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 degrees 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 150 250 degrees C. Then the exhaust gas is guided to a filter for dust removal.

Claims

1. A method for the denitrification of bypass exhaust gases in a plant for producing cement clinker, wherein the plant has a rotary kiln for the sintering of raw meal to cement clinker, and has a calciner for the deacidification of the raw meal, downstream of the rotary kiln in the kiln exhaust gas flow direction, the rotary kiln has a rotary kiln inlet chamber which is connected directly or via a kiln riser duct to the calciner, and the bypass exhaust gas is drawn off in the region of the rotary kiln inlet chamber, comprising the steps: passing of the bypass exhaust gas into a first mixing chamber, where the bypass exhaust gas is cooled in the first mixing 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, passing of the bypass exhaust gas from the first mixing chamber through a reaction section disposed in a conduit, where the residence time of the bypass exhaust gas in the reaction section is between 0.5 s and 3 s, and where ammonia, aqueous ammonia solution or ammonia-releasing substances are injected into the reaction section for the denitrification of the bypass exhaust gas by the process of selective non-catalytic reduction (SNCR), passing of the bypass exhaust gas from the reaction section into a second mixing chamber, where the bypass exhaust gas is cooled in the second mixing chamber to a temperature of between 150 C. and 250 C. to remove chloride compounds from the gaseous phase by deposition, and by passing of the bypass exhaust gas from the second mixing chamber to at least one filter for the dedusting of the bypass exhaust gas.

2. The method as claimed in claim 1, wherein the residence time of the bypass exhaust gas in the reaction section is between 1 s and 2 s.

3. The method as claimed in claim 1, wherein cooling media introduced into the first mixing chamber comprise one, two or three cooling media from the group consisting of fresh air, water, and cold raw meal, and into the second mixing chamber comprise at least one of water or fresh air.

4. The method as claimed in claim 1, wherein the bypass exhaust gas is cooled in the second mixing chamber to a temperature of between 180 C. and 220 C.

5. The method as claimed in claim 1, wherein the reaction section is fed with at least one sorbent for the additional, pollutant-removing purification of the bypass exhaust gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is illustrated with the FIGURE that follows.

(2) 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

(3) 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.

(4) 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.

(5) 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.

(6) 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.

(7) 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.

(8) 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.