Numerical modeling of a coal/ammonia Co-fired fluidized bed: Control and kinetics analysis of nitrogen oxides emissions

Abstract

The potential increase in nitrogen oxide emissions in the coal/ammonia co-firing can hinder the large-scale utilization of ammonia to reduce carbon emissions. In this work, a fluidized bed simulation model was established to investigate the NOx and N2O behaviors in the process of coal/ammonia co-firing. The effects of several variables on nitrogen oxides emission characteristics were studied, including the ammonia ratio, temperature, excess air ratio, and air/ammonia distribution strategies. The findings indicate that NOx and N2O concentrations rise and then decline with the NH3 co-firing ratio (CR-NH3) increased, peaking at 10 % and 5 % CR-NH3. The formation of N2O is insensitive to the addition of ammonia, while NOx emissions vary dramatically with different ammonia ratios. Higher temperatures enhance the formation of NOx but inhibit the generation of N2O within 750 °C–950 °C. As the temperature rises, the primary decomposition path of N2O shifts from N2O→N2H2→NNH→N2 to N2O→NO2→NO→N2. The generations of NOx and N2O are both enhanced due to the weakness of the reduced atmosphere with the excess air ratio increased. When the primary air ratio is raised, N2O gradually takes over as the main source of nitrogen oxides instead of NOx. The specific primary air ratio in the fluidized bed should be considered in the priority treatment of NOx or N2O in the process of lowering nitrogen oxides emissions. Ammonia distribution strategies have opposite effects on NOx and N2O emissions. With more NH3 introduced as a secondary fuel, the dilute phase area can change from the main source of NO to the consumption area of NO. The present findings can help control the emissions of nitrogen oxides during coal/ammonia co-combustion in coal-fired circulating fluidized bed power plants.