Novel and Rapid LSC method for the analysis of biogenic carbon in fuels

Biofuel analysis at the refinery

Abstract

A novel method based on radiocarbon dating and liquid scintillation counting (LSC) was developed to determine the biocarbon content in biofuels without fuel specific background sample. Hidex biofuel method is an easy-to-use measurement that requires no sample preparation, making it well suitable for fast in-house detection needs. It uses a patented algorithm to find the background count rate and counting efficiency of unknown biofuel samples with variable colour. Hidex biofuel method employs a liquid scintillation counter with triple-double coincidence ratio (TDCR) output and an external standard. The performance of the method was demonstrated by analysing several different types of biofuels with a bio-% ranging from 1 to 100%. Results showed an excellent correlation between Hidex biofuel method and accelerator mass spectrometry (AMS), with detection limit down to 1% biocarbon.

Challenge

Fossil based fuels are increasingly replaced by fuels of biologic origin, such as hydrotreated vegetable oil (HVO) and bioethanol. Percentage of biogenic component in fuel can be determined with accelerator mass spectrometry (AMS) and LSC measurement with benzene synthesis (ASTM D6866 and EN 16640 guidelines), both based on 14C dating principle. These methods provide accurate results but require expensive and time-consuming sample preparation. Direct detection by LSC is an attractive alternative as liquid fuel samples can be measured directly after mixing with scintillation cocktail (DIN 51637, ASTM D8473-22 guidelines and Hurt et.al., Energy Fuels, 35, 2, 1503–1510, 2021). Fuel samples are often highly variable and intense in colour, which is affecting both counting efficiency and background. This is a challenge especially with fuel blends, as a fossil-based background sample with similar quenching properties is rarely available.

Solution

Hidex Biofuel method is an improvement on the direct detection by LSC. The method includes an algorithm, that uses a combination of two different types of quench parameters (external standard TDCR = eTDCR and QPE) to detect simultaneously both colour and chemical quench in the sample. It calculates the background count rate without a fuel specific background sample. The same algorithm also finds the counting efficiency for each sample. The method employs a 3-PMT LS counter enabling triple-double coincidence ratio (TDCR) output and an external standard. The biofuel algorithm is derived from background and efficiency quench curves with chemical and colour quenchers. TDCR and external standard quench parameters are used for the calculation of background and counting efficiency of unknown samples. The performance of the method has been demonstrated in routine use for over two years at fuel refineries and contract research organisations.

Calibration of the biofuel method

The biofuel method requires a TDCR counter equipped with cooling and Eu-152 external standard source. The counter used in this study was Hidex 300SL super low-level instrument. The algorithm is derived from background and counting efficiency quench curves which are measured on-site. Each quench set includes 7 standards with variable amount of chemical quenching agent and 7 standards with variable amount of colour quenching agent. Quench curve sets are available in Teflon coated plastic vials or in glass vials. After calibration, the algorithm is installed to the instruments measurement protocol and is ready for use in routine analysis of biofuel samples.

Sample preparation

Biofuel sample is mixed directly with MaxiLight+ scintillation cocktail in a vial. The typical sample amount is 10 ml biofuel and 10 ml cocktail. Samples are weighed before measurement, and the carbon content of the biofuel is determined for the bio-% calculation. Very volatile samples (for example, naphtha and gasoline) can be difficult to weigh. To ease the sample preparation, such samples can be cooled before weighing. Dilution is recommended for intense coloured biofuel samples. Quenching can be reduced using less sample. Sample to cocktail ratio can be reduced from 10 + 10 ml to 5 + 15 ml, or experimentally even lower. Samples are always prepared in the same type of vials as used in the quench curve calibration.

LSC measurement

Long measurement times (5-7 hours) are recommended for low activity biofuel samples, i.e. for fossil and mid to low bio-% samples. Shorter measurement time (for example 1-2 hours) can be used for scanning purposes and for high bio-% samples. The LSC biofuel protocol will indicate with red flags if some of the samples is out of the optimised range of the algorithm (for example, too low quench parameter QPE and/or counting efficiency) and requires more dilution. All results are calculated in the LSC biofuel protocol. The counts are collected from optimised luminescence free triple coincidence ROI. Background count rate and counting efficiency are predicted using the algorithm. The fuel mass and the carbon content are entered to the biofuel protocol before or after the measurement. Results are reported as mass bio-% or pMC depending on the carbon content used.

Results

Hidex biofuel method has been tested with various types of fuels and their blends. Here we share the results from in-house prepared HVO-diesel samples (Table 1.) and from biofuel samples received as unknowns (Tables 2.). The samples were prepared by mixing 10 ml of fuel with 10 ml of MaxiLight+ cocktail and analysed for 7h.  

Conclusion

Our aim was to develop a novel, easy-to-use, direct LSC measurement for determining the bio-% of biofuels. The Hidex biofuel method was tested with different types of fuels whose bio percentage varied between 1% and 100%. The method is based on an algorithm which is derived from background and efficiency quench curves with chemical and colour quenchers. The algorithm uses TDCR and external standard quench parameters for finding the background and counting efficiency of unknown samples. A fuel-specific background sample is not required. The Hidex biofuel method expands the applicability of direct LSC. The results presented here show that it is suitable for a wide range of fuel types and their blends. as well as for coloured fuels. The bio percentages determined with the Hidex biofuel method were very well in-line with the corresponding AMS results.

Summary

• Direct in-house measurement of bio-%
• No need for a fuel specific background sample
• Minimal sample preparation
• Suitable for wide variety of fuel types and their blends
  with colour
• No luminescence interference
• Detection limit down to 1 % biogenic content

References

1. ASTM D6866-22. Standard Test Methods for Determining the Biobased Content of Solid. Liquid. and Gaseous Samples Using Radiocarbon Analysis.
DOI: 10.1520/D6866-22
2. SFS-EN 16640:2017. Bio-based products. Bio-based carbon content. Determination of the bio-based carbon content using the radiocarbon method.
3. DIN 51637:2014-02. Liquid petroleum products – Determination of the bio-based hydrocarbon content in diesel fuels and middle distillates using liquid scintillation method. https://dx.doi.org/10.31030/2071672
4. ASTM D8473-22. Standard Test Method for Determining the Biobased content of Liquid Hydrocarbon Fuels Using Liquid Scintillation Counting with Spiked Carbon-14. DOI: 10.1520/D8473-22
5. Matthew Hurt. Josephine Martinez. Ajit Pradhan. Michelle Young. and Michael E. Moir. Liquid Scintillation Counting Method for the Refinery Laboratory-Based Measurements of Fuels to Support Refinery Bio-Feed Co-Processing. Energy Fuels. 35. 2. 1503–1510 (2021). https://doi.org/10.1021/acs.energyfuels.0c03445
6. Aronkytö et al. (2021) Method for determining a background count rate in liquid scintillation counting (U.S. Patent No. 11520058).
7. Aronkytö et al. (2022) Method for determining a background count rate in liquid scintillation counting (European Patent No. 4071520).