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9. MTBE
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Ethanol and Decision Making
There are
much larger issues with ethanol than simply
whether it can be used to fill in for MTBE.
These
include:
- ENERGY
BALANCE: Does it take more fossil energy
to produce ethanol than ethanol itself
delivers? Expert views vary considerably,
but if so, ethanol would not be
renewable, and thus would not help
counteract global warming.
- FOOD
OR FUEL: What about the starving people
of the world? If we use food grains to
fuel up our BMWs and SUVs, what will
happen to them? On the other hand, corn
ethanol producers and farm associations
say ethanol comes from corn starch and
the protein in the corn is fed to cattle
as "distillers grains." If the
distillers grains are fresh, cows seem to
prefer them. How do we balance these
conflicting views?
- EFFECT
ON AGRICULTURE: Is ethanol a renewable
fuel if the agricultural system it is
based on is depleting soil and water
reserves in the Midwest? Of particular
concern is the Oglalla aquifer which is
used to irrigate some of the
nation’s most fertile corn growing
lands.
- LONG
TERM RESOURCE BASE: How close are we to
establishing a renewable liquid fuel
system using cellulosic biomass as a
feedstock instead of farm crops? The
California Energy Commission ethanol plan
of March, 2001 includes cellulose biomass
refineries, which also part of the Bush
national energy plan of May, 2001. Are
these types of facilities technologically
feasible?
- FUEL
ECONOMY: Although ethanol has fewer BTUs
than gasoline, its higher octane value
does allow for more efficient operation
in internal combustion engines with
higher compression ratios. In other
words, an ordinary gasoline engine may
get lower mileage, but a better adapted
engine would not. One interesting new
twist in this area is a proposal to allow
auto companies to meet fuel efficiency
requirements by creating cars that can
use either pure gasoline or pure ethanol.
The Sierra Club
calls this proposal a
"greenwash."
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The requirement for an
oxygen component in reformulated gasoline(RFG) has led
the oil industry to include two major fuel additives: MTBE and ethanol. Originally intended to clean
up the air, MTBE turned out to be what California finds a
serious water pollutant. Ethanol, on the other hand,
breaks down quickly if spilled and does not pollute water
supplies. Should the RFG program continue with ethanol
alone, or should it remove the requirement that RFG
contain oxygenated fuels?
To help determine
this, we need to consider the facts about ethanol.
Perhaps the most
important fact about ethanol is that it has been
controversial for most of the 20th century.
There is often an agrarian, prairie populist flavor to
pro-ethanol positions and an oil industry bias in many of
the anti-ethanol positions. The technical and scientific
questions about ethanol are often posed and fought for
political reasons that are not always obvious.
Technically and
economically, it is feasible to increase ethanol
production to substitute for MTBE’s position in the
RFG program. A little over 4 billion gallons of MTBE was
being blended into the 130 billion gallons of gasoline
sold in the year 2001. This 11 percent volume met the
Clean Air Act requirement of a two percent level of
oxygen in RFG. Ethanol, with twice the amount of oxygen,
could meet the same requirement with about 2.3 billion
gallons, which was approximately the projected industry
capacity at the end of 2001.
The drawbacks to
replacing MTBE with ethanol, according to the US
Department of Energy, include:
- Ethanol
containing fuel's higher volatility (which makes
it harder for refineries to meet the RFG
standard).
- A smaller
dilution effect (because 5.5 percent can be used
instead of 11 percent)
- Presence of
small amounts of sulfur in denaturing compounds
(which make the ethanol undrinkable).
- Can’t
ship ethanol via pipeline, so transportation
costs out of the Midwest are high.
None of these are
necessarily insurmountable problems, ethanol supporters
say. For example, ethanol is less photochemically
reactive when it evaporates, which means that volatility
is less of an issue with ethanol because it doesn’t
tend to form smog and ozone the same way that gasoline
does. They also say that ethanol has, indeed, been
shipped by pipeline.
No one disputes
that the price will be higher, but the DOE’s
estimate of 3.5 cents per gallon does not present
overwhelming financial difficulties.
Given this,
the EPA’s Blue Ribbon Commission on MTBE recommended
that the oxygen requirements be dropped from the Clean
Air Act. Congress would make the final choice between
oxygen with ethanol and a waiver of the oxygen
requirements.
California, the
state most affected by the MTBE / ethanol choice, backed
the waiver but said it could comply with an ethanol
– only standard. It even backed a plan to invest in
cellulose to ethanol production facilities – an echo
of Kettering’s original vision for auto fuels and
octane boosters.
Ethanol Containing Fuel - A
Volatility Paradox:
Ethanol containing fuels have a
higher volatility - a higher composite vapor
pressure than corresponding MTBE-containing
fuels. You
can see this data. But, as shown by its
boiling point and vapor pressure, ethanol
itself is much less volatile than MTBE.
So why would a gasoline
with 10% ethanol be more volatile - have a higher
vapor pressure than a similarly constituted
gasoline with 10% MTBE?
The answer is based on deviations
from Raoult's Law caused by variation in
intermolecular forces in pure alcohol and in
hydrocarbon solution.
Most gasoline components follow
Raoult's Law - that is their individual component
vapor pressure is the product of their pure
component vapor pressure times their mole
fraction. This Raoult 's Law behavior allows us
to predict the vapor properties of most gasoline
blends - each component will contribute according
to its concentration in the final blend.
But consider pure alcohol. At MW
46 it has a boiling point of 86 degrees Celsius,
far above what we would predict of a material of
that low molecular weight. Propane, MW 44, is a
gas! We explain the high pure ethanol boiling
point and low vapor pressure by the
intermolecular forces due to hydrogen bonding
between ethanol molecules. The hydrogen of one
molecule and the electron pair on oxygen on a
second molecule attract each other and additional
energy - more heat - is required to separate the
molecules to form a gas - to boil. And ethanol
has a high dipole moment - a skewed electron
distribution that establishes an additional
intermolecular attraction.
But in hydrocarbon solution, the
ethanol molecules are separated from each other
by the preponderence of nonpolar, hydrocarbon
molecules! Ethanol is soluble, but each polar,
hydrogen bonding molecule cannot find the ready
association with another ethanol that increases
boiling point and lower volatility. Ethanol acts
more like it has a MW of 46! Its partial vapor
pressure is a lot higher than we predict from
Raoult's Law. This deviation from linear,
"ideal" behavior that increases ethanol
containing gasoline volatility, is a common
phenomenon in chemistry.
Table
of Fuel Physical and Chemical Properties
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Additional
information:
Renewable Fuels Association (ethanol trade association)
National Corn Growers
Association
California Energy
Commission report on ethanol from
biomass
American Petroleum
Institute statement on alternative
fuels
Oxygenated Fuels Association (MTBE trade association)
US Department of Energy
Alternative Fuels
Data Center articles on ethanol
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