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Get fast results for S and Cl in Biofuels in compliance with ISO 20884

The demand for biofuels has increased in recent years and is expected to continue growing through the next decade.

Joseph Iaia
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Article Summary
In Europe specifically, the EU has made it a priority to have as much as 10% of the total transport fuel from every affiliated country come from renewable sources like biofuel by the year 20201. With over 220 biorefineries in operation across Europe, the industry is looking to quantify the sulphur and chlorine in their products in the most efficient and safe way possible.

Biorefineries producing biofuels or working with biofeedstocks will still be held accountable for the total sulphur count in their finished products as per ISO 20884. Likewise, the risk of corrosion from chlorine in these products is still just as much an issue as with any traditional crude oils. However, with biofuel samples there is typically a greater concentration of oxygen than in traditional samples. This is important to note because oxygen naturally absorbs XRF signals and as a result can cause analysers to report lower sulphur and chlorine concentrations than are actually present in the sample. For this reason, we have developed a case study that tests real world biofuel samples for sulphur and chlorine while using correction factors to eliminate bias from oxygen.

In this study, we will test three real world samples, showcase their concentrations, and then apply correction factors to mitigate biased results. The correction factors used were derived from the ASTM D7039 method, though these correction factors can still be applied for those in Europe adhering to the ISO 20884 method. As per Section 1 of ISO 20884, any sample with more than 3.7% oxygen content must be corrected for.

Application study
Three real world samples were analysed: a rapeseed oil sample, and two different palm oil samples. For this study, each sample was separated into 10 aliquots via pipette into a standard XRF cup. The samples were then sealed with sample film and vented. The two palm oil samples were heated to 42°C before being vented. See Figure 1 for the sample preparation process.

Each sample was then placed into a Sindie +Cl analyser and measured for 30 seconds and 300 seconds. Sindie +Cl was used in this study due to its convenient ability to measure total sulphur and chlorine concurrently. Sindie +Cl is not ISO 20884 method compliant due to its lack of a proportional counter, however, it still utilises the Monochromatic Wavelength Dispersive X-ray Fluorescence (MWDXRF) technology found in standalone Sindie and Clora analysers. Sindie and Clora analysers are ISO 20884 method compliant and can be used to obtain similar results as shown in this study. Sindie analysers deliver similar detection limits to Sindie +Cl, while Clora analysers can detect even lower chlorine levels than is achievable with Sindie +Cl.

Once the samples were analysed, data was compiled for each sample type as shown in Tables 1-3. The concentrations throughout this data are not corrected for oxygen content and therefore contain a bias. However, separate from the bias issue, we can see that the RSD (relative standard deviation) of this data is very low. This points to a high level of precision of the technique, and this will carry over once we apply our correction factors.

Each sample tested has its own oxygen content. The rapeseed oil has an oxygen content of 9.1%, the first palm oil sample has an oxygen content of 10.0%, and the other palm oil sample contained 11.6%. Knowing the oxygen content of the sample of interest is crucial to determine an accurate concentration count as the oxygen number is directly correlated to the correction factor. Correction factors for each sample were calculated using the numbers provided in Tables 4 & 5.

The use of correction factors require the user to know how much oxygen (as wt%) is in the sample. Differing oxygen contents in the biodiesel samples will require different correction factors. As noted in ASTM D7039 section 5.2.1, oxygen correction factors for sulphur in biodiesel can be found in Table 2 of the method (Table 4 in this paper) or calculated using Annex A1.

Refer to the Table 4 note for obtaining the correction factor that corresponds to the oxygen content in the measured sample. For example, 11 wt% oxygen corresponds to a correction factor of 1.1914. Apply the correction factor by multiplying the uncorrected measured value (reported by the analyser) by the correction factor to obtain the oxygen corrected value. Oxygen correction factors for chlorine in biodiesel are not included in the D7039 method, but they can be calculated using the same principles as mentioned above for sulphur. Table 5 is used in the same manner as the sulphur table. For example, the correction factor for chlorine in a biodiesel sample having 11 wt% oxygen is 1.1961.

Correction factors are ideal for laboratories that run biodiesels infrequently or don’t want to maintain multiple calibration curves for oxygenated and non-oxygenated samples. These correction factors will allow the user to run traditional diesel, biodiesel blends (e.g. B6-B20), and B100 samples on the same calibration curve instead of having three separate curves.

These correction factors were derived from ASTM D7039, and can be applied to any sample with known oxygen content even if the user is not complying specifically with D7039 methodology. The corrected data is shown in Tables 6-8 and as expected, we have a slight increase in total counts but have maintained our low RSD values and thus, our high level of precision.

With the push for renewable fuel sources on the rise in regions like Europe, petroleum professionals are looking to utilise technology that allows for quick, on-site measurement of biofuels. With analysers such as Sindie and Clora, users can analyse critical elements such as sulphur and chlorine while being method compliant with ISO 20884. Users less concerned with method compliance can utilise Sindie +Cl for precise measurement of both elements with one instrument. By quickly and simply applying a correction factor to the results, professionals can certify their biofuel products more efficiently than with other methods.

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