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Fossil-fueled Power
Non-Fossil Generation
End-Use Efficiency
Electricity T&D
Carbon Sequestration
Non-CO2 Reductions
Other GHG Reductions

Related topics in this section

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Landfill Methane
Nat. Gas Emissions
Coal Mine Methane
Animal Waste Methane
SF6 Reduction

 

 Animal Waste Methane Energy Recovery

 
 Background


Livestock manure is composed primarily of organic material and water. When manure decomposes in an anaerobic environment (i.e., in the absence of oxygen), methane is produced along with carbon dioxide and stabilized organic material. The major sources of U.S. livestock manure methane are large dairy and cattle operations and hog farms that use liquid manure management systems.

About ninety percent of manure is currently handled as a solid (e.g., in pastures or stacks on dry lots) but this portion produces only about twenty percent of total methane emissions from manure. Liquid manure management systems, such as lagoons, ponds, tanks, or pits, handle a much smaller portion of total manure but comprise 80 percent of total methane emissions from manure.

Dairy and swine farms are typically the only livestock farms where liquid and slurry manure systems are used. Beef, poultry, and other livestock farms generally don’t use liquid manure systems, and as a result produce much less methane.

The general trend in manure management - in part because of the trend towards larger automated farms - is toward increased use of the liquid systems that produce greater quantities of methane emissions. Liquid management is the more cost-effective option for manure management at large farms. As a consequence of this shift to liquid management, combined with changes in animal populations and feed consumption, methane emissions from manure management have been rising steadily over the past few years and are projected to continue rising.

Most of the methane emissions from this source category are from liquid systems. Two general options for reducing emissions from liquid systems are to: (1) switch from liquid management systems to dry systems; or (2) recover methane and use it to produce electricity, heat or hot water.  Switching from liquid to dry management systems is largely impractical for both environmental impact and process design reasons. Dry manure management systems can lead to significant surface and ground water pollution.  In addition, the liquid manure management systems at large dairy and swine farms are integrated with the overall production process, and switching to dry systems would require a fundamental shift in the entire production scheme.

With the use of liquid-based systems, the primary method for reducing emissions is to recover the methane before it is emitted into the air. Methane recovery involves capturing and collecting the methane produced in the manure management system. This recovered methane (a medium Btu gas with about 500-600 Btu/ft3) can be flared or used to produce heat or electricity. Three methane recovery technologies are available:

  Covered anaerobic digesters are the simplest form of recovery system, and can be used at dairy or swine farms in temperate or warm climates. In this system, manure is mixed with water and pumped into outdoor lagoons. The covered lagoons are air-tight and provide the anaerobic conditions under which methane is produced and recovered.
 
  Complete mix digesters present a methane recovery option for all climates. They are heated, constant-volume, mechanically-mixed tanks that decompose medium solids swine or dairy manure (3-8% total solids) to produce biogas and a biologically stabilized effluent. The manure is collected daily in a mixing pit where the percent total solids can be adjusted and the manure can be pre-heated. A gas-tight cover placed over the digester vessel maintains anaerobic conditions and traps the methane that is produced. The produced methane, representing about 8 to 11 percent of the total manure, is removed from the digester, processed, and transported to the end use site. [ Ref.: EPA, AgSTAR Technical Series: Complete Mix Digesters, EPA-430-F-97-004, February 1997]
 
  Plug flow digesters only work with dairy scraped manure and cannot be used with other manures. These are constant volume, flow-through units that decompose high solids dairy manure (>11% solids) to produce biogas and a biologically stabilized effluent. The basic plug flow digester design is a long tank, often built below ground level, with a gas-tight, expandable cover. A gas-tight cover collects the biogas and maintains anaerobic conditions inside the tank. The amount of methane produced is about 40 cubic feet per cow per day.


As noted, the amount of methane produced from aerobic decomposition (dry management) is small in comparison to the emissions from liquid management. Currently, no feasible options exist for reducing methane emissions from dry manure management.

The recovery of methane from manure management systems can significantly reduce the overall emission of greenhouse gases.  Utilities can work with large livestock producers to reduce overall emissions of methane from animal waste lagoons by encouraging producers to cover their lagoons and collect the methane for electricity generation or on-farm fuel.

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 Power Partners Projects

The AES Corporation formed a joint venture with AgCert International plc, called AES AgriVerde, to deploy AgCert’s GHG emissions-reduction technology in selected countries in Asia, Europe, and North Africa. By 2012, AES AgriVerde intends to create an annual production volume of 20 million metric tons of GHG emissions reductions through the reduction of methane. AES AgriVerde will capture methane from agricultural and animal waste products and either destroy it or use it to generate electricity or heat, reducing net GHG emissions from the manure management process by approximately 95 percent.

Dairyland Power Cooperative (DPC), in La Crosse, Wisconsin, is expanding its Evergreen Renewable Energy Program and is on track to reach 10 percent renewable generation by 2015. DPC has 17 MW of wind generation and 22 MW of hydroelectric power and owns a 10.4-MW landfill gas-to-energy plant. In addition, DPC’s animal waste-to-energy program utilizes manure from dairy and swine farms within the DPC system to produce methane for conversion to electricity. Currently, 3 MW of “cow power” are online, and DPC has plans to bring as much as 25 MW of additional capacity online over five years.

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 References, Sources, and Other Useful Data


Environmental Protection Agency, “The AgSTAR Program”
http://www.epa.gov/agstar/index.html

The AgSTAR Program is a program jointly sponsored by the U.S. Environmental Protection Agency (EPA), the U.S. Department of Agriculture, and the U.S. Department of Energy. The program encourages the use of methane recovery (biogas) technologies at the confined animal feeding operations that manage manure as liquids or slurries. These technologies reduce methane emissions while achieving other environmental benefits.

Environmental Protection Agency, AgSTAR Program, “AgSTAR Handbook and Software”
http://www.epa.gov/agstar/resources/handbook.html

This is a comprehensive manual developed to provide guidance on developing biogas technology at commercial farms. The Handbook also contains FarmWare, an expert decision support software package that can be used to conduct prefeasability assessments.

Environmental Protection Agency, AgSTAR Program, “Funding On-Farm Biogas Recovery Systems: A Guide to Federal and State Resources”
http://www.epa.gov/agstar/pdf/ag_fund_doc.pdf

This document provides information about programs and strategies, such as low-interest loans, grants, and tax incentives that can help parties interested in implementing anaerobic digestion technology overcome financial barriers to project development

Environmental Protection Agency, AgSTAR Program, “Managing Manure with Biogas Recovery Systems: Improved Performance at Competitive Costs”
http://www.epa.gov/agstar/pdf/manage.pdf

This 8-page brochure provides background information about anaerobic digestion, and explains how the methane produced from this process can be captured and used to generate heat, hot water, and electricity.

Environmental Protection Agency, “Ruminant Livestock”
http://www.epa.gov/methane/rlep/index.html

Globally, livestock are the largest source of methane from human-related activities – and in the U.S., the third largest source. Livestock production can also result in emissions of nitrous oxide, a very potent greenhouse gas, and carbon dioxide, the most abundant greenhouse gas. Fortunately, there are ways to reduce greenhouse gas emissions from livestock production through management strategies that improve production efficiency and result in lower emissions per unit of milk or meat produced. This website presents information about livestock emissions and how the adoption of improved livestock production practices can help to reduce the emission of greenhouse gases.

U.S. Department of Agriculture, Office of the Chief Economist, “Global Climate Change”
http://www.usda.gov/oce/climate_change/index.htm

The Global Change Program Office (GCPO) operates within the Office of the Chief Economist and functions as the Department-wide coordinator of agriculture, rural and forestry-related global change program and policy issues facing USDA. The Office ensures that USDA is a source of objective, analytical assessments of the effects of climate change and proposed mitigation strategies.

U.S. Department of Agriculture, Office of the Chief Economist,  “U.S. Agriculture and Forestry Greenhouse Gas Inventory: 1990-2001” (March 2004)
http://www.usda.gov/oce/global_change/gg_inventory.htm

The U.S. Agriculture and Forestry Greenhouse Gas Inventory: 1990-2001 (USDA GHG Inventory) is a comprehensive assessment of greenhouse gas emissions and sinks in U.S. agriculture and forests. The USDA GHG Inventory provides extensive, in-depth emissions and sinks estimates for livestock, cropland, and forests, as well as energy consumption in livestock and cropland agriculture.

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