Influence of the overfire air ratio on the NO_x emission and combustion characteristics of a down-fired 300-MW_e utility boiler

Down-fired boilers used to burn low-volatile coals have high NO_X emissions. To find a way of solving this problem, an overfire air (OFA) system was introduced on a 300 MW_e down-fired boiler. Full-scale experiments were performed on this retrofitted boiler to explore the influence of the OFA ratio (the mass flux ratio of OFA to the total combustion air) on the combustion and NO_x emission characteristics in the furnace. Measurements were taken of gas temperature distributions along the primary air and coal mixture flows, average gas temperatures along the furnace height, concentrations of gases such as O₂, CO, and NO_x in the near-wall region and carbon content in the fly ash. Data were compared for five different OFA ratios. The results show that as the OFA ratio increases from 12% to 35%, the NO _x emission decreases from 1308 to 966 mg/ Nm³ (at 6% O₂ dry) and the carbon content in the fly ash increases from 6.53% to 15.86%. Considering both the environmental and economic effect, 25% was chosen as the optimized OFA ratio.