Application of Model Predictive Control in a Dynamic System: An Application to BOF Steelmaking Process

摘要

The mechanism and the kinetics of oxidation reactions in the BOF are very intriguing and still not clearly understood. Prior to this work, a slag-droplet model based on multi-component mixed transport control theory was proposed in which it was assumed that the metal droplets ejected from the jet impact zone carry a thin layer of FeO on the surface and that the droplets were the main mechanism of transporting FeO to the top slag. Calcined lime was assumed to dissolve in the bulk slag through a normal process of formation of dicalcium silicate around lime particles, as also proven by laboratory experiments. In a subsequent work, the examination of morphology of the skull formed on the lance surface showed that the spherical droplets rich in FeO (>90% FeO) are ejected from the jet impact zone and they dissolve CaO and MgO (up to approximately 10 mass%). Since it is now confirmed that FeO rich droplets are ejected from the jet impact zone, it becomes necessary to consider the possibility of decarburization occurring in the region directly below the jet impact area. It is difficult to verify the contribution of decarburization occurring in the zone directly below the area of jet impact from that occurring in the ejected metal droplets and the decarburization taking place at the bulk slag-metal interface. If the decarburization must occur in the region directly below the jet impact zone then the contribution of droplets to the decarburization (after being ejected from a decarburized volume or surface) will be much smaller because they will be ejected from a region which is already decarburized. Also because of the presence of FeO on the surface of droplets the carbon content of metal on the surface of the droplets will be minimal (even though the bulk droplet may have a higher carbon content). In this paper, an attempt has been made to develop a model to understand the mechanism of oxidation reactions in BOF and predict the relative contributions of different possible mechanisms of decarburization, specially in the region of the turbulent zone(s) (the six different zones inside the bulk metal in the case of a six-nozzle tip which are termed in this paper as "reaction zone"). The dynamic reaction zone model thus developed is equally and more effectively successful in explaining and accounting for almost all the practically observed features of the BOF process including: the dynamic changes in the FeO content of slag, decarburization rate in different periods of a blow, evolution of slag composition, pattern of evolution of waste gas flow rate, effect of scrap size and retained slag on dynamic changes in metal and slag composition as well as on decarburization rate, and the evolution of bath temperature profile. One can estimate the reaction zone temperature. The reaction zone concept is also able to demonstrate how the limited oxygen supply in the reaction zone makes it appear as if the decarburization is a zero order reaction in the main blow period.