Biogas, produced by the anaerobic digestion of organic material, is primarily a mixture of methane and carbon dioxide typically in equal measures. It may have trace amounts of other compounds, depending on the source organic material. Landfills, sewage treatment plants, anaerobic digesters and manure storage facilities all produce biogas. This gas may be flared, either continuously to reduce the emissions of methane, or intermittently when the energy conversion equipment (e.g., genset, boiler) is not operating. Landfill gas is often burned in an enclosed flare, or an elevated flare with a wind shroud. In the agricultural setting, the flare equipment is generally a simple elevated pipe flare, perhaps equipped with a pilot burner and a wind shroud. The biogas may be provided at low pressure.
The Flare Test Facility (FTF) of Natural Resources Canada (NRCan) is a wind tunnel dedicated to testing flares. For the Biogas tests a simple pipe flare was used, 4 inches diameter, firing mixtures of natural gas with carbon dioxide or nitrogen. Dilution levels were 0%-v, 20%-v, 40%-v and 60%-v. The last two levels cover the likely range for biogas. The emissions were measured and combustion efficiency calculated.
Flares are commonly considered at least 98% efficient if the gas has heat content above 7.5 MJ/Nm3 (200 BTU/scf, a biogas with approximately 20%-v methane, balance inert gas). Our results show that this heat content threshold is not useful for biogas (methane/carbon dioxide mixture). Wind produces a significant decrease in combustion efficiency. The decrease in efficiency is indicated by unburned methane, a strong GHG. The combustion efficiency and destruction efficiency are correlated with a single dimensionless factor that incorporates the wind speed, pipe diameter, mass flow rate of the biogas, heat content (mass-basis) of the biogas, and the flammability limits. Natural gas/nitrogen mixtures have a separate correlation with the same dimensionless group. Sample calculations are presented to show how the annual emissions of methane can be estimated using this correlation and local wind speed data.