Anaerobic digestion plant

Renewable Energy

Anaerobic Digestion

The AD plant is owned and operated by Egmere Energy Ltd The AD plant requires approximately 35,000 tonnes of plant material per annum, which is primarily maize, hybrid rye, wildflowers and grass. The feedstock is typically grown at Holkham or by neighbouring farmers who can use maize as a viable break crop to extend crop rotations.

The process is a natural one which uses harmless natural bacteria to digest biomass in the absence of oxygen and convert soluble organic compounds present in the biomass into a stream of biogas and a sludgy fibrous material. The process is performed in sealed tanks so as to capture the biogas and the fibrous material. The AD plant itself is often referred to as a concrete cow. It requires continuous feeding and produces biogas (methane and carbon dioxide) and organic fertiliser. Crop is transferred from the silage clamps to a feed-hopper once a day and is then automatically fed into the fermenter.

The fermenter contains a series of vertical stirrers which rotate slowly in order to mix the contents. It is within this stage of the process where most of the biogas is produced. Digested material is then pumped to the post-digester, via a screw press. The purpose of the screw press is to separate the solids from the liquids, both of which are valuable organic fertilisers, known as digestate. The solid digestate is collected and stored on site prior to its distribution to the local fields, while the liquid digestate is stored within the post-digester. Within the post-digester the final stages of the digestion process take place, releasing the remainder of the biogas. Unlike the fermenter, the post-digester has a flexible domed roof where biogas from the process is stored. Finally there is a third tank, the storage tank, which as the name suggests is used for storage of both liquid digestate and biogas. The biogas is then upgraded into biomethane suitable for injection into the national gas grid, to be used by consumers industrial and domestic nationwide.

The plant now injects biomethane into the national gas grid and is equivalent to a 2MW electrical plant. This can provide enough gas to heat 2,500 homes during the winter and up to 40,000 homes during the summer. Approximately 25.000 tonnes of high quality organic fertiliser is produced to spread on local land.

Works Timetable

  • Planning permission granted - February 2013
  • Construction commenced – July 2013
  • First maize harvest – October 2013
  • Filling tanks with digestate to commence process – June 2014
  • Feeding of silage – July 2014
  • Start generating gas for the national grid – October 2014