Phosphor thermometry at the surface of single reacting large-diameter spherical coke particles to characterise combustion for packed bed furnaces

Packed-bed furnaces fired with large-diameter coke are important in high-temperature material processing industries such as lime and iron production. The combustion conditions are complicated by the presence of an ash layer surrounding the coke particle that remains intact during passage through the furnace and alters oxygen diffusion and heat transfer to the reacting particle core. The objective of this study is to determine the surface temperature of this ash layer using lifetime-based phosphor thermometry during combustion of single spherical 38 mm diameter coke particles in a high temperature tube furnace. Time traces of the coke particle core temperature and mass conversion rate do not significantly differ between experiments performed with and without the phosphor layer, indicating that the presence of the phosphor particles does not alter the overall combustion behaviour. Surface temperatures of up to 950 °C are measured and correlated with the fuel mass conversion rate. When the coke particle starts to react the surface temperature is up to 100 °C higher than that of the core. As the reaction front progresses toward the centre, the core temperature exceeds the surface temperature by 200 °C due to the insulating effect of the ash layer. The surface temperature of the ash layer decreases with time due to the steadily decreasing fuel mass conversion rate. The method and results can be used to provide key validation data for shrinking-core combustion models, for example by constraining the unknown transport properties of the ash layer, thereby assisting the development of complete packed-bed furnace simulations for process optimisation.