Factors influencing coke gasification with carbon dioxide

Of all coke properties the influence of the catalytic mineral matter on reactivity of metallurgical cokes is least understood. There is limited information about the form of minerals in the metallurgical cokes and no information about their relative concentration. A comprehensive study was undertaken for characterisation of mineral matter in coke (qualitative and quantitative), which enabled quantification of the effect of catalytic minerals on the reaction rate, and establishment of the effect of gasification on the mineral phases. Also, the relative importance of coke properties on the gasification reaction rate was determined. The reactivity experiments were performed at approximately 900ºC using 100% CO2 under chemically controlled conditions. The mineralogical composition of the investigated cokes was found to vary greatly as did the levels of catalytic mineral phases. These were identified to be metallic iron, iron sulfides and iron oxides. The gasification reaction rate at the initial stages was strongly influenced by the content of catalytic mineral phases and also by the particle size of the catalytic mineral matter. The reaction rate increased as the contact surface between catalyst and carbon matrix increased.Catalytic mineral phases showed a strong influence on the reaction rate at early stages of reaction. But their influence diminished during gasification. At later stages of reaction the influence of micropore surface area became more important.The influence of the catalytic mineral phases diminished during gasification because the catalyst was inactivated to some degree and the contact surface between the catalyst and carbon matrix diminished due to the strong gasification of the carbon around the catalyst particles. The partial inactivation of the catalytic mineral phases occurred because metallic iron and pyrrhotite were oxidised by CO2 to iron oxide, and in turn iron oxide reacted with other mineral phases, which it is associated with, to form minerals that are not catalysts. It is noteworthy that a significant percentage of the mineral matter present in the investigated cokes was amorphous (44 - 75%). The iron, potassium and sodium present in the amorphous phase did not appear to catalyse gasification, but their potential contribution to gasification could not be completely excluded