Optimizing Ammonia Injection in Bio-Fuelled CFBCs

The reduction of nitrogen oxides (NOx) via ammonia injection (NH₃) was investigated for circulating fluidized bed combustors (CFBCs) fuelled with mixtures of coal, peat, wood, bark, and logging residues. The reference boiler was the Alholmen Kraft, i.e. the largest co-fired unit in the world (550 MWth, 194 kg/s, 165 bar, 545 °C). The boiler featured ammonia injectors at the cyclone. Fuel composition, gas composition, and temperature were measured at suitable locations along the boiler while operating with diverse fuel mixtures. A chemical model was developed for analyzing the NOx reduction and was validated against measurements. The model accounts for the elemental composition of fuels, the composition of gases at the cyclone and in the stack, and the profile of temperature from cyclone to stack. The chemical reactions downstream the ammonia injection are described by gas-phase detailed chemical kinetics and accounting for ideal mixing. Measurements and simulations reveal that NOx reductions of over 50% are achievable for any fuel mixture and with moderate amounts of ammonia. Reductions are mainly affected by the temperature. All simulations show the existence of a maximum reduction vs. temperature, whose extent and location is affected by the concentrations of CO, NO, and the molar ratio [NH₃]_injected/[NO]_cyclone. Simulations also indicate that with fuel mixtures other than the mixtures used in the reference boiler the maximum reduction is also affected by the concentrations of H₂O, O₂, CxHy, and N₂O. Finally, simulations suggest an additional strategy for reducing emissions in co-fired CFBCs, where the N₂O formed from coal is used to support the NOx reduction by ammonia, thus reducing NOx while maintaining acceptable N₂O emissions. Further investigation is necessary for clarifying the details of this strategy. Under all circumstances, the only N-based pollutants predicted in detectable amounts are NO and N₂O.