Biofraction determination of mixed biogenic and fossil methane samples through stable isotope analysis of 13CH4 and CH3D

With the ongoing integration of biogas and other biofuels into existing energy infrastructure, there is a focus on the need to establish rigorous technical specifications for biogas and biomethane. Such regulatory framework requires the determination of the biogenic content of gases consisting of both biogenic and natural gas. One potential tool for making this determination is stable isotope analysis. The isotopic composition of natural gas and biogas varies significantly, with the isotopic ratios providing a valuable tool for source discrimination. Stable isotope methods have already found application in biogas production process monitoring, being used to characterise the dominant methanogenic pathways in biogas reactors and showing the potential to forecast reactor process failure.

We have developed a tunable diode laser spectrometer (TDLS) for simultaneous measurement of 13CH4 and CH3D in methane samples. We analysed biogenic methane samples originating from three different locations within Finland, and additionally prepared and analysed three mixtures of fossil and biogenic methane with different biofractions to assess the suitability of stable isotope analysis for biofraction determination. Using less than 2 ml of sample gas, the spectrometer was capable of determining the biogenic content to within 1.5%, provided the isotopic composition of the source materials was known. The performance of the developed methane isotope analyser for on-line and real-time measurement was tested, with the instrument demonstrating suitability for precise, long-term monitoring of biogas production processes.