CA2069218C - Process and an arrangement for reducing nox emissions - Google Patents

Abstract

The invention relates to a process for reducing NO x emission in the thermal reaction (combustion) of nitrogen-containing fuels or in the reduction of waste gases or vapors containing nitrogen oxides by a multistage reaction in which, in a first stage, all the nitrogen oxides present or spontaneously formed are reduced in a substoichiometric fuel-to-oxygen ratio and, in a following second stage or in following further stages, are after-reacted until all the oxidizable reaction components have been completely reac-ted. To accelerate the reaction, the reactants are addi-tionally activated in at least one of the reaction stages by activation regenerators which statically mix the reac-tants with one another and contribute towards the equaliza-tion of heat flow. A furnace internally comprising a regenerator wall with throughflow openings is provided for carrying out the process.

Description

1 ~.I
a ~~oc~ss ~rrD Art Aaa~aG~r~EaaT ~oR ~aEnucarr~ ~o= ~MassaoNs This invention relates to a process for reducing NO~
emissions in the thermal reaction of mass streams con-taining nitrogen oxides, for example Nz0 (laughing gas) and No (nitrogen monoxide), with fuel components or in the oxidation of nitrogen-containing fuels with oxygen or air by a multi-stage reaction, all the nitrogen oxides present or spontaneously formed being completely reduced in the first stage and then reacted in one or more following stages until all the reduction components have been com pletely oxidized.
Tn the combustion of nitrogen-bound fossil fuels and organic residues, for example organic liquids containing organically bound nitrogen, and in the post-oxidation of waste gases of vapors already containing high levels of nitrogen oxides, the emission of nitrogen oxides is con-siderably higher than in the combustion of the relatively nitrogen-free fuels, natural gas and light heating oil, with an excess of air or oxygen. Thus, nitrogen oxides are normally formed in a concentration of 5000 ppm although the permitted limit is only 100 ppm. This is because, where the fuels are burnt with an excess of oxygen, the nitrogen present in the fuel is completely or partly converted into nitrogen oxides and any nitrogen. oxide components already present pass through the reaction as quasi-inert components which produces the high emission levels.
Tt is known that NOX emissions can be reduced by so-called combustion in stages which is understood to be the division of the combustion process as a whole into a par-tial oxidation, i.e. incomplete combustion in the substoi-chiometric range with partial release of the heat of reac-tion and another or multistage after-reaction until all the oxidizable reaction components have been completely reac-ted.
Tie A 28 282 1 In the case of gas mixtures containing high percen-tages of nitrogen oxides, far example laughing gas or nitrogen monoxide, fuel has to be introduced in at least such a quantity - commensurate with these percentages by volume - that the entire oxygen component of the nitrogen oxides can be reduced. Since the enthalpies of formation of the nitrogen oxides are released in addition to the calorific value of the fuel components and since a stoichi-ometric reaction always occurs during the oxidation of the fuel component with the oxygen of the nitrogen oxides, relatively little nitrogen being available for reducing the theoretical reaction temperature in contrast to the reac-tion with air, an extremely high temperature is established which, in the presence of free oxygen, would lead to high NOx values in accordance with the equilibrium. Accordingly, steps have to be taken to ensure that, as in partial oxidation, a reducing atmosphere is maintained. This can only be achieved by the introduction of additional fuel or reducing components. Although endothermic decomposition reactions do reduce the temperature, depending on the quantity of fuel, the excess of fuel has to be kept very small for economic reasons which in turn leads to a low chemical potential and, in the absence of kinetic support, would require a very long residence time to complete the reaction.
In the partial oxidation of fuels, for example in the gasification of coal or oil, few - in any - nitrogen oxides are formed because, in these reactions, a reducing gas phase always limits the formation of nitrogen oxides to a considerable extent on account of the low partial pressure of oxygen atoms and, at the same time, the nitrogen oxide molecules already formed are reduced in statu nascendi.
This situation is determined by a number of reactions which, although together tending to establish a certain equilibrium, are all influenced to different extents and, he A 28 282 2 in some cases, even in different directions by the thermo-dynamic parameters of state, such as temperature, pressure or chemical potential. Because the heat of reaction is only partly released, in conjunction with endothermic reactions, the partial oxidation takes place at lower temperatures than in complete stoichiometric combustion so that, on the one hand, atomic oxygen is formed to only a relatively limited extent through the dissociation of reaction products, with the result that the NOx equilibrium concentration itself is relatively low at that temperature.
In addition, the high chemical potential of the reducing components ensures that the probability of a reaction to molecular oxygen is very high.
Complete oxidation of the reduction components, such as carbon monoxide and hydrogen, still present from the partial oxidation then takes place in the after-reaction stage following the partial oxidation. These reduction components are formed in the first stage and form the reactants for the reduction of NOX to molecular nitrogen and water.
Thus, where burning is carried out in stages, all the reaction stages take place at a thermodynamically lower level so that the kinetic reaction to complete conversion is impaired or takes place at a reduced rate in each reac-tion stage compared with single-stage combustion because the percentage of activated molecules decreases in accor-dance with the temperature level. To correct this defici-ency, a considerably longer reaction time is required for establishing an equilibrium between all the reaction prod-ucts. Unfortunately, however, this cannot be economically achieved. Since both exothermic and endothermic reactions take place simultaneously in the first stage and, together, determine a reaction temperature for the equilibrium, each element of space throughout the reaction zone should contain an equal quantity of reactants. However, this is Le ~ 28 282 3 only inadequately achieved, even by introducing the reactants into the reaction zone in one or more channels. Although this primary mixing can be optimized through the design of the inflow channels, this unfortunately does not meet the stringent requirements which the course of the reaction has to satisfy.
Accordingly, the problem addressed by the present invention was to improve the course of the reaction and, hence, economy in a process of the type mentioned at the beginning for reducing NOX emissions. Thus, the present invention provides a process for reducing NOX emissions in the thermal reaction of nitrogen oxide-containing product streams with fuel components or in the oxidation of nitrogen-containing fuels with oxygen or air by a multistage reaction, all the nitrogen oxides present or spontaneously formed being completely reduced in a first reaction stage fed with less than the stoichiometric quantity of oxygen and then being reacted in one or more following reaction stages until all the reduction components have been completely oxidized, wherein oxidation of the reduction components takes place in at least one after-reaction stage kinetically supported by activation regenerators in which the reactants are intensively mixed and a uniform temperature is established over the entire flow cross-section.
Another problem addressed by the invention was to provide a simple arrangement for carrying out this process.
Thus, the present invention also provides an arrangement for carrying out the process consisting of a furnace comprising a fuel feed pipe or a waste gas feed pipe and an oxygen or air feed pipe, wherein the furnace is divided by at least one activation regenerator which consists of a wall with throughflow openings into a first reaction stage and at least one following after-reaction stage, the first reaction stage comprising the fuel feed pipe or the waste gas feed pipe and an oxygen or air feed pipe and only one oxygen or air feed pipe opening as sole feed pipe into the after-reaction stage.
According to one aspect of the invention, the first of these two problems has been solved by a process which is characterized in that oxidation of the reduction components takes place in an after-reaction stage kinetically supported by activation regenerators, wherein an activation regenerator is defined by a plurality of throughflow channels or openings through which the gaseous reactants are conveyed and in which an intensive mixing of the reactants and a homogenization of the temperature across the total cross-section of flow are obtained.
The additional activation of the reactants enables the kinetic conditions under which the reaction takes place in the corresponding reaction stage to be improved to such an extent that substantially optimal reaction conditions are established. NOX emission can thus be almost completely suppressed or reduced.
In a preferred embodiment, the reactants are additionally activated in the first stage of the reaction.
In another embodiment of the process according to the invention, however, the reactants are additionally activated in all stages of the reaction. This optimizes the course of the combustion process as a whole because optimal conditions can be established in each stage of the reaction.
In another embodiment of the process according to the invention, the reactants are additionally activated after '9~~ ''~J~ ~
~a .,~ ~ J .~ .r_ the particular reaction stage.
Another preferred embodiment of the process according to the invention is characterized in that the reactants are mixed together in the corresponding arrangement used for their activation, the reactants preferably being mixed together both in the macro range and in the micro range.
In addition or alternatively, however, heat flow is equal-ized by the corresponding arrangement used for activation.
In this embodiment of the invention, therefore, the course of the reaction in the reaction zone - supported by an activation regenerator - is influenced to the extent that the reactants may be mixed (forcibly) both in the macro range and in the micro range. The resulting homogen eous distribution of all the reactants thus serves to accelerate the reaction and hence to shorten the residence time of the reactants in the reaction zone. In addition, the activation regenerator provides for rapid equalization of heat flow which is essentially achieved by an intensive exchange of heat by radiation. After-reactions can thus be influenced to the extent that they can still take place despite the low potential towards the chemical equilibrium which can be established at the reaction temperature. To be able to achieve the result of low NOX emission, it is of crucial importance to the invention spontaneously to activate the possible reactants under these relatively unfavorable thermodynamic conditions, which is achieved by mixing of the reactants in accordance with the invention and by the equalization of heat flow. Since the partial oxidation in the first stage of the reaction is determined by other reactions than the stoichiometric combustion reactions, different thermodynamic conditions have to be established accordingly. The course of the reaction is crucially determined in this regard by the heterogeneous and homogeneous water gas reaction, the Boudouard reaction and, in addition, the decomposition of all the hydrocarbons present in the gas. Since all the reaction equations are interdependent, the desired equilibrium between the result-Le A 28 282 ~~~~~~~_d ing reaction products at a certain temperature can be influenced by controlled introduction of the reactants. To this end, spontaneous activation conditions for the reac-tion also have to be established in the after--reaction stage. According to the invention, this is achieved in relatively short residence times by rapid mixing with the oxidation partner air or possibly or even oxygen, accom-panied by temperature equalization. In this post-oxidation stage, the activation regenerator as a mixer and heat reservoir also provides the kinetic support required in accordance with the invention. Although the exothermic oxidation reactions release heat, only slight increases in temperature occur locally because a large inert gas stream is always available for absorbing the heat. Accordingly, the temperature level remains in a range in which nitrogen oxides can only be formed as thermal BOX in the permitted limiting concentrations.
Accordingly, the success of the process according to the invention lies in the homogeneous distribution of all the reactants and in the establishment of the same tempera ture, which should be above 100°C, in each element of space.
In another very important embodiment of the process according to the invention, steam is additionally supplied as reactant to the reaction stages with substoichiometric combustion. This steam may be supplied either by injection of water into the reaction zone or directly as steam, the equilibrium concentrations being displaced towards a better reduction potential. Tn addition, the presence of steam in the reaction phase can prevent the separation of carbon crystals, thus avoiding a significant fall in the effective reduction potential.
Another embodiment of the process according to the invention is characterised in that, depending on the optimal reaction temperature, the reactants are used in the he A 28 282 corresponding ratio to one another in the particular reaction stage.
Finally, a further embodiment of the process according to the invention is characterized in that, in each reaction stage, the temperature is regulated with recycled, cold waste gas.
The arrangement for carrying out the process according to the invention is characterized in that at least one activation regenerator is arranged in the furnace, prefer ably having a wall with throughflow openings.
This wall is preferably arranged behind a reaction stage and extends over the entire cross-section of the furnace so that the reactants have to pass through the throughflow openings in the wall. If, for example, two reaction stages are provided, a wall of the type in ques-tion is arranged between these two reaction stages. In addition, a wall is also provided after the second reaction stage.
This wall forms a technically very simple activation regenerator and is designed in such a way that it acts as a static mixer for macromixing of the reactants and also performs the function of a heat reservoir. By virtue of its high heat capacity and the favorable exchange of heat by radiation, this wall establishes the thermal reaction conditions in a very short time and, in the case of exo-thermic reactions, re-absorbs heat which is than released again as and when required.
The activation regenerator and, more particularly, the wall may consist either of a temperature-resistant ceramic material or of a temperature-resistant metal, more par°
ticularly steel and, above all, special steel. These materials are capable of satisfying the requirements which an activation regenerator has to meet, in addition to which an activation regenerator of ceramic material may also act as a catalyst.
Le A 28 282 The wall is preferably in the form of checkerwork, particularly. where it is made of metal.
To enable it to be produced in a technically simple manner, the activation regenerator preferably consists of individual regenerator elements.
Where the wall is made in particular of ceramic material, it preferably consists of individual bricks or tiles which are joined firmly to one another in vertical layers. The wall can thus be made in a technically simple manner.
The bricks or tiles are preferably square in shape with throughflow openings.
In another embodiment, the bricks or tiles have additional protuberances or the Tike on their surfaces.
The resulting increase in surface area constantly influ ences boundary layer flows in such a way that the reactants undergo intensive micromixing which in turn provides for rapid and uniform heat and mass transfer.
Finally, in another embodiment, several activation regenerators are arranged one behind the other. This ensures that the reactants are present in optimally acti vated form in each of the stages so that the reaction follows an optimal course. Where the activation regenera tors are arranged one behind the other, several walls in the form of static mixers are preferably provided.
One embodiment of an arrangement according to the invention for reducing NOx emission in the oxidation of nitrogen-containing fuels or in the reduction of waste gases or vapors containing nitrogen oxides is described in detail in the following with reference to the accompanying drawings, wherein:
Figure 1 is a purely schematic side elevation of the arrangement for reducing NOx emission.
Figure 2 is another purely schematic side elevation of the arrangement shown in Fig. 1 in the form of a static Ls A 28 282 8 mixer.
Figure 3 is an elevation on a somewhat larger scale of an activation regenerator in the form of a wall provided with throughflow openings of the arrangement shown in Figs.
1 and 2.
A furnace 1 for the oxidation (combustion) of nitro-gen-containing foals or far the reduction of waste gases or vapors containing nitrogen oxides comprises a fuel supply pipe 2 for the nitrogen-containing fuels or reduction fuels ~.0 and a feed pipe 3 for the waste gases or vapors captaining nitrogen oxides. The furnace 1 further comprises a steam feed pipe 4 and an air feed pipe 5 which branches into the air feed pipes 5°, 5°' opening successively into the furnace 1.
The furnace 1 is a so-called multistage combustion furnace. It comprises a first reaction stage 51 and a following after-reaction stage S2. The air feed pipe 5' opens in the region of the reaction stage S1 while the air feed pipe 5 " opens in the region of the after-reaction stage S2. Finally, the furnace 1 comprises a waste gas outlet 6 at its exit end.
An activation regenerator 7 in the form of a wall occupying the entire cross-section of the furnace is arranged after the reaction stage S1 and the after-reaction stage S2. Each of these walls consists of individual regenerator elements 8 in the form of bricks which are substantially square in shape and which have openings underneath. These openings define throughflow openings 9 in the wall. Finally, the bricks have additional protuber ances 10 on their surfaces.
The arrangement operates as follows:
Either nitrogen-containing fuel and reduction fuel or waste gases or vapors containing nitrogen oxides are fed to the furnace 1 through the fuel feed pipe 2 or through the waste gas or vapor feed pipe 3. Whereas the nitrogen-Le A 2~ 2~2 containing fuels are to be oxidized, i.e. burnt, the waste gases or vapors are subjected to reduction.
The combustion or post-oxidation reaction takes place in stages. Air is fed to the first reaction stage S1 through the air pipe 5' in such a quantity that partial oxidation takes place in the substoichiometric range in this first reaction stage S1. In view of the incomplete combustion in this first reaction stage S1, oxidation is not complete so that carbon monoxide and hydrogen are l0 formed. The carbon monoxide and hydrogen in turn form the reactants for the reduction of NOx to nitrogen and water in the after-reaction stage S2.
The activation regenerator 7 in the form of the wall between the reaction stage S1 and the after-reaction stage S2 and the wall following the after-reaction stage S2 cause the reactants to be mixed both in the macro range and in the micro range. This is schematically illustrated in Fig.

2. In addition, the walls, which may be made bath of ceramic materials and of metals, provide for rapid equaliz-ation of heat flow which is essentially achieved by inten-sive exchange of heat by radiation.
Sy virtue on the one hand of the intensive mixing of the reactants, resulting in homogeneous distribution thereof, and by virtue on the other hand of the establish-ment of the same temperature in each element of space through the high heat capacity of the walls and the result ing intensive exchange of heat by radiation, the reaction taking place in the furnace 1 is accelerated through that NOX emission in the waste gas outlet 6 is reduced in an economically favorable manner.
The introduction of steam through the steam feed pipe 4 displaces the equilibrium concentration towards a better reduction potential. In addition, the presence of steam in the reaction phase can prevent the separation of carbon crystals, so that a significant fall in the effective reduction potential is avoided.
lLe A 28 282 10

Claims (12)

1. A process for reducing NO x emissions in the thermal reaction of nitrogen oxide-containing product streams with fuel components or in the oxidation of nitrogen-containing fuels with oxygen or air by a multistage reaction, all the nitrogen oxides present or spontaneously formed being completely reduced in a first reaction stage fed with less than the stoichiometric quantity of oxygen and then being reacted in one or more following reaction stages until all the reduction components have been completely oxidized, wherein oxidation of the reduction components takes place in at least one after-reaction stage kinetically supported by activation regenerators in which the reactants are intensively mixed and a uniform temperature is established over the entire flow cross-section.

2. A process as claimed in claim 1, wherein the reactants are passed through an activation regenerator in the first reaction stage also.

3. A process as claimed in claim 1 or 2, wherein steam is additionally supplied as reactant to the reaction stages with a reducing atmosphere.

4. A process as claimed in any one of claims 1 to 3, wherein depending on the optimal reaction temperature, the reactants are introduced into the particular reaction stage in the corresponding ratio to one another.

5. A process as claimed in any one of claims 1 to 4, wherein the temperature in each reaction stage is regulated with recycled, cold waste gas.

6. A process as claimed in any one of claims 1 to 5, wherein the nitrogen oxide-containing product streams are charged with laughing gas and/or nitrogen monoxide.

7. An arrangement for carrying out the process claimed in any one of claims 1 to 6, consisting of a furnace comprising a fuel feed pipe or a waste gas feed pipe and an oxygen or air feed pipe, wherein the furnace is divided by at least one activation regenerator which consists of a wall with throughflow openings into a first reaction stage and at least one following after-reaction stage, the first reaction stage comprising the fuel feed pipe or the waste gas feed pipe and an oxygen or air feed pipe and only one oxygen or air feed pipe opening as sole feed pipe into the after-reaction stage.

8. An arrangement as claimed in claim 7, wherein the activation regenerator consists of a temperature-resistant ceramic or metallic wall with throughflow openings.

9. An arrangement as claimed in claim 7 or 8, wherein the wall is a checkerwork of refractory ceramic material and consists of individual refractory bricks or tiles which are joined firmly to one another in vertical layers.

10. An arrangement as claimed in claim 9, wherein the bricks or tiles comprise additional protuberances on their surfaces.

11. An arrangement as claimed in any one of claims 7 to 10, wherein at least one activation regenerator is in the form of a static mixer.

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