Biogas is a mixture of gases formed anywhere organic material decomposes
in the absence of oxygen, such as underwater, deep in a landfill,
bubbling out of municipal solid waste, or in the guts of animals (including
you). Sometimes called swamp gas, biogas is produced through the biological and
the chemical process of anaerobic digestion (AD).
This is a natural process that happens without any assistance from you or me. Simply put, anaerobic digestion is the microbial decomposition (digestion) of carbohydrates in an oxygen-free (anaerobic) environment. It begins with a process similar to the fermentation of alcohol, but AD occurs in the absence of oxygen and continues past fermentation. In fact, oxygen is toxic to the process, in that it inhibits the growth of methane-producing microbes, also known as methanogens, which are ultimately what we want to encourage for the production of biogas
The Basics :
The main ingredient of biogas made in a controlled environment is methane. Methane (chemically known as CH4) is a hydrocarbon made up of one molecule of carbon and four molecules of hydrogen, and is lighter than air. Methane is also the primary component of natural gas, commonly used for cooking and heating, although biogas is not as energy-dense as natural gas.
The methane content of the biogas you make will probably range from 50 to 80 percent, compared to about 70 to 90 percent with utility-supplied natural gas. Natural gas also contains up to 20 percent other combustible gases, such as propane, butane, and ethane, while biogas does not. The exact makeup of biogas depends in part on the source of the gas, which is based on what is fed to the digester, and in turn what was fed to the producers of those ingredients. Noncombustible components of biogas can be considered impurities. These will be primarily carbon dioxide (CO2), along with small amounts of water vapor, nitrogen (N2), and possibly trace amounts of hydrogen sulphide (H2S). If air contaminates the process, nitrogen can dilute the biogas. Other trace impurities may be formed as well. You can remove these impurities if desired, but depending on how you intend to use the biogas, you may not need to.
Producing Biogas:
To produce biogas, you first mix water with organic material (often called feedstock) such as animal manure or vegetable material, add a starting culture, then close it all up in an airtight container. You maintain a temperature within the container that is close to the temperature inside an animal (around 100°F) and, in about a week, you should be generating biogas.
The airtight container where this process is captured and controlled is called an anaerobic digester or methane generator. I prefer the term generator for the system in general, because it implies the intention of producing something, while anaerobic digestion is a process that happens with or without our intention or intervention. While design specifics can vary, a methane generator usually contains a filler tube for feeding the digester vessel; an effluent outlet to remove digested solids and liquids (also called digestate); and a gas outlet. You can make a small generator from a single 55-gallon barrel, but any digester vessel smaller than 200 gallons should be considered experimental because it will not make enough biogas to be useful for any practical purpose.
Keeping Things Simple:
The biological and chemical processes of AD, along with all the nuances of feedstock variables, are complex. However, if you dwell on the complexity of the science, you may never get started.
Save that step for when you turn professional. Anaerobic digestion is a natural process of decay that wants to happen by itself — any encouragement you offer can only be helpful. In fact, you could probably ignore the rest of this chapter and find a sealed container, put a home brewer’s airlock on top, fill it halfway with water and halfway with any sort of organic material you can find, and have some success in making biogas within a week. But if you want to understand the process and be reasonably efficient about it, read on.
This is a natural process that happens without any assistance from you or me. Simply put, anaerobic digestion is the microbial decomposition (digestion) of carbohydrates in an oxygen-free (anaerobic) environment. It begins with a process similar to the fermentation of alcohol, but AD occurs in the absence of oxygen and continues past fermentation. In fact, oxygen is toxic to the process, in that it inhibits the growth of methane-producing microbes, also known as methanogens, which are ultimately what we want to encourage for the production of biogas
The Basics :
The main ingredient of biogas made in a controlled environment is methane. Methane (chemically known as CH4) is a hydrocarbon made up of one molecule of carbon and four molecules of hydrogen, and is lighter than air. Methane is also the primary component of natural gas, commonly used for cooking and heating, although biogas is not as energy-dense as natural gas.
The methane content of the biogas you make will probably range from 50 to 80 percent, compared to about 70 to 90 percent with utility-supplied natural gas. Natural gas also contains up to 20 percent other combustible gases, such as propane, butane, and ethane, while biogas does not. The exact makeup of biogas depends in part on the source of the gas, which is based on what is fed to the digester, and in turn what was fed to the producers of those ingredients. Noncombustible components of biogas can be considered impurities. These will be primarily carbon dioxide (CO2), along with small amounts of water vapor, nitrogen (N2), and possibly trace amounts of hydrogen sulphide (H2S). If air contaminates the process, nitrogen can dilute the biogas. Other trace impurities may be formed as well. You can remove these impurities if desired, but depending on how you intend to use the biogas, you may not need to.
Producing Biogas:
To produce biogas, you first mix water with organic material (often called feedstock) such as animal manure or vegetable material, add a starting culture, then close it all up in an airtight container. You maintain a temperature within the container that is close to the temperature inside an animal (around 100°F) and, in about a week, you should be generating biogas.
The airtight container where this process is captured and controlled is called an anaerobic digester or methane generator. I prefer the term generator for the system in general, because it implies the intention of producing something, while anaerobic digestion is a process that happens with or without our intention or intervention. While design specifics can vary, a methane generator usually contains a filler tube for feeding the digester vessel; an effluent outlet to remove digested solids and liquids (also called digestate); and a gas outlet. You can make a small generator from a single 55-gallon barrel, but any digester vessel smaller than 200 gallons should be considered experimental because it will not make enough biogas to be useful for any practical purpose.
Keeping Things Simple:
The biological and chemical processes of AD, along with all the nuances of feedstock variables, are complex. However, if you dwell on the complexity of the science, you may never get started.
Save that step for when you turn professional. Anaerobic digestion is a natural process of decay that wants to happen by itself — any encouragement you offer can only be helpful. In fact, you could probably ignore the rest of this chapter and find a sealed container, put a home brewer’s airlock on top, fill it halfway with water and halfway with any sort of organic material you can find, and have some success in making biogas within a week. But if you want to understand the process and be reasonably efficient about it, read on.
