Biogas, obtained from organic waste, is a composition of carbon dioxide (CO2), methane (CH4), and several other gases in small amount. While biogas primarily generates electricity and heat, it needs purification and upgradation to meet the quality standards to be used as fuel for vehicles or injected into natural gas grids.
The primary aim of upgrading biogas is to minimize the presence of carbon dioxide while maximizing methane content, thereby creating renewable natural gas (RNG). Generally, biogas is upgraded to RNG using four technologies, so let’s understand them without further due.
The membrane separation process involves using polymeric membranes to reduce the contents of carbon dioxide and other gases while increasing methane to meet the required standards.
The membranes work by restricting the movement of gases based on their size, which is why carbon dioxide, a smaller and ionically charged molecule, permeates it. On the contrary, methane, a large and nonpolar molecule, stays in the hollow center of the porous membrane.
Source: Biocycle.net
PSA, a popular acronym for Pressure Swing Adsorption, is a biogas upgrading technology that uses multiple vessels running parallel in pressure to separate molecules according to their size and weight. For this process of purification and upgradation, the vessels use adsorbent material that is generally molecular sieves or activated carbon.
CO2, a small molecule compared to methane, is adsorbed, whereas methane escapes the adsorber process because of its relatively bigger molecular size. Finally, the pressure in the vessels is reduced, and as a result, the impurities are released.
Source: Biocycle.net
It is a two-step process where the first approach is adsorption, and the second is desorption or stripping. It involves using a chemical solvent, generally an amine compound with the capacity to absorb carbon dioxide (CO2) from the stream.
In the adsorption column, biogas and chemical solvents come in contact. Here, amine reacts with carbon dioxide molecules, thereby retaining them in the solution. Conversely, methane remains unaffected by the chemical and passes the adsorption tower quickly.
In the second approach of the process, the scrubbing solution is boiled to reverse the reaction. Carbon dioxide in the tower is separated from the solution and released. The restored anime compound is cooled and recirculated to the adsorption tower for use.
Source: Biocycle.net
Water washing is a process of eliminating impurities and purifying biogas to increase its purity and energy content. The process involves a high-pressure reactor working in a countertrend fashion under which the cold water descends, whereas the biogas moves upward in high pressure. During the process, water absorbs CO2 along with other impurities.
The solution’s pressure in the depressurization tower is then released, which allows carbon dioxide to degas. Lastly, makeup water is added according to the requirement, and the drained water is eliminated from the system to ensure the pH and water quality.
Source: Biocycle.net
All of the above biogas upgrading technologies can achieve the much-needed quality standards for the treated biogas to be used as a renewable source of energy.
However, evaluating every project to determine the right size and configuration of the biogas upgrading system is essential to ensure its cost-effectiveness and hassle-free functioning in the long run.
Certified Company