An Experimental Study on NOx Emissions in Combustion of Pulverized Coal Preheated in a Circulating Fluidized Bed

This article describes an experimental study on NOx emissions in pulverized coal combustion after preheating and air staging. The process takes place in two stages: the pulverized coal is first preheated in a circulating fluidized bed, and then the resulting fuel gas and char particles are burned in a down-fired combustor under air-staging conditions. Experiments were conducted with three types of coal, three coal particle sizes, four preheating temperatures, and three air ratios in the circulating fluidized bed. With this process, combustion was stable, the temperature was uniform in the combustor after coal preheating, and the burning efficiency was high. The combination of air-staging combustion with pulverized coal preheated in a circulating fluidized bed was effective in reducing NOx emissions. NOx emissions from combustion of bituminous coal, anthracite coal, and semicoke were 368, 256, and 458 mg/N m(3) (@6% O-2), and the conversion rates of nitrogen in the fuel (fuel-N) were 28.66%, 23.94%, and 57.36%, respectively. The NOx emissions and fuel-N conversion rate initially decreased then increased with increasing preheating temperature. The NOx emissions and fuel-N conversion rate decreased with increasing pulverized coal size and air ratio in the circulating fluidized bed.