*1.6. Up-Gradation of End-Products*

Recent studies have laid much emphasis on agro-waste-AD systems, in which a mixture of 40–65% of methane (CH4), 35–55% of carbon dioxide (CO2), some traces of hydrogen sulphide (H2S), nitrogen gas(N2), H2, water vapor and other components (e.g., volatile hydrocarbons, chlorinated hydrocarbons, etc.) are produced as raw biogas. Similarly, the removal of contaminants, mainly H2S, CO2, and water vapor, in addition to some other toxic components from the biogas stream, is termed biogas up-gradation. This is usually performed to obtain a methane-rich gas of >96% CH4. In the biogas up-gradation process, three main techniques are usually used, i.e., pressure swing adsorption, absorption (physical and chemical), and membrane separation [42]. Pressure swing adsorption is carried out based on the molecular size to adsorb unwanted CO2, H2S, N2, and O<sup>2</sup> from the biogas

stream, and, as such, 96–98% of pure methane is obtained. The most commonly used adsorbent materials during biogas up-gradation techniques are activated carbon and zeolites. Another technique employed in biogas purification is the physical water scrubbing method based on the increased solubility of CO<sup>2</sup> and H2S in the water compared to CH4. Other separation techniques used to remove CO<sup>2</sup> and H2S from the biogas stream include amine scrubbing, caustic scrubbing, and amino acid salt solution usage. Examples of commonly used amines for chemical absorption are monoethanolamine, aminoethoxy ethanol, etc. On the other hand, the membrane separation technique involves using permeable membranes, which helps in trapping some other biogas constituents. In fermentation, the conventional method used for liquid biofuels up-gradation is distillation, which operates on the principle of the volatile nature of the substances in a mixture. The separation can be carried out primarily based on the less heavy products. Other classes of distillation that can be used for the product up-gradation include extractive, conventional, azeotropic, and molecular distillation. Hence, when an end-product undergoes an up-gradation process using the above processes, it can be utilized efficiently in various technology fields and serve as a promising alternative for renewable energy process development.
