Electric vehicle

120
Electric Vehicle Technology Explained, Second Edition
It is important to note at this stage that although the shift reaction (6.3) does occur at the same time as steam reforming, at the high temperatures needed for hydrogen generation the equilibrium point for the reaction is well to the left of the equation. The result is that by no means all the carbon monoxide will be converted to carbon dioxide. For fuel cell systems that require low levels of carbon monoxide, further processing will be required.
These reactions are the basis of the great majority of industrial hydrogen production,
using natural gas (mainly methane) as the fuel.
Hydrocarbons such as methane are not the only fuels suitable for steam reforming.
Alcohols will also react in a steam reforming reaction, for example methanol:
CH
3
OH
+ HO H+ CO
2
[
H = 49.7 kJ mol
−1
]
(6.4)
The mildly endothermic steam reforming of methanol is one of the reasons why methanol is finding favour with vehicle manufacturers as a possible fuel for fuel cell vehicles, a point which is considered further in Section 6.6.2 below. Little heat needs to be supplied to sustain the reaction, which will readily occur at modest temperatures (e.g.
250
◦
C) over catalysts of mild activity such as copper supported on zinc oxide. Notice also that carbon monoxide does not feature as a principal product of methanol reforming.
This makes methanol reformate particularly suited to PEMFCs, where carbon monoxide,
even at the parts per million level, can cause substantial losses in performance due to poisoning of the platinum catalyst. However, it is important to note that although carbon monoxide does not feature in reaction 6.4, this does not mean that it is not produced at all.
The water–gas shift reaction (6.3) is reversible, and carbon monoxide in small quantities is produced. The result is that the carbon monoxide removal methods described below are still needed with a methanol reformer used with a PEMFC.
6.3.3 Partial Oxidation and Autothermal Reforming
As an alternative to steam reforming, methane and other hydrocarbons maybe converted to hydrogen for fuel cells via partial oxidation:
CH
4
+
1 O CO + 2H
2
[
H = −247 kJ mol
−1
]
(6.5)
C
8
H
18
+ O CO + 9H
2
(6.6)

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