WO2010022731A1

WO2010022731A1 - Catalytic burner and method of catalytic combustion

Info

Publication number: WO2010022731A1
Application number: PCT/DK2009/050211
Authority: WO
Inventors: Rasmus Høyrup REFSHAUGE; Søren Nørgaard BERTEL
Original assignee: Dantherm Power A/S
Priority date: 2008-08-25
Filing date: 2009-08-24
Publication date: 2010-03-04

Abstract

The invention relates to a catalytic burner (1) being part of a fuel cell system, said burner comprises a hollow housing (2). The hollow housing (2) has a fuel/air mixture inlet (3) at a first end (13) and an exhaust outlet (6) at a second end (14). The hollow housing (2) further comprises a catalytic element (5) extending over substantially the entire cross-sectional area of the hollow housing (2). The catalytic element (5) is distributed in a longitudinal direction of the hollow housing (2) and the cross-sectional area of the hollow housing (2) increases in the longitudinal direction of the hollow housing (2) from the first end (13) to the second end (14).The invention also relates to a method of catalytic combustion.

Description

CATALYTIC BURNER AND METHOD OF CATALYTIC COMBUSTION

The present invention relates to a catalytic burner that is part of a fuel cell system, said burner comprising a hollow housing, the hollow housing having a fuel/air mixture inlet at a first end and an exhaust outlet at a second end, the hollow housing comprising a catalytic element extending over substantially the entire cross-sectional area of the hollow housing and being distributed in a longitudinal direction of the hollow housing.

US 2007/000175 discloses a reforming reactor for the conversion of a process fluid into hydrogen comprising a catalytic burner, wherein the housing of the catalytic burner surrounds several catalytic elements. Such a burner has limited flexibility, due to its inherent properties, when it comes to accomodating different system capacity requirements while maintaining its efficiency.

US 5,993,192 discloses a catalytic radiant burner having a conically shaped housing with an inlet and an outlet end and a catalyst layer provided at its outlet end, said burner being used for drying or heating different articles and substances. This is achieved by combusting a mixture of one particular gas and air at the outlet end of the burner. Said combusted mixture is subsequently used for drying or heating purposes. However, with this arrangement, it may be difficult to achieve a complete combustion of the fuel. One consequence thereof would be elevated emission levels of undesirable substances such as soot particles.

The object of the present invention is to provide a catalytic burner that retains high efficiency for different fuel flow velocities in the catalytic burner.

In view of this object, the catalytic burner is characterized in that a cross- sectional area of the hollow housing is increasing in the longitudinal direction of the hollow housing from the first end to the second end . Different system capacity requirements call for different flow velocities of the fuel/air mixture. However, an optimal catalytic combustion takes place at a well defined ratio between the flow velocity of the fuel/air mixture and the cross-sectional area of the hollow housing. Optimal combustion conditions are achieved for a wide range of flow velocities of the fuel/air mixture by means of the hollow housing with the increasing cross-sectional area since an increased cross-sectional area of the housing automatically causes a decrease in flow velocity of the fuel/air mixture. This allows for a given amount of the fuel/air mixture to be catalytically combusted upon contact with the catalytic element at a most suitable portion of the housing that comprises an adequately sized catalytic element. In this way, an extended turn-down ratio of the burner may be obtained. Turn-down ratio may be defined as the range of propagation velocities of the fuel/air mixture in which the burner is designed to operate. Hereby, a more flexible burner with regard to the system capacity requirements is achieved.

In an embodiment, the catalytic element is distributed across substantially the entire length of the hollow housing in the longitudinal direction. In this way, a better contol of the combustion process may be obtained. Hereby, an increased portion of the fuel/air mixture that has entered into the burner may be combusted. A more complete combustion is hereby achieved thus reducing the emission of undesirable substances such as soot particles.

In another embodiment, the catalytic element has a porosity of preferably at least 10%, more preferred at least 20 % and most preferred at least 30 %. Relatively free flow of the fuel/air mixture past the catalytic element may thereby be obtained.

In this way, the difference in the pressure of the fuel/air mixture upstream the catalytic element and the pressure of the combusted fuel/air mixture downstream the catalytic element may be reduced. Hereby, a blower fan with lower capacity may be used thus reducing power consumption of the blower fan and the system as a whole.

In yet another embodiment, the catalytic element comprises a plurality of wire meshes that are mutually spaced in the longitudinal direction of the hollow housing. The actual catalytic combustion takes place at the surface of the wire meshes. They may be coated with ceramics in order to increase their efficiency and may subsequently be coated with a catalyst such as platinum in order to further increase their efficiency. Wire meshes give a net-like structure to the element. As a consequence, relatively free flow of the fuel/air mixture past the catalytic element may be obtained. In this way, the difference in the pressure of the fuel/air mixture upstream the catalytic element and the pressure of the combusted fuel/air mixture downstream the catalytic element may be reduced. Hereby, a blower fan with lower capacity may be used thus reducing power consumption of the blower fan and the system as a whole.

In another embodiment, the distance between subsequent wire meshes is preferably smaller than 2 cm, more preferred smaller than 1 cm and most preferred smaller than 0.5 cm. In this way, the catalytic element extends in a substantially continuous way along the hollow housing. This contributes to a large active surface of the element. Hereby, a more efficient combustion of the fuel/air mixture may be achieved.

In an embodiment, the ratio of the cross-sectional area of the first wire mesh and a second, subsequent wire mesh in the longitudinal direction is preferably smaller than 1.25, more preferred smaller than 1.15 and most preferred smaller than 1.1.

In another embodiment, the catalytic element is provided with foam-like structure. The foam-like structure may be made in high temperature resistant metal and is highly porous. The actual catalytic combustion takes place at the surface portion of the structure. It may be coated with ceramics in order to increase their efficiency and may subsequently be coated with a catalyst such as platinum in order to further increase its efficiency. As a consequence, relatively free flow of the fuel/air mixture past the catalytic element may be obtained. In this way, the difference in the pressure of the fuel/air mixture upstream the catalytic element and the pressure of the combusted fuel/air mixture downstream the catalytic element may be reduced. Hereby, a blower fan with lower capacity may be used thus reducing power consumption of the blower fan and the system as a whole.

In a particular embodiment, the angle between a central axis extending in the longitudinal direction and the hollow housing is preferably smaller than 60°, more preferred smaller than 45° and most preferred smaller than 30°. In this way, it is allowed for movement of fuel/air mixture as well as combusted fuel/air mixture in radial direction. Thus, the turbulence across the burner is minimized and laminar, controllable and uniform flow is ensured. Hereby, an improved combustion of the air/fuel mixture may be achieved, regardless of the type of fuel used, in particular, when fuel portion of the mixture comprises several compounds.

In an embodiment, the ratio of the hollow housing diameter at the second end and at the first end of the catalytic burner is preferably smaller than 15, more preferred smaller than 8 and most preferred smaller than 3. As described above, this allows for movement of fuel/air mixture as well as combusted fuel/air mixture in radial direction and improves the overall combustion.

In another embodiment, the burner comprises a flash-back preventing device positioned upstream the catalytic element. In this way, an undesirable direction of propagation of the catalytic flame may be prevented. Hereby, safer operation of the catalytic burner may be obtained.

The present invention further relates to a fuel cell system comprising a catalytic burner as described above, wherein the burner discharges into a reformer adapted to produce hydrogen from hydrocarbons, and wherein the hydrogen is used as fuel for at least one fuel cell. The present invention further relates to a method of combusting fuel/air mixture in a catalytic burner. The method is characterized by that the fuel/air mixture is directed through a hollow housing from a first end to a second end thereby reacting with a catalytic element over substantially the entire cross-sectional area of the hollow housing and over a substantial part of the flow path from the fuel/air mixture inlet end to the exhaust outlet end and by that the flow velocity of the fuel/air mixture at the fuel/air mixture inlet end (3) is higher than flow velocity of the exhaust at the exhaust outlet end (6). Thereby, the above-discussed effects may be obtained.

In an embodiment, the catalytic element is distributed across substantially the entire length of the hollow housing in the longitudinal direction whereby the fuel/air mixture is catalytically combusted upon contact with the catalytic element over substantially the entire length of the hollow housing. Thereby, the above-discussed effects may be obtained.

In another embodiment, a flash-back preventing device is activated and the fuel/air mixture is brought into contact with the flash-back preventing device and ignited upon being in contact with the flash-back preventing device. Thereby, the above-discussed effects may be obtained.

The invention will now be explained in more detail by means of examples of embodiments with reference to the schematic drawings, in which

Fig. 1 is a perspective view of one embodiment of the present invention.

Fig. 2 is an axial section of the catalytic burner according to one embodiment of the present invention.

Fig. 3 is an axial section of the catalytic burner according to a different embodiment of the present invention.

Fig. 4 is a perspective view of the catalytic burner comprising the corresponding start-up system.

Fig. 5 is an axial section of the catalytic burner comprising the corresponding fuel reformer.

Figure 1 is a perspective view of an embodiment of the present invention. A hollow housing 2 is made in material that provides adequate electrical and thermal insulation such as Vermiculite or calcium silicate. There is a fuel/air mixture inlet 3 that is cut in the base 4 of the housing 2 at a first end of the hollow housing 2. The hollow housing 2 has a cross-sectional area that increases in a longitudinal direction. A catalytic element 5 that comprises two wire meshes 7 extends over substantially the entire cross-sectional area of the housing. Thus, the subsequent portions of the catalytic element 5 have different cross-sectional areas. Adequate portion of the element 5 is activated depending on the velocity of the fuel/air mixture that flows through the housing. In other words, if a high velocity fuel/air mixture flows through the housing then a section of the catalytic element 5 with corresponding large cross- sectional area is activated and the catalytic combustion takes place. The exhaust gas that is generated in the catalytic burner 1 may discharge into a fuel reformer (not shown) at an exhaust outlet 6. Figu re 2 is an axial section of the cata lytic burner 1 accord ing to an embodiment of the present invention. It shows a hollow housing 2 comprising a first 13 and a second end 14 that form part of the catalytic burner 1. The burner 1 further comprises a catalytic element 5 comprising a plurality of wire meshes 7. In this embodiment wire meshes 7 are made in high-temperature resistant material such as Kanthal. They may be coated with ceramics in order to increase their efficiency and may subsequently be coated with a catalyst such as platinum in order to further increase their efficiency. The wire mesh structure provides for large surface area of the catalytic element 5 thus enabling increased heat transfer from the catalytic element 5 to the fuel/air mixture. It further provides for low pressure losses in the housing 2. Furthermore, the burner 1 comprises a structure, positioned close to the first end 13 of the hollow housing 2, that acts as a flash-back preventing device 8. The flash-back preventing device 8 may be provided with electrical connections (not shown), connected to a current source and activated by means of an electric current. Flash-back may be defined as a condition where the catalytic flame passes backward out of the burner and into the source of the fuel/air mixture. One way to eliminate a potential flash-back is to design a process where the maximum catalytic flame velocity of the fuel/air mixture is lower than the propagation velocity of the fuel/air mixture. Catalytic flame velocity may be defined as the propagation velocity of the catalytic flame of the ignited fuel/air mixture. The catalytic flame would then be unable to propagate against the flow and thus it could only move downstream from the point of ignition. This solution requires an elevated propagation velocity of the fuel/air mixture and calls for blower fans with important power consumption. A flashback preventing device 8 provides for added control of the catalytic combustion, effectively reducing the need for unnecessarily high propagation velocity of the fuel/air mixture. In this embodiment the flash-back preventing device 8 is a wire mesh, but other alternatives are conceivable.

Figure 3 is an axial section of the catalytic burner 1 according to a different embodiment of the present invention. It shows a hollow housing 2 comprising a first 13 and a second end 14 that form part of the catalytic burner 1. The burner 1 further comprises a catalytic element 5 having a foam-like structure 9. The foam-like structure 9 may be made in high-temperature resistant metal. It may further be coated with ceramics in order to increase their efficiency and may subsequently be coated with a catalyst such as platinum in order to further increase its efficiency. The foam-like structure 9 is highly porous. Porosity may be defined as a measure of the void spaces in a material, and is normally measured as a percentage between 0- 100%. High porosity of the foam-like structure 9 provides for low pressure losses in the housing 2. The foam-like structure 9 provides for large surface area of the catalytic element 5 thus enabling increased heat transfer to the fuel/air mixture. Furthermore, the burner 1 comprises a structure, positioned close to the first end of the hollow housing 2, that acts as a flash-back preventing device 8.

Figure 4 is a perspective view of the catalytic burner 1 comprising the corresponding start-up system 10. The burner 1 may, however, operate independently of the start-up system 10. The performance of the catalytic burner 1 depends, inter alia, on the angle between a central axis 11 extending in the longitudinal direction and the hollow housing 2. This angle, as well as the cross- sectional shape and the size of the burner 1 may depend on the system requirements such as turn-down ratio. If burner 1, for instance, is designed to operate in the range 1 000 m3/h - 10 000 m3/h than its turn-down ratio is 10: 1. Furthermore, the shape and the size of the burner 1 may depend on types of fuel to be processed. The burner 1, according to the present invention, is designed to operate with several different types of fuel. The shape of the burner 1 has a particularly significant impact on the combustion process when at least two different fuels making up the fuel/air mixture, for instance natural gas and hydrogen, are to be processed simultaneously in the burner 1. Normally, the participating fuels of the fuel/air mixture have different maximum catalytic flame velocities. Typically, catalytic flame velocity of hydrogen is approximately ten times larger than the catalytic flame velocity of natural gas. With a convential burner this unbalance might introduce turbulence in the burner itself if hydrogen and the natural gas are to be processed at the same time. The turbulence in the burner 1 is undesirable since it may reduce the control of the combustion process and may negatively impact the efficiency of a subsequent fuel reformer (not shown). The burner 1 that comprises a hollow housing 2, the cross-sectional area of which is increasing in the longitudinal direction, allows, due to its inherent properties, for movement in radial direction of fuel/air mixture as well as combusted fuel/air mixture thus minimizing turbulence across the burner 1 and ensuring laminar, controllable and uniform flow of the combusted fuel/air mixture into the subsequent fuel reformer.

Figure 5 is an axial section of the catalytic burner 1 comprising the corresponding fuel reformer 12. The exhaust gas that is generated in the catalytic burner 1 may discharge into a fuel reformer 11 at a second end 14 of the hollow housing 1. Fuel reformers come in different shapes and since the burner 1 normally is directly connected to the reformer 6 the cross-sectional shape of the reformer 6 could dictate the cross-sectional shape of the catalytic burner 1. A round reformer 6 is advantageous since it facilitates the isolation of the reformer housing 15. Thus, a circular cross-section of the conical burner 1 is desirable. Furthermore, circular cross- section of the conical burner 1 would reduce pressure losses in the burner 1 due to reduced friction of the fuel/air mixture against the inner walls of the housing 2. However, other cross-sectional shapes, such as rectangular or elliptical, are to be considered for specific applications.

The catalytic burner normally forms part of a fuel cell plant. As a rule, the fuel cell plant requires the supply of both hydrogen and oxygen-rich gas such as air in order to produce electricity. For that purpose a reformer adapted to produce hydrogen-rich gas from hydrocarbons is integrated in the fuel cell plant. The reformer is usually provided immediately downstream the burner. The burner and the reformer convert a suitable hydrocarbon fuel acting as energy carrier, such as methane, liquid petroleum gas, gasoline, diesel or methanol, into a hydrogen-rich gas. This hydrogen- rich gas may then be conveyed through a hydrogen-enrichment unit and converted to substantially pure hydrogen gas before entering a fuel cell assembly. In the fuel cell assembly the substantially pure hydrogen gas reacts with the oxygen whereby the hydrogen dissociates into electrons and protons. The electrons are subsequently forced to travel through a circuit, thus constituting a current with a corresponding electrical energy. Although the invention above has been described in connection with different embodiments of the invention, it will be evident for a person skilled in the art that several modifications are conceivable without departing from the invention as defined by the following claims.

Claims

1. A catalytic burner (1) being part of a fuel cell system, said burner comprising a hollow housing (2), the hollow housing (2) having a fuel/air mixture inlet (3) at a first end (13) and an exhaust outlet (6) at a second end (14), the hollow housing (2) comprising a catalytic element (5) extending over substantially the entire cross- sectional area of the hollow housing (2) and being distributed in a longitudinal direction of the hollow housing (2), characterized in that the cross-sectional area of the hollow housing (2) is increasing in the longitudinal direction of the hollow housing (2) from the first end (13) to the second end (14).

2. A catalytic burner (1) according to claim 1, wherein the catalytic element (5) is distributed across substantially the entire length of the hollow housing (2) in the longitudinal direction.

3. A catalytic burner (1) according to claim 1 or 2 wherein, the catalytic element (5) has a porosity of preferably at least 10%, more preferred at least 20 % and most preferred at least 30 %.

4. A catalytic burner (1) according to any of the claims 1-3, wherein the catalytic element (5) comprises a plurality of wire meshes (7) that are mutually spaced in the longitudinal direction of the hollow housing (2). 5. A catalytic burner according to claim 4, wherein the distance between subsequent wire meshes (7) is preferably smaller than 2 cm, more preferred smaller than 1 cm and most preferred smaller than 0.

5 cm.

6. A catalytic burner according to claim 4 or 5, wherein the ratio of the cross- sectional area of a first wire mesh and a subsequent wire mesh is preferably smaller than 1.25, more preferred smaller than 1.15 and most preferred smaller than 1.10.

7. A catalytic burner (1) according to claim 1-3, wherein the catalytic element has a foam-like structure (9).

8. A catalytic burner (1) according to any one of the previous claims, wherein the angle between a central axis (11) extending in the longitudinal direction and the hollow housing (2) is preferably smaller than 60°, more preferred smaller than 45° and most preferred smaller than 30°.

9. A catalytic burner (1) according to any one of the previous claims having a hollow housing (2) with a second diameter at a second end and a first diameter at a first end, wherein the ratio of the diameter at the second end and at the first end is preferably not more than 15, more preferred not more than 8 and most preferred not more than 3 .

10. A catalytic burner (1) according to any of the previous claims, wherein the burner comprises a flash-back preventing device (8) positioned upstream the catalytic element (5).

11. A catalytic burner according to claim 10, wherein the flash-back preventing device (8) comprises at least one wire mesh (7) provided with a catalyst.

12. A fuel cell system comprising a catalytic burner (1) according to any of the previous claims, wherein the burner (1) forms part of a reformer (12) adapted to produce hydrogen from hydrocarbons, and wherein the hydrogen is used as fuel for the fuel cell system.

13. A method of catalytic combustion in a catalytic burner (1), wherein the fuel/air mixture is directed through a hollow housing (2) from a first end (13) to a second end (14) thereby reacting with a catalytic element (5) over substantially the entire cross-sectional area of the hollow housing (2) and over a substantial part of the flow path from the fuel/air mixture inlet end (3) to the exhaust outlet end (6) characterized by that flow velocity of the fuel/air mixture at the fuel/air mixture inlet end (3) is higher than flow velocity of the exhaust at the exhaust outlet end (6).

14. A method of catalytic combustion in the catalytic burner (1) according to claim 13, whereby the catalytic element (5) is distributed across substantially the entire length of the hollow housing (2) in the longitudinal direction and whereby the fuel/air mixture is catalytically combusted upon contact with the catalytic element (5) over substantially the entire length of the hollow housing (2).

15. A method of catalytic combustion in the catalytic burner (1) according to claim 13 or 14, the catalytic burner (1) comprising a flash-back preventing device (8) whereby the fuel/air mixture is brought into contact with the flash-back preventing device (8) and ignited upon being in contact with the flash-back preventing device (8).