Nitric oxide emissions during coal and coal/biomass combustion under air-fired and oxy-fuel conditions

Co-firing coal with biomass is becoming an increasingly important approach to the development of renewable energy sources for electricity production, especially in regions that do not have ample sources of wind and solar power. Cofiring may also result in lower emissions of pollutants, including nitrogen oxides (NOx). In this study of the co-firing of pulverized coal and sawdust under air-fired and oxy-fuel conditions, we investigate the effects of biomass particle size and nitrogen content on nitric oxide (NO) formation. Experiments were performed in a 30 kWth laboratory-scale coal combustor. Measurements of NO concentrations in the exhaust stream revealed that the NO formation is dependent upon the sawdust particle size. When compared to pure coal combustion, co-firing of up to 40% (wt.) sawdust that contained large particles (> 50 mesh) did not result in lower NO emissions, despite the much lower nitrogen content associated with the sawdust. The conversion of fuel-bound nitrogen to NO was found to increase as the percentage of sawdust co-fired also increased. However, reductions in NO were obtained when the sawdust particle size was reduced. Numerical simulation of the experiments show that for larger particles, the particles penetrate the flame and devolatilization is delayed, resulting in the release of fuel-bound nitrogen in a region that is more conducive to NO formation.