Application of laser surface detection technology in blast furnace gas ﬂ ow control and optimization

: The gas ﬂ ow distribution inside a blast furnace a ﬀ ects its smelting. Laser surface detection technology can control the gas ﬂ ow distribution and promote the stable and smooth running of blast furnace by observing and adjusting the surface condition in the blast furnace in real-time. The laser surface inspection technology in blast furnace gas ﬂ ow control and optimization was introduced, with Masteel 4,000 m 3 blast furnace as an example. The results showed that the 30 W high-power laser had good penetration performance and could obtain clear scanning images of the material surface. In the process of strengthening smelting, the laser detection system determined that the central air ﬂ ow of blast furnace was insu ﬃ cient, resulting in a decrease in furnace condition stability, and promptly took measures to adjust the angle of the mineral coke fabric equipment and lifting line. After adjusting the material line, the central air ﬂ ow was properly suppressed and stabilized, the angle of edge platform increased, the air ﬂ ow developed properly, and two suitable gas ﬂ ows were formed, which ﬁ nally stabilized the furnace condition. Meanwhile, the small duct air ﬂ ow other than the central air ﬂ ow was found in time through the laser detection system, which provided conditions for operators to take measures to e ﬀ ectively eliminate the duct air ﬂ ow, thereby avoiding big impacts on the blast furnace operation and technical and economic indexes. Therefore, the laser surface detection technology could e ﬀ ectively judge the gas ﬂ ow distribution in the furnace, which contributed to timely adjustment of process parameters and optimization of blast furnace operation.


Introduction
Blast furnace is a counterflow closed system with hightemperature, high-pressure, and high-speed gas flow.It takes only 3-8 s for the gas to be generated and discharged from the top of furnace, while the gas temperature is reduced from about 2,000°C to about 200°C [1][2][3][4].The main task of blast furnace operation is to maintain and control the gas flow distribution to avoid gas flow chaos [5].Although the blast furnace gas flow distribution is affected or controlled by many factors, the top fabric is the most common and effective way to regulate and control the gas flow in the furnace [6,7].At present, most blast furnaces in the process of adjusting the fabric matrix are "guessing the melon from the skin," which will cause the fabric matrix adjustment basis to be insufficient, not very high accuracy, and sometimes even the adjustment to be reversed, resulting in furnace condition fluctuations or malfunctions.In order to open the black box, domestic and foreign researchers have developed infrared scanner, microwave scanner, phased array radar, and many other new material surface detection devices, and have made some progress [8][9][10].However, these devices do not achieve the goal of real-time online inspection of furnace surface condition, nor do they provide a correct basis for blast furnace manipulation and adjustment.
An online laser inspection system is developed to symmetrically scan the real-time charge surface of the production blast furnace and perform laser video and image acquisition to establish a morphological model of the charge buildup in the furnace, which can provide visual image data for daily blast furnace operation adjustments [11][12][13].Currently, there are two major types of methods to achieve laser ranging: time-of-flight (TOF) ranging and non-TOF ranging [14,15].Among TOF ranging are the pulsed laser ranging and phased laser ranging.Non-TOF ranging is mainly triangular laser ranging.The phased TOF is more accurate in close range measurements and can be used in a whole line of sensors because it does not require time measurement circuitry and the circuit design is relatively simple.However, the phase TOF cannot distinguish whether the actual distance is within one or more measurement periods, and is therefore not suitable for long distance measurements.The maximum vertical and oblique distance between the top charge of the blast furnace leaving the chute and the material surface of the furnace throat is less than 100 m.Therefore, it is more reasonable to use the phase TOF laser distance measurement system.In addition, laser scanning detection of the material surface can display the shape of coke distribution in blast furnace.And by adjusting the shape of the material surface, the distribution of the furnace material can be adjusted, thereby controlling the radial gas flow distribution in the furnace, etc.However, there are still many problems that need to be explored in this technology.Although laser scanning technology has been widely used in blast furnace throat surface detection, most of them are in the stage of patent and commercial secrecy, and there are not many publicly published related papers.
In this work, a laser surface shape detection device was developed to meet the requirements of blast furnace working conditions, and to realize the special laser scanning device to project a certain power of laser into the blast furnace.At the same time, a special camera was used to capture the image of laser movement on the material surface, so that operators can observe the shape of material surface inside the furnace in real time during the blast furnace production, achieving the goal of opening the black box of blast furnace.Through the accurate detection of material surface shapes, it facilitated the blast furnace production and technicians to adjust the blast furnace fabric in time, so as to achieve more accurate and timely control of the blast furnace gas flow distribution and promote a more stable and smooth running of blast furnace.

Laser system installation and commissioning 2.1 Laser installation location openings
When the blast furnace is repaired, a hole is made in the top of the furnace for installation.The laser scanning device is installed approximately 300 mm down from the folding point of blast furnace shell.A 150 mm diameter hole is drilled horizontally using a hole opener and a short flange tube is welded to install the laser scanning device.In this study, a 150 mm diameter hole is drilled on the east and west side of the Masteel 4,000 m 3 blast furnace.

Laser camera system installation
The working environment inside the blast furnace is very harsh, which is a high-temperature and high-pressure smelting environment, with a high-speed airflow speed of up to 5-8 m•s −1 .In addition, high-speed gas flow can also carry a large amount of dust, and the dust content can reach 100-140 g•m −3 [16].Therefore, the laser and laser camera systems must undergo cooling and head blowing cleaning to ensure that the front of the quartz glass does not adhere to dust, to maintain the laser intensity and camera clarity.Figure 1(a) and (b) show the laser camera system and the appearance of furnace body, where the power of laser is maintained at 3-6 W. Furthermore, the measuring distance can be expressed as follows: where L is the measured distance, c is the speed at which light travels through the air, T is the period time of the modulated signal, and ϕ is the phase difference between the transmitted and received waveform.And the schematic diagram of phase laser ranging method is shown in Figure 1(c).However, this technology cannot be described in too much detail due to numerous confidentiality restrictions.

Laser and camera system equipment commissioning
The installation and commissioning of the data acquisition software were completed, and clear scanned images of the material surface during production were obtained, as shown in Figure 2. Abnormal conditions such as material surface offset and piping were found during the operation of the system, as shown in Figure 2(b), indicating that the application of laser system can detect abnormal conditions of the material surface inside the blast furnace in real time.
In the process of using the system, we found that we could get clear images of the material surface shape when the blast furnace was finished with ore, as shown in Figure 2(c), but we could not get clear images of the material surface shape after the coke was finished, as shown in Figure 2(d).There were two main reasons for this phenomenon.First, when the central air flow was squeezed by the  charge, the temperature of furnace top space was relatively low, and the brightness of camera in the furnace was low, the impact of laser on the furnace was very obvious.But after the cloth coke, the center airflow temperature was very high, and the brightness inside the furnace was also very high due to the influence of center airflow, resulting in difficulty to identify the laser in the furnace.Second, the coke used on site was mostly selfproduced dry quenched coke, so more soot was generated inside the furnace after the coke was laid.The main reason for this problem was that the power of laser used was not sufficient and the laser intensity could not meet the demand of 4,000 m 3 blast furnace production.To address this shortcoming, a new high-power laser emitter with adjustable output power was developed to meet the needs of on-site material surface inspection in very large blast furnaces.The new high-power laser had an adjustable laser output power, and the maximum power could reach 30 W, which was six times larger than the initial installation of 5 W laser, with greater penetration capability to ensure the use effect.As a result of these two improvements, the laser surface scanning system is now significantly more effective and provides a clear image of the material surface, as shown in Figure 3.As can be seen, clear images can be obtained in the blast furnace after laying out the ore and laying out the focus.
The use of high-power laser generators could obtain clear images, but it was found that as the laser power increased, the heat generation of the laser also increased dramatically, and the requirements for the cooling system also increased dramatically.Especially after entering summer, the output power of the laser decreased as the ambient temperature rose.By enhancing the power of thermo electric cooler (laser control and transmission) system and increasing the cooling capacity of laser, the laser was able to operate normally even when the external ambient temperature elevated in summer.In addition, the laser housing was newly designed and the thickness of laser base plate was increased, which was a substantial enhancement of the thermal conductivity of the laser and correspondingly improved the cooling efficiency of the laser, and the temperature of the laser generator during normal operation was reduced from 30 to 25°C to meet the requirements of the working temperature of the laser.After the cooling upgrade of high-power laser, the overall effect of the system was improved, and both laser scanning and imaging systems could obtain qualified scanning images.
3 Application in blast furnace gas flow control and optimization

Enhanced smelting adjustment of blast furnace
In order to strengthen the blast furnace smelting, the target air volume is 6,600 m³•min −1 and the oxygen enrichment is 13,000 m³•h −1 .The full coke load is adjusted on the blast furnace site by increasing the load by 0.05, and the adjusted full coke load is 4.50.Due to the adjustment of full coke load, the stability of blast furnace condition decreases, the central airflow is not stable enough, and the differential pressure increases from 170 kPa to a high level of 177 kPa.The air volume also decreases from nearly 6,600 to 6,550 m³•min −1 .
A comparison of parameters before and after the specific adjustment is shown in Table 1.
After the adjustment of the full coke load causes the decrease in furnace condition stability, the online laser detection system is used for material surface inspection.It can be seen from Figure 4 that the central air column is narrow, the slope is gentle, and the depth of central funnel is about 1.5 m.It is judged that the stability of furnace condition is reduced due to insufficient airflow in the center.
Due to the insufficient airflow in the center, the Z value drops significantly, and the air volume gradually decreases due to the increase in pressure difference in the furnace.In order to cope with the change in furnace condition, combined with the results of material surface scanning analysis, material system adjustment measures are taken, and simultaneously the angle of mineral coke fabric equipment is raised by 0.3°.Besides, in order to maintain the suitability of two gas streams, the material line is adjusted at the same time, and the material line is decreased from 1.50 to 1.45 m.The whole material system adjustment is shown in Table 2.The change in ore coke drop point after the angle and material line adjustment is shown in Table 3.

Laser measurement of material surface data change detection
Before and after the adjustment of the fabric system, the online laser detection system is used to scan the real-time material surface of the production blast furnace symmetrically to obtain the real-time material surface shape, which intuitively reflects the change in material accumulation shape in the furnace.It can be found from Figure 5 that after the cloth angle adjustment, the central air column is obviously widened, the slope is increased, the right platform is appropriately narrowed, the funnel depth is increased to nearly 2 m, the central airflow develops obviously, and the edge platform angle tends to be flat and the stability is enhanced, which achieves the expected effect of the material regime adjustment.The material line is adjusted so that the central airflow is properly suppressed and stabilized.The edge platform angle increases and the airflow develops appropriately, forming two suitable gas flows, as shown in Figure 6.It Application of laser surface detection technology in blast furnace  5 can be seen that the laser material level scanning system can give a good real-time situation of blast furnace material level adjustment.

Validation of the adjusted effect
The change in airflow in each ring band of cross temperature measurement after material system adjustment is shown in Figures 7-10.It can be seen that after the synchronous outward movement of angles from 4 to 11, the central airflow is obviously strengthened, and the average temperature of central four points of the crosstemperature measurement rises gradually from 275 to 350°C, and the Z-value also rises gradually from below 6.5 before the adjustment to above 7.0.The edge of airflow does not change significantly, the edge of four-point temperature fluctuates between 50 and 100°C, with the overall stability of the furnace and small fluctuations.W value is also maintained at about 0.41.The cross temperature  measurement of 2-5 ring temperature does not change significantly.The material system adjustment achieves the goal of strengthening the central airflow, and the edge airflow is not significantly suppressed.This corresponds to the results of increasing funnel depth and developing central airflow in the laser-detected material surface.
In order to properly maintain the two gas streams, the edge airflow is properly loosened and the feed line operation is performed.After the material line adjustment, the central airflow is properly suppressed, and the average temperature of central four points decreases to about 325°C.The Z value is maintained in the range of 7.0-7.5 due to the decrease in top temperature.The marginal airflow is not properly developed for a short time, and the average temperature of marginal four points gradually goes down to about 65°C and then gradually goes up to 80°C.The effect of material line adjustment is not obvious in the short time, and the edge airflow develops gradually after 24 h to form suitable two gas streams.
Table 4 shows the changes in some indicators of the blast furnace after the material system adjustment.It can be seen that after the adjustment of the material system, the short-term effect is not obvious, and the differential pressure is still maintained at a high level of 178 kPa.After 24 h of action, the central airflow is strengthened while maintaining proper marginal airflow, and Z/W gradually returns to about 20.The differential pressure inside the furnace gradually goes down, and the air volume gradually stabilizes from 6,500 to 6,600 m³•min −1 platform.The economic and technical indicators of the blast furnace are also improved, with the run-rate maintained at a high level of over 14,600 t and the fuel ratio at a good level of 495 kg•t −1 .By adjustment of the material system the improvement of the gas flow adjustment target and index has been achieved, and also, the good guidance of the line laser detection system for the blast furnace operation has been well verified.

Application of laser material surface system in the blast furnace gas flow anomaly
Between April 5 and April 10, 2017, the internal material surface of the blast furnace showed a small duct airflow in the southeast direction, as shown in Figure 11, which shows that the blast furnace operated normally on April 5 and the fabric was very regular; however, the results of the laser scan on April 6 showed that the central airflow was obviously strong, and at the same time, airflow appeared on the fabric slope in the southeast direction of the furnace throat, where it was the only airflow other than the center, and it was obvious that this is the airflow caused by a small duct, which may be due to a strong central airflow and a weak edge airflow, and also due to a high differential pressure control.After this anomaly was detected by the laser scanning system, the blast furnace operation immediately took countermeasures by pushing the cloth angle inward by 0.3°, while lifting the material line by 100 mm and controlling the differential pressure appropriately to cope with it.Due to the timely detection, the blast furnace only took more "gentle" operational measures to successfully eliminate the pipeline, the emergence of the blast furnace small pipeline, the blast furnace operation, and technical and economic indicators did not cause a large impact.However, without a laser surface shape measurement system, it would be difficult to accurately detect the problem and take appropriate countermeasures in a timely manner, and the stability of the blast furnace may fluctuate.This is because once a large blast furnace has a pipeline, it will be more difficult to eliminate, and sometimes even   requires sitting treatment, affecting the stable smooth operation of the blast furnace and deteriorating the techno-economic indexes [16][17][18].Therefore, the laser material surface detection system is applied on the blast furnace, which can effectively judge the distribution of gas flow in the furnace, timely adjust the process technical parameters, and optimize the blast furnace operation.

Conclusion
(1) The laser material surface scanning system could better monitor the internal material surface of the blast furnace.
For the 4,000 m 3 large blast furnace, a high-power laser generator was needed to eliminate the influence of dust on the image clarity in the fabric process.(2) The stability of blast furnace decreased due to the increase in full coke load in the process of strengthening smelting of Ma Steel blast furnace, which was found to be caused by insufficient airflow in the center of the furnace through the analysis of laser surface scanning system.(3) In the case of abnormal gas flow in the blast furnace, the laser charge surface scanning system promptly detected the small duct airflow other than the central airflow, and the blast furnace operator promptly took control measures, pushing the angle of ore distribution inward by 0.3°, while lifting the charge line by 100 mm and controlling the differential pressure appropriately, which effectively eliminated the duct airflow and avoided the unstable condition of the blast furnace.(4) The blast furnace charge surface laser scanning system was applied on the blast furnace, which could monitor the change in charge surface and gas flow in the blast furnace in real time, discover the abnormal situation on time, provide the operation basis for blast furnace operators, and provide technical support for the stable and smooth operation of blast furnaces and maintain good technical and economic indexes.

Figure 1 :
Figure 1: (a) and (b) Laser camera system composition and installation pictures and (c) principle diagram of phase laser ranging method.

Figure 2 :
Figure 2: Laser scanning images of the material surface inside the blast furnace.(a) The material surface image during normal production; (b) the material surface image at the time of piping on the southeast side of the material surface; (c) the material surface image after laying ore; and (d) the material surface image after laying coke.

Figure 3 :
Figure 3: Improved laser scan image of the furnace: (a) image of material surface shape after laying and (b) image of material surface shape after distributing coke.

Table 1 :Figure 4 :
Figure 4: Surface detection by online laser detection system.

Figure 5 :
Figure 5: Comparison of material surface shapes before (a) and after (b) angle adjustment.

Figure 6 :Figure 7 :Figure 8 :
Figure 6: Comparison of material surface shapes before (a) and after (b) material line adjustment.

3 Figure 10 :
Figure 10: Variation trend of Z and W values.Table 4: Index changes after material system adjustment Date Running ore volume (t) Fuel ratio (kg/t) Total coke load Air volume (m³•min −1

Table 2 :
Material system adjustment process

Table 3 :
Changes in coke drop point before and after adjustment of angle and feed line

Table 4 :
Index changes after material system adjustment Date Running ore volume (t) Fuel ratio (kg/t) Total coke load Air volume (m³•min −1