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| Fig. 1: Chart of average fuel economy in US, and extrapolation to include years of interest. [3] (Source: R. Weber) |
The Environmental Protection Agency (EPA) this month issued proposed biofuel standards for 2023-2025. [1] The goal of these new standards is to reduce total carbon emissions from transportation by using a higher percentage of clean biofuels and less gasoline. But how much of an impact will this make? Here we will dive into the numbers to compare how much carbon emissions could be reduced by the updated proposed standards in comparison with the status-quo. Specifically, we will examine their updated requirements for "Renewable fuel", as it is by far the largest quantity in their proposal. "Renewable fuel" in this case refers to a combination of cellulosic biofuel, biomass-based diesel, advanced biofuels, and (mostly) corn ethanol.
Table 1 below summarizes the calculations used to quantify the impact of the EPA's new proposal. The first column shows their proposed requirements for renewable fuel for 2023-2025. To evaluate this proposal, we will quantify projected emissions and compare it with total transportation emissions in the U.S. over the previous few years. According to the FHWA, the total vehicle miles traveled (VMT) in 2021 in the US was 3,228 billion miles. [2] The same agency estimates that the 30-year growth rate in VMT between 2019-2049 is 22%, resulting in an annual growth rate of 0.73%. [3] Using 2021 as a baseline, the VMT for each year is calculated with the following growth rate equation
This is done for years y = 2023,... , 2025 and the results are shown in Table 1. The next quantity of interest is the fuel efficiency, since we need to know how much total fuel is needed for the corresponding VMT. The EPA releases an annual report of the average fuel economy of the US fleet for the years since 1975. [4] Fig. 1 uses data from this report and shows the trend of these data over time. It is seen that since 2004, fuel efficiency has seen an upward trend. Though that trend has slightly decelerated since 2009 and has been roughly linear with some noise since. Thus, linear regression was used to determine a best-fit line for the data from 2009-2021, and this line was extrapolated to project what the average fuel economy for the years 2023-2025 would be, and this data is shown in Table 1. The projected VMT is then divided by the fuel efficiency for each year to get the total fuel needed. Note that for now we are ignoring EVs, as they make up only an estimated 1% of the U.S. vehicle fleet. While this number is obviously increasing, it will not have a major impact on evaluating the impact of total life cycle emissions from transportation for the years 2023-2025. The renewable fuel target is roughly 1/6 of all fuel needed, which is notably larger than the nationwide average ethanol concentration of gasoline of 10.36% in 2021.
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| Table 1: Summary of proposed volume requirements of renewable fuel for 2023-2025 and projected overall fuel need for those years. |
To find out how much of an impact these standards will have on total carbon emissions from transportation, we need to examine the "renewable fuel", which is broken down into four categories:
Cellulosic biofuel refers to cellulosic ethanol, which is a small amount of the renewable fuel targets, as seen in Table 2. This is because actual cellulosic ethanol production has been significantly less than targets because it has a hard time being economically competitive with gasoline, as the EPA admits in their draft analysis. [5] Actually, the term "cellulosic biofuel" is misleading, as the production of compressed natural gas and liquified natural gas (CNG/LNG) from landfills and municipal wastewater treatment facilities is allowed to count under this category - likely because the actual production of cellulosic biofuel is so low. In fact, CNG/LNG actually made up more than 98% of this category between 2013-2021, so this is what we will use to evaluate emissions. Note that for these proposed fuel targets, the EPA uses a unit called RINs—which are units with the equivalent energy content of one gallon of gasoline. For CNG/LNG 1 RIN is equivalent to 77,000 BTU (81.3 MJ) of biogas, and the reported life cycle emissions for CNG is 75 gCO2/MJ. [5] Thus, the emissions from this fuel type for 2023 are calculated by
Biomass-based diesel (biodiesel) is made from vegetable oils, animal fats or recycled restaurant grease. Since the various methods of production and the reliance on other industries (restaurants), it is not plausible to significantly scale up this industry, which is reflected in the slow growth of this fuel type in the proposed volume targets shown in Table 2. Also, estimation of life cycle GHG emissions from biodiesel vary considerably. [5,6] For this work, we will use a mean estimate of 40 gCO2/MJ biodiesel, recognizing that some will be much higher than this and others will be lower. Since the energy content for biodiesel is 121.5 MJ/gal, the emissions for the 2023 proposed value are calculated by:
Advanced biofuels are derived from non-food-based feedstocks and have varied widely over the years in their contribution to the overall biofuel mix since they often rely on global trends in other industries. [5] This makes them the most difficult to predict and quantify, and since they also represent such a small percentage of the renewable fuel target, we will ignore them for now and assume they have the same energy content and carbon intensity as conventional renewable fuels.
Conventional renewable fuel refers simply to corn ethanol, which is by far the biggest contribution to renewable fuel. Corn ethanol production is highly subsidized by the government because it is often of political interest but can be controversial because it is both energy negative (meaning it takes more energy to produce than it contains) and takes away corn that could be used in other ways - like food. [7] Because it's so energy intensive, the energy input into corn ethanol production has a dramatic effect on how carbon intensive the process is, so it can vary widely depending on where it is produced, and estimates range from 38-116 gCO2/MJ. [5] For this work, we will use a mean estimate of 75 gCO2/MJ for the production of corn ethanol, and the energy content is 89 MJ/gal.
After taking into account all renewable fuel type, the amount of conventional gasoline is found by subtraction the renewable fuel volume from the total fuel needed, which was calculated in Table 1. The assumed carbon footprint of conventional gasoline in 96 gCO2/MJ gasoline, and 1 gallon of gasoline is 131.76 MJ. [8] All the projected emissions from each fuel type are added together to get the final column in Table 2 showing the total project emissions under the proposed target scenario from the EPA.
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| Table 2: Summary of proposed volume requirements of different fuel types and total emissions. |
The EPA's proposal to increase biofuel requirements over current levels would have a slight impact on carbon emissions from driving, as the EIA estimates that 1.486 billion MT CO2 were emitted from fuel consumption in 2021. [9] The projected 1.421 billion MT CO2 emitted in 2025 would represent a 4% overall decrease compared to 2021 level, so this is not a bombshell proposal but simply more incremental progress. One important thing to note is that emissions from ethanol production are highly variable due to specific production practices. Lowering the use of coal in production would thus have a major impact. It should also be noted that some aspects of the proposal may be hard to meet. "Advanced biofuel" as discussed is highly variable year-over-year, and "cellulosic ethanol" has not demonstrated it has any potential to meet the amounts projected.
© Ross Weber. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] Z. Budryk, "EPA Announces New Biofuel Blending Mandate Proposal," The Hill, 1 Dec 22.
[2] "Traffic Volume Trends," U.S. Federal Highway Administration, December 2021.
[3] "2022 FHWA Forecasts of Vehicle Miles Traveled (VMT)," U.S. Federal Highway Administration, July 2022.
[4] "The 2021 EPA Automotive Trends Report," U.S. Environmental Protection Agency, EPA-420-R-21-023, November 2021.
[5] "Draft Regulatory Impact Analysis: RFS Standards for 2023-2025 and Other Changes," U.S. Environmental Protection Agency, EPA-420-D-22-003, November 2022.
[6] H. Xu et. al, "Life Cycle Greenhouse Gas Emissions of Biodiesel and Renewable Diesel Production in the United States," Environ. Sci. Technol., 56, 7512 (2022).
[7] D. Pimentel, "Ethanol Fuels: Energy Balance, Economics and Environmental Impacts Are Negative," Nat. Resour. Res. 12, 127 (2003).
[8] G. Cooney et. al, "Updating the U.S. Life Cycle GHG Petroleum Baseline to 2014 with Projections to 2040 Using Open-Source Engineering-Based Models," Environ. Sci. Technol. 51, 977 (2017).
[9] "Monthly Energy Review November 2022," U.S. Energy Information Administration, DOE/EIA-0035(2022/11), November 2022.