Stationary gas flow to a vertical extraction well in MSW landfill considering the effect of mechanical settlement on hydraulic properties
Résumé
Biogas recovery is a main concern in landfill management since it controls greenhouse gas emission from these facilities. Moreover, it is a key issue for the efficiency of energy from waste technologies associated with the emerging landfill bioreactor. In such enhanced biodegradation condition, the biogas production should be significantly higher than in classical landfill facilities. The design of biogas collection system thus requires a good understanding of biogas migration from the waste body to the extraction well. This paper presents a theoretical study of biogas flow in an axi-symmetric domain around a vertical extraction well taking into account various cap cover designs. A set of laboratory experimental data is used to describe the effect of mechanical settlement on the waste hydraulic properties. In order to focus on the effect of mechanical settlement, the coupling of biogas and leachate flow is neglected considering a constant gravimetric moisture content. The comparison of the resulting heterogeneous medium with an equivalent homogeneous medium exhibits a very sensitive effect of settlement on gas pressure and velocity distributions. A general trend to improve the well collection efficiency is observed but should be tempered because in situ and laboratory hydraulic properties may differ and because the effect of air entry on the anaerobic bioreaction should be considered. The effect of operating parameters (well suction, waste thickness, biogas generation rate) is evaluated with a parametric study that could be summarised using simple shape factors.