Appendix C. Ethanol and MTBE Emissions Comparison

The different properties of ethanol and MTBE cause refiners to have to make further investments in order to achieve the same level of toxic emissions reduction when switching from using MTBE to ethanol in reformulated gasoline. This section provides a comparison to better understand the issues. The complex formula developed by EPA is used to determine the emission characteristics of reformulated gasoline. It is based on empirical data that related various emissions to gasoline properties. Refiners use this model to develop gasoline blends that will meet the Federal RFG requirements.

Federal reformulated gasoline requirements for the most part are stated in terms of emission reductions required from an industry base gasoline that is defined in the final rule. Table 1 shows the properties of that base gasoline, and in the far right column, it shows what reductions are necessary from that baseline gasoline to meet reformulated gasoline requirements.

In the column next to the base gasoline is an RFG blend that uses MTBE. Using the complex model to estimate emissions, the 11.2 percent MTBE case shows that VOC’s are almost 26 percent lower than the industry baseline fuel, toxics are over 33 percent lower, and NOx emissions are reduced by about 8 percent. The Federal requirements are shown in the far right column. The Table shows that the MTBE blend far exceeds Federal requirements in everything except VOC’s, where it just meets that requirement.

Using the complex model, the next column of the table illustrates the emission changes that would occur when MTBE is removed and ethanol is put into the gasoline. No other changes are made to the gasoline.

Many refiners will only use 5.8 percent ethanol in their RFG, rather than higher amounts, both for economical and emission constraint reasons. With 5.8 percent ethanol, all emissions are higher, but only VOC emissions do not achieve Federal reformulated gasoline requirements. A refiner would have to remove other light, high-RVP components to bring the VOC’s within Federal limits. Although the toxics are within reformulated gasoline requirements, they violate MSAT for this refiner. The toxics in the 5.8 percent ethanol case at 61.4 milligrams per mile (mg/mi) are higher than in the MTBE case at 57.1 mg/mi, which violates MSAT’s anti-backsliding restriction. The major toxic component causing the difference is a large increase in acetaldehyde. This refiner, when switching from MTBE RFG to ethanol-blended RFG, would have to make further refinery changes to reduce toxics.

For a refiner that wants to use 10 percent ethanol in RFG, most emissions are higher than when using MTBE, as was the case when using 5.8 percent ethanol. Again, toxics and NOx fall within the Federal RFG requirements, while VOC emissions exceed Federal RFG limits. Since toxic emissions are greater than in the MTBE case, the 10 percent ethanol blend gasoline would not meet this refiner’s MSAT requirement. The 10 percent case provides an illustration of another dimension when using ethanol. Comparing the toxics in the 5.8 percent case to the 10 percent case, note that two effects are occurring: acetaldehyde increased when moving from 5.8 to 10 percent ethanol, but other toxics declined. A dilution effect occurs as the 10 percent ethanol dilutes the content of sulfur and aromatics, which are the main determinants of exhaust benzene, for example. So exhaust benzene (and most other toxic components) drop when moving from 5.8 to 10 percent ethanol, but they do not drop enough to match MTBE’s toxic performance in total.

Appendix C. Ethanol and MTBE Emissions Comparison - Tables