EP1723685A2

EP1723685A2 - Mixer/eductor for high temperature fuel cells

Info

Publication number: EP1723685A2
Application number: EP04821374A
Authority: EP
Inventors: Scott Blanchet; Alan Barlow; William J. Snyder; Dennis R. Farrenkopf; Robert J. Moffat
Original assignee: Fuelcell Energy Inc
Current assignee: Fuelcell Energy Inc
Priority date: 2004-02-17
Filing date: 2004-11-12
Publication date: 2006-11-22
Also published as: CN1906778A; CN100468826C; WO2005081709A3; WO2005081709A2; KR100842987B1; JP2007519204A; KR20060120245A; EP1723685A4

Abstract

A fuel cell system including a fuel cell stack having an anode side and a cathode side, the anode side having an inlet for receiving fuel and an outlet for discharging anode exhaust gas, and the cathode side having an inlet for receiving oxidant gas and an outlet for discharging exhaust oxidant gas, and including a mixer/eductor for mixing the exhaust anode gas with the oxidant supply gas. The mixer/eductor is adapted to create divergent jets or streams of oxidant supply gas so as to enhance mixing and promote pressure loss reduction. Specifically, the mixer/eductor includes elements each disposed at an angle relative to the path of incoming oxidant supply gas which act to establish the multiple oxidant gas streams.

Description

MIXER/EDUCTOR FOR HIGH TEMPERATURE FUEL CELLS BACKGROUND OF THE INVENTION Field of the Invention This invention relates to controlling the pressure differential between the anode or fuel and cathode or oxidant sides of a fuel cell stack and, in particular, to minimizing the pressure difference when mixing the anode-outlet or exhaust gas with inlet air or oxidant gas for supply to the stack cathode side. More particularly, the invention relates to anode tail gas mixing and differential pressure compensating for fuel cell systems. Description of the Related Art A fuel cell is a device that directly converts chemical energy stored in any hydrogen containing fuel such as hydrogen, methane or natural gas into electrical energy by means of an electrochemical reaction. This differs from traditional electric power generating methods which must first combust the fuel to produce heat and then convert the heat into mechanical energy and finally into electricity. The more direct conversion process employed by a fuel cell has significant advantages over traditional means in both increased efficiency and reduced pollutant emissions. In general, a fuel cell, similar to a battery, includes a negative or anode electrode and a positive or cathode electrode separated by an electrolyte that serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. To achieve this, gas flow fields are provided adjacent to the anode and cathode through which fuel and oxidant gas are supplied. In order to produce a useful power level, a number of individual fuel cells must be stacked in series with an electrically conductive separator plate between each cell and its neighbor. In high temperature fuel cell stacks, the fresh air provided to the oxidant or cathode-side of the stack needs to be heated from ambient temperature to the temperature of the fuel cell stack. One way to accomplish this is by burning the unused fuel from the anode-side exhaust in the incoming air. This "burning" may be done in a gas-phase combustor or in a catalytic reactor, the latter being more common in high temperature fuel cell applications. In order to ensure complete reaction of the fuel and to minimize temperature gradients in the catalyst, the anode-side exhaust must be completely mixed with the air. In terms of the process flow, this mixing process couples the anode-side gas pressure to the cathode-side gas pressure at the junction point of the two streams. The pressure at the exit of the anode-side is higher than the pressure at the inlet of the cathode-side by the amount required to overcome the pressure losses associated with any connection piping and with the oxidizer used to burn the gases. Any pressure difference creates a tendency for anode gas to leak into the cathode inlet space, and the higher the pressure difference the better the seals must be. Seals between anode and cathode in a fuel cell stack are typically formed by a mechanical force between two surfaces; in some cases, manifold seals involve a porous gasket material designed to allow an "acceptable" leak rate. Leak rate in both these examples is a function of the anode to cathode pressure differential. Therefore, minimizing the pressure differential is important to prevent excessive leaks. Ultimately, the leaks act to reduce system efficiency, increase pollutant emissions and reduce stack life. Minimizing the pressure differential has been achieved in past systems by attempting to equalize the pressure of the oxidant gas at the inlet of the cathode-side of a stack to the pressure of the exhaust fuel gas at the exit of the anode-side of the stack. This has to be realized in the face of the other operating requirements that tend to make the pressures unequal. Current fuel cell systems have taken a variety of approaches to solving both the mixing and differential pressure problems. Mixing is often achieved by allowing the two streams to flow in a long length of pipe prior to delivery to the catalyst.

However, the length of pipe required for adequate mixing is far too large to be used in commercial systems and results in excessive pressure differential. Long pipes can also produce undesirable heat losses from the gas streams. Other mixing systems employ commercially available "static" mixing plates which generate a high degree of fluid shear in the mixture and provide very uniform compositions. Unfortunately, these static mixers also generate large pressure losses that increase the imbalance in anode-to-cathode pressure on the fuel cell stack. To solve the pressure balance problem, one system utilizes a high temperature booster blower placed between the exit of the anode-side of the stack and the mixing point to overcome the pressure loss of the connection piping, mixer and oxidizer. This has the advantage of independently controlling the pressure balance but adds significant cost and reliability issues to a commercial system. Another system uses a downstream, hot recycle blower to draw both the anode exhaust gas and fresh air oxidant gas through a mixing device and oxidizer. This system configuration allows the gas pressure at the inlet on the cathode-side to run higher than the gas pressure at the exit on the anode-side, with some control over the difference. Disadvantages to this system are, again, the cost and reliability of the recycle blower as well as the overall complexity of the system hardware. A further approach to resolving the pressure differential is to simply allow the fuel pressure to run higher than the oxidant pressure. The experience in this case is that a multitude of operating problems can arise, such as non-uniform stack temperatures, reduced system efficiency and elevated exhaust pollutant emissions. In U.S. Application Serial No. 10/187,495, assigned to the same assignee hereof, a mixer/eductor is disclosed which is adapted to provide both the desired mixing of the fresh oxidant or air and the anode exhaust gas and a reduced pressure difference between the gas at the inlet at the cathode-side of a fuel cell stack and the gas at the outlet at the anode-side of the stack. The mixer/eductor uses energy from the incoming fresh air stream (provided by a fresh air blower) to reduce the pressure of the anode stream at the mixing point. Particularly, the mixer/eductor is designed to increase the velocity and decrease the pressure of the oxidant supply gas where the anode exit flow joins, and is mixed with, the oxidant gas stream. The high velocity mixture then passes through a diffuser section that reduces the mixture velocity and raises the mixture pressure. The increase in pressure of the mixed gases is controlled so that the gas pressure at the stack anode-side outlet is made substantially equal to the gas pressure at the stack cathode-side inlet. It is an object of the present invention to provide an improved mixer/eductor wherein the pressure loss of oxidant gas through the eductor is further reduced, the level of mixing of the oxidant gas fresh air with anode exhaust gas is optimized, and the pressure between the gas at the inlet of the cathode-side and the gas at the outlet of the anode-side of the stack is balanced over the entire operating range of the system and in a manner which avoids the above disadvantages.

SUMMARY OF THE INVENTION In accordance with the principles of the present invention, the above and other objectives are realized in a fuel cell system having a mixer/eductor of the above-type further adapted to create multiple jets for the oxidant supply gas, whereby a desired mixing is realized between the oxidant supply gas and the anode exhaust gas in the mixer/eductor and a more balanced pressure is realized between the oxidant gas at the cathode-side inlet and the anode exhaust gas at the anode-side outlet of the fuel-cell of the system. More particularly, in accord with the invention, the mixer/eductor includes elements arranged at an angle relative to the path of the oxidant supply gas to thereby produce a diverging set of jets of cathode gas that interact and mix efficiently with the anode out gas. This configuration of the mixer/eductor allows the desired mixing and balance of pressure to be realized for a given system design over a known range of operating points. Additionally, the flow of the oxidant input gas or the oxidant exhaust gas from an exhaust recycle blower can be controlled to help balance the pressures.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other features and aspects of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which: FIG. 1 is a block diagram showing a fuel cell system including a mixer/eductor in accordance with the principles of the present invention; FIG. 2 A is a cross-sectional view of a first embodiment of the mixer/eductor of FIG.1; FIG. 2B is an isometric cross-sectional view of a portion of the mixer/eductor of FIG. 2 A; FIG. 2C is a cross-sectional view of a portion of the mixer/eductor shown in FIG. 2B, showing the gas flow paths; FIG. 3 A is a cross-sectional view of a second embodiment of the mixer/eductor of FIG. 1; FIG. 3B is an isometric cross-sectional view of a portion of the mixer/eductor of FIG. 3 A; FIG. 3C is a cross-sectional view of a portion of the mixer/eductor shown in FIG. 3B, showing the gas flow paths; FIG. 4A is a cross-sectional view of a third embodiment of the mixer/eductor of FIG. 1; FIG. 4B is an isometric cross-sectional view of a portion of the mixer/eductor of FIG. 4 A; FIG. 4C is a cross-sectional view of a portion of the mixer/eductor shown in FIG. 4B, showing the gas flow paths; FIG. 5 A is a cross-sectional view of a fourth embodiment of the mixer/eductor of FIG. 1; FIG. 5B is an isometric cross-sectional view of a portion of the mixer/eductor of FIG. 5 A; FIG. 5C is a cross-sectional view of a portion of the mixer/eductor shown in FIG. 5B, showing the gas flow paths; FIG. 6 A is a cross-sectional view of the mixer/eductor of the aforementioned

'495 application; FIG. 6B is a cross-sectional view of a portion of the mixer/eductor of FIG. 6 A, showing the gas flow paths; FIG. 7 is a graph showing the measured air-side pressure loss improvement of the mixer/eductor of the present invention as compared to the mixer/eductor of the '495 application; FIG. 8 is a graph showing the measured anode-side suction improvement of the mixer/eductor of the present invention as compared to the mixer/eductor of the '495 application; FIG. 9 is a diagram showing the measured mixing uniformity achieved using the mixer/eductor of the present invention; and FIG. 10 is a diagram showing the measured mixing uniformity achieved using the mixer/eductor of the '495 application.

DETAILED DESCRIPTION FIG. 1 is a block diagram showing schematically a fuel cell system 1. The system 1 includes a fuel cell 2 having anode and cathode sections 2A and 2B respectively, an oxidizer 3 and a recycle blower 4. The fuel cell system 1 also includes a mixer/eductor 5 in accordance with the principles of the present invention. As shown, fresh air 6 and exhaust oxidant gas recycled by the recycle blower 4 from the cathode-side section 2B are combined to produce an oxidant supply gas 6 A. The oxidant supply gas 6A is fed to the mixer/eductor 5 at inlet 5 A. In passing through the mixer/eductor 5, the oxidant supply gas 6 A reduces the pressure and entrains anode exhaust gas entering the mixer/eductor 5 at inlet 5B. The mixture of oxidant supply gas and anode exhaust gas exits the mixer/eductor 5 at outlet 5C and continues into the inlet 3 A of the oxidizer 3. In the oxidizer 3, the remaining fuel in the anode exhaust gas of the mixed gases is burned, thereby raising the temperature of the gas stream. The outlet stream from the oxidizer is delivered to the cathode inlet 8 of the cathode section 2B. Oxidant exhaust gas is output from the cathode outlet 9 and, as stated above, a portion is recycled by the recycle blower 4 to the incoming air. Fuel gas is delivered to the anode section 2 A at the inlet 11. In general terms, one purpose of the mixer/eductor 5 in the system 1 is to cause the pressure difference between the anode exhaust gas at outlet 7 of the anode section 2 A and the oxidant gas at the inlet 8 of the cathode section 2B to be reduced or minimized. Another purpose of the mixer/eductor 5 is to achieve a desired mixing of the anode exhaust gas received at the inlet 5B of the mixer/eductor with the oxidant supply gas 6 A received at the inlet 5 A of the mixer/eductor for supply to the oxidizer 3. In accordance with the invention, the mixer/eductor 5 is adapted to increase the velocity and decrease the pressure of the oxidant supply gas at the location where the anode exit gas stream meets the oxidant supply gas stream. This decreases the pressure of the anode exhaust gas at the inlet 5B of the mixer/eductor. The anode exhaust gas mixes with the oxidant supply gas received at the inlet 5 A of the mixer/eductor. The pressure of the mixed gases is increased as it passes through and out of the mixer/eductor 5 to the oxidizer 3. In accordance with the invention, the mixer/eductor 5 is further adapted to include several elements, each arranged at an angle relative to the axis or path of the oxidant supply gas 6 A, whereby divergent jets of oxidant supply gas are established for the oxidant supply gas. In this manner, the increase in pressure of the mixed gases and the mixing of the gases can be better controlled to optimize the mixing and to minimize the pressure difference of the oxidant gas at the inlet 8 of the cathode section 2B and the anode exhaust gas at the outlet 7 of the anode section 2 A. Various embodiments of the mixer/eductor of the invention are described below. First Embodiment (FIGS. 2A-2Q: five nozzles FIGS. 2A-2C show a first embodiment of a mixer/eductor 100 of the invention. Referring to FIG. 2B, the mixer/eductor 100 includes an oxidant supply gas inlet 120 for oxidant supply gas and an anode exhaust gas inlet 121 for anode exhaust gas, the inlets forming a T-shaped mixing assembly 122. In an alternate configuration, shown in broken lines, the mixing assembly 122 can be modified to form a cross-shaped assembly by providing a second inlet 121 A for anode exhaust gas. Additional anode exhaust gas inlets could be added without changing the intent or function of this invention. It should be noted that the cross-sectional areas 101, 102 of the inlets 120, 121 shown in FIG. 2 A are selected based on the overall flow rates desired in the inlets. A nozzle arrangement 126 connected to the inlet 120 includes elements at divergent angles relative to the path of the incoming oxidant supply gas air so as to create divergent oxidant gas jets which enhance the mixing of the two streams and help minimize pressure loss. More particularly, as shown in FIGS. 2A-2C, five nozzle tubes 126A-126E are included in the nozzle arrangement, four of which 126B- 126E are disposed at angles relative to the incoming oxidant path 129 and one of which 126A is disposed along this path. In the illustrative case, the nozzle tube 126A is horizontally disposed, while a first pair of opposing tubes 126B-126C are at angles above and below the tube 126 A, and a second pair of opposing tubes 126D and 126E (not visible) are at angles to the right and left of the tube 126 A. Each of the nozzle tubes 126A-126E has a smooth filleted lead-in 139 (shown in FIG. 2B). The nozzle tubes 126A-126E accelerate the oxidant supply gas to a high velocity as depicted in FIG. 2C by velocity vectors 140, 141, 142 of the tubes 126A- 126C. The number and diameter 106 (shown in FIG. 2A) of the nozzle tubes 126A- 126E are selected to provide the necessary velocity required for the desired level of pressure. In this regard, each of the nozzles tubes 126A-126E may have a different diameter, depending on the specific requirements of the application. In addition, the orientation of the nozzles tubes 126A-126E can be different than shown. Generally the tubes may be arranged with one or more axes of symmetry, but may be arranged purposely to be asymmetric to suit the specific performance requirements of a given application. The length 107 of the nozzle tubes, moreover, is selected to be sufficient to ensure that the flow is directed at the desired angle 108 while preventing excessive frictional pressure loss in the tubes themselves. When the oxidant supply gas exits the nozzle tubes 126A-126E, it begins mixing with the anode exhaust gas in the mixing assembly 122. To this end, the configuration of the nozzle arrangement 126, such as the angles 108 at which the four nozzle tubes 126B-126E are oriented relative to the path 129 of incoming oxidant gas, is such as to maximize entrainment of the anode exhaust gas (depicted by vectors 150 and 151 in FIG. 2C) and mixing of the anode exhaust gas with the oxidant gas in the mixing assembly 122, and to delay the individual jets from coalescing into a single large jet. In addition, the rotational position 128 (FIG. 2B) of the nozzle tubes 126B- 126E is selected so as to maximize mixing of the two streams of gas. In addition, the position 109 of the ends of the nozzle tubes 126A-126E relative to the centerline (FIG. 2 A) of inlet 121 is selected so as to achieve the desired pressure distribution. The high velocity of the oxidant gas entrains the anode exhaust gas inside the mixing assembly 122 and the mixture moves into the mixing duct 123. The mixing duct 123 has a mouth opening 103, a length 104 and a flare or divergence angle 105 all selected to promote the desired mixing. Mixing between the two streams thus continues in the mixing duct and is further aided by a baffle plate 124, which acts to create turbulence and recirculation in the mixture. As shown in FIG. 2A, the position 112, angle 111 and length 110 of the baffle plate 124 are selected to maximize mixing while preventing excessive pressure loss. The gas mixture continues flowing down the mixing duct and enters the catalyst manifold 198, where it turns (as shown by arrow 159) and flows through the catalyst bed 199. Parameters usable for realizing the various components of the mixer/eductor of FIGS. 2A-2C are as follows: a nozzle tube divergence angle 108 of between 0 and 45 degrees; a nozzle tube position 109 of between -1 and +1 times the anode gas pipe diameter 102; a nozzle tube rotational clocking 128 of between -90 to + 90 degrees; a nozzle tube length 107 of between 1 and 10 times the nozzle tube diameter 106; a mixing duct flare angle 105 of between 0 and 30 degrees; a catalyst face orientation angle 159 of between 0 and 90 degrees; the use of five nozzle tubes 126; and the use of baffle plates 124.

Second Embodiment (FIGS. 3A-3C): four nozzles FIGS. 3A-3C show a second embodiment 200 of the mixer/eductor of the present invention. Referring to FIG. 3B, the mixer/eductor includes an oxidant supply gas inlet 220 for oxidant supply gas and an anode exhaust gas inlet 221 for anode exhaust gas, formed as a T-shaped mixing assembly 222. As discussed above with respect to the first embodiment of the mixer/eductor, in an alternate configuration, the mixing assembly 222 can be formed as a cross-shaped assembly by providing an additional inlet for the anode exhaust gas or further elaborated by adding additional inlets at various angular positions. The cross-sectional areas 201, 202 of the inlets 220, 221 shown in FIG. 3 A are selected based on the overall flow rates expected in the specific application. A nozzle arrangement 226 connected to the inlet 220 includes elements set at angles relative to the path of incoming oxidant gas so as to create divergent jets which enhance the mixing of the two streams and minimize pressure loss. More particularly, four nozzle tubes 226A-226D are included in the arrangement and are disposed at angles relative to the incoming oxidant path 229. In FIGS. 3A-3C, only vertically opposing nozzle tubes 226A-226B connected to the right side of the bulkhead that terminates the inlet 220 are shown. A similar pair of vertically opposed nozzle tubes 226C-226D (not visible) are connected to the left side of the bulkhead. Each nozzle tube 226A-226D has a smooth filleted lead-in 239 (shown in FIG. 3B). The nozzle tubes 226A-226D accelerate the oxidant supply gas to a high velocity as depicted in FIG. 3C by velocity vectors 240 and 241 of the tubes 226A- 226B. The diameter 206 (shown in FIG. 3 A) of the nozzle tubes 226A-226D are selected to provide the necessary velocity required for the desired level of vacuum. Each of the nozzles 226 may have a different diameter, depending on the specific requirements of the application. In addition, the nozzle tubes 226A-226D will generally be arranged with one or more axes of symmetry, but may be arranged asymmetrically to suit the specific vacuum and mixing requirements of the application. The length 207 of the nozzle tubes 226A-226D is selected to ensure that the flow is directed at the desired angle 208 while preventing excessive frictional pressure loss in the tubes themselves. When the oxidant supply gas exits the nozzle tubes 226A-226D, it begins mixing with the anode exhaust gas in the mixing assembly 222. The configuration of the nozzle arrangement 226, such as the angle 208 at which the nozzle tubes 226A- 226D are oriented, is such as to maximize entrainment of the anode exhaust gas (depicted by vectors 250 and 251 in FIG. 3C) and mixing of the anode exhaust gas with the oxidant gas in the mixing assembly 222, and to delay the individual gas flow paths from coalescing into a single large path. In addition, the rotational position 228 (FIG. 3B) of the nozzle tubes 226A-226D is selected so as to maximize mixing of the two streams of gas. In addition, the position 209 of the ends of the nozzle tubes relative to the centerline (FIG. 3 A) of inlet 221 is selected so as to achieve the desired pressure distribution. The high velocity of the oxidant gas entrains the anode exhaust gas inside the mixing assembly 222, and the mixture moves into the mixing duct 223. The mixing duct 223 has a mouth opening 203, a length 204 and a flare angle 205 all selected to promote the desired mixing. Mixing between the two streams thus continues in the mixing duct and is further aided by a baffle plate (not shown), which acts to create turbulence and recirculation in the mixture. The gas mixture continues flowing down the mixing duct and enters the catalyst manifold 298, where it turns (as shown by arrow 259) and enters the catalyst bed 299. Parameters usable for realizing the various components of the mixer/eductor of FIGS. 3A-3C are as follows: a nozzle divergence angle 208 of between 0 and 90 degrees; a nozzle tube position 209 of between -1 and +1 times the anode gas pipe diameter 202; a nozzle tube rotational clocking 228 of between -45 to +45 degrees; a nozzle tube length 207 of between 1 and 10 times the nozzle tube diameter 206; a mixing duct flare angle 205 of between 0 and 30 degrees; a catalyst face orientation angle 259 of between 0 and 90 degrees; the use of four nozzle tubes 226; and the use of baffle plates 124.

Third Embodiment (FIGS. 4A-4C): one nozzle, air spinner, mixing cone and mixture spinner FIGS. 4A-4C show a third embodiment 300 of a mixer/eductor of the present invention. Referring to FIG. 4C, the mixer/eductor 300 includes an oxidant gas inlet 320 for oxidant supply gas and an anode exhaust gas inlet 321 for anode exhaust gas, forming a T-shaped mixing assembly 322. In an alternate configuration (not shown in FIGS. 4A-4C), the mixing assembly 322 can be modified to form a cross-shaped assembly by providing an additional inlet for anode exhaust gas or further elaborated by adding additional inlets at various angular positions. The cross-sectional areas 301, 302 of the inlets 320, 321 shown in Fig. 4A are selected based on the overall flow rates expected in the specific application. A mixing arrangement 326 connected to the inlet 320 includes elements at an angle relative to the path of the incoming oxidant supply gas so as to create diverging flows which enhance the mixing of the two streams and minimize pressure loss. More particularly, as shown in FIG. 4B, the mixing arrangement includes a nozzle 326 A disposed along the path 329 of the incoming oxidant supply gas, and, preferably, also an air spinner 330 at the entrance to the nozzle. The air spinner 330 has bent blades 330A defining an angle of attack 338 and induces either a clockwise or counter-clockwise swirl to the incoming oxidant gas. As oxidant gas flows through the nozzle 326A, it is constricted and exits through an opening having a diameter 306 (shown in FIG. 4A) that is smaller than the diameter of the opening through which oxidant gas enters the nozzle. The constriction of gas flow by the nozzle accelerates the inlet oxidant gas to a high velocity, as depicted in FIG. 4C by velocity vectors 340 and 341. The diameter 306 of the nozzle opening is selected to accelerate the gas to the necessary velocity required for the desired pressure. In addition, the length 307 of the nozzle is selected to be sufficient to prevent excessive frictional pressure loss. When the oxidant gas stream of increased velocity exits the nozzle 326A it begins mixing with the anode exhaust gas in the mixing assembly 322, as shown in FIG. 4C. Also included in the mixing arrangement 326 is a mixing cone 327 having surfaces 327A angled with respect to the path 329 of the oxidant gas to split the oxidant gas into several distinct streams to improve gas mixing. A second spinner 331 also having bent blades 331 A defining an angle of attack 339 is provided within a sleeve 332 following the mixing cone 327. The spinner 332 also induces a clockwise or counter-clockwise swirl to the gas mixture in the diverse paths from the mixing cone 327, thereby further enhancing mixing. The configuration of the mixing arrangement 326, such as the dimensions 310, 311 of the mixing cone 327, is such that the entrainment of the anode exhaust gas (depicted by vectors 350 and 351 in FIG. 4C) and mixing of the anode exhaust gas with oxidant gas in the mixing assembly 322 is maximized, and prevents the individual flow streams from coalescing into a single large stream. Thus, for example, the distance 319 between the end of the nozzle 326 A and the base of the mixing cone 327 is selected so as to maximize mixing of the two streams of gas. In addition, the position 309 of the end of the nozzle tube 326 relative to the centerline (FIG. 4A) of inlet 321 is selected so as to achieve the desired pressure distribution. The high velocity oxidant gas entrains the anode exhaust gas inside the mixing assembly 322, and after flowing past the mixing cone 327 and the second spinner 331, the gas mixture moves into the mixing duct 323. The mixing duct 323 has a mouth opening 303, a length 304 and a flare angle 305 all selected to promote the desired mixing. This embodiment may also include a baffle plate such as baffle plate 124 shown in FIG. 2A. The gas mixture continues flowing through the mixing duct 323 and enters the catalyst manifold 398, where it turns (as shown by arrow 359) and flows through the catalyst bed 399. Parameters usable for realizing the various components of the mixer/eductor of FIGS. 4A-4C are as follows: air spinner 330 angle of attack 338 between -60 and +60 degrees; mixing spinner 331 angle of attack of between -60 and +60 degrees; nozzle position 309 of between -1 and +1 times the anode gas pipe diameter 302; a mixing cone position 319 of between 1 and 10 times the nozzle tube diameter 306; a mixing duct flare angle 305 of between 0 and 30 degrees; a catalyst face orientation angle 359 of between 0 and 90 degrees; the use of baffle plates 124. Fourth Embodiment (FIGS. 5A-5C): one nozzle, annular gap and mixing cone FIGS. 5A-5C show a fourth embodiment 400 of the mixer/eductor of the present invention. With reference to FIG. 5B, the mixer/eductor 400 includes an oxidant supply gas inlet 420 for oxidant supply gas and an anode exhaust gas inlet 421 for anode exhaust gas. The cross-sectional areas 401 and 402 of the inlets 420, 421 shown in FIG. 5 A are selected based on the overall flow rates expected in the specific application. A mixing arrangement 426 connected to the inlet 420 includes elements at an angle relative to the path of the incoming oxidant supply gas that divide the oxidant gas flow into two distinct streams which enhance the mixing of the anode exit flow and the oxidant supply gas and minimize pressure loss. More particularly, as shown in FIGS. 5A-5C, the mixing arrangement 426 includes a nozzle 426A disposed along the path 429 of the incoming oxidant gas and protruding into a mixing elbow 431. The nozzle 426A has a bellmouth having a diameter 412, a nozzle tube diameter 406 and an opening opposite the bellmouth having a diameter 413 that is smaller than the bellmouth diameter 412 and nozzle tube diameter 406. Incoming oxidant supply gas enters the nozzle 426A through the bellmouth, flows through the nozzle tube, and is constricted as it exits the nozzle through the opening into the elbow 431, as shown in FIG. 5C. The constriction of air flow by the nozzle 426 A accelerates the oxidant supply gas to a high velocity, as depicted in FIG. 5C by velocity vectors 440 and 442. Incoming oxidant gas that does not enter the bellmouth of the nozzle 426A flows around the nozzle and is constricted by an annular gap 424 formed at the entrance to the mixing duct 423. The constriction of this oxidant gas by the annular gap 424 also accelerates the oxidant gas passing through the gap to a high velocity, as depicted in FIG. 5C by velocity vectors 441 and 443. The combined flow area of the nozzle 426A (measured by diameter 413 of the nozzle opening) and the annulus 424 is selected to accelerate the oxidant gas to the velocity required for the desired pressure. In addition, the length 407 of the nozzle 426 A is selected to be sufficient to prevent excessive frictional pressure loss in the nozzle. As shown in FIG. 5B, the anode exhaust gas inlet 421 leads the anode exhaust gas into a constricted passage 430 that accelerates the anode exhaust gas as it flows into the mixing elbow 431 and is entrained by the oxidant gas stream of increased velocity leaving the nozzle 426A. As the oxidant gas stream of increased velocity exits the nozzle 426A it impinges upon a mixing cone 427 that is situated in the mixing elbow 431. The mixing cone 427 has surfaces 427 A at an angle relative to the path 429 of the oxidant gas so that the oxidant gas is split into several distinct flow streams. The interaction of these diverse gas streams improves the mixing of the oxidant exhaust gas and the anode exhaust gas as the latter is entrained, as shown in FIG. 5C. The configuration of the mixing arrangement 426, such as the dimensions 410, 411 of the mixing cone 427, is such as to maximize the entrainment of the anode exhaust gas (depicted by vectors 450, 451 in FIG. 5C) and the mixing of the anode exhaust gas with the oxidant gas in the mixing elbow 431, and to prevent the individual gas streams from coalescing into a single stream. Thus, for example, the distance 419 between the end of the nozzle 426 A and the base of the mixing cone 427 is selected so as to maximize mixing of the two streams of gas. In addition, the position 409 of the end of the nozzle tube 426A relative to the centerline (FIG. 5 A) of inlet 421 is selected so as to achieve the desired pressure distribution. The high velocity oxidant gas entrains the anode exhaust gas inside the mixing elbow 431 , and the mixture moves over the mixing cone 427 and into the mixing duct 423, where oxidant gas flowing through the annulus 424 is also introduced. The mixing duct 423 has a mouth opening 403, a length 404 and a flare angle 405 all selected to promote the desired mixing. This embodiment may also include a baffle plate such as baffle plate 124 shown in FIG. 2 A. The gas mixture continues flowing through the mixing duct 423 and enters the catalyst manifold 498, where it turns (as shown by arrow 459) and flows through the catalyst bed 499. Parameters usable for realizing the various components of the mixer/eductor of FIGS. 5A-5C are as follows: a nozzle tube position 409 of between -1 and +1 times the anode gas pipe diameter 402; a nozzle tube length 407 of between 1 and 10 times the nozzle tube diameter 406; a mixing cone position 419 of between 1 and 10 times the nozzle tube diameter 406; a mixing duct flare angle 405 of between 0 and 30 degrees; a catalyst face orientation angle 459 of between 0 and 90 degrees; the possible use of one or more baffle plates 124. '495 Application: FIGS. 6A and 6B FIGS. 6A and 6B show a mixer/eductor 500 of the type disclosed in the '495 application. Referring to FIG. 6A, the mixer/eductor includes an oxidant gas inlet 520 for oxidant gas and an anode exhaust gas inlet 521 for anode exhaust gas. The cross- sectional areas 501, 502 of the inlets 520, 521 are selected based on the overall flow rates expected in the specific application. As oxidant supply gas flows through the inlet 520, it is constricted or "pinched" through an annular gap 524 between the circular opening of an elbow section 521 A of the inlet 521 and the entrance to the mixing duct 523. The latter duct has a diameter 503, a length 504 and a flare angle 505. The constriction or "pinching" of air flow through the annular gap 524 accelerates the inlet oxidant supply gas to a high velocity in the mixing duct 523, as depicted by velocity vectors 540 and 541 in FIG. 6B. The area of the annular gap 524 is selected to provide the necessary velocity required for the desired pressure. In addition, the length 504 and width 505 of the mixing duct 523 are selected to be sufficient to prevent excessive pressure loss in the duct. The high velocity oxidant supply gas entrains the anode exhaust gas passing into the mixing duct 523 via the opening of the elbow section 521 A of the inlet 521. The flow of anode exhaust gas to the mixing duct 523 is represented by vector 550, shown in FIG. 6B. The gas mixture continues flowing through the mixing duct 523 and enters the catalyst manifold 598, where it turns (as shown by arrow 559) and flows through the catalyst bed 599. FIG. 7 is a graph showing the cathode side gas pressure loss from point 6 to point 8 of Fig. 1 for each of the four embodiments discussed above as compared with a system using the mixer/eductor of the '495 application. Particularly, relative pressures at the cathode-side gas inlet using the embodiments of the mixer/eductor of the invention and the mixer/eductor of the '495 application are compared at several levels of oxidant supply gas flow rate. Data points on the chart represent the oxidant gas supply pressure loss at different flow rates corresponding to the various embodiments shown and described and that of the '495 application. The distance 710 between data points relating to oxidant supply gas pressure loss in the mixer/eductor of the '495 application and data points relating to oxidant supply gas pressure loss for the first, second and third embodiments of the invention represents an approximately 40% reduction in oxidant gas pressure loss. The distance 711 between data points relating to oxidant gas pressure loss in the mixer/eductor of the '495 application and data points relating to the oxidant gas pressure loss for the fourth embodiment of the invention represents a reduction in oxidant supply gas pressure loss of approximately 15%. FIG. 8 is a graph showing the suction available at the anode exhaust of the mixer/eductor for each of the four embodiments of the invention and for the mixer/eductor of the '495 application. The suction available at the anode exhaust of the mixer/eductor is shown at several levels of oxidant supply gas flow rate fpr each of the four embodiments of the invention and for the mixer/eductor of the '495 application. More negative values along the y-axis of this graph indicate greater suction at a given flow rate, indicating the ability to overcome more flow restriction in the interconnect piping and oxidizer. As can be seen in this graph, the data show that the anode exhaust gas suction is approximately the same for the mixer/eductor of the '495 application and the mixer/eductor of the first and second embodiments. This is pointed out by the arrows 810 which indicate that the difference between the vacuum achieved by these embodiments and the vacuum achieved by the mixer/eductor of the '495 application is negligible. While the difference in suction is negligible, these embodiments of the invention nevertheless offer an advantage in that they provide a 40% reduction in oxidant supply gas pressure loss as compared to the mixer/eductor of the '495 application. The arrow 811 indicates the vacuum achieved by the fourth embodiment of the invention. Relative to the vacuum achieved by the mixer/eductor of the '495 application, as shown by arrows 810, this indicates that the fourth embodiment provides a 500% improvement in suction. At the same time, this fourth embodiment also provides a 15% improvement in the oxidant gas pressure loss relative to the '495 application. FIGS. 9 and 10 illustrate mixing uniformity over the face of the catalyst bed using the mixer/eductor of the first embodiment of the invention and the mixer/eductor of the '495 application, respectively. Values shown in FIG. 9 were obtained through measurements made in a laboratory by lacing the anode exhaust gas stream with helium and using a helium detector to measure concentrations at the oxidizer face. Deviation data are presented as a percentage departure from the average mixing value. As can be seen in FIG. 9, the variation of the mixing uniformity in the first embodiment is ±2%. Values in FIG. 10 were obtained through measurements made in the field of operating temperature distribution of the oxidizer. Temperature distribution was subsequently transformed into an estimated anode-outlet mixture distribution by subtracting the measured oxidizer inlet temperatures from the measured oxidizer outlet temperatures to obtain the distribution of exotherm in the oxidizer. The exotherm is then proportional to the anode exhaust gas distribution through the oxidizer. Deviation data in FIG. 10 are also presented as a percentage departure from the average mixing value. As can be seen in FIG. 10, the variation of the mixing uniformity is ±7 to 12%. Thus, the mixer/eductor of the invention provided a significant improvement in mixing of the gases as compared to the mixer/eductor of the '495 application. In all cases it is understood that the above-described arrangements are merely illustrative of the many possible specific embodiments which represent applications of the present invention. Numerous and varied other arrangements can be readily devised in accordance with the principles of the present invention without departing from the spirit and scope of the invention. Thus, for example, in the first and second embodiments of the invention, while four and five nozzle tubes have been used, respectively, a larger or smaller number of tubes can also be employed. A representative range for the number of tubes is from two to 16 tubes.

Claims

What is claimed is: 1. A fuel cell system comprising: a fuel cell stack having an anode-side and a cathode-side, said anode- side having an inlet for receiving fuel and an outlet for discharging anode exhaust gas containing fuel gas, and said cathode-side having an inlet for receiving oxidant gas and an outlet for discharging exhaust oxidant gas; and a mixer/eductor for mixing the anode exhaust gas from the outlet of said anode-side and oxidant supply gas, said mixer/eductor being adapted to mix said anode exhaust gas with the oxidant supply gas while also reducing the difference between the pressure of said anode exhaust gas at the outlet of the anode-side and the pressure of the oxidant gas at the inlet of the cathode-side; and said mixer/eductor being further adapted to create multiple streams of oxidant gas to promote the mixing of said oxidant supply gas and said anode exhaust gas and the reduction in pressure difference.

2. A fuel cell system in accordance with claim 1, wherein: at least first and second of said multiple gas streams are at an angle relative to one another.

3. A fuel cell system in accordance with claim 1, wherein: said mixer/eductor includes an assembly having elements at an angle with respect to the path of said oxidant supply gas to thereby establish one or more of said multiple gas streams.

4. A fuel cell system in accordance with claim 3, wherein: said mixer/eductor comprises: an anode exhaust gas inlet for receiving said anode exhaust gas and conveying said anode exhaust gas into and through a region of said mixer/eductor; an oxidant supply gas inlet for receiving said oxidant supply gas and conveying said oxidant supply gas into and through said region of said mixer/eductor; and a mixing duct following said region of said mixer/eductor for receiving gas passing through said region; and said assembly is disposed in said region.

5. A fuel cell system in accordance with claim 4, wherein: said assembly comprises: one or more nozzles disposed in said region and arranged at an angle relative to the path of said oxidant supply gas, portions of said oxidant supply gas passing through said one or more nozzles so that said one or more oxidant gas streams are established for said oxidant supply gas such that the oxidant supply gas in said one or more oxidant gas streams interacts with said anode exhaust gas in said region and then said gases pass into said mixing duct.

6. A fuel cell system in accordance with claim 5 wherein: said mixer/eductor further comprises: one or more baffles situated in said mixing duct for promoting mixing of the gas in said mixing duct.

7. A fuel cell system in accordance with claim 5, wherein: said one or more nozzles include: first and second opposing vertically spaced nozzles arranged at an first angle relative to one another; and third and fourth opposing vertically spaced nozzles arranged at a second angle relative to one another and horizontally spaced from said first and second nozzles.

8. A fuel cell system in accordance with claim 7, wherein: said mixer/eductor further comprises: one or more baffles situated in said mixing duct for promoting mixing of the gas in said mixing duct.

9. A fuel cell system in accordance with claim 7, wherein: said first and second angles are equal.

10. A fuel cell system in accordance with claim 5, wherein: said mixer/eductor further comprises: a further nozzle arranged along the path of said oxidant supply gas through which a portion of said oxidant supply gas passes to establish one of said oxidant gas streams for said oxidant supply gas.

11. A fuel cell system in accordance with claim 10, wherein: said one or more nozzles include: first and second opposing vertically spaced nozzles arranged above and below said further nozzle and at first and second angles relative to said further nozzle; and third and fourth opposing horizontally spaced nozzles arranged to the right and left of said further nozzle at third and fourth angles relative to said further nozzle.

12. A fuel cell system in accordance with claim 11, wherein: said mixer/eductor further comprises: one or more baffles situated in said mixing duct for promoting mixing of the gas in said mixing duct.

13. A fuel cell system in accordance with claim 11, wherein: said first, second, third and fourth angles are equal.

14. A fuel cell system in accordance with claim 4, wherein: said assembly comprises: a mixing cone arranged in said region and having surfaces arranged at an angle relative to the path of said oxidant supply gas, said oxidant supply gas impinging on said surfaces so that said one or more oxidant gas streams are established for said oxidant supply gas such that the oxidant supply gas in said one or more oxidant gas streams interacts with said anode exhaust gas in said region and then said gases pass into said mixing duct.

15. A fuel cell system in accordance with claim 14 wherein: said mixer/eductor further comprises: a nozzle arranged in the path of said oxidant supply gas preceding said mixing cone and through which said oxidant supply gas passes before impinging on said surfaces of said mixing cone.

16. A fuel cell system in accordance with claim 15 wherein: said mixer/eductor further comprises: first and second mixing spinners for circulating gas passing therethrough in one of a clockwise direction and a counterclockwise direction, said first spinner being situated in the path of said oxidant supply gas preceding said nozzle and said second spinner being situated following said mixing cone and in the path of the gas from said mixing cone.

17. A fuel cell system in accordance with claim 15, wherein: said spinners each have a plurality of bent blades.

18. A fuel cell system in accordance with claim 15, wherein: said mixer/eductor further comprises: one or more baffles situated in said mixing duct for promoting mixing of the gas in said mixing duct.

19. A fuel cell system in accordance with claim 4, wherein: said assembly comprises: a mixing cone arranged in said region and having surfaces arranged at an angle relative to the path of said oxidant supply gas, a first part of said oxidant supply gas impinging on said surfaces so that said one or more oxidant gas streams are established for said first part of said oxidant supply gas such that said first part of said oxidant supply gas is in said one or more oxidant gas streams and passes into said mixing duct; a nozzle arranged in the path of said oxidant supply gas preceding said mixing cone and through which said first part said oxidant supply gas passes before impinging on said surfaces of said mixing cone; a second part of said oxidant supply gas passing around said nozzle and said mixing cone and through a gap into said mixing duct; said inlet has an elbow section extending into said region to said mixing cone for carrying said anode exhaust gas to said mixing cone and into which said nozzle extends; and said first part of said oxidant supply gas in said one or more oxidant gas streams and said second part of said oxidant supply gas and said anode exhaust interacting in said mixing duct.

20. A fuel cell system in accordance with claim 19, wherein: said gap is formed in area exterior of the area in which said elbow engages a periphery part of a support for said mixing cone.

21. A fuel cell system in accordance with claim 19, wherein: said mixer/eductor further comprises: one or more baffles situated in said mixing duct for promoting mixing of the gas in said mixing duct.

22. A mixer/eductor for use with a fuel cell stack having an anode-side and a cathode-side, said anode-side having an inlet for receiving fuel and an outlet for discharging anode exhaust gas containing fuel gas, and said cathode-side having an inlet for receiving oxidant gas and an outlet for discharging exhaust oxidant gas, said mixer/eductor for mixing the anode exhaust gas from the outlet of said anode-side and oxidant supply gas; and said mixer/eductor being adapted to mix said anode exhaust gas and oxidant supply gas while also reducing the difference between the pressure of said anode exhaust gas at the outlet of the anode-side and the pressure of the oxidant gas at the inlet of the cathode-side; and said mixer/eductor being further adapted to create oxidant gas streams for the oxidant supply gas to promote the mixing of said oxidant supply gas and said anode exhaust gas and the reduction in pressure difference.

23. A mixer/eductor in accordance with claim 22, wherein: at least first and second of said oxidant gas streams are at an angle relative to one another.

24. A mixer/eductor in accordance with claim 22, wherein: said mixer/eductor includes an assembly having elements at an angle with respect to the path of said oxidant supply gas to thereby establish one or more of said oxidant gas streams.

25. A mixer/eductor in accordance with claim 24, wherein: said mixer/eductor comprises: an anode exhaust gas inlet for receiving said anode exhaust gas and conveying said anode exhaust gas into and through a region of said mixer/eductor; an oxidant supply gas inlet for receiving said oxidant supply gas and conveying said oxidant supply gas into and through said region of said mixer/eductor; and a mixing duct following said region of said mixer/eductor for receiving gas passing through said region; and said assembly is disposed in said region.

26. A mixer/eductor in accordance with claim 25, wherein: said assembly comprises: one or more nozzles disposed in said region and arranged at an angle relative to the path of said oxidant supply gas, portions of said oxidant supply gas passing through said one or more nozzles so that said one or more oxidant gas streams are established for said oxidant supply gas such that the oxidant supply gas in said one or more oxidant gas streams interacts with said anode exhaust gas in said region and then said gases pass into said mixing duct.

27. A mixer/eductor in accordance with claim 26 wherein: said mixer/eductor further comprises: one or more baffles situated in said mixing duct for promoting mixing of the gas in said mixing duct.

28. A mixer/eductor in accordance with claim 26, wherein: said one or more nozzles include: first and second opposing vertically spaced nozzles arranged at an first angle relative to one another; and third and fourth opposing vertically spaced nozzles arranged at a second angle relative to one another and horizontally spaced from said first and second nozzles.

29. A mixer/eductor in accordance with claim 28, wherein: said mixer/eductor further comprises: one or more baffles situated in said mixing duct for promoting mixing of the gas in said mixing duct.

30. A mixer/eductor in accordance with claim 28, wherein: said first and second angles are equal.

31. A mixer/eductor in accordance with claim 26, wherein: said mixer/eductor further comprises: a further nozzle arranged along the path of said oxidant supply gas through which a portion of said oxidant supply gas passes to establish one of said oxidant gas streams for said oxidant supply gas.

32. A mixer/eductor in accordance with claim 31 , wherein: said one or more nozzles include: first and second opposing vertically spaced nozzles arranged above and below said further nozzle and at first and second angles relative to said further nozzle; and third and fourth opposing horizontally spaced nozzles arranged to the right and left of said further nozzle at third and fourth angles relative to said further nozzle.

33. A mixer/eductor in accordance with claim 32, wherein: said mixer/eductor further comprises: one or more baffles situated in said mixing duct for promoting mixing of the gas in said mixing duct.

34. A mixer/eductor in accordance with claim 32, wherein: said first, second, third and fourth angles are equal.

35. A mixer/eductor in accordance with claim 25, wherein: said assembly comprises: a mixing cone arranged in said region and having surfaces arranged at an angle relative to the path of said oxidant supply gas, said oxidant supply gas impinging on said surfaces so that said one or more oxidant gas streams are established for said oxidant supply gas such that the oxidant supply gas in said one or more oxidant gas streams interacts with said anode exhaust gas in said region and then said gases pass into said mixing duct.

36. A mixer/eductor in accordance with claim 35 wherein: said mixer/eductor further comprises: a nozzle arranged in the path of said oxidant supply gas preceding said mixing cone and through which said oxidant supply gas passes before impinging on said surfaces of said mixing cone.

37. A mixer/eductor in accordance with claim 36 wherein: said mixer/eductor further comprises: first and second mixing spinners for circulating gas passing therethrough in one of a clockwise direction and a counterclockwise direction, said first spinner being situated in the path of said oxidant supply gas preceding said nozzle and said second spinner being situated following said mixing cone and in the path of the gas from said mixing cone.

38. A mixer/eductor in accordance with claim 37 wherein: said spinners each have a plurality of bent blades.

39. A mixer/eductor in accordance with claim 37, wherein: said mixer/eductor further comprises: one or more baffles situated in said mixing duct for promoting mixing of the gas in said mixing duct.

40. A mixer/eductor in accordance with claim 25, wherein: said assembly mixer/eductor further comprises: a mixing cone arranged in said region and having surfaces arranged at an angle relative to the path of said oxidant supply gas, a first part of said oxidant supply gas impinging on said surfaces so that said one or more oxidant gas streams are established for said first part of said oxidant supply gas such that said first part of said oxidant supply gas is in said one or more oxidant gas streams and passes into said mixing duct; a nozzle arranged in the path of said oxidant supply gas preceding said mixing cone and through which said first part of aid oxidant supply gas passes before impinging on said surfaces of said mixing cone; a second part of said oxidant supply gas passing around said nozzle and said mixing- cone and through a gap into said mixing duct; said inlet has an elbow section extending into said region to said mixing cone for carrying said anode exhaust gas to said mixing cone and into which said nozzle extends; and said first part of said oxidant supply gas in said one or more oxidant gas streams and said second part of said oxidant supply gas and said anode exhaust gas interacting in said mixing duct.

41. A mixer/eductor in accordance with claim 40, wherein: said gap is formed in area exterior of the area in which said elbow engages a periphery part of a support for said mixing cone.

42. A mixer/eductor in accordance with claim 40, wherein: said mixer/eductor further comprises: one or more baffles situated in said mixing duct for promoting mixing of the gas in said mixing duct.