New research supports the need for streamlining best practice in anaerobic digestion

This study suggests that, in France, the potential of anaerobic digestion (AD) to mitigate greenhouse gas (GHG) emissions depends on two factors: 1) it must be accompanied by post-fermentation gastight digestate storage; and 2) the efficiency of soil carbon storage at baseline (lower baseline soil carbon storage leads to greater GHG mitigation). The researchers also stress that direct injection of biomethane into the gas grid, rather than using biogas in a cogeneration unit for electricity and heat, would offer greater GHG emissions savings benefits in France. The average energy efficiency of converting biogas to electricity was set at 39%. GHG emission intensity of electricity production for France, given the large portion of nuclear power in the electricity mix, is fairly small (58 CO₂ eq/kWh) compared with the EU27 average of 265 CO₂eq/kWh, and even when compared with the target of 118 CO₂eq/kWh for 2030.  Countries where electricity production is associated with a higher greenhouse gas emission intensity would have a larger substitution effect with biogas-based electricity.

Renewable energy production is central to reducing dependency on fossil fuels and lowering GHG emissions. AD uses a variety of organic products such as food waste, livestock manure, food and feed crops and crop residues to produce biogas, that could be further upgraded to biomethane. AD has been promoted as a promising option to contribute to multiple sectors in achieving GHG emission reduction¹. It may also offer carbon sequestration when the digestate is spread on soils, and fertilisation that avoids emissions from synthetic fertiliser production and transport but only if there is limited use of food and feed crops as feedstock. In 2019, Europe had 19 000 AD plants cogenerating heat and power, producing 167 terawatt hours (TWh) of energy.

Many studies have looked to quantify the net emissions benefits of AD plants, mostly through life cycle analysis (LCA). However, researchers behind a new assessment argue that most of these studies inaccurately portray AD-plant net emissions, due to incomplete consideration of GHG emission sources – for example not evaluating their entire life cycle. In particular, they say the baselines to which AD emissions are compared are often poorly defined and assessed.

When not used in AD, organic products, such as crop residues, may also offer carbon benefits, for example soil carbon storage if directly buried. It is crucial to precisely compare the emissions from the different end uses of biomass when calculating the GHG emissions associated with AD.

Researchers therefore proposed a thorough assessment of GHG emissions from 30 cogeneration (heat and power) AD plants in France. Data were collected on substrate supply, processes, biogas and digestate disposal, and the plants’ performance in 2017–2018, prior to the revision of Renewable Energy Directive (RED) 2018/2001. They compared (by modelling) AD plant emissions to the net emissions that would be expected from the likely alternative use of the biomass – usually directly applied to agricultural soils. Rather than following an LCA protocol, they developed a specific GHG calculation tool², which modelled carbon and nitrogen fluxes in both scenarios.

In the sample of 30 plants, AD relied mostly on manure. The same amounts of biomass were considered for the AD and alternative scenario, so emissions from production of biomass were not considered. In AD, emissions and savings were estimated from substrate transport from field to plant, up to where biogas energy was substituted for fossil energy and the return of digestate to soil. In the alternative scenario, the majority of biomass that would be fed to digesters was considered to be returned to agricultural soils, and avoided emissions were calculated from soil carbon storage, substitution of fertilisers and soil management.

Findings showed that on average, in the French energy context and, without being compliant with the RED II provisions, the plants did not mitigate GHG emissions more than the baseline. On average, AD in fact produced 11% higher emissions than soil sequestration. In fact, soil sequestration led to 118 kg more avoided carbon emissions per year, per ton of carbon compared with AD, though this was not statistically significant. The higher emissions from the AD scenario resulted from emissions associated with transporting the biomass to the AD plants and then transporting the digestates to the agricultural fields for spreading and because AD specific emissions were higher than those related to composting the biomass. There was, however, great variability in the plants’ net emissions, and a third of the plants provided some mitigation.

The variation in emissions was due to differences in the efficiency of soil carbon storage in the alternative scenario, and plant management practices. For example, covering digestates and recovering residual biogas at storage facilities (as listed in the RED II methodology for calculating GHG emissions) drastically reduces methane (CH4) emissions and significantly improves the environmental performance of AD plants. Implementing optimal anaerobic digestion plant design and management, in line with the existing EU policy recommendations, is crucial for improving net emissions at AD plants, for example minimising biogas leakage. Under optimal conditions, biomethane/biogas plays an important role in decarbonisation.

Soil carbon storage is also variable, and AD performed relatively better in contexts where soil carbon storage was low (which was observed in one third of cases). Low soil carbon storage results when the biomass has low carbon density (such as feedstuffs) or is readily biodegraded. In these cases, AD may offer mitigation, and this was the chief factor involved in whether AD was superior, the researchers found. The researchers acknowledge that soil carbon storage is time limited, as it ultimately biodegrades releasing GHGs. This contrasts with avoiding emissions from fossil fuels (through AD) which offers a longer-term solution.

Footnotes:

1. At the EU level AD is promoted through the following: The Waste to Energy communication (2017) [available from: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A52017DC0034, accessed 22/5/2023]; The EU Methane Strategy (2020) [available from: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:52020DC0663, accessed 22/5/23]; The REPowerEU plan [available from: https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=SWD%3A2022%3A230%3AFIN, accessed 22/5/23] and the Best Available Techniques (BAT) reference document for intensive rearing of poultry and pigs [available from: https://op.europa.eu/en/publication-detail/-/publication/f673b352-62c0-11e7-b2f2-01aa75ed71a1/language-en, accessed 22/5/23]
2. The GHG emissions tool is based on a broad literature review and was used to reconstruct carbon and nitrogen fluxes throughout the two scenarios (AD and soil amendment) explored. The tool simulates carbon and nitrogen transformation in the digester and soils using anaerobic digestion. This is combined with a previously used soil carbon storage model.

Source:

Malet, N., Pellerin, S., Girault, R. and Nesme, T. (2023) Does anaerobic digestion really help to reduce greenhouse gas emissions? A nuanced case study based on 30 cogeneration plants in France. Journal of Cleaner Production, 384: 135578.

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“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.

Notes on content:

The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.