Msc: Sustainable Engineering : Chemical and Process Engineering
What is Biomethane?
Biomethane is a renewable energy source, which is derived from biogas, a byproduct of organic material decomposing in the absence of oxygen. It includes animal manure, municipal waste, and plant material. This process is the anaerobic digestion which refers to the decomposition of organic matter in the absence of oxygen and it is the main source of biogas which is mainly composed of methane and carbon dioxide. In order to carry out biogas-to-biomethane conversion that is suitable for practical applications, purification of the gas content is carried out to remove carbon dioxide and impurities, increasing the methane level thereby to a level comparable to that of natural gas.
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Several technologies available for upgrading biogas to biomethane:
Water Scrubbing
It is the high-pressure water which is used in this technique to eliminate carbon dioxide (CO2) and hydrogen sulfide (H2S) from the biogas and the only thing left is methane. The CO2 and H2S are then released as the pressure is reduced and the water is recycled (Rotunno et al., 2017).
Pressure Swing Adsorption (PSA)
This procedure involves the compression of biogas which is typically passed on through an activated carbon bed or a molecular sieves. CO2 and other removable contaminants get on the adsorbent material surface, while methane takes its way. The absorbent is then regenerated by reducing the pressure, hence the gases are released (Yellezuome et al., 2023).
Chemical Scrubbing
With this method, the gas generally condenses within a solution, usually amine, to pick up CO2. The CO2-containing solution is then heated up to turn into gaseous CO2, which is then sent for further treatment. The chemical scrubbing method can achieve a very high level of methane purity but it has a higher operating cost than other methods (Miltner et al., 2017).
Membrane Separation
During this stage, the biogas passes through a semi-permeable membrane which permits the transfer of CO2 and other gases while keeping methane stuck inside the membrane at the same time. The membrane system are able to generate methane of purity up to 99% and have minimal running costs (Martín-Hernández et al.,2020).
Cryogenic Separation
This method calls for the biogas to be cooled down to extremely low temperatures, which makes the CO2 and other impurities to condense and separate from the methane. Cryogenic separation techniques are capable of producing biomethane with very low impurities, but experience high power consumption and this method is usually reserved for large-scale projects (Nguyen et al.,2021).
Biological Upgrading
This new process with hydrogenotrophic methanogenic archaea is used for the conversion of CO2 in the bio-gas to methane. Hydrogen gas is introduced into the biogas, and the microorganisms utilize the hydrogen to reduce the CO2 to methane. Biological upgrading can turn methane into an incredibly pure product of up to 99% and allows lower operation costs than other methods do (Nguyen et al.,2021).
Reasons to Choose Water Scrubbing :
Simplicity and Reliability: Water scrubbing is a simple procedure that does not need the complex chemical reactions or sophisticated molecular separation technologies. Its mechanism operation is well-known and tested, involving high reliability and scaling-up (Rotunno et al., 2017).
Cost-Effectiveness: This is with the regard to the price of installing and operating it as compared to the control methods such as chemicals scrubbing and cryogenic separation. The main operational cost is related to the energy required for compressing the biogas and the water management system, which are usually lower than the cost for the chemical reagents or the extreme energy requirements for cryogenic temperatures.
Environmental Considerations: Water scrubbing is done with water as a medium of abrasive, as a water is non-toxic, readily available and biodegradable. A key difference between chemical scrubbing is that there are no hazardous compounds involved in the process, which diminishes the likelihood of environmental contamination in the case of proper storage.
Efficiency in Moderate Purity Requirements: For applications that are not a high methane purity like certain types of power generation or heating applications, water scrubbing is an efficient and effective solution. It, usually, accomplishes the methane purifications up to 96% to 98% level, which is sufficient for practical application in different areas (Rotunno et al., 2017).To learn more about water scrubbing please select the button below.
Advantages Over Other Processes:
Compared to PSA:
Concerning the Energy consumption, scrubbing water usually found to be superior to the PSA (Pressure Swing Adsorption). PSA is a significant energy consumer, as it repeatedly pressurizes and depressurizes the adsorption vessels. Water scrubbing solely demands energy for the first compression of gas and for the water pumping processes, therefore, it is more energy-efficient with the small-scale operational processes (Nazifa Rafa et al.,2021).