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Feedstock and Biodiesel Characteristics Report
The study was unique because it encompassed an extensive range of feedstocks and all feedstocks were pretreated, esterified, and transesterified using the same procedures and conditions allowing for uniform comparisons of critical fuel properties.
The research was conducted by Shannon D. Sanford, James Matthew White, Parag S. Shah, Claudia Wee, Marlen A. Valverde, and Glen R. Meier for the Renewable Energy Group.
A total of 36 feedstocks were evaluated and biodiesel was produced from 34 of them.
These feedstocks varied from traditional fats and oils to novel feedstocks from around the world.
The feedstocks used in the study were: algae (two samples), babassu, beef tallow, borage, camelina, canola, castor, choice white grease, coconut, coffee, distiller's corn, Cuphea viscosissima, evening primrose, fish, hemp, high IV and low IV hepar, jatropha, jojoba, karanja, Lesquerella fendleri, linseed, Moringa oleifera, mustard, neem, palm, perilla seed, poultry fat, rice bran, soybean, stillingia, sunflower, tung, used cooking oil, and yellow grease.
Jojoba and karanja were tested for feedstock quality but not made into biodiesel.
Each feedstock was tested for the characteristics of moisture, free fatty acid, kinematic viscosity, FAC colour, saponification value, moisture and volatile matter, insoluble impurities, unsaponifiable matter, MIU, oxidation stability, sulphur, phosphorous, calcium, and magnesium.
Each biodiesel was characterised according to the American Society for Testing and Materials (ASTM) D6751 and other properties. The characteristics were cloud point, cold filter plugging point, cold soak filtration, fatty acid profile, relative density, kinematic viscosity, sulphated ash, carbon residue, water and sediment, visual inspection, free and total glycerin, flash point, copper corrosion, phosphorous, calcium, magnesium, total acid number, moisture, sulphur, oxidation stability, and FTIR.
Moisture
The report says that moisture was a minor component found in all the feedstocks tested, but it can react with the catalyst during transesterification, which can lead to soap formation and emulsions.The study found that for the feedstocks with a moisture content of above 0.05 wt %, the application of heat and vacuum successfully lowered the moisture content.
The study said that fish oil, rice bran oil, and coconut oil were suspected to have significant amounts of volatile components because the respective moisture contents of the oils was much lower than the moisture and volatile matter results.
Free Fatty Acid
The interaction of Free Fatty Acid in the feedstock and sodium methoxide catalyst may form also emulsions, which make separation of the biodiesel more difficult; possibly leading to yield loss, the report says. Emulsions can also increase cost by introducing extra cleaning steps and replacement of filters.
To minimise the generation of soaps during the reaction, the target reduction for FFA in the feedstock was 0.5 wt % or less.
The research found that many of the feedstocks had FFA values that were above 0.5 wt %. These feedstocks were esterified using methanol and a special catalyst prior the transesterification step. Except for karanja oil, the FFA content was successfully lowered to below 0.5 wt%.
The study also found that coconut and babassu oil are types of feedstocks that are high in saturated fatty acids making them particularly stable towards oxidation. Some feedstocks, such as linseed, fish, and tung, present low oxidation stabilities since they contain high amounts of polyunsaturated fatty acids which are extremely susceptible to oxidation.
Kinematic Viscosity
The viscosity or the resistance to shear or flow was highly dependent on temperature the study found. It says the presence of strong or weak interactions at the molecular level can greatly affect the way the molecules of an oil or fat slide pass each other, therefore, affecting their resistance to flow.
The researchers found that Castor and Lesquerella oil presented the highest kinematic viscosities among the feedstocks studied. One possible reason for this, they say, is that these two oils contain high concentrations of hydroxy containing fatty acids (ricinoleic and lesquerolic acid) that are capable of forming hydrogen bonding.
They also found that tung oil contains high concentrations of a-eleostearic acid, an acid with naturally occurring conjugated double bonds that can interact with the double bond of adjacent fatty chains.
Saponification Value
The saponification value or the amount of potassium hydroxide (KOH) in milligrams required to turn one gram of fat or oil under the conditions specified soapy (to saponify), was based on the length of the fatty acids present in the triacylglycerol molecule and the weight of the triacylglycerol molecule.
The saponification value for the majority of the feedstocks was found to be in the range of 185 to 210 mg KOH/g. This range is typical for feedstocks having predominately fatty acids with a chain length between C16 and C18, the report says.
The research found that Babassu and coconut oil have a relatively higher saponification value of 258.5 and 267.6 mg KOH/g, respectively and that higher saponification values may indicate the presence of shorter chain lengths. The babassu and coconut oil also had a higher fraction of C12 and C14 fatty acids.
Jojoba and Lesquerella oil have lower than average saponification values of 106 and 173.9 mg KOH/g, respectively
Sulphur
Sulphur content in biodiesel is limited to 15 ppm maximum by ASTM D6751.
Therefore, it is important to know the original feedstock sulphur content since it can contribute to biodiesel sulphur content.
Removal of high levels of sulphur, such as in Lesquerella and neem, may require additional handling to meet the ASTM D6751 specification, the study says.
The rest of the feedstocks should be able to pass the ASTM D6751 specification using the pretreatment and transesterification procedures.
If a feedstock exceeded 10 ppm phosphorous, 5 ppm calcium and magnesium, it was pretreated using the phosphoric acid procedure and dried.
Cloud Point
The cloud point of biodiesel varies significantly with feedstock. Of the feedstocks evaluated, castor biodiesel has the lowest cloud point of -13.4ºC whereas beef tallow biodiesel and high IV hepar biodiesel have the highest cloud points of 16.0ºC.
Biodiesel made from feedstocks such as stillingia, tung, perilla, hemp, evening primrose, linseed, corn, borage, and soybean have a cloud point below or close to 0ºC because of the lower fraction of saturated fatty acids like palmitic and stearic. Biodiesel made from feedstocks such as beef tallow, yellow grease, and poultry fat, have a higher fraction of saturated fatty acids and therefore have higher cloud points.
Biodiesel made from Lesquerella and castor oil has cloud points of -11.6ºC and -13.4ºC, which may be due to the low amount of saturated fatty acids.
Cold Filter Plugging Point
Similar to cloud point, the CFPP of biodiesel also varies with the fatty acid distribution; with a lower fraction of saturated fatty acids resulting in a lower CFPP, and a higher fraction of saturated fatty acids resulting in a higher CFPP.
Usually, the CFPP of a fuel is lower than its cloud point. However, in the case of biodiesel made from castor and Lesquerella, the CFPP is higher than the cloud point.
To determine the CFPP of the biodiesel by ASTM D6371, the biodiesel passes through a 45 micron filtration device under a vacuum of 2 kPa. The biodiesel is cooled at 1°C intervals, and the temperature at which the fuel fails to pass through the test filter under the test conditions in a specified length of time is reported as its CFPP.39 In the case of biodiesel made from castor and Lesquerella oils, the reason for test filter plugging at temperatures higher than the cloud point could be due to the high viscosity of the biodiesel and not due to the crystallisation of biodiesel molecules. Babassu also has a cloud point lower than the CFPP.
Five biodiesel samples from castor, Lesquerella, neem, tung and poultry fat did not pass the ASTM Cold Soak Filtration specification of 360 seconds, all of which had filtration times longer than 720 seconds.
Further Reading
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November 2009
