A gas flow model for layered landfills with vertical extraction wells

摘要

This paper developed a two-dimensional axisymmetric analytical model for layered landfills with vertical wells. The model uses a horizontal layered structure to describe the waste non-homogeneity with depth in gas generation, permeability and temperature. The governing equations in the cylindrical coordinate system were transformed to dimensionless forms and solved using a method of eigenfunction expansion. After verification, the effects of different well boundary conditions and gas extraction systems on recovery efficiency were investigated. A dimensionless double-layer system, consisting of a cover and a waste layer, was also explored. The results show that a constant vacuum pressure boundary condition can be enough to describe a perforated pipe surrounded by drainage gravel with a reasonable value of well radius, such as half the radius of gravel fill. Also, the 7 independent variables (one marked with an asterisk is dimensionless) of a double-layer system can be integrated into 3 dimensionless ones: Cover permeability k_(v1)~*/(Vertical gas permeability of waste k_(v2)~* × Cover thickness h_1~*).-Vacuum pressure p_w × P_(atm)K_(v2)~*/(μR_gT_2 × Gas generation rate of waste s_2) and ln(Well radius r_w~*/(Anisotropy degree of waste k_2~*). The integration is based on the inherent mechanism of this flow system with certain simplification. The effects of these variables are then quantitatively characterized for a better understanding of gas recovery efficiency. Same recovery efficiency can be achieved with different variable combinations. For example, increasing h_1~* (such as doubling it) has the same effect with decreasing K_(v1)~* (such as halving it). Along with the reduction of variables by half, the integration can facilitate the preliminary design, and is a small but important advance in the consideration of MSW non-homogeneity.