Numerical Simulation of Combustion Characteristics in a Small-Scale Biomass Chain Grate Furnace under Different Primary Air Conditions

A small-scale biomass chain grate boiler utilizing biomass as fuel plays a pivotal role in enhancing combustion efficiency and reducing emissions, directly contributing to energy conservation and emission reduction. This study employs FLIC-FLUENT to develop a multiphysics coupled model for the solid-phase combustion on the grate and gas-phase combustion in the furnace of a small-scale biomass chain grate boiler. The model enables the simulation of gas-side processes of the small-scale biomass boiler. By varying the primary air distribution strategy, the study examines the combustion characteristics and pollutant emissions, providing theoretical insights for energy conservation and emission reduction. The results indicate that the optimized primary air distribution stabilized the furnace temperature distribution and reduced NO_x emissions. Although the solid-phase bed burnout rate under design air distribution is 80.44%, which is lower than 82.94% under delayed air distribution and 85.19% under uniform air distribution, it is still fully acceptable for a small-scale biomass boiler. Meanwhile, the peak bed temperature rises to 1531.5 K. Additionally, under the design air distribution, the furnace temperature distribution remains stable, reducing temperature deviations on the heat exchange surfaces and enhancing boiler safety. Finally, the optimized oxygen distribution leads to a reduction in NO concentration at the outlet from 143.79 mg/m³ (at 9% O₂) with uniform air distribution and 148.61 mg/m³ with delayed air distribution to 129.79 mg/m³.