Molecular dynamics investigation of the effect of ammonia on coal pyrolysis and the nitrogen transformation

Coal and ammonia (NH3) co-firing technology is a promising solution allowing the retrofitting of coal power facilities and contributing to global decarbonisation goals. Reactive force field (ReaxFF) molecular dynamic simulation was performed in this study to elucidate the effect of NH3 on coal pyrolysis and nitrogen transformation during coal/NH3 co-pyrolysis. The results indicate that the decomposition products of NH3 and coal would react with each other to form gaseous N-containing species, which is less reactive during the subsequent formation of char and tar. Meanwhile, the detailed decomposition process of NH3 was studied. It was found that the reaction NH3 + H ↔ NH2 + H2 is the primary pathway for NH2 radical generation. The concentration of NH2 radical increases with increasing temperature due to the promoted splitting reaction of NH3 and the dehydrogenation of coal. The increase in NH2 radical concentration promotes the reaction of NH2 and coal decomposition products. The cleavage of C-C and C-H bonds caused by higher temperatures also promotes the interaction between NH2 and coal fragments since it could generate more radical sites on coal fragments for attaching NH2. The nitrogen functionalities (N-functionalities) in the coal fragment would evolve into relatively stable N5 and N6 groups gradually. In addition, the morphological evolution routes of the N-functionalities in coal fragments were presented. This work provides molecular understandings on the mechanism of N transformation pathways during coal/NH3 co-pyrolysis.