Manure is an unavoidable by-product of housing livestock, and it can create problems by emitting methane, odors and carbon dioxide into the air.
Biogas recovery systems can help to solve these problems by turning manure, or feedlot biomass, into a biogas to produce power.
Some of the financial and environmental benefits of biogas recovery systems include reduced greenhouse gas emissions, lower fossil fuel costs and lower manure disposal costs.
AgSTAR, a voluntary program run by the U.S. Environmental Protection Agency, predicts that about 8,200 swine and dairy operators in the United States could support these biogas recovery systems, and these operators could generate more than 13 million megawatt-hours (MWh) of energy per year, with swine operators alone generating 6,341,527 MWh/year.
This could replace 1,670 megawatts (MW) of fossil fuel.
Some poultry operations could install these systems as well.
Anaerobic digestion refers to the process of placing wet cattle and hog manure into an oxygen-free environment such as a digester tank or covered lagoon.
As the organic matter decomposes, bacteria systematically break it down into biogas, which can then be captured and used to create electricity, heat and hot water.
Biogas recovery systems consist of four parts: a digester, a gas-handling system, a gas-use device and a storage tank for effluent manure.
The first step in the process is to place large amounts of liquid or semi-solid manure into the digester.
AgStar recommends that processors only use these systems on manure that has a maximum of 10-15 percent total solids in its makeup.
Biogas captured by the system generally contains 60-70 percent methane and 30-40 percent carbon dioxide.
According to research produced by Iowa State University, this biogas has a heating value of approximately 600-800 Btu/ft^3.
The gas-use device can use captured methane to run a generator to create power.
Types of digesters
A few different types of commercial biogas recovery systems currently exist for operations with different manure-handing needs.
A covered lagoon digester consists of a lagoon with a fitted cover designed to trap biogas. These digesters are ideal for operators who handle manure with 0.5-3 percent total solids content.
Complete mix digesters, on the other hand, work best with slurry manure that has a 3-10 percent total solids content.
These digesters consist of a heated concrete or steel tank with a tight cover. The slurry mixtures in these digesters should be mixed periodically.
Plug flow digesters are long tanks, most often placed below ground level. They work best for thick manure consisting of 11-13 percent total solids content. AgStar recommends only using plug flow digesters for dairy manure.
The cost of an open-air lined lagoon with treatment as well as storage functions can range between $200 and $400 per animal unit (1,000 pounds of live animal weight).
According to AgStar, these systems can begin to produce enough power and generate renewable energy credits to pay for themselves within 3 to 7 years.
After that time they can start to produce revenue, unlike conventional waste removal systems, which can range in price between $60 and $300 per AU.
Because anaerobic digesters result in reduced greenhouse gas emissions, many state and federal organizations offer grants, loans, tax credits and production incentives to install this technology.
One example is the federal Environmental Quality Incentives Program, which can provide up to 75 percent of the funding for conservation practices on an animal operation
Biogas can be used as a fuel source for boilers, chillers or heating systems.
It can also be combusted within an engine generator or purified into pipeline grade methane to produce electricity, which can then be resold to the local power grid or used to power on-farm operations.
Capturing biogas and using it for energy doesn’t just produce power; it also lowers methane-related greenhouse gas emissions.
According to AgStar, more than 30 states award operations using biogas recovery systems with renewable energy credits. Some operations can even receive payments for their reduced greenhouse gas emissions.
Anaerobic digestion systems also produce solids, which can be reused in fertilizers, compost and livestock bedding.
Much like the excess electricity that can result from these systems, these solids can also be used on an operation or sold to a third party.
Finally, biogas recovery systems also typically produce less odor than traditional manure management systems. Processors are less likely to need to resort to costly odor control methods like aeration.
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