Rapid and accurate analysis of sulphur and chlorine in biofuels by XRF

(White Paper) -Biofuels are fuels produced from feedstocks such as vegetable oil, animal fat, used cooking oil, biomass, or a blend of these. They are not produced from fossil fuels, but rather from contemporary, human-induced processes like transesterification and hydrogenation.

Julian Doug van Berkum

Article Summary

Biofuels are often used for blending with traditional fuels such as gasoil and gasoline. More recently, biofuels are being blended into aviation and marine fuels. However, it is possible to use certain biofuels independently without blending them with traditional fuels. It is important to note that all biofuels – blended or not – must meet certain sulphur regulatory limits. Although there are no regulations or methods for it currently, some biofuels may contain fairly high levels of chlorine which can cause corrosion damage during and after the production stages.
Biodiesels are first-generation biofuels that are made through the transesterification of vegetable oils, animal fats or used cooking oils.

Biofuels made through hydrogenation with non- food feedstocks are called second-generation biofuels or advanced biofuels.

Having the correct sulphur value for biofuels is critical as they are typically blended with fuels that have a maximum specification of 10 mg/kg for sulphur (15 mg/kg in the US). If a ULSD (Ultra Low Sulphur Diesel) is blended with a biodiesel, the maximum specification remains 10 mg/kg. Additionally, biofuel samples contain a greater concentration of oxygen than traditional fuel samples. This is important to note because oxygen absorbs XRF (X-ray Fluorescence) signals and as a result can cause analysers to report falsely low sulphur and chlorine concentrations. For this reason, we developed a study to test real-world biofuel samples for sulphur and chlorine while using correction factors to correct for the bias caused by  oxygen. To correct for oxygen content, it is important to know the actual concentration of oxygen in the sample because the correction factor is directly correlated to the amount of oxygen present in the sample.

X-ray Fluorescence (XRF) delivers rapid and accurate results for testing sulphur and chlorine in biofuels, backed by international standard test methods (ASTM, ISO, etc.). Advantages of XRF technology include its non- destructive nature, easy sample preparation process, and quick results, in addition to accuracy that is on par with alternative technologies like UVF (ultraviolet fluorescence). Some relevant specifications and methods include EN 14214 (Fatty Acid Methyl Esters (FAME) for use in diesel engines and heating applications), ASTM D6751 (Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels), and ASTM D7467 (Standard Specification for Diesel Fuel Oil, Biodiesel  Blend (B6 to B20)).

In this study, we will use Sindie +Cl to test sulphur and chlorine in eight real-world samples, including both first- and second-generation biofuels as well as the traditional biofuels they are typically blended with. We will then apply correction factors to mitigate biased results. Sindie +Cl was used in this study due to its convenient ability to measure total sulphur and chlorine concurrently. The correction factors used were derived from the ASTM D7039 method, though these correction factors can still be applied when using the ISO 20884 sulphur method. As per Section 1 of ISO 20884, any sample with more than 3.7% oxygen content must be corrected for.

For our biodiesel samples, the oxygen values have been calculated via the ester compositions of each sample. HVO does not contain oxygen as the hydrogenation process removes all functional groups, transforming the feedstock into paraffinic chains. Gasoil does not contain more than 3.7% oxygen and therefore does not need to be corrected for; however, it’s important to note that if a biofuel is blended with gasoil, the oxygen concentration does go up and may need to be corrected for in these scenarios.

For this experiment the following samples were collected and analysed for sulphur and chlorine:
•    B10 (ULSD with 10% FAME)
•    B20 (ULSD with 20% FAME)
•    Gasoil
•    Hydrotreated Vegetable Oil (HVO)
•    Rapeseed Methyl Ester (RME)
•    Soybean Methyl Ester (SME)
•    Tallow Methyl Ester (TME)
•    Used Cooking Oil Methyl Ester (UCOME)

Each sample was measured ten times under repeatability conditions; condition of measurement, out of a set of conditions that includes the same measurement procedure, same operator, same measuring system, same operating conditions, and same location, with replicate measurements on the same or similar objects over a short period of time. Each sample was separated into ten aliquots via pipette into ten standard XRF cups. The samples were then sealed with Etnom® sample film, placed into Sindie +Cl, and measured for 300 seconds on a mineral oil calibration curve.

For each biodiesel sample, the oxygen content has been calculated from the ester composition, which was obtained via GC (Gas Chromatography) analysis. The results can be found in Table 1 together with the correction factor that should be applied per element/sample. Tables 2 and 3 display the oxygen correction factors for samples containing 0-19 wt% oxygen measured on a mineral oil calibration. Note that these factors are specific to the analyser geometry of the Sindie® and Clora® analyser series.

With the continued development of biofuels and a push for higher concentrations in traditional fuels, petroleum professionals are looking to utilise technology that delivers rapid and accurate results for on-site biofuel measurement. Sindie +Cl is a viable solution, delivering total sulphur and chlorine in one measurement without the need for a matrix-matched calibration by simply applying a correction factor to the results. This allows professionals to certify their biofuel products more efficiently than with other methods. Sindie +Cl complies with ASTM D2622, and therefore meets the regulatory limit of ASTM D6751 and ASTM D7467. It also meets the precision requirements for ISO 20884 and can be used for EN 14214.

Using Mwdxrf to measure sulphur & chlorine in biofuels 
XOS’ proprietary technology, known as Monochromatic Wavelength Dispersive XRF (MWDXRF®) utilises high-performing   doubly curved crystal (DCC) optics coupled with a low-power X-ray tube creating a low maintenance, highly precise technology. MWDXRF is a simplified and highly robust X-ray technique which provides sub-1 ppm sulphur and chlorine detection. An MWDXRF analyser engine consists of a low power X-ray tube, a point-to-point focusing optic for excitation, a sample cell, a second focusing optic for collection and an X-ray detector. The first focusing optic captures a narrow bandwidth of X-rays from  the source and focuses this intense, monochromatic beam to a small spot on the sample cell.

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