Hydrocarbons for Hydrogen Generation - Hydrogen Generation, Storage, and Utilization - page 8

bond, significant energy is required to break it, which is often supplied by
heating to high temperature. Like in many chemical reactions, catalysis can
be used to weaken the C-H bond, facilitating the bond breaking for hydrogen
generation. One of the most common approaches is steam reforming, which
involves hydrocarbons and water steam for hydrogen generation, as dis-
cussed next.

2.2 STEAM METHANE REFORMING

Steam reforming of natural gas (CH 4 ) is the most common method for
making commercial bulk hydrogen. Most of the hydrogen manufactured in
the United States today is from steam reforming of natural gas. At high
temperatures (1000-1400 K or 700-1100°C), steam (water vapor) reacts
with methane to generate carbon monoxide (CO) and H 2 based on the fol-
lowing reaction:

CH + H O CO → + 3 H .

(2.1)

4 2 2

The standard heat of reaction for this reaction is ΔH 298 = +206.1 kJ/mol
and it is endothermic, requiring external energy input. Even though the reac-
tion is favored at low pressure based on Le Châtelier's principle, it is usually
conducted at high pressure (20 atm) since high pressure H 2 is the most mar-
ketable product, and purification based on pressure swing adsorption (PSA)
works better at higher pressures (e.g., 700-850°C) [1]. The external heat
needed is supplied by combustion of a fraction of the incoming natural gas
or from burning waste gases. The energy conversion efficiency, defined as
the ratio of hydrogen out/energy input, for large-scale reformers is typically
in the 75-80% range. Methane and water steam react in tubes filled with
catalysts, and the steam-to-methane mass ratio is 3 or higher to avoid carbon
buildup or “coking” on the catalyst.
Factors that affect the steam reforming reaction include pressure, tempera-
ture, and catalyst used. Usually Ni or the noble metals Ru, Rh, Pd, Ir, and
Pt are used as the active catalysts. Due to its low cost, Ni is the most widely
used catalyst even though it is less active and usually more prone to deactiva-
tion by, for example, carbon formation or oxidation [1]. The activity of a
catalyst usually increases with the metal surface area, and thus the catalytic
activity benefits from a high dispersion of metal particles. The metal catalysts
are usually supported on oxides such as MgO or Al 2 O 3 , and the support can
have substantial influence on the dispersion and catalytic activities of the
catalysts and thereby the reaction performance.