Chemical, Leaching, and Toxicity Characteristics of Coal Ashes from Circulating Fluidized Bed of a Philippine Coal-Fired Power Plant

Abstract

Characterization of the coal ash from a typical coal-fired circulating fluidized bed (CFB) power plant in the Philippines was done by studying physical and chemical properties as well as toxic elements content from Semirara and Indonesian fly and bottom ashes. Laboratory-scale experiment was carried out using serial batch leaching procedure (SBLP) to determine the leaching behavior of toxic elements from coal ashes and to mimic the environmental condition using sulfuric acid. From the X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses, the main compound present is CaO which makes the coal ash alkaline in nature. Moreover, SiO2, Fe3O4, and other trace minerals are also present. Also, toxicity characterization leaching procedure (TCLP) shows that more than 99 % of chromium and arsenic remain in the coal ashes matrix. The results of the chemical analysis of eluates deduced by the application of standard leaching tests according to TCLP method indicated that hazardous elements such as heavy metals and metalloids contained in fly and bottom ashes could potentially be transferred to the liquid phase. According to the Microtox analysis, the bottom ashes are less toxic than fly ashes due to the vaporization of toxic elements during the combustion and their subsequent adsorption on the surface of fly ashes. Furthermore, leaching of chromium from the coal ash samples was significantly affected by initial pH of the leachant adjusted with sulfuric acid. The highest leaching rate was reached using the combined condition of pH of 8, contact time of 8 h, and L/S of 5. With these conditions, the leaching rate of chromium from SBA is 0.059, from SFA is 0.070, from IBA is 0.054, and from IFA is 0.06 g Cr/g of ash per hour. Based on literature, the results are relatively comparable.