GB2348315A

GB2348315A - Solid oxide fuel cell burner systems an methods of generating heat therefrom

Info

Publication number: GB2348315A
Application number: GB0004972A
Authority: GB
Inventors: Joseph Jay Hartvigsen; Ashok C Khandkar
Original assignee: Sofco LP
Current assignee: Sofco LP
Priority date: 1999-03-18
Filing date: 2000-03-01
Publication date: 2000-09-27
Also published as: AU2227100A; JP2000306592A; DE10012844A1; GB0004972D0; CA2298970A1

Abstract

A burner system 10 comprises a solid oxide fuel cell (SOFC) 15, a partial reformer 20, an air preheater 25 and a heat energy recovery device 30. The reformer 20 is associated with the SOFC 15 and partially reforms fuel prior to introduction into the SOFC 15. The preheater 25 is associated with the SOFC 15 and preheats air for introduction into the reformer 20 and the SOFC 15. The heat energy recovery device 30 is associated with, and recovers heat energy from, the exhaust 36 of the SOFC 15.

Description

1 2348315 SOLID OXIDE FUEL CELL BURNER SYSTEMS AND METHODS OF GENERATING

HEAT THEREFROM The invention relates in general to solid oxide fuel cells (SOFC's), and more particularly to bumer systems for SOFC's and methods of generating heat therefrom.

SOFC's have the potential to convert any heating device using a gas bumer into a cogeneration system. To do so, heating performance must be unaffected in the combined system. Accordingly, the SOFC must maintain the duty cycle of the replaced burner if the heating performance is to be maintained. In addition, to maintain thermal efficiency, deviation from the air, fuel and heating rates as well as combustion product temperatures should be minimized. Fuel cell efficiency is not important in such applications, as heat is the primary product, while electric power is a byproduct. Indeed, energy not converted to electricity will be delivered as heat.

However, the additional cost of the fuel cell system must be offset by the value of the electric power produced within the then-nal system duty cycle.

Many of the characteristics attributable to an electrochemical burner are substantially different from those of a fuel cell power plant. In particular, fuel cell power plant designs allow for much longer start up times, with few required thermal cycles and a near 100% duty cycle. As electric power is the only product, high electrical efficiency is a requirement to compete on the basis of cost of electricity. In particular, such high efficiency designs incorporate recuperative air preheaters and fuel processors which boost electrical efficiency. The preheaters include heat exchangers which, for example, utilize stack heat to reform the fuel and to heat the fluid to desired elevated temperatures. Components such as these heaters, however, add to the cost and complexity of the system. Additionally, these components add substantial thermal mass to the system which makes frequent or rapid thermal cycling impractical.

Additionally, exhaust gas temperatures are much too low to enable efficient recovery of waste heat in a conventional heating system. An additional contributing factor which impedes the recovery of waste heat is the amount of excess air required to control temperature gradients in the fuel cell stacks. Conventional burners generally operate with an air to fuel ratio near stoichiometric, while a fuel cell may require air flow of five or more times greater than stoichiometric.

2 One aspect of the invention provides a burner system comprising:

a solid oxide fuel cell; means associated with the solid oxide fuel cell for partially refori-ning a fuel prior to introduction of the fuel into the solid oxide fuel cell; - means associated with the solid oxide fuel cell for preheating air prior to introduction of the air into the solid oxide fuel cell; and - means. associated with the solid oxide fuel cell for recovering heat energy from the exhaust of the solid oxide fuel cell.

Preferably, the partial reforming means sufficiently heats the fuel so as to eliminate the need for any fuel preheaters which are detrimental to the overall efficiency of the system.

In a preferred embodiment, the SOFC comprises a low temperature fuel cell stack or a monolithic cell, and the reforming means may comprise a POX reformer.

In another preferred embodiment, the air preheating means comprises means for providing a predetermined quantity of fuel to the preheating means. In such a preferred embodiment, the providing means directs between approximately 23% and 30% of the fuel supplied in the burner system to the preheating means.

It is also contemplated that the heat energy recovery means comprises at least one combustor, and, the air preheating means comprises an offstoichiometric (fuel lean) combustor. Furthermore, the reforming means may include means for accepting a predetermined quantity of air exiting the air preheating means.

Another aspect of the invention provides a method of generating heat, the method comprising the steps of - preheating a predetermined quantity of air in a preheater; - partially reforming a fuel stream in a reformer; passing at least a portion of the fuel stream and the air through a solid oxide fuel cell; and - combusting at least a portion of the exhaust of the solid oxide fuel cell.

In a preferred embodiment of the method, the step of preheating further 3) 0 includes the steps of. diverting a predetermined quantity of fuel to the preheater; and combusting the fuel in the preheater, to, in turn, preheat a predetermined quantity of 3 air. In such a preferred embodiment, the step of combusting the fuel further comprises the step of combusting the fuel in an off-stoichiometric bumer.

In another preferred embodiment, the step of reforming the fuel stream comprises the step of directing a predetermined quantity of preheated air from the preheater to the reformer.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which:

Fig. I is a schematic diagram showing the improved equipment configuration of an embodiment of the invention; and Fig. 2 is a graph showing the fuel fraction in the preheat bumer in mol/mol against the preheat temperature change in degrees Kelvin.

While this invention is susceptible of embodiment in many different forrns, there are shown herein in the drawings and will be described in detail several specific embodiments, with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated.

A bumer system 10 is shown in Fig, I as comprising a solid oxide fuel cell (SOFC) 15, means 20 for partially reforming fuel, means 25 for preheating air, and means 30 for recovering heat energy. As will be explained, the SOFC 15 combines air from the preheating means and fuel from the reforming means to generate electricity and heat, and, subsequently exhausts the products to a recovery means 30 which further captures some of the heat energy in the exhaust.

The SOFC 15 is shown in Fig. I as comprising a fuel inlet 32, an air inlet 34, an anode exhaust 36 and a cathode exhaust 39. The SOFC 15 may comprise any number of different SOFC constructions, including, but not limited to monolithic designs and low temperature stacks.

The partial reforming means 20 is shown in the Fig. I as comprising a POX reformer 40 having an air inlet 51, a fuel inlet 50 and an exhaust 52. The fuel inlet 50 is associated with a fuel supply, and the air inlet 51 is associated with an exhaust 46 of the preheat means 25. The POX reformer 40 may comprise an off- stoichiometric (fuel rich) reformer; however other refort-ners are likewise contemplated. Use of such a 4 partial reforming means to preheat the fuel eliminates the requirement for additional preheaters, which utilize, for example, steam to elevate the temperature of the fuel. In the present system, by utilizing the partial reforming means to heat the fuel, the additional costs and energy losses associated with the conventional heaters can be eliminated thereby improving overall performance and efficiency of the system.

The air preheat means 25 is shown in Fig. I as comprising an air inlet 42, a fuel inlet 44 and an exhaust 46. The fuel inlet 44 is associated with the system fuel supply and the air inlet 42 is associated with the system air supply. The air preheat means 25 may comprise an off-stoichiometric (fuel lean) combustor which increases the temperature of the air supply.

The heat recovery means 30, as shown in Fig. 1, comprises a heating device combustion chamber 48. The heating device combustion chamber 48 includes a cathode inlet 54 associated with the cathode outlet 38 of the SOFC 15, an anode inlet 56 associated with the anode outlet 36 of the SOFC 15 and an exhaust 58.

In operation, a fuel supply and an air supply are provided to the burner system 10. In particular, the fuel supplied may comprise natural gas, such as methane (CH4), although other fuels are likewise contemplated. In such an embodiment, the air supply comprises ambient air; however, for different fuels, different air mixtures may be required, having various constituents in various ratios.

The fuel supply is separated so that a predetermined portion is directed into the air preheating means 25. The portion directed to the air preheating means 25 will vary depending on the particular components and SOFC utilized. For a low temperature SOFC, approximately 23% to 30% of the fuel is directed to the air preheating means 25. As will be understood by those having ordinary skill in the art, higher preheat requirements may require approximately 35-40% or more of the fuel to be directed to the air preheating means.

As the fuel enters into the air preheating means 25, it is mixed with air and partially combusted. Since the combustion is off-stoichiometric, the combustion reacts the fuel with only a portion of the air. Thus, air and the combustion exhaust gasses exit through the exhaust 46 at a temperature greater than the air inlet temperature.

As the preheated air exits through the exhaust 46, a portion of the air is directed into the inlet 51 of the POX reformer 40. The POX reformer 40 accepts fuel from the inlet 50 and the air from the inlet 51 and partially reforms the fuel. The partially reformed fuel exits from the POX reformer 40 through the exhaust 52.

The fuel from the exhaust 52 is directed into the inlet 32 of the SOFC 15. Similarly, the portion of the air from the exhaust 46 of the air preheat means 25 is then directed to the inlet 34 of the SOFC 15. Within the SOFC, the air and fuel react to generate electricity, as is known in the art. The exhaust from the cathode side of the SOFC 15 is directed to the cathode exhaust 38 and the exhaust from the anode side of the SOFC 15 is directed to the anode exhaust 36.

Next, the cathode exhaust 38 is directed to the inlet 54 of the heating device combustion chamber 48, and the anode exhaust 36 is directed to the inlet 56 of the heating device combustion chamber 48. The two inlet streams further combust in the chamber 48 and the remaining exhaust gases exit through the exhaust 58. The chamber 48 of the heat recovery means 30 may comprise a substantially stoichiometric combustor.

In support of the above, analysis of one embodiment of the method was conducted. In such an embodiment, as shown in Fig. 2, approximately 23% of the fuel was required for air preheat. In this analysis, a POX reformer having a phi of 4 (phi is the fuel/air equivalence ratio) and a preheat means having a phi of 21 were utilized. When such a POX reformer was used, it was observed that approximately 58% of the fuels stream 61 remained for reaction in the SOFC 15. Utilization of the remaining fuel entering the SOFC 15 was then limited to approximately 50%. Limitation of the fuel was carried out by the conventional technique of controlling stack current, although other conventional limiting techniques can also be used. The remaining fuel resulted in an equivalent of 3.4 air stoichs relative to current. Stack heat is rejected by the mechanisms of partial internal reformation, conduction-radiation, and to the 44excess air". In such an example, the SOFC 15 operates in a temperature range from 600 C to 800 T so that it can take advantage of partial internal reformation as part of the overall SOFC cooling strategy. In such an example, the gross electrical efficiency (neglecting fan and power conversion losses) is calculated to be approximately 20% for a cell operating with a voltage of 0.70V. Of course, other embodiments may have even lower utilization, where such applications have lower electrical and thermal 6 requirements. Such lower utilization permits reduced SOFC sizes, and, in turn less expensive SOFC's.

The foregoing description and drawings merely explain and illustrate the invention and the invention is not limited thereto insofar as the appended claims are so limited, as those skilled in the art who have the disclosure before them will be able to make modifications and variations therein without departing from the scope of the invention.

7

Claims

1. A burner system comprising:

- a solid oxide fuel cell; - means associated with the solid oxide f-Liel cell for partially reforn- iing a fuel 5 prior to introduction of the fuel into the solid oxide fuel cell; - means associated with the solid oxide fuel cell for preheating air prior to introduction of the air into the solid oxide fuel cell; and - means associated with the solid oxide fuel cell for recovering heat energy from the exhaust of the solid oxide fuel cell.

2. A burner system according to claim 1, wherein the solid oxide fuel cell comprises a low temperature fuel cell stack.

3. A burner system according to claim 1, wherein the solid oxide fuel cell 15 comprises a monolithic cell.

4. A burrier system according to claim 1, claim 2 or claim 3, wherein the partial reforming means comprises a POX reformer.

5. A burner system according to any one of the preceding claims, wherein the fuel comprises natural gas.

6. A burner system according to any one of the preceding claims, wherein the air preheating means includes means for providing a predetermined quantity of fuel to the 25 air preheating means.

7. A bumer system according to claim 6, wherein the providing means directs approximately 23% to 30% of the fuel supplied in the burner system to the air preheating means.

8 8. A burner system according to any one of the preceding claims, wherein the recovering means comprises at least one combustor.

9. A burner system according to claim 8, wherein the recovering means comprises C a substantially stoichiometric combustor.

10. A burner system according to any one of the preceding claims, wherein the air preheating means comprises an off-stoichiometric combustor.

11. A burner system according to any one of the preceding claims, wherein the partial reforming means includes means for accepting a predetermined quantity of air exiting the preheating means.

12. A method of generating heat, the method comprising the steps of.

- preheating a predetermined quantity of air in a preheater; - partially reforming a fuel stream in a reformer; - passing at least a portion of the fuel stream and the air through a solid oxide fuel cell; and - combusting at least a portion of the exhaust of the solid oxide fuel cell. 20

13. A method according to claim 12, wherein the step of preheating comprises the steps of:

- diverting a predetermined quantity of fuel to the preheater; and combusting the fuel in the preheater, to in turn, preheat a predetermined 25 quantity of air.

14. A method according to claim 13, wherein the step of combusting the fuel comprises combusting the fuel in an off-stoichiometric burrier.

3 0

15. A method according to claim 12, claim 13 or claim 14, wherein the step of partially reforming the fuel stream comprises directing a predetermined quantity of preheated air from the preheater to the reformer.

9

16. A bumer system substantially as herein described with reference to and as illustrated in the accompanying drawings.

17. A method of generating heat substantially as herein described with reference to 5 and as illustrated in the accompanying drawings.