Supplying the UK's largest AD plant with a year round supply of quality feedstock

What is Anaerobic Digestion?

Anaerobic Digestion (AD) is a natural process whereby organic material, is broken down by micro-organisms in the absence of air. The origins of AD can be traced back hundreds of years but recent advances in technology have seen a rapid increase in the number of commercial AD plants, not only in Europe, but the UK.  AD plants can utilise a range of feedstocks including: food waste, manures and energy crops. By far the most efficient feedstock in terms of energy conversion is energy crops. Put simply energy produced is a direct result of energy put in. 

Working in partnership with the UK's largest crop fed AD plant we supply a year round supply of high quality feedstock, allowing the plant to run more productively and efficiently. 

Organic Material

The AD process can utilise a range of feedstocks in order to produce energy. By far the most efficient in terms of energy conversion is that of energy crops, put simply resulting energy is a direct result of energy put in. Demand for energy crops has lead to the development of varieties with high levels of ME (metabolisable energy), which in turn forms a direct correlation with energy released. 

Crops used in AD include: maize, rye, grass silage, wholecrop wheat & barley and sugar beet. To give an indication as to the significance of maize one hectare of maize will typically produce 55,000 kWh, or enough to run the average four bedroom family home for 2,155 days. A mix of crops used is integral to the success of an AD plant, with slow fermenting crops such as rye balancing fast fermenting crops like sugar beet. 


Once crops have been harvested they must be ensiled immediately to preserve their quality and maximise their gas yield potential. Storage of harvested crops can be done in two ways. Firstly crops can be "clamped" in purpose built facilities whereby the harvested crop is compacted until until no air exists within it. Storage of crops must be anaerobic, without the presence of oxygen therefore preserving the crop. 

The second from of storage involves utilising an "Ag-Bag" machine whereby harvested crop compacted into a 12ft diameter sealed tunnel, therefore allowing crops to be stored in locations where there is not a clamp. Provided crops are stored correctly it is possible for crops to remain in storage for a long period of time until they are required. 

Anaerobic Digestion

The process of anaerobic digestion itself can be described as the breakdown of organic material by micro organisms in the absence of oxygen. AD produces three main products: biogas, heat and digestate. Biogas is mainly formed of methane, carbon dioxide, nitrogen, ammonia and hydrogen. Gas yield and quality is determined by both the quality and quantity as well as combination of feedstock used. For example grass silage produces a cleaner source of methane than other feedstocks, however gas yields are not comparable to those of crops such as beet and maize. 

Using a continous flow of feedstock the digester is fed 24/7, this accounts for approximately 200 tons per day. Prior to entering the digester feedstock is mixed in the hydroliser enabling quicker gas release and allowing for greater automation as feedstock can be pumped in an out of the digester. 



Holding & Storage

It is possible for biogas to replace natural gas in most applications, however first it must go through a series of processes to make it suitable for use. The first stage is scrubbing whereby CO2, hydrogen sulphide and trace gases are removed. The removal of these elements and gases allow biogas to be used in usual applications such as heating and cooking. The second stage of processes is dehumidification, in its raw form biogas contains around 3-7% water vapour. In order to remove sufficient levels of water vapour from the biogas it is chilled to 5oc in order to remove enough water vapour. The third stage entails compressing the gas making it suitable for storage in a number of vessels including: cylinders and tanks as well as use in vehicles and directly pumped into the grid. 

Digestate is held in a lagoon where it is constantly agitated to improve homogenisation. Digestate can be used immediately or stored, storing in the short term does not adversely affect the composition or effectiveness of digestate. 

End Products

There are three end products of the AD process, these are: biogas, heat and digestate. Once processed biogas has propane added and is pumped directly into the gas grid. This enables biogas to be used for any purpose such as heating or gas for cooking. Due to the effects of millions of bacteria a large amount of heat is produced during the AD process. A proportion of this heat is retained and used to maintain a constant temperature within the digester itself. By maintaining a constant heat gas yields and quality is more predictable than leaving it to natural causes. 

The third product of the process is called digestate, digestate refers to the remnants of the digestion process usually in a liquid form. Digestate can be used as an effective fertiliser as it encompasses a wide range of nutrients and trace elements. Studies have shown that digestate applications can reduce artificial fertiliser applications by up to 75%. Digestate can be applied in many ways from surface applications through to injecting digestate, allowing increased nutrient uptake by crops. 

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