EP0784187A1 - Catalytic combustion system with staged fuel injection - Google Patents

Catalytic combustion system with staged fuel injection Download PDF

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Publication number
EP0784187A1
EP0784187A1 EP96402804A EP96402804A EP0784187A1 EP 0784187 A1 EP0784187 A1 EP 0784187A1 EP 96402804 A EP96402804 A EP 96402804A EP 96402804 A EP96402804 A EP 96402804A EP 0784187 A1 EP0784187 A1 EP 0784187A1
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Prior art keywords
monolithic
monolith
catalyst
fuel
combustion
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EP96402804A
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German (de)
French (fr)
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EP0784187B1 (en
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Jean-Hervé Le Gal
Gérard Martin
Patrick Euzen
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IFP Energies Nouvelles IFPEN
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IFP Energies Nouvelles IFPEN
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
    • F23C13/00—Apparatus in which combustion takes place in the presence of catalytic material
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
    • F23C6/00—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion
    • F23C6/04—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion in series connection
    • F23C6/045—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion in series connection with staged combustion in a single enclosure
    • F23C6/047—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion in series connection with staged combustion in a single enclosure with fuel supply in stages

Definitions

  • the present invention relates to a catalytic combustion system with stepped fuel injection and using a non-selective oxidation catalyst.
  • Catalytic combustion is an attractive solution to meet the increasing severity of pollutant standards.
  • the catalytic combustion chamber advantageously replaces conventional burners because it allows better control of total oxidation in a wide range of values of the air / hydrocarbon ratio, thus greatly reducing the emissions of nitrogen oxides, unburnt hydrocarbons and carbon monoxide. We can also mention that it burns a very wide variety of compounds.
  • the most common reactor configuration is a reactor comprising several catalytic zones: the inlet catalyst (s) being more specifically dedicated to initiating the combustion reaction, the following serving to stabilize the combustion reaction at high temperature; the number of catalytic stages (or zones) being adjusted according to the conditions imposed by the envisaged application.
  • Combustion catalysts are generally prepared from a monolithic substrate, ceramic or metal, on which a thin support layer is deposited, consisting of one or more refractory oxides with a surface and porosity greater than that of the monolithic substrate. On this oxide is dispersed the active phase mainly composed of platinum group metals.
  • the platinum group metals exhibit the highest catalytic activity for the oxidation of hydrocarbons and therefore initiate combustion at a lower temperature than the oxides of the transition metals. They are therefore preferably used in the first catalytic zones. However, due to the high temperatures reached either during the start-up phases or in steady state, these catalysts undergo a degradation which reduces their catalytic performance.
  • the sintering of the alumina-based support as well as the sintering of the active metallic phase and / or its encapsulation by the support are among the causes most commonly cited to explain this degradation.
  • the catalyst comprises platinum group metals or transition metals deposited on alumina, an oxide of a metal chosen from the group consisting of barium, lanthanum and strontium and an oxide of a metal chosen from the group consisting of tin, silicon, zirconium and molybdenum.
  • the critical point of the known multi-stage processes lies in the control of the temperature within the various catalytic stages. If the combustion reaction gets carried away, the temperature of the catalyst can quickly reach the temperature flame adiabatic. However, it is important to cover the entire load range of the gas turbine. From the ignition process to full load, passing the idle, the air-fuel ratio can vary significantly. The use of such a catalytic combustion chamber can therefore prove to be difficult.
  • the rest of the mixture is injected downstream of the catalytic zone in order to reach a combustion gas temperature compatible with the requirements of current combustion processes, ie 1200 ° C. to 1500 ° C. Due to the limitation of material temperatures to 1000 ° C, the catalyst is not deactivated.
  • This process is interesting insofar as it offers greater security than that of a process in which the temperature is controlled solely by the geometry of the monolith or of all of the monoliths. In the start-up phases, it is also more flexible to implement.
  • the subject of the present invention is a catalytic combustion system comprising an envelope having an inlet for an oxidizer such as air, several fuel injection means intended to carry out a staged injection of fuel, and at least a first monolithic element capable of being covered with a combustion catalyst placed downstream of a first fuel injection means relative to the direction of progression of an air-fuel mixture in the system, said first injection means performing a partial fuel injection.
  • an oxidizer such as air
  • first monolithic element capable of being covered with a combustion catalyst placed downstream of a first fuel injection means relative to the direction of progression of an air-fuel mixture in the system, said first injection means performing a partial fuel injection.
  • said system further comprises a second monolithic element disposed downstream of a second injection means, said second monolithic element being intended to stabilize combustion.
  • the second monolith element can be coated with a combustion catalyst, such as a hexaaluminate.
  • the first or the second monolith can comprise a plurality of elementary monoliths covered with catalysts based on precious metals.
  • the amount of fuel introduced by the second injection means is such that the adiabatic flame temperature does not exceed 1100 ° C. Furthermore, the quantity of fuel introduced by the first injection means is such that the adiabatic flame temperature does not exceed 900 ° C.
  • the first monolith element is such that the catalyst which covers it is capable of withstanding temperatures below 900 ° C. and that the temperature of the gases leaving the first monolith element is less than about 800 ° C.
  • the second monolith element is such that the catalyst which covers it is capable of withstanding temperatures below 1100 ° C. and that the temperature of the gases leaving the second monolith element is below 1000 ° C.
  • the catalyst combustion system comprises a third monolith element placed downstream of the second monolith element, with a third fuel injection means is disposed between the second and the third monolith element.
  • said catalyst essentially consists of a formula described by the applicant in French patent application FR-A-2 721 837, said formula being: A (1-x) B y C z Al (12-yz) O (19- d) in which A represents at least one element of valence X selected from the group formed by barium, strontium and rare earths; B represents at least one element of valence Y selected from the group formed by Mn, Co and Fe; C represents at least one element selected from the group formed by Mg and Zn; x having a value of 0 to 0.25, y having a value of 0.5 to 3 and z having a value of 0.01 to 3; the sum y + z having a maximum value of 4 and da a value which, determined according to the respective valences X and Y of the elements A and B and the value of x, y and z, is equal to 1-1 / 2 ⁇ (1-x) X + yY - 3 y -z ⁇ .
  • the catalyst covering the first monolith element comprises palladium oxide.
  • the catalyst covering the first monolith element can comprise a refractory oxide support and an active phase comprising, in weight percentage relative to said support: from 0.3 to 20% of cerium, from 0.01 to 3.5% iron and from 1 to 10% of at least one element chosen from platinum and palladium.
  • the envelope is essentially cylindrical, the inlet for the oxidant and the first injection means is located at one of its ends on a cross section, said second and / or third injection means open out. longitudinally in said cylinder.
  • At least one of the monolithic elements can be annular; for example, the first and second monolithic elements can be annular.
  • At least one of the monolithic elements is cylindrical and occupies the entire cross section of said cylinder.
  • all the monolithic elements are cylindrical.
  • the annular monolithic elements are arranged concentrically between an end disc equipped with oxidizer and fuel inlets and a bottom allowing the exit of gases, the inlet disc having a larger surface area. as the bottom, so that the movement of gases inside said cylinder is essentially radial.
  • a casing 1 has an inlet 2 through which are introduced a first oxidizing fluid (here air) and a fuel via a first injection means 3.
  • the first injection means 3 makes it possible to inject only part of the fuel necessary for total combustion.
  • a first monolithic element 4 Downstream of the first injection means 3, relative to the direction of progression of the fluids in the envelope 1 and indicated by the arrows A, a first monolithic element 4 is arranged.
  • the monolith element can comprise n sections (or sections) spaced or not spaced from each other.
  • the substrate of the monolith can be ceramic or metallic, with a honeycomb geometry.
  • the substrate is covered with a catalytic layer of precious metals such as palladium.
  • the first monolith element 4 thus allows a gradual increase in the temperatures of the gas passing through it and of the catalytic layer. Only a fraction of the fuel is burned there.
  • a second fuel injection means 5 intended to inject, according to this embodiment of the invention, the rest of the fuel necessary for total combustion.
  • a second monolithic element 6 is placed downstream of the second fuel injection means 5.
  • the second monolithic element 6 can comprise m sections (or sections) spaced or not spaced from one another.
  • the substrate can be ceramic or metallic and be covered with a catalytic layer preferably consisting of hexaaluminates.
  • the catalyst of the second monolith element must be able to withstand temperatures of up to 1100 ° C., in order to be able to bring the gases which pass through the second monolith to their auto-ignition temperature, ie around 1000 ° C.
  • the ignition initiation temperatures are then of the order of 700 ° C. almost temperature above 600 ° C.
  • the first catalytic stage must be such that the temperature of the gases at its outlet is between 650 ° C. and 800 ° C.
  • the catalyst used in the first monolith element 4 comprises palladium oxide. Knowing that this is reduced to metallic palladium above about 900 ° C. (value variable with pressure), the fuel flow rate is adjusted in 3 so that the temperature of the deposited catalyst is less than about 900 ° C, in order to avoid the transformation of palladium oxide. We will simultaneously try not to achieve self-ignition in monolith 4; thus the temperature of the gases leaving said monolith must then be less than approximately 800 ° C.
  • the catalyst which covers the first monolithic element comprises a refractory oxide support and an active phase comprising in weight percentage relative to said support: from 0.3 to 20% of cerium, from 0.01 to 3.5% of iron and from 1 to 10% of at least one element chosen from platinum and palladium.
  • the second combustion stage 5, 6 is followed (relatively to the direction of progression of the gases in the envelope 1) by a third combustion stage.
  • a third fuel injection means 7 is then placed behind the second monolith element 6. Downstream of the third fuel injection means, there is a third monolith element 8 which is preferably not coated with catalyst.
  • the third combustion stage thus defined is essentially intended to stabilize combustion when the second monolithic element 6 is itself covered with catalyst.
  • the monolith element presented here consists of three ceramic monoliths with a honeycomb structure, with a density of 350 cells / inch 2, ie approximately 54 cells / cm 2 juxtaposed with each other inside an envelope. .
  • the walls of the channels of the monoliths have a thickness of approximately 0.14 mm.
  • Each monolith has a length of 5 cm and a diameter of 20 cm.
  • the first monolith is covered with a catalyst based on palladium oxide on stabilized alumina, the last two are covered with a hexaaluminate.
  • the air arriving on the first monolith is preheated to 380 ° C under a pressure of 15 bars.
  • the fuel injected entirely upstream of the first monolith is natural gas (composition example: 98% CH 4 , 2% C 2 H 6 ) so that the richness of the mixture is equal to 0.4.
  • the air flow entering the combustion system is equal to 2880 kg / h.
  • the natural gas flow rate is 67 kg / h.
  • FIG. 3 shows the axial temperature profiles of the gas and of the substrate for this example.
  • Curves B and D relate to the temperatures of the mixture passing through the monoliths: between 0 and 0.05 m in the first monolith; between 0.05 and 0.10 m in the second monolith; and between 0.10 and 0.15 m in the third monolith.
  • Curves A and C give the temperatures of the substrate, that is to say of the catalyst and of the support, through the three abovementioned sections of monoliths.
  • Curve A indeed shows that the temperature of the substrate is always higher than 900 ° C., which will cause the deactivation of the catalyst of the monolith by the transformation PdO ⁇ Pd + 1/2 0 2 .
  • Curve A shows that the temperature of the substrate is higher than 900 ° C. This causes the decomposition of palladium oxide, which constitutes the active phase of the catalyst, into metallic palladium which is clearly less active and, consequently, the rapid drop in methane conversion.
  • the temperatures of the substrate in the second monolith are greater than 1350 ° C; this will cause the deactivation of the catalyst of the second and third monoliths.
  • the system priming temperature increased from 290 ° C to 450 ° C. That is to say, it is no longer possible to prime the catalyst under the operating conditions of a turbine.
  • the operating conditions are the same as those of Example 1.
  • the arrangement of the combustion chamber is different. It is always composed of three monoliths identical to those described in Example 1, but not all of the fuel is injected at the inlet of the reactor. Part of the fuel, here 55 kg / h is injected before the first monolith, the remaining 12 kg / h are injected immediately downstream of the third and last slice of monolith.
  • Curve A shows that the conversion is self-regulating in monoliths 4 and 6 since the temperature there is higher than 900 ° C.
  • Example 1 after 50 hours of operation the monoliths 4 and 6 have degraded and the system makes it possible to ensure the conversion of only 30% of the fuel injected by the first injector. At the time of the second injection the temperature of the fumes is not high enough to ensure self-ignition of the mixture.
  • the combustion chamber is made up of two monolithic elements.
  • the first includes two slices of monoliths of 5 cm in length each coated with a catalyst based on palladium oxide (and having the same cell density ).
  • the system then has a 2.5 cm space in which a fuel injection is carried out, then a second 2.5 cm long monolith covered with a catalyst of the hexaaluminate family: FIG. 1 illustrates such a structure.
  • the fuel injection is separated into two: 33.5 kg / h, or half of the fuel, is injected upstream of the first section of catalyst, the rest is injected before the last section.
  • the minimum injection at each monolithic stage (or section) is approximately 20% of the total load. This makes it easier to mix the fuel with the air or the fumes.
  • the improvement observed relates to the temperature of the substrate which does not exceed 900 ° C. in the first two sections. Similarly, in the last tranche, the temperatures do not exceed 1100 ° C, the maximum temperature that the hexaaluminates can withstand without significant deactivation. Combustion is completed in homogeneous phase immediately downstream of the last section. Under these conditions, the system operated for 400 hours without any significant deactivation of the catalyst or variation in the initiation temperature.
  • the present invention by significantly lowering the temperatures of the monoliths, makes it possible to preserve their integrity. Furthermore, catalytic combustion, which can only operate for temperatures below 900 ° C. is therefore, according to the invention, carried out in almost all monoliths.
  • Figure 6 relates to an embodiment of the invention which differs from that of Figure 2 only in the shape of the first two monoliths 4 and 6: instead of being cylindrical they are here annular with a central zone 12 preventing the 'flow. This allows a different flow of the mixture, which will be preferred if the reactor is to be used with operating conditions different from those of FIG. 2.
  • FIGS. 7A and 7B show an embodiment of the invention according to which the first and second monolithic elements 4, 6 are both annular, concentric and crossed radially by the air-fuel mixture.
  • the envelope 1 is a cylinder having a low height with respect to its diameter.
  • the air inlet is located on the inlet disc 9, near the axis of said cylinder, as is the first injection means 3.
  • the monolithic elements are arranged concentrically between the two end discs (9 , 10) of cylinder 1.
  • the input disc 9 has a larger surface area than the bottom 10.
  • the second injection means 5 here comprises several injection means which all open into the annular space 11 delimited by the first monolith element 4 and the second monolith element 6.
  • the injection means 5 make it possible to complete the combustion.
  • the second monolithic element 6 makes it possible to stabilize the combustion.
  • the gas mixture Downstream of the second monolith, the gas mixture leaves the cylindrical envelope 1 as indicated by the arrows A 'in FIG. 7A, that is to say by the end opposite to the inlet 2; this is made possible by the fact that the bottom 10 covers the second monolithic element 6 and that the input disc 9 has a larger surface than the bottom 10.
  • This embodiment of the invention will preferably be chosen when space problems prevent the choice of the embodiments described in relation to FIGS. 1 to 6.

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
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  • General Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
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  • Exhaust Gas Treatment By Means Of Catalyst (AREA)

Abstract

The system consists of a housing (1) with an inlet (2) for a combustive material such as air, and a number of injectors (3,5) positioned to give a staged injection of fuel. It has a first monolithic element (4) coated with a catalyst and positioned after the first fuel injector (3) in relation to the air/fuel mixture flow. At least a second monolithic element (6) is situated after a second injector (5) with the aim of stabilising the combustion. The second element is coated with a catalyst selected from hexa-aluminates. A third monolithic element can also be included. The first and second elements can be in the form of a number of sections coated with catalysts based on precious metals. The catalytic coating on the second element is able to withstand temperatures up to about 1100 deg C, with the temperature of its output gases below about 1000 deg C. The catalytic materials have different valencies and are selected from barium, strontium, rare earths, manganese, cobalt and iron, magnesium and zinc. The catalytic coating on the first element is of palladium oxide.

Description

La présente invention concerne un système de combustion catalytique avec injection étagée de combustible et utilisant un catalyseur d'oxydation non-sélective.The present invention relates to a catalytic combustion system with stepped fuel injection and using a non-selective oxidation catalyst.

La combustion conventionnelle, réalisée en présence d'une flamme, habituellement utilisée dans les procédés de combustion d'hydrocarbures, tels que le gaz naturel, est un processus difficilement contrôlable. Elle se produit dans un domaine de concentrations air/hydrocarbure bien déterminé et conduit, outre à la formation de dioxyde de carbone et d'eau, à la production de polluants tels que le monoxyde de carbone et les oxydes d'azote.Conventional combustion, carried out in the presence of a flame, usually used in the combustion of hydrocarbons, such as natural gas, is a process that is difficult to control. It occurs in a well-defined range of air / hydrocarbon concentrations and leads, in addition to the formation of carbon dioxide and water, to the production of pollutants such as carbon monoxide and nitrogen oxides.

En raison de la sévérisation accélérée des contraintes environnementales sur les polluants (oxydes d'azote, hydrocarbures imbrûlés, monoxyde de carbone) émis par les procédés de combustion, il devient nécessaire de trouver de nouvelles technologies permettant de diminuer fortement les émissions de ceux-ci. Plusieurs solutions conventionnelles sont bien connues de l'homme de l'art :

  • La réduction sélective des gaz d'échappement (SCR) : ainsi la réduction sélective des oxydes d'azote par l'ammoniaque permet de réduire les teneurs en NOx à environ 10 ppm. Mais cette solution nécessite la mise en place d'un réacteur particulier, le stockage et l'utilisation d'ammoniaque ; les frais d'installation et de fonctionnement d'un tel système sont donc élevés.
  • L'injection d'eau ou de vapeur d'eau : une telle injection abaisse la température atteinte par les gaz de combustion réduisant ainsi de façon significative les teneurs en NOx à environ 50 ppm. Le coût de l'addition d'un tel dispositif est faible. Mais les coûts de fonctionnement d'une telle installation sont élevées en raison notamment de la purification de l'eau préalablement à l'injection ; par ailleurs la surconsommation de combustible due à un abaissement du rendement énergétique élève aussi le coût de fonctionnement. En outre, si l'injection d'eau suffit pour passer les normes actuelles, elle ne permettra pas de satisfaire aux normes futures sur les NOx.
  • Une zone primaire à mélange pauvre : cette technologie repose sur l'amélioration de l'homogénéité du mélange air/combustible. Elle permet de faire chuter les émissions de NOx à environ 50 ppm mais cette diminution se fait au détriment des émissions de monoxyde de carbone et d'hydrocarbure imbrûlés, qui se trouvent accrues.
Due to the accelerated severity of environmental constraints on pollutants (nitrogen oxides, unburnt hydrocarbons, carbon monoxide) emitted by combustion processes, it becomes necessary to find new technologies that can greatly reduce their emissions. . Several conventional solutions are well known to those skilled in the art:
  • Selective reduction of exhaust gases (SCR): thus the selective reduction of nitrogen oxides by ammonia makes it possible to reduce the NO x contents to around 10 ppm. However, this solution requires the installation of a particular reactor, the storage and use of ammonia; the installation and operating costs of such a system are therefore high.
  • Injection of water or steam: such an injection lowers the temperature reached by the combustion gases, thereby significantly reducing the NO x contents to around 50 ppm. The cost of adding such a device is low. However, the operating costs of such an installation are high due in particular to the purification of the water prior to injection; moreover, the overconsumption of fuel due to a reduction in energy efficiency also raises the operating cost. In addition, if the injection of water is sufficient to pass the current standards, it will not meet future standards on NO x .
  • A lean mixture primary zone: this technology is based on improving the homogeneity of the air / fuel mixture. It allows NO x emissions to drop to around 50 ppm, but this reduction comes at the expense of unburnt carbon monoxide and hydrocarbon emissions, which are increased.

La combustion catalytique est une solution séduisante pour répondre à la sévérisation croissante des normes sur les polluants. En effet, la chambre de combustion catalytique remplace avantageusement les brûleurs conventionnels car elle autorise un meilleur contrôle de l'oxydation totale dans un large domaine des valeurs du rapport air/hydrocarbure réduisant ainsi fortement les émissions des d'oxydes d'azote, d'hydrocarbures imbrûlés et de monoxyde de carbone. On peut également mentionner qu'elle permet de brûler une très grande variété de composés.Catalytic combustion is an attractive solution to meet the increasing severity of pollutant standards. In fact, the catalytic combustion chamber advantageously replaces conventional burners because it allows better control of total oxidation in a wide range of values of the air / hydrocarbon ratio, thus greatly reducing the emissions of nitrogen oxides, unburnt hydrocarbons and carbon monoxide. We can also mention that it burns a very wide variety of compounds.

Ainsi que le décrivent notamment D.Reay dans "Catalytic Combustion : Current Status and Implications for Energy Efficiency in the Process Industries. Heat Recovery Systems & CHP, 13. n°5, pp 383-390, 1993" et D. Jones et S.Salfati dans "Rev. Gén. Therm. Fr. n°330-331, pp 401-406, Juin-Juillet 1989", les applications de la combustion catalytique sont multiples : panneaux et tubes radiants, réchauds catalytiques, turbines à gaz, cogénération, brûleurs, manchons catalytiques pour tubes de vapo-reformage, production de gaz chauds dans le domaine du chauffage par contact direct et réacteurs à plaques catalytiques.As described in particular by D. Reay in "Catalytic Combustion: Current Status and Implications for Energy Efficiency in the Process Industries. Heat Recovery Systems & CHP, 13. n ° 5, pp 383-390, 1993" and D. Jones and S .Salfati in "Rev. Gén. Therm. Fr. n ° 330-331, pp 401-406, June-July 1989", the applications of catalytic combustion are multiple: radiant panels and tubes, catalytic stoves, gas turbines, cogeneration, burners, catalytic sleeves for steam reforming tubes, production of hot gases in the field of direct contact heating and catalytic plate reactors.

Concernant les systèmes de combustion catalytique utilisés dans les domaines de la production d'énergie et de la cogénération, la configuration de réacteur la plus répandue est un réacteur comportant plusieurs zones catalytiques: le(s) catalyseur(s) d'entrée étant plus spécifiquement dédié à l'amorçage de la réaction de combustion, les suivants servant à stabiliser la réaction de combustion à haute température ; le nombre d'étages (ou de zones) catalytiques étant ajusté en fonction des conditions imposées par l'application envisagée.Regarding the catalytic combustion systems used in the fields of energy production and cogeneration, the most common reactor configuration is a reactor comprising several catalytic zones: the inlet catalyst (s) being more specifically dedicated to initiating the combustion reaction, the following serving to stabilize the combustion reaction at high temperature; the number of catalytic stages (or zones) being adjusted according to the conditions imposed by the envisaged application.

Les catalyseurs de combustion sont généralement préparés à partir d'un substrat monolithique, en céramique ou en métal, sur lequel on dépose une fine couche de support constituée d'un ou plusieurs oxydes réfractaires de surface et de porosité supérieures à celles du substrat monolithique. Sur cet oxyde est dispersée la phase active composée essentiellement des métaux du groupe du platine.Combustion catalysts are generally prepared from a monolithic substrate, ceramic or metal, on which a thin support layer is deposited, consisting of one or more refractory oxides with a surface and porosity greater than that of the monolithic substrate. On this oxide is dispersed the active phase mainly composed of platinum group metals.

Ainsi qu'il est connu de l'homme de l'art, les métaux du groupe du platine présentent la plus haute activité catalytique pour l'oxydation des hydrocarbures et amorcent donc la combustion à plus basse température que les oxydes des métaux de transition. Ils sont donc utilisés de façon préférée dans les premières zones catalytiques. Toutefois, en raison des températures élevées atteintes soit lors des phases de démarrage ou soit en régime établi, ces catalyseurs subissent une dégradation qui réduit leurs performances catalytiques. Le frittage du support à base d'alumine ainsi que le frittage de la phase métallique active et/ou son encapsulation par le support font partie des causes les plus couramment citées pour expliquer cette dégradation.As is known to those skilled in the art, the platinum group metals exhibit the highest catalytic activity for the oxidation of hydrocarbons and therefore initiate combustion at a lower temperature than the oxides of the transition metals. They are therefore preferably used in the first catalytic zones. However, due to the high temperatures reached either during the start-up phases or in steady state, these catalysts undergo a degradation which reduces their catalytic performance. The sintering of the alumina-based support as well as the sintering of the active metallic phase and / or its encapsulation by the support are among the causes most commonly cited to explain this degradation.

Il est connu que l'on peut stabiliser efficacement la chute de surface spécifique des supports à base d'alumine par un dopant approprié. Les terres rares et la silice sont souvent cités parmi les stabilisants les plus performants de l'alumine. Les catalyseurs préparés par cette technique sont décrits entre autres dans le brevet US-A- 4 220 559. Dans ce document, le catalyseur comprend des métaux du groupe du platine ou des métaux de transition déposés sur de l'alumine, un oxyde d'un métal choisi dans le groupe constitué par le baryum, le lanthane et le strontium et un oxyde d'un métal choisi dans le groupe constitué par l'étain, le silicium, le zirconium et le molybdène.It is known that one can effectively stabilize the drop in specific surface of alumina-based supports by an appropriate dopant. Rare earths and silica are often cited among the most effective stabilizers in alumina. The catalysts prepared by this technique are described inter alia in US-A-4,220,559. In this document, the catalyst comprises platinum group metals or transition metals deposited on alumina, an oxide of a metal chosen from the group consisting of barium, lanthanum and strontium and an oxide of a metal chosen from the group consisting of tin, silicon, zirconium and molybdenum.

En outre, afin de limiter le frittage de la phase métallique active, il a été proposé d'ajouter divers stabilisants à base essentiellement d'oxydes de métaux de transition tels que décrits par exemple dans brevet US-A- 4 857 499.In addition, in order to limit the sintering of the active metallic phase, it has been proposed to add various stabilizers essentially based on transition metal oxides as described for example in US Pat. No. 4,857,499.

Parmi les documents représentatifs des réacteurs de combustion comportant plusieurs zones catalytiques, on connait notamment :

  • la demande de brevet européen EP-A-198 948 qui décrit une première zone catalytique : Pd et Pt et NiO; et une deuxième zone catalytique : Pt et Pd ;
  • la demande de brevet japonais JP-A-04/197 443 qui enseigne une première zone catalytique : Pd et/ou Pt ; une deuxième zone catalytique : Sr0.8La0.2MnAl11O19-α ; et une troisième zone catalytique : Sr0.8La0.2MnAl11O19-α ;
  • les demandes de brevet international WO-A-92/9848 et WO-A-92/9849 montrent une première zone catalytique : Pd et (Pt ou Ag) ; une deuxième zone catalytique : Pd et (Pt ou Ag); et une troisième zone catalytique : pérovskite ABO3 ou oxyde de métal du groupe V(Nb ou V), du groupe VI (Cr) ou du groupe VIII (Fe, Co, Ni).
Among the documents representative of combustion reactors comprising several catalytic zones, we know in particular:
  • European patent application EP-A-198 948 which describes a first catalytic zone: Pd and Pt and NiO; and a second catalytic zone: Pt and Pd;
  • Japanese patent application JP-A-04 / 197,443 which teaches a first catalytic zone: Pd and / or Pt; a second catalytic zone: Sr 0.8 La 0.2 MnAl 11 O 19-α ; and a third catalytic zone: Sr 0.8 La 0.2 MnAl 11 O 19-α ;
  • international patent applications WO-A-92/9848 and WO-A-92/9849 show a first catalytic zone: Pd and (Pt or Ag); a second catalytic zone: Pd and (Pt or Ag); and a third catalytic zone: perovskite ABO 3 or metal oxide of group V (Nb or V), of group VI (Cr) or of group VIII (Fe, Co, Ni).

Le point critique des procédés multi-étages connus réside dans le contrôle de la température au sein des différents étages catalytiques. Si la réaction de combustion s'emballe, la température du catalyseur peut atteindre rapidement la température adiabatique de flamme. Or, il est important de couvrir l'intégralité de la plage de charge de la turbine à gaz. Depuis le processus d'inflammation jusqu'à la pleine charge en passant le ralenti, le rapport air-combustible peut varier dans des proportions importantes. L'utilisation d'une telle chambre de combustion catalytique peut donc s'avérer délicate.The critical point of the known multi-stage processes lies in the control of the temperature within the various catalytic stages. If the combustion reaction gets carried away, the temperature of the catalyst can quickly reach the temperature flame adiabatic. However, it is important to cover the entire load range of the gas turbine. From the ignition process to full load, passing the idle, the air-fuel ratio can vary significantly. The use of such a catalytic combustion chamber can therefore prove to be difficult.

On connaît par ailleurs le brevet américain US-A-4 731 989 de Furuya et al. qui décrit un procédé de combustion comportant comme caractéristique majeure une injection étagée de combustible. Le système associé dit "hybride" est composé d'une zone catalytique où est brûlée une fraction de combustible, cette zone catalytique étant suivie d'une zone de post-combustion en phase homogène où le reste du combustible est mélangé aux gaz chauds sortant du catalyseur et est brûlé sous forme de flamme de prémélange. Le rapport air/combustible du mélange entrant dans la zone catalytique est ajusté de façon à ce que la température adiabatique des gaz ne dépasse pas environ 1000°C en sortie de cette zone catalytique. Le reste du mélange est injecté en aval de la zone catalytique afin d'atteindre une température de gaz de combustion compatible avec les exigences des procédés de combustion actuels soit 1200°C à 1500°C. En raison de la limitation des températures de matériaux à 1000°C, le catalyseur ne subit pas de désactivation.We also know the US patent US-A-4,731,989 to Furuya et al. which describes a combustion process comprising as a major characteristic a staged injection of fuel. The associated "hybrid" system is composed of a catalytic zone where a fraction of fuel is burned, this catalytic zone being followed by a post-combustion zone in homogeneous phase where the rest of the fuel is mixed with the hot gases leaving the catalyst and is burned as a premix flame. The air / fuel ratio of the mixture entering the catalytic zone is adjusted so that the adiabatic temperature of the gases does not exceed approximately 1000 ° C. at the outlet of this catalytic zone. The rest of the mixture is injected downstream of the catalytic zone in order to reach a combustion gas temperature compatible with the requirements of current combustion processes, ie 1200 ° C. to 1500 ° C. Due to the limitation of material temperatures to 1000 ° C, the catalyst is not deactivated.

Ce procédé est intéressant dans la mesure où il offre une sécurité supérieure à celle d'un procédé où le contrôle de la température s'opère uniquement par la géométrie du monolithe ou de l'ensemble des monolithes. Dans les phases de démarrage, il est également plus souple à mettre en oeuvre.This process is interesting insofar as it offers greater security than that of a process in which the temperature is controlled solely by the geometry of the monolith or of all of the monoliths. In the start-up phases, it is also more flexible to implement.

Toutefois, plus récemment, dans un article de Furuya et al., "A study on combustion catalyst for gas turbine", Second Tokyo Conference on Advanced Catalytic Science and Technology (TOCAT), Tokyo, 21-26 août 1994, I.38, page 129-130, les auteurs indiquent que l'activité catalytique d'un catalyseur à base de palladium oscille entre 800°C et 1000°C en raison de l'équilibre suivant :

Figure imgb0001
Ce comportement instable du palladium est observé en cours de fonctionnement dans un tel réacteur de combustion à injection étagée ; Furuya et al. ajoutent que si ce problème d'instabilité des formulations à base de palladium peut être résolu, un catalyseur à base de palladium sera particulièrement adapté pour ce procédé à injection étagée.However, more recently, in an article by Furuya et al., "A study on combustion catalyst for gas turbine", Second Tokyo Conference on Advanced Catalytic Science and Technology (TOCAT), Tokyo, August 21-26, 1994, I.38, pages 129-130, the authors indicate that the catalytic activity of a palladium-based catalyst oscillates between 800 ° C and 1000 ° C due to the following balance:
Figure imgb0001
 This unstable behavior of palladium is observed during operation in such a staged injection combustion reactor; Furuya et al. add that if this problem of instability of palladium-based formulations can be resolved, a palladium-based catalyst will be particularly suitable for this staged injection process.

Une approche globale de la combustion catalytique prenant en compte à la fois les avantages et les inconvénients de la configuration du réacteur catalytique et la formulation catalytique devient donc nécessaire.A global approach to catalytic combustion taking into account both the advantages and disadvantages of the configuration of the catalytic reactor and the catalytic formulation therefore becomes necessary.

Par ailleurs, le problème de la combustion non étagée, où le combustible est injecté en un seul endroit (en amont du ou des étages catalytiques) est que les parois du réacteur atteignent alors la température adiabatique de la flamme ; ceci conduit rapidement à une désactivation du catalyseur.Furthermore, the problem of non-staged combustion, where the fuel is injected in one place (upstream of the catalytic stage or stages) is that the walls of the reactor then reach the adiabatic temperature of the flame; this quickly leads to deactivation of the catalyst.

Afin d'éviter ces températures élevées, il a déjà été proposé des procédés et des systèmes de combustion catalytique étagée, avec un fractionnement (ou étagement) de l'injection en combustible. Ainsi, le brevet US 4 731 989 déjà cité peut apparaître comme une solution à ce problème. Cependant, l'inconvénient cité plus haut concernant le comportement du palladium peut aboutir à la désactivation complète de ce dernier.In order to avoid these high temperatures, processes and systems of staged catalytic combustion have already been proposed, with a fractionation (or staging) of the fuel injection. Thus, the patent US Pat. No. 4,731,989 already cited may appear to be a solution to this problem. However, the drawback mentioned above concerning the behavior of palladium can result in the complete deactivation of the latter.

En outre la détérioration observée du catalyseur pose le problème du décalage de sa température d'amorçage. La fiabilité du procédé se trouve donc remise en cause.In addition, the observed deterioration of the catalyst poses the problem of shifting its initiation temperature. The reliability of the process is therefore called into question.

Ainsi, la présente invention a pour objet un système de combustion catalytique comprenant une enveloppe ayant une entrée pour un comburant tel que de l'air, plusieurs moyens d'injection de combustible destinés à réaliser une injection étagée de combustible, et au moins un premier élément monolithe susceptible d'être recouvert d'un catalyseur de combustion placé en aval d'un premier moyen d'injection de combustible relativement au sens de progression d'un mélange air-combustible dans le système, ledit premier moyen d'injection réalisant une injection partielle de combustible.Thus, the subject of the present invention is a catalytic combustion system comprising an envelope having an inlet for an oxidizer such as air, several fuel injection means intended to carry out a staged injection of fuel, and at least a first monolithic element capable of being covered with a combustion catalyst placed downstream of a first fuel injection means relative to the direction of progression of an air-fuel mixture in the system, said first injection means performing a partial fuel injection.

Selon l'invention, ledit système comprend en outre un deuxième élément monolithe disposé en aval d'un deuxième moyen d'injection, ledit deuxième élément monolithe étant destiné à stabiliser la combustion. Le deuxième élément monolithe peut être recouvert d'un catalyseur de combustion, tel qu'un hexaaluminate.According to the invention, said system further comprises a second monolithic element disposed downstream of a second injection means, said second monolithic element being intended to stabilize combustion. The second monolith element can be coated with a combustion catalyst, such as a hexaaluminate.

Conformément à l'invention, le premier ou le deuxième monolithe peut comprendre une pluralité de monolithes élémentaires recouverts de catalyseurs à base de métaux précieux.According to the invention, the first or the second monolith can comprise a plurality of elementary monoliths covered with catalysts based on precious metals.

La quantité de combustible introduite par le deuxième moyen d'injection est telle que la température adiabatique de flamme ne dépasse pas 1100°C. En outre la quantité de combustible introduite par le premier moyen d'injection est telle que la température adiabatique de flamme ne dépasse pas 900°C.The amount of fuel introduced by the second injection means is such that the adiabatic flame temperature does not exceed 1100 ° C. Furthermore, the quantity of fuel introduced by the first injection means is such that the adiabatic flame temperature does not exceed 900 ° C.

Par ailleurs, le premier élément monolithe est tel que le catalyseur qui le recouvre est susceptible de supporter des températures inférieures à 900°C et que la température des gaz en sortie du premier élément monolithe est inférieure à environ 800°C.Furthermore, the first monolith element is such that the catalyst which covers it is capable of withstanding temperatures below 900 ° C. and that the temperature of the gases leaving the first monolith element is less than about 800 ° C.

Le deuxième élément monolithe est tel que le catalyseur qui le recouvre est susceptible de supporter des températures inférieures à 1100°C et que la température des gaz en sortie du deuxième élément monolithe est inférieur à 1000°C.The second monolith element is such that the catalyst which covers it is capable of withstanding temperatures below 1100 ° C. and that the temperature of the gases leaving the second monolith element is below 1000 ° C.

Selon un mode de réalisation de l'invention, le système de combustion catalyseur comprend un troisième élément monolithe placé en aval du deuxième élément monolithe, avec un troisième moyen d'injection de combustible est disposé entre le deuxième et le troisième élément monolithe.According to one embodiment of the invention, the catalyst combustion system comprises a third monolith element placed downstream of the second monolith element, with a third fuel injection means is disposed between the second and the third monolith element.

Avantageusement ledit catalyseur consiste essentiellement en une formule décrite par la demanderesse dans la demande de brevet français FR-A-2 721 837, ladite formule étant : A(1-x)ByCzAl(12-y-z)O(19-d) dans laquelle A représente au moins un élément de valence X sélectionné dans le groupe formé par le baryum, le strontium et les terres rares ; B représente au moins un élément de valence Y sélectionné dans le groupe formé par Mn, Co et Fe ; C représente au moins un élément sélectionné dans le groupe formé par Mg et Zn ; x ayant une valeur de 0 à 0,25, y ayant une valeur de 0,5 à 3 et z ayant une valeur de 0.01 à 3 ; la somme y + z ayant une valeur maximale de 4 et d a une valeur qui, déterminée en fonction des valences X et Y respectives des éléments A et B et de la valeur de x, y et de z, est égale à 1-1/2 {(1-x)X + yY - 3 y -z}

Figure imgb0002
.Advantageously, said catalyst essentially consists of a formula described by the applicant in French patent application FR-A-2 721 837, said formula being: A (1-x) B y C z Al (12-yz) O (19- d) in which A represents at least one element of valence X selected from the group formed by barium, strontium and rare earths; B represents at least one element of valence Y selected from the group formed by Mn, Co and Fe; C represents at least one element selected from the group formed by Mg and Zn; x having a value of 0 to 0.25, y having a value of 0.5 to 3 and z having a value of 0.01 to 3; the sum y + z having a maximum value of 4 and da a value which, determined according to the respective valences X and Y of the elements A and B and the value of x, y and z, is equal to 1-1 / 2 {(1-x) X + yY - 3 y -z}
Figure imgb0002
 .

De façon particulière, le catalyseur recouvrant le premier élément monolithe comprend de l'oxyde de palladium.In particular, the catalyst covering the first monolith element comprises palladium oxide.

Plus précisément, le catalyseur recouvrant le premier élément monolithe peut comprendre un support d'oxyde réfractaire et une phase active comprenant en pourcentage poids par rapport audit support : de 0,3 à 20 % de cérium, de 0,01 à 3,5 % de fer et de 1 à 10 % d'au moins un élément choisi parmi la platine et le palladium.More specifically, the catalyst covering the first monolith element can comprise a refractory oxide support and an active phase comprising, in weight percentage relative to said support: from 0.3 to 20% of cerium, from 0.01 to 3.5% iron and from 1 to 10% of at least one element chosen from platinum and palladium.

Selon l'invention, l'enveloppe est essentiellement cylindrique, l'entrée pour le comburant et le premier moyen d'injection est située à l'une de ses extrémité sur une section transversale, lesdits deuxième et/ou troisième moyen d'injection débouchent longitudinalement dans ledit cylindre.According to the invention, the envelope is essentially cylindrical, the inlet for the oxidant and the first injection means is located at one of its ends on a cross section, said second and / or third injection means open out. longitudinally in said cylinder.

L'un au moins des éléments monolithes peut être annulaire ; par exemple le premier et le deuxième élément monolithes peuvent être annulaires.At least one of the monolithic elements can be annular; for example, the first and second monolithic elements can be annular.

Sans sortir du cadre de l'invention, l'un au moins des éléments monolithes est cylindrique et occupe toute la section transversale dudit cylindre.Without departing from the scope of the invention, at least one of the monolithic elements is cylindrical and occupies the entire cross section of said cylinder.

Selon l'un de ses modes de réalisation, tous les éléments monolithes sont cylindriques.According to one of its embodiments, all the monolithic elements are cylindrical.

Selon un autre mode de réalisation de l'invention les éléments monolithes annulaires sont disposés concentriquement entre un disque d'extrêmité équipé des entrées de comburant et de combustible et un fond autorisant la sortie des gaz, le disque d'entrée présentant une surface plus grande que le fond, de sorte que le mouvement des gaz à l'intérieur dudit cylindre est essentiellement radial.According to another embodiment of the invention, the annular monolithic elements are arranged concentrically between an end disc equipped with oxidizer and fuel inlets and a bottom allowing the exit of gases, the inlet disc having a larger surface area. as the bottom, so that the movement of gases inside said cylinder is essentially radial.

D'autres caractéristiques, avantages et détails de l'invention apparaîtront mieux à la lecture de la description qui va suivre, faite à titre illustratif et nullement limitatif, en référence aux dessins annexés sur lesquels :

  • La figure 1 est une coupe longitudinale simplifiée d'un premier mode de réalisation de l'invention;
  • La figure 2 est une coupe longitudinale simplifiée d'un deuxième mode de réalisation de l'invention;
  • Les figures 3 et 4 présentent des courbes de température en différents points de réacteurs selon l'art antérieur;
  • La figure 5 montre des courbes de température en différents points du mode de réalisation de l'invention illustré par la figure 2;
  • la figure 6 est une coupe longitudinale simplifiée d'un autre mode de réalisation de l'invention; et
  • les figures 7A et 7B illustrent respectivement par une coupe longitudinale et une coupe radiale, encore un autre mode de réalisation de l'invention.
Other characteristics, advantages and details of the invention will appear better on reading the description which follows, given by way of illustration and in no way limiting, with reference to the appended drawings in which:
  • Figure 1 is a simplified longitudinal section of a first embodiment of the invention;
  • Figure 2 is a simplified longitudinal section of a second embodiment of the invention;
  • Figures 3 and 4 show temperature curves at different points of reactors according to the prior art;
  • FIG. 5 shows temperature curves at different points of the embodiment of the invention illustrated by FIG. 2;
  • Figure 6 is a simplified longitudinal section of another embodiment of the invention; and
  • Figures 7A and 7B respectively illustrate by a longitudinal section and a radial section, yet another embodiment of the invention.

Selon la figure 1, une enveloppe 1 présente une entrée 2 par laquelle sont introduits un premier fluide comburant (ici de l'air) et un combustible via un premier moyen d'injection 3. Le premier moyen d'injection 3 permet d'injecter une partie seulement du combustible nécessaire à la combustion totale.According to Figure 1, a casing 1 has an inlet 2 through which are introduced a first oxidizing fluid (here air) and a fuel via a first injection means 3. The first injection means 3 makes it possible to inject only part of the fuel necessary for total combustion.

En aval du premier moyen d'injection 3, relativement au sens de progression des fluides dans l'enveloppe 1 et indiqué par les flèches A, est disposé un premier élément monolithe 4.Downstream of the first injection means 3, relative to the direction of progression of the fluids in the envelope 1 and indicated by the arrows A, a first monolithic element 4 is arranged.

L'élément monolithe peut comprendre n tranches (ou sections) espacées ou non les unes des autres. Le substrat du monolithe peut être céramique ou métallique, d'une géométrie en nid d'abeille. Le substrat est recouvert d'une couche catalytique en métaux précieux tels que du palladium.The monolith element can comprise n sections (or sections) spaced or not spaced from each other. The substrate of the monolith can be ceramic or metallic, with a honeycomb geometry. The substrate is covered with a catalytic layer of precious metals such as palladium.

Le premier élément monolithe 4 permet ainsi une augmentation progressive des températures du gaz qui le traverse et de la couche catalytique. On n'y brûle qu'une fraction du combustible.The first monolith element 4 thus allows a gradual increase in the temperatures of the gas passing through it and of the catalytic layer. Only a fraction of the fuel is burned there.

En aval du premier élément monolithe, est disposé un deuxième moyen d'injection de combustible 5 destiné à injecter, selon ce mode de réalisation de l'invention, le reste du combustible nécessaire à la combustion totale.Downstream of the first monolithic element, there is a second fuel injection means 5 intended to inject, according to this embodiment of the invention, the rest of the fuel necessary for total combustion.

En outre un deuxième élément monolithe 6 est placé en aval du deuxième moyen d'injection de combustible 5. Le deuxième élément monolithe 6 peut comprendre m tranches (ou sections) espacées ou non les unes des autres. Le substrat peut être céramique ou métallique et être recouvert d'une couche catalytique préférentiellement constituée d'hexaaluminates. Le catalyseur du deuxième élément monolithe doit pouvoir supporter des températures pouvant atteindre 1100°C, afin de pouvoir porter les gaz qui traversent le deuxième monolithe jusqu'à leur température d'auto-inflammation, soit vers 1000°C.In addition, a second monolithic element 6 is placed downstream of the second fuel injection means 5. The second monolithic element 6 can comprise m sections (or sections) spaced or not spaced from one another. The substrate can be ceramic or metallic and be covered with a catalytic layer preferably consisting of hexaaluminates. The catalyst of the second monolith element must be able to withstand temperatures of up to 1100 ° C., in order to be able to bring the gases which pass through the second monolith to their auto-ignition temperature, ie around 1000 ° C.

Avec cette configuration du deuxième monolithe, les températures d'amorçage de la combustion (au niveau du premier monolithe) sont alors de l'ordre de 700°C. presque température supérieure à 600°C.With this configuration of the second monolith, the ignition initiation temperatures (at the level of the first monolith) are then of the order of 700 ° C. almost temperature above 600 ° C.

Ainsi, de façon préférentielles, le premier étage catalytique doit être tel que la température des gaz à sa sortie est comprise entre 650°C et 800°C.Thus, preferably, the first catalytic stage must be such that the temperature of the gases at its outlet is between 650 ° C. and 800 ° C.

Selon un mode préféré de réalisation de l'invention, le catalyseur utilisé dans le premier élément monolithe 4 comprend de l'oxyde de palladium. Sachant que celui-ci se réduit en palladium métallique au-dessus d'environ 900°C (valeur variable avec la pression), on régule le débit de combustible en 3 de telle sorte que la température du catalyseur déposé soit inférieure à environ 900°C, afin d'éviter la transformation de l'oxyde de palladium. On cherchera simultanément à ne pas atteindre l'auto-inflammation dans le monolithe 4 ; ainsi la température des gaz en sortie dudit monolithe doit alors être inférieure à environ 800°C.According to a preferred embodiment of the invention, the catalyst used in the first monolith element 4 comprises palladium oxide. Knowing that this is reduced to metallic palladium above about 900 ° C. (value variable with pressure), the fuel flow rate is adjusted in 3 so that the temperature of the deposited catalyst is less than about 900 ° C, in order to avoid the transformation of palladium oxide. We will simultaneously try not to achieve self-ignition in monolith 4; thus the temperature of the gases leaving said monolith must then be less than approximately 800 ° C.

Plus précisément, le catalyseur qui recouvre le premier élément monolithe comprend un support d'oxyde réfractaire et une phase active comprenant en pourcentage poids par rapport audit support : de 0,3 à 20 % de cérium, de 0,01 à 3,5 % de fer et de 1 à 10 % d'au moins un élément choisi parmi la platine et le palladium.More specifically, the catalyst which covers the first monolithic element comprises a refractory oxide support and an active phase comprising in weight percentage relative to said support: from 0.3 to 20% of cerium, from 0.01 to 3.5% of iron and from 1 to 10% of at least one element chosen from platinum and palladium.

Ce catalyseur a été décrit dans la demande de brevet EN. 94/13739 déposée au nom de la demanderesse.This catalyst has been described in the patent application EN. 94/13739 filed in the name of the plaintiff.

Selon un autre mode de réalisation de l'invention, illustré par la figure 2, le deuxième étage de combustion 5, 6 est suivi (relativement au sens de progression des gaz dans l'enveloppe 1) d'un troisième étage de combustion.According to another embodiment of the invention, illustrated by FIG. 2, the second combustion stage 5, 6 is followed (relatively to the direction of progression of the gases in the envelope 1) by a third combustion stage.

Plus précisément, un troisième moyen d'injection de combustible 7 est alors placé derrière le deuxième élément monolithe 6. En aval du troisième moyen d'injection de combustible, on dispose un troisième élément monolithe 8 qui n'est préférentiellement pas revêtu de catalyseur.More specifically, a third fuel injection means 7 is then placed behind the second monolith element 6. Downstream of the third fuel injection means, there is a third monolith element 8 which is preferably not coated with catalyst.

Le troisième étage de combustion ainsi défini est essentiellement destiné à stabiliser la combustion lorsque le deuxième élément monolithe 6 est lui-même recouvert de catalyseur.The third combustion stage thus defined is essentially intended to stabilize combustion when the second monolithic element 6 is itself covered with catalyst.

Dans ce qui suit, des exemples comparatifs vont être donnés afin de montrer les différences entre l'art antérieur et la présente invention, et les avantages qui en découlent.In the following, comparative examples will be given in order to show the differences between the prior art and the present invention, and the advantages which result therefrom.

Exemple 1 (art antérieur) : Example 1 (prior art):

L'élément monolithe présenté ici est constitué de trois monolithes céramiques à structure en nid d'abeille, d'une densité de 350 cellules/pouces2 soit environ 54 cellules/cm2 juxtaposés les uns aux autres à l'intérieur d'une enveloppe. Les parois des canaux des monolithes présentent une épaisseur d'environ 0,14 mm. Chaque monolithe a une longueur de 5 cm et un diamètre de 20 cm. Le premier monolithe est recouvert d'un catalyseur à base d'oxyde de palladium sur alumine stabilisée, les deux derniers sont recouverts d'un hexaaluminate.The monolith element presented here consists of three ceramic monoliths with a honeycomb structure, with a density of 350 cells / inch 2, ie approximately 54 cells / cm 2 juxtaposed with each other inside an envelope. . The walls of the channels of the monoliths have a thickness of approximately 0.14 mm. Each monolith has a length of 5 cm and a diameter of 20 cm. The first monolith is covered with a catalyst based on palladium oxide on stabilized alumina, the last two are covered with a hexaaluminate.

L'air qui arrive sur le premier monolithe est préchauffé à 380°C sous une pression de 15 bars. Le combustible injecté en totalité en amont du premier monolithe est du gaz naturel (exemple de composition : 98% CH4, 2 % C2 H6) de telle manière que la richesse du mélange soit égale à 0,4. Le débit d'air entrant dans le système de combustion est égal à 2880 kg/h. Le débit de gaz naturel est de 67 kg/h.The air arriving on the first monolith is preheated to 380 ° C under a pressure of 15 bars. The fuel injected entirely upstream of the first monolith is natural gas (composition example: 98% CH 4 , 2% C 2 H 6 ) so that the richness of the mixture is equal to 0.4. The air flow entering the combustion system is equal to 2880 kg / h. The natural gas flow rate is 67 kg / h.

La figure 3 présente les profils axiaux de température du gaz et du substrat pour cet exemple.FIG. 3 shows the axial temperature profiles of the gas and of the substrate for this example.

Les courbes B et D concernent les températures du mélange qui traverse les monolithes : entre 0 et 0,05 m dans le premier monolithe ; entre 0,05 et 0,10 m dans le deuxième monolithe ; et entre 0,10 et 0,15 m dans le troisième monolithe.Curves B and D relate to the temperatures of the mixture passing through the monoliths: between 0 and 0.05 m in the first monolith; between 0.05 and 0.10 m in the second monolith; and between 0.10 and 0.15 m in the third monolith.

Les courbes A et C donnent les températures du substrat c'est-à-dire du catalyseur et du support, à travers les trois tranches de monolithes précitées.Curves A and C give the temperatures of the substrate, that is to say of the catalyst and of the support, through the three abovementioned sections of monoliths.

Les courbes A et B vont d'abord être analysées :Curves A and B will first be analyzed:

La courbe A montre en effet que la température du substrat est toujours supérieure à 900°C, ce qui va entrainer la désactivation du catalyseur du monolithe par la transformation PdO → Pd + 1/2 0 2 .

Figure imgb0003
Curve A indeed shows that the temperature of the substrate is always higher than 900 ° C., which will cause the deactivation of the catalyst of the monolith by the transformation PdO → Pd + 1/2 0 2 .
Figure imgb0003

La courbe A montre que la température du substrat est supérieure à 900°C. Ceci entraine la décomposition de l'oxyde de palladium, qui constitue la phase active du catalyseur, en palladium métallique nettement moins actif et, en conséquence, la chute rapide de la conversion du méthane.Curve A shows that the temperature of the substrate is higher than 900 ° C. This causes the decomposition of palladium oxide, which constitutes the active phase of the catalyst, into metallic palladium which is clearly less active and, consequently, the rapid drop in methane conversion.

D'autre part, les températures du substrat dans le deuxième monolithe (distance axiale comprise entre 0,05 et 0,10 m) sont supérieures à 1350°C; ceci va entrainer la désactivation du catalyseur des deuxième et troisième monolithes.On the other hand, the temperatures of the substrate in the second monolith (axial distance between 0.05 and 0.10 m) are greater than 1350 ° C; this will cause the deactivation of the catalyst of the second and third monoliths.

En outre, après 50 heures de fonctionnement dans ces conditions, la température d'amorçage du système est passée de 290°C à 450°C. C'est-à-dire qu'il n'est plus possible d'amorcer le catalyseur dans les conditions de fonctionnement d'une turbine.In addition, after 50 hours of operation under these conditions, the system priming temperature increased from 290 ° C to 450 ° C. That is to say, it is no longer possible to prime the catalyst under the operating conditions of a turbine.

Une solution pour réduire la température du catalyseur aurait pu être de faire varier la vitesse de l'air à l'entrée du système de combustion (par exemple en diminuant le diamètre de la chambre). Les courbes C et D montrent qu'en multipliant la vitesse de l'air par 4, on n'obtient pas de variation significative de la température du substrat, notamment au niveau du premier monolithe. Cette solution n'est donc pas à retenir, d'autant qu'elle augmente considérablement les pertes de charge (x16), à un niveau non réaliste.One solution to reduce the temperature of the catalyst could have been to vary the speed of the air entering the combustion system (for example by reducing the diameter of the chamber). Curves C and D show that by multiplying the air speed by 4, no significant variation in the temperature of the substrate is obtained, in particular at the level of the first monolith. This solution is therefore not to be retained, especially since it considerably increases the pressure losses (x16), at an unrealistic level.

Exemple 2 (art antérieur) : Example 2 (prior art):

Dans cet exemple, illustré par la figure 4, les conditions opératoires sont les mêmes que celles de l'exemple 1. Cependant, la disposition de la chambre de combustion est différente. Elle est toujours composée de trois monolithes identiques à ceux décrits dans l'exemple 1, mais tout le combustible n'est pas injecté en entrée du réacteur. Une partie du combustible, soit ici 55 kg/h est injectée avant le premier monolithe, les 12 kg/h restants sont injectés immédiatement en aval de la troisième et dernière tranche de monolithe.In this example, illustrated by FIG. 4, the operating conditions are the same as those of Example 1. However, the arrangement of the combustion chamber is different. It is always composed of three monoliths identical to those described in Example 1, but not all of the fuel is injected at the inlet of the reactor. Part of the fuel, here 55 kg / h is injected before the first monolith, the remaining 12 kg / h are injected immediately downstream of the third and last slice of monolith.

La courbe A montre que la conversion s'auto-régule dans les monolithes 4 et 6 puisque la température y est supérieure à 900°C.Curve A shows that the conversion is self-regulating in monoliths 4 and 6 since the temperature there is higher than 900 ° C.

Le complément de combustible injecté dans les gaz chauds en sortie de réacteur permet d'obtenir l'auto-inflammation du mélange et donc la conversion complète.The additional fuel injected into the hot gases at the outlet of the reactor makes it possible to obtain self-ignition of the mixture and therefore complete conversion.

Cependant, comme dans l'exemple 1, après 50 heures de fonctionnement les monolithes 4 et 6 se sont dégradés et le système permet d'assurer la conversion de seulement 30 % du combustible injecté par le premier injecteur. Au moment de la deuxième injection la température des fumées n'est pas assez élevée pour assurer l'auto-inflammation du mélange.However, as in Example 1, after 50 hours of operation the monoliths 4 and 6 have degraded and the system makes it possible to ensure the conversion of only 30% of the fuel injected by the first injector. At the time of the second injection the temperature of the fumes is not high enough to ensure self-ignition of the mixture.

Le même inconvénient que dans l'exemple 1 persiste donc.The same drawback as in Example 1 therefore persists.

Exemple 3 (selon l'invention) : Example 3 (according to the invention):

Dans cet exemple illustré par la figure 5, les conditions opératoires sont identiques à celles des deux exemples précédents. La chambre de combustion est composée de deux éléments monolithes. Le premier comprend deux tranches de monolithes de 5 cm de longueur chacune revêtue d'un catalyseur à base d'oxyde de palladium (et ayant une même densité de cellules...). Le système présente ensuite un espace de 2,5 cm dans lequel on réalise une injection de combustible, puis un deuxième monolithe de 2,5 cm de longueur recouvert d'un catalyseur de la famille des hexaaluminates : la figure 1 illustre une telle structure.In this example illustrated by FIG. 5, the operating conditions are identical to those of the two previous examples. The combustion chamber is made up of two monolithic elements. The first includes two slices of monoliths of 5 cm in length each coated with a catalyst based on palladium oxide (and having the same cell density ...). The system then has a 2.5 cm space in which a fuel injection is carried out, then a second 2.5 cm long monolith covered with a catalyst of the hexaaluminate family: FIG. 1 illustrates such a structure.

L'injection de combustible est séparée en deux : 33,5 kg/h, soit la moitié du combustible, est injecté en amont de la première tranche de catalyseur, le reste est injecté avant la dernière tranche.The fuel injection is separated into two: 33.5 kg / h, or half of the fuel, is injected upstream of the first section of catalyst, the rest is injected before the last section.

Selon l'invention, l'injection minimale à chaque étage (ou tranche) monolithique est d'environ 20 % de la charge totale. Ceci permet de réaliser plus facilement le mélange du combustible avec l'air ou les fumées.According to the invention, the minimum injection at each monolithic stage (or section) is approximately 20% of the total load. This makes it easier to mix the fuel with the air or the fumes.

Vis-à-vis des systèmes précédents, l'amélioration constatée concerne la température du substrat qui ne dépasse pas 900°C dans les deux premières tranches. De même, dans la dernière tranche, les températures ne dépassent pas 1100°C, température maximale que peuvent supporter les hexaaluminates sans désactivations importantes. La combustion est achevée en phase homogène immédiatement en aval de la dernière tranche. Dans ces conditions, le système a fonctionné 400 heures sans qu'on note de désactivation significative du catalyseur ou de variation de la température d'amorçage.Compared to the previous systems, the improvement observed relates to the temperature of the substrate which does not exceed 900 ° C. in the first two sections. Similarly, in the last tranche, the temperatures do not exceed 1100 ° C, the maximum temperature that the hexaaluminates can withstand without significant deactivation. Combustion is completed in homogeneous phase immediately downstream of the last section. Under these conditions, the system operated for 400 hours without any significant deactivation of the catalyst or variation in the initiation temperature.

Ainsi la présente invention, en abaissant significativement les températures des monolithes, permet de préserver leur intégrité. Par ailleurs, la combustion catalytique, qui ne peut s'opérer que pour des températures inférieures à 900°C est donc, selon l'invention, réalisée dans presque tous les monolithes.Thus the present invention, by significantly lowering the temperatures of the monoliths, makes it possible to preserve their integrity. Furthermore, catalytic combustion, which can only operate for temperatures below 900 ° C. is therefore, according to the invention, carried out in almost all monoliths.

La figure 6 concerne un mode de réalisation de l'invention qui ne diffère de celui de la figure 2 que par la forme des deux premiers monolithes 4 et 6 : au lieu d'être cylindriques ils sont ici annulaires avec une zone 12 centrale empêchant l'écoulement. Ceci permet un cheminement différent du mélange, qui sera préféré si le réacteur doit être utilisé avec des conditions opératoires différents de celles de la figure 2.Figure 6 relates to an embodiment of the invention which differs from that of Figure 2 only in the shape of the first two monoliths 4 and 6: instead of being cylindrical they are here annular with a central zone 12 preventing the 'flow. This allows a different flow of the mixture, which will be preferred if the reactor is to be used with operating conditions different from those of FIG. 2.

Enfin les figures 7A et 7B montrent un mode de réalisation de l'invention selon lequel les premier et deuxième éléments monolithes 4, 6 sont tous deux annulaires, concentriques et traversés radialement par le mélange air-combustible. Plus précisément, l'enveloppe 1 est un cylindre présentant une faible hauteur vis-à-vis de son diamètre. L'entrée pour l'air est située sur le disque d'entrée 9, près de l'axe dudit cylindre de même que le premier moyen d'injection 3. Les éléments monolithes sont disposés concentriquement entre les deux disques d'extrémité (9, 10) du cylindre 1. Le disque d'entrée 9 a une surface plus grande que le fond 10.Finally, FIGS. 7A and 7B show an embodiment of the invention according to which the first and second monolithic elements 4, 6 are both annular, concentric and crossed radially by the air-fuel mixture. More specifically, the envelope 1 is a cylinder having a low height with respect to its diameter. The air inlet is located on the inlet disc 9, near the axis of said cylinder, as is the first injection means 3. The monolithic elements are arranged concentrically between the two end discs (9 , 10) of cylinder 1. The input disc 9 has a larger surface area than the bottom 10.

Ainsi, le mélange air-combustible est dévié radialement à travers le premier élément monolithe annulaire 4. Le deuxième moyen d'injection 5 comprend ici plusieurs moyens d'injection qui débouchent tous dans l'espace annulaire 11 délimité par le premier élément monolithe 4 et le deuxième élément monolithe 6.Thus, the air-fuel mixture is deflected radially through the first annular monolith element 4. The second injection means 5 here comprises several injection means which all open into the annular space 11 delimited by the first monolith element 4 and the second monolith element 6.

Comme dans les modes de réalisation présentés ci-avant, les moyens d'injection 5 permettent d'achever la combustion. De même, le deuxième élément monolithe 6 permet de stabiliser la combustion.As in the embodiments presented above, the injection means 5 make it possible to complete the combustion. Similarly, the second monolithic element 6 makes it possible to stabilize the combustion.

En aval du deuxième monolithe, le mélange gazeux sort de l'enveloppe cylindrique 1 comme indiqué par les flèches A' sur la figure 7A c'est-à-dire par l'extrémité opposée à l'entrée 2 ; ceci est rendu possible par le fait que le fond 10 recouvre le deuxième élément monolithe 6 et que le disque d'entrée 9 présente une surface plus grande que le fond 10.Downstream of the second monolith, the gas mixture leaves the cylindrical envelope 1 as indicated by the arrows A 'in FIG. 7A, that is to say by the end opposite to the inlet 2; this is made possible by the fact that the bottom 10 covers the second monolithic element 6 and that the input disc 9 has a larger surface than the bottom 10.

Ce mode de réalisation de l'invention sera préférentiellement choisi lorsque des problèmes d'encombrement empêchent de choisir les modes de réalisations décrits en relation avec les figures 1 à 6.This embodiment of the invention will preferably be chosen when space problems prevent the choice of the embodiments described in relation to FIGS. 1 to 6.

Claims (15)

Système de combustion catalytique comprenant une enveloppe (1) ayant une entrée (2) pour un comburant tel que de l'air, plusieurs moyens d'injection (3, 5 ; 7) de combustible destinés à réaliser une injection étagée de combustible, et au moins un premier élément monolithe (4) susceptible d'être recouvert d'un catalyseur de combustion et placé en aval d'un premier moyen d'injection de combustible (3) relativement au sens de progression d'un mélange air-combustible dans le système, ledit premier moyen d'injection réalisant une injection partielle de combustible, caractérisé en ce qu'il comprend au moins un deuxième élément monolithe (6) disposé en aval d'un deuxième moyen d'injection (5), ledit deuxième élément monolithe (6) étant destiné à stabiliser la combustion, le deuxième élément monolithe (6) étant recouvert d'un catalyseur de combustion choisi parmi les hexaaluminates.Catalytic combustion system comprising a casing (1) having an inlet (2) for an oxidant such as air, several fuel injection means (3, 5; 7) intended to perform a staged fuel injection, and at least a first monolithic element (4) capable of being covered with a combustion catalyst and placed downstream of a first fuel injection means (3) relative to the direction of progression of an air-fuel mixture in the system, said first injection means performing a partial injection of fuel, characterized in that it comprises at least a second monolithic element (6) disposed downstream of a second injection means (5), said second element monolith (6) being intended to stabilize combustion, the second monolith element (6) being covered with a combustion catalyst chosen from hexaaluminates. Système selon la revendication 1, caractérisé en ce qu'il comprend un troisième élément monolithe (8) placé en aval du deuxième élément monolithe (6) et en ce qu'un troisième moyen d'injection de combustible (7) est disposé entre le deuxième (6) et le troisième élément monolithe (8).System according to claim 1, characterized in that it comprises a third monolith element (8) placed downstream of the second monolith element (6) and in that a third fuel injection means (7) is disposed between the second (6) and the third monolithic element (8). Système selon l'une quelconque des revendications précédentes, caractérisé en ce que le premier ou le deuxième monolithe (4) comprend une pluralité de monolithes élémentaires recouverts de catalyseurs à base de métaux précieux.System according to any one of the preceding claims, characterized in that the first or the second monolith (4) comprises a plurality of elementary monoliths covered with catalysts based on precious metals. Système selon l'une quelconque des revendications précédentes, caractérisé en ce que le deuxième élément monolithe (6) est tel que le catalyseur qui le recouvre est susceptible de supporter des températures inférieures à environ 1100°C, et que la température des gaz en sortie du deuxième élément monolithe est inférieure à environ 1000°C.System according to any one of the preceding claims, characterized in that the second monolithic element (6) is such that the catalyst which covers it is capable of withstanding temperatures below about 1100 ° C, and that the temperature of the gases leaving of the second monolith element is less than about 1000 ° C. Système selon la revendication 4, caractérisé en ce que ledit catalyseur consiste essentiellement en une formule A(1-x)ByCzAl(12-y-z)O(19-d), dans laquelle A représente au moins un élément de valence X sélectionné dans le groupe formé par le baryum, le strontium et les terres rares ; B représente au moins un élément de valence Y sélectionné dans le groupe formé par Mn, Co et Fe ; C représente au moins un élément sélectionné dans le groupe formé par Mg et Zn ; x ayant une valeur de 0 à 0.25, y ayant une valeur de 0.5 à 3 et z ayant une valeur de 0.01 à 3 ; la somme y + z ayant une valeur maximale de 4 et d a une valeur qui, déterminée en fonction des valences X et Y respectives des éléments A et B et de la valeur de x, y et de z, est égale à 1-1/2 {(1-x)X + yY - 3 y -z}
Figure imgb0004
.System according to claim 4, characterized in that said catalyst consists essentially of a formula A (1-x) B y C z Al (12-yz) O (19-d) , in which A represents at least one element of valence X selected from the group formed by barium, strontium and rare earths; B represents at least one element of valence Y selected from the group formed by Mn, Co and Fe; C represents at least one element selected from the group formed by Mg and Zn; x having a value of 0 to 0.25, y having a value of 0.5 to 3 and z having a value of 0.01 to 3; the sum y + z having a maximum value of 4 and a value which, determined according to the respective valences X and Y of the elements A and B and the value of x, y and z, is equal to 1-1 / 2 {(1-x) X + yY - 3 y -z}
Figure imgb0004
 . Système selon l'une quelconque des revendications précédentes. caractérisée en ce que le catalyseur recouvrant le premier élément monolithe (4) comprend de l'oxyde de palladium.System according to any one of the preceding claims. characterized in that the catalyst covering the first monolithic element (4) comprises palladium oxide. Système selon la revendication 6, caractérisé en ce que ledit catalyseur comprend un support d'oxyde réfractaire et une phase active comprenant en pourcentage poids par rapport audit support : de 0,3 à 20 % de cérium, de 0,01 à 3,5 % de fer et de 1 à 10 % d'au moins un élément choisi parmi la platine et le palladium.System according to Claim 6, characterized in that the said catalyst comprises a refractory oxide support and an active phase comprising in percentage by weight relative to the said support: from 0.3 to 20% of cerium, from 0.01 to 3.5 % iron and from 1 to 10% of at least one element chosen from platinum and palladium. Système selon l'une quelconque des revendications précédentes, caractérisé en ce que le premier élément monolithe (4) est tel que le catalyseur qui le recouvre est susceptible de supporter des températures inférieures à environ 900°C, et que la température des gaz en sortie dudit premier élément monolithe est inférieure à environ 800°C.System according to any one of the preceding claims, characterized in that the first monolithic element (4) is such that the catalyst which covers it is capable of withstanding temperatures below about 900 ° C, and that the temperature of the gases leaving said first monolith element is less than about 800 ° C. Système selon l'une quelconque des revendications précédentes, caractérisé en ce que l'enveloppe (1) est essentiellement cylindrique, l'entrée (2) pour le comburant et le premier moyen d'injection (3) est située à l'une de ses extrémité sur une section transversale, lesdits deuxième et/ou troisième moyen d'injection (5, 7) débouchent longitudinalement dans ledit cylindre (1).System according to any one of the preceding claims, characterized in that the casing (1) is essentially cylindrical, the inlet (2) for the oxidant and the first injection means (3) is located at one of its ends on a cross section, said second and / or third injection means (5, 7) open longitudinally into said cylinder (1). Système selon la revendication 9, caractérisé en ce que l'un au moins des éléments monolithes est annulaire.System according to claim 9, characterized in that at least one of the monolithic elements is annular. Système selon la revendication 9, caractérisé en ce que l'un au moins des éléments monolithes (4 ; 6 ; 8) est cylindrique et occupe toute la section transversale dudit cylindre (1).System according to claim 9, characterized in that at least one of the monolithic elements (4; 6; 8) is cylindrical and occupies the entire cross section of said cylinder (1). Système selon la revendication 10, caractérisé en ce que tous les éléments monolithes (4, 6 ; 4, 6, 8) sont cylindriques.System according to claim 10, characterized in that all the monolithic elements (4, 6; 4, 6, 8) are cylindrical. Système selon l'une quelconque des revendications 9 ou 10, caractérisé en ce que le premier élément monolithe (4) et le deuxième élément monolithe (6) sont annulaires.System according to either of Claims 9 and 10, characterized in that the first monolithic element (4) and the second monolithic element (6) are annular. Système selon l'une quelconque des revendications 11 ou 12, caractérisé en ce que le troisième élément monolithe (8) est cylindrique.System according to any one of claims 11 or 12, characterized in that the third monolith element (8) is cylindrical. Système selon l'une quelconque des revendications 9 ou 10, caractérisé en ce que les éléments monolithes (4, 6) annulaires sont disposés concentriquement entre un disque d'extrêmité (9) équipé des entrées de comburant (2) et de combustible (3) et un fond (10) autorisant la sortie des gaz, le disque d'entrée (9) présentant une surface plus grande que le fond (10), de sorte que le mouvement des gaz à l'intérieur dudit cylindre (1) est essentiellement radial.System according to either of Claims 9 and 10, characterized in that the annular monolithic elements (4, 6) are arranged concentrically between an end disc (9) fitted with oxidizer (2) and fuel (3) inlets. ) and a bottom (10) allowing the outlet of the gases, the inlet disc (9) having a larger surface than the bottom (10), so that the movement of the gases inside said cylinder (1) is essentially radial.
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FR2743616A1 (en) 1997-07-18
EP0784187B1 (en) 2002-07-17
US5797737A (en) 1998-08-25
FR2743616B1 (en) 1998-02-27
JPH09196307A (en) 1997-07-29
DK0784187T3 (en) 2002-09-02
DE69622361D1 (en) 2002-08-22
DE69622361T2 (en) 2002-11-14

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