Fuel cell system and operating method thereof

1. A method for operating a fuel cell system for generating power, the method comprising:

supplying to a fuel reformer of an anode recirculation loop, a fuel from a fuel supply device and a tail gas from an anode outlet of an anode of a fuel cell stack;

receiving a reformate at a reformer outlet of the fuel reformer, wherein the reformate is divided into a recirculated reformate and a slip reformate, and wherein the recirculated reformate is recirculated back to the anode inlet of the anode;

measuring a fuel flow rate of the fuel provided into a reformer inlet of the fuel reformer;

measuring a current drawn from the fuel cell stack by an electrical load;

measuring a recycle ratio in the anode recirculation loop defined as a flow rate ratio of the recirculated reformate to the reformate; and

estimating a fuel utilization of the fuel cell stack based on the measured fuel flow rate, the measured current and the measured recycle ratio; and

controlling the fuel cell system based on the estimated fuel utilization of the fuel cell stack.

2. The method of claim 1 ,

wherein estimating the fuel utilization of the fuel cell stack comprises:

calculating the fuel utilization of the fuel cell system based on the measured fuel flow rate, the measured current and a number of fuel cells of the fuel cell stack and

determining the fuel utilization of the fuel cell stack by comparing the calculated fuel utilization of the fuel cell system and the measured recycle ratio to a pre-established fuel utilization model.

3. The method of claim 2 , wherein controlling the fuel cell system prevents fuel starvation by comparing the calculated fuel utilization of the fuel cell system and a fuel utilization setpoint of the fuel cell system.

4. The method of claim 1 , wherein controlling the fuel cell system comprises generating a warning signal or sending a notification when the estimated fuel utilization of the fuel cell stack is close to a fuel utilization high limit or a fuel utilization low limit of the fuel cell stack.

5. The method of claim 1 , wherein controlling the system comprises:

adjusting one or more of the fuel flow rate provided into the reformer inlet of the fuel reformer, the current drawn from the fuel cell stack and the recycle ratio in the anode recirculation loop when the estimated fuel utilization of the fuel cell stack is close to a fuel utilization high limit or a fuel utilization low limit of the fuel cell stack.

6. The method of claim 1 , wherein adjusting the fuel flow rate provided into the reformer inlet of the fuel reformer comprises:

increasing the fuel flow rate when the estimated fuel utilization of the fuel cell stack approaches a fuel utilization high limit of the fuel cell stack and

decreasing the fuel flow rate when the estimated fuel utilization of the fuel cell stack approaches a fuel utilization low limit of the fuel cell stack.

7. The method of claim 1 , wherein adjusting the current drawn from the fuel cell stack comprises:

decreasing the current drawn when the estimated fuel utilization of the fuel cell stack approaches a fuel utilization high limit of the fuel cell stack and

increasing the current drawn when the estimated fuel utilization of the fuel cell stack approaches a fuel utilization low limit of the fuel cell stack.

8. The method of claim 1 , wherein adjusting the recycle ratio in the anode recirculation loop comprises:

increasing the recycle ratio when the estimated fuel utilization of the fuel cell stack approaches a fuel utilization high limit of the fuel cell stack and decreasing the recycle ratio when the estimated fuel utilization of the fuel cell stack approaches a fuel utilization low limit of the fuel cell stack.

9. A method for operating a fuel cell system, the method comprising:

an estimated fuel utilization of a fuel cell stack that is approaching at least one of a fuel utilization high limit or a fuel utilization low limit of the fuel cell stack,

controlling the fuel cell system to prevent fuel starvation by at least one of:

a) adjusting a fuel flow rate of a fuel supplied to a fuel reformer of the anode recirculation loop,

b) adjusting current drawn from the fuel cell stack by an electrical load connected thereto, or

c) adjusting a recycle ratio in the anode recirculation loop,

wherein the recycle ratio is defined as a flow rate ratio of a recirculated reformate to a reformate,

wherein the reformate is output by the fuel reformer and is divided into a slip reformate and the recirculated reformate, and

wherein the recirculated reformate is recirculated back to the fuel cell stack.

10. The method of claim 9 , wherein the estimated fuel utilization of the fuel cell stack is based on a measured fuel flow rate of the fuel supplied to the fuel reformer, a measured current drawn from the fuel cell stack, and a measured recycle ratio of the recirculated reformate to the reformate.

11. The method of claim 10 , wherein the fuel utilization high limit of the fuel cell system is predicted from a fuel utilization model comprising the measured recycle ratio and a pre-established fuel utilization high limit of the fuel cell stack, and wherein the fuel utilization model defines a mapping relationship between a fuel utilization of the fuel cell stack, a fuel utilization of the fuel cell system, and the recycle ratio in the anode recirculation loop.

12. The method of claim 10 , wherein the fuel utilization low limit of the system is determined based on a steam-to-carbon ratio (SCR) model.

13. The method of claim 12 , wherein the SCR model defines a mapping relationship between the fuel utilization of the fuel cell system and the anode recirculation loop.

14. The method of claim 9 , wherein controlling the fuel cell system comprises controlling the fuel cell system based on at least one of a fuel flow rate trim, a current trim, and a recycle ratio trim.

15. The method of claim 14 , wherein the fuel flow rate trim, the current trim, and the recycle ratio trim are based on a predicted fuel utilization high limit of the fuel cell system, a predicted fuel utilization low limit of the fuel cell system, and the calculated fuel utilization of the fuel cell system.

16. The method of claim 14 , wherein controlling the system comprises controlling the system based on at least one of a corrected fuel flow rate setpoint, a corrected current setpoint, and a corrected recycle ratio setpoint, wherein the corrected fuel flow rate setpoint defines a fuel flow rate setpoint corrected using the fuel flow rate trim, wherein the corrected current setpoint defines a current setpoint corrected using the current trim, and wherein the corrected recycle ratio setpoint defines a recycle ratio setpoint corrected using the recycle ratio trim.

17. A method for controlling a fuel cell system including a fuel cell stack, the method comprising:

estimating a fuel utilization of the fuel cell stack based on:

a) a measured fuel flow rate of fuel supplied to a fuel reformer of the fuel cell system,

b) a measured current drawn by an electrical load connected to the fuel cell stack, and

c) a measured recycle ratio in an anode recirculation loop,

wherein the measured recycle ratio is defined as a ratio between a reformate being output by the fuel reformer and a portion of the reformate being recirculated to the fuel cell stack; and

in response to the estimated fuel utilization of the fuel cell stack approaching one of a fuel utilization high limit or a fuel utilization low limit of the fuel cell stack,

controlling the fuel cell system to prevent fuel starvation by at least one of:

a) adjusting a fuel flow rate supplied to the fuel reformer,

b) adjusting the current drawn from the fuel cell stack, and

c) adjusting the recycle ratio in the anode recirculation loop,

wherein controlling the fuel cell system includes controlling the fuel cell system with respect to one of a fuel utilization high limit of the fuel cell system based on the measured recycle ratio and a pre-established fuel utilization high limit of the fuel cell stack and controlling the fuel utilization low limit of the system based on a steam-to-carbon ratio model.

18. The method of claim 17 , wherein the measured fuel flow rate is indicative of a flow rate of fuel supplied from a fuel supply device, and wherein the measured recycle ratio is a flow rate ratio of a recirculated reformate to the reformate in the anode recirculation loop.

19. The method of claim 17 , wherein estimating the fuel utilization of the fuel cell stack comprises:

calculating a fuel utilization of the fuel cell system based on the measured fuel flow rate, the measured current, and a number of fuel cells of the fuel cell stack and

determining the fuel utilization of the fuel cell stack from a pre-established fuel utilization model according to the calculated fuel utilization of the system and the measured recycle ratio.

20. The method of claim 17 , wherein adjusting the fuel flow rate provided to the fuel reformer comprises:

increasing the fuel flow rate when the estimated fuel utilization of the fuel cell stack approaches the fuel utilization high limit of the fuel cell stack and

decreasing the fuel flow rate when the estimated fuel utilization of the fuel cell stack approaches the fuel utilization low limit of the fuel cell stack.