The effect of H2 and NH3 on the pyrolysis of coal model compounds based on reactive molecular dynamics simulation

Abstract

The co-combustion of coal and ammonia (NH3) has been receiving considerable attention due to its potential for mitigating CO2 emissions. Consequently, comprehending the pyrolysis mechanism of coal under the NH3 atmosphere is of paramount significance. Using pyrrole and pyridine, which are crucial model compounds of coal, the pyrolysis mechanisms within N2/H2/NH3 is systematacially investigated with the help of chemical reaction kinetics simulations (ReaxFF MD). The results revealed that pyrrole performs inferior structural stability compared to pyridine, resulting in accelerated and shorter pyrolysis, alongside a lower activation energy for the reaction. As a result, the pyrolysis process of pyrrole is more prone to C-C and C-N bond cleavage, ultimately producing more HCN. Moreover, this investigation revealed that both H2 and NH3 atmospheres promote the pyrolysis of pyrrole and pyridine, with the NH3 atmosphere having a more pronounced influence. The presence of an NH3/H2 atmosphere increased the production of HCN, lowered the activation energy required for the pyrolysis of pyrrole and pyridine, and aided in the cleavage of C-C and C-N bonds. Additionally, this study demonstrated the reaction network of pyrrole and pyridine in N2/H2/NH3 atmospheres, focusing on the observation that NH2 radicals, during the initial phases of the reaction, stimulate the production of low-carbon fragments and assist the fracture process of the model compounds. Furthermore, this study presents the reaction network of pyrrole and pyridine in N2/H2/NH3 atmospheres, with a particular focus on the NH3 atmosphere. The existence of NH3 in the atmosphere facilitates the formation of low-carbon fragments during the cleavage phase of the model compounds. This leads to the production of low-carbon intermediates and an increase in the generation of HCN during the subsequent condensation stage. These findings highlight the significant impact of the ammonia atmosphere on the pyrolysis of the model compounds. The outcomes of this study elucidate the fundamental principles of nitrogen-containing compound pyrolysis at the mechanistic level, providing critical insights for subsequent experimental and computational research.