Sustainability 2022 , 14 , 2619
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may guide the practitioners’ choices were described in detail and structured. By using a multicriteria decision method called the analytic hierarchy process (AHP), the practitioners’ preferences and context related perceptions were explored. More specifically, the following research questions were addressed in the study: RQ 1: What are the distinct characteristics of allocation methods that may guide the practitioner’s choice of allocation methods in biorefinery process development? RQ 2: What preferences do practitioners in wood biorefinery process development have regarding the distinct characteristics of allocation methods? RQ 3: How are these preferences based on context related perceptions induced by: • product characteristics? • type of impact (economic or environmental)? • professional background? Overall, this study was carried out to reveal another facet of the allocation issue, in particular, with regard to its application in the practical context of firms that deal with multifunctional processes. By connecting life cycle assessment (LCA) literature about allo- cation methods with empirical research on practitioners’ viewpoints, we aim to contribute to more transparent and inclusive assessment approaches and, consequently, to cleaner processes and products from biorefineries and other multi-output production systems.
2. Materials and Methods 2.1. Wood Biorefineries with Multiple Product Outputs
On a larger scale, the separation of wood components takes place in wood pulping processes. These processes have, so far, mostly been developed and optimized for the extraction of cellulose [25]. The resulting lignin is available at an estimated quantity of about 40–50 million tons per annum globally (so called technical lignin), illustrating the global significance of this byproduct [26]. It is currently used mainly on site (about 95–98%) to obtain energy and recover the process chemicals [27], but is also expected to play major roles in biorefinery concepts in the future (e.g., in various kinds of material applications) [28]. Fiber fines are the smallest fraction of fibers (i.e., they can pass through a 200-mesh screen). These fines, which are generated during the pulping, bleaching and pulp-refining processes [29], account for only 3–8% of the kraft pulp [30] and were chosen as a complementary product to lignin in this study. The fines influence the properties of paper products; however, their separation is a topic of discussion (e.g., to save bleaching costs and for material applications) [29]. Currently, fiber fines are part of the pulp and are not (yet) separated from it. If this fiber fraction were separated, this would lead to a quantitatively small amount of obtained fines as compared to the biorefinery output of lignin (as lignin accounts for approximately 18–35% of wood) [31]. 2.2. Allocation in Multifunctional Process Assessments Frischknecht (2000) defined multifunctional processes as processes that contribute to multiple product systems. Coproducts are products of a joint production process that have a relatively high total sales value, while products that only have a low sales value as compared to others are referred to as byproducts [32]. A definition by Suh et al. (2010) suggests that the product should be considered as a byproduct if an increased demand for a product in joint production does not lead to an increase in the production volume due to its limited contribution to the total revenue [33]. However, the issue of allocating, for example, costs or environmental impacts of shared production to specific products prevails in such multiple product systems. While the question of cost allocation in production processes was raised early on, the allocation of environmental impacts emerged within the context of the LCA during the first half of the 1990s (see [34]). LCA practitioners commonly need to address allocation issues, and particularly when multifunctional processes such as multi-output systems exist [19,35]. Choosing which allocation procedure to apply is one of the most extensively debated and controversial topics discussed among LCA practitioners, especially because
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