MXPA96004721A - A method for purifying gases containing nitrogen oxides and an apparatus for purifying gases in a generation boiler of va - Google Patents

Abstract

A method is disclosed for reducing the level of nitrogen oxides in flue gases, emitted from combustion units, by introducing reducing agents, which come into contact with gases containing nitrogen oxides, in a first and second reductive stages. The first reducing stage is a non-catalytic stage (for example, at temperatures greater than 800 ° C), while the second stage is a catalytic stage (for example at temperatures of about 300-400 ° C). A boiler is also supplied for the generation of steam, with improved resources for the reduction of nitrogen. The amount of nitrogen oxides in the hot gases is reduced by the combination of the first and second reducing stages, while the steam is produced in the boiler system generating this steam, thus producing essentially nitrogen-free gasses, while eliminating the possibility of escape of NH3 (or other reducing agent) within the discharged flue gases. The heat transfer in a convection section is used to establish stabilized temperature conditions for the catalytic reduction

Description

A METHOD FOR PURIFYING GASES CONTAINING OXIDES FROM NITROGEN AND AN APPARATUS FOR PURIFYING GASES IN A STEAM GENERATION BOILER The present invention relates to a method for reducing the level of nitrogen oxides in flue gases emitted from combustion units and, more particularly, refers to the reduction of NOx levels, by the introduction of reducing agents in contact with the gases containing nitrogen oxides and at the end of the reduction bringing the gas to a catalytic reduction. The present invention also relates to a boiler for the generation of water vapor, which has improved resources for the reduction of nitrogen.

The invention relates to a method for decreasing the content of nitrogen oxides of flue gases, which emanate from the reactions of substantially any fuel, including solid fuels, sediments, gaseous fuels or the like. In particular, the invention provides an improved combustion process, fluidized bed, in which effluent gases from the stack can be controlled economically to meet current environmental standards.

Reducing emissions of nitrogen oxides from exhaust gases or flue gases, before releasing them into the atmosphere, has been a prolific topic of discussion in the field of environmental aspects of combustion energy production. of combustible materials. Because N0X emissions are related to several environmental problems, minimizing the release of N0X from combustion systems is of current interest.

It is evident that the emissions of the nitrogen oxides result from any combustion reaction, where the air is present and / or the spent fuel contains nitrogen. The combustion of fluidized bed fuel is a well-known practice and was found to be beneficial in reducing the emissions of nitrogen oxides, due to its relatively low operating temperature. In the combustion of fluidized bed, the air is introduced traditionally through a plenum, where it is distributed through the distribution grid for this air. The fuel, fluidizing solids and possibly sorbents [such as limestone or dolomite], are fluidized and they react in the furnace at temperatures normally in the approximate range of 700 to 12002C. Nitrogen oxides are generated by burning any co-fuel as a result of the thermal fixation of nitrogen in the air and the conversion of nitrogen from the fuel. The first reaction is favored at high temperatures (above about 9502C), while the latter is carried out mainly at lower temperatures, for example, those generally found in fluidized bed combustion systems. U.S. Patent No. 3,900,544 suggests the noncatalytic removal of nitrogen oxides from flue gases exiting a conventional furnace by injecting ammonia (NH3) into the effluent stream while at a approximate temperature from 871 to 1093dC. European patent publication 176,293 also discloses the use of NH3, for the control of NOx by the injection of ammonia into the co-fuel gas stream, before it enters a centrifugal separator. Many other patents have suggested the use of ammonia with a catalyst, so that this ammonia is injected into the gases, before the catalytic reduction. The patent of E. U. A., No. 4,393,031 suggests the injection of ammonia into the gases and, after mixing the gases with the ammonia, passing the mixture through a catalytic reactor.

The methods suggested by the prior art are advantageous, but still have several drawbacks. The non-catalytic reduction of NOx by the injection of ammonia into the flue gases, has limited the ability to reduce emissions of nitrogen oxides, while the molar ratio of NH3 / N0X can increase to a high level which occurs the "escape of NH3". This causes inconvenient emissions of ammonia with flue gases into the atmosphere, as does the possible binding of ammonia in the ashes. The suggested method of injecting ammonia into gases, before its contact with the catalyst, has better reduction capabilities. However, the catalytic reduction of nitrogen oxide requires a large amount of catalyst. As a result, containers, which occupy a large space, are necessary to support the catalytic layers. In commercial scale plants, this kind of catalytic vessel can normally be even larger than 7-10 meters. Substantial pressure losses also occur in this class of reduction systems.

According to the present invention, a method is provided for reducing the emissions of nitrogen oxides in the atmosphere from a combustion process, in which an efficient reduction is achieved and the disadvantages of the prior art methods are overcome. , discussed above. In accordance with the present invention, a method is provided for reducing N0X emissions from a combustion process for steam generation, in which efficient reduction is achieved with a compact sized steam generator. According to the present invention, a method is provided for reducing the N0X emissions from a combustion process, by the efficient reduction in a catalytic treatment, in which the pressure losses are low, typically less than about 400 Pa.

The present invention also comprises a boiler system for steam generation, fluidized bed, with better reduction resources for emissions of N0X compared to the prior art, and where efficient reduction is achieved with a bed reactor Fluidized, compact size. According to an exemplary method of the present invention, a method is disclosed for purifying combustion gases from a boiler system for the generation of steam, which comprises a fuel reaction chamber and a convection section of the gas of humer, operatively connected to the reaction chamber, which has heat transfer elements to extract the heat from the gases.

The method comprises the steps of: (a) maintaining the combustion reactions within the combustion chamber, which result in the production of hot gases containing nitrogen oxides; (b) discharging the hot gases from the reaction chamber and guiding them to the convection section; (c) cooling the gases in the convection section of the flue gases; (d) reducing the nitrogen oxides in a first reducing stage, bringing the hot gases in contact with a reducing agent; and then (e) reducing the nitrogen oxides in a second reducing stage, subjecting these gases, which contain the reducing agent of the first reducing stage, to a catalytic reduction of the N0X, in the convection section of the flue gases. The invention also relates to a boiler system for the generation of steam, which includes the following elements: A fuel reaction chamber and a convection section of the flue gases, operatively connected to the reaction chamber. This section of convection has elements of heat transfer, to extract the heat from the flue gases. Elements for introducing the reducing agent into the flue gases in the reaction chamber. And a catalytic reducing element of the nitrogen oxides, inside the convection section, on the opposite side of the introduction element of the reaction chamber. The invention also relates to a method for purifying the combustion gases of a fluidized bed steam generation plant, including a fluidized bed reaction chamber, a particle separator, connected to the reaction chamber, and a convection section of the flue gases, connected to the particle separator and having heat transfer elements, to extract the heat from the gases. This method comprises the steps of: (a) maintaining the combustion reactions in a fluidized bed of solids in the fluidized bed reaction chamber, which result in the production of hot flue gases; (b) discharging the hot gases and particles entrained by them from the reaction chamber and carrying the gases and particles within the particle separator: (c) separating the particles from the gases in the separator; (d) in a first reducing stage, bring the hot gases in contact with a reducing agent, which makes the reduction of the NOx content of the gases, under non-catalytic conditions: and then (e) transport these flue gases inside of the convection section of the flue gases and cool the gases there. And (f), in a second reduction stage, subject the gases, which contain the reducing agent of the first reducing stage, to a catalytic reduction of the N0X in the convection section of the flue gases, after the practice of the stage (e).

The amount of nitrogen oxides in the hot gases is reduced, according to the present invention, in a combination of two stages, arranged in series, while the steam is produced in the boiler system for the generation of the same, resulting in gases essentially free of nitrogen oxides and eliminating the possibility of leaks of NH3 (or a similar reducing agent) in the discharged flue gases. The present invention utilizes heat transfer surfaces in a convection section to establish stabilized temperature conditions for catalytic conversion. According to the present invention, the reducing agent of N0X, preferably ammonia, is injected into the hot combustion gases into the reaction chamber and / or into a section connecting the reaction chamber and the convection section, at a temperature higher than 8002C. This performs the non-catalytic reduction of the nitrogen oxides in the hot gases. The injection in the locations described above causes no additional pressure losses. According to the invention, it is preferable to adapt the injection location (s) to the steam generation load of the plant, thus ensuring that the injection temperature and the retention time of the ammonia are kept optimal, in the first stage of reduction, in all operating conditions of the boiler system, for the generation of steam, fluidized bed.

The gases - which will still contain nitrogen oxides and ammonia - are passed to the heat transfer surfaces in the convection section of the steam generation plant, thus lowering the temperature of these gases. After they have cooled to At an approximate temperature of 300 to 500ac, the gases are introduced into the second reduction stage, for the catalytic reduction of the nitrogen oxides. In this second stage, ammonia, previously injected into hot gases, if present, and essentially no additional reducing agent, is necessary in normal operation. The temperature is selected according to the requirements of the used catalyst and, once selected, the temperature must be kept stable within certain limits and ensure that the reduction takes place. According to the invention, it is preferable to cool the gases to the selected temperature, which is typically within the approximate range of 300 to 500SC, by the appropriate transfers of heat in the convection section, before the catalytic treatment. In this way, the temperature of the catalytic treatment can be stabilized and also preferably maintained in the second stage within the approximate range of ± 25ac of the optimum working temperature of the catalyst used. Stable temperature conditions are easily maintained by regulating the flow rate of the heat transfer medium at least in a heat exchanger, which precedes the catalyst in the convection section. This is preferably carried out according to the load of the steam generator, thus always obtaining an optimum working temperature of the catalyst, with variable loads of the plant.

Since the main reduction portion has taken place, in the first reduction stage, the catalyst in the second stage is preferably arranged in connection with the convection section, more preferably within the convection section, after the surfaces of the convection section. heat transfer. The dimension of the catalyst layer, according to the present invention, is so small that it can be disposed at an appropriate location within the convection section, so that the temperature of the catalyst can be maintained at its optimum level of operation. It has been found that the required reduction in nitrogen oxides results if the gases are arranged to flow through the catalyst over a linear flow distance of less than about 2 meters (e.g., less than 1 meter) in a commercial size plant. Thus, in accordance with the present invention, a much smaller catalytic surface than in the prior art methods is required. Consequently, pressure losses are lower. The catalyst required by the present invention results in pressure losses that are at least 50% lower than in the prior art methods (for example less than about 400 Pa of pressure reduction). This results in considerable savings in operating costs, while still providing an adequate reduction of NOx without excessive ammonia leaks.

Under some operating conditions, such as a low load, there may be an excess of the ammonia injected after the first reduction stage. In such cases, the second stage of reduction, according to the invention, while reducing the emissions of the nitrogen oxides, simultaneously eliminates the escapes of the gaseous N0X from the gases. This makes the present invention, with two reducing stages, even more attractive. It is possible to inject such amounts of ammonia that the reduction is at a maximum, without the risk of dangerous escapes of the NOx and their entrainment to the atmosphere together with the discharged flue gases.

It should be understood that any known nitrogen reducing agent can be used in connection with the present invention, but preferably this reducing agent is selected from the group consisting essentially of agents containing amines, ammonia or urea, or a precursor that produces ammonia.

Figure 1 shows a circulating fluidized bed steam generation plant, provided with the NOx reducing resources according to the present invention.

This Figure 1 illustrates a power plant system that incorporates a typical steam generation, fluidized bed, circulating type, with superheater and an economizer system, this system incorporates the present invention. The boiler system, generally designated by the reference number 1, comprises a combustor device 2, which has a combustion chamber 3, inside which the combustible material, the non-combustible material, additional additives or recycled material, air are fed. primary and secondary air. In the combustion chamber 3, the bed is maintained in a fluidized state, having the correct inventory of the bed material and the desired flow of air. This combustion chamber 3 is provided with a bottom 4 having a grid-like construction, through which the fluidizing air is introduced. The walls of the combustion chamber are preferably conventional membrane-type tube walls, which serve as the steam generating surfaces, with or without a refractory cover. The materials (particles / solids) of the combustion chamber are brought from this combustion chamber 3 by the hot exhaust gases via the conduit 5 to a separator 6 of hot particles (typically a cyclone separator), in which the solids they are separated from the gases for return by means of the particle recycling system, 7, 8 and 9, to the lower section of the combustion chamber, to be reused in the bed. The recycled particles can be passed through fluidized bed coolers, or the like (not shown) before returning to the combustion chamber 3.

The details of the circuit for the feedwater, the steam generation and the superheaters are not illustrated, since they do not form an essential part of the present invention, and they are conventional.

To reduce the content of the nitrogen oxides of the hot gases, in the first stage of reduction, the reducing agent - preferably the NH3 - is injected via the nozzles 15, 16 and / or 17, into the gases, where these gases are at a temperature higher than 8OO2C. At such a temperature, the reducing reactions between N0X and NH3 take place in a non-catalytic manner, and thus large, separate, large-sized catalyst beds are not necessary. The location of the injection points 15 to 17 can be controlled according to the load of the plant, so that optimum temperature and retention time are obtained for the non-catalytic reduction of N0X. A critical factor for effective reduction is to efficiently mix ammonia with hot gases. Therefore, nozzles in locations 16 or 17 provide good results due to efficient mixing in cyclone separator 6. The NOx content of the gases is significantly decreased in the first reducing stage by the injection of ammonia, this N0X content is preferably reduced to a level below about 60 ppm. The flue gases from the separator 6 pass through a conduit 19 to a convection section 10, where the first reduction stage is practiced. The stage 11 of the superheater can be placed inside the convection section 10 with, for example, the superheaters 12 and 13 placed downstream of the superheater 11 and upstream of the surfaces 14 of the economizer. The selection of the necessary heat transfer structures upstream of the economizer surfaces 14 depends on the fuels, reducing agents and other particular variables. The gases, while passing over the heat transfer structures 11, 12 and 13, are cooled and pass through the surfaces 14 of the economizer. After passing the surfaces 14 of the economizer, the gases are brought to the second reduction stage, according to the invention. In the second reduction step, the gases are subjected to a catalytic reduction, in the presence of a reducing agent, from the first reduction stage. The catalyst bed or section 18 in the second reduction stage completes the reduction and the amount of nitrogen oxides, after the second stage, is at a generally acceptable level, preferably below about 20 ppm. The required amount of the catalyst in the second stage is very small, thus being directly adaptable to the convection section 10 of the plant 1. The linear length of the catalyst section 18, ie the passage of the gases to flow through the section 18 of the catalyst is less than about 2 meters, preferably less than 1 meter (or, if more than one bed 18 is supplied, the total length of the linear passage is less than 2 meters). [Typical catalysts that can be used in the catalytic section / bed 18 include the V2O5 catalysts or the CuO catalysts.] This results in virtually no additional pressure losses in the second reduction stage, the loss of pressure in the Catalytic section 18 is less than about 400 Pa, preferably less than 200 Pa. Thus, the present invention provides an adequate reduction of nitrogen oxides, where there is virtually no significant loss of pressure, contrary to the prior art methods, which they can result in pressure losses as high as twice the present invention.

To ensure proper functioning of the second reduction stage, the temperature of the gases entering the catalyst section 18 is stabilized and maintained at a desired level, by the influence of the flow rate of the feed water to the economizer 14 The optimum reduction temperature for this illustrative embodiment and the commercially available V2O5 catalyst is 300 to 4002C. In the case of using a catalyst which has its optimum working temperature above 400 ° C, it is preferably disposed before the economizer 14, or between the first and second surfaces 14 of the economizer in the convection section 10. At a temperature of about 500SC, zeolite catalysts are preferred. According to the invention, it is possible to place the section 18 of the catalyst at its optimum working temperature, by selecting the appropriate location in the convection section 10 and, furthermore, to have the temperature maintained within the operating range of the catalyst, by the influence of the flow regime of the medium flowing through the heat transfer surfaces, which precede the section 18 of the catalyst. Stabilization of gas temperature is important. If the temperature of the gas entering the section 18 of the catalyst decreases substantially by more than 25 SC from the optimum working temperature of the catalyst, the reduction of the nitrogen oxides decreases radically. On the other hand, if the temperature is too high, a secondary oxidation reaction from S02 to S03 could occur, which would cause a corrosive condensation later in the process.

As illustrated in the dotted line in Figure 1, one or more catalyst sections 18 'may be disposed after the economizer surfaces 14, if the catalyst is selected so that its optimum working temperature is above 400 ° C. , for example, around 5002C. It is also possible to place the catalyst section 18 'between the economizer surfaces 14 if the temperature requirements are such that this location is in order. The present invention makes it possible to easily select the appropriate location in the convection section 10 of the catalyst sections 18, 18 'and still supply the temperature within the desired range.

Under some operating conditions, such as a low load condition, there may be an excess of ammonia injected after the first reduction stage. In such cases, the second stage of reduction, while reducing emissions of nitrogen oxides, simultaneously removes the escape of gaseous NH3 from the gases. This makes the present invention with two reductive stages even more attractive, due to its ability to provide a safe method of operation of the fluidized bed steam generator, even at low loads.

As a precautionary measure, the second reduction stage in the convection section 10 may be provided with an additional injection nozzle 20 of the reducing agent, upstream of the catalyst section 18, 18 '. This additional nozzle 20 for introducing the reducing agent can only operate in response to the temporary aberrant conditions in the first reduction stage [for example, if a blockage occurs in the injection nozzles of the reducing agent]. While circulating fluidized bed combustion has been described herein as a preferred embodiment, it should be understood that the present invention can be applied to various processes, for example, the convection section 10 can be arranged in association with, for example, a black liquor recovery boiler. It can also be associated with a waste heat boiler of a process in which the thermal N0X is produced in the flue gases, in this case, the convection section can be arranged in a horizontal position.

The present invention can utilize the existing soot blowing system for the heat exchangers in the convection section 10. Sections 1818 'of the catalyst can be kept clean using the soot blowers provided for the heat transfer surfaces in the convection section 10. In the case that the catalyst sections 18, 18 'require their own soot blowers, the use of the soot blowing system of the heat transfer surfaces in the convection section 10 can still be used. While the invention has been described in connection with what is currently considered to be the most practical and preferred embodiment, it will be understood that the invention is not limited to the described modality and, on the contrary, attempts are made to cover several modifications and equivalent arrangements included within the scope of the invention. spirit and scope of the appended claims.

Claims (28)

1. A method for purifying combustion gases from a steam generating plant, this plant comprises a fuel reaction chamber and a convection section of the flue gas, operatively connected to the reaction chamber, the convection section has transfer elements of heat to extract the heat from the gases, this method is characterized by the steps of: (a) maintaining the combustion reactions inside the reaction chamber, which results in the production of hot gases containing nitrogen oxides; (b) discharging the hot gases from the reaction chamber and carrying them into the convection section; (c) cooling the gases in the convection section of the flue gas; (d) reducing the nitrogen oxides in a first reducing stage, bringing the hot gases into contact with a reducing agent; and then (e) reducing the nitrogen oxides in a second reducing stage, by subjecting the gases, which contain the reducing agent of the first reducing stage, to a catalytic reduction of the NOX in the convection section of the flue gas.

2. A method, according to claim 1, characterized in that an economizer, having heat transfer surfaces, is provided within the convection section; and because stage (e) is practiced after the gases have passed through the heat transfer surfaces of the economizer, and step (d), before passing the gases through the heat transfer surfaces of the economizer .

3. A method, according to claim 1, characterized in that an economizer, having heat transfer surfaces, is provided within the convection section; and because step (e) is practiced before the gases pass through the heat transfer surfaces of the economizer.

4. A method, according to claim 1, characterized in that step (c) is practiced to cool the gases, inside the convection section, at a temperature of < 500sc by the steam heating surfaces of the convection section, before the catalytic reduction in the second reducing stage.

A method, according to claim 1, characterized in that step (c) is practiced to cool the gases within the convection section, at a temperature of about 300 to 400 ° C, before the second reducing stage, and step (d) is practiced before step (c).

A method, according to claim 1, characterized in that a further step of maintaining the temperature of the gases in the second reducing stage, at a level within ± 252C of the optimum working temperature of the catalyst used in the second stage reducing, by controlling the flow rate of the heat transfer medium in the convection section that precedes the second reducing stage.

7. A method, according to claim 6, characterized in that step (d) is practiced by injecting the reducing agent for its contact with the hot gases, in one or more positions consistent with the steam generation charge of the power generation plant. this steam

8. A method, according to claim 5, characterized in that step (d) is carried out by bringing the hot gases in contact with the reducing agent, at a temperature higher than 800 ° C, to perform the non-catalytic reduction of the NOx in the hot gases .

9. A method, according to claim 1, characterized in that step (e) is practiced to reduce any excess amount of the reducing agent in the gases, before these gases leave the convection section.

10. A method, according to claim 1, characterized in that step (e) is practiced to pass the hot gases through the catalyst bed, at a linear flow distance of less than about 2 meters.

11. A method, according to claim 1, characterized in that steps (a) - (e) are practiced so that the pressure loss of the catalytic treatment of step (e) is less than 50% of the pressure loss for conventional catalytic reduction.

12. A method, according to claim 1, characterized in that the pressure loss of the catalytic treatment, in step (e), is less than about 400 Pa.

13. A method, according to claim 1, characterized in that the reducing agent introduced in step (d), is selected from the group consisting essentially of agents containing amines, ammonia, urea, or a precursor that produces ammonia.

14. A boiler system for the generation of steam, which comprises: a fuel reaction chamber and a convection section of the flue gas, operatively connected to the reaction chamber; this section of convection has elements of heat transfer, to extract the heat from the flue gases; elements for introducing the reducing agent into the flue gases, inside the reaction chamber, before the heat transfer elements; characterized by a catalytic reducing element of the nitrogen oxides, inside the convection section, on the opposite side of the introducer element, from the reaction chamber.

15. A boiler system for the generation of steam, according to claim 14, characterized in that the catalytic reducing element of the nitrogen oxides comprises a catalytic bed, which has a linear length in the direction of the flow of the gases, a through the convection section, and because the length of the catalytic bed is less than 2 meters.

16. A boiler system for steam generation, according to claim 15, characterized in that the length of the catalytic bed is less than 1 meter.

17. A boiler system for the generation of steam, according to claim 14, characterized in that the catalytic reducing element of the nitrogen oxides is placed in a location in the convection section, where the gases are at a temperature which is substantially optimal for catalytic reduction.

18. A boiler system for generating steam, according to claim 14, characterized in that the reaction chamber comprises a fluidized bed combustor device, and further comprises a particle separator, between the reaction chamber and the catalytic reducing element.

19. A method for purifying combustion gases, according to claim 1, in which the steam generation plant comprises a fluidized bed reaction chamber, a particle separator connected to the reaction chamber and a gas convection section of a humer, connected to the particle separator and having heat transfer elements, to extract heat from the gases, characterized by the subsequent steps of: (a) maintaining the combustion reactions in a fluidized bed of solids in the reaction chamber of the fluidized bed, which results in the production of hot flue gases; (b) discharging the hot gases and particles entrained with them from the reaction chamber and carrying the gases and particles within the particle separator; (c) separating the particles from the gases, inside the separator; (d) in a first step of reduction, bringing the hot gases in contact with a reducing agent, which performs the reduction of the N0X content of the gases, under non-catalytic conditions; then (e) cooling the gases in the convection section of flue gases; and (f) in a second reducing step, subjecting the gases, which contain the reducing agent of the first reducing stage, to the catalytic reduction of the N0X in the convection section of the flue gas, after practicing step (e) .

20. A method, according to claim 19, characterized in that the plant includes an economizer with heat transfer surfaces within the convection section, and because the stage (f) is practiced after the gases have passed through the surfaces of heat transfer from the economizer.

21. A method, according to claim 19, characterized in that the plant includes an economizer with heat transfer surfaces within the convection section, and because the stage (f) is practiced before the gases pass through the surfaces of heat transfer from the economizer.

22. A method, according to claim 19, characterized in that step (e) is practiced to cool the gases within the convection section, at a temperature < 500ac, by the steam heating surfaces of the convection section, before the catalytic reduction in stage (f).

23. A method, according to claim 19, characterized in that step (e) is practiced to cool the gases in the convection section, from a temperature greater than 8002C to a temperature of approximately 300 to 400ac, before the catalytic reduction in the stage (f).

24. A method, according to claim 19, characterized by the further step of maintaining the temperature of the gases in the second reducing stage, at a level within ± 25ac of the optimum working temperature of the catalyst used in the second reducing stage, controlling the flow regime of the medium of. heat transfer in the convection section that precedes the second reducing stage.

25. A method, according to claim 19, characterized in that step (d) is practiced by bringing the hot gases in contact with the reducing agent, at a temperature higher than 800ac, to perform the non-catalytic reduction of the NOx in the hot gases .

26. A method, according to claim 24, characterized in that step (d) is practiced by injecting the reducing agent in contact with the hot gases, in one or more positions consistent with the steam generation load of the steam generating plant .

27. A method, according to claim 19, characterized in that step (f) is practiced to pass the hot gases through a catalyst bed over a linear distance of the flow of less than about 2 meters.

28. A method, according to claim 19, characterized in that step (d) is performed at a temperature higher than 800ac, to reduce the NOx content of the gases to approximately 60 ppm or less, and because the stage (f) it is practiced at a temperature of about 300 to 400ac, to reduce the N0X content of the gases to about 20 ppm or less, and because essentially the possibility of NH3 or a similar reducing agent is eliminated.