Insights on the initial stages of carbonization of sub-bituminous coal

Detailed studies were carried out on the initial stages of carbonization of sub-bituminous coal using reactive force field (ReaxFF) molecular dynamics simulations. Evolution of different gaseous species during carbonization were analyzed at different temperatures and densities. Elementary reactions were identified leading to the formation of small gaseous species. Cleavage of homolytic O-H bond was found to be the first step of sub-bituminous coal carbonization. CH4 formed mainly due to reaction of center dot CH3 with hydrogen radical/hydrogen abstraction. Among others, C9HmOn radicals were the most abundant species at any simulations condition considered here, where m = 9, 8 or 7 and n = 1, 2 or 3. Bond dissociation energy (BDE) of the identified reactions decreased by similar to 3.0 kcal/mol for endothermic reactions and increased by similar to 3.0 kcal/mol for exothermic due to change in enthalpy at higher temperature. It was observed that the formation of 5-membered carbon only rings and sp-hybridized carbon species played crucial role in the subsequent growth. The results agree with available experimental observations and computational studies. This work provides new insights on the carbonization mechanism of subbituminous coal. (C) 2021 Elsevier Inc. All rights reserved.