44 technical article - news of temperature & moisture … distribution in a blast furnace is ......

Technical ArticleM

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This article is available online at AIST.org for 30 days following publication.

Blast Furnace Above-Burden Infrared Camera

A newly developed on-line Above-Burden Infrared Camera can

monitor all the activities and events above the burden in blast furnaces.

Through it, three important phenomena were discovered;

actual burden distributions can be observed and optimized, and the operators will gain advance warning of adverse conditions

occurring in their furnaces.

Authors

D. (Frank) Huangscientist, process research, ArcelorMittal Global R&D – East Chicago, East Chicago, Ind., [email protected]

Marcelo Andrade manager, raw materials and ironmaking, ArcelorMittal Global R&D – East Chicago, East Chicago, Ind., [email protected]

David Whitedirector, process research, ArcelorMittal Global R&D – East Chicago, East Chicago, Ind., [email protected]

Don Zukeprocess engineer, ArcelorMittal Indiana Harbor, East Chicago, Ind., [email protected]

James Bobekoperations manager, ArcelorMittal Indiana Harbor, East Chicago, Ind., [email protected]

Osama Hassensenior engineer — steel producing quality, ArcelorMittal Cleveland, Cleveland, Ohio, [email protected]

Phillip Pergioperations manager, iron producing, ArcelorMittal Cleveland, Cleveland, Ohio, [email protected]

Bruce StackhouseArcelorMittal Cleveland, Cleveland, Ohio, [email protected]

Charles WalpoleArcelorMittal Cleveland, Cleveland, Ohio, [email protected]

Jonathan Maudeproject engineer, iron producing, ArcelorMittal Cleveland, Cleveland, Ohio, [email protected]

Converting iron ore into hot metal via an ironmaking blast furnace

consumes approximately 70% of the total energy for producing the shipped steel. The keys to increasing the fuel efficiency, productivity, and cooling stave life are being focused on high gas utilization, low heat loss and process stability. Gas utilization, heat loss, and permeability of the burden can be improved through modifying or refining the burden distribution (including redistribu-tion) and tuyere operating condi-tions in addition to continuously improving the burden quality. If the burden distribution, burden redis-tribution, uneven burden descent, gas distribution, gas channels, and all the events above the burden top can be monitored and visualized on-line and in real time, blast furnace operators will be able to detect the adverse conditions occurring in their furnaces and take proper actions in time to maximize the performance of their furnaces. Traditionally, the gas distribution in a blast furnace is monitored by above-burden1,2 and in-burden probes.3 Recently, a two-dimensional (2D) acoustic top gas temperature measurement device4 has been available as well. The probes and 2D acoustic gas tem-perature detector could only report the top gas temperature profile, and possibly gas composition profile. They are not able to monitor and help visualize the other phenomena that are also very important to blast furnace operators to operate their furnaces. Thus, there is strong inter-est among blast furnace operators to have a robust on-line and real-time visualization system to detect and

record these phenomena plus the ability to measure the 2D burden top temperatures. In principle, this mission could be well-performed through a high-quality infrared camera along with a proper lens and protection system. The protec-tion system was already designed.5 Using a proper infrared camera and the type of lens required per the protection system design,5 an infra-red monitoring system had been developed at ArcelorMittal Global R&D – East Chicago and it has been successfully implemented since June 2012. Because this system is based on an infrared camera that is installed inside the dome and above the burden top of a blast furnace, following the same logical name of above-burden probe, it is called the Above-Burden Infrared Camera (ABirC). In this paper, the applica-tions of the ABirC system will be presented along with the discoveries and achievements via its applica-tions after a short review of infrared cameras and the earlier efforts of using infrared thermal-viewer in the blast furnace tops.

Infrared Camera and Blast Furnace Burden Top Thermal Graphing

Infrared was discovered in 1800 by Sir William Herschel.6 Developments of the infrared camera spanned four generations. The first-generation infrared camera was a two-dimen-sional mechanical scanner based on a detector of a single element or multiple elements.7 The second-generation infrared camera was built upon vector detectors of 64

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or more elements.8 Introducing this type of vector detector, the two-dimensional scanning was reduced to one dimension (usually in vertical direction). The third-generation infrared camera consisted of two-dimensional arrays with several columns of elements. This type of infrared camera was still scanning in one direction. It performed a time delay integration (TDI)9 of the signal in the scanning direction for improving the signal-to-noise ratio. Fourth-generation infrared cameras are using focal plane array (FPA) as a sensor.10 An FPA is a two-dimensional array detec-tor. It works like the “digital film” in the common digital camera. After using an FPA sensor, the fourth-generation infrared cameras take the infrared images without any scanning. There two types of focal plane arrays: cooled11 and uncooled.12,13 In this paper, all the infrared cameras with the scanning components will be called infrared scanners and those using FPAs called infrared cameras. In 1947, the first infrared scanner took 1 hour to produce a single image (1 hour per frame).6 By the late 1970s, after the continuously improving the sensors and the mechanical scanners (Fig. 1),14 this speed was increased to 1.5 seconds per frame of the 200 × 200 points.15 Now using uncooled FPAs (Fig. 2), the infrared cameras have been able to take thermal images at the frequency of 60 frames per second (60 HZ) with the resolution of 320 × 240, 640 × 480 pixels, or even higher.

In the mid-1970s, Nippon Steel14 and NKK15 in Japan and ARVED Belval Works16 in Luxembourg had installed infrared scanners on their blast fur-naces. By the end of the 1970s, Nippon Steel had rolled out their infrared scanner systems to about a dozen blast furnaces inside and outside Japan.14 Similar infrared scanner systems were also installed at ArcelorMittal’s Indiana Harbor blast furnace No. 7 in 1980 and Tubarão blast furnace No. 1 in 1983. Due to the limited frequency (frame/second) and

low image resolutions, the infrared scanner systems designed for blast furnace use were in the status of

“normal off.” The large gate valve at the front of the infrared scanner would be opened and the system would be activated for taking two-dimensional tem-peratures of the burden top after the burden dump was completed and the burden top descended to the target stockline. The application of infrared scanner systems in blast furnaces was discontinued after the middle or late 1980s because of the high system cost, heavy maintenance and limited information for blast furnace operators. Infrared cameras with uncooled FPAs have been commercialized since 1995. In 2004, thyssenkrupp installed and commissioned a system based on this type of infrared camera on the top of Schwelgern No. 1 blast furnace.17 The camera was installed in a large port that was attached to the blast furnace dome from the outside. It has been used mainly to measure the two-dimensional temperature of the burden top. After a cycle of burden dumps were completed and the dust settled, the large gate valve was opened and the infrared camera was activated to take thermal images of the burden top for a short moment. In principle, the infrared camera system at Schwelgern No. 1 blast furnace is on-line but not monitoring the burden top continuously.

The Above-Burden Infrared Camera (ABirC)

The spectral range of the infrared camera for the ABirC system is from 7.5 to 13 μm. With this range, the infrared camera will be able to detect the objec-tive surface temperature from –20°C to 1,200°C or even higher. This measurement range is sufficient for measuring the top temperatures of the burden inside any blast furnace when its blast is on or off. In the ArcelorMittal Group, the first ABirC was piloted

A typical structure of infrared camera based on two-dimensional scanning.13

Figure 1

The appearance of a typical focal plane array, unclouded microbolometer (available in market).

Figure 2

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at ArcelorMittal Indiana Harbor blast furnace No. 7 (IH7) and it was commissioned successfully in June 2012. This system at IH7 has been running on-line and continuously since then. After the successful implementation of ABirC at IH7, this system was rolled out to Cleveland blast furnace No. 6 (C6) in November 2013 and Tubarão blast furnace No. 1 (AMT BF 1) in October 2014. The ABirC system will be installed in more blast furnaces of ArcelorMittal, such as ArcelorMittal Ostrava. Fig. 3 shows the appear-ances of the ABirCs at the tops of the three blast fur-naces and an example location for the infrared cam-era inside the blast furnace dome. From the location of the ABirC shown in Fig. 3d, the camera protection

device (or housing) remains inside the blast furnace dome and the camera window is open to the burden top all the time. Therefore, in addition to measur-ing the two-dimensional tempera-ture profiles of the burden top, the installed ABirC systems have been monitoring all possible phe-nomena from the burden top on-line and continuously. The heavy-duty housing was developed spe-cially for this application. Except the wearing parts, the housing should never need replacing. The ABirC system had kept the cam-era lens clean and view window fully opened for more than 9 months continuously without any maintenance (even after the blast furnace kicked seriously during this period) when the purging gas (nitrogen) and cooling water had been running at the required

flowrates all the time. Fig. 4 shows the layout of the ABirC system. This system can be accessed remotely via intranet and internet. Authorized users can view and operate the system from their offices or any place in the world.

The current main display of ABirC system is shown in Fig. 5. The real-time thermal image of the burden top is shown on the top left; the trend of average tem-perature of burden top or top gas (if it contains cer-tain amount of dust) within the white circle is plotted as the red line on the top right; and the temperature profile along a diameter of the throat (the white line) is displayed in the chart on the bottom of this figure. The white line simulates two above-burden probes

The appearances of the Above-Burden Infrared Camera (ABirC) system and the location of the infrared camera: ABirC at IH7 (a), ABirC at C6 (b), ABirC at ArcelorMittal Tubarão BF1 (c) and the typical location of the ABirC (d).

Figure 3

(a) (b) (c) (d)

Layout of the ABirC system (1 = switch: optical/Cat_6; 2 = check valve; 3 = infrared camera; 4 = camera housing).

Figure 4

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(ABPs). These two virtual probes, 180° apart, reach the center of the blast furnace all the time no matter how hot the central gas flow is. The concerns of over-heating and bending of above-burden probes would no longer exist if ABP were replaced by ABirC. Users can add as many white lines as they wish across any radial diameter of the throat to display the tempera-ture profile along those diameters. This indicates that the purpose of above-burden temperature probes can be fulfilled by ABirC as one of its features.

Fig. 6 shows the average temperatures of four uptakes that were measured by the thermocouples in them during the same time period as that shown in the chart of average top temperature of Fig. 5. The mean value of the curve of the averaged top tempera-ture (the red line) in Fig. 5 is about 120°C, and the mean value of uptake temperatures (the red line) in Fig. 6 is 110°C. Comparing these two red lines, it is found that the top temperature measured by the infrared camera inside the dome of a blast furnace is higher than the uptake temperature by approximately

10°C. The difference is mainly due to the heat loss through the dome and uptakes. The more important difference between these two red lines is that red line in Fig. 5 is significantly affected by the burden discharges and the red line in Fig. 6 is not. Blast furnace burden materials consist of coke, iron ores, fluxes, etc. These materials contain a certain amount of moisture (except the dry-quenched fresh coke) and their temperatures are lower than that of top gas. When charging these materials onto the top of the burden in a blast furnace, the top temperature will decrease due to a portion of its heat being

absorbed by the newly charged burdens for heating them and for vaporizing their moistures. Along the red line of Fig. 5, the peaks show the top temperatures when the burden has descended to the target stockline and before receiving new burden; the valleys show the top temperatures when a new burden discharge was completed. The infrared camera inside the dome detects the top temperature changes clearly because it measures the object surface temperature with almost zero response time. However, the thermocouples in the uptakes are hidden inside the protection sheathes, which are usually covered by a layer of deposit of very low thermal conductivity. Therefore, the uptake tem-perature barely responds to the burden charges.

Blast furnaces are equipped with burden top water spray systems. When the top temperature is over the upper limit, the water injection system will spray water into the blast furnace dome to cool down the top gas to avoid the blast furnace top from being overheated. When the deposit layer on the thermocouple sheath grows to certain thickness, the uptake thermocouples would not respond to the changes of top temperature quick enough. This potential issue may cause the blast furnace top being overheated or the burden top water being oversprayed. Overspraying burden top water would cause burden shooting. This phenomenon will be presented in the later section of this paper. ABirC system will report the top temperature changes imme-diately and issues of overheating and overspraying could be eliminated.

Two-Dimensional Temperatures From Blast Furnace Burden Top

The essential goal of ABirC system is to measure, on-line and continuously, the two-dimensional tempera-ture profiles above the burden inside blast furnaces.

Main display of ABirC.

Figure 5

Average four uptake temperatures measured by the thermocouples.

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With this type of information, the size, shape and location of the chimney, i.e., central gas flow, width of wall gas flow and gas channels, can be monitored at real time. Fig. 7 shows the different chimney sizes (the shapes are torch-like with the colors of white, red, etc.) of a blast furnace. The chimney sizes change from large to none. Too large or too small of a chimney would not give a low fuel rate nor stabil-ity to a blast furnace. For most large blast furnaces, especially those with pellets as their major burden, a stable medium-size chimney should be maintained. The typical chimney shapes observed are shown in Fig. 8. There are solid, hollow, and “6” or “G” shapes. In order to maintain the stable central gas flow, a solid cone chimney should be established. In addition to the size and shape, ABirC has also been monitor-ing the chimney location. Fig. 9 shows the different chimney positions observed. If the chimney is shifted too far away from the center, the blast furnace would not be stable, and if the blast furnace has multiple

tapholes, each taphole might have different drain-ing behaviors. The wall gas flow, gas channel in the region between the sidewall and central gas flow are shown in Fig. 10a. A serious wall gas channel is shown in Fig. 10b.

Burden-Shooting and Coke-Spraying

Through the ABirC systems, three new phenomena — coke-surging, coke-spraying and burden-shooting — have been discovered. Coke-surging will be presented in a later section. Fig. 11 shows the phenomenon of coke-spraying observed along with the chute and cen-tral gas flow. The spots of light blue and green colors are the coke particles that were sprayed out from the coke chimney. The droplets or spots of blue, light blue, and pink color with the tails are the coke par-ticles that fell back to the burden top after they were sprayed out of the coke chimney and flew upward for

Size of the central gas flow: large chimney (a), medium chimney (b), small chimney (c), no clear chimney (d).

Figure 7

(a) (b)

(c) (d)

Observed locations of central gas flow: not at center (a) and at center (b).

Figure 9

(a) (b)

Shape of the central gas flow: solid shape (a), hollow or donut shape (b), “6” shape (c), “G” shape (d).

Figure 8

(a) (b)

(c) (d)

Example of wall gas flow and gas channels: wall gas flow, gas channel and central gas (a) and central gas flow and serious wall gas channel (b).

Figure 10

(a) (b)

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a short time. This phenomenon occurred when wet coke was dumped to or rolled down to hot central gas flow. The wet coke was heated rapidly by the hot central gas flow and the moisture within it was vapor-ized quickly. This sudden gas expansion pushed the coke off from the central coke pile and caused the coke-spraying. Since coke-spraying pushes coke away from the coke central pile, when iron ore is dumped on top of the coke layer, the iron ore may roll down to the center of the coke layer, depending on the height of the remaining central coke pile. This would close up the coke chimney and seal off the central gas flow. When the central gas flow is reduced to the level that could not cause coke-spraying, the wet coke would remain at the center to build the chimney, then cause the coke-spraying again. This issue would result in a cycle of change in coke chimney size, from large to small and from small to large, and eventually in unsta-ble blast furnace operation. The solution for avoiding the coke-spraying is to set an upper limit for coke moisture, which is dependent on the temperature and strength of central gas flow of a given blast furnace.

When the burden meets a strong gas flow with a tem-perature higher than 150°C or 200°C, the moisture in the burden would be vaporized very rapidly. If the water vapor could not find sufficient space to release, it would build up pressure in the burden column and eventually shoot out the local burden. This phenom-enon is called burden-shooting. Burden-shootings are caused by sudden moisture vaporization at a certain depth under the burden inside a blast furnace. These issues happen in winter when the burden top temper-ature is extremely low or when the burden top water is oversprayed. Fig. 12 shows the burden-shooting that occurred inside a blast furnace the night of 16 December 2013. Before the burden-shooting, the top temperature was very low for quite a while. The top

temperature trend recorded by the ABirC system was between 50°C and 70°C. When the top gas tempera-ture is much lower than the water boiling point under the top pressure, a certain portion of moisture would remain in the burden until it descends to a certain depth. At that depth, the wet burden may meet the hot gas flow or gas channels and the burden-shoot-ing occurs. According to the highest temperature recorded in Fig. 12, the burden-shooting happened at the elevation where the burden had been heated to about 150°C. Fig. 13 shows the serious burden-shooting after burden top water-spraying for reducing its top temperature. Fig. 14 shows an even more severe burden-shooting after burden top water-spraying. In blast furnaces, the top water sprays are opened and closed by the uptake temperatures. Due to the long response time of the uptake thermocouples to the

Coke-spraying observed.

Figure 11

Burden-shooting occurred when the top temperature was too low (50–70°C) for a while.

Figure 12

Burden-shooting occurred after burden top water-spraying.

Figure 13

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top gas temperatures (comparing Fig. 6 versus Fig. 5), when the measured uptake temperatures dropped down to the setpoint, the burden top water had been oversprayed. According to the highest temperatures recorded in Figs. 13 and 14, the two burden-shootings occurred at the elevations where the burden had

been heated to 250°C and 330°C, respectively. Using top temperatures reported by the ABirC to control the burden top water spray might help to avoid these issues. Burden-shooting would mix coke layers together with iron ore layers. When the mixed burden descends to the cohesive zone, there would be no coke slots. This would seriously reduce the permeability of the burden column and might cause burden hang-ing and slipping. The severe burden-shooting could knock off the refractory materials (usually shotcrete) attached to the blast furnace dome.

Burden Discharging Flowrate and Shapes of Falling Streams

Fig. 15 shows that the width of the burden falling stream changed with the chute revolutions. During the first revolution (Fig. 15a), the width of the falling stream was much smaller than that of the fourth reso-lution (Fig. 15d). The second and third revolutions (Figs. 15b and 15c) were in between. The width of the burden falling stream is proportional to the bur-den flowrate. The changes in stream falling with the chute revolution indicate that the burden discharging

Severe burden-shooting occurred after burden top water-spraying.

Figure 14

The widths of burden (pellets) falling stream. The part at the top of each infrared image with the “H” shape is the tip of the rotating chute. The circle of red, yellow, green and light blue is the central gas flow. The rod at the 8 o’clock position is one of the above-burden probes. The two short nozzles are the burden top water sprayers. The blue stream from the chute tips are the burden falling streams.

Figure 15

(a) (b) (c) (d)

The shapes of burden falling stream: front view of falling stream when the chute rotates clockwise (a), back view of falling stream when the chute rotates clockwise (b), front view of falling stream when the chute rotates counterclockwise (c) and back view of falling stream when the chute rotates counterclockwise (d).

Figure 16

(a) (b) (c) (d)

Shape of the conical spring.

Figure 17

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flowrate changes as the chute rotates. Even though this issue might not happen to every discharge, in order to have a more accurate burden distribution, the setpoints for the ring practices should be based on the weight percentages of the burden batch instead of the revolution numbers or the percentage of dis-charging time. Therefore, installing and maintaining properly working load cells in the discharge hoppers would benefit the performance of a blast furnace significantly.

Fig. 16 shows the shapes of the burden (pellets) falling streams observed from a blast furnace. Similar to Fig. 15, the blue streams from the chute tips are the burden falling streams. From the recorded live videos, the shapes of falling streams can be clearly observed, but in order to better illustrate the falling streams, their borders are marked by two brown lines on both sides. Figs. 16a and 16b show the same falling stream of the same revolution with a clockwise chute rotation, and Figs. 16c and 16d show another stream of the same revolution with a counterclockwise chute rotation. From the four images in Fig. 16, when look-ing at the falling streams from their fronts (Figs. 16a and 16c), the upper portion of the falling stream is wider than the lower portion. However, when looking at those streams from the back (Figs. 16b and 16d), the top portions are narrower than the lower portions. Actually, the width of a falling stream is almost consis-tent along its length. After leaving the tip of a chute, the falling stream is twisted. This is the reason why, in the ABirC images, the pictured width of a falling stream is dependent on its view angle.

Instead of a simple downward parabola, the burden falling stream is of a three-dimensional shape. It is a conical spiral of twisted band, resembling a section of conical spring (Fig. 17), but the round wire would be replaced by a band that was twisted by 50° to 80° in the first quarter of pitch. The cross-section of the falling stream is a semi-circle or one-third of a circle. The conical shape of the falling stream is due to the centrifugal force generated by the chute rotation and gravity; the spiral shape due to the effects of chute

rotation and gravity; the twisted shape is due to the burden material being lifted up along the trailing sidewall of the chute caused by the centrifugal force and then the particles at different heights being thrown off the chute tip with different tangential momentum; and the shape of stream cross-section is formed by the chute shape. The actual falling stream shape inside a blast furnace during its full wind opera-tion was observed for the first time and recorded through this ABirC system. This discovery is very important to determine a proper method for the fall-ing curve measurements and a proper falling curve module for a burden distribution model.

Burden Distributions and Redistributions

In addition to a proper falling curve module, the angles of repose of the burden materials and the slope or profile of the burden top inside a blast furnace are also very important to assure a burden distribution model to turn out reasonable results. If a blast furnace is equipped with a burden top profile meter, the bur-den top profile (slope) might be estimated properly. Due to the size of the radar beam, the profile meter would not be able to detect edges of a burden ring for estimating the angles of repose of burden materials inside a blast furnace. In addition to estimating the burden top slope, ABirC system would also be able to estimate the actual angles of repose inside a blast furnace with blast on. Fig. 18 shows the width of a burden ring (with the colors pink and blue in between the sidewall and light blue circle) during the first revolution of a pellet discharging of a blast furnace. Based on this measured width of the burden ring, with the known stockline, burden top profile, bur-den impacting point or range, and associations of a proper burden distribution model, the actual range of repose of the burdens in a blast furnace with blast on could be estimated properly. If the angles of repose of the burden have been given, the burden top slope

The width of the burden ring of the first revolution of pellet discharge.

Figure 18

The ABirC view of the coke layer top immediately prior to discharge pellets.

Figure 19

The ABirC view of burden top after discharge of pellet on the coke layer.

Figure 20

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or the burden descending factors could be estimated through the measured ring width.

Coke-pushing means the coke layer is pushed by the iron ore batch when it is dumped onto the coke layer. It has been well-reported in literature that coke-push-ing is the source of burden redistribution. The ABirC system discovered another source that causes coke layer redistribution. Fig. 19 shows the ABirC view of the coke layer before dumping pellets on it, in which the central gas flow of proper size is presented. Fig. 20 shows the ABirC view of the burden top after dump-ing pellets onto the top of the coke layer. In this figure, the chimney size was increased after a layer of pellet was dumped on top of the coke layer. Observations via ABirC showed that the enlarged portion of the chim-ney was built mainly by the coke that was surged out from the coke layer while the pellets covered it. The spots of the light blue color are also the coke particles that were surged out from the coke layer.

Fig. 21 illustrates the mechanism of coke-surging. The coke layer that sits on the burden top was heated by the top gas and generates water vapor. While the pellets are being discharged, the pellet layer covers the coke layer from the sidewall to the center. When most of the outer area of the coke layer top is covered by the pellet layer (0.3 to 0.4 m thick), due to the low permeability of the pellet layer, a portion of the top gas and moisture vapor in the coke layer would be forced to its central region. The coke in the central region would be surged off when the pressure of the top gas and the steam was built up to a certain level. The coke-surging is another source of burden redis-tribution. Of course the central coke pile built by the coke particles that are pushed by the pellet layer and the surged coke would also contain a certain por-tion of pellet, especially in the edge of the enlarged chimney.

Blast Furnace Blow-Down and Blow-In

On 1 June 2014, IH7 was blown down to replace its copper staves and to repair its hearth sidewall. ABirC had been used for monitoring the entire blown-down process. After the blow-down was completed, the infrared camera lens was still clean. During the blow-down, ABirC at IH7 was on-line and continu-ously monitoring the top water spray, top temperature and the final burden top after blow-down. Since the ABirC system detects the burden top temperature or gas temperature (if the gas contains certain amount of dust) and its changes immediately, unlike the uptake thermocouple, which has a long response time, through ABirC, the top water spray would be adjusted in time and more accurately for preventing (1) blast furnace top from being overheated and (2) avoiding burden-shooting due to water overinjection. Fig. 22 shows the ABirC view when the blow-down

The coke-pushing and coke-surging.

Figure 21

An ABirC view of the end of a blow-down.

Figure 22

An ABirC view of a dry-up.

Figure 23

An ABirC view of a initial filling.

Figure 24

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was completed. The white spots on the edge of the red circle are the exposed raceways at the front of the tuyeres. This indicated the blow-down had been done well. Fig. 23 shows the patterns of the flames from the four burners that were installed for IH7 dry-up. ABirC also observed IH7 inner wall temperature profiles during the dry-up. Fig. 24 shows the initial filling of another blast furnace in November 2013. Usually, a burden distribution model is not designed for the super-low stockline. The ABirC views of the on-line falling streams would help operators to adjust the chute angle in time to avoid throwing burden onto the new sidewall. Building a burden column with preferred layer structures is difficult if too much material is rebounded from the sidewall. Mixing the coke layers together with the iron ore layers may cause some difficulties during blow-in. Also, the refractory materials on the new sidewall might be knocked off by the burden impacts, especially when the refractory material had not been cured properly. Therefore, dur-ing initial filling, the impact of burden materials on the sidewall should be minimized.

Burden Overflow and Spillage

The chute of a bell-less top usually rotates eight revolutions per minute. Due to the centrifugal force effect, the burden material will be lifted up along the chute sidewall of the trailing side to a certain height while the material slides down the chute. If the chute sidewall is not tall enough, there are no stop-plates on the top of its sidewall, the stop-plates are not wide enough, the discharging rate is too fast, or the major portion of burden stream from the spout does not drop down to the center of the chute bottom, the bur-den might be lifted over the chute sidewall and drop off the chute from its trailing sidewall rather than from its discharging tip. This phenomenon is called

the burden overflow in this paper. Fig. 25 shows the burden overflowing from the right side (trailing side) of the chute when the chute was rotating clockwise and the burden material was dumped to the chute from the lock hopper on left-hand side (leading side). Fig. 26 shows a similar scenario when the chute was rotating counterclockwise and receiving the mate-rial from the hopper on right-hand side (leading side). Fig. 27 shows the double falling streams during burden discharging. The major stream was from the discharging tip of the chute, another from the back of the chute. This phenomenon is called burden spillage. In this case, the back falling stream was the material falling off the back of the chute because the back plate of the chute had fallen off (Fig. 28).

After replacing the chute and the center ring above the spout, and reducing the discharging flowrate, the amount of burden overflow was minimized. In some blast furnaces, after a long period of service, the burden impacts might cut a hole through the chute bottom. A portion of the burden would fall through the hole. This would also generate a secondary falling stream from the middle of the chute and result in a similar type of burden spillage. With ABirC on-line, this kind of issue would be detected in time if the ABirC views are properly observed and analyzed.

Improved Blast Furnace Performances

At both C6 and IH7, the ABirC system is being used to monitor the top temperature profile, chimney size and locations, and gas channels; observing the bur-den distribution; and detecting all the other events presented in the preceding sections.

Since June 2012, when the ABirC was commis-sioned successfully at IH7, many important ABirC on-line views have been used to improve IH7 burden distribution and its performances, thereby reducing

Burden overflow when chute rotated clockwise.

Figure 25

Burden overflow when chute rotated counterclockwise.

Figure 26

Double falling streams (burden spillage).

Figure 27

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its hot metal cost. In October 2015, via ABirC system, the burden overflow and spillages were detected on-line. After the burden spillage was solved and the overflow was minimized, in addition to increasing the actual central chimney size, IH7 became more stable and in November and December of 2015 its total fuel rate had made a new historical low in its current cam-paign since 2003.

After ABirC implementation at C6 on 19 November 2013, it did not take long for the ABirC system to have an immediate, positive effect on the performance of C6 blast furnace. After adding nut coke to the bur-den and reducing furnace coke batches in December 2013, the furnace was experiencing upsets and the blast pressure had reached the blower limit of 241 kPa (35 psi). The ABirC view in Fig. 29 shows a hot ring in the mid-radius after the coke of step 4. C6 blast furnace is equipped with a compact bell-less top and it appeared that the coke charged near the wall in step 4 was not reaching the coke charged near the center in step 3. A ring change was made to better spread step 3 coke so that it forms a complete layer with step 4. After the change, the hot ring disappeared, the blast pres-sure dropped 21 kPa (3 psi) and the process became more stable.

Later in December 2013, burden-shooting was observed in the burden center column and it was determined that the probable cause was raw flux being discharged in the center of the furnace. The charging pattern was reversed to discharge it closer to the wall. This modification improved the burden permeability and reduced the hot blast pressure by 14–21 kPa (2–3 psi, Fig. 30).

But the biggest impact came a year and a half later. C6 has been prone to periods of unstable operation since the installation of a compact bell-less top in 1999. It has also been more sensitive to burden mate-rial changes than its double-bell sister furnace, C5. In

a plant that often utilizes up to four different coke suppliers and has limited mixing capability, this is an area of concern. It fell into a prolonged unstable period during May–July 2015 when the furnace was being pushed for maximum production. Natural gas injection had to be reduced and scrap taken out of the furnace burden to provide some stability. A burden distribution model was supplied to C6 in 2014 to help stabilize the furnace, but a model is only as good as the accuracy of the furnace parameters it uses to do the calculation. While falling curves were calculated from data obtained during the initial fill, and angles of repose measured in the raw material storage yard, the center-to-wall (C/W) descent ratio had to be esti-mated. Therefore, any results from the model were questionable. But by using the ABirC, this ratio could be back-calculated. By adjusting the temperature span, it was possible to see that some materials were not reaching the furnace center, as predicted using the estimated C/W ratio. The model was run many times with different ratios until the output matched the observations of the ABirC. The estimated ratio that had been used was 1.8. This ratio had been suc-cessfully used in another furnace’s model. The new C/W descent ratio was 0.8.

With renewed faith in the distribution model, a major change in the charging philosophy was calcu-lated and implemented on 28 July 2015. There was immediate improvement. Over the next few weeks, the natural gas was returned to the normal operating level, and scrap was put back in the burden. The fur-nace is no longer sensitive to burden material changes and the operation remains stable. Compared to May through July 2015, the 3-month period before modify-ing the burden distributions, the average top gas CO utilization from August to October 2015 was increased by about 3% and the equivalent coke rate was reduced accordingly. During these two time periods, the raw

The back plate of the chute missing.

Figure 28

The hot ring after coke of step 4.

Figure 29

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flux additions were almost the same (the difference was only 0.9 kg per metric ton of hot metal).

Conclusions

• The above-burden infrared camera (ABirC) system has been developed and implemented successfully.

• ABirC systems have been monitoring on-line and continuously the two-dimensional top tem-peratures and all the other events occurring from the burden top inside blast furnaces.

• ABirC system works on-line with low mainte-nance. It can work continuously for more than 9 months without maintenance.

• The actual shapes of burden falling stream had been discovered. It is a three-dimensional coni-cal spiral of twisted band, instead of a simple parabola. The width of a falling stream is almost consistent along its length.

• The phenomena of coke-spraying and burden-shooting had been discovered. Burden-shooting

could be caused by overspraying burden top water or too-low top gas temperature. Principle solutions were presented for preventing coke-spraying and burden-shooting.

• Coke-spraying and burden-shooting could result in an unstable blast furnace and lower its performance.

• The phenomenon of coke-surging was discov-ered, which is another source for burden redis-tribution in addition to coke-pushing.

• The ABirC system can detect the width of the burden ring of a single revolution and of an entire dump. These widths can be used to cali-brate the angles of repose of the burden materi-als inside a blast furnace with blast on. For given angles of repose of materials, the width had been used for calibrating the burden descend-ing factor, i.e., the burden top slope.

• The ABirC system responds to the tempera-tures of burden top or top gas immediately. It is a proper system for controlling the burden top water spray for preventing furnace tops from overheating and burden-shooting during the production period or blow-down of blast furnaces.

• The burden discharging flowrate may vary with the chute revolutions. It is better to control bur-den distribution based on the measured burden weight percentage via the lock hopper load cells rather than the discharging time or revolutions for each ring.

• ABirC performs the function of above-burden temperature probes as one of its features. It would save the costs and the maintenance work for ABPs.

• Using the information obtained via the ABirC system, blast furnaces stabilities and fuel rates have been improved significantly.

The hot ring after coke of step 4.

Figure 30

Utilizing the ABirC to calibrate the burden distribution model: model results with center/wall = 1.8 (a); ABirC after step 1 pellets (b); model results with center/wall = 0.8 (c).

Figure 31

(a) (b) (c)

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Acknowledgments

The ABirC system was developed successfully with the support and help provided by many people. The authors would like take this opportunity to thank all of them again, especially P. Chaubal, M. Atkinson, R. Allen (retired), T. Coyle, W. Ference of Global R&D; M. Dutler, S. Trenkinshu, M. Washo, D. Catherman, A. Ables, R. Ferguson, M. Saberniak of IH7; D. Cronin, J. Gromek, G. Lauer of C6; and D. Ruy, F. Gomes and M. Iwamura of ArcelorMittal Tubarão.

References

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2. B.D. Das et al., “Improvement of Gas Utilization of BF #7 of Bhilai Steel Plant Through Utilization of Over Burden Probe,” METEC & 2nd ESTAD 2015, Düsseldorf, 15–19 June 2015.

3. R. Timmer, “Quantitative Description of the Gas Flow Pattern in the Blast Furnace Throat Area,” Technical Steel Research, European Commission, ISSN 1018-5593, Office for Official Publications of the European Communities, Luxembourg, 1997, p. 16.

4. M. Tonteling, M. Brodeck and H. Rausch, “2D Blast Furnace Top Gas Temperature Measurement System — TMT SOMA,” Iron & Steel Technology, Vol. 10, No. 12, 2013, pp. 45–55.

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6. W. Herschel, “Experiments on the Refrangibility of Invisible Rays of the Sum,” Philosophical Transactions of the Royal Society of London, Vol. 90, 1800, pp. 284–292.

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Scan Thermal Imaging,” Infrared Physics, Vol. 22, 1982, pp. 31–42.9. R. Chapman, “Infrared Time Delay With Integration CTD Imager,” U.S.

Patent, No. 4,314,275, 2 February 1982.10. C. Corsi, “Smart Sensors,” Microsystem Technologies, 1995, pp.

149–154.11. A.M. Fowler and D. Bass et al., “Next-Generation in InSb Arrays:

ALADDIN, The 1024 x 1024 InSb Focal Plane Array Readout Evaluation Results,” Proceedings of SPIE, Vol. 2269, 1994.

12. T. Schimert and C. Hanson et al., “Advances in Small Pixel, Large Format a-Si Bolometer Arrays,” Proceedings of SPIE, Vol. 7298, 2009.

13. C. Hanson, “IR Detectors: Amorphous-Silicon Bolometers Could Surpass IR Focal-Plane Technologies,” Laser Focus Word, 1 April 2011.

14. Y. Togino and M. Tateoka et al., “Application of Thermoviewer at Blast Furnace,” ASME Proceedings of Ironmaking Conference, Vol. 38, March 1979, pp. 295–304.

15. M. Iizuka and H. Yoshida et al., “NKK Thermo Camera System for Furnace Top and Its Application to Blast Furnace Operation,” ASME Proceedings of Ironmaking Conference, Vol. 38, March 1979, pp. 313–326.

16. P. Tonteling, “Computer Analyzing of Infrared Blast Furnace Burden Data,” ASME Proceedings of Ironmaking Conference, Vol. 38, March 1979, pp. 327–335.

17. H. Rausch et al., “Investigation of 3D Charging Dissymmetries and Influence on the Blast Furnace Condition,” Technical Steel Research, European Commission, ISSN 1018-5593, Office for Official Publications of the European Communities, Luxembourg, 2006, pp. 110–118. F

This paper was presented at AISTech 2016 — The Iron & Steel Technology Conference and Exposition, Pittsburgh, Pa., USA, and published in the Conference Proceedings.

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