Heating – Processes of heating or heater operation – Including passing – treating or conveying gas into or through...
Reexamination Certificate
2001-09-28
2003-04-08
Lu, Jiping (Department: 3749)
Heating
Processes of heating or heater operation
Including passing, treating or conveying gas into or through...
C432S106000, C106S761000, C106S762000
Reexamination Certificate
active
06544032
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and to apparatus for the heat treatment of fine-grained material wherein pollutant content of the exhaust gases is reduced by means of ammonia-containing additives.
BACKGROUND OF THE INVENTION
In the heat treatment of fine-grained material, particularly in the production of cement clinker, a substantial proportion of the fuel is already added in the calcination zone. In this case, a very far-reaching deacidification of the preheated material takes place in the calcination zone.
In view of the intensification of environmental conditions, attempts have been made for about a decade to optimize the combustion conditions in the region of the calcination zone in order to reduce the pollutant content of the exhaust gases from the combustion zone, particularly the nitrogen oxide content (NO
x
). For this purpose, it is known to add a small quantity of fuel (at most approximately 10% of the total quantity of fuel) to the combustion zone in the inlet region in order to produce a reducing atmosphere in the uptake pipe which forms the calcination zone. The rest of the calcination fuel then burns with sub-stoichiometric addition of air. Complete secondary combustion of CO and other flue gas constituents takes place in a section of the calcination zone located above an inlet for exhaust air from the cooling zone (tertiary air).
In order to set a defined temperature window which is necessary for optimal NO
x
reduction, the addition of the preheated material as well as the delivery of the exhaust air from the cooling zone is carried out in stages. In this case, the lower introduction point for material and air lies approximately at the height of the fuel delivery to the calcination zone, and the second introduction point for the preheated material is located above the upper (second) inlet for the exhaust air from the cooling zone.
Such a method is the subject of EP 0 854 339 A1 filed by the present applicants. In that method, first of all, a hot reducing region for NO
x
reduction and then a hot oxidizing region for the breakdown of unburnt flue gas constituents are created in the calcination zone in the direction of flow of the gases.
According to a further proposal made in EP 0 854 339 A1, it may sometimes be advantageous if in addition ammonia (NH
3
), a corresponding ammonia solution or an effective additive is injected into the reaction chamber of the calcination zone for the purpose of further NO
x
reduction (so-called SNCR process, cf. “World Cement”, March 1992, pages 2 to 4). In this variant of the method described in EP 0 854 339 A1, the tertiary air branch pipe delivering the upper air is shut off. The combustion of the fuel introduced into the calcination zone takes place in an oxidizing atmosphere. The dosaged addition of NH
3
or similar agents is carried out after the delivery of the fuel and tertiary air. In this case, the optimal temperature window for the SNCR process is set by the distribution of the preheated material. An additional burner disposed in the kiln inlet (inlet region of the combustion zone) remains in operation. Since the NO
x
coming from the combustion zone is thereby partially reduced, the quantity of added NH
3
can be kept lower.
This known prior art which is described above does indeed already constitute a marked improvement over earlier proposed solutions. However, there is a need for further developments to lower the NO
x
level even more.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a method and apparatus for heat treatment of fine-grained material so that the NO
x
emission level is lowered even more drastically than hitherto, and at the same time, the consumption of ammonia-containing additives is reduced and the utilization of fuel in the calcination zone is improved.
This object is achieved according to the invention in that at least the predominant proportion of the ammonia-containing additives is only added in a region of the calcination zone in which the secondary combustion of the combustion products which are formed in the section which is operated under reducing conditions but are not completely reacted out is largely concluded.
Advantageously in this method;
a) the exhaust air from the cooling zone is introduced in two branch air streams at different points into the calcination zone, the delivery of the air and fuel being set in such a way that the section of the calcination zone lying between the first and second introduction points is also operating under reducing conditions, and
b) the ammonia-containing additives are only introduced into a section of the calcination zone which is operated under oxidizing conditions and adjoins the section operated under reducing conditions.
Unlike the above-mentioned variant of the method described in EP 0 854 339 A1 (in which as NH
3
is added, the tertiary air branch pipe delivering the upper air is shut off), in the method according to the invention, due to the staged delivery of tertiary air, a large part of the length of the kiln exhaust gas pipe is operated as a reduction zone. Thus, even before the ammonia-containing additives come into effect, a very drastic reduction of the NO
x
content of the flue gases can take place in this region. This preliminary lowering of the basic level of NO
x
, together with a targeted adjustability of the reaction conditions in the calcination zone, particularly by the choice of an optimal temperature window and a favorable O
2
content, makes possible a very effective further lowering of the NO
x
content with a reduced consumption of ammonia-containing additives as well as a favorable utilization of fuel in the calcination zone.
If the preheating zone is formed by a plurality of cyclone stages which are disposed one above the other and through which the material to be preheated passes in succession from top to bottom, then in an advantageous embodiment of the method according to the invention, the stream of material coming from the second-lowest cyclone stage is divided into two branch streams, of which the first branch stream is introduced into the section of the calcination zone which is operated under reducing conditions, preferably approximately at the height of the first introduction point for exhaust air from the cooling zone, whilst the second branch stream is introduced into the section of the calcination zone which is operated under oxidizing conditions, preferably above the second introduction point for exhaust air from the cooling zone.
The branch streams of the preheated material and the quantities of air are advantageously calculated so that in the section of the calcination zone which is operated under oxidizing conditions and adjoins the section of the calcination zone which is operated under reducing conditions, the gas temperature is between 900 and 950° C. and the O
2
content is between 3 and 5%.
The secondary combustion of the combustion products (CO, hydrocarbons, etc.) which are formed in the section of the calcination zone which is operated under reducing conditions also takes place in the section of the calcination zone which is operated under oxidizing conditions. OH radicals are necessary for this. However, these OH radicals are also necessary for the reduction process proceeding with the ammonia-containing additives. Thus, in the section of the calcination zone which is operated under oxidizing conditions, there is a competition situation which is unfavorable either for the NO
x
reduction or for the CO secondary combustion.
If at least the predominant proportion of the ammonia-containing additives is only added in a region of the calcination zone in which the secondary combustion of the combustion products which are formed in the section operated under reducing conditions but are not completely reacted out is largely concluded, then the competition situation described above can be effectively neutralized.
If the calcination zone comprises an ascending branch adjoining the combustion zone, a deflection zone and a descending branch, th
Krupp Polysius AG
Lu Jiping
Muramatsu & Associates
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