Anaerobic Digestion Plants

It is difficult to generalize but the illustration below hopefully demonstrates the likely sizes of your AD plants.

Size of Plant

Nethy Energy believes that it is at present sensible to consider four distinct sizes of AD plant.

  • Small CHP: 
    where the main/only ingredient is slurry/ manure. These AD plants are less cost effective from a capital viewpoint but because the feedstock is virtually free the overall return is good. It is likely that there will be very little excess heat after the digester has been heated so the revenue/return will be based on electricity and digestate/fertiliser, especially if negating the need to buy either from third parties.
  • Medium CHP: 
    These are suited to larger mixed farms probably with an element of rotation. The purpose grown crop may be 50% of the input but is probably responsible for 80% of the biogas. There will be some excess heat and it is important to try to use this in order to claim the RHi (presently the RHi is capped at 200kwth but this is expected to be increased in 2014).
  • Large CHP: 
    these will be suited to either the larger estates or a local consortium/club of farmers. From a capital expenditure perspective there will be substantial economies of scale but it may be difficult to use all the heat (presently the RHi is capped at 200kwth but this is expected to be increased in 2014).
  • Biomethane: 
    If the amount of feedstock would have commanded a CHP rating in excess of 2Mwh it is likely that biomethane will preferable financially. However, this requires extra capital outlay but means virtually none of the energy from the biogas is wasted and so a higher total subsidy claimed. Typically these AD plants are combined with a small CHP to provide heat and power for the digester. It will be possible to fund these plants through equity providers.
Although rare in the UK the use of biomethane as a vehicle fuel is widespread in many countries. Nethy Energy believes that in the UK, probably for Heavy Goods Vehicles particularly, this will be an effective use of biomethane.


The financing for these AD plants is currently difficult and is limited to; the owner's cash resources, raising additional debt, possibly partially through asset finance or in the case of the larger plants specialist equity providers. Nethy Energy can assist in this and is seeking opportunities for financial institutions.