Steam reforming free energies

Figure 8.1. Free energy change for steam reforming and related reactions, including those leading to deposition of carbon on the catalyst. The plot clearly illustrates why steam reforming needs to be carried out at high temperatures.

Fig. 5 Free energy changes in the steam reforming, decomposition, dehydrogenation and dehydration of ethanol. The data for water-gas-shift reaction also is included.

Fig. 11 Free energy changes in the oxidative steam reforming/autothermal reforming of ethanol, acetaldehyde and methane.

Logistic fuels, such as jet and diesel fuels, are readily available, but a compact and effective way to remove sulfur from these fuels is needed for portable hydrogen production. Consequently, for most portable applications, it is likely that sulfur-free fuels, such as methanol, will be used. An additional advantage of methanol is that it is easier to activate at low temperatures than other hydrocarbons. Therefore, a portable hydrogen production unit based on methanol steam reforming would be simpler and less costly than other alternatives. Methanol can also be considered an energy carrier as an alternative to liquefied natural gas... 

Clean hydrogen production processes, based on carbon-free energy sources and water splitting will definitely be encouraged by carbon taxes that may apply to steam methane reforming and utilisation of natural gas as an industrial heat source or reducing chemical agent. 

Of the many possible reactions in the steam reforming of methane, a consideration of the free energies leaves the following three reactions at temperatures in excess of 600°C. 

The free energy changes for the reforming of acetaldehyde, ethylene, and CH4, which could be formed as intermediates during the SRE reactions as discussed above, are shown in Figure 2.25. As can be seen, the decomposition of acetaldehyde to CH4 and CO (line 2 Eq. 2.87 is favorable at room temperature, while a temperature of above 250 °C is required for the steam reforming of acetaldehyde (lines 3 and 5 Eqs. 2.88 and 2.89) and steam reforming of ethylene (line 4 Eq. 2.90). On... 

Figure 2.25. Free energy changes in the steam reforming of acetaldehyde, ethylene, and methane. Adapted from Velu and Song.167...

A. lulianelli, T. Longo and A. Basile, Methanol steam reforming reaction in a Pd-Ag membrane reactor for CO-free hydrogen production, bit. J. Hydrogen Energy, 2008, 33, 5583-5588. 

Since the proton exchange membrane fuel cell (PEMFC) anode catalyst can be poisoned by CO at ppm levels of concentration, it is necessary to estimate how much CO will exist at each of the fuel processing steps. Let us use the steam reforming As an example to do some analyses. Table 3.2 lists the molar Gibbs free energy of formation of the species involved, plus values for CO2 and O2 for later analysis. 

Molar Gibbs Free Energy Change for Steam Reforming C Hi, CjHg, Methanol, and Ethanol... 

The results in Figure 6.4 are based on total energies. For steam reforming, it appears more relevant to apply the free energy as the entropy term becomes dominating at the high reaction temperatures (refer to Figure 1.7). 

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