Design considerations for effective control of an afterburner sub-system in a combined heat and power (CHP) fuel cell system (FCS)

Abstract

This article investigates various control strategies for a combined heat and power (CHP) fuel cell system (FCS), with a specific focus on the afterburner sub-system. The afterburner sub-system recovers heat and by-products from the excess fuel and oxidant not consumed within the fuel cell. The overall performance of a CHP FCS depends crucially on the control of the afterburner sub-system because the control of this sub-system (1) determines the extent of thermal energy recovered from the system, between 35 and 55% of fuel energy input; (2) establishes the rate limiting step in the control of the overall CHP FCS because the rate at which the afterburner can combust excess fuel and oxidant safely and raise steam affects the rate at which the fuel cell’s electrical power output can change; and (3) impacts upstream mass and energy flows strongly, such as the system’s overall water balance and also the raising of steam for the upstream fuel processor and cathode humidification, as this is the point in the system where the CHP FCS becomes closed loop for heat and mass flows. Using an Aspen Plus ® chemical engineering model of the CHP FCS, this article (1) identifies potential challenges in operating the afterburner sub-system, (2) discusses various options for ameliorating those challenges, and (3) recommends viable solutions. The two challenges it discusses in detail are (1) the danger of overheating the afterburner, and (2) the danger of overheating a downstream steam generator. Regarding the first challenge, in the low anode hydrogen utilization (AHU) range (66–85%) specified by some fuel cell manufacturers, the afterburner is in danger of overheating beyond its maximum rated operating point. Regarding the second challenge, also at low anode hydrogen utilizations, the steam generator is in danger of overheating beyond its maximum rated operating point. This article demonstrates that one solution for overcoming these challenges is to dilute the afterburner’s stream with exhaust gas from the cathode. This article shows the ratio of cathode exhaust flow rates that achieve the desired operating temperature regions for the afterburner and downstream sub-system components. Using this method, this article determines an optimal control strategy solution for the afterburner sub-system.