## 4 Emerging Biogas Upgrading Technologies
### Separation via Phase Creation  
The separation via phase creation method consists of creating a second phase by transferring energy to or from the process or by reducing pressure. Gas hydrate formation (also called clathrate hydrates of gases), as a novel technique for biogas upgrading, takes advantage of the capability of water to spontaneously organize itself around a gas molecule, forming a solid structure under certain conditions of temperature and pressure [129]. These \"cages\" are stabilized via the existing van der Waals forces between the water and the gas molecules [130]. Furthermore, it is possible to improve the hydrate formation rate using chemical additives (promoters), although they can reduce the hydrate's stability and the separation selectivity of the process [130; 131]. Gas hydrates were discovered because they caused plugging problems in NG transport via pipelines [129]. NG hydrates are also very common on the sea floor, where NG finds adequate conditions for hydrate formation. It is estimated that the quantity of existing NG hydrates in nature is more than twice the reserve of the other fossil fuels [129]. The captured CO\({}_{2}\) and impurities can then be stored in geological deposits of NG hydrates (NGH), leading to an exchange of in situ CH\({}_{4}\) hydrate for a CO\({}_{2}\)-dominated hydrate and a simultaneous release of CH\({}_{4}\) from these extensive natural energy deposits in permafrost sediments [132]. Thus, they can be considered an unconventional energy source [129]. The next step in gas hydrates' investigation focused on their applicability to gas storage and transport since they are safer and their formation occurs under mild temperature and pressure conditions, thus reducing the energy requirements of the process [131]. Lastly, efforts were made concerning their use for gas separation and purification [129]. It is also necessary to understand well the effects of associated gas impurities, such as SO\({}_{2}\) (from landfill gas), H\({}_{2}\)S, N\({}_{2}\), and H\({}_{2}\), in hydrate-based biogas purification [132]. For biogas upgrading, two different methods using gas hydrates (a clathrate compound, which is an ice-like solid that consists of methane that is trapped within the crystal structure of water; it has the formula of CO\({}_{2}\)-5.75H\({}_{2}\)O or 4 CH\({}_{4}\)-23H\({}_{2}\)O [133]) have been described: carbon dioxide hydrates and biogas hydrates [129]. The first consists of forming CO\({}_{2}\) hydrates selectively from biogas. This is possible since CO\({}_{2}\) forms hydrates at lighter conditions than methane [129]. Also, H\({}_{2}\)S is removed simultaneously with CO\({}_{2}\). This way, the upgraded biogas is obtained, and then the hydrate phase is dissociated to recover CO\({}_{2}\)[130]. The second method, biogas hydrates, forms hydrates of CO\({}_{2}\) and methane, and then it releases the components selectively: first the CO\({}_{2}\) and then methane [129]. This process requires more energy than forming only CO\({}_{2}\) hydrates, but the external energy required can be minimized using proper process integration [129]. H\({}_{2}\)S cannot be removed with this method since it will remain trapped along with the methane, so its pre-removal is highly recommended. This can be used, in addition to biogas upgrading, for biomethane storage, as gas hydrates are characterized by high storage capacity. Theoretically, a given volume of gas hydrates contains more than 150 times the same volume of gas at a standard temperature and atmospheric pressure [134]. The results obtained from different experiments are summarized in Table 8. The collected data correspond to the experiment with better results for each reference. The upgraded biogas does not reach enough purity with this method, so it can be used as a pretreatment step or in combination with other upgrading technologies. Apart from not reaching high methane concentrations, the methane losses are high [66].