Review on chemical upgrading of coal: Production processes, potential applications and recent developments

Coal is a major source of energy. In order to overcome the operational problems and environmental issues related to direct utilization of coal, coal upgrading research and development works are in progress in terms of moisture removal, demineralization, removal of harmful constituents such as sulfur, mercury etc. There have been two major approaches to achieve this: dissolving minerals out of coal – the product is termed as ultra clean coal (UCC) and solvent extraction of coaly matter – the product termed as ash-free coal (AFC). In this paper, current knowledge regarding chemical upgrading of coal has been reviewed. In particular, production process and effects of various parameters on yield and product quality have been discussed. The potential applications of the demineralized coals and the economic assessment with respect to commercialization of this process have also been presented.Chemical treatment processes have been demonstrated as the most effective techniques for removal of ash forming minerals from coal to produce coal with minimal or almost no ash. In general, the ash content of UCC is reported within the range of 0.15 to 0.6% either by treating bituminous coal with HF followed by HCl or HNO3. Most effective treatment available in literature is HF followed by HNO3, achieving ash content as low as 0.15%. Organic solvents like polar, non-polar and mixed solvent have been used to extract the organic matters from coal to produce AFC. In general, extraction yield has been found higher at high treatment temperature with polar solvents.The advantages of using UCC and AFC are that they emit lesser pollutants and cause less or no abrasion/erosion and fouling/slagging in the boilers. However, the literature on economic evaluation of AFC applications is scarce and needs to be carried out in future.