Page 336 - Adsorbents fundamentals and applications

METHANE STORAGE 321

4.0%
3.5%
3.0%
Q (g/g) 2.5%
2.0%
1.5%
1.0%
0.5%
0.0%
0 200 400 600 800 1000 1200
Pressure (psia)
Figure 10.30. Adsorption isotherm of hydrogen on MWNT/NiMgO system after external
◦
◦
pretreatment in 800 CH 2 followed by 500 C vacuum in the adsorption apparatus. The
corresponding value of desorption showed a maximum value of 3.6% (from Lueking and Yang,
2003, with permission).
straightforward due to different pressures and pretreatments, it is interesting to
speculate how residual metal content may have affected various hydrogen storage
reports. For example, the SWNT sample that had an estimated 5–10% hydrogen
storage value also had a ∼20 wt % cobalt content (Dillon et al., 1997). The 4%
hydrogen storage reported by Li et al. (2001) was for a sample with no attempt
to remove the iron catalyst. The pretreatment used by Liu et al. — an acid wash
followed by high temperature heat-treatment, which that led to a twofold increase
in hydrogen storage (Liu et al., 1999) — is surprisingly similar to the pretreatment
process that Chiang et al., showed to expose metal particles (Chiang et al., 2001).

10.4. METHANE STORAGE

Sorbent development for methane storage has been an active research area since
the 1980’s. The aim of the research is to develop a competitive storage system
for natural gas for vehicular use, known as adsorbed natural gas (ANG).
The relative abundance and clean-burning characteristics of natural gas make
it attractive as a vehicular fuel, especially for urban areas. Compared with gaso-
line, natural gas combustion produces less hydrocarbon emission as well as less
emissions of sulfur and nitrogen oxides. The history of natural gas vehicles
(NGV) dates back to the 1930’s when Italy launched an NGV program (Cook
et al., 1999). Today there are more than one million NGVs worldwide (mostly
in Argentina, Italy, and countries of the former Soviet Union, where the relative
price of gasoline/natural gas is high). These vehicles use compressed natural gas
(CNG), at about 3000 psig (20 MPa or 200 bar) and ambient temperature. Liq-
uefied natural gas (LNG) is usually stored at the boiling point of about 112 K
◦
(−161 C) in a cryogenic tank at 1 atm pressure. LNG is not used for passenger
vehicles due to safety hazards, leakage, and other problems, although over 1000
LNG fueled trucks and buses are in use worldwide (Cook et al., 1999). For CNG,