WO2013093425A1

WO2013093425A1 - Catalytic reactor and catalytic structure

Info

Publication number: WO2013093425A1
Application number: PCT/GB2012/053098
Authority: WO
Inventors: David James West
Original assignee: Compactgtl Limited
Priority date: 2011-12-22
Filing date: 2012-12-12
Publication date: 2013-06-27
Also published as: TW201338860A; GB201122193D0

Abstract

A reactor (10) defines first and second flow channels (16, 17) with a removable combustion catalyst insert (24) provided in each of those channels (17) in which a combustion reaction is to occur. The combustion catalyst insert comprising a first portion (Q2) including active catalytic material, a second portion (Q1) having a catalytic activity less than 0.2 times the catalytic activity of the first portion (Q2), and a third portion (P) including active catalytic material with an activity at least 0.3 times that of the first portion (Q2). The first portion (Q2), the second portion (Q1), and the third portion (P) are progressively further from the inlet; and they may be integral with each other.

Description

Catalytic Reactor and Catalyst Structure

The present invention relates to a reactor for performing chemical reactions which involve heat transfer, the reactor defining channels in which there is a catalyst structure for a combustion reaction, and to a catalyst structure for use in such a reactor.

The use of a catalytic reactor consisting of a stack of metal sheets that define first and second flow channels, where catalyst is provided on removable inserts such as corrugated foils within the flow channels, is described for example in WO

03/033131 , which describes use of such a reactor for performing various chemical reactions for example Fischer-Tropsch synthesis, steam methane reforming, or combustion. WO 2010/067097 also describes a catalytic reactor in which a catalyst insert may comprise one or more corrugated foils. The two sets of channels enable heat transfer to take place between the contents of those channels. For example steam methane reforming is an endothermic reaction that requires an elevated temperature, typically above 750 °C; and the requisite heat may be provided by a combustion reaction taking place in the other set of channels within the catalytic reactor.

According to one aspect of the present invention there is provided a reactor defining first and second flow channels within the reactor, each flow channel having an inlet and an outlet, with a removable combustion catalyst insert provided in each of those channels in which a combustion reaction is to occur, the combustion catalyst insert comprising a first portion including active catalytic material, a second portion having a catalytic activity less than 0.2 times the catalytic activity of the first portion, and a third portion including active catalytic material and having an activity at least 0.3 times that of the first portion, wherein the first portion, the second portion, and the third portion are progressively further from the inlet.

In use, a combustible gas mixture is supplied to the inlet, and flows successively past the first portion, the second portion and the third portion. The variation in catalytic activity of the successive catalyst portions controls the rate of combustion, and therefore controls the temperature distribution. There is no need to introduce additional components (such as fuel) at intermediate positions. Hence the section of the flow channel containing the combustion catalyst insert has only a single inlet. Although mention has been made of there being first and second flow channels, for first and second fluids, it will be appreciated that the reactor might define flow channels for more than two different fluids. The first portion, the second portion, and the third portion may be integral with each other; or alternatively they may be separate, and arranged end to end in the channel.

The catalytic activity of equal lengths of catalyst structure, for example of length 100 mm, can be compared in an experimental test in which a combustible gas mixture is passed over the catalyst structures placed in turn within a channel, holding the temperature at the channel wall at a constant value, for example l OCO, and holding the gas composition and flow rate constant. The catalytic activity can be assessed by determining the proportion of a combustible gas such as hydrogen which undergoes combustion.

The second portion may have a catalytic activity about 0.1 times the catalytic activity of the first portion, or it may have no catalytic activity. It has been found that an oxidised aluminium-containing steel alloy which has an alumina surface can have a catalytic activity between 0.075 and 0.130 times that of a normal combustion catalyst; whereas a steel alloy which does not contain aluminium, such as stainless- steel, is suitable where catalytic activity is not required.

The third portion may have a catalytic activity approximately the same as that of the first portion, as both these portions are intended to bring about combustion. For example the catalytic activity of the third portion may be between 0.5 and 5.0 times that of the first portion, or between 0.75 and 2.0 times that of the first portion. Indeed they may have equal catalytic activity. Furthermore the catalytic activity of the third portion may be graded along its length, for example by varying the concentration of active catalytic material.

In one embodiment the length of the first portion is between 5 mm and 100 mm; and the length of the second portion is between 20 mm and 300 mm; and the third portion occupies the remainder of the length of the channel. For example the first portion may be of length between 10 mm and 35 mm, for example 35 mm; while the second portion may be of length between 50 mm and 200 mm, for example 100 mm. The first portion may be between 0.5% and 15% of the total length of the catalyst within the channel, for example 1 % or 5% of the total length; the second portion may be between 2% and 45%, for example 5% or 20%; the third portion may be more than 50% of the total length, for example 75%. Each portion may be at least 5 mm long.

The effect of the first portion is to initiate the catalytic combustion reaction, but the length of the first portion is sufficiently short that no more than 15%, and more typically no more than 10%, of the combustible gas undergoes reaction.

Nevertheless the presence of the first portion raises the temperature of the combustible gas mixture. The second portion may have low catalytic activity, so catalytic combustion occurs at a low rate, maintaining an elevated temperature.

When the combustible gas mixture reaches the third portion, catalytic combustion occurs at a higher rate, so a larger proportion of gas undergoes reaction. The present invention is particularly applicable where an endothermic reaction is to take place in channels which are in thermal contact with the channels in which combustion occurs, the channels for the endothermic reaction incorporating a catalyst for the endothermic reaction, but in which a distributor region of the endothermic reaction channel contains no catalyst for the endothermic reaction. In such a context the first portion and the second portion of the catalyst insert in the combustion channels may be arranged adjacent to the distributor region of the endothermic reaction channel. The first portion and the second portion of the catalyst insert provide sufficient heat to suppress any heat loss from the gases supplied to the distributor region.

The reactor may also include a non-catalytic insert adjacent to the inlet of each channel for combustion, the non-catalytic insert comprising one or more foils which subdivide the flow channel adjacent to the inlet into a multiplicity of narrow flow paths which are narrower than the maximum gap size for preventing flame propagation. Such a non-catalytic insert may be made of a stainless-steel which does not form an alumina surface. It would typically be of length at least 30 mm, but no more than 100 mm long, for example 50 mm or 60 mm. The provision of such a non-catalytic insert is described in EP 2 015 865 B (CompactGTL pic). The first flow channels and the second flow channels may extend in parallel directions, within a reactor block, and the combustible gas mixture and the endothermic reaction mixture flow in the same direction (co-flow). The flow channels may be of length at least 300 mm, and may be at least 500 mm, but usually no longer than about 1000 mm. A length of between 500 mm and 700 mm, for example 600 mm may be advantageous. It has been found that co-flow operation gives better temperature control, and less risk of hot-spots.

In one embodiment each first flow channel (the channels for the combustion reaction) and each second flow channel (the channels for the endothermic reaction) contains a removable catalyst structure to catalyse the respective reaction, and each catalyst structure may comprise a metal substrate, and incorporate an appropriate catalytic material. Each such catalyst structure may be shaped so as to subdivide the flow channel into a multiplicity of parallel flow sub-channels. Each catalyst structure may include a ceramic support material on the metal substrate, which provides a support for the catalyst. The metal substrate provides strength to the catalyst structure and enhances thermal transfer by conduction. The metal substrate may be of a steel alloy that forms an adherent surface coating of aluminium oxide when heated, for example a ferritic steel alloy that incorporates aluminium (eg Fecralloy (TM)). The substrate may be a foil, a wire mesh or a felt sheet, which may be corrugated, dimpled or pleated; the preferred substrate is a thin metal foil for example of thickness less than 150 μηι, which is corrugated to define the longitudinal sub-channels. Depending on the size of the flow channels, the substrate may comprise either a single corrugated foils or an assembly of foils, which may be bonded together, for example by spot welding.

Where the foils are corrugated, the corrugations may be square, rectangular, trapezoidal or hexagonal in cross-section; or arcuate or sinusoidal; or they may be of zigzag shape, defining triangular corrugations, or a sawtooth shape, for example with sloping portions connected by flat peaks. The corrugations typically run parallel to the length of the foils. In some alternative configurations, the corrugations may be non- parallel or even perpendicular to the length of the foil.

In the case of a substrate which comprises an assembly of foils, if the corrugated foils have corrugations that would enable adjacent foils to intermesh then the corrugated foils may be spaced apart by foils that are flat or substantially flat, to ensure they do not intermesh. Such flat foils are not necessary if the adjacent foils have corrugations that are not parallel, or are otherwise shaped to ensure the foils they do not intermesh. For example if the peaks and troughs are defined by flat portions of the profile, and the flat portions in adjacent foils are aligned with each other, then the foils will not intermesh. Where flat foils are used, they may also be corrugated at a very small amplitude, for example to provide a total height of less than about 0.2 mm, for example 0.1 mm, as this makes them slightly less flexible and so easier to work with during assembly. The direction of the corrugation of the substantially flat foil may be lengthwise along the foil or, alternatively, may be transverse. The shape of the corrugations of the flat foils may be sawtooth or rippled. The foils may be of thickness in the range 20-150 μηι, for example 50 μηι. A thicker foil, for example 100 μηι thick, may provide benefits in enhanced heat transfer. By preventing intermeshing of the corrugated foils, either by the provision of flat foils or by the provision of adjacent foils with non-parallel corrugations, the overall height of the insert is more repeatable and controllable than a stack in which identical corrugated foils are deployed.

To form the catalyst structure the metal substrate would be provided with a catalyst on at least some of the surfaces. For example the substrate may be coated with ceramic support material, for example based on alumina, and this would be impregnated with active catalytic material appropriate for the reaction that is to take place in the corresponding channel. The ceramic coating may be applied by techniques such as dip coating, or spraying, to achieve a ceramic thickness between 10 μηι and 100 μηι, depending on the reaction. Where the channel contains a plurality of foils, the coating may be applied to separate foils before they are stacked together, or even after such foils have been bonded together.

The second portion of the catalyst insert may comprise a length of a steel alloy that forms an adherent surface coating of aluminium oxide when heated, for example a ferritic steel alloy that incorporates aluminium (eg Fecralloy (TM)), which has been oxidised to ensure that the surface is of alumina. Such an oxidised surface has a catalytic activity for combustion about 0.1 times that of a conventional combustion catalyst.

The first portion and the third portion, as mentioned above, preferably include a metal substrate and a ceramic support material, such as a coating of alumina, and this would be impregnated with active catalytic material. The active catalytic material may be platinum, or palladium oxide. Palladium and platinum have been previously suggested as suitable active catalytic materials for a combustion catalyst. Platinum is catalytically active in the metal form, rather than the oxide form, and is stable as the metal; and it has a higher light-off temperature than palladium. Platinum is active at high temperatures, and not particularly active at temperatures below about 600 °C. However, it tends to undergo deactivation at temperatures above about 795°C.

Palladium is catalytically active in the oxide form, and significantly less active in the metal form, and in the presence of a low oxygen partial pressure the transformation from the oxide to the metal may occur, with a consequential reduction in the catalytic activity, particularly at high temperatures. Conversely, palladium oxide is active at lower temperatures than platinum, for example between 400 °C and 700 °C.

Consequently the first portion may comprise palladium oxide, whereas the third portion may comprise palladium oxide and also platinum.

The reactor consists of a block, typically of metal, which defines the first and second flow channels. For example the first and second flow channels may be defined by grooves in plates arranged as a stack, or by spacing strips and plates in a stack, the stack then being bonded together. Alternatively the flow channels may be defined by thin metal sheets that are castellated and stacked alternately with flat sheets; the edges of the flow channels may be defined by sealing strips. The stack of plates forming the reactor is bonded together for example by diffusion bonding, brazing, or hot isostatic pressing. The stack of plates, after being bonded together, provides the requisite structure to ensure that the reactor can resist the differential pressures and thermal stresses that are applied during operation; the catalyst insert does not have to provide structural support. Consequently the catalyst inserts can be non-structural, as they do not have to hold the walls of the channels apart during operation.

The channels may be square in cross-section, or may be of height either greater than or less than the width, where the height refers to the dimension in the direction of the stack, that is to say in the direction for heat transfer. For example the plates might be 0.5 m wide and 1 .0 m long, or 0.6 m wide and 0.8 m long; and they may define channels 7 mm high and 6 mm wide, or 3 mm high and 10 mm wide, or 10 mm high and 5 mm wide. These dimensions are merely exemplary, and the skilled person will recognise that many different shapes and sizes are equally suitable. Arranging the first and second flow channels to alternate in the stack helps ensure good heat transfer between fluids in those channels. For example the first flow channels may be those for combustion (to generate heat) and the second flow channels may be for steam/methane reforming (which requires heat). The catalyst inserts are inserted into the channels, and can be removed for replacement.

In a further aspect, the present invention provides a combustion catalyst insert for use in such a reactor, the combustion catalyst insert comprising a first portion including active catalytic material, a second portion having a catalytic activity less than 0.2 times the catalytic activity of the active catalytic material, and a third portion including active catalytic material with an activity at least four times greater than that of the second portion, such that after insertion into a combustion flow channel the first portion, the second portion, and the third portion are progressively further from the inlet.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 shows a schematic perspective view, partly in section, of part of a reactor block suitable for steam/methane reforming and including catalyst inserts (the section being on the line 1 -1 of figure 2);

Figure 2 shows a side view of the assembled reactor block of figure 1 showing the flow paths;

Figures 3a and 3b show plan views of parts of the reactor block of figure 1 during assembly;

Figure 4 shows a side view of a catalyst insert of Figure 1 .

The invention is applicable to a process for making synthesis gas, that is to say a mixture of carbon monoxide and hydrogen, from natural gas by steam reforming. The synthesis gas may, for example, subsequently be used to make longer-chain hydrocarbons by a Fischer-Tropsch synthesis. The steam reforming reaction is brought about by mixing steam and methane, and contacting the mixture with a suitable catalyst at an elevated temperature so the steam and methane react to form carbon monoxide and hydrogen. The steam reforming reaction is

endothermic, and the heat may be provided by catalytic combustion, for example of hydrocarbons and/or hydrogen mixed with air, so combustion takes place over a combustion catalyst within adjacent flow channels within the reforming reactor.

Referring now to figure 1 there is shown a reactor block 10 suitable for use as a steam reforming reactor, or for use in a steam reforming reactor. The reactor block 10 defines channels for a catalytic combustion process and channels for steam methane reforming. The reactor 10 consists of a stack of plates that are rectangular in plan view, each plate being of corrosion resistant high-temperature alloy such as Inconel 625, Incoloy 800HT or Haynes HR-120. Flat plates 12, typically of thickness in the range 0.5 to 4 mm, in this case 2.0 mm thick, are arranged alternately with castellated plates 14 or 15, so the castellations define channels 16 or 17. The castellated plates 14 and 15 are arranged in the stack alternately. The thickness of the castellated plates 14 and 15, typically in the range between 0.2 and 3.5 mm, is in each case 0.9 mm. The height of the castellations, typically in the range 2-10 mm, is 6 mm in each case, and solid bars 18 of the same thickness are provided along the sides. The wavelengths of the castellations in the castellated plates 14 and 15 may be different from each other, but in the embodiment shown in the figure the wavelengths are the same, so that in each case successive fins or ligaments are 7 mm apart. The castellated plates 14 and 15 may be referred to as fin structures. At each end of the stack is a flat end plate 19, which in this case is also of thickness 2.0 mm.

Although only five channels are shown as being defined by each castellated sheet 14 or 15 in figure 1 , there might be many more, for example over seventy channels in a reactor block 10 of overall width about 500 mm. The stack of plates would be assembled and bonded together typically by diffusion bonding, brazing, or hot isostatic pressing. Into each of the channels 16 and 17 is then inserted a respective catalyst insert 22 or 24 (only one of each are shown in Figure 1 ), carrying a catalyst for the respective reaction. These inserts 22 and 24 comprise a metal substrate and a ceramic coating acting as a support for the active catalytic material. The metal substrate of each insert 22, 24 comprises a stack of corrugated foils and flat foils occupying the respective flow channel 16 or 17, each foil being of thickness less than 0.2 mm, for example 100 μηι; the stacks shown in figure 1 consist of three corrugated foils separated by two flat foils, bonded together. The channels 16 and 17 in this example are 6 mm high and 7 mm wide, while the catalyst inserts 22 and 24 in this case are 5.4 mm high and 6.6 mm wide, so providing a degree of clearance from the walls of the channels 16 and 17. This is necessary to allow for tolerances in manufacture of the reactor block 10.

Referring now to figure 2 there is shown a side view of the assembled reactor block 10. The gas mixture undergoing combustion enters a header 30 at one end of the reactor block 10 (top, as shown) and after passing through a baffle plate flame arrestor 31 follows the flow channels 17 that extend straight along most of the length of the reactor 10. Towards the other end of the reactor block 10 the flow channels 17 change direction through 90° to connect to a header 32 at the side of the other end of the reactor 10 (bottom right as shown), this flow path being shown as a broken line C. The gas mixture that is to undergo the steam methane reforming reaction enters a header 34 at the side of the one end of the reactor block 10 (top left, as shown), passes through a baffle plate 35 and then changes direction through 90° to flow through the flow channels 16 that extend straight along most of the length of the reactor block 10, to emerge through a header 36 at the other end (bottom, as shown), this flow path being shown as a chain dotted line S. The arrangement is therefore such that the flows are co-current; and is such that each of the flow channels 16 and 17 is straight along most of it length, and communicates with a header 30 or 36 at an end of the reactor block 10, so that the catalyst inserts 22 and 24 can be readily inserted before the headers 30 or 36 are attached. Each of the flat plates 12 shown in figure 1 is, in this example, of dimensions

500 mm wide and 1 .0 m long, and that is consequently the cross-sectional area of the reactor block 10. Referring now to figure 3a there is shown a plan view of a portion of the reactor block 10 during assembly, showing the castellated plate 15 (this view being in a plane parallel to that of the view of figure 2). The castellated plate 15 is of length 800 mm, and of width 460 mm, and the side bars 18 are of width 20 mm. The top end of the castellated plate 15 is aligned with the top edge of the flat plate 12, so it is open (to communicate with the header 30). One of the side bars 18 (the left one as shown) is 1 .0 m long, and is joined to an equivalent end bar 18a that extends across the end. There is consequently a 180 mm wide gap at the bottom right-hand corner (to communicate with the header 32). The rectangular region between the bottom end of the castellated plate 15 and the end bar 18a is occupied by two triangular portions 26 and 27 of castellated plate: a first portion 26 has castellations parallel to the end bar 18a, and extends to the edge of the stack (so as to communicate with the header 32), whereas the second portion 27 has

castellations parallel to those in the castellated plate 15.

Referring to figure 3b there is shown a view, equivalent to that of figure 3a, but showing a castellated plate 14. In this case the castellated plate 14 is again of length 800 mm, and of width 460 mm, and the side bars 18 are of width 20 mm. The bottom end of the castellated plate 14 is aligned with the bottom edge of the flat plate 12, so it is open (to communicate with the header 36). One of the side bars 18 (the right one as shown) is 1 .0 m long, and is joined to an equivalent end bar 18a that extends across the end. There is consequently a 180 mm wide gap at the top left- hand corner (to communicate with the header 34). In the rectangular region between the top end of the castellated plate 14 and the end bar 18a there are triangular portions 26 and 27 of castellated plate: a first portion 26 has castellations parallel to the end bar 18a, and extends to the edge of the stack (so as to communicate with the header 34), while the other portion 27 has castellations parallel to those in the castellated plate 14.

It will be appreciated that many other arrangements of portions of castellated plates may be used to achieve this change of gas flow direction. For example the castellated plate 15 and the portion of castellated plate 27 may be integral with each other, as they have identical and parallel castellations; and similarly the castellated plate 14 and the adjacent portion of castellated plate 27 may be integral with each other. Preferably the castellations on the triangular portions 26 and 27 have the same shape as those on the channel-defining portions 14 or 15. In some cases the triangular portions 26 and 27 may be omitted, to leave a gas distribution space between the flat plates 12 through which the gas flows between the end of the castellated plate 14 or 15 and the header 32, 34 at the side of the block 10. As mentioned previously, after the stack of plates 12, 14, 15 has been assembled, catalyst inserts 22 and 24 are inserted into the reaction channels 16 and 17. The catalyst inserts 24 for insertion in the channels 17 for combustion are of length 800 mm and incorporate active catalytic material along 600 mm of their length, corresponding to the bottom three-quarters of the straight channels as shown in plan in figure 3a, this portion being indicated by the arrow P. An adjacent portion Q1 of length 150 mm of the inserts 24 is of low catalytic activity, while the remaining portion Q2, closest to the inlet, of length 50 mm, incorporates active catalytic material.

Similarly in the channels 16 for the steam reforming gas mixture S the catalyst inserts 22 are of length 800 mm, and as indicated by the arrow R active catalytic material is provided along the portion occupying the upper three-quarters of the straight channels as shown in plan in figure 3b; the other 200 mm of the length of the inserts 22 as indicated by the arrow Q3 are non-catalytic. After inserting the catalyst inserts 22 and 24, a wire mesh or grille (not shown) may be attached across the bottom end of the reactor block 10 so that the catalyst inserts 22 do not fall out of the flow channels 16 when the reactor block 10 is in its upright position (as shown in figure 2). It will hence be appreciated that the major part of the active catalytic materials on the inserts 22 and 24 are present in those portions P and R of the flow channels 16 and 17 which are immediately adjacent to each other.

It will be appreciated that headers 30, 32, 34 and 36 might then be attached to the reactor block 10. Alternatively it may be more convenient to provide a reactor of larger capacity, and this may be achieved by combining several such reactor blocks 10 together, before attaching headers.

As indicated above the inserts 22 and 24 comprise a metal substrate and a ceramic coating acting as a catalyst or catalyst support. The metal substrate of each insert 22, 24 comprises a stacked assembly of corrugated foils and flat foils occupying the respective flow channel 16 or 17, each foil being of thickness less than 0.2 mm, for example 50 μηι or 100 μηι; the assemblies shown in figure 1 consist of three corrugated foils separated by two flat foils, bonded together. The total length of each insert 22 and 24 is 800 mm, and all the foils are 800 mm long in this example.

Referring now to figure 4 there is shown a side view of an insert 24 on a larger scale, and with the foils separated for clarity. The insert 24 consists of three corrugated foils 41 separated by two flat foils 42. Each foil is of the appropriate width to fit in the corresponding channel, 6.6 mm wide in this example. The foils are spot welded together at multiple positions 40. Each foil 41 and 42 is of Fecralloy steel alloy, and is heat treated an oxygen-containing atmosphere so as to have an alumina surface. The portion Q1 which is to be of low activity is not treated further, but the other portions P and Q2 are provided with a surface coating of ceramic, such as alumina, which bonds to the surface and provides a catalyst support. The portion Q2 is then impregnated with 10% palladium oxide, while the portion P is impregnated to a level of 10% with a mixture of platinum and palladium oxide.

Hence, in use, as the combustible gas mixture flows into the channels 17 for combustion, combustion is initiated in the portion Q2, so the combustion gas mixture is heated up. The next portion, Q1 , has only low catalytic activity, typically about 0.1 times as much that of the other portions Q2 and P, so combustion occurs only to a small extent, maintaining an elevated temperature. If the gas mixture to undergo reforming in the channels 16 is supplied at an elevated temperature, the reactor can be operated such that there is little or no loss of heat from the reforming gas mixture as it flows through the distribution region that links the header 34 to the channels 16. This is achieved by the limited degree of combustion due to the portions Q1 and Q2 in the combustion channels adjacent to the distribution region. In a modification the reactor 10 may also include a non-catalytic insert (not shown) adjacent to the inlet of each channel 17 for combustion, the non-catalytic insert comprising one or more foils which subdivide the flow channel 17 adjacent to the inlet into narrow flow paths which are narrower than the maximum gap size for preventing flame propagation. This non-catalytic insert may be made of a stainless- steel which does not form an alumina surface. It may for example be of length 50 mm. In this case the catalytic insert 24 would have to be 50 mm shorter, to provide space for the non-catalytic insert adjacent to the inlet, for example the portion Q1 may be of length 1 10 mm, while the remaining portion Q2 is of length 40 mm.

As is described in US 2006/0035182, one parameter for assessing if a reaction channel can experience flame propagation is known as the safe quenching distance or quenching gap, which is the maximum channel width that ensures suppression of all flame propagation at a specific pressure and temperature. If the channel gap is greater than the quenching gap, flame propagation may be possible, and a flame may become a deflagration, that is a combustion wave propagating at subsonic velocity. In practice the maximum gap (at which flame propagation is suppressed) is actually significantly larger than the quenching gap, at least for channels of a rectangular cross-section, and is approximately equal to the detonation cell size. Both of these parameters depend on the nature of the flammable material, on how close the composition is to the stoichiometric ratio, and on the temperature and pressure. By way of example, for a stoichiometric mixture of hydrogen and air (as a source of oxygen) at an initial state of 1 atmosphere and about 25°C, the quench gap is about 0.1 mm, but the maximum gap size is about 5 mm. The maximum gap size with hydrogen in oxygen is about 1 .2 mm. These values for maximum gap size decrease as the temperature increases, and decrease as the pressure increases. For other fuel mixtures the values are typically larger, for example for ethane in air the quench gap is about 1 .5 mm (and the maximum gap size is about 50 mm). The maximum gap at which flame propagation is suppressed may be referred to in this document as the "limiting gap size".

Hence the appropriate size of the flow paths defined by the non-catalytic insert depends, in principle, on the nature of the combustible gases, and the temperature and pressure. Typically the flow channels in the non-catalytic insert would be of smaller cross-sectional area and smaller width than those within the catalytic insert. However, in a modification, the non-catalytic insert is integral with the catalytic insert. For example each channel 17 for combustion may contain an insert of length 800 mm comprising a non-catalytic metal support, for example consisting of corrugated and flat foils of a non-aluminium-containing stainless-steel. One end portion P of length 600 mm is provided with a coating of a ceramic such as zirconia or alumina, which is impregnated with a combustion catalyst such as a Pd/Pt mixture, as described above. An adjacent portion Q1 of length 1 10 mm is uncoated, so it has negligible catalytic activity. An adjacent portion Q2 of length 40 mm is coated with a coating of a ceramic which is impregnated with a combustion catalyst such as palladium oxide. This leaves an end portion of length 50 mm which is uncoated, so it has negligible catalytic activity. The uncoated end portion acts as a flame suppressor, so as explained above it must subdivide the flow channel 17 adjacent to the inlet into narrow flow paths which are narrower than the "limiting gap size". Typically however the flow paths are the same throughout the length of the insert, as they are defined by the size of the corrugations, so the flow paths throughout the length of the insert are narrower than the "limiting gap size".

Claims

1 . A reactor defining first and second flow channels within the reactor, each flow channel having an inlet and an outlet, with a removable combustion catalyst insert provided in each of those channels in which a combustion reaction is to occur, the combustion catalyst insert comprising a first portion including active catalytic material, a second portion having a catalytic activity less than 0.2 times the catalytic activity of the first portion, and a third portion including active catalytic material and having an activity at least 0.3 times that of the first portion, wherein the first portion, the second portion, and the third portion are progressively further from the inlet.

2. A reactor as claimed in claim 1 wherein the first portion, the second portion, and the third portion are integral with each other.

3. A reactor as claimed in claim 1 or claim 2 wherein the second portion has a catalytic activity about 0.1 times the catalytic activity of the first portion.

4. A reactor as claimed in claim 1 or claim 2 wherein the second portion has negligible catalytic activity.

5. A reactor as claimed in any one of the preceding claims wherein the third portion has an activity between 0.5 and 2.0 times that of the first portion.

6. A reactor as claimed in any one of the preceding claims wherein the length of the first portion is between 5 mm and 100 mm; and the length of the second portion is between 20 mm and 300 mm; and the third portion occupies the remainder of the length of the channel.

7. A reactor as claimed in any one of the preceding claims also including a non- catalytic insert adjacent to the inlet of each channel for combustion.

8. A catalyst insert for insertion into a combustion channel as claimed in any one of the preceding claims.