WASTE GAS TREATMENT DEVICE AND METHOD FOR TREATING WASTE GAS

Abstract

An offgas treatment apparatus having a reduction catalyst and an oxidation catalyst downstream of the reduction catalyst may also include a temperature-affecting apparatus for the offgas positioned between the reduction catalyst and the oxidation catalyst. In some examples, the apparatus may include a second temperature-affecting apparatus for the offgas positioned upstream of the reduction catalyst. At least one of the first or second temperature affecting apparatuses may comprise a heat exchanger, a preheating apparatus, an auxiliary heater, or a mixing-in device for a fluid, for instance. In some examples, the apparatus may involve a dust filter positioned upstream of the reduction catalyst.

Claims

1.-19. (canceled)

20. An offgas treatment apparatus comprising: a reduction catalyst; an oxidation catalyst positioned downstream of the reduction catalyst; and a first temperature-affecting apparatus for the offgas positioned between the reduction catalyst and the oxidation catalyst.

21. The offgas treatment apparatus of claim 20 further comprising a second temperature-affecting apparatus for the offgas positioned upstream of the reduction catalyst.

22. The offgas treatment apparatus of claim 21 wherein at least one of the first temperature-affecting apparatus or the second temperature-affecting apparatus is a preheating apparatus.

23. The offgas treatment apparatus of claim 21 wherein at least one of the first temperature-affecting apparatus or the second temperature-affecting apparatus comprises an auxiliary heater.

24. The offgas treatment apparatus of claim 21 wherein at least one of the first temperature-affecting apparatus or the second temperature-affecting apparatus comprises a mixing-in device for a fluid.

25. The offgas treatment apparatus of claim 21 wherein at least one of the first temperature-affecting apparatus or the second temperature-affecting apparatus comprises a heat exchanger.

26. The offgas treatment apparatus of claim 25 wherein there is heat transfer from the offgas downstream of the oxidation catalyst in the heat exchanger.

27. The offgas treatment apparatus of claim 26 wherein there is heat transfer from a medium in addition to the offgas in the heat exchanger.

28. The offgas treatment apparatus of claim 27 wherein based on a direction of flow of the medium through the heat exchanger a first feed for the medium is positioned beyond a heat exchanger stage, wherein based on the direction of flow of the medium through the heat exchanger a second feed for the medium is positioned upstream of the heat exchanger stage, wherein the heat exchanger comprises a control unit for dividing the medium between the first and second feeds.

29. The offgas treatment apparatus of claim 25 wherein the heat exchanger comprises a heat storage means.

30. The offgas treatment apparatus of claim 29 wherein at least one of the first temperature-affecting apparatus or the second temperature-affecting apparatus comprises at least two heat storage means through which the offgas to be supplied to either the oxidation or reduction catalysts flows in alternation to be preheated, or the offgas that has left the oxidation catalyst flows to absorb heat energy.

31. The offgas treatment apparatus of claim 29 wherein the heat storage means comprises a ceramic heat storage material.

32. The offgas treatment apparatus of claim 20 further comprising a dust filter positioned upstream of the reduction catalyst.

33. The offgas treatment apparatus of claim 20 further comprising a metering device for a reducing agent positioned upstream of the reduction catalyst.

34. A method for treating offgas, the method comprising: guiding the offgas through a reduction catalyst; changing a temperature of the offgas after the offgas is treated in the reduction catalyst; and guiding the offgas through an oxidation catalyst after the temperature of the offgas is changed.

35. The method of claim 34 wherein the changing the temperature of the offgas comprises heating the offgas before treatment in the oxidation catalyst to a temperature of between 250 C. and 650 C.

36. The method of claim 34 further comprising changing a temperature of the offgas before the offgas is treated in the reduction catalyst.

37. The method of claim 36 wherein the changing the temperature of the offgas before the offgas is treated in the reduction catalyst comprises heating the offgas to a temperature of between 160 C. and 380 C.

38. The method of claim 34 further comprising employing the offgas when producing or processing raw materials.

Description

[0030] The invention is elucidated in detail hereinafter with reference to working examples illustrated in the drawings. The drawings show:

[0031] FIG. 1: in a schematic view, an offgas treatment apparatus of the invention in a first mode of connection;

[0032] FIG. 2: the offgas treatment apparatus according to FIG. 1 in a second mode of connection; and

[0033] FIG. 3: a plant for burning cement clinker comprising an offgas treatment apparatus of the invention.

[0034] FIGS. 1 and 2 show, in schematic form, an embodiment of an inventive offgas treatment apparatus 1 as may be used especially for treatment of offgas originating from a processing apparatus for mechanical and/or thermal processing of inorganic material, for example a (rotary) kiln for burning cement clinker.

[0035] The offgas treatment apparatus 1 comprises an oxidation catalyst 5 and a first preheating apparatus 3 assigned to the oxidation catalyst 5, a reduction catalyst 2, a second preheating apparatus 6 assigned to the reduction catalyst 2 for the offgas, and a metering apparatus 4 for a reducing agent arranged between the second preheating apparatus 6 and the reduction catalyst 2. The oxidation catalyst 5 is arranged beyond the reduction catalyst 2 in flow direction of the offgas. This is associated with the advantage that the reduction catalyst 2 can act as a filter or guard for the oxidation catalyst 5 with respect to particular pollutants (for example sulfur, alkalis, heavy metals). This is because these pollutants cause only a slight decrease in activity, if any, in the reduction catalyst 2, but would, without filtering by the reduction catalyst 2, lead to a significant decrease in activity of the oxidation catalyst 5.

[0036] Each of the preheating apparatuses 3, 6 comprises two heat exchangers 7, one of which in each case, by the offgas, is arranged directly before entry into the catalyst assigned in each case and the other in flow direction beyond the oxidation catalyst 5. This connection of the two heat storage means 7 of each of the preheating apparatuses 3, 6 can be switched over, as shown in the two FIGS. 1 and 2.

[0037] The heat exchangers 7 each comprise a ceramic heat storage material 15 which features a high heat capacity. The heat storage means 7 through which the offgas leaving the oxidation catalyst 5 flows are heated by the offgas and remove heat energy therefrom as a result. This heat energy is (for the most part) intermediately stored. By switching over the heat storage means 7 of the two preheating apparatuses 3, 6, it is then possible to integrate the two heat storage means charged by the offgas leaving the oxidation catalyst 5 into the stream of the offgas entering the catalyst assigned in each case, as a result of which the intermediately stored heat energy is partly transferred to the respective offgas stream and leads to the envisaged preheating of the offgas. In this context, the heat transfer from the offgas downstream of the oxidation catalyst 5 to the offgas upstream of the oxidation catalyst 5 (and partly also upstream of the reduction catalyst 2) which is achievable by means of the preheating apparatuses 3, 6 is based on the generation of heat energy by the exothermic oxidation of, in particular, carbon monoxide (CO) and organic hydrocarbons which proceeds in the oxidation catalyst 5 according to the following reaction equations:

2CO+O.sub.2.fwdarw.2CO.sub.2;

C.sub.nH.sub.m+(n+m/4)O.sub.2.fwdarw.nCO.sub.2+m/2H.sub.2O.

[0038] In the reduction catalyst 2, by contrast, there is reduction of nitrogen oxides (NOx) and especially of nitrogen monoxide (NO) in conjunction with the aqueous ammonia solution introduced into the offgas as reducing agent via the metering apparatus 4 according to the following reaction equation:

4NO+4NH.sub.3+O.sub.2.fwdarw.4N.sub.2+6H.sub.2O.

[0039] For maximum lowering rates of nitrogen oxides in the reduction catalyst on the one hand and carbon monoxide and hydrocarbons in the oxidation catalyst on the other hand, different temperatures of the offgas in the respective catalysts are envisaged. For example, a temperature of the offgas supplied to the offgas treatment apparatus 1 at, for example, about 150 C. on entry into the reduction catalyst 2 of about 240 C. may be optimal, whereas the offgas on entry into the oxidation catalyst 5 should have a temperature of about 380 C. The particular temperature increase is achieved by preheating of the offgas before entry into the reduction catalyst 2 by means of the second preheating apparatus 6 and before entry into the oxidation catalyst 5 by means of the first preheating apparatus 3. As a result of the difference in the temperatures of the offgas to be established for the reduction catalyst 2 on the one hand and the oxidation catalyst 5 on the other hand, it may advantageously be the case that the offgas leaving the oxidation catalyst 5 flows first through the heat exchanger 7 of the first preheating apparatus 3 which has just been connected accordingly and then, i.e. in the already more cooled-down state (at, for example, about 240 C.), through the heat exchanger 7 of the second preheating apparatus 6 which has just been connected accordingly (which the offgas then leaves again at, for example, about 150 C.).

[0040] Very exact attainment of the offgas temperatures to be established for the two catalysts can be achieved firstly via a corresponding design of the heat exchangers 7 of the preheating apparatuses 3, 6. At the same time, the heat transfer from the offgas to the heat storage means 7 downstream of the oxidation catalyst 5 and from the heat storage means 7 to the offgas upstream of the oxidation catalyst 5 (and in the second preheating apparatus 6 also upstream of the reduction catalyst 2) can be affected, for example, by the volumes of the respective heat storage materials 15, the sizes of the contact areas between the heat storage materials 15 and the offgas and/or the mean flow rate of the offgas through the heat storage materials 15.

[0041] On the other hand, for very exact attainment of the offgas temperatures to be established for the two catalysts, it may also be the case that the offgas is additionally preheated at the appropriate point if required by means of an auxiliary heater 8 in each case, in which a fuel, for example natural gas, mineral oil or coal can be combusted in order to generate heat energy. These auxiliary heaters 8 are actuated by a closed-loop control apparatus (not shown) which controls the additional generation of heat by means of the auxiliary heaters 8 as a function of the (target) offgas temperature envisaged in each case.

[0042] Preheating of the offgas upstream of the reduction catalyst 2 and between the reduction catalyst 2 and the oxidation catalyst 5 can be achieved, as an alternative or in addition to regenerative utilization of heat energy which is removed from the offgas treated itself, also by heat exchange with another medium, especially a fluid flow and more preferably a gas flow. Various options for this purpose are shown in FIG. 3.

[0043] FIG. 3 shows the integration of an inventive offgas treatment apparatus 1 into a plant for burning cement clinker. The plant comprises, as well as the offgas treatment apparatus 1, a rotary kiln 9 in which finely ground cement raw meal which has been preheated beforehand in a material preheater 10 in countercurrent by offgas leaving the rotary kiln 9 is burnt to give cement clinker. The offgas leaving the material preheater 10 then either flows through a cooling tower 11 or is utilized at least partly in a raw mill 12 for drying the cement raw meal in the course of grinding. Subsequently, the offgas is dedusted in a dust filter 13 and sent to the offgas treatment in the offgas treatment apparatus 1. In addition, the plant also comprises a clinker cooler 14 in which the cement clinker discharged from the rotary kiln 9 is cooled by means of cooling air.

[0044] FIG. 3 shows, by the dotted arrows, that it is possible, for example, to branch off a substream of the offgas directly upstream of, directly downstream of or from the material preheater 10 itself andoptionally after pretreatment by means of a pretreatment apparatus 16utilize it to preheat the offgas flowing through the offgas treatment apparatus 1. It is likewise possible to correspondingly utilize the cooling air from the clinker cooler 14, in which case branching-off is possible either before or after complete flow through the clinker cooler 14, which affects the temperature of the cooling air and hence the possible heat transfer to the offgas flowing through the offgas treatment apparatus 1.

REFERENCE NUMERALS

[0045] 1. offgas treatment apparatus [0046] 2. reduction catalyst [0047] 3. first preheating apparatus [0048] 4. metering apparatus for a reducing agent [0049] 5. oxidation catalyst [0050] 6. second preheating apparatus [0051] 7. heat exchanger [0052] 8. auxiliary heater [0053] 9. rotary kiln [0054] 10. material preheater [0055] 11. cooling tower [0056] 12. raw mill [0057] 13. dust filter [0058] 14. clinker cooler [0059] 15. ceramic heat storage material [0060] 16. pretreatment apparatus