Biodiesel is the only alternative fuel to have fully completed the emission characteristics and health effects testing requirements of the US EPA 1990 Clean Air Act Amendments.1 It has been demonstrated that using blends of 20% biodiesel and 80% diesel fuel in conventional diesel engines results in a reduction of total hydrocarbons by up to 30%, carbon monoxide by up to 20% and total particulate matter by up to 15%. In addition, the hydrocarbon emissions of pure biodiesel (B100) are associated with an ozone forming potential that is almost 50% less than that of diesel fuel.2
Biodiesel is nearly carbon neutral. Plants cultivated for biodiesel production absorb carbon dioxide from the atmosphere during growth. This balances the carbon dioxide emissions generated when biodiesel is combusted. Additionally, pure biodiesel fuel has a lower sulfur concentration compared to conventional diesel fuel, thereby reducing sulfur dioxide emissions to virtually zero.
Biodiesel can be either used in its pure format (B100) or blended with diesel fuel at varying concentrations. However, biodiesel manufacturers must ensure that their fuel is free of certain elemental contaminants in order to ensure optimum engine performance and avoid any potential failures.
The most common starting material for biodiesel production is plant oil, which contains comparatively high levels of phosphorous, the presence of which can cause the mechanical components of engines to corrode. Elevated sulfur concentrations can also accelerate engine wear. The total amount of phosphorous and sulfur in the final biodiesel fuel is essentially determined by the concentrations of these two elements in the starting materials.
Biodiesel is more susceptible to water contamination than petroleum diesel. The presence of water in biodiesel can lead to corrosion of fuel system components and growth of microorganisms.3 Other contaminants that are of significant importance for biodiesel final product quality are free and total glycerol and the products of oxidation. High levels of bound glycerin can cause crystallization and increased viscosity while oxidation can produce chemical compounds that improve the cetane number but also increase the acidity of the fuel.4
In order to optimize the biodiesel production process and ensure final product quality, the content of elemental contaminants is tightly controlled by fuel quality and environmental legislation.
Regulatory requirements
In February 2003, the European Committee for Standardization introduced the EN 14214 international standard to specify the minimum requirements and test methods for biodiesel. EN 14214 covers marketed and delivered fatty acid methyl esters (FAMEs) to be used either as automotive fuel for diesel engines at 100% concentration, or as an extender for automotive fuel for diesel engines.5
The American Society for Testing and Materials (ASTM) has established the D6751 standard, which is applicable in the United States and Canada, to cover pure biodiesel grades S15 and S500 for use as a blend component with middle distillate fuels. According to the standard, the specified biodiesel must contain FAMEs derived from vegetable oils and animal fats. The fuel must be tested for a variety of elements including methanol, water and sediment, kinematic viscosity, sulfated ash, oxidation stability, sulfur, copper strip corrosion, cetane and acid number, carbon residue, total and free glycerin, phosphorus, reduce pressure distillation temperature and atmospheric equivalent temperature.6
As an example, Table 1 details the maximum allowable concentrations of phosphorous and sulfur in biodiesel fuels.
In order to ensure regulatory compliance, it is important to identify a competent method to enable precise, dependable monitoring of elemental contaminants in biodiesel.
Biodiesel analytical methods
Biodiesel has traditionally been analyzed using chromatographic and/or spectroscopic methods. The major chromatographic methods are gas chromatography (GC) and liquid chromatography (LC). Although GC can be an effective technique, a few key downfalls are associated. Prior to GC analysis, biodiesel samples must be dissolved in low concentrations of organic solvent and derivatized with a specific reagent. Additionally, it is possible that some overlap in elution time will occur in complex mixtures, especially in the case of major components with similar properties.7
With LC, complex mixtures containing many different compounds are normally separated by classes of compounds and not by individual compounds. This is an important disadvantage as it decreases the accuracy of the method.
Contrary to chromatographic methods, atomic spectroscopy techniques analyze the entire sample at the same time. Uncertainties related to chromatographic methods are eliminated and no derivatization is required. Inductively Coupled Plasma (ICP) Emission Spectrometry is a popular spectroscopic method for biodiesel analysis, featuring an impressive analytical range that extends from parts per billion (ppb) to percent (%) levels.
Inductively coupled plasma (ICP) emission spectrometry
Both the radial and duo view configurations of ICP emission spectrometry can be used for this type of analysis, however the duo view is associated with carbon-based emission interferences that can impact analytical signal-to-background ratios negatively. This can be addressed using advanced radial plasma view technology. Offering excellent sensitivity and multi-element detection capability, this robust method is ideal for comprehensive biodiesel testing.
The radial view plasma configuration of ICP emission spectrometry achieves superior detection limits for lower concentrations of samples, being capable of providing accurate, reliable analysis of the important elements such as phosphorus, sulfur and lead. Detection limits are significantly lower than the maximum allowable limits given in the above standards. This is a significant benefit, given that the required detection limits are ten times below the regulated concentration levels in order to provide sufficient margin to ensure an accurate and confident measurement at the level of interest.
An experiment was developed to assess the capability of radial and duo view ICP emission spectrometry to perform accurate biodiesel analysis.
Materials and methods
A standard organics sample introduction kit and solvent flex pump tubing were used. The analysis was performed using a Thermo Scientific iCAP 6000 Series ICP emission spectrometer. A biodiesel sample derived from used cooking oil was obtained from a local supplier and was diluted by a factor of ten with kerosene. An additional aliquot of the sample was diluted, and a spike from oil-based standards of analyte elements to be analyzed was added. Multi-element standards were prepared using 300-mg/kg oil-based standards and a 5000-mg/kg potassium standard while 5000-mg/kg sulfur standards were used for preparing a single-element standard. All standards were diluted in kerosene and blank oil was added to match the diluted oil concentration of 10% by weight.
A method was developed containing the wavelengths of interest. Adjustments were made to the auxiliary and nebulizer gas flow to ensure the base of the plasma was approximately 1 mm to the right of the load coil on the duo view configuration and 1 mm below the coil on the radial view configuration. The nebulizer gas flow was further adjusted to ensure the sample channel extended approximately 3 mm from the load coil. Other sample introduction parameters, including pump speed and radial viewing height, were adjusted to provide the best signal-to-background ratio (Table 2).
Following method development, the instrument was calibrated and the samples were analyzed in a single run. The sub-array plots for each of the wavelengths were examined and the central integration region and background correction points were adjusted to minimize interference. A detection limit study was performed by measuring ten replicates of a matrix-matched blank. The resulting standard deviations were multiplied by three to provide the detection limits.
Results and discussion
Table 3 details the sample analysis results, spike recoveries and detection limits.
Sample results obtained for the ICP duo and radial view configurations were generally comparable and demonstrated that the samples were in compliance with ASTM and EN specification. Only sulfur concentration appeared slightly increased approaching 5.5 mg/kg. This result is most likely due to contamination during the manufacturing process. The result from the iCAP radial and the duo instruments for sulfur differ; this is most likely due to the presence of molecular interferences from oxygen based molecules that emit in the UV region of the spectrum, leading to a false negative. These interferences could be easily corrected with further optimization of the method. Other elements with elevated results were silicon, copper and iron. Magnesium silicate that is being used in the biodiesel production process could possibly cause the silicon contamination whereas contaminated production and storage vessels could generate the elevated iron and copper results.
Spike recoveries of both configurations were also comparable and within acceptable limits with the exception of calcium on the radial and phosphorus on the axial view of the duo instrument. Calcium was only slightly elevated and this could be attributed to contamination of the sample preparation flask. Phosphorus results in the axial view were close to the instrument detection limits, leading to an inaccurate spike recovery result.
Comparing the two viewing configurations, detection limits (Table 3, Figure 1) were similar for the majority of elements with less than a factor of 2 difference. The only significant differences were the detection limits observed for phosphorus and sulfur, with those obtained in the axial view of the duo configuration being poorer than the radial. This is due to the presence of molecular emission interferences in this region of the spectrum which are more marked with an axial plasma view than a dedicated radial view due to the greater viewing path length and the inability to select a viewing position that is optimized for minimum molecular emission in the axial view configuration. Therefore the radial iCAP instrument is ideal for this analysis due to its short path length in combination with the adjustable radial viewing height minimizing interferences on important elements.
Conclusion
Biodiesel is an environmentally friendly alternative to conventional diesel fuel, the production of which is strictly regulated to ensure optimum product quality for superior engine performance. Radial view ICP emission spectrometry is the method of choice for accurate detection of elemental contaminants in biodiesel and biodiesel blends. With effectively optimized radial view ICP design, the carbon-based emission interferences that hamper the determination of the important regulated elements sulfur and phosphorus are dramatically reduced, while analytical sensitivity is improved and unmatched detection limits are achieved. This makes an optimized, dedicated radial view ICP the ideal tool for performing such determinations.
References
1. The Official Site of the National Biodiesel Board, Recourses, Biodiesel Information, Biodiesel FAQs, www.biodiesel.org/resources/faqs/
2. The Official Site of the National Biodiesel Board, Recourses, Biodiesel Information, Fuel Fact Sheets, Health and Environmental, Environmental Benefits, www.biodiesel.org/pdf_files/fuelfactsheets/Enviro_Benefits.PDF
3. University of Kentucky, College of Agriculture, Biodiesel FAQ, Tim Stombaugh, Czarena Crofcheck, Mike Montross, Biosystems and Agricultural Engineering, www.bae.uky.edu/Publications/AENs/AEN-90.pdf
4. Determining the Influence of Contaminants on Biodiesel Properties, Jon H. Van Gerpen, Liangping Yu, Earl G. Hammond and Abdul Monyem, Iowa State University, www.sae.org/technical/papers/971685
5. Automotive fuels - Fatty acid methyl esters (FAME) for diesel engines - Requirements and test methods, www.acbiodiesel.net/docs/FAME%20Euro%20standard%20draft%20Oct02.pdf
6. American Society for Testing and Materials (ASTM) International, ASTM D6751 - 07be1 Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels, www.astm.org/Standards/D6751.htm
7. Biodiesel Analytical Methods August 2002–January 2004, J. Van Gerpen, B. Shanks, and R. Pruszko, Iowa State University, D. Clements, Renewable Products Development Laboratory, G. Knothe, USDA/NCAUR. www.nrel.gov/docs/fy04osti/36240.pdf
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