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Biodiesel refers to a diesel-equivalent, processed fuel derived from biological sources. Though derived from biological sources, it is a processed fuel that can be readily used in diesel-engined vehicles, which distinguishes biodiesel from the straight vegetable oils (SVO) or waste vegetable oils (WVO) used as fuels in some modified diesel vehicles.

In this article's context, biodiesel refers to alkyl esters made from the transesterification of both vegetable oils and/or animal fats. Biodiesel is biodegradable and non- toxic, and has significantly fewer emissions than petroleum-based diesel when burned. There is much debate about the extent to which biodiesel can safely be used in conventional diesel engines without modification. Using biodiesel in unmodified engines can lead to problems, particularly blocked injectors, which in turn can lead to serious engine damage. The majority of vehicle manufacturers say using 100% biodiesel can damage their engines. In the UK, for example, most manufactuers only maintain their engine warranties for use with maximum 5% biodiesel - blended in with 95% conventional diesel - although this position is generally considered to be overly cautious. Peugeot and Citroen are an exception in that they have both recently announced that their HDI diesel engine can run on 30% biodiesel. Scania are another exception as they allow most of their engines to operate on 100% biodiesel.

Biodiesel can also be used as a heating fuel in domestic and commercial boilers. A technical research paper No.7 published in the UK by the institute of plumbing and heating entitled "Biodiesel Heating, Sustainable Heating for the Future" by Andrew J. Robertson describes laboratory research and field trials project using pure biodiesel and biodiesel blends as a heating fuel in oil fired boilers. During the Biodiesel Expo 2006 in the UK, Andrew J. Robertson presented his biodiesel heating oil research from his technical paper and suggested that B20 biodiesel could reduce UK household CO2 emissions by 1.5 million tonnes per year and would only require around 330,000 hectares of arable land for the required biodiesel for the UK heating oil sector. The paper also suggests that existing oil boilers can easily and cheaply be converted to biodiesel if B20 biodiesel is used.

Biodiesel can be distributed using today's infrastructure, and its use and production are increasing rapidly. Fuel stations are beginning to make biodiesel available to consumers, and a growing number of transport fleets use it as an additive in their fuel. Biodiesel is generally more expensive to purchase than petroleum diesel but this differential may diminish due to economies of scale, the rising cost of petroleum and government tax subsidies.


Biodiesel is a light to dark yellow liquid. It is practically immiscible with water, has a high boiling point and low vapor pressure. Typical methyl ester biodiesel has a flash point of ~ 150 °C (300 °F), making it rather non-flammable. Biodiesel has a density of ~ 0.86 g/cm³, less than that of water. Biodiesel uncontaminated with starting material can be regarded as non-toxic.

Biodiesel has a viscosity similar to petrodiesel, the industry term for diesel produced from petroleum. It can be used as an additive in formulations of diesel to increase the lubricity of pure Ultra-Low Sulfur Diesel (ULSD) fuel, although care must be taken to ensure that the biodiesel used does not increase the sulfur content of the mixture above 15 ppm. Much of the world uses a system known as the "B" factor to state the amount of biodiesel in any fuel mix, in contrast to the "BA" or "E" system used for ethanol mixes. For example, fuel containing 20% biodiesel is labeled B20. Pure biodiesel is referred to as B100.

Technical standards

The common international standard for biodiesel is EN 14214.

There are additional national specifications. ASTM D 6751 is the most common standard referenced in the United States. In Germany, the requirements for biodiesel is fixed in the DIN EN 14214 standard. There are standards for three different varieties of biodiesel, which are made of different oils:

  • RME ( rapeseed methyl ester, according to DIN E 51606)
  • PME (vegetable methyl ester, purely vegetable products, according to DIN E 51606)
  • FME (fat methyl ester, vegetable and animal products, according to DIN V 51606)

The standards ensure that the following important factors in the fuel production process are satisfied:

  • Complete reaction.
  • Removal of glycerin.
  • Removal of catalyst.
  • Removal of alcohol.
  • Absence of free fatty acids.
  • Low sulfur content.

Basic industrial tests to determine whether the products conform to the standards typically include gas chromatography, a test that verifies only the more important of the variables above. More complete tests are more expensive. Fuel meeting the quality standards is very non-toxic, with a toxicity rating ( LD50) of greater than 50 mL/kg.

Biodiesel sample
Biodiesel sample


Biodiesel can be used in pure form (B100) or may be blended with petroleum diesel at any concentration in most modern diesel engines. Biodiesel will degrade natural rubber gaskets and hoses in vehicles (mostly found in vehicles manufactured before 1992), although these tend to wear out naturally and most likely will have already been replaced with Viton which is nonreactive to biodiesel. Biodiesel's higher lubricity index compared to petrodiesel is an advantage and can contribute to longer fuel injector life. Biodiesel is a better solvent than petrodiesel and has been known to break down deposits of residue in the fuel lines of vehicles that have previously been run on petrodiesel. Fuel filters may become clogged with particulates if a quick transition to pure biodiesel is made, as biodiesel “cleans” the engine in the process. It is, therefore, recommended to change the fuel filter within 600-800 miles after first switching to a biodiesel blend.


In warm climates, pure unblended biodiesel can be poured straight into the tank of any diesel vehicle. Some older diesel engines still have natural rubber parts which will be affected by biodiesel, but in practice these rubber parts should have been replaced long ago. Biodiesel has been noted to be linked to premature injection pump failures. While many vehicles have been using biodiesel for many years without ill effect, the uncanny correlation between several cases of pump failure and biodiesel cannot be dismissed. Pure biodiesel produced 'at home' is in use by thousands of drivers who have not experienced failure, however. The fact remains that biodiesel is a very new subject and will carry some risk until it is fully researched. Biodiesel sold publicly is held to high standards set by the ASTM.


The temperature at which pure (B100) biodiesel starts to gel varies significantly and depends upon the mix of esters and therefore the feedstock oil used to produce the biodiesel. For example, biodiesel produced from low erucic acid varieties of canola seed (RME) starts to gel at approximately -10 °C. Biodiesel produced from tallow tends to gel at around +16 °C. As of 2006, there are a very limited number of products that will significantly lower the gel point of straight biodiesel. One such product, Wintron XC30, has been shown to reduce the gel point of pure biodiesel fuels. Wintron XC30 is a blend of styrene copolymer esters in a toluene base. It reduces the tendency of the viscosity of biodiesel to increase as it is cooled. This is a key step in cold temperature crystallisation. In this way it acts to decrease both the temperature at which the crystals formed become large enough to block the pores of a fuel filter (cold filter plugging point or CFPP) and the lowest temperature at which the fuel will still flow (pour point). A number of studies have shown that winter operation is possible with biodiesel blended with other fuel oils including #2 low sulfur diesel fuel and #1 diesel / kerosene. The exact blend depends on the operating environment: successful operations have run using a 65% LS #2, 30% K #1, and 5 % bio blend. Other areas have run a 70 % Low Sulfur #2, 20 % Kerosene #1, and 10% bio blend or a 80% K#1, and 20 % biodiesel blend. According to the National Biodiesel Board (NBB), B20 (20 % biodiesel, 80 % petrodiesel) does not need any treatment in addition to what is already taken with petrodiesel.

Contamination by water

Biodiesel, although hydrophobic, may contain small but problematic quantities of water. Some of the water present is residual to processing, and some comes from storage tank condensation.

The presence of water is a problem because:

  • Water reduces the heat of combustion of the bulk fuel. This means more smoke, harder starting, less power.
  • Water causes corrosion of vital fuel system components: fuel pumps, injector pumps, fuel lines, etc.
  • Water freezes to form ice crystals near 0 °C (32 °F). These crystals provide sites for nucleation and accelerate the gelling of the residual fuel.
  • Water accelerates the growth of microbe colonies which can plug up a fuel system. Biodiesel users who have heated fuel tanks therefore face a year-round microbe problem.

Previously, the amount of water contaminating biodiesel has been difficult to measure by taking samples, since water and oil separate. However it is now possible to measure the water content using water in oil sensors.



Chemically, transesterified biodiesel comprises a mix of mono- alkyl esters of long chain fatty acids. The most common form uses methanol to produce methyl esters as it is the cheapest alcohol available, though ethanol can be used to produce an ethyl ester biodiesel and higher alcohols such as isopropanol and butanol have also been used. Using alcohols of higher molecular weights improves the cold flow properties of the resulting ester, at the cost of a less efficient transesterification reaction. A byproduct of the transesterification process is the production of glycerol. A lipid transesterification production process is used to convert the base oil to the desired esters. Any Free fatty acids (FFAs) in the base oil are either converted to soap and removed from the process, or they are esterified (yielding more biodiesel) using an acidic catalyst. After this processing, unlike straight vegetable oil, biodiesel has combustion properties very similar to those of petroleum diesel, and can replace it in most current uses.

Biodiesel feedstock

Soybeans are used as a source of biodiesel
Soybeans are used as a source of biodiesel

A variety of oils can be used to produce biodiesel. These include:

  • Virgin oil feedstock; rapeseed and soybean oils are most commonly used, though other crops such as mustard, palm oil, hemp, jatropha, and even algae show promise (see List of vegetable oils for a more complete list);
  • Waste vegetable oil (WVO);
  • Animal fats including tallow, lard, yellow grease, and the by-products of the production of Omega-3 fatty acids from fish oil.
  • Sewage. A company in New Zealand have successfully developed a system for using sewage waste as a substrate for algae and then producing bio-diesel.

Worldwide production of vegetable oil and animal fat is not yet sufficient to replace liquid fossil fuel use. Furthermore, some environmental groups object to the vast amount of farming and the resulting over-fertilization, pesticide use, and land use conversion that they say would be needed to produce the additional vegetable oil.

Many advocates suggest that waste vegetable oil is the best source of oil to produce biodiesel. However, the available supply is drastically less than the amount of petroleum-based fuel that is burned for transportation and home heating in the world. According to the United States Environmental Protection Agency (EPA), restaurants in the US produce about 300 million US gallons (1,000,000 m³) of waste cooking oil annually. Although it is economically profitable to use WVO to produce biodiesel, it is even more profitable to convert WVO into other products such as soap. Therefore, most WVO that is not dumped into landfills is used for these other purposes. Animal fats are similarly limited in supply, and it would not be efficient to raise animals simply for their fat. However, producing biodiesel with animal fat that would have otherwise been discarded could replace a small percentage of petroleum diesel usage.

The estimated transportation fuel and home heating oil used in the United States is about 230 billion US gallons (0.87 km³) (Briggs, 2004). Waste vegetable oil and animal fats would not be enough to meet this demand. In the United States, estimated production of vegetable oil for all uses is about 24 billion pounds (11 million tons) or 3 billion US gallons (0.011 km³), and estimated production of animal fat is 12 billion pounds (5.3 million tons). (Van Gerpen, 2004)

Biodiesel feedstock plants utilize photosynthesis to convert solar energy into chemical energy. The stored chemical energy is released when it is burned, therefore plants can offer a sustainable oil source for biodiesel production. Most of the carbon dioxide emitted when burning biodiesel is simply recycling that which was absorbed during plant growth, so the net production of greenhouse gasses is small.

Feedstock yield efficiency per acre affects the feasibility of ramping up production to the huge industrial levels required to power a signifcant percentage of national or world vehicles. The highest yield feedstock for biodiesel is algae, which can produce 250 times the amount of oil per acre as soybeans..

Feedstock US Gallons/acre Litres/hectare
Soybean 40 375
Rapeseed 110 1,000
Mustard 140 1,300
Jatropha 175 1,590
Palm oil 650 5,800
Algae 10,000 95,000

Efficiency and economic arguments

According to a study written by Drs. Van Dyne and Raymer for the Tennessee Valley Authority, the average US farm consumes fuel at the rate of 82 liters per hectare (8.75 US gallons per acre) of land to produce one crop. However, average crops of rapeseed produce oil at an average rate of 1,029 L/ha (110 US gal/acre), and high-yield rapeseed fields produce about 1,356 L/ha (145 US gal/acre). The ratio of input to output in these cases is roughly 1:12.5 and 1:16.5. Photosynthesis is known to have an efficiency rate of about 16 % and if the entire mass of a crop is utilized for energy production, the overall efficiency of this chain is known to be about 1 %. This does not compare favorably to solar cells combined with an electric drive train. Biodiesel out-competes solar cells in cost and ease of deployment. However, these statistics by themselves are not enough to show whether such a change makes economic sense. Additional factors must be taken into account, such as: the fuel equivalent of the energy required for processing, the yield of fuel from raw oil, the return on cultivating food, and the relative cost of biodiesel versus petrodiesel. A 1998 joint study by the U.S. Department of Energy (DOE) and the U.S. Department of Agriculture (USDA) traced many of the various costs involved in the production of biodiesel and found that overall, it yields 3.2 units of fuel product energy for every unit of fossil fuel energy consumed. That measure is referred to as the energy yield. A comparison to petroleum diesel, petroleum gasoline and bioethanol using the USDA numbers can be found at the Minnesota Department of Agriculture website In the comparison petroleum diesel fuel is found to have a 0.843 energy yield, along with 0.805 for petroleum gasoline, and 1.34 for bioethanol. The 1998 study used soybean oil primarily as the base oil to calculate the energy yields. Furthermore, due to the higher energy density of biodiesel, combined with the higher efficiency of the diesel engine, a gallon of biodiesel produces the effective energy of 2.25 gallons of ethanol. Also, higher oil yielding crops could increase the energy yield of biodiesel.

The debate over the energy balance of biodiesel is ongoing, however. Transitioning fully to biofuels could require immense tracts of land if traditional crops are used. The problem is especially severe for nations with large economies, since energy consumption scales with economic output. If using only traditional plants, most such nations do not have sufficient arable land to produce biofuel for the nation's vehicles. Nations with smaller economies (hence less energy consumption) and more arable land may be in better situations, although many regions cannot afford to divert land away from food production. For third world countries, biodiesel sources that use marginal land could make more sense, e.g. honge oil nuts grown along roads or jatropha grown along rail lines. More recent studies using a species of algae with up to 50 % oil content have concluded that only 28,000 km² or 0.3 % of the land area of the US could be utilized to produce enough biodiesel to replace all transportation fuel the country currently utilizes. Furthermore, otherwise unused desert land (which receives high solar radiation) could be most effective for growing the algae, and the algae could utilize farm waste and excess CO2 from factories to help speed the growth of the algae. The direct source of the energy content of biodiesel is solar energy captured by plants during photosynthesis. The website the positive energy balance of biodiesel:

When straw was left in the field, biodiesel production was strongly energy positive, yielding 1 GJ biodiesel for every 0.561 GJ of energy input (a yield/cost ratio of 1.78).
When straw was burned as fuel and oilseed rapemeal was used as a fertilizer, the yield/cost ratio for biodiesel production was even better (3.71). In other words, for every unit of energy input to produce biodiesel, the output was 3.71 units (the difference of 2.71 units would be from solar energy).

Biodiesel is becoming of interest to companies interested in commercial scale production as well as the more usual home brew biodiesel user and the user of straight vegetable oil or waste vegetable oil in diesel engines. Homemade biodiesel processors are many and varied. The success of biodiesel homebrewing, and micro-economy-of-scale operations, continues to shatter the conventional business myth that large economy-of-scale operations are the most efficient and profitable. It is becoming increasingly apparent that small-scale, localized, low-impact energy keeps more resources and revenue within communities, reduces damage to the environment, and requires less waste management.

Environmental benefits

Environmental benefits in comparison to petroleum based fuels include:

  • Biodiesel reduces emissions of carbon monoxide (CO) by approximately 50 % and carbon dioxide by 78 % on a net lifecycle basis because the carbon in biodiesel emissions is recycled from carbon that was already in the atmosphere, rather than being new carbon from petroleum that was sequestered in the earth's crust. (Sheehan, 1998)
  • Biodiesel contains fewer aromatic hydrocarbons: benzofluoranthene: 56 % reduction; Benzopyrenes: 71 % reduction.
  • Biodiesel can reduce by as much as 20 % the direct (tailpipe) emission of particulates, small particles of solid combustion products, on vehicles with particulate filters, compared with low-sulfur (<50 ppm) diesel. Particulate emissions as the result of production are reduced by around 50 %, compared with fossil-sourced diesel. (Beer et al, 2004).
  • Biodiesel produces between 10 % and 25 % more nitrogen oxide NOx tailpipe-emissions than petrodiesel. As biodiesel has a low sulphur content, NOx emissions can be reduced through the use of catalytic converters to less than the NOx emissions from conventional diesel engines. Nonetheless, the NOx tailpipe emissions of biodiesel after the use of a catalytic converter will remain greater than the equivalent emissions from petrodiesel. As biodiesel contains no nitrogen, the increase in NOx emissions may be due to the higher cetane rating of biodiesel and higher oxygen content, which allows it to convert nitrogen from the atmosphere into NOx more rapidly. Debate continues over NOx emissions. In February 2006 a Navy biodiesel expert claimed NOx emissions in practice were actually lower than baseline. Further research is needed.
  • Biodiesel has higher cetane rating than petrodiesel, which is insignificant in terms of emissions and performance.
  • Biodiesel is biodegradable and non-toxic - the U.S. Department of Energy confirms that biodiesel is less toxic than table salt and biodegrades as quickly as sugar. (See Biodiesel handling and use guidelines)
  • In the United States, biodiesel is the only alternative fuel to have successfully completed the Health Effects Testing requirements (Tier I and Tier II) of the Clean Air Act (1990).

Since biodiesel is more often used in a blend with petroleum diesel, there are fewer formal studies about the effects on pure biodiesel in unmodified engines and vehicles in day-to-day use. Fuel meeting the standards and engine parts that can withstand the greater solvent properties of biodiesel is expected to--and in reported cases does--run without any additional problems than the use of petroleum diesel.

  • The flash point of biodiesel (>150 °C) is significantly higher than that of petroleum diesel (64 °C) or gasoline (−45 °C). The gel point of biodiesel varies depending on the proportion of different types of esters contained. However, most biodiesel, including that made from soybean oil, has a somewhat higher gel and cloud point than petroleum diesel. In practice this often requires the heating of storage tanks, especially in cooler climates.
  • Pure biodiesel (B100) can be used in any petroleum diesel engine, though it is more commonly used in lower concentrations. Some areas have mandated ultra-low sulfur petrodiesel, which reduces the natural viscosity and lubricity of the fuel due to the removal of sulfur and certain other materials. Additives are required to make ULSD properly flow in engines, making biodiesel one popular alternative. Ranges as low as 2 % (B2) have been shown to restore lubricity. Many municipalities have started using 5 % biodiesel (B5) in snow-removal equipment and other systems.

Environmental concerns

Where the oil-producing plants are grown is of increasing concern to environmentalists, one of the prime worries being that countries will clear cut large areas of forest in order to grow such crops. This has already occurred in the Philippines and Indonesia, and both of these countries plan to increase their biodiesel production levels, which will lead to the deforestation of tens of millions of acres if these plans materialize.

The Levington Agricultural report highlights in section 4.6 that the use of biodiesel has an energy footprint 25 times bigger than the use of Pure Plant Oil (PPO) in suitably modified engines.

The Union of Concerned Scientists writes: "When it comes to buying a new car, gasoline-powered models are better than diesels on toxic soot and smog-forming emissions. The downside to current diesels is that they produce 10 to 20 times more toxic particulates than their gasoline counterparts, more than can be made up for with the use of biodiesel. Diesels fare even worse when it comes to smog-forming nitrogen oxide emissions, with greater than 20 times the emissions of a comparable gasoline vehicle."

An indepth look at some of the worrying problems with the pursuit of biodiesel can be found at Biofuelwatch, a leading watch dog for the unsustainable growth in the biodiesel international market.

Historical background

Transesterification of a vegetable oil was conducted as early as 1853, by scientists E. Duffy and J. Patrick, many years before the first diesel engine became functional. Rudolf Diesel's prime model, a single 10 ft (3 m) iron cylinder with a flywheel at its base, ran on its own power for the first time in Augsburg, Germany on August 10, 1893. In remembrance of this event, August 10 has been declared International Biodiesel Day. Diesel later demonstrated his engine and received the "Grand Prix" (highest prize) at the World Fair in Paris, France in 1900. This engine stood as an example of Diesel's vision because it was powered by peanut oil—a biofuel, though not strictly biodiesel, since it was not transesterified. He believed that the utilization of a biomass fuel was the real future of his engine. In a 1912 speech, Rudolf Diesel said "the use of vegetable oils for engine fuels may seem insignificant today, but such oils may become, in the course of time, as important as petroleum and the coal-tar products of the present time."

During the 1920s diesel engine manufacturers altered their engines to utilize the lower viscosity of the fossil fuel (petrodiesel) rather than vegetable oil, a biomass fuel. The petroleum industries were able to make inroads in fuel markets because their fuel was much cheaper to produce than the biomass alternatives. The result was, for many years, a near elimination of the biomass fuel production infrastructure. Only recently have environmental impact concerns and a decreasing cost differential made biomass fuels such as biodiesel a growing alternative.

Research into the use of trans-esterified sunflower oil and refining it to diesel fuel standard was initiated in South Africa in 1979. By 1983 the process to produce fuel quality engine-tested bio-diesel was completed and published internationally (SAE Technical Paper series no. 831356. SAE International Off Highway Meeting, Milwaukee, Wisconsin, USA, 1983). An Austrian Company, Gaskoks, obtained the technology from the South African Agricultural Engineers, put up the first pilot plant for bio-diesel in November 1987 and the erection of the first industrial bio-diesel plant on 12 April 1989, with a capacity of 30 000 tons of rapeseed per annum. Throughout the 1990s, plants were opened in many European countries, including the Czech Republic, France, Germany, Sweden. At the same time, nations in other parts of world also saw local production of biodiesel starting up and by 1998, the Austrian Biofuels Institute identified 21 countries with commercial biodiesel projects.

In the 1990s, France launched the local production of biodiesel fuel (known locally as diester) obtained by the transesterification of rapeseed oil. It is mixed to the proportion of 5 % into regular diesel fuel, and to the proportion of 30 % into the diesel fuel used by some captive fleets ( public transportation). Renault, Peugeot, and other manufacturers have certified truck engines for use with up to this partial biodiesel. Experiments with 50 % biodiesel are underway.

In September of 2005 Minnesota became the first state to require that all diesel fuel sold in that state contain part biodiesel. The Minnesota law requires at least 2% biodiesel in all diesel fuel sold.

Current research

There is ongoing research into finding more suitable crops and improving oil yield. Using the current yields, vast amounts of land and fresh water would be needed to produce enough oil to completely replace fossil fuel usage. It would require twice the land area of the US to be devoted to soybean production, or two-thirds to be devoted to rapeseed production, to meet current US heating and transportation needs.

Specially bred mustard varieties can produce reasonably high oil yields, and have the added benefit that the meal leftover after the oil has been pressed out can act as an effective and biodegradable pesticide.


From 1978 to 1996, the U.S. National Renewable Energy Laboratory experimented with using algae as a biodiesel source in the " Aquatic Species Program". A recent paper from Michael Briggs, at the UNH Biodiesel Group, offers estimates for the realistic replacement of all vehicular fuel with biodiesel by utilizing algae that have a natural oil content greater than 50%, which Briggs suggests can be grown on algae ponds at wastewater treatment plants. This oil-rich algae can then be extracted from the system and processed into biodiesel, with the dried remainder further reprocessed to create ethanol.

The production of algae to harvest oil for biodiesel has not yet been undertaken on a commercial scale, but feasibility studies have been conducted to arrive at the above yield estimate. In addition to its projected high yield, algaculture — unlike crop-based biofuels — does not entail a decrease in food production, since it requires neither farmland nor fresh water.

On May 11, 2006 the Aquaflow Bionomic Corporation in Marlborough, New Zealand announced that it had produced its first sample of bio-diesel fuel made from algae found in sewage ponds. Unlike previous attempts, the algae was naturally grown in pond discharge from the Marlborough District Council's sewage treatment works. In November 2006, a commercial-scale project was announced in South Africa. Using American-made, closed bioreactors, it is expected to produce 900 millions gallons a year (58 thousand barrels a day) of biodiesel within a couple of years.

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