Biodiesel can be made from a variety of sources including the oil from oil seeds such as soybeans, palm kernel and canola, to name only a few. It can also be made from animal fat-based feedstocks such as beef tallow and recycled cooking oils (e.g., French fryer oil). In the United States most plants are set up to run soybean oil as the feedstock, although some plants can run multiple feedstocks, including corn oil. The selection of soybean oil for U.S. plants is simply a reflection of the abundant U.S. supply of soybeans. In other countries, such as India and Malaysia, other feedstocks such as palm oil are used due to their greater availability in those nations.
Regardless of the feedstock used, the process to produce biodiesel is essentially the same. From a chemical standpoint, vegetable oils are triglycerides of fatty acids and possess properties that are not desirable in diesel fuel. For instance, they may cause injector coking. While diesel engines running on French fryer oil may make for a great media story, it does not make for great diesel fuel quality. Almost all engine manufacturers recommend against the use of raw pressed, or partially refined, vegetable oils that have not been processed through transesterification.
In order to overcome the unfavorable properties of vegetable oils and animal fats, they are reacted with an alcohol (usually methanol, but ethanol or other alcohols could be used). The triglycerides are combined with methanol and reacted with a catalyst to yield biodiesel and glycerin. Essentially this reaction uses the alcohol to remove the glycerin, which is undesirable in diesel fuel. This process is called transesterification, and the resulting biodiesel is technically a Fatty Acid Methyl Ester (FAME) when methanol is the alcohol used. The resulting biodiesel has chemical and physical properties similar to conventional diesel fuel.
Although biodiesel can be, and is in some cases, used as a fuel by itself, it is more commonly used as a blend component in conventional diesel. Biodiesel levels typically range from 2 volume percent (B2) to 20 volume percent (B20) of the total biodiesel blend, with the remainder of the blend being conventional diesel. While biodiesel production and use are increasing significantly in the U.S. it has been widely available only over the past few years. Engine and fuel system manufacturers have been engaged in various research projects to determine which biodiesel blend levels are appropriate for their products. In addition, as biodiesel blend sales have increased, manufacturers are rapidly gaining field experience to aid in such decisions.
ASTM Specifications
As is the case for diesel fuels, ASTM International has a specification for biodiesel. That specification is ASTM D 6751- Standard Specification for Biodiesel Fuel Blend-stock (B100) for Middle Distillate Fuels. This specification applies to biodiesel for use as a blend component in diesel. ASTM has not yet developed a specification for blends such as B5 and B20, but expects to do so in the future.
ASTM D 6751 includes limits on several of the same properties that ASTM D 975 requires for No. 2 diesel fuel. However, there are additional property requirements for biodiesel.
Additional requirements for biodiesel include a maximum limit of 5 ppm for calcium and magnesium (combined). Calcium and magnesium may be present as abrasive solids or soluble metallic soaps, so their presence must be limited, because abrasive solids would contribute to injector and pump wear as well as piston and ring wear. Soluble metallic soaps contribute to engine deposits. Calcium and magnesium may also collect in particulate filters, increasing back pressure, and may result in the need for shorter service intervals.
Alcohol content is limited in one of two manners. Either the alcohol content must not exceed mass percent, or the flash point must be 130ºC minimum. This is done to ensure that the alcohol used in the transesterification process is properly removed from the fuel.
While ASTM D 975 has a limit on ash, ASTM D 6751 specifies a “sulfated ash” maximum of 0.02 mass percent. In addition to limiting abrasive solids and soluble soaps (see above), this specification limits any unremoved catalysts from the biodiesel production process. Carbon residue is limited to 0.05 mass percent, but a different test method is used than that for No. 2 diesel fuels.
An acid number specification limit of 0.50 maximum mg KOH/g is placed on biodiesel to control the level of free fatty acids or processing acids. High acidity can increase fuel system deposits. It may also increase fuel system corrosion. High acid values may also be an indication of fuel degradation from oxidation.
Free glycerin is limited to 0.02 mass percent and total glycerin is limited to 0.24 mass percent. High levels of free glycerin can contribute to injector deposits and clogging of the fuel system. Free glycerin can also build up in the bottom of storage tanks. The total glycerin includes free glycerin plus any glycerin content of any unreacted or incompletely reacted oils or fats. Low levels of total glycerin confirm a high conversion rate of oils and fats. High levels of total glycerin can contribute to injector deposits and fuel filter plugging and could impact low-temperature operability.
A limit is placed on phosphorus, because it can damage exhaust after-treatment catalysts. The ASTM maximum limit for phosphorus in biodiesel is 0.001 mass percent (10 ppm). U.S. biodiesel (soybean oil based) has routinely been in the 1 to 2 ppm range, but biodiesel from other feedstock sources could contain higher phosphorus levels.
Sodium and potassium may be present as abrasive solids or soluble metallic soaps, resulting in problems similar to those described for sulfated ash.
Finally, an oxidation stability rating or 3 hours minimum under specified test conditions is required. This test provides an indication of the storage life of biodiesel, which is known to degrade more quickly than standard diesel fuel.
The remainder of the requirements are similar to those for standard diesel fuel.
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The only other major differences between the two fuels in the above table are viscosity and cetane. In the case of viscosity, it is desirable to express a range. Minimum viscosity is specified because of potential loss of power due to injector pump leakage and injector leakage. Maximum viscosity is limited because of practical considerations of the engine size and design, as well as the operating parameters of the injection system. The upper limit of viscosity for biodiesel is higher than for No. 2 diesel. Blending biodiesel which has a viscosity rating near the maximum limit could cause the biodiesel blend to exceed the maximum viscosity limit specified for No. 2 diesel, especially for higher concentration biodiesel blends.
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The cetane number requirement for biodiesel is 47 compared to 40 for No. 2 diesel. The impact this has on the blend depends on the blend level. For B2, the cetane number only increases to 40.1, and for B5 to a still-modest 40.3. For B20, the cetane number increases to 41.4
There are other properties of interest which are not specified in the ASTM Specification. These include the following:
Energy Content:
No. 2 diesel has a btu content of approximately 130,000 btu/gal. Biodiesel’s energy content is approximately 118,000 btu/gal. The impact this has on the biodiesel blend depends on the blend level. The following figure depicts the energy content of different blend levels using 129,500 btu/gal. for No. 2 diesel and 118,000 btu/ gal. for B100. The decrease in energy content for B2 is less than 0.2 percent, and for B5 still below 0.5 percent. For B20 blends, the energy content is 1.8 percent lower. B100 has about 8.9 percent lower energy content, which may result in lower power and noticeably lower fuel economy.
Lubricity:
There is not currently a lubricity specification in ASTM D 6751. However, biodiesel is known to improve the lubricity of No. 2 diesel. In fact, biodiesel is often added at the 2 volume per- cent level to improve the lubricity of No. 2 diesel, making B2 probably the most common biodiesel blend. The lubricity of No. 2 diesel is limited to 520 microns of wear in the HFRR test, while B100 typically achieves levels of below 300 microns.
Cloud Point/Pour Point:
As noted earlier, there are no limits set for cloud point or pour point for No. 2 diesel. It is known that B100 has a cloud point about 7ºC higher than No. 2 diesel and a pour point of 20ºC to 25ºC above No. 2 diesel. In other words, the low-temperature operability of B100 is poorer than that of No. 2 diesel. This is usually not noticeable at the B2 and B5 blend levels. However, at higher levels, biodiesel blenders may need to use additives to improve low-temperature operability.
It is important to again note that information presented here is based largely on soy methyl ester (SME). However, biodiesel (FAME) can be made from a variety of feedstocks, and the different feed- stocks may result in biodiesel with slightly different properties. For instance, SME can tolerate lower temperatures than animal-based biodiesel before reaching its cold filter plug point. Similarly, the cetane number can vary. This is largely a function of the carbon content of the final FAME, which can vary with feedstock type. However, for the B2 to B20 blend levels, these differences are rarely perceptible. ASTM D 6751 is intended to apply to all biodiesel produced from any feedstock.
Other Fuel Quality Considerations
Biodiesel (B100) possesses certain properties that make it more sensitive to storage conditions than No. 2 diesel. This makes proper storage, transport and blending of biodiesel very important. Biodiesel is hygroscopic, meaning it absorbs water. Over a period of time the water content of biodiesel can rise to its saturation point (around 1200 ppm). This can increase the risk of corrosion as well as microbial contamination. Fuel storage systems must be kept free of water. Additional steps could include treatment with moisture dispersants and biocides. Biodiesel has a greater solvency effect than No. 2 diesel. It will remove sediment and residual contaminants in storage tanks. Storage tank cleaning and preparation are very important. Biodiesel may deteriorate more quickly than No. 2 diesel. This necessitates steps such as monitoring product turnover as well as painting above-ground storage tanks a reflective color to reduce product temperature during storage.
The Benefits and concerns of Biodiesel and Biodiesel Blend Use
The performance and operability benefits of biodiesel include reduced net CO2 emissions, reduced HC and CO emissions and lower visible smoke. Biodiesel also has a higher cetane number and contains no aromatics. It has a low sulfur content and improves lubricity. Biodiesel is also nontoxic and biodegradable.
The performance and operability concerns most often expressed by the diesel engine and vehicle manufacturers include several topics. First, there are concerns about materials compatibility. Biodiesel may cause corrosion of certain metals. These include zinc, copper-based alloys, tin, lead and zinc. Certain elastomers and seal materials may also harden or swell. These effects may be more pronounced on older vehicles or equipment and may increase with biodiesel concentration in the blend.
There is also concern with the potential for increased water contamination, which could increase corrosion as well as the potential for microbial contamination. Biodiesel may also increase NOx emissions, especially at higher blend levels. The catalyst can typically handle small increases in engine-out NOx emissions. This is of particular concern with engines certified to the more stringent 2007 NOx emission standards, because it could have implications for the emission certification equipment, the life of which must be warrantied for very lengthy periods.
Biodiesel may negatively impact low-temperature operability unless additized. Additization may also be necessary to address manufacturers’ concerns that the thermal and oxidative stability of biodiesel is poorer than No. 2 diesel. At higher blend levels, and especially at the B100 level, power output and fuel economy are reduced. Manufacturers have also expressed concern about potential damage to paint due to biodiesel’s solvency effect. This solvency effect can also loosen sediments and contaminants in the fuel tank and plug fuel filters upon initial use of biodiesel.
Manufacturer Guidelines
US Manufacturers have only a limited amount of field experience, complemented by research and testing programs, upon which to base their recommendations for biodiesel use. As such, the guidelines currently vary from one manufacturer to another. Most manufacturers permit the use of B2 and B5. Some permit the use of B20 or permit its use under special circumstances. Only a few manufacturers permit the use of B100.
Manufacturer approval of biodiesel blend use may cover an entire product line, but more often applies only to specific vehicles, model years or engine families. The fuel injection equipment (FIE) manufacturers have thus far limited their use recommendations to the B5 level.
Regardless of the permitted blend level, nearly every manufacturer stipulates certain use requirements and usually increased maintenance or service intervals.
Most manufacturers require that the biodiesel used in the blend meet the specifications set forth in ASTM D 6751 (in some cases meeting the European Standard EN 14214 may also be required). Several also specify, or recommend, that the biodiesel or biodiesel blend be supplied by a BQ-9000 accredited producer or BQ-9000 certified marketer. Most also note that, similar to their position on diesel fuel and gasoline, warranty claims resulting from the use of off-specification or low-quality fuel may not be honored.
Several manufacturers recommend more frequent fuel filter changes, with some specifying certain filter media. In the case of farm tankage or storage tanks for fleet operations, some manufacturers recommend adding a biocide to the fuel, adding an in-line filter to the storage tank dispensing system, and adding a fuel/water separator. Some manufacturers also recommend keeping storage tanks as full as possible to reduce the potential for condensation of water on storage tank walls. Many also recommend adding an antioxidant to stored product to improve its shelf life or recommend that biodiesel not be stored for more than three months before it is used.
Certain manufacturers have expressed concern that uncombusted biodiesel (especially at higher blend levels) may get past piston rings and dilute engine oil. Such manufacturers recommend checking engine oil levels daily to see if the oil level increases (an indication of potential engine oil dilution). They may also recommend increased oil change intervals due to such concerns. However, there is considerable technical data indicating engine oil dilution should not be a problem for conventional fuel systems.
Other manufacturer recommendations include using lower blend levels, such as B2 or B5, during cold weather due to concerns about cold flow properties. An alternative recommendation offered by some is to add a cold flow improver additive to address low- temperature operability concerns. Additional recommendations may include warnings about spilling biodiesel on painted surfaces, because it could cause paint damage.
A number of manufacturers also provide recommendations about storing vehicles or equipment. Several recommend that their products not be stored for extended periods with biodiesel in them.
Finally, several manufacturers note that their biodiesel research and testing programs, especially at higher blend levels, are ongoing. They note that as they gain more field experience and complete additional testing, recommendations may be revised.