Plant Design & Project Phases

A. Design

There are several ways of designing the AD plant and the detailed design needs to be adapted around the existing facilities. Nethy Energy can procure the design of the AD plant in a cost effective manner.
At a minimum there will typically be:

  • Nethy Energy
    Nethy Energy

    A. Pre Storage for the feedstock.
    Tanks for the slurry/ manure
    Clamps for the harvested purpose grown crops.

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    Nethy Energy

    B. Feeder
    A method for putting the feedstock into the digester. This will depend on the characteristics of the feedstock and may include a macerator for further breaking down the feedstock.

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    Nethy Energy

    C. Digester/s
    The main part of the plant and where the anaerobic digestion process occurs. The digesters are heated, by cantilevered steel pipes, and the digestate will be stirred as well by propeller type stirrers which are designed to (a) ensure homogenous feedstock and (b) easy maintenance.

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    Nethy Energy

    D. Biogas storage
    The biogas is stored in a double skinned tensile dome above the digesters. The skins are separated by air which means that they are weather proof. The biogas is carefully monitored so that its supply to the CHP can be manipulated to maximise output.

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    E. Pump house
    The material has to be pumped between individual areas.

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    F. Digestate Storage
    Covered or uncovered storage facility for the digestate after anaerobic digestion but prior to disposal/spreading. There will probably be a type of digestate separator in order to separate the liquid and fibre digestate.

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    G. Office
    Containing the electrical control boxes/ computer facilities for monitoring.

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    H. CHP House
    The CHP virtually runs almost constantly. The CHP is normally adjacent to the rest of the AD plant but there may be reasons for it to be remote especially if seeking planning permission for ancillary development.

B.  Approvals:

1. Planning

Planning policy is in favour of sustainable development generally and in particular of renewable energy generation so in theory at least there should be a willingness to grant consent. That said Local Authorities are often unfamiliar with AD plant applications and planning is always a hurdle that should not be underestimated so it is wise to start discussions with the Local Authority at an early stage. Obviously there are some methods of improving planning prospects: keeping away from neighbours.

As from April 2012 subject to conditions some smaller English agricultural and forestry AD wholly "on site" plants have been granted some permitted development rights. Depending on existing facilities these could mean that the AD plant can be constructed immediately.

Of particular interest to Nethy Energy is the likelihood that an AD plant will in effect become enabling development perhaps meaning that a planning consent for a small business park or some houses will be possible. The biogas can be transported some distance to the CHP which can therefore be some distance from the rest of the AD plant.

2. Other Permits

Environmental permits can be confusing. However, almost all smaller AD plants where the feedstock and the digestate are confined to the farm will be exempt.

There may be other restrictions for example spreading of the liquid digestate in Nitrogen Vulnerable Zones.

The UK government does not want large areas of the countryside to switch to purpose grown crops. In the future there will be a voluntary code with the aim of stopping this.

Farm view
Farm view


3. Electric

Obviously a large amount of electricity will be exported into the grid and the local DNO will have to be contacted to see what upgrading works may be required. This might include a new transformer or substation on site and upgrading the line.

4. Gas

The Gas Grid covers 80% of the UK population but does not extend to the majority of rural areas. Theoretically there are no reasons why Biomethane cannot be inserted into the existing gas grid.

These include;

  • cost of insertion,
  • ownership of insertion equipment,
  • an agreed specification for the cleaned biomethane particularly the level of oxygen allowed
  • pressure parameters of the inserted Biomethane.


C. AD Plant Construction and Development Management

Depending on the client's budget Nethy Energy will facilitate the construction in the quickest most cost effective manner.

D. Commissioning

It takes on average at least 3 months for the new AD plant to achieve peak digestion and it's important to minimise the commissioning period.

E. Service and Asset Management

The plant should be considered almost as a living organism rather than a machine. In common with all farm livestock it cannot be left unattended for extended periods. The AD plant, particularly the CHP, require regular ongoing maintenance.

F: Process Control

There are a number of process parameters within the digester itself that must be maintained so that the methane producing bacteria are efficient. This involves weekly sampling of the active digestate itself and regular daily sampling of the biogas. The AD plant can be monitored remotely with various alarms and controls if it does not function at or close to peak efficiency.