Numerical Study of Landfill Gas Emissions through Three Earthen Landfill Covers

Abstract

The effects of different types of earthen landfill covers and vapor on landfill gas emissions are not well understood. In this study, an existing theoretical model of water and multicomponent gas (methane, carbon dioxide, nitrogen, and oxygen) flow was improved by incorporating vapor flow. The model was validated using a published soil column test simulating landfill gas emissions through landfill covers. Numerical simulations were conducted to investigate landfill gas emissions through a single clay-layer cover, a cover with capillary barrier effects (CCBE), and a three-layer cover during continuous drying. The CCBE retained the lowest water content after drying, whereas the bottom clay layer in the three-layer cover stayed almost saturated due to the protection afforded by the top two layers. Accordingly, methane emissions from the CCBE were the highest, whereas those from the three-layer cover were the lowest. Ignoring vapor will predict a drier soil state, and hence will cause methane emissions to be overestimated by 1–2.5 times. Effects of vapor became more significant as the overall gas coefficient of permeability of the cover increased.