SOLID OXIDE FUEL CELL WITH ANODE EXHAUST RECYCLE
BACKGROUND
This disclosure relates to a solid oxide fuel cell system. More particularly, the disclosure relates to a SOFC system with anode exhaust recycle.
SOFC systems typically utilize petroleum-based fuel that is processed by a desulfurizer and a reformer to provide reformate including hydrogen for a fuel cell. In some SOFC systems, anode exhaust is recycled for other purposes within the system to improve the overall efficiency of the SOFC system.
It is desirable to maintain the inlet temperatures of the reformer and fuel cell at relatively high temperatures so as to ensure proper operating conditions and avoid reducing their efficiency. In one prior art SOFC system, a heat exchanger is arranged between the desulfurizer and the reformer. The anode exhaust is reintroduced at a mixing node upstream from the heat exchanger. Heat is transferred from a heat distribution network to the anode exhaust and fuel entering the heat exchanger. Other SOFC systems incorporate a recycle blower that circulates the anode exhaust through the system. The recycle blower cannot withstand temperatures at which the anode exhaust exits the fuel cell so that the anode exhaust must be cooled. Another prior art SOFC system includes a combustor that receives the anode exhaust flow for combustion to produce heat that can be transferred to another component of the system, such as the reformer. However, the heat from the combustor could be more beneficially used elsewhere within in the system.
While prior art SOFC systems utilize the anode exhaust to improve the efficiency of the system, use of the anode exhaust and other gaseous flows within the system have not been sufficiently optimized to achieve a desired overall efficiency for the SOFC system. SUMMARY
A solid oxide fuel cell (SOFC) system includes an anode exhaust that flows through an anode exhaust recycle passage. The first portion of the anode exhaust flows through a reformate heat exchanger and then the anode recycle blower. This first portion of the anode exhaust flow is referred as the anode exhaust recycle flow. A flow control device regulates the distribution of the anode exhaust recycle flow to a desulfurizer inlet, providing hydrogen to the desulfurizer, and a desulfurized fuel passage that fluidly interconnects the desulfurizer and reformer, providing steam to the reformer. The second portion of the anode exhaust flows to a heat exchanger arranged between a desulfurizer and a reformer to heat desulfurized fuel entering the reformer. This second portion of the anode exhaust is then burnt in a catalytic burner. The burner outlet stream may be mixed with cathode exhaust and used to preheat process air provided to a cathode of the fuel cell.
These and other features of the disclosure can be best understood from the following specification and one or more drawings, the following of which is a brief description.
BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a highly schematic view of a solid oxide fuel cell system having an anode exhaust recycle arrangement.
DETAILED DESCRIPTION A solid oxide fuel cell (SOFC) system 10 is schematically shown in Figure 1.
The system 10 includes a fuel cell 12 having an anode 14 and a cathode 16. The anode 14 and the cathode 16 are separated by an electrolyte 18, which is a ceramic in one example. The fuel cell 12 uses fuel from a fuel source 20 and process air from a process air source 24 to produce electricity for a load 19.
Process air from the process air source 24, which is typically air from the surrounding environment, is supplied to the cathode 16 by a process air blower 26. The fuel source 20 is supplied to the anode 14 by a fuel pump 22. Typically, the fuel source 20 is a petroleum-based fuel, such as natural gas. The fuel must be converted to a pre- reformate containing hydrogen that is useable by the anode 14. In one example, a desulfurizer 28 receives the fuel and removes sulfur, which would otherwise inhibit desired operation of the fuel cell 12. The desulfurizer 28 is a hydro-desulfurizer (HDS) in one example, where the sulfur contained in the fuel supplied from fuel source 20 is removed by hydrogen through hydrodesulfurization reaction. HDS has lower maintenance requirements compared to absorbent bed type desulfurizers. The hydrogen required in HDS is supplied from anode exhaust recycle flow. The desulfurizer 28 receives the fuel at a desulfurizer inlet 30 and supplies desulfurized fuel to a desulfurized fuel passage 34 from a desulfurizer outlet 32. The desulfurized fuel passage 34 fluidly interconnects the desulfurizer 28 to a reformer 36. The reformer 36 includes a reformer inlet 38 that receives the desulfurized fuel. The reformer 36 converts the desulfurized fuel to a reformate composition that optimizes the system efficiency. In one example, the reformer 36 is a steam reformer.
A reformate supply passage 42 fluidly interconnects a reformer outlet 40 to an anode inlet 44. The reformate supplied to the anode 14 chemically reacts with oxygen from the cathode 16 to produce electricity for the load 19. Anode exhaust flow exits an anode outlet 46 to an anode outlet passage 48. The anode exhaust flow contains process water in the form of steam. Typically, the anode exhaust flow exits at approximately 8000C. The anode exhaust flow is split into two flows. The first portion is the anode exhaust recycle flow, which travels through a recycle passage 49 and the second portion is the anode exhaust flow, which travels through an exhaust passage 54. A reformate heat exchanger 50 is arranged in the reformate supply passage 42 and the anode exhaust recycle passage 49. As a result, heat from the anode exhaust flow is transferred to the reformate flow prior to entering the anode 14. Raising the temperature of the reformate flow entering the anode 14 to ensure the required fuel cell anode inlet temperature is achieved.
In one example, the steam reformer 36 is designed as an adiabatic equilibrium reactor for its simplicity. The conversion ratio depends upon the gas temperature (for example, about 6000C) and composition at the reformer inlet 38. Due to the endothermic steam reforming reaction, the reformate temperature (for example, about 5000C) in the reformer outlet 40 may be lower than the required fuel cell anode inlet temperature (for example, about 7000C). The reformate heat exchanger 50 is sized to raise the reformate temperature in the reformate supply passage 42 to the required temperature at the anode inlet 44.
A recycle blower 52 is arranged in the anode exhaust recycle passage 49 to circulate the anode exhaust flow through the system 10. In one example, the recycle blower 52 is a varying speed blower wherein the conversion ratio of the reformer and steam to carbon ratio is controlled by the speed of the blower. Transferring heat from the anode exhaust flow to the reformate flow reduces the temperature at the recycle blower 52, which enables current technologically available blowers to be used in the system 10. The anode outlet passage 48 splits into an anode exhaust recycle passage 49 and an anode exhaust passage 54.
Anode exhaust flow from the anode exhaust passage 54 flows to a mixing node 58 where the anode exhaust flow may be mixed with or introduced to a cathode exhaust flow. Alternatively, the anode exhaust flow may not be mixed with or introduced to a cathode exhaust flow at mixing node 58. The cathode exhaust is supplied from a cathode outlet 60 through a cathode exhaust passage 62. A reformer heat exchanger 56 is arranged in the desulfurized fuel passage 34. The anode exhaust flows through the reformer heat exchanger 56 and transfers heat to the desulfurized fuel to raise the temperature of the desulfurized fuel flow prior to entering the reformer 36. When cathode exhaust is mixed anode exhaust flow at mixing node 58, the mixed flow passes through the reformer heat exchanger 56.
The anode exhaust flow contains unused fuel, which should be mixed with portion of cathode exhaust flow upstream of the combustor 64. The combusted mixture from the combustor 64 flows through a combustion flow passage 70 to a mixing node 66. Cathode exhaust flow from the cathode exhaust passage 62 is supplied to the mixing node 66 before entering a capillary tube 68 that supplies the cathode exhaust flow to the mixing node 66 where it intermixes with the combusted mixture. The capillary tube 68 regulates the flow of cathode exhaust between the mixing nodes 58, 66.
The combusted mixture and additional cathode exhaust from the mixing node 66 flows through a preheater heat exchanger 72. The preheater heat exchanger 72 is arranged in a cathode supply passage 74 that supplies the process air from the process air source 24 to the cathode 16. The combusted mixture transfers heat to the process air flow to raise its temperature prior to entering the cathode 16, which provides the process air to the cathode at the desired temperature.
The combusted mixture from the preheater heat exchanger 72 may be supplied to a steam generator 78. The steam generator may supply the steam to a mixing node 82 through a steam passage 80. The steam may be used to reform the desulfurized fuel. The rest of the combusted mixture is exhausted from the steam generator 78. The steam generator 78 may provide steam to the reformer during start up.
Anode exhaust recycle flow from the recycle blower 52 is supplied to at least one of the desulfurizer 28 and the mixing node 82. A desulfurizer bypass passage 84 fluidly interconnects the anode exhaust recycle passage 49 and the desulfurized fuel passage 34. A flow control device 86, such as a valve, is arranged in the desulfurizer bypass passage 84, for example. A controller 88 is in communication with the flow control device 86 and is configured to send a command signal to the flow control device 86 to regulate the flow of anode exhaust between the fuel supply passage 92 and the mixing node 82 in the desulfurized fuel passage 34. The controller 88 may receive signals from temperature sensors or other devices (not shown) to achieve a desired desulfurizer inlet temperature and/or desired reformer inlet temperature by commanding the flow control device 86 to a desired position.
The system 10 includes a start-up burner 90 that receives fuel from the fuel source 20 through a fuel valve 94. A process air valve 96 regulates the flow of process air to the start-up burner 90. The start-up burner 90 supplies a combustion flow to the air preheater to warm up the stack and to the steam generator 78 to enable the reformer 36 to produce reformate during a fuel cell start-up procedure.
Although example embodiments have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.