Mitigating CO2 emission in pulverized coal-fired power plant via co-firing ammonia: A simulation study of flue gas streams and exergy efficiency

Coal-fired power plants are one of the major sources of CO2 emission, and the novel application of directly co firing the carbon-free fuel ammonia into the coal-fired power plants is emerging as a promising method of CO2 emission reduction. In this study, simulations of the boiler system of a 600 MWe coal-fired power plant unit were performed in both coal-fired condition and ammonia-coal co-firing conditions, and the impacts of ammonia co firing on the emission and thermal properties of flue gas, exergy loss, and exergy efficiency were analyzed. Two co-firing scenarios (identical adiabatic flame temperature scenario and identical excess air coefficient scenario) were compared, both assuming that the total heating value of the fuel into the furnace kept unchanged with different ammonia co-firing ratios (5%, 10%, 15%, 20% on heating value basis). The results showed that CO2 emission was effectively reduced by 24.08 t/h for every 5% increase in the ammonia co-firing ratio. The amount of total flue gas was also reduced while both the content and the emission rate of H2O were increased with ammonia co-firing. Consequently, the specific heat capacity of flue gas increased and further changed its temperature distribution profile. Compared with the coal-fired base case, the exergy loss in the furnace under the ammonia co-firing conditions increased continuously as the ammonia co-firing ratio increased, suggesting that the irreversible loss of the ammonia combustion process was greater than that of coal. By contrast, the exergy loss of heat exchangers and fans decreased in ammonia co-firing conditions. Finally, the exergy efficiency of the boiler system was only slightly reduced (53.82% vs. 53.27%-53.22%) in ammonia co-firing conditions, showing that co-firing ammonia in coal-fired power plant would not significantly deteriorate the thermal economy.