Competition between NH3-O2 reaction and char-O2 reaction and its influence on NO generation and reduction during char/NH3 co-combustion: Reactive molecular dynamic simulations

Abstract

To reduce CO2 emissions in power plants, the co-firing of ammonia (NH3) and coal in boiler has received increasing attention in industry and academia. In this work, char/NH3 co-combustion behaviors and NO generation and reduction characteristics under different temperatures, O2 equivalence ratios and NH3 co-combustion ratios are studied via the reactive molecular dynamic (ReaxFF MD) simulations. The initial reaction mechanisms of char oxidation and NH3 oxidation revealed by ReaxFF MD simulations conform to the models that have been proposed, which proves the accuracy of ReaxFF MD method in studying char/NH3 co-combustion. The char/NH3 co-combustion simulation results show that NH3 will compete with coal char for O2, resulting in an increase of NH3 oxidation rate and a decrease of char oxidation rate. The competition between NH3-O2 reaction and char-O2 reaction during char/NH3 co-combustion are largely depended on temperature, O2 equivalence ratio and NH3 co-combustion ratio. Compared to NH3 combustion, the concentration of O and OH radicals are higher while the concentration of NH2, NH and NNH intermediates are lower during char/NH3 co-combustion due to the competition between NH3-O2 reaction and char-O2 reaction, which enhances the NO generation reactions but weakens the NO homogeneous reduction reactions. Under oxygen-lean conditions, the homogeneous reduction of NO by NNH and the heterogeneous reduction of NO by unburnt char play an important role in reducing NO emissions, especially at high NH3 co-combustion ratios. The reaction paths of NO reduction by char during char/NH3 co-combustion are revealed. The results indicate that both the carbon atoms and the nitrogen atoms in char are responsible for NO reduction.