An open (elevated) flare is a sound choice for small biogas plants when the regulator does not require a high-temperature enclosed flare. It costs less, has fewer controls and lasts a long time. This post describes a 170 Nm³/h open flare with an integral knockout pot, specified for biogas from pig manure and palm oil mill effluent.
The challenge: a safe, simple flame for variable biogas
Small digesters need a flare for start-up, upsets and surplus gas. The conditions were:
- Methane of 25 to 60% and oxygen of 0 to 5%, saturated gas at about 30°C.
- Maximum flow 170 Nm³/h, with a typical flow of 135 Nm³/h and a minimum of 20 Nm³/h, which is a 10:1 turndown.
- A feed pressure of about 3 kPa at full flow, so no booster is included.
The approach: the AC/L range
- Stack and tip: a pre-aerated elevated stack, with a 310 stainless steel burner head suitable for continuous temperatures above 850°C. The supplier expects the tip to last at least 20 years. The height is set so that ground-level radiation is acceptable.
- Knockout pot: galvanised carbon steel, with a polypropylene demister, a level switch and a drain.
- Flame control: a manual air-to-fuel adjustment at the base of the riser gives a flame temperature of about 850°C.
- Safety: a self-checking UV flame sensor, which responds faster than a thermocouple, a PLC-controlled electric ignition, a slam-shut valve that closes on power loss, and a low-pressure switch.
- Controls: an IP56 panel and a local emergency stop.
Open versus enclosed
| Open flare (AC range) | Enclosed flare (SC/LM range) | |
|---|---|---|
| Temperature control | Manual air setting, about 850°C | Automatic, 900 to 1,000°C or more |
| Residence time | Not applicable | At least 0.3 s |
| Flame | Visible | Hidden in shroud |
| Materials | Galvanised steel and 310 stainless | Stainless steel with ceramic lining |
| Best for | Small plants with no emissions testing requirement | Permitted sites needing proven destruction |
What to consider for your own project
- Ask what the permit requires. Open flares generally cannot show emissions compliance.
- Check radiation to neighbours and staff.
- Plan drainage for the condensate from the knockout pot.
- Keep spares for the UV sensor and ignition components.
See the Environment Agency’s landfill gas guidance for how flaring is treated in the UK.
FAQ
What is an AC/L flare?
A stand-alone open flare with an integral knockout pot and no gas booster.
Why a UV sensor?
It detects flame-out immediately, with no lag.
Can it burn gas of 25% methane?
That is the lower end of the design range.
Does it need a booster?
Not if the gas arrives at about 3 kPa.