Gas flaring is a common occurrence at many Anaerobic Digestion facilities. Being able to flare off gas is important for the process and also from a safety perspective. Failure to flare off gas correctly can lead to catastrophic consequences to the process and its hardware.
What is Gas Flaring?
Gas flaring is the process of burning off excess gas within a process. The gas is often not needed due to surplus or there may be a buildup of gas that could potentially become dangerous to the plant and people.
Why is Flaring Needed?
When the Anaerobic Digestion process has taken place and the biogas has been produced, it is then used in two ways. This is either through using the energy to power a CHP unit or through direct injection back into the grid. Often at times there is more energy produced by the process than is needed for the output. This then presents problems not only from a safety perspective but also an environmental point of view.
The two main options are:
- Storage
- Flaring
It is possible for biogas to be stored, usually in lagoons, whilst it is waiting to be used up. Commonly, it is only stored for a few hours before moving through to the desired end use. Biogas can be compressed but this is a more costly process and is usually used if there is an upgrading phase.
Alternatively, a flare can be used to allow the excess gas to be safely burned off in the event of excess gas or if the plant has failed and become potentially dangerous.
Different Types of Flare
Open Flares
When a process uses an open flare it often comprises of a burner with a windshield around it to protect the flame. Often controlled manually by means of a valve. The use of an open flare can be inefficient and difficult to achieve the correct mixture of gases for complete combustion. Many do not meet the efficiency demands and regulations that countries are putting in place and therefore are not a popular choice.
Enclosed Flares
Due to the design of the enclosed flare it promotes a uniform burn with the emissions being much lower than that of the open flare. There is much closer monitoring of emissions within an enclosed system and there is also better control over the combustion ratio too. Due to these reasons the enclosed flare is a popular choice for system designers.
Typical Flare Design
There a many common features that a flare must have to make it operational but also to ensure the safety of the plant and people. These include:
- Knock out pot – the role of a knockout pot is to remove any condensed liquids or droplets from the flare gases
- Flow control valve – a flow control valve is designed to ensure there is the correct amount of gas flow into the flare so that the burn takes place as designed
- Gas booster/blower – a booster is used to pressurise the gas entering the flare in order to create better combustion
- Failsafe valve – cuts off the flow of gas to the flare in the case of an emergency
- Flame arrest – allows gases to pass through but prevents the transmission of a flame which reduces the risk of fire or explosion
- Flame shroud – a shroud is used as a safety barrier round the flare and burner to avoid anyone coming into direct contact
Reference – Flaring_4-4.PDF (iea-biogas.net)
