An enclosed flare is defined by a short list of design criteria: the flow range it can follow, the methane range it can burn, the temperature it holds and how long the gas stays at that temperature. For a 1,000 Nm³/h enclosed flare quoted for a landfill in southern Europe, those were a 5:1 turndown, 25 to 70% methane, at least 1,000°C and at least 0.3 seconds of residence time. This post explains what each of them means and why it matters when you compare quotations.
The challenge: reading a flare datasheet
Flare proposals open with a table of design criteria. The numbers look similar from one supplier to the next, but small differences decide whether the flare will burn your gas reliably. A buyer who understands the table can ask better questions.
The approach: the 1,000 Nm³/h flare, line by line
Flow and turndown
The flare was designed for 200 to 1,000 Nm³/h. The ratio, 5:1, is the turndown. Landfill gas production tends to fall as a site ages, so a flare with only a narrow range may have to be switched off or run outside its design. Heat release scales with flow: the burner capacity was about 7 MW at maximum and 1.4 MW at minimum.
Methane range
The design accepts 25 to 70% methane by volume and up to 5% oxygen. The 1,000°C figure applies when methane is above 25%. Below that, down to 20% methane, the flare continues to run but burns cooler. A site that expects low-methane gas should ask what the flare does at the bottom of its range.
Temperature and residence time
The flare holds a combustion chamber temperature of at least 1,000°C using an automatic controller that adjusts the combustion air louvre. Gas stays in the hot zone for at least 0.3 seconds. High temperature and sufficient time are what destroy trace compounds in the gas. In the UK, the Environment Agency’s guidance, LFTGN05, discusses the temperature and residence time expected of enclosed flares. The proposal also states compliance with the TA Luft emission classes and with the European standard for flares for combustion of biogas (BS EN ISO 22580).
Pressure arrangement
Because this flare was a blower package, the pressures are part of the criteria. The gas arrives at the plant at about 80 to 100 mbar below atmospheric pressure, the blower develops roughly 150 mbar of total head, and the flare is supplied at 30 to 50 mbar. These numbers tell you how far the plant can pull gas from the field.
Materials and protection
Pipework, knockout pot and shroud are in 304 stainless steel, and the skid is hot-dip galvanised carbon steel. The control panel is rated IP56. Electrical equipment is selected for hazardous-area zoning where required, and the blower motor is an explosion-protected type.
Why the criteria work together
Raising temperature needs more fuel and tighter air control. Lengthening residence time needs a taller or wider shroud. Widening turndown needs a burner that can run stably at low flow. No single figure tells you how good a flare is, but together they define its envelope. For how these figures differ between flare types, see ground flare vs elevated flare.
What to consider for your own project
- Quote your methane range honestly. Include the lowest value you expect.
- Check the residence time against your permit. Requirements differ between countries and regimes.
- Look at sampling provision. The design includes four DN125 sampling ports and an access platform, which makes emissions testing practicable.
- Budget for spares. The recommended list includes a UV bulb, ignition electrodes, thermocouples, an ignition transformer and a burner control unit.
- Look beyond the headline. A flare that meets the numbers but cannot be sampled or serviced safely is hard to run.
Our post on the scope of a packaged flare covers the inclusions and exclusions for a similar unit.
FAQ
What is turndown on a flare?
It is the ratio between the maximum and minimum gas flow the flare can burn properly. A 5:1 turndown means it can run at one fifth of full flow.
What is residence time?
It is how long the gas spends inside the hot part of the flare before it leaves the stack, usually stated in seconds.
Why does methane content affect temperature?
Gas with less methane releases less heat, so the flame runs cooler unless extra fuel is added.
Does the flare need a pilot?
This design uses a pilot line with its own solenoid valve and flame arrester, lit by an electric igniter at start-up.
Comparing flare quotations?
Send us the quotations and we will help you compare the design criteria like for like.