162
Water for Energy and Fuel Production
Also, at these temperatures and pressures, the following water–gas shift reaction is
favored:
CO + H O  CO + H
(6.2)
2
2
2
Figure 6.2 presents the Gibbs free energy (ΔG°/RT) associated with the steam reforming of a series of alkanes such as CH 4 , C 2 H 6 , C 3 H 8 , and C 6 H 14 normalized per mole of
CO produced along with that for the water–gas shift reaction. For a reaction to occur,
negative free energy value in Figure 6.2 is needed. These results show that while
water–gas shift reaction is favorable at low temperature, the thermodynamics of
steam reforming of alkanes is only favorable at higher temperatures (e.g., T > 675 K
for C 6 H 14 and T > 900 K for CH 4 ). Thus, at lower temperatures, lower alkanes cannot
be reformed to syngas.
The oxygenated hydrocarbons having a C/O ratio of 1:1 form carbon monoxide
and hydrogen according to the following reaction [1–7,16]:
C H O  zCO + yH
(6.3)
z 2 y z
2
The Gibbs free energy diagrams for some typical oxygenated compounds such as
methanol (CH 3 OH), ethylene glycol [C 2 H 4 (OH) 2 ], glycerol [C 3 H 5 (OH) 3 ], and sorbitol
[C 6 H 8 (OH) 6 ] are also shown in Figure 6.2. These results indicate that the APR of
these compounds at low temperatures are thermodynamically favorable. Sorbitol is
generally obtained by the hydrogenation of glucose [C 6 H 6 (OH) 6 ]. Thus, oxygenated
hydrocarbons can be reformed at much lower temperatures than the alkanes with
similar carbon number. A combination of aqueous (or steam) reforming of oxygenated carbohydrates and water–gas shift reaction will allow the production of hydrogen at low temperatures.
Figure 6.2 also illustrates the logarithms of vapor pressure as a function of temperature for methanol, ethylene glycol, glycerol, and sorbitol. For the first three substances, steam reforming (in the gas phase) can be carried out at temperatures of
≥550 K, while for sorbitol, vapor-phase steam reforming requires a temperature of at
least 750 K. Thus, at low temperatures (<750 K), reforming of sorbitol (and glucose)
can be carried out in the aqueous phase producing hydrogen and syngas. The favorable thermodynamics for APR of oxygenated compounds illustrated in this figure
prompted a significant research to evaluate favorable kinetic conditions to produce
hydrogen, syngas, and alkanes via the APR process [1–7,16].
Since the thermodynamics of steam reforming of alkanes at low temperatures
are not favorable, hydrogen and carbon dioxide formed from oxygenates at lower
temperatures are not stable and alkanes can be formed by the methanation and FT
reactions between hydrogen and carbon monoxide and carbon dioxide. For example,
at 500 K, the equilibrium constant for methanation reaction is favorable [1–7,16]:
CO 2 + 4H 2  CH 4 + 2H O
2
(6.4)
Thus, forming hydrogen selectively and inhibiting the formation of alkanes would
require a catalyst that promotes C–C scission followed by the water–gas shift reaction
and inhibits C–O scission followed by the hydrogenation.