ABC's of Biofuels
Hey students! Biofuels such as bioethanol and biodiesel can make a big difference in improving our environment, helping our economy, and reducing our dependence on foreign oil. This page tells all about biofuels research by the U.S. Department of Energy (DOE) Biomass Program. Read on to find out about:
- Biofuels for Transportation
- Bioethanol Feedstocks
- Bioethanol Production
- Biodiesel Feedstocks
- Biodiesel Production
- Biofuels and the Environment
- Biomass and Biofuels Science Fair Project Ideas
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Most cars and trucks on the road today are fueled by gasoline and diesel fuels. These fuels are produced from oil, which is a non-renewable fossil fuel. Non-renewable fuels depend on resources that will eventually run out. Renewable resources, in contrast, are constantly replenished and will never run out. Biomass is one type of renewable resource, which includes plants and organic wastes.
Biomass Program researchers are studying how to convert biomass into liquid fuels for transportation, called biofuels. The use of biofuels will reduce pollution and reduce the United States' dependence on non-renewable oil. For more information on how biofuels are used in vehicles on the road today, check out the Alternative Fuels Data Center.
DOE's Biomass Program is focusing on bioethanol and biodiesel production. Other DOE research programs are looking at using biomass to produce other types of clean energy and fuels. For more information about bioenergy in general, link to Biomass Energy Basics.
Bioethanol is an alcohol made by fermenting the sugar components of biomass. Today, it is made mostly from sugar and starch crops. With advanced technology being developed by the Biomass Program, cellulosic biomass, like trees and grasses, are also used as feedstocks for ethanol production. Ethanol can be used as a fuel for cars in its pure form, but it is usually used as a gasoline additive to increase octane and improve vehicle emissions. Check out What is Bioethanol? for more information on ethanol.
Biomass is material that comes from plants. Plants use the light energy from the sun to convert water and carbon dioxide to sugars that can be stored, through a process called photosynthesis. Organic waste is also considered to be biomass, because it began as plant matter. 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 the energy as simple sugars. These 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.
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 under consideration as a feedstock for bioethanol production. Specific feedstocks under consideration 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)
- Food processing and other industrial wastes (black liquor, a paper manufacturing by-product)
- 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. Its crystalline structure makes it resistant to hydrolysis, the chemical reaction that releases simple, fermentable sugars from a polysaccharide.
Hemicellulose is also a major source of carbon in biomass, at levels of between 20% and 40% by weight. It is a complex polysaccharide made from a variety of five- and six-carbon sugars. It is relatively easy to hydrolyze into simple sugars but the sugars are difficult to ferment to ethanol.
Lignin is a complex polymer, which provides structural integrity in plants. It makes up 10% to 24% by weight of biomass. It remains as residual material after the sugars in the biomass have been converted to ethanol. It contains a lot of energy and can be burned to produce steam and electricity for the biomass-to-ethanol process.
For more information about biomass in general, see What Is Biomass?
Key Reactions. Two reactions are key to understanding how biomass is converted to bioethanol:
Hydrolysis is the chemical reaction that converts the complex polysaccharides in the raw feedstock to simple sugars. In the biomass-to-bioethanol process, acids and enzymes are used to catalyze this reaction.
Fermentation is a series of chemical reactions that convert sugars to ethanol. The fermentation reaction is caused by yeast or bacteria, which feed on the sugars. Ethanol and carbon dioxide are produced as the sugar is consumed. The simplified fermentation reaction equation for the 6-carbon sugar, glucose, is:
|C6H12O6 —> 2 CH3CH2OH + 2 CO2|
| glucose ethanol carbon
Process Description. The basic processes for converting sugar and starch crops are well-known and used commercially today. While these types of plants generally have a greater value as food sources than as fuel sources there are some exceptions to this. For example, Brazil uses its huge crops of sugar cane to produce fuel for its transportation needs. The current U.S. fuel ethanol industry is based primarily on the starch in the kernels of feed corn, America's largest agricultural crop.
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 hydrolyze the cellulose fraction of 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. Cellulose hydrolysis is also called cellulose saccharification because it produces sugars.
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 is caused by yeast or bacteria, which feed on the sugars. As the sugars are consumed, ethanol and carbon dioxide are produced.
Pentose Fermentation. The hemicellulose fraction of biomass is rich in five-carbon sugars, which are also called pentoses. Xylose is the most prevalent pentose released by the hemicellulose hydrolysis reaction. In this step, xylose is fermented using Zymomonas mobilis or other genetically engineered bacteria.
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. The crystalline structure of cellulose makes it difficult to hydrolyze to simple sugars, ready for fermentation. Researchers are developing enzymes that work together to efficiently break down cellulose. Read more about Enzymatic Hydrolysis.
Pentose fermentation. While there are a variety of yeast and bacteria that will ferment six-carbon sugars, most cannot easily ferment five-carbon sugars, which limits ethanol production from cellulosic biomass. Researchers are using genetic engineering to design microorganisms that can efficiently ferment both five- and six-carbon sugars to ethanol at the same time.
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. Check out What is Renewable Diesel? for more information on biodiesel.
In the United States, most biodiesel is made from soybean oil or recycled cooking oils. Animals 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 reacts an alcohol (like methanol) with the triglyceride oils contained in vegetable oils, animal fats, or recycled greases, forming fatty acid alkyl esters (biodiesel) and glycerin. The reaction requires heat and a strong base catalyst, such as sodium hydroxide or potassium hydroxide. The simplified transesterification reaction is shown below.
|Triglycerides + Free Fatty Acids (<4%) + Alcohol ——> Alkyl esters + glycerin|
Pretreatment Reaction. Some feedstocks must be pretreated before they can go through the transesterification process. Feedstocks with less than 4% free fatty acids, which include vegetable oils and some food-grade animal fats, do not require pretreatment. Feedstocks with more than 4% free fatty acids, which include inedible animal fats and recycled greases, must be pretreated in an acid esterification process. In this step, the feedstock is reacted with an alcohol (like methanol) in the presence of a strong acid catalyst (sulfuric acid), converting the free fatty acids into biodiesel. The remaining triglycerides are converted to biodiesel in the transesterification reaction.
|Triglycerides + Free Fatty Acids (>4%) + Alcohol ——> Alkyl esters + triglycerides|
Acid Esterification. Oil feedstocks containing more than 4% free fatty acids go through an acid esterification process to increase the yield of biodiesel. These feedstocks are filtered and preprocessed to remove water and contaminants, and then fed to the acid esterification process. The catalyst, sulfuric acid, is dissolved in methanol and then mixed with the pretreated oil. The mixture is heated and stirred, and the free fatty acids are converted to biodiesel. Once the reaction is complete, it is dewatered and then fed to the transesterification process.
Transesterification. Oil feedstocks containing less than 4% free fatty acids are filtered and preprocessed to remove water and contaminants and then fed directly to the transesterification process along with any products of the acid esterification process. The catalyst, potassium hydroxide, is dissolved in methanol and then mixed with and the pretreated oil. If an acid esterification process is used, then extra base catalyst must be added to neutralize the acid added in that step. Once the reaction is complete, the major co-products, biodiesel and glycerin, are separated into two layers.
Methanol recovery. The methanol is typically removed after the biodiesel and glycerin have been separated, to prevent the reaction from reversing itself. The methanol is cleaned and recycled back to the beginning of the process.
Biodiesel refining. Once separated from the glycerin, the biodiesel goes through a clean-up or purification process to remove excess alcohol, residual catalyst and soaps. This consists of one or more washings with clean water. It is then dried and sent to storage. Sometimes the biodiesel goes through an additional distillation step to produce a colorless, odorless, zero-sulfur biodiesel.
Glycerin refining. The glycerin by-product contains unreacted catalyst and soaps that are neutralized with an acid. Water and alcohol are removed to produce 50%-80% crude glycerin. The remaining contaminants include unreacted fats and oils. In large biodiesel plants, the glycerin can be further purified, to 99% or higher purity, for sale to the pharmaceutical and cosmetic industries.
Global Warming. The combustion of fossil fuels such as coal, oil, and natural gas has increased the concentration of carbon dioxide in the earth's atmosphere. The carbon dioxide and other so-called greenhouse gases allow solar energy to enter the Earth's atmosphere, but reduce the amount of energy that can re-radiate back into space, trapping energy and causing global warming.
One environmental benefit of replacing fossil fuels with biomass-based fuels is that the energy obtained from biomass does not add to global warming. All fuel combustion, including fuels produced from biomass, releases carbon dioxide into the atmosphere. But, because plants use carbon dioxide from the atmosphere to grow (photosynthesis), the carbon dioxide formed during combustion is balanced by that absorbed during the annual growth of the plants used as the biomass feedstock—unlike burning fossil fuels which releases carbon dioxide captured billions of years ago. You must also consider how much fossil energy is used to grow and process the biomass feedstock, but the result is still substantially reduced net greenhouse gas emissions. Modern, high-yield corn production is relatively energy intense, but the net greenhouse gas emission reduction from making ethanol from corn grain is still about 20%. Making biodiesel from soybeans reduces net emissions nearly 80%. Producing ethanol from cellulosic material also involves generating electricity by combusting the non-fermentable lignin. The combination of reducing both gasoline use and fossil electrical production can mean a greater than 100% net greenhouse gas emission reduction. In the case of ethanol from corn stover, we have calculated that reduction to be 113%.
Vehicle Emissions. Petroleum diesel and gasoline consist of blends of hundreds of different hydrocarbon chains. Many of these are toxic, volatile compounds such as benzene, toluene, and xylenes, which are responsible for the health hazards and pollution associated with combustion of petroleum-based fuels. Carbon monoxide, nitrogen oxides sulfur oxides and particulates, are other specific emissions of concern. A key environmental benefit of using biofuels as an additive to petroleum-based transportation fuels is a reduction in these harmful emissions.
Both bioethanol and biodiesel are used as fuel oxygenates to improve combustion characteristics. Adding oxygen results in more complete combustion, which reduces carbon monoxide emissions. This is another environmental benefit of replacing petroleum fuels with biofuels. Ethanol is typically blended with gasoline to form an E10 blend (5%-10% ethanol and 90%-95% gasoline), but it can be used in higher concentrations such as E85 or in its pure form. Biodiesel is usually blended with petroleum diesel to form a B20 blend (20% biodiesel and 80% petroleum diesel), although other blend levels can be used up to B100 (pure biodiesel).
"Photosynthesis" This experiment, from Newton's Apple, demonstrates how plants make food from sunlight.
"What is Ethanol and how does it make a car run?" This experiment, from Newton's Apple, demonstrates how yeast ferments different types of food.
Greenhouse Gas Effect This experiment, from the California Energy Commission, demonstrates how the greenhouse gas effect keeps the earth warm.
Peanut Power This experiment, from the California Energy Commission, demonstrates how energy is stored in plants.
NREL's Renewable Energy Activities Choices for Tomorrow (REACT) Middle Level Grades 6-8 (PDF 3.80 MB)
- Which has more heat energy: vegetable oil or petroleum oil? This experiment demonstrates how different types of fuel produce different amounts of energy (p.61 of the pdf).
- Which grass produces more biomass? This experiment demonstrates how different types of plants generate different amounts of biomass (p.65 of the pdf).