Multicomponent landfill gas transport in soil cover: column tests and numerical modelling

A soil column test was carried out to investigate the mechanism of multicomponent landfill gas transport in loess. Four gas components, namely methane (CH4), carbon dioxide (CO2), oxygen (O-2) and nitrogen (N-2), were considered. Based on the dusty-gas model and mass balance equation, a one-dimensional transient model for multicomponent gas transport in landfill layered cover soils was then developed. Methane oxidation in the soil is also considered in the model. The numerical model was solved by the finite-element method-based program Comsol Multiphysics v5.1. The numerical results agree well with those observed from the soil column tests. The parameter analysis shows that ordinary diffusion plays an important role in the transport of multicomponent gases. For methane and carbon dioxide, ordinary diffusion contributes to 97% of the total transport flux in the shallow zone. The effect of ordinary diffusion decreases with the increase in the depth. Advection contributes to 37% of the flux at the bottom of the cover soil. The effect of Knudsen diffusion is relatively weak, which only contributes 0.5-12% to the total flux. This is due to the relatively high gas permeability of the soil. The contribution of advection is comparable to that of diffusion when the gas permeability increases up to 3 x 10(-13) m(2). The maximum methane oxidation rate is found to increase by a factor of 1.8, when influent flux increases from 1 x 10(-5) to 1 x 10(-4) (mol/m(2))/s. The numerical solution can be used for the analysis of multicomponent landfill gas transport in the soils and the design of landfill cover systems with respect to gas pollution control.