Parametric study of combined premixed and non-premixed flame coal burner

A combined gas/air mixture–coal burner was developed to include heat recirculation by utilizing a radiative solid material with premixed flame jets impinging onto the downstream side to preheat the fuel/air jet on the upstream side. Providing the heat recirculation mechanism at different air staging degrees enhanced the destruction rates of the fuel nitrogen oxides. Concentric elliptical premixed gas/air and coal/air jets had a stronger preheating effect and a consequent increased NOx reduction effectiveness as compared to concentric circular jets, where the inner elliptical jets enlarged the contact diffusion area and entrainment thus increasing the preheating time. The parametric variation in the feeding ports to the coal combustor affected the exhaust emissions, wherein the use of an inclined or shifted injection from the centre-line contributed to the NOx reduction. Increasing the jet angle in the upstream direction reduced the CO concentrations, while the NOx emissions varied depending on the degree of staging. The inverse/normal flame configuration was found more effective than the normal flame configuration with respect to NOx reduction that was enhanced at higher heat input ratios. Utilizing inverse triple flames led to a further NOx reduction since higher temperatures prevailed in the initial flame region with a five reaction zone structure. Finer particles produced less NOx, which was further reduced by blending the coal with biomass.