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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: 

z Biofuels for Transportation 

z Bioethanol Feedstocks 

z Bioethanol Production 

z Biodiesel Feedstocks 

z Biodiesel Production 

z Biofuels and the Environment 

z Links 

z Biomass and Biofuels Science Fair Project Ideas 

Some of the following documents are available as Adobe Acrobat PDFs. Download Adobe Reader. 

Biofuels for Transportation 

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 nonrenewable 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. 

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Bioethanol 

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. 

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Bioethanol Feedstocks 

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: 

z Agricultural residues (leftover material from crops, such as the stalks, leaves, and husks of corn 

plants) 

z Forestry wastes (chips and sawdust from lumber mills, dead trees, and tree branches) 

z Municipal solid waste (household garbage and paper products) 

z Food processing and other industrial wastes (black liquor, a paper manufacturing by-product) 

z Energy crops (fast-growing trees and grasses) developed just for this purpose 

The main components of these types of biomass are: 

z 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. 

z 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. 

z Lignin is a complex polymer, which provides structural integrity in plants. It makes up 10% to 24% 

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Biomass Program: ABC's of Biofuels Page 1 of 5

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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? 

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Bioethanol Production 

Key Reactions. Two reactions are key to understanding how biomass is converted to bioethanol: 

z 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. 

z 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: 

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. 

z 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. 

z 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. 

z 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. 

z 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. 

z 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. 

z 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. 

z 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. 

z 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 

C6H12O6 —> 2 CH3CH2OH + 2 CO2

 glucose ethanol carbon 

 dioxide

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researchers are working to improve the efficiency and economics of the ethanol production process by 

focusing their efforts on the two most challenging steps: 

z 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. 

z 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. 

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Biodiesel 

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. 

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Biodiesel Feedstocks 

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. 

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Biodiesel Production 

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. 

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. 

Process Description.

z 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. 

z 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. 

base

Triglycerides + Free Fatty Acids (<4%) + Alcohol ——> Alkyl esters + glycerin

acid

Triglycerides + Free Fatty Acids (>4%) + Alcohol ——> Alkyl esters + triglycerides

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z 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. 

z 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. 

z 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. 

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Biofuels and the Environment 

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). 

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Links 

Ethanol 

Ethanol-As a Fuel This is a comprehensive college-level curriculum, prepared by Northwest Iowa 

Community College, discussing details of ethanol production, use and environmental benefits. 

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Biomass and Biofuels Science Fair Project Ideas 

Middle School 

"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) 

z 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). 

z Which grass produces more biomass? This experiment demonstrates how different types of plants 

generate different amounts of biomass (p.65 of the pdf). 

Lesson Plans 

z Biofuel Production (PDF 197 KB) 

z Environmental Aspects (PDF 495 KB) 

z Photosynthesis and Biomass Growth (PDF 405 KB) 

z Biofuel Utilization (PDF 225 KB) 

High School 

NREL's Research Projects in Renewable Energy for High School Students (PDF 1.10 MB) 

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z What is the efficiency of ethanol production from various biomass sources? This experiment compares 

the relative yields of ethanol from different biomass sources, like sugar, fruit and corn (p. 13 of the

pdf). 

z What kinds of plants have the most heat energy in a given quantity of biomass? This experiment 

compares the heat energy in a variety of sources like peanuts, corn, milkweed (p. 15 of the pdf). 

z How much energy can be obtained from alcohol fuels? This experiment compares the heat energy in 

ethanol, methanol, and other types of alcohols (p.16 of the pdf). 

z What factors affect biomass growth? This experiment demonstrates how varying fertilizer use, soil 

type, light exposure, etc. affects biomass growth (p.16 of the pdf). 

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