FHWA Resource Center
AIR QUALITY TEAM
MOBILE6 Air Toxics-OXYGENATE and OXYGENATE FUELS Commands
Tianjia Tang
FHWA Resource Center, 61 Forsyth Street, SW, Suite 17T26, Atlanta, GA 30303
404-562-3673 (phone); 404-562-3700 (fax); Tianjia.tang@fhwa.dot.gov
According to EPA's October 2002 edition of User's Guide to MOBILE6.1 and 6.2, Section 2.8.10.7.f, maximum blending volumes for ethers are 20.7% for MTBE, 24.1% for ETBE, and 22.6% for TAME. The correct maximum blending volumes are 15.0% for MTBE, 17.6% for ETBE, and 16.5% for TAME. The maximum blending volume for ethanol (10.7%) is correct as stated in the User's Guide. The original ether blending data are based on the assumption that a 3.7% oxygen weight content waiver is granted to ether blended fuel. The correct information is that blending waiver is only granted to ethanol blending. The correction to the above information is important due to the fact that MOBILE6 will either not run or produce erroneous results without warning with the original maximum volume data.
In the same section of the User's Guide, TAME is expressed as "Tertiary Amine Methyl Ether". This is incorrect. The correct term for TAME is Tertiary Amyl Methyl Ether. Amine is referring to a nitrogen atom bonded to carbon and/or hydrogen atoms. Amyl is referring to the C5H11 radical. To add an amine compound to gasoline will most likely increase NOx production, which is not desirable.
Weighted oxygen weight content data for ether blended gasoline are required by the OXYGENATED FUELS command (Section 2.8.10.4). The equation listed below can be used to process data contained in the OXYGENATE command to obtain the weighted oxygen weight content data.
Equation:
Weighted oxygen weight content = (WT%MTBE x MARKET%MTBE + WT%ETBE x MARKET%ETBE + WT%TAME x MARKET%TAME)/(MARKET%MTBE + MARKET%ETBE + MARKET%TAME)
WT%MTBE: oxygen weight content of MTBE blended fuel
MARKET%MTBE: MTBE blended market share
WT%ETBE: oxygen weight content of ETBE blended fuel
MARKET%ETBE: ETBE blended market share
WT%TAME: oxygen weight content of TAME blended fuel
MARKET%TAME): TAME blended market share
Equation Constrains:
1: (MARKET%MTBE + MARKET%ETBE + MARKET%TAME) < or =(100% - market share of ethanol blended fuel).
2: Total market shares for both ether and ethanol blended fuels shall be no more than 100%.
Example 1: To compute weighted oxygen weight content
9.5% of fuel sold in an area is 10.0% volume MTBE blended; 3.0% of fuel sold in the area is 12.0% volume ETBE blended; and no fuel is blended with TAME. Alcohol blended fuel has a market share of 10.0%. What is the weighted oxygen weight content of ether blended fuel?
Solution:
Step 1: To verify market shares.
9.5% + 3.0% +0% + 10.0% = 22.5% < 100%
The market share data is valid.
Step 2: To compute oxygen weight contents from volume blending data.
WT%MTBE = 0.1786 X 10% = 0.0179
WT%ETBE = 0.1533 X 12% = 0.0184
WT%TAME = 0.1636 X 0% = 0
Step 3: To compute weighted oxygen weight content for ether blended fuels.
Weighted oxygen weight content = (9.5% x 0.0179 + 3.0% x 0.0184 + 0 x 0)/(9.5% + 3.0% + 0) = 0.018 < 2.7%
Since 0.018 < 2.7%, the result is valid.
FINAL ANSWER = 0.018 = 1.8%
Example 2:
To demonstrate how invalid ether blending data can still run through MOBILE6 without being warned.
9.5% of fuel sold in an area is 20% volume MTBE blended (20% is not valid); 70.0% of fuel sold in the area is 0.1% volume ETBE blended; and no fuel is blended with TAME. Alcohol blended fuel has a market share of 10%. To calculate the weighted oxygen weight content of ether blended fuel for the OXYGENATED FUELS command.
SOLUTION:
Step 1: To verify market shares.
9.5% + 70.0% +0% + 10.0% = 89.5% < 100%
The market share data is valid.
Step 2: To compute oxygen weight content from volume blending data.
WT%MTBE = 0.1786 X 10% = 0.01786
WT%ETBE = 0.1533 X 0.1% = 0.0001533
WT%TAME = 0.1636 X 0% = 0
Step 3: To compute weighted oxygen weight content for ether blended fuels.
Weighted oxygen weight content = (9.5% x 0.01786 + 70.0% x 0.0001533 + 0 x 0)/(9.5% + 70.0% + 0) = 0.0023 < 2.7%
FINAL ANSWER = 0.23%
Data in Example 2 will work for both the OXYGENATE and OXYGENATED FUELS commands. MOBILE6 will not be able to automatically detect the fact that MTBE exceeds the maximum blending ratio due to the fact the weighted oxygen weight content is below the 2.7% level.
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