Landfill Gas Flare

Landfill gas is saturated with moisture and typically contains 100 to 2,000 ppm of hydrogen sulphide, along with trace halogenated compounds. Combined, these attack untreated carbon steel over time, particularly at condensate low points and burner internals.

Wetted components and burner tips are commonly built in 304 or 316 stainless steel, with coated carbon steel used for the outer stack and structural sections. Material selection is matched to each site's actual gas analysis rather than a single default specification, since H2S concentration and moisture loading vary significantly between landfills.

Flare design and mechanical construction typically follow API 537 (Flare Details for Petroleum, Petrochemical, and Natural Gas Industries), with pressure-retaining components meeting applicable ASME codes.

On the emissions side, US landfills are governed by US EPA NSPS Subpart XXX (new and modified sites) and the corresponding Emission Guidelines for existing sites, both requiring 98% NMOC destruction efficiency or an outlet concentration below 20 ppmv. Sites outside the US typically reference the EU Industrial Emissions Directive (IED) or the equivalent national environmental regulation.

• API 537 (flare mechanical design and safety)
• ASME (pressure-retaining component codes)
• US EPA NSPS Subpart XXX / WWW (landfill gas emission control)
• EU IED (industrial emissions for EU-based sites)

Properly designed enclosed landfill gas flares achieve destruction efficiencies above 98% for methane and non-methane organic compounds, meeting the enclosed combustion device threshold set out in US EPA NSPS Subpart XXX and the corresponding Emission Guidelines for existing landfills.

Achieving that figure in practice depends on maintaining combustion temperature, residence time, and air-to-fuel ratio across the flare's full turndown range, not just at design flow. Automated control systems that hold these parameters steady as gas volume and composition shift are what keep destruction efficiency consistent over the life of the site.

Gas generation from a landfill cell peaks within the first few years after waste placement, then declines steadily over 15 to 30 years as the organic fraction is depleted. A flare sized only for that initial peak struggles to hold a stable flame once volume drops to a fraction of the design flow.

Enclosed landfill gas flares are typically specified with turndown ratios between 10:1 and 100:1, depending on the site's projected decline curve. CRA sizes turndown to each site's actual gas generation forecast rather than defaulting to a single ratio, so the flare stays stable from early peak flow through the long tail of a closed cell.

Landfill gas generation follows a predictable arc: it rises for several years after waste placement, peaks, then declines gradually over decades as the organic fraction is consumed. Sizing a flare only for peak flow leaves it oversized and unstable during the long decline that follows.

Sizing starts with a gas generation model built from waste-in-place tonnage, composition, and site-specific decay rates, then checks the flare's turndown range against both the projected peak and the multi-decade tail. Modular or multi-unit configurations are used where a single flare cannot cover the full range, or where phased cell development means gas volume arrives in stages rather than all at once.

Landfill gas carries hydrogen sulphide, mercaptans, and a range of non-methane organic compounds (NMOCs) produced as municipal solid waste decomposes. These compounds are the source of the odour associated with active and recently closed cells.

An enclosed flare destroys them through sustained high-temperature combustion inside a refractory-lined chamber, typically operating above 870°C with a residence time of at least 0.3 seconds. US EPA guidance requires enclosed combustion devices for landfill gas to reduce NMOC by at least 98% by weight, or to an outlet concentration below 20 ppmv as hexane. Burner and chamber design account for the specific mix of odorous compounds in landfill gas rather than treating it as a generic methane stream.

Both are enclosed, refractory-lined combustion units, and the core technology is the same. The difference is in the gas stream and the flow profile each is engineered around.

Digester biogas from an anaerobic digestion plant tends to have a steadier flow and methane content once the process stabilises. Landfill gas, by contrast, comes from a decomposing waste mass with a flow rate that declines over 15 to 30 years, higher variability in moisture and H2S content, and a broader mix of trace NMOCs and siloxanes.

CRA's Landfill Gas Flare is specified for that declining, variable profile and the corrosion risk it brings. The Biogas Enclosed Flare covers digester gas and general biogas applications where flow is comparatively stable.

In biogas and landfill applications, enclosed ground flares destroy methane and trace contaminants when the gas can't be productively used — during commissioning, maintenance, low-demand periods, or as a backup to upgrading or power generation systems. Methane destruction is critical for greenhouse gas reduction and odour control.

Typical biogas service:

• Backup destruction when CHP engines or upgrading units are offline
• Excess gas during high biogas production periods
• Commissioning of new digesters before downstream equipment is ready
• Odour control where venting is not permitted

Enclosed design is preferred at landfill and digester sites because they're frequently near communities, and methane destruction with no visible flame meets both compliance and community-acceptance requirements.