LANDFILL GAS EMISSION
Landfill gas contains a high percentage of methane due to the
anaerobic decomposition of organic matter, which can be utilised as a source of
energy. Previously, we will explain the composition and
properties, risks, migration and control of landfill gas.
Climatic and environmental conditions also influence gas
composition. Due to the heterogeneous nature of the landfill, some acid-phase
anaerobic decomposition occurs along with the methanogenic decomposition. Since
aerobic and acid-phase degradation give rise
to carbon dioxide and not methane, there may be a higher
carbon dioxide content in the gas generated than what would otherwise be
expected. Furthermore, depending on the moisture distribution, some carbon
dioxide goes into
solution. This may appear
to increase (artificially) the methane content of the gas measured in
the landfill. A typical landfill gas contains a number of components such as
the following, which tend to occur within a characteristic range:
Methane: This is
a colourless, odourless and flammable
gas with a density lighter than air,
typically making up 50 - 60% of the landfill gas.
dioxide: This is a
colourless, odourless and
non-inflammable gas that is denser than
air, typically accounting for 30 - 40%.
flammability of methane depends
on the percentage of oxygen.
It is, therefore, important to control oxygen
levels, where gas
abstraction is undertaken.
Nitrogen: This is essentially inert and
will have little effect, except to modify t explosive range of methane.
Landfill gas consists of a mixture of flammable, asphyxiating
and noxious gases and may be hazardous to health and safety, and hence the need
Some of the major hazards are listed below:
Explosion and fire: Methane
is flammable in air within the range of 5 - 15% by
volume, while hydrogen is flammable within the range of 4.1 - 7.5% (in
the presence of oxygen) and potentially explosive. Fire, occurring within the waste, can be difficult to
extinguish and can lead to unpredictable and uncontrolled subsidence as
well as production of smoke and toxic fumes.
Trace components: These comprise
mostly alkanes and alkenes, and
their oxidation products such
as aldehydes, alcohols
and esters . Many of them
are recognised as toxicants,
when present in air atconcentrations above occupational exposure standards.
warming: Known also as greenhouse effect, it is the warming of the
earth's atmosphere by the accumulation of gases (methane, carbon dioxide and
chlorofluorocarbons) that absorbs reflected solar radiation.
development, most of the gas
produced is vented
to the atmosphere, provided the
permeable intermediate cover has been used. While biological and chemical
processes affect gas composition through methane oxidation, which converts
methane to carbon dioxide, physical factors affect gas migration. The physical
factors that affect gas migration include:
Environmental conditions: These
affect the rate of degradation and
gas pressure build up.
Geophysical conditions: These
affect migration pathways. In the presence of fractured geological strata or a
mineshaft, the gas may travel large distances, unless restricted by the water
atmospheric pressure, rainfall
and water infiltration rate
affect landfill gas migration.
of void space
in the ground,
rather than permeability,
determines the variability of
gas emission. If the
escape of landfill
gas is controlled and proper extraction system is
designed, this gas
can be utilised as a source of
energy. If landfill gas
is not utilised,
it should be
burnt by means of flaring. However,
landfill gas utilisation can save
on the use of
fossil fuels since
its heating value is approximately
6 kWh/m3 and can be
utilised in internal combustion engines for production of electricity
important that landfill gas is
extracted during the operation phase.
It is extracted out of the
landfill by means of gas wells, which are normally drilled by auger and are
driven into the landfill at a spacing of 40 - 70 m. In addition, horizontal
systems can be installed during operation of the landfill. The gas wells
consist mainly of perforated plastic pipes surrounded by coarse gravel and are
connected with the gas transportation pipe with flexible tubing.
The vacuum necessary for gas
extraction and transportation is created by means of a blower. The most
important factors influencing planning and construction of landfill gas
extraction systems are settling of waste, water tables in landfills and gas