EP0809067A1 - Process and installation for reducing by recombustion of nitric oxides in exhaust gases from a primary combustion in a furnace - Google Patents

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

The invention relates to a process for reduction by recombination, nitrogen oxides contained in the fumes from primary combustion produced in an oven, as well as an installation for the implementation of this process.

It is known that the combustion of fuels of all kinds is at the origin of the production of nitrogen oxides by reaction with air, in more or less quantity depending on the type of fuel and the conditions of combustion. These nitrogen oxides emitted into the atmosphere are the cause of various types of pollution. They notably contribute to the formation of acid rain. In addition, combined with carbon monoxide and volatile organic compounds present in the atmosphere, they produce tropospheric ozone which is at the origin of the increase of respiratory affections (asthma, respiratory insufficiency, etc.).

It is therefore important to reduce or eliminate these nitrogen oxides.

For this purpose, a process known as reductive combustion or overcombustion is known.

This process consists in injecting a hydrocarbon according to very precise operating conditions, downstream of a first combustion during which the nitrogen oxides are produced. The purpose of injecting this hydrocarbon is to create a reducing atmosphere which has the consequence, when the temperature is sufficiently high (generally above 1000 ° C.), of cracking the components of the hydrocarbon and of producing radicals (CH ° , H °, ..) which will combine with nitrogen monoxide and the other precursors of nitrogen oxides from the main combustion (first combustion) by complex chemical transformations to form molecular nitrogen and oxygen. The zone in which the injection of this hydrocarbon is carried out is called the reduction or overcombustion zone.

The unburnt materials formed in the reductive overcombustion zone are then oxidized in a third step known as post-combustion. The hydrocarbon which will supply the radicals necessary for the destruction of nitrogen oxides, is thus used both as a reactive depollution agent and as energy.

• 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 entraînés 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 stage called main or primary combustion carried out in a first zone of a combustion furnace, said zone of main or primary combustion. This stage corresponds to the stage proper of combustion of fuels, stage 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.

• 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 step, a fuel 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 step called post-combustion carried out in a third zone of the combustion furnace downstream of the recombustion zone, consisting in injecting air into this third zone to complete the combustion.

The efficiency of a recombustion process depends on many factors such as the temperature, the residence time in the recombustion zone, the nature and mode of injection of the hydrocarbon, the quantity of hydrocarbon injected, the rate initial nitrogen oxides, etc.

However, the recombination process of the prior art described above only achieves efficiency rates for the reduction of nitrogen oxides of the order of 50 to 60%. The rate of effectiveness of the reduction of nitrogen oxides corresponds to the ratio of the number of moles of nitrogen oxides destroyed by the recombination / number of moles of nitrogen oxides before the recombustion.

However, theoretical kinetic studies show that if the mixture of the fuel introduced into the combustion zone and the fumes from the main combustion zone were perfectly homogeneous, an efficiency of reduction of nitrogen oxides could be reached of almost 90%. .

The present invention aims to overcome the above drawbacks by ensuring better penetration and distribution of the fuel in the combustion zone and thus obtaining satisfactory nitrogen oxide reduction rates even when the dimensions of the furnace do not did not allow a good mixture of fuel and fumes, as in the process of the prior art.

These objects are achieved by the invention which proposes a process for reducing the nitrogen oxides contained in the fumes from primary combustion, in an oven, by recombination of said fumes characterized by an injection of fuel into the recombustion zone of said oven according to at least two jets associated with relatively high and relatively low pressure respectively.

More precisely in this process the low pressure fuel jet is externally concentric with the higher pressure fuel jet.

Preferably, according to the invention, the fuel is a gas at a pressure of between a few millibars and a few hundred millibars for the low pressure jet and between a few hundred millibars and a few bars for the high pressure jet.

Also according to the invention, the pressure and the gas flow rates of the two above-mentioned jets are controlled so as to adapt to the dimensions of the combustion zone and to the characteristics of the fumes.

In addition, the furnace's recombustion zone is subjected to acoustic waves.

Preferably, the above-mentioned acoustic waves have a frequency of less than about 20 Hz.

The invention also provides an installation for implementing the method described above comprising an oven equipped with a primary combustion zone supplied by a main fuel and followed by a recombustion zone supplied by a secondary fuel, characterized in that that said recombustion zone is provided with at least two secondary fuel inlets at different pressures.

More specifically, the two aforementioned inputs are coaxial.

According to the invention, the two aforementioned inlets are respectively provided with a device for controlling the flow rate and the pressure of the secondary fuel forming the two aforementioned jets.

In addition, the installation according to the invention has an acoustic wave generator mounted on the wall of the furnace so as to be able to homogenize the mixture in the recombustion zone.

The invention will be better understood and other objects, characteristics, details and advantages thereof will appear more clearly during the explanatory description which follows and which is made with reference to the single appended drawing which represents a preferred embodiment. of an oven in which all of the above-described operations of main combustion, recombustion and post-combustion are carried out.

One of the problems encountered during the implementation of the recombination process of the prior art resides in the mixing of the fuel (natural gas or any other hydrocarbon used) with the fumes containing the nitrogen oxides originating from the combustion zone and arriving in the recombustion zone. Indeed, a good homogenization of the fuel and the fumes in the recombustion zone is essential from the point of view of the temperature and mass exchanges. In fact, if there are flow zones where the radicals (CH °, etc.) are not produced and diffused, no destruction of the nitrogen oxides will take place in these zones.

Likewise, at any point in the mixture, the temperature must be sufficient to crack the radicals which will react with the nitrogen oxides to give molecular nitrogen and oxygen.

Furthermore, it is crucial that the hydrocarbon used is dispersed and cracked as quickly as possible. 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 desired goal.

The homogenization of the mixture is all the more difficult to obtain that the dimensions of the furnace are large and that the volume of fuel injected represents only about 1% of the volume of the fumes of the main combustion.

In addition, the homogenization of the fuel and smoke mixture is often limited by the residence time of the fuel-smoke mixture in the combustion zone. Indeed, this residence time is limited by the dimensions of the oven which are most often limited in height.

By way of example, a combustion oven for implementing the process for reducing nitrogen oxides according to the invention is illustrated in FIG. 1. This oven has the three combustion zones mentioned above, that is to say say a first zone A of main combustion, a second zone B of recombustion and a third zone C of post-combustion. Above the post-combustion zone, there is a gas cooling zone D.

The main combustion zone A comprises supply orifices 3 for a mixture of a so-called fuel primary fuel and air. The primary fuel can be of any kind: coal, fuel oil, waste, wood, natural gas, etc.

Preferably in the invention, the primary fuel will be natural gas.

The aeration rate of this main combustion zone i.e., the ratio of the actual combustion air volume to the theoretical combustion air (stoichiometric combustion) generally varies in the range of 1.05 at 1.1.

The nitrogen oxides contained in the fumes from this primary combustion are then reduced in zone B of reductive combustion or overcombustion. The reduction of nitrogen oxides is achieved by injecting a fuel called secondary fuel by the feed marked 2 in FIG. 1. Preferably, the secondary fuel is injected through inlet 2 in a volume representing 10 to 20% of the volume of the primary fuel, in order to obtain a ventilation rate close to 0.9 in zone B.

In this way, the desired reducing atmosphere (lacking oxygen) is created in the recombination zone.

The secondary fuel used for the recombustion injected into the reducing zone B, can, like the primary fuel, be of any kind: coal, fuel oil, waste, wood, natural gas, etc. However, according to the invention, natural gas is preferably used.

The oxidation stage of unburnt materials is called the post-combustion stage, and is then carried out.

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.

To be effective, the injection of the secondary fuel, preferably natural gas, into the recombustion zone B must be carried out in a temperature range between 1100 and 1500 ° C. and it must be carried out at a corresponding flow rate and pressure. sufficient residence time in the recombination zone to allow satisfactory homogenization of the fuel-smoke mixtures as well as to allow reduction reactions to take place. The residence time must generally be of the order of 0.5 to 1 second depending on the operating conditions.

The reduction temperature used for the recombustion step, in the recombustion zone B, can be lowered to approximately 1000 ° C. but it is then necessary to increase the residence time.

If the rise in temperature increases the efficiency of the destruction of the precursors of nitrogen oxides in the recombination zone, there is however a risk in this case of producing nitrogen oxides at the level of the injection d air in the post-combustion zone above a certain temperature.

A balance must therefore be found between the residence time-temperature parameters, in order to obtain a satisfactory homogenization between the secondary fuel used in zone B of combustion and the fumes from zone A of primary or main combustion containing the oxides d nitrogen to reduce.

In fact the secondary fuel behaves like a jet in a transverse flow and the homogenization of a mixture of a jet (secondary fuel) in a transverse flow (fumes from primary combustion) is subject to antagonistic constraints, d 'all the more difficult as the volume flow rate of the jet on the wall of the furnace is low compared to the main transverse flow.

This is the case in the recombustion process where the volume of fuel, preferably natural gas, injected represents 1% and sometimes less than the flow rate of the fumes.

In order to improve homogenization, the present invention consists in injecting the secondary fuel, preferably natural gas, via a system with several pulses. More particularly, in the installation illustrated in FIG. 1, the secondary fuel is injected by a double pulse system into the recombustion zone.

The double pulse injection according to the invention has two gas supply circuits 2, 2 ′, a low pressure supply circuit 2 ′, adjusted and controlled by any suitable means noted 4 in FIG. 1, and a high pressure gas supply circuit 2 adjusted and controlled by any suitable means noted 5 in FIG. 1.

The double pulse makes it possible to improve the penetration of the jet of secondary fuel up to the center of the oven and also to distribute this fuel throughout the oven enclosure. The central high pressure jet also entrains part of the fuel injected at lower pressure, which also improves its mixing. This double pulse of gaseous fuel can be very easily adjusted, the variations in flow rate and pressure of the gaseous fuel are very easy to achieve in order to optimize the mixture, that is to say in order to adapt to the dimensions of the combustion zone, to the characteristics of the smoke, and at the desired reduction rates.

In the embodiment shown in FIG. 1, the two inlets 2, 2 ′ of the secondary fuel are coaxial and the inlet of the low pressure fuel is externally concentric with the inlet of the high pressure fuel.

The use of such a double-pulse injection system makes it possible to obtain a reduction of the nitrogen oxides contained in the fumes from the primary combustion of the order of 70 to 80%, in moles, whereas with the method and the device of the prior art comprising neither this step of double pulse injection of the secondary fuel and nor the injection system of the secondary fuel with several pulses, the NOx reduction rates are of the order from 50 to 60%, in moles.

The invention is therefore particularly advantageous in cases in which the dimensions of the furnace do not allow good mixing of the fuel in the flue gases, in particular.

The invention also makes it possible, by improving the mixture, to reduce the quantities of secondary fuel to be injected in order to obtain a given rate of reduction of nitrogen oxides.

This is particularly advantageous, for example, in the case of a boiler where the primary fuel is coal and the secondary or recombination fuel is natural gas.

Currently in a recombination process, the nitrogen reduction rates obtained are of the order of 50 to 60% in moles, by substituting, for coal, 20% in energy supply of natural gas.

With the use of the process and / or the installation of the invention, this same reduction rate can be obtained with only 10 to 15% in energy value of natural gas injected, which represents an economic advantage.

In order to further improve the mixing, in particular in the areas close to the walls of the reactor, and also between the layers of the high pressure jet and the low pressure jet of the gaseous secondary fuel, an infrasound generator denoted 6 in FIG. 1 appended is mounted on one of the walls of the combustion furnace at the level of the recombination zone B.

With the addition of this infrasound generator, the reduction rate of nitrogen oxides contained in the fumes from primary combustion is generally greater than 80%, more precisely in the range of about 80 to about 90%, in moles.

The process for reducing nitrogen oxides by recombining the fumes from primary combustion, according to the invention, therefore makes it possible to achieve an NOx reduction efficiency of the order of 70 to 80% by performing the injection. secondary fuel, preferably natural gas, in the combustion zone, following at least two jets associated with relatively high and relatively low pressure respectively.

In this process, the low pressure fuel jet is externally concentric with the higher pressure fuel jet, which improves the penetration of the secondary fuel to the center of the furnace thanks to the high pressure jet and also distributes the gas throughout the enclosure. The central high-pressure jet entraining part of the gas injected at a lower pressure, the mixture of the gaseous secondary fuel with the fumes of the primary combustion containing the nitrogen oxide to be reduced is thus improved.

By associating with this injection of secondary fuel, in the recombustion zone, following at least two associated jets, concentric, at relatively high pressure and relatively low pressure respectively, the jet being externally concentric with the fuel jet at higher pressure, a production of acoustic waves in the recombustion zone, the efficiency of the process of the invention reaches nearly 80 to 90%.

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.

The acoustic waves used have a frequency of less than about 20 Hz.

When regulatory constraints do not require achieving a reduction as large as that mentioned above, the use of multiple pulse injection and the production of infrasound in the nitrogen oxides recombination process makes it possible to achieve NOx reduction efficiency rate of around 50% but using less fuel. For example, the reduction in the quantity of secondary fuel is of the order of 50% by volume, which makes it possible to reduce the cost of the process.

Of course, the invention is in no way limited to the embodiments described and illustrated which have been given only by way of example.

Thus although a method and a device comprising a double-pulse injection have been described, devices and methods using a multiple-pulse injection of the secondary fuel in the recombination zone can be used. Thus, a triple pulse injection of the secondary fuel can be envisaged, in which the gaseous fuel will be injected according to two high pressure jets concentric with one another and a low pressure jet concentrically external to the two high pressure jets.

Consequently, the invention includes all the technical equivalents of the means described as well as their combinations if these are carried out according to the spirit.

Claims (10)

Process for the reduction of nitrogen oxides contained in the fumes from primary combustion, in an oven, by recombination of said fumes, characterized by an injection of a secondary fuel into the recombustion zone of said oven using at least two jets associated with relatively high and relatively low pressure respectively. Method according to claim 1, characterized in that the low pressure fuel jet is externally concentric with the higher pressure fuel jet. A method according to claim 1 or 2, characterized in that the fuel is a gas at a pressure between a few millibars and a few hundred millibars for the low pressure jet and between a few hundred millibars and a few bars for the high pressure jet. Process according to any one of Claims 1 to 3, characterized in that the pressure and the gas flow rates of the two aforementioned jets are controlled so as to adapt to the dimensions of the recombustion zone and to the characteristics of the fumes. Method according to any one of Claims 1 to 3, characterized in that the recombination zone of the oven is subjected to acoustic waves. Method according to claim 5, characterized in that the above-mentioned acoustic waves have a frequency of less than about 20 Hz. Installation for implementing the method according to any one of Claims 1 to 6, comprising an oven comprising a primary combustion zone (A) supplied with a main fuel and followed by a recombustion zone (B) supplied with a fuel secondary, characterized in that said recombustion zone is provided with at least two inlets (2, 2 ') of secondary fuel at different pressures. Installation according to claim 7, characterized in that the two above-mentioned inputs (2, 2 ') are coaxial. Installation according to claim 7 or claim 8, characterized in that the two aforementioned inlets are respectively provided with an apparatus for controlling the flow and pressure (4, 5) of the secondary fuel forming the two aforementioned jets. Installation according to any one of claims 7 to 9, characterized in that an acoustic wave generator (6) is mounted on the wall of the furnace so as to be able to homogenize the mixture in the recombination zone.

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