Study on the combustion characteristics of NH and hydrochar mixture using ReaxFF MD: Oxygen equivalence ratio, ammonia co-combustion ratio, and combustion environment.

Co-combustion of NH (ammonia) and hydrochar is a promising strategy to reduce fossil fuel usage and CO emissions. In this study, reactive molecular dynamics simulations (ReaxFF MD) were performed to investigate the effects of oxygen equivalence ratio (λ), ammonia co-combustion ratio, and combustion atmosphere on the combustion characteristics of the NH/hydrochar mixture. Firstly, Increasing the oxygen equivalence ratio from 0.5 to 1.5 accelerated the reaction and enhanced fuel conversion: the total molecule count rose by ∼56 % (from 2449 to 3810), and heavy coke species (C40 compounds) were completely eliminated at λ ≥ 1.0. A higher λ facilitated the conversion of H into HO and N into NO/NO, increasing HO, NO, and NO yields while suppressing H and N. At λ = 0.5, no CO or NO formed; at λ = 1.5, small amounts appeared, with CO produced being several-fold more abundant than CO. Correspondingly, the peak NH intermediate count increased from 181 to 233 as λ rose from 0.5 to 1.5, whereas NH intermediates (NH, NH, NH) declined - an oxygen-driven shift favoring NO formation over N. Secondly, increasing the ammonia co-combustion ratio from 62.5 % to 85 % led to higher final yields of CO, NO, and N, with a corresponding drop in residual unconverted carbon (UC). A greater NH proportion promoted the conversion of hydrochar into small molecules (C-C gases), thereby reducing tar and coke formation (fewer tar/coke species at 85 % NH than at 62.5 %). Mechanistic analysis showed that a hydrochar fragment (CHO) can be stepwise broken down by NH-derived radicals (e.g. NH, OH, H) into intermediate species such as CHO and CHO, ultimately producing CO. Finally, the combustion atmosphere had a notable impact on reaction heat release. Under both air (O/N) and pure O conditions, the co-combustion process began with an endothermic phase followed by exothermic heat release. The peak heat absorption at λ = 1.0 reached 69.18 Ha under pure O, much higher than the 16.78 Ha under air, indicating a more intense initial reaction in an oxygen-rich environment. By the end of 500 ps, however, the net energy released in air (20.62 Ha) slightly exceeded that in pure O (14.10 Ha). These results demonstrate that pure oxygen accelerates fuel consumption and product formation, whereas air yields a greater overall energy release - providing quantitative insight for optimizing practical NH-hydrochar co-combustion.