2025 Volume 65 Issue 12 Pages 1840-1849
A blast furnace is a metallurgical reactor that produces liquid hot metal from solid raw materials. The top section of the blast furnace comprises various process influencing parameters such as surface topography, central gas flow width and location, and temperature distribution. The blast furnace operator requires visibility and measurement of these parameters to optimize the furnace performance. This study describes the use of an infrared-based technology called Thermal Hawk in aiding the blast furnace operator to obtain insights into the blast furnace process to optimize the stability of the process. Thermal Hawk comprises a high-resolution uncooled microbolometer housed inside a cooling and purging enclosure. Globally, infrared-based measurement technology has been well-tested at the top cone of the blast furnace to obtain temperature distribution and central gas flow characteristics. In this study, Thermal Hawk has been tested at a novel installation site to continuously monitor various events, from the distribution of the raw materials to the flow of gases through the raw materials in real-time inside the blast furnace. The value propositions of Thermal Hawk are not limited to temperature measurement but also encapsulate numerous possibilities using the real-time video feed inside of the furnace. The feed obtained in real-time aids the blast furnace operator in diverse ways such as capturing abnormal events inside the blast furnace, enhancing process optimization, and enhancing safe operations.
The production of liquid-hot metal through the blast furnace process is the most common route of iron making. A blast furnace is a counter-current reactor involving gas-solid reactions to produce hot metal. At the top of the reactor, the solid raw materials such as coke and ferrous materials, called burden, are spirally charged through the rotating chute, while the hot gases get injected at the bottom. The spiral distribution of the burden inside the furnace, as shown in Fig. 1, forms a unique top surface profile. Repetitive charging of the burden, as per the burden charging matrix, results in maintaining the uniform profile throughout the bed of raw materials. The burden charging matrix comprises the number of rotating chute revolutions assigned to each radial ring for both coke and ferrous burden.1,2,3) The hot gases passing through the burden lead to the gas-solid reaction. The path of the hot gases depends on the burden distribution, primarily centre coke charging, inside the blast furnace. The central column, formed by centre coke, leads to the formation of the central gas flow. The central column also influences the gas-solid reactions, ultimately impacting the reducing gas (carbon monoxide) utilization.4,5,6,7,8) For regular monitoring, the blast furnace operators monitor the top gas composition and the temperature values above the burden surface. The lower the carbon monoxide gas utilisation, the higher the coke rate. Thus, the top burden surface profile, width and location of the central flow of gases exiting the bed, and top surface temperature distribution are key enablers that help the blast furnace operator stably run the furnace.6,7,9,10,11)
Continuous monitoring of the process influencing events enables the blast furnace operator to optimize furnace performance, subsequently impacting CO2 emissions. Among carbon-intensive industries, iron and steel is one of the biggest CO2 emitters, accounting for 7–9% of the global anthropogenic CO2 emissions.12,13,14) With the growing demands to maximize blast furnace efficiency and reduce CO2 emissions, there is a need for process measurement solutions, particularly in the top section of the furnace. This area is critical because the top part of the blast furnace encompasses many process-influencing events, such as burden charging and distribution, central gas flow management, and temperature distribution. Effective visualization in this section helps the blast furnace operators monitor the precise burden discharge and flow of materials, ensuring uniform distribution and reducing the likelihood of process abnormalities and other inefficiencies.15,16) By gaining a clear view of material behavior at the top of the furnace, operators can make real-time adjustments to optimize the burden distribution inside the furnace. The tuned burden distribution ensures better utilization of the reducing gas, resulting in improved overall efficiency and minimized CO2 emissions.
Various process measurement systems have been developed for the top part of the blast furnace, providing blast furnace operators with crucial insights into internal conditions. These systems include radar and laser-based burden surface analysers, trajectory measurement probes, stock-level monitoring sensors, ultrasonic gas thermometry, infrared-based furnace top cameras, and above-burden probes.15,16,17,18,19,20,21,22) The burden surface scanning technologies, such as radar and laser-based analysers, measure the profile of the top burden surface, offering data on material distribution and contour within the furnace walls. It maps the topography of the burden surface, allowing for tuning the burden distribution adjustments.9,17,18) During high ambient humidity, the incoming burden carries moisture, which vaporises in the presence of heat and generates steam. The conditions of high steam and heavy dust make it difficult for the radar and laser-based measurement systems to measure the top burden profile. The high-wavelength radar waves limit resolution while measuring the top burden profile in the blast furnace. The reliability of laser-based scanners in mapping centre coke is questionable as the lasers often fail to penetrate the central gas flow. The stock-level monitoring sensor tracks the level of burden within the blast furnace to ensure optimal loading and prevent overfilling, providing real-time data on the burden levels.16) The gas thermometry measurement system monitors gas temperature inside the furnace- providing critical information about gas-solid reactions.16) The Above Burden Probe (ABP) collects temperature, gas composition, and pressure data above the burden, providing necessary information for process oversight and optimization.22) Overall, each of these measurement systems contributes uniquely to the needs of the blast furnace operations. An exhaustive measurement of the internal details of the blast furnace would require the deployment of most, if not all, of these measurement systems. Each measurement system would also bring with it its utility and maintenance requirements to ensure 24/7 availability inside the furnace.
The infrared-based measurement system, however, stands out with its immense capability to offer multiple use cases.22,23,24,25) From the perspective of blast furnace operations, infrared thermography is particularly advantageous. Its ability to provide continuous, non-contact measurements makes it well-suited for the highly variable and harsh conditions typical of blast furnaces. Steel makers, such as ArcelorMittal Group, Nippon Steel, ThyssenKrupp, etc., have installed infrared-based measurement systems in blast furnaces. Huang et al. (2017) mentions the installation of infrared-based measurement at various blast furnaces of the ArcelorMittal group, such as Indiana Harbor, Cleveland, and Tubarão.22,26,27,28,29) The infrared camera system described in the study includes a microbolometer-based thermal camera housed in a specially designed cooling enclosure to withstand extreme conditions within the blast furnace. This non-invasive technique measures the infrared radiation emitted by the burden, translating it into temperature profiles that reveal critical information about the thermal state and gas flow inside the furnace. The measurement system, mounted on the top cone of the blast furnace, provides the temperature distribution inside the furnace and the real-time feed of the top burden surface and the furnace’s inner wall.22) The feed can be further utilized for image processing, which involves analysing visual data to detect patterns, identify irregularities, and monitor changes over time.
Globally, the infrared-based measurement system has been mounted at the top cone to monitor the process-influencing events.20,24,25,26,27) The top cone location offers a 2D view of the top burden surface- restricting observations related to burden tilt, burden slip, raw material scattering, and burden stock level. Many of these systems have failed to provide continuous online measurement. A high-resolution view impacts the granularity of the data captured, influencing the possibility of more use cases. Thus, the need is for a 24/7 high-resolution view of the various process influencing events through the hostile conditions inside the blast furnace. These observations led to an indigenous infrared-based measurement system called Thermal Hawk. This study details the development and commissioning of Thermal Hawk at a new location, the wall of the throat section, on the blast furnace. In the development of Thermal Hawk, extensive field-of-view studies were carried out to optimize installation positioning, resulting in a more detailed and comprehensive view of the blast furnace’s internal environment. The optimal positioning of an infrared camera can capture extensive data in both video and photo formats that can be further quantified using image processing algorithms, as mentioned earlier. The results and discussion section highlights all the process abnormalities and the insights captured using Thermal Hawk during the data acquisition phase post-installation. Some of these insights, also part of this study, are- centre coke scattering, increased burden stock level, and deviated central gas flow.
An infrared-based measurement system utilizes the principle that each body emits infrared radiation depending on its temperature. The measurement system captures the infrared radiation from the object under focus to form the heat map. Similarly, Thermal Hawk, when mounted on the wall of the blast furnace provides the visual heat map of the top burden surface in a blast furnace. The location of the Thermal Hawk is very close to the raw materials bed inside the blast furnace. The infrared sensor is susceptible to high temperatures, high pressure, dust, and steam. Thus, the enablers to a successful commissioning of Thermal Hawk were a) identification of a suitable infrared sensor to capture the process influencing events inside the blast furnace and b) ensuring 24/7 availability of the infrared sensor. Thermal Hawk solves the first barrier by utilising a high-resolution uncooled microbolometer, a type of Focal Plane Array (FPA) infrared sensor that can see through the hostile conditions inside the blast furnace. The microbolometer is a compact array of micro-electrical heat-sensitive resistors integrated into a silicon chip, which uses heat radiation from an object to image it. The essential parameters in choosing this were resolution, frame rate, object temperature range, and form factor.
A custom-designed cooling and dust-purging enclosure is required to ensure 24/7 availability of the uncooled microbolometer. The uncooled microbolometer offers a small form factor, which benefits a lean cooling enclosure design. The enclosure not only protects the microbolometer against the high temperature inside the blast furnace but also ensures no dust accumulation on the surface of the lens. The cooling and purging enclosure used for Thermal Hawk is modular, comprising a purging cap, a camera holding module, a length adjusting module, a nitrogen inlet module, and a gas sealing module. It utilises both nitrogen and water for optimum cooling, thereby ensuring the temperature of the microbolometer is maintained well below the maximum operable temperature range of the microbolometer. Figure 2(a) shows the geometry of the blast furnace box considered for the computational work performed using Ansys Fluent software. The inlet to the blast furnace box comprises the typical blast furnace gas composition. The furnace box is maintained at a pressure which is equivalent to blast furnace top pressure. Thermal Hawk is inserted at the bottom right corner to study the effect of parameters such as design of the purging cap, number of purging channels, and minimum flow rates of nitrogen & water. One such analysis of the effect of dual purging nitrogen channels, as shown in Fig. 2(b), shows a drastic increase in the normalized velocity of nitrogen at the exit of the purging cap, which is essential in avoiding dust accumulation on the lens’ surface. Development of an efficient purging & cooling enclosure involved several computational studies which would be part of a separate study. The developed enclosure has been tested at blast furnaces of varied working volumes across Tata Steel India.
Thermal Hawk can be installed either on the throat section or the top cone of the blast furnace. View from both these locations cover internal details of the furnace during burden charging and post-burden charging. Since infrared based measurement is susceptible to intense dust, a disturbed view is observed when the burden charging is in progress. The burden charging process lasts a few seconds, the disturbed view then transitions to a settled view for the post-burden charging case. Figure 3 compares the view obtained from the two distinct locations taken for two individual blast furnaces post-burden charging. Albeit the top cone location offers a complete view of the top burden surface, it limits the use cases, providing monitoring of the central gas flow and rotating chute. Qualitatively, the throat section location offers a horizontal view of the burden surface, above burden probes, rotating chute, and falling raw materials. Thus, the throat section location adds to the use cases offered by the top section location, providing the details related to the centre coke heap, burden surface level, burden distribution, and falling raw material trajectory. For this reason, the throat section location was considered for Thermal Hawk’s installation. The selection of the appropriate microbolometer involved fulfilment of three needs- 1) Object temperature measurement and Field of View, 2) 100% availability, and 3) Quantitative analysis. The first need was easily met with the available microbolometers. However, it took a few iterations to select the appropriate microbolometer based on resolution, frame rate, and form-factor. High resolution and high frame rate are key enablers in quantitative analysis. While a small form-factor is an enabler for 100% availability of the system. The finalization of microbolometer involved trade-offs among these parameters.
The specific installation site of Thermal Hawk on the throat section of the blast furnaces depends on the field-of-view of the infrared sensor, the blast furnace proper drawing, the thickness of the refractory material, and the burden stock level. The objective is to identify a site that provides a view of all the major process-influencing events, such as burden distribution, central gas flow, and temperature distribution. The output of the field-of-view study includes- the angle of insertion and insertion length of the measurement system. The final assembly utilizes a guide pipe, which is welded to the wall of the throat section of the blast furnace. The guide pipe is a permanent part of the blast furnace wall, as shown in Fig. 4.
The first successful Thermal Hawk was commissioned at a Blast Furnace of 1578 m3 working volume in 2023 and has been running online since then. The success led to the rollout of the measurement system to nine blast furnaces of Tata Steel India in the same calendar year. A commissioned Thermal Hawk provides both- a complete visualisation of the top surface inside the blast furnace and a thermographic video feed of the internal details of the blast furnace top. Figure 5 shows the feed obtained using Thermal Hawk at the throat section of the blast furnace. The feed is far superior to the feed obtained using a Charged Coupled Device (CCD) camera when placed at a similar position inside the blast furnace. The high-resolution feed provides a real-time view of different entities inside the blast furnace, such as the top burden surface, central gas flow, centre coke heap, stock rod, and Above Burden Probes (ABP), as shown in Fig. 5. Thus, the throat section of the blast furnace provides an unobscured view of the various process-influencing events- burden distribution, central gas flow, and temperature distribution. Most importantly, the cooling and purging enclosure ensures that the blast furnace operator gets access to a 24/7 view of the internal details of the blast furnace.
Ever since Thermal Hawk’s first pilot trial, the measurement system has been significant in generating insights into the blast furnace process that has led to improved blast furnace process- resulting in a reduction in the fuel rates and subsequent CO2 savings. The different ways in which the Thermal Hawk feed empowered the daily operations of the blast furnace operator have been mentioned in the section below. Please note that the coke rate is a key performance indicator of the blast furnace and is affected by several input parameters and process-related interventions. For this study, we have considered the normalized monthly average values of the coke rate, focusing on the period before and after the installation of Thermal Hawk.
3.1. Elimination of Centre Coke ScatteringDuring the pilot testing of Thermal Hawk at Blast Furnace of 1578 m3 working volume, the blast furnace operator captured an abnormal influence of the Above Burden Probe (ABP) on the centre coke charging stream. Usually, a blast furnace employs two to four ABPs, located diametrically opposite, for the temperature measurement above the burden surface. The measurement probe starts at the blast furnace wall and ends near the centre of the furnace for the radial capture of the temperature values. This results in interference with the trajectory of the incoming burden. The scattering of the incoming centre coke stream led to an abnormal distribution of the centre coke on the top burden surface, resulting in subsequent deviation in the central gas flow.
The centre coke is primarily responsible for maintaining a uniform central gas flow. The burden distribution process parameters were tuned using Thermal Hawk’s feed to ensure a uniform central gas flow. The first intervention included adjustments such as a decrease in the rotating chute angle, an increase in plunger opening, and a decrease in charging cycle time. These adjustments led to a reduction in the influence of the centre coke scattering on the fuel rate. The unprecedented insight was further analysed using the Discrete Element Method (DEM) simulations. The DEM analysis led to the second intervention, which involved adjustment in the length of one of the ABPs such that the gap- between the two ABPs- was maintained at 1200 mm.30) The reduced ABP length eliminated the centre coke scattering, as shown in Fig. 6. The smooth flow of the centre coke material led to an increase in the height of the centre coke heap, which led to a high percentage of the unutilized reducing gas leaving the burden surface. Subsequently, optimization of the percentage of centre coke to control the unutilized reducing gas led to better process stability, resulting in a reduction in the monthly average coke rate and an increase in the monthly average production rate, as shown in Fig. 7. The elimination of centre coke scattering helped realize the best-ever monthly average coke rate values and savings in CO2 emissions for the blast furnace. Thus, the application case establishes Thermal Hawk’s capability to empower the blast furnace operations in avoiding process deviations by providing real-time visibility.
The burden stock level inside the blast furnace is maintained using either a stock radar sensor or a stock rod sensor. Both measurement systems offer only the quantified data to the blast furnace operator without any visualization. The regular measurements using burden stock level sensors help maintain the burden level at the operating level inside the blast furnace. Thermal Hawk led to two extreme insights- when the burden stock level was higher and lower than the operating level. The operating level of burden is close to the ABP location inside the blast furnace. A subsequent increase in the operating burden level leads to the burden surface either contacting the ABP or submerging the ABP, as shown in Fig. 8. The submersion of the ABP inside the burden surface has led to frequent damage to the probe- incurring additional expenditures because of the frequent replacements. During the data acquisition in one of the blast furnaces, having working volume of 594 m3, the burden level was below the operating level- an opportunity to improve the furnace’s productivity, as shown in Fig. 9. The blast furnace operator gradually increased the burden stock level while monitoring the reducing gas utilization. The increase in the burden stock level led to the utilization of the unused volume of the furnace, thereby increasing the productivity of the furnace. The furnace realized a decrease in its normalized yearly average coke rate by 17% and an increase in the normalized yearly average production rate by 10% (excluding the furnace relining period), as shown in Fig. 10. In both scenarios, the stock radar sensor, which is the conventional measurement system, reported burden level at the operating height. Therefore, Thermal Hawk played an instrumental role in providing crucial insights into burden stock level monitoring- benefitting the blast furnace process by reducing the number of ABP replacements and reducing fuel rate, respectively.
The insights generated using Thermal Hawk are deployable horizontally to other blast furnaces. Another blast furnace, having a working volume of 3230 m3, saw the implementation of both the elimination of centre coke scattering and an increase in the burden stock level. The length of both ABPs was reduced by 1000 mm and the rotating chute angle was decreased by one degree. The unobstructed flow of the centre coke led to uniform heap formation at the centre of the top burden surface. To optimize the reducing gas utilization, the centre coke heap’s width & height were the controlling factors. The width of the heap was reduced by altering the burden charging matrix to discharge the centre coke in two chute revolutions. The height of the heap was optimized by reducing the percentage of input centre coke. Without top burden surface visibility, the burden level was assumed to be close to the rotating chute, thus eliminating any possibilities of a further increase in the level. Thermal Hawk’s real-time view ensured the burden level was sufficient distance below the chute. The burden stock level was increased by 0.5 meters. The utilization of the unused volume inside the furnace led to improved furnace productivity. However, the increase in burden level altered the landing points of the falling burden materials. The ferrous burden material landing near the wall rolled towards the centre coke heap. The real-time qualitative feed showcased a tilted top burden surface inside the blast furnace, as shown in Fig. 11. Process adjustments in parameters such as pellet discharge delay and burden matrix led to a flat top burden surface profile. Thermal Hawk-aided tuning of the blast furnace parameters positively influenced reducing gas utilization, resulting in significant improvement in the coke rate, as shown in Fig. 12.
The Above Burden Probe, as the name suggests, is mounted just above the top burden surface inside the blast furnace. The primary purpose of ABP is to measure the temperature above the top burden surface. Thermal Hawk, having an infrared sensor at its core, betters the temperature measurement proposition of ABP. It provides temperature distribution of the complete top burden surface. Figure 13 shows the layout of temperature measurement points for Thermal Hawk and ABP across the furnace diameter. The daily average top burden thermal profile obtained using Thermal Hawk, as shown in Fig. 14(a), was performed using measurement points across the furnace diameter. The number and direction of the measurement points are flexible. Only ten measurement points were available to capture the top burden profile using ABP. These ten points were not mapping the central region of the blast furnace. Also, since the ABP measures the temperature above the top burden surface, there is a decrease in temperature values. Thus, Thermal Hawk, in the true sense, estimates the top burden thermal profile.
The top burden thermal profile using Thermal Hawk is measured continuously, even when the burden material discharges through the rotating chute. Figure 14(b) captures the deviation in the hourly and daily average temperature values for points at three locations inside the furnace captured over an hour. The high standard deviation observed for the centre location is because of the burden distribution process. The centre coke material enters the furnace at ambient temperature. Its temperature gradually increases when heated by the hot ascending gases. Thus, the central region of the blast furnace, in comparison to the peripheral, operates in a bigger band of temperature values because of the cyclical burden distribution, as captured by the standard deviation. Also, the non-invasive feature of Thermal Hawk mitigates involvement with the incoming burden material from the rotating chute. Decommissioning ABP would ensure a uniform burden distribution as it would eliminate interference with burden charging. Thus, the non-invasive infrared measurement system offers a unique proposition to replace the ABP inside the blast furnace. The only point of caution is ensuring the timely calibration of the infrared sensor, usually every three years.
3.5. Safe Shutdown OperationThe measurement system has played a pivotal role in empowering the blast furnace operations during the shutdown and start-up phases of the blast furnace. During the shutdown, Thermal Hawk feed assists the blast furnace operator in initiating the top firing and its sustenance until the shutdown is complete, as shown in Fig. 15. Top firing ensures venting of the top gas for maintenance activities planned at the blast furnace top during the shutdown period. The shutdown feed from Thermal Hawk also provides insights into the condition of cooling staves. Figure 15 shows the ferrous burden material deposited on the cooling staves, which influences the heat losses through the wall. In case the deposited material peels off during furnace operation, it may damage the cooling staves as well. This insight helps the blast furnace operation team to act against the deposition and prevent process abnormalities. Hence, the Thermal Hawk feed not only ensures safe shutdown & start-up of the furnace but also empowers the furnace operator to tune the burden distribution using the insights obtained during the shutdown.
In summary, Thermal Hawk acts in a closed-loop feedback system with blast furnace operations. The real-time insights using Thermal Hawk enable blast furnace operators to take early action in stabilising the blast furnace, averting potential losses in coke rate. The economic benefits and the substantial savings in CO2 emissions realized due to these interventions make a strong case for Thermal Hawk’s installation at the throat section of the blast furnace. Thermal Hawk represents a significant advancement in industrial monitoring technology, offering enhanced performance, reliability, and environmental benefits in challenging operational conditions. Thus, such a measurement system has immense potential across blast furnaces of different sizes and is pivotal for sustainable iron making.
In this study, Thermal Hawk, an infrared-based measurement system, was developed and tested at the throat section of the blast furnace to monitor the processes influencing events inside the top section of the blast furnace. The main conclusions drawn from this study are as follows:
a. The throat section location offers continuous monitoring of burden distribution, falling burden trajectory, central gas flow, and temperature distribution.
b. An infrared-based measurement system, installed at the throat section of the blast furnace, offers a unique proposition to extract qualitative and quantitative results from its feed compared to other measurement systems- laser or radar-based.
c. The real-time Thermal Hawk feed comprises a view of the top burden surface, centre coke heap, central gas flow, Above Burden Probes, and rotating chute. Further, image processing techniques, such as object detection, can be operated on the Thermal Hawk feed to generate the quantitative output continuously.
d. The measurement system positively influences the daily operations of the blast furnace operator based on the diverse number of unprecedented insights it generates on a real-time basis. The role of such a technology is to ensure an optimum performance of the blast furnace process and to alert the operator in case of any deviation from the optimum.
e. The insights generated into the process- capturing abnormal events inside the blast furnace such as centre coke scattering, and increased burden stock level; enhancing process optimization such as tuning the burden distribution; and enhancing safe operations such as during furnace shutdown and start-up- empower the operator in taking early action to rapidly restore the blast furnace stability, averting losses in the fuel rate.
The authors declare no conflicts of interest regarding this manuscript.
