COMBUSTION EQUIPMENT | RENEWABLE ENERGY | Updated August 2026 | 8 min read

Biogas flares burn waste gas produced by anaerobic digestion of organic material at landfills, wastewater treatment facilities, agricultural digesters, and food processing operations. The combustion engineering for biogas service differs from natural gas applications in three important ways: methane concentration varies widely, the gas contains CO₂ that reduces heating value, and trace contaminants including siloxanes and H₂S require specific material and design choices that do not apply to clean natural gas flares.

Hero Process Solutions, founded in 2011 and headquartered in Kellyville, Oklahoma with operations in Midland, Texas, manufactures combustion equipment for biogas, landfill gas, and digester gas applications. This guide covers the engineering decisions that determine whether a biogas flare performs reliably and meets regulatory requirements from day one.

1. Why Biogas Is Different from Natural Gas in a Flare

Pipeline natural gas is a stable, high-BTU fuel with a consistent methane content typically above 90%. Biogas is not. Composition varies by source, feedstock, and operating conditions, and the variability creates combustion challenges that require specific design responses.

Methane content range. Landfill gas typically contains 40-55% methane, with the remainder largely CO₂ plus trace nitrogen, oxygen, and contaminants. Digester gas from wastewater treatment or agricultural anaerobic digesters typically runs 55-70% methane. The lower heating value (LHV) of biogas is roughly 300-600 BTU/scf, compared to natural gas at approximately 1,000 BTU/scf. This matters for flare pilot design because the minimum BTU required for stable pilot operation, and for the main flame to sustain combustion without blowout or flameout, is set by the lowest expected methane concentration.

CO₂ dilution effect. Carbon dioxide does not combust. In biogas, CO₂ acts as a diluent that reduces the heating value per unit volume and absorbs heat in the combustion zone. A biogas flare flame is cooler than a natural gas flame at the same flow rate and tip diameter. Combustion temperature must be sufficient to achieve the required destruction efficiency despite this dilution.

Siloxanes. Siloxanes are organosilicon compounds present in digester gas from municipal wastewater treatment plants and some landfills. When combusted, siloxanes convert to silicon dioxide (SiO₂), which deposits as a white abrasive residue on pilot tips, thermocouples, and internal combustion chamber surfaces. Siloxane deposits cause premature pilot tip failure and can block thermocouple sensor wells, causing false pilot-on readings while the actual pilot is extinguished. Biogas flares for siloxane-bearing gas require inspection and cleaning intervals not needed in natural gas service.

Hydrogen sulfide. Biogas from agricultural digesters and some landfills contains H₂S at concentrations ranging from low tens of ppm to several hundred ppm. At these concentrations, sour service metallurgy per NACE MR0175/ISO 15156 may not be required for the flare components themselves, but H₂S at any concentration accelerates corrosion of carbon steel pilot fittings and small-bore gas supply piping over time. Stainless steel small-bore components are the practical choice for biogas pilot systems to avoid accelerated corrosion in H₂S-containing gas.

2. Combustion Efficiency Requirements for Biogas Flares

The applicable combustion efficiency requirements for a biogas flare depend on the facility type and regulatory classification.

Landfill gas: EPA 40 CFR 60 Subpart WWW and Subpart XXX. Municipal solid waste (MSW) landfills that accept waste after November 8, 1987 and meet the size threshold are subject to Subpart WWW (existing sources) or Subpart XXX (new sources after July 17, 2014). These rules require landfill gas collection and control systems to achieve 98% reduction in NMOC emissions. Flares used as the control device must meet combustion efficiency standards and operational requirements including continuous pilot flame monitoring and records of flare operating parameters.

Wastewater treatment: state rules and EPA CAA requirements. Biogas flares at municipal wastewater treatment plants (WWTPs) are subject to state air quality permit requirements and, for large facilities, EPA New Source Review or Title V permit conditions. The specific combustion efficiency requirement depends on the permit, but 98% destruction efficiency is the standard performance target used in most permit applications.

Agricultural digesters and food processing. Digester gas flares at agricultural operations and food processing facilities are typically regulated under state minor source air permits. Combustion efficiency requirements vary by state, but 95-98% NMOC destruction is the range most states apply for enclosed combustors or controlled flares at these facilities.

For upstream oil and gas sites processing biogas or landfill gas as part of a gas treatment or recovery project, EPA 40 CFR 60 Subpart OOOOb may also apply if the facility meets the definition of a covered production site. The OOOOb compliance resources at Hero Process Solutions cover the intersection of upstream gas control requirements and biogas combustion equipment.

3. Enclosed Combustor vs. Open Flare for Biogas Service

For most biogas applications, an enclosed ground-level combustor is a better technical choice than an open elevated flare. The reasons are specific to biogas gas characteristics.

Combustion temperature control. An enclosed combustor maintains a defined combustion chamber temperature regardless of ambient wind conditions. For low-BTU biogas with variable methane content, maintaining adequate combustion temperature (typically 1,400°F or higher) requires a controlled environment that an open flare flame cannot consistently provide in crosswind conditions.

Supplemental fuel integration. Most enclosed combustors can integrate a supplemental fuel gas burner that automatically fires when the biogas BTU falls below the threshold required to maintain combustion chamber temperature. This auto-supplement feature is straightforward to implement in an enclosed combustor and much more complex to engineer reliably in an open flare tip design.

Visible flame elimination. Open flares produce a visible flame that may be objectionable or restricted at urban landfills, municipal WWTPs, or agricultural facilities near residential areas. Enclosed combustors eliminate the visible flame and substantially reduce noise and light emission compared to elevated open flares.

Lower regulatory reporting burden. Many state air permits for biogas flares specify combustion temperature as the compliance parameter for enclosed combustors, rather than requiring periodic EPA Method 25C combustion efficiency testing. Maintaining a documented combustion temperature above the permit threshold is operationally simpler than scheduling and passing periodic source tests.

The vapor combustors from Hero Process Solutions are available in configurations for biogas, landfill gas, and digester gas service, with optional supplemental fuel integration and combustion temperature monitoring outputs compatible with facility SCADA systems.

4. Sizing a Biogas Flare for Variable Flow

Biogas production rates are not constant. Landfill gas flow increases with waste mass and decreases in cold weather. Digester gas flow varies with feedstock loading, temperature, and retention time. A biogas flare must be sized to handle both the maximum expected flow and the minimum flow at which the system must maintain stable combustion without operator intervention.

Turndown ratio is the ratio of maximum to minimum controllable flow. A biogas flare with a 10:1 turndown can operate stably down to 10% of its maximum design flow. For landfill applications where gas production changes significantly across seasons and as the landfill ages, a high turndown ratio is operationally important. Modulating gas control valves, staged burner designs in enclosed combustors, and variable-speed blowers on air-assist systems are the engineering tools used to achieve high turndown in biogas service.

Pilot sizing for biogas flares must be based on the minimum expected BTU content of the waste gas, not the average. A pilot sized for 55% methane biogas will not reliably ignite 40% methane gas from the same landfill during a cold-weather low-production period. Contact Hero Process Solutions at sales@hero-ps.com or (918) 941-2166 to discuss turndown and pilot sizing for a specific biogas application.

5. Biogas Flare Equipment from Hero Process Solutions

Hero Process Solutions supplies combustion equipment for the full range of biogas and landfill gas applications. The product line relevant to biogas service includes:

Enclosed vapor combustors for digester gas and landfill gas in configurations from small municipal WWTP installations to large regional landfill projects. Available with supplemental fuel burners, combustion temperature monitoring, and variable-speed blower control. See the combustors page for configurations and specifications.

Utility and low-flow flare systems for smaller biogas streams where an enclosed combustor is cost-prohibitive and a permitted open flare is acceptable. The utility flares product line includes low-flow configurations appropriate for small agricultural digester applications.

Air-assist flare systems for larger landfill gas or digester gas streams where smokeless operation is required. The air-assist flares product line includes biogas-compatible configurations with stainless small-bore pilot fittings and ignition systems sized for low-BTU gas.

Field services from Hero Process Solutions cover startup, commissioning, and performance testing for new biogas combustion installations.

Article Summary

• Biogas flares combust methane-containing gas from landfills, wastewater treatment digesters, agricultural digesters, and food processing facilities, where methane content typically ranges from 40-70% and heating value is 300-600 BTU/scf.
• Variable methane concentration, CO₂ dilution, siloxane deposits, and low-level H₂S are the four biogas-specific design challenges that distinguish biogas flares from natural gas combustion equipment.
• EPA 40 CFR 60 Subpart WWW (existing) and Subpart XXX (new) require 98% NMOC destruction for landfill gas control devices. State air permits govern biogas flares at WWTPs, agricultural digesters, and food processing facilities, with 95-98% destruction efficiency as the typical requirement.
• Enclosed combustors are generally the better technical choice over open flares for biogas service because they provide controlled combustion temperature, support supplemental fuel integration for low-BTU periods, eliminate visible flame, and simplify permit compliance monitoring.
• Turndown ratio is a critical sizing parameter for biogas flares. Equipment must maintain stable combustion across the full seasonal and operational flow range, from maximum design production to the minimum expected flow.
• Pilot sizing must use the minimum expected methane concentration as the design case. A pilot sized for average BTU content will fail to ignite reliably during low-production, low-BTU periods.
• Hero Process Solutions manufactures enclosed combustors, utility flares, and air-assist flares for biogas and landfill gas service from its Kellyville, Oklahoma facility, with field commissioning support for new installations.

Frequently Asked Questions

What is a biogas flare and where is one required?
A biogas flare is a combustion device that destroys methane and associated combustible gases in biogas streams from anaerobic digestion of organic waste. Biogas flares are required at regulated landfills, municipal wastewater treatment plants, agricultural digesters, and food processing facilities where biogas cannot be used for energy recovery and must be destroyed to meet air quality permit requirements. EPA rules under 40 CFR 60 Subpart WWW and Subpart XXX mandate landfill gas collection and control for qualifying MSW landfills, with flares as an approved control device.

What combustion efficiency is required for a biogas flare?
For regulated landfills, EPA 40 CFR 60 Subparts WWW and XXX require 98% non-methane organic compound (NMOC) destruction efficiency. For wastewater treatment plants and agricultural digesters, state air quality permits set the applicable efficiency requirement, typically in the 95-98% range. Demonstrating compliance generally requires either periodic EPA Method 25C combustion efficiency testing or, for enclosed combustors, continuous combustion temperature monitoring with a permit-specified minimum temperature threshold.

Why do siloxanes cause problems in biogas flares?
Siloxanes are organosilicon compounds present in digester gas from municipal wastewater treatment. When combusted, they convert to silicon dioxide (SiO₂), a hard white abrasive that deposits on pilot tips, thermocouple wells, and combustion chamber surfaces. Deposits on pilot tips cause premature tip erosion and eventual failure. Deposits on thermocouple wells can produce false pilot-on signals while the actual pilot is extinguished. Biogas flares for siloxane-bearing gas require more frequent inspection and cleaning than natural gas flares.

Should I specify an open flare or an enclosed combustor for biogas service?
For most biogas applications, an enclosed combustor is the better choice. Enclosed combustors maintain controlled combustion temperatures despite variable gas BTU content, support supplemental fuel integration for low-BTU periods, eliminate visible flame for urban or residential-proximate installations, and typically offer a simpler compliance monitoring path under state air permits. Open flares are suitable for remote applications where a permit allows visible flame and gas composition is relatively stable.

How should a biogas flare be sized for variable flow?
Size a biogas flare using the maximum design flow for tip and combustion chamber capacity, and the minimum expected methane concentration for pilot sizing and combustion stability verification. Do not use average flow or average BTU content as the design case for stability analysis. A high turndown ratio (10:1 or greater) is important for landfill applications where gas production changes significantly across seasons and as the landfill matures. Modulating control valves and variable-speed blowers on air-assist systems are standard tools for achieving high turndown in biogas service.

Does Hero Process Solutions supply biogas flare systems?
Yes. Hero Process Solutions manufactures enclosed combustors, utility flares, and air-assist flare systems for biogas, landfill gas, and digester gas service. Equipment is available with supplemental fuel integration, combustion temperature monitoring, and control configurations appropriate for landfill, WWTP, agricultural digester, and food processing applications. Contact sales@hero-ps.com or (918) 941-2166 to discuss a specific biogas combustion application.