Gas aftertreatment

The function of the water-gas shift reactor in a fuel cell system is to reduce the concentration of carbon monoxide in the product gas of the autothermal reforming (reformat) from approx. 10 vol.-% to approx. 1 vol.-%. Excessive concentrations lead to an adsorptive poisoning of the catalytically active centers in the anode of the fuel cell, which is operated at 160–180 °C. This is manifested in a significantly poorer current-voltage curve of the fuel cell. The reaction equation for this is given below.

A series of reactor generations for the water-gas shift reaction were also designed, manufactured and experimentally tested at IET-4. There are some characteristics that these generations have in common:

• Use of a commercial precious metal/ZrO₂-CeO₂ catalyst
• Design with high temperature (400-450 °C) and low temperature stage (300-330 °C)
• Cooling between the stages by quenching the gas flow with cold water

For a fuel cell with an operating temperature of around 80 °C, the concentration of carbon monoxide in the reformate must be reduced from 1 % by volume to 10 - 100 ppm by means of a further process (CO fine purification). The preferential oxidation of CO in a hydrogen-rich gas mixture was investigated at IET-4 for this purpose. Various catalysts with ideal reformate and different concentrations of carbon monoxide were examined in a test rig in order to determine the optimum operating conditions for this reaction.

The catalytic burner has two essential functions. Firstly, in its catalytic part, the combustible components hydrogen, carbon monoxide and methane in the exhaust gas exiting from the anode of the fuel cell are converted into carbon dioxide and water, so that the fuel cell system does not emit any toxic or combustible substances (see diagram below). On the other hand, the enthalpy stream from the catalytic combustion is transferred to the three following fluid streams of a fuel cell system by means of integrated heat exchangers: (i) a significant part of the water flow needed for autothermal reforming, for its vaporization and overheating, (ii) the heat transfer fluid in the fuel cell for its temperature control and (iii) the air flow before entering the autothermal reformer for its preheating.