EP0809067B1 - Process and installation for reducing by recombustion of nitric oxides in exhaust gases from a primary combustion in a furnace - Google Patents

Process and installation for reducing by recombustion of nitric oxides in exhaust gases from a primary combustion in a furnace Download PDF

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Publication number
EP0809067B1
EP0809067B1 EP97400984A EP97400984A EP0809067B1 EP 0809067 B1 EP0809067 B1 EP 0809067B1 EP 97400984 A EP97400984 A EP 97400984A EP 97400984 A EP97400984 A EP 97400984A EP 0809067 B1 EP0809067 B1 EP 0809067B1
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Prior art keywords
zone
fuel
recombustion
pressure
combustion
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German (de)
French (fr)
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EP0809067A1 (en
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Thierry Ferlin
Jean-Charles Joigneault
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Engie SA
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Gaz de France SA
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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 
    • 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
    • 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
    • F23C2201/10—Furnace staging
    • F23C2201/101—Furnace staging in vertical direction, e.g. alternating lean and rich zones

Definitions

  • the invention relates to a reduction method by recombustion, nitrogen oxides contained in the fumes from primary combustion produced in an oven, as well as an installation for the implementation of this process.
  • This process consists of injecting a hydrocarbon according to very precise operating conditions, downstream of a first combustion during which the oxides nitrogen are produced.
  • the injection of this hydrocarbon has aim of creating a reducing atmosphere which has for consequence, when the temperature is sufficiently high (generally above 1000 ° C), cracking components of the hydrocarbon and produce radicals (CH °, H °, ..) which will combine with the monoxide nitrogen and other precursors of nitrogen oxides from main combustion (first combustion) by complex chemical transformations to form molecular nitrogen and oxygen.
  • the area in which the injection of this hydrocarbon is carried out is called the recombustion or overcombustion zone reductive.
  • the unburnt products formed in the overcombustion zone reductive are then oxidized in a third step called post-combustion.
  • the hydrocarbon that will supply the radicals necessary for the destruction of oxides nitrogen, is thus used both as a reactive agent of pollution and as energy.
  • the effectiveness of a recombination process depends on many factors such as temperature, time of stay in the recombustion zone, nature and mode injection of oil, the amount of oil injected, the initial nitrogen oxides rate, etc.
  • the reduction efficiency rate nitrogen oxides is the ratio of the number of moles of nitrogen oxides destroyed by the recombustion / number of moles of nitrogen oxides before recombustion.
  • the object of the present invention is to overcome the previous disadvantages in ensuring a better fuel penetration and distribution in the recombustion zone and thus obtain rates of reduction of satisfactory nitrogen oxides even when the dimensions of the oven did not allow a good mixture fuel and fumes, as in the process of prior art.
  • the invention proposes a process for the reduction of nitrogen oxides contained in fumes from primary combustion in an oven, by recombination of said fumes characterized by a fuel injection in the combustion zone of said oven following at least two jets associated with pressure relatively high and relatively low respectively so that the high pressure jet entrains part of the fuel injected at low pressure.
  • the jet of fuel low pressure is externally concentric with the jet of higher pressure fuel.
  • the fuel is a gas at a pressure between a few millibars and a few hundred millibars for the low jet pressure and between a few hundred millibars and a few bars for the high pressure jet.
  • the pressure and the flow rates of the gases of the two aforementioned jets are controlled so as to adapt to the dimensions of the recombination zone and to the characteristics of the fumes.
  • furnace's recombustion zone is subjected to acoustic waves.
  • the aforementioned acoustic waves have a frequency below about 20 Hz.
  • the invention also provides an installation for the implementation of the method described above comprising a oven equipped with a primary combustion zone supplied by a main fuel and followed by a zone of recombustion powered by a secondary fuel, characterized in that said recombustion zone is fitted with at least two secondary fuel inlets to a relatively high pressure and a relatively low pressure respectively so that the high pressure jet drives part of the fuel injected at low pressure.
  • the two aforementioned entries are coaxial.
  • the two aforementioned entries are respectively fitted with a flow control device and the pressure of the secondary fuel forming the two aforementioned jets.
  • the installation according to the invention has a generator of acoustic waves mounted on the wall of the oven so as to ability to homogenize the mixture in the recombustion.
  • the temperature is sufficient to crack the radicals which will react with nitrogen oxides to give molecular nitrogen and oxygen.
  • hydrocarbon used as quickly as possible dispersed and cracked. In otherwise, the hydrocarbon (fuel) will burn with the residual oxygen from the fumes from the main combustion and, create nitrogen oxides which is contrary to the aim sought.
  • the homogenization of the mixture is all the more delicate when get the dimensions of the oven important and that the volume of fuel injected represents only about 1% of the volume of combustion fumes main.
  • the homogenization of the fuel mixture and smoke is often limited by the residence time of the fuel-smoke mixture in the combustion zone. Indeed, this residence time is limited by the oven dimensions which are most often limited in height.
  • FIG. 1 a combustion furnace for setting up implementation of the nitrogen oxides reduction process according to the invention is illustrated in FIG. 1.
  • This oven comprises the three combustion zones mentioned above, i.e. a first main combustion zone A, a second recombination zone B and a third zone C of post-combustion. Above the post-combustion zone, there is a gas cooling zone D.
  • Main combustion zone A has orifices supply 3 of a mixture of a so-called fuel primary fuel and air.
  • Primary fuel can be of any kind: coal, fuel oil, waste, wood, natural gas, etc ...
  • the primary fuel will be natural gas.
  • the ventilation rate of this main combustion zone that is, the ratio of the actual air volume of combustion on theoretical combustion air (combustion stoichiometric) generally varies in the range of 1.05 to 1.1.
  • the nitrogen oxides contained in the fumes from this primary combustion are then reduced in the Reduction or overcombustion zone B.
  • the reduction of nitrogen oxides is achieved through the injection of a fuel called secondary fuel by the power supply marked 2 in FIG. 1.
  • the secondary fuel is injected through inlet 2 in one volume representing 10 to 20% of the fuel volume primary, in order to obtain a ventilation rate close to 0.9 in zone B.
  • the reducing atmosphere in default oxygen is created in the recombination zone.
  • Secondary fuel used for recombination injected into the reducing zone B can, like the primary fuel, being of any kind: coal, fuel oil, waste, wood, natural gas, etc. However, according to In the invention, natural gas is preferably used.
  • the oxidation stage of unburnt materials is called the post-combustion stage, is then performed.
  • the post-combustion stage is carried out thanks to the introduction, by the inlet marked 1 in FIG. 1, of air to complete the combustion.
  • fuel injection secondary, preferably natural gas, in the area of recombustion B must be within a range of temperature between 1100 and 1500 ° C and it must be performed at a flow and pressure corresponding to sufficient residence time in the recombination zone to allow homogenization of mixtures fuel-smoke satisfactory as well as to allow reduction reactions to occur.
  • Time to stay should generally be in the range of 0.5 to 1 second according to the operating conditions.
  • the reduction temperature used for the recombustion, in the recombustion zone B, can be lowered to around 1000 ° C but it is then necessary to increase the residence time.
  • a balance must therefore be found between the parameters residence time-temperature, in order to obtain a satisfactory homogenization between the fuel secondary used in zone B of recombustion and fumes from primary combustion zone A or main containing nitrogen oxides to reduce.
  • the secondary fuel behaves like a jet in a transverse flow and the homogenization of a mixing a jet (secondary fuel) in a stream transverse (fumes from primary combustion) is subject to antagonistic constraints, all the more difficult as the flow rate of the jet on the wall of the furnace is weak compared to cross flow main.
  • the present invention consists of injecting the secondary fuel, preferably natural gas, through a multiple pulse system. More specifically, in the installation illustrated in Figure 1, the fuel secondary is injected by a double pulse system in the recombustion zone.
  • the double pulse injection according to the invention has two gas supply circuits 2, 2 ', one circuit low pressure supply 2, adjusted and controlled by any suitable means noted 4 in Figure 1, and a high pressure gas supply circuit 2 'set and controlled by any suitable means noted 5 in Figure 1.
  • the double impulse improves the penetration of the jet of secondary fuel to the center of the furnace and also to distribute this fuel throughout the oven enclosure.
  • the high pressure central jet also causes part of the fuel injected into lower pressure, which also improves its mixed.
  • This double pulse of gaseous fuel can be easily adjusted, variations in flow and gas fuel pressures are very easy to be carried out to optimize the mixture, that is to say so as to adapt to the dimensions of the recombustion, smoke characteristics, and reduction rates sought.
  • the two secondary fuel inlets 2, 2 ' are coaxial and the low pressure fuel input is externally concentric with the fuel inlet at high pressure.
  • the invention is therefore particularly advantageous in cases in which the dimensions of the oven do not not allow proper fuel mixing in smoke, in particular.
  • the invention also makes it possible, by improving the mixing, reduce the amount of secondary fuel to inject to obtain a given rate of reduction of nitrogen oxides.
  • the levels of reduction of nitrogen obtained are of the order of 50 to 60% in moles, by substituting, for coal, 20% in intake energy from natural gas.
  • a generator infrasound noted 6 in Figure 1 attached is mounted on one of the walls of the combustion furnace at the level of the zone B of recombustion.
  • the rate of reduction of nitrogen oxides contained in the fumes from primary combustion is generally greater than 80%, more precisely in the meantime from about 80 to about 90%, in moles.
  • the process of reduction of nitrogen oxides by recombination of fumes from primary combustion therefore achieves an efficiency NOx reduction of around 70 to 80% by performing injection of secondary fuel, preferably natural gas, in the combustion zone, according to minus two jets associated with relatively high pressure and relatively low respectively so that the high pressure jet causes part of the fuel injected at low pressure.
  • secondary fuel preferably natural gas
  • the low pressure fuel jet is externally concentric with the fuel jet at higher pressure, which improves the secondary fuel penetration to the center of the oven thanks to the high pressure jet and also to distribute gases throughout the enclosure.
  • the central jet at high pressure causing part of the gas injected to lower pressure, the secondary fuel mixture gaseous with fumes from primary combustion containing the nitrogen oxide to be reduced is thus improved.
  • the gaseous fuel is injected for the low pressure jet at a pressure between a few millibars and a few hundred millibars and for the high pressure jet, between a few hundred millibars and a few bars.
  • the acoustic waves used have a frequency less than about 20 Hz.
  • the use of injection multiple pulse and infrasound production in the nitrogen oxides recombination process allows achieve NOx reduction efficiency rates of around 50% but using less fuel. For example, reducing the amount of fuel secondary is around 50% by volume, which allows reduce the cost of the process.

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

Abstract

The reduction of oxides of nitrogen contained in furnace smoke is carried out by recombustion of the smoke, using a secondary fuel injected into a recombustion zone in at least two jets at relatively high and relatively low pressure respectively. The low-pressure jet is situated outside and concentrically with the high-pressure jet. The secondary fuel is a gas at a pressure of a few millibars to a few hundred millibars for the low pressure jet, and a few hundred millibars to a few bars to the high-pressure one. The pressure and flow of the gas in the two jets are regulated according to the dimensions of the recombustion zone and the characteristics of the smoke. The recombustion zone is also subjected to acoustic waves at a frequency of below some 32 Hz. The procedure is carried out in a recombustion zone (B) fed with secondary fuel through two coaxial inlets (2,2') equipped with pressure regulators (4,5). One wall of the recombustion zone is equipped with an acoustic wave generator (6) to homogenise the mixture inside the zone.

Description

L'invention concerne un procédé de réduction par recombustion, des oxydes d'azote contenus dans les fumées issues d'une combustion primaire réalisés dans un four, ainsi qu'une installation pour la mise en oeuvre de ce procédé.The invention relates to a reduction method by recombustion, nitrogen oxides contained in the fumes from primary combustion produced in an oven, as well as an installation for the implementation of this process.

On sait que la combustion de combustibles de toute nature est à l'origine de la production d'oxydes d'azote par réaction avec l'air, en quantité plus ou moins importante selon le type du combustible et les conditions de la combustion. Ces oxydes d'azote émis dans l'atmosphère sont la cause de diverses pollutions. Ils contribuent notamment à la formation des pluies acides. Par ailleurs, combinés avec le monoxyde de carbone et les composés organiques volatils présents dans l'atmosphère , ils produisent de l'ozone troposphérique qui est à l'origine de l'augmentation des affections respiratoires (asthme, insuffisance respiratoire, etc.).We know that the combustion of fuels of all kinds is responsible for the production of nitrogen oxides by reaction with air, in more or less quantity depending on the type of fuel and the conditions of the combustion. These nitrogen oxides emitted into the atmosphere are the cause of various types of pollution. They contribute especially the formation of acid rain. Otherwise, combined with carbon monoxide and compounds volatile organic present in the atmosphere they produce tropospheric ozone which is the source increased respiratory conditions (asthma, respiratory failure, etc.).

Il est donc important de réduire ou d'éliminer ces oxydes d'azote.It is therefore important to reduce or eliminate these oxides nitrogen.

Dans ce but, on connaít un procédé dit de recombustion ou de surcombustion réductrice.For this purpose, we know a process called recombination or reductive overcombustion.

Ce procédé consiste à injecter un hydrocarbure selon des conditions opératoires très précises, en aval d'une première combustion au cours de laquelle les oxydes d'azote sont produits. L'injection de cet hydrocarbure a pour but de créer une atmosphère réductrice qui a pour conséquence, lorsque la température est suffisamment élevée (généralement supérieure à 1000°C), de craquer les composants de l'hydrocarbure et de produire des radicaux (CH°, H°, ..) qui vont se combiner avec le monoxyde d'azote et les autres précurseurs des oxydes d'azote issus de la combustion principale (première combustion) par des transformations chimiques complexes pour former de l'azote moléculaire et de l'oxygène. La zone dans laquelle l'injection de cet hydrocarbure est effectuée est appelée zone de recombustion ou de surcombustion réductrice.This process consists of injecting a hydrocarbon according to very precise operating conditions, downstream of a first combustion during which the oxides nitrogen are produced. The injection of this hydrocarbon has aim of creating a reducing atmosphere which has for consequence, when the temperature is sufficiently high (generally above 1000 ° C), cracking components of the hydrocarbon and produce radicals (CH °, H °, ..) which will combine with the monoxide nitrogen and other precursors of nitrogen oxides from main combustion (first combustion) by complex chemical transformations to form molecular nitrogen and oxygen. The area in which the injection of this hydrocarbon is carried out is called the recombustion or overcombustion zone reductive.

Les imbrûlés formés dans la zone de surcombustion réductrice sont ensuite oxydés dans une troisième étape dite de post-combustion. L'hydrocarbure qui va fournir les radicaux nécessaires à la destruction des oxydes d'azote, est ainsi utilisé à la fois comme agent réactif de dépollution et comme énergie.The unburnt products formed in the overcombustion zone reductive are then oxidized in a third step called post-combustion. The hydrocarbon that will supply the radicals necessary for the destruction of oxides nitrogen, is thus used both as a reactive agent of pollution and as energy.

Le procédé de recombustion comporte donc trois étapes :

  • une première étape dite de combustion principale ou primaire effectuée dans une première zone d'un four de combustion, dite zone de combustion principale ou primaire. Cette étape correspond à l'étape proprement dite de combustion des combustibles, étape au cours de laquelle les oxydes d'azote se forment et sont entraínés dans les fumées;
  • une deuxième étape dite de recombustion ou de surcombustion réductrice effectuée dans une deuxième zone, dite zone de recombustion ou de surcombustion réductrice, du four de combustion lui-même.
The recombustion process therefore comprises three stages:
  • a first step called main or primary combustion carried out in a first zone of a combustion furnace, called main or primary combustion zone. This step corresponds to the actual step of combustion of fuels, step during which nitrogen oxides are formed and are entrained in the fumes;
  • a second stage known as reductive combustion or overcombustion carried out in a second zone, known as reductive combustion or overcombustion zone, of the combustion furnace itself.

Dans cette étape, un combustible dit combustible secondaire est injecté dans la zone de recombustion pour réduire les oxydes d'azote contenus dans les fumées issues de la combustion principale; et

  • une troisième étape dite de post-combustion effectuée dans une troisième zone du four de combustion en aval de la zone de recombustion, consistant à injecter de l'air dans cette troisième zone pour achever la combustion.
In this stage, a so-called secondary fuel is injected into the combustion zone to reduce the nitrogen oxides contained in the fumes from the main combustion; and
  • a third so-called post-combustion step carried out in a third zone of the combustion furnace downstream from the recombustion zone, consisting in injecting air into this third zone to complete the combustion.

L'efficacité d'un procédé de recombustion dépend de nombreux facteurs tels que la température, le temps de séjour dans la zone de recombustion, la nature et le mode d'injection de l'hydrocarbure, la quantité d'hydrocarbure injectée, le taux d'oxydes d'azote initial, etc.The effectiveness of a recombination process depends on many factors such as temperature, time of stay in the recombustion zone, nature and mode injection of oil, the amount of oil injected, the initial nitrogen oxides rate, etc.

Mais, le procédé de recombustion de l'art antérieur décrit ci-dessus ne permet d'atteindre que des taux d'efficacité de la réduction des oxydes d'azote de l'ordre de 50 à 60%. Le taux d'efficacité de la réduction des oxydes d'azote correspond au rapport du nombre de moles d'oxydes d'azote détruits par la recombustion/nombre de moles d'oxydes d'azote avant la recombustion.However, the recombination process of the prior art described above only achieves rates efficiency of nitrogen oxides reduction in the range of 50 to 60%. The reduction efficiency rate nitrogen oxides is the ratio of the number of moles of nitrogen oxides destroyed by the recombustion / number of moles of nitrogen oxides before recombustion.

Or, les études théoriques de cinétique montrent que si le mélange du combustible introduit dans la zone de recombustion et des fumées issues de la zone de combustion principale était parfaitement homogène on pourrait atteindre une efficacité de réduction des oxydes d'azote de près de 90%.However, theoretical kinetic studies show that if the mixture of fuel introduced into the recombustion and fumes from the area of main combustion was perfectly homogeneous could achieve oxide reduction efficiency nitrogen of almost 90%.

La présente invention a pour but de pallier les inconvénients précédents en permettant d'assurer une meilleure pénétration et répartition du combustible dans la zone de recombustion et ainsi d'obtenir des taux de réduction des oxydes d'azote satisfaisants même lorsque les dimensions du four ne permettaient pas un bon mélange du combustible et des fumées, comme dans le procédé de l'art antérieur.The object of the present invention is to overcome the previous disadvantages in ensuring a better fuel penetration and distribution in the recombustion zone and thus obtain rates of reduction of satisfactory nitrogen oxides even when the dimensions of the oven did not allow a good mixture fuel and fumes, as in the process of prior art.

Ces buts sont atteints par l'invention qui propose un procédé de réduction des oxydes d'azote contenus dans les fumées issues d'une combustion primaire, dans un four, par recombustion desdites fumées caractérisé par une injection de combustible dans la zone de recombustion dudit four suivant au moins deux jets associés à pression relativement haute et relativement basse respectivement de façon que le jet à haute pression entraíne une partie du combustible injecté à basse pression.These objects are achieved by the invention which proposes a process for the reduction of nitrogen oxides contained in fumes from primary combustion in an oven, by recombination of said fumes characterized by a fuel injection in the combustion zone of said oven following at least two jets associated with pressure relatively high and relatively low respectively so that the high pressure jet entrains part of the fuel injected at low pressure.

Plus précisément dans ce procédé le jet de combustible basse pression est extérieurement concentrique au jet de combustible à pression plus élevée.More precisely in this process the jet of fuel low pressure is externally concentric with the jet of higher pressure fuel.

De préférence, selon l'invention le combustible est un gaz à une pression comprise entre quelques millibars et quelques centaines de millibars pour le jet basse pression et entre quelques centaines de millibars et quelques bars pour le jet haute pression.Preferably, according to the invention the fuel is a gas at a pressure between a few millibars and a few hundred millibars for the low jet pressure and between a few hundred millibars and a few bars for the high pressure jet.

Egalement selon l'invention, la pression et les débits du gaz des deux jets précités sont commandés de façon à s'adapter aux dimensions de la zone de recombustion et aux caractéristiques des fumées.Also according to the invention, the pressure and the flow rates of the gases of the two aforementioned jets are controlled so as to adapt to the dimensions of the recombination zone and to the characteristics of the fumes.

De plus, la zone de recombustion du four est soumise à des ondes acoustiques.In addition, the furnace's recombustion zone is subjected to acoustic waves.

De préférence, les ondes acoustiques précitées ont une fréquence inférieure à environ 20 Hz.Preferably, the aforementioned acoustic waves have a frequency below about 20 Hz.

L'invention propose également une installation pour la mise en oeuvre du procédé décrite ci-dessus comprenant un four équipé d'une zone de combustion primaire alimentée par un combustible principal et suivie par une zone de recombustion alimentée par un combustible secondaire, caractérisée en ce que ladite zone de recombustion est munie d'au moins deux entrées de combustible secondaire à une pression relativement haute et à une pression relativement basse respectivement de façon que le jet à haute pression entraíne une partie du combustible injecté à basse pression. The invention also provides an installation for the implementation of the method described above comprising a oven equipped with a primary combustion zone supplied by a main fuel and followed by a zone of recombustion powered by a secondary fuel, characterized in that said recombustion zone is fitted with at least two secondary fuel inlets to a relatively high pressure and a relatively low pressure respectively so that the high pressure jet drives part of the fuel injected at low pressure.

Plus précisément, les deux entrées précitées sont coaxiales.More specifically, the two aforementioned entries are coaxial.

Selon l'invention, les deux entrées précitées sont respectivement munies d'un appareil de commande du débit et de la pression du combustible secondaire formant les deux jets précités.According to the invention, the two aforementioned entries are respectively fitted with a flow control device and the pressure of the secondary fuel forming the two aforementioned jets.

De plus, l'installation selon l'invention a un générateur d'ondes acoustiques monté sur la paroi du four de façon à pouvoir homogénéiser le mélange dans la zone de recombustion.In addition, the installation according to the invention has a generator of acoustic waves mounted on the wall of the oven so as to ability to homogenize the mixture in the recombustion.

L'invention sera mieux comprise et d'autres buts, caractéristiques, détails et avantages de celle-ci apparaítront plus clairement au cours de la description explicative qui va suivre et qui est faite en référence au dessin unique annexé qui représente un mode de réalisation préféré d'un four dans lequel on effectue l'ensemble des opérations ci-dessus décrites de combustion principale, recombustion et post-combustion.The invention will be better understood and other aims, features, details and benefits of it will appear more clearly during the description explanatory which will follow and which is made with reference in the single appended drawing which represents a mode of preferred embodiment of an oven in which one performs all of the operations described above from main combustion, recombustion and post-combustion.

Un des problèmes rencontrés lors de la mise en oeuvre du procédé de recombustion de l'art antérieur réside dans le mélange du combustible (gaz naturel ou tout autre hydrocarbure employé) avec les fumées contenant les oxydes d'azote issus de la zone de combustion et parvenant dans la zone de recombustion. En effet, une bonne homogénéisation du combustible et des fumées, dans la zone de recombustion est indispensable du point de vue de la température et des échanges massiques. En effet s'il existe des zones d'écoulements où les radicaux (CH°, etc...) ne sont produits et diffusés, aucune destruction des oxydes d'azote n'aura lieu dans ces zones. One of the problems encountered during the implementation of the recombination process of the prior art resides in the fuel mixture (natural gas or any other hydrocarbon used) with the fumes containing the nitrogen oxides from the combustion zone and arriving in the recombustion zone. Indeed, a good homogenization of fuel and fumes, in the recombustion zone is essential from the point of view of temperature and mass exchanges. Indeed if there are flow zones where the radicals (CH °, etc ...) are not produced and distributed, no destruction nitrogen oxides will not occur in these areas.

De même, il faut qu'en tout point du mélange la température soit suffisante pour craquer les radicaux qui vont réagir avec les oxydes d'azote pour donner de l'azote moléculaire et de l'oxygène.Likewise, at any point in the mixture the temperature is sufficient to crack the radicals which will react with nitrogen oxides to give molecular nitrogen and oxygen.

Par ailleurs il est crucial que l'hydrocarbure utilisé soit le plus rapidement possible dispersé et craqué. Dans le cas contraire, l'hydrocarbure (combustible) va brûler avec l'oxygène résiduel des fumées issues de la combustion principale et, créer des oxydes d'azote ce qui est contraire au but recherché.Furthermore, it is crucial that the hydrocarbon used as quickly as possible dispersed and cracked. In otherwise, the hydrocarbon (fuel) will burn with the residual oxygen from the fumes from the main combustion and, create nitrogen oxides which is contrary to the aim sought.

L'homogénéisation du mélange est d'autant plus délicate à obtenir que les dimensions du four sont importantes et que le volume de combustible injecté ne représente que environ 1% du volume des fumées de la combustion principale.The homogenization of the mixture is all the more delicate when get the dimensions of the oven important and that the volume of fuel injected represents only about 1% of the volume of combustion fumes main.

De plus, l'homogénéisation du mélange du combustible et des fumées est souvent limitée par le temps de séjour du mélange combustible-fumées dans la zone de recombustion. En effet, ce temps de séjour est limité par les dimensions du four qui sont le plus souvent limitées en hauteur.In addition, the homogenization of the fuel mixture and smoke is often limited by the residence time of the fuel-smoke mixture in the combustion zone. Indeed, this residence time is limited by the oven dimensions which are most often limited in height.

A titre d'exemple, un four de combustion pour la mise en oeuvre du procédé de réduction des oxydes d'azote selon l'invention est illustré en figure 1. Ce four comporte les trois zones de combustion citées précédemment, c'est-à-dire une première zone A de combustion principale, une seconde zone B de recombustion et une troisième zone C de post-combustion. Au-dessus de la zone de post-combustion, il existe une zone D de refroidissement des gaz.For example, a combustion furnace for setting up implementation of the nitrogen oxides reduction process according to the invention is illustrated in FIG. 1. This oven comprises the three combustion zones mentioned above, i.e. a first main combustion zone A, a second recombination zone B and a third zone C of post-combustion. Above the post-combustion zone, there is a gas cooling zone D.

La zone de combustion principale A comporte des orifices d'alimentation 3 d'un mélange d'un combustible dit combustible primaire et d'air. Le combustible primaire peut être de toute nature : charbon, fioul, déchets, bois, gaz naturel, etc...Main combustion zone A has orifices supply 3 of a mixture of a so-called fuel primary fuel and air. Primary fuel can be of any kind: coal, fuel oil, waste, wood, natural gas, etc ...

De préférence dans l'invention, le combustible primaire sera du gaz naturel.Preferably in the invention, the primary fuel will be natural gas.

Le taux d'aération de cette zone de combustion principale c'est-à-dire, le rapport du volume d'air réel de combustion sur l'air de combustion théorique (combustion stoechiométrique) varie généralement dans l'intervalle de 1,05 à 1,1.The ventilation rate of this main combustion zone that is, the ratio of the actual air volume of combustion on theoretical combustion air (combustion stoichiometric) generally varies in the range of 1.05 to 1.1.

Les oxydes d'azote contenus dans les fumées issues de cette combustion primaire sont ensuite réduits dans la zone B de recombustion ou surcombustion réductrice. La réduction des oxydes d'azote est réalisée grâce à l'injection d'un combustible dit combustible secondaire par l'alimentation notée 2 en figure 1. De préférence, le combustible secondaire est injecté par l'entrée 2 en un volume représentant 10 à 20% du volume du combustible primaire, afin d'obtenir un taux d'aération voisin de 0,9 dans la zone B.The nitrogen oxides contained in the fumes from this primary combustion are then reduced in the Reduction or overcombustion zone B. The reduction of nitrogen oxides is achieved through the injection of a fuel called secondary fuel by the power supply marked 2 in FIG. 1. Preferably, the secondary fuel is injected through inlet 2 in one volume representing 10 to 20% of the fuel volume primary, in order to obtain a ventilation rate close to 0.9 in zone B.

De cette façon, l'atmosphère réductrice (en défaut d'oxygène) voulue est créée dans la zone de recombustion.In this way, the reducing atmosphere (in default oxygen) is created in the recombination zone.

Le combustible secondaire utilisé pour la recombustion injecté dans la zone réductrice B, peut, comme le combustible primaire, être de toute nature : charbon, fioul, déchet, bois, gaz naturel, etc. Cependant, selon l'invention, le gaz naturel est de préférence utilisé.Secondary fuel used for recombination injected into the reducing zone B, can, like the primary fuel, being of any kind: coal, fuel oil, waste, wood, natural gas, etc. However, according to In the invention, natural gas is preferably used.

L'étape d'oxydation des imbrûlés est dite étape de post-combustion, est ensuite effectuée. The oxidation stage of unburnt materials is called the post-combustion stage, is then performed.

L'étape de post-combustion est réalisée grâce à l'introduction, par l'entrée notée 1 en figure 1, d'air pour achever la combustion.The post-combustion stage is carried out thanks to the introduction, by the inlet marked 1 in FIG. 1, of air to complete the combustion.

Pour être efficace, l'injection du combustible secondaire, de préférence le gaz naturel, dans la zone de recombustion B doit se faire dans une plage de température comprise entre 1100 et 1500°C et elle doit être réalisée à un débit et une pression correspondant à un temps de séjour suffisant dans la zone de recombustion pour permettre une homogénéisation des mélanges combustible-fumées satisfaisante ainsi que pour permettre aux réactions de réduction de se produire. Le temps de séjour doit généralement être de l'ordre de 0,5 à 1 seconde selon les conditions opératoires.To be efficient, fuel injection secondary, preferably natural gas, in the area of recombustion B must be within a range of temperature between 1100 and 1500 ° C and it must be performed at a flow and pressure corresponding to sufficient residence time in the recombination zone to allow homogenization of mixtures fuel-smoke satisfactory as well as to allow reduction reactions to occur. Time to stay should generally be in the range of 0.5 to 1 second according to the operating conditions.

La température de réduction utilisée pour l'étape de recombustion, dans la zone de recombustion B, peut être abaissée à environ 1000°C mais il faut alors augmenter le temps de séjour.The reduction temperature used for the recombustion, in the recombustion zone B, can be lowered to around 1000 ° C but it is then necessary to increase the residence time.

Si l'élévation de la température augmente l'efficacité de la destruction des précurseurs des oxydes d'azote dans la zone de recombustion, il y a cependant un risque dans ce cas de produire des oxydes d'azote au niveau de l'injection d'air dans la zone de post-combustion au-delà d'une certaine température.If the rise in temperature increases the efficiency of destruction of the precursors of nitrogen oxides in the recombination zone, there is however a risk in this case of producing nitrogen oxides at injecting air into the afterburner beyond of a certain temperature.

Un équilibre doit donc être trouvé entre les paramètres temps de séjour-température, afin d'obtenir une homogénéisation satisfaisante entre le combustible secondaire utilisé dans la zone B de recombustion et les fumées issues de la zone A de combustion primaire ou principale contenant les oxydes d'azote à réduire. A balance must therefore be found between the parameters residence time-temperature, in order to obtain a satisfactory homogenization between the fuel secondary used in zone B of recombustion and fumes from primary combustion zone A or main containing nitrogen oxides to reduce.

En fait le combustible secondaire se comporte comme un jet dans un flux transversal et l'homogénéisation d'un mélange d'un jet (combustible secondaire) dans un flux transversal (fumées issues de la combustion primaire) est soumise à des contraintes antagonistes, d'autant plus difficile que le débit volumique du jet sur la paroi du four est faible en comparaison au flux transversal principal.In fact the secondary fuel behaves like a jet in a transverse flow and the homogenization of a mixing a jet (secondary fuel) in a stream transverse (fumes from primary combustion) is subject to antagonistic constraints, all the more difficult as the flow rate of the jet on the wall of the furnace is weak compared to cross flow main.

Ceci est le cas dans le procédé de recombustion où le volume de combustible, de préférence du gaz naturel, injecté représente 1% et parfois moins du débit des fumées.This is the case in the recombustion process where the volume of fuel, preferably natural gas, injected represents 1% and sometimes less of the flow of fumes.

Afin d'améliorer l'homogénéisation, la présente invention consiste à injecter le combustible secondaire, de préférence du gaz naturel, par l'intermédiaire d'un système à plusieurs impulsions. Plus particulièrement, dans l'installation illustrée en figure 1, le combustible secondaire est injecté par un système à double impulsion dans la zone de recombustion.In order to improve homogenization, the present invention consists of injecting the secondary fuel, preferably natural gas, through a multiple pulse system. More specifically, in the installation illustrated in Figure 1, the fuel secondary is injected by a double pulse system in the recombustion zone.

L'injection à double impulsion selon l'invention possède deux circuits 2, 2' d'alimentation en gaz, un circuit d'alimentation à basse pression 2, réglé et contrôlé par tout moyen approprié noté 4 dans la figure 1, et un circuit d'alimentation en gaz haute pression 2' réglé et contrôlé par tout moyen approprié noté 5 sur la figure 1.The double pulse injection according to the invention has two gas supply circuits 2, 2 ', one circuit low pressure supply 2, adjusted and controlled by any suitable means noted 4 in Figure 1, and a high pressure gas supply circuit 2 'set and controlled by any suitable means noted 5 in Figure 1.

La double impulsion permet d'améliorer la pénétration du jet du combustible secondaire jusqu'au centre du four et également de répartir ce combustible dans l'ensemble de l'enceinte du four. Le jet central à haute pression entraíne également une partie du combustible injecté à plus basse pression, ce qui améliore également son mélange. Cette double impulsion de combustible gazeux peut être très facilement réglée, les variations de débit et des pressions du combustible gazeux sont très faciles à réaliser pour optimiser le mélange, c'est-à-dire de façon à s'adapter aux dimensions de la zone de recombustion, aux caractéristiques des fumées, et aux taux de réduction recherchés.The double impulse improves the penetration of the jet of secondary fuel to the center of the furnace and also to distribute this fuel throughout the oven enclosure. The high pressure central jet also causes part of the fuel injected into lower pressure, which also improves its mixed. This double pulse of gaseous fuel can be easily adjusted, variations in flow and gas fuel pressures are very easy to be carried out to optimize the mixture, that is to say so as to adapt to the dimensions of the recombustion, smoke characteristics, and reduction rates sought.

Dans le mode de réalisation représenté en figure 1, les deux entrées 2, 2' du combustible secondaire sont coaxiales et l'entrée du combustible basse pression est extérieurement concentrique à l'entrée du combustible à haute pression.In the embodiment shown in FIG. 1, the two secondary fuel inlets 2, 2 'are coaxial and the low pressure fuel input is externally concentric with the fuel inlet at high pressure.

L'utilisation d'un tel système d'injection à double impulsion permet d'obtenir une réduction des oxydes d'azote contenus dans les fumées issues de la combustion primaire de l'ordre de 70 à 80%, en moles, alors qu'avec le procédé et le dispositif de l'art antérieur ne comportant ni cette étape d'injection à double impulsion du combustible secondaire et ni le système d'injection du combustible secondaire à plusieurs impulsions, les taux de réduction des NOx sont de l'ordre de 50 à 60%, en moles.The use of such a double injection system pulse provides reduction of oxides nitrogen contained in the fumes from combustion primary of the order of 70 to 80%, in moles, whereas with the method and the device of the prior art do not comprising neither this double pulse injection step secondary fuel and neither the fuel injection system secondary fuel with multiple pulses, the rates NOx reductions are around 50 to 60%, in moles.

L'invention est donc particulièrement avantageuse dans les cas dans lesquels les dimensions du four ne permettent pas un bon mélange du combustible dans les fumées, en particulier.The invention is therefore particularly advantageous in cases in which the dimensions of the oven do not not allow proper fuel mixing in smoke, in particular.

L'invention permet également, en améliorant le mélange, de réduire les quantités de combustible secondaire à injecter pour obtenir un taux donné de réduction des oxydes d'azote.The invention also makes it possible, by improving the mixing, reduce the amount of secondary fuel to inject to obtain a given rate of reduction of nitrogen oxides.

Ceci est particulièrement avantageux, par exemple, dans le cas d'une chaudière où le combustible primaire est du charbon et le combustible secondaire ou de recombustion est du gaz naturel.This is particularly advantageous, for example, in the case of a boiler where the primary fuel is coal and secondary or recombination fuel is natural gas.

Actuellement dans un procédé de recombustion les taux de réduction d'azote obtenus sont de l'ordre de 50 à 60% en moles, en substituant, au charbon, 20% en apport énergétique de gaz naturel.Currently in a recombination process the levels of reduction of nitrogen obtained are of the order of 50 to 60% in moles, by substituting, for coal, 20% in intake energy from natural gas.

Avec l'utilisation du procédé et/ou de l'installation de l'invention, on peut obtenir ce même taux de réduction avec seulement 10 à 15% en valeur énergétique de gaz naturel injecté, ce qui représente un avantage économique.With the use of the process and / or the installation of the invention you can get this same reduction rate with only 10 to 15% in energy value of gas natural injected, which is an advantage economic.

Afin d'améliorer encore le mélange, en particulier dans les zones proches des parois du réacteur, et également entre les couches du jet haute pression et du jet basse pression du combustible secondaire gazeux, un générateur d'infrasons noté 6 dans la figure 1 annexée est monté sur une des parois du four de combustion au niveau de la zone B de recombustion.In order to further improve the mixing, especially in areas close to the reactor walls, and also between the layers of the high pressure jet and the low jet pressure of secondary gaseous fuel, a generator infrasound noted 6 in Figure 1 attached is mounted on one of the walls of the combustion furnace at the level of the zone B of recombustion.

Avec l'ajout de ce générateur d'infrasons, le taux de réduction des oxydes d'azote contenus, dans les fumées issues de la combustion primaire, est généralement supérieur à 80%, plus précisément dans l'intervalle d'environ 80 à environ 90%, en moles.With the addition of this infrasound generator, the rate of reduction of nitrogen oxides contained in the fumes from primary combustion, is generally greater than 80%, more precisely in the meantime from about 80 to about 90%, in moles.

Le procédé de réduction des oxydes d'azote par recombustion des fumées issues d'une combustion primaire, selon l'invention, permet donc d'atteindre une efficacité de réduction des NOx de l'ordre de 70 à 80% en réalisant l'injection du combustible secondaire, de préférence du gaz naturel, dans la zone de recombustion, suivant au moins deux jets associés à pression relativement haute et relativement basse respectivement de façon que le jet à haute pression entraíne une partie du combustible injecté à basse pression. The process of reduction of nitrogen oxides by recombination of fumes from primary combustion, according to the invention, therefore achieves an efficiency NOx reduction of around 70 to 80% by performing injection of secondary fuel, preferably natural gas, in the combustion zone, according to minus two jets associated with relatively high pressure and relatively low respectively so that the high pressure jet causes part of the fuel injected at low pressure.

Dans ce procédé, le jet de combustible à basse pression est extérieurement concentrique au jet de combustible à pression plus élevée, ce qui permet d'améliorer la pénétration du combustible secondaire jusqu'au centre du four grâce au jet haute pression et également de répartir les gaz dans l'ensemble de l'enceinte. Le jet central à haute pression entraínant une partie du gaz injecté à plus basse pression, le mélange du combustible secondaire gazeux avec les fumées de la combustion primaire contenant l'oxyde d'azote à réduire est ainsi amélioré.In this process, the low pressure fuel jet is externally concentric with the fuel jet at higher pressure, which improves the secondary fuel penetration to the center of the oven thanks to the high pressure jet and also to distribute gases throughout the enclosure. The central jet at high pressure causing part of the gas injected to lower pressure, the secondary fuel mixture gaseous with fumes from primary combustion containing the nitrogen oxide to be reduced is thus improved.

En associant à cette injection du combustible secondaire, dans la zone de recombustion, suivant au moins deux jets associés, concentriques, à pression relativement haute et pression relativement basse respectivement, le jet étant extérieurement concentrique au jet combustible à pression plus élevée, une production d'ondes acoustiques dans la zone de recombustion, l'efficacité du procédé de l'invention atteint près de 80 à 90%.By associating this injection with secondary fuel, in the recombustion zone, following at least two jets associated, concentric, at relatively high pressure and relatively low pressure respectively, the jet being externally concentric with the pressurized fuel jet higher, production of acoustic waves in the recombination zone, the efficiency of the the invention reaches almost 80 to 90%.

De préférence, le combustible gazeux est injecté pour le jet à basse pression à une pression comprise entre quelques millibars et quelques centaines de millibars et pour le jet à haute pression, entre quelques centaines de millibars et quelques bars.Preferably, the gaseous fuel is injected for the low pressure jet at a pressure between a few millibars and a few hundred millibars and for the high pressure jet, between a few hundred millibars and a few bars.

Les ondes acoustiques utilisées ont une fréquence inférieure à environ 20 Hz.The acoustic waves used have a frequency less than about 20 Hz.

Lorsque les contraintes réglementaires ne nécessitent pas d'atteindre une réduction aussi importante que celle citée précédemment, l'utilisation de l'injection à impulsion multiple et la production d'infrasons dans le procédé de recombustion des oxydes d'azote permet d'atteindre des taux d'efficacité de réduction des NOx de l'ordre de 50% mais en utilisant moins de combustible. Par exemple, la réduction de la quantité de combustible secondaire est de l'ordre de 50% en volume, ce qui permet de réduire le coût du procédé.When regulatory constraints do not require achieve a reduction as large as that cited above, the use of injection multiple pulse and infrasound production in the nitrogen oxides recombination process allows achieve NOx reduction efficiency rates of around 50% but using less fuel. For example, reducing the amount of fuel secondary is around 50% by volume, which allows reduce the cost of the process.

Bien entendu l'invention n'est nullement limitée aux modes de réalisation décrits et illustrés qui n'ont été donnés qu'à titre d'exemple.Of course, the invention is in no way limited to described and illustrated embodiments which have not been given as an example.

Ainsi bien que l'on ait décrit un procédé et un dispositif comportant une injection à double impulsion, des dispositifs et des procédés utilisant une injection à impulsion multiple du combustible secondaire dans la zone de recombustion pourront être utilisés. Ainsi une injection à triple impulsion du combustible secondaire peut être envisagée, dans laquelle le combustible gazeux sera injecté selon deux jets à haute pression concentriques l'un avec l'autre et un jet à basse pression concentriquement extérieur aux deux jets haute pression.So although we have described a process and a device comprising double-pulse injection, devices and methods using injection to multiple pulse of secondary fuel in the area may be used. So a triple pulse injection of secondary fuel can be considered, in which the gaseous fuel will be injected using two high pressure jets concentric with each other and a low jet pressure concentrically external to the two high jets pressure.

Claims (10)

  1. Process for reducing the nitrogen oxides contained in exhaust gases from a primary combustion, in a furnace, by recombustion of said exhaust gases, characterized by an injection of a secondary fuel in the recombustion zone of said furnace according to at least two associated jets at a relatively high pressure and a relatively low pressure respectively, so that the high pressure jet drives a part of the fuel injected at low pressure.
  2. Process according to claim 1, characterized in that the low pressure fuel jet is outerly concentric to the higher pressure fuel jet
  3. Process according to claim 1 or 2, characterized in that the fuel is a gas at a pressure between some millibars and some hundreds of millibars for the low pressure jet and between some hundreds of millibars and some bars for the high pressure jet.
  4. Process according to anyone of claims 1 to 3, characterized in that the pressure and the outputs of the gas of said two jets are controlled in order to be adapted to the size of the recombustion zone and to the exhaust gas characteristics.
  5. Process according to anyone of claims 1 to 3, characterized in that the recombustion zone of the furnace is subjected to sound waves.
  6. Process according to claim 5, characterized in that said sound waves have a frequency that is smaller than about 20 Hz.
  7. Installation for carrying into effect the process according to anyone of claims 1 to 6, comprising a furnace including a primary combustion zone (A) fed by a main fuel and followed by a recombustion zone (B) fed by a secondary fuel, characterized in that said recombustion zone is provided with at least two inlets (2, 2') for the secondary fuel at a relatively high pressure and a relatively low pressure, respectively.
  8. Installation according to claim 7, characterized in that said two inlets (2, 2') are coaxial.
  9. Installation according to claim 7 or claim 8, characterized in that said two inlets are respectively provided with an apparatus for controlling the output and the pressure (4, 5) of the secondary fuel that forms said two jets.
  10. Installation according to anyone of claims 7 to 9, characterized in that a sound wave generator (6) is mounted on the wall of the furnace in order to homegenize the mixture in the recombustion zone.
EP97400984A 1996-05-21 1997-04-30 Process and installation for reducing by recombustion of nitric oxides in exhaust gases from a primary combustion in a furnace Expired - Lifetime EP0809067B1 (en)

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SI9730090T SI0809067T1 (en) 1996-05-21 1997-04-30 Process and installation for reducing by recombustion of nitric oxides in exhaust gases from a primary combustion in a furnace

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FR9606293A FR2749066B1 (en) 1996-05-21 1996-05-21 METHOD FOR REDUCING, BY RECOMBUSTION, THE NITROGEN OXIDES CONTAINED IN THE SMOKE FROM A PRIMARY COMBUSTION MADE IN AN OVEN, AND INSTALLATION FOR IMPLEMENTING IT
FR9606293 1996-05-21

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US5908003A (en) * 1996-08-15 1999-06-01 Gas Research Institute Nitrogen oxide reduction by gaseous fuel injection in low temperature, overall fuel-lean flue gas
FR2834774B1 (en) * 2002-01-16 2004-06-04 Saint Gobain Emballage BURNER AND METHOD FOR REDUCING NOx EMISSIONS IN A GLASS FURNACE
NL2001797C2 (en) * 2008-07-14 2010-01-18 Essent En Produktie B V Method for burning a second solid fuel in combination with a first solid fuel.

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JPS59173605A (en) * 1983-03-23 1984-10-01 Hitachi Zosen Corp Uniform feeding method of powder fuel in combustion furnace
DE3410945A1 (en) * 1984-03-24 1985-10-03 Steag Ag, 4300 Essen METHOD FOR REDUCING NO (ARROW DOWN) X (ARROW DOWN) FORMATION IN COMBUSTION PLANTS, IN PARTICULAR MELT CHAMBER FIREPLACES, AND COMBUSTION SYSTEM FOR IMPLEMENTING THE PROCESS
DE3441675A1 (en) * 1984-11-15 1986-05-22 L. & C. Steinmüller GmbH, 5270 Gummersbach METHOD FOR REDUCING NO (ARROW DOWN) X (ARROW DOWN) CONTENT IN COMBUSTION GASES
US4779545A (en) * 1988-02-24 1988-10-25 Consolidated Natural Gas Service Company Apparatus and method of reducing nitrogen oxide emissions
IT1247541B (en) * 1991-05-07 1994-12-17 Ente Naz Energia Elettrica PROCESS FOR REDUCING NITROGEN OXIDES IN COMBUSTION GASES
US5181475A (en) * 1992-02-03 1993-01-26 Consolidated Natural Gas Service Company, Inc. Apparatus and process for control of nitric oxide emissions from combustion devices using vortex rings and the like
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FR2749066B1 (en) 1998-08-21
HUP9700845A1 (en) 2000-05-28
DE69702950T2 (en) 2001-03-29
EP0809067A1 (en) 1997-11-26
FR2749066A1 (en) 1997-11-28
PL320054A1 (en) 1997-11-24
ES2152070T3 (en) 2001-01-16
CZ135297A3 (en) 1997-12-17
SK55397A3 (en) 1998-01-14
DE69702950D1 (en) 2000-10-05
HU9700845D0 (en) 1997-06-30
SI0809067T1 (en) 2001-04-30
PT809067E (en) 2001-02-28

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