WO1993025847A1

WO1993025847A1 - Catalytic safety burner, catalyst assembly and catalyst support suitable therefor

Info

Publication number: WO1993025847A1
Application number: PCT/NL1993/000118
Authority: WO
Inventors: André RIJFERS; Johannes Jacobus Smits; Germán Enrique KNOPPERS
Original assignee: Produktcentrum T.N.O.
Priority date: 1992-06-05
Filing date: 1993-06-04
Publication date: 1993-12-23
Also published as: AU4587693A; NL9200996A

Abstract

Catalytic safety burner at least comprising a selfsupporting catalyst assembly (1) and feed means for passing an air/fuel mixture (2) to the catalyst assembly (1), the assembly (1) comprising an oxidation catalyst which may be supported, and ignition means in the form of resistance heating means (3) with connection means adapted to be connected to a power source (6) for initiating a combustion reaction near the catalyst, said resistance heating means being designed in the form of the oxidation catalyst itself, the catalyst support, or a combination thereof.

Description

Catalytic safety burner, catalyst assembly and catalyst support suitable therefor.

The present invention relates to a catalytic safety burner at least comprising a selfsupporting catalyst assembly and feed means for passing an air/fuel mixture to the catalyst assembly, the assembly comprising an oxidation catalyst which may be supported, and ignition means in the form of resistance heating means with connection means adapted to be connected to a power source for initiating a combustion reaction near the catalyst.

Such a burner is for instance known from JP-A- 62-243908. Said Japanese application relates to a catalyst combustion device in which the fuel feed is divided into two separate feeds, one for a central area and one for an outer peripheral area in order to shorten the ignition time and to enable execution of clean combustion even during migration from ignition to steady combustion. The catalyst assembly is composed of an outer catalyst layer and an inner heat insulating layer, in which resistance heating means are embedded.

Said Japanese application provides a solution for inhomogeneous heating, which iε a problem occurring with all known catalytic safety burners, comprising a catalyst assembly with resistance heating means.

The operation of the device according to JP-A- 62-243908 provides an improvement over the earlier state of the art, in which during ignition of a catalytic burner an open flame was used, usually in the form of a pilot flame, spark or the like. During ignition in such catalytic burners always an open flame was formed as a result above the surface of the catalyst, which slowly disappeared up to the moment when the catalyst was completely operative. The combustion of fuel during ignition which gives to an open flame produces a considerably risk of explosion. The use of resistance heating means has overcome this problem, however, it is very difficult to heat the catalyst in a catalytic assembly throughout the entire mass thereof homogeneously.

The construction of the burner according to JP-A- 62-243908 is relatively complicated and rather expensive. Further the homogenity of the heating of the catalyst can still be improved.

The object of the invention is to overcome the abovementioned drawbacks and to provide a generally improved catalytic safety burner with resistance heating means, and to this end the invention is characterized in that the catalyst assembly comprises resistance heating means, which are designed in the form of the oxidation catalyst itself, the catalyst support, or a combination thereof.

By designing the resistance heating means according to the invention an extremely homogeneous and very rapid heating of the catalyst is provided throughout the entire catalyst assembly, without any unnecessary additional parts or a complicated feed of the air/fuel mixture to the catalyst assembly. The resistance of the part of the catalyst assembly used as the resistance heating means i.e. the catalyst, the support or both is preferably very high, as result of which a good temperature resistance characteristic curve is obtained and in addition it becomes possible to measure the temperature of the catalyst by measuring the current passage. Control of the ignition will be discussed below.

Further embodiments of the catalytic burner according to the invention are described in the subclai ε 2-7. The invention will be explained below in more detail with reference to the accompanying drawing, in which:

Figure 1 shows a diagrammatic representation of a catalytic burner according to the invention; Figure 2 shows an embodiment of a catalytic burner according to the invention; and

Figure 3 shows a top view of a gauze-like catalyst support with connecting lugs.

In Figure 1, 1 schematically indicates the catalyst assembly which, for example, may comprise a metal gauze (Fe, Al etc.) which serves as a support for an oxidation catalyst such as, for example, palladium, platinum or another noble metal. The catalyst may also be unsupported. 2 schematically indicates an air/ fuel mixture feed. This mixture may, for example, consist of an air/gas or air/vapour mixture which is to be burnt with the aid of the catalyst 1. 3 schematically indicates the resistance heating means in the catalyst 1, through which, via lines 4 and 5, a current can be passed with the aid of a power source 6. The resistance heating means can be designed as the support of an oxidation catalyst, the oxidation catalyst itself, while being supported or not or a combination thereof. Further the heating will always be very homogeneous, as either the support, which is in close contact with the catalyst throughout the catalyst assembly or the catalyst itself is heated. 7 indicates a temperature sensor which, via a line 8, is likewise connected to the adjustable power source 6, in order to be able to measure the temperature. Many versions thereof will be obvious to those skilled in the art. For example, the temperature can alternatively be determined by measuring the current. If the resistance heating means 3 consist of a conductive material having a very high temperature- dependent resistance, this can be carried out with the aid of the generally known temperature/resistance characteristic curve. Generally, when igniting a catalytic burner according to the invention, initially an air/fuel mixture is fed in, whereupon current is passed through the resistance heating means in order to bring the catalyst, to a suitable temperature by means of resistance heating, whereupon the catalyst is very quickly able to sustain combustion autonomously. In the process, there is never any explosion hazard, as the temperature of the catalyst can be kept below the ignition temperature of the air/ uel mixture in a simple manner.

With the catalytic safety burner according to the invention the catalyst assembly is preferably composed of an oxidation catalyst supported by an electrically conducted catalyst support material forming the resistance heating means. More preferably the catalyst assembly is composed entirely of the oxidation catalyst itself, forming the resistance heating means. In the description the process for manufacturing a catalyst assembly is not described, as this is well known in the art. Various processes are commonly known for producing oxidation catalyst which may be supported. A skilled person is able to produce a catalyst assembly for use in a catalytic safety burner according to the invention. The oxidation catalyst being supported or not, the support or both must be electrically conductive and have a sufficient high resistance, to be able, to form the resistance heating means. The oxidation catalyst advantageously comprises one or more noble metals, such as for example Ru, Rh, Pd, Ir and Pt, being the most preferred catalyst.

Figure 2 shows in more detail a particular embodiment of a catalytic burner according to the inven- tion, which is suitable, for example, to be used for heating purposes. Possible applications are the heating of water or the like in places where an open fire is undesirable. A burner of this type can also be incorpor¬ ated, for example, in a hair dryer or curler or the like. This catalytic burner comprises a metal casing 9 in which, near the bottom, an air/fuel mixture feed tube 10 is introduced through a side wall. Said air/fuel mixture feed tube 10 in this case -is particularly suitable for feeding in an air/gas mixture, since air feed orifices 11 are present. These can be used for mixing air with a gas stream 12. The outflow orifice 13 is advantageously situated near the wall of the casing 9, opposite the orifice through which the air/fuel mixture feed tube 10 is passed. As a result, turbulence of the inflowing air/gas stream is achieved, which is important for mixing the air/gas mixture. The air/gas mixture then passes through a plate 14 which is provided with one or more orifices 15, by means of which the air/gas mixture is distributed uniformly within the casing. Thereafter, the air/gas mixture passes through a metal gauze 16 to achieve a yet more uniform distribu- tion, and finally through the catalyst assembly 17. The catalyst assembly 17 in this case comprises a steel gauze which is coated with a Pt oxidation catalyst. The metal gauze is clamped in a steel ring 18, with the interposition of an electrically insulating material, in this case a layer of ceramic insulation paper 19.

20 and 21 indicate two connection terminals, which can be connected to a power source, so that, in order to ignite the catalytic burner, the catalyst assembly 17, in this case the metal gauze, can be brought to the appropriate temperature by resistance heating. This temperature is not very critical and is generally below the actual operating temperature of the catalytic burner. For example, in the case -of butane gas a temperature increase up to ± 200°C is sufficient to ignite the catalytic burner. The temperature of the latter automatically rises to about 450°C after ignition. The actual operating temperature can easily be controlled by controlling the gas flow.

The current to be passed through is not very critical; it goes without saying that it should advant- ageously be as high as possible, so that heating takes place very rapidly.

In the catalytic burner according to Figure 2, no temperature sensor is shown, although this could likewise be present. A sensor of this type could be used so as to be able to regulate, using a control device, the power supply during ignition of the catalytic burner or else to be able to regulate the gas supply to the burner when the latter is working. The last-mentioned control prevents excessive temperature increase of the catalyst while the catalytic burner is working, so that possible burning of the catalyst is prevented. Also a temperature increase to above the ignition temperature of the fuel is prevented.

Figure 3 shows a top view of an embodiment of a catalyst support to be used as resistance heating means for a burner according to the invention. This comprises a steel gauze (22) with two metallic connecting lugs (23, 24) which are joined to the gauze (22) over a large part of the circumference of the gauze. This embodiment of the connection terminals has the advantage that the heating of the catalyst support and thus of the catalyst during ignition of the burner takes place uniformly, minimising local temperature increases such as would occur in the case of point-like attachment of the connection terminals. It goes without saying that this manner of attaching the connection terminals is advantageous not only in the case of a gauze-shaped catalyst support. It is generally preferable to arrange for the resistance heating to take place as uniformly as possible and therefore to join the connection terminals to the resistance heating means over a considerable portion of the catalyst.

In the following a working example of a catalytic safety burner according to the invention is given.

-Catalyst assembly: ±25 cm² expanded metal gauze

(1.5x1.0x0.3x0.2 mm) obtained from ITALFI M, Pedrengo (IT) , coated with Pt -Power source (AC) : I = 80 A, V = 24 V, Freq= 5 kHz -Ignition time : 5 s

When using the above specifications of the catalyst assembly, having the form as given in fig. 3, said assembly is brought to a temperature of about 200°C in 5 s by resistance heating, then a butane/air mixture is flown through. Depending on the flow of the butane/air mixture the temperature of the catalyst assembly can be regulated from said 200 °C to the ignition temperature of butane. For the purpose of heating a water container for e.g. a coffee L-rolley in which hot beverages can be prepared fresh each time, the temperature of the catalyst assembly is brought to about 480°C. Temperature control is established by using one or more temperature sensors near the catalyst assembly which measure the temperature. Said values are then processed by a microprocessor which compares the values to set-values and changes the gasflow if necessary.

It will be evident that, starting from the description hereinabove, many versions of the catalytic safety burner and the catalyst assembly according to the invention will be obvious to those skilled in the art. The ignition means i.e. resistance heating means according to the invention, may likewise be used for a variety of other catalytic applications, since a temperature increase of the catalyst generally has a positive effect on a catalytic reaction. Furthermore, the size of the burner is not critical. The ignition mechanism according to the invention is suitable for small domestic burners such as camping stoves, burners for heating water and the like in aeroplanes or trains, hair dryers etc. as well as for large industrial burners.

Claims

1. Catalytic safety burner at least comprising a selfsupporting catalyst assembly and feed means for passing an air/fuel mixture to the catalyst assembly, the assembly comprising an oxidation catalyst which may be supported, and ignition means in the form of resistance heating means with connection means adapted to be connected to a power source for initiating a combustion reaction near the cata¬ lyst, characterised in that the catalyst assembly comprises resistance heating means, which are designed in the form of the oxidation catalyst itself, the catalyst support, or a combination thereof.

2. Catalytic safety burner according to claim 1, characterised in that the catalyst assembly is composed of an oxidation catalyst supported by an electrically conductive catalyst support material forming the resistance heating means.

3. Catalytic safety burner according to claim 1, cha¬ racterised in that the catalyst assembly is composed entirely of the oxidation catalyst itself, forming the resistance heating means.

4. Catalytic safety burner according to claims 1 or 2, characterised in that the catalyst assembly (17) comprises an oxidation catalyst supported on steel gauze (22) .

5. Catalytic safety burner according to one or more of claims 1-4, characterised in that the oxidation catalyst comprises one or more noble metals such as, for example, Ru, Rh, Pd, Ir and Pt.

6. Catalytic safety burner according to claim 5, charcterised in that the catalyst is a Pt oxidation catalyst.

7. Catalytic safety burner according to one or more of claims 4-6, characterised in that the connection means are constructed in th form of connecting lugs (23, 24) which are joined to the gauze (22) over a large part of the circumference of the gauze (22) .

8. Catalyst assembly which may or may not comprise a support and/or be provided with connection terminals, for a catalytic safety burner according to one or more of claims 1-7.

9. Catalyst support which may or may not be provided with connection terminals, for a catalytic safety burner according to one or more of claims 1-7, or in a catalyst assembly according to claim 8.