Biogas is odorous and strong greenhouse gas. No direct discharge of biogas into atmosphere is allowed in many countries.
The simplest flare is candlestick, which is being phased out due to the low destruction rate. The flame is open and susceptible to weather, and the flame temperature is usually too low for complete removal of methane and odor.
A simple improvement is to add windshield to enclose the flame. The windshield needs to be sized to accommodate the flame with enough residence time for combustion. Flame detection (UV or temperature) and automatic ignition is generally required for smooth operation. Flame arrestor and automatic slam shut valve is also compulsory ensure the safety.
If the biogas pressure, flow and/or composition is unstable, the flare may distinguish too often. In this case, a pilot fire, using biogas or LPG, can be added. The pilot fire is always burning in the flare to ensure the existence of ignition source.
For long term continuous operation, insulation layer is required to protect the steel structure from heat stress.
There are various design of biogas burners and venturi-type burner is the most commonly used as it provides good mixing of biogas and air. The biogas pressure and flow rate dictates the design of burners array.
Modern biogas flare can achieve flame temperature of 800-1200 degree C, which ensures high destruction rate while minimise NxO production.