The importance of biodiesel analysis

The use of biofuels is set to become more and more popular, but just how do you find out what is in your environmentally friendly juice?

Everyone is familiar with ethanol as a fuel alternative or blend component with traditional petroleum-based fuel. But are you familiar with biodiesel?  Biodiesel is a fuel derived from plant or biomass. Starting materials such as soy, rapeseed, corn, coconuts, and even used cooking oil are the basis for a simple reaction to create a fuel that is clean burning and low in sulphur. Biodiesel is popular as a pure fuel alternative (B100) or as a blend with petroleum-based diesel fuel, for example B20 indicates that 20% of the blend is biodiesel. Blending with petroleum-based diesel can also add lubricity, a desirable feature, decreased when sulphur is removed. Traditional diesel engines can use blends up to 20% biodiesel without modification. Europe is very progressive in incorporating biodiesel in fuel programs. In fact, the European Union has passed a directive (2003/30/EC) to encourage the use of biofuels in the transport industry. The UK produced approximately 51,000 tonnes of biodiesel in 2005, increasing 80% over the previous year, as reported by the European Biodiesel Board (www.ebb-eu.org). The entire EU is estimated to have produced 3,184,000 tonnes of biodiesel from 120 plants.  The US and Pacific Rim countries such as Malaysia are also rapidly growing in biodiesel production as petroleum-based fuel becomes more expensive.

Biodiesel is produced by the reaction of a biomass raw material, such as soybean or sunflower oil, with a catalyst in the presence of methanol. Biodiesel and glycerin by-product are produced. The quality of biofuels is critical to performance and acceptance in the market. Filter clogging, poor cold flow, and engine damage may result if the biodiesel quality does not meet the necessary limits for contaminants and residual reaction starting materials or by-products. Standards-setting organisations in Europe (EN 14214) and the United States (ASTM D-6751-03) have designated metrics for biodiesel quality.  Some of the tests specified are as follows: ? Flash point using a closed-cup test method. This is an indicator of the level of unreacted alcohol remaining in the fuel. ? Viscosity determined with a dynamic viscosity test method. Too high or too low a viscosity can result in power loss due to inefficiency in the engine injection pump. ? Acid number using a potentiometric titration test method. This is used to indicate the level of free fatty acids or processing acids in biodiesel. ? Sulphur using an ultraviolet (UV) fluorescence method: Sulphur degrades engine wear by leaving deposits on engine components. It also impacts emission-control systems performance. ? Phosphorus using an inductively coupled plasma optical emission spectrometer (ICP-OES) test method: High levels of phosphorus have been shown to damage catalytic converters used in emission control systems. ? Group I and Group II metals (Na, K, Ca, Mg) can be determined with atomic spectroscopy techniques such as atomic absorption or ICP-OES.  These elements may come from the catalyst used and may cause soaping problems if too much remains in the final product. ? Free glycerol using a gas chromatography (GC) test method: Free glycerol (or glycerin), which is a by-product of the transesterification process, causes injector deposits, which can clog the fuel system. It can also build up in the bottom of storage and fuel tanks. ? Total glycerol using a GC test method: This measures the level of free glycerin plus any unreacted oil or fats (mono-, di- or triglycerides) in the biodiesel. These unreacted glycerides can cause injector deposits and may adversely affect cold-weather operation.

Metallic contamination can arise from the raw material or be introduced in the production process. Although the methods specified for metallic elements, sulphur, calcium, magnesium, potassium, and sodium use different technology, a more efficient way to determine these elements is in a multielement analysis using one inorganic analysis technique such as inductively coupled plasma optical emission spectroscopy (ICP-OES).  Method prEN 14107 and prEN 14538 specify ICP-OES for the measurement of phosphorus, calcium, and magnesium. They can serve as the base for an expanded method to determine the full suite of elements in a more efficient manner. If additional metals may be present in the raw material and may affect the production process or final product, these may be monitored in the starting material, using the same methodology.  Typical detection limits using ICP-OES for the listed elements are compared with the regulated concentration in Table 1. Generally, detection limits ten times below the level where a decision will be made provides sufficient margin to ensure a sensitive measurement. In this case, it can be seen that the margin is generous and maximum levels could be reliably measured at lower concentrations in the future, if necessary.

Table 1.  Typical detection limits and regulated maximum levels for metallic contaminants (mg/kg)

	Detection Limit using ICP-OES	Biodiesel EU 14214	Biodiesel DIN V 51606	Biodiesel ASTM 6751	Diesel Fuel EU 590 (1999)
Sulphur	0.01	10	10	15	350
Phosphorus	0.04	10	10	10
Sodium	0.0005	Sum Na+K 5
Potassium	0.001	Sum Na+K 5
Calcium	0.00005	Sum Ca+Mg 5
Magnesium	0.00004	Sum Ca+Mg 5

An example of sulphur, one of the more difficult elements to measure in a diesel fuel and several soybean oil-based biodiesel final products, is shown in Table 2.

Table 2.  Sulphur measurements in diesel and biodiesel (mg/kg) determined using the PerkinElmer Optima ICP-OES

Sample	Mean Concentration	Standard Deviation
NIST 2723	10.9	0.07
Diesel 1	7.06	0.11
B8-10-1	17.5	0.06
B8-10-2	1.56	0.11
95081	4.89	0.10

The precision for three replicates is excellent and the NIST reference diesel fuel standard recovery is 99% of the true sulphur value. EU standard 14105 describes a method for quantitatively determining free and total glycerin in B100 methyl esters (biodiesel) by GC using flame ionisation detection (FID) technology. This is a critical quality measurement to ensure high engine performance and the detection range for this method is 0.005% – 0.05% for free glycerin. The sample is first derivatised with a silylating agent and then injected into an open tubular GC column packed with a 5% phenylpolydimethylsiloxane. Calibration is achieved with two internal standards (butanetriol and tricaprin) and four reference materials. Mono-, di- and triglycerides are determined by comparison with mono-olein, di-olein and tri-olein, respectively. Conversion factors are then applied to the results for mono-, di- and triglycerides to calculate the sample’s bonded glycerin content. The total glycerin represents the sum of the free and bonded glycerin.

Table 3 shows the quantitative results for a biodiesel sample.  It can be seen that the free glycerin is within the specified limit of 0.02 mass %. The total free and bound glycerides are also within the 0.25 mass % specified in the EU standard.

Table 3.  Quantitative results of free and total glycerides for a soybean-based biodiesel Sample

Time (min)	Component	Mass %
4.442	Glycerin	0.014
5.159	1,2,4-Butanetriol (ISTD)	--
15.53	Total Monoglycerides	0.142
18.95	Tricaprin (ISTD)	--
20.31	Total Diglycerides	0.011
22.44	Total Triglycerides	0.004
	Total Glycerides	0.171

Biodiesel is a growing renewable fuel that offers many advantages over conventional petroleum-based diesel. Regulations are being put into place in countries around the world to encourage the use of biodiesel in its pure form or as a blend component. The quality of biodiesel fuel is important to ensure good engine performance. Good cold flow characteristic is especially important in cooler climates. Adequate testing performed on an ongoing basis can ensure uninterrupted production and a compliant final product, satisfying our need for energy, while reducing the burden on the environment.

References

1. Lee Davidowski, Sulphur in Diesel and Biodiesel, ICP-OES Analysis, Webcast presentation, September 18, 2006, PerkinElmer event no. 83989, www.perkinelmer.com

2.  Timothy Ruppel and Gerald Hall, Free and Total Glycerin in B100 Biodiesel by Gas Chromatography, PerkinElmer application note 007701_01, 2006.

 

By Zoe Grosser and Matt Bowman. Zoe Grosser, Ph.D. is a Segment Marketing Manager for PerkinElmer Life and Analytical Sciences, a manufacturer of analytical instrumentation.