Simulation of the pyrolysis process of tar-rich coals in an H₂O atmosphere based on the reactive force field molecular dynamics

Objective Tar-rich coal resources, abundant in China, can be converted into energy products (e.g., chemicals and gas/liquid fuels) through pyrolysis, thus alleviating the country’s dependence on oil and gas imports. A thorough understanding of the product evolution and reaction mechanisms of the pyrolysis of tar-rich coals is crucial to research into the clean and efficient conversion processes of coals.

Methods Based on the reactive force field molecular dynamics (ReaxFF MD), this study simulated the pyrolysis process of tar-rich coals (long-flame coals) and explored the impacts of the H₂O atmosphere on the distribution of pyrolytic products, along with the mechanism behind the impacts.

Results and Conclusions  The results indicate that tar-rich coals (long flame coals) are pyrolyzed at temperatures ranging from 1200 K to 2800 K, involving two stages: pyrolysis (1200 K to 2000 K) and polycondensation (2000 K to 2800 K). In the pyrolysis stage, coal molecules decompose rapidly as the temperature rises, accompanied by gradually decreasing coke products and constantly increasing tar and gas products. The polycondensation stage witnesses the polycondensation among tar products, which generates coke and releases low-molecular-weight gas, leading to decreased tar products but increased coke and gas products. Therefore, more gas products can be obtained by increasing the pyrolysis temperature and prolonging the pyrolysis time, while the key to improving the tar yield is to inhibit the polycondensation. The introduction of the H₂O atmosphere into the pyrolysis during high-temperature polycondensation demonstrates that H₂O can effectively accelerate the cracking of coal molecules. Specifically, with an increase in the proportion of H₂O, the coal pyrolysis system exhibits decreasing C―C bonds but increasing C―H and C―O bonds. As revealed by the analysis of the interactions between coals and H₂O, the free radicals generated during coal pyrolysis react with H₂O, promoting the decomposition of H₂O molecules. In turn, the H• and OH• generated from H₂O decomposition further expedite coal cracking and react with the products of coal pyrolysis to produce more tar and gas. This study, contributing to a deeper understanding of the pyrolysis process of tar-rich coals, can serve as a guide for the clean and efficient utilization of coal resources.