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Abstract
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.
Keywords: CO2 emissions, blast furnace, mathematical model, process simulation,
steelmaking, recycling, CO2 storage.
Introduction
In recent years, an increasing concern has been given to climate change issues.
The necessity to reduce the emissions of CO2 has become a priority in
practically all segments of industry. This has been specifically acknowledged
by the steel industry, which contributes to about 6 % of the anthropogenic
greenhouse gas emissions, mostly through CO2 from the blast furnace.
In this context, the European project ULCOS, which stands for Ultra Low CO2
Steelmaking was launched in 2004 with a target of at least halving CO2
emissions compared to current levels. To reach such high levels of CO2
mitigation, it is necessary to consider new, breakthrough routes for making
steel. One of the promising technologies retained in ULCOS consists in
recycling the top gas evolved from the blast furnace after having removed and
stored most of the CO2 contained. In the present paper we investigate the
performance of this new technology using mathematical modeling.
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