Description
The U.S. EPA greenhouse reporting rules provide an option for site-specific methane oxidation values other than the default value of ten (10) percent. Methane oxidation values are now assigned to be 0, 10, 25, or 35 % depending on the methane flux in grams per square meter per day (g/m2/d) and on the extent and type of landfill cover. A few important issues in the new rule, however, have made it difficult for landfills to actually use these higher than the default 10% oxidation values.
Florida State University, in collaboration with Waste Management Inc., performed a field study to measure methane oxidation in soil covers at the Riverbend landfill in Oregon. The study provided a unique opportunity to monitor methane flow and methane oxidation through different soil profiles at a large scale and under field conditions. To the best of our knowledge, this study is also the only study in which LFG was introduced below soil profiles at a controlled rate. Test pads were constructed as lysimeters with a geomembrane at their bottom. LFG was continuously introduced at the bottom of the 10 x 20 m test pads and through a gas distribution layer placed above the geomembrane. The gas distribution layer consisted of a 15 cm thick gravel layer covered with a non-woven geotextile. A perforated PVC pipe network was also embedded in the gravel layer to distribute LFG across the bottom of the test pads.
The soil profile in Test Pad 1 consisted of a 45 cm (18 inches) thick layer of local soil (silt loam) to simulate an interim cover. This thickness is widely used in most states. The soil profile in Test Pad 2 consisted of a layer of 15 cm (6 inches) of local soil overlain with 30 cm (12 inches) of fresh mulch. Test Pad 2 was constructed to simulate a different type of interim cover (a daily cover overlain by 30 cm of mulch) for erosion control and possible additional oxidation of methane. In Test Pad 3, 15 cm (6 inches) of local soil was placed to simulate a daily cover. Note that due to excessive soil erosion, Test Pad 3 was abandoned 3 months after construction. After completion of test pad construction, LFG was obtained from a gas header and continuously introduced into the bottom of each test pad. The gas inflow rate was maintained at a low rate so that methane loadings into the soil profile of the test pads were in the range of three methane loadings listed in the U.S.EPA GHG Reporting Rule (0-10 g/m2/d, 10 -70 g/m2/d, more than 70 g/m2/d).
An extensive program was designed to monitor the mass balance of methane through each test pad. Site visits and methane emissions and oxidation monitoring started on July 15th 2015 and continued until March 2017. Weekly and then monthly surveys of the composition of the gas below each test section was performed using a field LFG analyzer, and the concentrations of CH4, CO2, and O2 were recorded. The flow rate and composition of the landfill gas entering each test pad, was also monitored on a weekly and then monthly basis. Three flux measurements were performed during each monitoring event. Flow into the test pads was regulated through the use of one valves and estimated by measuring the pressure drop across the orifice plates. The flow rate was then combined with methane concentration at the orifice plate to obtain the mass flux of methane entering each Test Pad. Adynamic chamber technique was used to determine the methane loading into the soil profile (from the gravel layer) at five locations in each test pad, using chambers permanently installed at the soil-gravel interface. Methane surface emission rates from the test pad surfaces were measured using static chambers located on the surface, right above the buried dynamic chambers. Gas samples from the inflow, the gravel layer, and the emitted gas were also obtained and shipped to the laboratory for composition and stable isotopic analysis.
The methane, oxygen and carbon dioxide concentrations in the injected LFG varied from 23.4% to 62.9%, for methane, 0.2% to 13.1% for oxygen and 14.2% to 38.1% for carbon dioxide.
Even though a gas distribution layer was placed at the bottom of each test pad, uneven distribution of methane concentration was observed in the gravel layer. The LFG tended to stay at the elevated end of the test pads and near the point where it was injected.
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