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How the Technology Works at Livestock Facilities
Most methane digesters and recovery systems employ one of three basic types of anaerobic digester technology: covered lagoon, plug flow, and complete mix.
Covered Lagoon System
With this type of system, an impermeable cover is placed over a manure lagoon, capturing methane gas. Covers are usually made from an industrial fabric that floats on the surface of the lagoon. The cover is held in place by rope anchors attached to cement footings on the edge of the lagoon. It is sealed by placing a weighted curtain on top of the cover. A perforated pipe inserted under the cover collects the biogas and sends it to a generator or boiler.
Plug Flow Digester System
Plug flow digesters are usually rectangular and built below ground, with an airtight cover made from expandable material or concrete. In a plug flow system, the manure flows to a mixing tank where the amount of total solids can be adjusted by adding water. Manure is added to the mixing tank (the plug) daily. As manure is pushed through the system, biogas accumulates at the top of the digester tank. Internal tank pressure or a pump then moves the biogas through a pipe located in the top of the tank to the end use application. Digester tanks for both plug flow systems and complete mix systems are often heated with recovered heat from engine/generator to improve gas production.
Complete Mix Digester System
Complete mix digester systems consist of an engineered tank called a mixing pit, located above or below ground. In cold-weather regions, the tank is usually installed below ground for better insulation. The digester tank is covered with a fixed lid, often from poured concrete. Manure is collected in the mixing pit either by a gravity-flow or a mechanical pump system, where it is preheated to improve the anaerobic process and diluted if necessary. The manure is then sent to a reactor digester tank to be mixed, where a mechanical prop or blade system is used to keep the manure solids in suspension. As the biogas builds at the top of the digester tank during the anaerobic process, methane is removed from the digester by pipe and transported to the end use application. The warmer the manure is in the mixing pit and digester, the shorter the retention time and the greater the biogas production.
Once methane is captured, several options are available for using the biogas as an energy source, depending on the facility's size, system costs, amount and quality of biogas produced, and the facility's energy needs.
Combustion Engine/Generator
An internal combustion engine that has been modified to operate on methane gas turns a generator, producing electricity. Many engine/generator applications can also provide for the capture of waste heat from the engine water jacket and exhaust.
Microturbine
A microturbine system incorporates a compressor, recuperator, combustor, turbine and permanent magnet generator to produce electricity. The rotating components are mounted on a single shaft, supported by bearings that rotate at up to 96,000 rpm. Microturbines are known for their ultra-low emissions, minimal maintenance and quiet operation.
Boiler
A boiler is often a round sealed tank used to burn fuel to heat water or create steam. A burner is placed at the bottom of the tank or against the tubes of a heat exchanger to transfer heat from the flame. At livestock confinement facilities, boilers are used to heat water for cleaning and radiant heat systems. Boilers are a good choice at small facilities where lower biogas production cannot support an electricity generation system.
Cooling and Refrigeration
Biogas may be used to operate chillers, an attractive option for dairy farmers where up to 30 percent of electricity needs are for cooling milk. Electricity can also be produced with a generator to operate conventional cooling equipment.
Fuel Cells
A fuel cell is an electrochemical device that converts hydrogen and oxygen into electricity and heat. Methane from landfills, animal waste or other fuels with high methane content can provide the hydrogen for the fuel cell.
Direct Combustion
In a direct combustion system, nearly dried manure is burned as a fuel source. Burners are designed for the various moisture levels found in recovered manure and the amount of contaminants or dirt in the fuel. Air quality can be a concern with direct combustion systems.
Methane Energy Recovery System Costs
In Iowa, systems have averaged $350,000 for a 700-cow dairy operation and $500,000 for a 5,000 head farrow-to-wean swine operation. Engineering and design work have averaged $31,000 per system. Costs of design and construction are often higher than other forms of construction because these systems are relatively new. Currently, few contractors or farm managers have experience in the design or construction of methane energy recovery systems. In the future, as the technology becomes more commercialized and expertise grows, system costs should decrease.
DNR Contact
Jim Bodensteiner
(515) 281-8416
Jim.Bodensteiner@dnr.state.ia.us
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