A fuel cell system containing a mixer/eductor for reducing the pressure difference
between the gases in the anode and cathode-sides of a fuel cell stack. The mixer/eductor
is placed at the mixing point between the anode tail or exhaust fuel gas and the
fresh air or oxidant supply gas inlet. The incoming oxidant pressure is converted
to a high momentum stream to form a suction on the fuel exhaust gas. This suction
compensates for pressure losses in the gas piping and the oxidizer used to burn
the exhaust fuel gas resulting in substantial balancing of the anode and cathode-side
gas pressures. The mixer/eductor can be in the form of an axial or annular jet
pump unit, both of which are advantageous in aggressively mixing the fuel exhaust
gas with the incoming oxidant or air. The mixer/eductor geometry can be varied
to provide the proper suction to balance pressures over a chosen range of operating
conditions. Additionally, the incoming fresh air flow rate or an oxidant exhaust
recycle flow rate can be varied to actively balance pressures at any operating point.
