Process for controlling carbon monoxide concentration out of a preferential oxidation reactor during turndown using staged multi-port air injection

A process of operating a multi-stage, isothermal, carbon monoxide preferential oxidation (PrOx) reactor. The reactor preferably including a plurality air injectors, positioned along the length of the H2-rich gas flow path through the reactor, for injecting varying amounts of air at each injector depending on the electrical load demand on an associated fuel cell stack, or depending on the concentration of CO exiting the reactor. During normal or peak operating conditions of the fuel cell stack a majority of the air is injected through injectors position closer to the reformate gas inlet to the reactor. Upon a turndown of the electrical load demand on the fuel cell stack, a majority of the air is injected through the injectors closer the to reactor outlet thus eliminating or substantially reducing a reverse water gas shift reaction and substantially reducing the reaction of oxygen and hydrogen flowing through the PrOx.