running on biofuels emit carbon dioxide
(CO2), the primary source of greenhouse
gas emissions, just like those running on
gasoline. However, because plants and trees
are the raw material for biofuels, and,
because they need carbon dioxide to grow,
the use of biofuels does not add CO2 to
the atmosphere, it just recycles what was
already there. The use of fossil fuels,
on the other hand, releases carbon that
has been stored underground for millions
of years, and those emissions represent
a net addition of CO2 to the atmosphere.
Because it takes fossil fuels – such
as natural gas and coal – to make
biofuels, they are not quite “carbon
Argonne National Laboratory has carried
out detailed analyses of the “well-to-wheels”
greenhouse gas emissions of many different
engine and fuel combinations. The chart
at right shows a few selected examples.
Argonne’s latest analysis shows reductions
in global warming emissions of 20% from
corn ethanol and 85% from cellulosic ethanol.
gas emissions in an E85 blend using corn
ethanol would be 17% lower than gasoline,
and using cellulosic ethanol
would be 64% lower.
A separate analysis found that biodiesel
reduces greenhouse gas emissions by 41%;
thus, a B20 blend would achieve a reduction
of about 8%.
achieves such high reductions for several
1. Virtually no fossil fuel is used
in the conversion process, because waste
biomass material, in the form of lignin,
makes an excellent boiler fuel and can be
substituted for coal or natural gas to provide
the heat needed for the ethanol process.
2. Farming of cellulosic biomass is much
less chemical- and energy-intensive than
farming of corn.
3. Perennial crops store carbon in the soil
through their roots, acting as a carbon
“sink” and replenishing carbon
in the soil. Switchgrass, for example, has
a huge root system that penetrates over
10 feet into the soil and weighs as much
as one year’s growth aboveground (6-8
tons per acre).
ethanol is at least as likely as
hydrogen to be an energy carrier
of choice for a sustainable transportation
– Natural Resources Defense
Council, Union of Concerned Scientists
Compared to conventional diesel fuel, the
use of biodiesel results in an overall reduction
of smog-forming emissions from particulate
matter, unburned hydrocarbons, and carbon
monoxide, as shown at right. Biodiesel slightly
increases nitrogen oxide emissions, by about
2% in a B20 blend.
Sulfur oxides and sulfates, which are major
components of acid rain, are not present
As for ethanol, the oxygen atom in the ethanol
molecule leads to more complete fuel combustion
and generally fewer emissions. E10 blends
have been credited with reducing emissions
of carbon monoxide by as much as 30% and
particulates by 50%.
However, mixing low levels of ethanol (2%
to 10%) with gasoline increases the blend’s
tendency to evaporate and contribute to
low-level ozone unless the gasoline itself
This problem diminishes with higher levels
of ethanol. At blends between 25% and 45%,
the fuel is equivalent to gasoline, and
at higher blends it is less evaporative.
E85 has about half the volatility (tendency
to evaporate) of gasoline.
The effect of E85 on air quality is almost
uniformly positive, with the exception of
increased emissions of aldehydes, such as
acetaldehyde. Conventional catalytic converters
control these emissions in ethanol blends
of up to 23%,
and it is expected that they could be readily
adapted to E85 blends.
A test of advanced emission control systems
in three conventional gasoline vehicles
found that advanced systems reduced formaldehyde
emissions by an average of 85% and acetaldehyde
by an average of 58%.
Even without advanced controls, the benefits
of reducing other toxic emissions outweigh
the effects of aldehydes. The National Renewable
Energy Laboratory tested a 1998 Ford Taurus
FFV running on E85 and reported: “Emissions
of total potency weighted toxics (including
benzene, 1,3-butadiene, formaldehyde, and
acetaldehyde) for the FFV Taurus tested
on E85 were 55% lower than that of the FFV
tested on gasoline.”
Emissions characteristics of E85*
Actual emissions will vary with
engine design; these numbers reflect
the potential reductions offered
by ethanol (E85), relative to conventional
Fewer total toxics are produced.
• Reductions in ozone-forming
volatile organic compounds of
• Reductions in carbon monoxide
• Reductions in particulate
emissions of 20%.
• Reductions in nitrogen
oxide emissions of 10%.
• Reductions in sulfate
emissions of 80%.
• Lower reactivity of hydrocarbon
• Higher ethanol and acetaldehyde
based on ethanol’s inherently
properties with an engine that
takes full advantage of these
– U.S. Environmental Protection
The principal contributor to toxic air pollution
from gasoline is a class of chemical compounds
called aromatics, which make up an average
of 26% of every gallon of gasoline. Blended
with gasoline to increase octane, aromatics
have the potential to cause cancer, and
they also result in emissions of fine particulates
and smog-forming gases that harm lung function
and worsen asthma.
was required by the Clean Air Act Amendments
of 1990 to seek “the greatest degree
of emission reduction achievable”
of air toxics in automobiles.
In response to recent litigation, the EPA
issued a rule to reduce one of these hazardous
air pollutants, benzene, but the agency
did not address the two other aromatic compounds,
toluene and xylene, which form benzene
during combustion. Using biofuels instead
of aromatics to improve octane would result
in public health benefits worth tens of
billions of dollars from the reduction in
emissions of small particles alone.
Native perennial grasses such as switchgrass
had to be tough to survive on the prairie.
They are deep-rooted and drought-resistant
and require less water than food crops.
They also need less fertilizer, herbicide,
insecticide, and fungicide per ton of biomass
than conventional crops.
Switchgrass is an approved cover crop under
the Conservation Reserve Program because
it prevents soil erosion and filters runoff
from fields planted with traditional row
crops. Buffer strips of switchgrass, planted
along stream banks and around wetlands,
can remove soil particles, pesticides, and
fertilizer residues from surface water before
they reach ground water or streams.
There are enough
varieties of prairie grass and other sources
of cellulosic biomass that farmers need
not all rely on a single energy crop –
so-called monocultures. Indeed, recent research
suggests that mixed prairie grasses may
be more productive than monocultures.
One study found that a diverse mixture of
grasses grown on degraded land would yield
51% more energy per acre than ethanol from
corn grown on fertile land.
general, perennial energy crops create more
diverse habitats than annual row crops,
attracting more species and supporting larger
populations. Switchgrass fields are popular
with hunters, as they provide habitat for
many species of wildlife, including cover
for deer and rabbits and a nesting place
for wild turkey and quail
– and pheasants, as shown at right.
As long as farmers avoid work that would
disturb the birds during nesting or breeding
seasons, their fields will remain popular