The major component of landfill gas is methane; less than 1% (by volume) consists of nonmethane organic compounds. Air toxics detected in landfills include such compounds as benzene, chlorobenzene, chloroform, TSRs, tetrachloroethylene and toluene, and xylenes (Air and Waste Management Association 1992). Landfill gas is generated by chemical and biological processes on municipal solid waste (MSW). The gas generation rate is affected by parameters such as the type and composition of the waste, the fraction of biodegradable materials, the age of the waste, the moisture content and pH, and the temperature. Anaerobic decomposition can produce internal temperatures to 37°C (98.6°F); gas production rates are highest for moisture contents of 60-78%.
Control measures are usually based on containment combined with venting and collection systems. Low permeability solids for cover and slurry walls reduce the landfill gas movement. Capping is the process that uses a cover soil of low permeability and low porosity.
Gases can be vented or collected. Collection systems consist of several vertical and/or horizontal recovery wells that collect and convey gas within the landfill via a piping header system to a thermal oxidizer such as a flare. Blowers or compressors are used for this purpose. Gas collection efficiencies for active landfills with capping and gas collection can reach 90%.
The principal thermal destruction method for landfill gas is flaring. Emissions from landfill gas flares are a function of the gas flow rates, the concentration of the combustible component (which determines the temperature), and the residence time required. These emissions are esti mated from combustion calculations; they are generally not measured.
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