Enhanced NO x Reduction by Combined Staged Air and Flue Gas Recirculation in Biomass Grate Combustion

Flue gas recirculation (FGR) is a conventional means of reducing NO x 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 NO x reduction, especially in biomass combustion. This article reports results on NO x 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 NO x reduction level from staged air combustion at the optimal primary excess air ratio is ∼70%, while employing FGR can reduce the NO x emissions by an additional 5%–10%. The optimal primary excess air ratio range is 0.9–1. However, FGR more effectively reduces NO x when employed outside of the optimum primary excess air ratio range, i.e., excess air ratios higher than 1 and less than 0.9. The experiments with FGR located above the grate exhibit higher reduction potential, while FGR located below the grate produces decreased reduction. The recycled-NO conversion factor, which gives a measure of maximal FGR efficiency, at the maximum point, is nearly 100% when FGR is applied below the grate and is 85%–100% in the case of recirculation above the grate.