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Energies 2020, 13, 420 57 of 96 Natural Gas Natural gas can be stored and transported in solid-phase adsorbed into solid structures or by forming gas hydrate [707]. The physisorption consists of the weak interaction (Van der Waals forces) between the hydrogen molecules and the adsorbent material [708]. Adsorbed natural gas is a promising gas storage technology in which natural gas is adsorbed into a porous solid material at relatively low pressure (35–40 bar) and room temperature [709]. Compared with compressed and liquefied natural gas storage, the energy consumption is lower, whereas safety is increased, especially for transport fuel application [710]. Adsorbent materials should have a high surface area that ranges between 1000 m2/g for low-grade materials and 3000 m2/g for high quality upgraded materials, high porosity and fast rate of charge and discharge [711]. Adsorbent materials considered viable for gas storage are activated carbon, metal-organic framework and activated carbon fiber. Physical and chemical activation are adopted to increase the adsorbent properties and develop micropores to enhance the specific surface area. Physical activation uses steam or carbon dioxide, while chemical activation exploits potassium hydroxide, potassium chloride or phosphoric acid treatments [712]. Thermal and hydrodynamic management are crucial issues that affect storage capacity. The heat of adsorption increases during the charging phase. The problem could be overcome by cooling the gas before the injection into the reservoir and by the recirculation of not adsorbed gas. Instead, during the discharge phase, the temperature drop has to be avoided [713]. However, the desorption of high molecular weight hydrocarbons is ineffective, and they have to be removed before the gas injection into the reservoir [714,715]. Natural gas hydrate is a crystalline ice-like solid in which the natural gas compound is surrounded by a cage of water molecules [716]. The hydrate structure is stabilized when gas molecules create a weak interaction (Van der Waals forces) with the liquid water leading to a crystalline phase. Gas hydrate store large quantities of natural gas up to 180 m3 per one volume of gas hydrate [717]. The storage of gas in hydrate occurs at low temperatures or high pressures. The natural gas hydrate can be formed at 80–100 bar and 2–10 ◦C [718]. Moreover, the hydrate slurry is chemically stable at temperatures below −10 ◦C [719]. Processes for hydrate formation includes mixing of gas-saturated water, fine water jet, wave impact on a water-bubble medium and vibratory and supersonic technologies [720]. High agitation, high surface area, miscible hydrate promoter introduction increases the storage capacity reducing the amount of water occluded by the solid hydrate [721]. Hydrogen The adsorption of a suitable amount of hydrogen into the solid structure requires cryogenic temperatures. Materials used for physical adsorption should have a high specific surface area, a low bulk density and high enthalpy. Therefore, porous solid materials are used such as carbon materials (activated carbon, nanotubes, nanofibers, aerogel and templated carbon), porous polymers, zeolites and metal-organic framework [722]. The average energy of the interaction between the hydrogen and the solid structure ranges between 4 and 8 kJ/mol at room temperature. However, the energy of the adsorption enthalpy has to be enhanced for a higher storage capacity since at higher temperatures the stored hydrogen is desorbed, and the cryogenic temperature is required for proper gas storage [723]. The satisfactory binding energy for room temperature storage ranges between 15 and 25 kJ/mol [724]. The storage capacity improvement is reached increasing the available adsorption surface adopting microporous materials for higher porosity or functionalizing the solid structure [725,726]. The structural functionalization is realized through the insertion of dopant materials (B, Ca, Co, Ni, Pd, Li) in the carbon structure. The hydrogen is adsorbed on the defect sites and diffuses into the structure, and the interaction strength is enhanced by the modification of the electronic structure [708,727]. Metal hydride is an efficient and safe way to store a large amount of hydrogen. The highest volumetric density reported is 150 kg/m3 in Mg2FeH6 and Al(BH4)3 [728]. Hydrogen injected in the lattice structure reacts with the metallic material forming hydrides (MH, MH2 or MH3) [729].PDF Image | Green Synthetic Fuels
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