Pulverized combustion under conventional (O2/N2) and oxy-fuel (O2/CO2) conditions of biomasses treated at different temperatures

Incorporation of biomass as a partial substitute of coal in industrial processes is one of the most important ways to reduce CO2 emissions to the atmosphere. The combination of its combustion with CO2 capture technologies, such as oxy-fuel, may lead to negative emissions, decreasing CO2 emissions of power and ironmaking plants. Densification of biomass by torrefaction and carbonization is necessary to improve its energetic characteristics and allows its incorporation in large-scale facilities. In this study, chips of three woody biomasses and an olive stone previously torrefied at 250 °C and carbonized at 450 °C were pulverized and combusted in a Drop Tube Furnace (DTF) at 1300 °C using different O2/N2 and O2/CO2 atmospheres with variable O2 concentrations. The characteristics of the chars were evaluated by its reactivity in air and CO2 in thermobalance, optical microscopy and adsorption isotherms techniques. The biomasses that were treated at lower temperatures yielded higher burnouts and compared to conventional combustion (O2/N2), the conversion results were slightly higher under oxy-fuel (O2/CO2) conditions for all biomasses. Torrefied biomass chars showed isotropic cenospheric particles with high porosity within the walls and chars from carbonized biomasses showed an isotropic fusinoid structure whereas olive stone showed massive particles. The pre-treatment temperature had showed relatively low impact in the distribution of porosity of the chars generated from them and surface areas of O2/CO2 chars were higher than those of O2/N2 ones. Apparent reactivities were in general higher for the chars generated from the torrefied biomasses and were generally close for chars from both conventional and oxy-fuel atmospheres.