EP1395776A1 - Thermal generator and combustion method for limiting nitrogen oxide emission by re-combustion of fumes - Google Patents

Thermal generator and combustion method for limiting nitrogen oxide emission by re-combustion of fumes

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Publication number
EP1395776A1
EP1395776A1 EP02743330A EP02743330A EP1395776A1 EP 1395776 A1 EP1395776 A1 EP 1395776A1 EP 02743330 A EP02743330 A EP 02743330A EP 02743330 A EP02743330 A EP 02743330A EP 1395776 A1 EP1395776 A1 EP 1395776A1
Authority
EP
European Patent Office
Prior art keywords
combustion
fuel
fumes
thermal generator
zone
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP02743330A
Other languages
German (de)
French (fr)
Inventor
Gérard Martin
Willi Nastoll
John Roesler
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
IFP Energies Nouvelles IFPEN
Original Assignee
IFP Energies Nouvelles IFPEN
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by IFP Energies Nouvelles IFPEN filed Critical IFP Energies Nouvelles IFPEN
Publication of EP1395776A1 publication Critical patent/EP1395776A1/en
Withdrawn legal-status Critical Current

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23M—CASINGS, LININGS, WALLS OR DOORS SPECIALLY ADAPTED FOR COMBUSTION CHAMBERS, e.g. FIREBRIDGES; DEVICES FOR DEFLECTING AIR, FLAMES OR COMBUSTION PRODUCTS IN COMBUSTION CHAMBERS; SAFETY ARRANGEMENTS SPECIALLY ADAPTED FOR COMBUSTION APPARATUS; DETAILS OF COMBUSTION CHAMBERS, NOT OTHERWISE PROVIDED FOR
    • F23M9/00—Baffles or deflectors for air or combustion products; Flame shields
    • F23M9/06—Baffles or deflectors for air or combustion products; Flame shields in fire-boxes
    • 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
    • F23C6/042—Combustion apparatus characterised by the combination of two or more combustion chambers or combustion zones, e.g. for staged combustion in series connection with fuel supply in stages
    • 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 
    • F23C2201/00—Staged combustion
    • F23C2201/30—Staged fuel supply

Definitions

  • the present invention relates to the field of thermal generators, in particular industrial boilers, making it possible to limit the emissions of nitrogen oxides by recombination of the fumes and a method for implementing such a generator.
  • the invention is applicable in any type of boiler such as, for example, boilers with smoke tubes, with water tubes, ambitubular boilers, and its realization is possible whatever the thermal power of said boiler.
  • This recombustion is a technique for reducing nitrogen oxides based on the staging of combustion. In a home usually these boilers and where this technique is implemented, we can distinguish three zones:
  • the remaining fuel is injected which consumes the excess oxygen of the fumes from the first zone.
  • the atmosphere in this second zone becomes reducing and the nitrogen oxides generated in the first zone are mainly transformed into molecular nitrogen, under the action of hydrocarbon radicals.
  • air is added in order to eliminate all the unburnt substances generated in the second zone and to have a standard excess of air of 5 to 15% as output.
  • the recombustion method as previously described generally makes it possible to reduce the NO x emissions by approximately 50 to 80%.
  • the main risk is to have a non-combustion of said fuel following an incident or a poorly controlled transient operation. In this case, there is a risk of explosion in the parts located downstream of the combustion zone (recovery boiler, filter, etc.).
  • the present invention proposes to remedy the above drawbacks by means of a thermal generator comprising a hearth tube in which a fuel is burned, recombination means making it possible to reduce the levels of nitrogen oxides present in said fumes, and means for recovering the heat of the fumes from said combustion, characterized in that it comprises containment means in which said recombination means are placed.
  • the confinement means are arranged between the hearth tube and the heat recovery means.
  • Said confinement means can be extractable and fixed on an access hatch.
  • the confinement means can comprise means making it possible to modify the speed profile of the fumes at the inlet of said confinement means.
  • a smoke box can be placed between the confinement means and said heat recovery means.
  • Said smoke can may include heat exchange tubes with the fumes.
  • said confinement means can comprise a ferrule inside which are successively arranged, in the direction of circulation of the fumes, means for injecting a combustion fuel and means for generating a pilot flame.
  • Said confinement means can further comprise air injection means allowing post-combustion.
  • the pilot flame can be positioned substantially equidistant from the injection point of the post combustion air and the injection point of the combustion fuel.
  • the invention also relates to a method for limiting the emissions of nitrogen oxides emitted by a thermal generator, characterized in that the following steps are carried out: a) a majority of the fuel is burned in a combustion zone, b) passing the fumes resulting from said combustion into a containment zone (11), c) mixing in said containment zone a minority of the fuel with said fumes.
  • air can be injected during a step d) into the fumes resulting from step c).
  • step d) can be carried out in said confinement zone.
  • step c) The fumes from step c) can be brought into contact with a pilot flame before step d).
  • step d a chemical reagent other than the fuel allowing the reduction of nitrogen oxides by selective non-catalytic route.
  • a chemical reagent other than the fuel allowing the reduction of nitrogen oxides by selective non-catalytic route.
  • the device and / or method according to the invention advantageously makes it possible to reduce the discharges of nitrogen oxides by 30 to 90% and preferably from 50 to 75% in industrial boilers of powers generally between 100 kWth (thermal kilowatts ) and 50 MWth (thermal megawatts), without requiring expensive and sophisticated equipment, since they are limited to a ferrule and means for injecting fuel and oxidant.
  • this method of reducing nitrogen oxide emissions does not use "reagents" other than the fuels used by the boiler.
  • the proposed solution is all the more advantageous since the installation of known specific burners and producing only small quantities of nitrogen oxide (called low-NO x burners) is often difficult, if not impossible, in homes. confined like those encountered with fire tube boilers or in instant steam boilers.
  • the solution provided by the present invention can also be envisaged in existing boilers, with a minor modification of the smoke box which connects the furnace tube and the smoke tubes. Thanks to the invention, the recombustion is carried out under optimal conditions, insofar as the flow of smoke to be treated is homogeneous in temperature and in concentration and thanks to the possibility of injecting the recombustion fuel and the afterburner air, in optimal conditions from the point of view of the mixture.
  • this recombustion is carried out without risk for the hearth pipe, which is a sensitive part of the boiler, and the confinement means (a ferrule for example) constitute a screen which protects said hearth pipe from possible risks of corrosion. and / or carbon deposits.
  • the furnace tube is thus never in contact with reducing gases and / or loaded with substances liable to create carbon deposits.
  • pilot burner eliminates the possibility of having significant quantities of unburnt fuel at the outlet of the combustion zone, and the possible risks of explosion in the downstream parts of the boiler which result therefrom.
  • the possible risks of coking of the recombination zone are also much more limited on the warmer walls of the confinement means (typically between 800 and 1100 ° C.) than on membrane walls such as those usually used or in the hearth tube, where the temperatures are usually limited between 250 and 400 ° C.
  • the presence of two fuel injection points in the furnace tube, a first point at the main burner and a second point at the recombination zone facilitates the control of the heat extraction in said tube-hearth, in particular in reduced operation, if one wishes to avoid too great a reduction in temperatures at the place where the post-combustion operation is carried out.
  • the proposed solution advantageously makes it possible to carry out a recombination operation with heavy petroleum products; which would certainly be difficult to do without means of containment, due to fouling problems.
  • the device and / or method according to the invention it is also possible to envisage a strategy for introducing multi-stage recombination fuel, which is certainly more suitable for fuels having a certain amount of nitrogen of constitution, in order to thus avoid the conversion of said nitrogen of constitution into NO x .
  • the maintenance operations on the recombustion zone are very simple, insofar as the confinement means can be easily removed from the hearth tube.
  • the containment means of the invention has a low mass compared to that of the boiler and therefore does not introduce inertia additional important. Start-up, shift or stop times are therefore not penalized.
  • the solution proposed in the present application for significantly reducing the level of NO x emission in the fumes finally discharged can also be applied to boilers with vertical cylindrical hearth, with instantaneous vaporization or not. It is also conceivable for boilers with water tubes having fireplaces with cylindrical or parallelepiped geometry.
  • FIG. 1 shows diagrammatically a boiler according to the invention.
  • FIG. 1 shows an industrial boiler with two-pass smoke tubes incorporating a recombination device according to the invention, but of course, the invention is not however limited to this type of boiler configuration.
  • This boiler comprises a burner 1, a cylindrical hearth tube 2, flue tubes 3 serving as means for recovering the heat of the flue gases from combustion, a cylindrical boiler body 4 in which the water to be heated is located and to be vaporized, smoke boxes 5 and 6, an evacuation to the chimney 7, a water supply 101, a steam outlet 102 and a member 103 for regulating the level of water in said boiler body 4.
  • the burner 1 is supplied, by line 8, with a gaseous or liquid fuel and by an oxidizer, here in the form of gas which can be air, by line 9.
  • This burner is placed in an opening 10 and produces a flame 29 which develops in the hearth tube 2 of substantially cylindrical shape and which will heat the water present around this hearth tube.
  • the hearth tube 2 is calculated so that the flame at full power does not occupy its entire length, but leaves, in its downstream part in the direction of the gas flow, a free space which corresponds for example to a third of the total volume of said focal tube
  • confinement means in the form of a substantially cylindrical shell 11, are placed in the downstream part of the hearth tube 2. It is in this shell that the fumes from the burner 1 will penetrate and that the recombustion and possibly post-combustion operation of said smoke is carried out.
  • the outside diameter of the ferrule is slightly smaller than the inside diameter of the focal tube, so as to create a passage 12 between said ferrule and said focal tube. This passage must be minimized according to any known technique so that a minimal fraction smoke crosses it but must be sufficient for disassembly of the shell by an access hatch 24 is possible.
  • the section of the passage 12 represents from 0.1 to 10% of the total passage section of the said tube-hearth, and preferably from 2 to 5%. Said ferrule can simply be placed in the focal tube on support elements 13, but other fixing means known to those skilled in the art are possible.
  • a concentric rod 17 supplied with fuel and oxidizer by lines 18 and 19 and which is used to create at its end 20 an annular pilot flame 31, multipoint or even single, and finally a last concentric rod 21 supplied with air by a line 22 and which makes it possible to inject the air afterburner 32 which is introduced for example through the calibrated orifices 23.
  • These fuel and oxidizer injection means are for example fixed on an access hatch 24. They can be dimensioned according to the rules of the art so that their cooling takes place solely thanks to the fuel, the oxidant and any fluids carriers they convey, but their cooling by an auxiliary fluid circulating in jackets (not shown in Figure 1) is also possible.
  • the ferrule 11 may optionally include in its upstream part (in the direction of the flue gas flow) means 25 intended to modify the velocity profile of the flue gases at the inlet of said ferrule.
  • These means include for example a grid or a perforated plate. The geometry of these means is defined by a person skilled in the art, so that in combination with the means used for injecting the combustion fuel, a very rapid and very homogeneous dispersion of the said combustion fuel is obtained in the smoke stream to be treated.
  • the means 25 can also serve as a heat shield and protect the means for injecting the combustion fuel and the post-combustion air from excessive radiation from the flame.
  • the means 25 can finally serve to homogenize the temperature in the shell 11.
  • the pilot flame 31 is positioned between the injection point of the combustion fuel 30 and the injection point of the post-combustion air 32, and preferably substantially at equal distance between the two points.
  • the ferrule 11 has orifices 26 through which the treated smoke leaves to go into the smoke box 5, then then into the smoke tubes 3.
  • the ferrule 11 can have a total opening in its downstream part (which opens into the smoke box 5).
  • the shell only houses the recombustion zone, the post-combustion operation being carried out in the smoke box and the post-combustion air being introduced to from the walls of said smoke box, and no longer from a central rod.
  • the smoke box can be with refractory walls or partially or totally provided with exchanger tubes 27 connected or not to the boiler body 4 according to an embodiment similar or equivalent to that described in "Engineering techniques, BE2, B1480- 7 ".
  • the exchanger tubes can be partially offset, in order to leave a free passage to the shell, so that it can be easily removed from the boiler if necessary.
  • the ferrule 11 is made of refractory metallic materials. It can be covered internally, partially or completely with insulating materials, in order to reduce heat exchange with the hearth tube. For example, all or part of the shell corresponding to the recombustion zone may be covered with a thin layer of a highly insulating material such as ceramics, for example, while the part of the shell where the the post-combustion will be substantially free of insulation.
  • the insulating materials will be deposited on the walls according to the rules of the art, taking into account in particular the differences in expansion between metal parts and ceramics.
  • the refractory linings of the combustion zone may also be chosen so that they limit the formation of coke, more especially when heavy petroleum fuels are used which are capable of generating large quantities of unburnt materials.
  • the injection of said combustion fuel will preferably take place in two stages (or even in several stages): a first injection immediately at the inlet of the shell, and a second injection located approximately halfway between the first entry point of the combustion fuel and the injection point of the post-combustion air.
  • the flow of recombustion fuel injected at the first point is calculated so as to consume all of the residual oxygen from the main combustion zone, without creating a zone that is truly rich in fuel.
  • the object of the second injection is to create a zone which is truly rich in fuel in the second part of the recombination zone.
  • the burner 1 uses, for example, natural gas, or heavy fuel oil, or petroleum residues, or else any type of fuel used by industrial boilers with smoke tubes. It is generally a conventional burner that generates a compact flame, and with which it is difficult to develop strategies for reducing nitrogen oxides at the burner. In fact, the most generally used hearth tubes are too narrow to receive burners with low nitrogen oxide emissions because these generally generate very large flames.
  • the burner 1 can have a means of partially or fully rotating the oxidizing gas (not shown in FIG. 1), in order to have flowing current of the smoke towards the outlet of the hearth tube, rather localized near the wall of said hearth tube, and thus facilitate the flow of a minor portion of the fumes into space 12 in the direction indicated in FIG. 1.
  • the burner 1 and the oxidant injection means can be designed to promote a strong axial pulse of the oxidant, so as to create recirculation currents along the walls of the hearth tube.
  • the direction of circulation of the smoke in the space 12 is opposite to that indicated by the arrows 33 in FIG. 1.
  • a fraction of the smoke present in the smoke box 5 could thus be recycled upstream of the shell 11
  • the advantage of this operating mode is that all of the fumes can be treated by recombination, while in the previously mentioned mode (burner with rotation of the oxidizer) the fraction of fumes which circulates in space 12 is not subject to recombination.
  • the excess air at the burner relative to the stoichiometry is adjusted so as to be typically between 5 and 25%.
  • the position of the ferrule 11 in the hearth tube 2 is fixed so that the temperature of the fumes at the inlet of said ferrule in nominal operation is between 1100 and 800 ° C and preferably between 1000 and 900 ° C .
  • the quantity of recombustion fuel introduced into the shell is between 5 and 30% of the total fuel consumed by the boiler, and preferably between 10 and 15%.
  • the fuel used by the pilot torch typically consumes only 1% of the total fuel.
  • the post-combustion air flow is calculated so that the excess air leaving the shell is between 5 and 25%.
  • the assembly for mixing the combustion fuel with the fumes to be treated constituted by the means 25 and the injection device 15 will be sized according to any technique known to those skilled in the art, so that said mixing is carried out in less 100 ms.
  • the injection may be carried out from a single head provided with a sufficient number of orifices, as shown in FIG. 1, but other modes of injection at one or more rings with a diameter greater than that of the rod 14 are also possible.
  • the injection head will be calculated according to the rules of the art, so that the particle size distribution and the initial droplet speeds ensure complete and homogeneous coverage of the vein of fumes to be treated, without contact of droplets not completely vaporized with the inner wall of the shell 11.
  • the residence time of the fumes between the point of injection of the combustion fuel and the point of injection of the post-combustion air is between 100 and 500 ms and preferably between 150 and 200 ms.
  • the role of the pilot flame 31 is to ensure the combustion of the combustion fuel in cases where the temperature at the entrance to the combustion zone would drop suddenly following an incident or a poorly controlled transient operation.
  • the pilot flame 31 essentially has a safety function and it is generally excluded to permanently maintain a recombustion operation where the recombustion fuel would not be partially or fully oxidized before the pilot burner.
  • a temperature probe not shown in FIG. 1 is placed on the rod 14, with one or more measuring points located between the end 15 of said rod 14 and the end 20 of the rod 17. According to an example of the operation, when the temperature measured at this or these points will be lower than a set value, which is for example between 500 and 1000 ° C, and preferably between 800 and 900 ° C, stopping the recombustion operation is immediately engaged.
  • the device for introducing the post-combustion air is calculated according to the rules of the art, so that the time for mixing said post-combustion air with the gases from the combustion zone is less than 100 ms.
  • Complementary means not shown in FIG. 1, such as for example venturi or diaphragm, can be installed before or at the point or points of injection of the post-combustion air, in order to favor the mixing of said post air. -combustion with gases from the recombustion zone.
  • the post-combustion air can optionally be added with reagents such as ammonia or urea or other compounds with equivalent effects, in order to add a reduction of nitrogen oxides by selective non-catalytic route, to the post-combustion operation proper.
  • reagents such as ammonia or urea or other compounds with equivalent effects
  • the recombustion fuel is only introduced into the shell once the following operations have been carried out:
  • the control device is adjusted so as to maintain a substantially constant temperature after the injection of the combustion fuel. For example, when the power of the boiler is reduced by half, it is possible according to a first operating mode to lower the fuel flow rates at the main burner and in the recombustion zone in the same proportions.
  • this approach has the disadvantage of reducing the temperature at the inlet of the shell and therefore in the recombustion zone, with the consequent risks of substantial reduction in the reduction efficiency of nitrogen oxides.
  • a strategy which consists of reducing the flow rate at the level of the main burner and increasing the flow rate of recombination fuel will advantageously be preferred, the sum of these two flow rates remaining identical to the flow normally required by partial operation. By doing so, the heat exchange in the upstream part of the hearth tube is reduced.
  • This mode of managing the transient phases advantageously makes it possible to maintain a substantially constant thermal level in the downstream part of said hearth tube and consequently a NO x reduction yield that is also substantially constant.
  • the adjustment of the thermal profile in the furnace tube 2 during variations in the operation of the boiler can also be obtained by moving the injection assembly of the combustion fuel and air of combustion, along the main axis of the ferrule 11. By proceeding in this way, it is thus possible to modify the quantity of heat extracted in the hearth tube.

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)

Abstract

The invention concerns a thermal generator comprising a furnace tube (2) wherein a fuel is burnt, re-combustion means (14, 15) for reducing the nitrogen oxide contents present in said fumes and means for recuperating the heat (3) of the fumes resulting from said combustion. The invention is characterised in that the re-combustion means (14, 15) are placed in confinement means (11).

Description

GENERATEUR THERMIQUE ET PROCEDE DE COMBUSTION PERMETTANT DE LIMITER LES EMISSIONS D'OXYDES D'AZOTE PAR RECOMBUSTION DES FUMEESTHERMAL GENERATOR AND COMBUSTION METHOD FOR LIMITING EMISSIONS OF NITROGEN OXIDES BY RECOMBUSTION OF FUMES
La présente invention concerne le domaine des générateurs thermiques, notamment de chaudières industrielles, permettant de limiter les émissions d'oxydes d'azote par recombustion des fumées et un procédé pour la mise œuvre d'un tel générateur.The present invention relates to the field of thermal generators, in particular industrial boilers, making it possible to limit the emissions of nitrogen oxides by recombination of the fumes and a method for implementing such a generator.
L'invention est applicable dans tout type de chaudière tel que par exemple les chaudières à tubes de fumées, à tubes d'eau, les chaudières ambitubulaires, et sa réalisation est possible quelle que soit la puissance thermique de ladite chaudière.The invention is applicable in any type of boiler such as, for example, boilers with smoke tubes, with water tubes, ambitubular boilers, and its realization is possible whatever the thermal power of said boiler.
11 est déjà connu des procédés de recombustion des fumées émises pour réduire les émissions de NOx, tels que ceux décrits dans les brevets US 5,139,755 et WO 97/2513411 is already known methods of recombining the fumes emitted to reduce NO x emissions, such as those described in US Patents 5,139,755 and WO 97/25134
Cette recombustion est une technique de réduction des oxydes d'azote basée sur l'étagement de la combustion. Dans un foyer que comporte habituellement ces chaudières et où cette technique est mise en oeuvre, on peut distinguer trois zones :This recombustion is a technique for reducing nitrogen oxides based on the staging of combustion. In a home usually these boilers and where this technique is implemented, we can distinguish three zones:
- la première zone dans laquelle on brûle environ 85 à 95 % en masse du combustible dans des conditions standards, c'est-à-dire avec des excès d'air d'environ 5 à 15 % quand on emploie des combustibles gazeux ou des combustibles liquides.- the first zone in which approximately 85 to 95% by mass of the fuel is burned under standard conditions, that is to say with excess air of approximately 5 to 15% when using gaseous fuels or liquid fuels.
- dans la seconde zone , située en aval de la première, on injecte le combustible restant qui consomme l'excès d'oxygène des fumées provenant de la première zone. L'atmosphère de cette seconde zone devient réductrice et les oxydes d'azote générés dans la première zone sont transformés en azote moléculaire pour l'essentiel, sous l'action de radicaux hydrocarbonés.- In the second zone, located downstream of the first, the remaining fuel is injected which consumes the excess oxygen of the fumes from the first zone. The atmosphere in this second zone becomes reducing and the nitrogen oxides generated in the first zone are mainly transformed into molecular nitrogen, under the action of hydrocarbon radicals.
- dans une troisième zone (ou zone de post-combustion), on ajoute de l'air afin d'éliminer toutes les substances imbrûlées générées dans la seconde zone et avoir en sortie un excès d'air standard de 5 à 15 %.- in a third zone (or post-combustion zone), air is added in order to eliminate all the unburnt substances generated in the second zone and to have a standard excess of air of 5 to 15% as output.
La méthode de recombustion tel que précédemment décrite permet en général de diminuer les émissions de NOx d'environ 50 à 80 %.The recombustion method as previously described generally makes it possible to reduce the NO x emissions by approximately 50 to 80%.
Si, dans son principe, la recombustion est attractive du point de vue des performances et ne fait appel à aucun réactif permettant la réduction des oxydes d'azote autre que le combustible lui-même, elle souffre néanmoins d'inconvénients non négligeables.If, in principle, recombustion is attractive from the performance point of view and does not use any reagent allowing the reduction of nitrogen oxides other than the fuel itself, it nevertheless suffers from non-negligible drawbacks.
Dans le cas d'un générateur thermique fonctionnant avec du gaz naturel, les difficultés rencontrées les plus importantes sont :In the case of a thermal generator operating with natural gas, the most significant difficulties encountered are:
- une forte corrosion dans la zone de recombustion et dans la zone de post-combustion dus à la présence d'une atmosphère réductrice ou à l'alternance d'atmosphères oxydante et réductrice. - une oxydation imparfaite du combustible de recombustion dans la zone de post-combustion avec comme corollaire la formation d'imbrûlés gazeux et solides, et dans les cas les plus sévères, des encrassements des surfaces d'échange situées en aval qui vont réduire le rendement énergétique global et exiger des équipements de nettoyage automatique plus sophistiqués et plus chers.- strong corrosion in the recombustion zone and in the post-combustion zone due to the presence of a reducing atmosphere or to the alternation of oxidizing and reducing atmospheres. - imperfect oxidation of the combustion fuel in the post-combustion zone with as a corollary the formation of gaseous and solid unburnt substances, and in the most severe cases, fouling of the exchange surfaces located downstream which will reduce the yield energy consumption and require more sophisticated and more expensive automatic cleaning equipment.
- une sécurité difficile à assurer due à l'étape d'injection d'un combustible dans la zone de recombustion. Le risque principal est d'avoir une non-combustion dudit combustible suite à un incident ou à une marche transitoire mal maîtrisée. Dans ce cas, il y a des risques d'explosion dans les parties situées en aval de la zone de recombustion (chaudière de récupération, filtre, etc.).security which is difficult to guarantee due to the step of injecting a fuel into the recombustion zone. The main risk is to have a non-combustion of said fuel following an incident or a poorly controlled transient operation. In this case, there is a risk of explosion in the parts located downstream of the combustion zone (recovery boiler, filter, etc.).
Dans le cas d'un générateur thermique opérant avec des produits pétroliers lourds, les difficultés de mise en oeuvre sont les mêmes que celles rencontrées avec le gaz naturel, mais elles sont souvent renforcées pour certaines. C'est en particulier vrai pour les imbrûlés solides qui sont produits en plus grandes quantités. Par ailleurs, le rendement de dénitrification peut être plus faible avec ces . produits lourds, à cause de la présence de composés azotés dans le combustible initial.In the case of a thermal generator operating with heavy petroleum products, the implementation difficulties are the same as those encountered with natural gas, but they are often reinforced for some. This is particularly true for solid unburnt products which are produced in larger quantities. Furthermore, the denitrification yield may be lower with these. heavy products, due to the presence of nitrogen compounds in the initial fuel.
La présente invention se propose de remédier aux inconvénients ci-dessus grâce à générateur thermique comprenant un tube foyer dans lequel est brûlé un combustible, des moyens de recombustion permettant de diminuer les taux d'oxydes d'azote présents dans lesdites fumées, et des moyens de récupération de la chaleur des fumées issues de ladite combustion, caractérisé en ce qu'il comprend des moyens de confinement dans lesquels sont placés lesdits moyens de recombustion. Avantageusement, les moyens de confinement sont disposés entre le tube- foyer et les moyens de récupération de chaleur.The present invention proposes to remedy the above drawbacks by means of a thermal generator comprising a hearth tube in which a fuel is burned, recombination means making it possible to reduce the levels of nitrogen oxides present in said fumes, and means for recovering the heat of the fumes from said combustion, characterized in that it comprises containment means in which said recombination means are placed. Advantageously, the confinement means are arranged between the hearth tube and the heat recovery means.
Lesdits moyens de confinement peuvent être extractibles et fixés sur une trappe d'accès.Said confinement means can be extractable and fixed on an access hatch.
De manière préférentielle, les moyens de confinement peuvent comprendre des moyens permettant de modifier le profil de vitesse des fumées à l'entrée desdits moyens de confinement.Preferably, the confinement means can comprise means making it possible to modify the speed profile of the fumes at the inlet of said confinement means.
De manière avantageuse, une boite à fumée peut être placée entre les moyens de confinement et lesdits moyens de récupération de chaleur.Advantageously, a smoke box can be placed between the confinement means and said heat recovery means.
Ladite boite de fumée peut comprendre des tubes d'échange de chaleur avec les fumées.Said smoke can may include heat exchange tubes with the fumes.
Préférentiellement, lesdits moyens de confinement peuvent comprendre une virole à l'intérieur de laquelle sont disposés successivement, dans le sens de circulation des fumées, des moyens d'injection d'un combustible de recombustion et des moyens de génération d'une flamme pilote.Preferably, said confinement means can comprise a ferrule inside which are successively arranged, in the direction of circulation of the fumes, means for injecting a combustion fuel and means for generating a pilot flame.
Lesdits moyens de confinement peuvent comprendre en outre des moyens d'injection d'air permettant la post-combustion.Said confinement means can further comprise air injection means allowing post-combustion.
La flamme pilote peut être positionnée sensiblement à égale distance du point d'injection de l'air de post combustion et du point d'injection du combustible de recombustion.The pilot flame can be positioned substantially equidistant from the injection point of the post combustion air and the injection point of the combustion fuel.
En outre, des moyens d'injection de l'air permettant la post-combustion peuvent être disposés en aval de ladite virole. L'invention concerne également un procédé de limitation des émissions d'oxydes d'azote émis par un générateur thermique, caractérisé en ce qu'on effectue les étapes suivantes : a) on brûle une majorité du combustible dans une zone de combustion, b) on fait passer les fumées résultantes de ladite combustion dans une zone de confinement (11), c) on mélange dans ladite zone de confinement une minorité du combustible avec lesdites fumées.In addition, air injection means allowing post-combustion can be arranged downstream of said shell. The invention also relates to a method for limiting the emissions of nitrogen oxides emitted by a thermal generator, characterized in that the following steps are carried out: a) a majority of the fuel is burned in a combustion zone, b) passing the fumes resulting from said combustion into a containment zone (11), c) mixing in said containment zone a minority of the fuel with said fumes.
Suivant un mode préférentiel, on peut injecter durant une étape d) de l'air dans les fumées résultantes de l'étape c).According to a preferred mode, air can be injected during a step d) into the fumes resulting from step c).
Suivant une autre variante, on peut effectuer l'étape d) dans ladite zone de confinement.According to another variant, step d) can be carried out in said confinement zone.
Suivant un autre mode, on peut brûler environ 70% à environ 95% de la masse du combustible total durant l'étape a), environ 5 à environ 30% de la masse du combustible total durant l'étape c) et on peut injecter une quantité d'air durant l'étape d) permettant d'avoir un excès d'air par rapport à la stœchiométrie d'environ 5 à environ 25%.According to another mode, one can burn approximately 70% to approximately 95% of the mass of the total fuel during stage a), approximately 5 to approximately 30% of the mass of total fuel during stage c) and one can inject an amount of air during step d) making it possible to have an excess of air relative to the stoichiometry of approximately 5 to approximately 25%.
On peut mettre en contact les fumées issues de l'étape c) avec une flamme pilote avant l'étape d).The fumes from step c) can be brought into contact with a pilot flame before step d).
On peut ajouter à l'air injecté durant l'étape d) un réactif chimique autre que le combustible permettant la réduction des oxydes d'azote par voie sélective non catalytique. Dans un mode avantageux, lors des marches réduites du générateur, on peut ajuster les débits de combustible injectés dans la zone de combustion et dans la zone de confinement de façon à maintenir une température sensiblement constante dans ladite zone de confinement.It is possible to add to the air injected during step d) a chemical reagent other than the fuel allowing the reduction of nitrogen oxides by selective non-catalytic route. In an advantageous mode, during reduced steps of the generator, it is possible to adjust the flow rates of fuel injected into the combustion zone and into the confinement zone so as to maintain a substantially constant temperature in said confinement zone.
Grâce à l'invention, une solution simple, efficace et sécurisée de la recombustion des fumées de chaudières est appliquée pour réduire de façon significative les émissions de polluants et plus particulièrement d'oxydes d'azote (NOJ.Thanks to the invention, a simple, effective and secure solution for the combustion of boiler fumes is applied to significantly reduce the emissions of pollutants and more particularly nitrogen oxides (NOJ.
Le dispositif et/ou le procédé selon l'invention permet avantageusement de réduire les rejets d'oxydes d'azote de 30 à 90 % et préférentiellement de 50 à 75 % dans des chaudières industrielles de puissances en général comprises entre 100 kWth (kilowatts thermiques) et 50 MWth (mégawatts thermiques), sans nécessiter des équipements onéreux et sophistiqués, puisqu'ils se limitent à une virole et des moyens d'injection du combustible et du comburant.The device and / or method according to the invention advantageously makes it possible to reduce the discharges of nitrogen oxides by 30 to 90% and preferably from 50 to 75% in industrial boilers of powers generally between 100 kWth (thermal kilowatts ) and 50 MWth (thermal megawatts), without requiring expensive and sophisticated equipment, since they are limited to a ferrule and means for injecting fuel and oxidant.
De plus, ce mode de réduction des émissions d'oxydes d'azote ne fait pas appel à des « réactifs » autres que les combustibles employés par la chaudière.In addition, this method of reducing nitrogen oxide emissions does not use "reagents" other than the fuels used by the boiler.
La solution proposée présente d'autant plus d'intérêt que l'implantation de brûleurs spécifiques connus et ne produisant que de faibles quantités d'oxyde d'azote (appelés brûleurs bas-NOx) est souvent difficile, voire impossible, dans les foyers confinés comme ceux que l'on rencontre avec les chaudières à tubes de fumées ou dans les chaudières à vaporisation instantanée. La solution apportée par la présente invention peut aussi être envisagée dans des chaudières existantes, moyennant une modification mineure de la boîte à fumées qui relie le tube-foyer et les tubes de fumées. Grâce à l'invention, la recombustion est réalisée dans des conditions optimales, dans la mesure où l'écoulement de fumées à traiter est homogène en température et en concentration et grâce à la possibilité de procéder à l'injection du combustible de recombustion et à l'air de postcombustion, dans des conditions optimales du point de vue du mélange. De plus, cette recombustion est effectuée sans risque pour le tube-foyer, qui est une pièce sensible de la chaudière, et les moyens de confinement (une virole par exemple) constituent un écran qui prémunit ledit tube-foyer d'éventuels risques de corrosion et/ou de dépôts carbonés. Enfin, le tube- foyer n'est ainsi jamais en contact avec des gaz réducteurs et/ou chargés en substances susceptibles de créer des dépôts carbonés.The proposed solution is all the more advantageous since the installation of known specific burners and producing only small quantities of nitrogen oxide (called low-NO x burners) is often difficult, if not impossible, in homes. confined like those encountered with fire tube boilers or in instant steam boilers. The solution provided by the present invention can also be envisaged in existing boilers, with a minor modification of the smoke box which connects the furnace tube and the smoke tubes. Thanks to the invention, the recombustion is carried out under optimal conditions, insofar as the flow of smoke to be treated is homogeneous in temperature and in concentration and thanks to the possibility of injecting the recombustion fuel and the afterburner air, in optimal conditions from the point of view of the mixture. In addition, this recombustion is carried out without risk for the hearth pipe, which is a sensitive part of the boiler, and the confinement means (a ferrule for example) constitute a screen which protects said hearth pipe from possible risks of corrosion. and / or carbon deposits. Finally, the furnace tube is thus never in contact with reducing gases and / or loaded with substances liable to create carbon deposits.
En outre, la présence d'un brûleur pilote supprime la possibilité d'avoir des quantités importantes de combustible imbrûlé à la sortie de la zone de recombustion, et les risques éventuels d'explosion dans les parties aval de la chaudière qui en découlent.In addition, the presence of a pilot burner eliminates the possibility of having significant quantities of unburnt fuel at the outlet of the combustion zone, and the possible risks of explosion in the downstream parts of the boiler which result therefrom.
Il est également possible de mettre en place des matériaux isolants sur certaines des parties internes du moyen de confinement pour ajuster le profil thermique en fonction des exigences de la recombustion. Ainsi, ce moyen de confinement peut aussi être recouvert intérieurement de matériaux spécifiques comme certaines céramiques qui limitent la formation de coke.It is also possible to install insulating materials on some of the internal parts of the confinement means to adjust the thermal profile according to the requirements of the recombustion. Thus, this means of containment can also be covered internally with specific materials such as certain ceramics which limit the formation of coke.
Les éventuels risques de cokage de la zone de recombustion sont aussi beaucoup plus limités sur les parois plus chaudes du moyen de confinement (typiquement entre 800 et 1100 °C) que sur des parois membranées telle que celles habituellement utilisés ou dans le tube-foyer, où les températures sont limitées le plus souvent entre 250 et 400 °C. De plus, la présence de deux points d'injection de combustible dans le tube- foyer, un premier point au niveau du brûleur principal et un second point au niveau de la zone de recombustion, facilite le contrôle de l'extraction de chaleur dans ledit tube-foyer, en particulier en marche réduite, si l'on veut éviter un abaissement trop important des températures à l'endroit où l'on réalise l'opération de post-combustion.The possible risks of coking of the recombination zone are also much more limited on the warmer walls of the confinement means (typically between 800 and 1100 ° C.) than on membrane walls such as those usually used or in the hearth tube, where the temperatures are usually limited between 250 and 400 ° C. In addition, the presence of two fuel injection points in the furnace tube, a first point at the main burner and a second point at the recombination zone, facilitates the control of the heat extraction in said tube-hearth, in particular in reduced operation, if one wishes to avoid too great a reduction in temperatures at the place where the post-combustion operation is carried out.
La solution proposée permet avantageusement de réaliser une opération de recombustion avec des produits pétroliers lourds ; ce qui serait certainement difficile de faire sans moyen de confinement, à cause des problèmes d'encrassement.The proposed solution advantageously makes it possible to carry out a recombination operation with heavy petroleum products; which would certainly be difficult to do without means of containment, due to fouling problems.
Grâce au dispositif et ou au procédé selon l'invention, il est permis également d'envisager une stratégie d'introduction du combustible de recombustion multi-étagée, qui est certainement plus appropriée pour les combustibles ayant une certaine quantité d'azote de constitution, afin d'éviter ainsi la conversion dudit azote de constitution en NOx.Thanks to the device and / or method according to the invention, it is also possible to envisage a strategy for introducing multi-stage recombination fuel, which is certainly more suitable for fuels having a certain amount of nitrogen of constitution, in order to thus avoid the conversion of said nitrogen of constitution into NO x .
Il est également possible de combiner éventuellement une opération de réduction sélective non catalytique à l'opération de recombustion, et ce, dans des conditions optimales pour les deux opérations. Avec une telle combinaison, les rendements peuvent dépasser les 90 %.It is also possible to optionally combine a non-catalytic selective reduction operation with the recombination operation, and this, under conditions optimal for both operations. With such a combination, yields can exceed 90%.
De par l'invention, les opérations de maintenance sur la zone de recombustion sont très simples, dans la mesure où le moyen de confinement peut être facilement retiré du tube-foyer.By the invention, the maintenance operations on the recombustion zone are very simple, insofar as the confinement means can be easily removed from the hearth tube.
Les moyens de confinement de l'invention présente une masse faible comparée à celle de la chaudière et n'introduisent donc pas d'inertie supplémentaire importante. Les délais de démarrage, de changement de marche ou d'arrêt ne sont donc pas pénalisés.The containment means of the invention has a low mass compared to that of the boiler and therefore does not introduce inertia additional important. Start-up, shift or stop times are therefore not penalized.
Avantageusement, la solution proposée dans la présente demande pour réduire de façon notable le niveau d'émission des NOx dans les fumées finalement rejetées peut également être appliquée à des chaudières à foyer cylindrique vertical, à vaporisation instantanée ou non. Elle est aussi envisageable pour des chaudières à tubes d'eau ayant des foyers à géométrie cylindrique ou parallélépipédique.Advantageously, the solution proposed in the present application for significantly reducing the level of NO x emission in the fumes finally discharged can also be applied to boilers with vertical cylindrical hearth, with instantaneous vaporization or not. It is also conceivable for boilers with water tubes having fireplaces with cylindrical or parallelepiped geometry.
Les autres caractéristiques et avantages du présent dispositif apparaîtront mieux à la lecture de la description ci-après d'un mode de réalisation non limitatif de l'invention, en se référant à la figure .1 unique qui schématise une chaudière selon l'invention.The other characteristics and advantages of the present device will appear better on reading the following description of a non-limiting embodiment of the invention, with reference to the single FIG. 1 which shows diagrammatically a boiler according to the invention.
La figure 1 montre une chaudière industrielle à tubes de fumées à deux passes incorporant un dispositif de recombustion selon l'invention mais bien entendu, l'invention n'est cependant pas restreinte à ce type de configuration de chaudière.FIG. 1 shows an industrial boiler with two-pass smoke tubes incorporating a recombination device according to the invention, but of course, the invention is not however limited to this type of boiler configuration.
Cette chaudière comprend un brûleur 1 , un tube-foyer cylindrique 2, des tubes de fumées 3 servant de moyens de récupération de la chaleur des fumées issues de la combustion, un corps de chaudière cylindrique 4 dans laquelle se trouve l'eau à chauffer et à vaporiser, des boîtes à fumées 5 et 6, une évacuation à la cheminée 7, une alimentation en eau 101 , une sortie vapeur 102 et un organe 103 de régulation du niveau d'eau dans ledit corps de chaudière 4.This boiler comprises a burner 1, a cylindrical hearth tube 2, flue tubes 3 serving as means for recovering the heat of the flue gases from combustion, a cylindrical boiler body 4 in which the water to be heated is located and to be vaporized, smoke boxes 5 and 6, an evacuation to the chimney 7, a water supply 101, a steam outlet 102 and a member 103 for regulating the level of water in said boiler body 4.
Le brûleur 1 est alimenté, par la ligne 8, avec un combustible gazeux ou liquide et par un comburant, ici sous forme de gaz qui peut être de l'air, par la ligne 9. Ce brûleur est placé dans un ouvreau 10 et produit une flamme 29 qui se développe dans le tube-foyer 2 de forme sensiblement cylindrique et qui va chauffer l'eau présente autour ce tube foyer.The burner 1 is supplied, by line 8, with a gaseous or liquid fuel and by an oxidizer, here in the form of gas which can be air, by line 9. This burner is placed in an opening 10 and produces a flame 29 which develops in the hearth tube 2 of substantially cylindrical shape and which will heat the water present around this hearth tube.
Le tube-foyer 2 est calculé de façon à ce que la flamme à pleine puissance n'occupe pas toute sa longueur, mais laisse, dans sa partie aval au sens de l'écoulement gazeux, un espace libre qui correspond par exemple à un tiers du volume total dudit tube-foyerThe hearth tube 2 is calculated so that the flame at full power does not occupy its entire length, but leaves, in its downstream part in the direction of the gas flow, a free space which corresponds for example to a third of the total volume of said focal tube
Cette disposition telle qu'elle vient d'être décrite est classiquement utilisée dans l'art antérieur dans le cas d'une chaudière industrielle à deux passes (Techniques de l'ingénieur, BE2, B1480-5 (1998)).This arrangement as just described is conventionally used in the prior art in the case of an industrial boiler with two passes (Engineering techniques, BE2, B1480-5 (1998)).
Selon l'invention, des moyens de confinement, sous la forme d'une virole sensiblement cylindrique 11 , sont placés dans la partie aval du tube-foyer 2. C'est dans cette virole que pénétreront les fumées issues du brûleur 1 et qu'est réalisée l'opération de recombustion et éventuellement de postcombustion desdites fumées. Le diamètre extérieur de la virole est légèrement inférieur au diamètre intérieur du tube-foyer, de façon à créer entre ladite virole et ledit tube-foyer un passage 12. Ce passage doit être minimisé selon toute technique connue de telle façon qu'une fraction minime des fumées la traverse mais devra être suffisant pour le démontage de la virole par une trappe d'accès 24 soit possible. La section du passage 12 représente de 0,1 à 10 % de la section de passage totale dudit tube-foyer, et de préférence de 2 à 5 %. Ladite virole peut être simplement posée dans le tube-foyer sur des éléments de support 13, mais d'autres moyens de fixation connus de l'homme du métier sont possibles.According to the invention, confinement means, in the form of a substantially cylindrical shell 11, are placed in the downstream part of the hearth tube 2. It is in this shell that the fumes from the burner 1 will penetrate and that the recombustion and possibly post-combustion operation of said smoke is carried out. The outside diameter of the ferrule is slightly smaller than the inside diameter of the focal tube, so as to create a passage 12 between said ferrule and said focal tube. This passage must be minimized according to any known technique so that a minimal fraction smoke crosses it but must be sufficient for disassembly of the shell by an access hatch 24 is possible. The section of the passage 12 represents from 0.1 to 10% of the total passage section of the said tube-hearth, and preferably from 2 to 5%. Said ferrule can simply be placed in the focal tube on support elements 13, but other fixing means known to those skilled in the art are possible.
A l'intérieur de la virole 11 , et de préférence en son centre, on trouve plusieurs moyens d'injection de combustible et de comburant. Ces moyens peuvent être concentriques comme représentés à la figure 1 , avec une première canne centrale 14 avec, en son extrémité 15, des moyens de pulvérisation mécanique ou pneumatique pour l'injection du combustible de recombustion 30 apporté par une ligne 16, puis, dans le sens de la périphérie, une canne concentrique 17 alimentée en combustible et comburant par des lignes 18 et 19 et qui sert à créer en son extrémité 20 une flamme pilote annulaire 31 , multipoints ou encore unique, et enfin une dernière canne concentrique 21 alimentée en air par une ligne 22 et qui permet d'injecter l'air de post-combustion 32 qui est introduit par exemple par les orifices calibrés 23.Inside the shell 11, and preferably at its center, there are several means for injecting fuel and oxidizer. These means can be concentric as shown in Figure 1, with a first central rod 14 with, at its end 15, mechanical or pneumatic spraying means for injecting the combustion fuel 30 supplied by a line 16, then, in the direction of the periphery, a concentric rod 17 supplied with fuel and oxidizer by lines 18 and 19 and which is used to create at its end 20 an annular pilot flame 31, multipoint or even single, and finally a last concentric rod 21 supplied with air by a line 22 and which makes it possible to inject the air afterburner 32 which is introduced for example through the calibrated orifices 23.
Ces moyens d'injection du combustible et du comburant sont par exemple fixés sur une trappe d'accès 24. Ils peuvent être dimensionnés selon les règles de l'Art pour que leurs refroidissements se fassent uniquement grâce au combustible, au comburant et aux éventuels fluides porteurs qu'ils véhiculent, mais leurs refroidissements par un fluide auxiliaire circulant dans des jaquettes (non représentées sur la figure 1) est aussi possible.These fuel and oxidizer injection means are for example fixed on an access hatch 24. They can be dimensioned according to the rules of the art so that their cooling takes place solely thanks to the fuel, the oxidant and any fluids carriers they convey, but their cooling by an auxiliary fluid circulating in jackets (not shown in Figure 1) is also possible.
La virole 11 peut éventuellement comporter dans sa partie amont (au sens de l'écoulement des fumées) des moyens 25 destinés à modifier le profil de vitesse des fumées à l'entrée de ladite virole. Ces moyens comprennent par exemple une grille ou une plaque perforée. La géométrie de ces moyens est définie par l'Homme de l'Art, afin qu'en combinaison avec les moyens mis en œuvre pour l'injection du combustible de recombustion, on obtienne une dispersion très rapide et très homogène dudit combustible de recombustion dans la veine de fumées à traiter. Les moyens 25 peuvent également servir d'écran thermique et protéger les moyens d'injection du combustible de recombustion et d'air de post-combustion d'un rayonnement trop important de la flamme. Les moyens 25 peuvent enfin servir à homogénéiser la température dans la virole 11. La flamme pilote 31 est positionnée entre le point d'injection du combustible de recombustion 30 et le point d'injection de l'air de post-combustion 32, et de préférence sensiblement à égale distance entre les deux points.The ferrule 11 may optionally include in its upstream part (in the direction of the flue gas flow) means 25 intended to modify the velocity profile of the flue gases at the inlet of said ferrule. These means include for example a grid or a perforated plate. The geometry of these means is defined by a person skilled in the art, so that in combination with the means used for injecting the combustion fuel, a very rapid and very homogeneous dispersion of the said combustion fuel is obtained in the smoke stream to be treated. The means 25 can also serve as a heat shield and protect the means for injecting the combustion fuel and the post-combustion air from excessive radiation from the flame. The means 25 can finally serve to homogenize the temperature in the shell 11. The pilot flame 31 is positioned between the injection point of the combustion fuel 30 and the injection point of the post-combustion air 32, and preferably substantially at equal distance between the two points.
La virole 11 possède des orifices 26 par lesquels sortent les fumées traitées pour aller dans la boîte à fumées 5, puis ensuite dans les tubes de fumées 3.The ferrule 11 has orifices 26 through which the treated smoke leaves to go into the smoke box 5, then then into the smoke tubes 3.
Selon un autre mode de réalisation, la virole 11 peut présenter une ouverture totale dans sa partie aval (qui débouche dans la boîte à fumées 5).According to another embodiment, the ferrule 11 can have a total opening in its downstream part (which opens into the smoke box 5).
Sans sortir du cadre de l'invention, il est également possible que la virole n'abrite que la zone de recombustion, l'opération de post-combustion étant réalisée dans la boîte à fumées et l'air de post-combustion étant introduit à partir des parois de ladite boîte à fumées, et non plus à partir d'une canne centrale.Without departing from the scope of the invention, it is also possible that the shell only houses the recombustion zone, the post-combustion operation being carried out in the smoke box and the post-combustion air being introduced to from the walls of said smoke box, and no longer from a central rod.
La boîte à fumées peut être à parois réfractorisées ou munie partiellement ou totalement de tubes échangeurs 27 reliés ou non au corps de chaudière 4 selon un mode de réalisation similaire ou équivalent à celui décrit dans « Les techniques de l'ingénieur, BE2, B1480-7 ». Sur la partie arrière 28 de la chaudière, les tubes échangeurs peuvent être en partie déportés, afin de laisser un libre passage à la virole, afin qu'elle puisse être facilement retirée de la chaudière si nécessaire.The smoke box can be with refractory walls or partially or totally provided with exchanger tubes 27 connected or not to the boiler body 4 according to an embodiment similar or equivalent to that described in "Engineering techniques, BE2, B1480- 7 ". On the rear part 28 of the boiler, the exchanger tubes can be partially offset, in order to leave a free passage to the shell, so that it can be easily removed from the boiler if necessary.
La virole 11 est constituée de matériaux métalliques refractaires. Elle pourra être recouverte intérieurement, partiellement ou totalement de matériaux isolants, afin de réduire les échanges thermiques avec le tube-foyer. Par exemple, tout ou partie de la virole correspondant à la zone de recombustion pourra être recouverte d'une fine couche d'un matériau très isolant comme, des céramiques par exemple, tandis que la partie de la virole où se déroule la post-combustion sera sensiblement exempte d'isolant. Les matériaux isolants seront déposés sur les parois selon les règles de l'Art en tenant compte en particulier des différences de dilatation entre des pièces métalliques et des céramiques.The ferrule 11 is made of refractory metallic materials. It can be covered internally, partially or completely with insulating materials, in order to reduce heat exchange with the hearth tube. For example, all or part of the shell corresponding to the recombustion zone may be covered with a thin layer of a highly insulating material such as ceramics, for example, while the part of the shell where the the post-combustion will be substantially free of insulation. The insulating materials will be deposited on the walls according to the rules of the art, taking into account in particular the differences in expansion between metal parts and ceramics.
Les revêtements refractaires de la zone de recombustion pourront également être choisis afin qu'ils limitent la formation de coke, plus spécialement quand on emploie des combustibles pétroliers lourds susceptibles de générer d'importantes quantités d'imbrûlés.The refractory linings of the combustion zone may also be chosen so that they limit the formation of coke, more especially when heavy petroleum fuels are used which are capable of generating large quantities of unburnt materials.
Lorsqu'on utilise des combustibles chargés en azote comme combustible de recombustion, l'injection dudit combustible de recombustion se fera de préférence en deux étapes (voire en plusieurs étapes) : une première injection immédiatement à l'entrée de la virole, et une seconde injection située approximativement à mi-distance entre le premier point d'entrée du combustible de recombustion et le point d'injection de l'air de postcombustion. Le débit de combustible de recombustion injecté au premier point est calculé de façon à consommer la totalité de l'oxygène résiduel issu de la zone de combustion principale, sans créer une zone véritablement riche en combustible. La seconde injection a au contraire pour objet de créer une zone véritablement riche en combustible dans la seconde partie de la zone de recombustion.When using fuels loaded with nitrogen as a combustion fuel, the injection of said combustion fuel will preferably take place in two stages (or even in several stages): a first injection immediately at the inlet of the shell, and a second injection located approximately halfway between the first entry point of the combustion fuel and the injection point of the post-combustion air. The flow of recombustion fuel injected at the first point is calculated so as to consume all of the residual oxygen from the main combustion zone, without creating a zone that is truly rich in fuel. The object of the second injection, on the contrary, is to create a zone which is truly rich in fuel in the second part of the recombination zone.
Le brûleur 1 utilise par exemple du gaz naturel, ou du fuel lourd, ou des résidus pétroliers, ou encore tout type de combustible employé par des chaudières industrielles à tube de fumées. C'est en général un brûleur classique qui génère une flamme compacte, et avec lequel il est difficile de développer des stratégies de réduction des oxydes d'azote au niveau du brûleur. En effet, les tubes-foyers les plus généralement utilisés sont trop étroits pour recevoir les brûleurs à basses émissions d'oxydes d'azote car ceux-ci génèrent le plus souvent des flammes très épanouies.The burner 1 uses, for example, natural gas, or heavy fuel oil, or petroleum residues, or else any type of fuel used by industrial boilers with smoke tubes. It is generally a conventional burner that generates a compact flame, and with which it is difficult to develop strategies for reducing nitrogen oxides at the burner. In fact, the most generally used hearth tubes are too narrow to receive burners with low nitrogen oxide emissions because these generally generate very large flames.
Le brûleur 1 peut avoir un moyen de mise en rotation partielle ou totale du gaz comburant (non représenté sur la figure 1), afin d'avoir des courants de circulation des fumées vers la sortie du tube-foyer, plutôt localisés à proximité de la paroi dudit tube-foyer, et ainsi faciliter l'écoulement d'une mineure partie des fumées dans l'espace 12 selon le sens indiqué sur la figure 1.The burner 1 can have a means of partially or fully rotating the oxidizing gas (not shown in FIG. 1), in order to have flowing current of the smoke towards the outlet of the hearth tube, rather localized near the wall of said hearth tube, and thus facilitate the flow of a minor portion of the fumes into space 12 in the direction indicated in FIG. 1.
Selon un autre mode de réalisation et de fonctionnement, le brûleur 1 et les moyens d'injections du comburant peuvent être conçus pour favoriser une impulsion axiale forte du comburant, de façon à créer des courants de recirculation le long des parois du tube-foyer. Dans ces conditions, le sens de circulation des fumées dans l'espace 12 est opposé à celui indiqué par les flèches 33 sur la figure 1. Une fraction des fumées présente dans la boîte à fumées 5 pourrait être ainsi recyclée en amont de la virole 11. L'intérêt de ce mode de fonctionnement est que l'intégralité des fumées peut être traitée en recombustion, alors que dans le mode évoqué précédemment, (brûleur à mise en rotation du comburant) la fraction des fumées qui circule dans l'espace 12 n'est pas soumise à la recombustion.According to another embodiment and operation, the burner 1 and the oxidant injection means can be designed to promote a strong axial pulse of the oxidant, so as to create recirculation currents along the walls of the hearth tube. Under these conditions, the direction of circulation of the smoke in the space 12 is opposite to that indicated by the arrows 33 in FIG. 1. A fraction of the smoke present in the smoke box 5 could thus be recycled upstream of the shell 11 The advantage of this operating mode is that all of the fumes can be treated by recombination, while in the previously mentioned mode (burner with rotation of the oxidizer) the fraction of fumes which circulates in space 12 is not subject to recombination.
L'excès d'air au brûleur par rapport à la stœchiométrie est réglé de façon à être typiquement compris entre 5 et 25 %.The excess air at the burner relative to the stoichiometry is adjusted so as to be typically between 5 and 25%.
La position de la virole 11 dans le tube-foyer 2 est fixée de façon à ce que la température des fumées à l'entrée de ladite virole en marche nominale soit comprise entre 1100 et 800 °C et de préférence entre 1000 et 900 °C. La quantité de combustible de recombustion introduite dans la virole est comprise entre 5 et 30 % du combustible total consommé par la chaudière, et de préférence entre 10 et 15 %. Le combustible employé par la torche pilote ne consomme typiquement que 1 % du combustible total. Le débit d'air de post-combustion est calculé de façon à ce que l'excès d'air en sortie de virole soit compris entre 5 et 25 %.The position of the ferrule 11 in the hearth tube 2 is fixed so that the temperature of the fumes at the inlet of said ferrule in nominal operation is between 1100 and 800 ° C and preferably between 1000 and 900 ° C . The quantity of recombustion fuel introduced into the shell is between 5 and 30% of the total fuel consumed by the boiler, and preferably between 10 and 15%. The fuel used by the pilot torch typically consumes only 1% of the total fuel. The post-combustion air flow is calculated so that the excess air leaving the shell is between 5 and 25%.
L'ensemble de mélange du combustible de recombustion avec les fumées à traiter, constitué par le moyen 25 et le dispositif d'injection 15 sera dimensionné selon toute technique connue de l'Homme de l'Art, afin que ledit mélange soit réalisé en moins de 100 ms. Dans le cas d'un combustible de recombustion gazeux, l'injection pourra être réalisée à partir d'une tête unique munie d'un nombre d'orifices suffisant, tel que représenté sur la figure 1 , mais d'autres modes d'injection au niveau d'une ou plusieurs couronnes d'un diamètre supérieur à celui de la canne 14 sont également possibles. Dans le cas d'un combustible de recombustion liquide, la tête d'injection sera calculée selon les règles de l'Art, de façon à ce que la distribution granulométrie et les vitesses initiales des gouttelettes assurent une couverture complète et homogène de la veine de fumées à traiter, sans qu'il y ait de contact de gouttelettes non totalement vaporisées avec la paroi intérieure de la virole 11.The assembly for mixing the combustion fuel with the fumes to be treated, constituted by the means 25 and the injection device 15 will be sized according to any technique known to those skilled in the art, so that said mixing is carried out in less 100 ms. In the case of a gaseous recombustion fuel, the injection may be carried out from a single head provided with a sufficient number of orifices, as shown in FIG. 1, but other modes of injection at one or more rings with a diameter greater than that of the rod 14 are also possible. In the case of a liquid recombustion fuel, the injection head will be calculated according to the rules of the art, so that the particle size distribution and the initial droplet speeds ensure complete and homogeneous coverage of the vein of fumes to be treated, without contact of droplets not completely vaporized with the inner wall of the shell 11.
Le temps de séjour des fumées entre le point d'injection du combustible de recombustion et le point d'injection de l'air de post-combustion est compris entre 100 et 500 ms et de préférence entre 150 et 200 ms.The residence time of the fumes between the point of injection of the combustion fuel and the point of injection of the post-combustion air is between 100 and 500 ms and preferably between 150 and 200 ms.
La flamme pilote 31 a pour rôle d'assurer la combustion du combustible de recombustion dans les cas où la température à l'entrée de la zone de recombustion chuterait brutalement suite à un incident ou à une marche transitoire mal maîtrisée. La flamme pilote 31 a essentiellement une fonction de sécurité et il est en général exclu de maintenir en permanence une opération de recombustion où le combustible de recombustion ne serait pas partiellement ou totalement oxydé avant le brûleur pilote. Une sonde de température non représentée sur la figure 1 est placée sur la canne 14, avec un ou plusieurs points de mesure situés entre l'extrémité 15 de ladite canne 14 et l'extrémité 20 de la canne 17. Selon un exemple de procédure de fonctionnement, lorsque la température mesurée en ce ou ces points sera inférieure à une valeur de consigne, qui se situe par exemple entre 500 et 1000 °C, et de préférence entre 800 et 900 °C, l'arrêt de l'opération de recombustion est immédiatement enclenché.The role of the pilot flame 31 is to ensure the combustion of the combustion fuel in cases where the temperature at the entrance to the combustion zone would drop suddenly following an incident or a poorly controlled transient operation. The pilot flame 31 essentially has a safety function and it is generally excluded to permanently maintain a recombustion operation where the recombustion fuel would not be partially or fully oxidized before the pilot burner. A temperature probe not shown in FIG. 1 is placed on the rod 14, with one or more measuring points located between the end 15 of said rod 14 and the end 20 of the rod 17. According to an example of the operation, when the temperature measured at this or these points will be lower than a set value, which is for example between 500 and 1000 ° C, and preferably between 800 and 900 ° C, stopping the recombustion operation is immediately engaged.
Le dispositif d'introduction de l'air de post-combustion est calculé selon les règles de l'Art, afin que le temps de mélange dudit air de post-combustion avec les gaz issus de la zone de recombustion soit inférieur à 100 ms. Des moyens complémentaires non représentés sur la figure 1 , tels que par exemple venturi ou diaphragme, peuvent être implantés avant ou au niveau du ou des points d'injection de l'air de post-combustion, afin de favoriser le mélange dudit air de post-combustion avec les gaz issus de la zone de recombustion.The device for introducing the post-combustion air is calculated according to the rules of the art, so that the time for mixing said post-combustion air with the gases from the combustion zone is less than 100 ms. Complementary means not shown in FIG. 1, such as for example venturi or diaphragm, can be installed before or at the point or points of injection of the post-combustion air, in order to favor the mixing of said post air. -combustion with gases from the recombustion zone.
L'air de post-combustion peut éventuellement être additivé avec des réactifs comme l'ammoniac ou l'urée ou d'autres composés aux effets équivalents, afin d'ajouter une réduction des oxydes d'azote par voie sélective non catalytique, à l'opération de post-combustion proprement dite.The post-combustion air can optionally be added with reagents such as ammonia or urea or other compounds with equivalent effects, in order to add a reduction of nitrogen oxides by selective non-catalytic route, to the post-combustion operation proper.
Le combustible de recombustion n'est introduit dans la virole qu'une fois effectuées les opérations suivantes :The recombustion fuel is only introduced into the shell once the following operations have been carried out:
- Allumage et mise en régime du brûleur principal 1 ;- Ignition and start-up of the main burner 1;
- Allumage de la flamme pilote 31 ;- Ignition of the pilot flame 31;
- Alimentation en air de post-combustion 32. Pour l'arrêt de la chaudière, les mêmes opérations sont effectuées, mais en sens inverse.- Post-combustion air supply 32. To stop the boiler, the same operations are carried out, but in reverse.
Lors des variations de marche de la chaudière, le dispositif de conduite est réglé de façon à maintenir une température sensiblement constante après l'injection du combustible de recombustion. Par exemple, lorsque la puissance de la chaudière est réduite de moitié, il est possible selon un premier mode de fonctionnement de baisser les débits de combustible au brûleur principal et dans la zone de recombustion dans les mêmes proportions. Cependant, cette approche présente l'inconvénient de diminuer la température à l'entrée de la virole et donc dans la zone de recombustion, avec des risques consécutifs de diminution substantielle du rendement de réduction des oxydes d'azote. Dans un deuxième mode de fonctionnement de l'invention, on préférera avantageusement une stratégie qui consiste à réduire de manière plus importante le débit au niveau du brûleur principal et à augmenter le débit de combustible de recombustion, la somme de ces deux débits restant identique au débit requis normalement par la marche partielle. En procédant ainsi, on réduit l'échange thermique dans la partie amont dû tube-foyer. Ce mode de gestion des phases transitoires permet avantageusement de conserver un niveau thermique sensiblement constant dans la partie aval dudit tube-foyer et par suite un rendement de réduction des NOx également sensiblement constant.During variations in the operation of the boiler, the control device is adjusted so as to maintain a substantially constant temperature after the injection of the combustion fuel. For example, when the power of the boiler is reduced by half, it is possible according to a first operating mode to lower the fuel flow rates at the main burner and in the recombustion zone in the same proportions. However, this approach has the disadvantage of reducing the temperature at the inlet of the shell and therefore in the recombustion zone, with the consequent risks of substantial reduction in the reduction efficiency of nitrogen oxides. In a second mode of operation of the invention, a strategy which consists of reducing the flow rate at the level of the main burner and increasing the flow rate of recombination fuel will advantageously be preferred, the sum of these two flow rates remaining identical to the flow normally required by partial operation. By doing so, the heat exchange in the upstream part of the hearth tube is reduced. This mode of managing the transient phases advantageously makes it possible to maintain a substantially constant thermal level in the downstream part of said hearth tube and consequently a NO x reduction yield that is also substantially constant.
Selon un autre mode de fonctionnement de l'invention, l'ajustement du profil thermique dans le tube-foyer 2 lors des variations de marche de la chaudière pourra également être obtenu en déplaçant l'ensemble d'injection du combustible de recombustion et d'air de recombustion, le long de l'axe principal de la virole 11. En procédant de cette manière, il est ainsi possible de modifier la quantité de chaleur extraite dans le tube-foyer. According to another operating mode of the invention, the adjustment of the thermal profile in the furnace tube 2 during variations in the operation of the boiler can also be obtained by moving the injection assembly of the combustion fuel and air of combustion, along the main axis of the ferrule 11. By proceeding in this way, it is thus possible to modify the quantity of heat extracted in the hearth tube.

Claims

REVENDICATIONS
1 ) Générateur thermique comprenant un tube foyer (2) dans lequel est brûlé un combustible, des moyens de recombustion (14,15) permettant de diminuer les taux d'oxydes d'azote présents dans lesdites fumées, et des moyens (3) de récupération de la chaleur des fumées issues de ladite combustion, caractérisé en ce qu'il comprend des moyens de confinement (11) dans lesquels sont placés lesdits moyens de recombustion.1) Thermal generator comprising a hearth tube (2) in which a fuel is burned, means of recombustion (14,15) making it possible to reduce the levels of nitrogen oxides present in said fumes, and means (3) of recovery of the heat of the fumes from said combustion, characterized in that it comprises containment means (11) in which said recombination means are placed.
2) Générateur thermique selon la revendication 1 , dans lequel les moyens de confinement (11) sont disposés entre le tube-foyer (2) et les moyens (3) de récupération de chaleur.2) A thermal generator according to claim 1, wherein the confinement means (11) are arranged between the hearth tube (2) and the means (3) of heat recovery.
3) Générateur thermique selon la revendication 1 ou 2, dans lequel lesdits moyens de confinement sont extractibles et fixés sur une trappe d'accès (24).3) A thermal generator according to claim 1 or 2, wherein said confinement means are extractable and fixed on an access hatch (24).
4) Générateur thermique selon l'une quelconque des revendications 1 à 3, dans lequel lesdits moyens de confinement comprennent des moyens (25) permettant de modifier le profil de vitesse des fumées à l'entrée desdits moyens de confinement.4) A thermal generator according to any one of claims 1 to 3, wherein said confinement means comprise means (25) for modifying the speed profile of the smoke at the inlet of said confinement means.
5) Générateur thermique selon l'une quelconque des revendications 1 à 4, dans lequel une boite à fumée (5) est placée entre les moyens de confinement (11) et lesdits moyens de récupération de chaleur.5) A thermal generator according to any one of claims 1 to 4, wherein a smoke box (5) is placed between the confinement means (11) and said heat recovery means.
6) Générateur thermique selon la revendications 5, dans lequel ladite boite de fumée comprend des tubes (27) d'échange de chaleur avec les fumées.6) A thermal generator according to claim 5, wherein said smoke box comprises tubes (27) for heat exchange with the fumes.
7) Générateur thermique selon l'une quelconque des revendications 1 à 6, dans lequel lesdits moyens de confinement comprennent une virole (11) à l'intérieur de laquelle sont disposés successivement, dans le sens de circulation des fumées, des moyens d'injection d'un combustible de recombustion (14,15) et des moyens de génération d'une flamme pilote (17, 20).7) A thermal generator according to any one of claims 1 to 6, wherein said confinement means comprise a ferrule (11) inside which are successively arranged, in the direction of circulation of the fumes, means for injecting a combustion fuel (14,15) and means for generating a pilot flame (17, 20).
8) Générateur thermique selon la revendication 7, dans lequel lesdits moyens de confinement comprennent en outre des moyens d'injection d'air (21 , 23) permettant la post-combustion.8) A thermal generator according to claim 7, wherein said confinement means further comprises air injection means (21, 23) for post-combustion.
9) Générateur thermique selon la revendication 7 ou 8, dans lequel la flamme pilote (20) est positionnée sensiblement à égale distance du point d'injection de l'air de post combustion (23) et du point d'injection du combustible de recombustion (15).9) A thermal generator according to claim 7 or 8, wherein the pilot flame (20) is positioned substantially equidistant from the injection point of the post combustion air (23) and the injection point of the combustion fuel. (15).
10) Générateur thermique selon l'une quelconque des revendications 1 à 7, dans lequel des moyens d'injection de l'air permettant la post-combustion (21 ,23) sont disposés en aval de ladite virole.10) A thermal generator according to any one of claims 1 to 7, wherein the air injection means allowing the post-combustion (21, 23) are arranged downstream of said shell.
11) Procédé de limitation des émissions d'oxydes d'azote émis par un générateur thermique, caractérisé en ce qu'on effectue les étapes suivantes :11) Method for limiting the emissions of nitrogen oxides emitted by a thermal generator, characterized in that the following steps are carried out:
a) on brûle une majorité du combustible dans une zone de combustion,a) a majority of the fuel is burned in a combustion zone,
b) on fait passer les fumées résultantes de ladite combustion dans une zone de confinement (11),b) the fumes resulting from said combustion are passed through a containment zone (11),
c) on mélange dans ladite zone de confinement une minorité du combustible avec lesdites fumées.c) a minority of the fuel is mixed in said containment zone with said fumes.
12) Procédé selon la revendication 11 , dans lequel on injecte durant une étape d) de l'air dans les fumées résultantes de l'étape c). 13) Procédé selon la revendication 11 ou 12, dans lequel on effectue l'étape d) dans ladite zone de confinement.12) Method according to claim 11, wherein air is injected during a step d) in the fumes resulting from step c). 13) The method of claim 11 or 12, wherein step d) is carried out in said containment zone.
14) Procédé selon l'une des revendications 1 1 à 13, dans lequel on brûle environ 70% à environ 95% de la masse du combustible total durant l'étape a), dans lequel on brûle environ 5 à environ 30% de la masse du combustible total durant l'étape c) et dans lequel on injecte une quantité d'air durant l'étape d) permettant d'avoir un excès d'air par rapport à la stcechiométrie d'environ 5 à environ 25%.14) Method according to one of claims 1 1 to 13, wherein one burns about 70% to about 95% of the mass of the total fuel during step a), wherein one burns about 5 to about 30% of the mass of the total fuel during step c) and into which an amount of air is injected during step d) allowing an excess of air relative to the stoichiometry of about 5 to about 25%.
15) Procédé selon l'une quelconque des revendications 11 à 14, dans lequel on met en contact les fumées issues de l'étape c) avec une flamme pilote avant l'étape d).15) Process according to any one of claims 11 to 14, in which the fumes from step c) are brought into contact with a pilot flame before step d).
16) Procédé selon l'une quelconque des revendications 11 à 15, dans lequel on ajoute à l'air injecté durant l'étape d) un réactif chimique autre que le combustible permettant la réduction des oxydes d'azote par voie sélective non catalytique.16) Process according to any one of claims 11 to 15, in which is added to the air injected during step d) a chemical reagent other than the fuel allowing the reduction of nitrogen oxides by a non-catalytic selective route.
17) Procédé selon l'une quelconque des revendications 11 à 16, dans lequel, lors des marches réduites du générateur, on ajuste les débits de combustible injectés dans la zone de combustion et dans la zone de confinement de façon à maintenir une température sensiblement constante dans ladite zone de confinement. 17) Method according to any one of claims 11 to 16, in which, during reduced steps of the generator, the flow rates of fuel injected into the combustion zone and into the confinement zone are adjusted so as to maintain a substantially constant temperature. in said containment zone.
EP02743330A 2001-05-30 2002-05-23 Thermal generator and combustion method for limiting nitrogen oxide emission by re-combustion of fumes Withdrawn EP1395776A1 (en)

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FR0107116 2001-05-30
FR0107116A FR2825448B1 (en) 2001-05-30 2001-05-30 THERMAL GENERATOR FOR LIMITING EMISSIONS OF NITROGEN OXIDES BY RECOMBUSTION OF FUMES AND METHOD FOR IMPLEMENTING SUCH A GENERATOR
PCT/FR2002/001749 WO2002097327A1 (en) 2001-05-30 2002-05-23 Thermal generator and combustion method for limiting nitrogen oxide emission by re-combustion of fumes

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US20040161716A1 (en) 2004-08-19
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US7249946B2 (en) 2007-07-31

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