American Biogas Council
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Frequent Questions

Is there a wholesale price for renewable natural gas (RNG) sold for vehicular fuel?
Typically, the producer can expect to receive avoided cost of cogeneration (COG, i.e, the commodity) plus a negotiated percentage of the value realized from sale of the RIN (renewable identification number attached to the gallon equivalent of fuel) and LCFS (low carbon fuel standard) credit, if in CA, associated with the amount of gas produced.

That price will of course be variable month to month. A fixed, wholesale price for RNG that will be sold as vehicle fuel doesn't exist at this time. To create it, any seller of RNG vehicle fuel would have to risk (or somehow hedge) their exposure to variable RIN, LCFS and NG commodity pricing (which is based on what the market will bear at any one time) in order to offer a fixed price to the producer.


What is "Anaerobic Digestion"?
Anaerobic digestion is the process by which naturally occurring bacteria, which can only live in places where there is no air, break down organic, biodegradable material over time and converts it to biogas and an inorganic fertilizer. In fact anaerobic means "no oxygen."


What is "Biogas"?
Biogas is the type of gas that is produced in an anaerobic digester. Biogas is mostly made of methane and carbon dioxide plus small amounts of some other gases. Biogas generally contains 55%-75% methane and 44%-24% carbon dioxide, with the other gases making up 1% or less of the mixture. Since biogas is made from organic material, it is sometimes also called "renewable natural gas."

What is organic material?
Organic material is something that was living and can decay. Wasted or spoiled food, plant clippings, animal manure, meat trimmings, and sewage are common types of organic material used with Anaerobic digestion. In contrast, inorganic material includes things like rocks, dirt, plastic, metal and glass.


What kinds of material can be digested in an Anaerobic Digester?
Theoretically, any organic material can be broken down through Anaerobic Digestion. Wasted or spoiled food, plant clippings, animal manure, meat trimmings, and sewage after it's been treated are especially well suited to this type of digestion. Inorganic material such as rocks and dirt, and man-made materials such as plastic, metal cans, and glass, will not be broken down through anaerobic digestion. In addition, woody wastes do not digest well because their strong fibers are naturally resistant to degradation and also the anaerobic bacteria.


biogas yields for different feedstocksWhat materials yield the most biogas?
When put into an digester, fats, oils and greases (industry lingo: FOGs) and food waste create the most biogas. For this reason, many dairy farms that have digesters add local food scraps to the manure in their digesters to increase the amount of biogas produced. Digesting different materials is called co-digestion. (Image source: Basisdaten Biogas Deutchland, The Climate Trust, BioCycle and OWS, Inc.)


What is an "Anaerobic Digester"?
An Anaerobic Digester is a large, air-tight container or tank that contains no oxygen. The tank is filled with organic material and maintained at an optimum temperature for anaerobic bacteria to digest the material. Depending on what you put into it, the contents can be wet or dry.


Does something specific have to be grown to feed into an Anaerobic Digester?
Certain crops like corn and grasses can be used to feed anaerobic digesters, but most organic waste materials like manure, and food scraps work even better than specially grown crops.


How big is an Anaerobic Digester?
From the size of a large refrigerator to the size of small building. The size of the digester depends on how much organic material will be fed into the digester, and how quickly that specific digester can break down the organic material.


What makes an Anaerobic Digester work?
The key is to keep the microscopic bacteria, which break down the material fed into the digester, living and constantly multiplying inside the digester. The bacteria require a sufficient amount of organic feedstock, vitamins, supplements, and some water to live. The bacteria also thrive in a warm environment without large fluctuations in the acidity levels. Like all living things, the bacteria work better without toxins.


Are digesters cold or hot?
To keep the right bacteria alive and multiplying, many of the larger digesters are kept at or above 30 to 38 degrees Celsius or 86 to 100 degrees Fahrenheit. Digesters can also work at temperatures that are both lower and higher than this. Because the bacteria working in the digester are very sensitive to temperature, cooler digesters take more time to break down the biodegradable feedstock, while hotter ones may break down the biodegradable feedstock more quickly.


How long does it take to break down the organic material in the digester?
It varies based on the type of organic material you feed into the digester and the design of the digester. Simpler organic compounds, such as simple sugars, fats and proteins, will digest fairly quickly. More complex organic compounds may take 30 plus days to completely digest, especially fibrous materials like cellulose, the major constituent of paper, paperboard, and card stock and of textiles made from cotton, linen, and other plant fibers), . The operating temperature of the digester also has a significant impact on the time it will take to break down the material.


How is Biogas used?
Biogas is lighter than liquid and rises to the top of the digester From the top of the digester, tubes and pipes allow the biogas to slowly release from the tank into a container so it isn't released into the air. Biogas can be used as fuel for a boiler to generate hot water or steam, or for an engine to power an electric generator to generate electricity. Biogas can be also be "scrubbed", meaning the carbon dioxide and other non-methane "contaminants" are removed, leaving purer methane gas. Methane is the prime component of natural gas so this scrubbed biogas can then be pressurized and injected in an existing natural gas pipeline, liquefied for other storage or even used as a vehicle fuel.


Is Biogas toxic?
You shouldn't breathe biogas. Biogas, due to its methane content, is flammable and should be dealt with in a safe and secure manner. Some of the trace gases that make up about 1% or less of biogas are acidic and can be corrosive to certain kinds of metals and need to be dealt with carefully.


Isn't methane a greenhouse gas?
Methane is a greenhouse gas that is more than 20 times as damaging to the environment as carbon dioxide, but it has to be released into the air and atmosphere to take part in the "greenhouse effect." That is why people who run anaerobic digesters are very careful to contain the biogas and not let it be released into the air—it should be used for energy! One of the benefits of anaerobic digestion is its ability to capture the methane that would normally be emitted into the air if the organic material was left to decompose in an uncontrolled landfill.


Is Biogas a kind of renewable energy?
Yes. Anaerobic digester technology is employed world-wide to create renewable energy. Biogas produced from an anaerobic digester is comprised primarily of methane gas, which can be used instead of fossil fuels to produce energy. This "renewable natural gas" can substitute for fossil fuel natural gas for any need including heating, cooking and driving. Biogas can also be used as fuel to make clean electricity. All of these options provide us with the opportunity to turn organic "waste" and into a valuable renewable energy resource in a sustainable manner.


By "Organic" do you mean "Certified Organic" food?
No. By "Organic" we mean what chemists refer to as an "Organic" material – the carbon-based material from which all living things are made. Man-made materials such as plastic and glass, and some other natural materials like rock, sand, metals, and dirt are called "Inorganic Materials". Inorganic materials will not be broken down in an anaerobic digester.


AgSTARHow many operational Anaerobic Digesters are there in the U.S.?
In the U.S., there are over 191 anaerobic digesters on farms and about 1,500 more operating at wastewater treatment plants. Of the 1,500 wastewater treatment plants (WWTPs) that produce biogas, about 250 use the biogas; for the other 1,250, the biogas is flared. For comparison, there are approximately 65,000 dairy farms in the U.S., although many other types of farms can benefit from digesters. In the wastewater sector, there are over 17,000 wastewater treatment plants, but only 3,500 that process more than 1 million gallons per day (MGD)--the minimum most experts agree is required for a digester to be economically viable for the owner/operator. Sources: AgSTAR, SW Farm Press, U.S. Census and a 2011 report yet to be released


How many Anaerobic Digesters are there in operation the rest of the world?
It is hard to say exactly, but the number is in the hundreds of thousands or over a million. Anaerobic digesters are at work on every continent except Antarctica. Countries like India, China, and some Western European countries have been employing anaerobic digesters for decades.


How can I get involved in promoting Anaerobic Digestion in America?
Contact us at info@americanbiogascouncil.org. If you would like to become a member of the American Biogas Council, or if you would care to learn more about our council, please click on "Become a Member" at the top of this page. Anyone can join and your membership dues will be used to advance biogas and anaerobic digestion as a renewable energy technology in America.
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American Biogas Council | 1211 Connecticut Ave NW, Suite 650, Washington, DC 20036-2701 | 202-640-6595 | info@americanbiogascouncil.org
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