Theoretical understanding of coal char oxidation and gasification using reactive molecular dynamics simulation

Abstract

Coal char possesses high carbon and low ash features, making it suitable for many industrial uses. Despite the extensive interest in coal utilization, it is the char that has the great influence on the combustion reactivity, where char gasification reaction by CO2 can affect the combustion characteristics during oxy-fuel combustion. In this work, a carefully established surrogate coal char model, coupled with the selected ReaxFF force field, is used to simulate the processes of char oxidation by O2, char gasification by CO2, and char combustion under O2/CO2 and O2/N2 conditions. In comparison with the oxidation process, high CO2 concentration hinders gaseous molecules (C2-C4) and light tar (C5-C13) from reacting with O radicals to form smaller carbonaceous molecules. The C–C bond breaking activation energies are 164 and 217 kJ/mol for char-O2 oxidation and char-CO2 gasification, respectively. Specifically, O radical and CO2 will adsorb and destroy the edge of representative poly-aromatic hydrocarbon molecules in the char model during char oxidation and gasification. The CO/CO2 molar ratio decays almost exponentially with the extent of char consumption due to the subsequent oxidation of CO in char oxy-fuel combustion. The C–C bond breaking activation energies are 196, 190 and 167 kJ/mol for char combustions under O2/CO2 ratios of 25%/75%, 50%/50% and 75%/25%, respectively. At the same O2 concentration, the oxidation ratio under the O2/CO2 environment is always higher than that under the O2/N2 environment, as a result of the additional O radicals provided by CO2. This work is devoted to deepen the understanding of the thermochemical conversion process of char at the atomistic level, and is expected to further optimize char utilization.