Direct Measurement of Landfill Emissions using the Tracer Gas Dispersion Method

Abstract

Methane is a significant greenhouse gas (GHG) with a 100-year global warming potential 28 times that of carbon dioxide. Anaerobic degradation of organic wastes in landfills creates a major source of anthropogenic methane which, when collected, is an important source of renewable energy. However, fugitive emissions of landfill gas are estimated to account for around a third of the total UK emissions of methane.Currently, UK landfill methane emission reporting is based on a national landfill gas generation model (MELMod). Landfill gas collected for energy generation or flaring can be measured, and this is subtracted from the predicted gas generation rate to give the combined amount of fugitive methane emissions and the methane bio-oxidised (to carbon dioxide) in the soils covering the landfill. The ratio of the amount of landfill gas collected to the amount generated is the landfill gas collection efficiency, which is calculated by MELMod. A recent trend in MELMod outputs, where predicted gas generation rates are falling, combined with no drop in the amount of gas collection reported by industry, has started to increase the calculated gas collection rates towards values (around 75%) previously considered unrealistically high. Higher gas collection efficiency rates are to be welcomed, but monitoring of emissions at a selection of landfills would provide evidence for the verification of GHG (MELMod) inventory modelling and allow validate individual site performance.National inventory reporting requires an annual mass of emitted methane. However, techniques for measuring methane emissions provide the emission rate only at the point of measurement. Methane emissions may vary due to a number of factors that include, for example, the season, meteorological conditions and operational controls at the site. Without a better understanding of the factors that influence how representative individual methane emission measurements are, simple extrapolations of such measurements to provide an annual averaged emission rate must be treated with caution.The aim of this research was to quantify the variability in whole site methane emission rates from landfill and to help understand how different operational, meteorological and other site conditions affect these emission rates.The method used to measure whole site methane emissions was the tracer gas dispersion method (TDM). The technique involves the controlled and measured release of a tracer gas (acetylene) from the landfill. Methane and acetylene gas concentrations are then measured downwind of the landfill using a mobile analytical instrument to obtain concentration values across the gas plume. TDM is based on the assumption that the tracer gas released at an emission source will disperse in the atmosphere in the same way as methane emitted from the landfill. The method is well-established internationally and considerable work has been undertaken to help understand the potential errors associated with the method (within approximately ±20%).Three landfills (one closed and restored, Site A; and two operational, Sites B and C) were surveyed under different meteorological and operational conditions using the TDM