A control arrangement in a fuel cell system for producing electricity with fuel cells, the fuel cell system including means for recirculating fuel through the anode sides of the fuel cells, and at least one system controller in a control processor for controlling the operation of the fuel cell system. The control arrangement includes means for performing a substantially asynchronous chemical reaction rates calculation process of at least one of fuel composition and fuel flow rate to accomplish information in a substantially iterative process on at least recirculation ratio of the fuel recirculation through anodes and means for generating, in a substantially synchronous process with the system controller process, fuel utilization (FU) information and Carbon formation information by utilizing the latest available recirculation ratio information provided by said asynchronous process.