NL1004051C2 - Catalytic radiation burner. - Google Patents

Description

Title: Catalytic radiation burner

The invention relates to a method for catalytic combustion of flammable gases, such as hydrocarbons (natural gas and the like), CO and / or H2. More particularly, the invention is directed to a method for catalytically burning flammable gases or gas mixtures with the production of indirect heat (radiant heat), wherein the gases to be burned are catalytically burned in one or more steps, after which in a last step a homogeneous gas phase combustion takes place. Radiant heat is emitted from this last step in particular.

Indirect heating, for example in ovens, using radiant burners is a suitable technique when both high temperature and clean heating are required. Gas-fired closed jet tube burners are known for this purpose, for example from applications in the metal industry. In addition, the gas flame is placed in a heat-resistant closed tube and the heat is released as heat radiation through the surface of the closed tube.

It is known that catalytic combustion of combustible gases has a number of advantages, especially with regard to reducing the formation of undesired by-products in the combustion gases. It would therefore be interesting if the combustion in a jet burner could be at least partially catalytic.

However, it has been found that this cannot simply be realized, since the combustion is not stable, that is to say that the temperature fluctuates strongly.

It is an object of the invention to provide a method for performing at least partial 1 0 0 40 5 1 2 catalytic combustion in a gas-fired radiant burner, whereby on the one hand the advantages of a catalytic combustion, such as low nitrogen oxides content, and on the other hand a stable combustion is obtained.

5 It also has the advantage of a low noise level and better heat transfer to the radiant surface.

The present invention relates to a process for the catalytic combustion of gaseous hydrocarbon, CO or H2 using an oxygen-containing gas, wherein the gases are successively burned in at least one catalytic combustion section in the presence of suitable catalytic materials and in which the final temperature of the gases after the last section is between 700 and 1000 ° C, the gases after the last section are fed to a combustion chamber, which is provided with means for emitting radiant heat, in which combustion chamber in the presence of a high temperature catalyst produces a homogeneous, at least partially non-catalytic, complete combustion occurs.

Surprisingly, it has been found that in this way an efficient, stable combustion can be obtained, whereby only a few undesirable compounds are formed.

The invention therefore consists in that after a possible preheating of the combustion gas mixture (air, combustible gas) in a catalytic part of the burner in the presence of one or more combustion catalysts, the combustible gas is partially burned, the temperature rising to a value of about 850 ° C to 1000 ° C. After this catalytic section, the gas ignites and a complete, homogeneous gas phase combustion occurs.

The catalytic section can consist of one section in which the desired final temperature is reached. This is possible if the initial temperature of the gas / gas mixture is already close to 700 ° C. In that case, it is sufficient to use one type of catalyst, a high-temperature combustion catalyst. If the initial temperature is lower, it will be necessary to use more than one type of catalyst.

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As is known, combustion catalysts have a rather limited range of optimum operating temperature due to the nature of the support materials used and the structure of the catalytically active material. The so-called 5 low-temperature catalysts have an optimal effect in the range of 300 to 750 ° C. These are usually supported palladium or platinum / palladium catalysts, the support being a ceramic material such as alumina, zirconia, silica, titania and the like. Optionally, the support 10 is stabilized against thermal and / or chemical degradation using lanthanum or silicon.

Catalysts in the range of 500 to 1000 ° C include platinum on stabilized ceramic supports, or transition metal oxides, such as copper, manganese, iron, nickel and / or cobalt, on stabilized ceramic supports. The stabilization of said carriers mainly concerns stabilization against degradation of the carrier by the high temperature and against reaction of the catalyst metal component with the carrier material using, for example, lanthanum.

Such a catalyst is described in European Patent No. 327177.

As stabilized supported transition metal oxides, catalysts for the range from 700 to 1200 ° C can also be used, as well as transition metal aluminates (such as Cu (II) aluminate, Mn (II) aluminate,

Mg-aluminate, but also CuO / Mg-aluminate), perosites (such as compounds of the type Lai-XAXB03, in which A - Sr, Ba, Ca and / or Ce, and B - Co, Mn and / or Fe) and metal hexa-aluminates (such as BaMyAli2-yOi9-a ,, where M = Al, Cr, Mn, Fe, Co and / or 30 Ni))

The catalytically active material can be arranged in the catalytic sections in the usual manner. It is of course important here that on the one hand a sufficient amount of catalyst is present, while on the other hand the pressure drop across the catalysts is small and the contact between the gas mixture and the catalyst surface is good.

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For these reasons, among others, it is therefore preferred that the catalysts be applied to the surface of porous structures such as monoliths, metal or ceramic foams, gauzes or gas mixers.

As indicated, a number of catalytic sections can be used. Seen in the direction of flow of the gas, these sections are arranged one behind the other, optionally between two sections being a non-catalytically active material to stabilize the reaction. However, it is also possible for combinations of two or more different types of catalysts to be used more or less homogeneously mixed together. Depending on the prevailing temperature, one type will be the most stable at a certain location in the catalyst bed, while another type will be more stable in another part of the bed.

At the outlet of the catalyst bed (catalyst beds) is the combustion chamber, in which the homogeneous gas phase combustion takes place. In order to promote combustion stability, it may be preferable to provide an inert porous material, for example a monolith of a non-catalytically active material, between the last catalyst bed and the combustion chamber. It has been found that such a construction is conducive to the stability of the combustion and prevents flashback.

The combustion chamber is provided with at least one wall of a material that can radiate heat to the environment through radiation. This is, for example, a wall at the end of the room or around the room. The material thereof must be able to withstand the temperature in the combustion chamber. Suitable materials include metals with good high temperature resistance and ceramic materials. More specifically, Hastalloy, Inconel 310, Fecralloy, SiC, S13N4 or corderite are used.

A homogeneous gas phase combustion occurs in the combustion chamber. This combustion can take place by spontaneous ignition of the gas-air mixture, which happens when the temperature of the gas mixture after the last catalyst bed is sufficiently high. It is preferred, however, that a high-temperature combustion catalyst is provided in the combustion chamber, which ensures ignition and stabilization. This catalyst may, for example, be applied to the wall of the chamber, or, and this is preferred, if present on a mesh in the combustion chamber.

The following figures serve to further illustrate the invention, in which various embodiments of the invention are shown in figures 1, 4 and 7, while figures 2, 3, 5 and 6 show graphical results.

Figure 1 shows a simple embodiment of a radiation burner. Air is preheated in electric preheater 1. After supplying natural gas, air and natural gas are mixed in gas mixer 2 and the resulting mixture is passed via low temperature catalyst 3 (monolith with Pd / ZrC> 2), medium temperature catalyst 4 (monolith with 20 Pt / ZrO 2), blank monolith 5 and high temperature catalyst 6 (monolith with BaMnAlnOig) fed to combustion chamber 7 fitted with a Sic radiation plate. The flue gases are discharged from the combustion chamber 7 via discharge.

Figures 2 and 3 give test results obtained using the arrangement according to figure 1.

Figure 4 shows another embodiment of the radiant burner according to the invention. This differs from the embodiment according to figure 1 in that the high-temperature catalyst is applied to a mesh 9, for example of FeCrAlloy, which is arranged in the combustion chamber between the supply of the gas and the discharge of the flue gases.

Figures 5 and 6 give test results obtained using the arrangement according to figure 4.

Figure 7 shows an application of the burner according to the invention, in which the surface for radiant heat emission is formed by a closed tube 8, which is situated around the combustion chamber 7. The air / natural gas mixture can be partially burned in a start-up burner 10 to obtain a suitable initial temperature, after which the gases are heated in heat exchanger 11.

In this figure, an already existing concept of a jet burner (so-called SER; Single Ended Recuparator) has been converted with high efficiency into a catalytic operating system.

Further improvements in the burner according to the invention are possible by increasing the heat transfer between the catalyst surface and the gas mixture in catalysts 3 and 4 by: 1) Increasing linear speed in monolith channels:

High linear gas velocities in catalyst 3 and 4 15 (5-50 m / sec). This high speed provides better heat and mass transfer between the catalyst and the gas mixture, which reduces the risk of overheating in the catalysts. However, if you want to achieve the same conversion, the length of the system will also have to be adjusted.

20 2) Increase turbulence of the gas mixture in the monolith channels:

Higher turbulence of the gas mixture in the catalytic system ensures better heat and mass transfer. This can be accomplished by several ways:

Using the so-called "entrance" effect. When the gas mixture enters the monolith, an increased turbulence of the gas mixture is created. As a result of this increased turbulence, better heat and mass transfer occurs in the beginning of the monolith channels, resulting in a lowering of the catalyst temperature.

This increased turbulence can be created by, for example:

Divide catalysts 3 and 4 into small monolith 35 segments of eg 1-2 cm length and place them in series with a distance of eg 0.5 cm.

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Instead of a mutual distance, the segments can also be rotated about the axial axis with an angle of, for example, 45 degrees.

Using a geometry other than 5 monoliths, such as gas mixers (especially the so-called Sulzer Mixer: Katapak), porous sintered metal, porous ceramic / metallic foam and gauze.

The efficiency of the system can be increased by 1) Increasing the radiation efficiency: 10 Increasing the radiation efficiency can be done by increasing the emission factor of the radiating surface.

SiC already has an emission factor of 0.9 ± 0.05 2) It can also be done by increasing the temperature of the radiant surface. The latter can be achieved by creating a better heat transfer between the gas mixture and the SiC plate by eg attaching fins to the inside of the plate along which the gas mixture is guided.

3) Another possibility is the catalyst.

Bring active mesh as close to the SiC plate as possible.

4) The best result is achieved by applying the high temperature catalyst directly to the inside of the SiC plate.

5) Increase total efficiency: 25 With the aid of a heat exchanger between the exiting gas mixture and the incoming (possibly premixed with natural gas) cold air, this air can be heated to the required stop temperature of the first catalyst.

The efficiency can be further increased by using a heat exchanger system in which the incoming mixture is heated to the attack temperature of the medium or high temperature catalyst. This makes the upstream low-temperature catalyst superfluous.

35 The burner can be started at a low air factor (1.1 <n <1.6) by an ignition mechanism in the combustion chamber. Once the temperature after the heat exchanger has reached the stop temperature of Catalyst 3, the required temperatures in the overall system can be adjusted by controlling the air supply.

Another possibility is to place a start-up burner in front of the heat exchanger and after the attack temperature of catalyst 3 has been reached, the burner can be switched off.

The monitoring system can consist of thermocouples in the most critical places, such as after the heat exchanger and 10 in the combustion chamber.

The invention is illustrated by means of a comparative example (Example 1) and an example according to the invention (Example 2).

15 Example 1

The following test was carried out using the burner according to Figure 1.

In preheated air (600 Nl / min) at 400 ° C, 20 natural gas is mixed to 3.5 vol.% And after passing a gas mixer, the combustion mixture is passed through a catalytic system at a linear speed of 20 m / s, followed by a cylindrical closed combustion chamber.

Before the gas mixture leaves the combustion chamber, part of the heat is transferred to a SiC plate. The catalytic system consists of three series-connected catalysts. To measure the temperature of the catalysts and of the effluent gas mixture, thermocouples are placed in the catalysts and a thermocouple placed in a blank monolith is also present between catalysts 4 and 6 to measure the temperature of the gas mixture at that location. The gas temperature is also measured after catalyst 6.

The results are shown in Figure 2.

The natural gas is 54% catalytically converted into catalysts 3 and 4. As a result, the temperature of these catalysts increases to 800 ° C and 1000 ° C, respectively. The temp. the outgoing gas mixture is then 860 ° C. This gas mixture is then further catalytically converted to a conversion of 64% 1 ö 4 0 1 1 9 9 and leaves the catalytic system at a temperature of 940 ° C. The temperature of catalyst 6 is 1000 ° C.

The residual amount of natural gas is then converted to 99.9% in the combustion chamber via the homogeneous gas phase 5 reaction and part of the heat created is transferred to the SiC radiation plate. The average external temperature of the plate is 650 ° C.

The drawback of this system is that it produces an irregular, oscillating combustion pattern. This is shown in figure 3. The result is that temperatures can rise to 1200 ° C in the catalysts. These temperatures are much too high, in particular for catalysts 3 and 4, which results in a decrease in the activity of the catalysts caused by sintering and evaporation of the catalytic material.

This oscillatory behavior can be caused by various effects, including the transition known from literature from the active palladium oxide to the inactive metallic palladium.

Another effect is the so-called blowback effect of the flame in the catalyst. This impact of the flame can be caused by the radiation of heat from the combustion chamber back to the catalysts. The catalysts thereby become so hot that the homogeneous gas phase reaction already takes place in the catalyst. This can create a run-away effect.

Example 2 30 To suppress this last effect, the following changes have been made in example 2 (see figure 4) ·

A blank monolith is placed between Catalyst 4 and the combustion chamber. The high temperature catalyst 6 is mounted on a mesh and placed in the combustion chamber.

The result is shown in figure 5.

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The temperatures of catalysts 3 and 4 are now 780 ° C and 860 ° C, respectively. This is considerably lower than in example 1.

The outgoing gas mixture has a temperature of 5740 ° C and a conversion of 35% has been reached. This residual gas mixture comes into contact with the catalytically active mesh in the combustion chamber and is completely converted by both the mesh and the homogeneous gas phase reaction. The temperature of the gauze (catalyst 6) is 1050 ° C and the temperature in the combustion chamber is on average 950 ° C. The external temperature of the Sic plate is 650 ° C.

The oscillatory behavior of the temperature in the catalytic system consisting of catalysts 3 and 4 has been greatly reduced as a result of these adaptations (see figure 6).

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Claims (16)

A method for the catalytic combustion of gaseous hydrocarbon, CO or H2 using an oxygen-containing gas, wherein the gases are successively burned in at least one catalytic combustion section in the presence of suitable catalytic materials and in which the final temperature of the gases after the last section is between 700 and 1000 ° C, the gases after the last section are fed to a combustion chamber, which is provided with means for radiating heat, in which combustion chamber in the presence of a high temperature catalyst produces a homogeneous, at least partly non catalytic, complete combustion occurs. 2. Process according to claim 1, wherein the combustion chamber contains a mesh, the surface of which is catalytically active for high temperature combustion. The method of claim 1, wherein at least a portion of the surface of the combustion chamber wall is catalytically active for high temperature combustion. Method according to claims 1-3, wherein the gases ignite in the combustion chamber. 5. Process according to claims 1-4, wherein as combustion catalyst use is made of one or more catalysts selected from the group consisting of palladium on a support, platinum / palladium alloy on support, oxides of transition metals, aluminates of transition metals and perovites. 6. Process according to claim 5, wherein a platinum / palladium or a palladium catalyst is used for a temperature range of 300 to 750 ° C. The method according to claim 5 or 6, wherein for a temperature range of 500 to 1000 ° C, a platinum or a transition metal oxide is used on a stabilized support. A method according to claims 5-7, wherein for a temperature range of 700 to 1200 ° C an aluminate of a transition metal or a perosite is used. 9. A method according to claims 1-8, wherein the combustion gases originating from the combustion chamber are used at least in part for preheating the gas and / or gas mixture to be burned, prior to the first catalytic section. 10. A method according to claim 9, wherein the heat transfer takes place via the wall of the combustion chamber and / or the catalytic sections. 11. Process according to claims 1-10, wherein the catalytic combustion sections consist of porous materials, preferably monoliths, sponges, meshes, gas mixers, the surface of which is catalytically active for the combustion of hydrocarbons. 12. Process according to claims 1-11, wherein a porous material, which is non-catalytically active material for combustion, is located between two catalytic sections and / or between the last catalytic section and the combustion chamber. A method according to claims 1-9, wherein the means for delivering radiant heat consist of at least one ceramic material and / or metals or metal alloys resistant to high temperatures. A method according to claim 13, wherein as ceramic material use is made of SiC, S13N4 and / or corderite. Method according to claim 13, wherein as metal or metal alloy use is made of Hastalloy, 310 and / or Fecralloy. A catalytic radiant burner for burning flammable hydrocarbon gases using the process of any one of claims 1-15, comprising means for supplying a flammable gas and an oxygen-containing gas to a first catalytic section, which contains a first combustion catalyst, means for supplying gases from said first catalytic section to a second catalytic section, which contains a second combustion catalyst, means for supplying gases from a last catalytic section to a combustion chamber having a surface catalytically active therein for combustion of hydrocarbon gases, and means for delivering radiant heat to the environment. 1 ü 0 G j -