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 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.
Biogas 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 when generating biogas. 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 generating biogas within a week. But if you want to understand the process and be reasonably efficient about it, read on.
How It Works
Once your digester is filled with organic material and water, biochemical processes begin to happen. First, the ingredients will break down and ferment, then acids will begin to form, followed by the desired methane production. There are four stages in the breakdown of organic material within a biogas generator. These four stages can be separated into two phases: acid formation and methane formation. The waste of one stage feeds the next. Once a generator is operating and producing gas, these processes happen simultaneously rather than as discrete sets of chemical reactions.
Hydrolysis starts when water is mixed with organic material. Hydrolysis is the enzymatic breakdown of complex proteins, carbohydrates, fats, and oils into amino acids, simple sugars, and fatty acids. The broken-down (depolymerized) material is in a chemically accessible form and ready to be fermented by acid-forming bacteria.
Acidogenesis, or fermentation, happens when acid-forming bacteria oxidize the simple compounds formed during hydrolysis to create carbon dioxide, hydrogen, ammonia, and organic acids.
Acetogenesis is the conversion of organic acids into acetic acid. Acetic acid is the main ingredient in vinegar and is the food for the final stage of decomposition within the generator. Acid-forming bacteria are fast-breeding and hearty, producing lots of CO2.
Methanogenesis is the creation of methane-producing microbes, or methanogens (single-celled, nonbacterial microorganisms from the group Archaea). Methanogens combine hydrogen and CO2 produced during the acid-forming phases to create methane. In contrast to the acid formers, methanogens are slow to reproduce and extremely sensitive to temperature, pH, and the presence of oxygen.
How Much Biogas Can You Make?
There are many variables in all processes of generating biogas. These include the type and quality of feedstock, the type of generator and how well it is maintained and fed, and other factors that you will read about later. I point this out because it is almost impossible to calculate exactly how much gas you can produce from any given “recipe.” There are, however, rules of thumb that offer enough guidance to point us in a generally useful direction.
Rule of thumb for biogas production: A well-managed generator may produce approximately its own volume of biogas each day. To put this in terms of energy production, a bit of math is required:
- A 55-gallon drum has a volume of about 7.35 cubic feet.
- One cubic foot of methane contains 1,000 Btus of energy.
- Biogas containing 60 percent methane offers 600 Btus of energy for each cubic foot.
- 7.35 cubic feet x 600 Btus per cubic foot = 4,410 Btus.
A typical gas cook stove burner might burn through 15,000 Btus of fuel per hour on maximum heat. At this rate, a 55-gallon methane generator can potentially produce enough gas in a day to supply the burner for about 18 minutes, allowing you to boil about 2 gallons of water (assuming a 60-percent transfer efficiency between the energy in the flame and the water in your pot).
This might be enough in some cases, but in a practical sense, a small family with modest daily cooking needs will require the output of a warm, well-fed, 200-gallon (27-cubic-foot) methane generator at a minimum. This much biogas represents about 16,000 Btus and offers about one hour of cooking time, or enough energy to boil around 8 gallons of water.
The quantity and quality of methane you make depends on the nutrient value of the feedstock and how well the microbes convert the available nutrients into methane. For practical purposes, biogas production and quality are functions of your specific recipe and generator management. Important things to understand about generating biogas are:
- Recipe development and the carbon-to-nitrogen ratio of ingredients
- Solids, liquids, and digestible quality
- Temperature
- Feeding rate
- Retention time
- pH
- Mixing
We’ll cover all of these topics, but as you can see, any estimate of methane yield for each unit of digestible material has quite a few variables. That means any lists you come across indicating specific values for any of the variables are only estimates. Your actual production will vary. I encourage you to not get lost in the numbers or details, and simply experiment. You can learn at least as much about how to make biogas by doing as by studying!
Reprinted with permission from The Homeowner’s Energy Handbook (Storey Publishing, 2013) by Paul Scheckel.
