Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States (2022) / Chapter Skim
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2 Fundamentals of Life-Cycle Assessment
2 Fundamentals of Life-Cycle Assessment
Pages 18-34
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From page 18... ...
This phase lays the foundation for an LCA, specifying the goal of conducting the analysis. For transportation fuels policy, this goal could be to assign GHG emissions-intensity scores that can be compared across a range of fuel options.
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Each stage in the fuel pathway also has its own supply chain, with corresponding upstream resource consumption and emissions. For example, petroleum refineries consume electricity, which is generated using multiple energy sources.
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TWO BROAD CATEGORIES OF LIFE-CYCLE ASSESSMENT In defining the goal and scope of an LCA, a practitioner may select from a wide variety of desired outcomes: perhaps two different products are being compared to inform the selection of one based on its environmental impact or an industrial production process is being assessed to identify opportunities for reducing its life-cycle impacts. However, there are two broad categories of LCA that are relevant to this report and require such fundamentally different approaches that they are important to discuss in greater detail: attributional LCA and consequential LCA.
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A consequential approach to GHG emissions accounting in products provides information about the GHG emitted, directly or indirectly, as a consequence of changes in demand for the product. This approach typically describes changes in GHG emissions levels from affected processes, which are identified by linking causes with effects.
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Attributional LCA: LCA aiming to describe the environmentally relevant physical flows to and from a life cycle and its subsystems. Consequential LCA: LCA aiming to describe how environmentally relevant flows will change in response to possible decisions.
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In the case of transportation fuels, it is common to include material extraction, transportation of raw material to point of processing, raw material conversion into fuel, transportation of fuel to points of distribution, and combustion of the fuel. In a system boundary that encompasses multiple fuel systems, this consideration will be more complex.
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The product system includes processes that are directly linked to the product by material, energy flows or services following a supply-chain logic." – "An attributional LCA accounts for only the direct emissions associated with the fuel lifecycle, including the emissions from production of energy and material inputs to the fuel lifecycle." – "The attributional approach attempts to provide information on what portion of global burdens can be asso ciated with a product (and its life cycle) ." Table 2-1 also declares that: – "A consequential approach to GHG emissions accounting in products provides information about the GHG emitted, directly or indirectly, as a consequence of changes in demand for the product.
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In EIO LCA models, emissions from each sector are based on average, rather than marginal, emissions in the sector, so these analyses generally do not estimate net emissions implications of changes in fuel use unless marginal emissions are similar to average emissions in the relevant economic sectors (e.g., if emissions are linear with economic output in the relevant sectors)
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Process-based, EIO, and hybrid process/EIO LCA all provide approaches to track environmental effects across fuel supply chains. Process-based LCA, for example, can be useful in informing the development of a new fuel production process (e.g., converting lignin to a hydrocarbon fuel)
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, even as the biofuel supply chains themselves are modeled using a predominantly ALCA approach. This practice of mixing ALCA and CLCA is discussed in Chapter 3.
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(2015) present two boundary maps showing how the system boundaries might differ between an ALCA model and a CLCA model of a municipal waste-to-ethanol production system.
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Attributional LCA: Both bottom-up and top- What emissions are attributable to a National, multi-regional or Hybrid Process/EIO down emissions process or product within the system global economy accounting boundaries, as approximated by a combination of supply chain and economic sector information? Consequential LCA Counterfactual emissions How will emissions change in Varies, but ideally as comparison response to a decision or action?
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Figure 2-4b illustrates that using average emissions estimates from an attributional LCA in this situation can produce poor estimates of the emissions consequences of the change unless emissions rise linearly with volume. Marginal emissions are shown in Figure 2-4c and represent the slope of the emissions curve at current levels.
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FIGURE 2-4 Illustration of the hypothetical relationships between attributional and consequential emissions and average and marginal emission factors for a single fuel, shown for one possible case when GHG emissions from increased production of this fuel are convex with fuel production volume. The figure is not intended to imply that GHG emissions are always convex with production volume.
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2021. Correcting remaining truncations in hybrid life cycle assessment database compila tion.
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methods for biofuel supply chains. Processes 8(2)
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2017b. Hybrid life cycle assessment (LCA)
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