Environmental Engineering Reference
In-Depth Information
DID YOU KNOW?
Of the estimated U.S. biomass resource of 590 million net tons, only 14 mil-
lion dry tons, or enough to supply about 3000 MW of capacity, are currently
available (USDOE, 2014b).
l
andFill
B
iogas
Landfills can be a source of energy. Some wastewater treatment plants with anaerobic
digesters located close to landfills harness the methane from the landfill and combine
it with methane from their anaerobic digesters to provide additional power for their
plant site. Landfills produce methane as organic waste decomposes in the same anaer-
obic digestion process used to convert wastewater and farm waste slurries into biogas.
Most landfill gas results from the degradation of cellulose contained in municipal and
industrial solid waste. Unlike animal manure digesters, which control the anaerobic
digestion process, the digestion occurring in landfills is an uncontrolled process of
biomass decay. To be technically feasible, a landfill must be at least 40 feet deep and
have at least a million tons of waste in place for landfill gas collection.
The efficiency of the process depends on the waste composition and moisture
content of the landfill, cover material, temperature, and other factors. The biogas
released from landfills, commonly called
landfill gas
, is typically 50% methane,
45% carbon dioxide, and 5% other gases. The energy content of landfill gas is 400
to 550 Btu per cubic foot.
Figure 5.12
s
hows a landfill energy system. Such a system consists of a series of
wells drilled into the landfill. A piping system connects the wells and collects the gas.
Dryers remove moisture from the gas and filters remove impurities. The gas typically
fuels an engine-generator set or gas turbine to produce electricity. The gas can also
fuel a boiler to produce heat or steam. Because waste-generated biogas is consid-
ered to be a dirty gas, as compared to natural gas, further gas cleanup is required to
improve biogas to pipeline quality, the equivalent of natural gas. Reforming the gas to
hydrogen would make possible the production of electricity using fuel cell technology.
IMPACTS OF BIOMASS CONSTRUCTION,
PRODUCTION, AND OPERATION
The combination of constructing biomass facilities (and associated ancillaries),
producing biomass feedstock, and operating biomass energy facilities may have
environmental impacts. For example, construction activities that could have an envi-
ronmental impact include ground clearing, grading, excavation, blasting, trenching,
drilling, facility construction, and vehicular and pedestrian traffic. Additionally,
potential environmental impacts could result from biomass feedstock production
activities such as the collection of waste materials and growth and harvesting of
woody and agricultural crops or algae, preprocessing, and transportation activi-
ties. Finally, operations activities that may have an environmental impact include
