Chapter 3 Goal and System Definition
3.4 System Definition
3.4.1 prinCiplesOf systeM MOdeling
The reference flows and subsequent impacts for each FU are calculated based on a well-defined system. System modeling is based on a holistic approach that provides a global understanding of the system by considering it as a whole, in all of its dynamics and complexity (Le Moigne 1990). The system is more than the sum of its elements.
The system modeling approach focuses on the relationships between elements that make up the system rather than the elements themselves. The system is then described in terms of these relationships and their significance to the function of the system. In LCA, the world can be schematically decomposed into the environment, the system providing a product or function, and the rest of the economic activity (Figure 3.3).
The system here is defined as a group of dynamically interacting elements, orga-nized to achieve one or more functions. It is identified by the elements it contains, called processes, the links between these elements, and the boundaries that delineate it from the surroundings (environment plus economy). The inputs of the environment into the system are the extracted resources, which include the energy and the land used; the out-puts of the system into the environment are emissions to air, water, and soil. The output of the system into the economic world is the service provided by the product.
The assessed and modeled system is built by linking different process modules.
The processes and elements required to fulfill the function are identified, and these are expressed as a series of unit processes (Figure 3.4), the smallest elements in the analysis, for each of which inputs and outputs are quantified. Unit processes are linked to one another within the system by intermediary flows, expressing the quan-tity of each unit process needed for the subsequent unit process. The outputted prod-uct flows to the economy are any prodprod-ucts that leave the system. Unit processes are linked to the environment by elementary flows, with input elementary flows corre-sponding to the use of natural resources, such as extracted raw material, energy, and land use. Elementary flows exiting a unit process are emissions to water, air, or soil.
Economy Environment
Assessed system
Input = 0 Emission (to air, water and
soil; of substances, noise, etc.)
Product Resource extraction
(minerals, land, water, etc.)
System boundary
FIGURE 3.3 Relationships and exchanges between the studied system, the economic world, and the environment.
Goal and System Definition 35
Figure 3.5 presents the intermediary and elementary flows of primary liquid aluminum, as described in ecoinvent 2.2. The manufacturing of liquid aluminum makes use of the following intermediary flows: aluminum oxide, electricity and quantities of anode and cathode for electrolysis, and heat produced by burning light fuel oil and natural gas. It also involves transoceanic freight and treatment of wastes from aluminum production. Direct air emissions during liquid aluminum manu-facturing results in some of the following elementary flows: benzo[a]pyrene and other polycyclic aromatic hydrocarbon emissions, carbon dioxide and chlorofluo-rocarbon-14 (CFC-14), sulfur dioxide, nitrogen oxides, and PM2.5, as well as waste heat. Each of the intermediary flows requires other intermediary flows and generates
Unit process Exiting elementary flows (emissions to the
environment) Input elementary flows
(extractions from the environment)
Intermediary product flow
Intermediary product flow Unit process
Unit process
FIGURE 3.4 Example of a set of unit processes in a system. (After ISO, ISO 14040 Environmental Management—Life Cycle Assessment—Principles and Framework, 2006.)
Aluminum, primary,
liquid 1 kg
Transport, transoceanic freight ship 3.8 tkm Disposal, filter dust Al electrolysis 2.00 g Electricity mix, aluminum industry 15.9 kWh
Heat, light fuel oil 0.089 MJ Heat, natural gas 0.084 MJ Aluminum electrolysis plant 1.54×10–10 p Aluminum oxide 1.92 kg Anode, aluminum electrolysis 0.448 kg Cathode, aluminum electrolysis 0.0181 kg
Disposal, red mud from bauxite digestion 1.36 kg
Nitrogen oxides 63.9 mg
Heat, waste 56.0 MJ Hydrogen fluoride 539 mg Benzo[a]pyrene 1.30 mg Carbon dioxide, fossil 1.50 kg Carbon monoxide, biogenic 91.7 g
PAH 45.7 mg
Particulates, < 2.5 μm 2.61 g Sulfur dioxide 8.83 g
FIGURE 3.5 Unit process flows associated with liquid primary aluminum at plant, ecoin-vent 2.2. Elementary flows from and to the environment are shown in italics.
36 Environmental Life Cycle Assessment
additional direct emissions. For example, the manufacturing of aluminum oxide will itself use aluminum hydroxide, as well as additional electricity and heat from light fuel oil and natural gas, and will generate some more waste heat.
The level of detail required in modeling a unit process depends on the objectives of the study. Each unit process can be further subdivided into other unit processes down to the level of necessary detail (Figure 3.6). Since this is a physical system, mass and energy balances can be carried out to check that unit processes and the global system respect conservation of mass and energy (see example in Figure 8.3).
The theoretically ideal system is one defined such that the economic world has no inputs to the system and only one output from the system, namely the product cor-responding to the studied function (Figure 3.3). All processes required to fulfill the system function should be part of the system. In practice, this is often not possible, either because of a lack of data or time to carry out the LCA. Moreover, the system may have outputs other than the studied product, resulting in coproducts of value to which a portion of the emissions must be allocated (Section 4.5).
3.4.2 flOwChart
The flowchart or flow diagram or process tree (such as the one depicted in Figure 3.6) provides a clear overview of the processes and their relationships. It depicts each unit process considered within the system and quantifies the intermediary flows linking these unit processes. The flow diagram is built starting from the reference flows
Aluminum oxide
FIGURE 3.6 Flowchart for the manufacturing of liquid primary aluminum at plant, based on ecoinvent 2.2.
Goal and System Definition 37
(what you need to buy for one FU), then identifying the first-tier intermediary flows (quantities of preceding unit processes) associated with each reference flow. The operation is then repeated, starting from the Tier 1 intermediary flow, yielding a second set of Tier 2 elementary flows. In practice, for a new study, the flow diagram will display all linkages from reference flows up to existing database unit processes, whose upstream and downstream links are described in further detail in the database itself and therefore do not necessarily need to be shown in the flowchart. Figure 3.6 presents the flowchart for liquid aluminum production. With the goal of displaying all key unit processes, the system boundaries also include the upstream Tier 2 pro-cesses, such as aluminum hydroxide.
3.4.3 desCriptiOnOf sCenariOs
Each scenario being compared in the LCA must have its own flowchart to be prop-erly visualized and broken down into unit modules. Each scenario must cover the same functional reality and yield the same FU, which means having the same pri-mary function. Of course, scenarios may share certain unit processes, as described in a case study in Chapter 8 (Figure 8.1).