Post-combustion and heat transfer at blowing of oxygen into a carbon monoxide containing slag foam

A combined post-combustion model (CPM) for smelting reduction processes was developed in a multi-national research project supported by the European Coal and Steel Commission. The project partners were CSM, Rome, Hoogovens, Ijmuiden, MPI, Düsseldorf, and TUB, Berlin. This paper reports about a heat transfer model developed by TU Berlin within this project. The batch-type smelting reduction reactor has a two-layered slag: an upper foamy and a lower less foamy slag. A bubble stream of (CO+H₂) gas originating from the iron oxide reduction reaction with coal in the lower slag flows upwards. The rising (CO+H₂) gas is post-combusted by three oxygen jets blown horizontally into the upper part of the slag. A flame zone, and above the flame a mixing and a bubble zone form, in which post-combustion reaction and transfer of the post-combustion heat to the slag take place. The modelling of the flame zone was the subject of a previous paper. The present report describes models of the mixing and the bubble zone and of the occurrences in the gas space above the slag. The macro-kinetics of the overall heat transfer process including slag recirculation and heat transfer from the upper foamy to the lower dense slag are presented further. The model calculations provide information about the distribution of the post-combustion and the heat transfer processes over the single zones as functions of the important internal process parameters. Further, the oxygen utilisation, the heat efficiency and the temperatures at various locations of the process are described as functions of the ratio of post-combustion oxygen flow rate to (CO+H₂) evolution rate. In all the calculations a specific gas through-put of carbon monoxide of 3 mol/t?s is assumed. This value corresponds to 510 mol/s for the assumed melt of 170 t. The model shows that heat transfer efficiencies of more than 90 % and slag temperatures of less than 1700°C are possible, if the slag circulation rate is 300 kg/s. Lower circulation rates lead to higher slag temperatures and worse heat transfer efficiencies. Controlled slag circulation is thus an important process tool.