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Engines 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 neutral.”

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. Thus, greenhouse 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%.

Cellulosic ethanol achieves such high reductions for several reasons:

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).

“Cellulosic ethanol is at least as likely as hydrogen to be an energy carrier of choice for a sustainable transportation sector.”
– Natural Resources Defense Council, Union of Concerned Scientists

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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 in biodiesel.

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 is adjusted. 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 gasoline.

• Fewer total toxics are produced.
• Reductions in ozone-forming volatile organic compounds of 15%.
• Reductions in carbon monoxide of 40%.
• Reductions in particulate emissions of 20%.
• Reductions in nitrogen oxide emissions of 10%.
• Reductions in sulfate emissions of 80%.
• Lower reactivity of hydrocarbon emissions.
• Higher ethanol and acetaldehyde emissions.

* Estimates based on ethanol’s inherently “cleaner” chemical properties with an engine that takes full advantage of these fuel properties.
– U.S. Environmental Protection Agency

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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.

The EPA 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.

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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.

In 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 with wildlife.

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