Landfill Methane Outreach Program
- How is landfill gas generated?
- What components make up landfill gas?
- How are nonmethane organic compounds generated in landfill gas?
- At what concentrations are nonmethane organic compounds typically found in uncontrolled landfill gas?
- Can landfill gas combustion be used as an energy resource?
How is landfill gas generated?
Landfill gas (LFG) is generated during the natural process of bacterial decomposition of organic material contained in municipal solid waste (MSW) landfills. A number of factors influence the quantity of gas that a MSW landfill generates and the components of that gas. These factors include, but are not limited to, the types and age of the waste buried in the landfill, the quantity and types of organic compounds in the waste, and the moisture content and temperature of the waste. Temperature and moisture levels are influenced by the surrounding climate.
What components make up landfill gas?
By volume, LFG is about 50 percent methane and 50 percent carbon dioxide and water vapor. It also contains small amounts of nitrogen, oxygen, and hydrogen, less than 1 percent nonmethane organic compounds (NMOCs), and trace amounts of inorganic compounds. Some of these compounds have strong, pungent odors (for example, hydrogen sulfide). NMOCs consist of certain hazardous air pollutants (HAPs) and volatile organic compounds (VOCs), which can react with sunlight to form ground–level ozone (smog) if uncontrolled. Nearly 30 organic hazardous air pollutants have been identified in uncontrolled LFG, including benzene, toluene, ethyl benzene, and vinyl chloride. Exposure to these pollutants can lead to adverse health effects. Thermal treatment of NMOCs (including HAPs and VOCs) and methane through flaring or combustion in an engine, turbine, boiler, or other device greatly reduces the emission of these compounds.
How are nonmethane organic compounds generated in landfill gas?
NMOCs are contained in discarded items such as household cleaning products, materials coated with or containing paints and adhesives, and other items. During the waste decomposition process, NMOC can be stripped from the waste by methane, carbon dioxide, and other gases and carried in LFG. Three different mechanisms are responsible for the production of NMOCs and their movement into LFG: (1) vaporization (the change of state from liquid or solid to vapor) of organic compounds until the equilibrium vapor concentration is reached, (2) chemical reaction of materials present in the landfill, and (3) biological decomposition of heavier organic compounds into lighter, more volatile constituents.
At what concentrations are nonmethane organic compounds typically found in uncontrolled landfill gas?
Concentrations of NMOCs in uncontrolled LFG can vary depending on several factors, including the type of waste discarded in the landfill, the climate surrounding the landfill, and the physical properties of the individual organic compound. EPA's Compilation of Air Pollutant Emission Factors (AP–42) presents the default concentration of NMOC as 595 parts per million by volume (ppmv). Of this total, 110 ppmv are considered HAP compounds, according to default concentrations in AP–42. Therefore, total uncontrolled concentrations of organic HAPs at MSW landfills are typically less than 0.02 percent of the total LFG. The Standards of Performance for New Stationary Sources (NSPS) and National Emission Standards for Hazardous Air Pollutants (NESHAP) regulations require combustion of NMOC, a surrogate for organic HAP, at a destruction efficiency of 98 percent or to an outlet concentration of 20 ppmv NMOC.
Can landfill gas combustion be used as an energy resource?
LFG can be an asset when it is used as a source of energy to create electricity or heat. It is classified as a medium-Btu gas with a heating value of 350 to 600 Btu per cubic foot, approximately half that of natural gas. LFG can often be used in place of conventional fossil fuels in certain applications. It is a reliable source of energy because it is generated 24 hours a day, 7 days a week. By using LFG to produce energy, landfills can significantly reduce their emissions of methane and avoid the need to generate energy from fossil fuels, thus reducing emissions of carbon dioxide, sulfur dioxide, nitrogen oxides, and other pollutants from fossil fuel combustion.