Natural gas, a mixture of light hydrocarbons has clean burning characteristics led to increment of its use in vehicles as a clean fuel. Natural gas contains mostly methane and ethane. These light gases have very high vapor pressure at ambient temperature and their storage require high pressure compression or reduction of temperature extremely.
In the first method, natural gas is pressurized to about 3000 to 3600 psi and stored in high pressure vessels. In such a process, natural gas volume will decrease to almost 1 percent of its original volume at standard conditions. Currently, the cars using CNG or compressed natural gas, use such vessels. Also more gas can be stored, high pressure reduces the safety of such method and there will be a high risk of explosion in case of car accidents. In the second method, the natural gas is cooled to almost −162 °C (−260 °F). As a result, it liquefies and the volume is decreased to about 1/600 of its primary volume, which is much more than in CNG process. The liquefied natural gas is commonly known as LNG. In this process, high pressure problem is solved (pressure is about 4 psi), but it is very expensive and required high technology equipment.
Recently, a new process is introduced to store natural gas, both efficiently and safely. Adsorbed natural gas, or ANG, is a new process developed to replace conventional natural gas storage methods. In this process, a porous sorbent is used to absorb natural gas at relatively low pressure (500 to 600 psi) and ambient temperature and consequently, both the high pressure and low temperature problems are solved. Surprisingly, if a suitable sorbent used, it’s possible to store more gas in a sorbent filled vessel comparing to an empty vessel at the same pressure. The amount of adsorbed gas depends on pressure, temperature and sorbent type. Since this absorption process is exothermic, pressure increase or temperature decrease will enhance the process efficiency. Besides, changing the sorbent can affect efficiency. Carbon porous materials have good properties to absorb natural gas. Activated carbon is one of the sorbents with high surface area that can be used in ANG storage tanks. Currently, researchers are developing new sorbents with higher absorption ratio to optimize this process. A not so far future scope is to produce high efficiency nano absorbers that can store enough amount of gases so that ANG can compete LNG and CNG.
CNG tanks are made in cylindrical shape so that they can be filled by high pressures. Since ANG does not need that high pressure, it’s possible to make the tanks in free shapes. As a result, there will be more space in cars or other natural gas consuming vehicle.
It is possible to mix the ANG and CNG technology to reach an increased capacity of natural gas storage. In this process known as high pressure ANG, a high pressure CNG tank is filled by absorbers such as activated carbon and stores natural gas by both CNG and ANG mechanisms.