A packaged enclosed flare is a ground flare, a gas blower and the safety and control equipment, all built on one skid and tested before it leaves the factory. In a proposal to an engineering contractor in Australia, a 1,000 Nm³/h unit with a 5:1 turndown was specified to burn at a minimum of 1,000°C for at least 0.6 seconds. Just as important is the list of what such a package leaves out.
The challenge: moving gas and burning it safely, with no existing equipment
Landfill gas does not move by itself. If a site has no suitable gas-moving equipment, the flare supplier has to include a blower, a way to remove condensate, and the safety devices that stop a flame travelling back down the pipe. Asking for these separately means several suppliers and several interfaces. A skid-mounted package puts them together.
The approach: one skid, three assemblies
The design duty
| Parameter | Design value | What it means |
|---|---|---|
| Gas flow | 200 to 1,000 Nm³/h | A 5:1 turndown, so the flare can follow a falling gas yield |
| Methane | 25 to 70% (design point 50%) | Rich enough to hold temperature; weaker gas runs cooler |
| Combustion temperature | 1,000°C minimum, up to 1,200°C | Applies when methane is above 25% |
| Residence time | At least 0.6 seconds | How long the gas stays hot inside the shroud |
| Noise | 70 dB(A) at 15 m | At full load, 2 m above ground |
Blower assembly
The gas first passes a condensate knockout pot with a polypropylene mesh filter. The clean demister causes a pressure drop of less than 3 mbar and removes 99% of particles larger than 6 microns. A single centrifugal gas blower then raises the pressure to feed the flare. Around it sit isolation valves, stainless steel bellows, an emergency stop, a low-pressure switch to indicate a blocked line or blower failure, and an ATEX-compliant flame arrester on the blower outlet.
The enclosed flare
The shroud is lined with 100 mm of ceramic blanket insulation. A PID controller adjusts a combustion air louvre to hold the temperature. Four sampling ports in the stack allow emissions testing. Ignition is automatic, using electric igniters, a UV flame sensor and a secondary air damper that is set mechanically at start-up and then handed to the temperature controller. Two fail-safe main gas valves sit in series with a manual bleed valve between them, a double-block-and-bleed arrangement.
Controls and data
The control panel, rated IP66 and housed in a walk-in rain shelter, has a PLC, a touch-screen display, a variable speed drive on the blower and a modem for remote access. A data logger records gas flow, vacuum, pressure and the methane, carbon dioxide and oxygen content of the gas. A gas analyser panel is included to supply these readings.
What is left out
The exclusions list is as important as the scope. The package does not include:
- Foundations and civil works, although the supplier provides the design.
- Gas piping and the electrical supply to the flare.
- Equipment to remove liquid water from the gas before the plant. The system is not designed to take liquid streams.
- Earthing and lightning protection.
- Third-party inspection, safety integrity level studies, hazardous area assessments, flue gas analysis and any authority acceptance test.
- Installation, site delivery and import duties.
In this offer the flare supplier also included support in getting the unit signed off by an accredited inspector for the state energy safety regulator. Commissioning was proposed as a training exercise: one engineer supervises while the contractor’s own staff do the work, so that they can commission the next unit themselves.
What to consider for your own project
If you are writing a tender, our guide to a landfill gas flare specification lists what to state up front.
- Decide who supplies the blower. If the site already has gas-moving equipment, a flare without a blower costs less. If not, specify a packaged unit.
- Dewater the gas first. A knockout pot removes droplets, not slugs of liquid.
- Ask for the exclusions in writing. Earthing, lightning protection and safety studies are often assumed to be included.
- Match the residence time to your regulator. This design offered 0.6 seconds. Other regimes accept 0.3 seconds. See the Environment Agency’s guidance on monitoring enclosed flares for the UK position.
- Plan for spares. The proposal lists a recommended spares kit (UV bulb, ignition electrodes, three thermocouples, ignition transformer and burner control unit), none of it included in the price.
For an alternative layout where the gas arrives under pressure, see our post on a flare without a blower.
FAQ
What is an enclosed ground flare?
It burns the gas inside an insulated shroud rather than at the top of an open stack, which allows the combustion temperature and air supply to be controlled closely.
Why is there a knockout pot before the blower?
Landfill gas is saturated with water. The pot and mesh filter remove droplets that would otherwise damage the blower or disturb combustion.
What does 5:1 turndown mean?
The flare can operate from one fifth of its maximum flow up to full flow, here 200 to 1,000 Nm³/h.
Is commissioning included?
Not in the package supplied here. A single engineer was proposed to supervise and train the client’s staff.
Specifying a packaged flare?
Send us your gas flow, methane range and site conditions and we will confirm what a packaged unit should include.