Fuel cell system and operation method thereof

Abstract

A fuel cell system (10) and a method of operating the fuel cell system (10) are provided. The fuel cell system (10) includes a fuel cell stack (12) having an anode outlet (24) and a cathode outlet (20), a burner (34) having a combustion chamber, at least one burner inlet (38), and a catalyst (52), and a fuel cell stack outlet flowpath (36) providing fluid communication between the anode outlet (24) and the at least one burner inlet (38) and between the cathode outlet (20) and the at least one burner inlet (38), the fuel cell stack outlet flowpath (36) supplying fuel cell stack off-gas from the anode outlet (24) and the cathode outlet (20) to the burner (34). The fuel cell stack off-gas includes fuel and oxygen. The burner (34) is configured to receive fuel and oxygen exclusively from the anode outlet (24) and the cathode outlet (20) to enable catalytic combustion of an off-gas fuel mixture of fuel and oxygen at a catalyst (52) and flame combustion of the off-gas fuel mixture in a combustion chamber upstream of the catalyst (52). The method includes forming an off-gas fuel mixture from fuel and oxygen supplied from the anode off-gas outlet (24) and the cathode off-gas outlet (20), where the off-gas fuel mixture is supplied exclusively by the anode off-gas outlet (24) and the cathode off-gas outlet (20), and combusting the off-gas fuel mixture either by catalytic combustion at the catalyst (52) or flame combustion in the burner (34).

Description

[Technical Field]

[0001] The present invention relates to a fuel cell system having a burner or tail gas burner capable of both flame combustion and catalytic combustion. [Background technology]

[0002] Electrochemical fuel cells use an electrochemical conversion process to oxidize fuel to generate electricity. Such fuel cell units may be arranged one on top of the other in a stacked arrangement, for example, 10 to 200 fuel cell units may be arranged in a stack. Each fuel cell unit operates to generate electricity when in operation.

[0003] One type of fuel cell system is the solid oxide fuel cell system. The technology behind solid oxide fuel cells (SOFCs) is based on a solid oxide electrolyte that conducts negative oxygen ions from a cathode to an anode, located on either side of the electrolyte. To achieve this, fuel, or a reformed fuel, contacts the anode (also known as the fuel electrode) of the fuel cell unit, and an oxidant, such as air or an oxygen-rich fluid, contacts the cathode (also known as the air electrode) of the fuel cell unit. Fluid passages within and between the cell units make this possible. There are other forms of electrochemical cell units as well.

[0004] Burners or tail gas burners that utilize off-gases from fuel cell stacks, particularly fuel-containing output from the anode outlet of the stack where the fuel is not completely consumed during the electrochemical process, are known in the art. See, for example, International Publication No. 2016 / 097687. Combustion of off-gases provides additional heat and is particularly advantageous for utilizing gases that would otherwise be waste gases from a fuel cell stack. The general trend toward improved legislation and environmental responsibility has led to increased interest in reducing emissions resulting from fuel combustion or chemical combustion in all operations. Particularly important in controlling emissions is the reduction of carbon monoxide (CO) and nitrous oxide (NOx) emissions. The air-fuel ratio (lambda, λ) of the mixture supplied to the tail gas burner can also affect combustion, often requiring auxiliary gas supplied directly to the burner to control combustion. Summary of the Invention [Problem to be solved by the invention]

[0005] To address this need, flameless catalytic combustion can be used instead of a flame combustor, which allows the off-gas to be burned with lower emissions. Prior art systems using catalytic combustors and flame combustors to utilize the off-gas are well known. See, for example, U.S. Pat. No. 9,343,758 (B2) and EP 2,127,009 (B1).

[0006] However, the operation of both types of combustion can be improved. It is particularly desirable to produce a burner that maintains low emissions and can handle varying airflows, especially a wide range of air-fuel ratios (lambda), without utilizing complex systems. [Means for solving the problem]

[0007] According to one aspect of the present invention, there is provided a method of operating a fuel cell system including a burner and a fuel cell stack including an anode inlet, a cathode inlet, an anode off-gas outlet, and a cathode off-gas outlet, the method comprising: forming an off-gas fuel mixture from fuel and oxygen supplied from an anode off-gas outlet and a cathode off-gas outlet of the fuel cell stack, wherein the off-gas fuel mixture for the burner is supplied exclusively by the anode off-gas outlet and the cathode off-gas outlet; combusting the off-gas fuel mixture by either catalytic combustion on a catalyst in the burner or flame combustion in the burner; A method of operation is provided, including:

[0008] In some embodiments, the off-gas fuel and oxygen are fed separately to the burner and mixing occurs within the burner, while in other embodiments, mixing occurs prior to entering the burner.

[0009] During operation of some embodiments of the fuel cell system, fuel from the anode off-gas outlet and oxygen from the cathode off-gas outlet are sent to a burner. In the burner, the off-gas fuel mixture formed from the fuel and oxygen from the outlets can be combusted by flame combustion or can react with a catalyst (the catalyst reacts with the fuel to cause catalytic combustion). Both forms of combustion result in the generation of heat. All of the fuel and oxygen supplied to the burner are supplied from the outlet of the fuel cell stack. This eliminates the need for a separate fuel supply to directly supply fuel to the burner for heat generation purposes. The heat can be used to heat the system, such as the fuel cell stack, thereby improving the efficiency of the system.

[0010] The heat generated by the flame or catalytic combustion heats the burner, which in turn heats the off-gas fuel mixture.

[0011] Preferably, the fuel supplied from the anode off-gas outlet comprises hydrogen. In some embodiments, ammonia (NH3) is supplied to the system and decomposed to form H2 for the stack and thus the burner. Other fuels may be used instead.

[0012] Preferably, the fuel cell system is a solid oxide fuel cell system comprising a solid oxide fuel cell stack.

[0013] Preferably, the method of operating a fuel cell system includes combusting fuel by catalytic combustion during startup of the fuel cell system, where startup is a period of operation of the fuel cell from a low-temperature, non-operating condition or state to a point or state where electrical current can be drawn from the stack. For example, in some embodiments, in solid oxide fuel cells, particularly intermediate-temperature solid oxide fuel cells, the startup process can begin at a low temperature until the temperature of the air-side outlet (typically the cathode off-gas outlet) of the stack reaches 450°C. This is generally the approximate temperature at which electrical current can be drawn from the entire stack, and thus the fuel cell becomes operational (as an electricity generating device). However, various different types of fuel cells, and various different electrochemically active components of the electrolyte, each have different temperatures at which they can begin to draw electrical current. The chemical nature of the electrochemically active components typically determines that temperature.

[0014] In some embodiments, catalytic combustion continues during extended start-up until the fuel cell is operating at the optimal temperature for maximum current draw. For example, in some embodiments, in solid oxide fuel cells, particularly intermediate-temperature solid oxide fuel cells, the extended start-up process can be from above 450°C until the temperature at the air-side outlet of the stack (typically the cathode off-gas outlet) reaches 620°C for the first time during the heating process, with only partial current / load. This is generally the approximate temperature at which maximum current can be drawn from the entire stack and above which available current begins to decline. However, again, various different types of fuel cells and various different electrochemically active components of the electrolyte each have different temperatures at which maximum current can be drawn. The chemical nature of the electrochemically active components typically determines that temperature.

[0015] Preferably, the method for operating a fuel cell system comprises: (i) During start-up of the fuel cell system; (ii) while the temperature of the off-gas fuel mixture in the burner is insufficient to cause auto-ignition of the fuel by flame combustion; and (iii) While the fuel concentration of the off-gas fuel mixture in the burner is insufficient to be combusted by flame combustion. In at least one of the above, further comprising the step of combusting the fuel by catalytic combustion.

[0016] More preferably, catalytic combustion occurs when the temperature is too low to allow autoignition due to flame combustion.

[0017] For the avoidance of doubt, these conditions are the standard operating pressure of the fuel cell.

[0018] During fuel cell startup, the stack is relatively cold compared to its operating conditions. Therefore, fuel consumption by the electrochemical reaction is low, and fuel supplied to the anode is sent through the fuel cell to the anode outlet. This results in a fuel-rich off-gas fuel mixture in the burner. The off-gas fuel mixture in the burner will not auto-ignite if its temperature is below the auto-ignition level. In the absence of other ignition sources, flame combustion will not occur, and instead, the off-gas fuel mixture will undergo catalytic combustion. Similarly, during operation, if the off-gas fuel mixture becomes too lean, flame combustion will not occur, and instead, catalytic combustion will occur. This allows the off-gas fuel mixture to be burned in the burner under various conditions. Therefore, even when a flame is not normally generated, heat can be generated without the need to supply additional fuel, i.e., fuel that does not pass through the stack. The use of catalytic combustion also controls emissions.

[0019] Preferably, the method for operating a fuel cell system further includes the step of flame-combusting the off-gas fuel mixture in the burner using heat generated by the catalytic combustion, whereby the catalytic combustion has a synergistic effect with the flame-combustion, allowing the off-gas fuel mixture to ignite by the heat generated exceeding the auto-ignition of the fuel in the off-gas fuel mixture.

[0020] This all happens in the same burner. The autoignition temperature (AIT) is the lowest temperature at which a substance will ignite spontaneously without an external ignition source such as a flame or spark. This is also called self-ignition.

[0021] Preferably, the method for operating a fuel cell system further includes a step of exclusively combusting the off-gas fuel mixture by catalytic combustion in a catalyst in the burner when one or both of the temperature of the off-gas fuel mixture in the burner and the concentration of fuel in the off-gas fuel mixture in the burner are insufficient for flame combustion. The ability to combust the off-gas fuel mixture by flame combustion can be achieved by the fuel concentration and temperature of the off-gas fuel mixture. If these do not reach the required levels, flame combustion cannot be sustained or initiated, and therefore only catalytic combustion occurs in the burner. No additional burners are provided for separate combustion; there is only a single burner.

[0022] Preferably, the method for operating a fuel cell system further includes combusting the off-gas fuel mixture by flaming combustion in the burner when both the temperature of the off-gas fuel mixture in the burner and the concentration of fuel in the off-gas fuel mixture in the burner are sufficient for flaming combustion. The off-gas fuel mixture is combusted when flaming combustion can occur, e.g., when there is sufficient fuel concentration and temperature. Thus, the burner allows for both flaming combustion and catalytic combustion, as allowed by the temperature and fuel concentration.

[0023] Preferably, the method for operating a fuel cell system further includes a step of reducing the concentration of fuel in the off-gas fuel mixture supplied to the burner. As the stack warms up, i.e., during operation, the fuel utilization rate for the electrochemical reaction increases. As a result, less fuel is present in the off-gas at the stack outlet (anode outlet). Therefore, the off-gas fuel mixture in the burner becomes increasingly lean. The reduction in fuel present in the off-gas can reduce the fuel concentration in the off-gas fuel mixture in the burner below the lower flammable limit (LFL). The lower flammable limit (LFL) is the lowest gas concentration that, when mixed with air, will sustain a self-igniting flame. Below the LFL, there is not enough fuel to sustain flame combustion, i.e., the off-gas fuel mixture is too "lean" to combust.

[0024] Preferably, the method for operating a fuel cell system further includes passing burner exhaust gas discharged from a burner outlet of the burner through a preheating heat exchanger to exchange heat between the burner exhaust gas in the burner exhaust passage and cathode inlet gas supplied to the cathode inlet. The cathode inlet gas contains oxygen supplied to the cathode inlet of the fuel cell stack. The electrochemical reaction is more efficient at high temperatures. The stack itself generates heat as part of the reaction. However, heating the inlet gas, in this case the cathode inlet gas, aids in the electrochemical reaction and reduces the heat that needs to be supplied from other sources. Therefore, using the burner exhaust gas for this heat exchange improves system efficiency.

[0025] Preferably, the method for operating a fuel cell system further comprises the step of heating the catalytic burner using an electric heater at start-up of the fuel cell system, warming up the catalyst improving the initiation of chemical combustion.

[0026] Preferably, the method of operating a fuel cell system further includes igniting the flaming combustion of the fuel mixture solely by a temperature in the burner that causes autoignition, or by catalytic combustion that causes ignition. The absence of an ignition device, such as a spark plug, simplifies manufacturing. Furthermore, the absence of an opening for an ignition device makes it easier to reliably seal the burner, and thus the combustion chamber. This improves burner efficiency, as heat loss is reduced. Furthermore, ignition of the fuel by temperature-induced autoignition of the off-gas fuel mixture, or by heat from the catalytic combustion itself, reduces the need for control means to manipulate ignition.

[0027] Preferably, the method of operating a fuel cell system further includes controlling inlet fuel supplied to a fuel supply path fluidly communicating between the fuel supply source and the anode inlet of the fuel cell stack in response to the burner outlet temperature. The fuel cell is operated in response to the burner outlet temperature. Because there is no separate fuel mixture feed to the burner and all fuel and oxygen for the burner is typically supplied as off-gas from the fuel cell stack for mixing in the burner, there is no need for a separate fuel feed or control of such a fuel feed. Instead, the burner exhaust temperature can provide a direct correlation with stack operation. Thus, the operation of the fuel cell system can be simplified.The controller can manipulate the main fuel supply to the stack to switch the type of combustion in the burner.

[0028] Preferably, the method for operating a fuel cell system comprises: (a) a mode of operation in which an off-gas fuel mixture is supplied into or to a burner, and the temperature of the off-gas fuel mixture in the burner is insufficient so that fuel in the off-gas fuel mixture is combusted only by catalytic combustion in a catalyst; (b) a mode of operation in which an off-gas fuel mixture is fed into or to a burner, and the fuel in the off-gas fuel mixture in the burner is at a sufficient temperature to ignite the fuel and combust the fuel mixture by flame combustion within the burner; and (c) a mode of operation in which an off-gas fuel mixture is supplied into or to a burner, and due to insufficient fuel concentration in the off-gas fuel mixture, the fuel in the off-gas fuel mixture is combusted only by catalytic combustion in a catalyst; The method further includes using one of:

[0029] Catalytic combustion occurs when the temperature is too low to allow the fuel to autoignite due to flame combustion. Thus, in operating mode (a), the fuel is not burned by flame combustion but instead by catalytic combustion, so-called low-temperature catalytic combustion. When the off-gas fuel mixture temperature increases, for example, from the operation of the fuel cell stack providing warm gases at the outlet and / or through catalytic combustion, the off-gas fuel mixture will ignite. This is operating mode (b). Ignition occurs near the catalyst, where some of the hot gases cause the fuel / off-gas fuel mixture to flash back at the burner inlet. Ignition can also occur at the burner inlet itself, for example, if the off-gas fuel mixture at the burner inlet already has a temperature above the AIT. Generally, flame combustion occurs upstream of the catalyst, typically near the burner inlet (since this is where the fuel enters the burner). When the fuel is consumed by flame combustion, catalytic combustion stops. Note that in some cases, there is an overlap of combustion modes, for example, when switching between combustion operating modes or when fuel is not completely consumed during flame combustion, such as in a particularly rich mixture where insufficient oxygen is present to combust all of the fuel. In such situations, the catalyst may still be catalytically combusted. In operating mode (c), the fuel cell system is hot and can operate at high efficiency. Thus, most of the fuel is consumed in the electrochemical process in the stack. When the fuel concentration of the off-gas fuel mixture in or supplied to the burner falls below a level where flame combustion can be sustained (e.g., LFL), catalytic combustion resumes, which is lean catalytic combustion.

[0030] The term off-gas fuel mixture means the mixture of fuel and oxygen from the outlet of the stack. When referring to the combustion of fuel or the combustion of an off-gas fuel mixture, this refers to the resulting fuel and oxygen supplied to the burner.

[0031] In a further aspect of the invention, a fuel cell stack having an anode outlet and a cathode outlet; a burner having at least one burner inlet and a combustion chamber containing a catalyst; a fuel cell stack outlet flow path providing fluid communication between the anode outlet and the at least one burner inlet and between the cathode outlet and the at least one burner inlet, the fuel cell stack outlet flow path being for supplying fuel cell stack off-gas from the anode outlet and the cathode outlet to the burner, the fuel cell stack off-gas comprising fuel and oxygen; A fuel cell system comprising: A fuel cell system is provided in which the burner is configured to receive fuel and oxygen exclusively from the anode outlet and cathode outlet of the fuel cell stack, and is configured to enable catalytic combustion of an off-gas fuel mixture comprising fuel and oxygen at the catalyst, and flame combustion of the off-gas fuel mixture upstream of the catalyst in the combustion chamber.

[0032] In some embodiments, during operation of the fuel cell, the off-gas fuel mixture is supplied from the anode off-gas outlet and the cathode off-gas outlet to a burner, or mixing occurs within the burner. Thus, fuel cell stack outlet flow paths are provided from the anode outlet and the cathode outlet of the fuel cell stack to the burner. The fuel cell stack outlet paths may comprise separate paths for each of the anode outlet or the cathode outlet, so that mixing occurs within the burner, or may be combined into a single path, i.e., mixing occurs primarily within the flow path.

[0033] The off-gas fuel mixture is combusted in the combustion chamber of the burner. The combustion can be either a flame combustion, i.e., combustion of the off-gas fuel mixture, or a catalytic combustion, i.e., flameless chemical combustion of the fuel, in which a catalyst reacts with the fuel.

[0034] Flame combustion typically, or preferably, occurs upstream of the catalyst.

[0035] All of the fuel and oxygen supplied to the burner comes from the fuel cell stack; there are no additional fuel or oxygen feeds. This eliminates the need for a separate fuel feed to directly supply fuel to the burner for heat generation, thus improving the efficiency of the system.

[0036] The heat generated by combustion can heat the burner, which in turn heats the off-gas fuel mixture. The heat can also be used to heat other elements of the system, such as the fuel cell stack or the fluid feed to the fuel cell stack. Recycling the heat improves efficiency. A heat exchanger can be used for this purpose.

[0037] Preferably, the fuel cell system is further configured such that the burner is configured to ignite the off-gas fuel mixture exclusively for auto-ignition flame combustion. Flame combustion occurs when the fuel mixture reaches its auto-ignition temperature. That is, flame combustion occurs when the temperature inside the burner reaches the auto-ignition temperature of the fuel mixture and the fuel concentration is adequate (since the fuel stack may still be using little or no fuel).

[0038] More preferably, the fuel cell system does not include an ignition device. No additional ignition device is required to initiate flame combustion. Instead, flame combustion is controlled by the temperature of the off-gas fuel mixture, i.e., by heat generated by catalytic combustion, or auto-ignition of the fuel can occur by heat from elsewhere, such as the fuel cell stack.

[0039] This auto-ignition does not depend solely on the fuel cell stack temperature, since chemical combustion in the catalyst can generate heat to achieve the temperature required for auto-ignition when the output temperature of the off-gas at the cathode off-gas outlet and / or anode off-gas outlet is lower than the auto-ignition temperature.

[0040] Preferably, the fuel cell system further includes a flame shield, which protects the catalyst from the flame generated by the flame combustion. Because the catalyst is in the same chamber where the flame is generated, the flame shield can be positioned between the burner inlet and the catalyst. Fuel and oxygen, or a mixture thereof (and the flame combustion gases) can pass around the flame shield during normal operation.

[0041] Preferably, the combustion chamber or burner defines a mixing volume between the burner inlet and the catalyst. Fuel and oxygen (or a mixture thereof) enter the burner at the burner inlet from the fuel cell stack outlet flow path. The mixing volume not only mixes the fuel and oxygen (even if supplied as a mixed stream before the burner inlet, further mixing is beneficial), but also provides a volume for mixing with the heat generated within the burner. This ensures efficient combustion, both on the catalyst and via flame combustion.

[0042] Preferably, the burner is configured so that the mixing volume is heated by catalytic combustion, so that within the mixing chamber, fuel and oxygen can be mixed with the heat generated by catalytic combustion, which may obviate the need for additional or external heating of the fuel and oxygen in the fuel cell stack outlet flowpath, or possibly the burner itself.

[0043] Preferably, the burner is configured such that the mixing volume is heated to the autoignition temperature of the fuel and oxygen by catalytic combustion. The catalyst may provide sufficient heat to raise the temperature of the fuel mixture to reach the autoignition temperature and self-ignite. Thus, chemical combustion ensures combustion of the off-gas fuel mixture and thus heat generation, allowing for flaming combustion for further heat generation.

[0044] Preferably, the fuel cell system further comprises a cathode inlet flow path between an oxygen source and a cathode inlet of the cathode of the fuel cell stack, a burner exhaust flow path between a burner outlet of the burner and an exhaust port of the fuel cell system, and a preheating heat exchanger arranged to exchange heat between the burner exhaust gas in the burner exhaust flow path and the oxygen source in the cathode inlet flow path. By heating the path provided at the cathode inlet, i.e., the oxygen source path, the oxygen can be preheated, thereby improving the efficiency of the fuel cell. In particular, using heat generated from the burner for this purpose can ensure higher temperatures at the stack and stack outlet, reducing the need for heating the cathode inlet from other sources. This further heats the fuel mixture, enabling more efficient combustion.

[0045] Preferably, the fuel cell stack off-gas includes an anode off-gas containing unused fuel from the anode outlet of the fuel cell stack, and a cathode off-gas containing unused oxygen from the cathode outlet of the fuel cell. The fuel source provided at the anode inlet is not supplied to (not directly connected to) the burner. Therefore, the fuel source for the fuel cell stack is dedicated to the stack. Similarly, the oxygen source provided at the cathode inlet is not supplied to (not directly connected to) the burner. Therefore, the oxygen source for the fuel cell stack is dedicated to the stack. In other words, no auxiliary fuel or oxygen is supplied to the burner. This provides a more compact and simple system than systems that require auxiliary fuel or oxygen supplies for the burner, and is achieved by incorporating a catalytic burner into the system.

[0046] Preferably, the fuel and oxygen supplied to the at least one burner inlet are supplied from the anode and cathode off-gas outlets of the fuel cell stack in all modes of operation, and thus during start-up, steady-state operation and shutdown, the burner inlet, and therefore the burner, is supplied with fuel and oxygen (or a mixture thereof) exclusively from the fuel cell stack.

[0047] Preferably, the fuel cell system further comprises an electric heater arranged to heat the catalyst. A warm catalyst undergoes catalytic combustion more efficiently. Therefore, the electric heater may be operated, such as during start-up, to initially warm the catalyst. As the heat generated by combustion or from the electrochemical reaction in the stack continues, there is no need to operate the electric heater, as the reaction or combustion provides the necessary heat.

[0048] Preferably, the fuel cell system further comprises one or more thermocouples, allowing the controller to control the oxygen-fuel ratio as a function of the temperature output from the thermocouple(s). More preferably, thermocouples are located at the fuel cell stack inlet, fuel cell stack outlet, and burner output. During warm-up (start-up), a higher fuel flow rate is used to increase warm-up speed. When the stack is at operating temperature, the fuel flow rate is reduced to optimize efficiency. The hotter the stack is, the more current it can draw and the more fuel it will utilize. The burner outlet temperature is controlled to ensure there is sufficient heat in the oxygen source supplying the stack as it goes through these operating conditions, i.e., by heat exchange with the cathode inlet flow path. Hotter air to the stack during warm-up comes from a hotter burner outlet, and as the stack warms up, a lower burner outlet temperature may be required. For example, for an intermediate-temperature solid oxide fuel cell, the stack is considered warm when the temperature at the air-side outlet (typically the cathode off-gas outlet) of the stack reaches 540°C-620°C at full current / load, or at the end of the extended start-up process described above. However, again, various different types of fuel cells, and various different electrochemically active components of the electrolyte, each have different temperatures at which they are considered warm (i.e., fully operational). The chemical nature of the electrochemically active components typically determines that temperature.

[0049] Preferably, the combustion chamber or burner includes or provides a diffuser or diffusing effect to reduce fluid velocity in the direction of fluid flow. The change in flow velocity within the combustion chamber or burner can improve the mixing of fuel, oxygen, and heat within the burner. It also ensures that flame combustion does not cause large flames that could damage the catalyst. Instead, the flame and heat are controlled or deflected by the reduction or change in flow velocity or flow direction.

[0050] Preferably, the catalyst comprises a material that induces chemical combustion between fuel and oxygen, which is a flameless combustion of fuel and oxygen. This is also called catalytic oxidation. Chemical combustion allows for the generation of heat at a lower fuel mixture temperature. This also reduces emissions associated with flame combustion, such as NOx. Reducing emissions is beneficial during highly fuel-inefficient operation of the fuel cell stack, such as during startup, where emissions may be particularly high. Thus, the present invention advantageously eliminates the need for a starting flame combustion system (black start or gray start), improving emissions, efficiency, and reducing complexity.

[0051] According to a further aspect of the present invention, there is provided a burner assembly for a fuel cell system, comprising: an elongated body extending along a central axis and defining a combustion chamber, the body having a first end and a second end at opposite ends of the central axis, the first end having a smaller cross-sectional area than the second end; at least one inlet into the body at the first end; an outlet from the body at the second end; a catalyst positioned in the body; a flame shield positioned between the first end and the catalyst; an inlet supply line connected at least one inlet, the inlet supply line having a first arm connected to the at least one inlet and a second arm connected to the first arm by an elbow, the first arm oriented along a central axis; A burner assembly is provided comprising:

[0052] As noted above for the previous embodiment, the burner assembly can operate with both catalytic combustion and flame combustion. However, it is important to protect the catalyst from direct exposure to the flame, as this can damage the catalyst.

[0053] In some embodiments, the flame combustion occurs at or near the first (burner) inlet. Therefore, a flame shield is provided downstream to prevent the flame from being directed toward the catalyst. The flame shield can further reduce contaminants from the flame clogging or adhering to the catalyst surface (coking), which would otherwise reduce the surface area and, consequently, the efficiency of catalytic combustion. Furthermore, the combustion chamber body is shaped with a first end that is smaller than the second end. This advantageous shape therefore reduces the flow rate of gas within the combustion chamber. This can promote mixing, but also results in a shorter flame. The shorter flame is less likely or unable to reach the catalyst or the flame shield (also known as the flame wall), further protecting the catalyst.

[0054] The gas supplied to the first inlet is directed through the elbow. Therefore, the gas is slowed down by the turbulence generated in the elbow. This has the advantage of shortening the flame length, as mentioned above. Furthermore, the flow entering the combustion chamber is not directed along the central axis. Therefore, the flame, and thus the hot gases, are directed approximately along the chamber wall, i.e., deflected from the central axis. Therefore, the catalyst can be positioned within the body away from the direction of the flame and hot gases. For example, the catalyst can be positioned along the central axis, away from the wall of the body.

[0055] The burner assembly may be compatible with or incorporated as a burner for the systems and methods described above, ie, may be supplied with its oxygen and fuel exclusively from the anode and cathode off-gas outlets of the fuel cell stack.

[0056] Preferably, the burner assembly is configured such that the length of the first arm is less than the distance between the first and second ends of the elongated body, ensuring that turbulent and non-axial flow enters the combustion chamber and does not become sufficiently laminar at a distance from the elbow.

[0057] Preferably, the burner assembly is configured such that the first arm is substantially perpendicular to the second arm, and the arms are "L" shaped, which optimally or advantageously changes the direction of gas flow as needed or desired.

[0058] Preferably, the burner assembly is configured such that the body is conical. Advantageously, the narrow portion of the cone is at the first end. The cone is an effective diffuser shape for reducing the flow velocity of the fluid passing therethrough. More preferably, the burner body is partially conical. In this case, the burner body can have a controlled flow path while still retaining the advantages of a cone.

[0059] Preferably, the burner assembly is configured such that the catalyst is a catalytic mesh that allows gases to pass through but provides a large surface area for the gases to react with the catalytic material of the catalyst, thus enhancing catalytic combustion.

[0060] Preferably, the burner assembly is such that the outlet line is connected to the outlet at the second end, the outlet line having an "L"-shaped portion. This configuration draws the flow out of the combustion chamber in a direction that is not along the central axis. This draws the flow around the catalyst, ensuring that direct flame contact is minimized and heat does not damage the catalyst. More preferably, the outlet line has a smaller diameter than the second end. Thus, the flow impinges on the second end of the combustion chamber, creating a region of turbulence. This can result in gas interacting with the catalyst from vortices created by the turbulence, and backgas flow, which can provide enough heat to support auto-ignition of the fuel mixture entering the burner.

[0061] Preferably, the burner assembly is configured such that the at least one inlet is in fluid communication with at least one of the anode off-gas outlet or the cathode off-gas outlet of the fuel cell stack, and more preferably, each of the at least one inlet is in fluid communication exclusively with at least one of the anode off-gas outlet or the cathode off-gas outlet.

[0062] Preferably, the burner assembly is configured such that the catalyst is positioned closer to the second end of the body than to the first end. Therefore, the distance between the flames, i.e., the distance between the flames at or near the first end, is increased, reducing the possibility of direct contact with the flame or the possibility of contaminants from the flame combustion interfering with catalyst performance. Therefore, the flame burner (i.e., where the flame is formed) is at the first end of the combustion chamber. This is where the flame and combustible gases enter the combustion chamber.

[0063] Preferably, the burner assembly further includes a second flame shield downstream of the catalyst, which can encourage heat from catalytic combustion to be distributed throughout the combustion chamber rather than being drawn straight out the second end. This can also help prevent flame-combusted off-gases from clogging or damaging the catalyst from the second end, as the flame-combusted off-gases are directed out of the combustion chamber.

[0064] Particular and preferred aspects of the invention are set out in the accompanying independent claims. Combinations of features from the dependent claims may be combined with features of the independent claims as desired and appropriate and not only in combinations explicitly set out in the claims. [Brief explanation of the drawings]

[0065] [Figure 1] 1 is a schematic diagram (P&ID) of a fuel cell system according to the present invention. [Figure 2]FIG. 2 is a schematic diagram of an embodiment related to FIG. 1. [Figure 3] FIG. 3 is a schematic side view of a burner used in the embodiment of FIG. 1 or 2. [Figure 4] FIG. 4 is a schematic diagram of the computational flow dynamics for the burner of FIG. 3. [Figure 5] FIG. 1 shows a chart of various combustion modes of a burner for a fuel cell system. [Figure 6] FIG. 6 is a diagram showing a first trend in the operation of the fuel cell stack and burner in the operating mode of FIG. 5; [Figure 7] 6 is a diagram showing a second trend in the operation of the fuel cell stack and burner in the operating mode of FIG. 5. FIG. [Figure 8] 6 is a diagram illustrating a third trend in the operation of the fuel cell stack and burner in the operating mode of FIG. 5. FIG. [Figure 9] 6 is a diagram showing a fourth trend in the operation of the fuel cell stack and burner in the operating mode of FIG. 5. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0066] A full and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification. Reference will now be made in detail to embodiments of the invention, one or more examples of which are set forth below. Each example is provided by way of explanation of the invention, but not as a limitation of the invention.

[0067] It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope of the invention. For example, features described as part of one embodiment may be used with another embodiment to yield a still further embodiment. Accordingly, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents.

[0068] Other objects, features, and aspects of the present invention are disclosed in the remainder of this specification. It should be understood by those skilled in the art that this discussion is merely a description of exemplary embodiments and is not intended as a limitation of the broader aspects of the present invention, which are embodied in the exemplary configurations.

[0069] A list of reference symbols used herein is provided at the end of the description. Repeat use of reference symbols in the present specification and drawings is intended to represent the same or similar features or elements.

[0070] 1, there is shown a fuel cell system 10. The fuel cell system 10 has a fuel cell stack 12, which is preferably a solid oxide fuel cell system comprising a solid oxide fuel cell stack.

[0071] The fuel cell stack 12 is where the electrochemical reactions take place to generate electrical current. The fuel cell stack 12 includes a plurality of fuel cells, each having a cathode 14 and an anode 15 on either side of an electrolyte. The fuel cell stack 12 further includes a cathode inlet 18 through which a fluid is supplied to the cathode 14 of each fuel cell. This fluid is typically oxygen. The fuel cell stack 12 also includes a cathode off-gas outlet 20 through which a fluid or gas exits the cathode 14 of each fuel cell.

[0072] The fuel cell stack 12 further includes an anode inlet 22 through which a fluid, typically a fuel, is supplied to the anode 16 of each fuel cell. The fuel cell stack 12 also includes an anode off-gas outlet 24 through which a fluid or gas exits the anode 16 of each fuel cell.

[0073] Thus, in the fuel cell system 10, a fluid is supplied to the stack 12, which undergoes an electrochemical reaction during operation, and the output (off-gas) from the anode and cathode following this reaction is output at the anode off-gas outlet 20 and the cathode off-gas outlet 24.

[0074] The anode inlet 22 is supplied with fluid from a fuel source 26. The fuel source 26 is connected to the anode inlet 22 by an anode supply channel 30. A variety of fuels may be supplied to the stack to enable the electrochemical reaction. For example, hydrogen may be used as the fuel. Other fuels may alternatively be used, as is known in the art.

[0075] Cathode inlet 18 is supplied with oxygen from oxygen source 28. Oxygen source 28 is in fluid communication with cathode inlet 18 by cathode supply passage 32. Various oxygen supplies can be provided, such as air, water or steam, or oxygen-enriched air.

[0076] The anode outlet 24 and the cathode outlet 20 are in fluid communication with the burner 34 via one or more fuel cell stack outlet passages 36, which are in fluid communication with the anode outlet 24 and the cathode outlet 20 and a burner inlet 38 of the burner 34. Thus, off-gas from the fuel cell stack 12 can be routed to the burner 34.

[0077] 1, the fuel cell stack outlet flow path 36 is shown as a single line, but individual lines can be formed from each of the anode outlet 24 and cathode outlet 20 to the burner inlet 38. This is shown in FIG. 2 (described below). Alternatively, the anode outlet 24 and cathode outlet 20 can be combined from multiple lines into a single line that forms the fuel cell stack outlet flow path 36 anywhere between the anode outlet 24 and cathode outlet 20 and the burner inlet 38.

[0078] The burner 34 has a burner outlet 40 in fluid communication with an exhaust port 46 via a burner exhaust passage 48. Thus, off-gases are supplied to the burner 34, and exhaust gases generated from the combustion are exhausted / vented from the system via the exhaust port 46. Thus, a flow direction is defined from the fuel source 26 and oxygen source 28, through the fuel cell stack 12, through the burner 34, and out the exhaust port 46.

[0079] 1, the only connection to the burner 34 at the inlet 38 is the fuel cell stack outlet flow passage 36. The fuel source 26 and the oxygen source 28 are not in direct fluid communication with the burner 34. Between these connections is the fuel cell stack 12.

[0080] In this embodiment, a preheat heat exchanger 42 is provided between the burner exhaust flow path 48 and the cathode supply flow path 32 to exchange heat therebetween. This allows the exhaust gas from the burner 34 to exchange heat with and warm the fluid in the cathode supply flow path 32. In some cases, such as when the temperature of the exhaust gas is lower than the temperature from the oxygen source 28, exchanging heat is not advantageous. Therefore, in this embodiment, an oxygen source bypass line 44 is provided to bypass the preheat heat exchanger 42 so that the oxygen source 28 is instead in direct fluid communication with the cathode inlet 18.

[0081] Thus, a fuel cell system 10 is provided in which the burner 34 is in exclusive fluid supply communication with the anode outlet 24 and the cathode outlet 20 of the fuel cell stack 12 .

[0082] Referring to Figure 2, an embodiment similar to that described above is shown, and similar reference numerals are used. Figure 2 shows that the fuel cell stack outlet flow path 36 can form individual lines from each of the anode outlet 24 and cathode outlet 20 to the burner 34.

[0083] Referring to Figure 3, a burner 34 is shown. The burner 34 is also referred to as a burner assembly 34. Two fuel cell stack outlet paths 36 are shown at the burner inlet 38. This embodiment is therefore in accordance with Figure 2, in which one of the fuel cell stack outlet paths is connected to the cathode outlet and the other fuel cell outlet path 36 is connected to the anode outlet 24. Various other configurations for the burner inlet 38 can be used. However, the fuel cell stack outlet path 36 will always be in communication with the fuel cell stack 12.

[0084] Burner 34 has a burner body 50, which is an elongated body extending between a burner inlet 38 and a burner outlet 40. The burner body has a central axis 58 extending between burner inlet 38 and burner outlet 40 and positioned centrally between the sides of burner body 50. Burner inlet 38 and burner outlet 40 are also referred to as a first end and a second end.

[0085] The burner body 50 defines a combustion chamber 50 in which combustion occurs. Within the burner body 50 is a catalyst 52 that extends across the burner body 50, i.e., perpendicular to the central axis 58. The burner 34 is a three-dimensional shape, and therefore, although reference is made to a two-dimensional representation in the drawings, it will be understood that the features each have depth and occupy more than one plane. Thus, for example, the catalyst 52 lies in a plane perpendicular to the central axis 58 when viewed from the side.

[0086] The burner body 50 is shown to be conical with a cylindrical portion nearest the burner outlet 40. However, various configurations are available in which the cross-sectional area of ​​the burner body 50 at the burner inlet 38 is smaller (e.g., smaller radius) than the cross-sectional area of ​​the burner body 50 at the burner outlet 40 (e.g., larger radius).

[0087] In the illustration, catalyst 52 is a dual-layer catalyst. However, catalyst 52 may be configured as a single layer, multiple layers, or different shapes, as desired. Catalyst 52 is typically made of a mesh material or supported on a mesh material. Catalyst 52 is made of a material that reacts with fuel to undergo chemical combustion and generate heat. Such catalyst materials are well known in the art.

[0088] In this embodiment, the burner body 50 defines a mixing chamber 56 in which off-gases from the fuel cell stack 12 supplied via the fuel cell stack outlet passage 36 are mixed within the burner 34 .

[0089] Flame shields 54 are positioned within the burner body 50 on either side of the catalyst 52. The flame shields 54 are likewise oriented in the same manner as the catalyst 52 and perpendicular to the central axis 58. The flame shields 52 can be circular or other shapes, but generally have a shape or profile similar to the cross section of the burner body 50 (or catalyst).

[0090] A gap is provided between the flame shield 54 and the sidewall of the burner body 50 to allow fluid to flow around the flame shield 54. The flame shield 54 is positioned to protect the catalyst 52 from flames generated at the burner inlet 38, i.e., near the entrance to the fuel cell stack outlet flowpath 36 from the anode outlet 24. Accordingly, various configurations of flame shields 54 may be provided. For example, one flame shield 54 may be provided between the catalyst 52 and the burner inlet 38. Similarly, the flame shield 54 may be larger or smaller as needed to protect the catalyst 52, i.e., the catalyst 52 may be sized so as not to contact the wall of the burner body 50.

[0091] The fuel cell stack outlet flow passage 36 is shown to have an elbow 60 before it meets the burner inlet 38. The elbow 60 changes the direction of the off-gas flow. In particular, the pipe is shown to have a vertical or "L" shaped bend at the elbow 60. While both fuel cell stack outlet flow passages 36 are shown to have elbows 60, in some configurations, only one of the fuel cell stack outlet flow passages 36 has an elbow 60. In such a situation, the flow from the other fuel cell stack outlet flow passage 36 caused by the elbow 60 is sufficient to disrupt the flow in the mixing volume 56.

[0092] The length from the elbow 60 to the burner inlet 38 is less than the length (along the central axis 58) from the burner inlet 38 to the burner outlet 40. In some cases, the length from the elbow 60 to the burner inlet 38 is less than half the length from the burner inlet 38 to the burner outlet 40.

[0093] Additionally, the burner exhaust flow path 48 is shown to have a second elbow 62 after exiting the burner outlet 40. The second elbow 62 changes the flow direction of the exhaust gases from the burner 34. In particular, the pipe is shown to have a vertical or "L" shaped bend at the second elbow 62. Similar to the elbow 60 of the fuel cell stack outlet flow path 36, the distance of the second elbow 62 to the burner outlet 40 is less than the distance from the burner inlet 38 to the burner outlet 40. In some cases, the length from the second elbow 62 to the burner outlet 40 is less than half the length from the burner inlet 38 to the burner outlet 40.

[0094] Figure 4 is a schematic diagram of the temperatures generated by the flame within burner 34. This schematic diagram may be generated using computational fluid dynamics (CFD). It is emphasized that burner 34 is shown upside down compared to burner 34 in Figure 3, i.e., burner inlet 38 is on the left side of the diagram.

[0095] In Figure 4, a combustion flame is shown at one end of the fuel cell stack outlet channel 36, i.e., the end connected to the anode outlet 24. This combustion flame is seen by the change in shading relative to the area surrounding the end of the fuel cell stack outlet channel 36. The area of ​​interest 64 is zoomed in. The point at one end of the fuel cell stack outlet channel 36 is the flame seat 66. It can be seen that the change in shading away from the flame seat 66 indicates the flame that has been generated. Because of the elbow 60, the cathode off-gas deflects (by pushing or pulling) the flame away from the central axis 58 and toward one of the walls of the burner body 50. In this embodiment, the cathode off-gas deflects the flame downward, in a direction opposite to the direction of rotation of the elbow 60.

[0096] This deflection causes the catalyst 52 to be out of line with the flame from the flame seat 66, i.e., the flame is not along the central axis 58 about which the catalyst is centered. This also protects the catalyst 52 from the hot gases as they are drawn along the walls of the burner body 50 and thus around the catalyst 52.

[0097] Thus, a burner 34 is provided in communication with the outlets 20, 24 of the fuel cell stack 12, the burner having a catalyst 52 to enable catalytic combustion and a flame seat 66 in which flame combustion occurs.

[0098] Referring to FIG. 5, a chart 100 is shown which is drawn to illustrate the different combustion modes implemented within the burner 34: flame combustion and catalytic combustion.

[0099] Chart 100 shows fuel concentration 102 on the x-axis and temperature 104 on the y-axis, where fuel concentration 102 and temperature 104 refer to the concentration and temperature of the off-gas fuel mixture in burner 34. The off-gas fuel mixture is mixed to some extent in mixing volume 56 to achieve concentration 102 and temperature 104.

[0100] Fuel concentration 102 refers to the amount of fuel compared to oxygen in the mixture. For example, a lean concentration 102 has less fuel and is therefore lower along the x-axis.

[0101] Extending from the y-axis 104 is an auto-ignition temperature line (Self-ign. T). This is the temperature at which the fuel in the mixture can self-ignite, also called the auto-ignition temperature (AIT). As shown in chart 100, below this auto-ignition temperature 106, the fuel cannot ignite. Therefore, in the absence of an ignition source, the fuel will not flame below the auto-ignition temperature 106.

[0102] This chart includes a low-temperature catalytic oxidation block 110. Referring to the low-temperature catalytic oxidation block 110, within this temperature and concentration range, i.e., at all fuel concentration levels 102 and below the fuel's autoignition temperature 106, the off-gas fuel mixture can undergo catalytic combustion. That is, the catalyst material oxidizes in the presence of fuel in the off-gas fuel mixture, generating heat. This is also referred to as low-temperature catalytic combustion 110. This low-temperature catalytic oxidation 110 mode can occur during start-up of the fuel cell system 10. In this situation, the off-gas fuel mixture's autoignition temperature 106 is not reached. Therefore, no flame is present.

[0103] Fuel and oxygen are supplied to stack 12 from fuel source 26 and oxygen source 28. The fuel and oxygen pass through anode 16 and cathode 14, respectively. At start-up, stack 12 is cold, so little, if any, fuel is used by the electrochemical process within stack 12. Therefore, as shown in Figures 1 and 2, a fuel-containing fluid (off-gas) output from anode 16, along with oxygen-containing off-gas output from cathode 14, is supplied to burner 34.

[0104] According to chart 100, the temperature 104 is below the autoignition temperature 106 of the fuel, and therefore low temperature catalytic oxidation 110 occurs at catalyst 52 within the range of temperature 104 and fuel concentration 102 shown.

[0105] When the temperature of the fuel 104 exceeds the autoignition temperature 106, the fuel ignites and combustion switches from low-temperature catalytic oxidation 110 to flame combustion 112, as shown by the flame combustion region 112. This occurs as the stack 12 warms up and the electrochemical reactions generate more heat, which in turn increases the temperature of the off-gas supplied to the burner 34, additionally or alternatively due to the heat generated by catalytic combustion in the low-temperature catalytic oxidation stage 110. Thus, this occurs during the warm-up phase of the fuel cell system 112, as well as during normal operation. Normal operation is when the fuel cell system 10 is at operating temperature. Within this normal operation, there can be power draw, where current is drawn from the fuel cell, and idle operation, where no current is drawn.

[0106] As shown in chart 100, there is an additional operating mode in which flame combustion 112 does not always occur above the temperature 104 for autoignition of gas 106. When fuel concentration 102 is relatively low but the temperature is above the autoignition temperature 106, burner 24 catalytically combusts the off-gas fuel mixture over catalyst 52. This mode is lean catalytic combustion 114, in which a lean off-gas fuel mixture is combusted over catalyst 52. On the x-axis of fuel concentration 102 is lower flammable limit (LFL) 108. The LFL is the lowest gas concentration that will sustain an autoignition flame when mixed with air. Below the LFL, there is not enough fuel to sustain combustion; the mixture is too "lean" to burn. The flame combustion region 112 extends below the lower flammable limit 108 of fuel concentration 102. This is because there are often fuel concentrations that do not match the average fuel concentration indicated by the x-axis fuel concentration 102. Thus, the flame combustion 112 is maintained slightly below the LFL 108. However, when the fuel concentration 102 drops sufficiently below the lower flammability limit 108, the flame extinguishes in the burner 34. However, the fuel concentration 102 is too low to re-autoignite, even if the temperature 104 is above the autoignition temperature 106. Therefore, the fuel mixture in the burner 34 undergoes lean catalytic combustion 114 with the catalyst 52.

[0107] With respect to fuel cell system 10, this lean catalytic combustion mode 114 occurs when fuel is being consumed at its highest rate within fuel cell stack 12. This is when fuel cell system 10 is fully warmed up and current draw from stack 12 is at high power draw operation. Thus, the fuel content of the off-gas at anode outlet 24 is low because fuel has been significantly consumed by electrochemical processes within fuel cell stack 12. However, it is still advantageous to burn lean fuel within burner 34, and lean catalytic combustion mode 114 provides a combustion mode for this purpose.

[0108] It is emphasized that when the burner 34 switches from flame combustion 112 to lean catalytic combustion 114, there may be a momentary region of low or no combustion because all of the fuel mixture must be consumed by the flame combustion 112 and pass from the burner inlet 38 to the catalyst 52.

[0109] Thus, the burner 34 allows combustion of the fuel mixture in all modes without supplying additional fuel (non-stack off-gas fuel) to the burner 34 .

[0110] Referring to FIG. 6, a first temperature versus time trend 120 is shown illustrating flame combustion 112 and lean catalytic combustion 114 in one example.

[0111] The x-axis shows time 118 and the y-axis shows temperature 116. Trend line 122 is the catalyst mesh inlet temperature, or the catalyst inlet temperature 122 without the mesh. Trend line 124 is the burner outlet temperature. Trend line 126 is the fuel cell stack outlet temperature.

[0112] Flame combustion mode 112 is shown to occur when the inlet catalyst temperature 122 is greater than the stack exit temperature 126 and greater than or equal to the burner exit temperature 124. Thus, the flame is generating heat upstream of the catalyst 52, i.e., heat is generated by the flame at the flame seat 66. Thus, the catalyst mesh inlet temperature 122 is higher due to combustion of the off-gas fuel mixture by the flame combustion.

[0113] The lean catalytic combustion mode 114 occurs when the catalyst inlet temperature 122 is approximately equal to the stack outlet temperature 126, and the burner outlet temperature 124 is higher than both the catalyst inlet temperature 122 and the stack outlet temperature 126. Therefore, the temperature of the off-gas supplied from the stack 12 is similar to the temperature of the off-gas fuel mixture supplied to the catalyst 52, and therefore, no flame combustion should occur. However, the temperature of the exhaust gas from the burner outlet 40 (burner outlet temperature 124) is higher than both the catalyst inlet temperature 122 and the stack outlet temperature 126, resulting in a catalytic combustion temperature that generates heat. Therefore, the switching of the combustion mode is confirmed by the efficient fuel consumption and reduced fuel concentration in the stack 12.

[0114] 7, a second temperature versus time trend 130 is shown illustrating low temperature catalytic oxidation 110 in one example. The second trend 130 has an additional x-axis scale showing fuel flow rate 128. A trend 132 shows the fuel flow rate to the fuel cell stack 12.

[0115] The low temperature catalytic oxidation 110 mode of operation occurs during start-up and the initial warm-up phase of the fuel cell system 10. After routine safety checks and pre-purge, the fuel shut-off valve is commanded to open, and the burner outlet temperature 124 rapidly reaches the target temperature. Once this is reached, a control system (not shown) reduces the fuel flow 132, allowing the fuel cell stack 12 to warm up. In this example, the burner outlet temperature 124 is maintained at 500°C.

[0116] A second trend 130 shows that the stack exit temperature 126 is equal to the catalyst inlet temperature 122. Therefore, there is no flame combustion and the temperature of the off-gas fuel mixture fed from the stack exit into or to the burner 34 is the same temperature as it reaches the catalyst 52. As soon as the burner exit temperature 124 reaches the target temperature, it rises rapidly until the fuel inlet flow rate 132 is reduced.

[0117] When the fuel flow rate 132 is stopped, there is an increase in the burner exit temperature 124 at point 134. This increase when fuel flow is stopped and therefore combustion should not occur is due to the reduction in gas flow while the catalytic mesh 52 is still hot, thus causing an instantaneous increase in temperature with less gas flow.

[0118] At this particular stage of low temperature catalytic oxidation 110, stack 12 is using little or no fuel, so all available fuel is sent to burner 34. Flame combustion occurs when the fuel concentration does not exceed LFL 108.

[0119] Referring to FIG. 8, a third temperature versus time trend 135 illustrating the flame combustion 112 in one example is shown.

[0120] Flame combustion 112 occurs when the temperature of the off-gas fuel mixture in the burner 34 reaches the auto-ignition temperature 106 and the fuel concentration is adequate (because the stack 12 may still be using little or no fuel). Trend 135 shows one type of flame trigger that occurs when the temperature of the off-gas fuel mixture exceeds the auto-ignition temperature 106 at the catalyst 52 due to heat generated by catalytic combustion. The hot gases (fuel mixture) cause flashback and ignition of the off-gas fuel mixture at the burner inlet 38.

[0121] Referring to the third trend 135, the stack outlet 126 and catalyst mesh inlet 122 temperatures steadily increase. Also increasing is the burner outlet temperature 124. Upon reaching the autoignition temperature 106, the catalyst mesh inlet temperature 122 increases, indicating that a flame is being generated upstream of the catalyst 52 within the burner 34. Thus, the flame combustion 112 is occurring at or near the burner inlet 38.

[0122] As mentioned above, the flame combustion 112 can also be caused by the hot gas mixture, i.e., the off-gas fuel mixture, exceeding the autoignition temperature 106. This is because the off-gas from the stack 12 is hot when it reaches the burner 34. However, the temperature of the mixing volume 56 is also affected by the backward-directed flow of gases that have already reacted on the catalyst 52, and the rich fuel mixture is then ignited.

[0123] Referring to FIG. 9, a third temperature versus time trend 140 illustrating lean catalytic combustion 114 in one example is shown.

[0124] Once the fuel cell system 10 has started up and warmed up, fuel utilization in the fuel cell stack 12 increases and more current is drawn from the fuel cell system 10. Therefore, the off-gas at the anode outlet 24 contains less fuel, which reduces the fuel concentration 102 of the off-gas fuel mixture in the burner 32.

[0125] As the temperature of the fuel cell stack 12 increases, more current can be drawn, more heat is generated by the fuel cell stack 12, and less and less heat is contributed by the burner 32. To control this, the amount of fuel supplied to the fuel cell system 10, and therefore the amount of fuel supplied to the anode 16, is reduced until the off-gas fuel mixture in the burner 32 becomes too lean to be combusted in a flame combustion, even though the temperature exceeds the auto-ignition temperature 106. In fact, due to the local concentration of fuel, the average temperature of the off-gas fuel mixture is lower than the LFL 108 when the flame combustion 112 actually ceases, as shown in FIG. 5.

[0126] Referring to the fourth trend 140, the stack fuel flow rate 132 decreases, causing a switch from flame combustion 112 to lean catalytic combustion 114. Therefore, the catalyst inlet temperature 122 decreases to match the temperature of the stack outlet temperature 126, indicating no increase in temperature and therefore no flame combustion 112. The increase in the burner outlet temperature 124 compared to the catalyst inlet temperature 122 is due to the lean catalytic combustion 114 in the catalyst 52.

[0127] 1 , the operation and mode of operation of the fuel cell system 10 may be controlled by the fuel provided at the fuel source 26, such as by controlling the flow rate through a control valve. This fuel flow rate may be controlled, for example, manually or automatically via an electrical or manual control device, depending on the temperature. A burner outlet thermocouple 70 is provided at or near the burner outlet 40 of the burner 34. The burner outlet thermocouple 70 measures the temperature of the exhaust gases exiting the burner 34, which is indicative of the combustion mode within the burner 34.

[0128] A fuel cell stack outlet thermocouple 72 may also or alternatively be provided at the cathode outlet 20 of the stack 12. A thermocouple may also or alternatively be provided at the anode outlet 24. Multiple thermocouples may also be provided at each of the cathode outlet 20 and anode outlet 24.

[0129] The fuel cell stack exit thermocouple 72 measures the temperature of the off-gas exiting the stack 12 and is therefore indicative of the fuel cell stack operating mode, e.g., start-up, warm-up, idle / power draw, etc. Therefore, the fuel flow rate can be varied accordingly to transition to lean catalytic combustion 114 when the fuel cell stack is fully operational.

[0130] Additionally, a fuel cell stack inlet thermocouple 74 is provided at an inlet of the fuel cell stack 12, such as the cathode inlet 18 or the anode inlet 22. Because the cathode supply flow path 32 is often heated by the preheat heat exchanger 42, it is beneficial to have a fuel cell stack inlet thermocouple 74 at each inlet. The fuel cell stack inlet thermocouple 74 measures the temperature of the fuel and oxygen supplied to the fuel cell stack 12. The fuel cell stack inlet thermocouple 74 may additionally or alternatively be used to determine the operating mode or desired fuel flow rate.

[0131] The operating modes of the fuel cell system made possible by the catalyst 52 in the burner 34 allow for easy switching between catalytic combustion and flame combustion without the need for complex systems. [Explanation of symbols]

[0132] 10 Fuel Cell System 12 fuel cell stacks 14 Cathode side 16Anode side 18 cathode inlet 20 Cathode off-gas outlet 22 Anode inlet 24 anode off-gas outlet 26 fuel sources 28 oxygen source 30 anode supply channels 32 cathode supply channels 34 Burner 36 Fuel cell stack outlet flow passage 38 Burner inlet 40 burner outlet 42 Preheating heat exchanger 44 oxygen source bypass line 46 exhaust outlet 48 burner exhaust passage 50 burner body 52 catalyst 54 Flame Shield 56 mixing volume 58 center axis 60 elbow 62 Second Elbow 64 Areas of Interest 66 Flame seat 70 Burner outlet thermocouple 72 Fuel cell stack outlet thermocouple 74 Fuel cell stack inlet thermocouple 100 Charts 102x axis fuel concentration 104y-axis fuel temperature 106 auto-ignition temperature Flammability level lower than 108 110 Low-Temperature Catalytic Oxidation 112 Flame Burning 114 Lean catalytic combustion 116y-axis temperature 118x axis time 122 The Second Trend 122 Catalyst mesh inlet temperature 124 Burner outlet temperature 126 Stack outlet temperature 128 fuel flow 132 The Second Trend 132 Fuel cell stack fuel inlet flow rate 134 Highlights 135 The Third Trend 140 The Fourth Trend

Claims

1. A method for operating a fuel cell system comprising a burner and a fuel cell stack, wherein the fuel cell stack comprises an anode inlet, a cathode inlet, an anode off-gas outlet, and a cathode off-gas outlet. The aforementioned method, A step of forming an off-gas fuel mixture for a burner from fuel and oxygen supplied from the anode off-gas outlet and cathode off-gas outlet of the fuel cell stack, wherein the off-gas fuel mixture for the burner is supplied exclusively to the burner inlet by the anode off-gas outlet and the cathode off-gas outlet, and the burner is configured to allow catalytic combustion of the off-gas fuel mixture in a catalyst and flame combustion upstream of the catalyst in the combustion chamber of the burner between the flame shield and the burner inlet, The step of burning the off-gas fuel mixture by either catalytic combustion in a catalyst within the burner, or flame combustion in the combustion chamber of the burner located between the flame shield and the burner inlet, wherein the method merely ignites the flame combustion of the off-gas fuel mixture with the temperature inside the burner, and the temperature causes autoignition. Methods that include...

2. (i) While the fuel cell system is starting up, (ii) While the temperature of the off-gas fuel mixture in the burner is insufficient to cause autoignition of the fuel by flame combustion, (iii) While the fuel concentration of the off-gas fuel mixture in the burner is insufficient to be burned by flame combustion The method according to claim 1, further comprising the step of burning the fuel by catalytic combustion in at least one of the steps.

3. The method according to claim 1, further comprising the step of using the heat generated by the catalytic combustion to ignite the flame combustion of the off-gas fuel mixture in the burner.

4. The method according to claim 1, further comprising the step of burning the off-gas fuel mixture exclusively by catalytic combustion in a catalyst in the burner when either or both of the temperature of the off-gas fuel mixture in the burner and the concentration of fuel in the off-gas fuel mixture in the burner are insufficient for flame combustion.

5. The method according to claim 1, further comprising the step of burning the fuel by flame combustion in the burner when both the temperature of the off-gas fuel mixture in the burner and the concentration of the fuel in the off-gas fuel mixture in the burner are sufficient for flame combustion.

6. The method according to claim 4, further comprising the step of reducing the concentration of fuel in the off-gas fuel mixture supplied to the burner.

7. The method according to claim 1, further comprising the step of passing the burner exhaust gas discharged from the burner outlet of the burner through a preheating heat exchanger to exchange heat between the burner exhaust gas and the cathode inlet gas supplied to the cathode inlet.

8. The method according to claim 1, further comprising the step of heating the catalyst burner using an electric heater when starting the fuel cell system.

9. The method according to claim 1, further comprising the step of controlling the inlet fuel supplied to a fuel supply path that fluidly communicates between a fuel source and the anode inlet of the fuel cell stack, according to the temperature of the burner outlet.

10. The following: (a) An operating mode in which the off-gas fuel mixture is supplied to or into the burner, wherein the temperature of the off-gas fuel mixture in the burner is insufficient, and the fuel in the off-gas fuel mixture is burned only by catalytic combustion in the catalyst. (b) Operating modes in which the off-gas fuel mixture is supplied to or into the burner, wherein the temperature of the off-gas fuel mixture in the burner is sufficient, causing the fuel in the off-gas fuel mixture to autoignite and burn by flame combustion in the burner, and (c) An operating mode in which the off-gas fuel mixture is supplied into or to the burner, wherein, due to an insufficient fuel concentration in the off-gas fuel mixture, the fuel in the off-gas fuel mixture is burned only by catalytic combustion in the catalyst. The method according to claim 1, wherein one of the following is used.

11. A fuel cell system, A fuel cell stack having an anode outlet and a cathode outlet, A burner comprising a combustion chamber, at least one burner inlet, a catalyst, and a flame shield disposed between the burner inlet and the catalyst, A fuel cell stack outlet passage that provides fluid communication between the anode outlet and the at least one burner inlet and between the cathode outlet and the at least one burner inlet, wherein the fuel cell stack outlet passage is for supplying fuel cell stack off-gas from the anode outlet and the cathode outlet to the burner, and the fuel cell stack off-gas contains fuel and oxygen, A fuel cell system equipped with, A fuel cell system wherein the burner is configured to receive the fuel and oxygen exclusively from the anode outlet and cathode outlet of the fuel cell stack, and to enable catalytic combustion of the off-gas fuel mixture containing the fuel and oxygen in the catalyst, and flame combustion of the off-gas fuel mixture upstream of the catalyst in the combustion chamber, and for the latter, the burner is configured to ignite the off-gas fuel mixture exclusively for flame combustion by autoignition.

12. The fuel cell system according to claim 11, wherein the burner is not equipped with an ignition device.

13. The fuel cell system according to claim 11, wherein the combustion chamber defines the mixing volume between the burner inlet and the catalyst.

14. The fuel cell system according to claim 13, wherein the burner is configured such that the mixed volume is heated by the catalytic combustion.

15. The fuel cell system according to claim 13, wherein the burner is configured such that the mixed volume is heated by the catalytic combustion to the autoignition temperature of the fuel and oxygen.

16. The fuel cell system according to claim 11, further comprising: a cathode inlet passage between an oxygen source and the cathode inlet of the fuel cell stack; a burner exhaust passage between the burner outlet of the burner and the fuel cell system exhaust port; and a preheating heat exchanger arranged within the cathode inlet passage and the burner exhaust passage to exchange heat between the burner exhaust gas in the burner exhaust passage and the oxygen source in the cathode inlet passage.

17. A burner assembly for a fuel cell system, wherein the burner assembly is A body having an elongated shape extending along a central axis and defining a combustion chamber, wherein the body has a first end and a second end at both ends of the central axis, and the first end has a smaller cross-sectional area than the second end, At least one inlet to the body at the first end, The outlet from the main body at the second end, A catalyst positioned within the main body, A flame shield positioned between the first end and the catalyst, An inlet supply line connected at at least one inlet, comprising a first arm connected to the at least one inlet and a second arm connected to the first arm by an elbow, wherein the first arm is oriented along the central axis, and the burner assembly is configured to be exclusively supplied with its oxygen and fuel from the off-gas outlets of the anode and cathode of the fuel cell stack, and such gases interacting with the catalyst provide enough heat to assist in the autoignition of the fuel mixture entering the burner, A burner assembly equipped with the following features.

18. The burner assembly according to claim 17, wherein the length of the first arm is less than the distance between the first end and the second end of the elongated body.

19. The burner assembly according to claim 17, wherein the burner assembly is a burner for a fuel cell system according to any one of claims 11 to 16.

20. The method according to any one of claims 1 to 10, wherein the burner is the burner assembly according to claim 17 or claim 18.

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