What about the anaerobic digestion of biomass to make biogas?

CowSome types of biomass can be used to make biogas, using a process known as anaerobic digestion. Suitable materials include animal manure, kitchen waste, garden waste, other wastes from agriculture and slaughterhouses, and even human excreta.

Anaerobic digestion can be seen as “composting without air”. Under normal conditions, such as in a compost bin, aerobic (oxygen breathing) organisms break down biodegradable organic materials into simpler forms of matter, producing carbon dioxide (CO2) in the process. But in anaerobic digestion, the biomass is decomposed with the exclusion of air. In the absence of oxygen, certain microorganisms break down the biomass to produce methane (CH4), a combustible gas.

What is biogas?

In practice, biogas produced through anaerobic digestion is usually a mixture of methane (45-85%) and carbon dioxide (15-45%) with small amounts of other gases. It can be purified to achieve higher methane concentrations. Chemically, purified biogas is very similar to some types of natural gas - as this is also mostly methane - so can be used with the same appliances. Only a small proportion of the total mass of the feedstock is converted into biogas. The remainder is converted into a nutrient-rich slurry which can be used as fertilizer. The energy value of biogas depends on the methane concentration. Pure methane gas has an energy value of around 10 kilowatt-hours per cubic metre (kWh/m3); biogas with a methane concentration of 60% can provide around 6 kWh/m3.

Is biogas a low carbon energy source?

The important difference between biogas and natural gas is that the latter is a fossil fuel that causes global warming, whereas biogas can be a carbon-neutral energy source - how close to neutral will depend on the 'ingredients'.

Burning pure methane produces carbon dioxide and water. All the carbon contained in biogas (whether in the form of carbon dioxide or methane) has previously been absorbed from the atmosphere by the plants that produced the feedstock. Hence burning biogas will only release as much carbon dioxide into the air as the plants have taken out of it. As long as biomass sources are replanted at the same or higher levels, biogas can be a relatively carbon-neutral source of energy. However, methane itself is a potent greenhouse gas, many times more powerful than carbon dioxide in causing climate change, so it is very important that all biogas is burned and no leakage occurs during production and distribution.

Another issue is that digestion is a good way of dealing with waste food and by-products of agriculture, but if crops are specificially grown to put into a digester then this risks taking good arable land away from food production, or deforestation to provide the land for those crops. Keeping digestion as a way of dealing with waste materials is generally the best option.

Can I have a domestic digester?

At the moment this is not really feasible in the UK. A typical UK household will not produce enough suitable biomass waste, and in addition, our cooler climate means that only large digestion systems can maintain the required temperature. Around 35°C is ideal for biogas production. Even at 20°C, gas yields will be significantly lower than at the optimal temperature. A small unit needs heat input to keep the process going in cold weather, and keeping the heat input lower than the energy output (as gas) will then be vital.

Bear in mind that it is still best to minimise food waste, due to the 'embedded' carbon emissions in food (the energy to grow, process and deliver food). Reducing food waste in the first place is much better than not worrying about food waste because it all goes to a digester.

In some areas of the UK there are collection schemes for cooked food waste (which is unsuitable for composting), and the collected waste is anaerobically digested - often mixed with agricultural waste. Many more community projects to process waste into biogas is likely to be a better way to go than individual household units.

What type of system is feasible?

UK biogas plants typically have digester volumes of 500 to 5000 cubic metres, and require several thousand tonnes of biomass input per year. At this scale, a digester will often produce more biogas than a single farm requires for cooking or heating purposes. Ideally, the biogas is burnt in the generator of a combined heat and power (CHP) unit to produce both heat and electricity.

One tonne of cow manure produces around 36 cubic metres of biogas. Figures from a demonstration biogas plant in Ludlow, Shropshire, suggest that one tonne of UK household kitchen waste produces 140 cubic metres of biogas. In South Shropshire, a digester uses 5,000 tonnes of household food waste per year to run a 195 kilowatt (kW) CHP. A proportion of the electricity (15%) is needed to run the plant. The rest – enough to supply a few hundred households – is sold to the national grid. Of the heat, 40% is needed to heat the digester and the excess is currently wasted - but there are plans to install a local district heating scheme.

This highlights a problem with the use of biogas for CHP generation - the plants are not normally located in densely populated areas. This makes piping of heat for district heating less viable. In Germany, where government subsidies have lead to the creation of thousands of farm-scale biogas plants, these have started to export purified biogas (biomethane) into the national gas grid.


Anaerobic Digestion Portal - www.biogas-info.co.uk
Information about anaerobic digestion, including case studies, funding options , regulations, etc. Funded by the UK government and run by the National Non-food Crops Centre (NNFCC).

Anaerobic Digestion & Bioresources Association - http://adbioresources.org
Trade association representing businesses involved in the anaerobic digestion industry. Lobbies on their behalf to promote the technology. List of members on web site, along with details of events and news items.

National Farmers Union - www.nfuonline.com
Membership organisation for farmers and growers in England and Wales. Advice for farmers on the potential for energy crops, anaerobic digestion and other biofuels.

WRAP (Waste & Resources Action Programme) - www.wrap.org.uk
Charity that helps governments, businesses and communities to improve resource efficiency (design, use, re-use and recycling of products & materials). Areas of work include food waste & anaerobic digestion. Also administer funding, including the Rural Community Renewable Energy Fund and On-Farm AD Fund.

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