Environmental Assessment of Bottom Ash from Canadian Coal-Fired Power Plants

Bottom ashes collected from ESP and baghouse of seven pulverized coal-fired power plants using subbituminous and bituminous coal and a fluidized bed combustor using crushed bituminous coal were examined for their mineralogy and elemental composition. The results presented in this paper are based on the average of three samples collected from each power plant. The mineralogy of each sample was determined using XRD and SEM/EDX; Elemental content was determined using INAA, ICPMS, and CVAAS; and the speciation of As, Cr, and Ni was determined by XANES. Bottom ash from the pulverized power plants consists of granular particles with a minor input of melted glassy fragments, while the bottom ash from the fluidized bed combustor consists entirely of granular particles. The sulphur and carbon contents of pulverized bottom ashes range from 0.03 to 2.32 wt % and 0.19 to 6.62 wt %, respectively. For the fluidized bed combustor, the sulphur and carbon contents were 5.27 wt % and 10.72 wt %. The concentrations of As, Cr, Hg, Ni, and Pb in bottom ash are related to sulphur content of coal and are higher for bottom ashes from high sulphur feed coals. Most of the elements associated with S (As, Hg, and Pb) are captured from fluidized bottom ash, more so than by the corresponding ESP fly ash. Most of the elements in bottom ash have enrichment (RE) factors of less than 0.7 indicating that they are not enriched in the bottom ashes as compared to the feed coals. Arsenic, Cd, Cr, Ni, and Pb have higher concentrations in granular bottom ash as compared to glassy bottom ash of the same power plant. The feldspars and quartz of feed coal are health hazards and are captured mostly by bottom ash and therefore prevent their emission from stack. The As, Cr, and Ni in form of: non-toxic As +3 , mostly beneficial Cr 3+ , and non-carcinogenic Ni 2+ are in coordination predominantly with oxygen. Mercury and Pb are low and have very little environmental impact.The hazardous elements (As, Cr, Ni, and Pb) are only leached by HCl, indicating that under normal condition they remain immobile and their impact on environment can be considered low.