A shrouded, open-flame flare with manual air adjustment is enough for a standby or emergency biogas flare that rarely runs, provided your permit does not require a guaranteed combustion temperature. For a 1,300 Nm³/h standby flare at a biogas site in southern Europe, the supplier offered exactly that, and set out clearly what it does not do: no automatic temperature control and no guaranteed flame temperature. An enclosed flare with temperature control was priced as an option for a 99.97% methane destruction efficiency.
The challenge: a flare that must work, but seldom runs
Many biogas sites burn nearly all of their gas in engines or boilers and keep a flare only for when those are down. Such a flare may stand idle for long periods. It must light reliably when needed, but it will not run for hours every day. The question is how much flare you need to buy for that duty.
The proposal understood the flare as a standby emergency unit, and the design reflects that.
The approach: the base offer and the options
The base flare
| Parameter | Value | What it means |
|---|---|---|
| Maximum flow | 1,300 Nm³/h | Rated at the design point of 50% methane |
| Minimum flow | 260 Nm³/h | A 5:1 thermal turndown |
| Thermal rating | About 6.5 MW at maximum | The energy in the gas the flare can burn |
| Methane range | 25% to 70% by volume | Design point 50% |
| Oxygen in the gas | 0% to 5% | Biogas can carry some air from extraction |
| Inlet pressure needed | 20 to 40 mbar g | No booster is included |
| Noise | About 80 dB(A) at 15 m | At full load, 2 m height |
The flare is an elevated, pre-aerated burner, with an insulated shroud around the flame. The burner head is made from 310 stainless steel, a high-temperature grade suitable for continuous use above 850°C. The supplier stated that the tip and shroud would last a minimum of 20 years. The shroud is 304 stainless steel lined with 100 mm of ceramic blanket insulation.
Manual flame control
The air:fuel mix is adjusted by hand at the base of the stack riser. This allows adjustment to about 850°C, but there is no automatic control and no guaranteed temperature. A flare tip height is chosen relative to the gas flow so that radiation at ground level stays acceptable.
Safety and ignition
- An inlet butterfly valve for isolation and flow adjustment.
- An ATEX-rated, electro-hydraulically actuated slam-shut valve on the feed, which fails closed on loss of power or an alarm.
- An ATEX-compliant flame arrester to EN 12874 with a stainless steel element.
- A low-pressure switch to indicate line blockage or pressure failure.
- A pilot line with solenoid valve and flame arrester, and electric ignition sequenced by a PLC.
- An ultraviolet flame sensor, self-checking and insensitive to sunlight. The proposal pointed out that a UV sensor responds immediately to flame-out, where a thermocouple has a noticeable lag.
The control panel is outdoor rated to IP56 and includes a burner control unit, emergency stop and alarm lamps.
Why it works, and where it stops
An open-flame flare can be a sensible way to meet a standby duty at a lower cost than a fully enclosed unit. The trade-off is clear in the proposal’s own wording: no guaranteed flame temperature, and only moderate control of combustion by manual adjustment of the air:fuel ratio.
If your permit or planning conditions require a guaranteed temperature, the same proposal listed an SC range enclosed flare with temperature control, which guarantees 99.97% methane destruction efficiency and needs 20 to 40 mbar g at the inlet, and a packaged version with the booster and flare on one skid under one control package. In the UK, regulators expect enclosed flares to reach at least 1,000°C with 0.3 seconds of residence, according to the Environment Agency’s monitoring guidance (LFTGN05), so an 850°C manual flare would be unlikely to satisfy a UK permit on its own. See our post on ground flare versus elevated flare for a comparison of the two temperature classes.
Supplied ready to use
The base unit is supplied “ready to use” and assumes the user has qualified engineers to install and start it. Commissioning was an option, priced at two days on site, and the proposal said it was strongly recommended if a flare with temperature control is chosen. The unit is built in Thailand with a lead time of around 12 to 14 weeks plus about 5 weeks of shipping, and is assembled on site from a skid-mounted base and an upper stack section, having been tested in the factory first.
Exclusions to read carefully
The supply excludes transport from port, offloading and installation; foundations and civil works; gas and electrical connections; dewatering of the gas before the flare (it is not designed to take liquid); condensate management; earthing and lightning protection; third-party inspection; safety integrity level analysis and DSEAR assessment; and flue gas analysis. Note in particular that wet gas must be dealt with before it reaches the flare.
What to consider for your own project
- Check the permit. If a minimum temperature is stated, a manual flare will not meet it.
- Check the gas pressure at the flare inlet. This flare needed 20 to 40 mbar g without a booster, and a booster of about 45 mbar was an option.
- Plan for liquid. Fit a knockout pot upstream if the gas is wet.
- Test the standby flare. A flare that never runs can fail unnoticed, so check ignition and the flame sensor on a schedule.
- Allow for commissioning. Particularly when you need someone to set the air:fuel mix.
FAQ
What is a standby flare?
It is a flare kept for use when the main gas users, such as engines, are out of service. It runs rarely, so cost and reliability matter more than fine control.
Does an open-flame flare have a guaranteed temperature?
Not in this offer. Temperature is set by hand through the air:fuel mix, to about 850°C, with no automatic control.
Why use a UV sensor rather than a thermocouple to detect flame?
A UV sensor responds immediately to flame-out. A thermocouple has a noticeable lag.
Do I need a gas booster?
Only if the gas does not reach the flare at 20 to 40 mbar g. A booster was offered with a pressure rise of about 45 mbar.
Choosing a standby flare?
Tell us your gas flow, pressure and permit conditions and we will recommend an open or enclosed design.