Lessons learned from a UAS survey of methane emissions from multiple biogas plants in France

An on-going campaign monitors the greenhouse gases emissions of biogas plants in the Grand Est region, in France, using airborne in situ CO₂ and CH₄ concentrations and wind measurements from Uncrewed Aerial System, associated with a mass balance method. During 16 days in 2024, we quantified the instantaneous emissions of 19 agricultural biogas plants, with installed methane productions ranging from 128 to 312 Nm³.h^-1_,producing biogas injected into the network mainly from manure, energy crops and agricultural wastes.

Observations obtained to date were used to quantify emissions either representative of the globality of a biogas plant or of specific targeted sources inside a site (inputs, effluents, digesters or biogas purification). Global plant methane emissions among all sites range from 1.5 to 26 kg.h^-1, with average emissions of 10 kg.h^-1_. Repeated measurements of emissions on the same site at different dates depict a significant temporal variability, however overwhelmed by the variability of emissions among all sites. We estimated instantaneous methane losses ranging from 1.7 to 10 %, comparing monitored emissions with the installed productions. Emissions of targeted sources among sites suggest that inputs and effluents might be the predominant methane sources on the sites, while biogenic CO₂ emissions might be mostly attributed to the biogas purification process.

This campaign highlighted several limits intrinsically linked with the mass balance method. One of them is the sensitivity to contamination by parasite sources, which has to be anticipated during the field campaign preparation. Another difficulty is the risk of measuring truncated plumes, as the mass balance method requires the monitoring of an entire plume cross-section to provide quantifications representative of the complete source emissions. These limitations could be overturned in the future by alternative quantification methods, such as inversion methods based on Large Eddy Simulation of the atmospheric transport, considering the highly variable nature of the turbulent plume. These new developments, associated with evolutions of the monitoring protocol, may improve the reliability and precision of the results.