Research and application of new control strategy for ball mill

Abstract: The research and application information of new control strategies for ball mills is provided by excellent flowmeter and flowmeter manufacturers. Introduction In our country's small and medium thermal power plants, the use of ball mill pulverizing system is very common, and this system is another important adjustment system in addition to the boiler combustion system in the power plant. The task of the pulverizing system is to grind coal with a small amount of limestone through steel balls. For more flowmeter manufacturers to select models and price quotations, you are welcome to inquire. The following is the details of the research and application of new control strategies for ball mills. Introduction In our country's small and medium thermal power plants, the ball mill pulverizing system is widely used, and this system is another important adjustment system in addition to the boiler combustion system in the power plant. The task of the pulverizing system is to grind the coal containing a small amount of limestone through the steel ball coal mill to make pulverized coal with a certain fineness, and dry, transport and separate it by the boiler wind, and finally store the pulverized coal into the silo. The pulverizing system includes raw coal bunker, vibrating coal feeding control equipment, coarse and fine powder separation equipment, ball mill, powder silo, auger powder feeding gate valve, powder discharge machine inlet control valve, ball mill hot air, cold air, isolation, recirculation control valve, tertiary air It is a relatively large air-powder mixing system. The installed capacity of the ball mill pulverizing system is large (the power of the ball mill alone is generally several hundred kilowatts). Due to the use of the intermediate storage system, the system operates intermittently and starts and stops frequently, so the disturbance to the boiler combustion system is large. For the pulverizing system, how to maintain the maximum output of the coal mill so as to minimize the power consumption per ton of coal is the goal pursued by the system control. But because the ball mill is a nonlinear system with serious multi-variable coupling, it is difficult to achieve the control goal by using the conventional control scheme. This paper takes the control of the ball mill pulverizing system of a thermal power station in Nantong as an example, and based on the analysis of the dynamic performance of the ball mill, an effective control strategy is proposed. The control strategy adopts the soft sensing technology to obtain the grinding load information, and realizes the control of the grinding load and the outlet temperature of the ball mill through the coordinated controller. The negative pressure at the inlet of the ball mill can be controlled by controlling the ratio of the amount of hot and cold air. The controller adopts the fuzzy PID control algorithm, which uses the fuzzy control to realize the effective control of the dynamic process, and uses the PID control to ensure the steady-state accuracy of the system. The actual operation results show that the control strategy can achieve satisfactory control effect. 1. Analysis of the dynamic characteristics of the ball mill The core equipment of the ball mill pulverizing system is the ball mill. The ball mill itself is a complex equipment including mechanical energy conversion and heat exchange. Figure 1 is a schematic diagram of the input/output of the ball mill. The input of the ball mill generally has the amount of cold air Fr, the amount of hot air Fh and the amount of coal feeding Fc. The controlled quantities have 5 inlet negative pressure P, outlet temperature T and wear load L. The significance of controlling the negative pressure at the inlet of the ball mill is to make the whole ball mill in a negative pressure state to prevent excessive air leakage of the pulverized coal and the ball mill. The outlet temperature of the ball mill refers to the temperature of the air-powder mixture at the outlet of the ball mill, which is an important parameter reflecting the drying output of the ball mill and preventing the deflagration or explosion of pulverized coal. The grinding load reflects the amount of coal stored in the drum during milling. During the operation of the ball mill, by adjusting the amount of coal feeding, hot air and cold air, the amount of coal stored in the drum of the ball mill (that is, the grinding load), the outlet temperature of the coal mill, and the inlet negative pressure are at the optimum values ​​respectively. nearby, so that the ball mill runs efficiently. The relationship between the motor power PD of the ball mill, the pressure difference between the inlet and outlet ΔP, the grinding output force Q and the coal storage amount N in the coal mill [4] is shown in Figure 2. In zone I of the characteristic curve, the power consumption of the ball mill is large and the output is small; zone III is easy to block coal; zone II, the motor power is not the largest, but the output of the ball mill is the largest, which is an ideal working area. It can also be seen from the characteristic curve that the pressure difference between the inlet and outlet can indirectly reflect the grinding load. From the above analysis, it can be concluded that the ball mill has the dynamic characteristics of multi-capacity high-order, large pure delay, and coupling between parameters. The structure of its mathematical model can be approximately expressed as where K is the gain; τ is the time constant; t is the pure lag time. From the time of the step response, the response of the negative pressure at the inlet of the coal mill is the fastest, and the response of the differential pressure at the inlet and outlet is the slowest, and the time ratio reaches several tens. The speed of these time responses is reflected in the time constant in the formula. The above formula is obtained by linearization under certain conditions. In fact, the dynamic characteristics of the ball mill are variable, which is mainly manifested in the difference between the characteristics of self-balancing ability and non-self-balancing ability and the difference in the length of signal delay time. The pressure difference between the inlet and outlet of the ball mill has the characteristics of self-balancing ability within the range of the maximum value of the amount of powder in the hot air, while it shows the characteristics of no self-balancing ability when it is outside the maximum value. The delay of inlet negative pressure and outlet temperature is much smaller than that of differential pressure. This variability of the dynamic characteristics of the ball mill greatly increases the control difficulty. In summary,The reason why the ball mill is difficult to control can be attributed to the following 4 points. a. The system has multi-variable strong coupling characteristics. For example, the coupling between the inlet negative pressure p of the ball mill and the outlet temperature T is very serious. b. The signal is difficult to measure. Grinding load is difficult to measure directly. At present, the pressure difference ΔP between the inlet and outlet of the coal mill is usually used to approximately reflect the load of the ball mill. However, the differential pressure signal is affected by the air volume in the cylinder, and the delay is large, so it cannot correctly reflect the amount of coal stored in time.

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