Generated from the breakdown of organic matter, biogas, and its upgraded form biomethane could help mitigate harmful carbon dioxide (CO2) emissions by providing a lower emission alternative to energy production.
Despite this, the ‘clean’ gases are not without their downsides and were found to be responsible for methane leaks at various points during production and distribution.
The study, which looked at existing biomethane and biogas methane emission data, showed that methane leaks throughout the supply chain have been underestimated, with most of the methane being emitted during anaerobic digestion (AD) – the process in which bacteria breaks down organic matter such as food waste, without oxygen.
By characterising the emissions profile of biomethane biogas supply chains, it was found that – although the biogas supply chain emits less methane than the oil and natural gas supply chain – the emission rate itself is higher and emissions could be more than two times higher than previously estimated.
Comparison of methane emissions along supply chain
Source: Methane emissions along biomethane and biogas supply chains are underestimated
It also reveals that 62% of the methane leaks were concentrated in a small number of facilities and pieces of equipment within the chain, dubbed ‘super-emitters’.
The scope of the study covered each part of the supply chain from biogas production and upgrading, transmission, storage, and distribution, and digestate storage.
Analysing data from over 50 previously published studies on methane measurements and site data from emission sources along the supply chain, researchers were able to create a model which they compared with off-site emissions reported from whole-site measurements in previously published studies.
It was discovered that the amount of CO2-equivalent methane per megajoule higher heating value released may account for 18.5 megatonnes of methane per year released, twice as high as the 9.1 megatonnes reported by the International Energy Agency (IEA) in 2021.
The reasons behind the leakiness of the biogas supply chain were found to be related to intermittent emission patterns, making them harder to track, underusing process equipment, and improper maintenance strategies.
Less-than-effective design also contributed to unnecessary emissions. It was discovered that the open storage of digestate has a major impact on emissions, potentially accounting for nearly 27% of global CO2 equivalent emissions.
To prevent this from occurring, researchers suggested that continual monitoring is required to detect any anomalies and that if biomethane is widely used in the future to achieve decarbonisation goals, biomethane supply chain emissions should be avoided.