Mechanisms of Solid Fuel Conversion by Chemical-Looping Combustion (CLC) using Manganese Ore: Catalytic Gasification by Potassium Compounds

Chemical-looping combustion (CLC) is an emerging technology that can be used to meet the growing demands for electrical energy production without CO2 emissions. In CLC with solid fuels, the gasification of the carbonaceous fuel by steam is envisaged to be performed directly in the fuel reactor. This requires high steam-gasification rates for the effective use of the solid fuel. Recently, it has been observed that the choice of oxygen carrier can have a profound effect on the char-conversion rates in the fuel reactor. More specifically, the charconversion rate with a Brazilian manganese ore was a factor of five higher than that with ilmenite. In this work, the reaction mechanism of the char gasification was investigated in the presence of this manganese ore with the aim to explain the high rates observed. Steam-gasification experiments with petroleum coke were performed by using a batch fluidized-bed reactor with manganese ore as the bed material. In addition, partial gasification experiments of petroleum coke were conducted, and detailed energy-dispersive X-ray spectroscopy (EDX) analyses were performed on the surface and interior of the fuel and manganese-ore particles. The effect of the possible gas-phase release of oxygen from the manganese ore was also investigated. It was found that the release of gas-phase oxygen by the oxygen carrier does not explain the high gasification rates observed. Instead, the transfer of a catalytically active material, potassium, from the bed material to the solid fuel was observed, which in turn catalysed the steam-carbon-gasification reaction. As the catalytically active compound is included in this naturally occurring bed material, it may offer cost-efficient, catalytic gasification in a CLC process.