two main choices: allocation of impacts among the co-products or
system expansion [8].
When allocation is performed—the impact of the system is
distributed among the multiple products—the LCA is called attributional. Priority should be given to allocation according to physical relations between the co-products, e.g., mass or energy content.
When this is not possible, other relations can be used, e.g., economic value.
Mass allocation requires impacts distribution according to the
mass of the products. Following the biorefinery example (Fig. 4),
fertilizers have the highest mass (due to high water content) and
receive 91% of the impacts, while bioethanol and protein receive 7%
and 1%, respectively (Table 2).
Energy allocation considers the energy content of the products.
In this case, bioethanol has the highest energy content and receives
88% of the impacts, protein 12%, and fertilizer 0%, since this is
mostly water and nutrients (Table 2).
Economic allocation considers the economic value of the products. Bioethanol has a lower price than proteins, but given the
higher production, he receives the 78% of the impacts while protein
the 22% (Table 2). Fertilizers have no commercial value, therefore
no impacts are allocated to it.
Whenever possible, the practitioner should use the system
expansion approach [8]. In the biorefinery example, when
seaweed-based fertilizer and protein enter the market, a decrease
in production of mineral fertilizers and soy proteins occurs, i.e.,
substituted products (Fig. 4). In the LCA calculation, the substituted product has a mathematically negative value on the total
impact. Therefore, the impact of bioethanol is equal to the total
impact of the system minus the avoided impact of the production of
soy protein and mineral fertilizer (Table 2).
Fig. 4 The diagram shows the system boundaries when performing allocation (unbroken line) or system
expansion (dashed line)
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system expansion [8].
When allocation is performed—the impact of the system is
distributed among the multiple products—the LCA is called attributional. Priority should be given to allocation according to physical relations between the co-products, e.g., mass or energy content.
When this is not possible, other relations can be used, e.g., economic value.
Mass allocation requires impacts distribution according to the
mass of the products. Following the biorefinery example (Fig. 4),
fertilizers have the highest mass (due to high water content) and
receive 91% of the impacts, while bioethanol and protein receive 7%
and 1%, respectively (Table 2).
Energy allocation considers the energy content of the products.
In this case, bioethanol has the highest energy content and receives
88% of the impacts, protein 12%, and fertilizer 0%, since this is
mostly water and nutrients (Table 2).
Economic allocation considers the economic value of the products. Bioethanol has a lower price than proteins, but given the
higher production, he receives the 78% of the impacts while protein
the 22% (Table 2). Fertilizers have no commercial value, therefore
no impacts are allocated to it.
Whenever possible, the practitioner should use the system
expansion approach [8]. In the biorefinery example, when
seaweed-based fertilizer and protein enter the market, a decrease
in production of mineral fertilizers and soy proteins occurs, i.e.,
substituted products (Fig. 4). In the LCA calculation, the substituted product has a mathematically negative value on the total
impact. Therefore, the impact of bioethanol is equal to the total
impact of the system minus the avoided impact of the production of
soy protein and mineral fertilizer (Table 2).
Fig. 4 The diagram shows the system boundaries when performing allocation (unbroken line) or system
expansion (dashed line)
Life Cycle Assessment
113