I started this thread because there was no scientific reason for the 10% ethanol blend. Now I found proof. This is an exerpt from a 280 page report put out this month from the U.S. EPA. I don't expect you to read all 280 pages ( but graph lovers, there's about 100 graphs! ) but these exerpts help explain why we see 10% at the pumps.

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PAGE 36
http://www.epa.gov/otaq/regs/fuels/r...r08002body.pdf

The increased use of ethanol has resulted in a higher percentage of RFG being oxygenated with ethanol,
a larger fraction of total oxygen in RFG being supplied by ethanol and an increase, on average, in the total oxygen content of RFG.
The total oxygen content has increased along with the ethanol share because most ethanoloxygenated
RFG supplied outside of California contains about 10 volume percent ethanol, providing about
3.5 weight percent oxygen, even though RFG was only required to have only 2 weight percent oxygen.
This occurred, in part, because, until a recent legislative change (American Jobs Creation Act, 2004), a
federal excise tax exemption available to ethanol was greatest when ethanol was blended at 10 volume
percent and pro-rated for blending at 5.7 or 7.7 volume percent (see the Oxygenates chapter).
Additionally, MTBE must be used at around 11 volume percent in order to provide the required 2 weight
percent oxygen. Thus, when ethanol is used to replace MTBE, blending at 10 volume percent replaces
much of the lost MTBE volume.

Taken from Pages 64-66

Oxygenates have been blended into gasoline in order to comply with regulatory requirements
intended to reduce air pollutant emissions from gasoline vehicles and engines. The Clean Air Act
required that reformulated gasoline (RFG) contain 2.0 weight percent oxygen. RFG was intended to
reduce emissions of toxics and ozone-forming pollutants. Oxygenates provided direct toxics and exhaust
VOC emission reduction benefits through the effect of oxygen on combustion, as well as indirect benefits
through dilution of, or partial substitution for gasoline blendstocks containing sulfur, aromatics or benzene
(constituents which can adversely impact emissions). Oxygen also directly reduces carbon monoxide
(CO) emissions through its effect on combustion. The Clean Air Act required that states adopt
oxygenated fuel programs for carbon monoxide non-attainment areas during the portion of the year
(typically winter) in which the area is prone to high ambient CO concentrations.
Oxygenates are also used in gasoline blending for reasons unrelated to emission reduction. Both
ethers and alcohols have a high blending octane value, making them at times economically attractive
blending component choices. Ethanol is a renewable fuel which, in the United States, is primarily
produced from corn. Federal tax incentives
have encouraged the use of ethanol in gasoline. These incentives included an excise tax exemption
available for gasoline alcohol blends. The amount of the exemption depended on the ethanol content of
the blend, with the maximum exemption available for 10% by volume ethanol blends and pro-rated
amounts for 7.7% and 5.7% blends (based on 190 proof or 95% pure ethanol). Additionally, blenders
receive a tax credit for blending ethanol into gasoline. The American Jobs Creation Act of 2004, signed
on October 22, 2004 changed the way the excise tax exemption operates. The amount of the exemption
is no longer based on these three blend levels.
The Energy Policy Act of 2005, repealed the RFG oxygen content requirement, effective
immediately in California and 270 days after enactment elsewhere. It added a gasoline renewable fuel
content requirement of 4 billion gallons in 2006, incrementally increasing to 7.5 billion gallons in 2012.
These recent legislative changes will clearly impact the composition of future RFG and CG. ( Reformulated Gasoline and Conventional Gasoline )

In order to provide gasoline meeting a specific oxygen content requirement, oxygenates such as MTBE or
ethanol are blended into a hydrocarbon blendstock. As noted, oxygenates contain hydrogen and carbon
as well as oxygen, and different oxygenates contain different amounts of oxygen. For example, a pound
of pure MTBE contains about 0.18 pounds of oxygen, while a pound of pure ethanol contains about 0.35
pounds of oxygen. Therefore, it would be necessary to blend about 11 pounds of MTBE with 89 pounds
of non-oxygenated blendstock to produce 100 pounds of 2.0 weight percent oxygenated gasoline, which
would contain 11% MTBE, by weight. Alternatively, it would be necessary to blend about 5.7 pounds of
pure ethanol with 94.3 pounds of non-oxygenated blendstock to produce 100 pounds of 2.0 weight
percent oxygenated gasoline, which would contain 5.7% ethanol, by weight.

Since gasoline and oxygenates are liquids, oxygenate concentrations are more commonly
expressed as volume percentages rather than weight percentages. There are no universal weight percent
to volume percent conversion factors because each gasoline blend is a different mixture of hydrocarbons.
In order to convert exactly between a weight and volume oxygenate concentration it is necessary to
know the density of the gasoline-oxygenate blend, which varies from blend to blend. (There are
procedures to measure the density of a gasoline-oxygenate blend and these measurements are included
in the reporting system data submitted to EPA.) However, even if the density of the specific gasolineoxygenate
blend is not known, a reasonably accurate weight to volume or volume to weight conversion
can be made because the densities of these blends vary over a fairly narrow range. A 5.7 weight
percent pure (anhydrous and 200 proof) ethanol blended gasoline would typically contain slightly less
than 5.7 volume percent of pure ethanol. However, a 5.7 volume percent ethanol concentration is often
nominally associated with a 2.0 weight percent oxygen concentration, 7.7 volume percent ethanol with
2.7 weight percent oxygen and 10 volume percent ethanol with 3.5 weight percent oxygen. The 2.0%
and 2.7% oxygen weights and the 10 volume percent ethanol content have significance with respect to
Clean Air Act requirements and EPA regulations.
As noted, (prior to the effective date of the repeal of this provision by the 2005 energy
legislation) the Clean Air Act required that RFG contain 2.0 weight percent oxygen. EPA's regulations
allowed refiners, importers and oxygenate blenders to meet this requirement through compliance with a
2.0 weight percent per gallon standard, or a 2.1 weight percent “averaged standard” and a 1.5 weight
percent per gallon minimum. The per gallon minimum applicable to certain suppliers was adjusted as a
consequence of RFG Survey failures for oxygen content, and for 2005 remained at 1.6 weight percent for
certain suppliers and covered areas. These RFG oxygen requirements applied throughout the year. The
Clean Air Act also required an oxygen credit program; i.e. parties using more than the required amount of
oxygen in RFG generate credits which may be transferred to other parties for use in meeting the oxygen
standard.
The Clean Air Act's oxygenated fuel requirements (Section 211(m)) are intended to address CO
non-attainment. States may also have “maintenance” programs. These oxygenated fuel programs are
seasonal and state-specific. The Clean Air Act specified that the oxygen content for the non-attainment
areas requiring such programs be at least 2.7 weight percent. While some areas currently have winter
oxygenated fuel programs a number of areas, including several areas in the RFG program, at one time
had programs but no longer implement them.

In addition to the requirements for oxygen use, there are several restrictions on the maximum
oxygen content and maximum amount of certain specific oxygenates that can be used. Both the Simple
Model and Complex Model have valid range limits of 4.0 weight percent for oxygen. However, these
range limits had little effect on the maximum oxygen content of gasoline since other requirements, in
place long before the RFG and Anti-Dumping programs, were as restrictive or more restrictive.
The Clean Air Act (‘211(f)(1)) prohibits fuel and fuel additive manufacturers from first introducing
into commerce, or increasing the concentration in use of any fuel or fuel additive for general use in light
duty motor vehicles which is not substantially similar to that utilized in the certification of motor vehicles
or engines under section 206 of the Act. A manufacturer may apply for a waiver of this provision for a
fuel or fuel additive which is not substantially similar. The Act, however, does not define “substantially
similar”, consequently EPA defined "substantially similar" as it applies to unleaded gasoline in an
interpretive rule, last revised in 1991 (56 FR 5352). 28
EPA's "substantially similar" definition allows certain alcohols (other than methanol) and ethers
including ethanol and MTBE, provided that the oxygen content does not exceed 2.7 weight percent. This
oxygen weight allows approximately 15% MTBE by volume, but the exact volume of MTBE allowed under
this definition is blend-specific. However, a waiver has also been granted which, under some conditions,
allows 15% MTBE by volume even if the oxygen content weight limit is exceeded. This oxygen weight
percent limit would only allow ethanol blending up to about 7.7 volume percent. However, the
“gasohol” waiver allows use of up to 10% by volume pure ethanol. This waiver is particularly significant
since ethanol is often blended at about 10 volume percent in order to take full advantage of the tax
incentives. This waiver allows approximately 3.7 weight percent oxygen, but again, this is blend-specific.
The definition and several waivers also allow use of methanol as an oxygenate, but the conditions for
methanol use are much more restrictive. Unless it is used with other oxygenates, methanol can only be
used at 0.3% by volume (about 0.16 weight percent oxygen). Additionally, health effects testing
requirements apply to methanol blends containing oxygen at 1.5 weight percent or greater. (Trace
quantities of methanol were often found in MTBE-oxygenated gasoline since methanol was used to
produce MTBE.) A document summarizing 211(f) waiver requests and EPA decisions is available on EPA's
website (EPA, 1995).
EPA's Oxygenate and Oxygen Data

One complication with this reporting is that ethanol blending typically does not occur at refineries,
but downstream at terminals. (This is necessary because gasoline is often shipped via pipeline and there
are problems associated with pipeline shipment of ethanol-blended gasoline due to ethanol's affinity for
water, which is present to some extent in pipelines.) Although this is less common, MTBE and other
ethers may also be blended downstream of the refinery. Thus, refiners generally produce and ship
reformulated blendstock for oxygenate blending (RBOB) rather than finished ethanol-oxygenated RFG.
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On that last part, I wanted to do some "MythBusting". People often say ethanol is bad because it cannot be shipped in existing piplines, and they say it is because ethanol puts water into the pipes. This is FALSE FALSE FALSE!!! Ethanol cannot be shipped in existint pipes because the pipes put water into the ethanol!!!

Ethanol is good for pipes, the pipes are bad for the ethanol!!!

-John