Biodiesel, also known as alkyl esters, are renewable, alternative fuels made from agricultural crops such as soybean, palm, rapeseed (canola), and even cooking oils. Fuel is derived by mixing these source oils with alcohol and a catalyst, most often a base such as KOH. The resulting biodiesel can be used alone (B100) or mixed with petroleum-based diesel (B2, B3. B4. B5) to power diesel engines. While vegetable oil alone can be used in diesel engines, biodiesel performs more efficiently and does not have damaging effects on the vehicle's engine. Biodiesel can be purchased commercially, but do-it-yourself home kits are becoming increasingly popular. This "diy" biodiesel is an especially green fuel source because, in addition to being a low-emission fuel, the home kits allow for recycling of used oil (frequently available for free at fast food restaurants).


Biodiesel is composed of just two ingredients: vegetable oil or animal fat and small alkyl alcohols.¹·² These ingredients are reacted via transesterification in the present of a catalyst to produce the long-chain alkyl ester structure of biodiesel. This section will briefly describe both of these ingredients and how their chemical structures play a role in the production of biodiesel.

Fat and Oil
The active chemical species in the oils and fats used in biodiesel production are triglycerides. Triglycerides are the product of the esterification of glycerol with three long chain fatty acids.³


The fatty acids can have varying chain lengths, leading to a multiplicity of triglycerides, and every oil can be made up of a variety of triglycerides leading to further diversity in physical properties in the finished biodiesel.³ For example, the biodiesel produced from soybean oil is composed of 6 different long-chain alkyl esters each with a differently sized chain.² Among the properties that are affected by this variety is lubricity, color, calorific value of combustion, and cloud point.⁴ The cloud point is the temperature at which the biodiesel starts to gel, or go solid, and is why many biodiesel vehicles require a separate, traditional fuel tank to initially generate the heat needed to “melt” the biodiesel.⁴

The alcohols used in the production of biodiesel are low-molecular weight mono-functional alcohols. Typically methanol is used due to its low cost, however ethanol, isopropanol, and butanol have been used. The choice of alcohol is something of a trade off in that the higher molecular weight alcohols produce biodiesel with better cloud point properties, but they are more difficult to react with the fat and oil and give less yield.⁴


Biodiesel is the product of a transesterification of fats and oils. This process was first used as early as 1853 and was used to produce a fuel as early as 1893.⁴ It was officially patented in 1937.⁴ Today, the production levels of useable biodiesel is growing rapidly with 460 million gallons being produced in the United States alone in 2007.² The actual capacity for biodiesel production in the United States is in the billion of gallons per year.² These numbers are likely higher for the rest of the world, especially in Europe, where the demand for biodesiel is higher.⁴

The biodiesel produced commercially is subject to stringent regulations and specifications to ensure quality and safety.² However, there are several internet sites which set out the process, in an almost cookbook recipe format, so that it can be done by anyone at home. (See for example The remainder of this section will describe this process and the chemistry within it.

The Biodiesel Process
In a nutshell, the transesterification reaction that produces biodiesel is a simultaneous undoing of the triglyceride ester bond which links the glycerol and fatty acid and the reesterification of the fatty acid with the alcohol. The reaction is depicted in the following scheme:


The reaction can be catalyzed with either an acid or base.¹·² However, most commercially viable biodiesel production uses a basic catalyst, either sodium or potassium hydroxide, as the reaction requires reasonable temperatures, pressures, and time under these conditions.² Typical reactions can be done at approximately 50 °C in 4-8 hours.¹·² A three-fold excess of alcohol above the stoichiometric amount required is also added to push the reaction forward.¹·²

The remainder of the process is depicted in the flow chart below.


Following the reaction, the batch is a mixture of 86% biodiesel, 10% glycerol from the triglyceride, and 4% excess alcohol.² The glycerol is more dense than the biodiesel and settles to the bottom allowing separation. The biodiesel portion is then washed and purified in warm water to remove residual catalyst and fatty acids and then dried to a finished product. Throughout the process the alcohol is stripped via evaporation or distillation to be reused in the next reaction.¹ The biodiesel produced in this way can be over 98% pure.

The glycerol layer also undergoes the same process resulting in an 80-88% pure glycerol product which can be used in the soap making and various other industries.¹·² In fact, the glycerol produced in the manufacture of biodiesel has largely replaced other processes for manufacturing glycerol.⁵


Basic Properties of Biodiesel (B100)

Heating Value or Calorific Value

Boiling Point

Vapor Pressure

Flash Point

Specific Gravity

Melting Point

Negligible in Water

No. 2 Diesel has a heating value of approximately 129,000 BTU/gal, which about 8-11% more than biodiesel. Material compatibility of biodiesel is a concern with using pure diesel in some conditions and applications. Gelling or thickening is common when biodiesel is exposed to colder temperatures (< 14 - 61 °F) depending on the origin of the biodiesel and its chemistry. There are admixtures developed and being refined to lower the gelling point. The use of two fuel tanks, one with petrodiesel (#2 Diesel) and the other with biodiesel, allow the vehicle to start using the petrodiesel and warm the biodiesel for consumption. Blends of petrodiesel and biodiesel such as B20 also are subject to gelling and create operation concerns in diesel vehicles. Water absorption is a concern when storing biodiesel and exposing it to the atmosphere. Biodiesel is a hygroscopic, which means it can absorb water from atmospheric moisture. The absorption of water can be reduced during production by adding addition methanol to insure a complete reaction. Biodiesel is also incompatible with oxygen and strong oxidizers such as chlorates, nitrates and peroxides.

Biodiesel’s most important property is that it is biodegradable, non-toxic and free of sulfer and aromatics. This allows the biodiesel to be safely handled and transported.

Negative characteristics of biodiesel are that it releases a larger amount of NOx, which can lead to increases in photochemical smog in industrial and high traffic areas, than petrodiesel (No. 2 Diesel).


There are many uses for biodiesel and more applications currently being researched to provide a sustainable fuel for the future. Biodiesel can be used in:
  1. vehicles (cars, trucks, buses, agricultural applications, and off-road/construction applications)
  2. airplanes
  3. trains
  4. marine equipment
  5. heating oil
  6. blends of fuel
Other applications include replacement of conventional and synthetic motor oil and other lubricants.

Biodiesel Vehicles
Vehicles with conventional unmodified diesel engines do not require modification to consume biodiesel, but problems arise when biodiesel is used. Vehicle manufacturers have been able to design modified engines to reduce some of side effects of running on biodiesel, which include fuel filter plugging, carbon deposits in the fuel injector, piston ring malfunction, seal breakage, and fuel line breakage. These effects are caused by gelling or crystallizing of biodiesel which do not allow it to be filtered or pumped. Soy methyl esters typically begin to gel and/or crystallize at temps of 22 °F; No. 2 Diesel has a gelling point of 1 °F.

Marine Applications
The use of B100, pure biodiesel, is very favorable in marine applications since biodiesel is non-toxic and biodegradable.

Fuel Blends
Blends comprised of biodiesel are designated as Bxxx, where the xxx can range from 100 to 0.01 percent of biodiesel. Blends are more favorable for use in commercial and domestic vehicles because it has a substantial impact on reducing emissions and also reduce the negative impacts of cold temperatures on biodiesel.
John Deere, agricultural equipment manufacturer, has been building and promoting the use of biodiesel in their engines(see video).⁷ They have developed a fuel conditioner that can be used in fuels with a B20 or higher grade to avoid adverse effects.

Below is chart that “shows data for the cold flow properties of biodiesel and blends of biodiesel with diesel fuel. The cloud point is the temperature at which crystals first start to form in the fuel and the pour point is the lowest temperature at which the fuel will still pour from a container. The cold filter plugging point (CFPP) is the lowest temperature at which at certain volume of fuel can be drawn through a metal screen filter. It usually correlates well with the lowest temperature that an engine will operate.”

Cloud Point (oF)
Pour Point (oF)
Soy Methyl Esters (soy)
Yellow Grease Methyl Esters (YG)
#2 low Sulfur Diesel Fuel (#2)
2% Soy in #2
5% Soy in #2
2% YG in #2
5% YG in #2
50% #2 / 50% #1 (50/50)
2% Soy in 50/50
5% Soy in 50/50
2% YG in 50/50
5% YG in 50/50
#1 Low Sulfur Diesel Fuel (#1)
< -30
2% Soy in #1
< -30
5% Soy in #1
< -30
2% YG in #1
< -30
5% YG in #1


When using biodiesel in a diesel powered vehicle, it retains similar horsepower and torque. Biodiesel contains a higher cetane number, but also a lower energy content than traditional diesel. These properties result in a more lubricated, better performing engine, but a 2-8% decrease in fuel economy.⁹

Energy Balance of Biodiesel
The long running controversy over biodiesel is the question of whether the fuel needed to harvest and process biodiesel is worth the fuel produced. Most of those against biodiesel state how the plants can only actually trap 3-6% of solar radiation through the process of photosynthesis.¹⁰ Compared to solar cells, which catch just under 20%, this seems very small. However, the other side of the debate states that biodiesel is much less expensive and there are already many cars that can use biodiesel; if one wants a solar/electric car they would have to get rid of their current vechile and buy a new electric car.

Another important factor is which plants should be used to produce biodiesel. While some plants are very cheap, they many not produce per acre as much fuel as another plant can produce. While soybeans are one of the most popular plants used for the production of biodiesel, they are the most inefficient. Palm oil production is becoming more and more popular in Malaysia and Indonesia as it is fairly inexpensive and has such a high yield of biodiesel per acre of the feedstock crop.¹¹
Food Vs Fuel
The use of land for the production of fuel doesn't seem like a waste to most people; however, to those in third world countries it is. Many poorer countries are now using land to grow agriculture for fuel, and not for the food needed by the people of those countries. Some suggest that only non-edible crops which can grow in more harsh conditions be used for biofuel. The idea behind this is that the fertile soil can still be used to grow crops to feed people, not fuel cars. The issue from this is that farmers will grow whatever will make them the most profit. If it is edible or not, they will make the most profit off of crops for biodiesel, so that is what they will grow.¹²

Environmental Effects

Biodiesel is seen as cleaner alternative to fossil fuels. It is often seen as being much more earth friendly because the carbon dioxide emissions are "recycled" by the plants that will be made into biodiesel. This is often seen in as cycle diagram, as shown below.
Biodiesel Cycle
This image is often seen as misrepresenting the amount of carbon dioxide emitted from biodiesel powered engines. While they do reduce emissions compared to standard fossil fuels, their emissions are not immediately reabsorbed by plants.


Greenhouse Gas Emissions
Biodiesel is the only alternative fuel that has had its emission and potential health effects completely evaluated by the EPA under the Clean Air Act. Overall, biodiesel was found to reduce emissions when compared to standard diesel.¹³

However, this is only a piece of the emissions. There is also energy used to grow, transport, and process the feedstocks; so if a farm uses energy inefficient tractors the overall emissions for the biodiesel is higher than a farm with more efficient machinery. Thus, even the local where the feedstocks are grown can effect the overall environmental impact of the biodiesel. In addition, biodiesel from one plant does not produce the same emissions as another. For example, most soy oil has a higher emission than most rapeseed (canola) oil and both have higher emissions than palm oil. Surprisingly, the biodiesel with the lowest amount of environmental impact is of cooking oil and tallow. The following chart shows the comparisons of biodiesels, as well as traditional fossil fuels.


Compared to diesel, biodiesel can reduce direct tailpipe emissions of sulfur, CO, hdyrocarbons, and other particulates. The benefits of the sulfur reduction could be a possible improvement of acid rain (which is caused by sulfur dioxide). Also, reduction of CO and hydrocarbons would improve air quality and potential health effects from breathing in emissions. However, using B100 biodiesel actually produces more nitrogen oxide (NOx) than standard diesel does. NOx is one of the main contributors to smog, so in areas like Los Angeles and Mexico city, which are known for car-produced smog, this could actually make the air quality worse.¹³·¹⁴

Demand for oil is vey high in tropical regions for the production of biodiesel. With this high demand comes the need for more space to grow the crops; and this land is coming at the price of tropical rainforests. Indigenous people are being forced to relocate, forests and waters are being contaminated by pesticides, and certain species of plants and animals are becoming endangered. This need for land to produce biodiesel is damaging ecosystems, creating social conflict, and destroying vegitation that could help reduce greenhouse gases.¹⁵

As seen in this chart, that as production for soybeans used for biodiesel has increased, the deforestation of the Amazon has increased accordingly.

A study performed by the University of Idaho found that biodiesel degrades twice as fast as petroleum-diesel. Two types of biodiesel (rapeseed and soybean oils) were tested against different types of petroleum diesel. Even in soil, the biodiesels degraded much more rapidly than all the different types of diesel.¹⁶


1.Wikipedia,“Biodiesel Production.” (April 21, 2009)
2.National Biodiesel Board, (April 21, 2009)
3.Wikipedia,"Biodiesel.” (April 21, 2009)
4.Wikipedia, “Triglyceride.” (April 21, 2009)
5. Wikipedia,“Glycerol.” (April 21, 2009)
6.Chevron Philips. "MSDS." 29, 2009)
7.John Deere, "What Does Biodiesel Mean for John Deere Engines?" 29, 2009)
8.BECON,"Biodiesel Education - What Do You Need to Know?." (April 29, 2009)
9.Environmental Protection Agency,"Biodiesel." October 2006 29, 2009)
10.Food and Agriculture Organization of the United Nations, "Biological Energy Production." 1997 29, 2009)
11.International Land Coalition, "U.S. biofuels policy drives deforestation in Indonesia, the Amazon." January 17, 2008. 29, 2009)
12.Malaysian National News Agency, "Palm Oil Based Biodiesel Has a Higher Chance of Survival." December 2006. 29, 2009)
13.National Biodiesel Board,"Biodiesel Emissions." 29, 2009)
14.U.S. Department of Energy,"Just the Basics - Biodiesel." August 2003. 29, 2009)
15.Greenpeace,"How the Palm Oil Industry is Cooking the Climate." 29, 2009)
16.Peterson, C.L., Moller, Gregory,"Biodegradability, BOD5, COD, and Toxicity of Biodiesel Fuels." 29, 2009)