Biogas Basics
Biogas is the result of gases released during the decomposition of organic matter by methanogenic bacteria. Biogas is comprised of multiple gases, the most dominant of which is methane (CH4) followed by a smaller concentration of carbon dioxide (CO2). It may also contain traces of hydrogen (H2), nitrogen (N2), water vapor (H2O), and hydrogen sulfide (H2S). Biogas turns what would be waste -- such as sewage, manure, or food scraps -- into a clean renewable energy. The gas burns cleanly, is smokeless, and is non-toxic. The premise behind biogas is a simple one – organic matter decomposes. The decomposition resulting from anaerobic digestion releases methane. Methane is a potent greenhouse gas and is estimated to be 21 to 23 times more damaging than the same volume of carbon dioxide (Sharma et al 2010; Themelis and Ulloa 2007). When methane (CH4) is burned, the carbon and hydrogen atoms combine with oxygen to create carbon dioxide (CO2) and water vapor (H2O) as the by-product. Technology systems, ranging from the very simple and affordable to the very complex and expensive, can capture methane from decomposing material
and convert it into a usable energy. This process can replace a range of other emission-producing energy sources from firewood to coal to fossil fuels. While burning methane still releases greenhouse gases, CO2 is less potent than the methane and it also avoids the release of CO2 currently sequestered in other sources. Thus, the emission reduction results from the combination of two factors: (1) The methane that would have been emitted regardless through natural processes of decomposition is now harnessed for energy, and (2) The emissions from the energy source replaced by biogas are eliminated, or at least reduced. The technology to harness the energy potential released by anaerobic digestion varies in scale, complexity, and feeder materials. Regardless of design, a biogas plant has three primary components: an inlet to get organic matter into the digestion chamber where anaerobic digestion and gas capture occurs and an outlet to remove the digested organic matter (Karki et al 2009; see Figure 1 for typical Nepali design). The gas use devices and equipment vary based on the intended end-use of the generated biogas. A biogas plant can be constructed at home with some ingenuity at minimal cost and can use methane from human sewage, animal excrement, food waste, or a combination thereof to provide cooking fuel or lighting (see Figure 2). A biogas plant can also be constructed at considerable expense costing millions of dollars (see the United States Environmental Protection Agency’s website for anaerobic digesters at
http://www.epa.gov/agstar/anaerobic/ad101/anaerobic-digesters.html
for diagrams and photographs of typical biogas facilities in the United States). Plants can serve as waste management facilities for concentrated animal feeding operations with thousands of animals. The biogas from such a plant can be used to generate electricity for on-farm use or for sale into the power grid, for heating, or for conversion into vehicle fuel. Alternatively, it can simply be flared to burn off the methane. Plants of many sizes and designs exist. For example, in Nepal an undergrad fixed-dome construction is common with a manual mixer, a ball valve above the underground dome to control gas flow, and an area to collect effluent (see Figure 1 for example). After anaerobic digestion is complete, a solid waste by-product remains that can be used for fertilizer; this is also depicted in the promotional poster in Figure 1. Thus, anaerobic digestion
provides an additional use-value before manure fertilizer reaches the field by capturing and using the methane produced through its decomposition as a renewable energy source (see AgSTAR’s basic anaerobic digester system flow diagram at
http://www.epa.gov/agstar/documents/digester_flow_diagram.pdf).
Biogas is the result of gases released during the decomposition of organic matter by methanogenic bacteria. Biogas is comprised of multiple gases, the most dominant of which is methane (CH4) followed by a smaller concentration of carbon dioxide (CO2). It may also contain traces of hydrogen (H2), nitrogen (N2), water vapor (H2O), and hydrogen sulfide (H2S). Biogas turns what would be waste -- such as sewage, manure, or food scraps -- into a clean renewable energy. The gas burns cleanly, is smokeless, and is non-toxic. The premise behind biogas is a simple one – organic matter decomposes. The decomposition resulting from anaerobic digestion releases methane. Methane is a potent greenhouse gas and is estimated to be 21 to 23 times more damaging than the same volume of carbon dioxide (Sharma et al 2010; Themelis and Ulloa 2007). When methane (CH4) is burned, the carbon and hydrogen atoms combine with oxygen to create carbon dioxide (CO2) and water vapor (H2O) as the by-product. Technology systems, ranging from the very simple and affordable to the very complex and expensive, can capture methane from decomposing material
and convert it into a usable energy. This process can replace a range of other emission-producing energy sources from firewood to coal to fossil fuels. While burning methane still releases greenhouse gases, CO2 is less potent than the methane and it also avoids the release of CO2 currently sequestered in other sources. Thus, the emission reduction results from the combination of two factors: (1) The methane that would have been emitted regardless through natural processes of decomposition is now harnessed for energy, and (2) The emissions from the energy source replaced by biogas are eliminated, or at least reduced. The technology to harness the energy potential released by anaerobic digestion varies in scale, complexity, and feeder materials. Regardless of design, a biogas plant has three primary components: an inlet to get organic matter into the digestion chamber where anaerobic digestion and gas capture occurs and an outlet to remove the digested organic matter (Karki et al 2009; see Figure 1 for typical Nepali design). The gas use devices and equipment vary based on the intended end-use of the generated biogas. A biogas plant can be constructed at home with some ingenuity at minimal cost and can use methane from human sewage, animal excrement, food waste, or a combination thereof to provide cooking fuel or lighting (see Figure 2). A biogas plant can also be constructed at considerable expense costing millions of dollars (see the United States Environmental Protection Agency’s website for anaerobic digesters at
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Homemade biogas digester on a rooftop in Kathmandu. Cow manure was used as the initial feeder material to begin the anaerobic digestion after which only food scraps have been added. The biogas is used to operate a kitchen burner. Photograph by author. |
http://www.epa.gov/agstar/anaerobic/ad101/anaerobic-digesters.html
for diagrams and photographs of typical biogas facilities in the United States). Plants can serve as waste management facilities for concentrated animal feeding operations with thousands of animals. The biogas from such a plant can be used to generate electricity for on-farm use or for sale into the power grid, for heating, or for conversion into vehicle fuel. Alternatively, it can simply be flared to burn off the methane. Plants of many sizes and designs exist. For example, in Nepal an undergrad fixed-dome construction is common with a manual mixer, a ball valve above the underground dome to control gas flow, and an area to collect effluent (see Figure 1 for example). After anaerobic digestion is complete, a solid waste by-product remains that can be used for fertilizer; this is also depicted in the promotional poster in Figure 1. Thus, anaerobic digestion
provides an additional use-value before manure fertilizer reaches the field by capturing and using the methane produced through its decomposition as a renewable energy source (see AgSTAR’s basic anaerobic digester system flow diagram at
http://www.epa.gov/agstar/documents/digester_flow_diagram.pdf).
