Nitrogenous Gas Emissions from Coal/Biomass Co-combustion under a High Oxygen Concentration in a Circulating Fluidized Bed

Oxy-fuel combustion of coal/biomass is able to realize negative CO2 emission. Increasing the total oxygen concentration in oxy-fuel combustion surely reduces the scale and cost of the flue gas recirculation system. The increase of the total oxygen concentration leads to considerable changes in the emission characteristics of nitrogenous gases (nitrogen oxides and their precursors). Here, coal/biomass co-combustion tests were conducted in a 0.1 MW oxy-fuel circulating fluidized bed combustion apparatus to investigate nitrogenous gas emissions from oxy-fuel combustion of coal/biomass under a high total oxygen concentration (50%). HCN was also detected in the flue gas, besides NO and N2O, especially in the mixed fuel tests. A lower excess oxygen ratio led to less NO and N2O emissions, the same as conventional air combustion. The secondary flow ratio affected emissions of nitrogenous gases depending upon fuel types and atmospheres. Under the same total oxygen concentration, a higher oxygen concentration in the primary flow led to lower emissions of HCN and N2O. The increase of straw share significantly improved the NO, N2O, and HCN emissions. O2/recycled flue gas combustion released less nitrogenous gases than O2/CO2 combustion thanks to its longer residence and reaction time. The temperature rise increased NO emission but reduced N2O and HCN emissions significantly.