Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
.
The invention relates generally to a process for
,
calcining raw materials, such as limestone, dolomite or
maynesite, in a uniflow regenerative shaft furnace having
at least two shafts, wherein simultaneous cooling of caLcined
lime in the cooling zones of`the shafts i5 performed.
A regenerative approach for strongly endothermic
processes such as for example, the calcining of limestone,
known from German AT-PS 211,214, has frequently been used~
for the construction of uniflow/counterflow shaft furnaces
having two or three shafts and i has also frequently been
described in the literature, among others, by E. SchieIe and
L.W. Berens in the book "Xalk" (lime) pages 147-151,
published by Verlay Stahl-Eisen, D~sseldorf (W. Germany).
The exclusively regenerative utilization of the
heat carrier in this calclning process has been very successful
because in the preheating zone of the shafts, not only the
material to be calcined, but also the combustion air are
preheated. The regenerative preheatina of the combustion air
constitutes the thermo-technological feature of such a furnace.
It would seem difficult to practlcall~i improve this calclning
process particularly with respect to heat -technology involved.
However, the process has been found not to meet all o~ the
requirements with respect to its operational parameters.
It is, therefore, the task of the present invention to provide
an improvement in the aforementioned calcining process in this
regard.
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SU~'LM~RY OF THE INVENTION
,
In accordance with the basic principles of the
invention, cooling air which is heated in the cooling zone
of the fuxnace by the calcined raw material is removed at
least partially fro~ the furnace at the end of the cooling
zone and its heat content is recuperatively utlli2ed
,
outside of the furnace shafts for preheating the raw materials
which are supplied to the furnace andjor for preheating the
combustion air to be supplied at the upper end of the pre-
heating zone.
The various features of novelty which characterize ~ :
: . the invention are pointed out with particularity in the claims
: . annexed to and forming a part of thls disclosure. For a better
: understanding of the invention, its operating advantages and
.
~ specific objects attained by its use, reference should be had
.
to the accompanying drawings and descriptive matter in which
there are illustrated and described preferred e~bodiments
o the invention.
D CRIPTION OF THE DR_WINGS
'
In the Drawings:
Fig. 1 is a schematic illustration of a two.shaft
. urnace with a square or rectang~lar cross-section;
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Fig. 2 is a pressure model for the cooling zone
of the counterflow shaf~ of a regenerative shaft furnacer
wherein there occurs removal of an amount of cooling air
at the outer side wall;
Fig. 3 is a pressure model for the cooling zone of
the uniflow shaft of a~ reyenerative shaft furnace, wherein
there occurs removal of an amount of coollng air at the
inner side wall; and
Fig. 4 is a schematic illustration of a two-shaft
furnace with circular shaft cross-section.
.
; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Illustrated in Fig. 1 is a two-shaft furnace
.
operate~ in uniflow wherein the shaft 1 operates as the
calcining shaft and wherein the shaft 2 operates as the
counterflow shaft. The shafts 1, 2 each define a preheating
zone V, a calcining zone B, an after-deacidification zone N
and a cooling zone K. Combustion air and the cooling air
are supplied by means of a blower 3 or 4, e.g. a rotary
compressor. Fuel enters the calcining shaft 1 in Fig. 1
thxough burners 5 approximately at the beginn;ng of the
calcining zone B~ Flue gases symbolically illustrated
by an arrow 6 and enriched by CO2 which emerge from the
~L~.Z4~ ~
limestone initially flow downwardly parallel with the
charge, then further in the counterflow shaft 2 and~ in the
'shaft 2, upwardly opposite the charging'movement to the
exhaust 7.
The cooling air 10 which is supplied through a line 8
to the two shafts 1, 2 over sliding tables 9 serving to
dlscharge the calcined material, flows upwardly in.the
cooli.ng zone K, ancl after preheating through the calcined
lime, it flows laterally outwardly through exhaust ducts 11
through a dust-collecting device, e.g., a cyclone, and through
a line 13 into a limestone preheater 14 or through an alr
recuperator 15 to the furnace exterior or to a dust-collecting
system (not shown).
The combustion air delivered by the'blower 4
th~rough a line 16 enters the shaft 1 at the top either
dlrectly through a line 17 or through the air regulator 15.
The limestone preheated and predried in the charging bucket or
.
ladle 18 is, as required, supplied to one or the other of
the shafts 1 or 2~ A change is made after the termination
of the calcining procedure in the shaft 1 and the shaft 2
becomes the calcining shaft and the shaft 1 become5 the
counterflow shat.
' Figs. 2 and 3 graphically depict flow conditions
in the after-deacidification zone N and in the cooling
zone K f the count~rflow sha-t 2 and the calcining shaft 1.
-- 5 --
13
The results determined in a model by means of the electrical
analogy method show that it is possible in the counterflow
shaft 2 as well as in the calcining shaft 1 -to exhaust
practically the entire amount of cooling air by means of
the bIower 3 if the exhaust ducts 11 for the counterflow
shaft are arranged at the outer side wall, i.e. the wall
facing away from shaft 1 and those for the calcining shaft 1
are arranged at the inner side, i.e., the wall facing toward
the shaft 2.
The design of the two-shaft furnace iIlustrated
in Fig. 4 is basically the same as the design of the furnace
in Fig. 1, except that the shaEts of the furnace of Fig. 4
are formed with a circular cross-section while the shafts
of the furnace of Fig.l have a quadrilateral cross-section.
Accordingly, the same parts are provided with the same
reference numerals and they are no~ again described~
The flue gases 6 enriched with the CO2 from the deacidlfication
procedure flow from the calcining zone B of the uniflow
shaft 1 through overflow ducts 20 into the counterflow shaft 2,
while the heated cooling air 10 flows off through a central
hollow cylinder 21. In furnaces having large shaft diameters,
between the central hollow cylinder 21 and the cooling zone
wall 22 there are advantag,eously provided roof-like cross-
vaults or beams 23 which facilitate a uniform removal of the
preheated cooling air. In the case of smaller shaft
diameters, a cover 24 is provided above the central hollow
cylinder 21.
.
-- 6
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The thermal balance of the preheating zone V is
demonstrated with the aid of an example in accordance with
the followiny: .
Let it be assumed that:
- the deacidification of the limestone beqins at
810C~I the temperature difference between limestone and
flue gas at the beginnlng of the calcining zone B is 30C and,
accordingly, the temperature of the flue gases entering the
preheating 20ne V from the calcining zone B~is 840C~;
- there is a free CaO of 94%;
- the heat loss of the furnace walls ln the
preheating zone is 10 kcal/kg lime; and
,
- the temperature of the cooling alr which;is
preheated and removed from the cooling zone K is 800C.
The amount of coollng air is assumed to be 0.6Nn3/kg*
lime~. This amount is completely Femoved.
The cooling air has a temperature at intake of 10C
and a temperature of 40C a~ter compression.
Heating ls performed by means of natural gas, the
heat consumption being 900 kcal/kg. The combustion of the
ormal cubic meter per kilogram
~.2~3
natural gas takes place with a theoretical air consumption
of 1007 Nm 3/kg and results in a flue gas amount of 1.135
Nm 3/kg. In addition, there are 0.365 Nm 3/kg expelled CO2
so that the entire amount of flue gas entering the preheat-
ing zone is 1.50 Nm 3 kg lime and its exhaust temperat~lre
is 100~.
Heat of the flue gas from the calcining z~one B may
be calculated as follows:
1~50 x 0.397 x 840 - ln5 x 100 x 0~349 - 10 -
437.87 Kcal/kg
Heat requirements for preheating the limestone may
be calculated as follows:
1.74 x 0.260 x (310 - 103 = 361.92 kcal/kg
Heat for preheating the combustion air may be
calculated as follows:
1.007 x 0.33 x (840 - 40) = 265.84 kcal/kg
The sum of the last two calculations is:
627.76 kcal/ky
The heat deicit in the preheating zone without
utilization of the preheated cooling air may be calculated
as follows:
627.76 - 437~87 = 189.89 kcal!kc~
.
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The removed and heated cooling air has a heat
content of 0.60 x 0.33 x 800 = 158.40 kcal/kg. Of this
amount, there are losses for h'eating the stone o~ltside of
the shafts at an ass~ned air exhaust temperature of 80C
of 0.6 x 0.33 x 80 ='15~84 kcal, further assumed losses in
walls and lines of 10.00 kcal, and losses for water evapora-
tion of the stone of 13.00 kcal with.'the
,
total losses being 38.84 kcal:. '
Accordingly, 158.40 - 38.84 = 119.56 kcalJkg
are picked up and recupera'ted ~rom the stone. This results
in a limestone heatiny of lI9.56 = 264.3C.
' 1.74x0.26
After adding the recuperative.heating of the stone,
the heat deficit in the preheating zone is
189.89 - 119.56 = 70.33 kcal/kg, which must be compensated
by an increased fuel supply. This heat deficit in the
preheating zone according to the process c~escribed above
is about the same as in the pure regenerative process
in which about 64 kcal/kg are generated in the combustion 3
with 20~ excess air, an amount of cooling air of 0.6 Nm3~kg
and an exhaust yas temperature of 80C.
In the regenerative process, the calcining cycle is
changed in certain time intervals. In accordance with the .
above-described process, it` is important to effect control .
in such a way that the limestone which is recuperatively
preheated outside of the furnace is charged into that shaft
_ 9 _ ,
.
in which the calc ning subsequentlv takes place in uniflow, so
that the cold combustion air meets the preheated limestone~
The advantages of the ahove-described pxocess reside
essentially in the fac-t that the regenerative system of the
furnace is relieved and that this results in significant
improvements with respect to operation, production and
utilization of the furnace.
Frequently, there is only available limestone
which is unwashed, significantly contaminated and also
very wet, such as, for example, chalk-like limestone with
10 to 20~ water content~ This leads to difficultles
during transport of the material into the furnace since a
substantial amount of fine material which cannot be separated
during sifting adheres to the stone and negatively influences
the furnace operation during calcining. During winter, there
is the additional problem that the wet limestone pieces can
.
freeze together into large lumps and can block -the chargina.
There also result problems during weighing of the limestone
to be charged, these problems becoming troublesome when
the water content of the material significantly varies
due to seasonal influences.
The moisture content of the limestone is usually
not measured before weighing. However, the amount of fuel
to be supplied should be adjusted to the water content
of the stone if a good and uniform quality of lime is
to be obtained. This disadvantage is eliminated b~ introducing
the limestone either completely dried or predried ~ith a
uniform moisture content.
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By directly conveying the heat of the cooling
air removed from the reyenerative system and heatcd by the
calcined lime to the limestone to be charged, or into a
container on the furnace, a heat utilization is achieved
which is about as good as ln the preheating zone of the
regenerated system. Furthermore, the undersized and
especially now also the dried fine material can be easlly
separated from the limestone grain size intended for calclning
before weighing and before the limestone is charged in the
furnace.
., . , .
Another opportunity for utilizing the process is
available whereby pellets of chalkstone prepared, ~or
example, on a granulating plate with the addition of water
can be dried and hardened by means of heated cooling air
before being charged in the furnace.
- A variation of the process resides in that an
entire or partial amount of the air heated in the cooling
zone of the furnace is partially conducted over a recuperator
and is used for preheating the combustion air to a temperature
of, for example, 150C or 200~C, while maintaining an
acceptable e~haust gas temperature. As a result, during i
start-up of the furnace the flue yases can be safely
prevented fro~ falling below the dew point, which would
otherwise create a disturbing influence on a subsequent
dust-collectiny system. -
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Another variation of the process resides in the
division of -the preheated cooling air to the recuperative
preheating of thé mater~al -to be calcined and to the
combustion air in such a manner that a portion of the preheated
cooling air is admixed with the cold air which is additionally
required for combustion.
Due to the complete or partial removal of the
heated cooling aLr, a further significant advantage of the
process resides in the fact that the partial pressure of the
carbon dioxide in the exhaust gas can be adjusted to a
desired-or a possible maximum value. Another advantage
resides in the fact that the a~ount of cooling àir can be
limited to the temperature desired for cooling the calclned
lime, to wit, to about 0.6 to 0.7 Nm3/kg llme. This is
because, under the hot and moist climatic conditions with
high humidity and a high temperature of the air entering
the coollng zone, the hydration of the calcined lime results
in a significant temperature increase of the lime to be
discharged. The heat consumption is thereby also increased,
the heat consumption being increased with an increase in
the amount of cooling air.
.
Due to the fact that the fuel ls supplied at the
beginning of the deacidification zone, the uniflow calcining
process facilitates the admission oE a very high heat which
can be fully utilized in accordance with the processes
previously described.
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However~ since, according to -the new process,
the cooliny alr does not flow off through the countershaft,
the pressure difference between the two shaft heads is .
reduced, whereby the heat supply and the deficiency of the
furnace can be raised.by about 30 -to 50%. .
Cooling air and combustion air are advantageously .
supplied by means of rotary compressors so that the furnace
can operate undèr pressure. In this manner, the circulation
of dust-containing gases in the compressors is avoidedO ~ .
further advantage resides in the fact that no cooling elements
are required in the process of.the invention.
While specific embodiments o~ the lnvention have
been shown and described in detail to illustrate the
application of the inventive principles, it will be under- .
stood that the invention may be embodied otherwise without
departlng from such prlnciples.
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