Also see the addendum containing a further simplification of the presented rules for avoiding allocation.
In a life-cycle assessment (LCA) involving only one of several products from the same process, how are the resource consumption and the emissions associated with this process to be partitioned and distributed over these co-products? This is the central question in co-product allocation, which has been one of the most controversial issues in the development of the methodology for life-cycle assessment, as it may significantly influence or even determine the result of the assessments. In this article, it is shown that in prospective life-cycle assessments, co-product allocation can always be avoided by system expansion. Through a number of examples, it is demonstrated how system expansion is performed, with special emphasis on issues that earlier have been a focus of the allocation debate, such as joint production (e.g., of chlorine and sodium hydroxide, zinc and heavy metals, and electricity and heat), the handling of “near-to-waste” by-products, processes simultaneously supplying services to multiple product systems, and credits for material recycling and downcycling. It is shown that all the different co-product situations can be covered by the same theoretical model and the same practical procedure, and that it is also possible to include the traditional co-product allocation as a special case of the presented procedure. The uncertainty aspects of the presented procedure are discussed. A comparison is made with the procedure of ISO 14041, “Life-cycle assessment—Goal and scope definition and inventory analysis,” the international standard.
System expansion addresses co-products by modelling the additional functions they provide, rather than dividing one process’s environmental exchanges between products using an allocation key. The product system is broadened to include the alternative production that would provide the co-product’s function; the corresponding inputs and emissions can then be accounted for in the comparison. This makes the model reflect the functions and processes relevant to the assessment. Weidema discusses examples such as joint production of chlorine and sodium hydroxide, electricity and heat, and credits for recycling and downcycling. The paper presents a common theoretical model and practical procedure for these situations, while also showing how traditional allocation can be treated as a special case.
