As was noted in the introduction to this chapter, different fuel cells put different demands on gas purity. CO removal is of particular concern to the operation of the PEMFC, less so for the PAFC. After reforming and water gas shift, the CO concentration in the reformer gas is usually reduced to 1-2%. When stable operation can be guaranteed, this level may well be acceptable to operate a PAFC reliably. A PEMFC will definitely require further cleanup down to levels in the lower ppm range. Another reason for having further CO cleanup stages is the risk of CO spikes, which may result from rapid load changes of the reformer system as expected in automotive applications.
There are a number of ways to clean the raw reformer gas of CO. Alternatively, ultra-pure hydrogen void of any contaminant can be produced using Pd-membrane technology. We will discuss these methods in turn.
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