Process for burning limestone, dolomite or the like and annular shaft furnace for performing the same

A process for burning and sintering material in lump form in a shaft furnace in which part of the cooling air flowing in a counterflow through the cooling zone is removed from the shaft furnace before the upper end of the cooling zone and is introduced into the column of material in the burning zone area, accompanied by a mixture of air and fuel. As a result a reducing furnace atmosphere is obtained, which extends up to the plane of the upper combustion chambers. The invention also relates to an annular shaft furnace for performing this process.

LIST OF REFERENCE NUMERALS 
10--Preheating zone 
12--Charging mechanism 
14--Upper burning zone area 
16--Lower burning zone area 
18--Cooling zone 
20--Shaft insert 
22--Peripheral wall 
24--Inner wall 
26--Annular shaft 
28--Base wall 
30--Cover 
32--Partition 
34--Lower portion of shaft insert 
36--Upper portion of shaft insert 
38--Cooling air opening 
40--Circulating gas opening 
42--Cooling air line 
44--Blower 
46--Bustle pipe 
48--Upper combustion chamber 
50--Combustion device 
52--Lower combustion chamber 
54--Combustion device 
56--Injector 
60--Outlet channel or port 
62--Bustle pipe 
64--Recuperator 
66--Line 
68--Compressor 
70--Primary air supply 
72--Flue gas feed line 
74--Furnace flue gas line 
76--Recuperator line 
78--Flue gas line 
The invention relates to an improvement in a process for burning and 
sintering material in lump form, which material is selected from a group 
consisting of limestone, dolomite and the like in a blast furnace in which 
the material successively passes through a preheating zone, a burning zone 
having an upper and a lower area and a cooling zone. At an upper end of 
the cooling zone of the furnace, a suction is placed on the cooling air, 
which is flowing from the bottom toward the top in a counterflow, and on 
fuel gases, which are uniformly flowing downward in the lower burning 
zone, to remove a mixture of the fuel gases and air, which mixture is 
reintroduced into the material column in the area of the lower burning 
zone. The reintroduced mixture has a part flowing upward and a downwrd 
flowing part which downward flowing part is again removed by the suction 
at the upper end of the cooling zone. Fuel is separately supplied in a 
controlled manner both to the fuel gases flowing upward to the upper area 
of the burning zone and to the gases which were reintroduced into the 
lower area of the burning zone in such a way that in the upper burning 
zone area and the lower burning zone area the combustion operation can 
take place with a different air excess or deficiency and consequently with 
a different temperature. The invention also relates to an improvement in 
an annular shaft furnace for burning and sintering material in lump form, 
which material is selected from a group consisting of limestone, dolomite 
and the like. The furnace has a preheating zone, a burning zone with an 
upper area and a lower area which areas are provided in two different 
planes, the furnace wall in each area has combustion chambers having 
burners supplied with fuel and combustion air, a cooling zone following 
the burning zone and having a lower cooling air is supplied. The furnace 
has a hollow shaft insert which is closed at the upper end which, between 
its outer peripheral wall and the inner wall of the furnace forms an 
annular shaft. The hollow shaft insert is subdivided by a partition into 
an upper and a lower portion with the lower end of the shaft insert being 
located in the cooling zone, whilst its upper end is positioned bove the 
upper burner plane. At the upper end of the cooling air zone, cooling air 
together with the fuel gases flowing downwards in uniform manner through 
the lower area of the burning zone enters the inner area of the insert via 
circulating gas openings and can be supplied from the inner air of the 
insert as a mixture of fuel and air to the combustion chambers of the 
lower burner plane, for performing the process. 
German Pat. No. 12 81 111 describes a process and an annular shaft furnace 
of the aforementioned type. In this reference a working in the lower part 
of the lower burning zone area takes place with the circulation of gases 
and the fuel is separately supplied in controlled a manner both to the 
fuel gases flowing upwards to the upper area of the burning zone and to 
the fuel gases partly flowing upwards and partly flowing downwards to the 
lower area of the zone in such a way that in the upper burning zone an 
intense deacidification of the material takes place with a low amount of 
air excess and high temperature, whilst in the lower burning zone area the 
residual deacidification takes place with a high amount of air excess and 
low temperature. 
It has been found that such a process and such an annular shaft furnace do 
not make it possible to produce burnt material and particularly lime with 
a very low sulphur content of the type which is required by many consumers 
and in particular the steel industry. 
SUMMARY OF THE INVENTION 
The object of the invention is to so further develop the process and 
annular shaft furnace of the aforementioned type that burnt material with 
an extremely low sulphur content can be obtained. 
According to the invention this object or task is solved with an 
improvement in a process of the aforementioned type. The improvement 
comprises that part of the cooling air flowing in a counterflow through 
the cooling zone is removed from the shaft furnace prior to the upper end 
of the cooling zone and is introduced into the column of material in the 
upper burning zone area, accompanied by the admixing of air and fuel. 
According to a preferred embodiment of the process according to the 
invention the quantity ratio of cooling air removed within the cooling 
zone on one hand and at the upper end of the cooling zone on the other 
hand can be adjusted for setting the CO-content in the upper burning zone 
area. Furthermore the air supplied to the lower and/or upper burning zone 
area can at least partly consist of flue or exhaust gases. 
According to an optional feature of the invention the proportions of 
primary air and flue gas in the supply air is adjustable to enable setting 
the CO-content in the lower burning zone area. 
Furthermore the aforementioned problem or object is solved in the case of 
an annular shaft furnace of the aforementioned type in that the lower 
portion of the shaft insert is sealed at its lower end from the inner area 
of the furnace to form a lower chamber, which is separated from the 
circulating gas openings in an upper chamber by the partition, that below 
the circulating gas openings within the cooling zone the shaft insert is 
provided with a cooling air opening for the partial suction of cooling air 
into the lower chamber, and that the cooling air which is sucked into the 
lower chamber of the insert can be supplied by means of a blower and 
optionally a bustle pipe to the combustion chambers of the upper burner 
plane. 
A preferred embodiment of the annular shaft furnace according to the 
invention is characterized by injectors for the drawing off the gases 
flowing into to the upper portion or chamber of the shaft insert and 
includes residual cooling air and fuel gases of the lower burner plane 
being substoichiometrically burnt within the lower burner zone area and 
having a reducing atmosphere. In order to introduce these gases into the 
combustion chambers of the lower combustion chamber plane, a compressed, 
preheated mixture of a controllable composition of furnace flue gases that 
were removed from the upper end of the shaft furnace and primary air is 
suppliable as an expanding agent to said injectors. 
In addition, the invention proposes an annular shaft furnace, characterized 
by a recuperator for preheating the mixture of furnace flue gases and 
primary air prior to being fed into the injectors of the lower combustion 
chamber group. A bustle pipe can be connected between the recuperator and 
the injectors. 
In the case of the annular shaft furnace according to the invention part of 
the furnace flue gases can flow through the recuperator. 
As a further development of the annular shaft furnace according to the 
invention, the proportion of the cooling air drawn off by the cooling air 
openings on the one hand and the circulating air opening on the other can 
be adjustable. 
Whereas in the prior art annular shaft furnace the fuel is burnt with an 
air excess in the lower burning zone area, i.e. in the vicinity of the 
lower combustion chamber plane, and whereas a substoichiometric burning 
takes place in the upper combustion chamber plane, the reverse procedure 
is adopted according to the present invention. The invention is based on 
the finding that it is possible to expel a high proportion of sulphur from 
sulphur-containing limestone during the actual burning process if there is 
a reducing atmosphere in the lower part of the burning zone, of a shaft 
furnace. Thus, according to the invention the known process is reversed in 
that the lower combustion chambers are operated substoichiometrically with 
a resulting air excess in the vicinity of the upper combustion chambers. 
As a result of the reducing operation in the lower combustion chambers, a 
reducing furnace atmosphere is obtained which, within the annular shaft if 
an annular shaft furnace is used, extends from the entrance of the 
circulating gases into the shaft insert up to the plane of the upper 
combustion chambers. Such a reducing, oxygen-free atmosphere with the 
resulting advantages attainable in the described manner, cannot be 
obtained with the hitherto known processes and shaft furnaces. 
Further features and advantages of the invention can be gathered from the 
following description in which an embodiment is described in detail 
relative to the diagrammatic drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As is shown by the drawing, an annular shaft furnace has from top to bottom 
a preheating zone 10 into which the material to be burnt can be charged by 
means of a charging mechanism 12, a burning zone comprising an upper 
burning zone area 14 and a lower burning zone area 16, as well as a 
cooling zone 18. A shaft insert 20 is supported within the annular shaft 
furnace fundamentally in the manner described in German Pat. No. 12 81 
111. An annular shaft or space 26 is formed between a peripheral outer 
wall or surface 22 of the shaft insert 20 and an inner surface or wall 24 
of the outer furnace shell. The annular shaft 26 receives the material to 
be burnt which passes through the furnace from top to bottom. Shaft insert 
20 which is substantially hollow is sealed at its lower end by a base wall 
28 and at its upper end by a conically shaped cover 30. The shaft insert 
20 is also subdivided into a lower hollow portion or chamber 34 and an 
upper hollow portion or chamber 36 by a partition 32. Annular shaft 26 is 
connected with the lower portion 34 of shaft insert 20 via cooling air 
openings 38, whilst at a higher point annular shaft 26 and the upper 
portion 36 of shaft insert 20 are connected by circulating gas openings 
40. 
A cooling air line or line 42 passes from the lower portion 34 of shaft 
insert 20 via a blower 44 to a bustle pipe 46 from which the preferably 
gas or oil-operated burners of upper combustion chambers 48 of an upper 
combustion chamber plane are supplied with air. The fuel is supplied to 
the combustion chambers 48 by means of combustion devices 50. Fuel is 
supplied to the lower combustion chambers 52 by means of combustion 
devices 54. The gases entering the circulating gas openings 40 are 
circulated by injectors 56. Outlet ports or channels 60 leading from the 
upper portion 36 of shaft insert 20 issue into injectors 56. A line 66 
passes through a heat exchanger or recuperator 64 and discharges into a 
bustle pipe 62 which supplies heated gas under pressure to each of the 
injectors 56. The line 66 is placed under pressure by the action of a 
compressor 68, which is connected to a primary air supply 70 and a flue or 
exhaust gas feed line 72. The drawing also shows a furnace flue gas line 
74 leading out of the annular shaft furnace at the upper end of the 
preheating zone 10 and the line 74 together with a recuperator line 76 
discharges into the flue gas feed line 72. A flue gas line 78 removes the 
flue gases not consumed in the process. 
The aforementioned annular shaft furnace operates in the following manner 
when performing the process according to the invention. A large part of 
the cooling air flowing upwardly in the cooling zone 18 in annular shaft 
26 is drawn off via cooling air openings 38 into chamber 34 by means of 
blower 44 and is uniformly supplied to each upper combustion chamber 48 
via bustle pipe 46. The remaining part of the cooling air continues to 
flow upwardly in the annular shaft 26 and, after entering the circulating 
gas openings 40, passes in per se known manner into the lower combustion 
chambers 52 where, unlike in the process known from German Pat. No. 12 81 
111, due to the greatly decreased quantity of cooling air which enters, a 
reducing, i.e. oxygen-free, atmosphere is obtained, which flows in the 
annular shaft in such a way that it extends therein from the level of 
circulating gas openings 40 roughly to the plane of the upper combustion 
chambers 48. This reducing atmosphere is further adjustably reinforced in 
that, unlike in the prior art, the injectors 56 are no longer exclusively 
supplied with primary air as the expanding agent and are instead supplied 
by bustle pipe 62 in part with furnace flue gas and, as required, more or 
less primary air and the quantity of the latter can be controlled. The 
CO-proportion in the atmosphere in the annular shaft furnace can easily be 
adjusted in a range of approximately 0.5 to 5% CO. This can be brought 
about by correspondingly fixing the quantity of the primary air addition 
and/or by varying the quantity ratio of total cooling air flowing within 
the coling zone and the cooling air removed at the upper end of the 
cooling zone. 
Part of the total furnace flue gas composed of the flue gases removed from 
the furnace by the furnace flue gas line 74 and by the flows via the flue 
gas feed line 72 into compressor 68. In the latter the flue gas portion is 
compressed together with a varying and correspondingly adapted quantity of 
primary air from the primary air supply line 70. The compressed flue gas 
and primary air is then preheated in the recuperator 64 and then 
introduced into the bustle pipe 62 from where the individual injectors 56 
are supplied. The furnace flue gases which are not required for the return 
process in the aforementioned process according to the invention 
constitute the true flue gases of the furnace and leave the furnace via 
flue gas line 78. The flue gases in line 78, contain an increased 
proportion of carbon dioxide, such as cannot be obtained with the hitherto 
known process in the known annular shaft furnace, particularly as 
described in German Pat. No. 12 81 111. Thus, the invention makes it 
possible for a soda manufacturer to use the flue gas having an increased 
CO.sub.2 - content leaving the annular shaft furnace via flue gas line 78. 
The features of the invention described in the above description, the 
drawing and claims can be essential to the realization of the different 
embodiments of the invention either singly or in random combinations.