Heating – Processes of heating or heater operation – Including passing – treating or conveying gas into or through...
Reexamination Certificate
2003-02-12
2004-05-11
Wilson, Gregory (Department: 3749)
Heating
Processes of heating or heater operation
Including passing, treating or conveying gas into or through...
C432S058000
Reexamination Certificate
active
06733283
ABSTRACT:
The present invention relates to a method for manufacturing cement clinker by which method cement raw meal is preheated in a preheater, calcined in a calciner comprising an upper end and a lower end, burned into clinker in a kiln and cooled in a subsequent clinker cooler. The invention also relates to a plant for carrying out the method.
A plant of the above-mentioned kind for manufacturing cement is well known from the literature. In some plants the calciner is specificially designed for using fuels having a low content of volatile constituents, such as petrocoke, whereas in other plants the calciner is specifically designed for attaining a low NO
x
emission level. There is strong evidence that henceforth more stringent requirements will be imposed in terms of acceptable NO
x
emission levels and that the utilization of fuels with a low content of volatile constituents will continue to be advantageous. Accordingly, there is a need for a method as well as a plant for manufacturing cement which will embody both of these beneficial effects at one and the same time.
A range of technologies are available for limiting NO
x
emission levels and for burning fuels with a low content of volatile constituents in plants of the aforementioned kind. It will be possible to limit NO
x
emission levels by primary methods, such as the appropriate design of burner equipment, appropriate design of calciner and preheater, or by secondary methods involving injection of extraneous substances such as ammonia and urea at designated locations in the preheater.
For example, a kiln plant of the type ILC low-NO
x
is known from Japanese patent application No. 155433-1978. This plant comprises a combustion chamber which is located between the kiln and the calciner and which in practice constitutes the lower part of the calciner. Limitation of NO
x
emission is achieved by injecting fuel into the combustion chamber which constitutes a so-called reducing zone, involving a combustion of fuel by reacting with NO
x
contained in the exhaust gases from the kiln, thereby reducing the NO
x
content. According to more recently known methods a small amount of preheated raw meal from the preheater may be introduced into this zone in order to limit the temperature. Preheated air from the clinker cooler as well as preheated raw meal from the preheater are subsequently fed to the calciner. In this known kiln plant, exhaust gases from the kiln are introduced at the bottom of the combustion chamber, thereby causing the gases to flow upwards through the combustion chamber and the subsequent calciner. In this type of calciner the raw meal must thus be directed up through the main part of the calciner suspended in the gas stream which flows upwards through the calciner. As for this type of calciner, it is a distinct disadvantage that a low NO
x
emission cannot be attained when burning fuels with a low content of volatile constituents in the calciner because of the excessive cooling of the gas/fuel suspension.
For example, kiln plants of the type SLC-D is known from U.S. Pat. No. 4,014,641 and PCT/DK97/00029. This type of plant incorporates a calciner which comprises a chamber configured as a downward-directed gas duct which at its lower end is connected to an upward-directed gas duct which is further connected to the exhaust gas discharge end of the kiln. Calcination essentially takes place in the downward-directed gas duct. The majority of the fuel used in the calciner is introduced axially at the top of the calciner. Preheated air from the clinker cooler and preheated raw meal are directed tangentially into the top of the calciner. Complete combustion of a substantial amount of the calciner fuel is thus attained in the calciner while the raw meal is simultaneously calcined during its passage down through the chamber. The suspension of exhaust gases, unburned fuel and at least partially calcined raw meal from the calciner are joined together in the upward-directed gas duct with an upward flow of kiln exhaust gases, to which should possibly be added a small amount of the calciner fuel and a small amount of preheated raw meal. The combined or intermingled exhaust gas/material suspension subsequently flows up through the upward-directed gas dust where any residual amount of raw meal and preheated air which have not yet been supplied from the clinker cooler can be added. The advantage of this known type of plant is that fuels with a low content of volatile constituents can be used due to the special embodiment of the calciner which will make it possible to attain a high degree of combustion efficiency even when using such fuels. Further, the production of NO
x
in the calciner is quite low. However, the disadvantage of this type of kiln plant is that it may become necessary to add a proportion of uncalcined raw meal directly into the exhaust gases from the kiln in order to fight coatings in the kiln smoke chamber and/or in the kiln riser duct. As a result the exhaust gas/fuel suspension is cooled excessively which entails limited possibilities of reducing the NO
x
which is produced in the kiln, when using fuel with a low content of volatile constituents.
It is the objective of the present invention to provide a method as well as a plant for manufacturing cement by means of which it will be possible to attain a high degree of combustion efficiency even when using fuel with a low content of volatile constituents and to attain a low level of NO
x
emission.
This is obtained by a method of the kind mentioned in the introduction and being characterized in that exhaust gases from the kiln are introduced into the upper end of the calciner, in that fuel is likewise introduced into the upper end of the calciner, in that the exhaust gas/fuel suspension is directed down through the calciner, in that preheated air from the clinker cooler and preheated raw meal from the preheater in combination or separately are fed into the calciner at a location under the zone where exhaust gases from the kiln and fuel are introduced, in that the preheated air is directed down through the calciner, being gradually mixed with the exhaust gas/fuel suspension, in that the raw meal under the action of gravity is directed down through the calciner along its wall, being suspended in the exhaust gases at the lower end of the calciner, and in that the exhaust gas/raw meal suspension is extracted from the lower end of the calciner and conveyed to a separating means for separating the raw meal which is subsequently directed to the kiln.
Hereby is obtained a significant reduction of the NO
x
which is led to the calciner together with the kiln exhaust gases, a high degree of fuel burn out even when using fuels with a low content of volatile constituents, and a low degree of conversion into NO
x
of the nitrogen which is led to the calciner together with the fuel. It will thus be possible to generate a NO
x
-reducing zone by introducing calciner fuel into the NO
x
-laden kiln exhaust gases while simultaneously ensuring a high combustion temperature within the range 900 and 1500° C. even when the combustion process is based on fuels with a low content of volatile constituents. As a result, the resultant NO
x
-formation will be at a much lower level than is attainable for prior art. The high combustion temperature is attained due to the fact that the raw meal, because of the downward-directed passage through the calciner, is routed down along the wall of the calciner, substantially under the action of gravity. This will reduce the cooling effect of the raw meal on the exhaust gas/fuel suspension during the combustion of the fuel. The high combustion temperature will also ensure, in addition to a low net formation of NO
x
, a rapid combustion of the fuel which is necessary for stable plant operation. The raw meal which is directed down along the wall of the calciner will also protect the latter against the high temperatures which may occur, thus reducing, as a spin-off effect, the level of coatings on the wall.
The kiln exhaust gases may be introduced into the uppe
Jensen Lars Skaarup
Thomsen Kent
F. L. Smidth A/S
Manelli Denison & Selter PLLC
Melcher Jeffrey S.
Wilson Gregory
LandOfFree
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