An introduction to combined heat and power digesters and why you should use them

What is a Combined Heat and Power (CHP) digester?

A CHP unit is a reciprocating gas engine that is fueled by Biogas, created from the digestion of plant derived fuel sources. This drives an electric generator producing power, Biogas typically consists of 50-75% methane, with much of the remainder being carbon dioxide.

It is called “combined” because power is produced from the generator and heat is created during the gas combustion process which is also recovered and reused.

Why is it called a digester?

Anaerobic digesters use plant material (biomass) to produce energy. Bacteria brake down the material to produce biogas methane that is then used for heat and power. Digestate is also a byproduct which can be used as a fertiliser and soil conditioner for your energy crops.

The amount of energy produced by anaerobic digestion varies depending on the material that goes into it and the digester itself, however, around one ton of food waste can generate approximately 300kw of energy. According to the Renewable Energy Association, if all the UK's domestic food waste was processed by anaerobic digestion, it would generate enough electricity for 350,000 households.

The main advantages of anaerobic digesters are that they:

• The value of the power and heat created by a CHP digester is far greater than the cost of the fuel
• Biogas fuel, avoids the use of fossil fuels
• Reduce waste material sent to landfill
• Digestate can be used as fertiliser, again avoiding the use of fossil fuel
• Help prevent climate change, as the methane produced is used as a fuel, not left to rot in landfill, which would create far higher levels of methane
• Community schemes can potentially benefit many people, including farmers and small villages
• CHP is zero carbon rated so this system helps to deliver ZERO Carbon buildings
• Create a revenue stream by selling surplus electricity back to the national grid

Various types of fuel that can be used

Biomass can include anything that is plant-derived, including:

• manure
• compost
• food waste
• waste water
• municipal solid waste
• crop residue
• paper