Biogenic Carbon Fraction of Biogas and Natural Gas Fuel Mixtures Determined with¹⁴C

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 14 C reference values for 100% biogenic carbon and on the 13 C-based isotope fractionation correction of the measured 14 C values. The separately (AMS) measured CO 2 and CH 4 fractions of 8 different biogas samples showed 14 C values between 102‰ and 116% (pMC). The δ 13 C values of these samples varied between –6‰ and +31‰ for the CO 2 fraction and between –28‰ and –62‰ for the CH 4 fraction. The uncertainty in calculated biogenic carbon fractions due to uncertainty in the 14 C 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 δ 13 C value of the mixed biofossil sample instead of the biogenic δ 13 C 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.