Pilot control strategy of biomass fuel feeding system of circulating fluidized bed boiler

Abstract

The application discloses a PID control strategy of a biomass fuel feeding system of a circulating fluidized bed boiler, and solves the problem of how to scientifically and rationally design the PID control strategy of the biomass fuel feeding system according to the characteristics of the circulating fluidized bed boiler of the biomass fuel; the application reversely judges the change of fuel feeding according to the change of oxygen content of flue gas at the tail of the boiler, that is, under the premise of constant air volume, if the oxygen content increases, it indicates that the biomass fuel feeding amount decreases, and if the oxygen content decreases, it indicates that the biomass fuel feeding amount increases; a PID control module is established with the oxygen content of flue gas at the tail of the boiler as an input value, the frequency of a fuel feeding screw feeder of the circulating fluidized bed boiler of the biomass fuel is regulated by the output value of the PID control module, and scientific and rational automatic control of the fuel feeding screw feeder is realized.

Description

Technical Field

[0001] This invention relates to a biomass circulating fluidized bed boiler, and more particularly to a PID control strategy for the fuel feeding system of a circulating fluidized bed boiler that burns biomass fuel. Background Technology

[0002] Biomass circulating fluidized bed boilers operate by burning biomass fuel, which is characterized by its variety and the blending of different fuels fed into the furnace. The biomass fuel feeding system in these boilers typically employs a screw feeder driven by a variable frequency drive (VFD) motor. Feeding is controlled on-site by adjusting the frequency of the VFD motor. However, fuel jamming frequently occurs during the conveying process. This is usually resolved by increasing the VFD motor's speed or stopping and reversing it. The problem is that the amount of biomass fuel entering the furnace varies, sometimes even intermittently, leading to significant fluctuations in the oxygen content within the furnace. When the oxygen content is low, sulfur dioxide levels in the flue gas increase; conversely, high oxygen levels increase nitrogen oxides. Therefore, designing a scientific and rational PID control strategy for the biomass fuel feeding system based on the characteristics of the circulating fluidized bed boiler, to achieve stable emissions of NOx, CO, and SO2 while maintaining stable thermal efficiency, is a problem that needs to be addressed on-site. Summary of the Invention

[0003] This invention provides a PID control strategy for a biomass fuel feeding system in a circulating fluidized bed boiler, solving the technical problem of how to scientifically and rationally design a PID control strategy for a biomass fuel feeding system based on the characteristics of a circulating fluidized bed boiler.

[0004] The present invention solves the above technical problems through the following technical solutions:

[0005] The overall concept of this invention addresses the limitations of existing metering methods and the fluctuating characteristics of biomass fuel, which prevent the accurate real-time measurement and display of the calorific value of biomass fuel in circulating fluidized bed boilers. This invention collects the oxygen content of the flue gas at the boiler's tail end, using this indicator to display the oxygen content during biomass fuel combustion in the furnace. Changes in the oxygen content of the flue gas at the boiler's tail end are used to infer changes in the fuel feed rate. Specifically, with a constant airflow, an increase in oxygen content indicates a decrease in the biomass fuel feed rate, while a decrease indicates an increase. A PID control module is established, using the oxygen content of the flue gas at the boiler's tail end as the input value. The output value of this PID control module is used to regulate the frequency of the screw feeder in the circulating fluidized bed boiler, achieving scientific and rational automatic control of the screw feeder's feeding.

[0006] A PID control strategy for a biomass fuel feeding system in a circulating fluidized bed boiler includes an operating circulating fluidized bed boiler, a flue gas oxygen content measurement sensor in the tail flue, a screw feeder for conveying biomass fuel to the furnace, and a central control room. The screw feeder uses a variable frequency speed-regulating motor to control the feeding rate, and a rate module is installed in the central control room. The system is characterized by the following steps:

[0007] The first step is to set up a feeding PID controller in the central control room. The control frequency of the variable frequency speed control motor of the screw feeder is used as the output of the feeding PID controller, and the oxygen content of the flue gas in the tail flue is used as the input of the feeding PID controller. The oxygen content of 5%-6% is set as the set value of the setting terminal SP of the feeding PID controller.

[0008] The second step is to input the oxygen content of the flue gas measured by the oxygen content sensor in the tail flue into the rate module, and obtain the rate of change of the oxygen content of the flue gas per second at the output of the rate module.

[0009] The third step involves collecting and calculating the average rate of change (VO) of the oxygen content in the flue gas per second for the 20 seconds preceding that reference time, and simultaneously collecting and calculating the average control frequency (F) of the variable frequency drive motor of the screw feeder for the 20 seconds preceding that reference time. 平均 The dynamic proportional coefficient P of the feed PID controller is calculated using the following formula: P = VO × F 平均 ×K; where K = 0.1 - 0.5;

[0010] Step 4: Embed the dynamic proportional coefficient P calculated in Step 3 into the feeding PID controller; input the real-time oxygen content of the flue gas measured by the oxygen content sensor in the tail flue into the input terminal of the feeding PID controller, and input the frequency signal of the output terminal of the feeding PID controller into the variable frequency speed control motor of the screw feeder. The feeding rate of the variable frequency speed control motor of the screw feeder is controlled by the feeding PID controller.

[0011] Collect the main steam pressure P of the circulating fluidized bed boiler in operation. 主 When the real-time main steam pressure P 主 When the main steam pressure exceeds the setpoint by 0.2 MPa, the setpoint SP of the feed PID controller will be increased by 2%. When the real-time main steam pressure P... 主 When the pressure is 0.2 MPa lower than the main steam pressure setting, the setting value of the setting terminal SP of the feed PID controller will be reduced by 2%.

[0012] This invention regulates the feed rate of biomass fuel by monitoring the oxygen content of flue gas in the tail flue, pioneering an "oxygen-based" feed rate regulation mode for circulating fluidized bed boilers using biomass fuel. This ensures the relative stability of the oxygen content in the furnace, thereby guaranteeing the stability of the boiler's environmental emission indicators and thermal efficiency indicators. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the feeding PID controller for regulating the frequency of the variable frequency speed control motor of the screw feeder according to the present invention. Detailed Implementation

[0014] The present invention will now be described in detail with reference to the accompanying drawings:

[0015] A PID control strategy for a biomass fuel feeding system in a circulating fluidized bed boiler includes an operating circulating fluidized bed boiler, a flue gas oxygen content measurement sensor in the tail flue, a screw feeder for conveying biomass fuel to the furnace, and a central control room. The screw feeder uses a variable frequency speed-regulating motor to control the feeding rate, and a rate module is installed in the central control room. The fuel for the circulating fluidized bed boiler is biomass fuel, and the feeding rate of the screw feeder is controlled by adjusting the frequency of the variable frequency speed-regulating motor of the screw feeder to change the feed rate to the boiler furnace. The specific steps are as follows:

[0016] The first step is to set up a feeding PID controller in the central control room. The control frequency of the variable frequency speed control motor of the screw feeder is used as the output of the feeding PID controller, and the oxygen content of the flue gas in the tail flue is used as the input of the feeding PID controller. The oxygen content of 5%-6% is set as the set value of the setting terminal SP of the feeding PID controller.

[0017] The second step is to input the oxygen content of the flue gas measured by the oxygen content sensor in the tail flue into the rate module, and obtain the rate of change of the oxygen content of the flue gas per second at the output of the rate module.

[0018] The third step is to use a specific moment as a baseline, collect and calculate the average rate of change (V) of the oxygen content in the flue gas per second over the 20 seconds preceding that baseline moment. O This involves collecting the output values ​​of the rate module within 20 seconds prior to the reference time and then calculating the average value; simultaneously, it also involves collecting and calculating the average value F of the control frequency F of the variable frequency speed control motor of the screw feeder within 20 seconds prior to the reference time. 平均 The dynamic proportional coefficient P of the feed PID controller is calculated using the following formula: P = VO × F 平均 ×K; where K=0.1-0.5; that is, the dynamic proportional coefficient P at each moment reflects the average factor of the rate of change of flue gas oxygen content in the previous 20 seconds and the average factor of the control frequency of the variable frequency speed control motor, so that the proportional coefficient of the feed PID controller always follows the trend of oxygen content change in the boiler furnace, and achieves close tracking of the combustion status in the boiler.

[0019] The fourth step involves embedding the dynamic proportional coefficient P calculated in the third step into the feeding PID controller; inputting the real-time oxygen content of the flue gas measured by the oxygen content sensor in the tail flue into the input terminal of the feeding PID controller; and inputting the frequency signal from the output terminal of the feeding PID controller into the variable frequency speed control motor of the screw feeder. The feeding PID controller then regulates the feeding rate of the variable frequency speed control motor of the screw feeder. Thus, by using the dynamic proportional coefficient P and the feeding PID controller to track changes in oxygen content, the control of boiler feeding is achieved, thereby stabilizing the oxygen content of combustion in the boiler.

[0020] Collect the main steam pressure P of the circulating fluidized bed boiler in operation. 主 When the real-time main steam pressure P 主 When the main steam pressure exceeds the setpoint by 0.2 MPa, the setpoint SP of the feed PID controller will be increased by 2%. In other words, when the main steam pressure P... 主 When increasing the pressure, it is necessary to reduce the biomass fuel supplied to the furnace. This can be achieved by increasing the set value of the feed PID controller; when the real-time main steam pressure P 主 When the main steam pressure is 0.2 MPa lower than the set value, the setting value of the feed PID controller SP is reduced by 2%. In other words, when the main steam pressure P decreases, the biomass fuel supplied to the furnace needs to be increased. This can be achieved by reducing the setting value of the feed PID controller.

Claims

1. A PID control strategy for a biomass fuel feeding system of a circulating fluidized bed boiler, comprising a circulating fluidized bed boiler in operation, a flue gas oxygen content measurement sensor in the tail flue, a screw feeder for conveying biomass fuel to the furnace, and a central control room, wherein the screw feeder uses a variable frequency speed control motor to control the feeding amount, and a rate module is installed in the central control room; characterized in that... The following steps: The first step is to set up a feeding PID controller in the central control room. The control frequency of the variable frequency speed control motor of the screw feeder is used as the output of the feeding PID controller, and the oxygen content of the flue gas in the tail flue is used as the input of the feeding PID controller. The oxygen content of 5%-6% is set as the set value of the setting terminal SP of the feeding PID controller. The second step is to input the oxygen content of the flue gas measured by the oxygen content sensor in the tail flue into the rate module, and obtain the rate of change of the oxygen content of the flue gas per second at the output of the rate module. The third step is to use a specific moment as a baseline, collect and calculate the average rate of change (V) of the oxygen content in the flue gas per second over the 20 seconds preceding that baseline moment. O Simultaneously, the average value F of the control frequency F of the variable frequency speed control motor of the screw feeder within 20 seconds prior to the reference time was collected and calculated. 平均 The dynamic proportional coefficient P of the feed PID controller is calculated using the following formula: P = V O ×F 平均 ×K; where K = 0.1 - 0.5; Step 4: Embed the dynamic proportional coefficient P calculated in Step 3 into the feeding PID controller; input the real-time oxygen content of the flue gas measured by the oxygen content sensor in the tail flue into the input terminal of the feeding PID controller, and input the frequency signal of the output terminal of the feeding PID controller into the variable frequency speed control motor of the screw feeder. The feeding rate of the variable frequency speed control motor of the screw feeder is controlled by the feeding PID controller. Collect the main steam pressure P of the circulating fluidized bed boiler in operation. 主 When the real-time main steam pressure P 主 When the main steam pressure exceeds the setpoint by 0.2 MPa, the setpoint SP of the feed PID controller will be increased by 2%. When the real-time main steam pressure P... 主 When the pressure is 0.2 MPa lower than the main steam pressure setting, the setting value of the setting terminal SP of the feed PID controller will be reduced by 2%.

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