Environmental Engineering Reference
In-Depth Information
VII.
L
ANDFILL
G
AS
E
NERGY
S
YSTEMS
With the recognition of the formation of landfill gas and its associated hazards, and the
potential to utilize the energy content of the gas, the modern landfill site is designed to trap
the gases for flaring or use in energy recovery systems, particularly for the landfilling of
biodegradable municipal solid waste in non-hazardous waste landfills. The priority for control
of the gases is to protect the environment and prevent unacceptable risk to human health, and
a landfill gas control system is therefore required. In addition, control mechanisms are
required to minimize the risk of migration of the gases out of the site.
Figure 8 shows a schematic diagram of a landfill gas energy recovery project. The energy
recovery technology is based around the gas collection system and the pre-treatment and
power generation technology. Each of those three systems is explained separately in details.
Figure 8. Schematic diagram of a landfill gas energy recovery scheme. Source: Brown and Maunder
1994.
A. LFG Collection System
Once gases are produced under the landfill surface, they generally move away from the
landfill. Gases tend to expand and fill the available space, so that they move, or "migrate,"
through the limited pore spaces within the refuse and soils covering the landfill. The natural
tendency of landfill gases that are lighter than air, such as methane, is to move upward,
usually through the landfill surface. Upward movement of landfill gas can be inhibited by
densely compacted waste or landfill cover material (e.g., by daily soil cover and caps). When
upward movement is inhibited, the gas tends to migrate horizontally to other areas within the
landfill or to areas outside the landfill, where it can resume its upward path. Basically, the
gases follow the path of least resistance. Some gases, such as carbon dioxide, are denser than
air and will collect in subsurface areas, such as utility corridors. Three main factors influence
