Char refiring under O2/N2 and O2/CO2 atmospheres. Implication for PCI injection in blast furnace

Abstract

It is attempt in pulverized coal injection (PCI) in blast furnace tuyeres to increase the injection rate without increasing the amount of unburned char inside the stack of the blast furnace. The unburned char can cause problems in the blast furnace operation, such as reduced permeability, undesirable gas/temperature distribution, excessive coke erosion and significant char carryover. In the near tuyere region the coal is injected with air but the resolidified char will burn in an atmosphere with a progressively lower oxygen content and higher CO2 concentration. In this study an experimental approach has been followed to address this situation trying to separate the combustion process into devolatilization and combustion steps. Initially coal has been injected in a drop tube furnace (DTF) operated at 1300 ºC in an atmosphere with low oxygen concentration to ensure volatile combustion and prevent soot formation. Then the char has been refired into the DTF at the same temperature under two different atmospheres O2/N2 (typical combustion) and O2/CO2 (oxycombustion) with the same oxygen concentration. Also coal injection under higher oxygen concentration for both typical combustion and oxy-combustion atmospheres was performed. The fuels tested comprised a petroleum coke, and coals ranging in rank from high volatile to low volatile bituminous currently used for PCI injection. A thermogravimetric analyser and microscopy techniques have been used to find out about char reactivity and appearance. Different trends of burnout have been observed between the coals which are related with the characteristics of the parent fuels and those of the resolidified chars.