Life cycle assessment (LCA) is an environmental management tool used for estimating the environmental burdens and impacts from a system — product, process or a service — over its whole life cycle. The life cycle stages normally included in LCA are extraction and refining of raw materials; product manufacture, distribution and use; disposal of wastes; and all transportation steps in between. The LCA methodology, as defined by the ISO 14040 and 14044 standards (ISO, 2006a, 2006b), is outlined in Fig 3.1A. It consists of four phases:

• Goal and scope definition;

• Life cycle inventory (LCI) analysis;

• Impact assessment (IA); and

• Interpretation.

Goal and scope definition is the first and most important phase of LCA. Here, the reasons for carrying out the LCA study as well as the intended audience are stated. The system boundary and the functional unit (unit of analysis) are defined as well as the impact assessment method to be used in the Impact assessment phase. Assumptions, limitations, cut-off rules, etc., are also described in this phase.

Life cycle inventory (LCI) analysis quantifies the environmental burdens in the system, i. e. materials and energy used and emissions discharged into the

environment. Allocation of environmental impacts is also carried out within LCI. Allocation is the process of assigning to each function of a multiple-function system only those environmental burdens that each functional output is responsible for. For example, if there are two or more co-products from a system, the environmental burdens should be allocated between them so as to reflect their contribution to those burdens. ISO 14044 (ISO, 2006b) recommends three methods for dealing with allocation:

• if possible, allocation should be avoided by disaggregating the given process into different sub-processes or by system expansion;

• if it is not possible to avoid allocation, then the allocation problem must be solved by using system modelling which reflects the underlying physical relationships among the functional units (e. g. mass or energy basis);

• where physical relationships cannot be established, other relationships, including economic value of the functional outputs, can be used.

The allocation method used will usually influence the results of the LCA study so that selection of an appropriate allocation method is crucial. Sensitivity analysis should be carried out in cases where the use of different allocation methods is possible to determine the influence of the allocation method on the results.

Impact assessment (IA) consists of several steps. First, categorisation of environmental impacts is carried out to determine which impacts will be considered.

Impact categories commonly considered in LCA are resource depletion, land use, global warming, acidification, ozone layer depletion, summer smog, eutrophication, human and eco-toxicity. This is followed by the characterisation step, to calculate the contribution of different burdens to the selected impact categories. The impacts are calculated by multiplying the ‘potency factors’ of each burden with its total life cycle emission. The potency factors indicate the potential of a burden to cause a particular impact and are expressed relative to a reference substance. For example, the potency factor for methane with respect to global warming is 25 kg CO2 eq./kg CH4, indicating that methane is 25 times more potent global warming agent than CO2, whose potency factor is defined as unity.

The remaining two steps in IA — normalisation and valuation — are optional. The former normalises each impact to the total impact in a region or the world over a certain period of time, normally one year. In the valuation step, the impacts are aggregated into a single environmental impact index by assigning the weights of importance to the different impacts. This is the most subjective step in LCA and requires elicitation of preferences by stakeholders or decision makers.

Interpretation is the final LCA phase, whereby the results are interpreted depending on the goal of the study. This may include identification of the most significant impacts, ‘hot spots’ in the system and opportunities for improvements. Sensitivity analysis is also carried out within this phase.

Generally, there are two types of LCA studies: attributional and consequential (Curran et al, 2001; Ekvall and Weidema, 2004). In attributional studies, the impacts are attributed to the system of interest (e. g. product) based on the flows in and out of the system as they are. For example in attributional LCA, impacts from the production of biofuel from wheat in the UK are estimated (attributed) based on the inputs and outputs from this system, not taking into account what happens with the other related activities in the economy, for example if the supply of wheat is constrained, e. g. due to its use for bread production. In consequential LCA studies, the aim is to estimate how the flows to and from the system would change as a result of different potential decisions. For example in the case of biofuels, a consequential LCA study would attempt to quantify the impacts of diverting wheat in the UK into biofuel production and having to supply food (e. g. bread) from alternative sources or from elsewhere in the world.

The attributional approach is used for labelling purposes (e. g. PAS2050 [BSI, 2008]) and certification systems (e. g. EU RED [EU, 2009]; Renewable Transport Fuel Obligation, RTFO [DfT, 2008]). Most biofuel LCA studies are also based on the attributional approach.

3.4 Sources of further information

CO2 tool for estimating GHG emissions from the production of transport fuels, electricity and heat from biomass

http://www. senternovem. nl/gave_english/co2_tool/index. asp

IEA Bioenergy

http://www. ieabioenergy-task38.org

Well-to-wheels evaluation of biofuels http://ies. jrc. ec. europa. eu/WTW

Biofuels sustainability scorecard

http://idbdocs. iadb. org/wsdocs/getdocument. aspx? docnum=2152669

Biofuels and sustainability in Europe: http://www. biofuelstp. eu/sustainability. html