In another post, we explained how a steel plant can reduce its CO2 emissions by injecting hydrogen in the blast furnace (BF), as a substitute for carbon fuels. Although there is evidence of its viability in the steel-making industry, the injection of H2 at an existing BF requires investments in machinery and in safety protocols due to its high flammability. Here, we will investigate how to reduce the blast furnace's CO2 emission by using sponge iron in the mix of raw materials.
As described in the cited post, CO2 is the result of several chemical reactions in the BF. Its emission level is closely related to the reduction requirement, meaning the proportion of iron oxides, like Fe2O3 and Fe3O4, in the charge. Since sponge irons have high metallic iron content, increasing their consumption reduces the fuel requirements of the BF, hence CO2 emission, without affecting the furnace production pace.
Beyond the lower stoichiometric carbon demand, the input of one metallic iron kilogram per hot metal tonne reduces the carbon demand for the process by about 0.4 kilograms of coke equivalent per hot metal tonne. The impact of the sponge iron consumption is shown below*.
The consumption of 200 sponge iron kilograms per hot metal tonne reduced the CO2 emission rate by 0.21 t CO2/t slab (nearly 11%). It is noteworthy that this alternative has operational constraints that naturally limit the usage of sponge iron.
The first one concerns the impact of the coke layer on the furnace permeability. The safe and efficient operation of the BF requires good permeability of the charge, to ensure proper gas flow inside the furnace. The coke charged in the BF positively contributes to keeping permeability at good levels. Therefore, even if we lower the need of carbon for reduction, the BF would still need coke to ensure permeability.
The other constraint concerns sponge iron smelting. This process requires heat, which demands more fuel to burn. At some point, the BF can no longer sustain the increase of sponge iron and fuel charge.
Finally, the sponge iron optimal consumption rate is subject to other economical and operational factors like material composition and price, environmental penalties and goals, and hot metal specifications. Those trade-offs are complex and the support of mathematical models for decision-making is key to tackle such issues.
* Note: data generated using Cassotis' Integrated Steel Plant model of a standard plant based on BF-BOF route
Co-author: Emmanuel Marchal - Managing Partner at Cassotis Consulting