Growing biomethane on peat emits three times more CO₂ than using natural gas, study finds

The cultivation of crops such as corn, which can produce biomethane as a cleaner alternative to fossil fuels and help achieve carbon neutrality, is expanding rapidly. But according to a study by the UK Centre for Ecology and Hydrology (UKCEH), some of this increased cultivation, on drained peatlands, emits three times more carbon dioxide than it avoids by not using natural gas.

The researchers also estimate that the area of ​​UK peatland used to grow maize, which is then processed in an anaerobic digester to produce biomethane, has tripled since 2015. But they say emissions from draining these carbon-rich wetlands for agricultural use have been largely overlooked.

The analysis focuses on maize and the UK, but the study team stresses that any crops grown on deeply drained peat will result in large amounts of greenhouse gas emissions. Its important findings highlight the need to consider soil carbon losses from all bioenergy crops grown on drained peat, everywhere, to enable better green energy decision-making.

“Biomethane is an important renewable energy source, but it seems unwise to use drained peatlands primarily to produce bioenergy in areas where this leads to higher CO emissions.₂ “The CO2 emissions are lower than those of the fossil fuel it replaces,” says Professor Chris Evans of UKCEH, who led the research, published in Nature Climate change.

Depleting our carbon reserves

Although burning any gas to produce energy emits greenhouse gases, the principle behind producing biomethane is that the carbon released during combustion has recently been removed from the air via photosynthesis, thus adding no additional CO.₂ in the atmosphere.

However, the carbon emissions from drained peatlands are significantly higher than those avoided by not using natural gas. That’s because draining these wetlands to grow crops or trees releases carbon that has been trapped in their soils for hundreds of years. This released carbon is exposed to atmospheric oxygen, forming CO₂ and resulting in significant additional amounts of greenhouse gases.

While each cubic meter of natural gas burned emits the equivalent of 2 kg of CO₂UKCEH field flux measurements show that soil carbon lost when growing maize for biogas production on drained peatlands results in emissions of up to 6kg per cubic metre of biomethane produced.

This figure does not include additional greenhouse gas emissions from applying fertilizer to corn fields, harvesting and transporting crops, or producing biomethane.

Strong increase in production

The study estimates that the area of ​​drained peat soils in the UK for all maize crops increased from around 6,000 ha in 2015 to more than 11,000 ha in 2021, while the proportion of the crop used for bioenergy, as opposed to food, increased from 20% to 34%, representing a three-fold increase in total.

The researchers stress that their findings do not mean that all forms of bioenergy production on drained peat soils will lead to increased emissions. For example, growing biomass crops on managed agricultural peatlands with higher water levels – paludiculture – is a potentially promising method for mitigating climate change.

According to Professor Evans, it is less harmful to use maize as a “cover crop” (plants included in crop rotation systems to reduce the risk of weeds, pests and diseases) than to take land out of food production to produce biomethane. This would have commercial value and help offset some of the CO emissions.₂ emissions associated with food production on peat.

The researchers add that corn grown on mineral soil has less impact on the soil’s long-term carbon balance, and the overall process in these non-peat areas may therefore be more effective in reducing emissions.

Improve decision making

UK biomethane production has quadrupled since 2000, largely due to government financial support for biogas production to support the decarbonisation of the energy sector, including the Green Gas Support Scheme and, before that, the Renewable Heat Incentive Scheme.

According to Dr Rebecca Rowe of UKCEH, co-author of the study, “The transition to net zero will not be entirely smooth. As well as successes, there will be failures and unintended consequences.”

“Our role as scientists is to support government, land managers and industry by providing them with the best up-to-date knowledge about the impacts of their actions so they can make informed decisions about energy crop production and land use.

“It’s about working together to ensure a sustainable future.”