A system and method for controlling the bleed valve in an anode
recirculation loop of a fuel cell system. The system uses a model to
determine the concentration of hydrogen in the recirculated gas by
calculating the volume flow of the recirculated gas through a fuel cell
stack, a pressure drop across the anode inlet and outlet of the stack,
and the density of the recirculated gas, and using a measured temperature
of the recirculated gas and a measured pressure drop across a
recirculation pump.