Assessment of Top Gas Recycling Blast Furnace: A Technology To Reduce CO2 Emissions in the Steelmaking Industry

摘要

We present a new mathematical model for simulating the operation of a blast furnace with top gas recycling. The model of the blast furnace, a physical-chemical model, was built using commercial process flowsheeting software. All of the important reactions and processes were taken into account, e.g. iron oxide reduction, coke and coal combustion, coal gasification, heat transfer. Coke and coal are the fossil solid fuels used in blast furnaces, the former being made by baking the latter. Steel industry contributes to about 6% of the anthropogenic greenhouse gas emissions, mostly through CO2. Reducing CO2 emission has become a priority in steel industry, as exemplified by the European ULCOS program, which targeted achieving mid-term >50% reduction through the use of new technologies. Recycling the exhaust top gas from blast furnace, the principal and most CO2 emitting steelmaking reactor, is one of the promising technologies selected by ULCOS. In a top gas recycling blast furnace, CO2 contained in the top gas is removed and the remaining stream, rich in reducing agents H2 and CO, is heated and re-injected into the blast furnace at two levels, the shaft and tuyeres, at different temperatures and flow rates. Captured CO2 is then piped to be stored geologically. We simulated different operations of the blast furnace, without top gas recycling and with recycling at one level (tuyeres) and at two levels (tuyeres and shaft). The higher the recycled flowrate, the lower the coke consumption. Up to 25% carbon (coke + coal) saving can be obtained with 90% recycling. These simulations were found to be in good agreement with reported data from a pilot blast furnace in Lulea, Sweden. By using top gas recycling coupled with the storage of CO2, the blast furnace CO2 emissions could be reduced by 75%. Besides, the model developed provides us with a full inventory of the flows, which respects mass and heat balances. The next step is to use these results as the inventory for life cycle assessment to evaluate the global environmental impact of the new process in different configurations.