Sustainability 2022 , 14 , 2619
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it can have a significant impact on the outcome of an LCA study [19,36]. Even though the allocation methods are applied from both environmental and economic perspectives, their application has not often been discussed from both perspectives in the scientific literature. The economic and environmental burdens of joint production processes can be categorized by allocation method [13,19]. Two major procedures are mentioned in the scientific literature, namely, system expansion and partitioning methods [13]. System expansion refers to the extension of the initial production system boundaries to include a possible alternative production of the co- and byproducts in question (e.g., [19]). Regarding biorefinery co- and byproducts, the production of fossil alternatives can be used as reference products (e.g., biofuel versus conventional fuel). Figure 1 illustrates the idea behind system expansion, contrasting the defined main product of the production system and the coproducts with the avoided production of their reference product.
Figure 1. Schematic illustration of system expansion with respect to a joint production process, including one main product and various coproducts, as well as the potentially avoided production of substitutes for the coproducts (adapted from [19]). When allocation is based on certain characteristics of the resulting co- and byproducts, this allocation is called partitioning [13]. Criteria frequently used for the partitioning method are the mass, volume, energy, exergy, or economic measures of the co- and byprod- ucts (and combinations thereof) [19]. Figure 2 illustrates how partitioning can be applied to define the share of the impacts of each co-/byproduct in the production process, based on the energy content.
Figure2. Schematic illustration showing how impacts are partitioned, based on the product energy content in a joint production process that includes three products (adapted from [19]). Ekvall and Finnveden (2001) cited examples in which partitioning offers a better solution than system expansion and vice versa [37]. Heijungs and Guinee (2007) stated that system expansion is based on too many assumptions and, therefore, its usefulness as a scientific tool is debatable [38]. Weidema (2000), on the other hand, preferred sys- tem expansion over partitioning as an allocation method [17]. Guidelines for allocation procedures in multi-output processes exist [39,40], but in many contexts, such as those
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