Digester gas at a wastewater treatment works needs a flare that burns at 900°C or above and a separate booster to push the gas to it. Splitting them into two packages gives flexibility, because the flare and booster can be sited, ordered and maintained separately. This post describes a 1,000 Nm³/h enclosed flare and a two-blower booster skid for a works in East Asia.
The challenge: surplus biogas and low pressure
Anaerobic digesters at sewage works produce biogas at only a few millibar. When the gas engines or boilers cannot take it all, the surplus must be flared. Here the design gas was 40 to 70% methane, with 25 to 40% carbon dioxide and up to 2% oxygen, at a maximum of 1,000 Nm³/h and a minimum of 200 Nm³/h.
The approach
The flare
- An enclosed stainless steel flare, with a 100 mm ceramic blanket lining.
- Combustion temperature of at least 900°C when methane is above 25%, falling for leaner gas down to 20%.
- Residence time over 0.3 seconds, thermal turndown of 5:1 and a destruction efficiency of at least 98%.
- Automatic temperature control with N-type thermocouples and actuated dampers.
- A UV flame sensor, a flame arrester, a low-pressure switch, an ultrasonic flow meter and a fail-safe slam-shut valve.
- A minimum inlet pressure of 30 mbar, a viewport, and a safety ladder and platform for access.
- A duty and standby PLC with a signal to the works’ SCADA system.
A revision of the proposal changed the shroud material from stainless steel to galvanised steel, a reminder that material choice is a design variable to discuss with the supplier.
Purge blower: a design choice
US flares often include a purge blower. European suppliers usually do not, relying on short idle times and natural draught to clear fugitive gas between ignition attempts. The proposal omitted it and offered it as an option.
The booster
Two centrifugal boosters on a single skid, 316 stainless steel pipework, lifting pressure from about -20 mbar at the inlet to about 60 mbar at the outlet, with a total head up to 80 mbar. The skid includes:
- A knockout pot with demister (pressure drop under 3 mbar clean) and a level switch.
- Flame arrester, non-return valves and flexible joints.
- Vacuum and pressure gauges and transmitters before the pot, and before and after each booster.
- Local emergency stop and low-pressure switches.
Why two packages
The booster is quoted on its own because on a works it may serve the existing flare or engines as well as a new unit. The interconnecting pipework between boosters and flare was outside the scope, so the interface needs clear definition early.
What to consider for your own project
- Define the gas pressure at the flare inlet. A flare with 30 mbar minimum needs a booster if the digester cannot supply that.
- Check the gas composition range. The temperature set point depends on the methane.
- Decide on purge and standby arrangements.
- Allow for the commissioning. Erection and commissioning of a flare this size typically takes one to two days, but the slab and connections need to be ready.
- Check exclusions: import duties, offloading, the flare compound, utility connections and gas analysis.
The UK guidance on enclosed flares (LFTGN05) is a useful benchmark.
FAQ
Why does a flare need a booster?
Digester gas often arrives at too low a pressure to flow through a flare’s valves and burner.
What is the destruction efficiency?
A minimum of 98% for this flare.
Is a standby booster included?
Two boosters are fitted on the skid.
Do I need analysers?
None were offered in this scope. They can be added separately.