Alkali emission during combustion of coals at the normal temperature range of PFBCs

Abstract

Four coals of three different ranks were combusted at 850 and 900/sup 0/C to investigate the emission of sodium and potassium compounds during combustion. One high volatile bituminous coal, two subbituminous coals and one lignite were combusted in a laboratory-scale, batch (50 grams of coal), fixed-bed combustor. Alkali emissions were found to be greater at 900/sup 0/C than at 850/sup 0/C. For coals with less than 9 wt % ash content on a dry basis, the sodium compound emission increased linearly with decreasing ash content. Sodium from coal was found to be fixed in the ash residue during combustion as either sulfate or water-insoluble compounds. The reaction of sodium compounds with the clay minerals in coal resulted in the formation of the insoluble compounds. Addition of sodium chloride to the three low-ash content coals (< 9 wt % ash) by aqueous solution impregnation markedly increased sodium chloride vapor emission. Addition of sodium chloride enhanced the release of potassium compounds also. The studies described in this report were undertaken to investigate the emission of sodium and potassium compounds during the combustion of coals and simulated high-chlorine coals at the normal temperature range of PFBCs, using a laboratory-scale, batch, fixed-bed combustor. Study of high-chlorine coal is of interest because earlier findings have shown that fouling and corrosive effects on the fire-side of a boiler furnace are closely related to the chlorine content of coal, and because thermodynamic calculations indicate that the chlorine content of coal is a major factor in determining the alkali level in flue gas during combustion.