Computational fluid dynamics and combustion modelling of HIsarna incinerator

Abstract

HIsarna technology combines the cyclone converter furnace (CCF) technology owned by Tata Steel and the HIsmelt technology owned by RioTinto. The CCF is mainly a prereduction vessel that prereduces and melts the iron ore particles, while the final reduction to metallic iron takes place in the smelt reduction vessel. The off-gases from the smelt reduction vessel undergo post-combustion in the CCF. Depending on the operating conditions of the HIsarna process, the off-gases may still contain small amounts of unburnt carbon and hydrogen; hence, for process safety and environmental reasons, they are passed through an incinerator that needs to be operated within a temperature window that will guarantee full combustion of the off-gases. In one of the HIsarna campaigns, it was observed that the temperatures in the incinerator dropped significantly during short periods of production. In order to avoid this phenomenon, the HIsarna process can be adjusted to meet the design conditions in the incinerator, but this is not a preferred option. A computational fluid dynamics study of the incinerator was carried out with different compositions of off-gases from the CCF with varying degrees of post-combustion and flowrates, in order to improve its design and operation. The combustion model predicted complete burn out of CO, CH₄ and H₂ when sufficient air/O₂ was injected. The computational fluid dynamics study showed that in all the cases, the flow pattern of the gases remained asymmetric. The temperature in the incinerator was generally higher if natural gas was mixed with the cyclone off-gas and much higher if oxygen was also injected. Modifications to the incinerator layout were recommended. In subsequent HIsarna trials, the new design was successfully implemented.