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The author looks at methods used to detect the presence of contaminants in biodiesel.
Biodiesel is a renewable fuel produced from vegetable oil, animal fats, and waste cooking oils. Feedstocks such as soy, canola, mustard, sunflower, coconut, palm, and cottonseed oil as well as beef tallow and fish oils have been used to manufacture biodiesel.
As a result of the world's energy demands, the use of biodiesel is increasing rapidly. Blends of biodiesel with conventional petroleum diesel represent a common use of biodiesel. In the United States, B20 (a blend of 20% biodiesel and 80% petroleum diesel) is recognized as an alternative diesel fuel. Biodiesel has a number of advantages, including:
Because the presence of contaminants can lead to operational problems, the American Society for Testing and Material (ASTM) and European Standard (EN) have developed standards to which pure biodiesel (B100) can be tested. The ASTM Standard is D-6751, shown in Table I.
Table I: ASTM D-6751
Physical, chromatographic, and spectroscopic methods are used to apply the standard. Here we look at the use of the FuelPro Biodiesel Metals Analyzer (Teledyne Leeman Labs, Hudson, New Hampshire) to apply the ASTM standard.
Accurate elemental analysis of biodiesel requires analytical methodology that is both sensitive and selective. The analyzer meets these requirements and can determine up to 70 different elements in a sample. The analyzer permits close monitoring of elemental content throughout processing, from the raw oil to the finished product. Trace metal analysis is an important part of quality control as well as quality checks of the finished products.
The effects of the biodiesel not meeting the specifications of the D-6751 standards follow:
Biodiesel samples were prepared by diluting 1:10 with kerosene. The analyzer was calibrated with standards prepared by diluting Plasma-Pure biodiesel stock standards. Standard concentrations were at the 0.00, 10.0, and 20.00 ppm levels for Na, K, Cu, Mg, P, and S.
The analyzer's preprogrammed method was followed for the set-up and analysis of biodiesel.
The values obtained as a result of the analysis of a B100 biodiesel sample are shown in Table II. All concentrations are given in ppm. The concentrations in the original sample are listed in the column labeled "Final Concentration (ppm)" ("ND" indicated that the analyte was not detected). The column labeled "Spike recovery (%)" contains the spike recovery data for a 3-ppm spike of all the analyte elements. (A separate sample was spiked with approximately 9 ppm sulfur to determine its recovery since the multielement stock standard contains sulfur as a matrix element.) The spike data show excellent recoveries, indicating that the 1:10 dilution is sufficient to eliminate any viscosity effects that may affect accuracy.
Table II: Analysis results
The results indicate that this biodiesel sample passes the ASTM D-6751 standard. The result for sulfur indicates that it successfully passes the S-15 low sulfur designation that all highway diesel fuels had to meet from 2007 onward.
Table III contains typical detection limits obtained in the oil matrix. The detection limits were determined by taking three replicates measurements in blank oil and multiplying the standard deviation by three.
Table III: Detection limits
Biodiesel is analyzed effectively using this method, and the detection limit capability of the instrument meets the requirements of the ASTM and EN Standards against which biodiesel must be measured.
Samples also are prepared effectively by dilution with a suitable solvent. Excellent recoveries are obtained from spiked biodiesel samples. This indicated that the method is suitable for the analysis of biodiesel fuels and that matrix interferences are not a problem.
Manny Almeida is with Teledyne Leeman Labs, Hudson, New Hampshire.
(2) Specification for Biodiesel (B100) 0 ASTM D6751.