THERMAL ENGINEERING
FEATURES OF THERMAL CONDITIONS AND CONTROL OF THERMAL PROCESSES IN BLAST FURNACE HOT BLAST STOVES WITH POURED CHECKERWORK L. N. Toritsyn
UDC 669.162.231
At Kosogorsk Metallurgical Plant two blocks of blast furnace hot blast stoves with a poured checkerwork were constructed and in 1989 placed into experimental production service. The checkerwork, made of 20-mm diameter spheres, has a height of 5.5 m and a diameter of 6 m. The proposal for the design was fulfilled by the All-Union Scientific-Research Institute for Metallurgical Thermal Engineering together with the Institute for High Temperatures and the Ukrainian Scientific-Research Institute for Refractories. During the decade and a half preceding this development, many scientific investigations were conducted in theAll-Union Scientific-Research Institute for Metallurgical Thermal Engineering on heat exchange, hydraulics, and mechanics. Developments were made in design, operating conditions of equipment with a poured checkerwork were determined, and economic effectiveness was evaluated. Despite a solid scientific background, during the design and preparation of the block for startup it was necessary to do additional work on the technology and means of drying and heating up the hot blast stoves since the normal method was found to be inapplicable for this purpose. With the use of a specially developed computer program [i] taking into consideration the basic features of the hot blast stoves tested at Kosogorsk Metallurgical Plant, an analysis of their thermal operation was made. It was found that the temperature of the bottom of the checkerwork increases very rapidly to the maximum allowable (350-400~ and, therefore, the equipment may be heated to operating temperatures with a large number of successive heatings and coolings (blowings). From Fig. 1 it may be seen that heating to the operating temperatures is possible only with very low consumptions of the heating heat carrier (1-3% of the nominal). With high consumptions of heat carrier the bottom of the checkerwork heats so rapidly that the temperature of the dome tdome during this time with following of the rate set by the instructions is not able to increase in comparison with that reached in the preceding stage of heating up (Fig. I, curves 3-6). The same analysis shows that even with low consumptions of heat carrier the occurrence of a dead-end situation is possible when with following of the specified heating rate of the dome during heating periods an increase in its temperature is impossible. In this case, the heating rate of the bottom of the checkerwork depends upon the degree of its preheating. If the temperature of the checkerwork cross section located at a distance of i m above the grid under the checkerwork is less than 600~ there will not be an increase in temperature of the bottom of the checkerwork. With a temperature in the same cross section of the checkerwork of more than 700~ there will be a gradual increase in temperature with reaching in about a day of a maximum average temperature of the bottom of the checkerwork of about 400~ During the subsequent time the temperature of the bottom of the checkerwork will drop. In this case, the temperature will be distributed nonuniformly in the cross section with a maximum in the center, which will be about 500~ and is reached in a week. Let us turn our attention to the high uncontrollable rates (40-60~ of reduction in tdome during the blowings (Fig. i), while the instructions specify much lower rates of about 10~ Calculations were also made of heating of the hot blast pipe. It was found that to heat it at acceptable rates (5-10~ rates of heating heat carrier exceeding 6% of the nominal rate continuously supplied to the hot blast pipe are necessary. The calculations show that to provide heating of the hot-blast pipe at the same rate as the stove rate, blast air of 20-50% of the nominal are required. Therefore, there is a contradiction. For heating of the stove small rates of heat carrier are necessary, and for heatinf of the hot-blast pipe large rates are necessary, which actually does not permit the necessary heating of the All-Union Scientific-Research Institute for Metallurgical Thermal Engineering. lated from Ogneupory, No. ii, pp. 34-41, November, 1990.
644
0034-3102/90/1112-0644512.50 9 1991 Plenum Publishing Corporation
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Time, days Fig. i. Theoretical thermal condition of high-temperature hot blast stoves with poured checkerwork during drying and heating up: H) heating; BI) blowing; i) temperature of the combustion products at the exit from the burner; 2) temperature of the surface of the dome; 3) temperature of the top of the checkerwork; 4) temperature of the bottom of the checkerwork under rated conditions; 5, 6) temperatures of the bottom of the checkerwork with an increase in burner power by 3 and I0 times, respectively, in comparison with the rated conditions; 7) consumption of diluting air. Parameters of the rated conditions (1-4, 7): consumption of cold blast in the blowings 70 ma/min; natural gas consumption in the burner during drying 29 m3/h (0.009 G n) and of air 280 m3/h and during heating 58 (0.0186 Gn) and 560 m3/h, respectively; G n) nominal capacity of the main burner. hot-blast pipe with air heated in blowings of the stove. It was not possible to develop a method without these disadvantages and, therefore, it was decided to dry and heat the high-temperature hot-blast stove with a low rate (1-3% of the nominal) of the burner with alternation of heating and cooling. It was decided to dry and heat the hot-blast pipe with air heated in the stove in the blowings because this is done without additional measures for controlling heating of the hot-blast pipe. During drying and the first portion of heating up of the stove the air heated during blowings cannot be supplied to the blast furnace because of its low temperature. Therefore, a pipe for withdrawal of the heated air during the blowings was placed on the end portion of the hot blast pipe. Based on the results of analysis of heat exchange, and taking into consideration the situation created, we have developed production instructions for drying, heating, and cooling of the stoves. The branch instructions of 1982 in effect at the time of development were taken as the basis. It should be noted that it did not consider checkerwork of corundum refractories and drying and heating of the hot blast pipe and had other disadvantages. The burner developed by the Institute of High Temperatures includes three igniters. The power of each of them is about 1% of the nominal power of the burner itself. The igniter power and temperature of the combustion products are practically uncontrolled, but remain constant. With operation with one, two, or three igniters it is possible to provide a stepped change in burner power in heating (i, 2, or 3% of the nominal) exactly within the proposed limits. Therefore, it was decided to dry and heat in operation of the igniters with supply to the combustion products of air supplied through the burner to obtain the required mixture temperature. The necessary heating rate obtained was provided by turning on one, two or three igniters and supplying the corresponding quantity of air. Therefore, it was necessary to supply combustion air with the capability of smooth control and measurement of its rate from nominal to zero. In general, the method of drying and heating was new and required development of a design for providing reliable control of the high-temperature hot-blast stove block and observation of safe operation. To measure the temperature of the bottom of the checkerwork
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Time Fig. 2. Drying and heating up of No. 1 high-temperature hot-blast stove of Kosogorsk Metallurgical Plant: I) start of operation with one igniter (initial consumption of diluting air 20,000 mS/h); II) start of. operation with two igniters; Ill) start of operation with three igniters; IV) switch to operation of the main burner; V) start of blowings into the furnace with low blast rates; VI) start of supply of blast to the furnace with nominal consumptions of it; VII) switch to continuous supply of blast to the blast furnace; temperature: l, 2) dome; 3) space below the checkerwork; 4) waste gases; i) method recommended by the instructions; 2-A) rules obtained in practice. two bayonette thermocouples measuring the temperature in the volume of the space under the checkerwork and two contact thermocouples measuring the temperature of the grids under the checkerwork were installed. It was decided to limit the maximum temperature of the outgoing gases to 350~ in order to decrease the probability of overheating the space below the checkerwork. In drying and heating of the high-temperature hot-blast stove (Fig. 2) the rules revealed in the theoretical analysis were confirmed. However, it was not possible to heat the stove with use of only one or two igniters but with operation of all three igniters on each stove it was possible to reach a dome temperature barely above 700~ It was assumed (Fig. i) that for full heating to 1300~ it is sufficient to turn on two igniters. As a result of the analysis two basic features which had a detrimental effect on heating-up of the stove in comparison with that expected were revealed: uncontrolled passage of combustion air through the completely closed valves (about I000 m3/h), and nonuniformity in distribution of the rates of the combustion products passing through the space beneath the dome and entering the checkerworko Further heating was done with operation of the burners at the minimum rates. At the same time, as was assumed earlier, heating at the rate specified by the instructions (4~ may be provided only if higher heating rates during the heating periods have been allowed. It was possible to heat the stove to tdome~1250~ with a combustion air consumption of 2025% of the nominal and lengths of the heating periods of 3-7 h and of the blowing periods of 1.5-6 h. In this case, the average rate of temperature increase of the dome was I0-15~ h, sometimes reaching 20~ and the average cooling rate of the dome during the blowing periods was 20-250C/h. The whole process of drying and heating took 44 days. Unfortunately, the heating method used is characterized by quite high local heating and cooling rates of the dome and does not make it possible to heat the stove according to a random curve. We have developed a new method of heating of stoves not having this disadvantage. At present a positive decision has been obtained on awarding it an author's certificate. We assume that the method and means of drying and heating the hot-blast pipe
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Fig. 3. Change in temperature of upper layer of the checkerwork (a) and in temperature of upper cross section of the checkerwork and thickness of its layer cooled by more than 50~ (b) in air blowings through the stove at the start and finish of the heating period: a) length of blowing shown on the curves; b) initial temperature of top of the checkerwork tt.ch and consumption of combustion air Vc. a with an initial temperature of the top of the checkerwork of 1500~ shown on the curves. must continue to be developed and accomplished. In one of the variations of such a method a separate system of heating the hot-blast pipe may be used. Since checkerwork with highly effective thermal characteristics is used, this has a significant influence on the change in temperature of its upper layer during the time or production blowing through of the stove with air before turning on and after turning off the burners. Figure 3 shows the changes in the average temperature across the cross section of the upper layer in relation to air consumption and blowing time calculated using the method of [2] (the calculated data on the change in temperature of the top of the checkerwork in blowings correspond to the data of observations on the stove). Observations of the surface of the checkerwork through a peephole in the dome showed that cooling of the top of the checkerwork occurs nonuniformly. On the surface of the checkerwork there are spheres which project upward the most and cool significantly more rapidly than the spheres of the uppe r layer located somewhat lower. In this case the cooling rate of the upper spheres of the checkerwork is an average of 10,000-30,000~ and of the spheres projecting upward up to 300,000~ Subsequent heatings of the upper layerafter blowings also occurred at high rates of 3000-10,000~ Determinations of the heat resistance of corundum spheres made using the method of [3] show that the upper spheres must gradually fail. From the material presented it is clear that the length of the blowings and the consumption of air during them must be kept to the minimum determined by safety requirements. The change in temperature of the top of the checkerwork influence the change in the dome temperature cycle (Fig. 4) and also the hot blast temperature at the outlet from the stove. The dome temperature was measured simultaneously with the dome thermocouple located at the tip of the dome and a pyrometer sighted on the checkerwork. This made it possible to separate the temperatures of the surface of the bell and of the top of checkerwork, and also to more accurately determine the change in temperature of these surfaces in the individual stages of the cycle. The theory of regenerative heat exchange suggests that in hotblast stoves with checkerwork having a high specific heating surface the surface temperatures of the dome and the checkerwork are close to one another. For example, in Fig. 4 it may be seen that in the heating period the temperatures of the top of the checkerwork and the 647
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Fig. 5. Temperature distribution in the height of the stove checkerwork at the start (a) and finish (b) of heating period of the checkerwork: i) nominal method (length of blast period ~b = 1 h; consumption of blast G b = 700 m3/min, temperature of hot blast before the bustle pipe t b = 1500~ A = i, B = i); 2) one of the characteristic methods used based on results of investigations of the initial stage of operation of the stoves (tdome = 1500~ A = 1.09, B = 1.28); 3) method providing the nominal G b and t b with double the length of the blast period (T b = 2 h, A = 1.009, B = 2). Fig. 6. Change in temperature of the domes (a) and spaces under the checkerwork (b) of stoves placed on long isolation. Stove No. 1 was placed on isolation after the heating period with reaching of an outgoing gas temperature of 350~ and stove No. 2 after a blast period shortened by 30%. Numbers at the curves are the rates of temperature change in ~ mind that the data of Fig. 5 were obtained without taking into consideration of blowings. The methods of Fig. 5 may be compared using the relationships
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where Q is the heat absorbed by the blast during the blast period, n is the number of the method of Fig. 5 (i, 2, or 3), G, c, and 9 are the consumption of heat carrier, specific heat of the heat carrier, and length of the heating period, the subscript cp refers to the combustion products and the subscript b to the blast. Curves 2 of Fig. 5 show that the real temperature distributions in the height of the checkerwork may differ strongly from the nominal and the checkerwork as a whole may have a high temperature. The maximum local change in checkerwork temperature tch.max during the heating period may be very large, especially in the lower portion of the checkerwork. For the method used (Fig. 5, curves 2) in the cross section of the checkerwork located at a distance of 1 m from its bottom, Atch.max = 700~ In this method there is a simultaneous reduction in the drop in blast temperature during the period of up to 10~ which makes it possible to supply blast to the furnace without controlling its temperature by addition of cold air. This is favorable since it makes it possible to increase the blast temperature by about 500C in comparison with method I (Fig. 5) with the same dome temperature. In operation of the block of high-temperature hot-blast stoves operation with 1;5-2 times longer periods than was planned was found desirable since, in this case, there is a decrease in the number of reversals of the valves. As the result of this the losses of blast temperature related to blowings of the stoves become less (Fig. 4). One of the possible methods with an increased blast period providing the nominal G b and t b is shown in Fig. 5 by curves 3 (the method with the given t b provides the minimum gas consumption). In this case the checkerwork temperature at the end of the heating period is 200-300~ higher than in method 1 and Atch.max = 900~ The maximum rate of change in checkerwork temperature except for its highest thin portion will be observed in the cross section with Atch.max and
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Fig. 7. Temperature distribution in the cross section (a-d) of the checkerwork of stove No. I during heating of it and the start of service: a) at a distance of i m from the bottom of the checkerwork; b) at 2 m; c) at 3 m; d) at 4 m; e) disposition of the thermocouple junctions in cross sections a-d; I) burner; II) hot blast pipe; o) location of a thermocouple junction; r) distance from the center to the thermocouple; R) radius of the checkerwork;., x) experimental data; .) without identification of the period; x) with identification of the period; S, F) start and finish of the heating period; numbers at the curves show the number of days from the start of drying the stove. is about 1000~ From an analysis of Figs. 4 and 5 it follows that in stoves with a poured checkerwork the service conditions of the checkerwork and wall refractories require greater heat resistance since there is an increase in the rate of change and the amount of the temperature changes during a cycle. In long shutdowns of stoves after heating periods of the checkerwork a gradual increase in temperature of the bottom of the checkerwork and the space under the checkerwork is observed (Fig. 6). In practice, the rules predicted by us in the theoretical analysis of the change in temperatures of the space under the checkerwork and the dome were completely confirmed qualitatively and quantitatively. In particular, it was confirmed that after the blast period there is no fear of a large increase in temperature of the space under the checkerwork. In five cross sections of a stove checkerwork at distances of i m from one another 22 thermocouples were installed. The locations of 20 of them in four cross sections are shown in Fig. 7. In the concluding stage of heating of the stove the thermocouples started to fail, shortly after the finish of heating almost all of them failed, and further temperature measurement of the checkerwork was discontinued. The data obtained indicate very nonuniform temperature distribution in the volume of the checkerwork. This is also indicated by the above-described differences in the realing heating process from the theoretical and
650
the results of temperature measurement of the space under the checkerwork (two thermocoupies) and of the exit gases (one thermocoupie) at the ~i~ish of the checkerwork heating period~ For example, for the two stoves the readings of these three thermocouples differed, although with uniform flow of the gases through the checkerwork they should be practically the same. For both of the stoves the readings of the thermocouples located in the space under the checkerwork on the portion of the wall opposite the burner were higher than the readings of the thermocouples located in the space under the checkerwork on the side of the burner. For stove No. 1 the temperature of the exit gases measured at the stack valve was approximately equal to the average for the space under the checkerwork, while for stove No. 2 it was 60-80~ more than the average. The temperature nonuniformity in the cross sections of the checkerwork may be caused by only one reason, nonuniform flow of gases in the checkerwork, which is confirmed by other data not considered in this article, such s the increased hydraulic resistance of the checkerwork. Existing information does not make it possible to accurately establish the distribution of gas flows in the cross section of the checkerwork, but there are bases for assuming that main gas flows pass through the portions of the checkerwork located between its center and the wall opposite the burner and close to the wall around the whole perimeter. The nonuniformity of flow of gases and blowing of the checkerwork with cold air reduced the thermal efficiency of the stoves. For example, for method 2 (Fig. 5) the actual efficiency is about 80% while the theoretical efficiency for this method is 87%. The presence of significant nonuniformities in the gas flows and temperatures in the checkerwork cross sections was completely unexpected since, previously, it was assumed that as the result of the very high hydraulic resistance of the checkerwork used these nonuniformities may not occur. During the heating period the exit gas temperature tex.g of the high-temperature hot-blast stoves changes differently than in normal hot-blast stoves. During a large portion of the period the temperature is significantly loer than with the use of normal hot-blast stoves, while at the end of the period it increases sharply with rates of 200-300~ rapidly reaching the established limit of 350~ Such a character of change in tex E decreases the difference between the average temperatures of the exit gases during the period at the finish of it with different values of tex g. A result of this is the weak influence of the temperature tex.g.max at the finish of the period (tex.g.max) on the efficiency. For example, in switching from method 1 to method 3 (Fig. 5) the theoretical efficiency decreases by only 0.9% although the tex g max increases from 170 to 270~ If it is also taken into consideration that the unfavorable influence of blowings on method 1 is stronger than on method 2, as the result of the greater frequency of blowings in method 1 (Fig. 5), then it may be assumed that the efficiencies of these methods are the same. During blowings of the stoves and ignitions of the burners the rates of change in temperature of the inner surface of the burner sleeve are 4000-6000~ with a temperature change during the period of 250-300~ and, therefore, the sleeve lining is under very severe conditions from the point of view of heat resistance in comparison with all the rest of the stove lining. The temperature of the dome surface is also subjected to thermal shocks~ During blowing of the stove the temperature of the dome inner surface decreases by about 40~ at a rate of 4000-5000~ and the initial rate of temperature increase at the start of supplying blast and at ignition of the burners reaches 300~ Let us recall that the portion of the wall lining under the most unfavorable conditions and in contact with the checkerwork is located at a distance of 1-1.5 m from the bottom of the checkerwork. The change in surface temperature of this portion of the lining during the period is 700-900~ and the rate of temperature change reaches 1000~ All these data show that the lining of high-temperature hot-blast stoves, as of normal hot-blast stoves, is under severe conditions with respect to heat resistance. In contrast to normal hot-blast stoves in high-temperature hot-blast stoves the lining of the checkerwork chamber walls operates under more severe Conditions and the lining of the hot-blast sleeve and the dome under somewhat more severe conditions. The top of the checkerwork of high-temperature hot-blast stoves experiences very severe temperature differentials and it is not possible to avoid partial failure of the uppermost spheres of the checkerwork. However, partial failure of the uppermost spheres is completely acceptable, since there is practically no change in the thermal and hydraulic operating characteristics of the stoves and the lower spheres of the checkerwork are protected from failure by the upper spheres assuming thermal shocks. 651
With respect to thermal resistance for the majority of indices types 2 and 3 methods are preferable to type 1 methods (Fig. 5). With the same efficiency of a high-temperature hot-blast stove it is possible to obtain a higher blast temperature (since addition of cold blast to the hot may be eliminated), to ease operating control conditions of the stove, and to increase service life of the stove with respect to the heat-resistance conditions of the refractories as a result of decreasing the number of operating cycles of the stove. In addition to the data described here, in tests of the high-temperature hot-blast stoves of Kosogorsk Metallurgical Plant important information was obtained on hydraulics, the operation of burners under the specific conditions of high-temperature hot-blast stoves, pressures of the checkerwork on the walls, and the formation of oxides of nitrogen and many other elements in the combustion products. The block of hot-blast stoves constructed actually has made it possible to decrease the amount of checkerwork and wall refractories used, to obtain high-blast and dome temperatures, and to save considerable coke (in experimental melting the saving in coke was more than 10%). However, there have been revealed significant features of high-temperature hotblast stoves the study of which must continue in order to obtain reliable methods of solution of specific problems in the design-and operation of hot-blast stoves with poured checkerwork. LITERATURE CITED i.
.
.
652
L. N. Toritsyn and O. V. Syroleva, "The theory and practice of thermal operation of metallurgical furnaces," in: Summaries of Papers, V. I. Guvinskii (ed.) [in Russian], Dnepropetrovsk Metal. Inst., Dnepropetrovsk (1988), p. 61. L. N. Toritsyn, "An effective method of calculation of nonsteady heat exchange in a layer,"Deposited in the All-Union Institute for Scientific and Technical Information, Sverdlovsk, May 16, 1989, No. 3252-V89. F. R. Shklyar, L. N. Toritsyn, and E. D. Lekomtseva, Ogneupory, No. 3, 15-18 (1989).
