Pyrite-enhanced coal spontaneous combustion: Insights from experiments and molecular simulations

Abstract

Pyrite is a principal factor affecting the spontaneous combustion of coal; however, the molecular-level structural evolution mechanism of pyrite-containing coal remains unclear. To investigate the mechanisms of pyrite on coal spontaneous combustion, experiments (temperature-programmed experiments, thermogravimetric-differential scanning calorimetry coupled with fourier transform infrared spectroscopy (TG-DSC-FTIR), low-temperature nitrogen adsorption, and in-situ infrared), molecular simulations (reactive force field, ReaxFF) and quantum chemical methods (density functional theory, DFT) were conducted. Experimental results showed that pyrite significantly promoted the generation of CO and CO₂, reduced the activation energy of coal samples (4.4%–13%), and increased heat release (14%–46%), thus enhancing the likelihood of coal spontaneous combustion. Additionally, pyrite increased the specific surface area of the coal samples and enhanced the content of active functional groups. ReaxFF molecular dynamics (ReaxFF MD) simulations revealed that pyrite accelerated the decomposition of tar, increased oxygen consumption, and increased the potential energy of the coal–oxygen reaction, thus further promoting coal–oxygen interactions. After the Fe-S bond breaks, Fe, serve as a carrier and repeatedly interacts with the carboxyl oxygen in the benzene ring, thus facilitating the opening of the benzene ring and generating more free radicals. Bond order and interaction force analyses indicated that the presence of Fe²⁺ reduced the C–C bond energy on the benzene ring, which is consistent with the results of MD simulation.