Metallurgical plants are plants for processing metal ore, wherein the central element of such a metallurgical plant is a blast furnace. These metallurgical plants have been known for a long time. A blast furnace is fed with raw materials which comprise metal ore, additives and heating material. Usually coal or coke is used as a heating material, wherein coal and coke produce heat by burning in the presence of air on the one hand and wherein coal and coke also function as reduction agent for the metal ore, as the metal ore is basically comprised of metal oxide. When reducing metal ore in a blast furnace, various gases are produced, which collectively are known as a furnace gas or flue gas. Said furnace gas usually contains a substantial amount of carbon dioxide (CO2). Carbon dioxide is a greenhouse gas, and during recent years more and more effort has been made to avoid or convert greenhouse gases, as these greenhouse gases are regarded as detrimental for the climate.
In the field of metal production, it is a general aim to use as few raw materials and heating materials as possible, as these materials are expensive and it is expensive to transport these materials. Much effort has been made to reduce the amount of coke/coal used in production. One approach was blowing coal dust into the blast furnace, and another approach was producing carbon monoxide as a reduction gas, either in the blast furnace itself or in a separate gasification reactor outside the blast furnace. From EP 09318401 A1 it is known to blow a portion of the carbon required for reducing the metal ore into the blast furnace in form of a substitute reduction material. In this sense, e.g. natural gas, heavy oil, fine coal and similar material having a high carbon content may be used as a substitute reduction material. These materials may be directly blown into the blast furnace shaft or may be gasified outside of the blast furnace shaft in a separate gasification reactor so as to form a reduction gas. Subsequently, such a reaction gas may be directed into the blast furnace shaft. The method known from EP 09318401 A1 may provide a possibility to reduce the consumed amount of coal or coke and may also provide a possibility to use materials, which are difficult to process as a substitute reduction material, however, the problem of high CO2 production in metal production has not been solved.
The prior art discloses methods wherein furnace gas or a particular component thereof is directed out of the blast furnace shaft and, after being processed in a CO2 converter, is re-directed into the blast furnace shaft. EP 2 543 743 A1 discloses a method wherein furnace gas is directed out of the blast furnace shaft and is directed to a separation device in which CO and CO2 are separated. Only the separated CO2 is subjected to reforming in a CO2 converter. Reforming produces mainly CO and H2O, wherein H2O is separated and CO is directed into the blast furnace shaft. WO 2011/087036 A1 also discloses a method wherein first furnace gas is directed to a separation device in which CO and CO2 are separated. In a CO2 converter, the CO2 is converted into O2 and CO. The CO from said conversion and the previously separated CO are jointly directed into the blast furnace shaft. U.S. Pat. No. 3,909,446 A discloses a method wherein furnace gas from a blast furnace shaft is mixed with coke oven gas in a CO2 converter. Thus, a gas mixture comprising CO and H2 is produced which is ro-directed into the blast furnace shaft. WO 2010/049536 A1 describes a similar method wherein also carbon containing particles are re-directed into the blast furnace shaft. U.S. Pat. No. 2,598,735 A discloses a method wherein furnace gas from a blast furnace shaft is mixed with carbon/coal and oxygen in a gas generator. A portion of the carbon is burnt in presence of the oxygen, and another portion of the carbon reduces the CO2 from the furnace gas and the CO2 from the burnt carbon to CO. Said CO is re-directed into the blast furnace shaft as a reducing agent. None of these documents discloses a method wherein further processing of a portion of the converted CO is carried out.