Hydrocarbons for Hydrogen Generation - Hydrogen Generation, Storage, and Utilization - page 23

200 120

180

100

160

140

80

120

100 60

80

Hydrogen
Carbon
Methane
Conversion

40

60

40

20

20

0 0

490 540 590 640 690 740 790 840 890 940 990

Temperature,°C

FIGURE 2.9 Equilibrium composition and conversion as a function of temperature. The figure shows
the equilibrium number of moles based on an initial 100 mol of CH 4 . Source : Reproduced with permis-
sion from Amin et al. [17].

Different mechanisms of methane cracking reaction have been proposed
[17, 19, 20]. Most of the proposed mechanisms involve step-wise removal
of H from the initial CH 4 , via radicals such as CH 3 , CH 2 , and CH, and finally
formation of H 2 from H atoms. The initial CH 4 and the reaction intermediates
or radicals are adsorbed on the catalysts that lower the activation energy of
reactions, leading to breaking of the C-H bonds. The initial CH 4 can be dis-
sociatively or nondissociatively adsorbed while the subsequent steps are
similar.

2.6.3 Other Decomposition Methods

There are other ways for generating hydrogen from hydrocarbons. For
example, methane and higher hydrocarbons can be directly decomposed to
hydrogen and carbon over molten magnesium (Mg) as a catalyst [21]. Hydro-
gen as well as micrometer-size carbon particles could be formed as products.
The catalyst loses activity because of the evaporation of metal Mg. The activ-
ity of the catalyst can be recovered by heating the upper cold section of the
reactor to circulate Mg back to the reactor bed. Mg 2 C 3 was has been identi-
fied as reaction intermediate in the reaction, as shown in the proposed reac-
tion mechanism in Figure 2.10. It was suggested that since magnesium is
more electropositive than carbon and hydrogen, the breaking of C-C and
C-H bonds should involve the valence electrons of magnesium. With the
evolution of hydrogen gas and the decomposition of the magnesium carbides,
metallic magnesium is regenerated.