Environmental Engineering Reference
In-Depth Information
Table 2. Typical Values for Gas Density and Viscosity at (00C)
Gas
Density (kg/m3)
Viscosity (Pa*s)
Air
1.29
1.71 × 10-5
Methane
0.72
1.03 × 10-5
Carbon Dioxide
1.9
1.39 × 10-5
50% CH4+ 50% CO2
1.35
1.21 × 10-5
60% CH4+ 40% CO2
1.19
1.17 × 10-5
Source: US Army Corps of Engineers (2008)
2. Heat value content
During the methanogenic stage, LFG can be expected to have a heating value of 18.6
MJ/m
3
under good conditions. This value is about half that of natural gas. The actual heating
value of the gas from a landfill is a function of the type age of the waste, the type of landfill
cover, and many other factors.
3. Non-methane organic compounds
If a landfill contains a significant amount of municipal solid waste, the gas produced will
consist of approximately 50 percent methane, 50 percent carbon dioxide, and trace amounts
of non-methane organic compounds (NMOC). The concentration of NMOC can range from
200 to 15,000 ppm according to research from the EPA. In the EPA study, ethane, toluene,
and methylene chloride were found at the highest concentrations in landfill gas with average
reported values of 143, 52, and 20 ppm, respectively. The most frequently detected
compounds reported were trichloroethene, benzene, and vinyl chloride. During the design
phase of a landfill closure, historical records or word of mouth information should be
obtained as to the type of wastes that were placed in the landfill and the potential for these
wastes to create off-gas emissions.
4. Water vapor
Gas created during the decomposition of organic compounds typically includes between
4 and 7 percent by volume water vapor. The actual water vapor content of LFG will depend
on the temperature and pressure within the landfill. Temperatures are typically elevated over
ambient during biological decomposition, increasing the evaporation of water into the LFG.
5. Others
Hydrogen is produced during waste decomposition, particularly during the initial
anaerobic conversion of mixed organic acids to acetic acid. Significant amounts of hydrogen
are later consumed in the formation of CH4. Hydrogen is flammable between 4 and 74
percent, by volume, in air. The presence of CO2 affects these ranges although little significant
change occurs near the lower limit of the range
