Method and plant for denitrifying bypass gases in a multi-stage system of mixing chambers in a plant for producing cement clinker

Abstract

A method and a corresponding plant for denitrifying bypass exhaust gases in a cement clinker production plant. Raw meal is sintered in a rotary kiln and deacidified in a calciner. A rotary kiln inlet chamber is connected to the calciner directly or by a riser duct. Bypass exhaust gas is drawn off near the inlet chamber. This exhaust gas is guided into a first mixing chamber, in which the exhaust gas is cooled to between 800 and 950 C., then the exhaust gas is guided through a reaction pipeline segment, wherein the dwell time is between 0.5 and 3 seconds and ammonia, aqueous ammonia solution, or ammonia-releasing substances are injected for denitrification. Then the exhaust gas is guided into a second mixing chamber, in which the exhaust gas is cooled to between 150 to 250 C. Then the exhaust gas is guided to a filter for dust removal.

Claims

1. A plant for the denitrification of bypass exhaust gases in the production of cement clinker, comprising: a rotary kiln for the sintering of raw meal to cement clinker, the rotary kiln having a rotary kiln inlet chamber, a calciner for the deacidification of the raw meal, the rotary kiln inlet chamber being connected directly or via a kiln riser duct to the calciner, and a takeoff device for drawing off the bypass exhaust gases from the region of the rotary kiln inlet chamber, wherein a first mixing chamber is provided for the cooling of the bypass exhaust gas, drawn off from the region of the rotary kiln inlet chamber, to a temperature of between 800 C. and 950 C. so that chloride compounds remain in a gaseous phase after the bypass exhaust gas is cooled, wherein downstream of the first mixing chamber in the gas flow direction, a reaction section is provided which is disposed in a conduit, wherein the conduit has dimensioning such that the residence time of the bypass exhaust gas in the reaction section is between 0.5 s and 3 s, and wherein in the region of the conduit, at least one device is provided for the injection of ammonia, aqueous ammonia solution or ammonia-releasing substances into the reaction section for the denitrification of the bypass exhaust gas by the process of selective non-catalytic reduction (SNCR), wherein downstream of the reaction section, a second mixing chamber is disposed, for the cooling of the bypass exhaust gas to a temperature of between 150 C. and 250 C. to remove chloride compounds from the gaseous phase by deposition, and wherein downstream of the second mixing chamber at least one filter is disposed for the dedusting of the bypass exhaust gas.

2. The plant as claimed in claim 1, wherein the conduit is dimensioned such that the residence time of the bypass exhaust gas in the reaction section is between 1 s and 2 s.

3. The plant as claimed in claim 1, wherein downstream of the at least one filter in the gas flow direction there is disposed a fan.

4. The plant as claimed in claim 1, wherein in the region of the conduit, at least one device is provided for feeding at least one sorbent into the reaction section for additional, pollutant-removing purification of the bypass exhaust gas.

5. An apparatus for the denitrification of bypass exhaust gases in the production of cement clinker, comprising: a rotary kiln configured to sinter raw meal and provide cement clinker, the rotary kiln having a rotary kiln inlet chamber; a calciner configured to deacidify raw meal, the rotary kiln inlet chamber being connected directly or via a kiln riser duct to the calciner; a first mixing chamber configured to receive bypass exhaust gas from the rotary kiln inlet chamber and cool the bypass exhaust gas to a temperature of between 800 C. and 950 C. so that chloride compounds remain in a gaseous phase after the bypass exhaust gas is cooled; a reactor, configured to receive the bypass exhaust gas after the bypass exhaust gas is cooled in the first mixing chamber, the reactor disposed in a conduit and dimensioned such that the residence time of the bypass exhaust gas in the conduit is between 0.5 s and 3 s, at least one device configured to inject ammonia, aqueous ammonia solution or ammonia-releasing substances into the reactor for the denitrification of the bypass exhaust gas by the process of selective non-catalytic reduction (SNCR), a second mixing chamber configured to receive the bypass exhaust gas and ammonia, aqueous ammonia solution or ammonia-releasing substances from the reactor and to cool the bypass exhaust gas to a temperature of between 150 C. and 250 C. to remove chloride compounds by deposition, and at least one filter, downstream of the second mixing chamber in the gas flow direction, for dedusting the bypass exhaust gas after it has been cooled in the second mixing chamber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The invention is illustrated with the FIGURE that follows.

[0020] The FIGURE shows a schematic representation of the method of the invention for the denitrification of bypass exhaust gases in a plant for producing cement clinker.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] In the FIGURE it is evident schematically that flue gas 1 flows into the rotary kiln inlet chamber 5 from a rotary kiln 2, in which raw meal 3 is sintered to form cement clinker which is then cooled in a clinker cooler 4. Following this in the direction of gas flow, in the working example depicted, are a kiln riser duct 6 and a calciner 7 for the deacidification of the raw meal 3. A fraction of the flue gas 1 (kiln exhaust gas) flows through kiln riser duct 6 and calciner 7 into the heat exchanger 8 (presently a multistage cyclone heat exchanger), which serves for the preheating of the raw meal 3 for cement production.

[0022] In accordance with the invention, a part of the flue gas stream 1 emerging from the rotary kiln 2 is drawn off as bypass exhaust gas 9 in the region of the rotary kiln inlet chamber 5, i.e., from the rotary kiln inlet chamber 5 or from the kiln riser duct 6. The bypass exhaust gas 9, which initially on emergence from the rotary kiln 2 has temperatures typically of around 1200 C. to 1300 C., and of around 1000 C. to 1200 C. at the takeoff location, is passed to the first mixing chamber 10. Cooling media fed into the first mixing chamber 10 may be fresh air 11, water 12 or cold meal 12a, or else hot meal, and also any desired mixtures of these. In the working example depicted, atmospheric fresh air 11, water 12 and cold meal 12a are injected, with cold raw mealthat is, raw meal which has not already been heated in the heat exchangeris advantageous particularly for a reduction in the amount of sulfur dioxide in the bypass exhaust gas. In the first mixing chamber 10, by extensive mixing of the bypass exhaust gas 9 with the cooling media, the bypass exhaust gas 9 is cooled to temperatures of between 800 C. and 950 C.

[0023] After emerging from the first mixing chamber 10, i.e., after the first cooling stage, the bypass exhaust gas 9 enters a conduit 13. Injected into the conduit 13 are ammonia, aqueous ammonia solution or ammonia-releasing substances 14. The flow rate of the bypass exhaust gas and the dimensioning of the conduit 13 are matched to one another in such a way as to result in a residence time for the bypass exhaust gas 9 in the conduit 13 of 0.5 s to 3 s, preferably between 1 s and 2 s. The temperature conditions of the bypass exhaust gas 9 and the residence time are therefore established in such a way that there is effective denitrification of the bypass exhaust gas by the process of selective non-catalytic reduction (SNCR) over a reaction section 15 which is formed within the interior of the conduit. Ammonia 14 here is converted by thermolysis into nitrogen and water. The temperatures of the bypass exhaust gas 9 without cooling or before cooling in the first mixing chamber 10 would be too high for such an SNCR, since the reducing agents would undergo combustion at such high temperatures. Additional feeding of sorbents which ensure further pollutant-removing purification of the bypass exhaust gas 9 is possible in the region of the reaction section 15.

[0024] Following completed denitrification by SNCR in the reaction section 15, the bypass exhaust gas 9 is passed into a second mixing chamber 16. In the second mixing chamber 16 it is rapidly cooled to the desired final temperature of between 150 C. and 250 C., preferably between 180 C. and 220 C. This second cooling stage is accomplished by injection of water 12 and/or fresh air 11 into the second mixing chamber 16. Rapid cooling to these temperatures minimizes the formation of dioxins and furans and leads to condensation of pollutants on the dust. The bypass exhaust gas 9 thus conditioned is subsequently dedusted in at least one filter 17. Suitability here is possessed by fabric filters/cloth filters/bag filters, and the use of electrostatic filters, and also a combination of different types of filter in series, may also be advantageous. In the working example, the purified bypass exhaust gas 9 subsequently passes through a fan 18, and is drawn off by a chimney 19 and released into the environment. In the bypass system as a whole, such as especially in the mixing chambers, effective commixing and a correspondingly uniform temperature field, and also a suitable bypass exhaust gas flow rate, are important for effective method steps. Depending on the arrangement and the associated path lengths, it may be advantageous in particular plants to provide internals in the gas pathway that ensure effective commixing, and also to provide additional fans in the bypass section, which introduce air into the bypass exhaust gas flow through continuous or discontinuous operation.

[0025] As a result of the construction according to the invention, altered relative to conventional procedures, and by the altered operating regime, the means of denitrification of the bypass exhaust gases is also effective and is also favorable in terms of acquisition and in operation, and removes the need for SCR catalysts to be used.

[0026] 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.