NO emissions under pulverized char MILD combustion in O2/CO2 preheated by a circulating fluidized bed: Effect of oxygen-staging gas distribution

MILD (moderate or intense low-oxygen dilution) combustion of pulverized char in O-2/CO2 was achieved by preheating fuel in a circulating fluidized bed (CFB). The purpose of this research was to study effects of oxygen-staging gas distribution on NO emissions under MILD combustion of pulverized char. First, the preheating characteristics were evaluated, including the conversion ratios of each fuel component and the composition of the coal gas. To calculate the nitrogen balance in the preheating process, the gas in the loop seal was changed to O-2/N-2. The results indicate that the conversion ratios of each component increased with the increasing oxygen concentration in the primary gas, and the conversion ratio of nitrogen was above 0.40. In addition, the reduced proportion of nitrogen increased with the increase of oxygen concentrations in the primary gas, surpassing 30.0%. The heating value of high-temperature coal gas also increased. Next, the effects of oxygen-staging gas distribution on NO emissions were investigated. Increasing oxygen concentrations in the primary gas and in the secondary gas as well as decreasing oxygen concentration in the tertiary gas all reduced NO emissions. The lowest NO emission was 26.8 mg/MJ, and the combustion efficiency was above 99.0%. These results demonstrate that the oxy-fuel preheating combustion technology with effective oxygen-staging gas distribution could achieve clean, efficient combustion of pulverized char.