Pyrolysis mechanisms of coal extracts based on TG-FTIR and ReaxFF MD study

Abstract

The decomposition of small molecules in coal dominates the initial pyrolysis stage, which further influences the ignition, combustion, and pollutant emissions. In this work, the detailed composition of the coal extracts was obtained through the gas chromatograph-mass spectrometer (GC–MS) and nuclear magnetic resonance (NMR) analyses. Furthermore, the coal extract molecular models were successfully constructed. Then, the microscopic pyrolysis mechanisms of coal extracts were elucidated through the reactive force field molecular dynamics (ReaxFF MD) simulations. More importantly, the evolution of specific chemical bonds and the decomposition of selected crucial molecular fragments were discussed. Meanwhile, the weight loss characteristics were investigated accordingly through TG-FTIR. The results indicate there are three main reaction stages including the low-temperature reaction, rapid thermal decomposition, and molecular condensation reaction. During the rapid thermal decomposition, the cleavages of CC and CO bonds cooperatively contribute to the formation of CH4 and CO2. As the temperature increases to the molecular condensation stage, the pyrolysis process is governed by the dehydrogenation condensation, accompanied by the increase of CC and HH bonds and decrease of CO bonds. Additionally, phenolic ethers can accelerate molecular condensation reactions. Furthermore, butylated hydroxytoluene primarily undergoes the demethylation reaction during rapid thermal decomposition.