Method and system for the denitrification of flue gases by means of SNCR (selective non-catalytic reduction) and downstream catalyst for ammonia decomposition

Abstract

A method for denitrification of flue gases and a system, wherein flue gases generated in a rotary kiln are conveyed to a calcining zone for the deacidification of raw cement meal. Aqueous ammonia solution, ammonia, or ammonia-releasing substances for denitrifying the flue gases injected into the calcining zone according to the method of selective non-catalytic reduction (SNCR), and the flue gas stream, together with an ammonia slip generated during the denitrification, is passed through a heat exchanger and through at least one dedusting device. The flue gas is guided through a exhaust gas line via a catalyst for the decomposition of excess ammonia with residues of nitrogen oxide in accordance with a method of selective catalytic reduction (SCR), wherein the catalyst is arranged in a reactor provided in the exhaust line, and is no larger than is required for a sufficient decomposition of the ammonia.

Claims

1. A method of denoxing flue gases comprising: guiding flue gases which arise in a rotary kiln for sintering of cement clinker into a calcination zone for deacidification of raw meal, injecting aqueous ammonia solution, ammonia (NH.sub.3) or substances that release ammonia for denoxing of the flue gases by the method of selective noncatalytic reduction (SNCR) into the calcination zone, and guiding the flue gas stream together with any ammonia slip that arises in the denoxing through a heat exchanger and at least one device for dedusting, wherein the flue gas stream is conducted from the heat exchanger through a heat exchanger offgas conduit via a catalyst for degradation of excess ammonia with residues of nitrogen oxide present in the flue gas over the catalyst in accordance with a method of selective catalytic reduction (SCR), and, wherein the catalyst is disposed in a reactor provided within the heat exchanger offgas conduit, and, wherein the flue gas stream, in the event of problems with, repair of, or maintenance of the catalyst, is guided through a bypass conduit to bypass the catalyst.

2. The method as claimed in claim 1, wherein the addition of ammonia is effected by injection of aqueous ammonia solution, ammonia or substances that release ammonia into the calcination zone at least two injection levels, each injection level being switched on and off individually depending on the fitting of the temperatures that exist at each injection level into a favorable temperature window for the SNCR process reactions.

3. The method as claimed in claim 1, wherein the amount of the aqueous ammonia solution, the ammonia or the substances that release ammonia injected is used to control the emission of nitrogen oxide, with release of the flue gas from the heat exchanger into the environment beyond the catalyst in gas flow direction, and the amount injected is adjusted such that the given limit in each particular case for the release of nitrogen oxides into the environment is attained or surpassed.

4. The method as claimed in claim 1, wherein a surface of the catalyst is cleaned to remove solid particles with compressed air.

5. A plant for denoxing flue gases comprising: a rotary kiln for sintering of cement clinker, a calcination zone for deacidification of raw meal, at least one device for injection of aqueous ammonia solution, ammonia (NH.sub.3) or substances that release ammonia into the calcination zone, and a heat exchanger having at least one device for dedusting of flue gas which follows on from the heat exchanger, and a heat exchanger offgas conduit which follows on from the latter, wherein in the heat exchanger offgas conduit there is disposed a reactor having a catalyst for degradation of ammonia with nitrogen oxides present in the flue gas stream from the heat exchanger, and, wherein the calcination zone is formed in a kiln riser duct, a calciner, or both.

6. The plant as claimed in claim 5, wherein at least two injection levels are provided for the injection of aqueous ammonia solution, ammonia or substances that release ammonia into the calcination zone, wherein one of the at least one device for injection is arranged at each injection level, each device for injection having at least one injection probe, a control unit for the flow rate of the at least one injection probe is provided at each injection level, the control unit for the at least one injection probe at one level being independent of the control unit of the injection probes at all other levels, and at least one device for measurement of the gas temperature in the corresponding section of the calcination zone is disposed at each injection level, the control of the flow rate of each injection probe being aligned to the gas temperature.

7. The plant as claimed in claim 5, wherein the catalyst is a honeycomb catalyst or a plate catalyst and only one catalyst layer is arranged in the reactor, with a device for receiving a second catalyst layer provided in the reactor.

8. The plant as claimed in claim 5, wherein a bypass conduit for bypassing the catalyst is arranged beyond the heat exchanger in flue gas flow direction, with at least one device for blocking the flow of the flue gas each provided both within the bypass conduit and directly upstream of the catalyst.

9. The plant as claimed in claim 5, wherein at least one dust blower for cleaning the catalyst surface is disposed in the reactor containing the catalyst.

10. The method of claim 4 wherein the cleaning is conducted at regular time intervals depending on the degree of contamination.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is elucidated in detail by the FIGURE which follows and wherein:

(2) FIG. 1 shows a plant of the invention for denoxing flue gases in a simplified schematic cross section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) FIG. 1 shows, in a plant of the invention for denoxing flue gas 1, in schematic form, the path of the flue gas 1 in that section of the cement production that relates to the denoxing of the flue gas 1. The flue gas 1 flows out of the rotary kiln 2 for sintering of cement clinker into the calcination zone 3 which, in the working example shown, extends over part of the kiln riser duct 4 and a calciner 5. In the SNCR method, sufficient ammonia 6 as an aqueous solution is injected into the calcination zone 3 through the injection probes 7 at a plurality of levels that, on flue gas emission 8, the limits that apply to nitrogen oxides are complied with. The injection is controlled at the individual levels according to the temperature windows measured (not depicted). The flue gas 1 that has been denoxed for the most part flows together with the excess proportion of ammonia 6 unconverted in the denoxing through the heat exchanger 9 that serves for preheating of the raw meal for the cement production, and through the device for dedusting 10, duplicated in the working example shown, in the form of cyclones here, into the heat exchanger offgas conduit 11.

(4) The flue gas 1 which has been largely freed of solid particles is guided through the heat exchanger offgas conduit 11 to the catalyst 12, which preferably takes the form of a honeycomb or plate catalyst and is disposed in a reactor 13. The catalyst 12 may have comparatively small dimensions and preferably consists of just one layer. In the method proposed here, in analogy to the SCR method, the ammonia slip is largely degraded over the catalyst 12 with nitrogen oxides to give nitrogen and water, the nitrogen oxides originating from the nitrogen oxide content still present in the flue gas 1. In this way, it is possible by the process proposed to achieve high-performance denoxing of flue gases 1 through excessive injection of ammonia 6 during the SNCR phase without leading to considerable emission 8 of ammonia 6 in the flue gas 1.

(5) The reactor 13 preferably has a device for accommodation of a second catalyst layer (not depicted). If the catalyst layer in operation has to be replaced owing to maintenance or wear, it is possible in an economically favorable manner to install a new catalyst layer into the reactor 13 simultaneously with the deinstallation of this layer in said device. During the phase of exchange of the catalyst layer and in the event of any other problems with the catalyst 12 or reactor 13, the stream of flue gas 1 can be diverted into a bypass conduit 14 that bypasses the reactor 13. A system of devices for blockage 15 of the flow of the flue gas 1, for example arrangements of barrier gate valves, barrier flaps, ballcocks or valves, regulates the flow rates of the flue gas 1 through the reactor 13 or bypass conduit 14.

(6) As is apparent from the foregoing specification, the invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. It should be understood that I wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of my contribution to the art.

LIST OF REFERENCE NUMERALS

(7) 1 Flue gas 2 Rotary kiln 3 Calcination zone 4 Kiln riser duct 5 Calciner 6 Ammonia 7 Injection probe 8 Flue gas emission 9 Heat exchanger 10 Device for dedusting 11 Heat exchanger offgas conduit 12 Catalyst 13 Reactor 14 Bypass conduit 15 Device for blocking