DK201670021A1 - Extrusion used for Production of a Reformer in a Fuel Cell System - Google Patents

Abstract

For the production of a reformer (6) in a fuel cell system (1), a profile (31) is extruded with guiding walls (17) for a meander-formed path (15) of the vapour during conversion into syngas for the fuel cell (2).

Description

Extrusion used for Production of a Reformer in a Fuel Cell System

FIELD OF THE INVENTION

The present invention relates to production methods for reformers in fuel cell systems. BACKGROUND OF THE INVENTION

When using liquid fuel for electricity-producing fuel cells, hydrogen for the fuel cells is generated by conversion of the liquid fuel into a synthetic gas, also called syngas, containing high amounts of gaseous hydrogen. An example of liquid fuel is a mixture of methanol and water, but other liquid fuels can also be used, especially, other alcohols, including ethanol. For the conversion, the liquid fuel is evaporated in an evaporator and subject to catalytic conversion in a reformer to provide syngas for the fuel cell. In order to reach the necessary conversion temperature of 250-300 degrees centigrade, the reformer has to be heated, for example by exhaust gas from a burner, the gas of which, typically, has a temperature of 350-400 degrees centigrade. A general example of a reformer is disclosed in US7976787 England. For a proper heat transfer between the hot exhaust gas and the reformer, the reformer is inserted into a radiator core of wound corrugated metal or extruded metal and brazed thereto. This approach is similar to the field of heat exchangers, where the general teaching is extrusion of a tube and thin fins brazed to the outer wall of the tubes for heat transfer from a medium flowing along the fins. Inside such extruded tubes, liquid flows in the same direction as the extrusion direction, which is in accordance with traditional thinking in the technical field of heat exchangers. From the general technology of heat exchangers, it is known to use claddings or powder spray onto an extruded aluminium core. An example is disclosed in EP0595601, where a spray coating of a powder braz- ing agent is used for brazing corrugated fins onto extruded aluminum core of a heat exchanger. Cladding corrugated fins prior to brazing is disclosed in EP0417894.

Chinese utility model CN202510702 discloses an extruded reformer module cylinder body with plural channels along the flow direction, which reflects the traditional thinking with respect to extrusion.

Whereas, extrusion is a generally used method for conduits where the flow of fluid is along a direction with constant cross section, extrusion is not an obviously advantageous method to use for reformers having more complex structure, where the flow is not along a direction with constant cross section.

High temperature proton exchange membrane fuel cells, also called high temperature proton electrolyte membrane (HTPEM) fuel cells, are advantageous in being tolerant to relatively high CO concentration and are therefore not requiring PrOx reactors between the reformer and the fuel cell stack, why simple, lightweight and inexpensive reformers can be used, which minimizes the overall size and weight of the system in line with the purpose of providing compact fuel cell systems, for example for automobile industry. However, in order to be efficient despite small size, the reformers need a sufficiently long reaction path, which typically is substantially longer than the outer dimensions of small reformers. A way to get combine light weight, small dimensions and long reaction path, the reaction path is potentially formed as a meander, such that the reactant has to follow a zig-zag curve through the reformer.

Such meander-formed path is also known from micro-reactors, for example as disclosed in US8574500, in which the production method is disclosed as milling or moulding. Typically, the milling is performed into a certain depth of a solid block, where the base remains as a solid plate with the structure extending from the basis. Milling or moulding are general methods proposed for meander-formed structures in fuel cell technology; an example is the production of solid bipolar plates with meander-formed flow channels, as disclosed in International patent application W02009/010067.

From these disclosures from various technical fields, it appears that extrusion is a useful and commonly used production method for tubes with constant profile along the axis of the tube, where the fluid-flow is along a direction with constant cross section, which is in the same direction as the extrusion direction, whereas milling is a typical method for more complicated profiles, where the flow is not along the extrusion direction, for example along meander-formed flow paths.

Milling is versatile and easy to adjust for complex profiles. However, it has the drawback of being relatively slow and therefore not optimum for large scale production. Also, it implies a large waste of material, as the material milled out of a block, for example aluminium block, is not useful and has to be discarded. Moulding is better for large scale production but is not suitable for closed structures that enclose a volume, why it puts constraints on production methods and requires additional assembly steps. Also, moulding requires different mould forms for different sizes, which increases the production costs.

It would therefore be desirable to find an alternative production method for fuel cell reformers with complex shapes, especially meander-formed paths.

DESCRIPTION / SUMMARY OF THE INVENTION

It is therefore the objective of the invention to provide an improvement in the art. In particular, it is an objective to provide an improved production method for fuel reformers in fuel cells systems, especially for complex forms deviating from straight tubes. One of the forms of interest for a reformed is a meander-formed path through the reformer, and an objective is to find an improved production method. This objective is solved by a method for production of a fuel cell system as described in the following.

For the production, a metal profile is provided as by extrusion, cut perpendicular to the profile for providing a reformer housing. Inside the reformer, a catalyst is provided, and the cut ends are covered with wall elements to close the volume. An evaporator is connected to one end and the fuel cell to the opposite end. Heat conducting lamellae are optionally provided on the outer surface in case of heating of the reformer by hot gas.

In more detail, an advantageous configuration is provided by the following. The extruded profile comprises two extruded opposite walls facing each other with a distance between the walls and a volume between the walls, the volume will finally be used for the conversion of fuel vapour to syngas. Each of the two walls integrally comprises a group of extruded, mutually-spaced guiding walls extending from each of the two walls towards the opposite wall by a distance of more than half of the distance between the walls. For example, the length of the guiding walls is between 60% and 90% of the distance between the opposite walls. The guiding walls form on each of the two walls a comb-shaped structure; the two comb-shaped structures facing each other and being laterally offset relatively to each other so as to intermesh and form a meander-formed flow path perpendicular to the extrusion direction. Thus, across the volume perpendicular to the extrusion direction, the gas cannot travel along a straight line but has to follow an extended path, which is meander-formed.

For example, the profile is extruded as a box-profile with the two opposite walls being connected by further walls, typically, two each other facing further walls in order to provide a box with a rectangular cross-sectional profile. Alternatively, the profile is provided as two extruded halves of a box profile which are then assembled into a box profile after extrusion. For example, each half comprises one of the two opposite walls and only one group of extruded mutually-spaced guiding walls extending from that one wall, and the assembly provides the guiding walls from opposite sides, creating the meander-formed flow path.

For example, if the guiding walls are equally spaced with a constant period and asymmetrically offset perpendicular to the extrusion direction by a quarter of a period in the half-profile, the two halves can be provided by extrusion of a single profile that is then cut into two equally long sections. For assembly, one section is provided in the extrusion direction and the other half rotated 180 degrees about the extrusion direction and 180 degrees about a line perpendicular to the extrusion direction. Due to the quarter period offset, the guiding walls will intermesh with half a period between the inter-meshed oppositely extending guiding walls.

An example of a useful material is aluminium or an aluminium alloy, for example wrought alloy. The heat conductivity is relatively high in order to conduct heat from an outer side of the profile into the volume inside the reformer, as the volume will be used for the catalytic conversion of fuel vapour to syngas, which typically requires temperatures above 250 degrees centigrade. In order to withstand the burner gas, for example at temperatures above 300 degrees, the alloy is advantageously a wrought aluminium alloy of the 5000-series containing aluminium and magnesium or 6000-series containing aluminium, magnesium, and silicon.

For flow of the vapour into the volume and flow of syngas out of the volume, a vapour inlet and a syngas outlet is provided in the profile on opposite ends of the meander-formed vapour flow path. For example, holes are drilled into the metal profile and conduits attached thereto. The conduits are later used for fluid-flow connecting a liquid-fuel evaporator to the vapour inlet and a fuel cell to the syngas outlet.

Depending on the desired width of the reformer, the profile is cut perpendicular to the extrusion direction and also perpendicular to the meander-formed flow path. Due to the extrusion, a single extruded profile can provide parts for multiple reformer housing, each one cut from the extruded profile in a length suitable for the purpose. For example, one cut piece of a profile can be short and used for a reformer that is connected to a single fuel cell stack, and another cut piece can be long and be used for a reformer providing syngas for multiple fuel cell stacks.

Thus, extrusion turns out not only to simplify production for reformers but also gives a large versatility with respect to size adjustment. In case of moulding, such versatility is not given, as each variation in size needs a different moulding form. The work involved for a small and a large reformer is not much different when using extrusion. This is also in great contrast to milling when used for production. Milling is generally versatile, but the larger the reformer, the longer it takes to perform the milling; and milling to a great depth is mechanically difficult with respect to precision and stability, why milling is also not suitable for large reformers, in addition to the disadvantage of providing a large amount of wasted milling material. All in all, extrusion for production of a reformer is advantageous in several aspects.

Catalytic material, for example catalytic pellets, is inserted into the volume, which is then closed by wall elements for providing a closed reformer housing for catalytic conversion of fuel vapour into syngas in the volume inside the housing.

During operation, the reformer housing is heated in order to provide the right temperature for the catalytic conversion process inside the housing, typically in the range of 250-300 degrees, if the fuel is a mix of methanol and water, and higher in case of higher alcohols or diesel as fuel.

Although, electrical heating is possible for adjusting the temperature in the reformer, a useful method of providing this necessary heat is by leading hot gas along the outer side of the metal profile, and the metal of the housing will conduct heat into the reformer volume. For an efficient heat transfer from the hot gas, parallel metal lamellae are brazed onto an outer side of the housing. For example, for providing such lamellae, thin sheet material is folded into a corrugated structure and provided with a brazing-agent cladding on its surface prior to being brazed to the outer side of the reformer housing. Optionally, the lamellae are provided on at least two sides of the profile, for example on opposite sides. Advantageously, for high heat transfer efficacy, one or more of the largest sides of the profile is/are covered with lamellae. For example, if the reformer volume has a rectangular cross section with two long sides and two short sides, the lamellae are provided on the long sides.

For guidance of hot gas along the lamellae, the reformer housing is enclosed by a casing that has a burner gas inlet at one end of the parallel metal lamellae and a burner gas outlet on an opposite end for the gas being guided along lamellae surfaces. The burner gas inlet is then fluid-flow connected to a burner.

Due to the profile being extruded, the walls can easily be made in various cross sectional shapes, including straight forms or bent forms, for example wavy forms, as long as the extruded profile has a constant cross section along the extrusion direction of the profile. The form of the guiding walls influences the flow speed of the vapour and syngas through the meander-formed path perpendicular to the extrusion direction. In addition, the guiding walls can optionally be provided with vanes to create turbulence and mixing and for guiding the gas towards the middle of the meander-formed path for optimization of the catalytic conversion process. For example, when seen in cross section, the guiding walls comprise a stem, straight or curved, that extends from one to the other of the two opposite walls and the stem comprises vanes extending from the stem, for example extending laterally from the stem, for providing obstructions and corresponding turbulence along the meander-formed vapour flow path. If multiple vanes are provided on the stems, an option is a fishbone shape when seen in cross section.

Examples of materials for the extruded profile are aluminium, aluminium alloys, typically well suited for extrusion are the 1000-6000-7000 series aluminium alloys.

Examples of materials for the lamellae are bent aluminium alloy foils with cladding layer of a brazing agent, meltable at a temperature in the range of 580-600 degrees centigrade.

Examples of the catalytic material are Platinum or Nickel based catalysts or CuZn based catalysts, which are typically well suited for the application of reforming methanol and other alcohols and hydrocarbons. For example, the catalyst is provided as cylindrical or spherical pellets with a size in the range of 0.5-10 mm in diameter and 0.5 to 10 mm in height. Alternatively, the pellets have similar size but other geometrical shape, for example with trapezoidal, cubical, hexahedral cross section, or combinations thereof with or without hollow parts

For the reasons given above, as compared to traditional milling or moulding of fuel cell reformers with meander-formed paths, use of extrusion in the production of such reformers implies multiple advantages in view of production speed and cost reduction as well as ease of size adjustment.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to the drawing, where FIG. 1 shows an example of a fuel cell system; FIG. 2 illustrates an example of a compact reformer with a meander-formed path; FIG. 3 illustrates an extruded reformer with brazed lamella, FIG. 4 illustrates different profiles for brazed lamella.

DETAILED DESCRIPTION / PREFERRED EMBODIMENT FIG. 1 discloses a fuel cell system 1. The fuel cell system 1 comprises a fuel cell stack 2 for which liquid fuel, for example a mixture of methanol and water is supplied from the fuel supply tank 3. In a liquid conduit 4, the liquid fuel is guided into an evaporator 5, in which the temperature of the liquid fuel is raised until evaporation of the fuel. The vapour is fed through a vapour inlet 23 into a reformer 6 that converts the vapour catalytically into syngas, for example by using a catalyser 7, optionally comprising copper. Syngas mainly consist of hydrogen and carbon dioxide and a small content of water mist and carbon monoxide. The syngas is supplied through a syngas outlet 24 and conduit 8 outside the reformer 6 to the fuel cell stack 2 anode side of the proton electrolyte membrane, while oxygen, typically from air, is supplied from an air supply 10 to the cathode side of the proton electrolyte membrane.

In order to reach the temperature relevant for the conversion process in the reformer 6, for example around 280 degrees centigrade, a burner 11 is advantageously employed, typically, using anode waste gas 12 for burning. For example, the exhaust gas 13 of the burner 11 has a temperature of 350-400 degrees centigrade and is used for heating the walls of the reformer 6, typically by guiding the exhaust gas 13 inside a casing 32 along an outer wall of the reformer 6, as illustrated. The gas 13 is also advantageously used for heating the evaporator 5.

Optionally, for evaporation in the evaporator 5, cooling-liquid 14 at a high temperature in the range of 120 to 200 degrees centigrade, for example at 170 degrees centigrade, is guided from the exit portion of the fuel cell stack 2 into the evaporator 5 for transfer of heat from the cooling-liquid to the liquid fuel for evaporation thereof.

As an example, the following parameters apply. For a HTPEM stack delivering 5 kW, typical dimensions are 0.5 m x 0.25 m x 0.15 m. For example, the entire fuel cell stack with burner, evaporator and reformer have a weight of around 20 kg, and an entire fuel cell system including electronics, cooling-liquid pump, first heat exchanger and valve weighs in the order of 40-45 kg. FIG. 2 illustrates a cross sectional drawing of a potential reformer 6 with a meander-formed path 15. The meander-formed path 15 is provided by two comb-shaped sets 16 of guiding walls 17 extending from two each other facing opposite walls 18, 19 and being mutually offset by half a comb-period 20. The length 21 of the guiding walls 17 is more than half of the distance 22 between the opposite walls 18, 19 in order to prevent a straight motion of the vapour and force it along the meander-formed path 15. For example, the length 21 of the guiding walls is between 60% and 90% of the distance 22 between the opposite walls 18, 19.

The reformer 6 comprises an inlet 23 for fuel vapour at one end of the meander-formed path 15 and an outlet 24 for syngas at the opposite end of the meander-formed path 15. Inside the reformer 6 along the meander-formed path 15, there is provided catalytic reformer material 25, for example provided as pellets as illustrated. The pellets 25 are only illustrated in one comer of the reformer 6 but are in reality distributed throughout and along the meander-formed path 15.

As an alternative, the meander-formed path 15 could be increasing in that the cross section of the meander-formed path 15 increases towards the syngas outlet 24, which would allow the vapour and produced syngas to gradually expand during conversion. In this case, the period 20 of the comb is not constant but increasing along the distance from the vapour inlet 23 to the syngas outlet 24.

The reformer 6 in FIG. 2 is provided as a box-shaped extruded profile 31. However, also possible is a reformer is made by assembly of two extruded profiles 31a, 31b, having an assembly contact area along a location along the profile 31 as indicated by break line 34. FIG. 3 is a three dimensional drawing of an extruded reformer 6 structure similarly to the reformer in FIG. 2. The metal profile 31 of the reformer 6 comprises the two comb-shaped set of guiding walls 17 offset by half of the constant period and with a length of the guiding walls corresponding to roughly 75% of the distance between the opposite walls. The reformer 6 comprises a vapour inlet 23 at one end and a corresponding syngas outlet at the opposite end, which is not visible in the image. The end 26 of the reformer 6 when seen from a direction perpendicular to the meander-formed path 15 is shown open but is normally closed with a wall element. The four comers 27 comprise stabilising quarter-circle profiles 28 that connect each of two wall sections that meet in one of the four comers.

As seen from a direction normal to the meander-formed path 15, the reformer 6 is rectangular with two opposite long sides from which the comb-formed sets of guiding walls 17 extend and short sides in which the vapour inlet 23 and the syngas outlet are provided, respectively. At the long sides, there are provided thin metal lamellae 29, typically brazed to the long sides of the extruded part of the reformer 6. These lamellae 29 assure a proper heat transfer from burner 11 exhaust gas 13 that is guided along the lamellae 29 and heating the lamellae 29, which then conduct the heat to the extruded part of the reformer 6. The heat is then conducted through the walls for providing the heat on the inner walls 18, 19 of the reformer.

For production, a profile is extmded in a direction parallel to the guide walls 17. Useful materials include aluminium and aluminium-containing alloys. Once extmded, the profile is cut perpendicular to the extrusion direction 30. The open ends 26 covered with a wall element prior to starting the reformer 6. At each end of the meander-formed path 15, an opening 23 is provided for a vapour inlet and a further opening for a syngas outlet. One side, or two sides, or even more sides are covered with thin metal lamellae 29 for increasing the surface area that can receive heat from how air 13 that is guided along the outer side of the reformer 6. For example, as illustrated in FIG. 3, the two largest sides are covered with lamellae 29. The lamellae 29 extend parallel to the meander-formed path 15, which is perpendicular to the direction 30 of extrusion and perpendicular to the guide walls 17. As seen in FIG. 3, at the end of the lamellae 29, there is provided a funnel-shaped guide 31 for collecting the exhaust gas before release thereof.

As the reformer 6 is extruded perpendicular to the meander-formed path 15 along which the vapour and syngas is flowing across the reformer 6, the reformer 6 can easily be cut into the correct width relatively the width of the fuel cell stack 2 or stacks 2. As indicated in FIG. 3, the reformer 6 provides syngas to three fuel cell stacks 2 and has a width corresponding to the total width of these three fuel cell stack in common. The open sides 26 that remain after cutting of the extruded profile are covered by wall elements (not shown) when preparing the reformer 6 for use.

As illustrated in FIG. 4, the lamellae 29 can be shaped into various configurations, for example plain, perforated, serrated or herringbone-type lamellae, the illustrations not being exhaustive for possible shapes. Although, the shown lamellae are folded into corrugation with rectangular cross-sectional profile, also triangular or quasi-sinusoidal cross-sectional profiles are possible. FIG. 5 illustrates a section of an embodiment, in which the guiding walls 17 comprise a stem 32 and vanes 33 extending laterally from the stem 32. The vanes 34 can have other shapes than the ones illustrated and other angles relatively to the stem 32. Alternatively, the stem 33 is curved and not straight as illustrated in FIG. 5.

Claims (8)

PATENT TRAVELER A method of producing a fuel cell system (1), comprising: providing a metal profile (31) by extrusion, wherein the extruded profile (31) comprises two extruded opposing walls (18, 19) at a distance (22). between the walls (18, 19) and a volume between the walls; each of the opposing walls (18, 19) comprising a built-in group of a plurality of extruded guide walls (17) spaced apart and extending from each of the two opposing walls (18, 19) at a distance (21) at more than half the distance (22) between the walls (18, 19), wherein the guiding walls (17) on each of the two walls (18, 19) form a comb-shaped structure; wherein the two cam-shaped structures face each other and are laterally displaced so as to engage each other and form a meander-shaped vapor flow path (15) perpendicular to the extrusion direction (30); providing a steam inlet (23) and a synthesis gas outlet (24) in the profile (31) at opposite ends of the meander-shaped steam flow path (15); the method further comprising cutting the profile (31) perpendicular to the direction of extrusion (30), inserting catalytic material (25) into the volume, and closing the volume with wall elements to provide a closed reformer housing for catalytic conversion of fuel vapors to the synthesis gas in the volume; soldering parallel metal slats (29) on an outer surface of the heat conduit housing from the metal slats (29) to the metal profile (31) and into the volume; enclosing the housing and metal slats (29) by means of a housing (32), wherein said housing (32) has a burner gas inlet at one end of the parallel metal slats (29) and a burner gas outlet at an opposite end; connecting a flow-the-fuel evaporator (5) to the steam inlet (23) and a fuel cell to the synthesis gas outlet (24) and a burner (11) to the burner gas inlet. The method of claim 1, wherein the length (21) of the guiding walls (27) is between 60% and 90% of the distance (22) between the opposing walls (18, 19). The method of claim 1 or 2, wherein, as seen in cross-section, the guide walls (17) comprise a stem (33) extending from the opposing walls (18, 19) and the stem comprising leaves (34) extending from the stem (33) to provide obstacles and corresponding turbulence along the meander-shaped vapor flow path (15). The method of claim 3, wherein the guide walls (17) are extruded in the form of a fish bone when viewed in cross section. A method according to any one of claims 1-4, wherein the profile (31) is extruded as a box profile, wherein the two opposing walls (18, 19) are connected to additional walls. A method according to any one of claims 1-4, wherein the profile (31) is provided as two extruded halves (31a, 31b) of a box profile (31), each half (31a, 31b) comprising one of the two opposing walls (18, 19) having a group of a plurality of extruded spaced guide walls (17), the method comprising joining two halves (31a, 31b) to a box profile (31) after extrusion. A method according to any one of the preceding claims, wherein the catalytic material (25) is provided as catalytic pellets and the method comprises filling the volume with the pellets between the conductive walls (17), the pellets having a size in the range of 0. , 5 to 11 mm. A fuel cell (1) prepared by a method according to any one of the preceding claims.