Fuel cell system and method of operating the same

Abstract

A fuel cell system (10) and a method of operating the same, the fuel cell system (10) comprising: a fuel cell stack (12) comprising an anode outlet (24) and a cathode outlet (20); a combustor (34) comprising a combustion chamber, at least one combustor inlet (38) and a catalyst (52); and a fuel cell stack outlet flow path (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 flow path (36) is for providing fuel cell stack off-gas from the anode outlet (24) and the cathode outlet (20) to the combustor (34). The fuel cell stack exhaust includes fuel and oxygen. The burner (34) is configured to receive the fuel and the oxygen only from the anode outlet (24) and the cathode outlet (20), and catalytically burn an off-gas fuel mixture of the fuel and the oxygen at the catalyst (52), and flame-combusting the exhaust gas fuel mixture upstream of the catalyst (52) in the combustion chamber. The method involves forming the off-gas fuel mixture from the fuel and the oxygen supplied from the anode off-gas outlet (24) and the cathode off-gas outlet (20) of the fuel cell stack (12), the off-gas fuel mixture being provided solely by the anode off-gas outlet (24) and the cathode off-gas outlet (20), and combusting the exhaust gas fuel mixture by catalytic combustion at the catalyst (52) or by flame combustion in the burner (34).

Description

Technical Field

[0001] The invention relates to a fuel cell system with a burner or tail gas burner, wherein the burner can perform both flame combustion and catalytic combustion. Background Art

[0002] Electrochemical fuel cells use an electrochemical conversion process that oxidizes a fuel to produce electrical energy. Such fuel cell units can be arranged in a stack, for example, 10 to 200 fuel cell units are arranged on top of each other in a stack. Each fuel cell unit works to produce electrical energy when in operation.

[0003] One type of fuel cell system is a solid oxide fuel cell system. The technology behind a solid oxide fuel cell (SOFC) is based on a solid oxide electrolyte that conducts negative oxygen ions from the cathode to the anode located on the opposite side of the electrolyte. To do this, a fuel or modified 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 channels within and between the cells allow this. There are other forms of electrochemical cells.

[0004] Burners or tail gas burners are known in the art that utilize the exhaust gases from a fuel cell stack, particularly fuel including the output from the anode outlet of the stack, where the fuel is not completely consumed in the electrochemical process. For example, see WO2016 / 097687. The combustion of the exhaust gases is particularly advantageous for providing additional heat and utilizing gases that would otherwise be wasted in the fuel cell stack. Legislation and the general trend toward improved environmental responsibility encourage people to reduce emissions generated by the burning or combustion of fuels in all operations. In terms of controlling emissions, reducing carbon monoxide (CO) and nitrogen oxides (NO x The air-to-fuel ratio (lambda, λ) of the mixture supplied to the tail gas burner can also have an effect on combustion, often requiring supplemental gas to be supplied directly to the burner to control combustion.

[0005] To address these requirements, flameless catalytic combustion can be provided instead of flame burners. This allows the exhaust gas to be burned with less emissions. Known prior art systems use catalytic burners and flame burners to utilize exhaust gas. See, for example, US9343758B2 and EP2127009B1.

[0006] However, the operation of both combustion types can be improved.It would be particularly desirable to produce a burner that does not use complex systems while maintaining low emissions and coping with varying airflows, and in particular a wide range of air to fuel ratios λ. Summary of the invention

[0007] According to one aspect of the present invention, there is provided a method of operating a fuel cell system, the fuel cell system comprising a burner and a fuel cell stack, the fuel cell stack comprising an anode inlet, a cathode inlet, an anode exhaust gas outlet, and a cathode exhaust gas outlet;

[0008] The method comprises the following steps:

[0009] forming an exhaust-fuel mixture from fuel and oxygen supplied from an anode exhaust outlet and a cathode exhaust outlet of the fuel cell stack, wherein the exhaust-fuel mixture for the burner is provided only from the anode exhaust outlet and the cathode exhaust outlet; and

[0010] The exhaust gas fuel mixture is combusted by catalytic combustion at a catalyst in the combustor or by flame combustion in the combustor.

[0011] In some embodiments, the exhaust fuel and oxygen are provided to the burner separately and mixed within the burner. In other embodiments, mixing occurs before entering the burner.

[0012] During operation of some embodiments of the fuel cell system, fuel from the anode exhaust outlet and oxygen from the cathode exhaust outlet are delivered to the burner. In the burner, the exhaust fuel mixture formed by the fuel and oxygen from the outlet can be burned by flame combustion or can react with a catalyst that reacts with the fuel to cause catalytic combustion. Both forms of combustion will result in the generation of heat. All fuel and oxygen provided to the burner come from the outlet of the fuel cell stack. This eliminates the need for a separate fuel feed to provide the fuel supply directly to the burner in order to generate heat. The heat can be used to heat the system, such as the fuel cell stack. Therefore, the efficiency of the system is improved.

[0013] The heat generated by the flame combustion or catalytic combustion heats the burner, thereby also heating the exhaust gas fuel mixture.

[0014] Preferably, the fuel supplied from the anode exhaust outlet comprises hydrogen. In some embodiments, ammonia (NH 3 ) is supplied to the system and cracked to form H for the stack and thus for the combustor 2 . Other fuels can be used instead.

[0015] Preferably, the fuel cell system is a solid oxide fuel cell system comprising a solid oxide fuel cell stack.

[0016] Preferably, the method of operating the fuel cell system includes burning the fuel by catalytic combustion during startup of the fuel cell system; startup is an operating period in which the fuel cell enters a point or condition from a cold, non-operating situation or condition where current can be drawn from the stack. For example, in some embodiments, in a solid oxide fuel cell, and in particular an intermediate temperature solid oxide fuel cell, the startup process can begin from a cold state until the temperature at the air side outlet of the stack (typically the cathode exhaust outlet) reaches 450 degrees Celsius. This is typically the approximate temperature at which current will be able to be drawn across the stack, and therefore the fuel cell will be operable (as a power generation device). However, various different types of fuel cells and various different electrochemically active components of the electrolyte will each have a different temperature at which current can begin to be drawn: the chemical properties of the electrochemically active components typically determine the temperature.

[0017] In some embodiments, catalytic combustion continues through an extended startup process until the fuel cell is operating at an optimal temperature for maximum current draw. For example, in some embodiments, in a solid oxide fuel cell, and in particular a medium temperature solid oxide fuel cell, an extended startup process may begin at above 450 degrees Celsius until the temperature at the air side outlet of the stack (typically the cathode exhaust outlet) reaches 620 degrees Celsius for the first time during the heating process while only at partial current / load. This is typically the approximate temperature at which the maximum current will be able to be drawn across the stack, and above this temperature the available current begins to decline. However, again, various different types of fuel cells and various different electrochemically active components of the electrolyte will each have a different temperature at which the maximum current can be drawn: the chemical properties of the electrochemically active components typically determine the temperature.

[0018] Preferably, the method of operating a fuel cell system further comprises: combusting the fuel by catalytic combustion during at least one of the following:

[0019] i) Start-up of the fuel cell system;

[0020] ii) the temperature of the exhaust gas fuel mixture in the burner is insufficient to cause auto-ignition of the fuel by flame combustion; and

[0021] iii) The exhaust gas fuel mixture in the burner has insufficient fuel concentration to burn through flame combustion.

[0022] More preferably, catalytic combustion occurs when the temperature is too low to automatically initiate flame combustion.

[0023] For the avoidance of any doubt, these conditions are at standard operating pressures for fuel cells.

[0024] At the start-up of the fuel cell, the stack is relatively cold compared to its operating conditions. Therefore, the fuel consumption of the electrochemical reaction is low, and the fuel fed into the anode is passed through the fuel cell to the anode outlet. This results in a fuel-rich exhaust gas fuel mixture in the burner. When the temperature of the exhaust gas fuel mixture in the burner is below the autoignition level, it will not autoignite. In the absence of other combustion sources, flame combustion will not occur, but the exhaust gas fuel mixture will undergo catalytic combustion. Similarly, if the fuel concentration of the exhaust gas fuel mixture is too lean during operation, flame combustion will not occur, but catalytic combustion will occur. This enables the exhaust gas fuel mixture to burn in the burner in various states. Therefore, heat can be generated even when a flame would not normally be generated, without providing additional fuel, that is, fuel that does not pass through the stack. Emissions are also controlled by using catalytic combustion.

[0025] Preferably, the method of operating a fuel cell system further comprises: using the heat generated by the catalytic combustion to initiate flame combustion of the exhaust fuel mixture in the burner. Thus, the catalytic combustion has a synergistic effect with the flame combustion, wherein the heat generated enables ignition of the exhaust fuel mixture by exceeding the autoignition temperature of the fuel in the exhaust fuel mixture. This all occurs in the same burner. The autoignition temperature (AIT) is the lowest temperature at which a substance spontaneously ignites without an external ignition source, such as a flame or spark. This is also known as autoignition.

[0026] Preferably, the method of operating a fuel cell system also includes, when one or both of the temperature of the exhaust gas fuel mixture in the burner and the concentration of the fuel in the exhaust gas fuel mixture in the burner are insufficient for flame combustion, burning the exhaust gas fuel mixture only by catalytic combustion at the catalyst in the burner. The ability of the exhaust gas fuel mixture to burn by flame combustion can be affected by the fuel concentration and temperature of the exhaust gas fuel mixture. When these do not reach the required levels, flame combustion cannot be maintained or started, so only catalytic combustion occurs in the burner. No additional burner is provided for separate combustion. There is a single burner.

[0027] Preferably, the method of operating the fuel cell system further comprises, when the temperature of the exhaust gas fuel mixture in the burner and the concentration of the fuel in the exhaust gas fuel mixture in the burner are both sufficient for flame combustion, burning the exhaust gas fuel mixture by flame combustion in the burner. When flame combustion can occur, such as when the fuel concentration and temperature are sufficient, the exhaust gas fuel mixture will be burned. Thus, the burner allows both flame combustion and catalytic combustion as allowed by the temperature and fuel concentration.

[0028] Preferably, the method of operating a fuel cell system further comprises: reducing the concentration of fuel in the exhaust gas fuel mixture supplied to the burner. As the stack warms up, i.e., during operation, the fuel utilization of the electrochemical reaction will increase. This results in less fuel being present in the exhaust gas at the stack outlet (anode outlet). As a result, the exhaust gas fuel mixture in the burner becomes increasingly leaner. The reduction in the fuel present in the exhaust gas can cause the fuel concentration in the exhaust gas fuel mixture in the burner to drop below a lower limit combustible concentration. The lower limit combustible concentration (LFL) is the minimum gas concentration that supports a spontaneous flame when mixed with air. Below the lower limit combustible concentration, there is not enough fuel to support flame combustion, i.e., the exhaust gas fuel mixture is too "lean" to burn.

[0029] Preferably, the method of operating a fuel cell system further comprises: passing a burner exhaust gas discharged from a burner outlet of the burner through a preheater heat exchanger so that heat is exchanged between the burner exhaust gas in the burner exhaust flow path and the cathode inlet gas supplied to the cathode inlet. The cathode inlet gas includes oxygen supplied to the cathode inlet of the fuel cell stack. The electrochemical reaction is more efficient at higher temperatures. The stack itself generates heat as part of the reaction. However, the heating of the inlet gas (in this case, the cathode inlet gas) contributes to the electrochemical reaction and reduces the heat that needs to be provided from other sources. Therefore, using the burner exhaust for this heat exchange improves system efficiency.

[0030] Preferably, the method of operating a fuel cell system further comprises: using an electric heater to heat the catalytic burner when the fuel cell system is started. The increase in temperature of the catalyst improves the start-up of chemical combustion.

[0031] Preferably, the method of operating a fuel cell system further comprises: initiating only flame combustion of the fuel mixture by temperature in the burner causing autoignition or catalytic combustion causing ignition. The absence of an igniter (such as a spark plug) simplifies manufacturing. It can also ensure that the burner and thus the combustion chamber are easier to seal because there is no opening for the igniter. This improves the efficiency of the burner because heat losses are reduced. Ignition of the fuel by autoignition of the exhaust fuel mixture due to temperature or ignition caused by heat from the catalytic combustion itself also reduces the need for control means for operating the ignition.

[0032] Preferably, the method of operating a fuel cell system further comprises: controlling the inlet fuel provided to a fuel supply path according to the temperature of the burner outlet, the fuel supply path being fluidly connected between the fuel supply and the anode inlet of the fuel cell stack. The fuel cell operates according to the burner outlet temperature. Since there is no separate fuel mixture fed to the burner, and all the fuel and oxygen of the burner are provided as exhaust gas from the fuel cell stack (typically for mixing in the burner), no separate fuel feed is required, or no control of such fuel feed is required. On the contrary, the burner exhaust temperature can provide a direct correlation between the operation of the stack. In view of this, the operation of the fuel cell system can be simplified. The controller can operate the main fuel supply of the stack to switch between the combustion types in the burner.

[0033] Preferably, the method of operating the fuel cell system further comprises using one of the following operating modes:

[0034] a) supplying the exhaust gas fuel mixture into or to a burner, wherein due to an insufficient temperature of the exhaust gas fuel mixture in the burner, the fuel in the exhaust gas fuel mixture is combusted only by catalytic combustion at the catalyst;

[0035] b) supplying the exhaust gas fuel mixture into or to a burner and igniting its fuel, since the temperature of the fuel in the exhaust gas fuel mixture in the burner is sufficient and the fuel mixture is burned by flame combustion in the burner; and

[0036] c) Supplying the exhaust gas fuel mixture into or to a burner, wherein the fuel in the exhaust gas fuel mixture is combusted only by catalytic combustion at the catalyst due to insufficient fuel concentration of the exhaust gas fuel mixture.

[0037] Catalytic combustion occurs when the temperature is too low to automatically ignite the fuel for flame combustion. Therefore, in operating mode (a), the fuel will not burn by flame combustion, but catalytic combustion, or so-called cold catalytic combustion, will occur. When the exhaust gas fuel mixture temperature has increased, for example, due to the operation of the fuel cell stack to provide warm gas at the outlet and / or by catalytic combustion, the exhaust gas fuel mixture will ignite. This is mode (b). Ignition can occur near the catalyst, where some of the hot gas will cause the fuel / exhaust gas fuel mixture supply at the burner inlet to flash back. Ignition can also occur at the burner inlet itself, for example, where the temperature of the exhaust gas fuel mixture at the burner inlet has exceeded the AIT. Typically, flame combustion will occur in the burner upstream of the catalyst, for example, usually near the burner inlet, because this is where the fuel enters the burner. When the fuel is consumed by flame combustion, catalytic combustion will stop. It should be noted that in some cases, the combustion modes will overlap, such as when switching between combustion operating modes, or when the fuel is not completely consumed during flame combustion, such as in a particularly rich mixture, where oxygen is not enough to burn all the fuel. In this case, the catalyst can still undergo catalytic combustion. In operation mode (c), the fuel cell system can be high temperature and run efficiently. Therefore, most of the fuel is consumed in the electrochemical process at the stack. The fuel concentration of the exhaust gas fuel mixture in the burner or supplied to the burner is reduced to a level below that can maintain flame combustion (e.g., LFL), and catalytic combustion occurs again, which is lean catalytic combustion.

[0038] The term exhaust gas fuel mixture refers to the mixture of fuel and oxygen from the outlet of the stack. When reference is made to fuel combustion or exhaust gas fuel mixture combustion, this refers to the resulting fuel and oxygen that has been supplied to the burner.

[0039] In another aspect of the present invention, there is provided a fuel cell system comprising:

[0040] a fuel cell stack comprising an anode outlet and a cathode outlet;

[0041] a burner comprising a combustion chamber including at least one burner inlet and a catalyst;

[0042] a fuel cell stack outlet flow path providing fluid communication between the anode outlet and at least one burner inlet and between the cathode outlet and at least one burner inlet, the fuel cell stack outlet flow path for providing a fuel cell stack exhaust gas from the anode outlet and the cathode outlet to the burner, the fuel cell stack exhaust gas comprising fuel and oxygen;

[0043] The burner is configured to receive fuel and oxygen only from the anode outlet and the cathode outlet of the fuel cell stack, and is configured to catalytically combust an exhaust gas fuel mixture including fuel and oxygen at the catalyst and to flame combust the exhaust gas fuel mixture upstream of the catalyst in the combustion chamber.

[0044] In some embodiments, during operation of the fuel cell, the exhaust fuel mixture is fed to the burner from the anode exhaust outlet and the cathode exhaust outlet, or mixing occurs in the burner. Therefore, a fuel cell stack outlet flow path is provided from the anode outlet and the cathode outlet of the fuel cell stack to the burner. The fuel cell stack outlet path may include a separate path for each anode outlet or cathode outlet so that mixing occurs within the burner, or may be combined into a single path, i.e., mixing initially occurs within the flow path.

[0045] The exhaust gas fuel mixture is burned in the combustion chamber of the burner. The combustion is either flame combustion, i.e., combustion of the exhaust gas fuel mixture, or catalytic combustion of the fuel by reaction of the catalyst with the fuel, i.e., flameless chemical combustion.

[0046] Flame combustion typically or preferably occurs upstream of the catalyst.

[0047] All fuel and oxygen supplied to the burner comes from the fuel cell stack. There is no additional fuel or oxygen feed. This eliminates the need for a separate fuel feed to provide the fuel supply directly to the burner in order to generate heat. Therefore, the efficiency of the system is improved.

[0048] The heat generated by the combustion can heat the burner, thereby heating the exhaust fuel mixture. The heat is also used to heat other elements of the system, such as the fuel cell stack or the fluid feed to the fuel cell stack. Recycling of heat improves efficiency. For this purpose, a heat exchanger can be used.

[0049] Preferably, the fuel cell system is further configured such that: the burner is configured to ignite the exhaust fuel mixture for flame combustion only by autoignition. Flame combustion will occur when the fuel mixture reaches the autoignition temperature. That is, when the temperature inside the burner reaches the autoignition temperature of the fuel mixture and the fuel concentration is sufficient (because the fuel stack can still use little or no fuel), flame combustion will be triggered.

[0050] More preferably, the fuel cell system does not include an igniter. No additional igniter is required to induce flame combustion. Instead, this is controlled by the temperature of the exhaust fuel mixture, i.e., by the heat generated by the catalytic combustion, or the fuel autoignition can be caused by heat from elsewhere (such as the fuel cell stack).

[0051] This autoignition is not solely dependent on the fuel cell stack temperature, because when the output temperature of the exhaust gas at the cathode exhaust outlet and / or the anode exhaust outlet is lower than the autoignition temperature, the chemical combustion at the catalyst can provide heat to reach the temperature required for autoignition.

[0052] Preferably, the fuel cell system also includes a flame shield. This protects the catalyst from the flame generated by the flame combustion. Since the catalyst is located in the same chamber where the flame is generated, the flame shield can be located between the burner inlet and the catalyst. The fuel and oxygen or a mixture thereof (and the flame combustion gas) can bypass the flame shield during normal operation.

[0053] Preferably, the combustion chamber or burner defines a mixing volume between the burner inlet and the catalyst. The 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 provides a volume for the fuel and oxygen to mix together (even if provided as a combined flow before the burner inlet, further mixing is also beneficial), but also mixes with the heat generated in the burner. This ensures both efficient combustion at the catalyst and efficient combustion of the flame combustion.

[0054] Preferably, the burner is configured so that the mixing volume is heated by catalytic combustion. Thus, within the mixing chamber, the fuel and oxygen can be mixed with the heat generated by the catalytic combustion. This can eliminate the need for additional or external heating of the fuel and oxygen within the fuel cell stack outlet flow path or in some cases the burner itself.

[0055] Preferably, the burner is configured so that the mixed volume is heated to the autoignition temperature of the fuel and oxygen by catalytic combustion. The catalyst can provide sufficient heat to increase the temperature of the fuel mixture to reach the autoignition temperature and autoignite. Thus, chemical combustion ensures the combustion of the exhaust fuel mixture, thereby generating heat to cause the flame to burn and further generate heat.

[0056] Preferably, the fuel cell system also includes: a cathode inlet flow path between an oxygen source and the cathode inlet of the cathode of the fuel cell stack, a burner exhaust flow path between the burner outlet of the burner and the exhaust of the fuel cell system, and a preheater heat exchanger arranged to exchange heat between the burner exhaust in the burner exhaust flow path and the oxygen source in the cathode inlet flow path. Heating the path provided by the cathode inlet, i.e., the oxygen source path, will improve the efficiency of the fuel cell because the oxygen can be preheated. Specifically, the heat generated from the burner is used for this purpose to ensure a higher temperature stack and stack outlet, and the need to heat the cathode inlet from other sources can be reduced. Therefore, the fuel mixture is further heated, and efficient combustion can occur.

[0057] Preferably, the fuel cell stack exhaust gas includes an anode exhaust gas and a cathode exhaust gas, the anode exhaust gas including unused fuel from the anode outlet of the fuel cell stack, and the cathode exhaust gas including unused oxygen from the cathode outlet of the fuel cell. The fuel source provided to the anode inlet is not provided to (directly connected to) the burner. Therefore, the fuel source of the fuel cell stack is specifically provided to the stack. Similarly, the oxygen source provided to the cathode inlet is not provided to (directly connected to) the burner. Therefore, the oxygen source of the fuel cell stack is specifically provided to the stack. This means that no supplementary fuel or oxygen is provided to the burner. Compared with a system that needs to provide supplementary fuel or oxygen costs for the burner, this provides a more compact and simpler system, and is achieved by including a catalytic burner therein.

[0058] Preferably, in all operating modes, the fuel and oxygen provided at the at least one burner inlet are provided from the anode exhaust outlet and cathode exhaust outlet of the fuel cell stack. Thus, during startup, steady state operation and shutdown, the burner inlet and therefore the burner are only supplied with fuel and oxygen (or a mixture thereof) by the fuel cell stack.

[0059] Preferably, the fuel cell system further comprises an electric heater arranged to heat the catalyst. The preheated catalyst catalytically burns more efficiently. Thus, the electric heater may be operated, such as during startup, to preliminarily preheat the catalyst. Once heat is generated by combustion or electrochemical reactions in the stack, the electric heater need not be operated, as the reaction or combustion will provide the necessary heat.

[0060] Preferably, the fuel cell system also includes one or more thermocouples so that the controller can control the oxygen and fuel ratio according to the temperature output from the thermocouple. More preferably, the thermocouples are arranged at the fuel cell stack inlet, the fuel cell stack outlet and the burner output. During the preheating (startup) period, a higher fuel flow rate is used to increase the preheating rate. When the stack is at operating temperature, the fuel flow rate will be reduced to optimize efficiency. The hotter the stack is, the more current it can draw, so the stack will use more fuel. The burner outlet temperature is controlled when it undergoes these operating states to ensure that there is enough heat for the oxygen source to enter the stack, that is, by heat exchange with the cathode inlet path. The air entering the stack during the preheating period is hotter, because the burner outlet is hotter, and once the stack is preheated, a lower burner outlet temperature may be required. For example, for a medium-temperature solid oxide fuel cell, the stack is considered to be preheated when the temperature of the air side outlet of the stack (usually the cathode exhaust outlet) reaches between 540 and 620 degrees Celsius at full current / load or at the end of the above-mentioned extended startup process. However, again, various different types of fuel cells and various different electrochemically active components of the electrolyte will each have a different temperature at which they are considered warmed up (ie fully operational): the chemical properties of the electrochemically active components will generally determine this temperature.

[0061] Preferably, the combustion chamber or burner includes or provides a diffuser or diffusion effect for reducing the velocity of the fluid in the direction of the fluid flow. The change in flow velocity within the combustion chamber or burner allows for improved mixing of fuel, oxygen and heat within the burner. It also ensures that the flame combustion does not produce a large flame that may damage the catalyst. Instead, the flame and heat are controlled or deflected by reducing or changing the flow velocity or direction.

[0062] Preferably, the catalyst includes a material that produces chemical combustion between the fuel and the oxygen, wherein the chemical combustion is a flameless combustion of the fuel and the oxygen. It may also be referred to as catalytic oxidation. Chemical combustion allows heat to be generated at a lower fuel mixture temperature. This also reduces emissions associated with flame combustion, such as NOx. During operation periods where the fuel efficiency of the fuel cell stack is extremely low, such as during startup, emissions can be particularly high, and reducing emissions is beneficial. Therefore, using the present invention, for example, the need for a startup flame combustion system (black start or gray start) is eliminated, with the benefits of improved emissions, efficiency, and reduced complexity.

[0063] According to another aspect of the present invention, there is provided a burner assembly for a fuel cell system, the burner assembly comprising:

[0064] an elongated body extending along a central axis and defining a combustion chamber, the body having a first end and a second end located at either end of the central axis, the first end having a smaller cross-sectional area than the second end;

[0065] at least one inlet into the body at the first end;

[0066] an outlet exiting the body at the second end;

[0067] a catalyst, the catalyst being located within the body,

[0068] a flame shield positioned between the first end and the catalyst; and

[0069] An inlet supply line is connected through the 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 at an elbow, the first arm being oriented along the central axis.

[0070] As described above with respect to the foregoing aspects, the burner assembly can operate 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.

[0071] In some embodiments, flame combustion will occur at or near the first (burner) inlet. Therefore, a flame shield is provided downstream, which will prevent the flame from directly reaching the catalyst. The flame shield can also reduce any pollutants in the flame from clogging the catalyst or adhering to the catalyst surface (coking), otherwise the surface area will be reduced, thereby reducing the efficiency of catalytic combustion. In addition, the shape of the combustion chamber body has a first end that is smaller than the second end. Therefore, this favorable shape reduces the flow rate of the gas in the combustion chamber. This can promote mixing, but it can also cause the flame to become shorter. The shorter flame is less likely or unable to reach the catalyst or the flame shield (also called a flame wall), thereby further protecting the catalyst.

[0072] The gas supplied to the first inlet is guided through the elbow. Therefore, the turbulence generated by the gas through the elbow is slowed down. This has the benefit of reducing the flame length as described above. In addition, the flow entering the combustion chamber will not be guided along the central axis. Therefore, the flame and thus the hot gas will be guided roughly along the wall of the chamber (offset from the central axis). Therefore, the catalyst can be positioned in the body in a direction away from the flame and the hot gas. For example, the catalyst can be placed along the central axis away from the body wall.

[0073] The burner assembly is compatible with or may be incorporated as a burner for the above-described systems and methods, ie, it may be supplied with its oxygen and fuel exclusively from the anode exhaust outlet and cathode exhaust outlet of the fuel cell stack.

[0074] Preferably, the burner assembly is such that the length of the first arm is less than the distance between the first and second ends of the elongated body. This ensures that turbulent and non-central axis oriented flow still enters the combustion chamber and does not become sufficiently stratified within the distance from the elbow.

[0075] Preferably, the burner assembly is such that the first arm is substantially perpendicular to the second arm. The shape of the arm is an "L" shape. This shape provides an optimal or beneficial change in the direction of gas flow as required or desired.

[0076] Preferably, the burner assembly is configured so that the body is tapered. The narrow portion of the taper is beneficially located at the first end. The taper is an effective diffuser shape for reducing the flow rate of fluid passing therethrough. More preferably, the burner body is partially tapered. Then, it can still retain the benefits of the taper while also having a controlled flow path therethrough.

[0077] Preferably, the burner assembly has the catalyst as a catalytic mesh. The mesh allows the gas to pass through, but has a larger surface area to allow the gas to react with the catalytic material of the catalyst. Thus, catalytic combustion is improved by the mesh.

[0078] Preferably, the burner assembly has an outlet line connected to the outlet at the second end, the outlet line having an "L" shaped portion. This arrangement draws the flow from the combustion chamber in a direction other than along the central axis. Therefore, the flow can be drawn around the catalyst to ensure that direct flame contact is minimized and the heat does not damage the catalyst. More preferably, the diameter of the outlet line is smaller than the second end. Therefore, the flow will hit the second end of the combustion chamber and cause a turbulent area. This can cause the gas to interact with the catalyst because the vortex and return air flow generated by the turbulence can provide sufficient heat to help the autoignition of the fuel mixture entering the burner.

[0079] Preferably, the burner assembly enables at least one inlet to be in fluid communication with at least one of the anode exhaust outlet and the cathode exhaust outlet of the fuel cell stack. More preferably, each of the at least one inlet is in fluid communication with only at least one of the anode exhaust outlet and the cathode exhaust outlet.

[0080] Preferably, the burner assembly is such that the catalyst is positioned closer to the second end of the body than to the first end. Thus, the distance between the flames, i.e., the distance at or near the first end, is increased to reduce the likelihood of direct flame contact or contaminants generated by the flame combustion damaging the catalyst performance. Thus, the flame burner (such as where the flame is formed) is located at the first end of the combustion chamber. This is where the flame combustion gases enter the combustion chamber.

[0081] Preferably, the burner assembly further comprises a second flame shield disposed downstream of the catalyst. This can cause the heat from the catalytic combustion to be distributed throughout the combustion chamber rather than being drawn directly from the second end. In addition, it can help prevent the exhaust gases from the flame combustion from clogging or damaging the catalyst from the second end, but rather direct them out of the combustion chamber.

[0082] Particular and preferred aspects of the invention are set out in the accompanying independent claims. Combinations of features from the independent claims may be combined with features of the independent claims as necessary and appropriate and not just as explicitly set out in the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Figure 1 shows a schematic diagram (P&ID) of a fuel cell system according to the present invention;

[0084] Figure 2 Shown with Figure 1 Schematic diagram of a related embodiment;

[0085] Figure 3 Shows Figure 1 or Figure 2 A schematic side view of a burner used in an embodiment of the present invention;

[0086] Figure 4Shows Figure 3 Schematic diagram of computational fluid dynamics of a burner;

[0087] Figure 5 a diagram showing various combustion modes of a burner of a fuel cell system;

[0088] Figure 6 The fuel cell stack and the burner are shown in Figure 5 The first trend of operation under the operation mode;

[0089] Figure 7 The fuel cell stack and the burner are shown in Figure 5 The second trend of operation under the operation mode;

[0090] Figure 8 The fuel cell stack and the burner are shown in Figure 5 a third trend of operation under the operating mode of ; and

[0091] Fig. 9 The fuel cell stack and the burner are shown in Figure 5 The fourth trend of operation under the operating mode. DETAILED DESCRIPTION

[0092] The complete and achievable disclosure of the present invention for those of ordinary skill in the art (including its best mode) is more specifically described in the remainder of the specification. Now with detailed reference to embodiments of the present invention, one or more examples thereof are described below. Each example is provided as an explanation of the present invention, rather than limiting the present invention.

[0093] It will be apparent to those skilled in the art that various modifications and variations may be made to the present invention without departing from the scope of the present invention. For example, a feature described as part of one embodiment may be used with another embodiment to yield yet another embodiment. Therefore, the present invention is intended to cover such modifications and variations as long as they fall within the scope of the appended claims and their equivalents.

[0094] Other objects, features and aspects of the present invention are disclosed in the rest of the 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 to limit the broader aspects of the present disclosure, which are embodied in the exemplary configurations.

[0095] A list of reference numerals used herein is given at the end of the specification.Repeat use of reference numerals in the present specification and drawings is intended to represent the same or analogous features or elements.

[0096] refer to Figure 1, a fuel cell system 10 is shown. The fuel cell system 10 has a fuel cell stack 12, and is preferably a solid oxide fuel cell system including a solid oxide fuel cell stack.

[0097] The fuel cell stack 12 is where the electrochemical reactions occur to produce electrical current. The fuel cell stack 12 includes a plurality of fuel cells, each having a cathode 14 and an anode 16 on either side of an electrolyte. The fuel cell stack 12 also includes a cathode inlet 18 through which fluid is provided to the cathode 14 of each fuel cell. This is typically oxygen. The fuel cell stack 12 also has a cathode exhaust outlet 20 through which fluid or gas leaves the cathode 14 of each fuel cell.

[0098] The fuel cell stack 12 also includes an anode inlet 22 from which a fluid is provided to the anode 16 of each fuel cell. This is typically fuel. The fuel cell stack 12 also has an anode exhaust outlet 24 through which a fluid or gas leaves the anode 16 of each fuel cell.

[0099] Thus, there is a fuel cell system 10 where the stack 12 has a fluid provided and when in operation the stack 12 undergoes an electrochemical reaction and the outputs (exhausts) from the anode and cathode after the reaction are output at the anode exhaust outlet 24 and the cathode exhaust outlet 20 .

[0100] The anode inlet 22 is provided with a fluid from a fuel source 26. The fuel source 26 is connected to the anode inlet 22 via an anode supply flow path 30. Various fuels can be supplied to the stack to allow electrochemical reactions. For example, hydrogen can be used as a fuel. As known in the art, other fuels can be used instead.

[0101] The cathode inlet 18 is supplied with oxygen from an oxygen source 28. The oxygen source 28 is in fluid communication with the cathode inlet 18 via a cathode supply flow path 32. Various oxygen supplies may be provided, such as air, water or steam or oxygen-enriched air.

[0102] 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 flow paths 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, exhaust gas from the fuel cell stack 12 may be passed to the burner 34.

[0103] Although in Figure 1 , the fuel cell stack outlet flow path 36 is shown as a single line, but the fuel cell stack outlet flow path 36 may provide separate lines from each of the anode outlet 24 and the cathode outlet 20 to the burner inlet 38. Figure 2Alternatively, the anode outlet 24 and cathode outlet 20 may be combined from multiple lines into a single line to form the fuel cell stack outlet flow path 36 anywhere between the anode outlet 24 and cathode outlet 20 and the burner inlet 38 .

[0104] The burner 34 has a burner outlet 40 that is in fluid communication with an exhaust section 46 via a burner exhaust flow path 48. Thus, exhaust gas can be supplied to the burner 34 and produce exhaust (i.e., exhaust from combustion) that is exhausted / exhausted from the system via the exhaust section 46. Thus, the flow direction is defined as from the fuel source 26 and the oxygen source 28, through the fuel cell stack 12, through the burner 34, and then out of the exhaust section 46.

[0105] like Figure 1 As shown, the only connection to the burner 34 at the inlet 38 is the fuel cell stack outlet flow path 36. The fuel source 26 and the oxygen source 28 are not in direct fluid communication with the burner 34. The fuel cell stack 12 is disposed between these connections.

[0106] In this embodiment, a preheater heat exchanger 42 is disposed in the burner exhaust flow path 48 and the cathode supply flow path 32 to exchange heat between the burner exhaust flow path 48 and the cathode supply flow path 32. Thus, the exhaust from the burner 34 can exchange heat to preheat the fluid within the cathode supply flow path 32. Exchanging heat may be disadvantageous, such as when the temperature of the exhaust is lower than the temperature of the oxygen from the oxygen source 28. Therefore, in this embodiment, an oxygen source bypass line 44 is provided to bypass the preheater heat exchanger 42 so that the oxygen source 28 is instead in direct fluid communication with the cathode inlet 18.

[0107] Thus, a fuel cell system 10 is provided whereby the burner 34 is in fluid feed communication exclusively with the anode outlet 24 and the cathode outlet 20 of the fuel cell stack 12 .

[0108] refer to Figure 2 , similar embodiments to those described above are provided, where the same reference numerals are used. Figure 2 The fuel cell stack outlet flow path 36 is shown to provide separate lines from each of the anode outlet 24 and the cathode outlet 20 to the burner 34 .

[0109] refer to Figure 3 , a burner 34 is provided. The burner 34 may also be referred to as a burner assembly 34. At the burner inlet 38, two fuel cell stack outlet paths 36 are shown. Therefore, this embodiment is different from Figure 2Consistently, one of the fuel cell stack outlet paths is connected to the cathode outlet, while the other fuel cell outlet path 36 is connected to the anode outlet 24. Various other configurations may be used at the burner inlet 38. However, the fuel cell stack outlet path 36 will always be in communication with the fuel cell stack 12.

[0110] The burner 34 has a burner body 50 which is an elongated body extending between the burner inlet 38 and the burner outlet 40. The burner body has a central axis 58 extending between the burner inlet 38 and the burner outlet 40 and centered between the defined sides of the burner body 50. The burner inlet 38 and the burner outlet 40 may also be referred to as a first end and a second end.

[0111] The burner body 50 defines a combustion chamber 50 where 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. It should be understood that the burner 34 is a three-dimensional shape, and therefore, although reference is made to a two-dimensional representation in the figures, each feature has depth and occupies more than one plane. Thus, for example, when viewed from the side, the catalyst 52 lies in a plane perpendicular to the central axis 58.

[0112] The burner body 50 is shown as being conical in shape with the cylindrical portion closest to 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., has a smaller radius) than the cross-sectional area of ​​the burner body 50 at the burner outlet 40 (e.g., has a larger radius).

[0113] In the figure, catalyst 52 is a double-layer catalyst. However, catalyst 52 can be adjusted as needed, adjusted to a single layer or multiple layers, or adjusted to different shapes. Catalyst 52 will usually be made of a mesh material, or will be carried on a mesh material. Catalyst 52 is made of a material that reacts with the fuel to chemically burn and thus generate heat. Such catalyst materials are well known in the art.

[0114] In this embodiment, the combustor body 50 defines a mixing chamber 56 wherein exhaust gases from the fuel cell stack 12 provided via the fuel cell stack outlet path 36 are mixed within the combustor 34 .

[0115] Flame shields 54 are located within the burner body 50 on either side of the catalyst 52. The flame shields 54 are also oriented in the same manner as the catalyst 52 (perpendicular to the central axis 58). The flame shields 52 may be circular or other shapes, but will generally have a shape or profile similar to the cross-section of the burner body 50 (or catalyst).

[0116] A gap is provided between the flame shield 54 and the side wall 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 the flame generated at the burner inlet 38 (i.e., near the inlet of the fuel cell stack outlet flow path 36 from the anode outlet 24). Therefore, various configurations of the flame shield 54 can be provided. For example, only one flame shield 54 can be provided between the catalyst 52 and the burner inlet 38. Similarly, the flame shield 54 can be larger or smaller as needed to protect the catalyst 52, i.e., the size of the catalyst 52 can be designed not to contact the wall of the burner body 50.

[0117] The fuel cell stack outlet flow path 36 is shown as having an elbow 60 before contacting the burner inlet 38. The elbow 60 provides a change in the flow direction of the exhaust gas. Specifically, the pipe is shown to have a vertical or "L" shaped bend at the elbow 60. Although both fuel cell stack outlet flow paths 36 shown in the figure have an elbow 60, in some configurations, only one fuel cell stack outlet flow path 36 has an elbow 60. In this case, the flow from the other fuel cell stack outlet flow path 36 affected by the elbow 60 will be sufficient to disrupt the flow within the mixing volume 56.

[0118] The length from elbow 60 to burner inlet 38 is shorter than the length (along central axis 58) from burner inlet 38 to burner outlet 40. In some cases, the length from elbow 60 to burner inlet 38 is less than half the length from burner inlet 38 to burner outlet 40.

[0119] In addition, the burner exhaust flow path 48 is shown as having a second elbow 62 after leaving the burner outlet 40. The second elbow 62 provides a change in the flow direction of the exhaust gas from the burner 34. Specifically, the conduit is shown to have a vertical or "L" shaped bend at the second elbow 62. Like the elbow 60 of the fuel cell stack outlet flow path 36, the distance from the second elbow 62 to the burner outlet 40 is shorter than the distance from the burner inlet 38 to the burner outlet 40. In some cases, the distance from the second elbow 62 to the burner outlet 40 is less than half the distance from the burner inlet 38 to the burner outlet 40.

[0120] Figure 4 A schematic diagram of the temperatures generated by the flame within the burner 34 is shown. This schematic diagram may be generated using computational fluid dynamics (CFD). It should be emphasized that the burner 34 is shown with Figure 3 The burner 34 in FIG. 3 is inverted vertically, ie the burner inlet 38 is now located on the left side of the figure.

[0121] exist Figure 4, a combustion flame is shown at the end of one of the fuel cell stack outlet flow paths 36, i.e., one end connected to the anode outlet 24. The combustion flame can be seen by the change in the shadow of the area around the end of the fuel cell stack outlet flow path 36. The area of ​​interest 64 is shown and magnified. The point at the end of one of the fuel cell stack outlet flow paths 36 is the flame seat 66. From this, it can be seen that the change in the shadow from the flame seat 66 indicates that a flame is generated. Due to the elbow 60, the cathode exhaust gas (by pushing or pulling) deflects the flame from the central axis 58 toward one of the walls of the burner body 50. In this embodiment, it deflects the flame downward, opposite to the direction of rotation of the elbow 60.

[0122] Due to the deflection, the catalyst 52 is not in line with the flame from the flame seat 66, i.e., the flame is not along the central axis 58 on which the catalyst is centered. This also protects the catalyst 52 from the hot gases, as the hot gases will be drawn along the walls of the burner body 50, thereby surrounding the catalyst 52.

[0123] 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 allow catalytic combustion and a flame seat 66 where flame combustion will occur.

[0124] refer to Figure 5 , a diagram 100 is shown for describing different combustion modes implemented in the combustor 34, namely, flame combustion and catalytic combustion.

[0125] Graph 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 exhaust fuel mixture within combustor 34. The exhaust fuel mixture may undergo some mixing within mixing volume 56 to obtain concentration 102 and temperature 104.

[0126] Fuel concentration 102 refers to the amount of fuel in the mixture compared to oxygen. For example, a lean concentration 102 would have less fuel and therefore be lower along the x-axis.

[0127] Extending from the y-axis 104 is a self-ignition temperature line (Self-ign.T) 106. This is the temperature at which the fuel of the mixture can self-ignite, also referred to as the auto-ignition temperature (AIT). Below the auto-ignition temperature 106, as shown in the graph 100, the fuel cannot ignite. Therefore, in the absence of a combustion source, the fuel will not burn with a flame below the auto-ignition temperature 106.

[0128] The diagram includes a cold catalytic oxidation block 110. Referring to the cold catalytic oxidation block 110, within this temperature and concentration range, i.e., all fuel concentration levels 102 and below the auto-ignition temperature 106 of the fuel, the exhaust fuel mixture can be catalytically combusted, i.e., the material of the catalyst will oxidize in the presence of the fuel of the exhaust fuel mixture, and heat will be generated. This is also referred to as cold catalytic combustion 110. This cold catalytic oxidation 110 mode can occur during startup of the fuel cell system 10. In this case, the auto-ignition temperature 106 of the exhaust fuel mixture is not reached. Therefore, there is no flame.

[0129] Fuel and oxygen are provided to the stack 12 from a fuel source 26 and an oxygen source 28. These pass through the anode 16 and cathode 14, respectively. At startup, the stack 12 will be very cold and the electrochemical processes within the stack 12 will use almost no fuel. Figure 1 and Figure 2 As shown, the fuel-containing fluid (exhaust gas) output from the anode 16 is provided to the combustor 34 along with the oxygen-containing exhaust gas output from the cathode 14 .

[0130] According to the graph 100 , its temperature 104 will be below the autoignition temperature 106 of the fuel, and thus in the region of temperature 104 and fuel concentration 102 shown, cold catalytic oxidation 110 will occur at the catalyst 52 .

[0131] When the temperature of the fuel 104 exceeds the autoignition temperature 106, the fuel ignites and combustion switches from the cold catalytic oxidation 110 to the flame combustion 112 as shown in the flame combustion region 112. This will occur as the stack 12 warms up and the electrochemical reaction produces more heat, and also or alternatively due to the heat produced by the catalytic combustion in the cold catalytic oxidation stage 110, thereby increasing the temperature of the exhaust gas provided to the burner 34. Therefore, this will occur during the warm-up phase of the fuel cell system 112 and during normal operation. Normal operation refers to the situation where the fuel cell system 10 is at operating temperature. During normal operation, there can be power draw, that is, current is drawn from the fuel cell, as well as no-load operation without current draw.

[0132] As can be seen from the graph 100, there is another mode of operation: flame combustion 112 does not always occur above the temperature 104 of autoignition of the gas 106. When the fuel concentration 102 is relatively low, but the temperature is above the autoignition temperature 106, the burner 24 will catalytically combust the exhaust fuel mixture at the catalyst 52. This mode is lean catalytic combustion 114, in which a lean exhaust fuel mixture is combusted at the catalyst 52. On the x-axis of the fuel concentration 102, there is a lower limit flammable concentration line (LFL) 108. The lower limit flammable concentration (LFL) is the lowest gas concentration that supports a spontaneous flame when mixed with air. Below the lower limit flammable concentration, the fuel is insufficient to support combustion, and the mixture is too "lean" to burn. The flame combustion region 112 extends below the lower limit flammable concentration 108 of the fuel concentration 102. This is because the fuel concentration is generally inconsistent with the average fuel concentration indicated by the x-axis fuel concentration 102. Therefore, the flame combustion 112 will be maintained slightly below the LFL 108. However, once the fuel concentration 102 drops sufficiently below the lower flammable concentration 108, the flame will extinguish within the burner 34. However, even if the temperature 104 is above the autoignition temperature 106, the fuel concentration 102 will be too low to autoignite again. Therefore, the fuel mixture in the burner 34 will undergo lean catalytic combustion 114 at the catalyst 52.

[0133] With respect to the fuel cell system 10, this lean catalytic combustion mode 114 will occur when fuel is being consumed at the highest rate at the fuel cell stack 12. This is when the fuel cell system 10 is fully warmed up and the current draw from the stack 12 is in high power draw operation. As a result, the exhaust gas at the anode outlet 24 is low in fuel content because the fuel has been largely consumed by the electrochemical processes in the fuel cell stack 12. However, it is still advantageous to burn lean fuel in the burner 34, and the lean catalytic combustion mode 114 provides a combustion mode for this purpose.

[0134] It is emphasized that when the burner 34 switches from flame combustion 112 to lean catalytic combustion 114 , low or no combustion regions may temporarily occur because all of the fuel mixture will be consumed by the flame combustion 112 and will need to pass from the burner inlet 38 to the catalyst 52 .

[0135] Thus, the combustor 34 allows the fuel mixture to be combusted in all modes without feeding any additional fuel (non-stack exhaust fuel) to the combustor 34 .

[0136] refer to Figure 6 , a first temperature versus time trend 120 is provided, showing flame combustion 112 and lean catalytic combustion 114 in the example.

[0137] The x-axis provides time 118 and the y-axis provides temperature 116. Trend line 122 is the catalyst mesh inlet temperature or catalyst inlet temperature without mesh 122. Trend line 124 is the combustor outlet temperature. Trend line 126 is the fuel cell stack outlet temperature.

[0138] The flame combustion mode 112 is shown to occur when the inlet catalyst temperature 122 is greater than the stack outlet temperature 126 and greater than or equal to the burner outlet temperature 124. Therefore, the flame generates heat upstream of the catalyst 52, that is, the heat is generated by the flame at the flame seat 66. Therefore, the temperature of the catalyst mesh inlet 122 is high due to the combustion of the exhaust fuel mixture by the flame combustion.

[0139] When the catalyst inlet temperature 122 is almost equal to the stack outlet temperature 126, and the burner outlet temperature 124 is greater than both, the lean catalytic combustion mode 114 is exhibited. Therefore, the temperature of the exhaust gas provided from the stack 12 is similar to the temperature of the exhaust gas fuel mixture supplied to the catalyst 52, so flame combustion does not occur. However, the temperature of the exhaust gas from the burner outlet 40 (burner outlet temperature 124) is higher than the catalyst inlet temperature 122 and the stack outlet temperature 126, so it is a catalytic combustion that generates heat. Therefore, the switching of the combustion mode due to the efficient consumption of fuel in the stack 12 and the reduction of the fuel concentration is confirmed.

[0140] See also Figure 7 , a second temperature versus time trend 130 is provided, in the example showing the cold catalytic oxidation 110. The second trend 130 has an additional x-axis scale, showing the fuel flow rate 128. The trend 132 shows the fuel flow rate to the fuel cell stack 12.

[0141] The cold catalytic oxidation 110 operating mode occurs at the start-up of the fuel cell system 10 and in the early stages of warm-up. After routine safety checks and pre-purging, when the fuel isolation valve is commanded to open, the burner outlet temperature 124 quickly rises to the target temperature. Once the target temperature is reached, the control system (not shown) reduces the fuel flow 132 and allows the fuel cell stack 12 to warm up. In this example, the burner outlet temperature 124 is maintained at 500°C.

[0142] The second trend 130 shows that the stack outlet temperature 126 and the catalyst inlet temperature 122 are equal. Therefore, there is no flame combustion, and the exhaust fuel mixture supplied to or entering the combustor 34 from the stack outlet is the same temperature as when it reaches the catalyst 52. The combustor outlet temperature 124 increases rapidly until the fuel inlet flow rate 132 is reduced once the target temperature is met.

[0143] When the fuel flow rate 132 stops, the combustor outlet temperature 124 increases at point 134. This increase is due to the reduction in gas flow while the catalyst screen 52 is still hot, thus momentarily increasing the temperature at low gas flow rates when the fuel flow has stopped and therefore combustion should not be occurring.

[0144] During this particular phase of cold catalytic oxidation 110, the stack 12 uses no fuel or very little fuel, so all available fuel is sent to the combustor 34. When the fuel concentration does not exceed the LFL 108, flame combustion occurs.

[0145] refer to Figure 8 , a third temperature versus time trend 135 is provided, showing flame combustion 112 in the example.

[0146] When the temperature of the exhaust fuel mixture within the burner 34 reaches the autoignition temperature 106, and the fuel concentration is sufficient (because the stack 12 may still be using little or no fuel), flame combustion is initiated 112. Trend 135 shows one type of flame initiation, which occurs when the temperature of the exhaust fuel mixture exceeds the autoignition temperature 106 at the catalyst 52 due to the heat generated by the catalytic combustion. The hot gases (fuel mixture) flash back and ignite the exhaust fuel mixture at the burner inlet 38.

[0147] Referring to the third trend 135, the temperature of the stack outlet 126 and the catalyst mesh inlet 122 steadily increases. The burner outlet temperature 124 also increases. When the autoignition temperature 106 is reached, the catalyst mesh inlet temperature 122 increases, indicating that a flame is generated upstream of the catalyst 52 in the burner 34. Therefore, flame combustion 112 occurs at or near the burner inlet 38.

[0148] As described above, flame combustion 112 may also be triggered due to the hot gas mixture (i.e., the exhaust fuel mixture) resulting in exceeding the autoignition temperature 106. This is because the exhaust gas from the stack 12 is very hot when it reaches the combustor 34. However, the temperature of the mixing volume 56 is also affected by the reverse flow of gases that have reacted on the catalyst 52, and then the rich fuel mixture is ignited.

[0149] refer to Fig. 9 , a third temperature versus time trend 140 is provided, showing lean catalytic combustion 114 in the example.

[0150] Once the startup and warm-up of the fuel cell system 10 are terminated, the fuel utilization in the fuel cell stack 12 increases and more current is drawn from the fuel cell system 10. As a result, the exhaust gas at the anode outlet 24 contains less fuel. As a result, the fuel concentration 102 of the exhaust gas fuel mixture in the burner 32 decreases.

[0151] As the temperature of the fuel cell stack 12 increases, more current can be drawn and more heat is generated by the fuel cell stack 12, requiring less and less heat contribution from the burner 32. To control this, the amount of fuel supplied to the fuel cell system 10, and thus the anode 16, is reduced until the exhaust gas fuel mixture within the burner 32 becomes too lean to be combusted by flame combustion, even at temperatures above the autoignition temperature 106. In fact, due to the local concentration of fuel, the average temperature of the exhaust gas fuel mixture is below the LFL 108, as the flame combustion 112 is effectively stopped, as shown in FIG. Figure 5 shown.

[0152] Referring to the fourth trend 140, the stack fuel flow rate 132 is reduced, resulting in a switch from flame combustion 112 to lean catalytic combustion 114. As a result, the catalyst inlet temperature 122 drops to match the stack outlet temperature 126, indicating no increase in temperature and therefore no flame combustion 112. The increase in the combustor outlet temperature 124 compared to the catalyst inlet temperature 122 is then caused by the lean catalytic combustion 114 at the catalyst 52.

[0153] refer to Figure 1 The operation and operating mode of the fuel cell system 10 can be controlled by the fuel supplied at the fuel source 26, such as by controlling the flow rate through a control valve. The fuel flow rate can be controlled in response to temperature, for example in response to manual control or automatic control, such as by an electrical or manual controller. A burner outlet thermocouple 70 is disposed at or near the burner outlet 40 of the burner 34. The burner outlet thermocouple 70 will provide the temperature of the exhaust gas leaving the burner 34. This will provide an indication of the combustion mode within the burner 34.

[0154] The fuel cell stack outlet thermocouple 72 may also or alternatively be provided at the cathode outlet 20 of the stack 12. The thermocouple may also or alternatively be provided at the anode outlet 24. Even multiple thermocouples may be provided at each of the cathode outlet 20 and the anode outlet 24.

[0155] The fuel cell stack outlet thermocouple 72 measures the temperature of the exhaust gas leaving the stack 12, thus providing an indication of the fuel cell stack operating mode, such as startup, warm-up, idle / power draw. Accordingly, the flow rate of the fuel may be varied in response thereto, such as switching to lean catalytic combustion 114 when the fuel cell stack is fully operational.

[0156] At the fuel cell stack 12 inlet, such as the cathode inlet 18 or the anode inlet 22, a fuel cell stack inlet thermocouple 74 is also provided. Since the cathode supply flow path 32 is typically heated by the pre-heater 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. This may also or alternatively be used to determine the operating mode or required fuel flow rate.

[0157] The operating mode of the fuel cell system is enabled by the catalyst 52 in the burner 34, allowing easy switching between catalytic combustion and flame combustion without the need for a complex system.

[0158] Reference numerals:

[0159] 10 Fuel Cell System

[0160] 12 Fuel Cell Stack

[0161] 14 Cathode side

[0162] 16 Anode side

[0163] 18 Cathode inlet

[0164] 20 Cathode exhaust gas outlet

[0165] 22 Anode inlet

[0166] 24 Anode exhaust gas outlet

[0167] 26 Fuel Source

[0168] 28 Oxygen Source

[0169] 30 Anode supply flow path

[0170] 32 Cathode supply flow path

[0171] 34 Burner

[0172] 36 Fuel cell stack outlet flow path

[0173] 38 Burner inlet

[0174] 40 Burner outlet

[0175] 42 Preheater Heat Exchanger

[0176] 44 Oxygen source bypass line

[0177] 46 Exhaust

[0178] 48 Burner exhaust flow path

[0179] 50 Burner body

[0180] 52 Catalyst

[0181] 54 Fire Shield

[0182] 56 Mixing Volume

[0183] 58 Central Axis

[0184] 60 elbow

[0185] 62 Second elbow

[0186] 64 Area of ​​Interest

[0187] 66 Flame Seat

[0188] 70 Burner outlet thermocouple

[0189] 72 Fuel cell stack outlet thermocouple

[0190] 74 Fuel cell stack inlet thermocouple

[0191] 100 Charts

[0192] 102 x-axis fuel concentration

[0193] 104 y-axis fuel concentration

[0194] 106 Autoignition Temperature

[0195] 108 Lower flammable concentration level

[0196] 110 Cold Catalytic Oxidation

[0197] 112 Flame Burning

[0198] 114 Lean Catalytic Combustion

[0199] 116y-axis temperature

[0200] 118x axis time

[0201] 120 Second Trend

[0202] 122 Catalyst screen inlet temperature

[0203] 124 Burner outlet temperature

[0204] 126 External temperature

[0205] 128 Fuel flow rate

[0206] 130 Second Trend

[0207] 132 Fuel cell stack fuel inlet flow rate

[0208] 134 Points of Interest

[0209] 135 The Third Trend

[0210] 140 The Fourth Trend.

Claims

1. A method of operating a fuel cell system, the fuel cell system comprising a burner and a fuel cell stack, the fuel cell stack comprising an anode inlet, a cathode inlet, an anode exhaust outlet, and a cathode exhaust outlet; The method comprises the following steps: forming an exhaust-fuel mixture from fuel and oxygen supplied from the anode exhaust outlet and the cathode exhaust outlet of the fuel cell stack, wherein the exhaust-fuel mixture for the burner is provided only from the anode exhaust outlet and the cathode exhaust outlet; and The exhaust fuel mixture is combusted by catalytic combustion at a catalyst in the combustor or by flame combustion in the combustor.

2. The method of operating a fuel cell system according to claim 1, further comprising: The fuel is combusted by catalytic combustion during at least one of: i) starting the fuel cell system; ii) the temperature of the exhaust gas fuel mixture in the burner is insufficient to cause auto-ignition of the fuel by flame combustion; and iii) the exhaust gas fuel mixture in the burner has insufficient fuel concentration to be combusted by flame combustion.

3. The method of operating a fuel cell system according to any one of the preceding claims, further comprising: Heat generated by the catalytic combustion is used to initiate the flame combustion of the exhaust gas fuel mixture in the burner.

4. A method for operating a fuel cell system according to any one of the preceding claims, wherein when one or both of the temperature of the exhaust gas-fuel mixture in the burner and the concentration of the fuel in the exhaust gas-fuel mixture in the burner are insufficient for flame combustion, the exhaust gas-fuel mixture is combusted only by catalytic combustion at the catalyst in the burner.

5. A method for operating a fuel cell system according to any one of the preceding claims, wherein the fuel is combusted by flame combustion in the burner when both the temperature of the exhaust gas-fuel mixture in the burner and the concentration of the fuel within the exhaust gas-fuel mixture in the burner are sufficient for flame combustion.

6. The method of operating a fuel cell system according to claim 4, comprising: The concentration of fuel in the exhaust gas fuel mixture supplied to the burner is reduced.

7. A method of operating a fuel cell system according to any one of the preceding claims, comprising: A burner exhaust discharged from a burner outlet of the burner is passed through a preheater heat exchanger to exchange heat between the burner exhaust and a cathode inlet gas supplied to the cathode inlet.

8. A method of operating a fuel cell system according to any one of the preceding claims, comprising: An electric heater is used to heat the catalytic burner when the fuel cell system is started.

9. A method of operating a fuel cell system according to any one of the preceding claims, comprising: The flame combustion of the fuel mixture is initiated only by: a temperature within the burner, the temperature causing autoignition; or Catalytic combustion.

10. A method of operating a fuel cell system according to any one of the preceding claims, comprising: Inlet fuel provided to a fuel supply path in fluid communication between a fuel supply and the anode inlet of the fuel cell stack is controlled based on a temperature of an outlet of the burner.

11. A method of operating a fuel cell system according to any one of the preceding claims, using one of the following operating modes: a) supplying the exhaust gas fuel mixture into or to the burner, wherein the fuel in the exhaust gas fuel mixture is combusted only by catalytic combustion at the catalyst due to insufficient temperature of the exhaust gas fuel mixture in the burner; b) supplying the exhaust gas fuel mixture into or toward the burner, wherein since the temperature of the exhaust gas fuel mixture in the burner is sufficient, the fuel in the exhaust gas fuel mixture self-ignites and burns through flame combustion in the burner; and c) supplying the exhaust gas fuel mixture into or to the burner, wherein the fuel in the exhaust gas fuel mixture is combusted only by catalytic combustion at the catalyst due to insufficient fuel concentration of the exhaust gas fuel mixture.

12. A fuel cell system comprising: A fuel cell stack, the fuel cell stack comprising an anode outlet and a cathode outlet; a burner comprising a combustion chamber and including at least one burner inlet and 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 for providing a fuel cell stack exhaust gas from the anode outlet and the cathode outlet to the burner, the fuel cell stack exhaust gas comprising fuel and oxygen; The burner is configured to receive the fuel and the oxygen only from the anode outlet and the cathode outlet of the fuel cell stack, and is configured to catalytically combust an exhaust gas-fuel mixture including the fuel and the oxygen at the catalyst and to flame-combust the exhaust gas-fuel mixture upstream of the catalyst in the combustion chamber. 13 . The fuel cell system according to claim 12 , wherein the burner is configured to ignite the exhaust gas fuel mixture for flame combustion only by auto-ignition.

14. The fuel cell system according to claim 12 or 13, wherein the burner does not include an igniter.

15. A fuel cell system according to any one of claims 11 to 14, wherein the combustion chamber defines a mixing volume between the burner inlet and the catalyst.

16. The fuel cell system of claim 15, wherein the burner is configured such that the mixing volume is heated by the catalytic combustion.

17. A fuel cell system according to any one of claims 15 or 16, wherein the burner is configured such that the mixing volume is heated to an auto-ignition temperature of the fuel and the oxygen by the catalytic combustion.

18. The fuel cell system according to any one of claims 12 to 17, further comprising: A cathode inlet flow path between an oxygen source and a cathode inlet of the fuel cell stack, a burner exhaust flow path between a burner outlet of the burner and an exhaust portion of the fuel cell system, and a preheater heat exchanger in the cathode inlet flow path and the burner exhaust flow path, and the preheater heat exchanger is arranged to exchange heat between the burner exhaust in the burner exhaust flow path and the oxygen source in the cathode inlet flow path.

19. A burner assembly for a fuel cell system, the assembly comprising: an elongated body extending along a central axis and defining a combustion chamber, the body having a first end and a second end located at either end 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 exiting the body at the second end; a catalyst located within the body, a flame shield, the flame shield being located between the first end and the catalyst; as well as An inlet supply line is connected through the at least one inlet, the inlet supply line comprising a first arm connected to the at least one inlet, and a second arm connected to the first arm at an elbow, the first arm being oriented along the central axis.

20. The burner assembly of claim 19, wherein the length of the first arm is less than the distance between the first end and the second end of the elongated body.

Images