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

• Char/NH 3 co-combustion characteristics were studied using ReaxFF MD. • The competition between NH 3 -O 2 reaction and char-O 2 reaction was demonstrated. • NH 3 oxidation rate increased but char oxidation rate decreased during char/NH 3 co-combustion. • Char-NO reaction played an important role in NO reduction under O 2 -lean conditions. • The reaction mechanisms of NO reduction by char were revealed. To reduce CO 2 emissions in power plants, the co-firing of ammonia (NH 3 ) and coal in boiler has received increasing attention in industry and academia. In this work, char/NH 3 co-combustion behaviors and NO generation and reduction characteristics under different temperatures, O 2 equivalence ratios and NH 3 co-combustion ratios are studied via the reactive molecular dynamic (ReaxFF MD) simulations. The initial reaction mechanisms of char oxidation and NH 3 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/NH 3 co-combustion. The char/NH 3 co-combustion simulation results show that NH 3 will compete with coal char for O 2 , resulting in an increase of NH 3 oxidation rate and a decrease of char oxidation rate. The competition between NH 3 -O 2 reaction and char-O 2 reaction during char/NH 3 co-combustion are largely depended on temperature, O 2 equivalence ratio and NH 3 co-combustion ratio. Compared to NH 3 combustion, the concentration of O and OH radicals are higher while the concentration of NH 2 , NH and NNH intermediates are lower during char/NH 3 co-combustion due to the competition between NH 3 -O 2 reaction and char-O 2 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 NH 3 co-combustion ratios. The reaction paths of NO reduction by char during char/NH 3 co-combustion are revealed. The results indicate that both the carbon atoms and the nitrogen atoms in char are responsible for NO reduction.