A mobile open flare for low-methane landfill gas needs two things a fixed flare often does not: a flare tip that folds down for transport, and a way to cover a very wide flow range. In a recent enquiry from an engineering contractor in Australia, both were solved by fitting two interchangeable flare tips to one skid, rated for roughly 10 to 100 Nm³/h and 100 to 500 Nm³/h of gas containing only 20 to 50% methane.

The challenge: weak gas, unknown flow, and a flare that has to move

The contractor needed a flare it could move between sites. The gas it expects to meet is weak: 20 to 50% methane, 45 to 60% carbon dioxide, up to 5% oxygen, with nitrogen making up the balance. Weak gas burns less readily than richer gas, and the flow from a gas field changes as the site ages.

That matters because a single burner cannot hold a stable flame across a 50:1 range of flows. From the smallest to the largest duty here, the range runs from about 10 to 500 Nm³/h. Mobile flares are also moved by road, so the stack has to be short enough to travel and quick to set up.

The approach: one skid, two tips, two measuring runs

Interchangeable hinged flare tips

The proposal is a skid-mounted open flare. The burner head, made from 310 stainless steel, is hinged and folds down into a cradle on the frame for transport. Two tips are supplied:

• A 500 Nm³/h tip for gas flows from about 100 up to 500 Nm³/h.
• A 100 Nm³/h tip for flows from about 10 up to 100 Nm³/h.

Each tip has an in-line plug. To change tips, the operator unplugs the connection and disconnects the tip at the hinge. The tip is chosen to suit the gas the site is producing at the time, rather than forcing one burner to work well at both ends of the range.

Two metered inlets

The knockout pot has two valved entries, one for each flow range. Each entry has its own orifice plate, and both feed a single differential pressure transmitter, with valves selecting which pressure signal it sees. A flow meter sized for 500 Nm³/h would be inaccurate at the bottom of the range, so each run is sized for its own part of it.

The gas train

The skid includes the equipment needed to move and burn the gas safely:

• A condensate knockout pot in 304 stainless steel with a polypropylene coalescing mesh filter.
• A centrifugal gas blower delivering about 85 mbar total head, driven by a 3 kW motor (2 kW absorbed).
• A deflagration-type ribbon flame arrester on the flare riser inlet.
• Two main gas solenoid valves in series, wired through in-line relays so that a single relay failure cannot leave them open. Both close on loss of power.
• A hard-wired, self-checking low-pressure switch on the blower outlet.
• Gas sampling points on the inlet, the knockout pot and the blower outlet.

Ignition and flame control

Ignition is by a high-voltage spark, with a UV sensor detecting the flame. If the flame is lost for any reason, the gas valve after the blower closes. Flame temperature is set by manually adjusting the air-to-fuel mix at the base of the riser, which allows a minimum of 850°C. This is a manual adjustment rather than an automatic control loop.

Why this design suits the job

An open flare is simpler and lighter than an enclosed one, and its tip can hinge down for the road. The trade-off is that it has less control over combustion air and a lower design temperature, so it is best suited to temporary or portable duties rather than to a permitted, long-term installation. The Environment Agency’s guidance on enclosed flares (LFTGN05) sets out the expectations for enclosed flares in the UK. Check what your regulator requires before choosing an open flare for permanent use.

The proposal assumes the buyer is an experienced engineering company. It is supplied ready to run, without a commissioning engineer, although one can be added as an option. A manual inlet butterfly valve sets the flow and pressure. As an option, a variable speed drive on the blower can hold a pressure chosen by the user at the control panel without the valve being adjusted by hand.

What to consider for your own project

1. Know your methane range. Gas at 20% methane behaves very differently from gas at 50%. State the lowest methane you expect, not the average.
2. Define the flow range, not only the peak. If your flows could span a factor of 50, one burner will probably not do. Ask about two tips or two flare sizes.
3. Decide whether manual control is acceptable. Manual air adjustment is fine for an attended, portable flare. An unattended site needs automatic temperature control.
4. Plan the interfaces. Foundations, the electrical supply connection and delivery from port to site were outside the quoted scope. A suitable electrical connection point is supplied, but the supply to it is the site’s job.
5. Allow for servicing. The warranty depends on satisfactory service arrangements, either a service contract or trained client staff keeping full records.

For a longer-term alternative, see our post on a hinged-chimney mobile ground flare, and for the permit side of landfill gas, the Environment Agency’s page on managing landfill gas.

FAQ

What is a mobile open flare?

It is a skid-mounted flare that burns gas at the top of a short stack without an enclosure, with a tip that folds down so the unit can be moved by road.

Can an open flare burn gas with only 20% methane?

This design was specified for 20 to 50% methane. Weak gas is harder to burn, so the burner and its air-to-fuel mix are chosen for the lowest methane the site will produce.

Why use two flare tips?

One tip cannot burn both a trickle of about 10 Nm³/h and a flow of 500 Nm³/h well. A small tip and a large tip each work in the range they were designed for.

What temperature does an open flare reach?

This one allows manual adjustment down to a minimum of 850°C. Enclosed flares typically run hotter, with tighter control.

Need a flare that can move?

Tell us your gas composition and flow range and we will recommend a flare size and tip arrangement.