Thermal Transformation of Coal Molecular Structures and Their Impact on Reactivity in Metallurgical-Grade Silicon Production

Coal has been widely used as a reducing agent for metallurgical-grade silicon (MG-Si). As a crucial raw material, the coal selection process represents one of the most critical steps in MG-Si production. However, due to the inherent complexity of coal, there is currently a lack of efficient and accurate methods for screening highly reactive coal in industrial production. This deficiency leads to significant carbon loss and excessive CO2 emissions, ultimately reducing the overall efficiency of industrial processes. To address this challenge, this study selected three types of bituminous coal with different performance characteristics from an MG-Si production plant for investigation. The aliphatic structures of the three coals were 69.85, 43.87, and 37.44%, respectively. The carbon-to-hydrogen ratio (C/H) of the coals exhibited an increasing trend with temperatures over the range of 0–1600 °C. The C/H of the No. 3 coal was the highest at different temperatures, with the highest degree of dehydrocyclization. The degree of ordering also increased with temperature for all three coals. The higher proportion of aliphatic structures in the No. 1 coal led to a long-range disordered microcrystalline structure. The No. 2 and No. 3 coals had higher percentages of aromatic structures and showed short-range ordered spikes. This study systematically investigated the correlation between stacking height (Lc), crystallite diameter (La), and C/H ratio. These results showed that C/H was positively correlated with the degree of ordering, and the correlation coefficient (R2) for La reached 0.87. Additionally, CO2 reactivity tests revealed a significant negative correlation between reactivity and both degree of ordering and C/H. These findings provided insights into the efficient utilization of coal for MG-Si production.