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.

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. 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.

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. The important difference is that natural gas 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 'ingedients'.

Burning methane produces carbon dioxide and water. However, all the carbon contained in biogas (in the form of carbon dioxide and 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 the biomass sources are allowed to regenerate, biogas is a 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 therefore important that all biogas is burned and no leakage occurs during production and distribution.

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, because of the 'embedded emissions' in food (the energy to grow, process and deliver it), rather then not worrying about food waste as it can go 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.

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 around 4,000 farm-scale biogas plants, and in the USA, biogas farms have started to export purified biogas (biomethane) into the national gas grid.

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