Numerical analysis of NOx reduction in large-scale MSW grate furnace through in-bed combustion optimization using multi-section fuel bed model with thermally thick treatment

•In-bed combustion optimization can largely reduce NOx emission of grate boiler.•A multi-section fuel bed model with thermally thick treatment is developed.•The combined influence of primary air distribution and grate velocity is studied.•Modeling results and real-furnace validation indicates a 53.58 % decrement of NOx. This study focuses on reducing NOx emissions in large-scale municipal solid waste (MSW) grate furnaces through in-bed combustion control. A multi-section fuel bed model with thermally thick treatment is developed to accommodate the configurations of large-scale MSW grate furnaces. The model incorporates detailed sub-grid models to simulate the intraparticle gradient, and the stochastic mixing is accounted to deal with the multi-section fuel beds with vertical drop-offs. The influences of primary air distribution and average grate velocity on combustion status and NOx emissions are investigated numerically and validated in a 600 t/d MSW grate boiler. The results indicate that combining a relatively low primary air ratio with a thickened fuel bed ensures both reduced NOx emissions and high burnout efficiency. By reducing the excess air ratio of primary air from 1.2 to 0.9 and distributing it through a rear-enhanced air mode, the overall NOx emission decreased from 398.50 mg/Nm3 to 215.05 mg/Nm3, although at the expense of an increased unburned carbon content from 1.995 wt% to 3.063 wt%. Further lowering the grate velocity to thicken the fuel bed allowed for leveraging the heterogeneous reduction effect of the char layer, reducing the NOx emissions to 184.95 mg/Nm3. Additionally, the unburned carbon content was also remarkably reduced to 1.560 wt%. The proposed strategies of delaying the primary air supply and thickening the fuel bed offer a cost-effective alternative to post-combustion measures, with which the MSW grate boilers can achieve lower NOx emissions while maintaining efficient and stable combustion.