A small biogas flare can save energy by using the gas’s own pressure when it is high enough, and switching on a booster only when it falls. A 250 m³/h elevated flare for a contractor in southern Europe was specified this way: a bypass around the booster, pressure switches that choose the feed route, and a flare tip at 850°C. This post explains how that arrangement works and when it suits a project.

The challenge: a flare for variable gas pressure

The enquiry was for an AC Range flare, the elevated pre-aerated type, for biogas at 250 m³/h. The site was still to be decided. With the site unknown, the unit had to cope with either gas arriving under pressure or gas that needs help to reach the flare.

That is common with small biogas sources. If a booster runs when it is not needed, it wastes electricity and adds wear. If it is missing when needed, the flare starves.

The approach: bypass, switches and a low-head booster

Two ways for gas to reach the flare

• Pressure feed: when the gas pressure is high enough, gas goes through a bypass line around the booster.
• Booster feed: when pressure drops, a booster driven by a hazardous-area (ExN) motor pushes the gas through. It was sized for a low total head of about 30 mbar.

Low and high pressure switches on the booster inlet detect which feed is needed, and the system switches automatically. Further pressure switches after the slam-shut valve watch the flare feed line. A forward-pressure regulator holds the pressure going to the flare steady.

Protecting the booster and the flare

• A condensate knockout pot with a polypropylene dewatering filter, so no liquid reaches the booster.
• Pressure gauges on the pot inlet and the booster inlet. The difference shows the pressure drop across the filter, so a clogged filter is easy to spot.
• Flame arresters with stainless steel elements on the booster inlet and outlet.
• A slam-shut valve on the feed line and a manual trim valve for flow.

The flare itself

• A 310 stainless steel flare tip and shroud, with the tip 6 m above ground.
• Manual flame temperature control, adjustable to 850°C by changing the air-to-fuel ratio.
• A UV flame sensor and automatic electric ignition, run by a PLC.
• Vent-only operation when the methane level cannot support a flame.
• An operator choice of continuing to run, shutting down, or venting if the flame goes out.

The knockout pot’s liquid seal sits in a lined hole beside the unit, at least 160 mm in diameter and 500 mm deep. The seal holds up to 50 mbar. Pumped removal or a buffer vessel were offered as alternatives for condensate.

Why it works

The bypass makes the booster optional. Running a booster against a small head of 30 mbar costs little, but running it when gas pressure is already high costs something for no benefit. The pressure switches make that decision automatic.

The 850°C setting reflects the type of flare. An elevated flare gives moderate control of combustion by manual adjustment of the air-to-fuel ratio. That is suitable where emission limits are not the main constraint. It would not normally meet the temperature expected of enclosed flares on UK landfill sites, where the Environment Agency expects at least 1,000°C. See our post on ground flare and elevated flare.

What to consider for your own project

1. Measure the gas pressure at the flare. If it is reliably high, a bypass could remove the need for a booster most of the time.
2. Know the regulation. Check which flare type your regulator accepts before choosing one.
3. Plan condensate removal. The knockout pot needs a drain, whether it is a seal pit, a pump or a buffer vessel.
4. Ask about shipping terms. This enquiry was for export, and the quote gave both a port-of-departure and a delivered option.

For biogas from landfill, the Environment Agency’s LFTGN03 sets out what is expected from flaring.

FAQ

When is a booster needed for a biogas flare?

When the gas arrives at too low a pressure to reach the flare, for example after pipework losses or filter pressure drop.

What does a knockout pot do?

It removes liquid from the gas before it reaches the booster, where water could cause damage.

What temperature does an elevated flare run at?

This one was adjustable to 850°C. A ground flare is typically designed for around 1,000°C.

What happens if methane is too low to burn?

The unit can run in vent-only mode.

Sizing a biogas flare?

Send us your gas details and we will suggest a flare and feed arrangement.