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Small-Scale Biodiesel Production

What is Biodiesel?

Figure 1 Canola Field

Biodiesel is an engine fuel made from vegetable oils or animal fats. The term biodiesel refers to 100% pure fuel (B100) that meets the requirements given by the American Society for Testing and Materials (ASTM) for biodiesel fuel in their D 6751 standard. A variety of oils can be processed into fuel for compression ignition internal combustion engines including canola, soybean, sunflower, palm, and safflower. Rendered animal fat and waste cooking oils can also be processed into biodiesel.

Table 1 Biodiesel Yields from Crops

Oil Extraction and Preparation

Seed Preparation

Small-scale biodiesel producers can purchase vegetable oils to process into biodiesel or extract oil from crop seeds. Producers who decide to extract oil from the crop seeds need specific equipment. Oil is removed from oilseed crops either by expeller pressing or solvent extraction. Small-scale biodiesel producers commonly use a press extraction method because it is a simpler, less expensive and safer process.

Extraneous material is first removed from the harvested crop seed by screening. This is particularly important to ensure materials such as small stones are removed prior to crushing and extraction. The crop seed should also be passed over a magnet to remove metal pieces.

Oil Extraction

Figure 2 Screw Press

Crushers are used to extract oil from oil seeds. Processors sometimes heat the seeds prior to processing to decrease the energy required to extract the oil from the seeds. Some seeds like soybeans, must be cooled to destroy antinutritional factors in the cake. The heated seeds can be crushed and flaked to further enhance oil extraction, or it can simply run through a cold press, also referred to as expeller pressing, to remove the oil. The material other than oil left after extracting the oil is referred to as cake. The extracted oil is referred to as "crude oil".

Oil Preparation

If animal fat feedstock is used to make biodiesel it must be stored in heated tanks at 55-60°C to prevent solidification. The animal fat feedstock is filtered to remove insoluble materials, such as meat and bone particles.

The crude vegetable oil is degummed before processing into biodiesel. Degumming involves removing phospholipids (gums). This is accomplished by mixing small quantities of water with the oil and then draining the water off the bottom of the oil. Several washings may be required to remove all the gums. Initially, the wash water appears milky-colored. The degumming is complete when the water appears clear. It is important to remove all the water from the oil before further processing because the presence of water will cause the fatty acids in the oil to form salts rather than biodiesel. Removing the water can be accomplished by heating the oil to 120°C to boil off the water.

Figure 4 Degumming Wash------------Figure 5 Clear Wash

Processing into Biodiesel (Transesterification)

Figure 5 Biodiesel (top) and Blycerin (bottom)

Biodiesel is the product of chemically reacting a vegetable oil or animal fat with an alcohol to produce fatty acid alkyl esters which is biodiesel. A catalyst such as sodium or potassium hydroxide is required. Without the catalyst the conversion process would not occur. Glycerin is produced as a byproduct. The proportions of the reaction are approximately ten parts oil to one part alcohol which yields ten parts biodiesel to one part glycerin, a sugar alcohol. In practice the process requires an excess amount of alcohol which then be recovered at the end of the processing.

A sample of the crude oil needs to be compared (titrated) with a standard solution of base in order to determine the concentration of free fatty acids present in the oil sample which will be the quantity of catalyst base required to cause the change from vegetable oil to biodiesel. Methanol is the most commonly used alcohol in producing biodiesel because it is currently the least expensive.

Biodiesel Quality Factors

Biodiesel is an excellent diesel engine fuel if it meets ASTM standards for biodiesel. The ASTM standard for biodiesel, D6751, specifies the parameters for 18 factors that affect biodiesel purity, storage, cold weather properties and use in engines. The ASTM D6751 standard is for B100, which is 100 percent biodiesel before it is blended with petroleum diesel. For biodiesel to consistently perform well in engines it must be completely converted from the source vegetable oil into biodiesel with any remaining catalyst and coproducts completely removed. The ASTM standard provides the assurance that this has been accomplished.

The ASTM standard for biodiesel, D6751, is available from several places on the Internet including at the National Biodiesel Board Web site at http://www.biodiesel.org. Commercial laboratories provide testing services to determine whether or not biodiesel meets the ASTM D6751 standards. Small-scale producers need to test the fuel they make to ensure it meets the D6751 standard both to insure the fuel quality for use in engines, and to qualify for government tax incentives available to biodiesel producers.

Perhaps the most important standards for biodiesel quality are the total glycerin and the free glycerin content. Glycerin is the major coproduct in biodiesel production and is higher in density than the fatty acid alkyl ester (biodiesel). The total g lycerin in biodiesel indicates the reaction did not proceed to completion. The free glycerin in biodiesel indicates incomplete washing after the conversion process. The free glycerin may separate out in storage, and cause problems when used in engines. The total acid number is also an important test for biodiesel. Total acid is an indicator of the level of free fatty acids present in biodiesel, as well as the presence of process acids. High acid values indicate poorly refined biodiesel, which may indicate methanol carryover in the final product, and can have negative effects on rubber seals and hoses in the engines.

Biodiesel Use in Engines

Biodiesel has less energy than diesel fuel. Summer petroleum diesel fuel typically contains about 140,000 British thermal units (Btu) per gallon, while biodiesel contains about 130,000 Btu per gallon. Fuels with a higher heat of combustion (Btu content) usually will produce more power per unit of fuel than lower-energy fuels. An engine using a lower-energy fuel will require more fuel to produce the same amount of power. Since biodiesel has less energy, it will require about 1.1 gallons of fuel to do the same work as 1 gallon of diesel fuel. This is about an 8 percent power reduction for B100 but proportionately less for blended fuels.

Several factors influence how well biodiesel works in diesel engines. The cetane number is a method for determining the ignition quality of a fuel. Most farm tractor engine manufacturers recommend a minimum cetane rating of 40. However, most fuel suppliers provide diesel fuel with a cetane rating of 45 to 50. A typical biodiesel cetane rating is 55. In general, high-cetane fuels permit an engine to be started more easily and provides for a faster engine warm-up without producing white smoke or misfiring. A high-cetane fuel also helps reduce the rate of varnish formation and carbon deposits in engines and reduces combustion roughness or engine knock.

Viscosity has an influence on the atomization of biodiesel fuel when it is injected into the engine combustion chamber. The viscosity of summer-blend petroleum diesel is generally about 3 CP, biodiesel about 5.7 CP and vegetable oil about 40 to 50 CP. The high viscosity of raw or partially refined vegetable oil may result in excessively high pressure in the injection system and cause poor atomization of the fuel in the combustion chamber which can cause deposits around the piston rings, valves and injectors.

Biodiesel is an excellent solvent and enhances removal of deposits in fuel systems, which in turn can plug fuel filters or cause deposits to accumulate in fuel tanks. Filters generally do not continue to plug after the initial use of biodiesel. Biodiesel may cause fuel lines, gaskets and fuel pump seals on older engines to deteriorate. Seals made from materials more biodiesel-tolerant are readily available.

Biodiesel (B100) provides excellent lubricity in diesel engine fuel systems. Since October 2006, most diesel fuel sold at retail locations in the U.S. is ultra-low sulfur diesel fuel that has different lubricating qualities than diesel previously available. The processing required to reduce sulfur also removes naturally-occurring lubricating agents in diesel fuel, resulting in increased wear on the various parts of the engine's fuel injection system. The ASTM D975 standard for diesel fuel lubrication sets the maximum amount of wear on materials when tested with specific fuels or blends of fuels. The test apparatus is called a High Frequency Reciprocating Test Rig (HFRR). Both No. 1 (winter blend) and No. 2 (summer blend) diesel test results cannot exceed 460 microns of wear when tested in an HFRR. A lower wear score indicates better lubrication. Biodiesel has been tested at varying concentrations with both ultra-low sulfur No. 1 and No. 2 diesel fuels. The results indicate that a 1 percent blend of biodiesel with No. 2 diesel is sufficient to reduce the HFRR micron score below the required 460 micron standard. However, a 2 percent biodiesel blend is required in No. 1 diesel to get the HFRR micron score down to 460 microns.

Cold Weather Issues

Two characteristics, the cloud point and the cold filter plugging point (CFPP), commonly characterize the low temperature operability of diesel fuel and are similarly important with biodiesel. The c loud point is the temperature of the fuel at which small, solid crystals can be visually observed as the fuel cools. CFPP is the temperature that a fuel filter plugs due to fuel components that have crystallized or gelled.

The cloud point of soybean biodiesel is about 30° F and the cloud point for No. 1 diesel is about -35° F. Usually, when the fuel nears the cloud point temperature, changes must be made to the fuel, such as the addition of anti-gel additives or No. 1 diesel fuel to prevent filters from clogging. B20 that is not treated with anti-gelling additives freezes about 3 to 5 degrees Fahrenheit higher than No. 2 petroleum diesel. Studies funded by the National Biodiesel Board indicate that blends of B2 or B5 have minimal or no effect on cold-flow properties of diesel blends. Biodiesel made from various crop oils have unique cold-weather characteristics that can vary up or down by as much as 5 degrees.

Several procedures can be used to enhance biodiesel performance in cold weather including the addition of fuel-line heaters or in-tank fuel heaters, using anti-gel additives, insulating fuel filters and fuel lines, and storing the diesel-powered equipment in heated buildings.

Engine Warranties

Most agricultural equipment manufacturers do not warranty their engines operated on B100 but they do allow blends of up to 5 percent with petroleum diesel. All engine warranties from the major tractor manufacturers also require both the petroleum diesel and the blended biodiesel to meet the American Society for Testing and Materials (ASTM) specifications for diesel and biodiesel fuels. Some manufacturers' warranties of farm tractors and equipment do allow higher blends, some even up to B100, so it is important to check with the equipment dealer or manufacturer for each engine model. Biodiesel “batch” producers need to be particularly concerned about the possibility of voiding engine warranties because of the difficulties of consistently producing high quality biodiesel. This underscores the need for small-scale producers to have fuel regularly analyzed to make sure it meets ASTM D6751 standards.

Engine Emissions

Generally engines operated on biodiesel produce fewer harmful emissions than engines operated on petroleum diesel. The U.S. Environmental Protection Agency (EPA) conducted a comprehensive analysis of biodiesel impacts on exhaust emissions in 2002. These analyses found tailpipe emissions from engines using biodiesel are significantly lower than emissions from similar engines operated on petroleum diesel. Hydrocarbon emissions from engines operated on B100 were about 67 percent less than petroleum diesel fuel emissions. Hydrocarbon emissions from engines contribute to ozone formation and are a key component of smog.

Particulate matter and carbon monoxide emissions from engines using biodiesel were 48 percent less than petroleum diesel. Particulate matter is very fine particles that can remain suspended in the atmosphere and contribute to smog. Carbon monoxide is a poisonous gas that is most dangerous in confined areas.

Nitrogen oxide emissions are about 10 percent higher using biodiesel. Reducing nitrogen oxide emissions is a crucial component of EPA's strategy for cleaner air and reducing acid rain. There are efforts to modify diesel engine combustion and exhaust systems to reduce nitrogen oxide emissions.

Emissions of carbon dioxide (CO2), a gas that is increasing in the atmosphere, are similar from engines operated on biodiesel or petroleum diesel. However, the carbon dioxide produced from burning petroleum diesel and emitted to the atmosphere comes from sources long sequestered in the earth. Oil seed crops actually take carbon from the atmosphere during their growth cycles and store that carbon in the ground; although the energy used to make biodiesel normally releases carbon into the atmosphere. When comparing the total life cycle of carbon emissions from the two fuels, petroleum diesel has a more negative effect on the environment. A study conducted by the U.S. Department of Agriculture and Department of Energy of the biodiesel and petroleum diesel life cycles jointly found that because biodiesel production requires such small amounts of fossil fuel, its CO2 life cycle emissions are much lower than those of petroleum diesel. Biodiesel reduces net CO2 emissions by more than 78 percent compared with petroleum diesel.

Biodiesel produces no sulfates when burned in diesel engines. Reducing sulfates is another part of the EPA's strategy to for cleaner air. Since there are zero sulfates in biodiesel, its use in diesel engines offers an excellent alternative to even the ultra-low sulfur diesel fuels mandated for use in over-the-road diesel-powered vehicles.

Storing Biodiesel

Figure 7 Biodiesel Storage

Biodiesel will have a storage life similar to diesel fuel, which usually is six months to one year. A two-year study completed at the University of Idaho found that biodiesel had slight deterioration and will store similarly to No.2 diesel fuel. Biodiesel mixtures of any blend should store during warm and cold months with little problem of separation of the biodiesel from the petroleum diesel.

Most biodiesel will begin to solidify if stored in cold conditions lower than 32°F. Since biodiesel is an excellent solvent is may loosen tank deposits. Biodiesel should be stored in dark conditions where there is no danger of water contamination. Biodiesel stored longer than six months should be analyzed to assure that it still meets ASTM D6751 standards.

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John Nowatzki, Extension Ag Machine Systems Specialist
Agricultural & Biosystems Engineering Department
North Dakota State University
John.Nowatzki@ndsu.edu
Telephone 701-231-8213