Effect of steam on ignition of pulverized coal particles in oxy-fuel combustion in a drop tube furnace

Abstract

Oxy-fuel combustion is recognized as a promising technology to reduce CO2 emissions from coal-fired power plants. Typical oxy-fuel combustion is O2/CO2 recycled combustion, in which the dilute gases are the mixture of O2 and CO2. Wet flue gas recycle combustion (O2/CO2/H2O atmospheres) should be promoted due to some merits over dry flue gas recycle combustion (O2/CO2 atmospheres). In this study the ignition of bituminous coal was studied using experimental and numerical methods under O2/CO2/H2O atmospheres. The experiments were conducted in a drop tube furnace using a high-speed camera with oxygen mole fractions of 21%, 30% and 40%. The substitution of CO2 by 5%, 10%, 20% and 30% steam were implemented to evaluate the effect of the mole fractions of steam on the ignition of coal in O2/CO2/H2O atmospheres. The experimental results reveal that the ignition distances are barely affected when low mole fraction of steam (5%, 10% and 20%) was added to O2/CO2 atmospheres. As the mole fraction of steam is increased to 30%, the ignition distance is significantly decreased. The effect of steam on the ignition of coal in O2/CO2/H2O atmospheres was particularly analyzed using a proposed CFD model. The numerical results indicate that the physical properties of steam lead to the ignition delay of coal when CO2 is replaced by steam in O2/CO2 atmospheres. The lower specific heat and higher diffusivity of steam compared to CO2 lead to the advanced ignition of coal, and the higher thermal radiation of steam leads to the ignition delay of coal. The chemical properties of steam are beneficial to the ignition. The steam shift reaction played a more important role in the advanced ignition of coal than the steam gasification reaction.