As
energy costs increase, wastewater treatment plants throughout
the United States are investigating several
opportunities to implement energy conservation and make greater
utilization of renewable energy sources. This presentation will
highlight studies performed for the Persigo Wash Wastewater Treatment
Plant (WWTP) which serves the City of Grand Junction, Colorado
and the C.C. Williams WWTP operated by the Mobile Area Water
and Sewer System (MAWSS). Both WWTPs use some of the digester
gas for heating feed solids. However, a significant portion of
the digester gas is flared. The heat content of the digester
gas produced approximately 635 BTU/CF. This compares to natural
gas, which typically has a heat content of 1000 BTU/CF. A range
of possible alternative uses were considered for the available
digester gas. These included heating the existing buildings with
a boiler/hot water system fuel by digester gas, producing electricity
using reciprocating engines, microturbines and fuel cells, and
further treatment and off site sale to other users or natural
gas providers. After an initial screening process, three of these
alternatives were analyzed in detail to determine if expenditures
to implement such a system are warranted. The three alternatives
were: 1) Conversion to electricity via microturbines, 2) Conversion
to electricity via fuel cells, and 3) Privatization of digester
gas processing with either sale to a power producer or creating
compressed natural gas fuel for city-owned cars/trucks.
Microturbines
are small, gas-fired, turbines that are packaged into a small
footprint. They have considerably higher efficiencies than
other mechanical electricity producing methods. These units
are designed to have low maintenance and operate on a variety
of fuels including natural gas, propane, digester and landfill
gas. When the gas is burned, it drives a turbine that produces
electricity.
A
fuel cell electrochemically converts hydrogen and oxygen
into water and electricity. They operate similarly to a battery,
where chemicals in the battery are consumed to produce
electricity. Fuel cells use digester gas and oxygen in the “battery”,
which are continually fed to the fuel cell so, unlike a
conventional battery, it can run continually. There are companies
specializing in privatization of digester gas from WWTPs. One
approach for these companies is to clean the gas and produce
pipeline-quality natural gas. |
