Biofuels such as ethanol and biodiesel can make a big difference in improving our environment, helping our economy, and reducing our dependence on foreign oil. This page discusses biofuels research supported by the Bioenergy Technologies Office.
Biofuels for Transportation
Most vehicles on the road today are fueled by gasoline and diesel fuels, which are produced from oil—a non-renewable fossil fuel, meaning the resources may eventually run out. Renewable resources, in contrast, are constantly replenished and are unlikely to run out. Biomass, which includes plants and organic wastes, is one type of renewable resource. The Bioenergy Technologies Office supports research on how to convert biomass into liquid fuels (biofuels) for transportation. Using biofuels will reduce pollution and U.S. dependence on imported oil. The Alternative Fuels Data Center provides more information on how biofuels are used in vehicles today.
Ethanol is an alcohol made by fermenting the sugar components of biomass. Today, it is made mostly from sugar and starch crops. With the Bioenergy Technologies Office developing advanced technology, cellulosic biomass—like trees and grasses—are also used as feedstocks for ethanol production. Ethanol is now used as a fuel additive for cars at a 10% blend, further reducing the need for oil imports. It is also blended in gasoline as an oxygenate to increase octane and improve vehicle emissions.
Through photosynthesis, plants use light energy from the sun to convert water and carbon dioxide to sugars. Researchers are studying how the sugars in the biomass can be converted to more usable forms of energy like electricity and fuels. Some plants, like sugar cane and sugar beets, store energy as simple sugars, which are mostly used for food. Other plants store the energy as more complex sugars, called starches. These plants include grains like corn and are also used for food and animal feed grains.
Another type of plant matter, called cellulosic biomass, is made up of very complex sugar polymers and is not generally used as a food source. This type of biomass is currently being developed as a feedstock for ethanol production. Specific examples include agricultural residues (leftover material from crops, such as the stalks, leaves, and husks of corn plants); forestry wastes (chips and sawdust from lumber mills, dead trees, and tree branches); municipal solid waste (household garbage and paper products); and energy crops (fast-growing trees and grasses) developed just for this purpose.
The main components of these types of biomass are:
- Cellulose is the most common form of carbon in biomass, accounting for 40%–60% by weight of the biomass, depending on the biomass source. It is a complex sugar polymer, or polysaccharide, made from the six-carbon sugar, glucose.
- Hemicellulose is also a major source of carbon in biomass, at levels from 20%–40% by weight. It is a complex polysaccharide made from a variety of five- and six-carbon sugars.
- Lignin is a complex polymer, which provides structural integrity in plants. It makes up 10%–24% by weight of biomass. It remains as residual material after the sugars in the biomass have been converted to ethanol and contains a lot of energy.
The basic processes for converting sugar and starch crops to ethanol are well-known and used commercially today. These processes include:
- Biomass Handling. Biomass goes through a size-reduction step to make it easier to handle and to make the ethanol production process more efficient. For example, agricultural residues go through a grinding process and wood goes through a chipping process to achieve a uniform particle size.
- Biomass Pretreatment. In this step, the hemicellulose fraction of the biomass is broken down into simple sugars. A chemical reaction called hydrolysis occurs when dilute sulfuric acid is mixed with the biomass feedstock. In this hydrolysis reaction, the complex chains of sugars that make up the hemicellulose are broken, releasing simple sugars. The complex hemicellulose sugars are converted to a mix of soluble five-carbon sugars, xylose and arabinose, and soluble six-carbon sugars, mannose and galactose. A small portion of the cellulose is also converted to glucose in this step.
- Enzyme Production. The cellulase enzymes that are used to break down the cellulose in the biomass are grown in this step. Alternatively the enzymes might be purchased from commercial enzyme companies.
- Cellulose Hydrolysis. In this step, the remaining cellulose is hydrolyzed to glucose. In this enzymatic hydrolysis reaction, cellulase enzymes are used to break the chains of sugars that make up the cellulose, releasing glucose.
- Glucose Fermentation. The glucose is converted to ethanol, through a process called fermentation. Fermentation is a series of chemical reactions that convert sugars to ethanol. The fermentation reaction, which is caused by yeast or bacteria that feed on the sugars, produces ethanol and carbon dioxide.
- Pentose Fermentation. The hemicellulose fraction of biomass is rich in five-carbon sugars, which are also called pentoses. Xylose, the most prevalent pentose released by the hemicellulose hydrolysis reaction, is fermented using Zymomonas mobilis or other genetically engineered bacteria in this step.
- Ethanol Recovery. The fermentation product from the glucose and pentose fermentation is called ethanol broth. In this step, the ethanol is separated from the other components in the broth. A final dehydration step removes any remaining water from the ethanol.
- Lignin Utilization. Lignin and other byproducts of the biomass-to-ethanol process can be used to produce the electricity required for the ethanol production process. Burning lignin actually creates more energy than needed and selling electricity may help the process economics.
Converting cellulosic biomass to ethanol is currently too expensive to be used on a commercial scale. So researchers are working to improve the efficiency and economics of the ethanol production process by focusing their efforts on the two most challenging steps, cellulose hydrolysis and pentose fermentation.
Biodiesel is a mixture of fatty acid alkyl esters made from vegetable oils, animal fats, or recycled greases. Biodiesel can be used as a fuel for vehicles in its pure form, but it is usually used as a petroleum diesel additive to reduce levels of particulates, carbon monoxide, hydrocarbons, and air toxics from diesel-powered vehicles.
In the United States, most biodiesel is made from soybean oil or recycled cooking oils. Animal fats, other vegetable oils, and other recycled oils can also be used to produce biodiesel, depending on their costs and availability. In the future, blends of all kinds of fats and oils may be used to produce biodiesel.
Key Reaction. The main reaction for converting oil to biodiesel is called transesterification. The transesterification process causes a reaction between an alcohol (like methanol) and triglyceride oils contained in vegetable oils, animal fats, or recycled greases, which forms fatty acid alkyl esters (biodiesel) and glycerin. The reaction requires heat and a strong base catalyst, such as sodium hydroxide or potassium hydroxide.
Renewable diesel is a good alternative to petroleum-based diesel. It is chemically similar to regular diesel, but releases the lowest emissions of the three. Unlike biodiesel, renewable diesel doesn't have an ester structure because it does not undergo transesterification.
Renewable Diesel Feedstocks
Renewable diesel can be produced from a variety of oil seeds, including canola oil, soy beans, algae, palm oil, and sunflower oil. Renewable diesel is also noted for its non-plant sources, such as industrial and solid waste and animal fats.
Renewable Diesel Production
Like biodiesel, renewable diesel can be made from animal fats and oils, but unlike biodiesel, renewable diesel is made by using a process called hydrotreating. Hydrotreating was once originally used to remove sulfur from petroleum-based diesel, but in this process, the hydrogen replaces sulfur, nitrogen, and oxygen atoms. There are other methods of making renewable diesel, such as biomass-to-liquid and pyrolysis, but these are used for different resources, such as hydrocarbons or cellulosic biomass.