Experimental investigation on the self-ignition behaviour of coal dust accumulations in oxy-fuel combustion system

• Both critical ignition temperature and ignition delay time of coal dusts clearly decrease with increasing oxygen mole fraction.
• Critical ignition temperature in 21% O2 + 79% CO2 is slightly higher than that in air.
• A modified F-K analysis is proposed to evaluate the self-ignition risk of coal dust in the oxygen-enriched O2/CO2 ambient.
• Both the reactivity and sensitivity of oxidation decrease as the maturity of coal sample increases.

For the oxy-coal combustion, the accumulation of coal dust in the system has a fire risk of self-ignition. Therefore, understanding the ignition dynamics of coal dust deposits in oxygen-enriched environment is essential for the prevention of fire and dust explosion. In this work, both hot-oven and hot-plate tests were conducted to study the self-ignition behaviour of coal dusts in O2/CO2 ambient with O2 mole fraction from 21% to 50%. Three coal dusts: Indonesian Sebuku coal, Pittsburgh No. 8 coal and South African coal were tested with different sizes. Experimental results revealed that the self-ignition risk increased significantly with the increasing O2 mole fraction: reducing both the critical ignition temperature (10 °C in hot-oven test and 40 °C in hot-plate test) and the ignition delay time. Comparatively, the inhibiting effect of CO2 was found to be small for self-ignition. In addition, a modified Frank-Kamenetzkii analysis was proposed to explain all measured critical ignition temperatures, and the genetic algorithm was used to determine kinetic parameters of the one-step global reaction. The analysis showed that as the coal maturity/rank increased, both the self-ignition risk and the sensitivity to oxidation decreased, along with the decreasing apparent activation energy and pre-exponential factor. Such trend did not change with the ambient oxygen condition for all three coal dusts. These results improve our understanding of the self-ignition behaviour and the fire risk of coal dust in the oxy-fuel combustion system.