Straight Vegetable Oil SVO as Diesel replacement fuel
What is Biodiesel
This article is about transesterified plant and animal oils. For alkane biodiesel, see Biomass to liquid. For unrefined vegetable oil used as motor fuel (incorrectly referred to as biodiesel), see straight vegetable oil.
Biodiesel refers to a diesel-equivalent, processed fuel derived from biological sources. Though derived from biological sources, it's 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. Biodiesel functions in current diesel engines, and is a possible candidate to replace fossil fuels as the world's primary transport energy source.
Biodiesel can be distributed using today's infrastructure, and its use and production is 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 can be made at home for much cheaper than either. 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, making it rather non-flammable. Biodiesel has a density of ~ 0.8 g/cm^3, 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.
The common international standard for biodiesel is EN 14214.
There are additional national specifications. The standard ASTM D 6751, which 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 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 petroleum. 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.
Pure (B100) biodiesel tends to gel at 4 °C (40 °F) or so, depending on the mix of esters. As of 2006, there is no available product that will significantly lower the gel point of straight biodiesel. A number of studies have concluded that winter operations require a blend of biodiesel, #2 low sulfur diesel fuel, and #1 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.
Biodiesel is hydrophilic. 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 of 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.
For more details on this topic, see Biodiesel around the World.
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.l
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 as a byproduct from the production of Omega-3 fatty acids from fish 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 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. Hence, 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 230,000 million US gallons (870 million m³) (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 23,600 million pounds (10,700,000 t) or 3,000 million US gallons (11,000,000 m³)), and estimated production of animal fat is 11,638 million pounds (5,279,000 t). (Van Gerpen, 2004)
For a truly renewable source of oil, crops or other similar cultivatable sources would have to be considered. Plants utilize photosynthesis to convert solar energy into chemical energy. It is this chemical energy that biodiesel stores and is released when it is burned. Therefore plants can offer a sustainable oil source for biodiesel production