Effect of coal rank, oxygen level and particle size on oxidation reactivity of typical Chinese coals

Highlights: • Effect of particle size on reactivity with decreasing O₂ shows a rank dependency. • Marginal revenue by reducing particle size is higher for low volatile coals. • In-furnace blending method is suggested when co-firing low volatile coal. • Linear relationship is obtained between reactivity and coal rank/oxygen level. • Smaller economic fineness can be established for low volatile coal in low NO_x combustion mode. The changeable and deteriorating coal quality in air staging mode threats the safe and economic operation of the boiler. Non-isothermal TGA experiment was performed with varying heating rates (5, 10, 15 and 20 K/min) on six Chinese coals at 20 % O₂ and on the lean coal with varying O₂ (5%, 10 %, 20 % and 40 %). Isoconversional method is adopted to determine the activation energy (E_α) and frequency factor (A_α). Influence of particle size at varying oxygen concentration is compared between lean coal and bituminous coal at 20 K/min. The characteristic temperature (T_0.1, T_0.5 and T_0.9) increases as the coal rank increases and oxygen level decreases. The slope of the line (characteristic temperature vs. O₂) increases from 1.6 to 3.6 as the conversion increases from 0.1 to 0.9, which means the oxygen diffusion has an increasingly important role as the oxidation proceeds. As O₂ decreases, the effect of particle size reduction on combustion improvement of low volatile coal is increasingly significant than that of high volatile coal. The smaller economic fineness of coal particle can be established for low volatile coal in low NO_x combustion mode compared with that for high volatile coal.