Page 45 - Materials Chemistry, Second Edition

2.2 Scope 29

or by several outputs in waste treatment), but no network. An exception is the
treatment of recycling, which is discussed in Section 3.3. Within a complete LCA,
the presentation ends at the disposal or at a point where co-products, by-products
or waste for reutilisation exceed the system boundary (thus leaving the product
system).
A special problem arises during the omission of parts of the life cycle. This can,
by all means, be justified, if, for example, a provisional estimation showed that the
overall system contribution is only very small (criteria: mass, energy, environmental
relevance) (see Section 2.2.2.1). However, it must always be examined whether,
within comparative studies, and thus in the majority of cases, no asymmetry of
systems results from omission. Here, particular attention should be paid to the
LCIA, because, compared to mass, very small emissions can nevertheless show
large effects. Within comparative LCAs, large parts of the life cycle may be omitted
in principle if they match accurately in all systems compared (black box method).
In Figure 2.1, for example, the construction elements on the right of the system
boundary (screws, Teflon foil, fittings, etc.) are not considered. This is adequate,
if, for instance, different windows (PVC, wood or aluminium windows) are to be
compared with each other and if these construction elements are used similarly
in all variants regarded. An estimation of relevance of the omitted sections
should nevertheless be made so that comparison of systems is not based on
completely insignificant differences. If, for example, two systems only differ in
waste treatment (End-of-Life Stage) and if these can be neglected in both, the ‘black
box’ approach is inadmissible: both systems are – within an error limit – identical
6)
in their environmental behaviour analysed in the specific LCA.
A precise description and quantification of material and energy flow is conducted
in the stage ‘life cycle inventory analysis’ (LCI) (see Chapter 3). Should details in the
context of LCI analysis indicate an inadequate description of the product system,
the description of the scope must be iteratively modified.

2.2.2
Technical System Boundary
2.2.2.1 Cut-Off Criteria
The specification of system boundaries is one of the most important steps in an
LCA. When two studies on a similar topic (e.g. single-use vs re-usable packaging)
contradict themselves, which may incidentally be the case, usually one or several
of the following reasons are responsible:

1. different methodology,
2. different data quality,
3. different system boundaries.

6) ‘Black box’ means a life cycle stage or unit process that may be omitted within comparative LCAs
because it is identical within all life cycles to be compared. It should nevertheless be applied
sparingly because some most important environmental aspects may be blinded out.