Reactivity to CO2 of chars prepared in O2/N2 and O2/CO2 mixtures for pulverized coal injection (PCI) in blast furnace in relation to char petrographic characteristics

Abstract

Pulverized coal injection (PCI) is employed in blast furnace tuyeres in order to increase the injection rate without increasing the amount of unburned char inside the stack. When coal is injected with air in the region of tuyeres, the resolidified char will burn in an atmosphere with progressively lower oxygen content and higher CO2 concentration. In this study, an experimental approach comprising refiring has been followed to separate the combustion process into two distinct devolatilization and combustion steps. A drop tube furnace (DTF) operating at 1300 °C in an atmosphere with low oxygen concentration was used to simulate devolatilization and then the char was refired into DTF at the same temperature under two different atmospheres O2/N2 (typical combustion) and O2/CO2 (oxy-combustion) with the same oxygen concentration. Coal injection was also performed under a higher oxygen concentration in both typical combustion and oxy-combustion atmospheres. The fuels tested comprised a petroleum coke and coals ranging in rank from high to low volatile bituminous, currently used for PCI injection. Specific surface areas, reactivity to CO2 and char petrography have been used to chars characterization. The morphology and appearance of the chars generated under oxy-fuel (O2/CO2) and conventional combustion (O2/N2) conditions with similar amount of oxygen were similar for each parent coal. Vitrinite-rich particles generated cenospheres with anisotropic optical texture increasing in size with increasing coal rank, whereas inertinite yielded a variety of morphologies and optical textures. The apparent reactivity to CO2 measured at high temperature (1000 °C) tended to increase with burnout reflecting the operation under a regime controlled by internal diffusion in which surface area also increased. This may have a significant effect in the reactivity to CO2 of the chars inside the stack of the blast furnace, even under oxygen lean atmosphere.