Methods for Transitioning a Fuel Cell System between Modes of Operation

A method of transitioning between operating modes of a fuel cell system, the fuel cell system comprising:

• a fuel cell stack comprising:
• a plurality of solid oxide fuel cells, each solid oxide fuel cell comprising an anode, a cathode, and an electrolyte;
• an anode loop comprising:
• an in-block fueling flowpath comprising a fuel supply manifold, a fuel exhaust manifold, and one or more fueling channels in fluid communication with said fuel supply manifold and said fuel exhaust manifold, wherein each anode is exposed to a fuel flowing in one or more of said fueling channels;
• an anode ejector having a fuel supply input, a fuel recycle input, and a combined fuel output;
• a fuel recycle conduit in fluid communication with said anode ejector fuel recycle input and said in-block fueling flowpath fuel exhaust manifold;
• and
• a combined fuel supply conduit in fluid communication with said anode ejector combined fuel output and said in-block fueling flowpath fuel supply manifold;
• a fuel supply conduit in fluid communication with said anode ejector fuel supply input;
• a source of SOFC fuel in fluid communication with said fuel supply conduit;
• a source of transition fuel in fluid communication with said fuel supply conduit;
• a cathode loop comprising:
• an in-block oxidizing flowpath comprising an oxidant supply manifold, an oxidant exhaust manifold, and one or more oxidizing channels in fluid communication with said oxidant supply manifold and said oxidant exhaust manifold, wherein each cathode is exposed to an oxidant flowing in one or more oxidizing channels;
• a cathode ejector having an oxidant supply input, an oxidant recycle input, and a combined oxidant output;
• an oxidant recycle conduit in fluid communication with said cathode ejector oxidant recycle input and said in-block oxidizing flowpath oxidant exhaust manifold;
• a combined oxidant supply conduit in fluid communication with said cathode ejector combined oxidant output and said in-block oxidizing flowpath oxidant supply manifold;
• and
• a heat source positioned to heat an oxidant flowing in the cathode loop;
• an oxidant supply conduit in fluid communication with said cathode ejector oxidant supply input;
• and
• an oxidant source in fluid communication with said oxidant supply conduit;
• the method of transitioning the fuel cell system from a hot standby mode with the anodes in an oxidized condition wherein:
• oxidant is flowing through the cathode loop at a hot standby temperature;
• no SOFC fuel is flowing in the anode loop;
• no RCB is applied to the fuel cell stack;
• and
• no transition fuel is flowing in the anode loop;
• to a hot standby mode with the anodes in a reduced condition wherein:
• oxidant is flowing through the cathode loop at a hot standby temperature;
• transition fuel is flowing from the source into the anode loop;
• RCB is applied to the fuel cell stack;
• and
• anode fuel utilization is in an anode reduction range;
• said method of transitioning comprising:
• maintaining flow of oxidant through the cathode loop;
• controlling the temperature of the fuel cell stack by controlling the mass flow rate and temperature of the oxidant flowing through the cathode loop;
• flowing transition fuel into the anode loop at a mass flow rate;
• applying RCB to the fuel cell stack;
• and
• controlling the anode fuel utilization by controlling the mass flow rate of the transition fuel and the magnitude of the RCB until the anodes are in a substantially reduced condition.