Current Methods for Life-Cycle Analyses of Low-Carbon Transportation Fuels in the United States (2022) / Chapter Skim
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6 Specific Methodological Issues Relevant to a Low-Carbon Fuel Standard
6 Specific Methodological Issues Relevant to a Low-Carbon Fuel Standard
Pages 93-124
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From page 93... ...
Petroleum transportation fuels are co-produced with a range of outputs including butane, gasoline, naphtha, kerosene, diesel fuel, heavy gas oil, and residual fuel oil. All of these may come from a single unit of refined crude oil.
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From page 94... ...
They also require the consideration of how to address market changes as a result of the policy itself. Allocation by economic ratio could be more philosophically consistent with the aims of marketbased approaches to reducing emissions, such as a low-carbon fuel standard (LCFS)
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From page 95... ...
For example, inedible tallow from beef production has a high market value and has a well-developed market for secondary uses, but it accounts for less than 2 percent of the overall value for beef production based on a study in 2015 (ICF International, 2015)
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. In the Renewable Transport Fuel Obligation, wastes and residues are considered to have zero life-cycle GHG emissions up to the process of collection of those materials.
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Policy or practice changes that reduce 4 Renewable Fuel Standard Program: Grain Sorghum Oil Pathway, Final Rule.
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. In addition to existing pathways for carbon sequestration, several stakeholders in the biofuels and agricultural sector are advocating for crediting of carbon removal through soil carbon sequestration associated with biofuel feedstock production (see comments from Gevo and Indigo Ag in CARB [2020]
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From page 99... ...
Changes in soil carbon caused by LUC can generate significant GHG emissions; the soil carbon section discusses different models and methods to estimate soil carbon change and the challenges in modeling and data collection. GHG emissions associated with growing and harvesting are discussed in Chapter 9.
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The assessment of carbon emissions of non-food crops used as feedstocks for biofuels may be considered to depend on the alternative use of the land on which they are grown. Future research should clarify how changes in carbon stock (including soil carbon change)
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With a single stand scale of accounting, increase in the harvest of trees for bioenergy will result in a carbon debt and then a dividend. This is because at a stand level, trees can take many years to grow back after harvest; there is a time interval between carbon release and re-absorption of that carbon from the atmosphere, which can temporarily increase GHG emissions in the atmosphere.
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From page 102... ...
. Forest Bioenergy In the case of forest bioenergy it is important to integrate both LCA of supply chain emissions during the production of the feedstock as well as biogenic carbon due to changes in forest carbon stocks (which could be biogenic carbon emissions or sinks, depending on the net forest carbon stock changes)
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From page 103... ...
More broadly, the results of LUC models that predict the amounts, types, and locations of land that would be used for feedstock production can be used in conjunction with soil organic carbon (SOC) modeling results to estimate soil carbon stock changes that would accompany widespread LUC.
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In sum, changes in SOC can be a significant contributor to the life-cycle GHG emissions of a biofuel. SOC modeling results are sensitive to parameters including soil depth and land use history.
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Specific Methodological Issues Relevant to a Low-Carbon Fuel Standard 105 FIGURE 6-2 Soil carbon sequestration rate changes with time. The rates were estimated for three soil depths: (a)
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Recommendation 6-4: Research should be conducted to collect existing soil organic carbon data from public and private partners in an open source database, standardize methods of data reporting, and identify highest priority areas for soil organic carbon monitoring. These efforts could align with the recommendations made in the 2019 National Academies report on negative emissions technologies to study soil carbon dynamics at depth, to develop a national on-farm monitoring system, to develop a model-data platform for soil organic carbon modeling, and to develop an ag ricultural systems field experiment network.
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From page 107... ...
. The combined global temperature change potential values are published in the IPCC Sixth Assessment Report and are to be applied to a change in emission rate rather than a change in emission amount, as it has been designed for cumulative emission frameworks such as national-level inventories, which is different from the LCA framework.
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From page 108... ...
Some biofuel pathways lead to a so-called carbon debt, increasing global warming impacts in the short-term. Given the urgency of climate change and the ambitious binding targets for GHG emission reduction set by climate policies such as the Paris Agreement (i.e., net zero CO2 emissions in 2050)
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From page 109... ...
to support the selection of an appropriate approach to account for the timing of GHG emissions and uptakes in LCA. VEHICLE–FUEL COMBINATIONS AND EFFICIENCIES Life-cycle GHG emissions of transportation fuels can be compared on a per-unit-energy basis (e.g., emissions could be measured per MJ of a fuel's energy)
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From page 110... ...
Recommendation 6-10: LCA of transportation fuels may include analysis using functional units based on the transportation service provided, such as passenger-mile or ton-mile, or otherwise be based on comparison of comparable transportation services. This may be reported in addition to an energy-based functional unit.
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From page 111... ...
Recommendation 6-11: When comparing life-cycle emissions of different transportation fuels, LCA studies that assess or inform policy should consider the range of vehicle efficiencies within each fuel type to ensure that the comparisons are made on comparable transportation services, such as passenger capacity, payload capacity, and performance. FIGURE 6-3 Range of vehicle energy consumption rates (inverse of efficiency)
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Figure 6-4 shows energy consumption rates for an internal combustion engine vehicle (ICEV) , hybrid electric vehicle (HEV)
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From page 113... ...
Use conditions, including driving style, climate, and energy source, all of which vary regionally, affect the emissions of vehicles for different fuel types differently and can affect which fuel type is estimated to have higher or lower life-cycle GHG emissions. Conclusion 6-7: If an LCA uses a single point estimate for efficiency of each vehicle type, its conclusions may vary substantially depending on which use conditions are assumed.
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From page 114... ...
Operating engines at higher compression can improve power and efficiency, although in modern engines this is just one variable of many.8,9 Fuel properties affect internal combustion engine performance, and engine design and controls affect fuel needs. The combined effects of both result in measurable differences in performance, fuel consumption, and tailpipe emissions.
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From page 115... ...
including vehicle production emissions in an LCA could affect its conclusion about which transportation fuels have the lowest carbon emission implications per unit of transportation services delivered. Recommendation 6-14: For regulatory impact assessment, LCA of transportation fuels and trans portation fuel policy should consider a range of estimates for possible changes in the emissions of vehicle production required to convert transportation fuels into transportation services, and the re sulting changes in vehicle fleet composition.
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2020. Climate and air quality impacts due to mitigation of non-methane near-term climate forcers.
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2018. Carbon Capture and Sequestration Protocol under the Low Carbon Fuel Standard.
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2020. Dynamic Stability of Soil Carbon: Reassessing the "Permanence" of Soil Carbon Sequestration.
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2010. Renewable Fuel Standard Program (RFS2)
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2015. Waste, Residue and By‐Product Definitions for the California Low Carbon Fuel Standard.
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2015. Review of recent lifecycle assessments of energy and greenhouse gas emissions for electric vehicles.
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2014. Soil carbon sequestration and land use change associated with biofuel production: Empirical evidence.
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2019. A global meta‐analysis of soil organic carbon response to corn stover removal.
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2015. Effects of regional temperature on electric vehicle efficiency, range, and emissions in the United States.
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