Experimental investigation and ReaxFF simulation on pore structure evolution mechanism during coalification of coal macromolecules in different ranks

This analysis revealed the alterations in the pore structure of large organic molecules in coal during the process of coal pyrolysis. Nine models of macromolecular structures in coals, representing distinct coal ranks, have been built. The research results show that along with the increasing coal rank, the average microporous volume of medium rank coal is 0.0287 cm³/g. The average microporous volume of high-grade coal is 0.0662 cm³/g. The micropore volume and specific surface area of coal samples decrease in the order of high rank, low rank, and middle coal. The experimental measurements align with the ReaxFF pyrolysis simulation calculations, indicating a decrease in the hydrogen to carbon ratio and oxygen to carbon ratio of all coal molecules. Additionally, the pore volume and specific surface area exhibit a pattern of initially decreasing and then increasing. The simulation results of gas probes indicate that a majority of the pores with a diameter larger than that of CH₄ molecules are found in the macromolecular structure models of low rank coal and medium to high rank coal. The conclusions are useful for us to understand the formation and development process of pores in coal reservoir. A two-dimensional representation of coal's macromolecular structure was constructed using ChemDraw software. The Forcite module in Materials Studio software was used to perform geometric optimization and annealing kinetics simulation of a two-dimensional macromolecular structure model. The ReaxFF-MD module in Amsterdam Modeling Suite (AMS) 2020 software to model the pyrolysis of XJ coal macromolecules.