Techno-economics of strategies to valorise off-streams from the iron and steel industry

The iron and steel industry ranks among the largest contributors to industrial carbon emissions, posing significant challenges to environmental sustainability. Carbon capture technologies are constantly improving. However, the necessity of infrastructures for transporting and storing compressed gas poses a significant challenge in CO2 sequestration. This thesis aims to analyse a possible solution to valorise the CO2-rich off-streams from the iron and steel industry. The solution consists of the methanation of CO2 and the subsequent pyrolysis of CH4 to obtain solid carbon. Carbon component inside the blast furnace gases is captured in such a way as to be stored or reutilised inside a blast furnace as carbon coke. The overall process balances and layouts are evaluated using the Aspen Plus® process simulator, focusing on heat integration among process units and streams. The process was designed for a medium-scale steel manufacturing facility producing 3.242 Mthm/yr. The process can produce 239.1 kg/thm of solid carbon, needing a make-up of 1,661 Nm3/thm of H2. An economic analysis is introduced to assess the schemes’ profitability through different economic indices, particularly the effect of different locations and raw material costs. The process is studied considering an internal return rate IRR of 25% and a discounted payback period DPBP of 6.19 years. Considering current technologies, the carbon selling price would be 5.270 $/kg, making the solution not profitable when compared to current market prices. Considering that future improvements in hydrolysis technologies could reduce the H2 cost, and carbon taxation could incentivise low-carbon emitting solutions, the process would be able to produce solid carbon at 1.184 $/kg, deeming the solution profitable in potential future scenarios. Considering only the CO2 methanation decoupled from the pyrolysis section to produce CH4 from CO2 and H2 is not profitable since CH4 produced this way can be sold in the most optimistic scenario at 0.503 $/Nm3, which is higher than the current selling price of CH4 in Europe and United States.