Enhanced NOx Reduction by Combined Staged Air and Flue Gas Recirculation in Biomass Grate Combustion

Abstract

Flue gas recirculation (FGR) is a conventional means of reducing NOx emissions that involves lowering the peak flame temperature and reducing the oxygen concentration in the combustion region. Staged air combustion is also an effective means of NOx reduction, especially in biomass combustion. This article reports results on NOx emissions in a set of experiments combining FGR and staged air combustion in a grate-fired laboratory-scale reactor. Two different compositions of the recirculated flue gas were used: CO2 and CO2 + NO. The CO2 concentration varied between 0–8 vol % of the total inlet flow rate and the NO concentration varied between 0 and 64 ppm. Two different FGR locations were also tested: above and below the grate. The results are compared with a reference experiment performed without FGR. The NOx reduction level from staged air combustion at the optimal primary excess air ratio is ∼70%, while employing FGR can reduce the NOx emissions by an additional 5%–10%. The optimal primary excess air rati...