With the increasing stakeholder pressure for lower carbon footprint practices, all businesses are searching for economically viable technologies that can bridge the current emission gap to a greener future. This transition is harder for companies in traditional process industries, which commonly deal with high competitiveness in a global market, like steel making. This post quantitatively shows the environmental impacts of an alternative technology in the classic blast furnace (BF) - basic oxygen furnace (BOF) route.
For many years, the industry invested in the research and development of technologies that could meet ESG goals while guaranteeing market competitiveness. It has been well documented that an integrated steel plant based on the BF-BOF route has emission levels of around 1.7 to 1.9 tonnes of CO2 per tonne of slab produced. By nature, the reduction processes (the blast furnace in particular) are responsible for the majority of emissions, generating CO2 in the process itself or producing an energetic gas that will generate CO2 in other processes where it will be burnt. Considering this logic, see below the breakdown of the CO2 emissions by process.
It is important to understand how the blast furnace process generates CO2 to tackle the emission causes. The reduction reactions of iron oxides by CO and H2 produce metallic iron, CO2, and H2O, respectively, as described in the equations below.
Fe2O3+3H2→2Fe +3H2O (g)
Fe2O3 +3CO→2Fe +3CO2 (g)
Both reactions are occurring simultaneously in the process. However, when charging the BF with coke and coal, the reduction reaction based on CO happens in a higher proportion, due to the composition of the solid fuels, generating more CO2. The alternative to reduce the generation would be to favor the reduction of iron by hydrogen, which would generate water instead.
This can be done by injecting hydrogen gas (H2) in the blast furnace. Empirical tests showed that each kilogram of H2 consumed replaced around 3.9 kilograms of coke. This replacement concept is known in the industry as coke equivalence and all blast furnace fuels, like coals, charcoals, and natural gas, have their own values. The images below show the impact of this technology on fuel consumption* and consequently on CO2 emission. The optimization suggests that 10 kg of H2 per tonne of hot metal (t HM) reduces the CO2 emission by 0.123 t CO2 per t of slab.
Additionally, the consumption of hydrogen has other operational impacts, which cannot be neglected. The main impacts are on the reduction yields of CO and H2, on the adiabatic flame temperature (AFT), and on the blast furnace productivity. The increase in H2 injection increases the reduction yield of CO (which means that the carbon is better used) as well as the blast furnace productivity. However, it reduces the H2 reduction yield (the proportion of reaction involving H2 actually compensates this), the AFT, and the permeability (due to the reduction of coke consumption). The last two are limiting factors of H2 injection. Check the numerical impacts:
Finally, it is important to review its economical impact. Although it has many operational and environmental benefits, H2 gas can be very expensive to acquire and its specific consumption can fluctuate from case to case. The figure below shows how the specific consumption of H2 changes according to its price in two different conditions: with or without emission penalties. It is clear that the viability of hydrogen consumption is closely related to the penalties and prices paid, but the chart shows that the trade-offs occur in a stepwise behavior that is not linear, and, especially, not trivial.
At a price of 22,700 eur/kg of H2, we observed an increase of 11.75 eur/t of slab in the production cost by injecting 10 kg/t HM.
The H2 injection is a great example of all the aspects that need to be considered before changing technologies. Beyond the environmental effect, every alternative to reduce emissions must estimate the collateral economical and operational impact in all the production chain.
* Note: All 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