The sulfation process during biomass combustion and cofiring, by converting KCl into K2SO4, can affect particulate matter (PM) formation, ash deposition, and corrosion in a furnace. In this study, the effects of temperature, SO2 concentration, O2 concentration, and oxy-combustion atmosphere on the sulfation and PM formation were investigated in an entrained flow reactor. Results show that the particle size distribution (PSD) of PM10 from biomass combustion is bimodal and that PM10 is dominated by PM1.0 consisting of KCl and K2SO4. Enhanced sulfation by SO2 addition generally increases the particle size of PM1.0 and the K2SO4 content in PM1.0, but its effect on PM1.0–10 is marginal. The effect of sulfation on PM1.0 formation strongly depends on the temperature: at high temperature (e.g., 1300 °C), sulfation is not favorable, thereby having negligible influence on PM1.0 formation; while at moderate temperature (e.g., 1100 °C), sulfation is significantly promoted, resulting in a larger size and higher yield ...