Kinetic and thermodynamic analyses of multigranular low-rank coal: Synergistic effect of particle size on reactivity and coke properties

This study performed kinetic and thermodynamic analyses on samples of multigranular low-rank unbound coal (LNC) with three different particle size ranges: 40–80 mesh, 80–150 mesh, and 150–300 mesh. The study investigated the pyrolytic coke yield of multigranular LNC and its reaction characteristics under a CO₂ atmosphere. The synergistic effect of particle size and coke properties on the reactivity was highlighted using a combination of characterization techniques (BET, SEM, XRD, Raman, and FTIR) and the Friedman, Kissinger-Akahira-Sunose, Flynn-Wall-Ozawa, and Starink methods. The results showed that reducing the particle size significantly increased the specific surface area and pore volume of the coal samples. Upon increasing the pyrolysis temperature, the coke yield decreased from 91.2–93.64 wt% at 300 °C to 64.8–66.92 wt% at 900 °C. The fine-grained samples had slightly lower yields because they underwent more thorough volatilization. At the same temperature, the I_D/I_G values of the fine-grained samples (150–300 mesh) were significantly higher than those of the coarse-grained samples, and they also exhibited greater reactivity. Their gasification reaction index (GRI), comprehensive pyrolysis index (CPI), and comprehensive gasification characteristic index (S) values were respectively 0.77 %, 20.21 %, and 31.36 % higher than those of the coarse-grained samples (40–80 mesh). The kinetic analysis indicated that the average activation energy (E_a) of the fine-grained samples was 169.19 kJ/mol, which was significantly lower than that of the coarse-grained samples. The reaction mechanism was dominated by diffusion and nucleation, and ΔH, ΔG, and ΔS confirmed that the fine-grained samples were more reactive. This study provides guidance for optimizing the particle size during the classification and utilization of low-rank coal to regulate the coke yield for the large-scale application of CO₂.