During the last years natural gas is increasingly used as an alternative fuel for internal combustion engines, mainly because of the concern for the environment and the high gasoline prices.
Natural gas can be stored by liquefaction, compression, or adsorption. When it is used in vehicles, natural gas is usually stored as compressed natural gas, (CNG), at pressures of about 20 MPa. Reducing the storage pressure is interesting as the energy necessary for compression is strongly reduced, the filling stations become less expensive and the necessary safety regulations are less stringent.
With the adsorption of natural gas on activated carbon (ANG), the storage pressure is reduced to 5 MPa or less, and the designers get more freedom in tank design.
In this paper, first an overview is given of the adsorption and desorption process and of the filling and emptying procedures of the tank. The equations of the isotherms are analysed and extended for multi component adsorption (guard bed). The compressibility effects are also considered.
The flow resistance within the adsorbent is examined (modified Darcy equation). Tests are carried out on a flow bench to obtain the correct Darcy coefficients. Results of the amount of delivered gas are shown in function of the temperature and pressure in the tank.
The pressure and temperature differences within the tank as well as the temperature difference between the tank and the surroundings cause a significant lost of capacity compared to the ideal case of isothermal filling and emptying. Therefore a detailed equilibration analysis is made, which shows that the pressure equilibration behaves completely different during filling and emptying. To examine the influence of the temperature changes the heat conductivity of the activated carbon is analysed and the amount of gas stored in function of the thermal conductivity is calculated.