US2799576A

US2799576A - Process for operating shaft blast furnaces

Info

Publication number: US2799576A
Application number: US467596A
Authority: US
Inventors: Gumz Wilhelm; Becker Ernst
Original assignee: Strikfeldt & Co W
Current assignee: Strikfeldt & Co W; W Strikfeldt & Co
Priority date: 1953-11-11
Filing date: 1954-11-08
Publication date: 1957-07-16
Anticipated expiration: 1974-07-16

Description

July 16, 1957 w. GU MZ ET AL 2,799,576

PROCESS FOR OPERATING SHAFT BLAST FURNACES Filed Nov. 8, 1954 s Sheets-Sheet 1 July 16, 1957 w. GUMZ ET AL 2,799,576

PROCESS FOR OPERATING SHAFT BLAST FURNACES med Nov. 8. 1954 s Sheets-Sheet 2 i I [6 \7 I 7 9 C w I 7 D g 7 l5 5 3 1 f P 6 1 I g July 16, 1957 w. GUMZ EI'AL 2,799,576

PROCESS FOR OPERATING SHAFT BLAST FURNACES Filed Nov. 8, 1954 3 Sheets-Sheet 3 lm emor's N G am 2 v .E. E3 011151 United States Patent Patented July s, 1957 ice PROCESS FOR OPERATING SHAFT BLAST FURNACES Wilhelm Gumz, Essen-Bredeney, and Ernst Becker, Gummersbach, Germany, assignors to W. Strikfeldt 8; C0., Gummersbach, Germany, a firm Application November 8, 1954, Serial No. 467,596

Claims priority, application Germany November 11, 1953 11 Claims. (Cl. 75--41) A good cupola furnace is necessarily a bad shaft furnace for the reduction of ores because it is endeavoured to keep down as far as possible the quantity of CO produced. A good shaft furnace for the reduction of ores is, on the other hand, a smelting furnace with a very low degree of efliciency because the furnace charge has a high CO (or Co+H2) content, which means a considerable fuel consumption, as the CO contained in the waste gas is a loss for the furnace process proper. The combination of a smelting furnace with a reduction furnacetherefore demands new measures in order to meet equally well the requirements of a good smelting furnace and a good reduction furnace.

Cupola furnaces are known in which the attainment of a low CO content and a low waste gas temperature and at the same time hot running bottom as the result of the i use of hot blast airand the introduction of cooled flue gases above the smelting zone of the cupola furnace, particularly the flue gases which emanate from the air heater plant. Apart from the fact that this arrangement applies solely to smelting furnaces, it also possesses certain disadvantages, such as, for example, that, due to the reverse reaction soot deposits occur which reduce the resistance of the charge column. shaft furnace and especially by the modification according to the invention herein described, it .is possible to obtain new effects in a shaft furnace which is used both for the reduction of oxide ores and for the smelting of By applying this known process to a pig and scrap, in that the carbon deposit, which is entirely the reduction of the ores. For this purpose it is, however, necessary to control the separation of the carbon so that it assists in the reduction work without increasing too much the total resistance of the charge column.

The gases vary somewhat in their content dependent on the fuel burned and the degree of combustion. With good control of the fire, however, the flue gas may be expected to contain about 76% nitrogen, 16% carbon 1 dioxide, 6% water vapor and the balance is carbon 'perature in the middle zone that suflicient CO is produced for the reduction work and sufficiently high temperatures are maintained in order to allow the reaction to be acdisadvantageous in purely smelting furnaces, is utilized for celerated, but that the equilibrium temperatures on the other hand are, by the addition of cold flue gases, maintained at such a level and by the addition of further quantities of flue gases this level is reduced so far that appreciable quantities of CO no longer occur in the throat. According to the invention, the disadvantage of an excessive carbon separation is eliminated in that within controllable limits flue gases are injected not only at one plane into the charge column but at two or more planes one above the other. By influencing the temperature and the quantity of flue gases introduced, the procedure taking place within the blast or shaft furnace can be controlled in such a manner that the bottom into which strongly preheated blast air is injected operates as a smelting zone, that in the middle portion, by the introduction of cold flue gases at different levels, temperatures and CO content are maintained which are required for the reduction of the oxide ore and the regeneration of CO with moderately strong carbon segregation, and above this portion, by further additions of cold or cooler flue gases, the C0 contents are still further reduced and the reaction is caused to freeze up at low temperature.

The invention is illustrated diagrammatically with the aid of the accompanying drawings, in which:

Figs. 1 and 2 are charts illustrating the working of the furnace under different conditions, and

Figs. 3 to 7 are diagrams illustrating the different methods of carrying out the invention.

The operation. in the shaft furnace can be imagined as follows with the aid of Fig. 1 of the drawings. Every shaft furnace, like every gas generator with a charge column composed chiefly of fuels containing carbon, for example coke, operates in such a manner that the oxygen introduced by the blast burns with the carbon to form CO2. This zone is called the oxidation zone. In the following reduction zone this CO2 is reduced on the excess of carbon to form CO and this CO forms the most important reducing agent for the iron oxides, the CO combining with the oxygen of the ores to form CO2, but then, according to the temperature, it is again reduced by the coke to CO and so forth. The quantity of CO, which adjusts itself in each case, is a function of the equilibriumternperature, whereas the equilibrium temperature in turn results from the heat balance of the reaction zone, that is from the quantity of heat led off in the form of gas and waste heat by the charge and by the heat introduced bythe blast. The adjustment of the final CO content coordinated to the state of equilibrium does not, it is true, take place immediately but requires a certam time. Consequently, the state of equilibrium is not attained in the reduction zone but only at the end thereof. In Fig. l the reduction zone is shown, the CO being indicated as function of the path or of the time. The curve 1 represents for example in principle the course of the CO content in a layer of coke, the CO content strives asymptotically towards the state of equilibrium. If on the other hand the heat balance of the process were to be influenced by increasing the heat carried off by introducing cold flue gases or by other means, or by reducing the heat supplied, a different state of equilibrium would set in, for example one which corresponds to curve 2. If with the same means the temperature level were to be again reduced at third state of equilibrium would be established. The CO content would strive asymptotically towards this new state of equilibrium according to-the curve 3. Now if a shaft furnace according to the present invention is observed, the heat equilibrium will, in this furnace, be disturbed by again introducing cold flue gas and the level of equilibrium changes, for example from 1 to 2 and then from 2 to 3. Again if the 00 content at the point a on the curve 1 has attained a value which is higher than the state of equilibrium of the curve 2 the reaction must be reversed in accordance with the equation 2CO- CiCOz that is a carbon segregation takes place as indicated by the cross-hatched area. The carbon thus separated out will deposit partly on the coke and partly on the ore at this point, a process which is particularly desirable to this extent because this carbon can now be utilized as direct reducing agent without increasing unnecessarily the resistance of the furnace by excessive clogging of the flow cross-section. Once again if the temperature level is reduced from 2 to 3, for example by the introduction of cold flue gases, a separation of carbon takes place once more until the new state of equilibrium is reached. In Fig. 2 the same conditions are illustrated for a repeated influencing of the temperature level in a shaft furnace by the introduction of cold flue gases and it can be seen that it is possible to influence the rate of carbon segregation by this repeated splitting up of the flow of gas in reverse direction into the shaft, because in this manner the carbon segregation is distributed within considerable limits and over a greater height of the shaft.

Figs. 3, 4, 5, and 6 show diagrammatically several embodiments of the invention. In each case 1 designates the shaft furnace, 2 the air heater, 3 the hearth for heating the air heater, 4 the hot air conduit and 5 and 6 the return conduits through which cold flue gases passing out from the air heater 2 are forced back into the charge. In the example illustrated only two return feed points are shown, but according to the invention more may be provided.

Fig. 3 shows the simplest example in which a shaft furnace 1 is operated with air which is heated in an air heater 2 by direct heat from a hearth 3. 4 is a hot air conduit leading to the shaft furnace, only one feeding point being shown on the drawing whereas actually an annular conduit with a suitable number of air feed nozzles, possibly arranged at different levels, should be provided. The same applies for the return feed of the cold flue gases into the shaft furnace. The waste gases from the fuel burnt on the hearth, any kind of fuel may be used and it will generally be a cheaper kind than the coke used in the shaft furnace l, is forced, after it leaves the air heater 2, through the

return conduits

5 and 6 into the charge column of the shaft furnace. In this manner the above described operation explained in connection with Figs. 1 and 2 will take place in the furnace charge. If more waste gases are present than are actually required or can be accommodated in the shaft furnace this excess can be exhausted directly into the atmosphere through the conduit 7. For the purpose of a better temperature control within the shaft furnace, the waste gas can be regulated both according to quantity and also according to temperature. Thus, Fig. 4 shows a shaft furnace with a two-piece air heater 2a and 2b which is heated directly by the hearth 3. A portion of the waste gases is forced back through the conduit at a still slightly higher temperature into the shaft furnace and the remainder of the flue gases is then forced back into the furnace at a higher level at a lower temperature so that a very accurate temperature control is attained at every point of the furnace column.

In Fig. 5 another modification of the arrangement is shown in which the adjustment of the gas admission temperature from the hearth 3 into the air heater 2 is at tained by returning a portion of the Waste gases from the furnace with the aid of the return conduit 8 and circulating blower 9.

Instead of circulating the furnace gases in order to adjust the temperature in t .e furnace to the desired degree, a portion of the waste gas may, according to Fig. 6, be returned through the conduit 8 and the circulating blower 9. Another modification of the process is possible by introducing small quantities of water into the shaft furnace with the blast, this, by properly measuring the quantities of water, can improve in known manner the heat transfer conditions in the lower part of the shaft. However, additional quantities of CO and H2 are produced which are undesirable in the ordinary cupola furnaces but which are advantageous in the combination shaft furnace for the reduction and smelting of iron because the lowering of the level of equilibrium carried out according to the invention reconverts the excess CO and H2 into CO2, H20 and C in the zone following the reduction zone.

A particular advantage is to be found in the fact that the portion of the heat consumption which is introduced into the shaft furnace by the hot air can be produced from a fuel which is cheaper than the ordinary shaft furnace fuel. For this purpose inferior quality coals, coal dust, gas, residue oils and the like come into question. Moreover, it is possible to use in the shaft furnace itself fuel sizes such as are normally not permissible in such shaft furnaces because they would lead to high CO contents. It is likewise possible by the apparatus described to work with a small sized coke, such as, for example, screened oven coke which still further reduces the fuel costs. Another advantage associated with the use of small sized coke is the reduction in the height of the charge because that portion of the height of the charge which is formed by the coke itself can be reduced in direct proportion to the mean diameter of the fuel grain. This results in lower furnaces and lower capital investment.

Another construction according to the invention enables the use of gases also for heating the air heater, which gases are drawn from the shaft furnace itself below the smelting zone, as illustrated, for example, in broken lines in Fig. 5. This opens the way for many further constructional possibilities both with the measures herein described as also with other measures already known in the art of shaft furnace operation. It is also possible to run the shaft furnace itself with cheap infcrior quality fuels and to dispense with, entirely or partly, any foreign heating for the air heater and nevertheless attain the advantages of the main idea of the invention described at the outset.

In the shaft furnace above described it is assumed that this is normally run with the ordinary fuel, for example coke. In some cases, however, an ore is available which is suitable for running the furnace, whereas there is, for example, no suitable coal available for producing the coke. It is therefore desirable under certain circumstances to run the shaft furnace for smelting pig iron and the reduction of the ore also with other fuels, for example, with liquid fuel such as oil or with gaseous fuels such as natural gas.

Processes are known for the reduction of oxide iron ores by gas mixtures which are produced, for example, by a re-forming process from oil or natural gas using heat treatment or partial combustion with oxygen. Hitherto the sole aim of such processes has been the reduction of the ores, the product being a so-called iron sponge which is melted down outside the furnace, either in an electric furnace or in an open-hearth furnace, and further worked up into steel. If the iron ores used are not very pure the iron must be separated from the gangue in a separate operation. Due to this method of procedure the output and economy of the steel production were subject to considerable limitations. The output of the furnace in particular was limited by the fact that temperatures must not be raised above the sintering temperature of the ores or the iron if a perfect discharge from the furnace is to be ensured. The separation of the reducing and smelting operations is also accompanied by additional losses of heat.

The process according to the invention for a smelting and reduction furnace fired with solid fuels can, however, be applied with a few modifications to the operation with gas of such a smelting and reduction furnace. The proposed type of furnace and manner of operation open up a number of possibilities for the reduction of gas and also ;'*5 for the reduction of gas in conjunction with the reduction of solid fuels. The principle of the process will now be explained with the aid of an example:

In the illustration 1 designates the reduction and smelting furnace which, as indicated by the capital letters, is divided into a smelting zone S, a reduction zone R, a carbon separation zone C and a preheating zoneV. Beside the furnace there is a heat exchanger 2 which is heated by a combustion chamber 3. It is now assumed by way of example that only natural gas is available for the reduction and smelting. At the point 4 the natural gas is fed to the combustion chamber 3 and is there burned with oxygen. In order at the same time to prevent the occurrence of too high a temperature, cooler waste gases or flue gases are introduced through the conduit 6 into the combustion chamber thereby regulating the desired temperature. In the heat exchanger 2 the gases from combustion give up their heat to the returning gases which are withdrawn from the upper part of the shaft furnace and with the air of the circulating blower 9 are forced through the conduit 8 into the heat exchanger whence they are introduced into the shaft furnace. The nozzles of the shaft furnace then receive natural gas through the conduit 5, oxygen through the conduit 7 and preheated return gas through the

conduits

8 and 10. By dosing these three gas mixture components it is possible to adjust any desired smelting temperature in the hearth of the smelting furnace and on account of the return gases being heated to a high temperature only a minimum quantity of oxygen is required. The waste gases from the combustion process are forced back into the shaft furnace at several points through the conduits 11 in a similar manner to that described in connection with the shaft furnace run with solid fuel, for the express purpose of cooling above the reduction zone the rising shaft gases to such an extent that the formation of CO is to a great extent suppressed. During this operation a separation of carbon takes place due to the decomposition of C0, the quantity and temperature of the gases returned into the shaft furnace being so regulated that in this instance the most intensive separation of carbon sets in. This carbon settles with the ore and the admixtures in the shaft furnace and moves into the region where the hot gases preferably charged with CO2 and H20 rise out of the smelting zone in upward direction. The CO2 is therefore reacted with the carbon which has been formed, producing thereby CO and H2 and the CO thus formed together with the H2 formed in a similar manner serve as reduction agents within the reduction zone. When an excess of flue gases is available this can be conducted off through the conduit 12.

With the same apparatus the operation of the combination smelting and reduction furnace can be carried out with air. In this case the air is introduced in front of the heat exchanger 2 and is heated therein. However, in order to avoid concentration of nitrogen ballast, the waste gas of the shaft furnace itself is not, amongst other things, returned through the conduit 9 but other gases, for example CO2 obtained by washing the waste gases or by some other process or from foreign sources, are used in known manner for increasing the CO2 content of the gases introduced into the shaft furnace. Consequently, in this case natural gas is fed through the conduit and burned with this highly heated mixture of CO2 and air in the smelting zone of the furnace. The heating of the heat exchanger 2 is also effected in this instance by burning the natural gas with air; the Waste gases of this process of combustion are forced back into the shaft furnace through the conduits 11 and excesses led off through the conduit 12.

In the drawing these conduits 11 are shown. In principle a plurality of conduits and a plurality of admission points are desirable but for practical reasons it will probably be necessary to reduce to a few the number of these gas return points.

These two methods of operation with oxygen and with air and the operation not specifically described with air enriched with oxygen enable a further modification of the process in which one is not restricted to the separation of carbon in the zone C of the shaft furnace but in which a relatively small quantity of coke is charged into the furnace together with the ore. In this manner the carbon supply within the furnace is independent of the additional carbon formed. In districts where there is a lack of suitable coke or coking coal, oil coke can be used. The quantity of charged coke is, however, at the same time only a fraction of the quantity of coke which would be required for carrying out methods of operation using exclusively solid fuels. As the above described process shows, this quantity of coke can be reduced to zero as may be desired.

It is further possible to use instead of natural gas other gases, especially those rich in hydrocarbons. It is likewise possible to use liquid fuels such as petroleum and petroleum products which take the place of the natural gas. This would be impossible in a furnace of the usual type because an important condition for carrying out the process is the preheating to a high temperature the air for combustion and the return gases and for reasons of economy the return of the cold flue gases into the shaft furnace in order to restrict the formation of CO and to favor the separation of carbon.

The manner of operation of the smelting and reduction furnace described can also be modified in that above the smelting zone highly preheated reduction gases containing CO and H2 are introduced as indicated by the conduit 13 shown in dash lines. These reduction gases can be obtained from the natural gas or hydrocarbonaceous gases or oil by decomposition by heat treatment or by partial combustion with oxygen. By introducing them through the conduit 13 the smelting zone of the furnace is not affected. The gases introduced mix with the gases containing CO2 and H20 rising from the smelting zone and serve exclusively for reducing the oxides in the reduction zone. The effect is intensified by the re-formation of CO and H2 from the CO2 and H20 from the smelting zone together with the carbon separated out in the zone C and sinking towards the bottom.

Another modification of the process consists, especially when working with oxygen, in the possibility of dispensing with the heat exchanger and forcing back cold gases from some other source into the shaft of the shaft furnace. For this purpose preferably flue gases come into consideration from a boiler plant fired with the same gaseous or liquid fuels, namely either oxygen with a suitable addition of return gases in order to control the temperature peak or by combustion with air. In this case the return gases from the upper part of the shaft are not preheated but introduced directly into the smelting zone of the shaft furnace with the fuel and oxygen.

We claim:

1. A method of operating a shaft furnace for smelting ore and iron and provided with direct fired air heater, consisting in leading the cooled flue gases leaving the air heater back into the charge column of the shaft furnace above the combustion zone.

2. Method as set forth in claim 1, wherein the cold flue gases are introduced into the charge column at at least two levels thereof.

3. Method as set forth in claim 1, consisting in introducing the flue gases leaving the air heater into the charge column at different heights and at different temperatures.

4. Method as set forth in claim 1, consisting in returning waste gases from the upper part of the charge column to regulate the heating temperature of the air heater.

5. Method as set forth in claim 1, consisting in recirculating the waste gases from the air heater to regulate the heating temperature of the air heater.

6. Method as set forth in claim 1, consisting in injecting Water in closed quantifies into the shaft furnace to influence the ratio of heat transmission. I

7. Method as set forth in claim 1, consisting in feeding the fuel in lump sizes which are smaller than usually employed in shaft furnaces.

8. Method as set forth in claim 1, consisting in using gases drawn off from below the smelting zone of the shaft furnace for heating the air beaten 9. Method as set forth in claim 1, consisting in preheating to a high temperature at least a portion of the Waste gases from the shaft furnace by a direct fired heat exchanger and at the same time introducing a suitable quantity of additional oxygen With the fuel into the smelting zone of the shaft furnace.

10. Method as set forth in claim 1, consisting in introducing the combustion air heated to a high temperature into the smelting zone of the shaft furnace simultaneously with at least a portion of the Waste gases from the furnace.

11. Method as set forth in claim 1, consisting in charging a small quantity of reduction fuel into the shaft furnace to participate in the reaction together with the gaseous reaction agents.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. A METHOD OF OPERATING A SHAFT FURNACSE FOR SMELTING ORE AND IRON AND PROVIDED WITH DIRECT FIRED AIR HEATER, CONSISTING IN LEADING THE COOLED FLUE GASES LEAVING THE AIR HEATER BACK INTO THE CHARGE COLUMN OF THE SHAFT FURNACE ABOVE THE COMBUSTION ZONE.