278 A. Brekke et al.
consequential LCA is a useful tool for exploring alternative states of the
world, while attributional LCA is a useful tool to assess the environmental (or
sustainability) performance of different stable product systems. Thus, attributional LCA is used as a basis for the calculation method in the Renewable
Energy Directive, although there might be controversies related to how stable
the different production systems are.
14.3.4 Dividing impacts on several inputs or outputs
The chapter started by asking the question of how sustainable a product or
service is and it has presented LCA as a useful tool to provide answers.
However, whenever a process produces more than one output or a waste
handling process transforms more than one input, there are difficulties in relating the environmental performance of the system to single inputs or outputs.
As part of a strategy for a circular bioeconomy, the concept of biorefinery
is often mentioned as a technical approach to making the most of organic
resources and diversifying product portfolios. Whereas the idea of a biorefinery is to diversify output and minimise waste, a much- debated attribute in
LCA is how to deal with systems which involve more than one output. As
examples, burning wood can result in both heat and power, and anaerobic
digestion of food in both biogas and biofertiliser (digestate). In order to
produce results for one output only, e.g. the environmental impacts of biogas,
it is necessary to divide the impacts from anaerobic digestion between the
biogas and the digestate. If possible, such divisions should be avoided, but in
other cases the ISO standard 14044 prescribes two different method options:
primarily to use system expansion, and secondarily to allocate impacts
between the two outputs (biogas and biofertiliser).
In the biogas system, system expansion could entail expanding the function
studied to include both outputs, which makes it impossible to separate the
environmental impacts of the biogas from those of the biofertiliser. An option
which is often regarded as a case of system expansion (e.g. in the ILCD handbook) is to consider the biofertiliser as substituting another type of fertiliser,
and therefore the impact of that fertiliser is subtracted from the anaerobic
digestion system. The idea is that the biofertiliser is dealt with, and the resulting environmental impact can be attributed to the biogas. Such a subtraction
requires two conditions to be met: (1) an alternative product can clearly be
defined, and (2) life cycle inventory data for the alternative product exists.
The choice of which product is substituted and how can be of high importance to the final results, biogas included (Hanserud, Cherubini, Øgaard,
Müller & Brattebø, 2018; Lyng et al., 2015), and the approach of substitution
is not undisputed (Heijungs & Guinee, 2007). With an increasing number of
outputs, such as in a biorefinery, the complexity of the model and its results
may increase.
As an alternative, the total environmental impacts of the biogas system can
be divided between, and allocated to, each co- product. The division can be
consequential LCA is a useful tool for exploring alternative states of the
world, while attributional LCA is a useful tool to assess the environmental (or
sustainability) performance of different stable product systems. Thus, attributional LCA is used as a basis for the calculation method in the Renewable
Energy Directive, although there might be controversies related to how stable
the different production systems are.
14.3.4 Dividing impacts on several inputs or outputs
The chapter started by asking the question of how sustainable a product or
service is and it has presented LCA as a useful tool to provide answers.
However, whenever a process produces more than one output or a waste
handling process transforms more than one input, there are difficulties in relating the environmental performance of the system to single inputs or outputs.
As part of a strategy for a circular bioeconomy, the concept of biorefinery
is often mentioned as a technical approach to making the most of organic
resources and diversifying product portfolios. Whereas the idea of a biorefinery is to diversify output and minimise waste, a much- debated attribute in
LCA is how to deal with systems which involve more than one output. As
examples, burning wood can result in both heat and power, and anaerobic
digestion of food in both biogas and biofertiliser (digestate). In order to
produce results for one output only, e.g. the environmental impacts of biogas,
it is necessary to divide the impacts from anaerobic digestion between the
biogas and the digestate. If possible, such divisions should be avoided, but in
other cases the ISO standard 14044 prescribes two different method options:
primarily to use system expansion, and secondarily to allocate impacts
between the two outputs (biogas and biofertiliser).
In the biogas system, system expansion could entail expanding the function
studied to include both outputs, which makes it impossible to separate the
environmental impacts of the biogas from those of the biofertiliser. An option
which is often regarded as a case of system expansion (e.g. in the ILCD handbook) is to consider the biofertiliser as substituting another type of fertiliser,
and therefore the impact of that fertiliser is subtracted from the anaerobic
digestion system. The idea is that the biofertiliser is dealt with, and the resulting environmental impact can be attributed to the biogas. Such a subtraction
requires two conditions to be met: (1) an alternative product can clearly be
defined, and (2) life cycle inventory data for the alternative product exists.
The choice of which product is substituted and how can be of high importance to the final results, biogas included (Hanserud, Cherubini, Øgaard,
Müller & Brattebø, 2018; Lyng et al., 2015), and the approach of substitution
is not undisputed (Heijungs & Guinee, 2007). With an increasing number of
outputs, such as in a biorefinery, the complexity of the model and its results
may increase.
As an alternative, the total environmental impacts of the biogas system can
be divided between, and allocated to, each co- product. The division can be