Page 109 - Materials Chemistry, Second Edition

94 A. Bjørn et al.

process, compared to a primary product with a high market value. In the extreme
situation where the value of the co-product is zero, its allocated share of the inputs
and outputs also becomes zero in accordance with the fact that a zero-value output
is not a co-product but waste and should be modelled as such.

8.5.3 LCI Modelling Framework: Attributional
and Consequential LCA

Traditionally, there have been two main LCI modelling frameworks: attributional
and consequential modelling. In the ILCD guidelines, these were adapted to match
the four decision context situations (i.e. A, B, C1 and C2). Understanding the
difference between attributional and consequential modelling and when to use what
has been one of the most difficult aspects of LCA, and there is still no consensus on
this issue within the LCA community. In addition, some aspects of the terminology
defined in the ILCD guidelines, in particular with regard to the definition and
settings of attributional modelling, are inconsistent with the traditional views within
the LCA community, thus adding more confusion to the matter (Ekvall et al. 2016).
Below we first offer an explanation of the two modelling frameworks, including
their handling of multifunctional processes and the use of average or so-called
marginal LCI data (to be explained below). Where relevant we specify discrep-
ancies between the ILCD guidelines and the traditional views. Table 8.3 sum-
marises the explanation and discrepancies. We then provide guidance in compliance
with the ILCD guidelines for selecting the LCI modelling framework with con-
sideration to the goal definition.
Attributional LCI modelling was initially the common practice when LCA
development caught pace in the early-mid nineties. The overall aim of attributional
modelling is to represent a product system in isolation from the rest of the tech-
nosphere or economy. The question addressed by attributional LCA can be said to

Table 8.3 The meaning of the attributional and consequential modelling frameworks and their
handling of multifunctionality
LCI Question to be answered Handling of multifunctional Modelling of
modelling processes when subdivision background
framework is not possible system
Before ILCD
ILCD
Attributional What environmental Allocation System Average
impact can be attributed expansion or processes
to product X? allocation
Consequential What are the System System Marginal
environmental expansion expansion processes
consequences of
consuming X?