Biogas produced by the process of Anaerobic Digestion is considered a ‘dirty’ gas and requires cleaning up before it can be used for electricity generation. Electricity generator manufacturers are very specific about the standard of biogas quality required at their engine. Consideration must be given to the following areas:
Pressure (typically required at the engine 50-200mbar)
Anaerobic digesters generally operate at pressures between 3mbar and 30mbar therefore biogas pressure must be boosted to meet the engine requirements.
Biogas conditioning equipment can produce pressure drops of up to 15mbar in the system. To overcome pressure drops and avoid developing negative pressure in the pipeline, its worth considering installing a small biogas fan.
Temperature – (typically 20-30C at the engine) and Water content / Relative Humidity – (typically 80% relative humidity required at the engine)
As soon as biogas leaves the digester between 35C – 40C and at 100% relative humidity, cooling starts and water begins to condense into the pipeline. In order to achieve 80% relative humidity the biogas must first be cooled using a tube/shell heat exchanger and a cold water chiller. This removes a large amount of water from the biogas. Condensate pots are used to collect the water and remove it from the system.
The biogas must then be reheated. There are a number of options for reheating the biogas:
- pass the biogas through another tube/shell heat exchanger to enable heat extraction from the incoming hot gas. This method offers ‘free’ heat recovery but depends on sufficient biogas temperature of the incoming biogas being available at all times
- pass the biogas through another tube/shell heat exchanger which utilises hot water from the CHP generator or a hot water boiler.
- use an electrical immersion heater heat exchanger.
- use the temperature rise across the biogas booster used to feed the engine.
Hydrogen Sulphide – typically 500-2000ppm
If H2S is present in the biogas at the point of combustion it will convert into sulphuric acid which is highly corrosive. Therefore, engine suppliers usually specify maximum figures of around 50ppm to 250ppm to be present in the biogas.
Removing H2S from biogas, successfully, can be an expensive process and one that requires an understanding of exactly how much H2S there will be in your system before deciding on a solution. There are several options for removing H2S from biogas including air injection, ferric chloride dosing and our range of stand alone H2S Scrubbing equipment.
Siloxanes – typically 50mg/m3
Siloxanes are derived from contamintents such as lubricants and personal care products. This is a common occurance within the biogas produced by waste water treatment plant anaerobic digesters. Siloxanes harden during combustion and form deposits within the combustion chamber of a gas engine or boiler greatly shortening its life.
(See Biogas Siloxane Scrubber for more information)
No one system is the same, therefore, Biogas Products Ltd design and manufacture biogas conditioning solutions to meet your specific requirements. We can work with you to achieve the required biogas quality for your engine manufacturer.