Effects of biomass co-firing on combustion and NOx conversion performance of a 660MW wall-tangentially fired lignite boiler

Abstract

Co-firing biomass can reduce CO2 emissions from coal-fired units, but it inevitably alters in-furnace combustion and NOx conversion characteristics. This work numerically investigates the effects of co-firing ratio and biomass type in a 660MW tangentially-fired coal boiler. The results demonstrate that the combustion behavior under biomass co-firing is dominated by a competition between the kinetic enhancement driven by high volatile matters and the thermodynamic suppression caused by flue gas dilution and moisture evaporation. When co-firing corn straw at a 10% blending ratio, the overall performance is optimal, where the kinetic enhancement leads to a marked temperature increase in the primary combustion zone, while CO and unburned carbon concentrations and NOx emissions decrease. As co-firing ratio increases to 20% and 30%, local oxygen deficiency intensifies, resulting in delayed combustion, and CO and unburned carbon levels re-increases. Meanwhile, NOx emission is lowest at 20% co-firing ratio owing to its strong reducing atmosphere and appropriate temperature window. Among different biomass types, corn straw yields the largest temperature rise and NOx reduction, while and palm kernel shell exhibits the smallest temperature rise but the highest unburned carbon and NOx emissions. These findings elucidate the variation of combustion and NOx conversion during biomass co-firing in a 300MW boiler, providing practical guidance for optimizing co-firing operations.