Attribution of Global Warming Potential impacts in a multifunctional metals industry system using different system expansion and allocation methodologies

Abstract

Abstract Purpose In order to reach a more circular economy, materials previously classified as waste can be upgraded and turned into valuable co-products, with associated environmental benefits. The generation of co-products raises many questions around the multifunctionality issue from a life cycle perspective. This article explores the attribution of Global Warming Potential (GWP) impacts for an ironmaking process, HIsarna, which additionally produces two co-products: zinc-rich process dust and slag, suitable for the zinc and cement sectors, respectively. Methods A wide range of LCA allocation methodologies are applied to attribute impacts between the main product, hot metal, and the two co-products. These include system expansion, physical allocation, economic allocation and zero burden allocation. Each method attributes a different GWP to each co-product. Additionally, different perspectives are explored to consider the most suitable methods according to the co-product user and the co-product producer. For instance, it might be in the co-product user’s interest that the co-product GWP was minimised, and lower than other material inputs performing a similar function. Conversely, the co-product producer may be incentivised to lower its primary product’s GWP by attributing the greatest possible burden to the co-products. Results and discussion The GWP impacts for zinc-rich process dust range from 0 to 3.71 kg CO2 eq. per kg. At the higher end, the GWP of zinc-rich dust would be higher than that of primary zinc concentrate. A similarly wide range is applicable for slag, 0 to 1.27 kg CO2 eq. per kg. This impacts the final GWP applied to HIsarna hot metal, which has an initial GWP of 1.72 kg CO2 eq. per kg but could decrease to 1.17 kg CO2 eq. per kg depending on the allocation methods employed. This would be a substantial reduction of over 30%, larger than many decarbonisation options that are predicted to provide. This scenario would also heavily burden the co-products and could be in conflict with interests of a co-product user seeking to utilise low emissions feedstocks as part of a decarbonisation strategy. Conclusions The reduction in GWP impact attributed to hot metal with the different approaches highlights the relevance of harmonizing the allocation methods used for co-products. The appropriateness of each of the approaches for attributing GWP impacts has been explored, offering insights as to how the benefits of such systems could be assessed and attributed in the future as circularity strategies and valuable co-products become more prevalent.