Effects of AAEMs on the char heterogeneous structure evolution during Zhundong coal pyrolysis: Insights from micro-Raman spectroscopy

The effects of alkali and alkaline earth metallic species (AAEMs) on the heterogeneous evolution of coal/char structure during pyrolysis are rarely investigated. In this study, micro-Raman mapping technology was devel-oped to characterize the effects of AAEMs on the distribution and heterogeneous evolution of the chemical structure of coal/char particles on micro-scale during pyrolysis. The pyrolysis chars from raw coal, leached coal and loaded coal samples were prepared by using a thermogravimetric analyzer, and more than 300 Raman spectra for each sample were analyzed by micro-Raman mapping technology. The results indicate that the water-soluble AAEMs inhibit the release of volatiles during the whole pyrolysis process. The ion-exchangeable AAEMs promote the release of volatiles at the early stage of pyrolysis and then inhibit the release. For integral char structure evolution, water-soluble and ion-exchangeable AAEMs enhance the average relative amount of small aromatic rings obviously at the devolatilization stage and aromatization polymerization stage, respectively. Besides, both water-soluble and ion-exchangeable AAEMs firstly decrease the average relative amount of active structures obviously and then slightly. For heterogeneous evolution, at the devolatilization stage, the water-soluble AAEMs promote the crack of polyolefin and substitutional groups from active components, and the transformation of these species to small aromatic rings. Meanwhile, water-soluble AAEMs promote the break-down of the cross-linked structures of the large aromatic ring system from inert components and decrease the heterogeneity of the chemical structure of char particles obviously. Besides, the ion-exchangeable AAEMs pro-mote the breakdown of chemical bonds between aromatic rings and the release of the active components. At the aromatization polymerization stage, the ion-exchangeable AAEMs inhibit the condensation of small aromatic rings and increase the heterogeneity of the chemical structure of char particles. In the later stage of pyrolysis, char chemical structure tends to be stable and AAEMs have little effect on the chemical structure of char particles.