EP0090745A1 - Burner for a catalytic heater - Google Patents

Abstract

This burner is of the type comprising a heating member (13), in the form of a metal basin, equipped with a gas supply system (14), in which there is mounted a support (15) of the catalytic mass, the active mass or catalytic mass and a holding frame (17) for the assembly. According to the invention, the properties of the catalytic mass (16) are such that the latter also carries out the function of diffusion system. <IMAGE>

Description

The present invention relates to a burner for catalytic heating apparatus.

A catalyzed heating appliance essentially comprises a burner allowing the catalytic oxidation at low temperature of a gaseous fuel or one brought into the gaseous state, such as a hydrocarbon.

Figure 1 of the accompanying schematic drawing shows a mobile catalysis heater (2), comprising a housing (3) inside which is mounted, behind a protective grid (4), a burner which is shown in more detail , in exploded perspective, in Figure 2. This burner comprises a heating body (5) consisting of a metal bowl, in the bottom of which is fixed a gas supply system consisting of a coil (6) pierced with orifices . In the heating body is fixed a metal support (7) consisting of a perforated sheet, on which rests a diffusion system (8) consisting of a mattress of mineral fibers. On the diffusion system (8) is pressed the catalytic mass or active mass (9) generally constituted by refractory mineral fibers or other materials, on which the catalyst is deposited. A metal frame (10) carrying a grid (12) constituted by longitudinal elements and transverse elements keeps the assembly in place.

• Le gaz combustible s'écoule par le serpentin placé au fond du corps de chauffe (5), à l'intérieur du vide ménagé entre le fond du corps de chauffe et le support métallique (7), qui joue le rôle de chambre de détente et contribue à la répartition du flux gazeux au sein du brûleur. Le gaz pénètre ensuite dans le matelas de fibres minérales, qu'il sature entièrement avant de passer dans la masse active ou masse de contact dans laquelle la réaction a lieu.

The operation of such a device is as follows:

• The combustible gas flows through the coil placed at the bottom of the heating body (5), inside the vacuum formed between the bottom of the heating body and the metal support (7), which acts as an expansion chamber. and contributes to the distribution of the gas flow within the burner. The gas then enters the mineral fiber mat, which it completely saturates before passing into the active mass or contact mass in which the reaction takes place.

In order for the catalytic reaction to start and then to maintain itself, it is necessary to have recourse to a device consisting in preheating the catalytic mass, which is generally carried out using gas passing through the catalytic mattress.

For this purpose, the gas flow rate is considerably increased until reaching a value corresponding to three times the nominal burner flow rate, for a period of time between 30 seconds and 1 minute. This ignition flow is called the choke flow.

As soon as it appears, on the surface of the catalytic mass, the gas flow is ignited, usually by a pilot, the flames then disappearing to make way for flameless catalytic combustion.

This technique used by all manufacturers has drawbacks due to the need for a perfectly suitable texture of the diffusing material, the excess gas during the ignition period and the permeability of the system.

Indeed, the texture of the diffusing material should be neither too compact nor too light, but perfectly adapted to the texture of the active mass, in order to avoid irregularities in the passage of gas resulting in a poor distribution of the charge in gas in the active mass, and consequently by a significant drop in the yield of the catalytic reaction.

Excess gas during the start-up period is essential if the catalytic mass is to be preheated under conditions acceptable to the user. Indeed, too limited a gas flow during the start-up period can result in a significant delay in ignition, resulting in an unpleasant explosion. The significant increase in gas flow during the start-up period quickly creates a significant concentration of gas at the base of the burner, which ignites before the dispersion of the gas flow is too great.

However, obtaining a gas flow rate greater than the nominal burner flow rate requires the use of a specific system and sometimes the creation of a special gas circuit. This leads to an increase in the complexity of the burner valves, which is not favorable from an economic point of view. In addition, a gas flow rate higher than the nominal burner flow rate appreciably prolongs the start-up period, since a catalytic burner cannot, even for a short time, properly digest an excess of gas and only strong proportions of unburnt gas are released into the atmosphere. As a result, the user is often obliged to completely interrupt the flow of gas for a period of time of the order of 1 minute. This intervention is often felt by the user as an easement characterizing catalytic heaters.

Even with a judicious choice of the gas diffusion system, this causes more or less significant compaction of the entire catalytic burner. This settlement printed on the active mass removes to the latter a certain degree of permeability, affecting the penetration of the combustion air within the mass, which results in a slowing down of the catalytic activity in the deep zone of the active mass. As part of this mass is not used correctly, the possibilities of the burner are considerably reduced.

The present invention aims to remedy these drawbacks.

To this end, as shown in Figure 3 of the accompanying schematic drawing which is an exploded perspective view, the burner of the catalyzed heating appliance to which it relates, of the type comprising a shaped heating body (13) metal bowl, equipped with a gas supply system (14), in which is mounted a support (15) for the catalytic mass, the active mass or catalytic mass and a retaining frame (17) for the assembly , is characterized in that the properties of the catalytic mass (16) are such that it also fulfills the function of diffusion system.

According to a characteristic of the invention, the catalytic mass (16) is mounted without compression inside the burner. Therefore, the frame ensuring its holding consists of a simple peripheral frame without longitudinal or transverse bars.

In addition, the gas supply system is significantly simplified since it consists of a straight ramp, or a simple nut mounted in the bottom of the bowl forming the heating body.

The active mass can rest without compression on a metal support constituted by a perforated sheet (15), or have an integrated reinforcement ensuring its own resistance independently of any external element.

In order to have good diffusion properties, the catalytic mass comprises elements such as fibers, beads, son, chips, any agglomerates, etc., of refractory materials, regularly distributed in a mattress of 5 to 15 millimeters thick, having a specific surface at least equal to 1 m ² / g, having good chemical and mechanical stability in an oxidizing atmosphere up to at least 800 ° C, the elements being arranged so as to provide 40 to 60 vacuum volumes for a full volume .

Insofar as the support of the active mass consists of ceramic fibers, these have a diameter of two to three microns and a density between 2.5 and ³ kg / dm ³ .

The operation of this burner is as follows: the combustible gas is distributed in the bottom of the heating body (13) by a straight tube (14a) pierced with small diameter orifices, or by a simple pierced nut (14b), as shown in mixed lines in the drawing. The gas fills the vacuum existing between the catalytic mass (16) and the bottom of the heating body (13) and comes into contact directly with the catalytic mass, the uniformly distributed gas flow passing through the latter, in contact with which the reaction takes place. catalytic oxidation.

The elimination of the diffusion mattress allows the entire contact mass to work under optimal oxygenation conditions, which results in particular from the absence of compression on the catalytic mass.

In addition, the penetration of the combustion air within the contact mass is favored by a judicious choice of the density and the thickness of the catalyst support.

Therefore, the catalytic reaction no longer takes place only on the surface of the contact mass, but also in depth, on the internal face of the contact mass, and propagates over the entire thickness of the mattress.

Therefore, for the same power per cm ² , the working temperature is 50 ° C higher than that of the burner described above. This results in a significant increase in the catalytic yield which commonly reaches 99.5%.

This improvement in efficiency also makes it possible to lower the level of the idle speed of the burner, which has an advantage for the user as well as an economic advantage.

Another advantage of the device according to the invention relates to the ignition and, consequently, the warming up of the contact mass. The suppression of the gas diffusion layer and consequently the greater permeability of the contact mass allows an extremely rapid reaction initiation and a great aptitude for propagation. This gain in catalytic activity is directly linked to the greater ease given to the reactants to mix as soon as they come into contact with the catalytically active substance.

This ease of initiation and propagation of the catalytic reaction allows the commissioning of a burner without choke function, that is to say without increasing the gas flow, the quantity of corresponding gases. satisfying the rated power of the device being sufficient. The elimination of the choke function therefore results in a simplification of the fittings.

For the user, ignition is simplified and the risks of misfires which result in the release of unburnt gases and, consequently, of odors, are considerably reduced.

The suppression of a higher gas flow during the ignition phase makes it possible to prolong the activity of the catalyst and to increase the reliability of the burner. Indeed, repeated ignitions of the catalytic mass using large quantities of gas lead, due to the abnormal rise in temperature at certain points in the mass, phenomena of premature recrystallization of the catalyst resulting in a loss of partial activity.

As regards the industrial use of catalytic burners, the new ignition possibilities bring important simplifications in terms of automatic gas distribution valves, the choke valve, timers and the electrical circuit associated with them being eliminated, which translates into substantial savings.

As is apparent from the above, the invention brings a great improvement to the existing technique by providing a catalytic heating device from hydrocarbons, such as methane, ethane, butane, propane, heptane, having a simplified structure and excellent performance, suitable for domestic, industrial and agricultural applications.

It goes without saying that the invention is not limited to the sole embodiments of this heating appliance described above by way of examples; on the contrary, it embraces all of its variant embodiments.

Thus, in particular, the active mass could not be associated with a support constituted by a perforated sheet, but present an internal frame, without thereby departing from the scope of the invention.

Claims (8)

1.- Burner for catalytic heating apparatus, of the type comprising a heating body (13), in the form of a metal bowl, equipped with a gas supply system (14), in which a support (15) is mounted ) of the catalytic mass, the active mass or catalytic mass and a retaining frame (17) of the assembly, characterized in that the properties of the catalytic mass (16) are such that it also fulfills the function of system broadcast. 2.- Burner according to claim 1, characterized in that the catalytic mass (16) is mounted without compression inside the burner. 3.- Burner according to any one of claims 1 and 2, characterized in that the frame (17) ensuring the maintenance in the assembled position of the various constituent parts of the burner is constituted by a simple peripheral frame without longitudinal or transverse bars. 4.- Burner according to any one of claims 1 to 3, characterized in that the gas supply system in the heating body (13) is constituted by a straight ramp (14a). 5.- Burner according to any one of claims 1 to 3, characterized in that the gas supply system in the heating body (13) is constituted by a nut (14b). 6.- Burner according to any one of claims 1 to 5, characterized in that the support of the catalytic mass is constituted by a frame integrated into this catalytic mass. 7.- Burner according to any one of claims 1 to 6, characterized in that the catalytic mass comprises elements such as fibers, balls, son, chips, any agglomerates, etc., of refractory materials, regularly distributed in a mattress from 5 to 15 millimeters thick, having a specific surface at least equal to 1 m ² / g, having good chemical and mechanical stability in an oxidizing atmosphere up to at least 800 ° C, the elements being arranged so as to spare 40 to 60 volumes of vacuum for a full volume. 8. Burner according to claim 7, characterized in that, insofar as the support of the active mass consists of ceramic fibers, these have a diameter of two to three microns and a density of between 2, 5 and 3 kg / dm ³ .

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