METHOD AND SYSTEM FOR THE CATALYTIC CLEANING OF EXHAUST GAS

Abstract

A method of closed-loop control of a catalytic operation to clean offgas from a plant for processing of a raw material may involve determining an offgas composition downstream of a catalyst and varying an entrance temperature of the offgas entering the catalyst based on the determined composition. By varying the entrance temperature, the offgas composition downstream of the catalyst may be brought or maintained within a target range. A plant for performing such methods may include a processing apparatus for a raw material that produces an offgas, a catalyst, a measuring apparatus for determining a composition of the offgas downstream of the catalyst, a temperature-affecting apparatus for varying the entrance temperature of the offgas entering the catalyst, and a closed-loop control apparatus that actuates the temperature-affecting apparatus based on the composition of the offgas downstream of the catalyst.

Claims

1.-20. (canceled)

21. A method of closed-loop control of a catalytic operation to clean offgas from a plant for processing a raw material, the method comprising: determining a composition of an offgas downstream of a catalyst; and varying an entrance temperature of the offgas entering the catalyst based on the composition of the offgas downstream of the catalyst so as to bring or maintain the composition of the offgas downstream of the catalyst within a target range.

22. The method of claim 21 further comprising determining a variability that affects or reflects the composition of the offgas or a temperature of the offgas downstream of the catalyst.

23. The method of claim 21 wherein varying the entrance temperature is effected by at least one of auxiliary firing, mixing-in of a fluid, or exchanging heat with a heat exchange medium.

24. The method of claim 21 wherein the entrance temperature of the offgas is varied between 150 C. and 600 C.

25. The method of claim 21 further comprising conducting a measurement that reflects a composition or a temperature of the offgas upstream of the catalyst.

26. The method of claim 21 further comprising determining a heat exchange performance of a heat exchanger used with the offgas, wherein the heat exchange performance is a variability that affects a composition or a temperature of the offgas upstream of the catalyst.

27. The method of claim 21 further comprising determining a flow rate of a medium used in processing the raw material, wherein the flow rate is a variability that affects a composition or a temperature of the offgas upstream of the catalyst.

28. The method of claim 21 further comprising determining a performance of an apparatus used to process the raw material, wherein the performance of the apparatus is a variability that affects a composition or a temperature of the offgas upstream of the catalyst.

29. The method of claim 21 further comprising recording a catalyst action of the catalyst as a function of the determined composition.

30. The method of claim 21 wherein when a catalyst effect drops below a limit, the catalyst is baked, which comprises overriding or suspending the closed-loop control based on the composition of the offgas downstream of the catalyst.

31. A plant comprising: a processing apparatus for a raw material that produces an offgas; a catalyst; a measuring apparatus for determining a composition of the offgas downstream of the catalyst; a temperature-affecting apparatus for varying an entrance temperature of the offgas entering the catalyst; and a closed-loop control apparatus that actuates the temperature-affecting apparatus based on the composition of the offgas downstream of the catalyst so as to bring or maintain the composition of the offgas downstream of the catalyst within a target range.

32. The plant of claim 31 wherein the measuring apparatus is a first measuring apparatus, the plant further comprising a second measuring apparatus for determining a variability that affects a composition or a temperature of the offgas upstream of the catalyst.

33. The plant of claim 31 wherein the temperature-affecting apparatus comprises at least one of an auxiliary heater, a mixing-in device for a fluid, or a heat exchanger.

34. The plant of claim 31 further comprising a material preheater disposed between the processing apparatus and the catalyst, wherein the material preheater transfers heat from the offgas to the raw material.

35. The plant of claim 34 wherein the material preheater comprises one or more heat exchanger stages, wherein a first feed for the raw material is disposed beyond a heat exchanger stage in a direction in which the raw material passes through the material preheater, wherein based on the direction in which the raw material passes through the material preheater a second feed for the raw material is disposed upstream of the heat exchanger stage, wherein the plant further comprises a control unit for dividing the raw material between the first feed and the second feed.

36. The plant of claim 31 wherein the catalyst comprises an oxidation catalyst.

37. The plant of claim 36 wherein the temperature-affecting apparatus is a first temperature-affecting apparatus, the plant further comprising a second temperature-affecting apparatus, wherein the closed-loop control apparatus actuates the second temperature-affecting apparatus based on the composition of the offgas downstream of the catalyst so as to bring or maintain the composition of the offgas downstream of the catalyst within the target range, or bring or maintain a composition of the offgas upstream of the catalyst within a target range.

38. The plant of claim 31 wherein the catalyst is a first catalyst, the plant further comprising a second catalyst connected in series with the first catalyst.

39. The plant of claim 38 wherein the first and second catalysts comprise an oxidation catalyst and a reduction catalyst.

40. The plant of claim 31 further comprising a reduction catalyst that has an upstream metering apparatus for a reducing agent.

Description

[0035] The invention is elucidated in detail hereinafter with reference to a working example illustrated in the drawing. The drawing shows:

[0036] FIG. 1: a plant of the invention in a schematic view.

[0037] The plant shown in FIG. 1 is a plant for production of cement clinker. It comprises a rotary kiln 1 in which finely ground cement raw meal which has been preheated beforehand in a material preheater 2 by the offgas which has left the rotary kiln 1 is burnt to give cement clinker. The offgas which has left the material preheater 2 then either flows through a cooling tower 3 or isat least partlyutilized for drying of the cement raw meal during grinding in a raw mill 4. Subsequently, the offgas is dedusted in a dust filter 5 and sent to a catalyst apparatus 6 for offgas cleaning. In addition, the plant also comprises a clinker cooler 7 in which the cement clinker discharged from the rotary kiln 1 is cooled by means of cooling air.

[0038] The catalyst apparatus 6 may comprise one or more catalysts, especially an oxidation catalyst and/or a reduction catalyst. Each of the catalysts may have one or more layers. Merely by way of example, in the present working example of the catalyst apparatus 6, only one oxidation catalyst is provided, by means of which, in particular, lowering of the concentrations of carbon monoxide (CO) and organic hydrocarbons (C.sub.xH.sub.y) in the offgas is to be achieved.

[0039] The gas which has left the dust filter, prior to entry into the catalyst apparatus 6, is first conducted through a first heat exchanger 8 designed as a gas-gas heat exchanger, in which this gas is preheated by heat exchange with the offgas leaving the catalyst apparatus. The temperature differential between the offgas which has left the catalyst apparatus 6 and the offgas which has left the dust filter 5 that enables such heat transfer results from an increase in temperature for the offgas in the catalyst apparatus 6 owing to exothermic oxidation of, in particular, carbon monoxide and organic hydrocarbons 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.

[0040] The offgas which has left the first heat exchanger 8 is subsequently conducted through a second heat exchanger 9. Further preheating of the offgas by heat exchange with a transfer medium 10 can be achieved therein. For this purpose, the transfer medium 10 has been heated beforehand in a third heat exchanger 11 by cooling air which has left the clinker cooler 7.

[0041] After the second heat exchanger 9, the offgas flows through a temperature-affecting apparatus in the form of an auxiliary heater 12 before it then enters the catalyst apparatus 6. In the auxiliary heater 12, the offgas can be heated further. For this purpose, a fuel, for example natural gas, is combusted and the heat energy thus generated is transferred very substantially to the offgas.

[0042] Closed-loop control of the fuel converted in the auxiliary heater 12 and hence the temperature of the offgas on entry into the catalyst apparatus 6 is effected by means of a closed-loop control apparatus 13 as a function of the offgas composition, specifically the concentrations of carbon monoxide and organic hydrocarbons, which is measured by means of a first measurement apparatus 14 integrated into the offgas line downstream of the catalyst apparatus 6 and also downstream of the first heat exchanger 8.

[0043] Depending on the configuration of the catalyst apparatus 6, sufficiently high degrees of lowering are only achieved above a particular limiting temperature. This limiting temperature can vary considerably depending on the offgas composition. In addition, there may be an upward restriction in the offgas temperature, in order firstly to avoid any adverse effect on the plant components and especially on the catalyst apparatus as a result of overheating, and secondly in order to minimize the fuel requirement of the auxiliary heater 12. A temperature range thus arises for the offgas entering the catalyst apparatus 6, within which the offgas temperature should be varied by adjusted heating by the auxiliary heater 12, in order to achieve high degrees of lowering for carbon monoxide and organic hydrocarbons. In this case, the degrees of lowering to be achieved, however, need not correspond to what is technically feasible, but may instead also be guided by emissions regulations.

[0044] In the operation of the plant, a multitude of variabilities affect the offgas composition and also the offgas temperature. These variabilities may be disturbance and/or auxiliary variables for the closed-loop control of the fuel supply 24 and hence the temperature of the offgas on entry into the catalyst apparatus 6. The variabilities include the flow rate of the cement raw meal supplied from the raw mill 4 to the material preheater 2 and hence the rotary kiln 1, which is determined by means of a corresponding second (flow rate) measuring apparatus 15. The varying composition of the cement raw meal is another variability of this kind. In addition, it is possible by means of a third (offgas composition) measuring apparatus 16 to ascertain the offgas composition, especially the concentration of carbon monoxide and oxygen of the offgas leaving the kiln, and by means of a fourth (offgas composition) measuring apparatus 23 to ascertain the offgas composition, especially the concentration of carbon monoxide of the offgas leaving the material preheater 2. Moreover, the temperature of the offgas leaving the dust filter 5 and hence also the potential heat transfer performance of the first heat exchanger 8 can be ascertained by means of a fifth (temperature) measuring apparatus 17. By means of a sixth (temperature) measuring apparatus 18, it is additionally possible to ascertain the temperature of the offgas between the second heat exchanger 9 and the auxiliary heater 12, and, by means of a seventh (temperature) measuring apparatus 19, it is possible to ascertain the temperature of the cooling air which has left the clinker cooler 7 before it is supplied to the third heat exchanger 11 (and hence a potential heat exchange performance of the third heat exchanger 11).

[0045] Further variabilities, especially relating to the offgas composition, may arise from the possible integrated operation of the cooling tower 3 and the raw mill 4, wherein at least a portion of the offgas is conducted through the raw mill 4. For example, in integrated operation, the oxygen content in the offgas may be higher than in direct operation (i.e. without supply of offgas to the raw mill 4), which can have a positive effect on the oxidation in the oxidation catalyst of the catalyst apparatus 6. In addition, the partial water vapor pressure may differ between direct and integrated operation. A higher water vapor content in the offgas may have a tendency to reduce the degrees of lowering. Operation of the raw mill 4 fundamentally also affects the pollutant concentrations in the offgas, since both outgassing and chemical and physical adsorption processes of pollutants take place, which could be lowered in the catalyst apparatus 6, but could also affect the activity of the catalyst. For example, sulfur dioxide is converted by temperature-dependent catalytic oxidation to sulfur trioxide and subsequent reaction with ammonia to ammonium bisulfate. Ammonium bisulfate can adversely affect a catalyst activity. In the operation of the raw mill 4, sulfur dioxide present in the offgas, however, is incorporated to a relevant degree and hence removed from the offgas. This could enable contacting of the catalyst apparatus 6 with lower offgas temperatures without any significant decrease in the degree of lowering as a result. Moreover, in the operation of the raw mill, outgassing of hydrocarbons is possible, such that higher concentrations thereof can occur in the offgas in integrated operation.

[0046] Operation of the raw mill 4 and possibly also the specific performance of a drive 21 of the raw mill 4 can be ascertained via an eighth (performance) measuring apparatus 22 and taken into account in the closed-loop control.

[0047] All these disturbance variables can be utilized to proactively affect the closed-loop control of the fuel conversion in the auxiliary heater 12, by drawing conclusions about the offgas composition and the offgas temperature between the second heat exchanger 9 and the auxiliary heater 12 based on the disturbance and/or auxiliary variables measured and controlling the fuel conversion in the auxiliary heater 12 by closed-loop control such that, based on the predicted offgas composition and offgas temperature, the introduction of heat to be established for the attainment of the envisaged degree of lowering is likely to be achieved by means of the auxiliary heater 12, and hence the target temperature of the offgas on entry into the catalyst apparatus 6.

[0048] The actual attainment of the (variable) target temperature of the offgas on entry into the catalyst apparatus 6, determined by the proactive and emissions-based closed-loop control, is monitored by means of a ninth (temperature) measuring apparatus 20 (measurement of the actual temperature) and used in an internal control loop for closed-loop control of the fuel conversion by means of the closed-loop control apparatus 13.

[0049] In the closed-loop control of the above-described working example, the fuel conversion in the auxiliary heater 12 is the manipulated variable for the closed-loop control of the temperature of the offgas on entry into the catalyst apparatus 6 and hence the offgas composition downstream of the catalyst apparatus 6. The manipulated variable is affected not only by the controlled variable, i.e. the offgas composition downstream of the catalyst unit 6 measured by means of the first measurement apparatus 14, but also by the disturbance variables described, which are caused by the variabilities of the operation of the plant. It is also possible, alternatively or additionally to the fuel conversion in the auxiliary heater 12, to utilize at least some of the variabilities as manipulated variables, such that it is possible, for example, to adjust the heat transfer from the cooling air originating from the clinker cooler 7 to the offgas by means of a variable configuration of the heat exchange system comprising the second heat exchanger 9 and the third heat exchanger 11. In addition, it is possible, for example, to vary the temperature of the offgas downstream of the dust filter 5, ascertained by means of the fourth measurement apparatus 17, by altering an injection of water in the cooling tower 3.

LIST OF REFERENCE NUMERALS

[0050] 1. rotary kiln [0051] 2. material preheater [0052] 3. cooling tower [0053] 4. raw mill [0054] 5. dust filter [0055] 6. catalyst apparatus [0056] 7. clinker cooler [0057] 8. first heat exchanger [0058] 9. second heat exchanger [0059] 10. transfer medium [0060] 11. third heat exchanger [0061] 12. auxiliary heater [0062] 13. closed-loop control apparatus [0063] 14. first measuring apparatus [0064] 15. second measuring apparatus [0065] 16. third measuring apparatus [0066] 17. fifth measuring apparatus [0067] 18. sixth measuring apparatus [0068] 19. seventh measuring apparatus [0069] 20. ninth measuring apparatus [0070] 21. drive [0071] 22. eighth measuring apparatus [0072] 23. fourth measuring apparatus [0073] 24. fuel supply