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 NO_x 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 NO_x 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 NO_x emissions vary dramatically with different ammonia ratios. Higher temperatures enhance the formation of NO_x 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 NO_x 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 NO_x. The specific primary air ratio in the fluidized bed should be considered in the priority treatment of NO_x or N₂O in the process of lowering nitrogen oxides emissions. Ammonia distribution strategies have opposite effects on NO_x 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.