Apparatus for thermal processing of raw materials in dust form

A cement clinker manufacturing installation a rotary kiln (10) having secondary firing and a grate cooler (16) with a high degree of heat recuperation. Instead of placing the grate cooler (16) in-line with the rotary kiln (10), the grate cooler (16) is positioned with its clinker transport direction (16a) transverse to the longitudinal axis of the rotary kiln (10). This angular positioning of the grate cooler (16) permits a discharge of hot air from the grate cooler as combustion air for the secondary firing of the calcination state by way of a discharge opening (20 or 21) on the lateral half of the kiln discharge housing (11) in which the beginning of the recuperation zone of the grate surface (15) of the grate cooler (16) is located. Thus somewhat cooler secondary air (18) flows into the rotary kiln (16) thereby avoiding excessively high temperatures at the rotary kiln discharge without reduction in overall thermal efficiency of the cement plant.

TECHNICAL FIELD 
This invention relates to an apparatus for thermal processing of raw 
materials in dust form, in particular for the production of cement clinker 
from raw meal. Raw meal is preheated in at least one preheater through 
which the flue gas of a rotary kiln flows, calcined in a calcination stage 
supplied with hot air from the clinker cooler and supplementary fuel and, 
if appropriate, rotary kiln flue gas, and burned to cement clinker in the 
sintering zone of a rotary kiln. The cement clinker is cooled in a 
downstream grate-type clinker cooler, having a kiln discharge housing 
arranged between the discharge end of the rotary kiln and the recuperation 
zone of the grate cooler. Part of the hot air from the cooler is routed 
into the kiln discharge housing and into the sintering zone of the rotary 
kiln as secondary air and another part of the hot air from the cooler 
serves as tertiary air by being routed to the calcination stage via a 
tertiary air line. 
INFORMATION DISCLOSURE STATEMENT 
In installations for the manufacture of cement clinker from cement raw 
meal, in order to avoid uneconomically long and/or large-diameter rotary 
kilns, and to keep the specific heat requirement of the cement clinker 
manufacturing process low, it is known to connect a calcination stage 
upstream of the rotary kiln as viewed in the direction of material flow, 
which calcination stage is equipped with a secondary firing (in addition 
to the firing in the rotary kiln). The cement clinker burned in the rotary 
kiln, in the glowing hot condition, is discharged downward, via a 
stationary kiln discharge housing enclosing the end of the rotary kiln, 
onto a grate cooler that follows the rotary kiln as viewed in the 
longitudinal direction of said rotary kiln, such as shown in KHD Humboldt 
Wedag AG Brochure No. 7-330, pages 4 and 5. The grate cooler may also be 
installed beneath the rotary kiln having its transport direction opposite 
thereto. In the grate cooler, the hot cement clinker, for example at 
1000.degree. C., is cooled by means of cool air. The hot air from the 
cooler trapped in the kiln discharge housing is utilized in two ways for 
the cement clinker burning process, namely, in one part, as secondary air 
for the rotary kiln firing and, in another part, as tertiary air for the 
second firing in the calcination stage. Because of the previously known 
geometrical arrangement of the grate cooler relative to the rotary kiln, 
the combustion air to the rotary kiln (secondary air) is hotter, namely by 
approximately 50.degree. to 100.degree. C. hotter, than the combustion air 
to the calcination stage (tertiary air). 
In recent years, appreciable efforts have been made to increase the 
recovery of heat (recuperation) in the grate coolers connected downstream 
of the rotary kilns by means of improvement of the efficiency of the 
coolers. This has led to still higher temperatures of the hot secondary 
air flowing in from the grate cooler via the kiln discharge housing into 
the rotary kiln. There are, however, several reasons for wanting to lower 
an excessively high secondary air temperature, which in modern grate 
coolers may be, for example, about 1100.degree. C. (formerly approximately 
800.degree. C.), namely: 
At excessively high secondary air temperatures, the refractory materials of 
the rotary kiln discharge as well as the kiln discharge housing approach 
the limit of their capacity. A change to materials with higher capacity 
would drastically increase purchase costs. For these reasons, a reduced 
temperature stress on the refractory materials of the kiln discharge 
housing as well as the metallic rotary kiln discharge segments and the 
burner lance in the rotary kiln discharge zone is desirable. 
Excessively high secondary air temperature makes precooling of the glowing 
hot cement clinker in the rotary kiln discharge zone more difficult and 
promotes the formation of a melt phase and the formation of deposits in 
the rotary kiln discharge zone. 
Excessively high secondary air temperatures and hence excessively high 
flame temperatures of the rotary kiln burner promote the undesirable 
formation of NO.sub.x in the rotary kiln flue gas. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is a primary object of the invention, in a cement clinker production 
plant using a rotary kiln having primary and secondary firing and grate 
coolers with a high degree of heat recuperation, to provide a lower 
temperature secondary air flowing from the recuperation zone of the grate 
cooler via the kiln discharge housing into the rotary kiln without it 
being necessary to cool the excessively hot secondary air with fresh air 
or infiltrated air, thereby preserving the desired maximum possible heat 
recovery. 
In the cement clinker manufacturing installation in accordance with the 
invention, having rotary kiln firing, secondary firing in the calcination 
stage, and a grate cooler, a control of the quantity of secondary air 
coming from the recuperation zone of the grate cooler and flowing through 
the kiln discharge housing into the rotary kiln, is made possible with 
respect to its temperature level, in that the offtake of the tertiary air 
from the kiln discharge housing (for the secondary firing) is designed 
such that there is an effect on the secondary air temperature. 
Specifically, in accordance with the invention, the secondary air and the 
tertiary air are interchanged, with respect to their temperature level, in 
comparison with the previously known designs and methods. For this 
purpose, the grate-type clinker cooler is not progressively aligned with 
the rotary kiln axis, but instead is positioned so its clinker transport 
direction is transverse to the longitudinal axis of the rotary kiln 
whereby the transport direction of the grate cooler makes an angle .alpha. 
of approximately 70.degree. to 150.degree. to the rotary kiln axis. This 
changes the flow patterns of the recuperation air in the kiln discharge 
housing. If the offtake of the hot air from the cooler as combustion air 
for the secondary firing (tertiary air) is installed on the lateral half 
of the kiln discharge housing in which the beginning of the recuperation 
zone of the grate surface of the grate cooler is located, then a cooler 
secondary air is left over for the rotary kiln. The depth of the bed of 
cement clinker falling, glowing hot, from the rotary kiln discharge onto 
the grate is greatest at the beginning of the recuperation zone of the 
grate surface of the transversely positioned grate cooler, and the hottest 
hot air from the cooler is extracted as tertiary air in this zone. The 
cooler substream quantity of the hot air from the cooler remains as 
secondary air for the rotary kiln, the cooler substream having flowed 
through a clinker bed that has already experienced cooling on the 
beginning section of the grate, so that the secondary air is cooler than 
the tertiary air. With the cooler secondary air obtained in this fashion 
in accordance with the invention, the disadvantages of prior art modern 
grate coolers can be avoided. 
In accordance with the invention, the opening for the offtake of hot air 
from the cooler as combustion air for the calcination state (tertiary air 
for the secondary firing) is arranged in the side wall and/or overhead 
wall of the kiln discharge housing at a point adjacent to the side of the 
kiln discharge from which the kidney of in-process material is discharged. 
Also the temperatures of the secondary air and the tertiary air taken from 
the kiln discharge housing can be influenced by the selection of the 
direction of rotation of the rotary kiln.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to the drawings, the discharge end of the rotary kiln (10) of a 
cement clinker production plant is enclosed by a stationary kiln discharge 
housing (11). The rotary kiln (10) rotates in the rotation direction (12), 
so that the kidney of in-process material (13) is formed in the rotary 
kiln. From this kidney of in-process material (13), glowing hot cement 
clinker is discharged onto the beginning of the grate surface (15) of a 
grate cooler (16), through which grate surface cool air (14) flows. The 
clinker transport direction (16a) of the grate cooler is transverse to the 
rotary kiln longitudinal axis such that the grate cooler transport 
direction makes an angle .alpha. of 70.degree. to 150.degree. to the 
rotary kiln axis. In the exemplary embodiment of FIG. 2, 
.alpha.=90.degree.. The flow lines of the hot air obtained from the cooler 
in the recuperation zone of the clinker cooler are shown in FIG. 1 as 
tertiary air (17). 
It can be seen clearly that a discharge opening (20) for tertiary air (17) 
is installed on the lateral half of the kiln discharge housing (11) in 
which the beginning of the recuperation zone of the grate surface (15) of 
the grate cooler (16) is located. Also located there is the greatest bed 
depth (19) of the glowing hot cement clinker dropping out of the rotary 
kiln discharge onto the cooling grate; that is, the hottest hot air from 
the cooler is extracted there as tertiary air (17), so that only the 
cooler substream quantity of the hot air from the cooler obtained in the 
recuperation zone of the clinker cooler remains as secondary air (18) 
flowing into the discharge end of rotary kiln (10). Referring to FIG. 2, 
the discharge opening (20) for the discharge of the tertiary air (17) via 
a tertiary air line 31 leading to the second burner stage of a calcinator 
32 is arranged in the side wall of the kiln discharge housing (11) at a 
point adjacent, depending on the rotation direction of the rotary kiln 
(10), to the kidney of in-process material (13) formed therein. Raw meal, 
indicated by arrow 33, is calcinated by firing in the calcinator 32 and 
the calcinated raw meal is delivered from the calcinator 32 to the rotary 
kiln 10 by line or conduit 34. Alternatively, a discharge opening (21) for 
the discharge of tertiary air can be formed in the overhead wall of the 
kiln discharge housing (11). 
In accordance with the invention, the control of the temperature level of 
the secondary air (18) flowing to the rotary kiln firing can also be 
influenced by virtue of the fact that, according to a further feature of 
the invention, the first longitudinal section (15) of the cooling grate of 
the grate cooler, with which the recuperation zone of said cooler begins, 
is designed variable in tilt to the horizontal. By means of the tilt 
adjustment, the coverage of the grate with clinker as well as the 
residence time of the clinker in the first cooler zone is changed. In this 
way, heat transfer from the hot clinker to the cooling air is also 
influenced, which cooling air then flows either to the rotary kiln (10) as 
secondary air (18) or to the second firing of the calcination state as 
tertiary air (17). 
There is also the possibility of rotating the rotary kiln (10) in the 
rotation direction (22) instead of rotation direction (12) and thus to 
influence the discharge characteristic of clinker (13) from the kiln onto 
the cooling grate. Also in this way, the air temperature in the cooler 
sections of the first cooler zone is influenced. 
In FIG. 2 it can be seen that, in the kiln discharge housing (11), the 
width (23) of the cooling grate in the clinker discharge zone is 
approximately 1/3 to 1/2 smaller than in the other longitudinal sections 
of the grate cooler (16). 
Finally, it can also be seen from FIG. 1 that the kiln discharge housing 
(11) at the transition (24) from its cover to the overhead wall of the 
grate cooler housing (16) arranged transversely to the rotary kiln (10) is 
designed or expanded in such fashion that there is no increase in gas 
velocity at this housing transition (24), by which means the offtake of 
tertiary air (17) via the discharge opening (20 or 21, respectively) might 
be hindered.