Abstract
The handling of multi-outputs in life cycle assessment (LCA) is a controversial topic in both LCA application practice and the literature. Although there is a prescribed hierarchy of how to deal with multi-outputs, which favors system expansion (including co-products and their upstream chains) as the most scientific solution over the allocation approach (dividing environmental impacts based on an allocation factor), the latter is still most common. In this study, system expansion, mass, economic, and energy-based allocation are studied within a biosystem engineering case study, which provides two marketable outputs, biogas and nano-cellulose. The global warming potential (GWP) is used as basis for comparison. There is hardly any difference between system expansion and economic allocation, but it is also shown how problematic the latter approach is for newly introduced products because the market price is still very high and not representative. Results show that the GWP with mass allocation has the lowest impact; however, this approach is inappropriate since the outputs are a solid product and biogas, which are hard to compare based on a mass basis. Overall, based on the studied approach, results can be significantly different. Hence, it is recommended to avoid comparison and interpretation of results when using different modeling approaches.