AD consists of a series of processes in which micro-organisms break down biodegradable material in the absence of oxygen and has been widely applied for the treatment of wastes from manure to industrial and municipal wastewaters for over 100 years.
A key benefit of AD is a volume and mass reduction of the input material through the biological conversion of organic materials into biogas. This biogas produced consists predominantly of methane (CH4) and carbon dioxide (CO2) and is suitable for energy production, helping to replace energy generated from fossil fuels. The process also produces stabilised compost that is nutrient rich and can be suitable for use as a fertilizer on agricultural land.
The two basic forms of anaerobic digestion are mesophilic and thermophilic.
Mesophilic: In the mesophilic process the digester is heated to 25-35 degrees C and the feedstock residence time is typically 15-30 days. Mesophilic digestion tends to be more robust and tolerant than the thermophilic process, but gas production is less and it requires larger digestion tanks.
Thermophilic: In the thermophilic process the digester is heated to 50-60 degrees C and the residence time is typically 12-14 days. Thermophilic digestion systems deliver higher methane production, faster throughput, and better pathogen and virus 'kill' than the mesophilic process, though they require more expensive technology, greater energy input and a higher degree of operation and monitoring.
Most agricultural biogas plants are operated at mesophilic temperatures whilst large scale Centralized Anaerobic Digester (CAD) systems often use thermophilic temperatures.
At the back end of the biogas production, various processes and techniques are used to clean up the biogas e.g. pressure swing ad/bsorption, membrane separation, cryogenic separation and water ad/bsorption. These processes will help to raise the quality of the gas to grid injection standards.
For gas to be injected into the UK gas distribution network, there are certain quality specifications that it has to meet. These are outlined in the Gas safety Management regulations (GS(M)R) which specifies the calorific value (CV) range for grid injected gas. However, since biogas naturally does not have as high a CV as required, it becomes necessary for the biogas to be injected with a higher CV gas such as propane to enable it meet the specified CV levels. However, the need for propane enrichment is more of a billing issue than a gas quality issue.
The gas quality is then monitored downstream of the propane injection point and the signal fed back to the supervisory system to control the calorific value of the biomethane. The fiscal metering should be prior to the odorant addition.
Prior to the injection of gas into the gas distribution network, the addition of odorant (80 per cent tertiarybutyl mercaptan, 20 per cent dimethyl sulphide) is required to impart the characteristic smell to the gas as required by GS(M)R. Routine monitoring carried out by rhinologists would confirm that acceptable odour intensity was being maintained.
The odourization stage is hugely important from a gas safety point of view as too much smell would lead to more wrong gas escape reports and too little odour would lead to unreported gas escapes which could potentially lead to gas explosions.