Environmental Engineering Reference
In-Depth Information
B. Landfill Gas Composition
LFG is the product of microbiological decomposition of land-filled garbage. The
microorganisms turn complex organic compounds in garbage into methane, carbon dioxide,
and trace amounts of other compounds. The composition of landfill gas depends on the
activity of the bacteria involved, the available substrate and other factors. Landfill gases can
be classified into three groups: (1) major components which consist of methane and carbon
dioxide; (2) minor components which consist of ammonia, hydrogen, hydrogen sulfide,
nitrogen, and carbon monoxide; and (3) trace compounds known as 'trace gases', mainly
volatile organic compounds (VOC) (Tchobanouglous et al. 1977). Table 3 shows the
composition of major and minor compounds in landfill gases, whereas Table 4 shows the
concentration of various VOC in landfill gases (Tchobanouglous et al. 1993).
Methane comprises about 50- 55% of LFG, while 40-45% of LFG is carbon dioxide.
Methane is the most reduced organic molecule. In other words, no further conversions to
simpler organic molecules are possible once methane has been produced. It is produced as an
end product of anaerobic metabolism. Methane is a short-lived GHG with an atmospheric
lifetime of approximately 12 years compared to over 100 years for carbon dioxide. It is 21
times more potent as a GHG, kilogram for kilogram, than carbon dioxide.
The balance of the input rate and the removal rate determines atmospheric concentrations
of GHG. There will be a greater impact by concentrating on methane in the medium-term
because it is short lived in the atmosphere and has a high global warming potential (GWP).
Some 60 percent of methane emissions come from anthropogenic sources, with around 40
percent from natural sources.
Over 550 trace gases have been identified to date, and doubtless more will yet be
discovered. The trace components have chemical or physical properties that differ
significantly from the bulk gases. Also, it is known that some of these trace components,
when present above threshold concentrations, cause physiological effects and thus have
potential health impacts. Just sixty two landfill gas trace substances were then more recently
identified within the landfill gas source-term as those from the list of 500, being likely to be
present at a significant concentration and to be worthy of further consideration.
Table 3. Typical Composition of Landfill Gas
Component
Percent (volume basis)
Methane
45 - 65
Carbon dioxide
40 - 60
Nitrogen
2 - 5
Oxygen
- 1
Sulfides
0 - 1
Ammonia
- 1
Hydrogen
0 - 0.2
Carbon monoxide
0 - 0.2
Trace constituents
0.01 - 0.6
(Source: Tchobanouglous et al., 1993)
