Biogas is produced when bacteria break down organic matter in an atmosphere with little or no oxygen.
This will occur anywhere that there is decomposition, including in the soil. It also occurs in slurry stores and, under controlled conditions, in an anaerobic digester.
Biogas comprises about 60% methane and 35% carbon dioxide, as well as some other gases including the noxious hydrogen sulphide. Methane and carbon dioxide are both greenhouse gases that are damaging to the environment. However, methane is a much more potent greenhouse gas than carbon dioxide, its impact on climate change being over 20 times greater over a 100 year period. The anaerobic digestion process captures these gases, which normally would be lost to the atmosphere as slurries and plants decompose.
Once captured, the biogas can be burned in a boiler to produce heat, or in a combined heat and power generator to produce both heat and electricity. The biogas can also be cleaned to use as a road fuel for converted vehicles or could be used for mains gas.
A single dairy cow can produce around 50 kg of slurry per day
- Each cow’s slurry will yield about 1 m3 of biogas per day, around 6 kg CO2 equivalent.
- 500 dairy cows could yield enough biogas to give savings of up to 1000 tonnes CO2 equivalent each year
- After digestion, the resulting digestate has:
- Same fertiliser value
- More plant-available nutrient
- Lower dry matter
- Less odour
- Fewer viable weed seeds
- Considerably fewer pathogens
- Consider all feedstock options potentially available from onfarm enterprises.
- Evaluate the cost effectiveness and sustainability requirements of feedstock options, especially growing crops for AD.
- Identify any off-farm waste streams such as foodprocessing and catering wastes.
- Aim to use all heat and power onsite to maximise viability through offsetting purchased energy.
- Keep up to date with changes in regulations and payment schemes.
Biogas plants are widespread across Europe and can use a variety of feedstocks. Manure or slurry from livestock is usually used and this reduces gaseous releases compared to conventional storage and field application. However, as the material has already been digested by the livestock the biogas potential is relatively low. To boost gas production, energy crops such as maize or silage have often been added to the mix.
Other organic wastes including food processing or catering waste can also be added. This not only boosts the gas output but could also generate a gate fee, which will provide additional income. However, this will increase the administrative and financial complexity of the site and can add to the capital cost of the plant.
Any feedstocks used must comply with government sustainability regulations. Requirements vary depending on whether the feedstocks are classed as wastes, residues or products. Slurry or manures are classed as wastes and are therefore deemed sustainable.
Potential income streams are available but maximising use of both the generated electricity and heat on farm to offset energy bills will aid the financial viability of sites.
Crops grown specifically for AD were discouraged by criteria and feedstock restrictions which apply to all new AD installations from May 2017, limiting payments (e.g. RHI) on biogas and biomethane not derived from waste and residue feedstocks to 50% of the total biogas yield on an annual basis. Utilising ‘wastes’ will also help to reduce green house gas emissions, keep land in food production and aid the circular economy.
Around a third of the heat produced from the biogas is used to maintain the digester temperature and a small amount of the electricity will be required to run the plant. However, the remaining heat and power are available for electricity for export for use on farm or generating heat for the farm or a district heating scheme.
Finding a use for the heat generated is often important for the project to be financially viable. Instead of a boiler or CHP, the biogas could be used for road vehicle fuel (after scrubbing) or mains gas injection (after upgrading; capital intensive, viable only on large scale plants).
Renewable Transport Fuel Obligation (RTFO) payments may be available if supplying fuel for the transport sector.
Why consider AD?
Stored slurry will emit both methane and ammonia gases, which can accelerate climate change.
By using AD, the methane is captured and can be used on-farm to produce electricity and heat.
The resulting digestate is nutrient-rich and less smelly than slurry, reducing odour problems.
Anaerobic Digestion for farm slurries can:
Reduce your fertiliser bills
Generate electricity and heat to offset your use or to create an income
Reduce Greenhouse Gas emissions
Potential biogas yield (m3/tonne)
|15 – 25||Pig slurry|
|15 – 25||Dairy slurry|
|30- 100||Pig slurry|
|160 – 200||Grass silage|
|200 – 220||Maize silage|
|580 – 1000||Crude glycerine|
|Up to 1000||Fats|
Points to note
Stability – the digester needs careful management to keep the bacterial population stable. The plant can be time-consuming and costly to get back on track after a crash.
Automation – plants that are fully automated are easier to manage. Automation needs a high gas output and low feedstock costs for best payback.
Waste Streams – high-yielding feed streams improve viability. Note that over-reliance on a non-guaranteed alternative feed stream could be commercially risky.
Maintenance – biogas can be corrosive to pumps and valves. Feedstock contaminants can build up in the digester. Regular maintenance is essential for efficient operation.
Access to Finance – capital requirements will depend on the scale and feedstocks. Farm scale plants can be in the order of £300k to £1.5M and beyond.
Scale – large-scale plants are likely to be more viable than small-scale ones. Collaborative ventures are an option and using more than one source of feedstock also spreads the risk.