NUMERICAL ANALYSIS OF GAS FLOW WITHIN A MUNICIPAL SOLID WASTE LANDFILL
Numerical analysis and modelling of gas flow within a municipal solid waste (MSW) landfill has the potential to significantly enhance the efficiency of design and performance of operation for landfill gas (LFG) extraction and utilization projects. Over recent decades, several numerical models and software packages have been developed and used to describe the movement and distribution of fluids in landfills. However, for the most part, these models have failed to gain widespread use in the industry because of a number of limitations. Available simple models implement a simple theoretical basis in the numerical solution, for example various approaches that treat the landfilled waste fill purely as a porous medium with the conservation of mass inherently assumed and the ongoing gas generation within the landfill thus ignored. Alternatively, researchers have developed some highly complex models whose use is rendered difficult as a result of the uncertainty associated with the large number of required inputs. Ultimately, more efficient design and operation of well-fields for LFG extraction will provide a benefit in the cost of construction and efficiency of operation as well as with a reduced potential of underground fire resulting from over-pumping or localized excessive wellhead vacuum. Thus, the development of tools for improved understanding and prediction of the gas flows and pressure distribution within MSW will potentially enhance operation of LFG extraction systems at existing landfills as well as the design for new LFG projects. In the simplest terms, a more accurate estimate of the relationship between flow rates and wellhead vacuum will allow for improved analysis of the network of LFG extraction wells, header pipes and valves and blowers. The research described in this thesis was intended to evaluate the error introduced into estimates of the intrinsic permeability of waste from LFG pumping tests if the ongoing gas generation within the landfilled MSW is ignored when the pumping tests are evaluated to yield estimates of the permeability. The City of Saskatoon’s Spadina Landfill Site was chosen as the research site. The landfill contains over six million tonnes of waste of age ranging from days and weeks old to almost 60 years and which continue to generate significant landfill gas. It was determined that during the pumping tests, the estimated volume of gas generated within the radius of influence of each well during the time of pumping ranged from 20% to 97% of the total volume of the gas actually pumped during the time of the pumping, depending on the well and the flow rate in question. Fleming (2009) reported that the landfill is estimated to generate about 250[m3/hour] of landfill gas containing up to 60% of methane. Two different approaches were used to simulate gas flow in the unsaturated waste fill. The first modelling tool was the GeoStudio 2007 software suite. The second approach used a very simple 1-dimensional axisymmetric finite difference (1-D FD) solution to estimate the radial distribution of pressures within the waste associated with gas flow toward an applied wellhead vacuum under conditions where the waste fill is estimated to generate landfill gas at a constant rate per unit volume. This 1-D FD solution was used to compare the flow rate and pressure distribution in the waste with that predicted using widely-available geotechnical software for 2-dimensional axisymmetric flow in an unsaturated porous medium. It is proposed that the correction charts so developed may represent a first step toward a reliable method that would enable such widely used software to be used with a correction factor to enable improved simulations of flow and pressure within the system. The results from both approaches support the previously reported intrinsic permeability values determined for the Spadina Landfill. The 1-D FD solution results show that there is some effect of the gas generation on the best-fit estimates of the value of intrinsic permeability. In addition, the 1-D FD solution shows better fit to the field data (when the gas generation is taken into account) compared with simulations carried out using AIR/W (GeoStudio 2007). Nevertheless, the AIR/W computer package was found to be simple, powerful and intuitive for simulating two-phase flow toward LFG extraction wells. The addition of an option to include a gas generation term in the commercial software package would enable more accurate results for evaluation of flow of landfill gas, however as a first step in this direction, charts of correction factors are proposed.
DegreeMaster of Science (M.Sc.)
DepartmentCivil and Geological Engineering
CommitteeHawkes, Christopher; Sharma, Jitendrapal; Ferguson, Grant
Copyright DateJuly 2012
landfill, gas generation, gas flow, municipal solid waste, numerical analysis.