**2. Methods of gas storage**

Natural gas storage and transport methods include underground gas storage (UGS) and adsorbed natural gas (ANG) [1]. Compression (compressed natural gas – CNG) and liquefaction (liquefied natural gas – LNG), but these methods are complex and expensive. In compression, the gas is stored as a supercritical fluid at room temperature but at high pressures of about 2 <sup>10</sup><sup>4</sup> – <sup>3</sup> <sup>10</sup><sup>4</sup> kPa, reaching a density that is about 230 times higher (230 v/v) than the density of natural gas at standard temperature and pressure (STP) conditions [2]. In this case, the energy density is approximately 25% of that of gasoline. The disadvantages associated with this storage method include the risk of carrying highly pressurized tank in transit, high energy requirement for raising the pressure to 2 <sup>10</sup><sup>4</sup> – <sup>3</sup> <sup>10</sup><sup>4</sup> kPa, the costs associated with acquiring the heavy thick-walled steel cylinders that can withstand such pressure, safety valves requirements, and the cost of transporting these heavy cylinders containing highly pressurized gas [2]. In the liquefied natural gas storage method, the gas is stored at a temperature of 112 K in a tank with a pressure of 1 10<sup>3</sup> kPa, in which case the gas has 72% of the total energy density of gasoline. Natural gas can be stored in CNTs, which are lightweight containers stuffed in a pressure vessel of about 2 <sup>10</sup><sup>4</sup> – <sup>4</sup> 104 kPa. This method reduces cost; it reduces the risks associated with other storage methods and is a possible alternative for large-scale transportation of natural gas. A comparison between the natural gas storing capacity of CNT and CNG has shown that although CNT stores less amount of natural gas, it however does this at 83% storage pressure lower than CNG, which is a huge advantage that can be exploited to efficiently and economically store and transport natural gas [2]. Due to the problems of CNG, adsorbed natural gas (ANG) storage was introduced as a good alternative to CNG. In the ANG process, gas storage is done at a lower pressure than CNG. To be used on cheaper ships, work safety must be increased. Natural gas is absorbed through the embedded pores of the adsorbents. This adsorption process is performed at room temperature. The adsorption that an adsorbent performs depends primarily on the properties of the adsorbent. Materials that can be used to adsorb natural gas include highly porous materials such as activated carbon, zeolite, silica gel, activated alumina, carbon nanotubes, and a variety of material artificial adsorbents [1]. Activated carbon is usually used in ANG vessels. The maximum capacity of gas storage of activated carbon at 1500 psi and 298 K is reported to be 160 v/v, which compared with CNG has a lower storage volume. The amount of the pure methane adsorbed is more than when natural gas is used [1]. Studies show that under the same conditions, adsorption on carbon nanotubes is often greater than adsorption on activated carbon. It is also agreed that the nanoparticles used in nanotubes are the best option for absorbing and storing natural gas [1]. As already mentioned, the efficiency of ANG is lower than that of CNG. To solve this problem, natural gas storage can be considered in nanostructures. Another disadvantage of the adsorption process on

activated carbon, which has made these materials less popular, is their isothermal heat. Adsorption is a thermal phenomenon, and the high conductivity of activated carbon makes it act as an insulating material. Researchers try to use nanoscale adsorbents to adsorb more gas [1].
