Biodiesel refers to a diesel-equivalent, processed fuel derived from biological sources (such as vegetable oils), which can be used in unmodified diesel-engine vehicles. It is thus distinguished from the straight vegetable oils (SVO) or waste vegetable oils (WVO) used as fuels in some diesel vehicles.
In this article’s context, biodiesel refers to alkyl esters made from the transesterification of vegetable oils or animal fats.
On August 31, 1937, G. Chavanne of the University of Brussels (Belgium) was granted a patent for a ‘Procedure for the transformation of vegetable oils for their uses as fuels’ (fr. ‘Procédé de Transformation d’Huiles Végétales en Vue de Leur Utilisation comme Carburants’) Belgian Patent 422,877. This Patent described the alcoholysis (often reffered to as transesterification) of vegetable oils using ethanol (and mentions methanol) in order to separate the fatty acids from the glycerol by replacing the glycerol by short linear alcohols. This appears to be the first account of the production of what is known as ‘Biodiesel’ today.
Biodiesel is biodegradable and non-toxic, and typically produces about 60% less net carbon dioxide emissions than petroleum-based diesel, as it is itself produced from atmospheric carbon dioxide via photosynthesis in plants. Pure biodiesel is available at many gas stations in Germany.
Some vehicle manufacturers are positive about the use of biodiesel, citing lower engine wear as one of the benefits of this fuel. However, as biodiesel is a better solvent than standard diesel, it ‘cleans’ the engine, removing deposits in the fuel lines, and this may cause blockages in the fuel injectors. For this reason, car manufacturers recommend that the fuel filter be changed a few months after switching to biodiesel (this part is often replaced anyway in regular servicing). Most manufacturers release lists of the cars that will run on 100% biodiesel.
Other vehicle manufacturers remain cautious over use of biodiesel. In the UK many 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 Citroën are exceptions in that they have both recently announced that their HDI diesel engine can run on 30% biodiesel. Scania and Volkswagen are other exceptions, allowing most of their engines to operate on 100% biodiesel.
Biodiesel can also be used as a heating fuel in domestic and commercial boilers. Existing oil boilers may require conversion to run on biodiesel, but the conversion process is believed to be relatively simple.
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. In Germany, biodiesel is generally cheaper than normal diesel at gas stations that sell both products.
Description
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 ~ 0.88 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 current 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, which is advantageous because it has virtually no sulfur content. 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.
Biodiesel is a renewable fuel that can be manufactured from algae, vegetable oils, animal fats or recycled restaurant greases; it can be produced locally in most countries. It is safe, biodegradable and reduces air pollutants, such as particulates, carbon monoxide and hydrocarbons. Blends of 20 percent biodiesel with 80 percent petroleum diesel (B20) can generally be used in unmodified diesel engines. Biodiesel can also be used in its pure form (B100), but may require certain engine modifications to avoid maintenance and performance problems. The industry standard for the amount of time it takes to produce biodiesel used to be 4 hours, but a San Antonio based company is currently experimenting, and has claimed to produce biodiesel fuel in a fraction of what it formerly was, with a 1.4 minute contact time
Technical standards
Biodiesel sample
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 and Canada. In Germany, the requirements for biodiesel are fixed in the DIN EN 14214 standard and in the UK the requirements for biodiesel is fixed in the BS EN 14214 standard, although these last two standards are essentially the same as EN 14214 and are just prefixed with the respective national standards institution codes.
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:
- 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. Tests that are more complete are more expensive. Fuel meeting the quality standards is very non-toxic, with a toxicity rating (LD50) of greater than 50 mL/kg.
Applications
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 FKM, which is nonreactive to biodiesel.
Biodiesel’s higher lubricity index compared to petrodiesel is an advantage and can contribute to longer fuel injector life. However, 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. As a result, fuel filters and injectors may become clogged with particulates if a quick transition to pure biodiesel is made, as biodiesel “cleans” the engine in the process. Therefore, it is recommended to change the fuel filter within 600-800 miles after first switching to a biodiesel blend.
Use
Pure, non-blended biodiesel can be poured straight into the tank of any diesel vehicle. As with normal diesel, low-temperature biodiesel is sold during winter months to prevent viscosity problems. 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 is used by millions of car owners in Europe (particularly Germany).
Research sponsored by petroleum producers has found petroleum diesel better for car engines than biodiesel. This has been disputed by independent bodies, including for example the Volkswagen environmental awareness division, who note that biodiesel reduces engine wear. Pure biodiesel produced ‘at home’ is in use by thousands of drivers who have not experienced failure, however, the fact remains that biodiesel has been widely available at gas stations for less than a decade, and will hence carry more risk than older fuels. Biodiesel sold publicly is held to high standards set by national standards bodies.
Gelling
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. 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 an 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.
Some people modify their vehicles to permit the use of biodiesel without mixing and without the possibility of gelling. This practice is similar to the one used for running straight vegetable oil. They install a second fuel tank (some models of trucks have two tanks already). This second fuel tank is insulated and a heating coil using engine coolant is run through the tank. There is then a temperature sensor installed to notify the driver when the fuel is warm enough to burn, the driver then switches which tank the engine is drawing from.
Heating applications
Biodiesel can also be used as a heating fuel in domestic and commercial boilers. A technical research paper published in the UK by the Institute of Plumbing and Heating Engineering entitled “Biodiesel Heating Oil: 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.
Demand and Availability
Global biodiesel production reached 3.8 million tons in 2005. Approximately 85% of biodiesel production came from the European Union.
“The nascent U.S. market for biodiesel has grown from 25 million gallons per year in 2004 to 78 million gallons by the beginning of 2005. The amount of biodiesel produced in the United States then grew more than threefold to 240 million gallons over the course of that year. By the end of 2006, biodiesel production was estimated to increase fourfold to more than 1 billion gallons” writes energy consultant Will Thurmond in the July-August 2007 issue of THE FUTURIST magazine.
Were the United States and other nations to begin mass producing new feedstocks of algae-based biodiesel, Thurmond forecasts that the fuel could meet 20% or more of the transportation-energy needs of the United States, Brazil, China, and India by the year 2020. Thurmond’s article can be downloaded from THE FUTURIST magazine.
Production
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 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.
A byproduct of the transesterification process is the production of glycerol. For every 1 tonne of biodiesel that is manufactured, 100kg of glycerol are produced. Originally, there was a valuable market for the glycerol, which assisted the economics of the process as a whole. However, with the increase in global biodiesel production, the market price for this crude glycerol (containing 20% water and catalyst residues) has crashed. Research is being conducted globally to use this glycerol as a chemical building block. One initiative in the UK is The Glycerol Challenge.
Biodiesel feedstock
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, soybean oil alone accounting for about ninety percent of all fuel stocks; It also can be obtained from field pennycress and Jatropha other crops such as mustard, flax, sunflower, canola, 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, chicken fat, and the by-products of the production of Omega-3 fatty acids from fish oil.
- Sewage. A company in New Zealand has successfully developed a system for using sewage waste as a substrate for algae and then producing bio-diesel.
- Thermal depolymerization is an important new process that reduces almost any hydrocarbon based feedstock, including non oil based feedstocks, into light crude oil.
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 gases is small.
Feedstock yield efficiency per acre affects the feasibility of ramping up production to the huge industrial levels required to power a significant 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.
Yields of common crops
| Crop | kg oil/ha | litres oil/ha | lbs oil/acre | US gal/acre |
|---|---|---|---|---|
| corn (maize) | 145 | 172 | 129 | 18 |
| cashew nut | 148 | 176 | 132 | 19 |
| oats | 183 | 217 | 163 | 23 |
| lupine | 195 | 232 | 175 | 25 |
| kenaf | 230 | 273 | 205 | 29 |
| calendula | 256 | 305 | 229 | 33 |
| cotton | 273 | 325 | 244 | 35 |
| hemp | 305 | 363 | 272 | 39 |
| soybean | 375 | 446 | 335 | 48 |
| coffee | 386 | 459 | 345 | 49 |
| linseed (flax) | 402 | 478 | 359 | 51 |
| hazelnuts | 405 | 482 | 362 | 51 |
| euphorbia | 440 | 524 | 393 | 56 |
| pumpkin seed | 449 | 534 | 401 | 57 |
| coriander | 450 | 536 | 402 | 57 |
| mustard seed | 481 | 572 | 430 | 61 |
| camelina | 490 | 583 | 438 | 62 |
| sesame | 585 | 696 | 522 | 74 |
| safflower | 655 | 779 | 585 | 83 |
| rice | 696 | 828 | 622 | 88 |
| tung oil tree | 790 | 940 | 705 | 100 |
| sunflowers | 800 | 952 | 714 | 102 |
| cocoa (cacao) | 863 | 1,026 | 771 | 110 |
| peanuts | 890 | 1,059 | 795 | 113 |
| opium poppy | 978 | 1,163 | 873 | 124 |
| rapeseed (Canola) | 1,000 | 1,190 | 893 | 127 |
| olives | 1,019 | 1,212 | 910 | 129 |
| castor beans | 1,188 | 1,413 | 1,061 | 151 |
| pecan nuts | 1,505 | 1,791 | 1,344 | 191 |
| jojoba | 1,528 | 1,818 | 1,365 | 194 |
| jatropha | 1,590 | 1,892 | 1,420 | 202 |
| macadamia nuts | 1,887 | 2,246 | 1,685 | 240 |
| Brazil nuts | 2,010 | 2,392 | 1,795 | 255 |
| avocado | 2,217 | 2,638 | 1,980 | 282 |
| coconut | 2,260 | 2,689 | 2,018 | 287 |
| oil palm | 5,000 | 5,950 | 4,465 | 635 |
| Chinese tallow | 5,500 | 6,545 | 4,912 | 699 |
| Algae* | 79,832 | 95,000 | 71,226 | 10,000 |
* Algae yields are projected based on the peak daily yields of the NREL’s aquatic species program. Sustainable average yields were half as much.
