• allocation;
  • ISO 14041;
  • joint production;
  • life-cycle assessment (LCA);
  • recycling;
  • system expansion

Abstract: 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.