Removal of mineral matter and trace elements by chemical coal cleaning, and use of the reaction product as desulfurization agent

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To suppress emission of toxic heteroatoms during coal combustion, it is important to understand the mode of occurrence and behavior of heteroatoms during combustion. In this project, we aim to propose the methods to suppress the emission of sulfur oxide and toxic trace elements. Detailed objectives of this project are as follow : 1) desulfurization by loading calcium with simple methods, 2) chemical cleaning using lime for removal of trace elements as well as mineral matter, And 3) development of method for suppression of trace elements emission. The following results were obtained from this project.1) SOx emission during combustion was effectively suppressed by loading hydrated calcium on coal. The level of SOx emission was much lower when compared with the combustion of the physical mixture of coal with limestone. Furthermore, desulfurization yield was increased when loading calcium under ultrasonic irradiation.2) Removal yields of trace elements such as B, F, Cd, and As by chemical coal cleaning with lime were attained to about 70%. The yield was not so high in comparison with that of mineral matter, indicating the presence of organically associated trace elements.3) Several insights on the mode of occurrence of trace elements were obtained by the comprehensive leaching experiments. We proposed a novel method, leaching of coal char as well as raw coal with HNO_3 and HCl, to elucidate the mode of occurrence of trace elements. We found that Pb, Cr and As were volatilized by heating up to 1100℃ and the volatilization behavior strongly depended on the heating rate. The heating rate dependency on the volatilization was related to the presence of unburnt carbon at high temperature region. Finally, we proposed a method for capture of As through the understanding of its volatilization behavior.