Comparison of Fine Ash Emissions Generated from Biomass and Coal Combustion and Valuation of Predictive Furnace Deposition Indices: A Review

Abstract

To address important ash-related issues associated with burning solid biomass fuels for power generation, this paper reviews results of studies performed at the Northeastern University (NU) Combustion and Air Pollution laboratory and elsewhere under well-characterized conditions. It compares the physical and chemical characteristics of fine ash emissions generated from the combustion of pulverized biomasses to those from pulverized coals, since biomass is considered as a substitute fuel for coal in power generation, and assesses their furnace surface deposition propensities. Comparisons show that combustion of some biomasses may generate disproportionally higher emissions of submicron ash particles than combustion of coals (0.03–1.1 versus 0.04–0.06 kg/GJ, respectively). The high submicron emissions of biomass are problematic, as conventional particulate control devices have low collection efficiencies for such small particles. Moreover, the chemical composition of submicron particles of biomass typically contain large amounts of alkalis (potassium and sodium), chlorine, sulfur and, often, phosphorous, whereas those collected from combustion of coal contain large amounts of silicon, aluminum, iron, and sulfur. The composition of biomass ashes renders them more amenable to deposition on furnace surfaces, as calculations based on published empirical surface deposition indices show. These calculations, as well as experiences elsewhere, indicate that the slagging and, particularly, the fouling deposition prospects of most biomasses are significantly higher than those of coals.