Comparison of chars obtained under oxy-fuel and conventional pulverized coal combustion atmospheres

The combustion of coal in conventional power plants produces large amounts Of CO2 which contributes to the greenhouse effect. One of the ways to approach the CO2 emissions abatement is to burn the coal in an O-2/CO2 atmosphere which eliminates the need of a separation step. In this study, two coals of different rank (a high volatile and a low volatile bituminous) have been burned in a drop tube reactor using O-2/N-2, and O-2/CO2 mixtures with increasing oxygen content from 0 to 21%. Various oxygen concentrations have been selected for each set of experiments in order to follow both the progress of combustion and the influence of oxygen content in the devolatilization behavior of coal. Results show that a higher amount of O-2 in CO2 than in N-2 is needed to achieve similar burnout levels. Significant differences were found in the influence of oxygen content on the devolatilization behavior of the lower and higher rank coal. The limited amount of oxygen in the reacting atmosphere resulted in volatile release inhibition for the high volatile bituminous coal, whereas the more plastic low volatile coal was hardly affected. The presence of variable amounts of oxygen in CO2 had a small influence on the char particle appearance. The chars from both the combustion series (O-2/N-2) and the oxy-fuel series (O-2/CO2) were similar for each parent coal in terms of reactivity and micropore surface area measured by CO2 adsorption. The main difference between both series of chars relied on the surface area determined by N-2 adsorption (S-BET) and on the size distribution of pores which was shifted to a larger size for the oxy-fuel series. The difference between both series of chars was larger for the high volatile bituminous coal chars than for the low volatile bituminous coal chars. This might have important implications for combustion under the diffusion-controlled regime.