A denitration inlet flue gas temperature control method considering energy storage of boiler tail heating surface

Abstract

The application discloses a denitration inlet flue gas temperature control method considering energy storage of a boiler tail heating surface, which adopts a boiler economizer flue gas bypass damper and an economizer feedwater bypass valve to adjust the SCR denitration inlet flue gas temperature. The set value of the SCR denitration inlet flue gas temperature is obtained from the coal-fired unit power and the SCR denitration system inlet NOx concentration, the deviation of the SCR denitration inlet flue gas temperature and the set value is obtained by PID calculation to obtain the instructions of the economizer flue gas bypass damper and the feedwater bypass valve, and the boiler tail heating surface inlet flue gas temperature is used as the feedforward of the instructions of the flue gas bypass damper and the feedwater bypass valve. The energy storage change of the horizontal low-temperature reheater is used as the feedforward of the economizer flue gas bypass valve instruction, and the energy storage change of the low-reheat economizer is used as the feedforward of the economizer feedwater bypass valve instruction. The control method can effectively solve the problem that the SCR denitration inlet flue gas temperature fluctuates greatly during frequent and rapid load change of the coal-fired unit.

Description

Technical Field

[0001] The present invention relates to the technical field of coal-fired power generation, and in particular to an SCR denitrification inlet flue gas temperature control strategy taking into account the energy storage of a boiler rear heating surface. Background Art

[0002] Coal-fired power plants are the ballast of my country's secure and stable energy supply. In the future, they will assume greater responsibility for peak and frequency regulation to accommodate increased wind and solar power. Consequently, they will frequently experience wide load variations and rapid load rate changes, which will inevitably impact the operation of their denitrification systems.

[0003] SCR denitrification technology is the most widely used denitrification technology in coal-fired power plants. When coal-fired units are in frequent load-variable operation or low-load operation, the flue gas temperature at the SCR inlet will fluctuate greatly, and the temperature will frequently and long-term be lower than the efficient operating range of the SCR denitrification reaction temperature. This poses new challenges to the safe and efficient operation of the SCR denitrification system. Summary of the Invention

[0004] In order to solve the problems existing in the above-mentioned prior art, the purpose of the present invention is to provide an SCR denitrification inlet flue gas temperature control method taking into account the energy storage of the boiler tail heating surface, so as to ensure that the SCR denitrification inlet flue gas temperature can be better controlled during frequent load changes and low load operation of the unit.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for controlling the flue gas temperature at the denitrification inlet taking into account the energy storage of the boiler tail heating surface. The flue gas temperature instruction at the inlet of the SCR denitrification system is obtained by the load of the coal-fired unit and the NOx concentration at the inlet of the SCR denitrification system:

[0007]

[0008] Where, F1(x) is the command of the inlet flue gas temperature of the SCR denitrification system, ℃; NO is the NOx concentration at the inlet of the SCR denitrification system, mg m -3 ; is the NOx concentration at the inlet of the SCR denitrification system under the rated load of the coal-fired unit, mg m -3 ;P e is the load of coal-fired units, MW; is the rated load of the coal-fired unit, MW; k1 is the calculation coefficient, which is a fixed value;

[0009] The boiler economizer flue gas bypass damper opening instruction is obtained by PID calculation based on the deviation between the SCR denitrification inlet flue gas temperature and its instruction. Its feedforward instruction is obtained by the change of the boiler tail heating surface inlet flue gas temperature and the horizontal low-temperature reheater energy storage:

[0010] F3(x)=k2*T gin +k3*Y sd

[0011] Where, F3(x) is the feedforward of the boiler economizer flue gas bypass damper opening instruction; T gin Y is the flue gas temperature at the inlet of the boiler tail heating surface, ℃; sd is the change of energy storage in the horizontal low-temperature reheater, kJ; k2 and k3 are calculation coefficients, which are constants;

[0012] The boiler economizer feedwater bypass valve opening instruction is obtained by PID calculation based on the deviation between the SCR denitrification inlet flue gas temperature and its instruction. Its feedforward instruction is obtained by the inlet flue gas temperature of the boiler rear heating surface and the change of the economizer energy storage on the low-temperature reheater side:

[0013] F2(x)=k4*T gin +k5*Y dz

[0014] Where, F2(x) is the feedforward of the boiler economizer feedwater bypass valve opening instruction; Y dz is the change of the economizer energy storage on the low-temperature reheater side, kJ; k4 and k5 are calculation coefficients, which are constants;

[0015] During the load variation process of the coal-fired unit, the inlet flue gas temperature of the SCR denitrification system can be controlled by coordinating the opening of the economizer flue gas bypass damper and the economizer feed water bypass valve.

[0016] Preferably, when the deviation between the inlet flue gas temperature of the SCR denitration system and its instruction is greater than α1, the inlet flue gas temperature of the SCR denitration system is adjusted by the opening of the economizer flue gas bypass damper, and α1 is 8-12°C. When the deviation between the inlet flue gas temperature of the SCR denitration system and its instruction is less than α1, the inlet flue gas temperature of the SCR denitration system is adjusted by the opening of the boiler economizer feedwater bypass valve.

[0017] Preferably, the command F1(x) of the flue gas temperature at the inlet of the SCR denitrification system satisfies the following relationship:

[0018]

[0019] Where M1 is the minimum value of the SCR denitrification inlet flue gas temperature instruction, generally 300-320℃; It is the maximum value of the SCR denitrification inlet flue gas temperature instruction, generally 380~400℃.

[0020] Preferably, the feedforward F3(x) of the boiler economizer flue gas bypass damper opening instruction satisfies the following relationship:

[0021]

[0022] In the formula, M2 is generally taken as 0; Generally, it is taken as 0.3~0.5.

[0023] Preferably, the feedforward F2(x) of the boiler economizer feedwater bypass valve opening instruction satisfies the following relationship:

[0024]

[0025] In the formula, M3 is generally taken as 0; Generally, it is taken as 0.2~0.4.

[0026] When economizer feedwater bypass and flue gas bypass are used to regulate the SCR inlet flue gas temperature, the energy storage of the boiler tail heating surface is not considered. This is called the original control strategy for regulating the SCR inlet flue gas temperature. The control strategy of the present invention is called the optimized control strategy. Compared with the original control strategy, the control strategy of the present invention has the following specific advantages:

[0027] 1) The inlet flue gas temperature control strategy of the SCR denitrification system of the present invention can automatically switch the regulation of the flue gas bypass damper and the water bypass valve.

[0028] 2) The inlet flue gas temperature control strategy of the SCR denitrification system of the present invention takes into account the energy storage changes of the heating surface at the rear of the boiler, which can make the flue gas bypass baffle and the feed water bypass valve more accurate in adjusting the inlet flue gas temperature of the SCR denitrification system, and the inlet flue gas temperature adjustment of the SCR denitrification system is smoother.

[0029] Under these two control strategies, the variation of the flue gas temperature at the inlet of the SCR denitrification system during the load change from 75% to 50% THA of the coal-fired unit is as follows: Figure 2 As shown in the figure, it can be seen that after adopting the optimized control strategy, the fluctuation of the flue gas temperature at the inlet of the SCR denitrification system is significantly reduced. At the same time, the optimized control strategy proposed by this invention allows the flue gas temperature at the inlet of the SCR denitrification system to reach a stable value more quickly. Therefore, the optimization control strategy significantly improves the regulation effect of the flue gas temperature at the inlet of the SCR denitrification system. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Schematic diagram of the control method of the present invention.

[0031] Figure 2 This is the regulation effect of the SCR inlet flue gas temperature after adopting the optimization control method proposed in this invention. DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] like Figure 1 As shown, the present invention proposes a method for controlling the flue gas temperature at the denitrification inlet taking into account the energy storage of the boiler tail heating surface. The command F1(x) of the flue gas temperature at the inlet of the SCR denitrification system is obtained by the load of the coal-fired unit and the NOx concentration at the inlet of the SCR denitrification system:

[0034]

[0035] Where, F1(x) is the command of the inlet flue gas temperature of the SCR denitrification system, ℃; NO is the NOx concentration at the inlet of the SCR denitrification system, mg m -3 ; is the NOx concentration at the inlet of the SCR denitrification system under the rated load of the coal-fired unit, mg m -3 ;P e is the load of coal-fired units, MW; P e 0 is the rated load of the coal-fired unit, MW; k1 is the calculation coefficient, which is a fixed value;

[0036] The command for the flue gas bypass damper opening of the boiler economizer is obtained by PID calculation based on the deviation Δ1 between the flue gas temperature at the SCR denitrification inlet and its command. Its feedforward command F3(x) is obtained from the change in the flue gas temperature at the inlet of the boiler rear heating surface and the energy storage of the horizontal low-temperature reheater:

[0037] F3(x)=k2*T gin +k3*Y sd

[0038] Where, F3(x) is the feedforward of the boiler economizer flue gas bypass damper opening instruction; T gin Y is the flue gas temperature at the inlet of the boiler tail heating surface, ℃; sd is the change of energy storage in the horizontal low-temperature reheater, kJ; k2 and k3 are calculation coefficients, which are constants;

[0039] The boiler economizer feedwater bypass valve opening instruction is obtained by PID calculation based on the SCR denitrification inlet flue gas temperature and its instruction deviation Δ1. Its feedforward instruction F2(x) is obtained by the inlet flue gas temperature of the boiler rear heating surface and the change of the economizer energy storage on the low-temperature reheater side:

[0040] F2(x)=k4*T gin +k5*Y dz

[0041] Where, F2(x) is the feedforward of the boiler economizer feedwater bypass valve opening instruction; Ydz is the change of energy storage of low-recycle economizer, kJ; k4 and k5 are calculation coefficients, which are constants;

[0042] During the load variation process of the coal-fired unit, the inlet flue gas temperature of the SCR denitrification system can be controlled by coordinating the opening of the economizer flue gas bypass damper and the economizer feed water bypass valve.

[0043] When the deviation between the SCR denitration system inlet flue gas temperature and its command exceeds α1, the SCR denitration system inlet flue gas temperature is regulated by the economizer flue gas bypass damper opening, with α1 set to 8 to 12°C. When the economizer flue gas feedwater bypass valve is used to regulate the SCR denitration system inlet flue gas temperature, its adjustment range is limited. Setting α1 to 8 to 12°C ensures that the economizer feedwater bypass valve is preferentially used to regulate the SCR denitration system inlet flue gas temperature within its adjustment range.

[0044] When the deviation between the inlet flue gas temperature of the SCR denitrification system and its instruction is less than α1, the inlet flue gas temperature of the SCR denitrification system is regulated by the opening of the boiler economizer feed water bypass valve.

[0045] The command F1(x) of the inlet flue gas temperature of the SCR denitrification system satisfies the following relationship:

[0046]

[0047] Where M1 is the minimum value of the SCR denitrification inlet flue gas temperature instruction, generally 300-320°C, which is the minimum temperature required by the SCR denitrification system catalyst; It is the maximum value of the SCR denitrification inlet flue gas temperature instruction, generally 380-400℃. This value is the maximum temperature required by the SCR denitrification system catalyst.

[0048] The feedforward F3(x) of the boiler economizer flue gas bypass damper opening command satisfies the following relationship:

[0049]

[0050] In the formula, M2 is generally set to 0, indicating that the economizer flue gas bypass damper is fully closed; Generally, it is set to 0.3 to 0.5, which can ensure that the economizer flue gas bypass damper does not affect the safe operation of the coal-fired unit during the process of adjusting the flue gas temperature at the inlet of the SCR denitrification system.

[0051] The feedforward F2(x) of the boiler economizer feedwater bypass valve opening command satisfies the following relationship:

[0052]

[0053] In the formula, M3 is generally set to 0, indicating that the economizer feedwater bypass valve is fully closed; Generally, it is set to 0.2~0.4, which can ensure that the economizer feed water bypass valve does not affect the safe operation of the coal-fired unit during the process of adjusting the flue gas temperature at the inlet of the SCR denitrification system.

[0054] When economizer feedwater bypass and flue gas bypass are used to regulate the SCR inlet flue gas temperature, the energy storage of the boiler tail heating surface is not considered. This is called the original control strategy for regulating the SCR inlet flue gas temperature. The control strategy of the present invention is called the optimized control strategy. Compared with the original control strategy, the control strategy of the present invention has the following specific advantages:

[0055] 1) The inlet flue gas temperature control strategy of the SCR denitrification system of the present invention can automatically switch the regulation of the flue gas bypass damper and the water bypass valve.

[0056] 2) The inlet flue gas temperature control strategy of the SCR denitrification system of the present invention takes into account the energy storage changes of the heating surface at the rear of the boiler, which can make the flue gas bypass baffle and the feed water bypass valve more accurate in adjusting the inlet flue gas temperature of the SCR denitrification system, and the inlet flue gas temperature adjustment of the SCR denitrification system is smoother.

[0057] Under these two control strategies, the variation of the SCR denitrification inlet flue gas temperature during the 75% to 50% THA load change of the coal-fired unit is as follows: Figure 2 As shown in the figure, it can be seen that after adopting the optimized control strategy, the fluctuation of the flue gas temperature at the inlet of the SCR denitrification system is significantly reduced. At the same time, the optimized control strategy proposed by this invention allows the flue gas temperature at the inlet of the SCR denitrification system to reach a stable value more quickly. Therefore, the optimization control strategy significantly improves the regulation effect of the flue gas temperature at the inlet of the SCR denitrification system.

Claims

1. A method for controlling the flue gas temperature at the denitrification inlet taking into account the energy storage of the boiler rear heating surface, characterized by: The command of the flue gas temperature at the inlet of the SCR denitrification system is obtained by the load of the coal-fired unit and the NOx concentration at the inlet of the SCR denitrification system: Where, F1(x) is the command of the inlet flue gas temperature of the SCR denitrification system, ℃; NO is the NOx concentration at the inlet of the SCR denitrification system, mg m -3 ; is the NOx concentration at the inlet of the SCR denitrification system under the rated load of the coal-fired unit, mg m -3 ;P e is the load of coal-fired units, MW; is the rated load of the coal-fired unit, MW; k1 is the calculation coefficient, which is a fixed value; The boiler economizer flue gas bypass damper opening instruction is obtained by PID calculation based on the deviation between the SCR denitrification inlet flue gas temperature and its instruction. Its feedforward instruction is obtained by the change of the boiler tail heating surface inlet flue gas temperature and the horizontal low-temperature reheater energy storage: F3(x)=k2*T gin +k3*Y sd Where, F3(x) is the feedforward of the boiler economizer flue gas bypass damper opening instruction; T gin Y is the flue gas temperature at the inlet of the boiler tail heating surface, ℃; sd is the change of energy storage in the horizontal low-temperature reheater, kJ; k2 and k3 are calculation coefficients, which are constants; The boiler economizer feedwater bypass valve opening instruction is obtained by PID calculation based on the deviation between the SCR denitrification inlet flue gas temperature and its instruction. Its feedforward instruction is obtained by the inlet flue gas temperature of the boiler rear heating surface and the change of the economizer energy storage on the low-temperature reheater side: F2(x)=k4*T gin +k5*Y dz Where, F2(x) is the feedforward of the boiler economizer feedwater bypass valve opening instruction; Y dz is the change of the economizer energy storage on the low-temperature reheater side, kJ; k4 and k5 are calculation coefficients, which are constants; During the load variation process of the coal-fired unit, the flue gas temperature at the inlet of the SCR denitrification system is controlled by coordinating the opening of the economizer flue gas bypass damper and the economizer feed water bypass valve.

2. A method for controlling the temperature of flue gas at the denitrification inlet taking into account the energy storage of the boiler rear heating surface according to claim 1, characterized in that: When the deviation between the inlet flue gas temperature of the SCR denitration system and its instruction is greater than α1, the inlet flue gas temperature of the SCR denitration system is adjusted by the opening of the flue gas bypass damper of the economizer, and α1 is 8~12℃; When the deviation between the inlet flue gas temperature of the SCR denitrification system and its instruction is less than α1, the inlet flue gas temperature of the SCR denitrification system is regulated by the opening of the boiler economizer feed water bypass valve.

3. A method for controlling the temperature of flue gas at the denitrification inlet taking into account the energy storage of the boiler rear heating surface according to claim 1, characterized in that: The command F1(x) of the inlet flue gas temperature of the SCR denitrification system satisfies the following relationship: Where M1 is the minimum value of the SCR denitrification inlet flue gas temperature instruction, which is 300-320℃; The maximum value of the SCR denitrification inlet flue gas temperature instruction is 380~400℃.

4. A method for controlling the temperature of flue gas at the denitrification inlet taking into account the energy storage of the boiler rear heating surface according to claim 1, characterized in that: The feedforward F3(x) of the boiler economizer flue gas bypass damper opening command satisfies the following relationship: Where, M2 is 0; Take 0.3~0.

5.

5. The method for controlling the flue gas temperature at the denitrification inlet taking into account the energy storage of the boiler rear heating surface according to claim 1, characterized in that: The feedforward F2(x) of the boiler economizer feedwater bypass valve opening command satisfies the following relationship: Where, M3 is 0; Take 0.2~0.4.

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