Gas Capture Rates: Uncertainty Involving Landfill Gas Emissions

Significant uncertainty - and controversy - surrounds the topic of estimating methane emissions from landfills, both whole-landfill emissions in a given year ("waste in place") and even more so, the lifetime emissions resulting from landfilling a ton of any given waste ("methane commitment"). Proponents and opponents of landfills and landfill gas waste-to-energy have each set forth summaries of research and other position papers arguing their cases. The authors of this toolkit have not taken it upon ourselves to review all of the claims and counterclaims and to report a conclusion, other than to note that there is significant controversy and uncertainty, involving estimates of how much methane is produced by waste in landfills, the timing of that methane generation, and by extension, how much methane is actually emitted to the atmosphere, after gas collection and oxidation.

ThisĀ report, prepared for the Solid Waste Industry for Climate Solutions by SCS Engineers, summarizes a number of research papers and provides the solid waste industry's position on the topics of gas collection efficiency, methane oxidation, and carbon storage.

  • Specific to gas generation and collection efficiency, the report states "The USEPA, state, and local regulators use assumed gas collection efficiencies to calculate landfill emissions in regulating and setting policies for landfills. These assumed efficiencies, usually 75%, are much lower than what many gas system operators believe is typically achieved. In many cases, the lower than actual collection efficiencies results from the use of USEPA models that likely over estimate landfill gas generation, particularly . . . (in) drier climates . . . this reported default collection efficiency is not based on test data and is somewhat dated . . . the industry position supports a landfill gas collection efficiency greater than the 75% default value." The report goes on to propose values, based on cited literature, ranging from 50 - 99% gas collection efficiency, depending on the type of landfill and gas collection system.
  • Specific to methane oxidation (at the landfill surface, where methane is converted to carbon dioxide by biological activity in surface soils), the report states "A report conducted by the USEPA in 2004 stated that 'average oxidation of methane . . . (ranges) from 10 percent to over 25 percent . . ." Due to the uncertainty involved and the lack of a standard method to determine oxidation rate, the USEPA recommends the default factor of 10% by volume methane oxidation for landfills with low permeability cover systems. . . . the industry position on methane oxidation in cover soils is much greater than the default 10%." The report proposes, again based on cited literature, values of 23 - 55% (or 45 g/m2/day - 181 g/m2/day), depending on cover and soil types.

A contrary view is summarized in Appendix A of thisĀ report, prepared by the Sierra Club Landfill Gas to Energy Task Force. The points raised include the following:

  • Landfill industry and EPA estimates of gas capture rates are based on gas collected during "active collections". Significant amounts of gas may be generated and emitted prior to installation of gas collection, and also after the gas collection system is turned off. Thus, "lifetime" or "integrated" gas collection efficiency (gas collected divided by lifetime gas generation) may be much lower than the industry's estimates.
    • Once covered, modern landfilling prevents moisture from penetrating waste. In the absence of moisture, methane production is suppressed. However, the potential to produce methane remains (the waste hasn't fully decomposed). Once the landfill is closed and the cover ultimately fails, more moisture will then enter the landfill and trigger a second wave of gas production, which will go largely unmanaged.
  • Surface measurements of methane are often unreliable. The greatest emissions can occur once a cover develops tears or holes, resulting in localized emissions plumes that may go largely undetected by traditional surface measurements.

Critics also point out that estimates of landfill gas collection efficiency that rely on estimates of gas generation suffer from significant uncertainty in gas generation models. For example, the California Air Resources Board conducted a study that estimated gas capture rates at 46 California landfills in 2006 by dividing gas collection by gas generation, as modeled using first-order decay models. The agency found implied gas collection efficiencies ranging from 6% to 257% (257% of gas assumed to be generated was being collected), indicating that first-order decay models can have significant error.