170 Introduction to Renewable Biomaterials
Table 5.4Greenhouse gas lifetime before
decomposition and corresponding global warming
potential (GWP) for a 20-year time horizon and a
100-year time horizon (Myhreet al., 2013).
Lifetime years GPW20 GPW100
CH 4 12.4 86 34
HFC-134a 13.4 3790 1550
CF-11 45 6900 5350
N 2 O 121 268 298
CF 4 50,000 4950 7350
such as Ecoinvent or the TRACI Method. Both of these methods are further discussed
in the following sections.
Characterized flow=flow(inventory unit)×Char. factorcharacterized units
inventory units
5.3.2.13 Optional Elements
Normalization Normalization is an optimal step of LCIA according to the ISO
standards; however, it can provide some additional insights into the relevancy of certain
emissions. Since midpoint indicators represent different measure and use different
units, relating the quantity of a midpoint indicator to another midpoint indicator can
be difficult. Furthermore, it is not always possible to know how significant a 50 kg of
CO 2 eq. emitted is compared to 50 kg H+equivalents.Youmightaskis50kgofCO 2
and H+huge emission in comparison to what is currently being emitted in the world?
Relating midpoint indicators to emissions of an exterior reference system is referred
to as external normalization. For instance, dividing the emission of 50 kg CO 2 by the
CO 2 emissions of a human in the United States over the course of a year allows a
comparison of the current system to the total impact that a person may have in a
year. This external normalization, defined as impact of study scenario divided by an
external reference value in the same units, provides a unitless value that can be further
examined in the next optional steps of impact assessment. External normalization
is commonly used; however, it has a major weakness. As an example, when the CO 2
emissions of a product are normalized by the CO 2 emissions of a US citizen over a year,
the impacts of the product are often trivial in comparison to the yearly total. This yearly
total emission that is used for normalization does not have any relation to what the
earth or natural environment can sustainably accommodate; rather it is the current and
often unstainable level of emissions. By using often large and unsustainable emissions
number to normalize midpoint indicators, the influence of certain impact categories
is often diminished in the final single score. In this example, 50 kg CO 2 divided by
24,000 kg (emissions of a US citizen for CO 2 per year,) yields 0.021, somewhat trivial
and small number (Table 5.5).
Another method of normalization for comparative LCA studies is internal normal-
ization. In such a comparative study, the midpoint indicators between an option A and
