Community Food Waste AD

With Greenfinch, and supported by Charles Banks at the University of Southampton, Michael Chesshire started developing the anaerobic digestion of food waste in 1996; first, with a feasibility project processing food waste from a cafe-bar; second, with a demonstration project, in 1999, digesting source-segregated food waste from 1200 households in South Shropshire; and third in 2004, with the design, construction and operation of the Defra demonstration digester in Ludlow, known as “Biocycle”.

Since March 2014, Lutra has been operating a small community food waste AD plant near Ludlow in Shropshire; the plant is known as “BMAD” – Barrett’s Mill Anaerobic Digester. Lutra has been collecting about one tonne per week of food waste from local pubs, restaurants and schools as part of a privately-funded feasibility study into the economics and logistics of small-scale food waste AD. The food waste has also been co-digested with other feedstocks – potatoes, poultry manure and grass cuttings.

BMAD is comprehensively monitored with daily records taken of many key parameters. Plant performance is verified using the principle of mass balance – in the 124-week period of operation when the only feedstocks were food waste and grass cuttings, the mass balance was accurate to within 1.5%. Methane production has been higher than expected, possibly due to the plug-flow design, and over the same 124-week period the methane production was 128m3 per tonne of food waste.

BMAD has been temporarily closed for redesign.

Power-to-Methane

The concept of P2M is based on the microbiology of anaerobic digestion, which is a multi-stage process with intermediate products. One of these stages is acetogenesis, by which volatile fatty acids are converted to acetic acid, hydrogen and carbon dioxide. The final stage, methanogenesis, converts acetic acid, hydrogen and carbon dioxide into methane. The microbes which convert hydrogen and carbon dioxide into methane are known as hydrogenotrophic methanogens. The principle of P2M is to use these same hydrogenotrophic methanogens to combine an external source of hydrogen with an internal or external source of carbon dioxide to produce methane. P2M has the potential to address the two major challenges referred to above: by using surplus or low-cost electricity to produce hydrogen through electrolysis; by using this hydrogen to produce methane; and by injecting the methane into the national grid which has a large storage capacity. This process therefore enables energy storage and electricity grid balancing to be achieved by using the gas grid as storage as well as by batteries.

Theory and Concept: The efficiency of the conversion of electricity to hydrogen is about 75% (excluding heat production) and the efficiency of the conversion of hydrogen to methane (excluding heat production is about 80%). The efficiency of the combination of electrolysis plus biomethanation is therefore about 60%, the balance being heat.

Formula for the electrolysis of water:

2H₂O + electricity → 2H₂ + O₂ + heat

Formula for biomethanation:

4H₂ + CO₂ = CH₄ + 2H₂O + heat

Combined formula for electrolysis plus biomethanation:

2H₂O + CO₂ + electricity = CH₄ + 2O₂ + heat

Ex-Situ Biomethanation: Most commercial research and development into P2M has been focussed on “ex-situ” biomethanation. This involves an anaerobic reactor in which hydrogenotrophic methanogens are cultivated and into which hydrogen and carbon dioxide are injected. Ex-situ biomethanation has the significant benefits that it can be switched on and off, and that the methane quality is not adversely affected by variable flow of input gases.

In-Situ Biomethanation:In-Situ biomethanation involves the injection of hydrogen into a stable working AD plant which is being fed in the normal way. The hydrogen combines with the carbon dioxide fraction of the biogas to form methane, with the potential to produce biogas with a concentration of 95% methane. The advantages of in-situ are that no special reactor is required and that the process replaces biogas upgrading. The disadvantages are that there are biological issues to be overcome and that the process needs a continuous rather than intermittent source of hydrogen if a consistent gas quality is to be achieved.

IB Catalyst H₂AD: The University of Southampton, in partnership with the University of York and the University of Sheffield, successfully bid for a BBSRC (Biotechnology and Biological Sciences Research Council) research grant under the “Industrial Biotechnology Catalyst Early Stage Translation” scheme to carry out research into the biomethanation of carbon dioxide in anaerobic digestion plants – “H₂AD”. The four-year project started in June 2015 and is due to be completed by the end of 2019: Southampton is researching reactor biology through the operation of eight 1-litre laboratory digesters; York is researching genome identification of the hydrogenotrophic microbes; and Sheffield is researching process modelling and optimisation. As well as the universities, there are four industrial partners H₂AD – United Utilities, Fera, ITM Power, and Lutra.

Lutra H₂AD Pilot: As a supplement to the BBSRC project, the University of Southampton successfully applied for grant funding to set up and operate a 1500-litre pilot plant, which has been set up and operated by Lutra in Shropshire. The feedstock is cow slurry collected by Lutra from a local dairy farm.

Biopropane

Michael Chesshire has been appointed as a consultant to C3 Biotechnologies, a partner in eForFuel (www.eforfuel.eu).
eForFuel is developing industrial biotechnology that uses electricity and microorganisms to convert CO₂ into hydrocarbon fuels, thus providing a sustainable replacement of fossil carbons without the need for sugar-based biofuels. One potential pathway is the electrochemical reduction of CO₂ to produce formic acid which is converted to propane or isobutene by E.coli strains.

Biochar