https://www.linkedin.com/posts/conrad-spindler_i-have-a-question-to-the-pcf-iso-14067-activity-7361739572969431040-M5Ju?utm_source=share&utm_medium=member_desktop&rcm=ACoAACfod8sBoAfMBAmhAN0NsiM2HjQSlLgciqQ
The difference between fossil methane and biogenic methane is referenced in the IPCC 2021 report (https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07.pdf) and is due to the ratio of the molar masses for CO2 (ca. 44 g/mol) and CH4 (ca. 16 g/mol), which is 2.75. So the difference is ca. 2.8 for complete oxidation. Or it is, as written in the IPCC 2021 report, a partial oxidation of 75% (0.75 * 2.75) plus an equivalent sink of CO2 from biogenic methane soil uptake and removal (0.7 kg CO2 eq.), which is again 2.8. Long story short, the difference between fossil methane and biogenic methane is 2.8. On top of this, also biogenic methane itself can be either 27 or 29.8, depending if biogenic carbon is accounted for in the greenhouse gas emissions or not.
The IPCC 2021 report mentions that "Methane from fossil fuel sources has therefore slightly higher emissions metric values than those from biogenic sources (high confidence). The CO2 can already be included in carbon emissions totals (Muñoz and Schmidt, 2016) so care needs to be taken when applying the fossil correction to avoid double counting.".
To avoid double counting, the biogenic methane factor is 27 in the 0/0 approach, where no biogenic carbon is explicitly tracked and accounted for, and the biogenic methane factor is 29.8 in the -1/+1 approach, where biogenic carbon is explicitly tracked and accounted. This is also the only difference between the "EF 3.1" LCIA method for PEF studies and the "EN15804 (based on EF 3.1)" LCIA method for EPDs. The -1/+1 approach to explicitly track biogenic carbon is also used in ISO 14067, therefore the characterization factor for biogenic methane has to be equal to the one for fossil methane which is 29.8.
