Fuel cell system including an ejector

Abstract

1. A fuel cell system comprising: a fuel cell module including an anode section configured to output an anode exhaust stream, a first junction configured to split the anode exhaust stream into an anode recycle stream and a system outlet stream, and an ejector. The ejector includes a low pressure inlet configured to receive a suction stream including a first portion of the anode recycle stream, a start-up inlet configured to receive a start-up stream including a second portion of the anode recycle stream, and an outlet configured to output an ejector output stream. The anode section is configured to receive an anode input stream including the ejector output stream.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of and priority to U.S. Provisional Application No. 63 / 285,274, filed December 2, 2021, which is incorporated herein by reference in its entirety. [Background technology]

[0002] FIELD OF THE DISCLOSURE This disclosure relates generally to the field of electrochemical cells, such as fuel cells and electrolyzer cells, and more particularly to fuel cell systems having exhaust recycling systems.

[0003] Generally, a fuel cell includes an anode, a cathode, and an electrolyte layer that together drive a chemical reaction to produce electricity. Multiple fuel cells can be arranged in a stack to produce a desired amount of electricity. A fuel, such as hydrogen gas or a hydrocarbon gas, is supplied to the anode, while an oxidant is supplied to the cathode. The fuel and oxidant are consumed in an electrochemical reaction as they flow over the anode and cathode, respectively.

[0004] To avoid depleting the reactant gases before they reach all areas of the cell, more fuel and oxidant are provided than can be reacted before the gases pass through the cell and exit the stack. To avoid waste, unreacted gases can be recycled back to the input of the fuel cell stack.

[0005] The exhaust from the anode of a solid oxide fuel cell reaches temperatures of approximately 750 degrees Celsius. Recycle systems often include specialized high-temperature blowers to pressurize the gas in the recycle stream. Such blowers are expensive in materials and manufacturing, but are still necessary to adequately cool the exhaust. Summary of the Invention

[0006] Certain embodiments of the present disclosure may address the above-mentioned problems associated with previous fuel cell systems.

[0007] In one specific embodiment, a fuel cell system includes a fuel cell module including an anode section configured to output an anode exhaust stream, a first junction configured to split the anode exhaust stream into an anode recycle stream and a system outlet stream, and an ejector. The ejector includes a low-pressure inlet configured to receive a suction stream including a first portion of the anode recycle stream, a startup inlet configured to receive a startup stream including a second portion of the anode recycle stream, and an outlet configured to output an ejector output stream. The anode section is configured to receive an anode input stream including the ejector output stream.

[0008] In some embodiments, the fuel cell system further includes a cooler configured to cool and remove water from the second portion of the anode recycle stream.

[0009] In some embodiments of the fuel cell system, the cooler is configured to spray a low-temperature water stream onto the second portion of the anode recycle stream to cool and condense vapor from the second portion of the anode exhaust stream.

[0010] In some aspects of the fuel cell system, the start-up stream further comprises a fresh fuel stream.

[0011] In some aspects, the fuel cell system further includes a compressor configured to receive and compress the startup stream before it is received by the startup inlet.

[0012] In some embodiments, the fuel cell system further includes a cooler configured to reduce the temperature of the second portion of the anode recycle stream such that the start stream received by the compressor has a temperature within a range of 55 degrees Celsius to 80 degrees Celsius.

[0013] In some aspects of the fuel cell system, the system outlet stream is discharged from the fuel cell system.

[0014] In some embodiments of the fuel cell system, the anode exhaust stream is split at a first junction so that the system outlet stream comprises 25% to 35% of the anode exhaust stream, and the anode recycle stream is further split at a second junction so that a first portion of the anode recycle stream comprises 12% to 22% of the anode exhaust stream and a second portion of the anode recycle stream comprises 48% to 58% of the anode exhaust stream.

[0015] In some aspects, the fuel cell system further includes an anode preheater configured to receive and heat the ejector output stream.

[0016] In some aspects, the fuel cell system further includes a carbon dioxide separation stage configured to remove carbon dioxide from the start-up stream, the carbon dioxide separation stage including a molten carbonate electrolyzer cell or an amine scrubber system.

[0017] In some aspects, the fuel cell system further includes a pre-reformer configured to at least partially reform the methane in the ejector output stream.

[0018] In some embodiments of the fuel cell system, the ejector is configured such that the mass ratio of the ejector output stream to the starting stream within the ejector is within a range of 2.0 to 3.0, and the ejector has a starting pressure within a range of 20.0 psi to 30.0 psi at nominal operating conditions.

[0019] In one particular embodiment, a method for recycling anode exhaust of a fuel cell is provided, the method including separating an anode exhaust stream from a fuel cell module into a system outlet stream, a suction stream, and a dryer stream, discharging the system outlet stream away from the fuel cell module, directing the suction stream to a low-pressure inlet of an ejector, directing at least a portion of the dryer stream to a start-up inlet of the ejector, and directing the ejector output stream from an outlet of the ejector to an anode inlet of the fuel cell module.

[0020] In some embodiments, the method further comprises cooling and removing water from the dryer stream.

[0021] In some embodiments, the method further comprises removing carbon dioxide from a portion of the dryer stream.

[0022] In some embodiments, the method further comprises pressurizing a portion of the dryer stream.

[0023] In some aspects, the method further includes mixing the fresh fuel stream with a portion of the dryer stream prior to compressing the portion of the dryer stream.

[0024] In some aspects, the method includes cooling a portion of the dryer stream before compressing the portion of the dryer stream, and heating the ejector output stream before directing the ejector output stream to the anode inlet.

[0025] In some embodiments of the method, the dryer stream comprises 12% to 22% anode exhaust stream, the suction stream comprises 48% to 58% anode exhaust stream, and the system outlet stream comprises 25% to 35% anode exhaust stream.

[0026] In some aspects of the method, the ejector is configured to have a mass ratio of the ejector output stream to the starting stream within the ejector in the range of 2.0 to 3.0 and a starting pressure in the range of 20.0 psi to 30.0 psi at nominal operating conditions. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 is a schematic diagram of a fuel cell anode recycling system with an ejector according to an exemplary embodiment.

[0028] [Figure 2] FIG. 1 is a schematic diagram of an ejector in accordance with an exemplary embodiment.

[0029] [Figure 3] FIG. 1 is a schematic diagram of a basic anode recycling system.

[0030] [Figure 4] FIG. 1 is a schematic diagram of an anode recycling system with an ejector according to an exemplary embodiment.

[0031] [Figure 5] FIG. 1 is a schematic diagram of an ejector in accordance with an exemplary embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0032] In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, like symbols typically identify like elements unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not intended to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that aspects of the present disclosure, as generally described herein and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are expressly contemplated and made a part of this disclosure.

[0033] Certain embodiments of the present disclosure provide fuel cell and electrolyzer cell systems with improved efficiency. Specifically, efficiency is improved by using a blower-driven ejector in the process recycle stream. An ejector is a mechanical device that accelerates a high-pressure gas stream or motive stream through a nozzle to entrain a low-pressure gas stream to form a pressurized output stream. In some embodiments, a high-temperature blower can be replaced with a low-temperature blower and ejector combination in the recycle stream of a fuel cell system or electrolysis cell system. An ejector can be advantageous over a high-temperature blower because it has no moving parts and can operate at high temperatures.

[0034] Referring to FIG. 1 , a fuel cell system 10 according to an exemplary embodiment is shown. The fuel cell system includes an anode section and a cathode section. A fuel cell module 15 includes one or more fuel cells, each having an electrolyte sandwiched between an anode and a cathode. The fuel cells may be, for example, solid oxide fuel cells. A fuel, such as hydrogen or a hydrocarbon fuel, may be delivered to the anode of the fuel cell, while an oxidant may be delivered to the cathode of the fuel cell. An anode exhaust stream 20 containing unreacted fuel may exit the anode section of the fuel cell module 15 and be split at a junction 21. A portion of the anode exhaust stream 20 may exit the fuel cell system 10 as a system outlet stream 22 and may be used for other purposes or vented to the atmosphere. Another portion of the anode exhaust stream 20 may be recycled or partially recycled to the fuel cell module 15 as an anode recycle stream 23. In some embodiments, the system outlet stream 22 may comprise 10% to 50% or 25% to 35% of the anode exhaust stream 20. Anode recycle stream 23 is circulated to junction 25 by the combined suction of ejector 30 and compressor 45, with a first portion being circulated toward ejector 30 as suction stream 26 and a second portion being circulated toward cooler 35 as dryer stream 27. Suction stream 26 may comprise 48% to 58% anode exhaust stream 20, and dryer stream 27 may comprise 12% to 22% anode exhaust stream 20.

[0035] In some embodiments, the dryer stream 27 may be cooled in the cooler 35 until the water vapor in the exhaust condenses into liquid water, which can be removed from the fuel cell system via the discharge stream 63. In some embodiments, the cooler 35 may cool the dryer stream 27 to a temperature above the condensation point of water, so that less or no water may be removed, while the compressor 45 is still able to compress the gas and not be damaged by heat. The cooler 35 simultaneously reduces the temperature of the dried recycle stream 42 to protect the compressor 45 and, optionally, to knock out excess water from the dryer stream 27. Removing water from the recycle stream may result in improved efficiency of the fuel cell module 15 by reducing fuel dilution effects. As discussed above, the dryer stream 27 may contain 12% to 22% of the anode exhaust stream 20. Thus, compared to a typical anode recycle stream where all of the gas may be directed to a dryer, in fuel cell system 10 a relatively small amount of the gas is dried and used to pressurize the remaining gas via ejector 30.

[0036] After being dried with the dryer stream 27, the dried exhaust then exits the cooler 35 as a dry recycle stream 42 and may be combined with fresh fuel from the fresh fuel stream 40 to form a combined fuel stream 43. The combined fuel stream 43 may then be compressed by a compressor 45. The fresh fuel stream 40 is at a lower temperature and is drier than the dry recycle stream 42, so the combined fuel stream 43 is cool enough and dry enough to be compressed by the compressor 45. If the cooler 35 is configured to condense the water in the dryer stream 27, the temperature of the combined fuel stream 43 may be within the range of 55°C to 80°C, or up to 200°C, depending on the compressor technology. It should be understood that the terms "blower" and "compressor" are used interchangeably herein and refer to devices configured to compress gases. Because the combination of fresh fuel and dried anode exhaust may be at a sufficiently low temperature, the compressor 45 may be relatively inexpensive compared to compressors that may be required for high-temperature compression. Pressurized gas from the compressor 45 may be directed to the ejector 30, where the pressurized gas may act as a motive stream 51. The motive stream 51 may pass through a constricting nozzle of the ejector 30, which accelerates the gas and creates a low-pressure zone inside the ejector.

[0037] The suction stream 26, which may be directed from the anode exhaust stream 20 to the ejector 30 without passing through the cooler 35 or compressor 45, may be a low-pressure gas stream entrained by the priming stream 51 in the ejector 30. The ejector 30 may combine the priming stream 51 and the suction stream 26 and output a combined gas as the ejector output stream 52. The ejector output stream 52 may include gases from the fresh fuel stream 40, the suction stream 26, and the dry recycle stream 42. The ejector output stream 52 may then be directed to the fuel cell module 15 and delivered to the anode as part or all of the anode input stream 17. The anode input stream may be heated to a temperature within a range of 630°C to 730°C.

[0038] The gas output from the compressor 45 is a relatively high carbon dioxide, relatively low humidity, relatively low temperature, and somewhat pressurized stream. Alternatively, the gas output from the cooler 35 has a higher carbon dioxide concentration but lower pressure. At this point, a carbon dioxide separation stage 46 can be added to the system. For example, the gas can be input to the anode of a molten carbonate electrolyzer cell, which can allow the carbon dioxide to cross the electrolyte while allowing hydrogen to pass through the anode without crossing the electrolyte. Alternatively, an amine scrubber system can be used to capture the carbon dioxide. Regardless of the method used, a high concentration of carbon dioxide can facilitate carbon capture.

[0039] Referring to FIG. 2 , an ejector 30 according to an exemplary embodiment is shown. A startup stream 51 may enter the ejector 30 through a startup inlet 205. The startup stream 51 may be a combination of the fresh fuel stream 40 and the dryer stream 27. The dryer stream 27 may be cooled to remove some or all of the water before the water is combined with the fresh fuel to form the startup stream 51. The dryer stream 27 may be a first portion of the anode recycle stream 23. A low-pressure gas, such as the suction stream 26, may enter the ejector 30 through a low-pressure inlet 210. The suction stream 26 may be a second portion of the anode recycle stream 23. The startup stream 51 may pass through a constricting nozzle 215, where the velocity of the startup stream 51 may increase. This increase in velocity may reduce the pressure of the startup stream 51 according to Bernoulli's principle. This reduction in pressure may cause the suction stream 26 to be entrained by the startup stream 51. The startup stream 51 and the suction stream 26 may be combined in the ejector 30 and output from the outlet 24 of the ejector 30 as the ejector output stream 52. The ejector 30 may have a startup pressure in the range of 20.0 to 30.0 psi and a mass ratio of the ejector output stream to the startup stream in the range of 2.0 to 3.0 at nominal operating conditions. The ejector 30 may have no moving parts and may be relatively tolerant to high temperatures. Unlike the use of a conventional recycle blower, the need to provide a recuperative heat exchanger to protect the ejector 30 may be eliminated (or limited). Additionally, by recirculating a significant portion of the recycle stream 23 without cooling (i.e., without the suction stream 26), the combined anode input stream 17 may be significantly hotter than if the entire stream were cooled through a conventional blower. This reduces or potentially eliminates the need for gas preheating / recuperation at the stack module inlet. Recuperative heat exchangers can contribute significantly to the overall cost of a system.

[0040] While it is possible to use an ejector 30 with only the fresh fuel stream 40 as the startup stream 51, this may produce suboptimal results in certain cases. Ejectors have specific performance profiles that dictate the ratio of startup flow to intake flow, which may conflict with the system's operating goals. Specifically, an ejector may be designed to be optimized for a specific flow rate and pressure, but may have insufficient or even inoperable conditions if off-design. When the fuel cell system 10 operates at part-load conditions, there may not be enough fresh fuel entering the system to act as the startup stream 51. Embodiments of the present disclosure avoid this problem by using a portion of the recycled stream in the startup stream 51 instead of using only the fresh fuel stream 40, which would be at a fixed pressure and have a fixed target flow rate for a given system operating point.

[0041] A common limitation of a pure ejector-driven system is turndown, for example, when the fuel cell is operating at reduced power. As the start-up flow decreases, the ejector's ability to provide a useful start-up to output mass flow ratio decreases. This means that a certain minimum ejector start-up flow is required to maintain useful ejector performance. The present system significantly expands its operability window through two mechanisms. First, because the start-up stream 51 is provided by the compressor 45, it can be independent of the input speed of the fresh fuel stream 40, even at part-load conditions. Second, because a portion of the fresh fuel and recycle streams are provided directly by the compressor 45, the system 10 can continue to operate even when ejector 30 performance is degraded due to a reduction in start-up flow. As system turndown increases, a larger portion of the anode input stream 17 is provided by the compressor 45.

[0042] The anode exhaust stream 20 may contain unreacted fuel as well as reaction products, including water. By removing water from a first portion of the anode exhaust (e.g., dryer stream 27), additional efficiency can be gained, thus increasing the reactant concentration in the recycle stream (e.g., in the anode input stream 17). Because only a portion of the anode exhaust (e.g., dryer stream 27) is cooled, the cooler 35 can be sized accordingly and can cool the exhaust portion relatively quickly. Removing water vapor from this portion of the anode exhaust provides a sufficient improvement in reactant concentration at the fuel cell inlet (e.g., in the anode input stream 17) to provide a significant efficiency improvement in the range of 2% to 4%. Water vapor can be removed in a number of ways. Direct contact spray towers can be used where appropriate. If a coolant stream is available, the water can be cooled and condensed using a liquid to liquid heat exchanger. If a coolant stream is not available, a liquid-to-air heat exchanger can be used. If freezing is a concern, a gas-air condenser or gas-glycol loop can be used to keep the condensed water above the freezing temperature. Cooling the dryer stream 27 below the condensation temperature of the water results in a decrease in the temperature of the resulting ejector output stream 52. This may require heating the gas before it reaches the fuel cell, for example, by an anode preheater 47. Some of this heat may be reheated from radiant heat inside the module or may heat the gas before it enters the fuel cell module 15. Nevertheless, removing water from the dryer stream 27 by additionally heating the anode input stream 17 before it reaches the fuel cell provides efficiency gains that may outweigh any losses.

[0043] System Model A fuel cell system simulation model was created to compare the predicted efficiency of the ejector-based recycle system according to the exemplary embodiment with a baseline design incorporating a high-temperature blower without an ejector. The model was constructed to target a total DC system power output of 61.4 kW. The first system was modeled with a conventional anode recycle blower without an ejector. Referring to FIG. 3 , a portion of the first system model 300 is shown. A fuel cell module 315 receives an anode input stream 317 of fuel and outputs an anode exhaust stream 320 containing fuel that did not react within the fuel cell module 315. The fuel cell module 315 also receives a cathode inlet stream 312 and outputs a cathode outlet stream 313. The anode exhaust stream 320 is split in a mixer 321 into a system outlet stream 322 and an anode recycle stream 323. The system outlet stream 322 is not returned to the fuel cell module but may be used elsewhere in the system, vented to the atmosphere, or used for other purposes. The anode recycle stream 323 is combined with the fresh fuel stream 340 in a mixer 341 to form a combined fuel stream 342. The combined fuel stream 342 is directed to a compressor 345, which is modeled at 75% efficiency, to compress the fuel and move it toward the fuel cell module 315. The combined fuel stream 342 is heated by an anode preheater 344, and the methane in the combined fuel stream 342 is at least partially reformed to hydrogen in a prereformer 348. The anode preheater 344 may be a heat exchanger, and heat may be extracted from other parts of the system to heat the ejector output stream 452. The combined fuel stream 342 is then directed to the fuel cell module 315.

[0044] According to one exemplary embodiment, the second system was modeled with an ejector-based anode recycle stream. Referring to FIG. 4, a portion of a second system model 400 is shown. Elements identified by reference numbers in FIG. 1 are the same or similar to elements identified by reference numbers in FIG. 4, with corresponding numbers in FIG. 4 being 400 higher than those in FIG. 1 (e.g., fuel cell module 15 corresponds to fuel cell module 415). Fuel cell module 415 receives an anode input stream 417 of fuel and outputs an anode exhaust stream 420 containing fuel that did not react within fuel cell module 415. Fuel cell module 415 also receives a cathode inlet stream 412 and outputs a cathode outlet stream 413. Anode exhaust stream 420 is split in mixer 421 into a system outlet stream 422 and an anode recycle stream 423. Gas in system outlet stream 422 is not returned to the fuel cell module but may be used elsewhere in the system, vented to the atmosphere, or used for other purposes. Anode recycle stream 423 is split into ejector suction stream 426 in splitter 425 and dryer stream 427 in splitter 432. Ejector suction stream 426 is directed to ejector 430.

[0045] Dryer stream 427 is directed to splitter 432 and split into water knockout stream 431 and bypass stream 429. The water knockout stream is directed to water knockout cooler 435. Cold water stream 461 is directed to atomizer 460 and outputs cold water atomizer stream 462. Cold water in cold water atomizer stream 462 is sprayed onto the gas from water knockout stream 431 to cool the gas and condense water vapor from stream 431. Dry gas is output from the top of cooler 435 via dry recycle stream 428, and water from cold water atomizer stream 462 and condensed water from water knockout stream 431 exit cooler 435 via discharge stream 463. Dry recycle stream 428 is then recombined with bypass stream 429 in mixer 433. The proportion of dryer stream 427 that is split into water knockout stream 431 and bypass stream 429 can be controlled based on how much water is desired to be removed from dryer stream 427. For example, if 90% of the water in dryer stream 427 is desired to be removed, then 90% of dryer stream 427 can be directed to water knockout stream 431 and 10% can be directed to bypass stream 429. Alternatively, in practice, cooler 435 can be selectively configured to remove less than all of the water from the stream, and dryer stream 427 may not need to be split. For example, dryer stream 427 can be directed directly to cooler 435 without being split in splitter 432. Cooler 435 can then remove 90% of the water from dryer stream 427. Table I shows the amount of water expected to be discharged via discharge stream 463, the excess heat removed from the ejector system, and the amount of waste heat required to re-evaporate the condensed water if the liquid water cannot be treated on-site. [Table 1]

[0046] The dried recycle stream 428 is recombined with the bypass stream 429 in the mixer 433 to form a combined dryer stream 442. The combined dryer stream 442 is further combined with fuel from the fresh fuel stream 440 in the mixer 441 to form the fuel stream 443. The fuel stream 443 is directed to the compressor 445. The compressor 445 was modeled with a pressure ratio of approximately 2.7 and an efficiency of 75%. The blower inlet temperature was modeled at a maximum of 74 degrees Celsius, which is within the range of standard or near-standard components. Cooling the dryer stream 427 before combining it with the fresh fuel stream 440 allows for the use of a much lower-cost blower / compressor than the blower / compressor that may be required to compress the hot anode recycle stream. The compressor 445 compresses the fuel stream 443 and outputs the motive stream 451, which is directed to the ejector 430. The ejector submodel is shown in detail in FIG. 5.

[0047] 2, with the motive stream 451 directed to the motive inlet 205 and the suction stream 426 directed to the low-pressure inlet 210. The motive stream 451 accelerates as it passes through the constricting nozzle 215, entraining the suction stream 426. The ejector 430 outputs an ejector output stream 452, which has a pressure between the higher pressure of the motive stream 451 compressed by the compressor 446 and the lower pressure of the suction stream 426. For a fuel cell system of this size, a motive stream flow rate of approximately 2.65 scfm and a suction stream flow rate of approximately 4.0 scfm are required.

[0048] The ejector output stream 452 may be heated by an anode preheater 444, and the methane in the combined fuel stream 342 may be at least partially reformed to hydrogen in a prereformer 448. The anode preheater 444 may be a heat exchanger that may remove heat from other parts of the system to heat the ejector output stream 452. The ejector output stream 452 is then at least partially reformed in the prereformer 448. The prereformer outputs the reformed fuel as an anode input stream 417. The second system model 400 also includes a pressure drop simulator 406 to compensate for any inefficiencies in the system. Portions of the models 300, 400 that are not shown may be the same or essentially the same between the models.

[0049] FIG. 5 illustrates a sub-model 500 of the ejector 430. Rather than fully simulating the ejector mechanism, the model is configured to determine the ejector outlet pressure and flow rate based on the pressures of the motive stream 451 and the suction stream 426. The motive stream is directed to an expander 570. The outlet pressure of the expander 570 is set to the pressure of the suction stream 426, and the pressure drop energy is directed to a compressor 580. The motive stream 451 and the suction stream 426 are combined in a mixer 575 and directed to the compressor 580. The pressure drop energy in the expander 570 is applied to the combined motive stream 451 and suction stream 426, and the ejector output stream 452 is output from the compressor 580. Thus, energy from the motive stream 451 is used to pressurize the ejector output stream 452 to a pressure between the pressures of the motive stream 451 and the suction stream 426. The ejector 430 was modeled with an expander efficiency of 99% and a compressor efficiency of 25%, corresponding to an overall efficiency of approximately 25%. A starting pressure of 25.3 psi and a mass ratio of the ejector output stream to the starting stream of 2.5 were selected. These performance characteristics are within reasonable expectations of ejector performance.

[0050] In the first configuration, the second system was modeled to maintain all conditions as close as possible to the first (baseline system) model. For example, the ejector model targeted the same system fuel utilization, the same stack fuel utilization, the same temperature, the same recycle ratio, etc. Next, in the second configuration, the ejector system was modeled with an ejector-based anode recycle stream according to an exemplary embodiment, with an eye toward possible performance improvements. The second configuration was modeled to target a higher system fuel utilization without significantly increasing the stack fuel utilization. Typically, the stack fuel utilization is kept below 100% to prevent fuel depletion before it reaches all parts of the fuel cell. In both the baseline system (i.e., first system model 300) and ejector system (i.e., second system model 400) configurations, 15% pre-reforming by the pre-reformer 448 was assumed. The anode inlet temperature was targeted at 680°C. In both cases, the fuel must be heated before entering the fuel cell module. In the baseline system, 4.9 kW of energy must be added to the fuel to heat it to this temperature. The first and second configurations of the ejector system required 7.2 kW and 7.6 kW of energy, respectively, to heat the fuel to 680 degrees Celsius. Additional heat is required because a portion of the recycle stream is cooled and dried in cooler 435.

[0051] Initially, it was anticipated that there would be an efficiency penalty due to the combination of the blower and ejector, but that this penalty would be so small that it would be worth including the ejector to enable the use of a low-temperature blower. However, when the system was evaluated, the results showed that the ability of the water knockout cooler 435 to reduce the steam content in the recycle stream and improve system fuel utilization actually improved system efficiency. Table II shows a comparison of the results between the base system (i.e., first model 300), the matched ejector system (i.e., first configuration of second system model 400), and the improved ejector system (i.e., second configuration of second system model 400). [Table 2]

[0052] As discussed above, a total power output of 61.4 kW was targeted for each case. The base system and matched ejector system were each targeted for a system fuel utilization of 85%, which was the optimum fuel utilization for the base system. The improved ejector system was optimized for a system fuel utilization of 90%. Due to the higher performance of the ejector model flow, less fresh fuel needed to be added via fresh fuel stream 440. This is shown in the row labeled "Chemical Power Input and Methane Inlet Flow." The stack fuel utilization (stack uf) and percent direct internal reforming (%DIR) were similar in all cases. [Table 3]

[0053] Table III illustrates the power requirements for the three cases. Because the base system model does not include cooling the anode recycle stream, the blower inlet temperature for the base system is much higher than that for the ejector system. In practice, cooling is almost always used to protect the blower and operate it at a lower, more efficient temperature. For efficiency calculation purposes, the base model assumes that specialized blowers are available that can operate at high temperatures and high efficiencies. This can make the base system model unrealistically favorable compared to the ejector system. Because the base system avoids the ejector, the base blower outlet pressure does not need to be very high. Because the ejector system involves cooling and drying a portion of the anode recycle stream, the fuel entering the blower is very cold, allowing a commercially available blower / compressor to be used. Furthermore, because only a portion of the anode recycle stream is directed to the blower, the blower can be much smaller than that of the base system, in which all of the recycle stream passes through the blower.

[0054] As shown in the row labeled "Recycled Power Consumption," there is an efficiency penalty for the ejector system because the blower in the ejector system requires more power than the base system. This results in an overall loss in net efficiency of 0.5% and 0.8% for the matched and improved ejector systems, respectively. However, the ejector systems consume 2.5% and 4.5% less fresh fuel than the base system, respectively. Overall, this results in a 2.0% increase in net efficiency for the matched ejector system and a 3.7% increase in net efficiency for the improved ejector system.

[0055] Carbon deposition The system model presented represents a natural gas-fed solid oxide fuel cell system. In these systems, carbon activity in the fuel stream must be considered. High levels of carbon activity increase the risk of carbon buildup, which can have a devastating effect on the operation of the system. Table VI compares carbon activity at three points in the system, comparing the baseline system with 85% uf and 90% uf ejector systems. [Table 4]

[0056] The carbon activity data shows that all systems have acceptable gas compositions in the anode recycle loop of the system. Carbon activity is highest when the fresh fuel stream is mixed with the recycle loop. However, the ejector system has lower carbon activity than the baseline system at this point, and the carbon activity in the ejector start-up stream is very low. The ejector system should not pose any additional challenges due to the carbon activity already not present in the baseline system.

[0057] Configuration of an exemplary embodiment As used herein, the terms "approximately," "about," "substantially," and similar terms are intended to have a broad meaning consistent with commonly accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of ordinary skill in the art who review this disclosure that these terms are intended to enable description of the particular features described and claimed without limiting the scope of those features to the precise numerical ranges provided. Accordingly, these terms should be interpreted to indicate that insubstantial or insignificant modifications or variations of the subject matter described and claimed are considered to be within the scope of the invention as recited in the appended claims.

[0058] As used herein, the terms "coupled," "connected," and the like refer to the joining of two members directly or indirectly to one another. Such joining may be fixed (e.g., permanent) or movable (e.g., removable or releasable). Such joining may be achieved by the two members or the two members and an additional intermediate member being integrally formed with one another as a single, unitary body, or by the two members or the two members and an additional intermediate member being attached to one another.

[0059] References herein to the location of elements (e.g., "top," "bottom," "up," "down," etc.) are merely used to describe the orientation of various elements in the figures. It should be noted that the orientation of various elements may differ according to other exemplary embodiments, and such variations are intended to be encompassed by the present disclosure.

[0060] It is important to note that the construction and arrangement of the various exemplary embodiments are illustrative only. While only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications (e.g., changes in the size, dimensions, structure, shape, and proportions of various elements, parameter values, mounting arrangements, use of materials, color, orientation, etc.) are possible without substantially departing from the novel teachings and advantages of the subject matter described herein. For example, elements shown as integrally formed may be composed of multiple parts or elements, the positions of elements may be reversed or otherwise changed, and the nature or number of individual elements or positions may be varied or changed. The order or sequence of any process or method steps may be changed or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes, and omissions may also be made in the design, operating conditions, and arrangement of the various exemplary embodiments without departing from the scope of the present invention.

Claims

1. 1. A fuel cell system, comprising: a fuel cell module including an anode section configured to output an anode exhaust stream; a first junction configured to split the anode exhaust stream into an anode recycle stream and a system outlet stream; a second junction configured to split the anode recycle stream into a first portion and a second portion; a cooler configured to cool and remove water from the second portion of the anode recycle stream; a compressor configured to receive and compress a startup stream including the second portion of the anode recycle stream before the startup stream is received by a startup inlet; a carbon dioxide separation stage configured to remove carbon dioxide from the start-up stream, the carbon dioxide separation stage comprising a molten carbonate electrolyzer system or an amine scrubber system; an ejector, wherein the ejector a low pressure inlet configured to receive a suction stream comprising a first portion of the anode recycle stream; an initiation inlet configured to receive the initiation stream; an outlet configured to output the ejector output stream; The fuel cell system, wherein the anode section is configured to receive an anode input stream that includes the ejector output stream.

2. 2. The fuel cell system of claim 1, wherein the cooler is configured to spray a low-temperature water stream onto the second portion of the anode recycle stream to cool and condense vapor from the second portion of the anode exhaust stream.

3. The fuel cell system of claim 1 , wherein the start-up stream further comprises a fresh fuel stream.

4. 10. The fuel cell system of claim 1, wherein the cooler is configured to reduce the temperature of the second portion of the anode recycle stream so that the start stream received by the compressor has a temperature within a range of 55 degrees Celsius to 80 degrees Celsius.

5. 2. The fuel cell system of claim 1, wherein the first junction is configured to split the anode exhaust stream so that the system outlet stream comprises 25% to 35% of the anode exhaust stream, and the anode recycle stream is further split at a second junction so that the first portion of the anode recycle stream comprises 12% to 22% of the anode exhaust stream and the second portion of the anode recycle stream comprises 48% to 58% of the anode exhaust stream.

6. 10. The fuel cell system of claim 1, further comprising an anode preheater configured to receive and heat the ejector output stream.

7. 10. The fuel cell system of claim 1, further comprising a pre-reformer configured to at least partially reform methane in the ejector output stream.

8. 2. The fuel cell system of claim 1, wherein the ejector is configured such that a mass ratio of the ejector output stream to the starting stream within the ejector is in the range of 2.0 to 3.0, and a starting pressure of the ejector is in the range of 20.0 psi to 30.0 psi at nominal operating conditions.

9. The fuel cell system of claim 1, further comprising: a third junction configured to separate a bypass stream from the second portion of the anode recycle stream; and a mixer configured to re-mix the bypass stream from the second portion of the anode recycle stream after water is removed from the second portion of the anode recycle stream in the cooler, wherein the bypass stream bypasses the cooler.

10. 1. A method for recycling anode exhaust of a fuel cell, said method comprising: Separating an anode exhaust stream from the fuel cell module into a system outlet stream, a suction stream, and a dryer stream; discharging the system outlet stream in a direction away from the fuel cell module; directing the suction stream to a low pressure inlet of an ejector; removing water from the dryer stream to produce a dried recycle stream; pressurizing a start-up stream containing the dried recycle stream; removing carbon dioxide from the start-up stream; directing the compressed priming stream to a priming inlet of the ejector; directing an ejector output stream from an outlet of the ejector to an anode inlet of the fuel cell module.

11. The method of claim 10 , further comprising mixing a fresh fuel stream with the dry recycle stream before pressurizing the start-up stream.

12. The method of claim 10, further comprising heating the ejector output stream before directing the ejector output stream to the anode inlet.

13. 11. The method of claim 10, wherein the dryer stream comprises between 12% and 22% of the anode exhaust stream, the suction stream comprises between 48% and 58% of the anode exhaust stream, and the system outlet stream comprises between 25% and 35% of the anode exhaust stream.

14. 11. The method of claim 10, wherein the ejector is configured such that a mass ratio of the ejector output stream to the starting stream within the ejector is in a range of 2.0 to 3.0, and a starting pressure is in a range of 20.0 psi to 30.0 psi at nominal operating conditions.

15. The method of claim 10, wherein removing water from the dryer stream comprises separating the dryer stream into a water knockout stream and a bypass stream, cooling the water knockout stream to remove water, and recombining the dried water knockout stream and the bypass stream.

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