A Static Flux Chamber Design for Evaluation of Gas Flux Through Composite Cover Systems
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EXECUTIVE SUMMARY Validated measurement of landfill gas (LFG) emissions can quantify the effectiveness of landfill cover and LFG collection systems for the management and containment of gases, ranging from trace (< 1%) constituents, such as hydrogen sulfide (H2S), volatile organic compounds (VOCs), and volatile fatty acids (VFAs) to major constituents, such as methane (CH4) and carbon dioxide (CO2). When emitted into the surrounding area, each of these LFG compounds can threaten public health and safety as well as be a nuisance. For example, CH4 and CO2 are strong greenhouse gases (GHGs) and because their emissions from landfills are not insignificant, they can be serious contributors to global warming. CO2 and CH4 are strong GHGs because of their ability to absorb energy and the duration that they stay in the atmosphere. CO2 has a global warming potential (GWP) of 1, while CH4 has a GWP of 28 to 36 over a 100-year period. More, H2S, and some VOCs, gives off a potent “rotten-egg” odor that can be irritating. LFG can also contribute to heat generation and accumulation within landfills; this excess heat has several unwanted implications such as potential damage to gas collection, landfill cover, and landfill liner systems. On a more positive note, gas management and containment can have several benefits, as LFG can be turned into a valuable resource. For example, CH4 can be turned into usable electricity as well as converted into compressed natural gas (CNG), an eco-friendly vehicle fuel, just as CO2 could be captured and sold as compressed gas or dry ice. Landfill cover systems are designed to facilitate gas management and containment, as they retard the migration of LFG to the surrounding environment, reduce nuisance odors, and limit precipitation into the waste body. Such systems are low-permeability (less than 10-7 m/s) barriers at the top of the landfill and often consist of fine-grain, low-hydraulic conductivity soils and sometimes geosynthetics, such as in a composite barrier. The properties of the barrier layer(s), especially those of the geomembrane (GM) used in a composite barrier, can influence gas transport (primarily diffusive flow) through the cover system. There are several techniques used to evaluate the emission rates through landfill cover systems, including indirect measurements, direct measurements, and laboratory simulations. The flux chamber (FC) technique – a direct measurement approach – has been a proven method of landfill emission measurements. Fundamentally, a FC seals a volume above a gas-emitting surface and, as gas diffuses through the surface, gas accumulates within the chamber and concentrations can then be measured over time. FCs can have either a static (closed flux) or dynamic (open flux) design: a static chamber technique does not involve air exchange throughout the chamber, while a dynamic chamber technique involves a clean sweep air. Whether static or dynamic, the FC method is a simple and cost-effective approach to determine emissions at a particular location; however, there are some disadvantages to FC use. Primary issues of FC implementation include the spatial variability and general heterogeneity of emissions at landfills as well as the temporal variability, as atmospheric conditions influence emission rates. These deficiencies tend to underestimate emissions determined from FCs. Another set of deficiencies surround the fact that many of the FC designs presented in literature lack supporting documentation on the methodology and design justifications. While there are some guidelines, there is not a single standard on FC design or FC approaches. The primary objective of this research was to design, construct, and protype a series of static FCs to evaluate very low flux rates of various gas species through a composite cover system. Very low flux is assumed, due to the estimated diffusion coefficients of the cover systems being on the order of 10-13 m2 s-1. This design methodology started first with a constructive review of existing FCs and methods, with a focus on detailing the limitations and deficiencies of historical approaches of FCs; most FCs presented in literature were designed and employed to measure relatively higher rates of gas flux from porous media (e.g., LFG emissions through interim soil cover profiles at municipal solid waste (MSW) landfills). Second, a novel FC design was developed based on this review and the primary objective of low flux evaluation. The considerations in the FC design included the chamber materials, geometry, size, and volume. The final design was chosen to be a static FC, consisting of an acrylic dome stacked on top of layered washers of aluminum, ethylene propylene diene terpolymer (EPDM), and GM. Three different sized FCs of this basic design were developed, in which each FC size was chosen to vary in outer diameter, inner diameter, and height but also chosen to have relatively equal volume-to-area (VA) ratios (ranging between 0.112 m and 0.127 m). These different sized FCs allowed for scale effects to be observed (in addition to flux evaluation). Third, a large-scale (1.0-m height, 1.2-m diameter) gas flux testing apparatus was designed and constructed to perform a series of FC validation tests in the laboratory. This apparatus consisted of a gas source, a soil column above the gas source, and a GM sealed above the soil column (to simulate a composite barrier system). The FCs could then be sealed on the GM and used to monitor the gas diffusion through the reproduced cover system. This apparatus was designed such that each variable of the system could be changed with relative ease, allowing for gas fluxes to be tested in a variety of scenarios. Variables of the system that could be adjusted include FC size, GM type, soil type(s), soil column thickness, soil moisture, soil compaction, gas source composition, and gas pressure. Fourth, a preliminary model of the large-scale testing apparatus was developed using a finite-element method (FEM) and used to validate the laboratory testing of the FCs. Following the validation from the large-scale testing apparatus and supplemental model, the FCs could be implemented for field-scale evaluation. Overall, the FC presented in this thesis was designed to evaluate very low flux rates of various gas species through a composite cover system. While there are some deficiencies, the design methodology of the FC was well-detailed and supported through a review of FCs. Ongoing and future validation efforts, as described, will be used to support this FC design and its implementation for very low flux rate evaluation.
landfill cover system