Fuel cell system and tail gas burner assembly and method

Abstract

The present invention is concerned with improved swirl burners, particularly, but not limited to, swirl burners used in fuel cell systems.

Claims

1. A fuel cell system comprising a fuel cell stack and a burner assembly: the burner assembly comprising: (i) a hollow longitudinally elongate body extending along a central axis and having a first end and a second end, (ii) a burner wall located between said first end and said second end, and defining a first volume from said first end to said burner wall, and a second volume from said burner wall to said second end, (iii) an oxidant inlet into said first volume, said oxidant inlet for providing an oxidant flow therethrough, (iv) at least one hollow elongate burner abutting said burner wall or extending through an opening in said burner wall from said first volume to said second volume, and comprising: (a) a burner plate or mixer having a first side opening into said first volume, and a second side opening into said second volume, (b) a first fuel inlet into said first volume for feeding a first fuel from a first fuel passageway to said first volume; and (c) a second fuel inlet into said second volume for feeding a second fuel from a second fuel passageway to said second volume; wherein said fuel cell system further comprises: a first fuel source for supplying the first fuel to the first fuel passageway, the first fuel being a high calorific value fuel and neither a reformate gas from a reformer of the fuel cell system nor an anode off gas from an anode of the fuel cell stack; a second fuel source for supplying the second fuel to the second fuel passageway, the second fuel being a reformate gas from a reformer of the fuel cell system or an anode off gas from an anode of the fuel cell stack; and at least one connection upstream of the first fuel passageway and the second fuel passageway to selectively connect fuel from the first fuel supply also into said second fuel passageway such that said second fuel is mixed with high calorific value fuel that is neither a reformate gas from a reformer of the fuel cell system nor an anode off gas from an anode of the fuel cell stack, prior to delivery to said second fuel inlet.

2. The system of claim 1, wherein said at least one connection comprises a three-way valve.

3. The system of claim 1, wherein said at least one hollow elongate burner comprises said mixer, and said mixer is an axial swirl mixer comprising a plurality of vanes having a first side opening into said first volume, and a second side opening into said second volume.

4. The system of claim 1, wherein said at least one hollow elongate burner comprises said burner plate, and said burner plate comprises a plurality of passageways extending between said first volume and said second volume.

5. The system of claim 1, further comprising an off-gas pipe system connecting an outlet of an anode of a fuel cell stack to said second fuel inlet for delivery of said second fuel to said second fuel inlet, wherein said second fuel passageway forms part of said off-gas pipe system.

6. The system of claim 1, further comprising a first gas pipe system connecting a fuel gas source to said first fuel inlet for delivery of said first fuel gas to said first fuel inlet, wherein said first fuel passageway forms part of said first gas pipe system and wherein said first fuel is any one or more of mains supplied gas, natural gas, start-up fuel and top-up fuel.

7. The system of claim 1, wherein said connection comprises a bypass line that extends to said second fuel passageway for selectively diverting said first fuel from said first fuel inlet to said second fuel inlet.

8. The system of claim 1, wherein at least one of said first fuel inlet and said second fuel inlet is an orifice in, respectively, said first fuel pipe or said second fuel pipe.

9. The system of claim 8, wherein said first, and/or second, fuel inlet is not positioned at an end of its respective fuel passageway, but is positioned along its respective fuel passageway.

10. A fuel cell system comprising a fuel cell stack and a burner assembly, the burner assembly comprising: (i) a hollow longitudinally elongate body extending along a central axis and having a first end and a second end, (ii) a burner wall located between said first end and said second end, and defining a first volume from said first end to said burner wall, and a second volume from said burner wall to said second end, (iii) an oxidant inlet into said first volume, said oxidant inlet for providing an oxidant flow therethrough, (iv) at least one hollow elongate burner abutting said burner wall or extending through an opening in said burner wall from said first volume to said second volume, and comprising: (a) a burner plate or mixer having a first side opening into said first volume, and a second side opening into said second volume, (b) a first fuel inlet into said first volume; and (c) a second fuel inlet into said second volume; wherein said fuel cell system further comprises: a first fuel passageway containing a first fuel, the first fuel being a high calorific value fuel and neither a reformate gas from a reformer of the fuel cell system nor an anode off gas from an anode of the fuel cell stack, and the first fuel passageway being connected to the first fuel inlet; a second fuel passageway containing a second fuel, the second fuel being a reformate gas from a reformer of the fuel cell system or an anode off gas from an anode of the fuel cell stack, and the second fuel passageway being connected to the second fuel inlet, and at least one connection for selectively connecting the first fuel passageway and the second fuel passageway to mix fuel of said first fuel passageway with said second fuel in the second fuel passageway, such that said second fuel is mixed with high calorific value fuel that is neither a reformate gas from a reformer of the fuel cell system nor an anode off gas from an anode of the fuel cell stack, prior to delivery to said second fuel inlet.

11. A method of operating a fuel cell system according to claim 1, said method comprising the steps of: (i) directing said oxidant flow to said oxidant inlet; (ii) selectively directing said first fuel from said first fuel source to said first fuel passageway and selectively directing said second fuel from said second fuel source to said second fuel passageway; and (iii) selectively combusting said selectively directed fuel or fuels in said second volume after exiting one of: a. said burner plate or mixer; b. said second fuel inlet; or c. said burner plate or mixer and said second fuel inlet.

12. The method of claim 11, comprising use of said connection for selectively connecting said first fuel to said second fuel passageway for delivering a mixture of said first and second fuels to said second fuel inlet, whereby said mixture of said first and second fuels is combusted in said second volume after exiting said second fuel inlet.

13. The method according to claim 11, wherein said fuel cell system is selectively operable in: (i) a first mode in which said first fuel is directed to said first fuel inlet via said first fuel passageway, said oxidant and said first fuel thus converging and mixing in said first volume between said first fuel inlet and said burner plate or mixer, and wherein said second fuel is not supplied to said second fuel inlet; (ii) a second mode in which said first fuel is directed to said first fuel inlet via said first fuel passageway, said oxidant and said first fuel thus converging and mixing in said first volume between said first fuel inlet and said burner plate or mixer, and said second fuel is supplied to said second fuel inlet, said oxidant and said second fuel thus converging and mixing in said second volume; and (iii) a third mode in which said first fuel is directed to said second fuel inlet via said at least one connection, and said second fuel is also directed to said second fuel inlet, said first fuel and said second fuel thus mixing for exiting said second fuel inlet as a mixture of said first and second fuels, wherein said oxidant and said mixture converge and mix in said second volume for combustion.

14. The method of claim 13, said system further having a selectable fifth mode in which said first fuel and said second fuel are both directed to said second fuel inlet, said first and second fuels thus converging and mixing, said mixture converging and mixing with said oxidant in said second volume for combustion, and wherein said first fuel is also directed to said first fuel inlet for mixing with said oxidant also in said first volume.

15. The method of claim 14, wherein a ratio of said mixture of said first and second fuels is variable and controlled by a processor.

16. The method of claim 14, wherein a ratio of flow rates of said first fuel to said first fuel inlet and said second fuel inlet is variable and controlled by a processor.

17. The method of claim 13, said system further having a selectable fourth mode in which said second fuel is supplied to said second fuel inlet, said oxidant and said second fuel converging and mixing in said second volume for combustion, wherein said first fuel is not supplied to either said first fuel inlet or said second fuel inlet.

18. The system of claim 1, wherein at least one of said first fuel inlet and said second fuel inlet is a nozzle and wherein said at least one nozzle is defined by at least one hole in its said respective fuel inlet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a partial cut-away plan view schematic of a swirl burner assembly according to the present invention;

(2) FIG. 1A shows a first end view of the swirl burner assembly of FIG. 1 (FIG. 1 is taken along line A-A of FIG. 1A);

(3) FIG. 1B shows a second end view of the swirl burner assembly of FIG. 1;

(4) FIG. 2 shows a detailed schematic of the feature marked as ‘A’ in FIG. 1;

(5) FIG. 3 shows a partial cut-away plan view schematic of a swirl burner assembly according to the present invention and further comprising an air curtain feature;

(6) FIG. 4 shows a detailed schematic of the feature marked as ‘B’ in FIG. 3;

(7) FIG. 5 shows a partial cut-away plan view schematic of a swirl burner assembly according to the present invention and further comprising a bypass feature;

(8) FIG. 6 shows a detailed schematic of the feature marked as ‘C’ in FIG. 5;

(9) FIG. 7 shows a detailed partial cut-away plan view schematic of a swirl burner assembly according to the present invention comprising an air curtain and bypass feature;

(10) FIG. 8A shows a trend from test result data of a phase of operations of a swirl burner according to the present invention. Temperature and Emissions are shown vs. Time, where the swirl burner is initially in a start-up operation mode and then a steady-state operation mode;

(11) FIG. 8B shows a trend from test result data from the phase of operations of FIG. 8A. Fuel Flow is shown vs. Time;

(12) FIG. 8C shows a trend from test result data from the phase of operations of FIG. 8A. Lambda is shown vs. Time;

(13) FIG. 9A shows a trend from test result data of a phase of operations of a swirl burner according to the present invention. Temperature and Emissions are shown vs. Time, where the swirl burner in steady-state operation is subjected to a step change;

(14) FIG. 9B shows a trend from test result data from the phase of operations of FIG. 9A. Fuel Flow is shown vs. Time;

(15) FIG. 9C shows a trend from test result data from the phase of operations of FIG. 9A. Lambda is shown vs. Time;

(16) FIG. 10A shows a trend from test result data of a phase of operations of a swirl burner according to the present invention. Temperature and Emissions are shown vs. Time, where the swirl burner under goes a number of hot starts;

(17) FIG. 10B shows a trend from test result data from the phase of operations of FIG. 10A. Fuel Flow is shown vs. Time;

(18) FIG. 10C shows a trend from test result data from the phase of operations of FIG. 10A. Lambda is shown vs. Time;

(19) FIG. 11 shows a schematic of an axial burner assembly suitable for use in the present invention;

(20) FIG. 12A shows a schematic view of a fuel cell system according to the present invention and including the burner assembly of FIG. 1 and FIG. 11; and,

(21) FIGS. 12B-D show respective schematic views of alternative fuel cell systems to that of FIG. 12A.

(22) A fully and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification. Reference now will be made in detail to the embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, not limitation of the invention.

(23) It will be apparent to those of ordinary skill in the art that various modifications and variations can be made in the present invention without departing from the scope of the invention. For instance, features described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents.

(24) Other objects, features, and aspects of the present invention are disclosed in the remainder of the specification. It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary constructions.

(25) A listing of reference symbols used herein is given at the end of the description. Repeat use of reference symbols in the present specification and drawings is intended to represent the same or analogous features or elements.

(26) For the purposes of this description, the term burner, swirl burner, swirl burner assembly, axial burner, axial burner assembly and tail-gas burner (TGB), are to be understood to refer to the burner assembly of the invention, and where appropriate, they are readily interchangeable.

(27) In the specific embodiment below, the fuel cell system is an IT-SOFC (intermediate temperature solid oxide fuel cell) system comprising at least one fuel cell stack, where the fuel cells of the at least one fuel cell stack typically operate in the range 450-650 deg C. In other embodiments, other fuel cell systems are used with corresponding operational temperature ranges.

(28) Referring to FIG. 1, a swirl burner assembly 10 is shown. The swirl burner assembly 10 comprises a generally cylindrical (i.e. predominantly cylindrical) swirl burner body 12 having a central axis 12′, swirl burner body top end wall 16 and swirl burner body bottom end wall 14, where swirl burner body bottom end wall 14 defines swirl burner body downstream end 30.

(29) Swirl burner assembly 10 is segmented by a burner wall 40 which intersects the body 12 radially across its cylindrical shape. Burner wall 40 has a downstream face 42 which faces the swirl burner body downstream end 30. Burner wall 40 also has an upstream face 44 which faces swirl burner body top end wall 16. The portion of body 12 between body top end wall 16 and burner wall 40 defines a first section referred to herein as burner tube 50. The portion of body 12 between burner wall 40 and body bottom end wall 14 defines a second section which is generally cylindrical and has a body inner surface 64 and a body outer surface 66.

(30) First volume 52 is defined by (i.e. is defined between) burner wall upstream face 44, inner face 54 of swirl burner body top end wall 16, and burner tube inner surface 56. Similarly, second volume 62 is defined by (i.e. is defined between) body inner surface 64, swirl burner body bottom end wall 14 and burner wall downstream face 42.

(31) Burner unit 100 has a burner unit first end 20 and a burner unit second end 124. Burner unit first end 20 (the upstream end) protrudes from the swirl burner assembly 10 and particularly from the first volume 52 through opening 16′ in swirl burner body top end wall 16. Burner unit second end 124 (the downstream end) protrudes from first volume 52 to second volume 62 through opening 40′ in burner wall 40.

(32) Burner wall 40 and swirl burner body top end wall 16 have openings (opening 40′ and opening 16′ respectively) defined in them to allow the passage or placement of burner unit 100 through them. This allows the manufacture of the burner unit 100 separate to swirl burner body 12. Therefore, assembly simply requires the placement of burner unit 100 through opening 16′ in swirl burner body top end wall 16 and opening 40′ in burner wall 40.

(33) Shoulder 112 of burner unit 100 abuts burner wall 40 and prevents burner unit 100 progressing further into swirl burner body 12 and second volume 62. Burner unit 100 is then constrained in place by the joining of burner unit 100 to swirl burner body 12 at swirl burner body top end wall 16 by welding. In other embodiments other joining techniques are used, including soldering, brazing, tacking or any other joining techniques known in the art. This results in the creation of a seal between burner unit 100 and swirl burner body top end wall 16 such that the first volume (first volume 52) is enclosed. Similarly, with shoulder 112 abutting burner wall 40, a seal is effected between them.

(34) Although a single burner unit is described below, in other embodiments (not shown) multiple burner units 100 are used where they pass through the swirl burner body 12 (for example through swirl burner body top end wall 16), through first volume 52, through burner wall 40 and into second volume 62.

(35) In the swirl burner assembly 10 as shown in FIG. 1, burner unit 100 passes through first volume 52 and is positioned mostly equidistant from burner tube inner surface 56. Part of the burner tube inner surface 56 has an opening to allow the feeding of air through air inlet 70 through the swirl burner body 12 into the first volume 52. Similarly passing through the swirl burner body 12 is igniter opening 82 through which igniter 80 protrudes into second volume 62.

(36) The positioning of igniter 80 and air inlet 70 are shown in FIG. 1 to be opposed to one another across an axis of swirl burner body 12. In other embodiments (not shown), the positioning of air inlet 70 and igniter 80 are varied, air is fed into the first volume 52, and initial ignition occurs in the second volume 62 due to a sparking of igniter 80.

(37) Second volume 62 defines a flame tube, wherein the combustion of gases is to occur.

(38) Swirl burner body exhaust 15 (which is positioned proximal swirl burner body bottom end wall 14 and which exhausts gases from, i.e. is in fluid flow communication with, second volume 62) is shown in FIGS. 1A and 1B. For simplicity and convenience it is not shown in FIG. 1.

(39) Referring to FIG. 2, a more detailed view of the swirl burner assembly 10 and burner unit 100 is shown. The portion of burner unit 100 passing through first volume 52 has a burner unit outer body 110 which is mostly cylindrical and is aligned in the same cylindrical direction (on central axis 12′) as swirl burner body 12. Burner unit 100 has a burner unit top inner surface 111 which faces in the general direction of burner wall 40. The end of the burner unit 100 which passes through opening 40′ in burner wall 40 into second volume 62 is burner unit second end 124 (i.e. a burner unit bottom end). Burner unit outer body 110 is a walled body and has a thickness. The inner surface of burner unit outer body 110 is inner face 114. Burner unit inner volume 116 is defined by (i.e. defined between) inner face 114, burner unit top inner surface 111 and burner unit second end 124.

(40) Burner unit outer body 110 protrudes through opening 40′ in burner wall 40 into second volume 62. Where burner unit outer body 110 protrudes through burner wall 40, burner unit outer body 110 has shoulder 112. Shoulder 112 is stepped remote from burner unit first end 20 such that said wall thickness of burner unit outer body 110 is reduced (in the assembled swirl burner assembly 10, this is at the point where the burner unit 100 reaches the burner wall downstream face 42 before protruding through burner wall 40). The portion of burner unit outer body 110 with a reduced thickness wall is outer collar 140, where outer collar 140 shares the same inner face 114 and has outer collar outer surface 144. Outer collar 140 protrudes through burner wall 40 into second volume 62 as far as the burner unit second end 124.

(41) Shoulder 112 is restrained against burner wall downstream face 42, this, advantageously, prevents shoulder 112 from passing through burner wall upstream face 44 when burner unit 100 is positioned through the openings in the burner wall 40 and the swirl burner body top end wall 16. When assembling the swirl burner assembly, this allows the simple insertion of burner unit 100 into swirl burner body 12, without the need for measurement of how far it should be positioned through first volume 52. This allows the machining of burner unit 100 and positioning of shoulder 112 to define the position of burner unit 100 and results in a more uniform positioning of burner units 100 relative to the swirl burner body 12 regardless the number of swirl burner assemblies 10 that are manufactured. It also results in a faster assembly process of a swirl burner assembly 10, since no additional measurements is required to position the burner unit 100 if the manufacturing is uniform.

(42) Burner unit outer body 110 has at least one air inlet hole 115 (in this embodiment, a plurality of air inlet holes 115) adjoining first volume 52 and burner unit inner volume 116 through inner face 114. These air inlet holes 115 allow the passage of gases from first volume 52 into burner unit inner volume 116 (or in an opposite direction, however, the operation of the swirl burner assembly 10 should discourage this). Air inlet holes 115 are cylindrical in shape and they are arranged around the circumference of the cylindrical shape of the outer body 110. In other embodiments (not shown) other geometries of shapes are possible for the air inlet holes 115.

(43) Aside from air inlet holes 115, first volume 52 is normally sealed from burner unit inner volume 116 within it. This ensures that air from air inlet 70 must travel through air inlet holes 115 before flowing into second volume 62.

(44) Running parallel and positioned radially internal to burner unit outer body 110 is HCV fuel tube 120. HCV fuel tube 120 protrudes through burner unit top inner surface 111 within burner unit 100 into burner unit inner volume 116. HCV fuel tube 120 is a walled cylinder with HCV fuel tube inner surface 121 and HCV fuel tube outer surface 122. At the downstream end of HCV fuel tube 120 is HCV inlet 125.

(45) Running parallel and positioned radially internal to the HCV fuel tube 120 is the LCV fuel tube 130. Fingers 130′ extend from LCV fuel tube 130 and centralise it within HCV fuel tube 120. The LCV fuel tube 130 protrudes through burner unit top inner surface 111 passes through the HCV tube internal volume 123, through HCV inlet 125, through burner unit second end 124 (through opening 40′ in burner wall 40) and into second volume 62. LCV fuel tube 130 is predominantly a walled cylinder with inner surface 131 and outer surface 132. At the downstream end of LCV fuel tube 130 is LCV inlet 135.

(46) HCV tube internal volume 123 is defined by (i.e. defined between) HCV fuel tube inner surface 121, LCV tube outer surface 132, HCV inlet 125 and burner unit first end 20. LCV tube internal volume 133 is defined by (i.e. defined between) LCV tube inner surface 131, LCV inlet 135 and burner unit first end 20. Although not shown in the figures, the end of the HCV fuel tube 120 which continues in the upstream direction will be connected to an HCV fuel supply, in particular referring to FIGS. 1A and 1B it can be seen that HCV fuel tube 120 is shown to approach swirl burner assembly 10 from a direction perpendicular to burner unit 100 before reaching burner unit first end 20. Likewise, the end of the LCV fuel tube 130 which continues in an upstream direction will be connected to an LCV fuel supply.

(47) HCV inlet 125 is positioned within the burner unit inner volume 116, upstream of burner wall 40, and LCV inlet 135 is positioned in second volume 62. HCV inlet 125 is on a radial plane with shoulder 112, i.e. a plane perpendicular to the axis of the cylinder of swirl burner body 12. LCV inlet 135 is further in the downstream direction, i.e. further toward swirl burner body downstream end 30 than the burner unit second end 124.

(48) LCV fuel tube 130 has no opening leading directly to HCV fuel tube inner volume 123. That is to say that HCV tube internal volume 123 is sealed aside from the opening at HCV inlet 125 which is an opening to burner unit inner volume 116. Likewise, the only opening within swirl burner assembly 10 for LCV fuel tube 130 is the opening at LCV inlet 135 into second volume 62, i.e. LCV tube internal volume 133 is sealed aside from LCV inlet 135. As previously discussed, although not shown, the ends of HCV fuel tube 120 and LCV fuel tube 130 continuing in the upstream direction will be connected to appropriate fuel supplies.

(49) Such sealing ensures that there is no mixing of the flows through the fuel pipes or the air within the internal volumes of each pipe. In operation there will be a flow through the pipes in the downstream direction which will further ensure that no flow of fuel or air can flow back down the pipes when there is a flow due to the pressure of the flow.

(50) Downstream of HCV fuel inlet 125, i.e. further toward swirl burner body downstream end 30 and upstream of LCV fuel inlet 135, i.e. further away from swirl burner body downstream end 30 is swirl mixer 150. Swirl mixer 150 has vanes 155 for directing a flow which passes through them. Vanes 155 extend from inner face 114 of outer collar 140 to inner collar 160, and more specifically inner collar outer surface 162. Inner collar 160 is positioned inward of outer collar 140, outward of the LCV fuel tube 130 and extends from the centre of the swirl mixer 150 in a downstream direction toward swirl burner body downstream end 30. The inner collar 160 extends no further in the downstream direction than the burner unit second end 124, which is the same as the outer collar 140. The LCV fuel tube 130 passes between the inner collar inner surface 163.

(51) Swirl mixer 150 is an axial-swirl swirl mixer. Vanes 155 are any number of vanes which influence the flow that passes through them, such that they cause an axial-swirl. The axial-swirl is important for reducing the flame length since a recirculation zone is created within the flame tube (i.e. second volume 62).

(52) Outer collar 140 and inner collar 160 advantageously have an effect on the flow of oxidant and fuel into second volume 62, and in the positioning of the recirculation zone formed by swirl mixer 150. This results in an improved swirl for reducing the flame length and controls the flame seat such that it is close to swirl mixer 150 but not exposed to it. This protects vanes 155 and LCV inlet 135 from being exposed to direct combustion thus preventing deformation such as pitting on the vane surface or inlet surface.

(53) Referring to FIGS. 3 and 4, swirl burner assembly 200 is shown, which is similar to that of FIGS. 1 and 2. However, passing through burner wall 40, there are air split openings 210. Air split openings 210 are through-holes arranged radially around swirl mixer 150.

(54) Air split opening 210 which adjoins second volume 62 to first volume 52 allows the air flow from air inlet 70 to pass to second volume 62 without passing through swirl mixer 150, and, when there is flow through HCV fuel tube 130, there will be limited mixing in second volume 62 of air which passes through air split opening 210 with the HCV fuel through HCV fuel tube 130.

(55) Such a feature allows the air which flows through air split opening 210 to form an air curtain along body inner surface 64. The air curtain provides a boundary between the combustion and body inner surface 64. This air curtain can be used where it is preferred to reduce the temperature of body inner surface 64 and consequently body outer surface 66.

(56) The air split openings 210 are configured so that about 10% of the total flow through air inlet 70 passes through them.

(57) Referring to FIGS. 5 and 6, there is provided swirl burner assembly 300 similar to that seen in FIGS. 3 and 4 where swirl burner assembly 300 is a multi-walled body. Extending from burner wall 40 to swirl burner body bottom end wall 14 is inner wall 360 having inner wall inner surface 364 (i.e. swirl burner body 12 inner surface) and inner wall outer surface 366. Second volume 362 is defined by (i.e. defined between) swirl burner body bottom end wall 14, burner wall 40 and inner wall 360. Outer wall 310 has outer wall inner surface 361 and extends from burner wall 40 to swirl burner body bottom end wall 14 and is outward of inner wall 360. Third volume 363 is defined by (i.e. defined between) burner wall 40, swirl burner body bottom end wall 14, inner wall outer surface 366 and outer wall inner surface 361.

(58) Second volume 362 is a flame tube, i.e. combustion of gases occurs in this volume.

(59) Through burner wall 40 radially outward inner wall outer surface 366 and radially inward of outer wall inner surface 361 there is bypass opening 320.

(60) Air from air inlet 70 within the first volume 52 may pass through bypass opening 320 and into third volume 363. Inner wall 360 prevents the movement of the bypass air into the combustion zone (i.e. second volume 362), and no mixing of the fuel from the fuel inlets and the air in third volume 363 occurs. Mixing of the bypass air and burner combustion products may occur in a downstream direction of second volume 362 and third volume 363, i.e. downstream of body bottom end wall 14.

(61) This feature is known as an air bypass. Such a feature allows the complete bypass of air through the third volume 363 with no interference with the combustion of the fuel. This may be useful where the swirl burner assembly 10 is required to function with an air-fuel ratio greater than the burner unit 100 is designed for, and, as such, the air can be bypassed through the third volume 363, yet emissions will still be within design limits.

(62) This can allow the swirl burner assembly 10 to function with much higher air to fuel ratio, such as from 2 to 18 lambda.

(63) In some embodiments (not shown) the bypass of air need not be a permanent feature, but bypass opening 320 can be enabled as required, for instance by the opening of bypass opening 320 in burner wall 40. Therefore, the operating mode may dictate if the bypass is required.

(64) FIG. 5 shows that the igniter opening 82 for swirl burner assembly 300 protrudes through third volume 363, such that the igniter 80 is positioned within the second volume 362. The extension of the igniter opening 82 is necessary to allow the igniter 80 to be able to cause a spark in the same volume as the combustible gases (i.e. to form a flame tube).

(65) FIG. 7 shows a swirl burner assembly 400 similar to that as hereinbefore described, incorporating both the air split opening feature (for the air curtain( ) and the bypass feature. Thus, third volume 363 is provided, and a plurality of air split openings 210, thus combining the features in a single burner. The presence of bypass opening 320 means that about 5% of the total flow through air inlet 70 passes through air split openings 210.

(66) The air flow through third volume 363 has the secondary effect of cooling the inner wall 360. However, where additional cooling is required, the air curtain provided by air split opening 210 can be combined with bypass opening 320 as shown in FIG. 7, thereby cooling inner wall 360 by the flow of air over inner wall outer surface 366 and inner wall inner surface 364.

(67) The temperature at the burner outlet is measured downstream of the combustion zone, i.e. in a downstream direction from second volume 62 beyond swirl burner body bottom end wall 14. In the configurations where an air bypass is utilised, the temperature at the burner outlet is the temperature of the combined flow of exhaust gases from the second volume and third volume. In the configurations where an air bypass is utilised, the mixing of the bypass air and combustion products may occur downstream of swirl burner body downstream end 30.

(68) When used in a fuel cell system, the burner has four modes of operation:

(69) 1) Warm-Up, Non-Reforming:

(70) Where the fuel cell system is cold, it is necessary to heat the stack prior to reaching the operational state. This initial phase raises the temperature of the fuel cell stack outlet to greater than 275 deg C., more preferably 300 deg C. The fuel may be gaseous or vaporised, but in this mode it is HCV fuel which is directly fed to the burner.

(71) Considering swirl burner assembly 10 of FIGS. 1 and 2, in this mode, the HCV fuel is fed into the burner through HCV fuel tube 120 of the burner unit 100. The HCV fuel exits HCV fuel tube 120 at HCV inlet 125. Simultaneous to this operation, air is fed into first volume 52 through air inlet 70. The air inside this volume passes through air inlet holes 115 into burner unit inner volume 116 and flows in the downstream direction toward swirl burner body downstream end 30.

(72) Prior to reaching the swirl mixer 150, i.e. upstream of swirl mixer 150, the HCV fuel and the air are exposed to one another for the first time since entering swirl burner body 12. It is here that initial pre-mixing of the HCV fuel and the air takes place. The HCV fuel and air mixture passes through swirl mixer 150 and the greatest degree of mixing between the HCV fuel and the air occurs through swirl mixer 150 and just into the second volume 62. This area just downstream of the swirl mixer 150 is the mixing zone. A high degree of mixing of the HCV fuel with the air is important to allow complete combustion and reduce the amount of unwanted emissions, such as CO and NO.sub.x.

(73) Although the term ‘air’ has been used, ‘oxidant’ is also a commonly used term to describe the oxygen carrying medium, along with other terms used in the art. As such air and oxidant are interchangeable for the purposes of this specification.

(74) The mixture of HCV fuel and air is then ignited via igniter 80. Swirl mixer 150 is an axial-swirler, which results in a reverse flow region or recirculation zone within the second volume 62. The recirculation zone is such that it impacts not only the combustion zone, but also the mixing zone. This has a number of benefits: ideally combustion of the HCV fuel mix should occur in this zone since the mixing will be most intense; also this reverse flow has the effect of reducing the length of the flame. As a result of the recirculation zone, the flame seat is just downstream of the swirl mixer 150.

(75) During this operation mode, the air flow rate is controlled by the control system which, amongst other measurements, measures the inlet temperature to the burner. The HCV fuel flow is controlled by the control system using a proportional control valve which varies the HCV fuel flow rate according to the temperature at the burner downstream end. The air flow rate through the burner in this mode can vary from 70 to 116 SLM. The HCV fuel flow rate is expected to be between 0.8 to 6 SLM. Where the air-fuel equivalence ratio (lambda) is equal to or less than 4.

(76) The varying of the arrangement and positioning of HCV fuel inlet 125 along with the size of the holes in the inlet can have an effect on the combustion and functioning of the burner such as producing different emissions which are beyond the regulated limits.

(77) 2) Warm-Up. Reforming.

(78) The second mode of operation for swirl burner assembly 10 occurs at fuel cell stack temperatures greater than 275 deg C., more preferably 300 deg C. This mode transitions the fuel from a directly fed HCV fuel to a LCV fuel from the fuel cell stack. That is to say that the LCV fuel may be a reformate gas or anode off gas from the reaction of the fuel cell.

(79) The LCV fuel is fed into swirl burner assembly 10 through LCV fuel tube 130. This LCV fuel tube 130 passes through the centre of the inner diameter of swirl mixer 150 and into second volume 62. It is only at this point that the LCV fuel is fed into second volume 62 through LCV inlet 135. Notably this is downstream of the flame seat of the HCV fuel.

(80) Since the LCV fuel does not pass through swirl mixer 150, there is a less intense mixing area with air in second volume 62 and only a small amount of mixing with the air prior to combustion occurs when compared to the HCV fuel. However, for the LCV fuel, this is preferred since the composition does not favour a high degree of mixing pre-combustion to result in the lower emissions of CO and NO.sub.x.

(81) Combustion occurs downstream of LCV fuel inlet 135. There is a complementary effect of the swirl mixer 150: the combustion of an LCV fuel typically results in a longer flame, i.e. a greater length than a HCV flame, this is partly due to the less intense combustion and greater volumetric flow; the reverse flow region from the swirl mixer 150, reduces the flame length of the LCV fuel flame. Such a reduction in flame length is useful for space saving, allowing a shorter, more compact swirl burner body 12, but also for protecting instrumentation toward the downstream end of, or even beyond the downstream end of the swirl burner assembly 10 (i.e. downstream of body bottom end wall 14).

(82) As the temperature of the fuel cell stack increases toward 550 deg C., the control system reduces the HCV flow and hence the mixed operation of the HCV and LCV fuels moves toward a solely LCV fuel operation as the fuel cell undertakes electrochemical reactions.

(83) 3) Idle/Power Draw

(84) In the third mode of operation, the fuel cell stack is typically at about 550 deg C. (the exact temperature of individual fuel cells and individual fuel cell components will vary; the fuel cells of the fuel cell stack will operate in the range of about 500-610 deg C.). This is a predominantly LCV fuel situation. In this mode, the LCV fuel continues to be fed into the burner through LCV tube 130. However, the LCV fuel flow rate is now determined by the fuel cell stack, and the electrical output required by the fuel cell system.

(85) The air flow through the fuel cell system during this operation mode is controlled by the temperature of the fuel cell stack. The outlet temperature of the burner is monitored, and if it drops below a certain threshold, additional HCV fuel is added to increase the temperature of the system which will maintain or increase the temperature of the fuel cell stack. However, the system is ideally designed such that only LCV fuel is required in this mode.

(86) 4) Shut Down

(87) In the fourth mode of operation, the LCV fuel flow is reduced to reduce fuel cell stack and fuel cell system temperatures until fuel cell stack reaches around 450 deg C., HCV fuel flow to the fuel cell system is stopped, which in turn stops the flow of LCV fuel through LCV fuel inlet 135 is stopped and combustion ceases. The fuel cell system is then left to cool down naturally.

(88) Referring to FIGS. 8A to 10C, a number of trends showing empirical results are shown. The labels of the trends are summarized as listed.

(89) NO.sub.x Air Free—Air free NO.sub.x emissions from the burner,

(90) Data points shown as an upward pointing triangle;
CO Air Free—Air free CO emissions from the burner, Data points shown as a downward pointing triangle;
tAirTgbOut—Temperature of air out of the burner, Data points shown as a square;
tAirTgbIn—Temperature of air into the burner, Data points shown as a circle;
dmolFuelRef—LCV fuel flow into the burner, Data points shown as a vertical bar;
dmolFuelTgb—HCV fuel flow into the burner, Data points shown as an asterisk;
lambda—The fuel to air ratio of the burner at the burner inlets, Data points shown as a solid diamond.

(91) FIGS. 8A to 10C show the graphical plots of the results of the real operation of the swirl burner assembly in a number of modes of operation along with the swirl burner assembly reaction to various events. The three Figures per operation (i.e. A, B and C) are all showing the same operating period for a swirl burner assembly according to the present invention. The time period for this data acquisition (x axis measured in hours) is not shown to start from zero and represents different operating phases of the burner of the present invention during a continuous test.

(92) The trends in FIGS. 8A, 9A and 10A show the air temperature in and out of the swirl burner assembly and is provided to show the swirl burner assembly is producing heat itself—the temperature out of the burner—and that the fuel cell stack is being heated by the burner operation and the hot off-gases are being fed back into the swirl burner assembly, i.e. the temperature into the swirl burner assembly. The top most trend also shows the carbon monoxide (CO) and Nitrous Oxides (NO)) within the combusted gases, i.e. gases leaving the swirl burner assembly. These are measured in parts per million (ppmv)—which is the typical unit of measurement in the art for such gases and are the air-free measurements, i.e. the adjusted values to simulate oxygen-free conditions in the combustions gases. The CO, NO.sub.x and other combustion products are collectively known as emissions, since they are the primary products that are produced from combustion of gases that are known to be undesirable from an environmental perspective. As such, the reduction of emissions is the subject of much legislation concerning combustion of gases. For the purposes of the invention, emissions will normally refer to just the CO and NO.sub.x since these are the primary products which the invention seeks to reduce.

(93) The trends in FIGS. 8B, 9B, and 10B show the fuel flow of the HCV fuel and LCV fuel. This is fuel flow into the swirl burner assembly and will indicate what mode the swirl burner assembly is operating in. For instance, when there is LCV flow, it is likely from the fuel cell stack which has reached a sufficient temperature to produce anode off-gases which may now be combusted. A HCV fuel flow shown on the trend indicates that there is a HCV fuel flow to the swirl burner assembly. HCV fuel flow is possible in any mode of operation, since its feed is independent of fuel cell operation.

(94) The trends in FIGS. 8C, 9C, and 10C show a ratio of the air to the fuel, where an equal proportion of air to fuel will have a lambda of 1, and as the proportion of air increases such that a mixture of air and fuel becomes more lean, the lambda will increase. The lambda trend shows the total lambda of the fuel and oxidant flow at the swirl burner assembly inlets, i.e. the flow at air inlet 70, LCV inlet 135 and HCV inlet 125. The lambda shown includes a calculation of the depletion of oxygen in the air stream when the fuel cell is operating. The lambda of the combustion reactants is important since the oxidant flow is controlled by the fuel cell stack. It is therefore desirable to have a swirl burner assembly which is able to operate over a large lambda range such that the oxidant flow does not need to be compensated by additional fuel flow for the burner to produce a stable combustion. Note, where the fuel flow has stopped, the lambda on the trend will increase off the scale, this is because with no fuel flow ratio of air to fuel is infinitely great. This is typically seen on the trends where the lambda increases above 20.

(95) Referring to FIGS. 8A, 8B and 8C, initially we can see in FIG. 8B that the fuel flow starting at 8 SLM is a HCV fuel. The temperature into the swirl burner assembly shown in FIG. 8A is initially quite low and certainly below the 275 deg C. required for the reforming operation of the fuel stack to commence. This is therefore mode 1: warm-up, non-reforming. The swirl burner assembly is in a purely HCV mode and the lambda is quite low, around 3 to 4 lambda, as seen in FIG. 8C, that is to say that the fuel mix is quite rich to create the heat necessary for heating up the fuel cell stack. Notably, although this is a purely HCV fuel mode in a warm-up phase, the emissions are still very low and even below the required limits. Note that for emission limits, it usual that the emissions are averaged over a period, start-up is an expected period where emissions are known to be greater.

(96) As the fuel cell stack temperature increases, the system is able to start reforming and LCV fuel is available for the swirl burner assembly. This is seen by the increase in the air temperature entering the swirl burner assembly and the initiation of LCV fuel flow. There is a brief increase in emissions at this stage, but as the lambda drops and the temperature rises, the emissions quickly drop to far below the target. The burner is in mode 2: warm-up reforming. This is a dual fuel operation, two flows are being combusted by the same burner in the same flame tube (i.e. second volume 62) and the resulting emissions are low.

(97) The temperature of the fuel stack then reaches a nominal level, seen by the levelling off of the temperature into the swirl burner assembly. This is now mode 3: steady state. In this mode the swirl burner assembly is predominantly fuelled by LCV fuel supplied from the fuel cell. The design of the burner results in very low emissions, the NO.sub.x emissions being around a tenth of the limit and the CO being even lower.

(98) FIGS. 8A to 8C clearly demonstrate that the geometry and positioning of the fuel inlets have resulted in a swirl burner assembly which can cope with various fuels with very different combustion requirements, yet still have low emissions.

(99) Note that where the trend key is shown in the top right hand corners of FIGS. 8A and 8C, the data points continue mostly in the manner in which they did prior to this key and there are no untoward data points being obscured.

(100) FIGS. 9A to 9C show a steady state operation with a step change, such a step change can occur due to a different current draw from the fuel cell stack. This will result in a different fuel flow to the swirl burner assembly and a different mix of HCV and LCV fuel. It is a known issue that step changes can result in emissions spikes due to different combustion characteristics and incidences such as flame switching. In this case the swirl burner assembly was running with very low emissions. When the step change occurs, i.e. when the fuel flow changes, the emissions do increase slightly, but still well below the limits. This shows the resilience the swirl burner assembly has to varying fuel flows when at steady state.

(101) Note that where the trend key is shown in the top right hand corners of FIG. 10A, the data points continue mostly in the manner in which they did prior to this key and there are no untoward data points being obscured.

(102) It is a known issue that hot starts can cause issues for burners and fuel cell systems. Combustion characteristics due to the high air inlet temperature can be very different resulting in instability in the flame and as a result emissions can very high. A fuel cell stack can take 12 to 16 hours to cool down to cold start conditions, whereas the fuel cell is often required more frequently. It is therefore desirable for a swirl burner assembly to be able perform a hot start-up yet maintain low emissions. In FIGS. 10A to 10C such a situation is shown, the system is restarted when the temperature into the swirl burner assembly is still high, approx. 300 deg C., yet in each the case, the emissions do not greatly exceed the limits, with the CO being very low.

(103) Note that where the trend key is shown in the top right hand corners of FIG. 10A, the data points continue mostly in the manner in which they did prior to this key and there are no untoward data points being obscured.

(104) Overall the design of the swirl burner assembly results in lower emissions when fuelled by various fuels in single mode and mixed mode, along with operating over a large lambda range and having a small flame length allowing for a compact design.

(105) FIG. 11 shows an alternative arrangement of the swirl burner assembly 10 of FIGS. 1 and 2. A burner assembly that is axial burner assembly 10′ is shown. The axial burner assembly 10′ has the same features described above with reference to the swirl burner assembly 10 other than the swirl mixer 150. Therefore, like reference numerals are used to describe the axial burner assembly 10′ and the description with reference to the swirl burner assembly 10 applies to the axial burner assembly 10′.

(106) The axial burner assembly 10′ has a burner wall 40 that defines the first volume 52 and second volume 62. The LCV fuel tube 130 passes through the first volume 52 and is connected to the LCV inlet 135 that is positioned in the second volume 62 for introducing fuel directly to this second volume 62 without introducing the fuel to the first volume 52 and mixing with an oxidant. Likewise, the oxidant inlet and HCV inlet each respectfully introduce oxidant and HCV fuel into the first volume 52.

(107) The burner 100 is integrally formed within the assembly and has a second end 124 that faces the second volume 62. At the second end of the burner unit 124 there is provided a burner plate 156. The burner plate 156 is positioned across the burner wall 40 and faces the first volume 52 on one side and the second volume 62 at the other side. Therefore, the burner plate 156 replaces the swirl mixer 150 of the earlier embodiment.

(108) The burner plate 156 has a plurality of openings 157 that define burner plate channels or passage ways 157 between the first volume 52 and second volume 62. Therefore, an oxidant and fuel that mixes in the first volume 52 passes through the plurality of burner plate passageways 157 and combusts in the second volume 62.

(109) The plurality of burner plate passageways 157 allows the flame seat to be very close to the second end 124 of the burner unit 100.

(110) Referring to FIG. 12A, this is a schematic view of part of a piping and instrumentation diagram of the fuel cell system 800. This shows a number of fluid flow inputs and outputs of a tail-gas burner 400 that optionally has the features of the swirl burner assembly 10 discussed above or a burner without a swirl mixer 150, such as the axial burner assembly 10′ discussed with reference to FIG. 11.

(111) A fuel cell stack 405 is shown schematically. Each fuel cell in the stack has a cathode side 60, an anode side 401, and an electrolyte layer 501 between the cathode side 60 and the anode side 401. A fuel is provided to the fuel cell stack 405. The fuel provided can be the HCV fuel, such as a fuel gas or natural gas. The fuel can be reformed through a reformer prior to entry to the fuel cell stack 405.

(112) The fuel cell stack 405 has common outlets from the cathode sides 60 and the anode sides 401 of all the respective cells. The outlet from the cathode side 60 is the cathode off-gas pipe system D, this provides a cathode off-gas fluid flow path D between the outlet of the cathode side 60 and the cathode off-gas inlet 83 of the tail gas burner 400. It is noted that the cathode off-gas inlet 83 is preferred to be the oxidant inlet 70 discussed above with reference to the earlier figures.

(113) Whilst it is shown that the cathode off-gas fluid flow path D is connected directly between the cathode side 60 and the tail-gas burner 400, in some embodiments, the cathode off-gas fluid flow path D may pass through further systems such as heat exchangers. The air to the burner may also be provided from other sources than the cathode off-gas fluid flow path D. For instance, the oxidant inlet 70 and cathode off-gas inlet 83 may be separate and provide different sources of air.

(114) The anode side 401 is in fluid communication with the anode off-gas inlet 821 through anode off-gas pipe system B which forms the anode off-gas fluid flow path B. The anode off-gas inlet 821 is the inlet to the LCV fuel tube 130 that leads to the LCV inlet 135 as discussed above with reference to the swirl burner assembly 10 or axial burner assembly 10′. The LCV fuel and anode off-gas fuel terms are interchangeable.

(115) A fuel source 250 for a fuel gas (i.e. a HCV fuel) is provided. The fuel source 250 is connected to a fuel gas inlet 805 on the tail gas burner 400 through a fuel gas fluid flow C that is a fuel gas pipe system C. The fuel gas inlet 805 is the inlet of the HCV fuel tube 120 that leads to the HCV inlet 125 as discussed above with reference to the swirl burner assembly 10 and or axial burner assembly 10′. The HCV fuel, fuel gas and natural gas terms are interchangeable.

(116) The fuel source 250 may be provided to various other parts of the fuel cell system 800, such as providing fuel for the fuel cell stack 405. This is not shown in the partial system view of FIG. 12A.

(117) On the fuel gas pipe system C there is provided a valve 810. The valve 810 is a three-way valve in communication with the fuel source 250, the fuel gas pipe system C, and a bypass line A. The bypass line A forms a fuel gas to anode off-gas fluid flow path A between the fuel source 250 and the anode off-gas pipe system B. Therefore, the bypass line A allows the delivery of fuel gas to the anode off-gas inlet 821.

(118) A portion of the anode off-gas pipe system B that is downstream from the connection with the bypass line A (i.e. between the bypass line and the off-gas inlet 821), will therefore allow the flow of a fuel gas and an anode off-gas. This portion of the connection is the fuel gas and anode off-gas fluid flow path B′ and therefore mixing of the two fuels can occur in this mixing portion of the anode off-gas piping system B′. Therefore, the bypass line A allows the delivery of a fuel gas (i.e. HCV fuel) to the LCV inlet 135. This delivery is controlled as discussed in detail below.

(119) Whilst mixing of the fuel gas and the anode off-gas is described as occurring in the mixing portion of the anode off-gas piping system B′, a mixer may be provided to allow mixing of the fuels. The mixer may be in the form of a pipe that encourages turbulent flow, or a larger chamber for the containing of the fuel, or other combinations to result in the mixing of fuels.

(120) The tail-gas burner 805 has a tail-gas burner exhaust 81 where combusted fuel, i.e. a fuel gas, anode off-gas or a combination thereof with an oxidant, is exhausted from the tail-gas burner 805. The exhaust gas is then passed through a tail-gas burner off-gas fluid flow path E for use elsewhere—which may be out of the fuel cell system, such as for CHP systems.

(121) As discussed above, the three-way valve 810 can deliver the fuel flow to the anode off-gas pipe system B and to the fuel gas pipe system C and the inlets and thus HCV inlet 125 and LCV inlet 135 connected to these. The three-way valve 810 is not in operation at all times. Instead, it can be operated to direct all the fuel gas flow to the fuel gas pipe system C, or all the flow to the anode off-gas pipe system B. The operation of the valve 810 is dependent as to what mode the fuel gas system 800 is operating, such as the start-up, warming, operating, shut-down and at what temperature the fuel cell system 800 and particularly the fuel cell stack 405 is operating at.

(122) In some embodiments, the three-way valve 810 is operable to divert a portion of flow to the anode off-gas pipe system B, thus allowing simultaneous fuel gas flow to both the fuel gas inlet 805 and the anode off-gas inlet 821. A controller may control the (variable) flow ratio to that valve.

(123) FIG. 12A (and FIGS. 12B, 12C and 12D) is a piping and instrumentation diagram (P&ID), therefore it will be appreciated that the inlets to the tail-gas burner 400 shown in these figures are merely illustrative, that is to say, for instance, that the anode off-gas inlet 821 is shown entering the tail-gas burner at the side in FIG. 12A. However, FIG. 2 would require the anode off-gas inlet 821 to be positioned at the first end of the burner assembly 10 due to the location of LCV fuel tube 130. Therefore, FIG. 12A does not limit to the connection positions shown, but rather shows how components are being connected. This likewise applies to the fuel gas inlet and the cathode off-gas inlet 83. Likewise, the symbol used for a tail-gas burner 400 of FIG. 12A does not show a burner unit 100, swirl mixer 150 or burner plate 156. However, the tail-gas burner 400 can be the swirl burner assembly 10 or axial burner assembly 10′ or other burner assemblies as previously discussed.

(124) Referring to FIGS. 12B, 12C and 12D, these show respective schematic views of alternative fuel cell systems with minor modifications over that of FIG. 12A. All reference numerals depict the same devices and pipe systems. FIG. 12B merely shows a two-way on/off fuel supply valve upstream of the three-way valve 250, but is otherwise the same layout as FIG. 12A, although the arrangement is depicted differently. FIGS. 12C and 12D merely replace the three-way valve 250 of FIG. 12A at which the pipe systems A and C are connected together with respective two-way, on/off valves 812 one respectively in each of pipe systems A and C, where those meet upstream at a permanently open joint connection. FIG. 12D additionally has a two-way on/off fuel supply valve upstream of the open joint connection.

(125) The pipe systems described can be any form suitable for the transfer of fluids, particularly fuels, air, oxidant and off-gases. The pipe systems can be a form of tubular pipes, flexible pipes, etc. The pipe systems may need to withstand temperature fluctuations, including high temperature flows.

(126) the fuel cell system, the tail-gas burner has four main modes of operation:

(127) 1) Warm-Up, Non-Reforming:

(128) Where the fuel cell system is cold, it is necessary to heat the stack prior to reaching the operational state. This initial phase raises the temperature of the fuel cell stack outlet to greater than 275 deg C., more preferably 300 deg C. The fuel may be gaseous or vaporised, but in this mode it is (only) HCV fuel which is directly fed to the burner.

(129) Considering burner assembly 10 or 10′ of FIGS. 1, 2, and 11 and the fuel cell system 800 of FIG. 12A, in this mode, the HCV fuel (fuel gas) is fed into the burner through HCV fuel tube 120 of the burner unit 100. Therefore, the three-way valve 810 is directing all of the HCV fuel from the fuel source 250 to the fuel gas inlet 805 via the fuel gas pipe system C. The HCV fuel exits HCV fuel tube 120 at HCV inlet 125. Simultaneous to this operation, air is fed into first volume 52 through air inlet 70, this is preferably cathode off-gas via the cathode off-gas pipe system D. The air inside this volume passes through air inlet holes 115 into burner unit inner volume 116 and flows in the downstream direction toward burner body downstream end 30.

(130) Prior to reaching the swirl mixer 150 or burner plate 156, i.e. upstream of swirl mixer 150 or burner plate 156, the HCV fuel and the air are exposed to one another for the first time since entering burner body 12. It is here that initial pre-mixing of the HCV fuel and the air takes place. The HCV fuel and air mixture passes through swirl mixer 150 or the burner plate 156. The greatest degree of mixing between the HCV fuel and the air occurs through swirl mixer 150 and just into the second volume 62. This area just downstream of the swirl mixer 150 is the mixing zone. A high degree of mixing of the HCV fuel with the air is important to allow complete combustion and reduce the amount of unwanted emissions, such as CO and NO.sub.x.

(131) Although the term ‘air’ has been used, ‘oxidant’ is also a commonly used term to describe the oxygen carrying medium, along with other terms used in the art. As such air and oxidant are interchangeable for the purposes of this specification.

(132) The mixture of HCV fuel and air is then ignited via igniter 80. Swirl mixer 150 is an axial-swirler, which results in a reverse flow region or recirculation zone within the second volume 62. The recirculation zone is such that it impacts not only the combustion zone, but also the mixing zone. This has a number of benefits: ideally combustion of the HCV fuel mix should occur in this zone since the mixing will be most intense; also this reverse flow has the effect of reducing the length of the flame. As a result of the recirculation zone, the flame seat is just downstream of the swirl mixer 150. Likewise, the burner plate 156 has the effect of confining the flames to a smaller region close to the burner plate 156. This is due to the plurality of burner plate passageways 157 that result in an increased number of flames with a shorter length.

(133) During this operation mode, the air flow rate is controlled by the control system which, amongst other measurements, measures the inlet temperature to the burner. The HCV fuel flow is controlled by the control system using a proportional control valve which varies the HCV fuel flow rate according to the temperature at the burner downstream end. The air flow rate through the burner in this mode can vary from 70 to 116 SLM. The HCV fuel flow rate is expected to be between 0.8 to 6 SLM. Where the air-fuel equivalence ratio (lambda) is equal to or less than 4.

(134) The varying of the arrangement and positioning of HCV fuel inlet 125 along with the size of the holes in the inlet can have an effect on the combustion and functioning of the burner such as producing different emissions which are beyond the regulated limits.

(135) 2) Warm-Up. Reforming. With Warm Tail-Gas Burner.

(136) The second mode of operation for swirl burner assembly 10 or axial burner assembly 10′ occurs at fuel cell stack temperatures greater than 275 deg C., more preferably 300 deg C. This mode transitions the fuel from a directly fed HCV fuel to a LCV fuel from the fuel cell stack 405. That is to say that the LCV fuel may be a reformate gas or anode off gas from the reaction of the fuel cell.

(137) The LCV fuel (anode off-gas) is fed into the tail gas burner 400 through LCV fuel tube 130. As shown in FIGS. 12A to 12D, the anode off-gas is supplied from the anode side 401 of the fuel cell stack 405 through the anode off-gas pipe system B to the anode off-gas inlet 821. This LCV fuel tube 130 passes through the centre of the inner diameter of swirl mixer 150 or inner region of the burner plate 156 and into second volume 62. It is only at this point that the LCV fuel is fed into second volume 62 through LCV inlet 135. Notably this is downstream of the flame seat of the HCV fuel.

(138) Since the LCV fuel does not pass through swirl mixer 150, there is a less intense mixing area with air in second volume 62 and only a small amount of mixing with the air prior to combustion occurs when compared to the HCV fuel. Likewise, because the LCV fuel does not pass through the burner plate 156, there is also only a small amount of mixing with oxidant prior to combustion. However, for the LCV fuel, this is preferred since the composition does not favour a high degree of mixing pre-combustion to result in the lower emissions of CO and NO.sub.x.

(139) Combustion occurs downstream of LCV fuel inlet 135. There is a complementary effect of the swirl mixer 150: the combustion of an LCV fuel typically results in a longer flame, i.e. a greater length than a HCV flame, this is partly due to the less intense combustion and greater volumetric flow; the reverse flow region from the swirl mixer 150, reduces the flame length of the LCV fuel flame. Such a reduction in flame length is useful for space saving, allowing a shorter, more compact swirl burner body 12, but also for protecting instrumentation toward the downstream end of, or even beyond the downstream end of the swirl burner assembly 10 (i.e. downstream of body bottom end wall 14).

(140) Warm-Up. Reforming. With Hot Tail-Gas Burner.

(141) As the temperature of the stack increases toward 550 deg C. and is in the range of 500 deg C. to 550 deg C., a sub mode of this operating mode occurs.

(142) The HCV fuel (fuel gas) is redirected and fed into the burner through the LCV fuel tube 130 of the burner unit 100. Therefore, the three-way valve 810 or two-way valves 812 are moved to direct all of the HCV fuel from the fuel source 250 to the anode off-gas inlet 821 via the bypass line A that connects the fuel source 250 to the anode off-gas pipe system B. The LCV fuel and HCV fuel is thus fed into swirl burner assembly 10 through LCV fuel tube 130. The anode off-gas is supplied from the anode side 401 of the fuel cell stack 405 through the anode off-gas pipe system B to the anode off-gas inlet 821. This LCV fuel tube 130 passes through the centre of the inner diameter of swirl mixer 150 and into second volume 62. It is only at this point that the HCV fuel and LCV fuel is fed into second volume 62 through LCV inlet 135.

(143) The HCV fuel and LCV fuel, therefore, converge in the mixing portion of the anode off-gas pipe system B′ and mixing of the fuels occurs in this area and through the LCV tube 130. Since the mixture of HCV fuel and LCV fuel does not pass through swirl mixer 150, there is a less intense mixing area with air in second volume 62 and only a small amount of mixing with the air prior to combustion. However, due to the high temperature in this sub-mode there would be a tendency for coke formation if HCV fuel was fed into HCV fuel inlet 125, whereas feeding the mixture of LCV fuel and HCV fuel into the LCV inlet 135 reduces the likelihood of coke formation (because hydrogen is present in the LCV fuel) and therefore the tendency for blocking of the HCV fuel lines.

(144) The provision of the bypass line A therefore allows more flexibility in terms of how the system is operated and hence improves system performance (e.g. response times): the continued use in this sub-mode of a mix of HCV fuel as well as LCV fuel can improve warm-up time in this mode, because total fuel flow is increased.

(145) To explain, when the system is in the final stages of warm up it is necessary to flow more fuel than during steady state operation, the fuel cell is consuming the same or more fuel than at steady state, but producing less power, and the heat input to the fuel cell is higher in-order to raise its temperature. While just providing more LCV fuel is possible, it may not be the optimal choice for fastest warm-up, with higher flow rates and larger system components required; for example, the reformer may need to be larger if only LCV was supplied to achieve the same warm-up time. Furthermore, the warm-up may be slowed because of increased flow of fuel into the fuel cell because of the endothermic reaction of reforming within the stack itself.

(146) As the temperature of the fuel cell stack increases toward 550 deg C., the control system selectively reduces the HCV flow and hence the mixed operation of the HCV and LCV fuels moves toward a solely LCV fuel operation as the fuel cell undertakes electrochemical reactions.

(147) 3) Steady State. Full Power.

(148) In the third mode of operation, the fuel cell stack is typically at about 550 deg C. (the exact temperature of individual fuel cells and individual fuel cell components will vary; the fuel cells of the fuel cell stack will operate in the range of about 500-610 deg C.). This is a predominantly LCV fuel situation. In this mode, the LCV fuel continues to be fed into the burner through LCV tube 130. However, the LCV fuel flow rate is now determined by the fuel cell stack, and the electrical output required by the fuel cell system.

(149) Steady State. Low Power

(150) The air flow through the fuel cell system during this operation sub-mode is controlled by the temperature of the fuel cell stack. The outlet temperature of the burner is monitored, and if it drops below a certain threshold, additional HCV fuel is added to increase the temperature of the system which will maintain or increase the temperature of the fuel cell stack.

(151) Where additional HCV fuel is required, this is again fed into the burner through the LCV fuel tube 130 of the burner unit 100. Therefore, the three-way valve 810 or two-way valves 812 are moved to direct all of the HCV fuel from the fuel source 250 to the anode off-gas inlet 821 via the bypass line A that connects the fuel source 250 to the anode off-gas pipe system B. The mixing of the fuels occurs in the mixing portion of the anode off-gas pipe B′ and the in the LCV tube 130.

(152) This low power sub-mode is also a sub-mode where, because of the higher operating temperatures, there could be a tendency for coke formation if HCV fuel was fed into the HCV inlet 125. The tendency for coking to occur reduces with HCV fuel and LCV fuel mixing and supplied through the LCV inlet 135 partly because the of the humidity of the LCV fuel which when mixed with the HCV fuel aids with the reduction of coke. Feeding HCV fuel as well as LCV fuel can also improve reforming water use, because water is used in steam reforming which is the flow into the fuel cell stack 405. Feeding HCV fuel as well as LCV fuel can allow the fuel cell stack 405 to be hotter or for the system to operate more efficiently, because total fuel flow can be increased.

(153) Thus, the provision of the bypass line A again allows more flexibility in terms of how the system is operated thereby improving system performance.

(154) 4) Shut Down

(155) In the fourth mode of operation, the LCV fuel flow is reduced to reduce fuel cell stack and fuel cell system temperatures until fuel cell stack reaches around 450 deg C., HCV fuel flow to the fuel cell system is stopped, which in turn stops the flow of LCV fuel through LCV fuel inlet 135 is stopped and combustion ceases. The fuel cell system is then left to cool down naturally.

(156) Table 1 below summarises how the present invention improves operation of the system in the four main operating modes; in particular, it shows the two sub-modes where it is advantageous to flow HCV fuel into the tail gas burner post-swirl mixer (or post burner plate), so as to minimise the tendency for carbon formation. By pre-swirl or post-swirl mixer feed is meant where the fuel in question enters the tail gas burner (i.e. before or after the swirl vanes or the burner plate in the axial burner assembly).

(157) ‘Hot TGB’ is typically when the air supply/cathode off-gas (stream D) to the tail gas burner is >500-550 C, whereas ‘Warm TGB’ is typically when the air inlet to the tail gas burner is <500-550 C. Transition between modes could be gradual and so it may be beneficial to have a gradual transition of HCV fuel from pre to post-swirl mixer (pre to post burner plate).

(158) TABLE-US-00001 TABLE 1 System with System with Interconnected Separate HCV and HCV and LCV Supplies LCV Supplies Operating Mode HCV fuel LCV fuel HCV fuel LCV fuel Ignition and warm-up no Pre-swirl None Pre-swirl None reforming mixer mixer Warm-up with reforming Pre-swirl Post-swirl Pre-swirl Post-swirl and ‘warm TGB’ mixer mixer mixer mixer Warm-up with reforming None Post-swirl Post-swirl Post-swirl and ‘hot TGB’ mixer mixer mixer Steady State - Full Power None Post-swirl None Post-swirl mixer mixer Steady State - Low None Post-swirl Post-swirl Post-swirl Power mixer mixer mixer Shutdown and ‘hot TGB’ None Post-swirl None Post-swirl mixer mixer Shutdown and ‘warm Pre-swirl Post-swirl Pre-swirl Post-swirl TGB’ mixer mixer mixer mixer

(159) The present invention is not limited to the above embodiments only, and other embodiments will be readily apparent to one of ordinary skill in the art without departing from the scope of the appended claims.

REFERENCE SIGNS

(160) 10 Swirl burner assembly 10′ Axial burner assembly 12 Swirl/axial burner body 12′ Central axis 14 Swirl/axial burner body bottom end wall 15 Swirl/axial burner body exhaust 16 Swirl/axial burner body top end wall 20 Burner unit first end 30 Swirl/axial burner body downstream end 40 Burner wall 42 Burner wall downstream face 44 Burner wall upstream face 44 Burner tube 50 First volume 52 Inner face 54 Burner tube inner surface 60 Cathode side 62 Second volume 64 Body inner surface 66 Body outer surface 70 Air inlet 80 Igniter 81 Tail gas burner exhaust 82 Igniter opening 83 Cathode-off gas inlet (oxidant inlet) 100 Burner unit/Burner 110 Burner unit outer body 111 Burner unit top inner surface 112 Shoulder 114 Inner face 115 Air inlet holes 116 Burner unit inner volume 120 HCV fuel tube 121 HCV fuel tube inner surface 122 HCV fuel tube outer surface 123 HCV tube internal volume 124 Burner unit second end 125 HCV inlet 130 LCV fuel tube 130′ Finger 131 LCV fuel tube inner surface 132 LCV fuel tube outer surface 133 LCV tube internal volume 135 LCV inlet 140 Outer collar 144 Outer collar outer surface 150 Swirl mixer 155 Vanes 156 Burner plate 157 Burner plate passageways 160 Inner collar 162 Inner collar outer surface 163 Inner collar inner surface 200 Swirl burner assembly with oxidant curtain 210 Air split opening 150 Fuel source 300 Swirl burner assembly 310 Outer wall 320 Bypass opening 360 Inner wall 361 Outer wall inner surface 362 Second volume (air bypass) 363 Third volume 364 Inner wall inner surface 366 Inner wall outer surface 400 Swirl burner assembly/tail gas burner 401 Anode side 405 Fuel cell stack 501 Electrolyte layer 800 Fuel cell system 805 Fuel gas (HCV fuel) inlet 810 Three-way valve 812 Two-way valve 821 Anode off-gas inlet A fuel gas to anode off-gas fluid flow path—bypass line B anode off-gas fluid flow path—anode off-gas pipe system B′ fuel gas and anode off gas fluid flow path—mixing portion of anode off-gas pipe system C fuel gas fluid flow path—fuel gas pipe system D cathode off-gas fluid flow path—cathode off-gas pipe system E tail-gas burner off-gas fluid flow path