Adsorbed Natural Gas Storage Research Articles

Impact of gas composition on natural gas storage by adsorption

AbstractAdsorption storage is the most promising low‐pressure alternative for storing natural gas, but some operational difficulties hinder the success of this technology. From a modeling perspective, this article addresses the impact of gas composition on the cyclic behavior of adsorptive natural gas storage systems. The cyclic operation of an onboard storage reservoir is modeled as a series of consecutive two‐step processes, each consisting of charge with a fixed composition gas mixture followed by discharge at constant molar flow rate. Special attention is given to the composition and calorific value of the gas delivered by the storage system and to performance quantifiers that measure net deliverable capacity as a function of cycle number. Evidence of a cyclic steady state is given, and the numerical results are compared with experimental measurements. The results show the need for identifying and evaluating economical means of removing the higher molecular‐weight hydrocarbons and other highly adsorbed species from the gas stream before charging the storage reservoir.

Behavior and performance of adsorptive natural gas storage cylinders during discharge

Abstract Adsorbed natural gas (ANG) has the potential to replace compressed natural gas in mobile storage applications, such as in vehicles. Although a substantial research effort has been devoted to ANG, very few studies evaluate the impact of heat of adsorption on system performance. This paper concentrates on the impact of heat of adsorption on ANG performance during discharge, while the gas outflow rate is dictated by the energy demand of the application. The temperature drop and performance loss was measured with commercially available ANG cylinders under realistic conditions. Data show as high as a 37°C temperature drop at high discharge rate, with a performance loss approaching 25% of isothermal capacity. The performance loss is expected to be 15–20% at moderate discharge rates. Analysis of data and predictions of a simple model indicate that the ANG system is neither adiabatic nor isothermal during discharge; the thermal capacity of the vessel wall and external heat transfer conditions have a significant effect on system behavior. The poor thermal conductivity of packed adsorbent is a major obstacle for the utilization of these energy sources. Changing the flow direction during discharge from axial to radial by a perforated tube inserted at the center of the cylinder significantly reduces the performance loss by increasing the heat transfer from the wall to the central region. At intermediate discharge rates, where the inserted tube has the largest impact, the performance loss is reduced to 12% from 22% without the tube under identical conditions.

Storage of natural gas by adsorption on activated carbon

Natural gas may be stored by liquefaction, compression, or adsorption. For use as a transportation fuel, liquefaction is impractical and compression requires high pressure (20 MPa) and an expensive multi-stage compression facility. At relatively low pressure (3–4 MPa) achievable by single-stage compression, adsorbed natural gas (ANG) has nearly the capacity of compressed natural gas (CNG). Monte Carlo calculations were performed to simulate the adsorption of natural gas on activated carbon. The model is pure methane intercalated between parallel planes of graphite at a slit of width 11.4 Å, optimized for ANG storage. The simulations predict that the maximum delivered energy density of ANG is 0.25 for monolithic carbon and 0.17 for pelletized carbon, compared to 0.29 for CNG and 1.0 for gasoline.