Heating – Processes of heating or heater operation – Including passing – treating or conveying gas into or through...
Reexamination Certificate
2000-02-02
2001-09-11
Lu, Jiping (Department: 3749)
Heating
Processes of heating or heater operation
Including passing, treating or conveying gas into or through...
C432S058000, C432S106000
Reexamination Certificate
active
06287110
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is directed to a system for the thermal treatment of meal-like raw materials, particularly in the manufacture of cement clinker from raw meal, whereby the raw meal is thermally treated in a burning process by pre-heating, calcining, sintering and cooling, and the exhaust gas stream of the sintering unit (rotary kiln) and an exhaust air stream (tertiary air) of the cooling unit (clinker cooler) are used in such a way for calcination of the raw meal in the calcining unit that at least one burning location is present both in the exhaust gas channel of the rotary kiln as well as in the tertiary air channel, whereby the gas/raw meal/fuel suspension coming from both channels is redirected in the calcining unit and is introduced into the lowest cyclone of the suspension-type cyclone pre-heater system for the purpose of separating the calcined raw meal from the gas stream.
Systems for the manufacture of cement clinker from raw meal of the type set forth above having a calcining stage that precedes the rotary tubular kiln and that is equipped with secondary firings must be in the position of producing a raw meal calcined to a great extent before introduction into the rotary tubular kiln. At the same time, the builders and operators of cement clinker production lines are confronted with stricter and stricter demands for low emission values of pollutants such as NO
x
and CO.
For reducing such noxious emissions, it is known (for example, EP-B-0 222 044 as well as EP-B-0 526 770) in cement clinker production systems of the type initially cited to burn the fuel in the ascending rotary kiln exhaust gas conduit sub-stoichiometrically, i.e. starved for oxygen, for the purpose of creating a CO-containing reduction zone for reduction of the pollutant NO
x
that, in particular, has been formed due to the high-temperature burning in the rotary tubular kiln (thermic NO
x
). In the neighboring channel, what is referred to as the tertiary air channel and through which a portion of the hot clinker cooler exhaust is conducted, fuel is burned over-stoichiometrically, i.e. with an excess of oxygen. The heat released in the fuel combustion is used for calcination of the raw meal in any case. As viewed in a downstream direction of the suspension flow, the CO not consumed in the NO
x
reduction zone is burned with excess oxygen from the tertiary air channel after being merged with the rotary kiln exhaust gas channel, whereby the residual burning is additionally promoted by the flow redirection of the suspension in the calcining unit.
Whereas the firing in the rotary tubular kiln is a high-temperature combustion with long burner flame, whereby mainly thermic NO
x
enters into the exhaust, the temperature in the usually flameless burning in the calcination unit does not exceed the calcination temperature of approximately 850° C., whereby it is mainly fuel NO
x
that arises. This burning temperature/calcination temperature, however, is too low for burning inert fuels that are difficult to ignite or, respectively, difficult to burn (what are referred to as secondary fuels) such as, for example, inferior coal, etc., so that such substances have been hitherto burned in the rotary tubular kiln itself or were not capable of being utilized in the production of cement clinker.
It is also known (trade periodical “World Cement”, October 1998, pages 83-88) to burn inert secondary fuels outside the rotary tubular kiln, namely in a vertically arranged, cylindrical calcining reactor, through which the suspension flows from bottom to top. In this modification, wherein the raw meal is introduced in two separate height levels and the fuel is introduced into the calcining reactor at the bottom, this functions in the fashion of a fluidized bed reactor; for dependable operation thereof given utilization of a large-sized secondary fuel, it would be necessary to grind the secondary fuel. Specific measures are not implemented for achieving low NO
x
emissions. In another known modification, a vertically arranged, cylindrical calcining combustion chamber has the tertiary air/raw meal/fuel suspension flowing through it from top to bottom. Inert fuels are thereby employed. A central flame, which extends from top to bottom in the combustion chamber, heats the entire reaction space such (for example, 1000° C.) that the substances that are difficult to burn ignite better. Apart therefrom that the inert fuels—particularly when they are not ground—drop rapidly from top to bottom through the combustion chamber with short dwell time, so that they do not have enough time to completely burn out, this known calcination system also does not address specific measures for achieving an optimally low-pollution burning of secondary fuels outside the rotary tubular kiln given reduced emissions of pollutants such as, in particular, NO
x
and CO.
SUMMARY OF THE INVENTION
In cement clinker production lines of the type described above, the invention is based on the object of creating a calcinator preceding the rotary tubular kiln and equipped with secondary firings in which inert fuels that are difficult to ignite or, respectively, difficult to burn (secondary fuels) can be beneficially disposed of by burning for the purpose of calcination of the cement raw meal and a complete burn-out of CO gas strands and other fuel constituents as well as an exhaust gas with low NO
x
are nonetheless assured.
A first characteristic of the inventive calcination unit is that the tertiary air channel coming from the clinker cooler is fashioned—in the region of the calcination unit—as a combustion chamber that expands the channel cross section and through which the cooler exhaust/raw meal/fuel suspension flows from bottom to top, and at whose lower end the tertiary air enters with a torsion component. An easily ignitable fuel is also centrally introduced at this lower combustion chamber end, whereby a high-temperature zone that extends centrally from bottom to top and has a hot core flame proceeds from this burning location. An inert fuel that is difficult to ignite or, respectively, difficult to burn (secondary fuel) is introduced into the flame space of the high-temperature zone of the combustion chamber, this dependably igniting thereat and burning and thus being disposed of At least a sub-stream of the raw meal from the second-lowest cyclone stage of the suspension-type cyclone pre-heater systems is introduced in the peripheral space between the flame space and the wall of the combustion chamber. In this peripheral space, the raw meal is entrained by the tertiary air entering with a torsion component and is conveyed from bottom to top on a spiral path around the hot flame space with the tertiary air stream in the combustion chamber and is thereby calcined to a high degree. In contrast to a combustion chamber with a material flow from top to bottom, particularly the coarser parts dwell longer in the reaction space given the suspension flow directed from bottom to top in the inventively arranged combustion chamber, i.e. the dwell time both for the burning of the secondary fuel in the hot flame space as well as for the calcination of the raw meal in the peripheral space between the flame space and the wall of the combustion chamber is noticeably lengthened, which ultimately leads to an optimally complete burn-out as well as to a high degree of calcination. At the same time, the solid material transported upward pneumatically on spiral paths in the peripheral space between the hot flame space and the wall of the combustion chamber acts as thermal insulation and, thus, as protection against thermal overload of the wall of the combustion chamber and against caking.
The advantage of the long dwell time in the combustion chamber connected with the suspension flow proceeding from bottom to top in the inventive combustion chamber is especially true of the coarse-sized parts of the secondary fuel introduced into the combustion chamber, which need not be finely ground before being introduced. If coarse-sized parts of the secondary fuel h
Brachthäuser Michael
Hand Andreas
Kuhnke Stephan
Ramesohl Hubert
KHD Humboldt Wedag AG
Lu Jiping
Sonnenschein Nath & Rosenthal
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