Separative Refining of Coals through Solvolytic Extraction under Milder Conditions: A Review

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.