Anaerobic Digestion

Anaerobic Digestion

What is Anaerobic Digestion (AD)?

Anaerobic Digestion is a process by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for converting energy crops and waste streams into biogas. Biogas is a mixture of approximately 55% methane and 45% carbon dioxide with traces of hydrogen sulphide and ammonia.

The raw biogas is used as a fuel source generating electricity through a combined heat and power unit (CHP), further processing of the biogas allows a 97% pure methane stream for injection into the gas network.

This polishing process also produces a liquified CO2 at a 99% purity.

At Vale Green Energy we utilise the biogas for the following:

  • Heat production (process heating, pasteurisation of liquid digestate, and exportable heat to a neighbouring poultry producer).
  • Electricity production (process power, to reduce reliance on grid electricity with the ability to export surplus generated power).
  • Transport fuel in vehicles designed to run on compressed gas.
  • Injection into the mains gas grid for domestic or industrial use.

Springhill AD Plant

Our Springhill anaerobic digester plant was commissioned in August 2013 and was the first gas to grid facility within the Wales and West network.

The plant size is 1.5mw consisting of 1x500kw CHP engine and Pentair gas to grid technology, exporting up to 7800m³/day of methane into the national grid. This is Pentair’s first biogas upgrading and CO2 capture plant in the UK. Our CHP engine is based on the site, which supplies power and heat for the daily operations of the plant.

The plant has pair of digester tanks with the combined capacity of 4200m³. Each digester is designed to produce 300m³ of biogas/hr with a retention time of 25-30 days a secondary digester extends our retention time to 50 days, this also acts a gas storage bag.

Our eco bag has a capacity of 5000m³ and is used to store separated liquid which is used as a fertiliser on our fields.

Rotherdale AD Plant

Our Rotherdale anaerobic digester plant was commissioned in May 2015 and was designed and built in house by our Vale Green Energy team. The plant size is 4.5mw consisting of 2 x 500kw CHP engines and Pentair gas to grid technology, exporting up to 22000m³/day of methane into the national grid. We currently capture 30,000kg of biogenic CO2 daily.

One of our CHP engines is based on the site, which supplies power and heat for the daily operations of the plant. The second CHP engine is a satellite unit installed on a neighbouring farm suppling heat & power for their chicken sheds. Any extra power that is generated is exported to the national grid.

The plant has completed with Phase 2 in 2017, by installing the additional 2 digesters and also commissioning the Biogas upgrade to full capacity of 900m³/hr. Currently each digester is designed to produce 600m³ of biogas/hr with a residence time of 25 – 30days. We have a secondary digester which extends our retention time to 50 days and produces Methane rich Biogas. The secondary Digester also doubles as additional gas storage of 8000m³.

Our covered lagoon is used to store separated liquid prior to spread on our fields.

Digestate

Digestate is the spent feedstock at end the digestion process. The whole digestate is split into a liquid and solid. The fibrous solid material can be used as a soil conditioner with the liquid being utilised a fertiliser. The addition of heat to the liquid fraction produces a pasteurised product suitable for organic production.

Biogas

Biogas from the Digesters is produced from rotational crops consisting mainly of grass, maize, sugar beet and wheat. We feed these tanks collectively up to 280 tonnes per day. A high percentage of our feedstock is produced in-house as we currently grow on over 4500 acres of land. Alongside this we have a close-knit network of farmers in the area that also grow crops for our plants, ensuring that we have a constantly supply of feed all year round. We always ensure high standards and the right quality of crops are selected.

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