Biofuels for Transportation

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Biofuels for Transportation
U.S. Department of Energy, Office of Science

What is biomass?[edit]

Biomass is renewable biological material, primarily plant matter or products derived from plant matter. Edible biomass includes sugars from sugar cane or sugar beets, starches from corn kernels and other grains, and vegetable oils. The fibrous, woody, and generally inedible portions of plants are called "cellulosic" or "lignocellulosic" biomass because they contain cellulose, hemicellulose, and lignin—key structural components of plant cell walls. Cellulosic biomass is the most plentiful biological material on earth.

Cellulose and hemicellulose are complex carbohydrates that can be broken down into simple sugars convertible to ethanol by microorganisms. Lignin is a complex, noncarbohydrate polymer that binds cellulose and hemicellulose and gives plant cell walls their rigidity. Dry cellulosic biomass consists of about 75% carbohydrates and 25% lignin. See Understanding Biomass: Plant Cell Walls for an illustrated description of plant cell-wall structure.

Sources of cellulosic biomass include stalks and leaves of corn and other crops; tree limbs or vegetation removed to reduce forest fire hazards; wood chips or sawdust from lumber and paper processing; municipal solid waste (e.g., discarded wood or paper products, yard trimmings, food scraps); and grassy or woody crops grown specifically for biofuels production.

What are biofuels and how much does the United States produce?[edit]

Biofuels are liquid, solid, or gaseous fuels derived from renewable biological sources. Biomass can be burned directly for thermal energy or converted to other high-value energy sources including ethanol, biodiesel, methanol, hydrogen, or methane. Currently, ethanol from corn grain and biodiesel are the only biofuels produced in the United States on an industrial scale.

Most of the 4 billion gallons of ethanol produced in 2005 came from 13% of the U.S. corn crop (1.43 billion bushels of corn grain). This represents a 17% increase from the 3.4 billion gallons produced in 2004. Ethanol is widely used as a fuel additive. The oxygen contained in ethanol improves gasoline combustibility. E10 (10% ethanol and 90% gasoline blend) is available from gas stations all over the United States E85 (85% ethanol and 15% gasoline blend) is available mainly in corn-producing states. E85 can be used as a substitute for gasoline in vehicles that have been modified to use this biofuel.

A commercial industry based on converting cellulosic biomass to ethanol does not yet exist in the United States, however, the technology is ready to be deployed in pilot or demonstration facilities. Iogen Corporation, a biotechnology company in Canada, operates the largest demonstration facility, which annually produces about one million gallons of cellulosic ethanol from wheat straw.

Biodiesel is a biologically derived diesel fuel substitute created by chemically reacting vegetable oils or animal fats with alcohol. Most biodiesel in the United States comes from soybean oil or restaurant greases. Biodiesel is readily used by vehicles with diesel engines. In 2005, about 75 million gallons of biodiesel were produced, tripling the 25 million gallons produced in 2004.

What are current levels of gasoline and diesel consumption?[edit]

In 2004, the United States consumed about 140 billion gallons of gasoline, more than any other country. Consumption averages about 380 million gallons of gasoline per day in 2004 and is reaching 400 million gallons per day in 2006. The 3.4 billion gallons of ethanol produced in 2004 represent about 2% of gasoline consumption. The 75 million gallons of biodiesel produced in 2005 represent a tiny fraction of roughly 40 billion gallons of diesel used each year for on-road transportation. (Sources: Annual Energy Outlook 2006 and U.S. Heating Oil, Diesel Fuel, And Distillate Data from the Energy Information Administration) What is current U.S. oil consumption, and how much of that is imported?

In 2004, the United States consumed 20.7 million barrels of petroleum products per day (about 7.5 billion barrels per year). A barrel contains 42 gallons, so total petroleum consumption in 2004 was about 318 billion gallons. Roughly 60% (~190 billion gallons) of petroleum consumed was imported, with about 13% (~40 billion gallons) coming from Persian Gulf countries. The United States primarily imports crude oil but also imports petroleum products including finished motor gasoline, aviation fuel, and fuel oil. The United States imported about 15 billion gallons of finished motor gasoline and gasoline-blending components in 2004. (Source: Annual Energy Outlook 2006 from the Energy Information Administration)

How many gallons of gasoline come from a barrel of oil?[edit]

Crude oil is a complex mixture of hydrocarbons (chemical compounds containing only hydrogen and carbon atoms) with different physical and chemical properties. Refineries separate crude oil into its hydrocarbon components, which are used to create a variety of refined petroleum products including gasoline, diesel, heating oil, jet fuel, and other products. Due to the lower densities of many petroleum products, a 42-gallon barrel of crude oil can generate roughly 44 gallons of petroleum products. About 20 gallons of gasoline and 7 gallons of diesel are produced from each barrel of crude oil. (Source: Where Does My Gasoline Come From?, Energy Information Administration) How much ethanol can we get from an acre of bioenergy crops?

Currently, perennial grass and woody crops have an average yield of about 5 dry tons per acre. Ethanol yield from a dry ton of biomass is about 67 gallons, so today we can obtain roughly 335 gallons of ethanol from an acre of bioenergy crops. If average biomass yields of about 10 to 15 dry tons per acre and ethanol yields of 80 to 100 gallons per dry ton of biomass could be achieved, an acre of bioenergy crops could generate 800 to 1500 gallons of ethanol.

Can one gallon of ethanol displace one gallon of gasoline?[edit]

No. Ethanol has about 70% the energy content of gasoline per unit volume, so for every gallon of gasoline consumed, 1.4 gallons of ethanol would be needed to displace it. Ethanol, however, has a higher octane rating than gasoline — about 113 for ethanol compared to 87 for regular gasoline. The higher the octane rating, the better a fuel is at preventing engine "knocking" caused by inefficient fuel combustion. In other words, the higher-octane fuel provides better performance because it is used more efficiently to generate power rather than heat. If engines were optimized to take advantage of the higher octane rating of ethanol, they could achieve fuel economy more similar to that of gasoline engines.

Can ethanol be used by existing fuel-distribution infrastructure?[edit]

Ethanol and gasoline-ethanol blends cannot be transported by existing pipelines that carry gasoline. Water present in petroleum pipelines can pull ethanol out and cause ethanol-gasoline blends to separate into two phases. Ethanol must be transported by train, barge, or truck within an independent distribution system to ensure handling separate from the ethanol-production facility to distribution terminals, where ethanol is blended with gasoline just before delivery to retail stations.

Can ethanol be used in colder northern U.S. climates?[edit]

Due to ethanol's lower vapor pressure, engine ignition is more difficult in colder weather for vehicles running on fuels with high ethanol content. During winter months, gasoline is added to E85 (85% ethanol and 15% gasoline blend) to make E70 (70% ethanol and 30% gasoline), which has a vapor pressure that improves starting in cold weather. Although current practice is to "blend-down" E85, the cold-start issue is a technologically solvable engineering problem for vehicle manufacturers.


This work is in the public domain in the United States because it is a work of the United States federal government (see 17 U.S.C. 105).