Oxidation mechanism of single char particles: Insights from chemical imaging by micro-Raman mapping technique in a heating stage

Abstract

This study developed a Raman-mapping method to visualize individual char particles during oxidation, tracking the spatiotemporal evolution of their chemical structure. 3 series of experiments were conducted: (i) micro-scale mapping of 1000 °C chars before oxidation to quantify inherent heterogeneity of the char particles, (ii) Raman mapping at room temperature after stepwise oxidation to resolve the progressive structural evolution during the char oxidation, and (iii) high-temperature in situ Raman spectroscopy of single char particle and char particle clusters to follow real-time carbon framework evolution and functional group formation during the oxidation sequence. The results show that Hongshaquan coal char pyrolyzed at 1000 °C exhibits intra- and inter-particle structural variation, with individual heterogeneity peaking when the particle's average structure approaches that of the bulk char. Besides, oxidation reactivity of the char correlates more strongly with char surface heterogeneity than with average structural parameters. During the char oxidation, its structures proceed in 3 stages: (i) O₂ attacks side chains and small aromatic rings, cleaving large aromatic rings and raising the fraction of oxygen-containing functional groups and small aromatic rings. (ii) These species accumulate to a peak and then deplete. (iii) A mineral-rich ash shell forms in the later stage of the oxidation, hindering oxygen diffusion and slowing down the oxidation of residual carbon. Single-particle oxidation results can capture the complete oxidation process and its chemical structure evolution, whereas particle clusters burn layer-by-layer, easily overlooking the early activation process. This study can provide a deep understanding on the oxidation mechanism of char single-particles.