Construction of a coal char model and its combustion and gasification characteristics: Molecular dynamic simulations based on ReaxFF

In present work, an atomistic coal char representation is constructed through the pyrolysis of the classical Given bituminous coal model using the reactive molecular dynamics (ReaxFF MD) simulation. The combustion and gasification characteristics of the char are also investigated via ReaxFF MD simulations. The coal pyrolysis activation energy obtained from ReaxFF simulation is 403 kJ/mol, which is in good agreement with recent experimental data. The char fragments are extracted from the pyrolysis products and the 3D representation of the char model is constructed. The chemical properties of the char examined are the H/C and O/C ratio; aromaticity; distribution of oxygen and nitrogen; aromatic and aliphatic ring structures; and the physical features of char model including the helium density, radial distribution function of carbon atoms, and pore size distribution. The char combustion and gasification simulation results show that the activation energy for char-O2 oxidation, char-CO2 gasification, and char-H2O gasification are 129, 320, and 283 kJ/mol, respectively. By analyzing the distribution of the products and radicals as well as the evolution of char molecular structure, the char combustion and gasification mechanisms are revealed. It is also found that char-H2O gasification leads to the generation of large amounts of C2 species.