Two studies conducted by my firm, Transport Energy Strategies (TES), and THiggins Energy Consulting show that gasoline blended with ethanol lowers carbon intensity (CI) even more than what is modeled today and what is shown in recent studies. The first study addresses tailpipe carbon emissions, the second well to wheels GHG effects. A key finding in both studies is that blended ethanol not only displaces some of the gasoline but enables a reduction of aromatics in all of the gasoline in the blend. Aromatics have a high CI, and their reduction further decreases the GHG impact of the E10. This advantageous blending attribute is due to ethanol’s high octane rating and has been neglected in prior literature.
Refinery modeling, economic considerations, availability of feedstocks and examination of gasoline properties supported the conclusion in Quantifying Ethanol CI Benefits in Gasoline Composition that as ethanol is blended into gasoline, so aromatics are reduced to maintain a constant octane rating. CI reductions based on fuel composition were found for ethanol blending using a surrogate three-component mixture of ethanol, toluene and iso-octane.
Similar chemical CI reductions were also confirmed for market fuel compositions representative of real-world gasoline blending and production, determined using a model. The model examined a variety of scenarios and E10, E15 and E20 blends. Scenarios included maintaining current refinery production, adjusting refinery production down as ethanol volume increased, and splash blending E15 and E20 using an E10 BOB. CI reduction relative to E0 ranged from 1.41% for E10 to 3.04% for E20 with a dedicated BOB, under the scenario of maintaining production with increasing exports.
The CI reduction quantified in this study is often overlooked in assessments of ethanol CO2 impacts. While percent changes in CI are small, the resulting implications for U.S. CO2 emissions are far from insignificant. The US gasoline consumed represents about 700 billion pounds of carbon annually, which yields 1.3 billion short (US) tons of CO2. From the fuel-based CI expected from an informed refinery model, a reversion from very high E10 penetration to a high aromatic E0 gasoline would raise the US CO2 inventory by 18.3 million tons per year.
Conversely, just from fuel CI effects on tank to wheels (TTW) emissions, a move to E20 would offer a beneficial reduction as high as 33 million tons per year at the tailpipe relative to E0 use. Taking into account vehicle efficiency effects embodied in the EPA 1.66% value (NPRM, 2020) for moving from Tier 2 E0 to Tier 3 E10 fuel suggests that E10 currently offers a national TTW reduction of 21.6 million tons of CO2 annually.
Aromatic reduction, explained in this report, not only decreases CI but also reduces PMI, with implied tailpipe PM 2.5 reduction. Gasoline exhaust PM is considered a major health care concern and has gained greater visibility as a result of GDI adoption and the relative improvement of diesel exhaust PM reduction. PMI is not directly correlated with CI, because the weight and structure of the aromatic components have a complex effect on PMI, but efforts to reduce CI have favorable PMI effects. In summary, ethanol blending into gasoline offers an attractive route for reduction of light duty vehicle TTW (tailpipe) GHG emissions while maintaining or raising gasoline octane rating, and while reducing PMI of the fuel
In Well-to-Wheels Carbon Intensity for Ethanol Blended Fuels, the TES study team found that when both direct displacement and reduction of aromatics in the blend are attributed to ethanol as the enabling additive, a “reduced blending CI” (BCI) of 43.4 g/MJ is found for ethanol when it is used in E10. Similarly, for anticipated market blending, the BCI for E20 is 44.8 g/MJ, a greater reduction than the 55.5 g/MJ from unblended ethanol. If the 52.4 g/MJ CI for unblended ethanol from the GREET estimate alone is used, the BCI for ethanol in E10 is about 40 g/MJ. The ethanol BCI is lowered to below 40 g/MJ for E10 if a CO2 correction for engine efficiency from the recent EPA Tier 3 certification fuel study is also attributed to the agency of ethanol. The low BCI of ethanol in E10, E15 and E20 encourages optimized blending of ethanol and gasoline motor fuels for immediate GHG reductions.
A web conference for Transport Energy Outlook members and other interested stakeholders will be held Wednesday, October 20, 2021 from 10-11 am ET (4-5 pm CET). To RSVP, please contact Tammy Klein at firstname.lastname@example.org.