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2000 |
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Department of Energy EERE Factsheet of consumer level information on methane and anaerobic digestion. |
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<http://www.eere.energy.gov/consumerinfo/factsheets/ab5.html> |
Page no longer available on EERE website
Methane (Biogas) from Anaerobic Digesters
Methane is a gas that contains molecules of methane with one atom of carbon and four atoms of hydrogen (CH4 ). It is the major component of the "natural" gas used in many homes for cooking and heating. It is odorless, colorless, and yields about 1,000 British Thermal Units (Btu) [252 kilocalories (kcal)] of heat energy per cubic foot (0.028 cubic meters) when burned. Natural gas is a fossil fuel that was created eons ago by the anaerobic decomposition of organic materials. It is often found in association with oil and coal.
The same types of anaerobic bacteria that produced natural gas also produce methane today. Anaerobic bacteria are some of the oldest forms of life on earth. They evolved before the photosynthesis of green plants released large quantities of oxygen into the atmosphere. Anaerobic bacteria break down or "digest" organic material in the absence of oxygen and produce 'biogas" as a waste product. (Aerobic decomposition, or composting, requires large amounts of oxygen and produces heat.) Anaerobic decomposition occurs naturally in swamps, water-logged soils and rice fields, deep bodies of water, and in the digestive systems of termites and large animals. Anaerobic processes can be managed in a "digester" (an airtight tank) or a covered lagoon (a pond used to store manure) for waste treatment. The primary benefits of anaerobic digestion are nutrient recycling, waste treatment, and odor control. Except in very large systems, biogas production is a highly useful but secondary benefit.
Biogas produced in anaerobic digesters consists of methane (50%-80%), carbon dioxide (20%-50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide. The relative percentage of these gases in biogas depends on the feed material and management of the process. When burned, a cubic foot (0.028 cubic meters) of biogas yields about 10 Btu (2.52 kcal) of heat energy per percentage of methane composition. For example, biogas composed of 65% methane yields 650 Btu per cubic foot (5,857 kcal/cubic meter).
Digester Designs
Anaerobic digesters are made out of concrete, steel, brick, or plastic. They are shaped like silos, troughs, basins or ponds, and may be placed underground or on the surface. All designs incorporate the same basic components: a pre-mixing area or tank, a digester vessel(s), a system for using the biogas, and a system for distributing or spreading the effluent (the remaining digested material).
There are two basic types of digesters: batch and continuous. Batch-type digesters are the simplest to build. Their operation consists of loading the digester with organic materials and allowing it to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated.
In a continuous digester, organic material is constantly or regularly fed into the digester. The material moves through the digester either mechanically or by the force of the new feed pushing out digested material. Unlike batch-type digesters, continuous digesters produce biogas without the interruption of loading material and unloading effluent. They may be better suited for large-scale operations. There are three types of continuous digesters: vertical tank systems, horizontal tank or plug-flow systems, and multiple tank systems. Proper design, operation, and maintenance of continuous digesters produce a steady and predictable supply of usable biogas.
Many livestock operations store the manure they produce in waste lagoons, or ponds. A growing number of these operations are placing floating covers on their lagoons to capture the biogas. They use it to run an engine/generator to produce electricity.
The Digestion Process
Anaerobic decomposition is a complex process. It occurs in three basic stages as the result of the activity of a variety of microorganisms. Initially, a group of microorganisms converts organic material to a form that a second group of organisms utilizes to form organic acids. Methane-producing (methanogenic) anaerobic bacteria utilize these acids and complete the decomposition process.
A variety of factors affect the rate of digestion and biogas production. The most important is temperature. Anaerobic bacteria communities can endure temperatures ranging from below freezing to above 135° Fahrenheit (F) (57.2° Centigrade [C]), but they thrive best at temperatures of about 98°F (36.7°C) (mesophilic) and 130° F (54.4°C) (thermophilic). Bacteria activity, and thus biogas production, falls off significantly between about 103° and 125°F (39.4° and 51.7°C) and gradually from 95° to 32°F (35° to 0° C).
In the thermophilic range, decomposition and biogas production occur more rapidly than in the mesophilic range. However, the process is highly sensitive to disturbances such as changes in feed materials or temperature. While all anaerobic digesters reduce the viability of weed seeds and disease-producing (pathogenic) organisms, the higher temperatures of thermophilic digestion result in more complete destruction. Although digesters operated in the mesophilic range must be larger (to accommodate a longer period of decomposition within the tank [residence time]), the process is less sensitive to upset or change in operating regimen.
To optimize the digestion process, the digester must be kept at a consistent temperature, as rapid changes will upset bacterial activity. In most areas of the
Other factors affect the rate and amount of biogas output. These include pH, water/solids ratio, carbon/nitrogen ratio, mixing of the digesting material, the particle size of the material being digested, and retention time. Pre-sizing and mixing of the feed material for a uniform consistency allows the bacteria to work more quickly. The pH is self-regulating in most cases. Bicarbonate of soda can be added to maintain a consistent pH, for example when too much "green" or material high in nitrogen content is added. It may be necessary to add water to the feed material if it is too dry, or if the nitrogen content is very high. A carbon/nitrogen ratio of 20/1 to 30/1 is best. Occasional mixing or agitation of the digesting material can aid the digestion process. Antibiotics in livestock feed have been known to kill the anaerobic bacteria in digesters. Complete digestion, and retention times, depend on all of the above factors.
Producing and Using Biogas
As long as proper conditions are present, anaerobic bacteria will continuously produce biogas. Minor fluctuations may occur that reflect the loading routine. Biogas can be used for heating, cooking, and to operate an internal combustion engine for mechanical and electric power. For engine applications, it may be advisable to scrub out hydrogen sulfide (a highly corrosive and toxic gas). Very large-scale systems/producers may be able to sell the gas to natural gas companies, but this may require scrubbing out the carbon dioxide.
Using the Effluent
The material drawn from the digester is called sludge, or effluent. It is rich in nutrients (ammonia, phosphorus, potassium, and more than a dozen trace elements) and is an excellent soil conditioner. It can also be used as a livestock feed additive when dried. Any toxic compounds (pesticides, etc.) that are in the digester feedstock material may become concentrated in the effluent. Therefore, it is important to test the effluent before using it on a large scale.
Economics
Anaerobic digester system costs vary widely. Systems can be put together using off-the-shelf materials. There are also a few companies that build system components. Sophisticated systems have been designed by professionals whose major focus is research, not low cost. Factors to consider when building a digester are cost, size, the local climate, and the availability and type of organic feedstock material.
In the
Below-ground, concrete anaerobic digesters have proven to be especially useful to agricultural communities in parts of the world such as
Resources
AgSTAR Program
World Wide Web: www.epa.gov/agstar/
The AgSTAR Program is a voluntary effort jointly sponsored by the U.S. Environmental Protection Agency, the U.S. Department of Agriculture, and the U.S. Department of Energy. The program encourages methane recovery at confined livestock operations that manage manure as liquid slurries. A variety of resources are available from the Web site, including the AgSTAR Handbook and Software.
General Bioenergy
Phone: (256) 740-5634; Fax: (256) 740-5535
Email: info@bioenergyupdate.com
World Wide Web: www.bioenergyupdate.com
General Bioenergy distributes the Handbook of Biogas Utilization (2nd edition) on behalf of the Southeastern Regional Biomass Energy Program. The Handbook is a technical resource for commercial agricultural operations. The price of the Handbook is approximately $25.00 plus shipping and handling. Contact General Bioenergy for specific ordering information.
German Appropriate Technology Exchange (GATE)
Deutsche Gesellschaft fuer Technische Zusammenarbeit (GTZ)
OE 4454 - GATE, Postfach 5180
65726 Eschborn, Germany
Fax: 49-6196 797352
Email: gate-id@gtz.de
World Wide Web: www.gtz.de/gate
GATE is a service provided by Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ). They have several publications on biogas, including Biogas Plants (1982, 66 pp., 10.00 EUR) and Engines for Biogas (1998, 133 pp., 12.50 EUR). The former provides design and construction details for simple biogas plants of the type widely used in
Mid-West Plan Service
122
Phone: (800) 562-3618; Fax: (515) 294-9589
World Wide Web: www.mwpshq.org
This Service has several publications on farm manure management and utilization.
Natural Resource Agricultural Engineering Service
Cooperative Extension
152 Riley-Robb Hall
Ithaca, NY 14853-5701
Phone: (607) 255-7654; Fax: (607) 254-8770
Email: NRAES@cornell.edu
World Wide Web: www.nraes.org
This service has numerous publications related to livestock manure management, including a 68 page booklet, Anaerobic Digestion for Dairy Farms, which includes information on how to build them, how much output to expect per pound of manure, and how to modify gas appliances to use the gas. Single copies are $10.00, plus $3.75 shipping and handling.
Volunteers in Technical Assistance (VITA)
Phone: (703) 276-1800
World Wide Web: www.vita.org
VITA has various publications on small-scale biogas systems, including blueprints and designs of digesters used in
This brief was reviewed for accuracy and updated in March 2002.
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