Mechanistic Insights into Multi-Stage NOx Reduction and Operational Optimization of Biomass Gasification Syngas-Coal Co-Firing in a 600 MW Tangential Pulverized Coal Boiler

This study investigates the co-combustion dynamics of biomass-derived syngas and coal in a 600 MW tangential pulverized coal (PC) boiler through integrated experiments and numerical simulations, with a focus on NO emission mechanisms and operational optimization. Besides, this paper treats biomass gasification gas as a core process parameter, comprehensively investigating and elucidating the effects of various common operating conditions-including gas flow rate, injection location, and boiler loads-on boiler performance and pollutant formation characteristics. Key findings reveal that NO concentration profiles along the furnace height exhibit a multi-stage trend: initial increase due to volatile nitrogen release and thermal NO formation, followed by reduction in transition zones via homogeneous/heterogeneous reactions, and a final stabilization as fuel-NO dominates. Introducing biomass syngas enhances post-SOFA zone temperatures while advancing coke nitrogen release and amplifying NO reduction through gas-phase reactions, ultimately lowering total NO emissions by 8.03 % compared to pure coal combustion. Notably, reduced boiler load intensifies the syngas-induced NO reduction effect (up to 7.3 % at 13 % load) due to prolonged NO reduction residence times and enhanced gas-solid interactions. Optimal performance is achieved at 6-8 % biomass thermal ratio, with lower injection positions maximizing utilization of transition zone reductants, thereby suppressing 8 % of peak NO formation. These results establish actionable strategies for emission control, demonstrating that strategic biomass syngas integration enables simultaneous NOx mitigation (248 -> 191 mg/Nm3) and operational flexibility across load ranges, providing critical insights for retrofitting conventional coal boilers toward sustainable co-firing systems.