Gas Storage

The target gases for storing are mainly methane and hydrogen.

Natural gas gathers attention as a clean energy source and is expected to be an automobile fuel [133]. Compression and liquefaction are the currently available techniques to store it and are called CNG (compressed natural gas) and liquefied natural gas (LNG), respectively. Gas is charged in a container at 25 MPa in CNG, so that the pressure resistance is required for the container and the valves, which lead to restricted application areas. In the LNG application, natural gas is liquefied at 111 K, which requires incidental facilities for lower temperatures. This technique is not also suitable for vehicle uses. ANG stands for adsorbed natural gas. In the ANG technique methane is usually adsorbed on a suitable adsorbent at a relatively lower pressure (3.5 MPa) at ambient temperature. The key for the practical uses of this technology is to find effective and cheap adsorbents. Activated carbons, porous polymers [134-137], silica gels, and zeolites [138-141] have been examined as candidates for the adsorbents.

As the critical point of methane is Tc = 190.55 K, Pc = 4.595 MPa, the substance is in the subcritical or in the supercritical states. Supercritical gases tend to selectively adsorb in the pores of which pore width is about 4-5 times the molecule size [133]. It is also well known that the equilibrium amount decreases with the increase in pore width. So the adsorbent should not have pores larger than 2 nm. Further the materials are required to be microporous with large density, because the adsorption ability is evaluated by the amount of adsorption per unit volume of the adsorbent rather than by the amount per weight of the material. The researchers are targeting the value of 150 Vm/Va at 298 K and 3.5 MPa, where Vm and Va stand for the volume of adsorbed methane and the volume of the adsorbent, respectively. The Gas Research Institute presented more severe conditions for practical automobile applications, 200 Vm/Vs (STP) and less than $4.41 per a kilogram [142]. Preparing microporous carbon materials with high packing density at cheaper prices can satisfy these requirements. From a theoretical calculation, the adequate pore sizes are 1.12 [143] or 1.14 nm [144]. If the density of an adsorbent is 0.67 g/ml, which is a typical value for a monolithic carbon, the maximum methane uptake is estimated to be 220 Vm/Va at 298 K.

The carbon materials that selectively possess micropores are ACFs. Alcaniz et al. examined ACFs with different pore characteristics prepared by different activating agents and conditions for methane storing ability [145]. They found that the methane uptake has a good relation between the micropore volumes and obtained the highest value such as 163 Vm/Va for adsorption and 143 Vm/Va for desorption of methane.

Coal, biomass, and used tires are cheaper raw materials for use in porous carbons for ANG. Sun et al. conducted a study with coal and found that KOH activated carbon has a larger methane uptake than the steam activated one on the weight basis, but the uptake by the steam activated one was larger than the KOH activated one, because of the smaller density of the KOH activated carbon [146]. Catula et al. found that the activated carbon made from peach kernel with zinc chloride activation showed a high adsorption ability for hydrocarbons [147]. MacDonald and Quinn extended the study to examine the methane storing ability by using phosphoric acid as an activator [148]. The methane storing ability was less than the master curve for other carbons. This was probably caused by the presence of abundant surface functional groups. Used tires seem to be a promising candidate for the adsorbents, but Muller et al. could obtain only a 47 Vm/Va storing index [149]. As the utilization of waste materials is a very important subject of contemporary engineering, efforts like these must proceed.

Hydrogen must be the ultimate fuel for automobiles because it emits only water after combustion of the fuel. Whether the power generator is direct combustion or fuel cells, the common elemental technique is storing hydrogen. The US Department of Energy Hydrogen Plan has provided a commercially significant benchmark for the amount of reversible hydrogen adsorption [150]. The benchmark requires a system-weight efficiency (the ratio of stored hydrogen weight to system weight) of 6.5 wt% hydrogen and a volumetric density of 62 kg-H2/m3. Three types of storing technology are considered: (1) cryogenic liquid hydrogen, (2) compressed gas storage, and (3) metal hydride storage technology. However, these approaches are not sufficient to satisfy the previous requirements [150]. Since Dillon et al. found the reversible hydrogen storage capacity for SWNT [127], many research groups started to conduct hydrogen storage experiments and have made some noticeable progress. The carbons examined for this approach were essentially so-called nanocarbons, like SWNTs, MWNTs, carbon nanofibers (CNFs), and nonstructured graphite. The amount of hydrogen adsorbed and the measurement conditions are listed in Table 3 [127, 151-165].

The results reported so far are varied and controversial due to poor or no reproducibility of the data. Recently David et al. measured the adsorption capacity using a variety of materials: carbon nanotubes, carbon aerogels, activated carbons, and so on. In the case of the nanotube samples all of the SWNTs and MWNTs tested showed poor adsorption values (less than 0.5 wt%) at 300 K and 10 MPa [166]. Also Takagi et al. reported the adsorption ability of hydrogen for ACF, a SWNT, and a zeolite [167]. In the study they improve the reliability of the data by confirming the reversibility between the adsorption and desorption at each equilibrium pressure. The values for the ACFs which showed a high adsorption amount in the samples, are around 0.2 wt% at 303 K and 3 MPa. At the moment, therefore, the capacitance level of hydrogen adsorption seems not to be so high, compared with the data listed in Table 3. In order to obtain accurate and reliable amounts of the adsorption, special attention should be paid to the preparation and purification of samples as well as the design of high pressure apparatus without leakage of hydrogen. Moreover, the adsorption

Table 3. Summary of reported hydrogen storage capacities in nanocarbons.

Material

Amount of hydrogen storage (wt%)

Temperature (K)

Pressure (MPa)

Ref.

SWNT

5-10

133-

0.04

[127]

SWNT

2-4

133-

0.04

[151]

SWNT

4.2

R.T.

10-12

[152]

SWNT

0.9

295

0.1

[153]

SWNT

2.1

77

0.1

[153]

SWNT

8

80

12

[154]

SWNT

6.5

77

1.5

[155]

SWNT

0.05

296

3.6

[156]

SWNT

0.1

300-

0.1

[157]

SWNT

1.6

R.T.

[158]

MWNT

3.4

290

10

[159]

MWNT

0.03

296

3.6

[156]

CNF

12.8

R.T.

11

[160]

CNF

ca. 5

300

10.1

[161]

CNF

1.2

296

10

[162]

CNF

0.7

R.T.

10.5

[163]

CNF

0.1

308

10

[164]

Nanostructured

7.4

-300

1.0

Note: R.T. is room temperature.

mechanism of hydrogen in the nanoporous sites has to be elucidated for the improvement of the adsorption capacity.

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