US3074706A - Method for carrying out endothermic processes in a shaft furnace - Google Patents

Description

Jan. 22, 1963 A. HMID ETAL I METHOD FOR A RYING OUT ENDOTHERMIC PROCES I SHAFT FURNACE Fi J 17. 1959 United States Patent 3,074,706 METHOD FOR CARRYING OUT ENDOTHERMIC PROCESSES IN A SHAFT FURNACE Alois Schmid, 46 Gegor Mendelstrasse, Vienna XIX,

Austria, and Hermann Hofer, Wopfing, Lower Austria,

Austria Filed July 17, 1959, Ser. No. 827,968 Claims priority, application Austria Aug. 9, 1958 3 Claims. (Cl. 263-52) This invention relates to a method for carrying out endothermic processes and to a shaft furnace for carrying out said method.

In hitherto known shaft furnaces for strongly endothermic processes, such as smelting furnaces, furnaces for deacidifying carbonates, firing charnotte, and the like, the thermal efficiency of the furnace is strongly reduced, due to high waste gas losses. The losses are the more extensive, the higher the amount of excess air which is used. The reduction of thermal efiiciency is particularly high when powdered, liquid, or gaseous fuel is used. The reason for this consists in that the waste or burnt gases leaving the reaction zone-in which the actual combustion takes place at very high temperaturescarry such large amounts of heat with them that the.

material in the preheating zone is not sufiicient for taking up .this heat, because only the temperature difiference up to the reaction temperature is available.

Moreover, if the material to be heated is heat-sensitive (e.g. the material heated in lime-burning kilns or dolomite-calcining kilns), the necessary low temperatures can be maintained only at high excess of air or by the addition of large amounts of inert gases (e.g. waste gas of the furnace). However, in this manner the amount of gas used for transporting heat from the reaction zone to the preheating zone, is essentially still further increased and the thermal efliciency of the combustion process is further impaired and results also in an increase of the waste gas temperature.

In order to avoid the above outlined disadvantages, a process has been suggested, in which between reaction zone and preheating zone hot waste gas is drawn off and is reintroduced into the reaction zone at a suitable point, if desired in mixture with cold air. Although it is true that in this process a considerable part of the losses usu-, ally resulting from waste gas circulation, is avoided, radiation losses occur in the bypass line. Furthermore, the necessary blast apparatus is under very high stress and liable to disturbances. Finally, the waste gas losses resulting from the use of excess air necessary in gas, oil or pulverized-coal firing cannot "be eliminated.

The known principle of -firingtemperature-sensitive materials in direct current, hasbeen likewise suggested for shaft furnaces in such a manner that between cooling zone, reaction zone and preheating zone sluice zones are inserted, whereby the airor waste gas is conducted by means of by-pass conduits in such a manner that the cooling zone and preheating zone are operated in counter current and the reaction zone in direct current. Up to date, this suggestion could not be used in practice, because there were considerable difliculties in the construction of the sluice zones which are supposed to operate in the range of normally high reaction temperatures. Furthermore, due to the length of the sluice zones and by-pass lines radiation losses must be also expected. Finally, this system offers no possibility of eliminating the waste gas heat losses caused by the excess air, which is always necessary for bringing about unobjectionable combustion.

Finally, furnaces have been built, into which fuel is introduced at several burner levels, instead of only one.

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The essential object of this method of operation was again to avoid injurious temperatures, whereby, however, waste gas losses could not'be eliminated. Moreover, such furnaceswhich are operated e.g. with generator gassuffer from constant clogging or obstruction of the upper burners, so that in fact they are operated with burners arranged in only one plane so that, in addition to high heat consumption, they yield, for example, a non-unirformly burnt lime.

Recuperators and heat exchangers have not come into commercial use up to date, in view of their low efiiciency, or high costs.

The main object of the present invention is to provide a process, in which the above-outlined difficulties are avoided.

According to this invention, an endothermic process which is suit-able for example for producing burnt lime is carried out in a shaft furnace, in which the material to be burnt or calcined is preheated in a first zone, is then subjected to the direct action of a fuel in a subsequent Zone and then to cooling in a further zone. It is essential in this process that the cooling zone and/ or the preheating zone, is passed by at least a part of the heat carrier needed for the burning process, alternately in direct current and counter current, so that the excess heat obtained in a conventional shaft furnace, is re-utilized again in the preheating zone for the burning process. In this manner any desired amount of excess air, or cold inert gas (waste gas) can be added, without the occurrence of losses. It depends then only on the correct adjustment of the furnace to tune the amounts of heat, which are passed during both periods of operation through the furnace, in such a manner that cold burnt material can be discharged and only cold waste gases escape from the shaft furnace.

Some specific embodiments of and best modes for carrying out the invention will now be described by way of example in the use of suitable shaft furnaces diagrammatically illustrated in the appended drawings likewise by way of example.

In the drawing,

The FIGURE illustrates diagrammatically in vertical section a two-shafted furnace according to the invention.

The twin furnace shown in the drawing has at its mouth a shift or switching bell G which is changed over every ten minutes to permit alternate charging of twin furnace shafts 3 and 4. Shafts 3 and 4 are divided into an upper or preheating zone V, and intermediate or burning zone B, and a lower or cooling zone K. The material treated e.g. limestone is introduced at A into furnace shaft 4 the upper or preheating zone V. While descending therein cooled waste gas passes in the opposite direction and escapes through the chimney S. At the same time, primary air passes from blast device R over the conduit L through the mouth to shaft 3 which contains limestone heated in the preceding period through the heat of Waste gases. The primary air is heated by hot limestone in the preheating zone V and proceeds downwardly to the centrally disposed burning zone B where it mixes with fuel supplied thereto by conduit C, which is located at the top of such burning zone.

The burning fuel heats the limestone in the combustion or burning zone and the partially consumed products of combustion mixed with secondary air from below pass through a conduit 5 to shaft 4. The conduit 5 is located between the burning zones B and the cooling zones K. The treated limestone then descends through the cooling zone K and is eventually discharged through any suitable port at the lower end of shaft 3. The mixture of gases or the waste gas already formed has been already previously mixed with secondary air which is supplied to the cooling zone K of shaft 3 from below over the conduit M likewise by the blast device R. Thereby the secondary air cools the heated limestone in the cooling zone K of shaft 3 and takes on heat therefrom. The mixture of Waste gases and heated secondary air pass up- Wardiy through the burning and preheating zones of shaft 4 and the fresh limestone introduced into the preheating zone V of the same so that the limestone is heated to about 900 C. The air supplied by blast device R is divided in intervals of 10 seconds into a secondary air stream and a primary air stream by a mechanically-operated shift or change valve K Thus the air is supplied to the shafts from above and from below intermittently and discontinuously. A second valve K serves for switching over the secondary air to shaft 4 when the operating period is changed over after every ten minutes, whereby the switching bell G at the throat is likewise changed over.

In operating the furnace shown in the drawing, the charge is e.g. supplied (in intervals of 10 minutes) to one of the shafts, for instance the shaft 4, in the amount of 1150 kg. of limestone (CaCO During this period 450 m. of primary air and 440 m. of secondary air are introduced into the other shaft 3. Into the latter, at the point marked C 58.5 m. natural gas with 9000 kcal./m. are introduced and of this gas about A is burned in this shaft 3. The rest is burned in shaft 4.

The limestone introduced into the furnace is heated to 900 C. in the preheating zone V and to 1150 C. in the combustion zone B. Each charge of 1150 kg. of limestone yields 632 kg. calcium oxide and 518 kg. of carbon dioxide. For obtaining one kg. of CaO only 830 kcal. or 0.092 m? natural gas is consumed which indicates a very low heat consumption. The calcium oxide cools in the cooling zone K to about 50 C., whereby its heat is extracted by the secondary air which then passes through conduit 5 to the burning zone B of the other shaft.

The waste gases leave the furnace with a temperature of about 50 C.

It will be understood from the above that this invention is not limited to the designs, conditions, proportions and other details specifically described above and can be carried out with various modifications without departing from the scope of the invention as defined in the appended claims.

What is claimed is:

1. A method of operating an endothermic shaft furnace having first and second shafts for the treatment of materials disposed therein, which consists of alternately charging the shafts with material at their upper ends thereof, first passing the material delivered to each shaft downwardly by gravity and through an upper preheating zone, alternately introducing fuel at points in the shafts between the upper preheating zone and a subsequent burning zone, secondly alternately passing the material downwardly by gravity through the subsequent burning zone to be subjected to the direct action of the burning of the fuel therewithin, thirdly alternately lowering the residual material into a subsequent cooling zone, and thereafter discharging the residual material from the cooling zone, said first and second shafts being interconnected with each other by a conduit at the level of the lower ends of the burning zones, said method further consisting of initiating a first operating period which includes introducing primary combustion supporting air into the upper end of said first shaft and passing the said primary air through the first preheat zone and downwardly over the material that has been previously treated thereby to preheat the said primary air, thereafter passing said primary air to the burning zone therebelow to support the combustion of the fuel introduced thereinto, and thereafter passing the combustion products produced from the combustion within the burning zone of the first shaft through said conduit to the burning zone of said second shaft and upwardly through burning and preheating zones of the second shaft in a direction counter to the lowering of the material therewithin to preheat the same, thereafter discharging combustion products from the upper end of the second shaft to the atmosphere, simultaneously during this first operating period introducing secondary combustion air into at least one of the cooling zones of the respective first and second shafts, mixing the secondary air with the combustion products within the lower end of the burning zones at the conduit level, passing the mixture through the burning zone and preheating zones of the second shaft to further support the combustion of the combustion products being passed therethrough and insure the preheating of the material therewithin; and thereafter initiating a second operating period by switching the furnace in order to operate the second shaft in the manner corresponding to the operation of the first shaft in the first operating period, and to operate the first shaft in the manner corresponding to the operation of the second shaft in the first operating period, the operation in said first period and the operation in said second period being repeatedly switched in a cycle of predetermined periods of time in order to retain and utilize in the shaft heat present in the primary and secondary combustion air and in the burnt material treated.

2. A method as claimed in claim 1, in which the primary air is mixed with an inert gas.

3. A method as claimed in claim 1, in which the amount of primary air being introduced into the first shaft in the first operating period is insuificient for combustion of the entire amount of fuel introduced into said first shaft in said first operating period thereby leaving a residual amount of said fuel in the first shaft, and thereafter burning said residual fuel in the second shaft.

References Cited in the file of this patent UNITED STATES PATENTS 685,064 Schubert Oct. 22, 1901 688,651 Kirk Dec. 10, 1901 1,177,680 Brown Apr. 4, 1916 2,532,335 Royster Dec. 5, 1950 FOREIGN PATENTS 520,966 Belgium July 15, -3

Claims (1)

1. A METHOD OF OPERATING AN ENDOTHERMIC SHAFT FURNACE HAVING FIRST AND SECOND SHAFTS FOR THE TREATMENT OF MATERIALS DISPOSED THEREIN, WHICH CONSISTS OF ALTERNATELY CHARGING THE SHAFTS WITH MATERIAL AT THEIR UPPER ENDS THEREOF, FIRST PASSING THE MATERIAL DELIVERED TO EACH SHAFT DOWNWARDLY BY GRAVITY AND THROUGH AN UPPER PREHEATING ZONE, ALTERNATELY INTRODUCING FUEL AT POINTS IN THE SHAFTS BETWEEN THE UPPER PREHEATING ZONE AND A SUBSEQUENT BURNING ZONE, SECONDLY ALTERNATELY PASSING THE MATERIAL DOWNWARDLY BY GRAVITY THROUGH THE SUBSEQUENT BURNING ZONE TO BE SUBJECTED TO THE DIRECT ACTION OF THE BURNING OF THE FUEL THEREWITHIN, THIRDLY ALTERNATELY LOWERING THE RESIDUAL MATERIAL INTO A SUBSEQUENT COOLING ZONE, AND THEREAFTER DISCHARGING THE RESIDUAL MATERIAL FROM THE COOLING ZONE, SAID FIRST AND SECOND SHAFTS BEING INTERCONNECTED WITH EACH OTHER BY A CONDUIT AT THE LEVEL OF THE LOWER ENDS OF THE BURNING ZONES, SAID METHOD FURTHER CONSISTING OF INITIATING A FIRST OPERATING PERIOD WHICH INCLUDES INTRODUCING PRIMARY COMBUSTION SUPPORTING AIR INTO THE UPPER END OF SAID FIRST SHAFT AND PASSING THE SAID PRIMARY AIR THROUGH THE FIRST PREHEAT ZONE AND DOWNWARDLY OVER THE MATERIAL THAT HAS BEEN PREVIOUSLY TREATED THEREBY TO PREHEAT THE SAID PRIMARY AIR, THEREAFTER PASSING SAID PRIMARY AIR TO THE BURNING ZONE THEREBELOW TO SUPPORT THE COMBUSTION OF THE FUEL INTRODUCED THEREINTO, AND THEREAFTER PASSING THE COMBUSTION PRODUCTS PRODUCED FROM THE COMBUSTION WITHIN THE BURNING ZONE OF THE FIRST SHAFT THROUGH SAID CONDUIT TO THE BURNING ZONE OF SAID SECOND SHAFT AND UPWARDLY THROUGH BURNING AND PREHEATING ZONES OF THE SECOND SHAFT IN A DIRECTION COUNTER TO THE LOWERING OF THE MATERIAL THEREWITHIN TO PREHEAT THE SAME, THEREAFTER DISCHARGING COMBUSTION PRODUCTS FROM THE UPPER END OF THE SECOND SHAFT TO THE ATMOSPHERE, SIMULTANEOUSLY DURING THIS FIRST OPERATING PERIOD INTRODUCING SECONDARY COMBUSTION AIR INTO AT LEAST ONE OF THE COOLING ZONES OF THE RESPECTIVE FIRST AND SECOND SHAFTS, MIXING THE SECONDARY AIR WITH THE COMBUSTION PRODUCTS WITHIN THE LOWER END OF THE BURNING ZONES AT THE CONDUIT LEVEL, PASSING THE MIXTURE THROUGH THE BURNING ZONE AND PREHEATING ZONES OF THE SECOND SHAFT TO FURTHER SUPPORT THE COMBUSTION OF THE COMBUSTION PRODUCTS BEING PASSED THERETHROUGH AND INSURE THE PREHEATING OF THE MATERIAL THEREWITHIN; AND THEREAFTER INITIATING A SECOND OPERATING PERIOD BY SWITCHING THE FURNACE IN ORDER TO OPERATE THE SECOND SHAFT IN THE MANNER CORRESPONDING TO THE OPERATION OF THE FIRST SHAFT IN THE FIRST OPERATING PERIOD, AND TO OPERATE THE FIRST SHAFT IN THE MANNER CORRESPONDING TO THE OPERATION OF THE SECOND SHAFT IN THE FIRST OPERATING PERIOD, THE OPERATION IN SAID FIRST PERIOD AND THE OPERATION IN SAID SECOND PERIOD BEING REPEATEDLY SWITCHED IN A CYCLE OF PREDETERMINED PERIODS OF TIME IN ORDER TO RETAIN AND UTILIZE IN THE SHAFT HEAT PRESENT IN THE PRIMARY AND SECONDARY COMBUSTION AIR AND IN THE BURNT MATERIAL TREATED.

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