FIELD OF THE INVENTION

The present invention relates to a fuel cell system having a burner or tail gas burner, where the burner is capable of both flame and catalytic combustion.

BACKGROUND

Electrochemical fuel cells use an electrochemical conversion process that oxidises fuel to produce electricity. Such fuel cell units may be arranged overlying one another in a stack arrangement, for example 10-200 fuel cell units in a stack. Each fuel cell unit operates to generate electricity when in operation.

One type of fuel cell system is a solid oxide fuel cell system. The technology behind solid oxide fuel cells (SOFCs) is based upon a solid oxide electrolyte that conducts negative oxygen ions from a cathode to an anode located on opposite sides of the electrolyte. For this, a fuel, or reformed fuel, contacts an anode of the fuel cell unit (aka the fuel electrode), and an oxidant, such as air or an oxygen rich fluid, contacts a cathode of the fuel cell unit (aka the air electrode). The fluid passageways inside and between the cell units permit this. There are other forms of electrochemical cell unit also.

Burners or tail gas burners are known in the art that utilize the off-gases from the fuel cell stack, particularly the fuel containing output from an anode outlet of the stack where the fuel is not fully consumed during the electrochemical process. See for instance, WO20 16/097687. The combustion of the off-gases is particularly advantageous to provide additional heat and to utilize the otherwise wasted gas from the fuel cell stack. Legislation and the general trend of improved environmental responsibility encourages an interest in reducing the emissions produced by the burning or combustion of fuel in all operations. Of particular importance in controlling emissions is the reduction of carbon monoxide (CO) and nitrous oxides (NO _X ) emissions. The air to fuel ratio (lambda, A) of the mixture that is provided to the tail-gas burner can also have consequences on the combustion, often requiring supplemental gases directly provided to the burner to control the combustion.

To address the requirements, flameless catalytic combustion can be provided instead of the flame combustors. This allows for a combustion of the off-gases with fewer emissions. Prior art systems are known that use a catalytic combustor and a flame combustor to use off-gases. See for example, US9343758B2 and EP2127009B1 .

However, the operation of the two types of combustion can be improved. It is particularly desirable to produce a burner without utilizing complex systems, whilst maintaining low emissions and coping with the varying airflows and, in particular, a wide ranging air to fuel ratio, lambda.

SUMMARY OF INVENTION

According to an aspect of the invention, there is provided a method of operating a fuel cell system comprising a burner and a fuel cell stack, the fuel cell stack comprising an anode inlet, a cathode inlet, an anode off-gas outlet and a cathode off-gas outlet; the method comprising the steps of: forming an off-gas fuel mixture from fuel and oxygen supplied from the anode offgas outlet and the cathode off-gas outlet of the fuel cell stack, wherein the off-gas fuel mixture for the burner is exclusively provided by the anode off-gas outlet and the cathode off-gas outlet; and combusting the off-gas fuel mixture by either catalytic combustion at a catalyst in the burner or by flame combustion in the burner.

In some embodiments, the off-gas fuel and oxygen are provided separately to the burner, with mixing taking place within the burner. In other embodiments the mixing occurs before entering the burner.

During operation of some embodiments of the fuel cell system, fuel from the anode offgas outlet and oxygen from the cathode off-gas outlet are passed to the 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 provided to the burner is from the outlets of the fuel cell stack. This removes the need for separate fuel feeds to provide a fuel supply directly to a burner for the purpose of generating heat. The heat can be used for heating the system, such as the fuel cell stack. Therefore, the efficiency of the system is improved. The heat generated by the flame or catalytic combustion heats up the burner and thus also the off-gas fuel mixture.

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

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

Preferably, the method of operating the fuel cell system comprises combusting the fuel by catalytic combustion during start-up of the fuel cell system; start-up is the period of operation of the fuel cell from a cold non-operating situation or condition into a point or condition where electric current can be drawn from the stack. For example, in some embodiments, in a solid oxide fuel cell, and particularly an intermediate temperature solid oxide fuel cell, the start-up process may be from cold until the temperature at the air-side outlet of the stack (usually the cathode off-gas outlet) reaches 450 degrees C. This is commonly the approximate temperature at which a current will be able to be drawn across the stack, and the fuel cell will thus be operational (as an electricity producing device). However, the various different types of fuel cell, and the various different electrochemically active components of the electrolyte, will each have different temperatures at which a current can start to be drawn: The chemistry of the electrochemically active components typically dictate that temperature.

In some embodiments, the catalytic combustion continues through an extended start-up - to when the fuel cell is operating at an optimum temperature for maximum current draw. For example, in some embodiments, in a solid oxide fuel cell, and particularly an intermediate temperature solid oxide fuel cell, the extended start-up process may be from the above 450 degrees C until the temperature at the air-side outlet of the stack (usually the cathode off-gas outlet) reaches 620 degrees C for the first time in the heatup process, while only at a partial current/load. This is commonly the approximate temperature at which a maximum current will be able to be drawn across the stack, and above which the available current starts to fall. However, again the various different types of fuel cell, and the various different electrochemically active components of the electrolyte, will each have different temperatures at which a maximum current can be drawn: The chemistry of the electrochemically active components typically dictate that temperature.

Preferably, the method of operating the fuel cell system further comprises combusting the fuel by catalytic combustion during at least one of: i) start-up of the fuel cell system; ii) a temperature of the off-gas fuel mixture in the burner being insufficient to cause auto-ignition of the fuel by flame combustion; and iii) a fuel concentration of the off-gas fuel mixture in the burner being insufficient to be combusted by flame combustion.

More preferably, when the temperature is too low to auto-ignite for flame combustion, catalytic combustion occurs.

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

On start-up of the fuel cell, the stack is relatively cold compared to its operating condition. Therefore, the consumption of fuel by 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 off-gas fuel mixture in the burner. When the temperature of the off-gas fuel mixture in the burner is below an auto-ignition level, it will not auto-ignite. With no other source of ignition, flame combustion will not occur and instead the off-gas fuel mixture is catalytically combusted. Likewise, if during operation the fuel concentration of the offgas fuel mixture is too lean, flame combustion will not occur and catalytic combustion occurs instead. This enables the off-gas fuel mixture to be combusted in the burner in various states. Therefore, heat can be generated even when a flame would not normally be produced without the need to provide additional fuel, i.e. fuel that does not pass through the stack. Emissions are also controlled by the use of catalytic combustion.

Preferably, the method of operating the fuel cell system further comprises using heat generated by the catalytic combustion to ignite flame combustion of the off-gas fuel mixture in the burner. Therefore, the catalytic combustion has a synergistic effect with the flame combustion where the heat generated enables ignition of the off-gas fuel mixture by exceeding the auto-ignition temperature of the fuel in the off-gas fuel mixture. This all occurs in the same burner. The auto-ignition temperature (AIT) is the lowest temperature at which a substance spontaneously ignites without an external source of ignition, such as a flame or spark. This is also called self-ignition.

Preferably, the method of operating the fuel cell system further comprises, when one or both of the temperature of the off-gas fuel mixture in the burner and a concentration of the fuel in the off-gas fuel mixture in the burner is insufficient for flame combustion, exclusively combusting the off-gas fuel mixture by catalytic combustion at the catalyst in the burner. The ability for the off-gas fuel mixture to be combusted by flame combustion can be effected by the fuel concentration and the temperature of the off-gas fuel mixture. Where these do not reach the required levels, flame combustion cannot be sustained or started, therefore only catalytic combustion occurs within the burner. No additional burner is provided for separate combustion. There is a single burner.

Preferably, the method of operating the fuel cell system further comprises, when both the temperature of the off-gas fuel mixture in the burner and a concentration of the fuel of the off-gas fuel mixture in the burner is sufficient for flame combustion, combusting the off-gas fuel mixture by flame combustion in the burner. When flame combustion can occur, e.g. sufficient fuel concentration and temperature, the off-gas fuel mixture is combusted. Therefore, the burner allows for both flame and catalytic combustion as allowed by the temperature and fuel concentration.

Preferably, the method of operating the fuel cell system further comprises 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 of the electrochemical reaction will increase. This results in less fuel being present in the off-gases of 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-gases can cause the fuel concentration in the off-gas fuel mixture in the burner to fall below the lower flammable limit. The lower flammable limit (LFL) is the lowest gas concentration that will support a self-emitting flame when mixed with air. Below the lower flammability limit, there is not enough fuel to support flame combustion, i.e. the off-gas fuel mixture is too “lean” to burn.

Preferably, the method of operating the fuel cell system further comprises passing burner exhaust gas emitted from a burner outlet of the burner to a pre-heater heat exchanger such that heat is exchanged between the burner exhaust gas in the burner exhaust flow path and a cathode inlet gas supplied to the cathode inlet. The cathode inlet gas comprises oxygen that is 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 gases, in this case the cathode inlet gases, assists with the electrochemical reaction and reduces the heat that needs to be provided from other sources. Therefore, the use of the burner exhaust gas for this heat exchange increases system efficiency.

Preferably, the method of operating the fuel cell system further comprises using an electric heater to heat the catalytic burner at start-up of the fuel cell system. The warming of the catalyst improves the initiation of the chemical combustion.

Preferably, the method of operating the fuel cell system further comprises only igniting flame combustion of the fuel mixture by the temperature within the burner, that temperature causing auto ignition, or catalytic combustion causing ignition. The lack of an igniter, such as a spark plug, simplifies manufacture. It can also ensure that the burner and hence combustion chamber, is more readily sealed as there is no opening for an igniter. This increases the efficiency of the burner as the heat loss is reduced. The igniting of the fuel by the auto-ignition of the off-gas fuel mixture due to the temperature, or the ignition from the heat from the catalytic combustion itself, also reduces the need for control means for operating ignition.

Preferably, the method of operating the fuel cell system further comprises controlling an inlet fuel provided to a fuel supply path in fluid communication between a fuel supply and an anode inlet of the fuel cell stack in accordance with a temperature of an outlet of the burner. The fuel cell is operated in accordance with the burner outlet temperature. As there is no separate fuel mixture feed to the burner and all fuel and oxygen for the burner is provided as off-gas from the fuel cell stack, usually for mixing in the burner, there is no requirement for a separate fuel feed, or for such a fuel feed to be controlled. Instead, the burner exhaust temperature can provide a direct correlation between the operation of the stack. In view of this, operation of a fuel cell system can be simplified. A controller can operate the main fuel supply to the stack to switch between the type of combustion in the burner. Preferably, the method of operating the fuel cell system further comprises using one of the following modes of operation: a) supplying the off-gas fuel mixture in or to the burner, wherein due to the insufficient temperature of the off-gas fuel mixture in the burner, the fuel in the off-gas fuel mixture combusts only by catalytic combustion at the catalyst; b) supplying the off-gas fuel mixture in or to the burner, and igniting the fuel thereof, due to the sufficient temperature of the fuel in the off-gas fuel mixture in the burner and combusting the fuel mixture by flame combustion in the burner; and c) supplying the off-gas fuel mixture in or to the burner, wherein due the insufficient fuel concentration of the off-gas fuel mixture, the fuel in the off-gas fuel mixture combusts only by catalytic combustion at the catalyst.

When the temperature is too low to auto-ignite the fuel for flame combustion, catalytic combustion occurs. Therefore, in mode (a) of operation, the fuel is not burnt with flame combustion and instead undergoes catalytic combustion, or so-called cold catalytic combustion. When the off-gas fuel mixture temperature has risen, for instance from operation of the fuel cell stack that provides a warm gas at the outlet, and/or through the catalytic combustion, the off-gas fuel mixture will ignite. This is mode (b). Ignition can occur near the catalyst, where some of the hot gases backfire the fuel I off-gas fuel mixture supply at the burner inlet. Ignition can also occur at the burner inlet itself - e.g. where the off-gas fuel mixture at the burner inlet is already at a temperature that exceeds the AIT. Generally, the flame combustion will be occurring in the burner upstream of the catalyst, e.g. generally near a burner inlet, as this is where the fuel enters the burner. As the fuel is being consumed by flame combustion, catalytic combustion will cease. It is noted that in some cases, there is an overlap of combustion modes, such as when switching between combustion operation modes, or when the fuel is not fully consumed during flame combustion, such as in a particularly rich mix where there is insufficient oxygen to burn all the fuel. In such a situation, the catalyst may still undergo catalytic combustion. In operation mode (c), the fuel cell system can be hot and running at high efficiency. Therefore, much of the fuel is being consumed in the electrochemical process at the stack. The fuel concentration of the off-gas fuel mixture in or supplied to the burner decreases to below the level that flame combustion can be sustained, (e.g. the LFL) and catalytic combustion occurs once again, this is lean catalytic combustion. The term off-gas fuel mixture is to mean the mixture of the fuel and oxygen from the outlets of the stack. When reference is made to the fuel combusting or the off-gas fuel mixture combusting, this is to mean the resultant fuel and oxygen that has been supplied to the burner.

In a further aspect of the invention, there is provided a fuel cell system comprising: a fuel cell stack comprising an anode outlet and a cathode outlet; a burner comprising a combustion chamber 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 being for providing fuel cell stack offgas from the anode outlet and the cathode outlet to the burner, the fuel cell stack off-gas comprising fuel and oxygen; wherein the burner is configured to receive the fuel and oxygen exclusively from the anode outlet and the cathode outlet of the fuel cell stack, and configured to enable catalytic combustion of an off-gas fuel mixture comprising the fuel and oxygen at the catalyst, and flame combustion of the off-gas fuel mixture upstream of the catalyst in the combustion chamber.

In some embodiments, during operation of the fuel cell, off-gas fuel mixture is fed to the burner from the anode off-gas outlet and the cathode off-gas outlet, or the mixing occurs in the burner. Therefore, there is provided a fuel cell stack outlet flow path from the anode outlet and the cathode outlet of the fuel cell stack to the burner. The fuel cell stack outlet path can comprise a separate path for each of the anode or cathode outlets, so that mixing occurs within the burner, or can be combined into a single path, i.e. so mixing initially occurs within the flow path.

The off-gas fuel mixture is combusted in the combustion chamber of the burner. The combustion is either flame combustion, i.e. the burning of the off-gas fuel mixture, or the catalyst reacts with the fuel so that the fuel undergoes catalytic combustion, i.e. flameless chemical combustion.

The flame combustion typically or preferably occurs upstream of the catalyst. All of the fuel and oxygen provided to the burner is from the fuel cell stack. There is no additional fuel or oxygen feed. This removes the need for separate fuel feeds to provide a fuel supply directly to a burner for the purpose of generating heat. Therefore, the efficiency of the system is improved.

The heat generated by the combustion can heat up the burner and thus the off-gas fuel mixture. The heat is also used for heating other elements of the system, such as the fuel cell stack, or fluid feeds to the fuel cell stack. The recycling of heat increases efficiency. For this purpose, heat exchangers can be used.

Preferably, the fuel cell system is further configured such that the burner is configured to ignite the off-gas fuel mixture for flame combustion exclusively by auto-ignition. Flame combustion will occur when the fuel mixture reaches an auto-ignition temperature. That is to say, when the temperatures inside the burner reaches an auto-ignition temperature for the fuel mixture, and fuel concentration is adequate (as the fuel stack may still be using little or no fuel), flame combustion is triggered.

More preferably, the fuel cell system does not comprise an igniter. There is no need for additional igniters to cause flame combustion. Instead, this is controlled by the temperature of the off-gas fuel mixture, i.e. by the heat produced by the catalytic combustion, or the fuel self-ignition may occur with heat from elsewhere, such as the fuel cell stack.

This auto-ignition does not rely solely on fuel cell stack temperature as the chemical combustion at the catalyst can provide the heat to achieve the temperature necessary for auto-ignition when the output temperature of off-gases at the cathode off-gas outlet and/or the anode off-gas outlet is below the auto-ignition temperature.

Preferably, the fuel cell system further comprises a flame shield. This protects the catalyst from the flame produced by flame combustion. As the catalyst is in the same chamber in which the flame is produced, the flame shield can be positioned between the burner inlet and the catalyst. Fuel and oxygen, or the mixture thereof, (and flame combusted gases) can pass around the flame shield during normal operation. Preferably, the combustion chamber or burner defines a mixing volume between the burner inlet and the catalyst. The fuel and oxygen (or the mixture thereof) enters the burner at the burner inlet from the fuel cell stack outlet flow path. The mixing volume provides a volume for the fuel and oxygen not only to mix together (even if provided as a combined stream prior to the burner inlet, further mixing is beneficial), but also to mix with the heat generated within the burner. This ensures an efficient combustion both at the catalyst and by flame combustion.

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

Preferably, the burner is configured such that the mixing volume is heated to an autoignition temperature of the fuel and oxygen by the catalytic combustion. The catalyst can provide enough heat to increase the temperature for the fuel mixture to reach the autoignition temperature and self-ignite. Therefore, the chemical combustion ensures the combustion of the off-gas fuel mixture and thus generation of heat to enable the flame combustion for the further generation of heat.

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 a fuel cell system exhaust, and a pre-heater heat exchanger that is arranged to exchange heat between burner exhaust gas in the burner exhaust flow path and the oxygen source in the cathode inlet flow path. The heating of the path provided to the cathode inlet, namely the oxygen source path, will increase fuel cell efficiency as the oxygen can be pre-heated. In particular, using the heat generated from the burner for this purpose ensures a higher temperature stack and stack outlet and can reduce the requirement for heating of the cathode inlet from other sources. Therefore, the fuel mixture is further heated and the efficient combustion can occur.

Preferably, the fuel cell stack off-gas comprises an anode off-gas comprising the unused fuel from the anode outlet of the fuel cell stack; and a cathode off-gas comprising 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 for the fuel cell stack is exclusively provided to the stack. Likewise, the oxygen source provided to the cathode inlet is not provided to (directly connected to) the burner. Therefore, the oxygen source for the fuel cell stack is exclusively provided to the stack. This means that no supplementary fuel or oxygen is provided to the burner. This provides a more compact and less complex system than one that requires supplementary fuel or oxygen fees for the burner, and is achieved by incorporating the catalytic burner therein.

Preferably, the fuel and the oxygen provided at the at least one burner inlet is provided from the anode off-gas outlet and the cathode off-gas outlet of the fuel cell stack in all modes of operation. Therefore, during start-up, steady state operation and shut-down, the burner inlet and hence the burner is supplied with fuel and oxygen (or a mixture thereof) from the fuel cell stack exclusively.

Preferably, the fuel cell system further comprises an electric heater arranged to heat the catalyst. A warm catalyst undergoes catalytic combustion more efficiently. Therefore, an electric heater can be operated, such as during start-up, to initially warm the catalyst. The electric heater is not required to be operated once the heat generated by the combustion, or from the electrochemical reaction in the stack, is underway as the reaction or combustion will provide the necessary heat.

Preferably, the fuel cell system further comprises one or more thermocouple to enable control, by a controller, of the oxygen and fuel ratio as a function of the temperatures output from the thermocouple(s). More preferably, thermocouples are arranged at the fuel cell stack inlet, the fuel cell stack outlet and the burner output. A higher fuel flow rate is used during warm-up (start-up) to increase the rate of warm-up. The fuel flow rate is reduced when the stack is at operating temperature to optimise efficiency. The hotter the stack, the more electric current it can draw so the stack will utilise more fuel. The burner outlet temperature is controlled as it goes through these operational states to ensure that there is sufficient heat for the oxygen source going to the stack, i.e. by heat exchange with the cathode inlet flow path. Hotter air to the stack during warm-up results from a hotter burner outlet, and a lower burner outlet temperature might be required once the stack is warm. For example, for an intermediate temperature solid oxide fuel cell, the stack is considered warm when the temperature at the air-side outlet of the stack (usually the cathode off-gas outlet) reaches between 540 and 620 degrees C at full current/load, or at the end of the extended start-up process mentioned above. However, again the various different types of fuel cell, and the various different electrochemically active components of the electrolyte, will each have different temperatures at which it is considered warm (i...e. fully operational): The chemistry of the electrochemically active components typically dictate that temperature.

Preferably, the combustion chamber or burner comprises or provides a diffuser or a diffusing effect for reducing a fluid velocity in the direction of flow of the fluid. The change in flow velocity within the combustion chamber or burner allows for the improved mixture of fuel, oxygen and heat within the burner. It also ensures that the flame combustion does not draw a large flame that can damage the catalyst. Instead the flame and heat is controlled or deflected by the reduction or change in flow velocity or direction.

Preferably, the catalyst comprises a material that creates a chemical combustion between the fuel and oxygen, where the chemical combustion is the flameless combustion of the fuel and oxygen. It can also be referred to as catalytic oxidation. The chemical combustion allows the generation of heat at a lower fuel mixture temperature. This also reduces emissions associated with flame combustion, such as NOx. The reduction of emissions is beneficial during highly fuel-inefficient operation of the fuel cell stack where emissions would otherwise be especially high, e.g. during start-up. With the present invention, therefore, the need for a start-up flame combustion system (blackstart or grey-start), for instance, is obviated, with the benefit of improved emissions, efficiency and reduced complexity.

In accordance with a further aspect of the invention, there is provided a burner assembly for a fuel cell system, the burner assembly comprising: an elongate shaped body extending along a central axis and defining a combustion chamber, the body having a first end and a second end 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 out of the body at the second end; a catalyst positioned in the body, a flame shield positioned between the first end and the catalyst; and an inlet supply line 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 orientated along the central axis.

As hereinbefore described for the preceding aspects, the burner assembly can operate with both catalytic combustion and flame combustion. However, it is important to protect the catalyst from being directly exposed to a flame as this can damage the catalyst.

In some embodiments the flame combustion will occur at or near the first (burner) inlet. Therefore, a flame shield is provided downstream that will prevent a flame from being directed onto the catalyst. The flame shield can also reduce any contaminants from the flame from clogging the catalyst or becoming adhered to the surface of the catalyst (coking), which would otherwise reduce the surface area and thus efficiency of the catalytic combustion. Furthermore, the shape of the combustion chamber body has a smaller first end than that of the second end. Therefore, this advantageous shape reduces a flow rate of the gases within in the combustion chamber. This can encourage mixing, but also results in a shorter flame. A shorter flame is less likely, or cannot, reach the catalyst or flame shield (also known as a flame wall), further protecting the catalyst.

The gases supplied to the first inlet are directed through an elbow. Therefore, the gases are slowed by the turbulent flow generated at the elbow. This has the benefits of reducing a flame length as discussed above. Furthermore, the flow entering the combustion chamber will not be directed along the central axis. Therefore, the flame and thus hot gases will be directed generally along a wall of the chamber - deflected from the central axis. Therefore, the catalyst can be positioned within the body away from the direction of flame and hot gases. For instance, the catalyst can be placed along the central axis away from the walls of the body.

The burner assembly is compatible with, or may be incorporated as the burner of, the systems and methods described above, i.e. it can be exclusively supplied its oxygen and fuel from the fuel cell stack’s anode and cathode off-gas outlets.

Preferably, the burner assembly is such that the length of the first arm is less than the distance between the first end and the second end of the elongate body. This ensures that the turbulent flow and non-central axis directed flow is still such as it enters the combustion chamber and has not become sufficiently laminar in the distance from the elbow.

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 optimum or beneficial change of direction in the flow of gases, as required or desired.

Preferably, the burner assembly is configured such that the body is cone shaped. The narrow part of the cone is beneficially at the first end. The cone is an effective diffuser shape for reducing flow velocity of a fluid passing therethrough. More preferably, the burner body is partially cone shaped. It can then still retain the benefit of a cone while also having a controlled flow path therethrough.

Preferably, the burner assembly is such that the catalyst is a catalytic mesh. A mesh allows gases to pass through but having a large surface area for the gases to react with the catalytic material of the catalyst. Thereby, the catalytic combustion is improved by the mesh.

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 arrangement draws the flow out of the combustion chamber in a direction that is not along the central axis. Therefore it can be drawn around the catalyst to ensure that direct flame contact is minimised and heat does not damage the catalyst. More preferably, the outlet line has a smaller diameter than the second end. Therefore, the flow will hit a second end of the combustion chamber and cause an area of turbulent flow. This can result in gases interacting with the catalyst from the swirl created by the turbulence and the flow of back- gases that can provide sufficient heat for helping auto-ignition of the fuel mixture entering the burner.

Preferably, the burner assembly is such that at least one inlet is in fluid communication with at least one of an anode off-gas outlet or a cathode off-gas outlet of a fuel cell stack. More preferably each at least one inlet is exclusively in fluid communication with at least one of the anode off-gas outlet or the cathode off-gas outlet. Preferably, the burner assembly is such that the catalyst is located closer to the second end than the first end of the body. Therefore, the distance between the flame, i.e. at or near the first end, is increased to reduce the likelihood of direct flame contact or contaminants from flame combustion disrupting performance of the catalyst. Therefore, the flame burner (such as where the flame is formed) is at the first end of the combustion chamber. This is where the flame combustible gases enter the combustion chamber.

Preferably, the burner assembly further comprises a second flame shield provided downstream of the catalyst. This can encourage the heat from catalytic combustion to be distributed throughout the combustion chamber rather than being drawn straight out of the second end. Furthermore, it can help prevent flame combusted off-gases from clogging or damaging the catalyst from the second end and instead they get directed out of the combustion chamber.

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 merely as explicitly set out in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 2 shows a schematic of a related embodiment to Figure 1 ;

Figure 3 shows a side view schematic of a burner used in embodiments of Figures

1 or 2;

Figure 4 shows a schematic of computation flow dynamics for the burner of Figure 3;

Figure 5 shows a chart for the various combustion modes of the burner for the fuel cell system; Figure 6 shows a first trend of the operation of the fuel cell stack and burner in the operation modes of Figure 5;

Figure 7 shows a second trend of the operation of the fuel cell stack and burner in the operation modes of Figure 5;

Figure 8 shows a third trend of the operation of the fuel cell stack and burner in the operation modes of Figure 5; and

Figure 9 shows a fourth trend of the operation of the fuel cell stack and burner in the operation modes of Figure 5.

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

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

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

A listing of reference symbols used herein is given at the end of the description. Repeat use of reference symbols in the present specification and drawings is intended to represent the same or analogous features or elements. Referring 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 comprising a solid oxide fuel cell stack.

The fuel cell stack 12 is the location wherein the electrochemical reaction occurs for the generation of electrical current. The fuel cell stack 12 comprises a plurality of fuel cells, with each fuel cell having a cathode 14 and an anode 16 on either side of an electrolyte. The fuel cell stack 12 further comprises a cathode inlet 18, through which a fluid is provided to the cathode 14 of each fuel cell. This is typically oxygen. The fuel cell stack 12 also has a cathode off-gas outlet 20 through which fluid or gases leaves the cathode 14 of each fuel cell.

The fuel cell stack 12 further comprises an anode inlet 22, where a fluid is provided to the anode 16 of each fuel cell. This is typically a fuel. The fuel cell stack 12 also has an anode off-gas outlet 24 through which fluid or gases leaves the anode 16 of each fuel cell.

Therefore, there is a fuel cell system 10 where the stack 12 has fluid provided, and the stack 12, when operating, undergoes an electrochemical reaction and the outputs from the anodes and cathodes following this reaction (off-gases) are output at anode and cathode off-gas outlets 20, 24.

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 by an anode supply flow path 30. Various fuels can be supplied to the stack to allow for the electrochemical reaction. For instance, hydrogen can be used as the fuel. Other fuels can instead be used, as known in the art.

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 by a cathode supply flow path 32. Various supplies of oxygen can be provided, such as air, water or steam, or oxygen enriched air.

The anode outlet 24 and the cathode outlet 20 are in fluid communication with a burner 34 via one or more fuel cell stack outlet flow path 36 which is in fluid communication with the anode outlet 24 and the cathode outlet 20 and a burner inlet 38 of the burner 34. Therefore, the off-gasses from the fuel cell stack 12 can be passed to the burner 34.

Whilst in Figure 1 , the fuel cell stack outlet flow path 36 is shown as a single line, the fuel cell stack outlet flow path 36 can provide individual lines from each of the anode outlet 24 and the cathode outlet 20 to the burner inlet 38. This is shown in Figure 2 (discussed below). Alternatively, the anode outlet 24 and the cathode outlet 20 can combine from multiple lines into a single line forming the fuel cell stack outlet flow path 36 at any position between the anode outlet 24 and the cathode outlet 20 and the burner inlet 38.

The burner 34 has a burner outlet 40 in fluid communication with an exhaust 46 via a burner exhaust flow path 48. Therefore, the off-gases can be supplied to the burner 34 and an exhaust gas produced, i.e. from combustion, which exhaust gas is exhausted/vented from the system via the exhaust 46. A flow direction is therefore defined from the fuel source 26 and oxygen source 28, through the fuel cell stack 12, through the burner 34, and out of the exhaust 46.

As shown in Figure 1 , 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 provided between these connections.

In this embodiment, a pre-heater heat exchanger 42 is provided in the burner exhaust flow path 48 and the cathode supply flow path 32 to exchange heat between these. Therefore, an exhaust gas from the burner 34 can exchange heat to warm a fluid within the cathode supply flow path 32. It may not be advantageous to exchange heat, such as when the temperature of the exhaust gas is lower than that from the oxygen source 28. Therefore, in this embodiment there is provided an oxygen source bypass line 44 to bypass the pre-heater heat exchanger 42, such that the oxygen source 28 is instead in direct fluid communication with the cathode inlet 18.

Therefore, there is provided a fuel cell system 10 whereby a burner 34 is in fluid-infeed communication with the anode outlet 24 and cathode outlet 20 of a fuel cell stack 12 exclusively. Referring to Figure 2, a similar embodiment is provided to that described above, with like for like reference signs used. Figure 2 shows that the fuel cell stack outlet flow path 36 can provide individual lines from each of the anode outlet 24 and the cathode outlet 20 to the burner 34.

Referring to Figure 3, a burner 34 is provided. The burner 34 can also be referred to as a burner assembly 34. At the burner inlet 38 there are shown to be two fuel cell stack outlet paths 36. Therefore, this embodiment is in accordance with Figure 2 where one of the fuel cell stack outlets 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 at 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.

The burner 34 has a burner body 50, which is an elongate body extended 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 burner outlet 40 and centrally positioned between the sides of the burner body 50 defined thereof. The burner inlet 38 and burner outlet 40 can also be referred to as a first end and second end.

The burner body 50 defines a combustion chamber 50 in which the combustion occurs. Within burner body 50 is a catalyst 52 extending across the burner body 50, i.e. perpendicular to the central axis 58. It will be appreciated that the burner 34 is a three- dimensional shape and therefore whilst reference is being made to the two dimensional presentation of the figures, the features each have a depth and occupy more than one plane. Therefore, for instance, the catalyst 52 sits on a plane perpendicular to the central axis 58 when viewed from the side.

The burner body 50 is shown to be cone shaped with a cylindrical section closest to the burner outlet 40. However, various configurations are available where 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).

In the figures, the catalyst 52 is a double layer catalyst. However, the catalyst 52 can be adapted as required - single layer or multiple layers, or different shapes. The catalyst 52 will generally be of a mesh material, or will be carried on a mesh material. The catalyst 52 is made from a material that reacts with a fuel to undergo chemical combustion and thus produce heat. Such catalyst materials are well known in the art.

In this embodiment the burner body 50 defines a mixing chamber 56 in which the off gases from the fuel cell stack 12 provided via the fuel cell stack outlet paths 36 undergo mixing within the burner 34.

Flame shields 54 are positioned within the burner body 50 at either side of the catalyst 52. The flame shields 54 are likewise orientated the same way as the catalyst 52, being perpendicular to the central axis 58. The flame shields 52 can be circular in shape, or other shapes, but generally will have a similar shape or profile to the cross section of the burner body 50 (or the catalyst).

A gap is provided between the flame shield 54 and the side wall of the burner body 50 to allow the flow of a fluid around the flame shield 54. The flame shield 54 is positioned to protect the catalyst 52 from a flame produced 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 flame shield 54 can be provided. For instance, only one flame shield 54 might be provided between the catalyst 52 and the burner inlet 38. Likewise, the flame shield 54 can be larger or smaller as required to protect the catalyst 52, i.e. the catalyst 52 can be sized to not be in contact with the walls of the burner body 50.

The fuel cell stack outlet flow path 36 is shown to have an elbow 60 prior to being in contact with the burner inlet 38. The elbow 60 provides a change in direction of the flow of the off-gases. In particular, it is shown that the pipe thereof has a perpendicular or ‘L’ shape bend at the elbow 60. Whilst both of the fuel cell stack outlet flow paths 36 are shown to have an elbow 60, in some configurations, only one of the fuel cell stack outlet flow paths 36 have an elbow 60. In such a situation, the flow from the other fuel cell stack outlet flow path 36 that is effected by the elbow 60 will be sufficient to disrupt the flow within the mixing volume 56.

The length from the elbow 60 to the burner inlet 38 is shorter than the length from the burner inlet 38 to the burner outlet 40 (along the central axis 58). 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. 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 provides a change in direction of the flow of the exhaust gas from the burner 34. In particular, it is shown that the pipe has a perpendicular or ‘L’ shape bend at the second elbow 62. As with the elbow 60 of the fuel cell stack outlet flow path 36, the second elbow 62 has a distance to the burner outlet 40 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 as the distance from the burner inlet 38 to the burner outlet 40.

Figure 4 shows a schematic of temperature produced by a flame within the burner 34. The schematic can be produced using computation flow dynamics (CFD). It is highlighted that the burner 34 is shown vertically reversed to the burner 34 of Figure 3, namely, the burner inlet 38 is now on the left of the figure.

In Figure 4, a combustion flame is shown at the end of one of the fuel cell stack outlet flow paths 36, namely the one connected to the anode outlet 24. This combustion flame can be seen by the change in the shading to the area around the end of that fuel cell stack outlet flow path 36. An area of interest 64 is shown and enlarged. The point at the end of one of the fuel cell stack outlet flow paths 36 is the flame seat 66. Here is it seen that the change in shading coming from the flame seat 66 is indicative of the flame produced. Due to the elbow 60, the cathode off-gases deflect (by pushing or pulling) the flame from the central axis 58 toward one of the walls of the burner body 50. In this embodiment, it deflects it downwards, opposing the rotation direction of the elbow 60.

Due to the deflection, the catalyst 52 is not in direct line of the flame from the flame seat 66, i.e. the flame is not along the central axis 58 on which the catalyst is centred. This also protects the catalyst 52 from hot gases as they will be drawn along the wall of the burner body 50 and thus around the catalyst 52.

Therefore, there is provided a burner 34 in communication with an outlet 20, 24 of a fuel cell stack 12, the burner having a catalyst 52 to allow catalytic combustion and a flame seat 66 where flame combustion will occur. Referring to Figure 5, a chart 100 is shown to depict for describing the different combustion modes implemented within the burner 34, i.e. the flame combustion and the catalytic combustion.

The chart 100 shows on the x-axis the fuel concentration 102 and on the y-axis the temperature 104, where the fuel concentration 102 and temperature 104 refers to that of the off-gas fuel mixture within the burner 34. The off-gas fuel mixture may have undergone some mixing within the mixing volume 56 to obtain the concentration 102 and temperature 104.

The fuel concentration 102 refers to the amount of fuel compared to the oxygen in the mixture. For instance, a lean concentration 102 will have less fuel and thus be lower along the x-axis.

Extending from the y-axis 104 there 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 that self-ignition temperature 106, as shown in the chart 100, the fuel is unable to ignite. Therefore, without an ignition source, the fuel will not undergo flame combustion below that self-ignition temperature 106.

The chart includes a cold catalytic oxidation block 110. Referring to the cold catalytic oxidation block 110, in this temperature and concentration range, i.e. all fuel concentration levels 102 and below the self-ignition temperature 106 for the fuel, the offgas fuel mixture can by catalytically combusted - i.e. the material of the catalyst will oxidise in the presence of the fuel of the off-gas fuel mixture, and will produce heat. This is also referred to as cold-catalytic combustion 110. This mode of cold-catalytic oxidation 110 can occur during start-up of the fuel cell system 10. In this situation, the self-ignition temperature 106 of the off-gas fuel mixture is not reached. Therefore, there is no flame.

Fuel and oxygen is provided to the stack 12 from the fuel source 26 and oxygen source 28. These pass through the anode 16 and cathode 14 respectively. As at start-up the stack 12 will be cold, very little if any fuel is used by the electrochemical process within the stack 12. Therefore, as seen in Figures 1 and 2, the fuel containing fluid (off-gas) outputted from the anode 16 is provided to the burner 34, along with the oxygen containing off-gas outputted from the cathode 14. As per the chart 100, the temperature 104 thereof will be below the self-ignition temperature 106 of the fuel, and thus cold catalytic oxidation 110 will occur at the catalyst 52 in the temperature 104 and fuel concentration 102 regions shown.

When the temperature of the fuel 104 passes the self-ignition temperature 106, the fuel ignites and as indicated by the flame combustion region 112, the combustion switches from cold catalytic oxidation 110 to flame combustion 112. This will occur as the stack 12 warms up and the electrochemical reaction produces more heat, thus raising the temperature of the off-gases provided to the burner 34, and also or alternatively due to the heat produced by the catalytic combustion in the cold catalytic oxidation phase 110. This, therefore, will occur during stages of warm-up of the fuel cell system 112 and also during normal operation. Normal operation is where the fuel cell system 10 is at operating temperature. Within this normal operation, there can be power draw, where current is being drawn from the fuel cell, and idle operation where there is no current draw.

As can be seen in chart 100, there is a further mode of operation: flame combustion 112 does not always occur above the temperature 104 for self-ignition of the gas 106. When the fuel concentration 102 is comparatively low, but the temperature is above self-ignition temperature 106, the burner 24 will catalytically combust the off-gas fuel mixture at the catalyst 52. This mode is the lean catalytic combustion 114, where lean off-gas fuel mixture is combusted at the catalyst 52. On the x-axis for fuel concentration 102, there is a lower flammable limit line (LFL) 108. The lower flammable limit (LFL) is the lowest gas concentration that will support a self-emitting flame when mixed with air. Below the lower flammability limit, there is not enough fuel to support combustion and the mixture is too “lean” to burn. The flame combustion region 112 extends below the lower flammable limit 108 for the fuel concentration 102. This is because there are often fuel concentrations that do not align 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 limit 108, the flame will be extinguished within the burner 34. However, the fuel concentration 102 will be too low for it to self-ignite again even though the temperature 104 is above the self-ignition temperature 106. Therefore, the fuel mixture in the burner 34 will undergo lean catalytic combustion 114 at the catalyst 52. In terms of the fuel cell system 10, this lean catalytic combustion mode 114 will occur when the fuel is being consumed in the fuel cell stack 12 at the highest rate. This is when the fuel cell system 10 is fully warmed and the electric current draw from the stack 12 is high - power draw operation. Therefore, the fuel content of the off-gas at the anode outlet 24 is low because the fuel has been greatly consumed by the electrochemical process in the fuel cell stack 12. However, it is still advantageous to combust the lean fuel in the burner 34, and the lean catalytic combustion mode 114 provides a combustion mode for this.

It is highlighted that there may be momentarily a region of low or no combustion when the burner 34 switches from flame combustion 112 to lean catalytic combustion 114 as all 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.

Therefore, the burner 34 allows for the combustion of the fuel mixture in all modes without any additional fuel (non-stack off-gas fuel) being fed to the burner 34.

Referring to Figure 6, there is provided a first temperature against time trend 120 showing the flame combustion 112 and lean catalytic combustion 114 in an example.

The x-axis provides the time 118 and the y-axis provides the temperature 116. Trend line 122 is the catalyst mesh inlet temperature or catalyst inlet temperature 122 where there is no mesh. Trend line 124 is the burner outlet temperature. Trend line 126 is the fuel cell stack outlet temperature.

The flame combustion mode 112 is shown to be occurring when the inlet catalyst temperature 122 is greater than stack outlet temperature 126, and greater or equal to the burner outlet temperature 124. Therefore, the flame is producing the heat upstream from the catalyst 52, i.e. the heat is being produced by the flame at the flame seat 66. Therefore, the temperature of the catalyst mesh inlet 122 is higher due to combustion of the off-gas fuel mixture by flame combustion.

The lean catalytic combustion mode 114 is demonstrated 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. Therefore, the temperature of the off-gas provided from the stack 12, is similar to the temperature of the off-gas fuel mixture supplied to the catalyst 52, thus no flame combustion must be occurring. However, the temperature of the exhaust gas from the burner outlet 40 (the burner outlet temperature 124), is higher than the both the catalyst inlet temperature 122 and the stack outlet temperature 126, and thus is catalytic combustion producing the heat. Therefore, the switching of the combustion modes, due to high efficiency consumption of the fuel in the stack 12 and a reduction in fuel concentration, is confirmed.

Referring to Figure 7, there is provided a second temperature against time trend 130 showing the cold catalytic oxidation 110 in an example. The second trend 130 has an additional x-axis scale showing a fuel flow rate 128. Trend 132 shows the fuel flow rate to the fuel cell stack 12.

The cold catalytic oxidation 110 operation mode takes place at the start-up and during the early stages of warm-up of the fuel cell system 10. After the routine safety check and a pre-purge, when fuel isolation valves are commanded open, the burner outlet temperature 124 goes rapidly to the target temperature. As soon as this 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.

The second trend 130 shows the stack outlet temperature 126 and catalyst inlet temperature 122 are equal. Therefore, there is no flame combustion and the off-gas fuel mixture supplied in or to the burner 34 from the stack outlet is the same temperature when it reaches the catalyst 52. The burner outlet temperature 124 increases rapidly, until the fuel inlet flow rate 132 is reduced once the target temperature is met.

When the fuel flow rate 132 is stopped, there is an increase in burner outlet temperature 124 at point 134. This increase, when fuel flow has stopped and thus no combustion should be occurring, is due to a reduction in the gas flow when the catalyst mesh 52 is still hot, hence increasing the temperature momentarily with low gas flow.

In this specific phase of the cold catalytic oxidation 110, the stack 12 is not using the fuel or using very little, therefore all the available fuel is sent to the burner 34. The flame combustion occurs as the fuel concentration does not exceed the LFL 108. Referring to Figure 8, there is provided a third temperature against time trend 135 showing the flame combustion 112 in an example.

When the off-gas fuel mixture’s temperature inside the burner 34 reaches self-ignition temperature 106, and fuel concentration is adequate (as the stack 12 may still be using little or no fuel), flame combustion 112 is triggered. Trend 135 shows one type of flame triggering, taking place when the temperature of the off-gas fuel mixture exceeds the self-ignition temperature 106 at the catalyst 52 due to the heat generated by catalytic combustion. The hot gases (fuel mixture) backfire and ignite the off-gas fuel mixture at the burner inlet 38.

Referring to the third trend 135, the temperature of the stack outlet 126 and catalyst mesh inlet 122 are steadily increasing. There is also an increase in the burner outlet temperature 124. When the self-ignition temperature 106 is reached, catalyst mesh inlet temperature 122 increases, indicating that there is a flame produced upstream of the catalyst 52 in the burner 34. Therefore, flame combustion 112 is occurring at or near the burner inlet 38.

As discussed above, flame combustion 112 can also be triggered by the self-ignition temperature 106 being exceeded due to hot gas mixing, i.e. the off-gas fuel mixture. This is due to the off-gases from the stack 12 being of a high temperature when they reach the burner 34. However, the temperature of the mixing volume 56 is also influenced by the backwards directed flow of gas that already reacted on the catalyst 52, the high concentration fuel mixture is then ignited.

Referring to Figure 9, there is provided a third temperature against time trend 140 showing the lean catalytic combustion 114 in an example.

Once the start-up and warm-up of the fuel cell system 10 are terminated, the fuel utilization in the fuel cell stack 12 increases, and more electric current is drawn from the fuel cell system 10. Therefore, the off-gas at the anode outlet 24 contains less fuel. Therefore, the fuel concentration 102 of the off-gas fuel mixture in the burner 32 reduces.

As the fuel cell stack 12 temperature increases, more electric current can be drawn, and more heat is generated by the fuel cell stack 12, requiring less and less heat contribution by 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 off-gas fuel mixture inside the burner 32 becomes too lean to burn with flame combustion, even if the temperature is above the self-ignition temperature 106. In practice, due to the local concentration of fuel, the average temperature of the off-gas fuel mixture is below the LFL 108 when the flame combustion 112 actually stops, as shown in Figure 5.

Referring to the fourth trend 140, the stack fuel flow rate 132 is reduced causing the switch from the flame combustion 112 to the lean catalytic combustion 114. Therefore, the catalyst inlet temperature 122 drops to match that of the stack outlet temperature 126 showing that there is no increase in temperature and thus no flame combustion 112. The increase in burner outlet temperature 124 compared to the catalyst inlet temperature 122 is then by lean catalytic combustion 114 at the catalyst 52.

Referring to Figure 1 , the operation and operation modes 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. This fuel flow rate can be controlled in response to temperatures e.g. manually in response, or automatically, such as through electrical or manual controllers. A burner outlet thermocouple 70 is provided 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.

A fuel cell stack outlet thermocouple 72 can also or instead be provided at the cathode outlet 20 of the stack 12. The thermocouple can also or instead be provided at the anode outlet 24. It is even possible for multiple thermocouples to be provided, even at each of the cathode and anode outlets 20, 24.

The fuel cell stack outlet thermocouple 72 measures the temperature of the off-gases leaving the stack 12 and therefore provides an indication of the fuel cell stack operation mode, e.g. start-up, warm-up, idle I power draw. Therefore, the flow rate of the fuel can be varied in response thereto, such as to move to lean catalytic combustion 114 when the fuel cell stack is fully operating. There is also provided a fuel cell stack inlet thermocouple 74 at the fuel cell stack 12 inlet, such as the cathode inlet 18 or the anode inlet 22. As the cathode supply flow path 32 is often heated by the pre-heater heat exchanger 42, it is beneficial to have fuel cell stack inlet thermocouples 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 too or instead can be used to determine a mode of operation or a desired fuel flow rate.

The fuel cell system’s operation modes, enabled by the catalyst 52 in the burner 34, allow for easy switching between catalytic combustion and flame combustion without requiring a complex system.

Reference signs:

10 Fuel cell system

12 Fuel cell stack

14 Cathode side

16 Anode side

18 Cathode inlet

20 Cathode off-gas outlet

22 Anode inlet

24 Anode off-gas outlet

26 Fuel source

28 Oxygen source

30 Anode supply flow path

32 Cathode supply flow path

34 Burner

36 Fuel cell stack outlet flow path

38 Burner inlet

40 Burner outlet

42 Pre-heater heat exchanger

44 Oxygen source bypass line

46 Exhaust

48 Burner exhaust flow path

50 Burner body

52 Catalyst

54 Flame shield 56 Mixing volume

58 Central axis

60 Elbow

62 Second elbow

64 Area of interest

66 Flame seat

70 Burner outlet thermocouple

72 Fuel cell stack outlet thermocouple

74 Fuel cell stack inlet thermocouple

100 Chart

102 x-axis fuel concentration

104 y-axis fuel temperature

106 Self-ignition temperature

108 Lower flammability level

110 Cold catalytic oxidation

112 Flame combustion

114 Lean catalytic combustion

116 y-axis temperature

118 x-axis time

120 Second Trend

122 Catalyst mesh inlet temperature

124 Burner outlet temperature

126 Stack out temperature

128 Fuel flow rate

130 Second Trend

132 Fuel cell stack fuel inlet flow rate

134 Point of interest

135 Third trend

140 Fourth trend