METHOD AND CATALYST FOR REDUCING THE NITROGEN OXIDE OF FLUE GASES

Abstract: Process for lowering the concentration of nitrogen oxide in flue gases by reducing the nitrogen oxides present in hot flue gases (1), by using ammonia (10) in the presence of particles (8) which catalyze the reduction of nitrogen oxides. In order to obtain a maximally good contact between the gases (1) and the catalyst (8), small catalyst particles (8) are mixed with the hot flue-gas flow, the particles being so small that they form with the flue gases a suspension flow, from which the catalyst particles are separated (3, 4) after the reduction of the nitrogen oxide, and are returned (6, 7) to the beginning of the hot flue-gas flow (1). The invention additionally relates to the catalyst used in this process.

Description: Method and catalyst for reducing the nitrogen oxide of flue gases. The present invention relates to a process for lowering the concentration of nitrogen oxides in flue gases, and especially to a process in which the nitrogen oxides present in hot flue gases are reduced by means of ammonia or the unburnt constituents of the flue gases, in the presence of particles which catalyze the reduction of the oxides of nitrogen. In addition, this invention relates to a catalyst intended for use in this process. There are several possibilities for removing from flue gases the oxides of nitrogen, mostly NO, which form during the burning of fuels such as coal, gas, oil, etc. One process which has advanced to the stage of commercial application, and which has been developed especially in Japan, is based o the injection of ammonia into flue gases and on the use of a selective catalyst. However, the method has two disadvantages. First, it is difficult to achieve complete mixing of ammonia and flue gases and to bring them into contact with a selective catalyst, for which reason part of the ammonia fed into the flue gases leaves the flue unreacted. Furthermore, the catalyst cell system used in the technical applications is made up of small passages through which the flue gases flow and which are easily soiled, which results i a lowering of the effectiveness of the catalyst. The catalys is soiled especially during the burning of solid fuels such as coal. Attempts have been made to eliminate this drawback by placing the removing device of nitrogen oxides at a point subsequent to the sulfur-removing device, but in such cases the temperature of the flue gas has been too low for the reactions. In the process according to the present invention, the disadvantageous factors presented above can be decreased decisively and, furthermore, when the process is applied specifically to fluidized-bed burning, a simple structure can be achieved. There are various previously known selective catalysts for removing nitrogen oxides from flue gases, such as cell catalysts, pellet catalysts, and cylinder catalysts. Cell catalysts are used in a solid bed, whereas pellet and cylinder catalysts are used as a moving bed so that the incoming flue-gas flow can pass through the pellet bed. However, pellet catalysts are not suitable for a heavy load of dust, since the dust (fly ash) will deposit on the surface of the catalysts, whereupon the activity of the catalyst will decrease and, when the operating time increases, the loss of weight will become immoderately large. Solid-bed catalysts made on a ceramic base (cell or bar catalysts) have proved to be successful in removing nitrogen oxides from flue gases derived from the burning of heavy fuel oil, whereas, if the fuel is coal, a reduction degree of only 60 % has been achieved with these catalysts. The object of the present invention is thus also to provide an improved catalyst for reducing the nitrogen oxides present in hot flue gases, by using ammonia and/or the unburnt constituents of the flue gases, the catalyst retaining its activity also in conditions in which they are subject to a high degree of soiling. The main charactertistics of the invention are given in the accompanying claims. In the process according to the present invention, an effective contact between the catalyst and the gases is obtained by mixing, with the hot flow of flue gases, catalys particles which are so small, preferably of a particle size of approximately 20-200 m, that they form together with the flue gases a suspension flow in which the catalyst particles additionally, owing to a turbulent flow, continually,_impinge against both one another and the conduit walls, so that the surfaces of the catalyst particles are continuously cleaned, whereby the activity of the catalyst remains at an effective level. The catalyst particles can be separated from the gas flow after the reduction of the nitrogen oxides and be returned to the beginning of the hot flue-gas flow. Although the catalyst particles are small, they are, nevertheless, preferably substantially larger or heavier tha the fly-ash particles possibly present in the flue-gas flow, so that the catalyst particles can be removed selectively from the flue-gas flow and the fly ash present in it after the reduction of the nitrogen oxides. The particles catalyzing the reduction of nitrogen oxides ar preferably mixed with a flue-gas flow having a temperature o at maximum 1000 °C, for example approximately 300-600 °C. The catalyst according to the invention thus differs from th previously known catalysts in that it flows along with the flue-gas flow, for example, as a part of the solids cycle of the fluidized-bed reactor. The actual reduction takes place at a temperature of approximately 350-450 °C. During strong circulation, the catalyst is regenerated and the reaction surface remains active. It is also possible to circulate the catalyst as an actual fluidized bed in which reactivation takes place. The catalyst can be prepared on a iθ2 base and be impregnated with the following elements: W, V, Ca, Mo, Fe, Cr, and/or Cu. In a preferred embodiment of the invention the catalyst is prepared by mixing Tiθ2 and O3 with each other at a ratio of 3 to 1, at a temperature of 20-100 °C. To this powder mixture there are added nitrates of the above-mentioned elements in an aqueous solution, to which saltpeter acid approximately 5 % has been added. The mixture is heated at 20-100 °C, until it attains a high viscosity. The catalyst mixture is dried at a temperature of 100-150 °C for 24-48 h. The catalyst particles are screened to form fractions having a diameter of approximately 200 μm. The composition of the catalyst is preferably as follows: 70 - 90 % Tiθ2 20 - 5 % 0 3 1 - 0.5 % V 3 - 2 % Ca 1 - 0.5 % Cr 1 - 0.5 % Cu 4 - 1.5 % Fe The catalyst prepared in the above-mentioned manner proved to be very effective in treating nitrogen-oxide-containing flue gases by means of ammonia, by using the apparatus depicted in the accompanying drawings, in which Figure 1 depicts a cross-sectional side view of one apparatus for carrying out the process according to the present invention, and Figure 2 depicts, also as a cross-sectional side view, an alternative apparatus for carrying out the process according to the present invention. In Figure 1, the nitrogen-oxide-containing hot flue-gas flow is indicated by reference numeral 1. Finely-divided selective catalyst 8 according to the invention is fed into this flue-gas flow by means of feeding device 9 and, at a point immediately thereafter, in order to reduce the nitrogen oxides present in the flue gases, ammonia 10 by means of feeding device 11. The mutual order of the points at which the catalyst 8 and the ammonia 10 are fed in does not affect the working of the process according to the invention. The catalyst 8 and the ammonia 10 can also be fed into the flue gases 1 by means of a common feeding device resembling a burner. Furthermore, the ammonia 8 can be left out completel or partially, if the flue gases 1 are derived from a burning which has been regulated so that the gases contain unburnt gases, such as hydrocarbons or carbon monoxides, in an amount sufficient for reducing the nitrogen oxides present in the flue gases. At the point 11 at which the ammonia 10 is fed in, the temperature of the flue gases must be below 1000 oC » preferably below 500 °C, in order for the course of the reaction to be maximally advantageous. Thereafter the suspension flow made up of the flue gases 1 and the ammonia 10, plus the catalyst 8, flows past the heat surfaces 2 of the reactor, whereupon the ammonia or unburnt gases present in the flue gases reduce the nitrogen oxides present in the flue gases to nitrogen by means of the catalyst. The coarse fraction of the catalyst is separated i the reactor funnel 3, from where it is blown or conveyed in some other manner back to the feeding-in point 9. The finer fraction of the catalyst is separated from the flue gases either in a mechanical separator or in an electro-filter 4, in order to ' be returned as flow 7 to the feeding-in point 9. The separator 4 has been dimensioned so as to separate mainl catalysts but to allow fine fly ash to pass through. For thi reason the particle size of the catalyst should be substantially larger than that of the fly ash, for example, more than 30 m. At the same time the particle size of the catalyst must, however, be so small that it flows along with the flue gases 1, forming a suspension with it. It has been observed in practice that a particle size under 200 μm is sufficient for this purpose. The process according to the invention can be applied to all types of reactors and furnaces, both vertical and horizontal structures, although the vertical structure has certain advantages in terms of the circulation of the catalyst. The process according to the invention can also be applied to a fluidized-bed reactor operating according to the circulating-bed principle, depicted in Figure 2, in which most of the catalyst 8 circulates from a furnace 16 via a cyclone 12 and a bed-material return 14 back to the furnace 16. Part of the bed material can be removed as an underflow 13 from the bottom of the furnace 16. The flue gases which contain nitrogen oxide are directed as a flow 1 into the lower part of the furnace 16, from where they flow through the bed to the upper part of the furnace 16, where ammonia 10 is injected into them by means of a nozzle 11. At a point subsequent to the furnace 16 there is the cyclone 12, from which most of the catalyst is returned through the pipe 14 to the furnace 16, whereas the gases leave from the upper part ' of the cyclone 12 and pass into a reactor 2 fitted at a point subsequent to the cyclone; the finer fraction of the catalyst is returned from the funnel 3 of the reactor 2 and/or from the cones 15 of a separating device 4 located at a point subsequent to it, in the form of flows 6 and 7, back to the furnace 16. The separating device 4 used can be, for example, a mechanical separator such as a cyclone, or an electro- or fiber filter. The separator 4 can be designed so as to separate primarily catalyst dust and to let fly ash pass through, whereby an unreasonable increase in the amount of dust in the cycle is avoided. Such a separator 4 can be, for example, a cyclone of the type used as a coarse separator in connection with so-called bubbling fluidized-bed reactors, and from which separator the unburnt fuel is returned to the furnace. The ammonia 10 required for the reduction of the nitrogen oxides of the flue gases 1 is fed into the furnace 16 throug a nozzle 11 fitted in its upper part, or alternatively into the entrance conduit of the cyclone 12, in order to obtain maximal mixing between the ammonia and the flue gases. The nozzle 11 can also be fitted in the cyclone 12 or at a point subsequent to it, but in that case the mixing effect of the cyclone 12 cannot be exploited in the best possible manner. Instead of ammonia it is possible to use for the reduction unburnt gases, which are obtained by carrying out incompletely the burning from which the flue gases are derived.

Claims: Claims 1. A process for lowering the concentration of nitrogen oxides in flue gases by reducing the nitrogen oxides present in the hot flue gases (1) by means of ammonia (10) and/or the unburnt constituents of the flue gases in the presence of particles which catalyze the reduction of nitrogen oxides, c h a r a c t e r i z e d i n that with the hot flue gas flow (1) there are mixed catalyst particles (8) so small that they form with the flue gases a suspension flow from which the catalyst particles (6, 7) are separated (3, 15) after the reduction of the nitrogen oxides, and are returned (8) to the beginning of the hot flue-gas flow (1). 2. A process according to Claim 1, c h a r a c t e r ! z e i n that with the hot flue-gas flow which contains fly ash there are mixed catalyst particles which are substantially larger or heavier than the fly-ash particles and which, after the reduction of the nitrogen oxide, can be separated from the flue-gas flow and the fly ash present in it. 3. A process according to Claim 1 or 2, c h a r a c t e r i z e d i n that particles which catalyze the reduction of nitrogen oxides and which have a particle size of approximately 20-200 μm are mixed with the hot flue-gas flow. 4. A process according to any of the above claims, c h a r a c t e r i z e d i n that the particles which catalyze the reduction of nitrogen oxides are mixed with a flue-gas flow having a temperature of at maximum approximately 1000 °C, preferably approximately 300-600 °C. 5. A catalyst for the reduction of the nitrogen oxides present in hot flue gases by using ammonia and/or the unburnt constituents of the flue gases, c h a r a c t e r i z e d i n that the particle size of the catalyst is so small that it will travel along with the flue-gas flow, but at the same time so large that it can be separated (3, 15) from the fly ash possibly present in the flue gas. 6. A catalyst according to Claim 5, c h a r a c t e r i z e i n that its particle size is less than 200 m, and preferably more than 20 μm. 7. A catalyst according to Claim 5 or 6, c h a r a c t e r i z e d i n that it contains, on a iθ2 base, WO3 and possibly V, Ca, Mo, Fe, Cr, and/or Cu. 8. A catalyst according to Claim 7, c h a r a c t e r i z e i n that it contains, as calculated from the weight of the catalyst, 70-90 % Tiθ2, 20-5 % O3, 1-0.5 % V, 3-2 % Ca, 1-0.5 % Cr, 1-0.5 % Cu, and 4-1.5 % Fe. 9. A process for the preparation of a catalyst usable in the reduction of the nitrogen oxides of flue gases by using ammonia and/or the unburnt constituents of the flue gases, c h a r a c t e r i z e d i n that Tiθ2 and O3 are mixed together at a weight ratio of 2.5-3.5 to 1 at approximately 20-100 °C whereafter an acidic aqueous solution of a V, Ca, Mo, Fe, Cr and/or Cu salt is possibly added to the powder mixture thus obtained, whereafter the mixture is heated at 20-100 °C to form a viscous mixture, which is dried at 100-150 °C and calcinated at 350-500 °C, and finally the catalyst particles are screened to an approximate particle size of 200 μm. 10. A process according to Claim 9, c h a r a c t e r i z e i n that the catalyst particles are screened to an approximate particle size of over 20 μm.

Inventors: JAANU KEIJO, SAVIHARJU KARI, JAANU, KEIJO, SAVIHARJU, KARI
Applicants: EKONO OY
Application year: 1986
Main classification: B01D53
IPC codes: B01D53, B01J23