Introduction to Life-Cycle Assessment and Decision Making Applied to Forest Biomaterials 157
the technology used is representative of the technology used to produce the product
under study.
5.3.2.4 Coproduct Treatment – Allocation
Some production processes produce more than one product, and the emissions of the
process cannot be easily attributed to a single product from the process. Coproduct
treatment methods as defined by ISO 14044 are used to properly account for emissions
from the production of multiple products. The ISO standard states that wherever
possible, allocation should be avoided by
- dividing the unit process to be allocated into two or more subprocesses and collecting
the input and output data related to these subprocesses;
- expanding the product system to include the additional functions related to the
coproducts.
If allocation cannot be avoided by these two methods, the allocation method should
- partition inputs and outputs of the system between its different products or functions
in a way that reflects the underlying physical relationships between them;
- partition input and output data between coproducts in proportion to the economic
value of the products.
The first route to avoiding allocation by process subdivision can remove the need
to account for multiple products from the overall system by splitting it into additional
subprocesses. With a more detailed process flow diagram and data, some products may
be produced through separate subprocess and thus can be accounted for individually.
Process subdivision is not always possible and expanding the product system may
be necessary to account for the coproducts. Using this method, all impacts associated
with production are assigned to the primary product of interest and a credit or negative
emission is used to account for the displacement of the coproduct production in other
manufacturing processes. The system expansion method works only when the manu-
facturing process is not the primary route to the coproduct. The life-cycle handbook
(Curran, 2012) gives the example of a hydrocracking unit that produces ethylene,
propylene, other hydrocarbons, fuel gas, and heat. In this example, energy and gas can
be accounted for using system expansion by giving a displacement credit. However,
system expansion does not work on the other products, as hydrocracking is the primary
commercial route to these products. The other products of the hydrocracking process
are accounted for using allocation.
When allocation is to be performed, physical parameters and relationships should be
used to attribute the total impacts to individual products. Such parameters may include
mass of final product, raw material ratio required to produce the final products, energy
content of products, or other physical relationships. If a physical relationship cannot
be determined, economic allocation can be performed by attributing the impacts in
accordance with the revenue associated with each product. For instance, in a process
producing products A, B, and C, if product A generates 98% of the revenue from a
process, 98% of the total impacts would be assigned to product A. Economic allocation
should be avoided as product prices can change and thus changes the overall LCA
results even when the production process and overall emissions remain constant.
