Page 169 - Materials Chemistry, Second Edition

154 A. Bjørn et al.

processes, which means that it is not possible to modify them to increase their
representativeness for the study. When aggregating processes, the LCI database
providers have made choices on how to handle multifunctional processes and how
to cut-off the life cycle of the process’ reference flow because including all pro-
cesses is not practically achievable in process-based LCI modelling (see Sect. 9.1).
These choices also relate to solving the issue of closed loops between processes,
which occurs if two processes need each other’s outputs as inputs. This issue is
commonly solved by matrix inversion (Heijungs and Suh 2002).
The way system expansion is performed in the construction of the inventory
model depends on the LCA software used, but it is usually simple to implement for
the LCA practitioner. For example, in GaBi it is performed by connecting the
avoided process as input but with a scaling factor of −1 so that it is computed
negatively as a crediting. In SimaPro, system expansion is performed by making a
direct link to the avoided process in the flow category ‘Avoided products’, and the
software automatically accounts for it negatively when processing the assessment.
In OpenLCA, which is a free LCA software, system expansion is modelled as an
avoided output of a unit process, in practice marking an output flow as ‘avoided
product’ by checking a mark in the process.

9.5.2 Calculation of LCI Results

The LCI results are the compilation of elementary flows over all the processes that
are part of the LCI model (scaled to the reference flow of the functional unit). For
the simplified product system in Fig. 9.8 the results would simply be the sum of
each of the resources and emissions across all the processes, see Fig. 9.9 describing
final LCI results.

Fig. 9.9 LCI results for the Emission A Emission B
product system in Fig. 9.8. 432kg 528kg
The aggregated elementary
flows of product W (72 kg),
that are not shown in Fig. 9.8,
are 100 kg of resource A and
28 kg of emission B
Product X
100kg
Product system

Resource A Resource B
960kg 100kg