Characterization of Ash Formation and Associated Interactions during the Combustion of Pulverized Biomass Blends

Combustion of biomass blends is a promising technology to promote the use of agricultural residues in pulverized fuel-fired systems through fuel blending to mitigate ash-related problems. The present work was done to investigate ash formation during the combustion of an agricultural residue blended with selected biomass fuels, with the focus on the interactions in ash chemistry between blended fuels and their impacts on ash formation. Experiments were carried out to burn wheat straw (WS) blended with municipal sewage sludge (SS), rice husk (RH), and pine sawdust (PS), each at three ratios, i.e., 20, 40, and 60 wt %, and the WS, SS, RH, and PS individually in a drop-tube furnace at 1300 °C. The resulting residual ashes were characterized by laser diffraction, scanning electron microscopy (SEM), and X-ray diffraction, and particulate matter (PM with an aerodynamic diameter of <10 μm) was analyzed with an electrical low pressure impactor and SEM equipped with energy-dispersive X-ray spectroscopy. The combustion of biomass blends and the blending ratio were observed to influence the characteristics of residual ash formed, including the size distribution and mineral composition of ash particles. The likely cause is the gas–solid interactions of aluminosilicates/silicates, mainly from SS, RH, or PS, capturing alkalis from WS. The combustion of biomass blends greatly reduces submicron ash formation relative to that of WS combustion due to the dilution effect and particularly the impact of the alkali-capturing interactions. Such interactions significantly reduce the submicron ash yield and the associated K and Cl concentrations, while gas–gas interactions of chloride sulfation/phosphation processes change the composition of alkali salts in the submicron ash. The interactions may also affect the K and Cl concentrations of supermicron PM, depending on the ash composition of the component fuels and blending ratio.