The process of producing biogas
Producing biogas is a straightforward process.
It occurs in a closed system called a biogas plant or digester.
A typical biogas plant consists of four main components: the input system, the digester, the gas storage system, and the output system. The digester system requires a constant supply of oxygen (O2), whereas compressed air may be utilized in other systems.
Let’s take a close look at how biogas is produced, and what role compressed air and oxygen play in each step of the process.
It all starts with the input system where the organic matter, also known as feedstock, is introduced into the biogas plant. The feedstock can include:
Agricultural waste,
Animal waste,
Food waste, and
Sewage sludge.
The digester is the heart of the biogas plant. This is where the anaerobic digestion process occurs in a tank sealed off from light and air, creating an oxygen-free environment.
The feedstock is then mixed with water and heated to an optimal temperature to promote the growth of microorganisms that produce methane (CH4).
As the bacteria break down the organic matter, biogas is produced and collected at the top of the digester.
Biogas consists mostly of methane (CH4) and carbon dioxide (CO2) as well as other gases like hydrogen (H2), nitrogen (N2), and hydrogen sulphide (H2S).
The latter is an undesirable byproduct of biogas production that needs to eliminated.
Next, the biogas produced in the digester is stored in a gas storage system for further treatment.
This system is responsible for cleaning and upgrading the biogas before it is used as a fuel. The output system is where the biogas can be used in one of two ways.
First, it can go to a gas engine connected to a generator to produce electricity or heat. Alternatively, it can be transported to a gas treatment plant for upgrading to biomethane, which is a refined form of biogas that is chemically identical to natural gas.
Biogas is a mix of carbon dioxide (CO2) and methane (CH4) which can contain various other gases like hydrogen sulphide (H2S), nitrogen (N2), oxygen (O2), hydrogen (H2), and Volatile Organic Compounds (VOC). This biogas can be upgraded into biomethane (the CH4 content of the biogas) by using different processes like water scrubbing, amines scrubbing, membranes, Pressure Swing Adsorption (PSA), and cryogenic distillation. All these methods of upgrading require a compressor to push and increase the pressure of the biogas from the digester to the upgrading process from usually atmospheric pressure up to 15 barg.
This can be done by using the Atlas Copco GG screw compressor. Once upgraded, the biomethane can be reinjected into the gas grid by eventually using an additional compressor, depending on the gas grid pressure. Using the same accounting principles of green electricity, biomethane can be taken from the grid to fill vehicles on a CNG refueling station, using compressors with outlet pressure up to 250 barg. This can be done with Atlas Copco’s CU or BBR CNG piston compressors.
When upgraded to biomethane, biogas can be used as fuel in vehicles that run on compressed natural gas (CNG) or injected into the natural gas grid.
