**1. Introduction**

The well-known alternative fuels for crude oil-based liquid products are natural gas and hydrogen. Nevertheless, these gaseous fuels require a special storage technique for effective utilization due to their lower volumetric energy density values. Natural gas with wide availability and improved production technology is considered to be an important energy source for the future and a cleaner form of energy compared to that of the higher hydrocarbon-based fuels such as gasoline and diesel due to its low C/H ratio. This consists of mainly methane (55–98 vol%) as the primary component along with other gases such as ethane (2–4 vol%), propane (0.5–2 vol%), and butane (0.25–0.5 vol%) in minor amounts along with few acid gases in trace amounts [1]. An estimate has shown that natural gas produces 55.9 kg CO2/GJ of energy, which is lower than that of anthracite coal (91.3 CO2/GJ), gasoline (78.5 CO2/GJ), and diesel (73.3 CO2/GJ) [2]. Therefore, among the several areas of attention for further technological advances in the natural gas domain, storage, utilization, and supply chain are prominent in the downstream sector. The concept of natural gas storage is old and typically produced and, in some cases,

the processed gas is stored in an underground facility in the vicinity of the supply center. The stored gas is regularly monitored for potential loss and emission reliability of the supply chain to meet the customer demand is an important parameter for this form of energy [3]. The current issues related to the environment has led to adopting effective measures to handle major pollution sources and the transportation sector is considered as one of the important one among these. The use of natural gas owing to wide availability and its lower carbon emission compared to that of the higher hydrocarbons as fuel for vehicular application is projected to control the pollution level to a certain extent. Natural gas in two forms are utilized as vehicle fuel, i.e. compressed natural gas (CNG) and liquefied natural gas (LNG). The CNG is especially important for light vehicle transport such as cars and other cargo transporting vehicles, whereas the LNG form is utilized in industries and manufacturing along with domestic applications. However, these systems suffer from the limitations of high cost, low storage efficiency, and safety issues.

To improve the efficiency of the process, the concept of adsorbed natural gas (ANG) originated, in which the natural gas was stored in a comparatively high amount in a porous adsorbent system under ambient temperature conditions. In this process, the methane is believed to be adsorbed in molecular form in the nanosized pores of the adsorbent network and the density in the adsorbed form exceeds the bulk density. The adsorption process being exothermic typically depended on the thermodynamic conditions. The amount of adsorption increased with a decrease in temperature and an increase in pressure. The ANG is considered to be a cost-effective alternative compared to that of liquefaction and energy-intensive compression. The storage pressure in this scenario can be decreased to ~35 bar compared to that of the utilized in CNG technology (200 bar) [4]. The target set by the Department of Energy (DOE) is 263 cm3 /cm3 working capacity under standard conditions for the adsorbent material to be commercially viable. The value is gravimetrically equivalent to 0.5 g/g of adsorbent and amount of CNG at 200 bar and 25°C pressure and temperature respectively. Furthermore, the low energy density value of the natural gas also requires improvement which may be achieved by highdensity packing so that it mimics the energy density value of LNG, i.e. ~22.2 MJ/L. Therefore, efficient adsorbents with high adsorption and desorption efficiency are desirable to utilize this ANG technology in an affordable manner for commercial implementation.
