Modelling of coal combustion in a drop tube furnace in air and oxy fuel environment

Abstract

The present energy crisis and the growing concern over emission lead to the development of various low emission technologies for efficient utilization of fossil fuels. These technologies offer the benefits to continue using fossil fuels without harming the environment. Oxyfuel combustion technology is one of the promising low emission combustion techniques that have the advantage of being a better retrofit option as well. A non comprehensive modelling study on coal combustion in air and oxy fuel mode was carried out using Fluent, a CFD tool. A numerical study to investigate the effects of balance gas N2 and CO2 and the effects of oxygen concentration on coal burning have been carried out. A drop tube furnace with higher heating rate simulates the real time furnace to some extent was chosen for modelling. The computational results show that, for the same oxygen concentration, the particle and burning temperatures were much lower and the burning of volatiles and char are delayed in oxy case compared to air case. It implied that coal particles take longer to burn completely in the presence of CO2. This will affect the entire processes such as heat transfer, flame stability, emission and ash characteristics inside the furnace. The less intense or slower burning in oxy case is attributed to the higher molar specific heat of CO2 and lower thermal and mass diffusivity in CO2. This ill effects in oxy case can be overcome by increasing the oxygen concentration above 21% so that the burning characteristics as in air (with 21% O2) case can be achieved. The trends obtained from this study are in agreement with the findings from the literature.