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Water for Energy and Fuel Production
act as catalysts for steam gasification reactions. The order of catalyst activity of alkali
metals on coal gasification reaction is Cs > Rb > K > Na > Li. For coal gasification,
coke deposition during gasification on active catalyst sites also affects the catalyst life.
The coking can be reduced by increasing the steam-to-carbon ratio [1–3].
4.2.2 meChAniSm oF STeAm reForming
As mentioned earlier, the steam reforming of methane is very attractive because methane contains the largest hydrogen/carbon (H/C) ratio in any hydrocarbon. Unfortunately,
methane molecule is very stable with C–H bond energy of 439  kJ/mol. Such high bond
energy makes methane resistant to many reactants and reactions. Furthermore, C–H bond
in methane is very strong. Methane molecule can be, however, activated by group 8–10
transition metals and can be oxidized to produce syngas. Further conversion of CO by
water–gas shift reaction generates the final product with large concentrations of hydrogen and carbon dioxide. Once the carbon dioxide is removed from the mixture of carbon
dioxide and hydrogen by adsorption, absorption, or membrane separation process, pure
hydrogen is obtained. The adsorption process allows the purity of hydrogen of about
999.999% at 25  bar feedstock pressure. In the recent years, the use of ceramic ion transport membranes (ITMs) with reformers has opened up the possibilities of the production
of high-quality and low-cost hydrogen [5,6,33–35] (Barrio et al., 2012, pers. comm.).
Methane reforming by steam is an endothermic reaction and favored at lower
pressures. While noble metal catalysts have been tested and used in the past, most
commercial operations use nickel catalyst because of its low cost and high activity,
stability, and selectivity. The activity of the catalyst depends on the catalyst surface
area and the temperature (around 400°C–1000°C) for steam pressure up to 30 atm.
The activity of the catalyst is usually described by the turnover frequency (TOF) that
is generally 0.5 s −1 at around 450°C. This number corresponds to about 10% methane conversion. High conversion rate demands higher temperature because the reaction is limited by thermodynamics that is favored at higher temperature. Very high
conversion requires the reactor to be operated at temperatures higher than around
900°C. Often the catalysts in the reformer are poorly used because heat transfer
between gas and solid is a limiting factor in the reaction. The reactor design plays
an important role in the performance of the reactor, which will be discussed later.
Numerous studies on mechanism of methane reforming have been reported
and these are well reviewed by Wei and Iglesia [36], Rostrup-Nielsen et al. [37],
and Bradford and Vannice [22,23]. The following discussion closely follows these
reviews. Wei and Iglesia [36] have shown that the rate-limiting step for steam reforming is C–H bond activation. They proposed the following mechanism:
H 2 O + * → O*(a) + H 2 (g)
(4.14)
CH 4 (g) + 2* → CH 3 * (a) + H * (a)
(4.15)
CH 3 * (a) + * → CH 2 * (a) + H * (a)
(4.16)
CH 2 * (a) + * → CH * (a) + H * (a)
(4.17)
CH * (a) + O* (a) → CO* (a) + H * (a)
(4.18)