between 70 and 92% carbon, vitrinite changes from plastic to elastic. Microhardness data was used to develop an empirical formula to calculate Elasticity Index (EI) which according to Kruszewska and du Cann285 along with Fluorescence measurements showed very good agreement in the evaluation of oxidation levels and both appear to be more reliable than chemical or rheological methods. Gas chromatography (GC), gas chromatography-

mass spectrometry (GCMS) Pisupati et al.

14 analysed the volatiles released from flash pyrolysis of both fresh and oxidised (natural and laboratory) coal by GCMS and observed that the volatiles released by oxidised coals, compared to fresh coals, were comparatively poorer in combustibles and richer in CO2, water, phenolic and acidic species. Smith et al.217 observed that among the aliphatic, aromatic and polar fraction (generated by column chromatography) of dichloromethane /MeOH extract of both coking coal and the same after aerial oxidation at various temperatures, the composition of both aliphatic and aromatic fractions for both fresh

and oxidised coals as studied by GCMS remained nearly same suggesting minor role of both in the process and major role of the polar fraction. Change in the aromatic fraction were mainly small but significant increase in phenanthrene in case of oxidised coals was observed suggesting dealkylation process during aerial oxidation. Puttman et al.

299 also obtained similar results. Lu et al.

300 used MTI fast-response gas chromatography to determine inter

alia quantitative relation between quantity and composition of index gas and coal bed temperature under simulated experimental conditions. They concluded that for prevention of simultaneous detection of CO and C2H4 and their regular rise indicated immediate risk of spontaneous combustion of coal and therefore immediate preventive measures. They further observed from the index gas experiments that MEA-1A (solid high molecular retardant of Yichuang Sience and Technology Corp., China) was effective in prevention of spontaneous combustion. Alkali extraction test of oxidised coals by

Ultraviolet/Visible(UV/VIS) spectroscopy Atkinson and Hyslop304, as early as 1961, extracted oxidised coal with sodium hydroxide and correlated the extractability with oxygen content in an attempt to assess incipient oxidation of coking coals. Standard test method recommended by ASTM301 is rather inexpensive and rapid method for determining the relative degree of oxidation in bituminous coal by alkali extraction. This test is not sensitive to thermally oxidised coal but intended for coals that have been oxidised due to weathering. This is for determination of extent of oxidation of bituminous coals. This standard method involves extraction of oxidised coal with boiling 1 N NaOH and measuring the transmittance of the extract at 520 nm. The results are reported as % transmittance. It is generally accepted that coals having less than 80% transmittance are unfit for metallurgical purposes. This method is based on original procedure developed by Lowenhaupt and Gray268, and was recommended as a standard test for oxidised coals by US Steel prior to being accepted as ASTM standard301. The alkali extraction test has been used to correlate changes in other coal properties with oxidation. Gutierrez and Aplan302 have correlated froth floatation recovery with alkali extraction test and found that froth floatation recovery decreased with degree of oxidation, as measured by alkali extraction test. In a study on weathering and


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laboratory oxidation of two high volatile bituminous coals Lin et al.

303 observed fairly good correlation between alkali extraction test results and Gieseler plastometric measurements. Gray et al.

193 have, however, opined that alkali solution will not extract the oxygen functional groups produced at the beginning of coal weathering. In view of the repeatability limit of extraction test, like FSI, does not work well for detecting early stages of oxidation. Giroux et al.30 after applying this test on coals stored under different conditions have expressed the same opinion. Though this is a standard method not much data or studies on Indian coals exist in this area, which leaves a lot of scope to generate data vis a vis Indian coals. Infrared /Fourier transform infrared spectroscopy

(IR/FTIR) & diffuse reflectance infrared fourier

transform (DRIFT) spectroscopy Although considered303 to an extent less sensitive than rheological tests e.g. Gieseler plastoemetry and dilatometry, infrared spectroscopy particularly after the introduction of Fourier Transform Infra Red Spectroscopy (FTIR) is generally considered the most widely used technique employed in the study of coal oxidation/ weathering since it provides information on the structural changes as a result of coal oxidation and over the years has provided immense valuable published information about various structural changes15,31,81,84,86,189,198,204,305,309,310,315,316,318,320,322,323,325,

326,329,333,344,346 that are brought about by aerial oxidation. These changes, (discussed in this section), are observed in the infrared spectrum of oxidised coals by the way of increase/decrease of various bands and also generation of new bands. Most bands are assigned on the basis of various available standard published data on group frequencies e.g. Bellami337, Jones350 and Socrates353 and results obtained by other investigators147,309,322,345. Oxidation at near ambient temperatures lead to subtle changes in the FTIR transmission/DRIFT spectrum which can be interpreted better using techniques81,189,322,332,333,340 such as spectral subtraction, band ratioing and curve fitting techniques. As a result of aerial oxidation of coal, progressive reduction of alkyl group is generally observed15,23,197,315,317,318,322,324,331,332 and is the preferred reaction322,331,332,343 during initial stages of coal oxidation at 150°C. However during oxidation of Beypazari lignite at 150°C Azik et al.

84 observed progressive increase in iron (II) sulphate bands with

no increase in aliphatic CH2 CH3 bands for the first 24 h after which increase in these bands were observed, suggesting that formation of iron (II) sulphate by pyrite oxidation was the preferred reaction in initial stages of coal oxidation. Rhoads et al.

322 have attributed loss of Gieselar fluidity to decrease in aliphatic groups. They observed rapid reduction of aliphatic C-H (2995-2750 cm-1) on exposure of coal to air at 50°C after which it slows down. This reduction is substantially greater at 140°C, thus indicating this process to be temperature dependent. Iglesias et al.

13 have opined that -CH2 groups to be more reactive towards oxidation than –CH3 groups based on observation that band at 1454 cm-1 disappears much faster than the band at 1375 cm-1, during oxidation at 200°C. Contrary to the opinion of most authors315,317,331,332 that aromatic moieties in coal are not affected by low temperature oxidation, Anderson and Johns329 using FTIR spectroscopy have found substantial degradation of aromatic moieties. It is now generally accepted that low temperature oxidation of coal initially leads to the formation of intermediate peroxide and hydroperoxide bodies which decompose to form various oxygenated groups31,189,319,322,323,325,326,328,329,343 namely hydroxyl, carboxyl, carbonyl, and ether/ester linkages. The most significant changes due to aerial oxidation of coal are observed in the range of 1900-1400 cm-1 which involves formation of various oxygenated groups. These changes at various frequencies documented in literature include 1771 cm-1 for aryl esters84-86,318,331,332 and for anhydrides15, 1734 cm-1 for different esters332, 1732 cm-1 for alkyl esters322, 1684 cm-1 for aryl aryl ketones332, 1655 cm-1 again for aryl aryl ketones84-86, 1540 cm-1 for carboxylate anions81,84-86, around 1640 cm-1 for highly conjugated C=O possibly quinone15,81 and around 1655 cm-1 for similar spectral interpretation332, 1833 cm-1 for anhydrides 13,15,84-86,317,318,327, 1711 cm-1 for carboxylic groups81,332, 1690 cm-1 again for carboxylic groups86,322 and 1785 cm-1 for aromatic esters85 and for anhydrides318. The reduction of band in oxidised coal at 1680 cm-1 observed in fresh coal323 assigned to aromatic aldehyde is presumably due to its oxidative conversion to carboxylic groups. Applying FTIR spectroscopy on thin section of coal Gethner325,326,334 postulated that oxidation of coal below 100°C involved at least three simultaneous interdependent reactions involving (i) addition of oxygen to form carbonyl species via hydroperoxides (ii)


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decomposition of hydroperoxides to carbon oxides and water at 100°C and (iii) direct decomposition of carboxyl and carbonyl groups and resultant formation of ether linkages. Aerial oxidation has been found to substantially increase the broad envelope centered at 1200 cm-1. Unfortunately this portion of the IR spectrum of both fresh as well as oxidised coals unlike some other discussed above bands does not have a well-defined identity. Although assigned to ethers by Solomon and co-workers347-349 after careful deconvolution and also by Dereppe et al.

89, assignment of various contributing bands remain complicated147 because this region essentially involves the C-O stretch of ethers and phenols81 (C-C stretch and O-H bending) due to highly mixed and coupled vibrations147. According to Dereppe et al.

89, the increase in the FTIR bands centered around 1200 cm-1 is mainly due to of C-O-C stretching vibrations from both aliphatic, aromatic ethers with very small contribution of carboxylic acids. Seki et al.

195 attributed the increased absorptions at around 1200 cm-1 to anhydride formation. Both Liotta et al.

31 and Gethner325,326 observed increase in broad band around 1100-1090 cm-1 and 1040- 1020 cm-1. Marinov312 found the band at 1040 cm-1 as the dominant band in coals oxidised at 170°C. Although such increases were attributed to oxygenated groups no specific assignments were made. Furthermore, clay minerals like kaolinite and illite have well defined bands near 1040 cm-1 322, which further complicate matters. Bandopadhyay and Sen351 observed that the spectral changes observed in the difference spectra on in situ heating of coal in high temperature DRIFT accessory at 110°C disappeared i.e. the spectra reverted to its original state on cooling in ambient for at least 5 h signifying that these changes were due to removal of moisture rather than oxidation and that no significant chemical change takes place during moisture determination in proximate analysis. In a study of coal weathering using FTIR and NMR techniques, Liotta et al.

31 observed no increase in carbonyl species in any form and that all the oxygen incorporated was in the form of ether linkages, which they opined, were responsible for increase in crosslink density resulting in loss of plastic properties. This observation has been contradicted by Painter et al.315 who have ruled out the formation of ether linkages since like Albers et

al.310 they could not detect any significant increase in

hydroxyl group which made chances of ether

formation via condensation of hydroxyl groups remotely possible. Painter et al.

315 explained loss of plastic properties on the basis of formation of ester cross links via condensation of carboxylic groups formed and already existing hydroxyl groups. Fredericks and Moxon328 found a good correlation between integrated area (after normalizing for ash and moisture) of 3100-2800 cm-1 and crucible swelling index. Sanches et al.

343 observed that FTIR spectra of an oxidised caking coal and that of a non caking coal is similar indicating that loss of caking properties due to oxidation is related to introduction of oxygenated groups. Tognotti et al.

338 studied the FTIR spectra of coal, solvent extract of coal and residue for both fresh and oxidised coal and observed that the pyridine extract FTIR spectra presented a better separation of band contributions involved in low temperature oxidation, particularly in the region 1800-1500 cm-1. Landias and Rochdi352 using micro FTIR on thin sections of coal oxidised at 140°C for 1 to 384 h observed increases in carbonyl, carboxyl and ether content. They further observed that different macerals behave differently when subjected to increasing oxidation. Xiao et al.

308 correlated the loss of hydrophobicity of coal on oxidation, as determined using Hallimond tube, to loss of alkyl groups as determined by DRIFT spectroscopy. Changes in the mineral matter content due to low temperature aerial oxidation306,340 include appearance of new bands at 1153, 1087, 675 and 594 cm-1 attributed to the formation of bassanite which has been proposed as an indicator of in seam weathering. Some authors189,346 have detected presence of iron (II) sulphate szomolnokite as the principal product of pyrite oxidation by bands at 1090 and 980 cm-1. Azik et al.

85 observed that removal of mineral matter in a lignite facilitated the oxidation compared to the original. Huggins et al.

189 observed that the ratio CO/(Ar+Al) [ratio of absorbance between 1850-1636 cm-1 (carbonyl C=O stretching region) to absorbance between 3194-2746 cm-1 (aromatic plus aliphatic C-H stretch region)] in DRIFT spectra increases with oxidation. Similar increase with oxidation is observed in CO/(Ar+Al) [ratio of absorbance between 1300-1100 cm-1 (carbonyl C-O- stretching region to absorbance between 3194-2746 cm-1 (Aaomatic plus aliphatic C-H stretch region)]. They also observed that increase in these ratios is nearly 5 times higher for coal oxidised at 50°C compared to ambient weathering and also that


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carbonyl bands only appear clearly on substantial oxidation which generally happens above 80°C. Wu et al.

197,198,330, also reported similar observations. One group204 have used decreases in the index (H3040/H2920) (Ratio of maximum absorbance (H) of bands centered around 3040 and 2950 cm-1 attributed to aliphatic and aromatic C-H stretching) while others341,342,344 have used the index (A1445/A1375) (ratio of integral areas of bands centered around 1445 and 1375 cm-1 attributed to methylene and methyl groups) as approximate indicators of the changes introduced by natural weathering. Bowman and Fericks316 during study of low temperature oxidation of coal in a coal pile suggested that intensity of 1700 cm-1 expressed as I1700/I1580 can be used as qualitative measure of carbonyl content change due to oxidation. Ibarra et al.

340 used the Hal to Car ratio (A2970-2840/A1620) for depletion of aliphatic moieties during weathering. Stankevich et al.

321 evaluated degree of oxidation (K0) from IR spectroscopic characteristics [K0 = D1690/(D1260+D3040)] where D values signify absorbance values at the respective frequencies. Assuming air oxidation of coal as first order reaction Albers et al.

310 calculated rate constants and apparent activation energies. TeVrucht and Griffiths307 obtained activation energy and rate constants for the air oxidation of a suit of coal samples of three particle sizes at 150, 200 and 250°C by tracking the decrease in the intensity of the aliphatic C-H band centered near 2920 cm-1 in the DRIFT spectra. The air oxidation was found to obey pseudo 1st order reaction kinetics when oxygen is available in unlimited supply. Rate of coal oxidation was found to depend on particle size, rank and temperature but was independent of geographical region and mineral matter content. Activation energies were calculated between 25.6 and 26.6 kcal/mole. Although FTIR equipments are presently quite common in various scientific laboratories in India, FTIR studies on coal particularly on aerial oxidation of coal are few. Considering the serious problems of spontaneous combustion and weathering being faced by the Indian coal industry considerable work needs to be done in this area. High temperature in situ DRIFT technique is particularly useful in detailed monitoring of the chemical/structural changes in coal due to aerial oxidation.

Photo acoustic infrared spectroscopy (PA-FTIR) Photo acoustic infrared spectroscopy is special technique in FTIR spectroscopy, which has certain

advantages over conventional FTIR, which include nondestructive sample technique requiring minimum sample preparation, and it being purely a surface technique. However, in spite of these advantages, PA-FTIR studies on coal particularly on coal oxidation have been few during last seventeen years. Rockley and Devlin360 first used PA-FTIR for generation of various carbonyl absorptions such as ketones carboxylic acids and anhydrides in a study of low temperature laboratory oxidation of coal. Later Chien et al.

358,359 and Zerlia356 obtained similar results. Lynch and coworkers354,355,357 were first to provide spectroscopic evidence for detection of peroxide species generated due to aerial exposure of coal using PA-FTIR after chemical treatment. Cimadevilla et al.

10,12 used PA-FTIR to record the structural changes in coals oxidised at 140°C. A semi quantitative assessment of the structural changes was done by evolving a series of parameters such as Hal/Car, CH3/CH2, CO/Car, COO-/Car defined as ratios of integrated absorbencies of selected regions in the PA-FTIR spectra. Various structural changes taking place during the oxidation process were evaluated by plotting change in the above parameters against oxidation time and loss of Gieselar MF(%). They concluded that considerable reduction of aliphatic hydrogen content along with shortening of the alkyl chains with weathering/oxidation time are the principal causes for concomitant reduction of plastic properties. Correlations between PA-FTIR data and coal properties such as initial oxygen content and log of Gieseler fluidity were studied by Lynch and co workers354,355. In most cases equations were developed based on linear correlations obtained. Gentzis et al.

283 observed that the aromaticity of all coals examined by them by PA-FTIR spectroscopy, did not change significantly with oxidation temperature up to 200°C. No record of any published information exists on PAS studies on oxidation of Indian coals. Considering the high cost and paucity of such equipments in India, scope of such studies continues to be bleak.

Thermo gravimetric analyser- fourier transform

infrared spectroscopy (TGA-FTIR) The TG-FTIR technique was developed by Solomon et al.

361, who applied it to the analysis of coals. MacPhee et al.

362 using TG-FTIR obtained total organic oxygen of fresh and oxidised coal and after studying the evolution profiles of CO2, CO and water concluded that the low temperature oxidation in the


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early stages involves the formation of organic peroxides which decompose at low temperature. They further concluded that most of the oxygen incorporated into the coal structure during oxidation produced water as a result of pyrolysis and that TG-FTIR could be used to detect and follow the progress of coal oxidation. FT-IR Photothermal beam deflection spectroscopy

(PBDS) Low and coworkers363 compared various characterisation techniques for studying low temperature oxidation of coal. They opined that though FTIR was less sensitive to empirical tests like Geisler plastometer particularly in the early stages, it permits the detailed study of the chemical changes in coal due to oxidation, which was not possible with techniques like Giesler plastometer or FSI. They364,365

used PBDS due to its excellent performance in case of strong infrared absorbers e.g. high temperature chars and carbons. They finally concluded that for studies of coal oxidation, though PBDS is more useful technique than conventional transmission–absorption method, it was not more advantageous than techniques like DRIFT or PAS. Major changes detected363 as a result of coal oxidation are generation of anhydrides particularly conjugated anhydrides and phenolic type esters (or even lactones). Ethers were not detected. Infrared thermography

Fierro et al.366 used infrared thermography to locate

hot spots in coal piles subjected to atmospheric oxidation and measure the temperature of the slopes of the coal pile and using various equations evaluated total heat flux and daily calorific value losses which was further used to evaluate yearly calorific losses due to weathering. This technique uses a watching thermal camera and measuring thermal camera and comparison of the temperatures measured by both cameras are used to detect hot spots in coal piles. Nuclear magnetic resonance spectroscopy (NMR) Solid state 13C NMR was used for the first time as a probe for the characterization of low temperature oxidation of coal by MacPhee and Nandi367 who reported decrease in aromaticity for coals subjected to artificial oxidation at 105°C implying that aromatic structures get oxidised in preference to aliphatic structure. This was in direct contradiction of

preceding FTIR based work315,317 which reported that during low temperature oxidation predominantly aliphatic part is attacked which effects mainly hydrocarbon bonds leading to the decrease in the aliphatic content which results in the proportionate increase in aromaticity as the aromatic structures are not affected. In the literature followed immediately32,324 and later15,84,85,89,370,371 authors using 13C NMR with Magic Angle Spinning (13C MAS NMR) and 13C MAS NMR with cross polarisation (13C CP/MAS NMR) and in some cases combined with dipolar dephasing techniques (DPD), contradicted McPhee and Nandi while supporting the earlier FTIR findings. Kalemma and Gavalas20 using FTIR have opined that oxidation of aromatic carbons can take place at temperatures as low as 175°C. In spite of contradicting MacPhee and Nandi367, Fredricks et al.

23 have suggested against dismissing their findings out right and have pointed out certain circumstances where such phenomenon may be possible. Thus, there is some degree of varied opinion regarding the final products of low temperature oxidation of coal as investigated by NMR. To some extent this is due to variation in the experimental conditions used by various authors. Fredericks et al.

23 used 13C CP/MAS NMR followed by dipolar dephasing experiments and concluded that ketones, carboxylic acids were formed in the solid oxidation residues during natural in situ oxidation of high volatile bituminous coals and during oxidation there is very little change in the degree of substitution of aromatic rings. Contrary to their findings obtained from FTIR studies on oxidised coals, Havens et al.

324

did not detect any carboxyl or carbonyl groups by 13C CP/MAS NMR. Yoshida and Maekawa370 while studying low temperature oxidation at 200°C using 13C CP/MAS NMR opined aromatic carbon was stable and that carbon loss was due to loss of aliphatic carbon especially CH2 resulting in significant increase in polar oxygenated such as carboxylic phenolic and carbonyl groups. Dereppe et al.

89 using similar techniques as Havens et al./Fredericks et al. reported, contrary to their findings, predominantly the formation of phenolic esters and ethers/anhydrides while formation of carboxylic acids and phenols were relatively subordinate. Liotta et al.

13 subjected fresh Illinois 6 coal to different time periods of oxidation in ambient to deutro O methylation and analysed them with 13C NMR. They concluded that in case of laboratory weathered coal the functional group


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formed is not carboxyl but ether group which lead to cross linking. Pisupati and Scaroni15 studied in situ and laboratory oxidation for a set of US coals. Using DRIFT and SS NMR, they observed that while phenolic esters, ketones and aliphatic esters were predominant in case of coal oxidised at 200°C, carboxylic acids were predominant for crop coal. Azik et al.

84-86 in their studies on structural changes during oxidation of lignite at 150°C observed formation of mainly carboxylic acids and esters. The ester band grows with progressive oxidation at the cost of phenolic band, which after a point becomes insignificant. Dunlop and Jons372 observed minimal change in the carbonyl region of the 13C NMR spectra but showed substantial increase when dipolar dephasing pulse sequence is applied. They also observed in both CP and DPD spectra significant formation of lactonic and anhydridic groups. Yokono et al.

368,369 correlated changes in spin-spin relaxation times (T1) and spin-lattice relaxation times (T2)m, (T2)r obtained from 1H NMR proton relaxation techniques to the progressive changes with oxidation in coking properties and further correlated it with ESR data and opined that coking and caking coals exhibit different effects of oxidation with perhaps phenols and quinones in caking coals acting as inhibitors to the growth of stable free radicals. X Ray photoelectron spectroscopy (XPS) X-ray Photo Electron Spectroscopy (XPS) is a surface technique capable of determining the elemental composition of a surface and at the same time, distinguish between different chemical environments (valence states) of these elements. It is generally observed that thermoplastic properties of coals as determined by Gieseler plastometry and dilatometry disappear during early part of oxidation although, during the same time period, little or no changes are observed with techniques like DRIFT and Mossbauer spectroscopy. Considering that weathering is a surface phenomenon and assuming that it will prove to be a sensitive technique to study the early stages of coal oxidation, several authors applied XPS for studying aerial oxidation of coal mainly to study incorporation of oxygen at coal surface but also to follow changes in the oxygen functional groups generated on the surface of coal as a result of aerial oxidation. Considering that the oxygen determination by difference method is prone to errors, Frost et al.

373 attempted to directly measure oxygen content of coal

using XPS. However contrary to their expectations, although carbon and sulphur determined by XPS were found to reasonably agree with the conventional ultimate analysis results, no such agreement was observed in case of oxygen. According to Perry and Grint374 coal oxidation occurs initially by way of the exterior surface of coal particles, which can be detected by XPS at a very early stage when oxygen incorporation is too low to be detected by conventional methods. They observed that surface oxygen as measured by XPS in coal heated at 100°C in flowing air increased by 6.5% as compared to bulk oxygen, which increases concomitantly by only 1.5%. They concluded that aerial oxidation occurs via the external surface producing a range of carbon oxygen functional groups. Single bonded C-O groups such as phenols, alcohols and ethers dominate over carbonyl and carboxyl. They also concluded that stable carboxyl groups are generated in high concentration only above 250°C. Clark and Wilson375 observed the surface oxygen concentration measured for weathered coals using XPS were higher than the bulk values for the samples, which they attributed to weathering. They opined that ESCA (XPS) can be used not only to observe the extent of oxygen uptake at the coal surface, but also to know the details of chemical bonding on the oxidised surface. On the contrary, Huffman et al.

189 opined that as compared to DRIFT, ESCA (XPS) was much less sensitive as they reported no detection of surface oxygen groups on oxidation of coal in air at 50°C for 353 days. Wu et al.

197,198,330 used XPS to monitor the structural changes in the coal surface due to weathering and correlated it with floatation and thermoplastic properties. Using the C1S peak, they could detect surface oxygenated groups at higher temperature (80°C, 313 days) but failed to detect the same at 25°C. In an oxidation study on two U.S. coals at 295 K and 398 K Kelemen and Freund376,377 found that XPS could observe no change in surface oxygen content even after oxidation at both 295 K for 170 days and at 398 K for several weeks. They however, observed significant surface enrichment of organic sulphur and suggested the use of this change as a rough estimate for extent of weathering. Kelemen and Kwiatek378 observed that oxidation of a U.S. coal at 125°C for various time periods resulted in progressive increase in carboxylic groups with concomitant decrease in hydroxyl content in case of sub-bituminous and low rank coals. From the XPS data, which was corroborated by SIMS data,


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Gong et al.379 opined that hydroxyl groups are

products of early stages of oxidation of coal. Carbonyl groups which are formed at intermediate stage and in final stages carboxyl groups are the predominant groups. Although XPS is a surface technique, contrary to expectations it has not proved to be very useful. Considering the steep cost of XPS equipments much cheaper equipments e.g. FTIR have proved to be much more useful. Secondary ion mass spectrometry (SIMS) Martin et al.

380,381 used SIMS to follow changes in oxygen functionalities in coal as a result of air oxidation and studied oxygen adsorption on coal by measuring the concentration of 18O at coal surfaces being subjected to oxidation in an atmosphere of 18O2. They also measured the activation energy of the low temperature oxidation of coal. According to Gong et al.379 the data provided by static SIMS, corroborates the XPS data. However, compared to XPS, SIMS data is more surface sensitive as only the outermost atomic layers of the surface under study is scanned. Tomodensitometry (Non destructive X scanner

technique) X-scanner technique, a tomodesitometric technique, has been used by Kister et al.

382 for studying coal structure and reactivity of coal which included reactivity towards oxidation. According to them X-scanner is a powerful non-destructive technique, which can reveal the molecular distribution and changes in molecular distribution in materials. Kister and co-workers studied inter alia aerial oxidation of a sub-bituminous coal at 200°C. Using this technique they explored the possibility of visualising the preferential path of oxidation and fault formation due to loss of water. Pyroysis mass pectrometry (PyMS) Survey of literature indicates that though FTIR is the most extensively used technique for study of structural changes due to oxidative weathering, pyrolysis mass spectrometry, particularly Pyrolysis- low voltage mass spectrometry (py-LVMS) has also been widely used to study such changes. Among the pyrolysing techniques Curie point pyrolysis techniques has been the most widely used. Furimsky et al.

383 while studying the effects of oxidation on the chemical nature and distribution of

low temperature pyrolysis product from bituminous coal observed significant changes in both yield and composition of volatile pyrolyzate. Applying various techniques like time resolved techniques and multivariate analysis generating discriminant spectra, major structural changes were observed by various authors32,84-86,383-387 including decrease in dihydroxy benzenes, alkyl substituted dihydroxy benzenes,alkyl substituted phenol, alkyl napthalenes, alkylbenzenes, tetralins; the last three indicate decrease in alkyl groups due to aerial oxidation. Concomitant increase observed included CO, CO2 and carboxylic (short chain fatty acids) and carbonylic groups as well as anhydrides of aromatic acids32. Discriminant spectra on weathering385 in case of lignite show strongly reduced dihydroxybenzene and moderately reduced phenol series, the sub-bituminous coals show equal reduction of both. In contrast the HV bituminous coal show a pronounced loss of alkylnapthalenes as well as a moderate loss of phenols and other hydroxyl and/or hydroxyl aromatic compounds which indicate these changes to be rank dependant. Similar, if not exactly same, observations have been made by Jakab et al.

388. Loss (sometimes four to five fold) of thermally extractable components such as alkyl benzenes, alkyl napthalenes and tetralin384 and hydroxyl aromatic series primarily alkylphenols387,388 in coals after weathering is attributed to their being part of the mobile phase (guest molecules) the reduction of which is attributed to linking of these with network phase (host) through grafting reactions and, are therefore unable to evaporate at lower temperature and therefore less available in the pyrozilate of the weathered coals. In a detailed study, using inter alia Py-MS, involving oxidation at 150ºC of raw, demineralised, and pyridine extracted Turkish Beypazari lignites, Azik et al.

84-86 observed that although overall effect of oxidation in raw and demineralised lignite were similar the intensities of some network components were greater in case of demineralised lignite while changes in case of pyridine extracted lignite were much less. Landias et al.

389 in a study on the effects of preheating, oxidation and oxidation after preheating of coal using Rock Eval pyrolysis and GC Py-MS indicated that oxidation induces loss of oil potential and increases in thermostability and that preheated coals are more susceptible to oxidation compare to raw coals. In a study involving distribution of pyrolysis products of coal oxidised by isotope labeled


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oxygen18O2 Ignasiak et al.390 opined that oxidation of

alkyl groups to carboxyl group is dominated by oxidation of oxygenated groups especially carbonyl groups to carboxyl groups. According to Jakab et al.

387,388, Py-MS is a sensitive technique for detecting weathering of coal and could even detect changes due to oxidation at 100°C for as less as one day, where changes in FSI are not even observed. Electon spin resonance spectrometry/Electron

paramagnetic resonance spectrometry (ESR/ EPR) Coal contains indigenous stable free radicals trapped in the solid matrix. Their concentration depends upon coal rank, maceral type and mineral content. Following the first ESR studies on coal391,392, various groups have made attempts to find a relationship between various ESR parameters and inter alia, aerial oxidation of coal. According to Dack et al.

397,398 two types of free radicals are present, a narrow width ESR signal (Lande g value 2.0027) characteristic of polycyclic aromatic hydroarbon free radical system with substantial delocalization and broad width ESR signal (Lande g value between 2.0032 and 2.0035) which is attributed to heteroatom free radical structures396. The narrow band disappears when coal is exposed to oxygen, but could be regenerated by evacuation, a reversible phenol- menon attributed to formation of radical oxygen complex393-395,396 which has been suggested by Marinov314 to be a preliminary step in the coal oxidation process. He further observed that during aerial oxidation while for higher rank coal oxidative dehydrogenation was accompanied by increase in free radical concentration, for lower rank coals it is just the reverse signifying that different mechanisms operated for different rank of coal. Similar observation was made by Yokono et al.

369 who measured from 1H NMR spin lattice relaxation times (T1) and spin-spin relaxation times (T2) as a function of degree of oxidation and correlated these with corresponding ESR data. Marinov314 on the basis of free radical data proposed, through acid base mechanism, the conversion of the adsorbed molecular oxygen to oxygen superoxide ion. Dack et al.

398,399 also opined that the changes in the broad signal during oxidation of coal was due to interaction between oxygen and free radical sites in coal. They also observed that coal/lignite on drying leads to, after immediate reduction, an increase in radical concentration which is considered as one of the

important factors for increased propensity of coal towards oxidative attack by molecular oxygen which was followed by a decrease again. Carr et al.

404 opined that this increase in radical concentration is due to formation and breakdown of hydroperoxy species and the decrease is due to their conversion of non radical products. Dack et al.

398,399 also opined that when equilibrium moisture was removed peroxide content decreased and spin concentration increased and this happened on the hetero atoms. Seki et al.

195 also studied the variation of various ESR parameters like relative spin concentration, line width (G), g-factor and microwave power at which signal saturates (Pmax). They observed that Pmax is related to 1H NMR relaxation times by the relation 1/(Pmax)∝ T1T2. On the basis of ESR and NMR measurements they suggested not only the formation of cross-linking by oxidation but also the cleavage of cross-linkages after prolonged oxidation. The group of Kudynska and Buckmaster400-403 studied the low-temperature oxidation of Alberta coals of various rank under various conditions whilst subjected to dry air flow in the temperature interval 20-250°C and observed changes in the character and spin concentrations of the free radical species present using new experimental technique called dynamic in situ 9 GHz continuous wave-electron paramagnetic resonance (c.w.-e.p.r.) spectroscopy. The e.p.r. line-shape changes were analysed using gradial difference a0d0-Argand diagrams. They observed400 that for sub-bituminous and high volatile (hv) bituminous coals free radical generation as a function of temperature was different and for the later it was determined by the partial pressure of oxygen over the coal. They also observed401 that compared to the raw hv bituminous coal its demineralised sample produced lesser number of free radicals when heated implying that presence of mineral matter played an important role in promoting oxidative self heating. They also observed402,403 that for the hv bituminous coal with enriched exinite content the c.w.-e.p.r. spectral parameters and the relative spin concentration exhibited maxima around 100°C and this was more pronounced in the moisture saturated sample. In a later publication405 they explored the potential of using the Arrhenius diagrammatic analysis of dynamic in situ 9 GHz c.w.- e.p.r, data taken in constant time-temperature intervals to calculate 'activation' energies characterizing stable free radical reactions that contribute to the various stages of the


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low-temperature oxidation process. In a study involving a lignite and a bituminous coal Cole et al.

406 used ESR to study coal oxidation and suggested that the free radical reactions are initiated or influenced by Fe(II) or Fe(III) sulphates. In an ESR study on oxidation of high volatile (hv) bituminous coals at 150°C up to 48 h Khan et al.407 observed that the pyridine extract of the coal was more sensitive to oxidation compared to the residue. Thermal analysis methods

Thermo gravimetric analysis (TGA), TGA-gas

chromatography and TGA- mass spectrometry (TGA-MS)

Mass changes due to coal oxygen interaction take place during aerial oxidation of coal and have been studied as a function of temperature by various authors using thermogravimetric analysis (TGA). Oreshko408 applying TGA under slow heating rate observed four distinct stages (below 70°C, between 70 and 150°C, between 150 and 230°C and above 230°C respectively). Jakab et al.

409 also observed several such distinct stages and attempted to interpret the chemical mechanism of these mass changes. They opined that lower rank coals having wider range of devolatilisation temperature are more susceptible to oxidation. Marinov313 observed using TGA that while heating coal sample from room temperature to 300°C at a constant specified heating rate it gains weigh between 80-100°C which indicated formation of solid complexes on the coal surface in spite of gaseous emission. In the opinion of several authors410-413 TGA-Mass spectrometry is a very sensitive technique in detecting subtle changes in coal structure as a result of low temperature oxidation. Apart from study of mass changes it was opined on the basis of increasing yield of acetic acid in the evolution profile that carboxylic groups are primarily formed by attack on aliphatic hydrocarbon groups. While studying combustion characteristics of weathered coal Pisupati et al.

14,16

observed that crop (weathered) coals showed higher char reactivity as determined by TGA. Izuhara et al.

414

used the change of maximum weight loss rate in TG as a wethering parameter. Worasuwannarak et al.

415 while studying effect of oxidation of coal at various temperatures on its weight decreasing behaviour and gas formation patterns on carbonization by TGA MS observed that during carbonization firstly weight of preoxidised coal decreased more gradually than raw coal and secondly formation rates of CO and CO2

increased while those of CH4 and tar decreased with

increasing degree of preoxidation which is attributed to formation of carbonyl groups and decrease in aliphatic groups during preopxidation. While studying devolatilization behaviour of naturally weathered and laboratory oxidised bituminous coals, Pisupati et al.

14 observed that compared to fresh coals, the volatile release rates as measured by TGA were slower and the volatiles released over a much wider range in case of oxidised coals. Pisupati and Scaroni416 in a study on influence of both natural weathering and low temperature laboratory oxidation on combustion behaviour using thermogrvimetric analysis and Drop tube furnace observed that the char reactivities of both weathered and laboratory oxidised coals were higher than corresponding fresh coals. They also observed that the char reactivity of acid washed coals in most but not all cases were lower than the unwashed counterparts indicating the positive catalytic activity of certain inorganic species in the coals. They further observed from burning profile curves that combustion for weathered coal is expected to complete at lower temperatures than their fresh counterparts under identical experimental conditions.

Differential scanning calorimetry (DSC)

Using DSC Mackinnon and co-workers417,418 studied role of glass transitions in low temperature air oxidation. In their opinion below glass transition temperatures coals are in a glassy state in which diffusion and acceptability to oxygen is restricted. However, above glass transition temperature coals reach a rubbery state and coal structure is much more relaxed which dramatically enhances oxygen diffusion giving rise to spontaneous combustion. Garcia et al.

419 observed that compared to total oxidation enthalpies the onset temperature of oxidation increased more systematically with increasing oxidation and proposed it to be a better parameter to monitor propensity of coals to oxidation.

Differential thermal analysis (DTA)

Marinov314 has used differential thermal analysis for studying the air oxidation reaction mechanism of coal while Banerjee and Chakravorty420 used this analysis for studying spontaneous heating susceptibility of coal. Banerjee and Chakravorty420 recognised three distinct transition stages in the DTA thermograms carried up to 300°C. The first being endothermic and is associated with water loss which is followed by exothermic and finally very exothermic


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stage. In a study of the self-heating of fresh and oxidised coals using differential thermal analysis, Pis et al.

421 observed that coal oxidation gives rise to an important modification both in self-heating and end of combustion temperatures and pointed out a relationship between total heat flow and ASTM calorific value. Clemens et al.

422 using isothermal DTA observed that the major feature of the DTA traces is an immediate sharp exotherm when oxygen contacts the dry coal. They155 detected this isotherm even at temperatures as low as 30°C. They further observed that the coal which is least prone to spontaneous combustion has significantly smaller DTA response than other coals.

X-ray diffraction (XRD) Pearson and Kwong423 observed that the amount of bassanite (CaSO4.1/2H20) determined in the low temperature ash (LTA) varied inversely with the free swelling index of the corresponding oxidised coal. Painter et al.

315 in a FTIR study correlated the above findings with FTIR data related to maceral oxidation as explained in infrared spectroscopy section above. Herman et al.

424 using a combination of XRD and SEM-EDX suggested similar opinion. Cole et al.

425

using XRD in conjunction with mossbauer spectroscopy examined the transformation of iron minerals during coal oxidation and its catalytic action on the low temperature oxidation of coal and suggested two possible reaction mechanisms involving the catalytic action of iron minerals and their oxidation products. The first suggested mechanism involves the reaction of Fe(III)-Fe(II) system with the peroxides in the coal analogous to oxidation reaction of organic substances with Fentons reagent. The second possible mechanism is based on the catalytic effect of ferrous sulpahte on graphite oxidation426. Garcia et al.

427 using XRD in conjunction with FTIR observed that chlorite, anatase calcite dolomite and ankerite present in the unoxidised (underground) mined coal were not observed in the XRD traces of its weathered counterpart (surface mined coal) and similar trends were observed with FTIR. The absence of carbonate minerals in the weathered coal is attributed to their dissolution in acid (H2SO4) waters generated due to pyrite oxidation. However contrary to this observation, during study on forced coal oxidation at 200°C Iglesias et al.

13 using XRD in conjunction with FTIR analysis of low temperature ash observed decrease in siderite and

increase in calcite content and opined that the thermal oxidation follows a reaction pathway different from weathering in so far as alteration of carbonate minerals is concerned. They did not observe significant change in mineral matter content due to oxidation and opined that changes due to oxidation are mainly due to changes in the organic matrix.

X-Ray absorption fine structure (XAFS) spectroscopy

Huggins et al.428,429 observed that under extreme

weathering conditions the abundance of calcium, which they measured using XAFS spectroscopy, increases in the oxidised part when compared with Ca content in unoxidised part.

Energy dispersive X-ray microanalysis (EDXMA)

MacPhee et al.430 used EDXMA on polished

surfaces of coal macerals to measure surface oxidation rates of a Canadian bituminous coal. They measured the O/C peak ratio changes due to oxidation by heating coal at 75, 90 and 105°C up to 120 h. They concluded that vitrinite and semifusinite have comparable reactivity and also that the oxidation kinetics above and below 90°C are governed by different reaction mechanisms.

Mossbauer spectroscopy In a study on the alteration of mineral matter in coal Huggins et al.

431,432,428 observed that pyrite was the most readily altered mineral during low temperature oxidation of coal and eventually results in the formation of goethite (α-FeOOH) and also in certain cases lepidocrocrite (γ-FeOOH) after initially giving rise to variety of iron sulphates which include szmolokonite in pyrite rich and jarosite [KFe3(SO4)2(OH)6] in pyrite poor coals. These sulphates were more abundant in final oxidation product of laboratory-oxidised coal where goethite was not observed in contrast to natural oxidation, which leads to goethite with sulphates being a minor phase. This indicates that oxidative alteration of pyrite due to natural weathering is difficult to replicate under laboratory conditions. They431 however did not encounter the presence of α-Fe2O3. Lin et al.

303 observed that oxidative transformation of pyrite in coal is rather a complex procedure and the differences in the products obtained were due to differences in sulphur contents and mineralogies. According to Huggins et al.

189,191 mossbauer detection of goethite is more sensitive than FSI in detecting weathering in


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coal with the added advantage that the corresponding unoxidised coal is not required. In a study of oxidised coals, Huggins et al.

428 observed a much better resolved spectrum of goethite in oxidised coal when recorded at cryogenic temperatures. Gracia et al.

434

using Mossbauer spectroscopy observed that in spite of different laboratory conditions applied to two Spanish coals jarosite was the only product of pyrite oxidation. They436 also observed that only a small fraction of Fe2+ from pyrite was oxidised to Fe3+ under laboratory conditions, even after heating in air to 468 K and concluded that artificial oxidation experiments hardly reproduce the conditions met by coal when submitted to natural weathering. In a study on coal oxidation in temperature range 413-673 K, Cole et al.

425 observed that pyrite was stable to oxidation up to 523 K after which ferrous sulphate monohydrate is produced which on further oxidation with increasing temperature proceeded through ferric hydroxyl sulphate and ferric oxysulphate to the final oxidation product α-Fe2O3 at 673 K. According to Shyu et al.

433 oxidation of pyrite in Illinois 6 coals occurs in three steps, pyrite to iron sulphate between 25 to 31°C being the first. They did not observe any goethite. Kolker et al.

437 studied the progressive oxidation of pyrite in coal using Mossbauer spectroscopy and observed that samples stored in wet air showed the maximum degree of weathering as demonstrated by maximum presence of jarosite forms compared to little difference observed in coals store in dry conditions when compared with unoxidised coal. Garcia et al.

427 using Mossbauer spectroscopy on an underground mined (unoxidised coal) and surface mined coal (oxidised coal) observed when compared to underground mined coal reduced in amount of pyrite with concominant increased appearance of jarosite and lepidocrocrite in the surface mined coal. They however did not report presence of goethite in the surface mined coal. Shrivastava et al.

435 observed the presence of α-Fe2O3 along with goethite and opined that formation of fine grained goethite can cause further cracking in coal allowing greater oxygen penetration resulting in more self heating. Huggins et al.

200 observed the presence of goethite in coals stored in argon atmosphere indicating that even coal stored under such safe environment is not totally immune to weathering.

Flotation test/studies Baranov et al.

438 and Sarikaya439 employed floatation test for investigating the initial stages of

oxidation on the basis of floatation recovery which decreased with degree of oxidation. Decrease in floatation recovery yields during floatation test were observed by Garcia et al.

427 which they attributed to increase in surface oxygenate functional groups e.g. phenolic and carboxylic due to weathering. Similar observation/conclusions have been made by some authors440-443 during their studies on effect of coal oxidation on floatation. Somasundaram et al.

440 using inter alia floatation test further observed that soluble inorganics generated as a result of pyrite oxidation absorb on the coal surface and modify its floatability. Fuerstenau et al.

441 also observed inter alia that even coal stored under inert condition show signs of oxidation, as indicated by the reduced flotation which he attributed to oxidation by the very small residual oxygen in the coal pores and in the purged storage container. Diao and Fuerstenau444 observed, using film floatation test, that with increasing oxidation, particularly above 100°C, coal becomes hydrophilic. While comparing Gieseler fluidity results and floatation tests, Wu et al.

197,334 opined that the decrease in floatation recovery on oxidation of a coking coal at 150°C was much slower (nearly half) than decrease in Gieseler fluidity values. Though much less sensitive than Gieseler plastometry, floatation recovery, if standardized, can be an extremely useful technique to assess degree of oxidation of oxidized coal, particularly coking coal, considering the simplicity of technique and relatively low cost.

Phase inversion test

Phase inversion test developed by Gray et al.262 is a rapid and simple test used to assess the floatability of coal prior to washing. In this test 2 tablespoons of -16 mesh coal followed by 5 drops of No. 2 fuel oil is added to 1 quart of water and blended in an electric blender for 2 min and poured into a graduated cylinder. If all the coal floats the coal floatation recovery is good and the coal will be good for beneficiation. If all the coal sinks the floatation recovery is judged poor and is unfit for beneficiation. Coal oxidation has an adverse effect on froth floatation recovery of coal and oxidised coal can be easily detected by the phase inversion test. Zeta potential and contact angle measurements Wen and Sun445 during their study on the electrokinetic behaviour of oxidised coal found that


127

increased degree of coal oxidation led to increase in the negative value of the zeta potential and decrease in isoelectric point and decrease in the contact angle. PZR (point of zeta potential reversal) of unoxidised coal is reduced due to oxidation and depending upon the degree of oxidation is reduced by 1 to 3 units. Correlation between zeta potential of oxidized (weathered) coal and its floatation response has been investigated by Yarar and Leja446, and Bolat et al.

447 using Zeta potential and floatation data concluded that floatability detoriated with oxidation but could be increased by using electrolytes at low concentration in the floatation media.

Wet oxidation methods

Reactions of coal with wet oxidising agents have been employed by some authors as an alternative method for assessment of the tendency of coal towards self-heating. Tarafdar and Guha448 used oxidation of coal by alkaline permanganate solution and measured (i) the differential temperature at different base temperatures at a constant heating rate and (ii) potential difference between saturated calomel electrode and carbon electrode immersed in coal - alkaline KMnO4 mixture after defenite reaction time and compared them with crossing point temperature (CPT). Maciejasz in Poland employed exothermicity of coal oxidation with H2O2 for comparing spontaneous combustion liability. Method of Guha and Tarafdar448 (carried out on few Indian coals) could prove to be a very useful technique considering its simplicity and low cost. However, extensive studies involving a large number of coals need to be done to standardize this method for Indian coals.

Miscellaneous techniques

Electrostatic charge device

Oxidised coal acquires electrostatic charge when impinges on a metal plate. A device using this principle, designed and patented (British Patent 73300, July 6, 1955) by British National Coal Board for determining degree of oxidation using this principle is attributed to Walton et al.

449. X-Ray fluorescence spectrometry

Garcia et al.427 observed using quantitative XRF of

ashes of both unoxidised coal and its weathered counterpart that although SiO2/Al2O3 were similar in both cases concentration of Fe, Mn, Mg and

especially Ca were reduced in weathered coal. The significant reduction in Ca in the weathered coal is attributed to dissolution and removal by leaching as discussed above in the XRD section. Drop tube furnace

Discard (oxidised) coal samples were tested by Wagner450,451 for their combustion efficiency in a drop tube furnace, and the combustion results and correlations to the degree of weathering resulting during stockpiling were reported. Pisupati and Scaroni416 during study of effect of both natural and low temperature laboratory oxidation using DTF and TGA correlated burning profile data observed inverse relationship between combustion determined by DTR and initial temperature ITbp in TGA burning profiles (which lowers on oxidation).

Computer-controlled scanning electron microscopy (CCSEM)

Huffman and his group189 used CCSEM to study alteration of minerals other than iron bearing minerals (accurately determined by Mossbauer spectroscopy) due to aerial oxidation. They observed that after oxidation for 200 days at 5°C and 65% humidity except for that about a third of the calcite was converted to gypsum, no other changes in non iron minerals were observed. However, no gypsum was observed in stockpiled coals after same length of time.

Temperature programmed oxidation

In a study of coal oxidation using used temperature programmed oxidation with mass spectral gas analysis, Clemens et al.

155 observed that evolution of gaseous products were insignificant at lower temperatures (no evolution detected up to 5 h oxidation at 60°C), both oxygen consumption and the evolution of product gases increased with temperature above 90°C. Gas flow test in assessing weathering propensity

Boyapati et al.452 used a gas-flow test to assess the

relative weathering propensities of these coals in the laboratory under accelerated weathering conditions. The principle of the test is that the pressure drop of a stream of nitrogen gas flowing through a coal sample during continuous heating is a function of the coal’s coking propensity. The test results were found to reflect the rate of deterioration in coke strength due to weathering in experimental stockpiles leading to the possibility of using the test to detect oxidised coal.


128

ESR Imaging

Liang et al.453 applied ESR imaging to study the

dynamic oxidation processes in solid coal and observed a homogenous radical concentration distribution across the sample.

Conclusions Although the process of coal oxidation is extremely complicated as discussed above and in spite of varied opinion in certain cases there has been significant advances in attempts to understand the phenomenon leading to certain aspects being now universally accepted. However, considerable grey areas continue to exist and the exact phenomenon of aerial oxidation of coal continues to be rather poorly understood. As a consequence, though large number of techniques have been used, some extensively e.g. FTIR and some in isolation involving only few studies, it is generally accepted that no single technique is able to, on its own, give complete information on this phenomenon. Considering that large number of grey areas exist, even a combination of techniques fail in this aspect. Certain differences of opinion do exist amongst various authors on the comparative merits of various techniques. However, as a general opinion some techniques are extremely useful. DRIFT spectroscopy is considered to be one of the extremely useful techniques due to its sensitivity combined with the high level of chemical information generated. Though Gieseler plastometry is the most sensitive technique it does not provide chemical information and is therefore useful for detection purpose. Mossbauer spectroscopy according to Huffman189 is the second most sensitive technique after Gieseler pastometry. Other useful techniques include pyrolysis mass spectrometry, ESR, slurry pH, thermal analysis and microscopic stain test. Certain tests are important to specific areas of caol utilization e.g. floatation tests for studying coal oxidation effects on coal preparation methods. New techniques involving the refinement of the existing, development of new instruments and accessories and experimental techniques tailor-made to study this phenomenon are being developed leading to quantum advancements in the generation of knowledge in this area. However, further work needs to be done in this area to address various impeading issues and to obtain a near complete understanding of this phenomenon. Particular methodologies have to be developed to obtain complete understanding of the parallel but competing reactions and the exact role of

water. Better and tailor-made techniques need to be developed to understand the role of surface properties like porosity and surface area. Acknowledgements The authors are thankful to Dr Gora Ghosh for extremely useful discussions and Director CIMFR for his kind permission to publish this paper.

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Aerial oxidation of coal-analytical methods, instrumental techniques and test methods: A survey

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