Methane/Biogas

Animal manure, human sewage, or food waste can produce methane during anaerobic digestion. Here, an animal scientist measures a cow for respiration of oxygen and carbon dioxide and production of heat and methane.

Biogas is a renewable natural gas containing approximately 70 percent methane (CH4) and roughly 30 percent carbon dioxide and trace amounts of other gases. Potential agricultural feedstocks for biogas production include manure (hog, dairy, beef, and poultry), food processing (byproducts of meat processing, potato, dairy, cheese whey, sugar beet, and vegetables), and energy crops cut as silage (wheat, barley, triticale, clover, alfalfa, ryegrass, turnips, and corn). Commercial products from biogas production include methane, electricity, heat, steam, fertilizer, chemical recovery, odor reduction, water recycling, carbon dioxide, and, potentially, carbon credits and greenhouse gas credits.

Anaerobic digestion of wastes provides biogas. Biogas contains about 70 percent methane that can be used to generate electricity or as fuel for vehicles or for heating. Any animal manure, human sewage, or food waste can produce methane during anaerobic digestion. Biogas can also be “cleaned” to yield purified methane that can be used in natural gas pipelines. Methane from biogas is an excellent alternative energy source. Using methane for energy [Page 293]helps the environment by replacing the use of nonrenewable fossil fuels with renewable energy and by taking methane out of the atmosphere. Methane is a greenhouse gas (GHG) with 21 times the heating effect of carbon dioxide. Biogas methane is renewable, unlike natural gas that is mined from underground wells and is a nonrenewable fossil fuel. Methane yields from agricultural feedstocks are in the range of 50 to 70 percent. Manure has the lowest yield, while energy crops and food processing have the highest yields. Blending feedstocks can achieve desirable methane yields while solving environmental issues.

Incentivizing the production of renewable natural gas, or biomethane, from sources that include animal manure, landfills, renewable biomass, and agricultural wastes will support expanding the role of renewable energy sources in the existing energy sector, where little opportunity exists today. It will also create new business investment prospects for renewable project developers and the potential to expand rural economies while supporting existing industrial jobs and dramatically reducing carbon emissions. The benefits of using biomethane are as follows:

• Renewable biomethane is a versatile form of bio-energy. It can be used directly at the site of production or placed in the pipeline to support a variety of residential, commercial, or industrial applications.
• Renewable biomethane produced from renewable sources including animal manure, landfills, renewable biomass, and agricultural wastes can be produced at high efficiencies ranging from 60 to 70 percent. Additionally, the technology components to produce renewable gas from this variety of sources exist today.
• Renewable biomethane can be delivered to potential customers via any existing pipeline infrastructure.
• Renewable biomethane can provide a renewable option for many heavy industries, which could save existing industrial jobs in a carbon-constrained economy while creating new rural green jobs to produce renewable biomethane.
• Renewable biomethane production in digesters provides the agricultural sector additional environmental benefits by improving waste management and nutrient control.

Biogas is produced from organic wastes by concerted action of various groups of anaerobic bacteria. Anaerobic biodigesters can be either wet fermentation or dry fermentation. Wet biodigesters, with a life expectancy of approximately 10 years, need to be cleaned out every one to three years. Dry biodigesters, however, do not require cleaning as frequently as wet biodigesters and have a life expectancy of approximately 20 years. Capital cost for a biogas facility that produces one megawatt of power is in the range of $4 to $6 million, depending on the level of infrastructure at the site. Both solid and liquid systems need to develop the infrastructure to increase manure collection and handling to feed the digesters. The frequencies of manure collection need to increase to at least four times a year to feed digesters with relatively fresh material. However, the availability of turnkey technology, expected to develop in the future, will help to reduce the costs involved.

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