Electric vehicle
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| 133 Table 6.6 Liquids that might be used locally to store hydrogen gas for fuel cells Name Formula Percentage Density Vol. (l) Notes H 2 (kg l) to store kg H 2 Liquid H 2 H 2 100 0.07 14 Cold, −252 ◦ C Ammonia NH 3 17.8 0.67 Toxic, 100 ppm Liquid methane CH 4 25.1 0.415 9.6 Cold, −175 ◦ C Methanol CH 3 OH 12.5 0.79 10 – Ethanol C 2 H 5 OH 13.0 0.79 9.7 – Hydrazine N 2 H 4 12.6 1.01 Highly toxic sodium borohydride solution NaBH 4 + HO 1.06 Expensive, but works well 6.6.2 Methanol Methanol is the ‘human-made’ carrier of hydrogen that is attracting the most interest among fuel cell developers. As we saw in Section 6.3, methanol can be reformed to hydrogen by steam reforming, according to the following reaction: CH 3 OH + HO CO+ 3H 2 (6.9) The equipment is much more straightforward, though the process is not so efficient, if the partial oxidation route is used, for which the reaction is 2CH 3 OH + O CO+ 4H 2 (6.10) The former would yield 0.188 kg of hydrogen for each kilogram of methanol, the latter kg of hydrogen for each kilogram of methanol. We have also seen in Section that autothermal reformers use a combination of both these reactions, and this attractive alternative would provide a yield somewhere between these two figures. The key point is that whatever reformation reaction is used (6.9 or 6.10) the reaction takes place at temperatures of around C, which is far less than those needed for the reformation of gasoline, as described in Section 6.3 (reaction 6.3 or 6.6). Also, the amount of carbon monoxide produced is far less, which means that far less chemical processing is needed to remove it. All that is needed is one of the four carbon monoxide cleanup systems outlined in Section Leading developers of methanol reforming for vehicles at present are Excellsis Fuel Cell Engines (DaimlerChrysler), General Motors, Honda, International Fuel Cells, Mitsubishi, Nissan, Toyota and Johnson Matthey. Most are using steam reforming although some organisations are also working on partial oxidation. DaimlerChrysler developed a methanol processor for the Necar 3 experimental vehicle. This was demonstrated in September as the world’s first methanol-fuelled fuel cell car. It was used in conjunction with a Ballard kW fuel cell stack. Characteristics of the methanol processor are given in Table 6.7. |