Effect of air staging on NOx emissions in biomass combustion in a bubbling fluidized bed

NOx emissions belong to the most critical gaseous emissions in thermal conversion of biomass and non-recyclable waste. Yet the role of air staging on NOx reduction is not fully understood in most combustion systems. This study investigates the effect of air staging on the NO_x reduction in an industrial bubbling fluidized bed by means of a detailed kinetic mechanism and assuming gradual entrainment of flue gases (with NOx precursors) into the air jets. The computed data is compared to unique non-published experimental industrial-scale data, from a 107 MW_th biomass-fired fluidized bed boiler. The air was staged via primary air (through the bed), and secondary and tertiary air jets with a high inlet velocity. NO, HCN and NH₃ concentrations were measured inside the furnace at a depth of 1.8 m, below the secondary air jets, above the secondary air jets and above the tertiary air jets. The computed NO_x values were in good agreement with the experimental values. Above the bed, the total NO + HCN + NH₃ concentration was 1169 ppm. After the tertiary air staging the total NO + HCN + NH₃ concentration was 76 ppm. The conversion of fixed nitrogen (NO + HCN + NH₃) to N₂ reached 65 % after the secondary air jets, and 90 % after the tertiary air jets. The study shows that exceptional reduction of NO_x emissions can be achieved with air staging in this type of industrial combustion systems for biomass.