Experimental investigation of particle size effects on high temperature thermal modification characteristics of granular coal

This study systematically examined the modification characteristics of coal particles across four size fractions (0–0.355 mm, 0–1 mm, 0–2 mm, and 0–4 mm) in a circulating fluidized bed (CFB) preheating combustion system operating at 850-950℃. Results revealed that volatile release, pore evolution, and emission profiles exhibit strong particle-size dependence. Notably, 0–4 mm particles achieved peak combustion efficiency (96.77 %) and gas calorific value (2.86 MJ/Nm³) at 850-900℃, benefiting from progressive fragmentation that extended residence time while suppressing thermal NO_x formation. By contrast, 0–2 mm particles developed optimized pore structures (42 % surface area increase) and enhanced reactivity at 900℃, though with elevated NO_x emissions (231 mg/m³) due to intensified fuel-nitrogen oxidation. At 950℃, thermal treatment induced structural homogenization, narrowing combustion efficiency variations to < 2.3 % but exacerbating NO_x generation in 0–4 mm particles. Advanced characterization identified key mechanistic relationships: 0–0.355 mm particles suffered pore degradation under thermal stress, whereas Raman spectroscopy revealed increased carbon defect concentrations in fragmented 0–4 mm particles, correlating with reactivity enhancement. Critical trade-offs were observed between combustion performance and emissions, with 0–4 mm particles enabling staged NOx control but requiring residence time optimization, while smaller particles favored rapid kinetics but demanded fuel-nitrogen conversion mitigation.