A Critical Review of Coal Demineralization and Its Implication on Understanding the Speciation of Organically Bound Metals and Submicrometer Mineral Grains in Coal

The state-of-the-art of two coal demineralization technologies, acid/alkali leaching for ultraclean coal (UCC) and solvent extraction for hypercoal (HPC), has been critically reviewed in this paper. UCC or HPC here refers to a coal-derived solid fuel with overall ash content in the order of 0.1 wt %, which has the potential to burn directly in gas turbine combined cycle (GTCC) systems with a net power generation efficiency of no less than 48% on the higher heating value (HHV) basis. The UCC or HPC can also be potentially used in direct carbon fuel cell (DCFC) systems with a net power generation efficiency larger than 60% on HHV basis. Its gasification-derived syn-gas is more energy-intensive, and can be used as a hydrogen source for high-efficiency electricity generation with zero emissions and as a feedstock for the synthesis of value-added chemicals and liquid fuels. In this paper, two typical processes for the generation of UCC and HPC have been comprehensively reviewed to address both fundamentals of the elution of metals and the practical feasibilities. In particular, direct information related to the properties of the inorganic metals remaining in HPC has been intensively addressed. Its implications to the speciation of original metals in coal, especially those embedded as organically bound metals and/or submicrometer particles in coal matrix, were summarized. Finally, the research requirement for the generation of ultraclean coal from low-rank coal was proposed.