A start-up transition process is disclosed for a fuel cell system operation state, which includes utilization of predefined first and second temperature limits for the fuel cells, specifying a low temperature operating state of cells below the first limit, at which presence of carbonaceous species at the cells is precluded, a transition temperature range of cells above the first and below the second limit at which fuel flow supply is initiated to the fuel system in a mixture with air, combined with anode tail gas recirculated at a recirculation rate over 70, and an intermediate temperature operating state of the cells above the second temperature limit, at which free oxygen at the anodes.