A New Tank Configuration for Large Scale Storage of Natural Gas in Adsorbed Form

Natural Gas (NG) is an inexpensive and clean burning fuel. Hence it has attracted the attention of the policy makers in several countries including China and India for its commercial and industrial use [1]. However the challenge of safe and compact storage of NG, due to its low energy density need to be addressed before it can be considered for its large scale use in various portable as well as stationary applications. Among the various storage methods available at present such as compressed natural gas (CNG), liquefied natural gas (LNG) and adsorbed natural gas (ANG), the last option (i.e., ANG) offers many benefits interalia safety, design flexibility of the storage tank, low cost etc. [2-7]. On the other hand the exothermic and endothermic nature of the adsorption and desorption processes with poor transport properties of the adsorbent particles strongly affects the storage and delivery capacity, thereby reducing the driving range of the vehicles substantially, when ANG is used for transport applications. Chang & Talu [7] observed a reduction of 35% storage capacity under adiabatic conditions, as compared to an isothermal charge case. In view of the importance of effective heat and mass transfer, many researchers have carried out studies on heat and mass transfer management of the adsorbent bed undergoing charge and discharge cycles. Different heat transfer enhancement techniques such as provision of internal and external fins, insertion of tubes carrying hot water during desorption, multi cylinder ANG tanks with centrally located heat pipes etc. have been proposed [817]. Studies are also carried out on prediction of the structural, thermo-physical as well as adsorption characteristics of different activated carbons [18,19]. Several cities in India and elsewhere have already introduced CNG based buses and other vehicles. There is also a growing interest in running locomotives using NG. Attempts were also made to use natural gas for fishing boat engines [20]. An important requirement of large scale storage of NG is seen in the filling stations. In most of these applications, design of ANG reactors with fast charge/discharge characteristics is essential. Since charging is exothermic and discharge is endothermic, use of suitable thermal energy storage system can minimize the costs associated with cooling and heating during charge-discharge processes. However, no studies on such ANG systems are available in open literature. In this paper a shell and tube type reactor suitable for large scale storage of NG is considered. Due to high storage pressure (35 bar) of NG, the adsorbent (Activated carbon) and adsorbate (NG) are confined to the tube side, while the external fluid (water) flows on the shell side in a closed loop. The water on the shell side acts as an energy carrier absorbing heat of adsorption from the bed during charging and supplying the same to the bed during discharging. A mathematical model based on reactor heat transfer and kinetics is developed to simulate the performance of this ANG system under variable charge-discharge conditions. Effects of important design and operating parameters on the performance of the reactor are studied in detail so as to suggest means for controlling the system performance. It is expected that this study will be useful in the practical design and evaluation of ANG systems for large scale storage of NG. Volume 2 Issue 7 2017