2. Materials and Methods
2.1. The Reason for Carrying Out the Study
2.2. The Perspective of the LCA Practitioner
2.3. The Functional Unit
2.4. The Intended Application of the Results
- Quantifying the impacts of a product or service, in order to use these values in another LCA study in which this product or service is used. Furthermore, quantified impacts can be used for the communication of environmental impacts, for example via environmental labeling, or periodic monitoring.
- Identifying opportunities to improve the environmental performance of the product or service, for example via innovation. For this type of application, a contribution analysis, or hotspot analysis aids in identifying the products and processes that contribute most to the environmental impacts of the functional unit.
- Making a decision to select between multiple alternative options. In the end, most LCA studies have the purpose to compare two alternative products or services that provide the same function. Furthermore, it is interesting to compare the impacts of a product before and after the application of an adjustment that is supposed to reduce the products’ environmental impacts.
2.5. Relationship between the Elements of the Goal and Scope
3.1. Updated Framework for Goal-Dependent Allocation
3.2. Archetypes of Goal and Scope Definitions
4.1. Untangling System Expansion, Substitution, and the Cut-Off Approach
- Questions 1 and 2 of Table 3 cannot be answered via the operation of subtracting functions instead of the addition of functions.
- Equivalence between system expansion and substitution is only achieved for question 3, and only with regard to the absolute difference in environmental impacts (e.g., “the alternative system is accountable for 20 kg CO2-eq more than the system under study”). Any other comparison between the two product systems, such as percentages of improvements, is distorted due to the subtraction of functions instead of the addition of functions.
- The results can be easily misinterpreted by the suggestion that the alternative production routes are substituted, while in an attributional process-oriented LCA, a comparison between two product systems can be done for the mere purpose of benchmarking.
- Keeping the functional unit as “1 kg of recycled yttrium” suggests that a product-oriented LCA is conducted. Results are easily used for downstream studies in which yttrium is used, while the results do not refer to the cradle-to-gate impacts attributed to yttrium, as the applied approach is a process-oriented LCA.
- System expansion: Including the additional functions related to the co-products in the functional unit.
- Substitution: Allocation of inputs and outputs between the products and functions in a way that reflects economic relationships.
- In an attributional product-oriented LCA, when partitioning is applied, it could appear that the co-product or recycled product has a very low mass (for mass allocation) or economic revenue (for economic allocation) compared to the other co-products of the system. In that case, the allocation factor for the low-value product could approach, or be, 0%. With an allocation factor of 0%, the product becomes burden free, which is in line with the cut-off approach . However, the cut-off approach is not appropriate when the allocation factor is low in an economic allocation due to the fact that the co-product, or recycled product, is produced in a low quantity, while the unit price is relatively high, as in this case the LCI per unit of product is non-negligible.
- In a consequential product-oriented or process-oriented LCA (questions 10–15), it is possible that the flows which are modeled by substitution generate relatively low impacts. In that case, it could be argued that the environmental consequences due to the multifunctional flow are negligible and are cut-off.
- Alternatively, if an LCA practitioner argues that the phosphorous powder is burden-free, because it is a waste, he/she might aim to conduct a process-oriented LCA where the flow of ingoing waste is a flow of interest. Instead of referring to this flow as “burden-free”, the flow should be included in the functional unit. As shown in Figure 3, functional flows do not “carry” an impact, as the overall impact of these flows is the subject of analysis.
4.2. Perspectives for the Use of Attributional and Consequential Approaches in Sustainability Assessments
Conflicts of Interest
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|Item of the Goal and Scope Definition||Parameters of Interest||Integration into Table 2||LCA Modeling Approach|
|The reason for carrying out the study||The production/treatment||α||Process-oriented LCA|
|The consumption||Product-oriented LCA|
|The perspective of the LCA||Accountability for impacts||β||Attributional LCA|
|Consequences on global impacts||Consequential LCA|
|The functional unit||The subject of the LCA||γ||Partitioning/Substitution|
|The subject of the LCA and additional functions||System expansion|
|Intended Application of the Results||Research Question|
|Quantifying environmental impacts||What is/are the β of α of γ?|
|Identifying opportunities for improvement||How can we decrease the β of α of γ?|
|Decision-making||Does α of γ have (a) lower β than its alternative?|
|LCA Approach||Functional Unit||Research Question||Modeling Specifics|
|Attributional process-oriented LCA||The treatment of 10.8 kg of phosphorous powder, and the production of 1 kg of yttrium, 2.2 kg of glass, 0.2 kg of lanthanum, 0.1 kg of cerium, 0.1 kg of terbium, and 0.1 kg of europium||Only the LCI of the foreground subsystem is calculated based on attributional background data. No allocation is necessary within the foreground subsystem, as system expansion is applied (Figure 3).|
|Attributional product-oriented LCA||The consumption of 1 kg of recycled yttrium||Partitioning must be applied to identify the cradle-to-gate inventory that is attributed to recycled yttrium. More information is required from the product system that supplies the phosphorous powder, as the recycling process is part of this product system (Figure 4).|
|Consequential process-oriented LCA||The treatment of 10.8 kg of phosphorous powder, and the production of 1 kg of yttrium, 2.2 kg of glass, 0.2 kg of lanthanum, 0.1 kg of cerium, 0.1 kg of terbium, and 0.1 kg of europium||Only the LCI of the foreground subsystem is calculated based on consequential background data. No allocation is necessary within the foreground subsystem, as system expansion is applied (Figure 3).|
|Consequential process-oriented LCA||The production of 1 kg of recycled yttrium||The LCI of the foreground subsystem is calculated based on consequential background data. The ingoing flow of phosphorous powder is modeled by the substitution of its marginal waste treatment process. The outgoing flows of glass, lanthanum, cerium, terbium, and europium are modeled by the substitution by their marginal users (Figure 4).|
|Consequential product-oriented LCA||The consumption of 1 kg of recycled yttrium||If the supply of recycled yttrium is constrained, the LCI represents the substitution of recycled yttrium by its marginal user. If the supply is unconstrained, the LCI of the foreground subsystem is calculated. The ingoing flow of phosphorous powder is modeled by the substitution of its marginal waste treatment process. The outgoing flows of glass, lanthanum, cerium, terbium, and europium are modeled by the substitution by their marginal users (Figure 5).|
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