Environmental Engineering Reference
In-Depth Information
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Gas production.
A landfill must be producing gas, and this gas must contain
chemicals that are present at explosive levels.
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Gas migration.
The gas must be able to migrate from the landfill. Underground
pipes or natural subsurface geology may provide migration pathways for landfill gas.
Gas collection and treatment systems reduce the amount of gas that is able to escape
from the landfill.
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Gas collection in a confined space.
The gas must collect in a confined space to a
concentration at which it could potentially explode. A confined space might be a
manhole, a subsurface space, a utility room in a home, or a basement. The
concentration at which a gas has the potential to explode is defined in terms of its
lower and upper explosive limits (LEL and UEL). The concentration level at which
gas has the potential to explode is called the explosive limit. The LEL and UEL are
measures of the percent of a gas in the air by volume. At concentrations below its
LEL and above its UEL, a gas is not explosive. However, an explosion hazard may
exist if a gas is present in the air between the LEL and UEL and an ignition source is
present.
Methane is the constituent of landfill gas that is likely to pose the greatest explosion
hazard. Methane is explosive between its LEL of 5% by volume and its UEL of 15% by
volume. Because methane concentrations within the landfill are typically 50% (much higher
than its UEL), methane is unlikely to explode within the landfill boundaries. As methane
migrates and is diluted, however, the methane gas mixture may be at explosive levels.
Table 6. Potential Explosion Hazards from Common Landfill Gas Components
Component
Potential to pose an explosion hazard
Methane is highly explosive when mixed with air at a volume between its LEL
of 5% and its UEL of 15%. At concentrations below 5% and above 15%,
methane is not explosive. At some landfills, methane can be produced at
sufficient quantities to collect in the landfill or nearby structures at explosive
levels.
Methane
Carbon dioxide
Carbon dioxide is not flammable or explosive.
Nitrogen dioxide
Nitrogen dioxide is not flammable or explosive.
Oxygen
Oxygen is not flammable, but is necessary to support explosions.
Ammonia is flammable. Its LEL is 15% and its UEL is 28%. However,
ammonia is unlikely to collect at a concentration high enough to pose an
explosion hazard.
Ammonia
Potential explosion hazards vary by chemical. For example, the LEL of
benzene is 1.2% and its UEL is 7.8%. However, benzene and other NMOC
alone are unlikely to collect at concentrations high enough to pose explosion
hazards.
NMOC
Hydrogen sulfide is flammable. Its LEL is 4% and its UEL is 44%. However,
in most landfills, hydrogen sulfide is unlikely to collect at a concentration high
Enough to pose an explosion hazard.
Source: Cheremisinoff 2003
Hydrogen sulfide
Also, oxygen is a key component for creating an explosion, but the biological processes
that produce methane require an anaerobic, or oxygen-depleted, environment. At the surface of
the landfill, enough oxygen is present to support an explosion, but the methane gas usually
