TREATMENT OF EXHAUST GASES FROM CEMENT CLINKER PRODUCTION

Abstract

Method for the purification of exhaust gas from the production of cement clinker in a rotary kiln, in which raw materials are ground in a mill to form raw meal, raw meal is preheated in countercurrent in a preheater with exhaust gas from the rotary kiln and optionally precalcined, preheated and optionally precalcined raw meal is supplied to the rotary kiln and burned in the rotary kiln to form cement, the exhaust gas from the rotary kiln is denitrified before entering the preheater of a selective non-catalytic nitrogen oxide reduction with a reagent which provides ammonia, and wherein, according to the invention, the exhaust gas from the preheater is subjected to gas conditioning and catalytic oxidation of ammonia. The object is further solved by means of a device for gas conditioning and catalytic oxidation which is arranged between the preheater and the mill.

Claims

1-18. (canceled)

19. A method for the purification of exhaust gas from the production of cement clinker in a rotary kiln, comprising the following steps: grinding raw materials in a mill to form raw meal, preheating the raw meal in countercurrent in a preheater with exhaust gas from the rotary kiln and optionally precalcinating the raw meal, supplying the preheated and optionally precalcined raw meal to the rotary kiln and burning the raw meal in the rotary kiln to form cement clinker, denitrifying the exhaust gas from the rotary kiln before entering the preheater by a selective non-catalytic reduction with a reagent which provides ammonia, wherein the molar ratio of ammonia to nitrogen oxides in the non-catalytic reduction is adjusted to 1.1 to 2.5, and subjecting the exhaust gas from the preheater to catalytic oxidation and gas conditioning.

20. The method according to claim 19, wherein a gas purification with alkaline substances, preferably calcium hydroxide, is carried out as gas conditioning.

21. The method according to claim 19, wherein aqueous ammonia or urea solution is used as the reagent.

22. The method according to claim 19, wherein the addition of reagent is measured in such a way that the nitrogen oxide concentration in the exhaust gas after the non-catalytic reduction is below 450 mg/m.sup.3, preferably below 200 mg/m.sup.3, in particular below 150 mg/m.sup.3.

23. The method according to claim 19, wherein the molar ratio of ammonia to nitrogen oxides in the non-catalytic reduction is adjusted to 1.3 to 1.6.

24. The method according to claim 19, wherein the addition of reagent takes place at a point at which the exhaust gases have a temperature of 800 to 1000 C., preferably from 830 to 950 C.

25. The method according to claim 19, wherein the cross section and type of oxidation catalyst and the flow rate of the exhaust gas through the catalyst are selected in such a way that the ammonia concentration in the exhaust gas after catalytic oxidation is below 30 mg/m.sup.3, preferably below 10 mg/m.sup.3.

26. The method according to claim 19, wherein the oxidation catalyst is blown clear from time to time, typically every 0.2 to 2 hours, by means of compressed air from a rotating unit.

27. The method according to claim 26, wherein the rotating unit is driven by the compressed air.

28. The method according to claim 26, wherein the compressed air is preheated, preferably to 150 to 300 C., in particular to 170 to 240 C.

29. A device for catalytic oxidation and gas conditioning in a plant for the production of cement clinker, comprising: a mill, in which raw materials are ground to form raw meal, a preheater, in which the raw meal is preheated in countercurrent with exhaust gas from a rotary kiln and optionally precalcined, the rotary kiln, in which the preheated and optionally precalcined raw meal is burned to form cement clinker, and a section between the rotary kiln and preheater, in which nitrogen oxides are reduced with a reagent which provides ammonia, said section being arranged between the mill and preheater, wherein the oxidation catalyst contains titanium dioxide with 1 to 4% by weight vanadium pentoxide and/or tungsten trioxide and the molar ratio of ammonia to nitrogen oxides in the non-catalytic reduction is adjusted to 1.1 to 2.5.

30. The device according to claim 29, wherein catalytic oxidation and gas conditioning are arranged in one housing.

31. The device according to claim 29, wherein the oxidation catalyst is doped with one or more of precious metals, copper and magnesium.

32. The device according to claim 29, wherein the oxidation catalyst consists of elements which are held in cartridges, the cross sections of which being equilateral triangles, wherein the side length of the cartridges is adapted to the radius of the housing.

33. The device according to claim 29, wherein a compensating distributor is arranged on the gas inlet into the housing.

34. The device according to claim 29, wherein a dust removal device is arranged before the gas outlet from the housing.

35. The device according to claim 29, wherein a rotating unit with offset nozzles varying in cross section is present to introduce compressed air, by means of which the catalyst can be blown clear by compressed air.

36. The device according to claim 35, wherein a heat exchanger is provided, by means of which the compressed air is preheated, preferably by the exhaust gas.

Description

[0033] Here are shown:

[0034] FIG. 1 a schematic overview of the method according to the invention

[0035] FIG. 2 a schematic depiction of a plant for cement production

[0036] FIG. 3 a device according to the invention for gas conditioning and catalytic oxidation.

[0037] FIG. 1 illustrates the method steps. The solid material flows are shown by solid arrows, the gas flows with outlined arrows. Raw materials are ground in a mill in a known manner. The raw meal produced is supplied to preheating, wherein storage in a raw meal silo is generally interposed. Preheating may include precalcination. The preheated and optionally precalcined raw meal is supplied to the rotary kiln, where it is burned to form cement clinker. The subsequent steps of cooling, grinding etc. are not shown. Fuel is supplied (dashed arrow) and burned to burn the raw meal in the kiln. Nitrogen oxides are produced both here and in the entire hot region of the kiln from oxygen and nitrogen. In the kiln, air flows in countercurrent to the raw meal such that the hot exhaust gas containing nitrogen oxides and other pollutants leaves the kiln at the point at which the raw meal is fed. From there, it is guided towards preheating, where the reagent for the SNCR is admixed according to the invention. In the preheating process, the denitrified exhaust gas heats the raw meal and is then guided into gas conditioning and catalytic oxidation. There, ammonia is oxidised to form nitrogen and, if applicable, further pollutants are oxidised and/or removed by gas purification. The denitrified exhaust gas, which is largely freed of ammonia, is guided either into the mill or into the exhaust gas filter which is arranged behind the mill.

[0038] FIG. 2 shows a typical plant for cement production. Raw materials are supplied to a mill 1 where they are ground to form raw meal. The raw meal reaches a silo 2, the exhaust gas from the mill a filter 3. Dust deposited in the filter 3 is supplied to the raw meal or for another use. The exhaust gas purified in the filter 3 is discharged through a stack 4. The raw meal is guided from the silo 2 into a preheater 5. The preheater is typically formed of a 4- to 6-stage cyclone heat exchanger. In the preheater 5, the raw meal is heated by hot exhaust gas from the rotary kiln 6 and the exhaust gas is cooled. In some plants, precalcination takes place in addition to preheating, usually with the addition of additional energy. The raw meal, which is preheated in this way and optionally precalcined, is supplied to the rotary kiln 6 and is gradually heated there such that it is sintered to form cement clinker. The cement clinker is already partially cooled in the kiln outlet, but in any case in the clinker cooler 7 by the supply of air. The air heated as a result is used as combustion air in the kiln 6 and leaves this as exhaust gas at the point at which the raw meal is supplied. According to the invention, a reagent which provides ammonia for the non-catalytic reduction of nitrogen oxides is supplied to the exhaust gas before entering the preheater 5. The denitrified exhaust gas is supplied to the preheater 5 and heats the raw meal. According to the invention, it is guided from the preheater 5 into a device 8 for gas conditioning and catalytic oxidation. The device 8 can be formed as two separate devices; it is preferably a unitary device. The exhaust gas is firstly brought into contact with an oxidation catalyst in the device 8. Gas conditioning then takes place, in which at least an adjustment of temperature and humidity of the exhaust gas, preferably also a gas purification, takes place. The exhaust gas is preferably purified with an aqueous solution of an alkaline substance, in particular calcium hydroxide. If fuels and raw materials contain little sulphur, gas conditioning without alkaline substances is completely sufficient. The exhaust gas is now denitrified and contains little to no ammonia. In this way, it can be supplied to the filter 3 without problems in direct operation. In normal operation, it is fed into the mill 1.

[0039] FIG. 3 shows the structure of the preferred device 8 for gas conditioning and catalytic oxidation and a sectional enlargement of the oxidation catalyst. The device 8 is accommodated in a housing 9. At its inlet end, a compensating distributor 10 is provided which ensures homogeneous gas and dust inflow. The exhaust gas then passes through a heat exchanger 11 in which it emits part of its heat to the compressed air for cleaning the catalyst, said compressed air being preheated as a result. The preheated compressed air is supplied to a rotating unit 12 which blows the inflow surfaces of the subsequent oxidation catalyst 13 clear from time to time. As can be seen in the sectional enlargement, the oxidation catalyst 13 is held in cartridges, the cross section of which corresponds to an equilateral triangle having a side length which is adapted to the radius of the housing 9. Gas conditioning 14 follows the oxidation catalyst 13. Depending on the sulphur content in the raw materials and fuels, an alkaline substance is additionally contained in the water is injected for gas conditioning for gas purification. Here, dust removal 15 is also arranged subsequent to the gas conditioning 14.

[0040] The exhaust gas leaving the housing 9 is largely freed of pollutants. The nitrogen oxides were removed in the SNCR, ammonia and optionally sulphur oxides were converted to nitrogen and CaSO.sub.3/4 or removed in the SCO and gas conditioning. Other oxidisable pollutants such as, for example, mercury were transferred into a form which is more easily separable in the filter 3 or converted into harmless substances by oxidation.