Fuel-NO formation in biomass-coal co-combustion at high temperature: Further insights into temperature and synergistic effect mechanisms

Co-combustion of biomass in pulverized coal boilers has a broad prospect due to the urgent need for CO₂ emission reduction in the power industry. However, the mechanism of NO formation in co-fired pulverized coal boilers (with temperatures reaching up to 1600 °C) remains unclear, which limits the development of co-combustion technology. Therefore, this study investigates the influence of high temperature and synergistic effects, which origin from the interaction from biomass and coal during co-combustion, on fuel-NO formation characteristics to explore the NO generation mechanisms during co-combustion. Two combustion methods (direct combustion and staged combustion) were employed to examine the formation characteristics of volatile-NO and char-NO under high temperature of 1000–1600 °C. Furthermore, combined with reaction kinetics calculations, the mechanisms by which temperature and synergistic effects influence NO formation during co-combustion were further revealed. The results indicate that increasing temperature suppresses the formation of both volatile-NO and char-NO. Even though the conversion of fuel-N to volatile-N increases under high temperatures, volatile-NO still decreases significantly. This suggests that higher temperatures promote the conversion of fuel-N to volatile-N while simultaneously inhibiting NO formation through homogeneous reactions, thereby reducing overall NO emissions. Simulation results explain this phenomenon: when the temperature rises from 1000 °C to 1600 °C, the rate of the elementary reaction HCO + NO=CO + HNO increases significantly, indicating that the reduction in NO is due to the enhanced reduction rate of NO by volatiles at higher temperatures. Additionally, synergistic effects may further inhibit fuel-NO formation. Among the three types of biomasses, rice husk-coal co-combustion exhibits NO suppression effects, while corn stalk/wheat straw -coal blends may show promotion effects. This inhibitory effect shows a significant positive correlation with the cellulose content in biomass fuels. Among the three lignocellulosic components of biomass, cellulose exhibits the strongest NO reduction capability. Compared to lignin, cellulose exhibits a 32.58 % higher total rate of production (ROP) in the NO reduction process. The findings in this study on the mechanisms of temperature and synergistic effects can provide practical guidance for system design and operation to further control NO emissions in co-combustion furnaces.