1 Introduction Coal blending is a large-scale industrial production method. According to different coal-fired uses, artificially created coal is the most ideal and economical coal that natural coal does not possess. Blended coal is a blended coal made up of several different kinds and types of coal blended in a certain proportion. Although it has some characteristics of its component coal, its comprehensive performance has changed, and it is actually a new "coal species" processed by man. The basic principle of power blending coal is to use the differences in nature of various coals and to “complement each other” to give full play to the advantages of each coal type, and finally to achieve the “best performance” of the blended coal to meet the requirements of the user. .
For coal for power generation of industrial power generation boilers, if the temperature of the coal's heat is too high, the equipment will be wasted; once the temperature of the heat is not up to the requirements, the working efficiency of the equipment will not go up. For such considerations, some low-quality coal that does not meet the design requirements of the power plant and high-quality coal whose heat output exceeds the design requirements of the power plant are mixed to achieve the best combustion effect with the requirements closest to the design index. It is "power blending coal". The goal of power blending is to mix some low-quality coal with some high-quality coal to achieve the required configuration. For example, in the southeastern coastal areas, due to the large power supply demand of industrially developed power plants, but due to the low calorific power and low calorific power of imported Indonesian coal, if it can be matched with the high sulfur and high heat in the interior, it can achieve both heat increase and sulfur reduction. The purpose of the points. Because of the cheap price of imported coal and stable supply, it will be very competitive in terms of price. In addition, from the coal utilization point of view, the coal blending project meets the development policy of clean coal, adopts a refined and fully automatic control and discriminating system, and the quality of the product blended out is also more stable and easier to control.
2 coal blending system design Power blending coal was proposed very early, but the technology is still relatively extensive. With the marketization of coal prices, coal production plants and clients have begun direct contact, and many companies have begun to engage in the coal blending industry. However, most companies in China are adopting extensive coal blending methods and are not sophisticated enough. With low technical content, the quality and stability of coal blended are poor. The heterogeneity of coal determines that sampling is relatively difficult and the error of compounding is relatively large.
2.1 Process principle
Industrial coal blending and coal blending usually uses a multi-bind coal blending system. According to the set ratio, the blending control is achieved so that the coal in the coal bin is mixed together. The system equipment includes 6 silos, and different coals are loaded into different silos. Each silo can hold 9,000 tons of coal. According to the requirements of users and different coal sources, a suitable coal blending scheme is proposed and realized through PLC automation technology. The coal blending ratio is controlled accurately and satisfies the user's requirements. The equipment can mix the coal in each silo in a set proportion to make the coal quality meet the industrial requirements. The key to automatic coal blending is the control of coal output in each coal bunker. The automated coal blending system based on Emerson CT's Commander SK series variable frequency drive is shown in Figure 1.
(1) System requirements: The silo coal output is 160T/h~1600T/h, and the corresponding motor speed is 3~50HZ.
(2) Driver parameters: SK6402 is a vector control driver with a rated voltage of 380~480V (±10%) and a rated output current of 210A. It can operate at 150% overload current for 60 seconds, rated power at 110KW, and output frequency at 0~1500Hz.
(3) Motor parameters: rated power 90KW; 3P; rated voltage 400V; rated current 95.3A; rated speed 1475RPM; power factor 0.89.
2.2 Electronic Control System
The electronic control system is integrated by the variable frequency drive and the PLC system. There is a scraping knife at the bottom of the silo. The position of the scraping coal cutter by two symmetrically rotating deceleration drive gears is symmetrical. The two motors are driven by a 110-kw Commander SK series variable-frequency drive SK6402, and the SK6402 is controlled by a PLC. Emerson CT variable frequency drive is used to drive the scraper motor at the bottom of the silo. The rotating scraper discharges the silo coal out of the silo and controls the coal output by adjusting the speed of the coal storage scraper blade. Each silo contains coal with different requirements. When the scraper rotates, the coal in the silo will be discharged from the coal outlet. After being mixed on the same conveyor belt, the finished coal will be sent directly to the cargo ship for export.
Fig.1 Schematic diagram of the mixer
2.3 Application Design of SK6402 Variable Frequency Drives Emerson CT is a world-famous manufacturer of direct AC drives and is part of the Emerson Electric Group. The SK6402 110/132kW AC400V 3Phase variable frequency drive belongs to the Commander SK series of Emerson CT's latest AC high-performance vector control drive products. SK6402 is equipped with the appropriate options and integrated to complete the frequency conversion control, saving design and operating costs. Its features are: high-performance vector control drive; 150% continuous overload for 60 seconds; auto-tuning of the complete motor model where the motor shaft does not rotate; the IP54 LED keyboard can be used for remote control; the built-in filter can be disconnected and recovered Can be used for closed-loop control, built-in lifting macro; optional and SP series variable-frequency drive common, SMARTSTICK can make your debugging more flexible; voltage 220V and 380V optional, power range 0.25kW-110kW range.
Commander SK series variable frequency drive SK6402 variable frequency drive electrical principle shown in Figure 2. Because the variable frequency drive drives two motors at the same time, when setting the parameters, it should be noted that the motor current needs to be set as the sum of the currents of the two motors, that is, 190.6A. Instead of the vector control, the V/F control mode is used instead.
Figure 2 SK6402 principle wiring diagram
2.4 Low speed and high torque frequency conversion system debugging
(1) Stall failure phenomenon. When the coal blender system is operating, the coal is discharged from the silo by a scraper. Due to the uneven size of the coal, or due to the weather caused by wet conditions, the blade force is not uniform, which leads to unstable motor load, the current fluctuations in the range of 90 ~ 120A, when the motor is running at low speed (3HZ below), If the load is too large, the scraper will get stuck and the driver output current will be too high. The driver will automatically limit the current and reduce the current. The state of the current is detected by the software CTscope. The waveform of the stall current monitoring is shown in Figure 3. At this time, when the scraper is stuck and the motor cannot rotate, the maximum output current of the driver can reach 250A. After that, the driver will limit the current output and when it drops to 0A, it will increase the current output again. When this happens, if left unchecked, the driver will display “OVL.d” and accumulate the overload current until the “It.AC” over-current alarm fault occurs, causing the system to stop production.
Figure 3 Waveforms Monitored When Motor Stalls
(2) Analysis of reasons for blocking. Because we use the V/F control mode to drive the motor, when the motor runs at a low frequency, the output voltage of the driver is relatively low, which is affected by the voltage drop caused by the resistance of the stator winding of the motor and it cannot provide enough torque output to the motor.
Figure 4 VF curve of the driver output
(3) Solve the problem of stalling. In view of this situation, we need to increase the voltage boost to compensate for the starting torque. The specific parameter settings can be determined by monitoring the current and motor speed waveforms to select a better curve. The relationship between driver output voltage, frequency, and voltage boost is shown in Figure 4. After adjusting the voltage boost value, the system can operate normally at low frequency (2HZ). Figure 5 shows the monitored current and motor speed curve. The current is stable between 90~120A, and the speed curve is stable, achieving the intended purpose.
Figure 5 Waveforms Monitored During Normal Operation
3 Conclusion The system requires the motor to operate at low frequency for a long time. This requirement applies to the SK6402's ability to output the rated torque at low frequency. The SK series driver can drive the motor to reach the maximum torque within the range of 1HZ to the rated frequency of the motor. We can also monitor the status of the drive through monitoring software CTsoft to ensure optimal system commissioning. The commissioned coal mixer equipment has greatly improved the production efficiency. The coal output rate is 1600T/h during normal operation. Manpower resources are saved and all operations can be controlled in the central control room. The finished product coal is evenly mixed and the coal quality is excellent.
