The valorisation of steel mill off-gas via chain elongation into higher value-added products: A prospective life cycle assessment

Abstract

Nowadays, one of the greatest challenges for the steel industry is decarbonisation to decrease its pressure on the environment. For this purpose, deploying carbon capture and utilisation technologies such as syngas fermentation can be instrumental, as they allow carbon in steel mill off-gas to be converted into ethanol. However, as ethanol is of relative low value, technologies that can upgrade it towards higher-value chemicals are becoming attractive. One such a technology is chain elongation, as it can convert ethanol effluents from syngas fermentation into caproic acid, which can subsequently be used as a feedstock to produce chemicals such as plasticisers. As information regarding the environmental sustainability of chain elongation is limited, in this study, a prospective environmental hotspot analysis on chain elongation was performed, assessing climate change and resource footprint indicators, from a Life Cycle Assessment perspective, based on a theoretical scale-up. In addition, climate change impacts of a novel basic oxygen furnace gas (BOFG) to plasticiser value chain integrating the theoretically scaled-up chain elongation technology (PLAST) were assessed and compared with two other valorisation routes, considering a US and a European scenario: BOFG incineration with electricity production (ELEC) and syngas fermentation for the production of ethanol for use as a fuel (ETOH). The hotspot analysis showed that electricity and chemicals consumption, mainly K-acetate, were the largest contributors to both the climate change and resource consumption footprint. For the US scenario, the novel PLAST route showed similar climate change impacts to the ETOH route, while the impacts were 15% higher than those of the ELEC pathway. In the European scenario, with a greener electricity mix, the PLAST route showed similar impacts compared to both the ELEC and ETOH routes. Although uncertainties lie within the results, this study highlights the potential of chain elongation as a part of an alternative carbon capture and utilisation pathway for the steel industry. For this however, further technological advances are required, such as changes in process configurations and increases in caproic acid productivity, which should be the focus of future research.