Primary air ratio affects coal utilization mode and NOx emission in lignite pulverized boiler

Abstract

Through a previously validated numerical model, seven combustion scenarios were simulated to study the effects of primary air ratio (PAR) on coal utilization mode and NOx formation characteristics of a wall-fired lignite boiler. Distribution profiles of combustion temperature, concentration of gas components, mole fractions of NO and volatile N-intermediates were discussed. Results show that the effects of PAR on coal utilization mode, i.e. char-O2 oxidation and char-H2O gasification process, differ in two separate ranges. When PAR is slightly increased from 0.35 to 0.41, char-O2 oxidation deteriorates in the lower furnace region while char-H2O gasification is enhanced. However, when PAR is largely increased from 0.44 to 0.53, char-O2 oxidation behavior gets re-improved but char-H2O gasification becomes weaker. Besides, NOx emission at the furnace exit rises monotonously with PAR increasing. Variation of coal utilization mode is attributed to the changes of mixing process between coal particles and secondary air, while the increase of NOx emission is ascribed to the increased initial oxygen around coal particles. The results thoroughly reveal the influence of air distribution between primary and secondary air on coal utilization process and NOx-related characteristics, which can be used as theoretical guidance for the practical operation of lignite-fired boilers.