What is biogas and how is it made?
Biogas used as fuel
‘Biogas’ refers to several useable gases attained using bioreactors. While the mechanics and chemistry behind biogas production are complex, put simply, several bacteria working together feed on waste, producing several gases as byproducts. One of these, methane, is useful as it can be burned for heat or light, so the methane is siphoned off and saved for later use.
The Gas
The main components of biogas are carbon dioxide (CO2) and methane (CH4). The ratio between the two varies depending on a number of conditions (including what species of bacteria are in the reactor and what is being put into it), but the normal range is 50– 70% methane and 30– 40% carbon dioxide with 60% CH4:40% CO2 being the average.
Other possible substances include hydrogen (H2) at 5– 10%, nitrogen (N2) at 1– 2%, water vapor (H2O) at 0.3%, and trace amounts of hydrogen sulphide (H2S). The easiest variable to control that produces the most methane and the least byproducts is the C:N ratio, or carbon-nitrogen ratio. If you were to look at the metabolism biochemistry of the bacteria (inside a bioreactor), you would find that for every nitrogen atom they consume, they need 30 carbon molecules. What this means is that if all the waste you put in (mixed together) has a total of 30 times more carbon than it does nitrogen, the bacteria will use up (almost) all of the carbon and nitrogen. Thus, there will be less stuff left-over to form other byproducts.
If, you are unable to achieve a CN value of 30 to prevent a reactor from producing a lot of (for example) H2S (which is a very dangerous gas in large quantities), there are simple ways to filter out any unwanted compounds. For example, ferrous materials—like from natural soils or certain iron ores—will react with and remove H2S when biogas is passed through it.
Properties
The following table summarizes some of methane’s measured qualities.
| Specific Gravity (relative to air) | 0.55 |
| Critical Temperature | 82.5°C |
| Pressure for Liquefaction | 5000 psi |
| Air Requirement for Combustion | 9.33m³/m³ |
| Ignition Temperature | 650°C |
The part of the gas that is burned when used as fuel is methane. Methane burns with a clear blue flame, is non-toxic, and doesn’t produce smoke. As a gas, it’s colorless in bright sunlight and virtually odorless (in fact, a strong odor is one of the signs that something is wrong with a bioreactor).
In its pure form, methane produces more heat than charcoal, wood, or kerosene. However, because the methane in unpurified biogas is diluted by a large volume of carbon dioxide, the total energy produced per unit volume of biogas is less (dependent on the ratio of CO2 to CH4). This is the reason optimizing the CN ratio is important; the closer to 30 you are, the greater the volume% of CH4 you’ll get per volume of biogas (and thus more energy).
How it’s Made
Bacteria
Bacteria can be broken up into three (broad) classes—based on the temperatures that they thrive in—as follows: psychrophilic bacteria, who like temperatures near-freezing (capable of growth between -15°C and +10°C); mesophilic bacteria, who enjoy room-temperature conditions (25– 40°C), and thermophilic bacteria, who enjoy thrive in very high temperatures (45– 80°C, or 113– 176°F). Psychrophiles are extremely rare, and surprisingly little is known about them (compared to the other two) in the scientific community. Between mesophiles and thermophiles, both are regularly used in bioreactors to generate methane gas. However, thermophiles tend to be unstable and difficult to maintain, so they are only used in settings where a high degree of control and precision are possible (i.e. not this project).
Most bioreactors (especially those in developing countries) use mesophilic bacteria. Although it took many years to figure out (see ‘History of Biogas’), we now know that it requires various bacteria—grouped into three types: acetobacter, methanobacter, and fermentative—for the full process to occur. These three types utilize what is known as anaerobic respiration, which is respiration without oxygen (the same process that yeast use to make alcohol). As a point of comparison, humans and animals have to breathe in oxygen to get energy (which is called aerobic respiration). There are many different ways that anaerobic respiration can work, each one resulting taking in and giving back different chemicals. To generate biogas, the three types of bacteria mentioned above must work together in a process.
The Process
Fermentative process chart
The first step is for the three major compounds in the substrate—proteins, saccharides (sugars), and lipids (fats)—to be broken down via hydrolysis. This is performed by fermentative bacteria; the term covers a very large group, which is broken down into sub-groups depending on what kinds of compounds they can process. To generate biogas, you need a different species for each of the compounds (though several fall into more than one group, so you could get away with only two distinct species if you got the right ones). The protein and lipids are hydrolyzed into organic acids, hydrogen, and carbon dioxide; and the saccharides are broken down into monosaccharides (simple sugars).
Acetogenesis process chart
The next step is acetogenesis, which requires the acetobacters. Like the fermentative bacteria, there are different groups of acetobacters, and usually two different groups are required. The first one takes the simple sugars produced in the previous step, and metabolizes them into acetic acid, ethanol, and organic acids (the same kind that were produced by some of the fermentative bacteria groups in the first step). The second group of acetobacters takes all the organic acids that have been produced, and turns them into acetic acid, hydrogen, and carbon dioxide.
Methanogenesis process chart
The final step is methanogenesis, which is performed by the methanobacters. There are three different groups of methanobacters which work at the same time, each one utilizing products of the other groups in addition to the chemicals produced in previous steps. The first group is able to take both carbon dioxide and ethanol, and break it down into acetic acid and methane. The second group takes all the acetic acid that has been formed up to this point, and breaks it down into carbon dioxide and methane. The final group takes the all the carbon dioxide and hydrogen that have been formed throughout the entire process and combines them to make methane and water.
