Fuel feeding automatic control strategy for biomass circulating fluidized bed boiler

Abstract

This invention discloses an automatic fuel feeding control strategy for a biomass circulating fluidized bed boiler, solving the problem of how to automatically control the rational supply of biomass fuel in the furnace to ensure stable boiler operation. This invention uses changes in the oxygen content of the flue gas at the boiler tail end to infer changes in the furnace fuel feed. Under the premise of constant air volume, an increase in oxygen content indicates a decrease in biomass fuel feed, while a decrease in oxygen content indicates an increase in biomass fuel feed. A PID control module is established with the oxygen content of the flue gas at the boiler tail end as the input value. The output value of this PID control module is used to adjust the frequency of the fuel feed screw feeder in the biomass circulating fluidized bed boiler. Furthermore, considering the unstable characteristics of biomass fuel, when the oxygen content in the furnace reaches a limit, the feedforward control mode and the lockout function mode of the PID controller are fully utilized to achieve stable boiler operation.

Description

Technical Field

[0001] This invention relates to a biomass-fueled circulating fluidized bed boiler, and more particularly to an automatic fuel feeding control strategy for a biomass-fueled circulating fluidized bed boiler. Background Technology

[0002] Biomass fuels used in circulating fluidized bed boilers are characterized by a wide variety of types and blends of 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 often gets stuck in the screw feeder when transporting biomass fuel to the furnace. On-site solutions typically involve increasing the VFD motor speed or stopping and reversing the VFD motor to resolve this issue and restore fuel delivery. This results in inconsistent amounts of biomass fuel entering the furnace, sometimes even intermittently, directly causing significant fluctuations in oxygen levels within the furnace. Low oxygen levels in the furnace lead to an increase in sulfur dioxide in the flue gas, while high oxygen levels lead to an increase in nitrogen oxides. Increased emissions of NOx, CO, and SO2; how to scientifically and rationally design a PID control strategy for the biomass fuel feeding system based on the characteristics of the circulating fluidized bed boiler fueled by biomass fuel, to achieve stable compliance with emission standards and stabilize the thermal efficiency of the circulating fluidized bed boiler, has become a problem that needs to be solved on-site; in thermal power plants using biomass circulating fluidized bed boilers with back-pressure units for heating, under the heat-driven power generation mode, the thermal power plant operates in a "furnace-following-the-generator" manner, making combustion control of the circulating fluidized bed boiler fueled by biomass very difficult; when the furnace temperature of the circulating fluidized bed boiler is too high or too low, especially when there are large fluctuations in the continuity of biomass fuel supply and sudden changes in fuel type, how to automatically control the reasonable supply of biomass fuel in the furnace to ensure the stable operation of the boiler has become a difficult problem that needs to be solved on-site. Summary of the Invention

[0003] This invention provides an automatic fuel feeding control strategy for a biomass circulating fluidized bed boiler, solving the technical problem of how to automatically control the rational supply of biomass fuel in the furnace to ensure the stable operation of the boiler.

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

[0005] The overall concept of this invention is as follows: This invention uses changes in the oxygen content of the flue gas at the boiler tail end to infer changes in the fuel feed in the furnace. That is, under the premise of constant air volume, an increase in oxygen content indicates a decrease in the biomass fuel feed, and a decrease in oxygen content indicates an increase in the biomass fuel feed. A PID control module is established with the oxygen content of the flue gas at the boiler tail end as the input value. The output value of this PID control module is used to regulate the frequency of the fuel feed screw feeder in the circulating fluidized bed boiler using biomass fuel. Furthermore, considering the unstable characteristics of biomass fuel, when the oxygen content in the furnace reaches a limit, the feedforward control mode and the lockout function mode of the PID controller are fully utilized to achieve scientific and reasonable automatic control of the fuel feed screw feeder, thereby achieving stable boiler operation.

[0006] An automatic fuel feeding control strategy for a biomass circulating fluidized bed boiler includes an operating circulating fluidized bed boiler, a flue gas oxygen content measuring 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. A bed temperature measuring sensor is installed on the circulating fluidized bed boiler, and a main steam flow measuring sensor is installed at the main steam output port of the biomass circulating fluidized bed boiler. A rate module is installed in the central control room. The strategy is characterized by the following steps:

[0007] The first step is to build a feeding PID controller. 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 the flue gas in the tail flue is set to 5%-6% 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 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;

[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] When the rate of change of oxygen content in the flue gas in the tail flue is V O When the value is above +0.06% or below -0.06%, the proportional gain P of the feed PID controller is 1.2 × V. O ×F 平均 ×K; where K = 0.1-0.5.

[0012] When the bed temperature measured by the bed temperature sensor is higher than 850℃, or when the bed temperature rises by more than 8℃ within 30 seconds, the output value of the feeding PID controller is increased by a lockout function, ensuring that the output value of the feeding PID controller is not greater than the output value when the lockout function is activated. When the bed temperature measured by the bed temperature sensor is lower than 750℃, or when the bed temperature drops by more than 6℃ within 30 seconds, the output value of the feeding PID controller is decreased by a lockout function, ensuring that the output value of the feeding PID controller is not less than the output value when the lockout function is activated.

[0013] When the difference between the oxygen content of the flue gas measured by the oxygen content sensor in the tail flue and the set value of the setting terminal SP of the feed PID controller is greater than 1%, and the increase in the oxygen content of the flue gas per second is greater than 0.04%, the main steam flow measured by the main steam flow sensor at this time is multiplied by the increase in the oxygen content of the flue gas per second at this time, and then multiplied by -0.15. The resulting product is used as the first feedforward quantity and superimposed on the output terminal of the feed PID controller. The frequency obtained after superposition is used to control the variable frequency motor of the screw feeder.

[0014] When the oxygen content of the flue gas measured by the oxygen content measuring sensor in the tail flue is less than 3%, and the oxygen content of the flue gas is still decreasing per second, the main steam flow measured by the main steam flow measuring sensor at this time is multiplied by the decrease in oxygen content of the flue gas per second at this time, and then multiplied by 0.3. The resulting product is used as the second feedforward quantity and superimposed on the output of the feed PID controller. The frequency obtained after superposition is used to control the variable frequency motor of the screw feeder.

[0015] Using the percentage of oxygen content in the flue gas measured by the oxygen content sensor in the tail flue as the abscissa and the negative frequency value (in Hertz) as the ordinate, the following coordinate points are marked on this coordinate: (3% 0.5), (2% 1), (1% 1.5), and (0.5% 3). Connect these four points with straight lines to form a broken line curve function of flue gas oxygen content versus the variable frequency motor control frequency of the screw feeder. Use this broken line curve function of the variable frequency motor control frequency as the third feedforward quantity, superimpose it on the output of the feeder PID controller, and use the frequency obtained after superposition to regulate the variable frequency motor of the screw feeder.

[0016] This invention uses "constant oxygen content" as the criterion to regulate the feed rate of biomass fuel, pioneering a new approach for circulating fluidized bed boilers using biomass fuel. By adjusting the feed rate of biomass fuel in real time, it ensures the relative stability of the oxygen content in the tail flue gas, thereby guaranteeing the stability of the boiler's environmental emission indicators and thermal efficiency indicators. Furthermore, it develops strategies to cope with significant changes in boiler bed temperature and substantial increases or decreases in furnace oxygen content, comprehensively regulating the delivery of biomass fuel. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the PID controller for controlling the frequency of the variable frequency speed control motor of the screw feeder according to the present invention. Detailed Implementation

[0018] The present invention will now be described in detail with reference to the accompanying drawings:

[0019] An automatic fuel feeding control strategy for a biomass circulating fluidized bed boiler includes an operating circulating fluidized bed boiler, a flue gas oxygen content measuring 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. A bed temperature measuring sensor is installed on the circulating fluidized bed boiler, and a main steam flow measuring sensor is installed at the main steam output port of the biomass circulating fluidized bed boiler. A rate module is installed in the central control room. The strategy is characterized by the following steps:

[0020] The first step is to build a feeding PID controller. 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 the flue gas in the tail flue is set to 5%-6% as the set value of the setting terminal SP of the feeding PID controller.

[0021] 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.

[0022] 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; 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.

[0023] 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.

[0024] When the rate of change of oxygen content in the flue gas in the tail flue is V O When the value is above +0.06% or below -0.06%, the proportional gain P of the feed PID controller is 1.2 × V. O ×F 平均 ×K; where K=0.1-0.5; by increasing the proportionality coefficient to suppress rapid changes in oxygen content.

[0025] When the bed temperature measured by the bed temperature sensor is higher than 850℃, or when the bed temperature rises by more than 8℃ within 30 seconds, the output value of the feeding PID controller is increased by a lockout function, ensuring that the output value of the feeding PID controller is not greater than the output value when the lockout function is activated. When the bed temperature measured by the bed temperature sensor is lower than 750℃, or when the bed temperature drops by more than 6℃ within 30 seconds, the output value of the feeding PID controller is decreased by a lockout function, ensuring that the output value of the feeding PID controller is not less than the output value when the lockout function is activated. The lockout function of the PID controller is used to suppress overshoot of biomass fuel.

[0026] When the difference between the oxygen content measured by the oxygen content sensor in the tail flue and the set value of the feed PID controller's setpoint SP is greater than 1%, and the increase in oxygen content per second is greater than 0.04%, the product of the main steam flow rate measured by the main steam flow rate sensor at this time multiplied by the increase in oxygen content per second at this time, and then multiplied by -0.15, is used as the first feedforward quantity and superimposed on the output of the feed PID controller. The frequency obtained after superposition is used to control the variable frequency motor of the screw feeder. When the oxygen content increases drastically, the normal adjustment of the feed PID controller can no longer achieve the target. At this time, the frequency of the variable frequency motor is directly intervened by the feedforward quantity to stabilize the oxygen content in the furnace.

[0027] When the oxygen content of the flue gas measured by the oxygen content measuring sensor in the tail flue is less than 3%, and the oxygen content of the flue gas is still decreasing per second, the main steam flow measured by the main steam flow measuring sensor at this time is multiplied by the decrease in oxygen content of the flue gas per second at this time, and then multiplied by 0.3. The resulting product is used as the second feedforward quantity and superimposed on the output of the feed PID controller. The frequency obtained after superposition is used to control the variable frequency motor of the screw feeder. When the oxygen content in the furnace drops drastically, the frequency of the variable frequency motor is also directly intervened through the feedforward quantity to suppress the drastic drop in oxygen content.

[0028] Using the percentage of oxygen content in the flue gas measured by the oxygen content sensor in the tail flue as the abscissa and the negative frequency value (in Hertz) as the ordinate, the following coordinate points are marked on this coordinate: (3% 0.5), (2% 1), (1% 1.5), and (0.5% 3). Connecting these four points with straight lines forms a piecewise linear curve function of flue gas oxygen content versus the variable frequency motor control frequency of the screw feeder. This piecewise linear curve function of the variable frequency motor control frequency is used as the third feedforward quantity and superimposed on the output of the feeder PID controller. The frequency obtained after superposition is used to regulate the variable frequency motor of the screw feeder. By using the form of the piecewise linear function as a feedforward quantity to directly intervene in the frequency of the variable frequency motor, the extreme drop in oxygen content is smoothly suppressed.

Claims

1. An automatic fuel feeding control strategy for a biomass circulating fluidized bed boiler, comprising an operating circulating fluidized bed boiler, a flue gas oxygen content measuring 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. A bed temperature measuring sensor is installed on the circulating fluidized bed boiler, a main steam flow measuring sensor is installed at the main steam outlet of the biomass circulating fluidized bed boiler, and a rate module is installed in the central control room; characterized in that... The following steps: The first step is to build a feeding PID controller. 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 the flue gas in the tail flue is set to 5%-6% 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. Third step, with a certain time as the benchmark, collect and calculate the average value V of the oxygen content of the flue gas per second change rate within 20 seconds before the benchmark time O At the same time, collect and calculate the average value F of the control frequency of the frequency conversion speed motor of the screw feeder within 20 seconds before the benchmark time 平均 According to the following formula, the dynamic proportional coefficient P of the feeding PID controller is calculated: P=V O ×F 平均 ×K; wherein 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. When the difference between the oxygen content of the flue gas measured by the oxygen content sensor in the tail flue and the set value of the setting terminal SP of the feed PID controller is greater than 1%, and the increase in the oxygen content of the flue gas per second is greater than 0.04%, the main steam flow measured by the main steam flow sensor at this time is multiplied by the increase in the oxygen content of the flue gas per second at this time, and then multiplied by -0.

15. The resulting product is used as the first feedforward quantity and superimposed on the output terminal of the feed PID controller. The frequency obtained after superposition is used to control the variable frequency motor of the screw feeder.

2. The automatic fuel feeding control strategy for a biomass circulating fluidized bed boiler according to claim 1, characterized in that, When the rate of change of oxygen content in the flue gas in the tail flue is V O When the value is above +0.06% or below -0.06%, the proportional gain P of the feed PID controller is 1.2 × V. O ×F 平均 ×K; where K = 0.1-0.

5.

3. The automatic fuel feeding control strategy for a biomass circulating fluidized bed boiler according to claim 1 or 2, characterized in that, When the bed temperature measured by the bed temperature sensor is higher than 850℃, or when the bed temperature rises by more than 8℃ within 30 seconds, the output value of the feeding PID controller is increased by the interlock function to ensure that the output value of the feeding PID controller is not greater than the output value when the interlock function is activated. When the bed temperature measured by the bed temperature sensor is lower than 750℃, or when the bed temperature drops by more than 6℃ within 30 seconds, the output value reduction interlock function of the feeding PID controller is activated to ensure that the output value of the feeding PID controller is not less than the output value when the interlock function is activated.

4. The automatic fuel feeding control strategy for a biomass circulating fluidized bed boiler according to claim 1, characterized in that, When the oxygen content of the flue gas measured by the oxygen content measuring sensor in the tail flue is less than 3%, and the oxygen content of the flue gas is still decreasing per second, the main steam flow measured by the main steam flow measuring sensor at this time is multiplied by the decrease in oxygen content of the flue gas per second at this time, and then multiplied by 0.

3. The resulting product is used as the second feedforward quantity and superimposed on the output of the feed PID controller. The frequency obtained after superposition is used to control the variable frequency motor of the screw feeder.

5. The automatic fuel feeding control strategy for a biomass circulating fluidized bed boiler according to claim 1, characterized in that, Using the percentage of oxygen content in the flue gas measured by the oxygen content sensor in the tail flue as the abscissa and the negative frequency value as the ordinate, in Hertz, the following coordinate points are marked on this coordinate: (3% 0.5), (2% 1), (1% 1.5), (0.5% 3). Connect these four points with straight lines to form a broken line curve function of flue gas oxygen content and variable frequency motor control frequency of the screw feeder. Use the broken line curve function of variable frequency motor control frequency as the third feedforward quantity, superimpose it on the output of the feeder PID controller, and use the frequency obtained after superposition to regulate the variable frequency motor of the screw feeder.

Images