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.