Controlling surface flux in landfill-well coupled flow in perforated horizontal wells

Throughout the existence of human settlements disposing of waste materials historically caused numerous environmental problems. Sanitary landfills are now known as one of the most efficient ways to treat waste with minimal environmental impact. When operated correctly, the landfill can be regarded as a potential source of renewable energy. Past research addressed different aspects of sanitary landfill sites. This study focuses on the simulation of a flow of landfill gas through the porous medium of the waste matrix and in a collection well, with a special emphasis on the influence of the well geometry on the surface mass flux. For this purpose a model containing three layers, namely cover, waste, and gravel is constructed. Two fluid flow types govern the coupled system: porous medium flow and unobstructed flow obeying Darcy’s law and Navier-Stokes equations respectively. The COMSOL solver is utilized to perform the numerical simulations. The GNU Octave software is used for postprocessing and visualization of both solutions. Different well geometries in terms of perforation size and position along the well are evaluated. In the beginning, gravity is excluded in order to focus on the sole impact of well geometry. A two-step assessment is conducted: in the first stage a 1D analytical solution of a simplified problem is utilized to validate the 2D numerical model setting. Then for different well geometry modifications pressure profiles in the well, velocity profiles at the surface and surface mass flux are investigated. Studied modifications and their potential combinations are prioritized based on the observed impact of each modification on the surface mass flux and pressure values throughout the landfill. Prior to stage two, there is a validation process for the utilized computational meshes to guarantee the accuracy of all simulations. In stage two for prioritized modifications and their combinations several sensitivity analyses are conducted on cover permeability, waste permeability, and pump pressure to understand the behavior of the landfill-well system. In the end, the effect of gravity on the system is investigated. The results suggest that well geometry modification–in terms of changing well perforation size and longitudinal distribution–influences the pressure field in the landfill as well as the surface flux. Changing perforation size and position are the first and second priorities, respectively. Moreover, a synergistic effect can be derived from the combination of these modifications. The surface flux increases with larger perforations or a higher number thereof unless the cover permeability is extremely tight. Well production capability decreases as a result of decreasing the number of perforation sets. In terms of landfill-well system properties, cover permeability and suction strength are the main means of control of landfill pressure and surface flux, while waste permeability is not. The gravity effect is shown to be of considerable importance.