Study on the combustion characteristics of NH3 and hydrochar mixture using ReaxFF MD: Oxygen equivalence ratio, ammonia co-combustion ratio, and combustion environment.

Co-combustion of NH3 (ammonia) and hydrochar is a promising strategy to reduce fossil fuel usage and CO2 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 NH3/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 H2 into H2O and N2 into NO/NO2, increasing H2O, NO, and NO2 yields while suppressing H2 and N2. At λ = 0.5, no CO2 or NO2 formed; at λ = 1.5, small amounts appeared, with CO produced being several-fold more abundant than CO2. Correspondingly, the peak NH2 intermediate count increased from 181 to 233 as λ rose from 0.5 to 1.5, whereas N2Hx intermediates (N2H2, N2H3, N2H4) declined - an oxygen-driven shift favoring NO formation over N2. Secondly, increasing the ammonia co-combustion ratio from 62.5 % to 85 % led to higher final yields of CO, NO, and N2, with a corresponding drop in residual unconverted carbon (UC). A greater NH3 proportion promoted the conversion of hydrochar into small molecules (C1-C4 gases), thereby reducing tar and coke formation (fewer tar/coke species at 85 % NH3 than at 62.5 %). Mechanistic analysis showed that a hydrochar fragment (C49H40O5) can be stepwise broken down by NH3-derived radicals (e.g. NH2, OH, H) into intermediate species such as C49H38O5 and C49H37O5, ultimately producing CO. Finally, the combustion atmosphere had a notable impact on reaction heat release. Under both air (O2/N2) and pure O2 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 O2, 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 O2 (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 NH3-hydrochar co-combustion.