Insight into structural evolution and detailed non-isothermal kinetic analysis for coal pyrolysis

In this study, the pyrolysis of low-ash coal (SF) and high-ash coal (NM) was investigated using a thermogravimetric analyzer. The semi-pyrolyzed char was collected to analyze its structural characteristics, and different model-fitting approaches were adopted for non-isothermal kinetics. Partial demineralized coal was also prepared to clarify the mineral–organics–gas correlation. The results showed that coal pyrolysis was a two-step sequence in which the aliphatic chain decomposed first (<600 °C), and then the aromatic ring condensed (>600 °C) with the formation of H2. Moreover, the amorphous structure in coal char was generated before the graphitization. HF leaching can promote the generation of H2 during the secondary pyrolysis of high ash coal. Inner minerals can promote graphitization under high-temperature pyrolysis. The nth-order model showed the best fitting performance, and the activation energy (Ea) increased with temperature based on piecewise analysis. Moreover, the Ea distribution became more concentrated after demineralization, especially for high-ash coal. Finally, the coal pyrolysis mechanism was summarized based on the solid structural evolution with consideration of the gas release property and inner minerals effect.