METHOD AND APPARATUS FOR SELECTIVE REDUCTION OF THE NO-CONTENT OF EXHAUST GASES FROM A KILN PLANT

Abstract: For selective reduction of the NO-content in exhaust gases from a kiln plant, a method is described according to which a mixture of ammonia and water is introduced into the exhaust gases. According to this method the ammonia and water is mixed just prior to being sprayed into the kiln plant by the ammonia being dosed into a preferably even stream of water. Hence an inexpensive and efficient removal of the NO-content in the exhaust gases is obtained.

Description: METHOD AND APPARATUS FOR SELECTIVE REDUCTION OF THE NO-CONTENT OF EXHAUST GASES FROM A KILN PLANT The invention relates to a method for selective reduction of the NO-content of exhaust gases from a kiln plant by means of liquid ammonia, according to which method the ammonia is introduced into the NO-containing exhaust gases in the presence of oxygen (0 2 ) and at a temperature between 700 and 1100 β C. The invention also relates to an apparatus for carrying out the method. A method of the aforementioned kind is known from the description of US patent No. 3900554. This patent provides a general description of the method in terms of the necessary quantities of ammonia and oxygen which must be present and the temperature range within which the process must take place. However, the patent does not provide any solutions for the practical implementation of the method. By use of this method a number of problems will occur. For example, if liquid ammonia from a pressure tank is used, in which tank the ammonia is normally kept at its boiling point, then the ammonia will tend to boil up when it is conveyed to the point of application, which will typically be located at a higher level than the tank, which again, besides giving rise to cavitation problems, makes it difficult to control the supply rate of ammonia to the exhaust gases. This problem may simply be solved by increasing the hydraulic pressure of the ammonia, for example by means of a pump, but this would require that the pressure is lowered again, for example in a nozzle injecting the ammonia, but it has then turned out that the nozzle employed for spraying the ammonia into the exhaust gases has a very limited service life. Practical experience has further shown that introduction of pure ammonia into an exhaust gas will not lead to any reduction of the NO-content; instead, the NO-content will be increased due to the fact that the ammonia is burnt off SUBSTITUTESHEET in the exhaust gas duct, giving rise to NO-formation. Finally, it has turned out to be difficul to obtain a uniform and constant flow of ammonia into the exhaust gases to be cleaned, a satisfactory distribution of the ammonia in the exhaust gases and besides to have the ammonia charged into the exhaust gases at the locations where the temperature and the oxygen content is optimal for the chemical processes. One method suggested for solving the distribution problems involves vapourization and dilution of the ammonia in a large airstream, which is subsequently distributed to a number of spraying points. However, this solution has a distinct disadvantage in that the amount of false air in the kiln system is substantially increased. In connection with this solution there is also a risk that a portion of the ammonia may be burnt off, giving rise to NO formation, as a result of the high oxygen percentage at certain locations, and, therefore, the utilization efficiency of the ammonia is low. Also, a significant amount of heat is used for vapourizing, and the vapourizer is a highly complex and expensive apparatus. The utilization efficiency of the ammonia is defined as the ratio of the number of NO molecules being removed per time unit to the number of NH 3 molecules being dosed per time unit. Another known method for solving the distribution problems has shown to be the use of ammonia mixed in water, since a liquid mixture such as ammonia water, by simple means, can be dosed evenly and distributed in effective manner in the exhaust gases, so that a high utilization efficiency of the ammonia is achieved. Furthermore, the ammonia will not be burnt off and cause NO to be formed when diluted with SUBSTITUTE SHEET water. However, the drawback of this solution is that the cost of ammonia water is relatively high, inter alia due to the fact that distilled water is used for the manufacture, and, therefore, this method of obtaining ammonia is very expensive. What is more, the costs of transportation per kg NH 3 from the point of manufacture to the point of application is relatively higher for ammonia water than for liquid ammonia. Besides, a further disadvantage is that the concentration of the ammonia has been determined beforehand so that continuous variations cannot be made during operation. It is the object of the present invention to provide a method and an apparatus for reducing the NO-content of an exhaust gas by means of liquid ammonia, by which method the aforementioned disadvantages are remedied, providing, at the same time, a high utilization efficiency of the ammonia and a low cost of the ammonia. According to the invention, this object is achieved by a method of the kind described in the introduction, being characterized in that the liquid ammonia, before being introduced into the exhaust gases, is dosed to a collecting point where it is mixed with a stream of water, such as mains water, sewage water and/or other waste water. By mixing the ammonia with water prior to its introduction into the exhaust gases, as suggested according to the invention, a higher utilization efficiency of the ammonia and a greater reduction of the NO-content is achieved compared with the known method, since the ammonia will not be burnt off and cause NO formation, when it is diluted with water. The method also provides enhanced capability of obtaining a satisfactory distribution of the ammonia in the exhaust gases without use of large amounts of air and without resulting in excessive wear of the nozzle spraying means. A further advantage of the method according to the invention is that liquid ammonia in the technical quality being used for agricultural purposes may be employed, this being by far the cheapest option, and that the purchase and transport costs are low as related to the costs involved when using ammonia water purchased from external source in finished state. By dosing the ammonia while it is still in liquid form, this will primarily result in a steady input of ammonia (the same quantity of kg NH 3 per hour). If, instead, the option involving dosage of ammonia water had been chosen, this might well result in variations in the NH 3 -supply in event of changes in the water/ammonia mixing ratio arising in the course of time. It is further an advantage that the NH 3 concentration can be varied continuously for attainment of the optimum utilization efficiency for the NO-cleaning. At plants, where the method according to the invention is to be carried out, there will typically be a substantial difference between the elevation of the storage location of the ammonia and the water and the location where the mixture is charged into the exhaust gases, and according to the invention it is therefore preferred in order to prevent boiling of the ammonia and the ammonia water, respectively, and to convey the ammonia water to the location where it is charged into the exhaust gases, that the hydraulic pressure and the volume flow of the water as well as the liquid ammonia are controlled, preferably by means of separate pumps, before mixing is effected at the collecting point, and that the mixture of ammonia and water is cooled in a heat exchanger immediately after being mixed. It is further preferred that the supply of ammonia and water, respectively, to the collecting point is controlled in such a manner that a weight ratio of water/ammonia in SUBSTITUTESHEET the mixture between 90/10 and 60/40, preferably between 85/15 and 75/25, is achieved. However, according to this invention, it is not necessary to maintain exactly the same ammonia/water ratio at all times. A constant amount of water may be applied while the input of ammonia is varied in consistency with the NO-content in the exhaust gases and which is to be removed. In order to obtain the best possible distribution of the ammonia in the exhaust gases and hence an optimum reduction of the NO- content in the gases, it is according to the present invention preferred that the mixture of ammonia and water is introduced into the exhaust gases via a spraying nozzle, preferably a compressed air spraying nozzle, and that compressed air is injected into the ammonia water just before the introduction of the latter into the exhaust gases. An apparatus for carrying out the method according to the invention must comprise at least a tank for liquid ammonia, a water reservoir, pipelines for conveying ammonia and water, respectively, to a collecting point or mixing chamber and for conveying the ammonia water mixture from this point and onward to the location where it is charged into the exhaust gases, a heat exchanger for cooling the ammonia water mixture and a spraying means for injecting the ammonia water mixture into the exhaust gases. In order to control the hydraulic pressure of the ammonia and ammonia water, respectively, to thereby prevent boiling of the latter and to control the volume flows in the pipelines it is preferred that the apparatus further comprises a number of liquid conveying means, preferably pumps. It has been found that the ammonia water is best SUBSTITUTESHEET distributed in the exhaust gases if it is sprayed into the gases by means of a nozzle, into which compressed air is injected, and, therefore, the apparatus preferably comprises a compressed-air spraying nozzle and a compressed air supply. The compressed air nozzle may comprise a tubular duct for conveying the ammonia water, and a surrounding annular duct, which is closed at the end and adapted for conveying the compressed air, an outlet mouth and a number of connecting openings between the two ducts being arranged at a distance from the outlet mouth to allow injection of compressed air into the ammonia water. The invention will now be described in further details with reference to the accompanying drawing, being diagrammatical and where Fig. 1 shows an embodiment of an apparatus according to the invention. Fig. 2 shows a compressed-air spraying nozzle which is used for carrying out the method according to the invention, Fig. 3 shows one example of a plant where the method according to the invention may be carried out, and Fig. 4 shows another example of a plant where the method according to the invention may be carried out. The apparatus 1 shown in Fig. 1 comprises a pressure tank 2 for storing liquid ammonia, a water reservoir 3 containing water supplied from the water mains system via a shut-off valve 5, two pumps 7, 8 for conveying liquid ammonia and water, respectively, a throttle valve 9, a SUBSTITUTESHEET collecting point or mixing chamber 11 for mixing of ammonia and water, a heat exchanger 13 for cooling the ammonia water, nozzles 15, 15' for spraying the ammonia water into the exhaust gases in an exhaust gas duct 16, a compressedair supply 17, and ducts 19, 20, 21 and 22. In Fig. 2 is shown a compressed-air nozzle 15 which may advantageously be used for spraying ammonia water into the exhaust gases. The nozzle 15 comprises a tubular duct 31 for conveying the ammonia water, a surrounding annular duct 32, closed at the end and adapted for conveying compressed air, an outlet mouth 33 and a number of connecting openings 34, 34' between the two ducts. Fig. 3 shows an example of a SLC-S kiln plant 41 for burning of cement clinker where the method according to the invention may be exploited to great advantage since this plant incorporates features for adjusting the temperature as well as the 0 2 level for optimizing the cleaning efficiency, without having any effect whatsoever on the capacity of the plant or its operating economy. As is shown in the figure, the kiln plant 41 comprises a cyclone preheater string 42 with four cyclones, a precalciner 43, a rotary kiln 44, a clinker cooler 45 and a fan 46. In principle, the ammonia water from the apparatus 1 can be sprayed into the exhaust gases at two locations, viz. into the exhaust gases leaving the precalciner 43 of the kiln plant via an exhaust gas duct 47 or into the exhaust gases leaving the rotary kiln 44 of kiln plant via an exhaust gas duct 48. Fig. 4 shows an example of a SLC kiln plant 51 which is also used for burning cement clinker where the method is also applicable. This kiln plant 51 comprises two cyclone preheater strings 52, 52' each provided with four cyclones, SUBSTITUTESHEET a precalciner 53, a rotary kiln 54, a clinker cooler 55 and two fans 46, 46'. In this kiln plant the injection of the ammonia water from the apparatus l can take place at 57 and 58. The oxygen required for the process is present in the exhaust gas. It may be assumed that the kiln is running with a normal excess of air, leaving 1-3 % oxygen at the point, where the ammonia water is injected. The air introduced during the spraying is such a small mass flow compared to the smoke gas mass flow, that it does not increase the %0 2 significantly. SUBSTITUTESHEET

Claims: CLAIMS 1. A method for selective reduction of the NO-content of exhaust gases from a kiln plant by means of liquid ammonia, according to which method the ammonia is introduced into the NO-containing exhaust gases in the presence of oxygen (0 2 ) and at a temperature between 700 β C and 1100 β C, c h a r a c t e r i z e d i n, that the liquid ammonia, before being introduced into the exhaust gases, is dosed to a collecting point (11) where it is mixed with a stream of water, such as mains water, sewage water and/or other waste water. 2. A method according to claim 1, c h a r a c t e - r i z e d i n, that the hydraulic pressure and the volume flow of the water as well as the liquid ammonia are controlled, preferably by means of separate pumps (7, 8) before mixing is effected at the collecting point (11). 3. A method according to claims 1-2 c h a r a c t e ¬ r i z e d i , that the water is supplied to the collecting point (11) in a smooth uniform flow and that the ammonia is dosed to the collecting point (11) as a function of the NO-content in the exhaust gases to be cleaned. 4. A method according to claims 1-3 c h a r a c t e ¬ r i z e d i n, that the mixture of ammonia and water, immediately after being mixed, is cooled in a heat exchanger in order to avoid boiling. 5. A method according to claims 1-3 c h a r a c t e ¬ r i z e i , that the input of ammonia and water, respectively, to the collecting point (11) is controlled in such a manner that a weight ratio of water/ammonia in the mixture between 90/10 and 60/40, preferably between 85/15 and 75/25, is achieved. SUBSTITUTESHEET 6. A method according to claims 1-5 c h a r a c t e ¬ r i z e d i n, that the mixture of ammonia and water is introduced into the exhaust gases via a spraying nozzle (15), preferably a compressed-air spraying nozzle. 7. Apparatus for carrying out the method according to claim 1-6 c h a r a c t e r i z e d i n, that it comprises a tank (2) for liquid ammonia, a water reservoir (3), pipelines (19, 20, 21) for conveying ammonia and water, respectively, to a collecting point or mixing chamber (11) and for conveying the ammonia water mixture from this point and onward to the location where it is charged into the exhaust gases, a heat exchanger (13) for cooling the ammonia water mixture, and a spraying means (15) for injecting the ammonia water mixture into the exhaust gases. 8. Apparatus according to claim 7, c h a r a c t e ¬ r i z e d i , that it further comprises a number of liquid conveying means, preferably pumps. 9. Apparatus according to claim 7 or 8, c h a r a c ¬ t e r i z e d i n, that it comprises a compressed-air spraying nozzle (15) and a compressed-air supply (17). 10. Apparatus according to claim 9, c h a r a c t e ¬ r z e d i , that the compressed-air nozzle (15) comprises a tubular duct (31) for conveying the ammonia water, and a surrounding annular duct (32), which is closed at the end and adapted for conveying compressed air, an outlet mouth (33) and a number of connecting openings (34,34* ) between the two ducts (31,32) being arranged at a distance from the outlet mouth (33). SUBSTITUTESHEET

Inventors: HUNDEBOL SOREN, HUNDEBOL, SOREN
Applicants: HUNDEBOL SOREN, SMIDTH & CO AS F L, F.L. SMIDTH & CO. A/S
Application year: 1993
Main classification: B01D53
IPC codes: B01D53, C04B7, F23J15