In light of tighter restrictions on vehicle emissions and the existence of abundant resources, natural gas represents a highly attractive transportation fuel. Worldwide, there are currently more than 976,000 natural gas-vehicles, most of which are located in Europe and South America. At present, Compressed Natural Gas (CNG) is the technology most widely used, which stores gaseous fuel at very high pressures (about 3,000 psig). However, short driving ranges and questions of safety and cost have limited the penetration of Natural Gas Vehicles (NGV) into the transportation market in the U.S. This is in spite of the relatively lower price of natural gas compared with other fuels on an energy basis. A new technology called Adsorbed Natural Gas (ANG) is being developed, which contains the gaseous fuel in a dense form at a moderate pressure (500 psig). The lower pressure utilized by ANG allows the usage of conformable tanks which overcomes the short driving range of CNG. Nonetheless, the hope for ANG vehicles depends on further development in the areas of heat management and gas conditioning, in order to preserve the gas tank capacity.
Comprising more than 90% methane by volume (Table 1), natural gas is extremely volatile. It remains as vapor regardless of the system pressure at ambient temperature. Conventionally, natural gas has been stored in tanks either as a liquid at cryogenic temperatures or as a gas at very high pressures. A recent application of solid adsorbents has been developed for NGV on-board storage systems. Activated carbons, among other materials such as: molecular sieves, silica gel, zeolites and alumina, have given the highest natural gas adsorption per unit weight and volume. When carbon is produced, it has interparticular voids as well as porous spaces consisting of micropores, mesopores and macropores (Fig. 1). Natural gas storage levels are the sum of the adsorbed phase and the compressed gas phase. The adsorbed phase is defined as the increase in particle density over gas phase density because of the presence of a wall or walls in the case of micropore filling. Consequently, the amount of gas stored by adsorption is greater than that obtained by compression in the range of low to moderate pressures (Fig. 2). Gas adsorption increases with pressure until, at a certain limiting value, the rate of increase levels off. For this reason, the filling pressure chosen for ANG systems is 500 psig. At this pressure, the coverage of natural gas on the activated carbon surfaces is roughly 75% of the saturation value. Natural gas storage capacities of 180-190 V/V at 500 psig and 77 F have been reported on activated carbons, with delivery volumes up to 155 V/V. In these tests, carbon monoliths were used to minimize voidage and mese-macropore volumes. This amount of gas delivered by ANG is 67% of that for CNG. The lower pressure required by ANG allows the use of larger and conformable gas tanks, which can fit the vehicle shape. Taking advantage of all available vehicle spaces, more fuel storage volume and longer driving ranges can be achieved by ANG. The performance of ANG tanks may be adversely affected under certain operating conditions, such as: fast filling and discharging, changes of ambient temperature, and the presence of heavy hydrocarbons and impurities in the gas. Further development in these areas is required to provide ANG with an integral design that is technically and economically superior to that of CNG.
The abundance of natural gas in North America (Table 2) and the existence of vast transmission and distribution systems facilitates the use of NGV as a substitute for liquid fuel powered vehicles.