A flare for anaerobic digester biogas usually does not need a gas booster, because the digester generates its own pressure. In a quotation for a 1,000 m³/h biogas flare, the unit was designed for low pressure loss through the burner and fed at roughly 10 inches water gauge (about 25 mbar) at full flow. This post explains what that means for the design and what to check before you leave the booster out.

The challenge: a safe outlet for digester gas

Digesters produce biogas continuously, and a flare is needed to burn it when it cannot be used, for example during engine maintenance or surges. The enquiry came from an engineering company supplying a digester scheme in southern Europe, and asked for a flare of 1,000 m³/h.

The approach: a low-loss elevated flare

The supplier proposed an elevated, pre-aerated flare (the AC Range) designed specifically for biogas from an anaerobic digester, with no gas booster. A booster could be added on request, so the enquirer should state whether the gas will arrive at enough pressure.

What the package includes

• A condensate knockout pot with a polypropylene dewatering filter, which stops liquid reaching the flare line and gives coarse filtration before the flame arrester.
• A manual trim valve for flow adjustment, and an electrically actuated slam-shut valve for isolation.
• A flame arrester with a stainless steel element on the inlet.
• A flare stack with a 321 stainless steel tip and shroud, about 6 m to the tip, with manual flame temperature adjustment up to 850°C.
• A self-checking UV flame sensor, automatic electric ignition under PLC control, and a pressure switch to detect gas flow interruption.
• A weatherproof control panel (IP65) and a galvanised skid with four lifting eyes.

Moderate combustion control

Air and fuel are balanced by manual adjustment, which gives moderate control of combustion. That is a common trade-off with elevated flares. A tightly controlled enclosed flare costs more and needs more pressure to push gas through it.

Why it works

Biogas leaves the digester at a modest pressure, so a flare with low resistance through the burner can use that pressure directly. Fewer moving parts means fewer items to maintain. The drawback is that every pressure drop upstream counts. Long pipework, a clogged flame arrester or a full knockout pot can reduce the pressure available at the burner.

What to consider for your own project

1. Calculate the pressure at the flare inlet at full flow, including pipe losses. Compare it with the burner’s requirement.
2. Plan for condensate. Biogas is wet, so provide drainage and a knockout pot.
3. Think about hydrogen sulphide. Biogas often contains it, which affects materials. This quotation did not address it, so raise it with your supplier.
4. Check whether an elevated flare meets your permit. The Environment Agency’s guidance on biogas plants is a useful starting point.

FAQ

Does biogas need a booster to flare it?

Often not. The digester pressure can be enough if pressure losses are low. Add a blower if it is not.

Why a knockout pot?

To stop condensate entering the flare line.

How hot does the flame run?

Up to 850°C with manual adjustment.

Sizing a biogas flare?

Send us your digester output and pressure and we will confirm whether a booster is needed. Read also ground or elevated flare.