What Is Anaerobic Digestion (AD) ?
AD is a special treatment applied to organic materials. It can be applied to a range of natural biodegradable materials but we at Ixora use mainly manure from cows, pigs or chickens. This is blended together with energy crops which are sustainably grown for the AD process to form a feedstock material. The feedstock is fed into a digester which is a large sealed tank without oxygen but containing naturally occurring micro-organisms and bacteria. The mixture is then heated up and gently stirred. The mixture stays in the digester for a number of days while the bacteria break down the feedstock and undergo a four stage process – hydrolysis, acidogenesis, acetogenesis and methanogenesis. During this process it emits a new gas – known as biogas. Biogas is a methane-rich gas, comprising around 60 per cent methane and 40 per cent carbon dioxide.
Initially the biogas from the process is transferred to a storage holder. Most often this tends to be a large dome. This dome tends to be what makes AD facilities most recognisable. From there the gas is transferred to either a Combined Heat and Power (CHP) engine or to a gas clean up facility. This depends on whether the site is generating either renewable gas or electricity.
- Renewable gas site – The gas is sent to a clean up system to extract the bio-methane. The bio-methane is carefully monitored for its purity and pressure and then can be exported into the national gas network. The gas is pumped directly into the network that supplies the local neighbourhood meaning that houses around the plant receive green energy gas.
- Renewable electricity site – The site will direct all of the Biogas to a single or series of CHP engines. These are rather like large car engines but in this instance generate electricity from the gas. The electricity from the engine is firstly used to power the site and then the rest is exported to the National Grid. During the generation process the CHP also emits heat from the engine which can be captured and also put to use in drying processes or for heating.
Once the gas has been extracted from the digestion process there remains a liquid material called digestate. This material is rich in nutrients and can be used as a direct replacement for artificial or manufactured fertilisers. The material is lower in odour than raw animal manure and is taken from the site and spread on fields to enhance future sustainable crop production.