Page 350 - Materials Chemistry, Second Edition

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The Sanitary Landfill 321
Passive Gas
Venting vent flare Barrier
trench wall

E E E
E

FIGURE 10.16 Passive gas control
systems showing venting to the atmos-
phere by convective forces, and a barrier
wall (U.S. EPA, 1994).

systems are more commonly used and extract gas from a landfill by using a blower to pull
gas out. The gas may be recovered for energy conversion, treated, or combusted in a flare system
(Figures 10.17a–c)(U.S. EPA, 1985). Gas extraction wells may be installed within the landfill cells
or beyond the landfill, in nearby extraction trenches (Figure 10.18). Active systems are not as sen-
sitive to freezing or saturation of cover soils as are passive systems.
The capital, operation, and maintenance costs of active gas systems are clearly higher than for
passive systems. These costs continue throughout the postclosure period. It is possible to convert
active gas controls into passive systems when gas production diminishes.
When designing the gas control system, several other practical issues must be taken into
account. For example, construction materials may be indirectly affected by the elevated tempera-
tures within a landfill unit as compared with the relatively cooler ambient air. Leachate water con-
taining corrosive and toxic constituents may condense within the plumbing and adversely affect
construction materials. Provisions for managing condensate should be incorporated to prevent accu-
mulation. The condensate can be returned to the landfill.

10.4.17 GAS UTILIZATION
Current EPA regulations under the Clean Air Act require many larger landfills to collect and com-
bust landfill gas. Several compliance options are available, including flaring the gas or installing a
landfill gas recovery and utilization system. There are a number of environmental and economic
benefits to recovering landfill gas. Gas recovery systems reduce landfill gas odor and migration,
reduce the danger of explosion and fire, and may be used as a source of revenue that may help to
reduce the cost of closure. Raw landfill gas, requiring removal of only water and particulates, may
be used for heating small facilities. A fairly concentrated and cleaned gas can be used for both water
and space heating as well as lighting, electrical generation, co-generation, and as a fuel for indus-
trial boilers. Landfill gas is also upgraded to pipeline standards and can be sold to local utilities
(SWANA, 1992).
According to the U.S. EPA (2002), 0.9 million MT (1 million tons) of MSW in a landfill gen-
3
3
erates about 8.5 m min (300 ft /min, cfm) of landfill gas that can then generate 7,000,000 kWh
(kilowatt hours) per year, energy sufficient to power 700 homes for a year. In a broader environ-
mental sense, utilizing 300 cfm/year of landfill gas yields the same reduction in greenhouse gases
as removing 6100 automobiles from the road for 1 year.