Anaerobic Digestion (AD) is a biological process that uses bacteria to break down biodegradable material to produce biogas (mainly methane and carbon dioxide). The biogas can be used through a combined heat and power (CHP) engine to produce electricity and heat. The electricity can be exported into the national grid and the heat can be used in a local district heating system. Some AD plants also have the ability to inject the biogas directly into the national gas grid, as a replacement for natural gas.
AD is commonly used to convert waste food, energy crops and animal slurries into renewable biogas. AD consists of four stages, with the first three stages being undertaken by bacteria and the fourth stage undertaken by archaea. Each of the four stages must be completed in turn, to create biogas. If one stage is not completed, incomplete digestion will occur. The process is operated anaerobically (without oxygen) as the bacteria within the digester cannot tolerate oxygen.
Digester designs vary depending on what biodegradable materials are being broken down. The process can also be operated at different temperatures, depending on the requirements of the system and the waste stream being treated. Psychrophilic AD systems operate around 20°C and require the lowest energy input for heating; however the digestion process is the slowest and is typically used only in hot countries. Mesophilic AD systems operate around 35°C and usually produce higher biogas yields compared to the other operating temperatures. AD systems are typically operated at mesophilic temperatures. The highest temperatures occur for thermophilic AD systems which operate around 55°C. Although thermophilic systems require more energy to heat the digester, the biodegradable material can be broken down faster, increasing the capacity of the system.
As well as biogas, the AD process also produces a second product called ‘digestate’. Digestate is the liquid product remaining after the AD process and once pasteurised, it can be applied to agricultural land as a replacement for artificially manufactured fertilisers. Artificial fertilisers are energy intensive to manufacture and by using digestate as a substitute for artificial fertilisers, this energy demand can be reduced.
Society produces a wide range of different wastes, some of which are suitable as feedstocks for AD. It is important to use these wastes as efficiently as possible to generate electricity and to reduce the volume of wastes sent to landfill. Of the wastes not suitable for AD, pyrolysis and/or gasification may be suitable treatment processes for these wastes.
Researchers at EBRI are investigating linking different energy generation processes together to increase the net energy yield which can be obtained from original waste and residue streams. Initial research has demonstrated that by linking anaerobic digestion and intermediate pyrolysis together, wastes can be utilised more effectively.
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