Developing micro-Raman spectroscopy for char structure characterization in the scale of micro- and bulk: A case study of Zhundong coal pyrolysis

Heterogeneity of coal in micro-scale is significant and can affect coal thermal behavior. In this study, microRaman spectroscopy was developed for characterizing coal/char in the scale of micro- and bulk with a case study of Zhundong coal pyrolysis. More than 150 particles in micro-scale were analyzed for each sample by micro-Raman mapping technology controlled by a computer program. Individual Raman spectrum of each particles in micro-scale and average Raman spectrum for each sample were analyzed, respectively. The results indicate that the evolution characteristics of aromatic rings, C-H in methyl or methylene, and C = O groups can be revealed by combined deconvolution of first- and second-order Raman spectrum. The fluorescence interference in Raman spectrum defined as drift coefficient alpha can quantitate volatiles content and H/C atom ratio of individual coal/char particle in micro-scale. The results reveal that the chemical structures of the Zhundong coal in micro-scale are significantly heterogeneous. The structural evolution of char particle in micro-scale differs from each other, especially in the early stage of the Zhundong coal pyrolysis. The chemical structure of char particles in micro-scale tends to converge as the pyrolysis progresses. A more severe pyrolysis condition can generally lead to a char with higher heterogeneity. For the bulk structural evolution, the Zhundong coal pyrolysis can be mainly divided into two stages. Firstly, the C-H in methyl or methylene releases rapidly, and the relative amount of small aromatic rings and C = O groups decrease mildly. And then, the aromatization degree of char increases rapidly along with the significant release of C = O groups.

Automated summary

This study utilized microRaman spectroscopy to investigate the chemical structures of coal at a micro-scale, revealing significant heterogeneity in the micro-scale. The evolution of char particles in micro-scale differs from each other, especially in the early stage of pyrolysis, but tends to converge as the process progresses. The results suggest that a more severe pyrolysis condition can lead to a char with higher heterogeneity, and the pyrolysis of Zhundong coal can be mainly divided into two stages in terms of bulk structural evolution.