Introduction to Biogas
Biogas is produced from the biological process of anaerobic digestion, which occurs when microorganisms decompose biodegradable material in an oxygen free environment. There are multiple digestion technologies for converting the feedstock, consisting of agricultural waste, livestock manure, treated municipal and industrial wastewater, municipal solid waste and other organic substances, into biogas. Digestion technology is composed of four general categories1:
- Complete mix digester – A heated, enclosed tank containing a motor driven device, recirculating pump or compressed biogas to mix the organic waste. These systems are best used for livestock manure with 3-10 percent solid waste.
- Covered anaerobic lagoon digester – This method uses a flexible cover to allow for expansion and contraction of the gases. The recovered methane is then piped to a combustion device for electrical generation or stored for later use.
- Dry digester – Large, silo like containers made from concrete or steel with a rigid cover. This system operates with 20-42 percent total solid waste so drier agricultural materials and manure can be combined with diluted liquid substances.
- Plug flow digester – Consists of a partially or fully buried long, narrow concrete tank and is typically used for the digestion of livestock manure.
Once digestion has occurred and biogas has been produced, it must be processed due to humidity and the presence of impurities. First the saturated biogas is dried by going through a heat exchanger and/or a chiller. If more drying is required a desiccant, such as silica gel, is added. Next, the impurities must be removed as the combustion device can become corroded quickly if they are left in the system. Multiple scrubbing methods are used to treat the biogas depending on the type of impurities present, which consist mostly of sulfide and siloxane. These methods include silica gel, activated carbon, sulfatreat, biofiltration using microbes and an iron sponge (generally wood chips impregnated with iron oxide). After the scrubbing process is completed a medium Btu biogas now exists and can be transported to a combustion generator for electrical production or go through another scrubbing process to remove carbon and other remaining impurities. The second scrubbing process produces a high Btu biogas, or renewable natural gas, that meets pipeline standards. This renewable natural gas is compressed for use in vehicles or shipment via pipeline or tube trailers2.
Biogas as an Alternative Fuel
Several uses for biogas as an alternative fuel exist, such as heat and electrical production, cogeneration, vehicle fuel, fuel cells, hydrogen production, and injection into the natural gas grid. Depending on the method of utilization, varying degrees of biogas quality are required, with the most stringent being pipeline grade renewable natural gas and the least being heat production. Raw biogas typically consists of 50-80% methane and 20-50% carbon dioxide along with trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide3. Although, biogas production can prove to have lower methane levels if the anaerobic digestion process is inefficient. As the necessity increases for more impurities to be removed from the raw biogas to enhance the concentration of methane, investment and production cost become significantly higher.
When consumed for direct heating purposes, biogas can be used for absorption, space and water heating. Utilized in this way, the biogas can contain as little as 20% methane; however, the heating value will be low, reducing efficiency, and corrosion can occur during combustion due to the impurities4. As a result, alterations must be made to the combustion systems to increase durability. The limited scrubbing requirements for biogas heating is beneficial for rural applications, where it can even be used for cooking and lighting.
Methane concentrations must be higher, above 40%, when biogas is used for electrical generation in gas engines and fuel cells; although, a higher methane content is much better as it will increase efficiency5. As of 2008, there were over 420 operating landfill gas recovery plants and 82 under construction. Landfill gas is around 50% methane and requires limited scrubbing, most of the recovered gas is used for on site generation or sold to the grid6. Biogas fuel cells, although still an emerging technology, are starting to make a transition to commercial use. Apple is installing the largest fuel cell system outside of the utilities industry at their data center facility in Maiden, North Carolina, which will consist of 24 200 kW fuel cells powered by biogas from animal waste supplied by surrounding farms. The project is scheduled for completion in November of 20127.
High Btu biogas, or renewable natural gas, is also a viable option for vehicle transportation and has been gaining more momentum over the past decade. For biogas to be suitable as a vehicle fuel, it must be upgraded to approximately 96% methane and compressed between 3000-3600 psi. As of 2009, there were 114,270 CNG vehicles in the United States, mostly buses, and 873 CNG refueling sites as of February 2011, including a CNG station in Anchorage. The amount of natural gas vehicles worldwide in 2011 was 14.8 million, mainly located in the Asia-Pacific and Latin American regions8.
Biogas in Alaska
Typically for anaerobic digestion to occur, hotter temperatures are preferred, which is a major drawback in Alaska. Optimal digestion takes place during the Mesophilic and Thermophilic stages, which range between 70º-104º F and 105º-150º F, respectively. Although Alaska may not be the most ideal environment, one study shows that it is possible to produce biogas in cooler climates. Psychrophiles are microbes that live in permafrost, as they metabolise organic material they release methane as a by-product. High school students in Cordova worked with the University of Alaska Fairbanks to research using Psychrophiles to produce methane that could be used as a heating fuel. To read more about this project, please click here.
At the Anchorage Regional Landfill, methane gas is recovered as bacteria decomposes waste. This gas is then used at a power plant to provide electricity to the Elemendorf and Richardson joint bases (JBER).
The power plant has four 1500 kW GE Jenbacher internal combustion engine generators. These generators have been specifically designed to utilize recovered landfill methane which can various contaminants. The power plant produces enough power to supply JBER's entire emergency load needs, should the base ever lose access to it's normal energy supply.
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