Blast Furnace Gas Based Combustion Systems in Steel Reheating Furnaces

Abstract

The usage of steelmaking process gases in thermoprocessing plants is the most efficient way to reduce the natural gas dependence and therefore both fuel costs and carbon footprint in steelworks. Furthermore NOx emissions can be cut by firing lean gases considering their low adiabatic flame temperatures. A European funded project aiming to enhance the usage of blast furnace gas (BFG) in steel reheating furnaces has been successfully accomplished by a multidisciplinary international consortium of research centers (ArcelorMittal Global R&D Asturias and Maizières, Centro Sviluppo Materiali, Swerea MEFOS, VDEh-Betriebsforschungsinstitut), burner suppliers (Tenova, AGA Linde) and end-users (ArcelorMittal). Three innovative preheated fuel gas burner technologies, namely double regenerative air-fuel, oxy-fuel and flat-flame burners, have been designed and manufactured for 100% BFG firing, so that the inherent constraints of burning very lean gases have been overcome. On the one hand, flameless oxy-fuel combustion with central preheated BFG, so that capital expenditures are limited, allowing easily furnaces retrofitting and potentially carbon capture. On the other hand, for air burners a dual honeycomb regenerator that preheats both air and BFG streams at the burner level, so that operating costs are reduced. Finally, an oxy-regenerative flat-flame burner that transfers homogeneously radiant heat to the load, combining the two beforehand mentioned technologies that are used mainly for high capacity burners (higher than 0.5-1 MW). These systems have been tested (long-term) at both pilot plant and industrial scale in order to define the guidelines for a safe application in the industrial environment, to address technical and economic issues and to put forward guidelines for retrofitting existing furnaces. Moreover, computational fluid dynamics (CFD)modelling has been carried out and validated with the pilot testing results, so that a numerical set-up has been defined for BFG firing. For this purpose a new radiation model for the radiative properties of the products of combustion of BFG has been developed, considering the low water vapor - carbon dioxide ratio. By means of preheating BFG with the waste heat content in flue gases stream, the typical operating temperatures of reheating furnaces (1350ºC) have been achieved without natural gas enrichment, keeping the NOx emissions level below the European regulation threshold.