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Anaerobic Digestion

Towers-on-a-farm

The rise in the number of Anaerobic digestion plants in recent years has been a massive success story.

The industry has grown from nothing to 576 plants registered on the Anaerobic Digestion and Bioresources website (http://adbioresources.org/map). There are also many others that are not registered on the website.

The process takes organic matter and uses a biological digestion process to turn it into methane and carbon dioxide. The process takes place inside a sealed airtight container. It is called anaerobic because the digestion process must take place in an oxygen free environment. The favourite feedstocks are food waste and animal slurries.

The industry struggled initially trying to get the right mix to maximise the efficiency but over the years the process has been refined to a level where you can almost buy a plant straight off the shelf. You can have almost any size you like from a major sewage treatment plant facility to a small on farm plant. The giant golf balls you see at the sewage treatment works are usually the biomethane gas storage tanks.

Anaerobic-digestion-plant

Having generated the methane there are various options for the plants.

Electricity
Simple plants take the methane / carbon dioxide mixture and feed it into gas engines. These are similar to diesel engines, but they are converted to run on the biomethane fuel. The engine is coupled to an electrical generator which produces electricity. They are usually linked to the national grid and feed their renewable energy into the grid, reducing the demand for fossil fuels.

Thermal energy
The simplest usage is the burning of the gas to produce heat. It can be burnt for domestic or industrial heat generation either directly, or by heating water in boilers. Some of the heat energy is also required to maintain the digestion process, which performs more efficiently when it is warmed gently to a little above ambient temperature. The thermal energy can also be captured from the engine exhausts on the gas engines. If they generate electricity and capture the thermal energy then these are referred to as combined heat and power facilities (CHP).

Gas to grid
The most modern plants have the capability to export methane to the domestic gas grid. This technology is growing and has only been possible because of recent technological advances.  Before the methane can be injected into the National Gas Grid it has to meet a tight specification. The key element is the removal of the carbon dioxide from the biogas mixture, purifying the methane.
The second element is the calorific value. Town gas is high quality methane and has a slightly higher calorific value than the purified biogas, so propane or butane is added to increase the energy content to meet the grid specifications.
The major benefit of this process is that it displaces the imported fossil fuel gas from overseas and allows it to be transported to other locations for usage, like household heating systems.

Problems
The Environment Agency became concerned about the growth of poorly controlled Anaerobic Digestion (AD) plants where the highly odorous liquids used in the process had escaped into the environment causing bad odours and potentially polluting the surrounding land. There are still some concerns, but on the whole, the industry has got its act together and is developing a better reputation.

The government has set out a vision for AD, with an estimate of potential that could reach between 3-5 Terra Watt hours for heat and electricity by 2020. AD is now a significant contributor to the renewable energy resources for the UK.

Anaerobic-digestion-graph

Growth
The industry has grown beyond all expectation, but it has issued a few growth warnings recently. They have said that the availability of food waste to fuel the process is becoming less freely available. As more plants came on line the demand for waste increased and the market appears to be almost saturated with supply matching demand. Growth may continue but not at the rate it has in recent years.

The story so far
Overall the AD story is one of success and it is contributing significantly in our quest to replace fossil fuels with renewable energy. It is just one aspect of renewable energy supply, but it has become a significant and reliable contributor.
Renewable energy is now providing around 7.5% of our total energy needs and of this it provides around 20% of our electricity needs. There is a long way to go but we are well on our way to self-sufficiency and sustainability.

 

If we can help you with applying for an environmental permit, sustainable development, resource efficiency or The Circular Economy, please contact us:- www.wasteplansolutions.co.uk , email [email protected] , or call 01604 859961.

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