Adsorption Storage

The most promising method for NG storage at low pressure is adsorption storage [3, 4], which consists of retaining the gas inside a porous adsorbent. Due to the strong attractive surface forces of the adsorbent, the distance between gas molecules inside the pores of the adsorbent is much shorter that of the molecules of the compressed gas at the same pressure. For this reason, the density of the gas contained inside the pores of the adsorbent is

Figure 1. Molecular simulation of methane adsorption (P = 1 atm, T = 298 K) in a graphitic slit-shaped pore of width 10 A: (a) gaseous phase (pCH4 = 0.67 kg/m3); (b) adsorbed phase (pcH4 = 27.4 kg/m3). The attractive forces exerted by the adsorbent reduce the distance between the gas molecules inside the pores, thereby increasing considerably the density of the adsorbed phase.

much higher than the density of the compressed gas. This way, the storage capacity can be increased.

Any porous solid whose average pore diameter is smaller than 2 nm (2 x 10-9 m) can adsorb gas in an amount which is proportional to its pore volume. Activated carbons, due to their large microporous volume, their ability to be efficiently compacted and their low production cost, are the adsorbents with the most favorable characteristics for NG storage [5]. These adsorbents are usually produced under the form of small particles by the carbonization of several precursors, such as coal, organic matter (wood, fruit shell, resin), and polymeric matter.

There are currently several commercial activated carbons that can store at a pressure of 35 atm the same amount of gas that is stored by compression at a pressure 2.5 times larger. With the advancement of materials sciences in the fields of adsorbent production and modification, is is expected that in the near future it will be possible to store an amount close to that obtained by standard compression of the gas at 130-150 atm.

Molecular simulation of methane adsorption in a model carbon adsorbent made of parallel graphite plates has enabled us to predict the optimum pore size that maximizes the density of the stored gas (Figure 1). Under these ideal conditions, a storage reservoir by adsorption at 35 atm has a storage capacity of 137 v/v, if the carbon is in granular form, and 195 v/v if the carbon is compacted to form a monolith [6] (Table 2; see Table 3 for comparison with other fuels). The actual production of such ideal carbon is a technological challenge that has not yet been attained.

TABLE 2. Comparison of the net capacity of various NG storage systems. The capacity is usually measured on a volume basis because the limiting factor is the available storage space in the vehicle. The more commonly employed quantifier is defined by the volume of NG, measured under normal pressure and temperature conditions, that is stored per unit volume

TABLE 2. Comparison of the net capacity of various NG storage systems. The capacity is usually measured on a volume basis because the limiting factor is the available storage space in the vehicle. The more commonly employed quantifier is defined by the volume of NG, measured under normal pressure and temperature conditions, that is stored per unit volume

of reservoir (v/v).

Storage method

Charge pressure (atm)

Net capacity (v/v)

Pure compression

207

216

A.C. granular

35

137 theoretical

A.C. monolith

35

195 limit

Target to achieve

35

150

A.C. granular

35

110 obtained in

A.C. monolith

35

140 laboratory

A.C. = activated carbon.

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