Life cycle assessment

Certain parts of this section are adapted from the project preceding this thesis (Galaaen, 2019). Only the parts of the LCA methodology used in this thesis are described. For further explanation of the LCA methodology, International Organization for Standardization (2006a) and International Organization for Standardization (2006b) should be accessed. LCA is a tool for identifying potential direct and indirect environmental impacts of a product or a service throughout its life cycle. It can be used for comparing different alternatives with regard to environmental performance, identifying ways of improving envi-ronmental performance, informing decision-makers and producing envienvi-ronmental product declarations (EPDs), among other purposes (International Organization for Standardization, 2006b). LCA is an iter-ative technique with four phases called goal and scope definition, life cycle inventory (LCI), life cycle impact assessment (LCIA) and interpretation, as illustrated in Figure 2.1 on the following page. As the International Organization for Standardization (2006b) establishes, the four phases should be thoroughly planned and coordinated, and phases often need to be revised as the analysis is carried out and unforeseen aspects emerge. In the following paragraphs, the different LCA phases are explained separately, before the mathematics behind the LCA methodology are presented.

2.1.1 Goal and Scope

The goal and scope definition is the first phase of the LCA methodology, and creates the foundation for the analysis. The goal definition should present the objective of the LCA and include the reasons for carrying out the study as well as the intended application and audience (International Organization for Standardization, 2006b). The scope definition should consider relevant aspects of the product or service, the functional unit, the system boundaries, allocation procedures, LCIA methodology, interpretation, data

Figure 2.1:The phases of the LCA methodology

requirements, assumptions, value choices, limitations and type and format of the study report, according to International Organization for Standardization (2006b). The functional unit and the system boundaries are two of the most important aspects of the scope, and these are therefore outlined in more detail in the following paragraphs.

Functional unit

The functional unit of an LCA should reflect the function of the product or service under study in a precise and quantitative way, and works as the reference for the LCA as all environmental impacts are related to it (International Organization for Standardization, 2006b). The functional unit can be based on distance, time, size or other parameters, dependent on the function of the product or service, and should also fit the objective of the desired analysis. When performing comparisons based on LCA, the functional unit is also the basis for comparison. Different systems need to have similar functional units in order to be comparable in a fair and productive way. A reference flow is also often defined in addition to the functional unit, describing the measure of process outputs necessary to fulfil the product function established in the functional unit (International Organization for Standardization, 2006b).

System boundaries

The system boundaries of an LCA describe which processes and flows that are included in the analysis, and can be related to life cycle phases, geographical borders, technical systems, time or other measures

2.1 Life cycle assessment (Arvesen, 2019b). The system boundaries should reflect the rest of the goal and scope and be carefully identified, since they affect the calculations and results of the study (International Organization for Stan-dardization, 2006a). LCA researchers often label system boundaries as cradle-to-gate or cradle-to-grave, based on which life cycle phases are assessed. The word cradle symbolises resource extraction, and grave symbolises end of life treatment. A cradle-to-gate system boundary considers everything from resource extraction up until the final delivery of the product ready for use, while a cradle-to-grave system boundary in addition includes the end of life treatment.

The system boundaries can also be defined based on other parameters than life cycle phases. They can be dependent on time in the form of the system being analysed for a given time period, and they can be dependent on geographical borders for instance when analysing production in a specific country.

Other system boundaries can also be defined, and the specifics of the system boundaries are unique for each LCA, which is essential when comparing different LCAs. Generally, the system boundaries should be consistent with the established goal of the analysis (Graedel and Allenby, 2015).

2.1.2 Life Cycle Inventory Analysis

In the LCI phase, quantitative data are used to identify resources consumed throughout the life cycle of the product or service under study, and an inventory is constructed (Graedel and Allenby, 2015). The quantitative data can include input and output materials, energy, emissions, resources and wastes, and need to be identified and systematised (Arvesen, 2019d; International Organization for Standardization, 2006b). Such data can either be measured directly, calculated, or estimated based on literature and databases (Strømman, 2010). Transport is a central part of the LCI. According to Strømman (2010), transport in LCA can be modelled as receiver aggregated, which aggregates transport of products and the end process into a new process, or receiver input, which considers the transport and outputs necessary for the end process separately. Data collected by the practitioner concerning the specific case analysed are called primary data, while data collected by others not necessarily specific for the case analysed are called secondary data. Data used in LCA should preferably be reliable, relevant and accessible (Arvesen, 2019d). The LCI analysis is an iterative procedure as new aspects arising during data collection may lead to changes or adjustments (International Organization for Standardization, 2006a). The LCI phase normally results in a flowchart of the system and a list of resources consumed and emissions to air, water and soil with information on mass flows and chemical specifications (Graedel and Allenby, 2015). Both foreground data and background data are needed in order to obtain a robust inventory. Foreground data concern processes and flows defined in the study, while background data cover upstream processes and flows linked to these, obtained from a generic database (Arvesen, 2019a).

The Ecoinvent database

Background data from databases are needed to compile the LCI. Strømman (2010) emphasised that the data quality is dependent on how well the data represent the processes under study and link with other rel-evant processes to create a robust image. A database is created by compiling various individual studies, and different LCA databases are available. One of the most commonly used databases is Ecoinvent con-taining more than 10,000 data sets including various process categories (Arvesen, 2019d). The Ecoinvent 3 database includes different system models, among others the cut-off system model (Ecoinvent, n.d.a).

The cut-off system model allocates the burdens of waste treatment to the producer, but does not grant the producer the impacts or benefits of recycling at end of life (Ecoinvent, n.d.a). The Ecoinvent 3.2

database includes both market processes and transforming processes. Transforming processes trans-form inputs to outputs and consider a specific activity in a specific geographical area (Ecoinvent, n.d.b).

Market processes represent consumption mixes and include both raw materials extraction, production, average transport and losses (Wernet et al., 2016). They are intended to be used when specific supply chain information is missing.

2.1.3 Life Cycle Impact Assessment

The LCIA phase relates the results from the LCI phase to environmental impacts (Graedel and Allenby, 2015). It should include selection of impact categories, indicators and characterisation models, clas-sification, i.e. assignment of LCI results to environmental impact categories, and characterisation, i.e.

calculation of indicator results (International Organization for Standardization, 2006b). Transparency is important in LCIA because its characteristics affect the results of the LCA (International Organization for Standardization, 2006a). Environmental stressors, e.g. emissions, wastes and resource use, are converted to environmental impacts using a characterisation model with factors characterising the relation between stressors and impacts based on physical and chemical properties (Arvesen, 2019c). Impact assessment can be performed at themidpointor theendpointlevel; midpoint indicators reflect environmental phe-nomena like global warming, while endpoint indicators reflect environmental damage like damage to human health (Arvesen, 2019b). Various cultural perspectives can be used in LCIA, representing dif-ferent viewpoints and positions regarding time and technology. The hierarchist perspective represents a controlling consensus model and is the most frequently used cultural perspective (PR´e, n.d.; Goedkoop et al., 2013). Several LCIA methodologies exist, varying regarding calculations, characterisation factors, definitions of impact categories and whether impacts are assessed at the midpoint level or the endpoint level (Woods, 2019). ReCiPe is a commonly used impact assessment method applied by the Ecoinvent database. Structural path analysis (SPA) is a technique that investigates the production or value chain of a final product or service, and identifies where specific environmental impacts occur (Peters and Hertwich, 2006). It can be performed after or as part of the LCIA phase in order to better understand the envi-ronmental performance of the system, obtaining valuable information for the interpretation. The LCIA phase also includes optional elements, among others data quality analysis (International Organization for Standardization, 2006b). Data quality analysis covers techniques for improving the understanding of the results, including sensitivity analysis (International Organization for Standardization, 2006b). Sensitiv-ity analysis is a method for identifying how changes in data and methodological choices affect results (International Organization for Standardization, 2006b), and can be valuable in the interpretation phase.

2.1.4 Life Cycle Interpretation

In the interpretation phase, the results from the previous LCA phases are interpreted, and conclusions and recommendations are made (Graedel and Allenby, 2015). The phase also concerns identifying significant issues, evaluating the completeness, sensitivity and consistency of the study, and discussing limitations (International Organization for Standardization, 2006b).

2.1.5 The mathematics behind LCA

The LCA technique is based on vector and matrix calculations for moving from data input to environmen-tal impacts. These mathematical aspects are described in the following paragraphs, based on Strømman

2.1 Life cycle assessment (2010).

The y vector expressed in Equation (2.1) describes the external demand of the processes in the sys-tem, typically representing the functional unit of the study.

y =

The total production output, including both the external demand and the additional requirements for producing the external demand, i.e. the intermediate demand, is described in the x vector expressed in Equation (2.2).

To address the interdependencies between the different system processes in the LCI and describe the intermediate demand per unit output, the A matrix is used. The A matrix is called the requirements matrix, and can be divided into four sub-matrices: A_ff, A_fb, A_bf and A_bb. A_ff considers the demands between foreground processes, and A_bbsimilarly considers the demands between background processes.

A_bfrepresents flows from the background system to the foreground system, while A_fbsimilarly describes flows from the foreground system to the background system. A_fb is generally set equal to zero in most LCAs because these flows are neglected as the flows between the foreground system and the background system are assumed to be unidirectional from the background to the foreground. Equation (2.3) illustrates the A matrix.

A coefficient of requirement, a_ij, represents the amount of process i needed per unit output of process j, as shown in Equation (2.4).

a_ij = amount of i required

output of j (2.4)

The total production is equal to the sum of the intermediate demand and the external demand of the system. This is called the production balance in LCA. Equation (2.5) illustrates the production balance and how it can be rearranged as a function of L, expressed in Equation (2.6).

x = Ax + y = Ly (2.5)

L = (I – A)^–1 (2.6)

L is the Leontief inverse matrix. Each coefficient in L, l_ij, represents the amount of process i

nec-essary to obtain one unit external demand of process j. I is the identity matrix, i.e. a square matrix the same size of the A matrix with ones along the diagonal and zeros elsewhere.

Contribution analysis is carried out to calculate stressors and environmental impacts. As described earlier in this chapter, environmental stressors are identified in the LCI phase. The S matrix, modelled in Equation (2.7), is a matrix describing the stressor intensities of the different processes, meaning how much of a stressor is resulting from a certain amount of a process. The stressor matrix can also be split into foreground and background sub-matrices like the requirements matrix. The foreground stressor intensity matrix S_f concerns stressors created by the foreground system while the background stressor intensity matrix S_bconcerns stressors created by the background system.

S =

The e vector represents the total stressors calculated in the LCI phase. It equals the product of the stressor intensities and the total production output, as expressed in Equation (2.8).

e = Sx (2.8)

To analyse which processes the total stressors of different types are caused by, the E matrix is created, as shown in Equation (2.9).ˆx is a diagonalised form of the x vector in the form of a square matrix.

E = Sˆx (2.9)

In order to convert environmental stressors to environmental impacts in the LCIA phase, the char-acterisation matrix C is applied. The charchar-acterisation matrix contains charchar-acterisation factors describing relations between stressors and impacts as introduced earlier in this chapter. Equation (2.10) illustrates the C matrix.

The total impacts equal the product of the characterisation matrix and the total stressors. The d vector represents the total impacts, and it is presented in Equation (2.11).

d = Ce (2.11)

To investigate how different processes contribute to various impacts, the D_pro matrix is used. It is equal to the product of the C matrix and the E matrix, as illustrated in Equation (2.12).

Dpro =

To analyse how different stressors contribute to various impacts, the Dstr matrix is used. It is the product of the C matrix and the e vector, as illustrated in Equation (2.13). ˆe is a diagonalised version of