Coal still remains an important source of power generation world over. Along with its usage, comes unwanted generation of noxious gas emissions, toxic metal releases into wastewater and other pollutants which ultimately lead to environmental concerns. So cleaning of coal through physical or chemical processes becomes utmost important. There are several coals which cannot be cleaned by physical beneficiation techniques to produce low ash cleaner coals. Such coals can be cleaned only through chemical cleaning techniques. The present paper reviews the chemical demineralisation and desulphurisation of coals over the years using various inorganic and organic acids, alkalis, oxidants, leachants and various acids and alkali-acid combinations to reduce the ash and sulphur contents in coals. As high as 90% demineralisation and desulfurization could be achieved with the use of these cheap inorganic acids as compared to the expensive solvents used for solvent extraction processes, a parallel approach of cleaning and refining coals.

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Not only cleaning but refining of coal has become an area of utmost importance both industrially and academically because of stringent environmental norms being imposed globally for arresting the climate change. However, coal is a polluting fuel, therefore, there is a grave need of deep refining of coal to obtain ashless coal for increasing its efficiency of utilization in order to minimize its impact on environment. The presence of ash in coal also greatly reduces the coal utility because of its deleterious effect on the boiler parts and gasification equipment. Solvent extraction (organo-refining) techniques produce Super Clean Coal (SCC) or Ultra Clean Coal (UCC) with almost zero ash content. The utilization of the SCC in the Integrated Gasification Combined Cycle (IGCC) Power Generation would increase the availability of this otherwise efficient and cleaner coal technology. Research work on different processes of solvent extraction of coal under varying conditions of temperature, pressure, and the nature of the solvent used and other parameters, their effect on the extraction yield have been discussed presently. Extraction of coal under milder ambient pressure conditions is desirable as this affords the recovery of coal without its fractionation, structural dislocations, or changes. The effect of the use of single solvent, mixture of two solvents (of solvent and cosolvent), i.e., synergism in enhancing the extraction yield under milder atmospheric pressure conditions have been reviewed. Utilization of high boiling solvents and coal derived solvents along with successive sequential solvent extractions for obtaining the enhanced extraction yields mainly under ambient pressure conditions have been discussed in the present paper. Hypercoal production under high pressure conditions, its characteristics, and utilization have also been described. Use of ionic liquids as potential solvents for coal extraction (especially of lignites) has also been discussed. Physical and chemical pretreatments greatly enhance the coal solvent extraction yield. Different potential uses of SCC such as in production of carbon nanotubes as a super clean fuel dust for car engines, synthesis of chemicals, etc. have been discussed. Separate discussion on nanotechnology of coals as related to solvent extraction of coal has also been included presently.

Indian coals have difficult to formidable washability characteristics. Presently, five Indian coals were subjected to solvent extraction under ambient pressure conditions using Nmethylpyrrolidone (NMP) containing a small amount of Ethylenediammine (EDA) (termed as e,N solvent system). The coals and their solvent refined super clean coal (SCC) products obtained after solvent extraction have been extensively characterised for calorific value, ATR-FTIR spectral analysis, XRD measurements and ICP-MS analysis. The organo-refining of coals through the use of e,N extraction resulted in the production of Super clean coals (SCCs) having 0.2-1.5% ash. Studies have been carried out for the first time to analyse the concentration of minor elements in super clean coal. However, the ATR-FTIR analysis, XRD measurements and ICP-MS analysis also revealed the reduction of major elements such as Na, K, Al, Mg, Ca and Fe. The ATR-FTIR and XRD studies of SCC clearly demonstrated deashing of coals. As high as 94% improvement in calorific value in one of the high ash coals, Pandra coal could also be achieved on extraction with e,N solvent system.

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