The treatment of co-products is one of the key methodological issues for most LCA studies and is especially true for the worldsteel LCI study because of the process gases, slags and many other co-products arising from the steelmaking processes. Therefore, how to account for these useful co-products and obtain the LCI of steel only is a critical issue.
There are essentially two main methods of accounting for co-products: allocation and system expansion.
‘Allocation’ consists of allocating the process in/outputs proportionally to the product and to the co-products, according to a parameter such as mass, feedstock energy or even monetary value.
For instance, if mass allocation were to be used: assuming the BOF generates 3 mass units of steel and 1 mass unit of recovered slags, three quarters of every BOF in/output is allocated to the steel and the remaining quarter to the slags.
‘System expansion’ assumes credits for co-products that render it unnecessary to produce functionally equivalent products by other means. For example, process gases that are exported and used to generate electricity are assumed to replace coal mining and combustion for electricity generation. The difference between the environmental burden associated with the coal and process gas combustion are then credited, or in some cases debited, to the steel LCI.
System expansion was applied in the worldsteel study primarily because ISO recommends the use of system expansion and the avoidance of allocation whenever possible. In addition, it is considered that there is no scientifically justifiable parameter to allocate in/outputs of the steelmaking process. In view of these and other relevant factors, worldsteel found the system expansion method to be the most scientific and fair way of accounting for most steel by-products.
Most importantly, it does not result in unfairly favourable or misrepresentative LCI results for the steel industry.