Environmental Engineering Reference
In-Depth Information
2. LFG Transport
2.1. LFG transport mechanisms
Transport of landfill gas occurs by the two principal mechanisms of diffusion and
advection. Transport conditions both within the landfill and for the subsurface surrounding
the landfill must be considered. These transport mechanisms are discussed in the following
paragraphs.
Diffusion
Molecular diffusion occurs in a system when a concentration difference exists
between two different locations. Diffusive flow of gas is in the direction in which its
concentration decreases. The concentration of a volatile constituent in the LFG will almost
always be higher than that of the surrounding atmosphere, so the constituent will tend to
migrate to the atmosphere. Wind often serves to keep the surface concentration at or near
zero, which renews the concentration gradient between the surface and the interior of the
landfill and thus promotes the migration of vapors to the surface. Geomembranes in landfill
covers will significantly reduce diffusion because the geomembrane prevents gases from
diffusing to the atmosphere.
Specific compounds exhibit different diffusion coefficients. Diffusion coefficients are the
rate constants for this mode of transport and quantify how fast a particular compound will
diffuse. Published diffusion coefficients have been calculated using open paths between one
vapor region (concentration) and another. This type of test is not very representative of the
conditions found in a landfill. In landfills, gases must travel a tortuous path around all the
solids and liquids in its path; thus, the published diffusion coefficients must be used with care.
Advection
.
Advective flow occurs where a pressure gradient exists. The rate of gas
movement is generally orders of magnitude faster for advection than for diffusion. Gas will
flow from higher pressure to lower pressure regions. In a landfill, advective forces result from
the production of vapors from biodegradation processes, chemical reactions, compaction, or
an active LFG extraction system. Variations in water table elevations can create small
pressure gradients that either push gases out (rising tide) or draw gases in (falling tide).
Changes in barometric pressure at the surface can also have an impact on the advective flow
of gas.
2.2. Factors affecting LFG transport mechanisms
LFG transport is affected by the following factors:
Permeability.
The permeability of waste has a large influence on gas flow rates and gas
recovery rates. Coarse-grain wastes exhibit large values of gas permeability and more
uniform gas flow patterns. By contrast, fine-grained and heterogeneous wastes are
characterized by small values of gas permeability and gas flow patterns that are not uniform
throughout the waste mass. Permeability of refuse is often reported in Darcys. One Darcy =
9.85×10-9 cm
2
. Reported values for the apparent permeability of municipal solid waste are in
the range of 13 to 20 Darcys. Water competes with air to occupy pore space within the solid
matrix and ultimately reduces the effective porosity and ability of vapors to migrate through
