Natural gas comprising more than 20 percent of methane has been used as a fuel for vehicles and that’s because of the low cost and less environmental effects relative to gasoline. Methane, the main component of natural gas, is in gas phase at STP condition. As it has very low density, for unit volume of methane, low amount of energy will be produced, requiring high amounts to be stored in tanks.
Liquidation is one of the methods to increase the density of methane. But as its critical temperature is almost 191k (far less than room temp.), it can’t be liquefied by compression only. It produces 23 MJ/Lit of energy which is 600 times higher than the energy per unit volume of gas phase methane. As the condition needed to produce or store LNG is low pressure and temperature, it is not usual to use LNG in normal vehicles.
Another method to increase mass and energy density of methane is to compress it to higher pressures relative to LNG to produce Compressed Natural Gas or CNG. In fact, CNG is supercritical methane stored at room temperature and pressure of 20-25 MPa. It has 8.8 MJ of energy per unit liter, which is 230 times higher than unit volume of gas phase methane. Besides these advantages of CNG, it has the problem of storage at high pressures in strong and thick storage tanks and also, multistage compressors are requires for the production process.
The best condition for methane storage is at room temperature and atmospheric pressure. This condition will be satisfied if the gas gets adsorbed on solids. Natural gas can be adsorbed on porous solids and this process is called ANG or Absorbed Natural Gas. Using this method, methane can be stores in thanks of adsorbents (also called sorbents) at room temperature and pressure of 3.5 – 4 MPa and consequently, it may be used as the future fuel for vehicles. The amount of adsorbed and delivered natural gas strongly depends on the type of adsorbent solids used. There are two important numbers: 213 v/v is the maximum theoretical capacity of activated carbon, and 120 v/v that is the capacity of usual adsorbents. Reaching to the maximum theoretical capacity from the low normal capacity of adsorbents has been the activating energy to expand ANG industry. It is notable that 150 v/v is known to be economical capacity for industrial adsorbents. In the next article, more information will be provided about adsorbents.
by G. Gholipur – Persian Gulf university of Iran
Director: Dr. M. Mofarahi
Translated by: P. Jowkar
Editor: M. A. Makarem