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

Abstract

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 NOx 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 MWth 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 NOx 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 NOx emissions can be achieved with air staging in this type of industrial combustion systems for biomass.