# The Effects of Methane Storage Capacity Using Upgraded Activated Carbon by KOH
## 1 Introduction  
The use of alternative energies, such as hydrogen (H2) and methane (CH4), has been studied. Studies on H2 are usually focused on the production process [1; 2; 3], while those on CH4 are mainly concentrated on its storage and transport. Methane is one of the most significant sources of energy from natural or purified biogas due to large resources with minimal environmental impacts. Liquefaction and compression techniques are generally applied for the storage of natural gas, but these are not practical for transportation purposes as automobile fuels because they require additional systems to lower the temperature to 111 K for liquefaction and pressurization over 20 MPa for compressing natural gas [4; 5]. As an alternative method for natural gas storage, adsorbed natural gas (ANG) systems, which can store natural gas at a relatively lower pressure (3-4 MPa), have been considered and intensively studied [6; 7; 8; 9; 10]. The CH4 storage capacity is the same or even higher than that of compressed natural gas (CNG) at 20 MPa. Since adsorption of CH4 involves the interaction between the gas phase and adsorbents at a certain pressure, the gas storage capacity is strongly dependent on the adsorbent'ssurface area and pore structures. Previous researchers have studied adsorbents such as activated carbon (AC), high surface activated carbon (HSAC), metal organic frameworks (MOF), and active carbon fiber [11; 12]. Among the various adsorbents, activated carbons (ACs) have been considered the most suitable adsorbents due to their high surface area, microporosity, and regenerative capacity [13; 14; 15]. In this respect, previous works have been conducted to enhance the properties of adsorbents with high surface areas to develop micropores using physical and chemical activation [16]. In the literature, there are few studies on physical activation using steam or carbon dioxide (CO\({}_{2}\)), and on chemical activation using potassium hydroxide (KOH), potassium chloride (KCl), phosphoric acid (H\({}_{3}\)PO\({}_{4}\)), etc. [17; 18; 19]. Particularly, the CH\({}_{4}\) storage capacity of activated carbon by H\({}_{3}\)PO\({}_{4}\) (14 mmol CH\({}_{4}\)/g-AC) followed by chemical activation is higher than that of KCl (9 mmol CH\({}_{4}\)/g-AC) [8]. [PERSON] et al. [20] mentioned that the diffusion rate of the potassium in KOH is more effective than the potassium in K\({}_{2}\)CO\({}_{3}\) due to high micropore volume and a larger surface area [21; 22; 23]. One of the difficulties in ANG techniques is the management of thermal effects. The heat of adsorption significantly affects ANG storage capability even for adsorbents with high surface areas. According to the computational results of [PERSON] et al., it was suggested that gas should be cooled before it is injected into vessels filled with adsorbents [24]. Owing to adsorbents having poor thermal conductivity, heat transfer to the surroundings is limited. Thus, it can lead to the suppression of adsorption storage capacity. According to the experimental results from [PERSON] and [PERSON] [25], gas delivery capacity decreased by more than 8% when the temperature decreased to 268 K, and its dynamic loss fluctuated from 15 to 25% due to changes in the heat capacity of the carbon bed. Thus, many researchers have developed transient heat and mass transfer models to simulate the performance of adsorption systems [26; 27; 28]. For instance, [PERSON] et al. [27] proposed an ANG cylinder with water passing through the tubes to effectively control the heat of adsorption. In addition, heat transfer on the adsorbent has an influence on gas delivery during discharge. The heat of adsorption generally decreases during the discharge. Thus, the amount of delivered gas decreases when the discharge flowrate is higher. Due to these reasons, a large amount of natural gas still remained after discharging in the previous adsorption systems. Currently, to overcome the limitations, MOF with flexible structures have been developed [29].