Mechanisms of gas generation from conventional and microwave pyrolysis of coal slime

Microwave pyrolysis can efficiently achieve the clean upgrading of solid waste value fuels, such as coal slime (CS). However, the mechanism of pyrolysis gas release remains unclear. This study investigated the effects of the pyrolysis temperature and methods on pyrolysis gas generation. The results demonstrated that microwave pyrolysis exhibited a higher percentage of gas phase and a lower percentage of liquid phase and promoted the generation of pyrolysis gas, while inhibiting the formation of tar compared to conventional pyrolysis. The gas composition was quantified by micro gas chromatography, and kinetic calculations revealed that microwaves decreased the activation energy of H2 and CO generation by 23.74% and 36.01%, respectively, and the release of H2 and CO increased. The analysis of tar by gas mass spectrometry revealed that microwaves can reduce secondary reactions and promote the transfer of oxygen-containing functional groups to the liquid phase. The increase in temperature further stimulated the cyclization and condensation of tar, leading to the transfer of numerous H and O radicals in tar to the gas phase, as more H2 and CO were released. This is further explained in terms of heat and mass transfer. Density functional theory calculations of the R-COO-R' reaction pathway showed that microwaves could generate tremendous energy to promote the thermal decomposition of ester groups in tar, generating more H2 and CO. Therefore, a possible reaction path for generating H2 and CO by the thermal decomposition of CS under microwave intensification is proposed, which provides a feasible idea for the efficient quality improvement of CS.