CN115025616A - Automatic control method for urea method SCR denitration technology of thermal power generating unit - Google Patents

Abstract

The invention discloses an automatic control method for a urea method SCR denitration technology of a thermal power generating unit, which comprises the steps of proportional-integral-derivative control PID cascade control, wherein the first-stage PID control is used for adjusting the deviation between the average concentration value of nitrogen oxide NOx at an outlet and a set value of the average concentration value, and the output of the first-stage PID control is part of a second-stage PID urea flow instruction; secondly, performing predictive control, wherein the output of the predictive control is the other part of the second-stage PID urea flow instruction, and the two parts are added to form a urea flow instruction; the second stage PID control is used for adjusting the deviation between the urea flow instruction and the actual urea flow; in addition, the Smith prediction circuit calculates the action command of the urea spray gun, and the action command and the output command of the first part of PID series are subjected to subtraction calculation to obtain the final action command of the urea spray gun; the opening of the urea spray gun is controlled by the action command of the urea spray gun, and finally, the automatic control of the whole urea method SCR denitration technology is realized. The invention realizes the advanced adjustment of the urea flow spray gun and avoids the disturbance caused by large inertia.

Description

A kind of automatic control method of urea method SCR denitration technology for thermal power unit

technical field

The invention relates to the technical field of automatic control of thermal power plants, in particular to an automatic control method of urea method SCR denitration technology for thermal power units.

Background technique

my country's energy consumption is mainly dominated by coal, and most of the carbon dioxide, nitrogen oxides, and soot emissions in air pollution emissions come from the combustion of coal. The discharged pollutants cause great harm to the ecological environment and have a great impact on human health. With the rapid development of the economy, the government has become more and more aware of the necessity and urgency of environmental protection, and nitrogen oxides (NO _X ), as a major source of air pollution, mainly refers to the types of NO and NO ₂ Environmental pollutants, directly or indirectly caused by acid rain, greenhouse effect, ozone layer destruction, photochemical smog, etc. are the key issues of ecological environment governance.

More than 90% of nitrogen oxides (NO _x ) in the flue gas emissions of thermal power units are in the form of NO. SCR denitration technology uses chemical reactions to reduce the pollutants in this part of the flue gas to harmless ones. The nitrogen (N ₂ ) and water (H ₂ 0) of nitrogen oxides (N 2 ) and water (H 2 0 ) are reduced to reduce the emission of nitrogen oxides (NO _X ) within the national standard, which relieves the pressure on the environment to a certain extent. The main advantage of this denitration technology is that the conversion rate of nitrogen oxides (NO _X ) is high, and the reaction products are safe and harmless, and it has been widely used in various types of thermal power plants.

The reducing agent used in SCR denitration technology is usually:

1) Liquid ammonia

Liquid ammonia is a colorless gas with a pungent odor at room temperature. Chemically unstable, toxic, and easy to burn, easily combustible and explosive. When ammonia leaks, it will seriously damage the health of the human body.

2) Ammonia water

Ammonia water is an aqueous solution of about 20% to 30%, which is relatively safe. Because of the large transportation volume, the transportation cost is high. Ammonia is weakly alkaline and strong antiseptic, which is harmful to the human body, especially when it reaches a certain concentration in the air, it will explode.

3) Urea

As a new type of environmentally friendly reducing agent, urea is white solid particles or crystals at room temperature. In industrial applications, pyrolysis of urea is often used to produce ammonia vapor. Compared with liquid ammonia and ammonia water, the biggest advantage of urea is its safety.

No matter which reagent is used as the reducing agent, the essence of SCR denitration technology is a chemical reaction with a certain reaction process. This process requires a certain reaction time to complete the reduction of nitrogen oxides (NO _X ) emissions. From the perspective of , this controlled system is called a control system with large delay and large inertia, which is characterized by difficult control and poor control accuracy. Compared with the liquid ammonia denitration technology, the urea denitrification technology adds the process engineering of pyrolysis of urea into ammonia vapor, and further prolongs the reaction time of reducing nitrogen oxides (NO _x ). For automatic control, It will be more difficult to control.

At present, the vast majority of newly built thermal power units use urea denitrification technology. Among the old thermal power units, a large number of units have changed their liquid ammonia denitration technology to urea denitration technology for safety reasons. In the future, urea as a reducing agent will occupy an increasing proportion in the SCR denitration technology of thermal power units, and the automatic control of this type of unit will also become more and more important.

SUMMARY OF THE INVENTION

In order to solve the characteristics of large delay and large inertia in the above-mentioned urea method SCR denitration technology, the purpose of the present invention is to propose an automatic control method for the urea method SCR denitration technology for thermal power units, and adopt a more reasonable control strategy to ensure the urea method SCR denitration technology. The nitrogen oxides (NO _X ) at the outlet of the controlled unit of the technology are stable and meet the economical and safe operation index of the unit.

To achieve the above object, the present invention is implemented by the following technical solutions:

An automatic control method for urea method SCR denitration technology for thermal power units, comprising the following steps:

Step 1: proportional integral derivative control PID cascade control, the first stage PID control is to correct and adjust the deviation between the average value of the outlet nitrogen oxide NOX concentration and the set value of the outlet nitrogen oxide NOX concentration, and its output is used as the second stage. It is a part of the urea flow command of PID level PID; secondly, predictive control, according to the change trend of coal volume, air volume and oxygen content inside the boiler, predicts the change trend of outlet nitrogen oxide (NOX) concentration, and obtains the prediction control loop calculation required Finally, according to the change trend of the average value of nitrogen oxide (NOX) concentration at the inlet of the denitration equipment, calculate the urea flow command required for one route, and add these parts of the urea flow command to obtain the final required urea flow command. , through the second-stage PID and the actual urea flow to correct and adjust, and obtain the action command of the urea spray gun;

Step 2: The Smith predictive control loop calculates the action command of a urea spray gun, and calculates by subtraction with the output command of the first part of the PID series pole to obtain the final action command of the urea spray gun;

Step 3: The opening degree instruction 18 of the urea flow adjustment spray gun is used to control the opening degree of the urea flow adjustment spray gun 4, and finally the automatic control of the entire urea method SCR denitration technology is realized.

Compared with the prior art, the present invention has the following advantages:

In view of the characteristics of large delay and large inertia of urea method SCR denitrification technology, the optimal prediction point should be found as much as possible to judge the change trend of nitrogen oxide (NOX) concentration in advance.

Creatively divide the effect of oxygen on nitrogen oxide (NOX) concentration into rapid coarse adjustment and accurate accuracy fine adjustment, introduce the change of air-to-coal ratio, and quickly predict the change trend of nitrogen oxide (NOX) concentration in advance , On this basis, the oxygen quantity of the boiler is quoted again to finally accurately judge the changing trend of nitrogen oxides (NOX) concentration, and control the nitrogen oxides (NOX) concentration from a technological point of view to ensure that the problem is fundamentally solved.

The cascade control strategy is adopted, considering the deviation between the concentration of nitrogen oxides (NOX) and its concentration setting value, and also taking into account whether the urea flow rate meets its flow rate setting value requirements, ensuring that each loop meets the requirements, and finally realizes the control of nitrogen oxides. Precise control of oxide (NOX) concentration.

Combined with the chemical reaction process of the urea method SCR denitration technology, a similar model is constructed, and the Smith prediction control algorithm is used to realize the early callback of the urea flow adjustment spray gun, avoiding the disturbance caused by the large inertia.

The concentration of nitrogen oxides (NOX) under the control of this strategy can realize the automatic full input of the urea flow adjustment spray gun, adapt to various lifting and load conditions of the unit, greatly reduce the operating burden of the operators, and provide further support for the intelligent operation of the power plant. Assure.

Description of drawings

FIG. 1 is a schematic diagram of the control system of the present invention.

Description of reference numbers:

1——High temperature hot air from the primary hot air outlet; 2——Urea pyrolysis furnace;

3——The urea solution from the urea pump; 4——The urea flow adjustment spray gun;

5——Measuring point of urea flow signal; 6——Original flue gas from economizer;

7—reactors on both sides of A and B; 8—flue gas to air preheater flue;

9——Signal measurement points of nitrogen oxide (NO _X ) concentration at the inlet on both sides of A and B;

10——Measurement points of nitrogen oxide (NO _X ) concentration signal at the outlet on both sides of A and B;

11——The set value of outlet nitrogen oxide (NO _X ) concentration;

12—the average value of nitrogen oxide (NO _X ) concentration at the outlet;

13—the average value of the inlet nitrogen oxides (NO _X ) concentration;

14 - total air volume; 15 - total coal volume; 16 - boiler oxygen volume;

17——Total urea flow into the pyrolysis furnace;

18 - urea flow adjustment spray gun opening command;

19—the first PID controller; 20—the first subtractor;

21—the first differential operator; 22—the first function generator;

23 - the second differential operator; 24 - the first addition operator;

25 - division operator; 26 - third differential operator;

27 - the fourth differential operator; 28 - the second adder;

29—the third adder; 30—the second PID controller;

31—the second subtractor; 32—the pure lag operator;

33 - inertia calculator;

Detailed ways

An automatic control system for urea method SCR denitration technology for thermal power units, including high-temperature hot air 1 from a primary hot air outlet, urea solution 3 from a urea pump, and urea solution 3 from the urea pump through four horizontally arranged urea flow adjustment spray guns The distribution of 4, the required urea is sent into the urea pyrolysis furnace 2, and the urea is pyrolyzed into ammonia steam by the high-temperature hot air 1 from the primary hot air outlet in the urea pyrolysis furnace 2, and the ammonia steam from the urea pyrolysis furnace 2 Divided into the reactors 7 on both sides of A and B respectively, the raw flue gas 6 from the economizer enters the reactors 7 on both sides of A and B respectively through the flue on both sides of the boiler, and the reactors 7 on both sides of A and B respectively. Inside, the ammonia vapor is mixed with NO in the original flue gas, and the reduction and denitration reaction is carried out under the action of the catalyst. Finally, the flue gas that meets the national standard after denitration is sent to the air preheater through the flue gas to the air preheater flue 8 Flue; 4 horizontally arranged urea flow regulating spray guns 4 to control the urea demand of the entire reaction process, the first PID controller 19, the second adder 28, the second PID controller connected with the urea flow regulating spray gun 4 30 and the second subtractor 31, by calculating the opening degree instruction 18 of the urea flow adjustment spray gun, to control the opening degree of the urea flow adjustment spray gun 4; a corresponding urea flow adjustment spray gun 4 is arranged at the outlet of each urea flow adjustment spray gun 4. Urea flow signal measuring point 5, a nitrogen oxide (NO _X ) concentration signal measuring point 9 is arranged at the inlet of the reactor on both sides of A and B, and the outlet of the reactor on both sides of A and B is arranged. A nitrogen oxide (NO _x ) concentration signal measuring point 10 is arranged at the outlet on both sides of A and B.

The first PID controller 19 and the second PID controller 30 work together as a set of cascade control loops, the first PID controller 19 is used as the main controller, and its input signal includes two channels, the first channel is the outlet nitrogen Oxide (NO _X ) concentration setting value 11, the implementation of this setting value is to directly set by the operator according to the demand; the second path is the average value 12 of the outlet nitrogen oxide (NO _X ) concentration that needs to be controlled and adjusted, which is determined by The nitrogen oxide (NO _x ) concentration signal measuring points 10 at the outlets on both sides of A and B are calculated by taking the average value; the first PID controller 19 includes proportional P and integral I functions, and its output is a part of the urea flow command. The PID controller 19 adjusts the deviation of the concentration of nitrogen oxides ( _NOx ) at the outlet in a small range, and the large _- scale adjustment is completed by the predictive control loop; Concentration deviation value prediction loop, the control idea is: outlet nitrogen oxides ( _NOx ) concentration average value 12 and outlet nitrogen oxides ( _NOx ) concentration set value 11 are calculated by the first subtractor 20 to obtain outlet nitrogen oxides (NO _X ) concentration deviation value, and then calculated by the first differential calculator 21 to obtain the urea flow command corresponding to the change in the outlet nitrogen oxide (NO _X ) concentration deviation value; the second path is the inlet nitrogen oxide (NO X ) concentration deviation value. _X ) concentration prediction loop, this loop can be understood as proportional + differential (PD) link, the control idea is: the average concentration of inlet nitrogen oxides (NO _x ) is 13, from the inlet nitrogen oxides (NO _x ) on both sides of A and B The concentration signal measuring point 9 is calculated by taking the average value, and the average value 13 of the inlet nitrogen oxide (NO _X ) concentration after the averaging process is calculated by the first function generator 22 to obtain the inlet nitrogen oxide (NO _X ) concentration. The urea flow command corresponding to the average value can be understood as a proportional link, and the average value 13 of the inlet nitrogen oxide (NO _X ) concentration is calculated by the second differential calculator 23 to obtain the inlet nitrogen oxide (NO _X ) The urea flow command corresponding to the differential of the average concentration value, this part is a differential link, the output value of the first function generator 22 and the output value of the second differential operator 23 are added by the first adder 24 to obtain the entry The final urea flow command corresponding to the average value of nitrogen oxide (NO _X ) concentration; the third route is the air-to-coal ratio prediction control loop, and the control idea is: the total air volume 14 signal and the total coal volume 15 signal are introduced, and the division operator 25 Calculate the instantaneous air-to-coal ratio value, and then calculate through the third differential calculator 26 to obtain the urea flow command corresponding to the change of the total air volume 14 and the total coal volume 15; the fourth route is the boiler oxygen volume prediction control loop, The control idea is: introduce the boiler oxygen quantity 16 signal, and obtain the urea flow command corresponding to the change of the boiler oxygen quantity 16 through the calculation by the fourth differential calculator 27; Obtain the urea flow prediction control loop command, the first PID control The urea flow command output by the controller 19 and the urea flow prediction control loop command are added through the third adder 29 to obtain the final urea flow command; the second PID controller 30 is used as a secondary adjustment controller, and its input signal includes two channels. , the first road is the urea flow command calculated by the third adder 29; the second road is the total urea flow rate 17 entering the pyrolysis furnace, and the total urea flow rate 17 entering the pyrolysis furnace is adjusted by the urea flow rate of the four urea The urea flow signal measuring points 5 at the outlet of the spray gun are added to obtain; the second PID controller 30 includes proportional P and integral I functions, and its output is the urea flow adjustment spray gun opening command.

The input signal of the second subtractor 31 includes two paths, the first path is the output of the second PID controller 30 calculated through the cascade control loop; the second path is a Smith prediction algorithm control loop. , the urea flow adjustment spray gun opening command 18 is calculated by the delay compensation of the pure lag calculator 32 and the inertia calculator 33 to obtain the urea flow adjustment spray gun opening compensation value that needs to be compensated, and the two signals pass through the second subtraction calculator 31 Calculate the difference value of , and obtain the final control output urea flow adjustment spray gun opening command 18 .

A control method for an automatic control system of urea method SCR denitration technology for thermal power units, the control method is composed of two parts, one part is PID cascade control, the average value of outlet nitrogen oxide (NO _X ) concentration and the outlet nitrogen oxide (NO _X ) are controlled. ) The deviation between the concentration setting values is corrected and adjusted, and according to the reaction characteristics of the boiler of the thermal power unit and the denitration equipment, the parameters that mainly affect the change of the nitrogen oxide (NO _x ) concentration at the outlet are extracted, and the prediction control is carried out in advance to overcome the urea concentration. The difficulty in controlling the inertia of the denitrification technology is large, and the other part is the Smith prediction algorithm control. The chemical process time of the urea reduction and denitrification reaction is simulated by establishing a model, and the control system is adjusted in advance to avoid the delay disturbance caused by the large inertia; Among them, the urea flow adjustment spray gun 4 controls the amount of urea entering the urea pyrolysis furnace 2 to adjust the outlet nitrogen oxide (NO _x ) concentration through the cascade control mode. It is the second PID controller 30, and the average value 12 of the outlet nitrogen oxide ( _NOx ) concentration is calculated by taking the average value of the outlet nitrogen oxide ( _NOx ) concentration signal measuring points 10 on both sides of A and B, which is the first PID control. The controlled object of the controller 19, the set value is the outlet nitrogen oxide (NO _X ) concentration set value 11. The realization of the set value is that the operator directly sets it according to the demand. The adjustment of the first PID controller 19 includes: Proportional P action, integral I action, when the outlet nitrogen oxide (NO _X ) concentration average value 12 increases, the outlet nitrogen oxide (NO _X ) concentration average value 12 and the outlet nitrogen oxide (NO _X ) concentration set value There is a positive deviation between 11 and 11, so that the proportional P action and the integral I action of the first PID controller 19 start to act, and an action command to increase the output of the first PID controller 19 is issued; similarly, when the outlet nitrogen oxide (NO _x ) concentration When the average value 12 decreases, a negative deviation occurs between the average value 12 of the outlet nitrogen oxide (NO _X ) concentration and the set value 11 of the outlet nitrogen oxide (NO _X ) concentration, so that the proportional P of the first PID controller 19 acts, The integral I action starts to act, and an action command to reduce the output of the first PID controller 19 is issued; the predictive control loop is designed with four circuits in total; The inertial controlled object, the use of differential control can effectively alleviate the characteristics of its hysteresis, the outlet nitrogen oxide (NO _X ) concentration average value 12 and outlet nitrogen oxide (NO _X ) concentration set value 11 through the first subtractor 20 Calculate the outlet nitrogen oxide (NO _x ) concentration deviation value, and then calculate through the first differential calculator 21 to obtain the urea flow command corresponding to the change in the outlet nitrogen oxide (NO _x ) concentration deviation value. When the average value 12 of the concentration of oxides (NO _x ) increases, the deviation value of the concentration of nitrogen oxides (NO _x ) at the outlet will also increase, and the differential action of the first differential calculator 21 starts to act, and an increase is issued. Add the action command output by the first differential calculator 21; similarly, when the outlet nitrogen oxide (NO _x ) concentration average value 12 decreases, the outlet nitrogen oxide (NO _x ) concentration deviation value will also decrease, the first differential calculator The differential action of 21 starts to act, and an action command to reduce the output of the first differential calculator 21 is issued; the second predictive control loop: the inlet nitrogen oxide (NO _X ) concentration prediction loop, for the denitration system, when the ammonia vapor flow rate is constant. Under the premise, the change trend of the inlet nitrogen oxide (NO _X ) concentration is completely consistent with the change trend of the outlet nitrogen oxide (NO _X ) concentration. The inlet concentration increases, and the outlet concentration also increases after a certain reaction time. Similarly, When the inlet concentration decreases, the outlet concentration will also decrease after a certain reaction time. Therefore, the change of the inlet nitrogen oxide (NO _X ) concentration is selected to intervene in advance, so that the urea flow reaches the demand value in advance, so as to ensure the outlet nitrogen oxide (NO _X ) ) concentration is maintained near its set value; the specific implementation method is that the average value of the inlet nitrogen oxide (NO _X ) concentration is 13, and the average value is obtained from the inlet nitrogen oxide (NO _X ) concentration signal measuring points 9 on both sides of A and B. After calculation, the average value 13 of the inlet nitrogen oxide (NO _x ) concentration after the averaging process is calculated by the first function generator 22 to obtain the urea flow command corresponding to the average value of the inlet nitrogen oxide (NO _x ) concentration , the specific setting parameters of the first function generator 22 are shown in Table 1:

Table 1: The average value of the inlet nitrogen oxide (NO _X ) concentration corresponds to the urea flow command function table

The meaning of the setting of the first function generator 22 is to provide the required urea flow command in real time according to the change of the average value 13 of the inlet nitrogen oxide (NO _X ) concentration, and maintain the average value of the outlet nitrogen oxide (NO _X ) concentration in advance. 12 is stable; in addition, the average value 13 of the inlet nitrogen oxide (NO _x ) concentration is calculated by the second differential calculator 23 to obtain the urea flow command corresponding to the differential of the average value of the inlet nitrogen oxide (NO _x ) concentration, when When the average value 13 of the inlet nitrogen oxide (NO _x ) concentration increases, the differential action of the second differential calculator 23 starts to act, and an action command to increase the output of the second differential calculator 23 is issued; similarly, when the inlet nitrogen oxide (NO x ) _X ) when the concentration average value 13 decreases, the differential action of the second differential calculator 23 starts to act, and sends out an action command to reduce the output of the second differential calculator 23; the output value of the first function generator 22 is related to the second differential calculator 23 The output value of the urea is added by the first adder 24 to obtain the urea flow command corresponding to the average value of the inlet nitrogen oxide (NO _x ) concentration; The main factors for the production of (NO _X ) are high temperature and oxygen enrichment. Considering that the temperature does not change much, oxygen enrichment is the most important factor for the production of nitrogen oxides (NO _X ). The amount of oxygen depends on the amount of air and coal. The relative proportion, and the change of air volume and coal volume will directly affect the change of nitrogen oxides (NO _X ), so the total air volume 14 signal and the total coal volume 15 signal entering the boiler furnace are selected here, which can realize the control of nitrogen oxides (NO _X ). ) change the earliest preliminary judgment, the total air volume 14 signal and the total coal volume 15 signal, the instantaneous wind-coal ratio value is calculated by the division operator 25, and then calculated by the third differential operator 26 to obtain the change of the wind-coal ratio The urea flow command corresponding to the amount of urea, when the dividing calculator 25 calculates that the instantaneous air-to-coal ratio value increases, it means that the change of the air volume is instantaneously greater than the change of the coal amount, and the differential action of the third differential calculator 26 starts to act. , issue an action instruction to increase the output of the third differential calculator 26; similarly, when the instantaneous air-to-coal ratio value is reduced by the division calculator 25, it means that the instantaneous change of the air volume is smaller than the change of the coal volume, and the third differential operation The differential action of the controller 26 starts to act, and an action command to reduce the output of the third differential calculator 26 is issued; the fourth predictive control loop: the above-mentioned predictive control of the air-to-coal ratio mainly realizes rapidity, which belongs to the effect of oxygen on nitrogen oxides (NO _X ) is a kind of rough adjustment, so on this basis, considering the complexity of the combustion inside the boiler, the oxygen signal of the boiler is introduced to accurately characterize the change of nitrogen oxides (NO _X ), and the accuracy of adjustment can be achieved. A kind of rough adjustment of nitrogen oxides (NO _X ); the signal of boiler oxygen quantity 16 is introduced, and after calculation by the fourth differential calculator 27, the urea flow command corresponding to the change of the boiler oxygen quantity 16 is obtained, when the boiler oxygen quantity 16 liters When it is high, the differential action of the fourth differential operator 27 starts to act, and it sends out Increase the action command output by the fourth differential calculator 27; similarly, when the boiler oxygen volume 16 decreases, the differential action of the fourth differential calculator 27 starts to act, and an action command to reduce the output of the fourth differential calculator 27 is issued; four-way prediction The control loop is calculated by the second adder 28 to obtain the urea flow prediction control loop command. The urea flow command and the urea flow prediction control loop command output by the first PID controller 19 are calculated by the third adder 29 to obtain the final urea flow. Flow command; the second PID controller 30 is used as a secondary controller, and its input signal includes two channels, the first channel is the urea flow command calculated by the third adder 29; the second channel is the total flow rate entering the pyrolysis furnace. Urea flow 17, the second PID controller 30 includes proportional P and integral I functions, and its output is the urea flow adjustment spray gun opening command, when the urea flow command calculated by the third adder 29 increases, the third addition There is a positive deviation between the urea flow command calculated by the calculator 29 and the total urea flow 17 entering the pyrolysis furnace, so that the proportional P action and the integral I action of the second PID controller 30 start to act, and an increase in the second PID control is issued. Similarly, when the urea flow command calculated by the third adder 29 decreases, the difference between the urea flow command calculated by the third adder 29 and the total urea flow rate 17 entering the pyrolysis furnace There is a negative deviation between the two, so that the proportional P action and the integral I action of the second PID controller 30 start to act, and an action command to reduce the output of the second PID controller 30 is issued; The denitration technology is a controlled object with large delay and large inertia. The pure lag operator 32 simulates the pure lag link in the algorithm, and the inertial operator 33 simulates the capacity lag link in the algorithm, and the loop after the compensation is controlled by the Smith prediction algorithm. , will not adversely affect the system, but only time-shift the output signal of the original second PID controller 30. The working principle of the pure lag operator 32 is to delay the output of the input signal, and the delay time is the pure lag operator. 32 is the delay time set inside the inertial calculator 33. The working principle of the inertial calculator 33 is to pass the input signal through a certain transition time to make the output value equal to the input value. The transition time is the inertial time set inside the inertial calculator 33. Simulate the chemical process time of the urea reduction and denitration reaction with the inertial calculator 33; when the urea flow adjustment spray gun opening command 18 changes compared to the previous moment, through the delay compensation calculation of the pure lag calculator 32 and the inertial calculator 33, the required The compensated urea flow adjustment spray gun opening compensation value is then calculated with the output value of the second PID controller 30 through the second subtractor 31 to obtain the real-time urea flow adjustment spray gun opening command 18, and there is no Smith prediction algorithm control. Compared with the loop, the system can perform callbacks in advance, avoiding disturbances caused by large inertia.

Claims (9)

1. An automatic control method for a urea method SCR denitration technology of a thermal power generating unit is characterized by comprising the following steps:

the method comprises the following steps: PID cascade control is carried out by proportional integral derivative control, wherein the first stage PID control is used for correcting and adjusting the deviation between the average value of the concentration of the NOx in the outlet nitrogen oxide and the set value of the concentration of the NOx in the outlet nitrogen oxide, and the output of the first stage PID control is used as a part of a second stage PID urea flow instruction; secondly, forecasting the change trend of the concentration of outlet Nitrogen Oxide (NOX) according to the change trends of the coal quantity, the air quantity and the oxygen quantity inside the boiler, obtaining a urea flow instruction required by the calculation of the forecasting control loop, calculating a path of required urea flow instruction according to the change trend of the average value of the concentration of the inlet Nitrogen Oxide (NOX) of the denitration device, adding the urea flow instructions to obtain a final required urea flow instruction, and correcting and adjusting the actual urea flow through a second-stage PID (proportion integration differentiation) to obtain an action instruction of the urea spray gun;

step two: the Smith predictive control loop calculates an action command of a urea spray gun, and the action command and the output command of the first part of PID series are subjected to subtraction calculation to obtain a final action command of the urea spray gun;

step three: the opening of the urea flow adjusting spray gun 4 is controlled by the opening instruction 18 of the urea flow adjusting spray gun, and finally, the whole urea SCR denitration technology is automatically controlled.

2. The automatic control method for the urea-process SCR denitration technology of the thermal power generating unit according to claim 1, characterized in that,

the method comprises the following steps: PID cascade control refers to proportional-integral-derivative control, wherein deviation between an average outlet Nitrogen Oxide (NOX) concentration value and a set outlet Nitrogen Oxide (NOX) concentration value is corrected and adjusted through a proportional-integral function of PID, and according to reaction characteristics of the interior of a boiler of a thermal power unit and denitration equipment, parameters mainly influencing outlet Nitrogen Oxide (NOX) concentration change are extracted, and prediction control is performed in advance; the predictive control loop includes four paths: the first path of prediction control loop: a differential control loop for the deviation between the average outlet nitrogen oxide (NOx) concentration and a set outlet nitrogen oxide (NOx) concentration value; the second predictive control loop: an inlet nitrogen oxide (NOx) concentration prediction loop; a third predictive control loop: a wind coal ratio prediction control loop; the fourth prediction control loop: the oxygen amount prediction control loop in the boiler, the four prediction control loops are calculated by a second addition arithmetic unit 28 to obtain the urea flow prediction control loop instruction; the urea flow instruction output by the first PID controller 19 and the urea flow prediction control loop instruction are calculated by a third addition arithmetic unit 29 to obtain a final urea flow instruction; the second PID controller 30 is used as an auxiliary controller, and its input signal includes two paths, the first path is the urea flow instruction calculated by the third addition operator 29; the second path is the total urea flow 17 entering the pyrolysis furnace, the second PID controller 30 comprises the proportional P and integral I functions, the output of the second PID controller is a urea flow adjusting spray gun opening instruction, when the urea flow instruction calculated by the third addition operator 29 is increased, a positive deviation occurs between the urea flow instruction calculated by the third addition operator 29 and the total urea flow 17 entering the pyrolysis furnace, the proportional P function and the integral I function of the second PID controller 30 start to act, and an action instruction for increasing the output of the second PID controller 30 is sent; similarly, when the urea flow command calculated by the third adder 29 decreases, a negative deviation occurs between the urea flow command calculated by the third adder 29 and the total urea flow 17 entering the pyrolysis furnace, so that the proportional P action and the integral I action of the second PID controller 30 start to act, an action command for reducing the output of the second PID controller 30 is sent, and finally, an opening command of the urea flow regulation spray gun of the PID cascade prediction control loop is obtained.

3. The automatic control method for the thermal power generating unit urea method SCR denitration technology according to claim 2, characterized in that the PID cascade control is mainly implemented by a first PID controller 19 and a second PID controller 30, the average value of the concentration 12 of the outlet nitrogen oxide (NOx) obtained by averaging the signal measuring points 10 of the concentration of the outlet nitrogen oxide (NOx) at both sides A, B is the controlled object of the first PID controller 19, the set value is the set value 11 of the concentration of the outlet nitrogen oxide (NOx), the set value is implemented by directly setting by an operator according to the requirement, the adjustment of the first PID controller 19 comprises a proportional P function and an integral I function, when the average value 12 of the concentration of the outlet nitrogen oxide (NOx) rises, a positive deviation occurs between the average value 12 of the concentration of the outlet nitrogen oxide (NOx) and the set value 11 of the concentration of the outlet nitrogen oxide (NOx), starting the proportional P action and integral I action of the first PID controller 19, and sending an action command for increasing the output of the first PID controller 19; similarly, when the average outlet Nitrogen Oxide (NOX) concentration 12 decreases, a negative deviation occurs between the average outlet Nitrogen Oxide (NOX) concentration 12 and the set outlet Nitrogen Oxide (NOX) concentration 11, the proportional P action and the integral I action of the first PID controller 19 start to operate, and an operation command for decreasing the output of the first PID controller 19 is issued.

4. The automatic control method for the thermal power generating unit urea method SCR denitration technology according to claim 2, characterized in that a first prediction control loop: the differential control law has a lead characteristic, the characteristic that the differential control can effectively relieve the lag is adopted, the outlet Nitrogen Oxide (NOX) concentration average value 12 and the outlet Nitrogen Oxide (NOX) concentration set value 11 are calculated by a first subtraction operator 20 to obtain an outlet Nitrogen Oxide (NOX) concentration deviation value, then a urea flow instruction corresponding to the variation of the outlet Nitrogen Oxide (NOX) concentration deviation value is obtained by calculation of a first differential operator 21, when the outlet Nitrogen Oxide (NOX) concentration average value 12 is increased, the outlet Nitrogen Oxide (NOX) concentration deviation value is also increased, the differential action of the first differential operator 21 starts to act, and an action instruction for increasing the output of the first differential operator 21 is sent; similarly, when the average outlet Nitrogen Oxide (NOX) concentration 12 decreases, the outlet Nitrogen Oxide (NOX) concentration deviation value also decreases, the differentiating action of the first differential operator 21 starts to operate, and an operation command for decreasing the output of the first differential operator 21 is issued.

5. The automatic control method for the urea-process SCR denitration technology of the thermal power generating unit as claimed in claim 2, wherein the second predictive control loop comprises: an inlet Nitrogen Oxide (NOX) concentration prediction loop selects the change of the inlet nitrogen oxide NOX concentration and intervenes in advance to make the urea flow reach a required value in advance so as to ensure that the outlet nitrogen oxide NOX concentration is maintained near a set value; the specific implementation mode is as follows: the inlet nitrogen oxide NOx concentration average value 13 is obtained by averaging inlet nitrogen oxide NOx concentration signal measuring points 9 on two sides of A, B, the averaged inlet nitrogen oxide NOx concentration average value 13 is calculated through a first function generator 22, and a urea flow instruction corresponding to the inlet nitrogen oxide NOx concentration average value is obtained, wherein the first function generator 22 is set to provide a required urea flow instruction in real time according to the change of the inlet nitrogen oxide NOx concentration average value 13, and the stability of the outlet nitrogen oxide NOx concentration average value 12 is maintained in advance; the inlet nitrogen oxide NOx concentration average value 13 is calculated by a second differential operator 23 to obtain a urea flow instruction corresponding to the differential of the inlet nitrogen oxide NOx concentration average value, and when the inlet nitrogen oxide NOx concentration average value 13 rises, the differential action of the second differential operator 23 starts to act to send an action instruction for increasing the output of the second differential operator 23; similarly, when the inlet nitrogen oxide NOX concentration average value 13 decreases, the differentiating action of the second differential operator 23 starts to operate, and an operation command for decreasing the output of the second differential operator 23 is issued; the output value of the first function generator 22 and the output value of the second differential operator 23 are added by the first adder 24 to obtain a urea flow rate command corresponding to the average value of the concentration of the inlet Nitrogen Oxides (NOX).

6. The automatic control method for the thermal power generating unit urea method SCR denitration technology according to claim 2, characterized in that a third predictive control loop: the air-coal ratio prediction control loop is characterized in that the main generation factors of nitrogen oxide NOX are high temperature and oxygen enrichment, and under the condition of considering that the temperature change is not large, the oxygen enrichment is the most main generation factor of the nitrogen oxide NOX, the amount of oxygen depends on the relative proportion of air quantity and coal quantity, and the change of the air quantity and the coal quantity can directly influence the change of the nitrogen oxide NOX, so that a total air quantity 14 signal and a total coal quantity 15 signal entering a boiler furnace are selected, a primary judgment of the earliest change of the nitrogen oxide NOX can be realized, the total air quantity 14 signal and the total coal quantity 15 signal are calculated by a division arithmetic unit 25 to obtain an instantaneous air-coal ratio value, a urea flow instruction corresponding to the variation of the air-coal ratio is obtained by calculation of a third differential arithmetic unit 26, when the instantaneous air-coal ratio value calculated by the division arithmetic unit 25 is increased, the variation of the air quantity is instantaneously larger than the variation of the coal quantity, the differential action of the third differential operator 26 starts to operate, and an operation command for increasing the output of the third differential operator 26 is issued; similarly, when the divider 25 calculates that the instantaneous wind/coal ratio value is decreased, it indicates that the amount of change in the air flow is instantaneously smaller than the amount of change in the coal flow, and the differentiation operation of the third differential operator 26 starts, and an operation command for decreasing the output of the third differential operator 26 is issued.

7. The automatic control method for the urea method SCR denitration technology of the thermal power generating unit according to claim 2, characterized in that a fourth prediction control loop: the wind-coal ratio prediction control mainly realizes rapidity, belongs to coarse adjustment of oxygen content to nitrogen oxide NOX, introduces an oxygen content signal of a boiler in consideration of complexity of combustion inside the boiler, accurately represents change of the nitrogen oxide NOX, realizes adjustment accuracy, and belongs to coarse adjustment of the oxygen content to the nitrogen oxide NOX; introducing a boiler oxygen amount 16 signal, obtaining a urea flow instruction corresponding to the variation of the boiler oxygen amount 16 through calculation of a fourth differential operator 27, and when the boiler oxygen amount 16 is increased, starting the differential action of the fourth differential operator 27 to send an action instruction for increasing the output of the fourth differential operator 27; similarly, when the boiler oxygen amount 16 decreases, the differentiating action of the fourth differential operator 27 starts to operate, and an operation command for decreasing the output of the fourth differential operator 27 is issued.

8. The automatic control method for the urea-process SCR denitration technology of the thermal power generating unit according to claim 1, it is characterized in that in the second step, the smith estimation control is performed, wherein the pure lag operator 32 simulates a pure lag link in the algorithm, the inertia operator 33 simulates a capacity lag link in the algorithm, a loop compensated by the smith estimation control is not adversely affected, only the time shift is performed on the output signal of the original second PID controller 30, the pure lag operator 32 operates by delaying the input signal for output, the delay time is the delay time set in the pure lag operator 32, the inertia operator 33 operates by passing the input signal for a certain transition time, so that the output value is equal to the input value, and the transition time is the inertia time set in the inertia operator 33, simulating the chemical process time of the urea reduction denitration reaction by a pure lag arithmetic unit 32 and an inertia arithmetic unit 33; when the opening command 18 of the urea flow regulating spray gun changes at a moment, the delay compensation calculation of the pure delay arithmetic unit 32 and the inertia arithmetic unit 33 is carried out to obtain the opening command compensation value of the urea flow regulating spray gun needing to be compensated.

9. The automatic control method for the thermal power generating unit urea method SCR denitration technology according to claim 1, wherein in the third step, the compensation value of the opening instruction of the urea flow regulation spray gun needing compensation in Smith predictive control obtained by the inertia arithmetic unit 33 and the opening instruction of the urea flow regulation spray gun of the PID cascade predictive control loop output by the second PID controller 30 are calculated by the second subtraction arithmetic unit 31 to obtain the real-time opening instruction 18 of the urea flow regulation spray gun, and compared with the control loop without the Smith predictive algorithm, the system can be adjusted back in advance to avoid disturbance caused by large inertia.

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