A two-dimensional gas flow model for layered municipal solid waste landfills

Recovery and utilisation of landfill gas (LFG) can not only reduce the greenhouse effect but also permit the generation of electricity. A good understanding of gas migration from the waste body to the LFG extraction system is required to permit efficient gas recovery. This paper presents an analytical two-dimensional gas flow model to predict the distribution of gas pressure, the CH4 emission flux, the distance of influence and recovery efficiency in landfills. The model is indicative of the flow towards a combined extraction system of vertical wells and horizontal gravel-filled trenches. Moreover, the model has a horizontal layered structure to accommodate anisotropy of municipal solid waste (MSW) and vertical variations in both gas generation rate and permeability. The relevant governing equations for multiple homogenous layers were combined using continuity conditions of gas pressure and flux at the layer interfaces, subjected to realistic boundary conditions, and then solved using an eigenfunction expansion approach. The solution was compared with data available in the literature, and a parametric evaluation was performed to understand the roles of the relevant variables. The results show that, for a non-homogeneous model, the maximum pressure appears at the upper layer of the landfill rather than at the bottom. The location of the maximum pressure depends on the assumptions regarding the vertical distributions of the LFG generation rate and permeability. Additionally, the results can be used to form preliminary estimates of LFG recovery efficiency and overall landfill stability for different waste compositions.