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 the14C reference values for 100% biogenic carbon and on the13C-based isotope fractionation correction of the measured14C values. The separately (AMS) measured CO2and CH4fractions of 8 different biogas samples showed14C values between 102‰ and 116% (pMC). The δ13C values of these samples varied between –6‰ and +31‰ for the CO2fraction and between –28‰ and –62‰ for the CH4fraction. The uncertainty in calculated biogenic carbon fractions due to uncertainty in the14C 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.