Effects of CO₂ atmosphere on low-rank coal pyrolysis based on ReaxFF molecular dynamics

Pyrolysis of low-rank coal in CO₂ atmosphere can reduce carbon emissions while comprehensively utilizing coal resources. Based on ReaxFF molecular dynamics (ReaxFF-MD), the pyrolysis processes of low-rank coal in inert and CO₂ atmosphere are simulated. By comparing the evolution of pyrolysis products, the influences of CO₂ on the pyrolysis characteristic and product distribution are analyzed. It is found that CO₂ slightly inhibits the conversion of char to tar in the early stage of pyrolysis. In the later stage, CO₂ significantly promotes the decomposition of char and increases the yield of tar and pyrolysis gas. According to the different bond breaking behaviors of coal molecules, the pyrolysis process can be divided into pyrolysis activation stage, initial pyrolysis stage, accelerated pyrolysis stage and secondary pyrolysis stage. The reforming reaction of CO₂ with alkanes generates free hydrogen radicals, which promotes the cleavage of ether bond, C_ar-C_ar bridge bond and aliphatic C-C bond. Compared with in inert atmosphere, final yield of light tar in CO₂ atmosphere increases from 17.98% to 20.68%. In general, the CO₂ atmosphere helps to improve the tar yield and tar quality of low-rank coal pyrolysis.