Study on NOx formation and ash characteristics during co-combustion of semi-coke and biomass under O2/CO2 conditions

Oxy-fuel co-combustion of semi-coke and biomass has emerged as a promising and environmentally sustainable avenue for reducing carbon emissions and realizing the resource utilization of solid waste. However, the underlying mechanisms of N-containing pollutants release and ash formation during co-combustion process remain somewhat unclear. Herein, the intricate characteristics of NO_x formation and ash-related behavior during oxy-fuel co-combustion of semi-coke (SC) and three typical biomass (rice hull (RH), pine wood (PW) and walnut shell (WS)) were investigated based on the effects of various parameters, including blending ratio, temperature and atmosphere in this research. The results show that the minerals in biomass could efficiently inhibit the NO_x formation through facilitating the reduction of NO_x to N₂, and increasing the biomass blending ratio would lead to a significant decrease of NO_x emission. Additionally, the conversion of fuel-N to NO_x demonstrated a consistent upward trend with increasing temperature from 900 °C to 1350 °C. Significant decreases of NO_x are detected when adjusting the ratio of O₂/CO₂, with an optimal O₂ concentration at 30 %. The biomass type and combustion atmosphere contributed greatly to chemical compositions and slagging propensity of the ash. In comparison with RH, PW and WS exhibited notably higher slagging propensity of ash under test conditions. Moreover, increasing the O₂ concentration in O₂/CO₂ atmosphere would elevate the slagging propensity of ash by promoting the formation of calcium sulfate, which subsequently fostered the development of additional low-temperature eutectics. This study provides theoretical support for developing the fuel value of biomass and semi-coke, enabling collaborative disposal approaches and ultimately realizing carbon peak in thermal power industry.