Page 138 - Materials Chemistry, Second Edition

9 Life Cycle Inventory Analysis 123

System Expansion
System expansion is second in the ISO hierarchy. As explained in Sect. 8.5.2,
system expansion is mathematically identical to crediting the studied product sys-
tem with the avoided production of the secondary function(s) that would alterna-
tively have been produced and delivered somewhere else in the technosphere.
When modelling a life cycle inventory, the technique used to perform crediting
varies between LCA software (see Sect. 9.5). The identification of avoided pro-
cesses depends on the decision context. For Situation A and C1 a market mix is
used, which corresponds to the average process used to supply the entire market
(see Sect. 8.5.4). To calculate a market mix, one needs to know the amount of
product or service that is produced and delivered to the relevant market by each
process at the time when the secondary function is delivered by the studied product
system (see Fig. 8.13). So, for example, if recycled steel is a co-product of a studied
life cycle and the two processes for producing steel, electric arc furnace (EAF) and
a basic oxygen furnace (BOF), delivered 60 and 140 million tonnes, respectively,
in the relevant market and reference year, then the market mix would be 30% EAF
and 70% BOF (World Steel 2015). The LCI model should thus be credited with a
constructed process composed of 30% of the flow quantities associated with the
production of 1 unit of EAF steel and 70% of the flow quantities associated with the
production of 1 unit of BOF steel. It is important to identify the correct market for
each system expansion. The correct market must reflect the geographical and
temporal scope (see Sect. 8.7). Note that some goods and services are sold in global
markets due to the low cost of transportation relative to their value (e.g. gold), while
other goods and services are sold on local or regional markets due to high trans-
portation cost (e.g. some biomaterials and water) or regulation. Information on
volumes produced and delivered to markets can often be obtained from reports or
databases of industry organisations (e.g. the World Steel Association in the example
of recycled steel). In consequential modelling (parts of Situation B, see Sect. 8.5.4),
the avoided process is not a market mix, but the marginal process (or a mix of
marginal processes) and its identification is explained in Sect. 9.2.3.
Allocation
Allocation is the third and last option in the ISO hierarchy. As mentioned in
Sect. 8.5.2 allocation should, when possible, be based on (1) causal physical
relationship, followed by (2) a common representative physical parameter and, as a
last resort, (3) economic value.
The causal physical relationship approach is possible when the ratio between
quantities of co-products can be changed. Consider again the above example of a
factory producing two products (x and y), where only the total electricity con-
sumption is known. Here it would be possible to derive the electricity consumption
of x and y by collecting data on production volumes and total electricity con-
sumption at two points in time, where the relationship between the produced
quantities are different. This could lead to the following simple system of equations: