Biogenic Carbon Fraction of Biogas and Natural Gas Fuel Mixtures Determined with 14C

Abstract

This study investigates the accuracy of the radiocarbon-based calculation of the biogenic carbon fraction for different biogas and biofossil gas mixtures. The focus is on the uncertainty in the ¹⁴C reference values for 100% biogenic carbon and on the ¹³C-based isotope fractionation correction of the measured ¹⁴C values. The separately (AMS) measured CO₂ and CH₄ fractions of 8 different biogas samples showed ¹⁴C values between 102‰ and 116% (pMC). The δ¹³C values of these samples varied between –6‰ and +31‰ for the CO₂ fraction and between –28‰ and –62‰ for the CH₄ fraction. The uncertainty in calculated biogenic carbon fractions due to uncertainty in the ¹⁴C reference values depends on the available information about the origin of the used biogenic materials. It varies between ±0.5% and ±3.5% (absolute) depending on the type of biogas. A method is proposed to minimize this uncertainty for different groups of biogases. The calculated biogenic carbon fraction deviates up to ±2.5% for biofossil gas mixtures, if the applied isotope fractionation correction is based on the δ¹³C value of the mixed biofossil sample instead of the biogenic δ¹³C value. Combination of both error sources shows that the uncertainty in the calculated biogenic carbon fraction varies between ±0.7% and ±4.5%, depending on the type of biogas in the sample.

DOI: 10.2458/56.16514