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Blast furnaces are an indispensable part of the iron industry. After iron ore is mined, it goes directly to the furnace to be smelted, or refined. But blast furnaces produce an incredible amount of carbon dioxide. A recent study showed that 252.5 million tons of CO2 was produced in the period from 2005-2008. Putting it another way, blast furnaces produce between four and seven percent of all the man-made CO2 in the world. This is an effect that cannot be ignored. But before entering that topic, let’s take a look at the purpose of blast furnaces.
Iron isn’t mined as pure nugget. Iron ore contains iron oxides such as hematite, goethite, limonite, and siderite with differing chemical composition. This mix of oxides is sent to the furnace to be smelted. Blast furnaces work by using carbon monoxide to remove the oxygen from the iron. The oxygen bonds with the carbon monoxide and is released as carbon dioxide.
ULCOS = Ultra Low CO2 Steelmaking
The ULCOS project is being carried out by a consortium of 48 European companies and organizations from 15 European countries. Its aim is to cooperate in finding methods and workflow steps which can reduce CO2 emissions from steelmaking facilities and other best routes by at least 50%. Scientists have come up with some ways to reduce the CO2 emission of the furnaces. One of these methods is to use hydrogen instead of carbon monoxide to extract the oxygen: hydrogen, when bonded with oxygen, produces non-toxic water. A few other procedures are listed below.
Top Gas Recycling
Top gas recycling involves recycling the hydrogen and CO in the waste gas, or off-gas, back into the furnace. This procedure also uses oxygen instead of hot air in the furnace chamber: this decreases the concentration of nitrogen in the off-gas.
Direct Iron Smelting Reduction
Direct Iron Smelting Reduction, or DIOS, is a procedure developed by the Japan Iron and Steel Federation in collaboration with eight other steelmakers and the Coal Utilisation Centre. This procedure involves the use of two fluidized bed reactors which are placed in series. The first of these preheats the iron ore, and the second pre-reduces it to 15-25%. Fines from the gases, off-gas, and dust removed from the smelter, are injected back into the smelter. A small amount of coal fines, about 50 kilograms per ton of molten metal, is mixed with the off-gas before it is injected back into the smelter. This cools it down as well as providing additional carbon monoxide and hydrogen for the pre-reduction process in the first fluidized bed furnace.
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The Corex Process
The Corex process was invented by Siemens VAI. It consists of two main parts: the reduction shaft and the melter-gasifier.
In the reduction shaft, the iron ore is mixed with limestone and dolomite additives and reduced into 95% direct reduced iron, or DRI. The DRI is then channelled into the melter-gasifier by six discharge screws.
The melter-gasifier has three stages: the gaseous free board zone, the char bed, and the hearth zone. First, the melter-gasifier creates reducing gas by gasifying fuel coal with oxygen and then cooling it. This produces reducing gas (carbon monoxide and hydrogen), which is then channeled to the reduction shaft. After the reduction process, the direct reduced iron is redirected to the char bed, where the iron and slag are melted.
Any byproducts from the creation of reducing gas are captured in the slag. This mixture is then channeled to the final chamber, the hearth zone. The heat of the melter-gasifier keeps the phenol concentration low, which keeps it out of the the atmosphere. Any remaining hot gas is channeled into the reduction shaft as Corex export gas, which is used to control the pressure in the plant. Many of the gases resulting from this process can be recycled or used to produce electricity. Any dust particles in the gas are recycled by four dust burners in the melter-gasifier.
Corex-Related Processes
Other similar processes include the CCF, AISI, and HISmelt operations.
In the CCF process, lump iron ore or fines are fed into the converter with lump coal and flux agents, while fine coal, O2 and air are injected at the top for submerged combustion. The oxidation is controlled by adjusting the fuel , air and coal ratios. A single reactor is used.
In the HISMELT process, coal injection is from the bottom and the carbon dissolves quickly, to form carbon monoxide and iron on reacting with the oxygen from the ore, in an endothermic process energized by the reaction of carbon monoxide with oxygen from the air injected from the top. The hot gas from the reaction is used to preheat and pre-reduce incoming iron ore.
The smelting reduction (HIsarna) technology uses very little coke, instead using materials such as biomass, for instance, for smelting. This reduces CO2 emissions. Charcoal injection may be the best substitute for fossil fuels, and along with torrefied wood and bioSNG, it has a moderate carbon footprint but high energy release. It is more costly than coal, but the use of bioreducers is quite feasible when compared to having to use other methods of eliminating carbon dioxide emissions. Biomass availability is another issue which seems to be resolvable in many parts of the world where steelmaking is carried on. Recycling the off-products and manufacturing other useful byproducts from the biomass after use in the furnace are other ways to improve the economic feasibility.
Iron Ore Electrolysis
Here iron and oxygen are produced by electrolysis of iron ore, and thus CO2 emission is zero. This is, however, among the least developed low-carbon dioxide technologies.
Conclusion
These are some of the ways by which CO2 emissions from blast furnaces can be reduced. Hopefully, most of these procedures and techniques will be implemented on a large scale by 2020. Considering how essential iron and steel are to our very way of life, it is surely just as essential that in the process of obtaining these materials, we do not sacrifice the safety of our environment.
Sources and Further Reading
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