A flare for sewage biogas can be simpler than one for landfill gas, because the gas is cleaner, richer in methane and often already under pressure. In a 500 m³/h design for a wastewater works with gas holders, the booster was left out, the pipework and knockout pot were stainless steel, and pressure switches took over the job of deciding when to flare. This post compares the two flare types using that design.
The challenge: a flare beside the gas holders
The client was a public services department with a sewage treatment works. Its sludge digesters make biogas, which is stored in a pair of gas holding tanks. When the stored gas cannot be used it must be burned safely. The flare would sit next to the holders.
A near-identical flare was being built at the supplier’s works for a wastewater plant elsewhere, so the client could see it before dispatch.
The approach: what the biogas version includes
- No booster. The gas holders supply the pressure.
- A stainless steel knockout pot. The condensate pot is 4 mm 304 stainless steel with a stainless float trap. It has a polypropylene dewatering filter, and it sits before the main slam-shut valve.
- Three pressure switches. One before the slam-shut valve detects that gas is available. If pressure is low, the valve will not open. One after it detects excess flow, and one detects low flow. Each starts a control action.
- A flame arrester with a stainless steel element after the slam-shut valve.
- An SC Range ground flare at 500 m³/h with a 7 m shroud, running at 1,000°C or above. The maximum design methane concentration is 70%.
- A 304 stainless steel shroud lined with 100 mm of ceramic blanket, fixed with a block-building system so that the fixings are not exposed to the heat.
- A 25 mm biogas pilot line to give a high flow to the pilot and so reliable ignition.
- A 120-second actuator on the combustion air louvre for temperature control.
Why the differences exist
| Landfill gas flare | Sewage biogas flare | |
|---|---|---|
| Gas supply | Drawn from wells by a booster | Usually delivered under pressure from digesters or gas holders |
| Methane range | Often low, down to about 20% | Higher, here up to 70% by design |
| Contaminants | Wide range of trace compounds | Cleaner gas |
| Pipework | Typically galvanised steel | Stainless steel for the gas train in this design |
| Control | Booster and flare controls | Pressure switches and slam-shut valve |
Biogas from digesters is usually moist and corrosive, hence the stainless steel. It needs no booster as long as the pressure at the flare stays within the burner’s working range. For a similar biogas flare at a wastewater scheme, the burner worked between roughly 5 and 30 mbar gauge, and the design left out a back-pressure regulator and a booster for that reason.
The supplier’s own view, expressed in correspondence on another wastewater project, was that landfill gas is the harder duty for a flare because of its many constituents, particularly when emission standards must be met. Cleaner biogas flares therefore need not be as technically advanced.
What to consider for your own project
- Measure the pressure. If the gas arrives between the burner’s minimum and maximum, you may not need a booster.
- Look at the methane range. Design for the highest concentration, not just the lowest.
- Choose materials for moisture. Stainless steel in the gas train resists corrosion from wet biogas.
- Use the pressure switches as safeguards. They stop the flare opening without gas and react to flow that is too high or too low.
- Check your regulator. Permit conditions may set a temperature and residence time.
See also our post on delivering and commissioning a biogas flare. For landfill sites, the Environment Agency’s LFTGN03 remains the main reference.
FAQ
Does a biogas flare need a booster?
Not if the gas arrives at the burner’s working pressure. This design had none.
Why are pressure switches used on the feed line?
They check that gas is available before the main valve opens, and they detect flow that is too high or too low.
Why use stainless steel?
Wet biogas can corrode galvanised or plain carbon steel.
Can a ground flare burn 70% methane?
Yes. This one was designed for it, with a maximum design methane of 70%.
Specifying a biogas flare?
Send us your gas details and we will suggest a flare for your digester or gas holder.