Landfill sites present a unique and often underappreciated hazard: gas migration into surrounding land and buildings. When methane concentrations drift into the explosive range, site managers face a genuine dilemma – extract the gas and risk dangerous conditions within the extraction equipment itself, or leave it in the ground and risk it migrating into nearby properties. The EXG (Explosive Gas) range of flare stacks was designed specifically to resolve this dilemma, safely drawing in, feeding forward, and destroying gases that sit within the explosive range.
The Problem: Gas in the Explosive Range
Methane is explosive at concentrations of roughly 5% to 15% methane-in-air. What’s less widely understood is that these explosive limits shift depending on the presence of diluent gases such as carbon dioxide, nitrogen, and oxygen – all of which are commonly present in landfill gas.
In a migration control system, it’s a common occurrence for gas concentrations to approach this explosive window. This creates a genuine operational bind:
- Draw the gas down and you risk creating dangerous conditions inside the extraction equipment itself.
- Leave the gas in place and it may migrate into buildings, forming explosive mixtures within occupied spaces.
While physical barriers like cement-bentonite slurry walls offer some mitigation, they don’t provide the certainty of a complete negative-pressure curtain achieved through active extraction.
Methane’s explosive range of approximately 5-15% methane-in-air is not fixed – the presence of CO₂, nitrogen, and oxygen in landfill gas can shift these limits, making standard extraction equipment unsuitable for handling gas in this zone.
The Solution: Purpose-Built for Explosive Gas Handling
The EXG range is engineered from the ground up to safely handle gas within the explosive range, rather than avoiding it. This is achieved through a combination of intrinsically safe equipment design and rigorous process safety controls.
Key Safety Features
- Zone 1 rated equipment throughout, suitable for hazardous atmosphere classification
- No spark-creating equipment anywhere in the gas flow path
- Optional blower rated to withstand an internal explosion, providing a critical layer of physical protection
- Dual oxygen-cell sensors for accurate, redundant gas monitoring
- Safety Integrity Level 2 (SIL2) as standard
- Full HAZOP/HAZAN (Hazard and Operability Study / Hazard Analysis) included as standard with every unit
- Automatic flame temperature control, maintaining a minimum design temperature of 1,000°C to 1,200°C
- Optional fully stainless steel construction for extended service life
- Optional catalytic and low-methane combustors for handling weaker gas streams
These features work together to ensure that gas which would be unsafe to process through conventional equipment can instead be extracted and destroyed in a controlled, non-hazardous manner.
How Combustion Performance Is Balanced
The core engineering principle behind any landfill gas flare is a trade-off between time and temperature when destroying trace contaminants in the gas stream. This trade-off is expressed as the Destruction Efficiency Rating (DER, sometimes referred to as DRE – Destruction Removal Efficiency).
The higher the combustion temperature, the shorter the residence time needed to achieve full destruction of contaminants. For reference, municipal waste incinerators are typically required to maintain temperatures greater than 850°C for a duration of 2 seconds. Landfill gas flares, by contrast, can achieve higher temperatures, allowing for shorter residence times.
There is, however, a ceiling to this approach. At approximately 1,200°C, thermal NOx formation begins to increase sharply. While higher temperatures do improve destruction efficiency for trace gases, the resulting increase in NOx emissions can negate that benefit – which is why the EXG range operates within a carefully controlled temperature band.
At the 1,200°C mark, thermal NOx formation increases sharply enough to offset any gains in contaminant destruction efficiency – a key reason the EXG system’s temperature control is automated rather than left to manual adjustment.
Technical Specifications
| Parameter | Specification |
|---|---|
| Flow rate (standard range) | 100 to 15,000 m³/hour |
| Pressure rise across gas booster | 150 mbar |
| Flame temperature | 1,000°C minimum |
| Retention time | 0.3 seconds |
| Minimum methane concentration for combustion | 25% |
| Number of inlets (standard) | 2 flanged inlets (additional available on request) |
| Flow control | Chemical duty butterfly valves |
| Pipework finish | Hot dip galvanised to industry standard |
| Burner material | High temperature stainless steel |
| Flame arrestor location | Gas booster inlet and outlet |
| Flame detection | Self-checking UV sensor |
| Standard colour | Battleship Grey (or customer specification) |
Control Panel and Monitoring Flexibility
Modern landfill gas management increasingly requires integrated gas analysis, and the EXG range’s control panel is designed with this in mind. The panel accepts both methane and oxygen analysers, providing dual-stage trigger levels for either alarm or automatic shutdown.
At the heart of the system is a fully programmable PLC (Programmable Logic Controller), which allows delay times, re-ignition sequences, and other operational parameters to be tailored to site-specific requirements.
Panels are also built with a 20% expansion factor as standard, allowing for future retrofits – such as adding a telemetry unit – without requiring a full panel replacement. The standard wiring configuration is designed to facilitate this kind of extension, so that the root cause of any alarm can be isolated within the PLC logic and reported remotely, for example via telephone line.
Construction and Materials
All pipework is fully galvanised, and the ground flare shroud is either galvanised or finished with high-temperature paint, depending on flow rate and the resulting size of the shroud. For sites wanting extended service life and improved aesthetics, fully stainless steel combustion chambers are available as an option.
Where the EXG Range Fits
The EXG range represents the entry point for handling explosive-range gas safely, providing minimum flame temperatures of 1,000°C for a minimum retention time of 0.3 seconds. This meets baseline Destruction Efficiency Rating requirements for typical landfill gas applications.
For sites requiring higher combustion temperatures – for example, where trace contaminant profiles demand greater destruction efficiency – the related MC class of flare units (see Data Sheet ODSF03) offers a step up in specification.
Key Takeaways
- The EXG range is purpose-built to safely extract and destroy landfill gas that falls within the explosive range for methane (approximately 5-15% methane-in-air, modified by diluent gases).
- All equipment is Zone 1 rated with no spark-creating components in the gas flow path, backed by SIL2 safety integrity and full HAZOP/HAZAN documentation as standard.
- Combustion operates at 1,000-1,200°C, balancing destruction efficiency against thermal NOx formation.
- Flow rates span 100 to 15,000 m³/hour, with a 0.3 second retention time and flexible, PLC-based control and monitoring.
- Optional stainless steel construction, catalytic combustors, and expandable control panels allow the system to be tailored to specific site conditions.
Get in Touch
If your site is managing landfill gas migration and you’re weighing the risks of extraction versus containment, the EXG range offers a proven, safety-certified way to extract and destroy explosive-range gas without compromising on site safety. Contact our team today to discuss your flow rates, gas composition, and site requirements, and let us help you specify the right flare stack solution for your project.