Boiler steam-water system for improving flexibility of coal-fired unit and control method

Abstract

The invention discloses a boiler steam-water system for improving the flexibility of a coal-fired unit and a control method. The boiler steam-water system comprises an economizer, a water cooling wall, a steam-water separator and more than two stages of superheaters which are connected in sequence, a pressure regulating valve is arranged on the steam guide pipe between the two stages of superheaters, and a pressure sensor is arranged on the steam guide pipe at the inlet end of the pressure regulating valve; the control method comprises the following steps: A) when the coal-fired unit runs, carrying out PID negative feedback calculation on the deviation between a pre-valve pressure set value and a pre-valve pressure measured value of the pressure regulating valve, and carrying out PID positive feedback calculation on the deviation between an active power set value and an active power measured value; b, the two PID calculated values are added to obtain a comprehensive value, and the opening degree of a pressure adjusting valve is adjusted according to the comprehensive value to adjust the load of the coal-fired unit. Decoupling of boiler heat storage and main steam pressure can be achieved, the boiler heat storage amount is adjusted and controlled on the premise that the Rankine cycle efficiency is not reduced, the variable load rate of the coal-fired unit is increased, and the system is simple in structure and low in cost.

Description

Technical Field

[0001] The present invention relates to a boiler steam-water system and a control method for a coal-fired unit, and in particular to a boiler steam-water system and a control method for improving the flexibility of a coal-fired unit. Background Art

[0002] In recent years, renewable energy sources such as wind and solar power have seen significant development. However, due to the temporal and spatial disparities between wind and solar resource availability and residential electricity consumption, this rapid growth has also led to increasing issues with wind and solar power curtailment in resource-rich regions. Coal-fired power plants are the primary source of peak load regulation in the power system, and improving their flexibility is one of the most direct and effective means of addressing this issue.

[0003] The load-variation rate is a key indicator of coal-fired unit flexibility. Limited by factors such as the pulverizing system's response speed and the safety of the metal on the heating surfaces, the load-variation rate of conventional coal-fired units is typically 1%-2%Pe / min. Power plant boilers often operate using a sliding pressure mechanism, meaning that the main steam pressure increases with load. Therefore, when the boiler is ramped from low to high load, the internal pressure and steam density rise, and the steam quality and heat storage within the boiler also increase with the load. This increase in internal heat storage causes the heat output to the turbine to be lower than the heat absorbed from the fuel, thereby slowing the unit's load ramp-up rate. Taking a 300MW subcritical unit as an example, when the load is increased from 50%THA to THA at a rate of 4%Pe / min, about 68MW of heat load is used to fill the boiler heat storage, while the average heat load used for power generation during the entire load change process is only about 530MW. Due to the change in boiler heat storage, an additional 12.8% of fuel needs to be input. Especially in the early stage of load change, due to the slow response speed of the pulverizing system and untimely heat input, the load deviation is too large or the main steam pressure is too low, which makes it difficult for the unit to pass the AGC acceptance. If the boiler adopts a constant pressure operation mode, the main steam pressure and internal heat storage of the boiler basically do not change with the load, but the throttling loss of the turbine main steam valve increases, and the efficiency of the Rankine cycle decreases accordingly. In order to increase the unit's load change rate, various types of coupled energy storage schemes have emerged. Although these schemes can increase the unit's load change rate, the control logic of the newly added energy storage equipment is complex and the investment is relatively high. Summary of the Invention

[0004] The purpose of the present invention is to address the above-mentioned deficiencies in the prior art and to provide a boiler steam-water system and control method for improving the flexibility of coal-fired units. The system can decouple boiler heat storage and main steam pressure, regulate boiler heat storage without reducing Rankine cycle efficiency, and improve the load variable rate of the coal-fired unit. The system has a simple structure and low cost.

[0005] To achieve the above objectives, the present invention provides a boiler steam-water system for improving the flexibility of a coal-fired unit, comprising an economizer, a water-cooled wall, a steam-water separator, and two or more stages of superheaters connected in sequence. The system is characterized in that a pressure regulating valve is provided on the steam conduit between the two stages of superheaters, and a pressure sensor is provided on the steam conduit at the inlet of the pressure regulating valve. The present invention uses a pressure regulating valve to increase the steam pressure and density in the steam space upstream of the valve, thereby increasing the steam quality and total heat storage inside the boiler. This regulation process does not affect the main steam pressure, thereby achieving decoupling of boiler heat storage and main steam pressure, and being able to freely adjust boiler heat storage while maintaining the main steam pressure unchanged. The pressure sensor can measure the inlet pressure of the pressure regulating valve, facilitating automatic control of the pressure regulating valve. As a further improvement of the present invention, the two or more superheaters include a ceiling-wall superheater, a low-temperature superheater, a platen superheater, and a high-temperature superheater connected in sequence. The low-temperature superheater is a serpentine heat exchanger, and the pressure regulating valve is disposed on the steam conduit between the low-temperature superheater and the platen superheater. Most coal-fired boilers are equipped with the above four superheaters, and the volume of the serpentine heat exchanger is relatively large. When the steam pressure changes, the heat storage change of the ceiling-wall superheater and the low-temperature superheater exceeds the majority of the total heat storage change in the boiler steam space. The pressure regulating valve is disposed after the low-temperature superheater to utilize most of the boiler's heat storage capacity. As a further improvement of the present invention, a primary desuperheater is provided between the low-temperature superheater and the platen superheater, and a secondary desuperheater is provided between the platen superheater and the high-temperature superheater; the two desuperheaters can maintain the inlet steam parameters of the subsequent superheater unchanged, thereby preventing the superheater from overheating due to an increase in working fluid pressure; A control method for a boiler steam-water system for improving the flexibility of a coal-fired unit according to the present invention comprises the following steps: A) while the coal-fired unit is operating, performing PID negative feedback calculation on the deviation between a set value of the inlet pressure of a pressure regulating valve and a measured value of the inlet pressure of the pressure regulating valve in a superheated steam pressure circuit; and performing PID positive feedback calculation on the deviation between a set value of the active power and a measured value of the active power in an active power circuit for power generation of the coal-fired unit, wherein the set value of the active power is the preset output power of the coal-fired unit, and the measured value of the active power is the actual power generated by the steam generated by the boiler entering the steam turbine to drive the generator; B) adding the PID calculated values ​​of the two circuits to obtain a comprehensive value, and adjusting the opening of the pressure regulating valve according to the comprehensive value to adjust the load of the coal-fired unit; When the pressure regulating valve is in the constant pressure operation mode, the pressure setting value before the valve is a fixed value; this setting value is between the safety pressure of the steam-water system and the main steam pressure setting value; When the pressure regulating valve is in sliding pressure operation mode, the valve inlet pressure setting value should be higher than the main steam pressure setting value and increase with the load. The valve inlet pressure setting value should be greater than or equal to the main steam pressure setting value plus the pipeline resistance; In the control method of the present invention, the opening of the pressure regulating valve is jointly determined by the active power of the unit and the pressure before the valve, wherein the active power loop is positive feedback, that is, when the useful power measurement value is lower than the set value, the pressure regulating valve will open wider; the pressure loop before the valve is negative feedback, that is, when the pressure measurement value before the valve is lower than the set value, the pressure regulating valve will close narrower, and the PID calculated values ​​of the two loops are used as output signals to control the opening of the pressure regulating valve; when the boiler load increases, the active power measurement value and the pressure measurement value before the valve are both lower than the set value, but the calculations of the two PID loops are combined to determine the opening of the pressure regulating valve. The pressure regulating valve is automatically opened, the boiler heat storage is consumed, the steam inlet to the turbine is increased, and the active power is improved; in the initial stage of load increase, during the response delay period of the pulverizing system, opening the pressure regulating valve can significantly improve the response speed of the unit to the load instruction; when the load increase process is completed, the active power measurement value is basically equal to the set value, and the pressure measurement value before the valve is lower than the set value, the pressure regulating valve is automatically closed, and the boiler heat storage is restored; the present invention regulates the boiler heat storage without reducing the efficiency of the Rankine cycle, thereby improving the load change rate of the coal-fired unit, and the system structure is simple and the cost is low.

[0006] In summary, the present invention can achieve the decoupling of boiler heat storage and main steam pressure, regulate boiler heat storage without reducing Rankine cycle efficiency, and improve the load variable rate of coal-fired units. The system structure is simple and the cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 This is a structural block diagram corresponding to the furnace body of an embodiment of a boiler steam-water system of the present invention.

[0008] Figure 2 This is a logic diagram of the boiler steam-water system control method of the present invention.

[0009] Figure 3 This is a curve diagram of the valve front pressure setting value according to an embodiment of the present invention. DETAILED DESCRIPTION

[0010] The present invention will be further described below with reference to the accompanying drawings, taking a 300MW subcritical unit and a π-type pulverized coal boiler as an example.

[0011] like Figure 1As shown, a boiler steam-water system for improving the flexibility of a coal-fired unit in this embodiment includes an economizer 1, a water wall 2, a steam-water separator 3, a ceiling-wall superheater 4, a low-temperature superheater 5, a platen superheater 8, and a high-temperature superheater 9, which are connected in sequence. The low-temperature superheater 5 is a serpentine tube heat exchanger. A pressure regulating valve 6 is provided on the steam conduit between the low-temperature superheater 5 and the platen superheater 7, and a pressure sensor 8 is provided on the steam conduit at the inlet end of the pressure regulating valve 6. A first-stage desuperheater 11 is provided outside the steam conduit between the low-temperature superheater 5 and the platen superheater 8, and a second-stage desuperheater 12 is provided outside the steam conduit between the platen superheater 8 and the high-temperature superheater 9. The outlet of the high-temperature superheater 9 is connected to the unit's steam turbine 13. The boiler's steam-water system uses a pressure regulating valve 6 to increase the steam pressure and density in the steam space upstream of the valve, increasing the steam quality and total heat storage within the boiler. This regulation process does not affect the main steam pressure, thus decoupling boiler heat storage from main steam pressure, allowing for free adjustment of boiler heat storage while maintaining a constant main steam pressure. A pressure sensor 7 measures the inlet pressure of the pressure regulating valve 6, facilitating automatic control of the pressure regulating valve. Because the ceiling-wall superheater 4 and the low-temperature superheater 5 are wall-type heat exchangers and serpentine heat exchangers, respectively, with relatively large volumes, the change in heat storage in these two units exceeds 80% of the total change in heat storage in the boiler's steam space (including the reheater) when steam pressure fluctuates. Therefore, placing the pressure regulating valve 6 after the low-temperature superheater 5 fully utilizes the boiler's heat storage capacity. The two desuperheaters 11 and 12 maintain the inlet steam parameters of the subsequent superheater constant, preventing overheating of the subsequent superheater due to increased working fluid pressure. The present invention provides a control method for a boiler steam-water system for improving the flexibility of a coal-fired unit, such as Figure 2 As shown, the method includes the following steps: A) when the coal-fired unit is in operation, for the superheated steam pressure circuit, PID negative feedback calculation is performed on the deviation between the valve inlet pressure setting value of the pressure regulating valve and the valve inlet pressure measurement value; for the active power circuit of the coal-fired unit power generation, PID positive feedback calculation is performed on the deviation between the active power setting value and the active power measurement value, wherein the active power setting value is the preset output power of the coal-fired unit power generation, and the active power measurement value is the actual power generated by the main steam produced by the boiler entering the steam turbine to drive the generator; B) the PID calculated values ​​of the two circuits are added to obtain a comprehensive value, and the opening of the pressure regulating valve is adjusted according to the comprehensive value to adjust the load of the coal-fired unit; like Figure 3As shown in the figure, when the pressure regulating valve is in the constant pressure operation mode, the valve upstream pressure setting value is a fixed value; this setting value is between the safety pressure of the steam-water system and the main steam pressure setting value; when the pressure regulating valve is in the sliding pressure operation mode, the valve upstream pressure setting value should be higher than the main steam pressure setting value and increase with the increase of load. The higher value can be selected according to the variable load rate requirement and the commissioning test results. This value also increases with the decrease of the unit load. Theoretically, 0.5-6MPa can meet the 4% variable load rate requirement. The valve upstream pressure setting value should also be greater than or equal to the main steam pressure setting value plus the pipeline resistance. The formula for calculating a single PID is: u(t)=Kp*e(t)+Ki*∫e(t)dt+Kd*de(t) / dt; u(t) is the control signal output by the controller at time t (that is, the output of the controller), Kp is the proportional coefficient (that is, the proportional gain), Ki is the integral coefficient (that is, the integral gain), Kd is the differential coefficient (that is, the differential gain), e(t) is the error between the input signal of the controller and the desired value, de(t) / dt is the derivative of the error, that is, the rate of change of the error; In the control method of the present invention, the opening of the pressure regulating valve 6 is jointly determined by the active power of the unit and the pressure before the valve, wherein the active power loop is positive feedback, that is, when the useful power measurement value is lower than the set value, the pressure regulating valve 6 will open wider; the pressure before the valve loop is negative feedback, that is, when the pressure before the valve measurement value is lower than the set value, the pressure regulating valve 6 will close narrower, and the PID calculated values ​​of the two loops are used as output signals to control the opening of the pressure regulating valve 6; when the boiler increases the load, the active power measurement value and the pressure before the valve measurement value are both lower than the set value, but the calculation of the two PID loops is combined to realize the automatic opening of the pressure regulating valve, consume the boiler heat storage to increase the steam intake of the turbine, and improve the active power; in the initial stage of load increase (low load), during the response delay of the pulverizing system, opening the pressure regulating valve can significantly improve the response speed of the unit to the load instruction; when the load increase process is completed, the active power measurement value is basically equal to the set value. Value, the pre-valve pressure measurement value is lower than the set value, the pressure regulating valve 6 is automatically closed, and the boiler heat storage is restored; this control method is used for the boiler of this embodiment. At 50% THA load, the outlet pressure is about 11 MPa lower than the original design. The pressure is increased to 16.8 MPa through the pressure regulating valve 6. The corresponding heat storage of the ceiling wall superheater 4 and the low-temperature superheater 5 is increased from 108.3 GJ to 130.6 GJ, an increase of 22.3 GJ. If the set load increase rate is 4% and the pulverizing system response time is 2 minutes, the unit needs to generate an additional 1.44 GJ of electricity due to the load increase within two minutes. When the coal amount remains unchanged, it is only necessary to open the pressure regulating valve 6, consuming about 3.5 GJ of boiler heat storage. After that, the pulverizing system completes the response, the amount of coal entering the furnace increases, and the boiler heat storage is no longer consumed. In addition, because the initial pressure of the boiler is high and the heat storage is large, the amount of powder required to be provided by the pulverizing system is also lower than that of the conventional unit. It can be seen that the present invention can regulate the boiler heat storage capacity without reducing the efficiency of the Rankine cycle. When the load changes, the boiler heat storage capacity can be quickly utilized to adjust the active power of the unit, thereby increasing the load change rate of the coal-fired unit and improving the flexibility of the coal-fired unit. In addition, the system structure is simple and the cost is low.

[0012] The arrangement position of the pressure regulating valve 6 in other furnace types may refer to this embodiment, that is, the design principle is to maximize the use of the boiler steam space and not increase the pressure of the high-temperature heating surface.

[0013] As a preference, a dead zone may be set for the active power circuit to prevent the pressure regulating valve 6 from frequently operating when the active power deviation is small (e.g. less than 0.5% of the rated load), thereby interfering with the automatic control of the main steam valve opening; The above embodiments have been described, but it should be understood that the above embodiments are only for the purpose of illustration and description, and are not intended to limit the present invention to the scope of the described embodiments.

Claims

1. A boiler steam-water system for improving the flexibility of a coal-fired unit, comprising an economizer, a water-cooled wall, a steam-water separator, and two or more stages of superheaters connected in sequence; characterized by: A pressure regulating valve is provided on the steam pipe between the two stages of superheaters, and a pressure sensor is provided on the steam pipe at the inlet end of the pressure regulating valve.

2. A boiler steam-water system for improving the flexibility of a coal-fired unit according to claim 1, characterized in that: The two or more stages of superheaters include a ceiling-wall superheater, a low-temperature superheater, a platen superheater and a high-temperature superheater connected in sequence. The low-temperature superheater is a serpentine tube heat exchanger, and the pressure regulating valve is arranged on the steam pipe between the low-temperature superheater and the platen superheater.

3. A boiler steam-water system for improving the flexibility of a coal-fired unit according to claim 2, characterized in that: A primary desuperheater is provided between the low-temperature superheater and the platen superheater, and a secondary desuperheater is provided between the platen superheater and the high-temperature superheater.

4. A method for controlling a boiler steam-water system for improving the flexibility of a coal-fired unit, characterized by: The following steps are involved: A) When a coal-fired unit is operating, a PID negative feedback calculation is performed on the deviation between the set value of the pressure regulating valve upstream of the superheated steam pressure circuit and the measured value of the pressure upstream of the valve. A PID positive feedback calculation is performed on the deviation between the set value of the active power circuit used for generating the coal-fired unit and the measured value of the active power. The set value of the active power is the preset output power of the coal-fired unit, and the measured value of the active power is the actual power generated by the steam generated by the boiler entering the steam turbine to drive the generator. B) The PID calculated values ​​of the two circuits are added together to obtain a comprehensive value, and the opening of the pressure regulating valve is adjusted according to the comprehensive value to adjust the load of the coal-fired unit.

5. The method for controlling a boiler steam-water system for improving the flexibility of a coal-fired unit according to claim 4, characterized in that: When the pressure regulating valve is in constant pressure operation mode, the pressure setting value before the valve is a fixed value.

6. The method for controlling a boiler steam-water system for improving the flexibility of a coal-fired unit according to claim 4, characterized in that: When the pressure regulating valve is in sliding pressure operation mode, the valve upstream pressure setting value should be higher than the main steam pressure setting value and increase with increasing load. The valve upstream pressure setting value should be greater than or equal to the main steam pressure setting value plus the pipeline resistance.

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