Detection and control scheme for the material level in the feed hopper of the J-CONTROL pellet ring cooler

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Detection and control scheme for the material level in the feed hopper of the pellet ring cooler. Pellets are an important raw material for iron production, and the main methods for producing pellets include shaft furnace pellets, belt-type pellets, and chain grate-rotary kiln-ring cooler pellets (hereinafter referred to as CHRC pellets). At present, shaft furnace pellets and belt-type pellets have been largely replaced by CHRC pellets in China. As shown in the figure below: 1. The necessity of detecting and controlling the material level in the feed hopper of the ring cooler. The function of the ring cooler is to cool the finished pellets, which have a temperature of around 1000°C at the exit of the rotary kiln, down to around 100°C, before transporting them to the finished product system. The working principle is as follows. The fixed screen in the high-temperature finished pellet plant is used to remove oversized lumps; the qualified material passing through the screen enters the feed hopper of the ring cooler, where it forms a pile. Below this pile lies a flat layer of material. The finished pellets are spread out on the trolley of the ring cooler using this flat layer of material, and then cooled by the ring cooler fans. After cooling, they are discharged from the ring cooler through the discharge chute, and subsequently transported to the finished product system via a plate feeder. Figure 1 shows a simple schematic of this process. Figure 1: Schematic diagram of the material level detection process in the feed hopper of the ring cooler. During production, if the stack of material is too high and exceeds the barrier, the material will spill over this barrier; as a result, the layer of material on the table of the ring cooler will be too thick and uneven, which affects the cooling efficiency of the ring cooler and makes it difficult to cool the material to the desired temperature ； If a material column is not formed, the flat material mass cannot come into contact with the material, and thus it cannot perform the function of leveling the material. The material on the tables of the ring cooler will also not be level, and in some areas air leakage may occur, which similarly affects the cooling efficiency of the ring cooler. Therefore, the height of the material column should be maintained within a certain range. Therefore, it is necessary to monitor the height of the material column; continuous measurement is preferred, and at the very least, measurements of the upper and lower material levels should be in place. 2. Methods for detecting the material level in the feed hopper of the ring cooler. The temperature of the material in the feed hopper of the ring cooler is around 1000°C, and the temperature of the recirculated air is also close to 1000°C. In addition, there is about 200 mm of fire-resistant insulating material present. There is also a fixed sieve at the discharge point of the rotary kiln; contact-based measurement methods such as guided wave radar, radio frequency admittance, and weight hammers cannot withstand such high temperatures ； Non-contact measurement methods such as ultrasonic, radar, and laser level gauges allow for external cooling measures to be applied, but it is difficult to find a suitable installation location that can avoid both the fixed screens and the material itself ； The structure of the feed hopper itself also makes it impossible to use a weighing method. Currently, some pellet plants abroad use high-temperature thermal cameras or high-temperature cameras to monitor the material level, and one plant in China has also adopted such systems. This method is an optical imaging technique that uses optical lenses on-site to convert thermal images into electrical signals, which are then transmitted to a centralized control room to provide visual reference information for operators. A high-temperature thermal imager monitors the temperature of a certain area, which can indirectly indicate the level of the material bin; however, its output consists of temperature values at several specific locations, and there is no linear relationship between these values and the bin level, making it difficult to establish a closed-loop control system with the speed of the ring cooler. After studying the drawings of the ring cooler equipment, conducting on-site inspections of the pellets production area, and undergoing thorough analysis, it was determined that, in addition to using a high-temperature thermal imager, two microwave level switches can be installed at the feed trough of the ring cooler. When there is no material at those level switches, microwaves are emitted by the transmitter, pass through the refractory layer, and reach the receiver; upon receiving these microwaves, the receiver sends a signal indicating that there is no material present ； Conversely, if the level switch indicates that there is material present, the microwaves penetrate the refractory layer but are blocked by the material and absorbed by it; as a result, the intensity of the microwaves received by the receiver decreases or even disappears, thereby generating a signal indicating the presence of material. The installation location of the level switch is shown in Figure 2. 3. Control and monitoring of the material level in the ring cooler feed hopper: The purpose of controlling the material level in the ring cooler feed hopper is to maintain it within specified upper and lower limits. There are two approaches to controlling the material level ； One is to change the discharge volume, and the other is to change the feed volume. 3.1 Changing the discharge volume can be achieved by adjusting the speed of the ring cooler, thereby altering the discharge volume from its feed hopper. When the material level reaches the feeding level, the ring cooler gradually accelerates and maintains this speed for a certain period of time. It then checks again whether the material level is still at the feeding level; if not, the ring cooler stops accelerating and keeps its current speed until the material level drops to the discharging level ； If it is still at the feeding level, continue to accelerate, maintain the speed, and make judgments until the feeding level condition disappears and normal operation is restored; at that point, the ring cooler will stop accelerating and maintain its current speed until the discharging level condition appears ； When the material level reaches the discharge level, contrary to what happens at the loading level, the ring cooler gradually slows down and remains at this reduced speed for a certain period of time. It is then checked again whether the material level is still at the discharge level; if not, the ring cooler stops slowing down and maintains its speed until the loading level is reached ； If it is still at the discharge level, continue to reduce speed, maintain the current setting, and make assessments until the discharge level condition disappears and the material level returns to normal; at that point, the ring cooler will stop reducing speed and will maintain its current speed until the discharge level condition reappears. 3.2 Changing the feed rate: As can be seen from the above, it seems that controlling the material level in the feed hopper of the ring cooler can be achieved simply by adjusting its speed, without the need to change the feed rate. However, this is not the case; when the thickness of the material layer on the table of the ring cooler remains constant and the air supply volume of the fan used in the ring cooler stays unchanged, changes in the speed of the ring cooler will directly affect the discharge temperature of the material from it. If the speed of the ring cooler is too high, the discharge temperature rises; once it exceeds a certain level, it can damage the conveyor belt used for transporting the finished ore ； Conversely, if the speed of the ring cooler is too low and the discharge temperature is too low, the capacity of the ring cooler becomes excessive and production decreases; the equipment’s capacity is not fully utilized, resulting in waste of investment and energy. Therefore, the speed of the ring cooler can only be adjusted within a certain range. After the speed of the ring cooler is adjusted to the upper or lower limit of this range, if the material level in the feeder of the ring cooler cannot return to the normal range within a short period of time, it is necessary to adjust the feeding amount to the feeder. The equipment upstream of the ring cooler includes a pelletizing dosing feeder, a pelletizer, transfer belts, swing conveyors, wide belts, multi-roller spreaders, grate kilns, rotary kilns, etc. To change the feeding amount from the feed hopper, it is necessary to adjust the feeding amount of the pelletizing dosing feeder. Since the feeding amount of each pelletizing dosing machine is determined through pelletizing experiments, it should not be changed arbitrarily; the total feeding amount of all pelletizing dosing machines can only be adjusted by increasing or decreasing the number of pelletizing machines in operation. Furthermore, there are many pieces of equipment between the pelletizing metering feeder and the feed hopper of the ring cooler, resulting in a certain delay time, which must be taken into account when determining the speed adjustment range of the ring cooler. In summary, the detection and control of the material level in the feed hopper of the ring cooler is an important aspect of the control in the chain-return global ball process, and it plays a crucial role in this process. 4. Introduction to the material level switch for the feeder hopper of the J-CONTROL ring cooler feeder. The J-CONTROL microwave level switch is manufactured by Jike Industrial Control Systems (Tianjin) Co., Ltd., a Sino-foreign joint venture. The product has the following advantages: ① It meets CE certification, SIL3 certification, and ATEX explosion-proof certification ； ②The level switch uses a contactless microwave principle for measurement, with a frequency of 24GHz; its horizontal detection range is 0~110m ； ③The microwave receiver features a 10-segment LED LCD display for easy adjustment of the reception strength ； ④The transmitter and receiver have at least 4 communication channel selection options, ensuring that interference between different communication channels is avoided when they are installed at high, very high, low, and very low positions ； ⑤Microwave switches can penetrate insulating materials such as glass, ceramics, and dry refractory bricks (materials with low electrical conductivity), and by taking advantage of this property of microwaves, they can pass through the walls of insulated containers to detect the level of material inside ； ⑥.The product is suitable for material handling under conditions of heavy loads, high temperatures, and high pressures ； ⑦. The receiver has a delay adjustment function; via the delay knob, a delay setting within the range of 0~10 seconds can be achieved ； ⑧The microwave receiver features a function for adjusting the operating threshold value; through the LED display panel, different threshold values can be set to suit various materials with different dielectric constants ； ⑨Both the microwave receiver and transmitter are powered by a power supply with a wide voltage range of 110~270VAC. The receiver can output N0 or NC signals, and this wide voltage input range helps to prevent the switches from being damaged due to fluctuations in the power supply ； ⑩The housing connecting the transmitter and receiver is made of aluminum alloy, the mounting connectors are made of SUS304 material, while the heads of the receiver and transmitter sections are made of imported PEEK material. 5. Application of J-CONTROL microwave level switch in primary dust removal for dry quenching of coke (1200°C operating condition)