Dynamic Simulation of the Fuel Processor

Generally, it is preferable to utilise steam as an energy carrier instead of air, because of its higher heat capacity. 

Springmann et al. [152] performed dynamic simulations of a combination of a monolithic autothermal reformer reactor with a tubular heat-exchanger for feed preheating and a high temperature water-gas shift reactor (diameter 70 mm 1600 cells per square inch or about 630-p,m chaimel size) [389]. Both reactors were metallic monoliths. The reformer has already been described in Section 5.1.3. During normal operation, air and steam were fed at 200 °C to the heat-exchanger and heated therein to approximately 650 °C. The reformate leaving the reformer was cooled in the 

To further reduce the start-up time demand of the water-gas shift reactor, addition of air (20Lmin ) was assumed upstream of the reactor after it had reached a temperature of200 °C [389]. Thus the carbon monoxide and hydrogen present in the reformate were partially combusted in the water-gas shift reactor. In this way, the time demand for pre-heating the high temperature water-gas shift reactor could be reduced to 3 min according to the calculations. 

However, it is questionable if this is really a viable option, because local overheating of the water-gas shift catalyst is likely. This catalyst is sensitive to temperature excursions at least when highly active precious metal catalyst formulations are applied (see Section 4.5.1). Additionally, the introduction of another device for air feeding into the system increases its complexity and cost. 

Taking into consideration that another two reactors (low temperature shift and preferential oxidation reactors) are required downstream of the high temperature water-gas shift, at least in a conventional fuel processor/low temperature PEM fuel cell system, the calculations demonstrate that it is difficult to pre-heat a chain of reactors without an excessive time demand. Other measures and functionalities become possible when plate heat-exchanger technology is applied, which may help to solve this problem. 

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