Effects of Interaction between Axial and Radial Secondary Air and Reductive Intensity in Reduction Region on Combustion Characteristics and NOx Emission of Coal Preheated by a Self-Preheating Burner

The study focused on the effects of the interaction between axial and radial secondary air and the reductive intensity in reduction region on combustion characteristics and NOx emission in a 30 kW preheating combustion system. The results revealed that the interaction and reductive intensity influenced the combustion in the down-fired combustor (DFC) and NOx emission greatly. For the temperature distribution, the interaction caused the position of the main combustion region to shift down as R2–12 (ratio of axial secondary air flow to radial secondary air flow) decreased or λ2 (total secondary air ratio) increased, and there was the interplay between both of their effects. As R3–12 (ratio of first-staged tertiary air flow to second-staged tertiary air flow) increased, the decrease in the reductive intensity also caused the above phenomenon, and the peak temperature increased in this region. For the NOx emission, the interaction affected the NOx reduction adversely when λ2 or R2–12 was higher, and the range of this effect was larger, so that the NOx emission increased obviously as they increased. The decrease in the reductive intensity caused the NOx emission increased under the homogeneous reduction mechanism, while was unchanged at a high level under the heterogeneous reduction mechanism. For the combustion efficiency, the interaction improved the combustion efficiency as λ2 increased when R2–12 was lower, while reduced it as λ2 increased excessively when R2–12 was higher. The proper decrease in the reductive intensity caused the combustion efficiency increased obviously, while was hardly improved further when the intensity decreased excessively. In this study, the lowest NOx emission was only 41.75 mg/m3 without sacrificing the combustion efficiency by optimizing the interaction and reductive intensity.