CN106468446B - Heating furnace control and combustion optimization method - Google Patents

Abstract

The invention discloses a method for controlling a heating furnace and optimizing combustion, which solves the problem that a heating furnace system is difficult to control and adopts the combination of an advanced control algorithm and a conventional control algorithmThe control strategy ensures the continuous and stable operation of the heating furnace system; by the use of O₂CO switching control is carried out to realize low-oxygen combustion; the national standard simplified formula is taken as an optimization target, and the self-optimization algorithm is adopted to realize the real-time optimization of the oxygen content of the flue gas, so that the aim of economic combustion is fulfilled.

Description

A heating furnace control and combustion optimization method

technical field

The invention relates to the technical field of industrial automatic control, in particular to a heating furnace control and combustion optimization method.

Background technique

Heating furnace is one of the main equipment in petrochemical refineries and chemical production plants, and is a kind of thermal heating equipment that is often used. The most important task of the heating furnace is to heat the raw oil to a relatively high temperature (sometimes up to 1000°C) to meet the requirements of the subsequent production process (fractionation or reaction processes). Heating furnace not only consumes a lot of energy, but also is one of the main sources of carbon dioxide, nitrogen oxides and other pollutant emissions.

The main factors affecting the thermal efficiency of the heating furnace are the heat loss of exhaust gas and the heat loss of incomplete combustion of the gas, and the adjustable parameter affecting these two main factors is the excess air coefficient, which determines the performance of the heating furnace, especially the heating furnace. Therefore, in the process of designing and operating the heating furnace, the excess air coefficient is an important index for our reference. If the excess air coefficient is too small, incomplete combustion will occur, which will increase the heat loss of incomplete combustion and reduce the thermal efficiency of the furnace; when the value is too large, the excess air will be discharged out of the furnace and enter the atmosphere to take away a lot of heat, and it will also bring a lot of heat to the environment. In addition, the size of the value affects the size of the resistance of the flue gas; the excessive oxygen content caused by the excess air in the flue gas can cause the furnace to decrease. The oxidation of internal components is intensified, and excess oxygen also leads to intensified oxidation of the surface of the furnace tube, which directly affects the service life of the furnace tube. Furthermore, excess oxygen can also increase the conversion of SO ₂ to SO ₃ , which increases the chance of low-temperature dew point corrosion of the flue gas.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a heating furnace control and combustion optimization method, aiming at safe and economical operation, aiming at the operation characteristics of the heating furnace system, adopting a control strategy combining advanced control algorithm and conventional control algorithm, and using _O2 and CO switching control realizes low-oxygen combustion. Through the combustion efficiency self-optimization algorithm, the oxygen content of flue gas is optimized in real time, which further improves combustion efficiency, reduces pollutant emissions, and improves the safe service life of equipment.

To achieve the above object, the technical scheme adopted in the present invention is:

1) The control strategy combining conventional control and advanced control is adopted to realize conventional automatic control of medium outlet temperature, flue gas oxygen content and furnace negative pressure to ensure the stability of heating furnace control; realize medium outlet temperature, flue gas oxygen content The advanced control of volume and furnace negative pressure ensures the rapid response and safety of the system under variable load and variable working conditions;

2) Aiming at the combustion characteristics of most of the heating furnaces in oxy-fuel combustion, the switching control of O ₂ and CO is adopted to realize low-oxygen combustion, reduce pollutant emissions, and improve the safe service life of equipment;

3) Using the simplified formula of the national standard, the efficiency optimization function of the heating furnace is constructed, and the oxygen content of the flue gas during the combustion process of the boiler is optimized in real time through the combustion efficiency self-optimization algorithm to ensure that the system operates in the best combustion area.

The control strategy combining conventional control and advanced control is adopted to realize automatic control of medium outlet temperature, oxygen content of flue gas, and furnace negative pressure to ensure the stability of heating furnace control. The conventional control algorithm is implemented in the process control station, which ensures the safety and stability of the control.

In the medium temperature control process, cascade control is adopted. The outer loop adopts the medium outlet temperature control loop as the main control loop, and the inner loop adopts the fuel flow control loop as the auxiliary control loop. When the medium temperature increases, the amount of fuel decreases, and when the medium temperature decreases, the amount of fuel increases;

In the process of controlling the oxygen content of the flue gas, single-loop control is adopted. Through the deviation between the oxygen set value and the process value, enter the controller to carry out PID control operation, and obtain the air supply baffle command. The combustion conditions change due to changes, but the situation becomes poor due to the slow change of the intake air volume, which improves the fast response characteristics of the air supply system;

In the furnace negative pressure control process, single-loop control is adopted. Through the deviation between the furnace negative pressure setting value and the process value, enter the controller to carry out PID control operation, and obtain the baffle control command. Since the negative pressure value is easily affected by the air supply volume, the feedforward control strategy is adopted for predictable disturbances. , the introduction of air supply flow feedforward to eliminate air supply interference in advance, so that the furnace negative pressure can quickly stabilize.

The control strategy combining conventional control and advanced control is adopted to realize the advanced control of medium outlet temperature, oxygen content of flue gas, and furnace negative pressure, so as to ensure the rapid response and safety of the system under variable load and variable working conditions. It is characterized by:

1) The switch between conventional control and advanced control is mainly realized by state judgment and selection. When the conventional control is in the automatic state and the operator chooses to input, the conventional control is switched to the advanced control;

2) For the medium outlet temperature control with large delay, the model-free adaptive (Anti-DelayMFA, Model-Free Adaptive) control of the large delay process is adopted, so that the system can still ensure stability and fast response characteristics under variable load and variable working conditions , whose characteristics are as follows:

2-1) Generate a dynamic feedback signal yc(t) as a feedback signal through the lag predictor, and generate an e(t) for the controller;

2-2) The MFA anti-lag controller does not require an accurate mathematical model, but only needs a lag time for lag prediction, and combines the strong adaptive ability and robustness of the MFA to control the process;

2-3) In order to quickly adapt to the influence of the change of the fuel quantity on the outlet temperature of the medium, the feedforward control of the fuel quantity is adopted to realize the stable and accurate control of the circuit.

3) Aiming at the fact that the oxygen content of the flue gas and the negative pressure of the furnace are multi-variable and strongly coupled systems, the multi-variable model-free adaptive (MIMO MFA, Model-Free Adaptive) control is adopted, so that the system can still be operated under variable loads and working conditions. To ensure stability, and fast response characteristics, the characteristics are as follows;

3-1) The MIMO MFA system consists of 2×2 MFA controllers, including two main controllers C11, C22 and two compensation controllers C21 and C12, and the process objects include four sub-processes G11, G21, G12 and G22;

3-2) The process detection variable flue gas oxygen content y1 and furnace negative pressure y2 are used as feedback signals of the two main loops to compare with the set value of flue gas oxygen content r1 and the set value of furnace negative pressure r2 to generate deviation signals e1 and e2 is input to two controllers respectively, and the outputs of the two controllers are combined with the output of the other compensator to produce control signals u1 and u2. It can be seen from the nature of the 2×2 process that the inputs u1 and u2 of the process are mutually Affecting the outputs y1 and y2, a change in one input will change both outputs at the same time.

3-3) The control objective of the 2×2 MFA control system is to generate output control signals u1(t) and u2(t) to force the process variables y1(t) and y2(t) to track their respective setpoints r1(t) and r2(t), the deviation signals e1(t) and e2(t) are minimized.

Aiming at the combustion characteristics of most of the heating furnaces in oxy-fuel combustion, the switching control of O ₂ and CO is adopted to realize low-oxygen fuel, reduce pollutant emissions, and improve the safe service life of equipment. It is characterized by:

1) First, a laser analyzer is added to monitor the online measurement of O2 and CO/CH4 in the combustion process in real time. Based on laser CO online measurement, it can be used to measure incomplete combustion, and at the same time, it can make up for the insensitivity of O2 measurement in low oxygen state, and provide reliable measurement basis for low oxygen combustion;

2) Secondly, improve the DCS control logic, realize the switching control of oxygen and CO, adopt the CO control strategy in the low oxygen state, and adopt the oxygen control method in the set CO control point. The two methods are automatically determined by the combustion state. switch, and finally achieve the control target of low oxygen combustion;

3) Finally, through the cross control logic in the control scheme, the fuel amount is limited by the air amount when the load is changed, and the air amount is always greater than the fuel amount in the combustion state to ensure the stability of combustion.

The simplified formula of the national standard is used to construct the efficiency optimization function of the heating furnace. Through the self-optimization algorithm of combustion efficiency, the oxygen content of the flue gas during the combustion process of the boiler is optimized in real time to ensure that the system operates in the best combustion area. The main features are as follows:

1) Determine the stability of the combustion system, and record the current boiler optimization objective function value and the oxygen content of the flue gas when the combustion system is stable;

2) Increase the oxygen content of flue gas by a preset step value of oxygen content of flue gas (range 0.2~0.5). After the system is stabilized, it can be divided into the following two situations:

3) In the first case, if the value of the objective function increases, the next step will continue to increase the oxygen content of the flue gas by a step value until the value of the objective function decreases, indicating that the boiler system at this time is already in the optimal combustion area, and this process ends. Sub-optimization, wait for the working conditions to change for the next optimization;

4) In the second case, if the value of the objective function decreases, the optimization step is reversed, and the oxygen content of the flue gas is reduced by one step. After the system is stabilized, if the value of the objective function increases, the next step continues to decrease. A step value of flue gas oxygen content; until the objective function value decreases, it means that the boiler system at this time is already in the optimal combustion area, end this optimization, and wait for the working conditions to change for the next optimization.

Compared with the prior art, the present invention has the advantages of safety and stability, convenient operation, energy saving and consumption reduction. On the one hand, the continuous, stable, safe and economical operation of the control system of the heating furnace can be ensured; on the other hand, the efficiency of the boiler can be improved, and the emission of pollutants can be reduced at the same time; the application of the present invention can also reduce the labor intensity of operators and improve the automatic input rate , so it has significant economic benefits and good social benefits.

The present invention has the following characteristics:

security and stability

The large amount of carbon dioxide produced in the furnace exhaust makes it impossible to determine the optimum air volume through oxygen control alone. The safe and stable combustion of the furnace can be ensured by online measurement of O ₂ and CO/CH ₄ and fast flow control of air.

extend your life

Excess air has a significant impact on furnace life. The excess air dilutes the _CO2 and _H2O concentrations in the flue gas, reducing radiative heat transfer, with the result that the heater requires more fuel to maintain the COT (furnace tube outlet temperature). This requires adding more heat in the convection section, so that the shield section temperature (between the convection and radiant sections) may exceed the design temperature, affecting tube coking and furnace life. By improving combustion measurement and control, this problem will be completely solved.

Improve efficiency

Energy efficiency is further improved by reducing excess air while maintaining safe combustion conditions by implementing combustion optimization in the furnace; at the same time, minimal excess air is passed through the exhaust gas to reduce heat loss and to maintain radiation by avoiding dilution of carbon dioxide and water vapor in the exhaust gas Heat transfer to achieve the goal of saving fuel.

reduce emissions

The heating furnace is the main source of nitrogen oxides and carbon dioxide emissions. Using a more reasonable air supply to achieve optimized combustion can reduce the emission of pollutants.

Description of drawings

Figure 1 block diagram of medium outlet temperature control;

Fig. 2 control block diagram of flue gas oxygen content;

Fig. 3 furnace negative pressure control block diagram;

Figure 4 Anti-Delay MFA system block diagram;

Figure 5 MIMO MFA system block diagram;

Fig. 6 _O2 and CO switching control algorithm logic diagram;

Fig. 7 Logic diagram of self-optimization algorithm.

Detailed ways

The content of the invention will be further described below in conjunction with the accompanying drawings and embodiments.

Referring to Figure 1, it is a block diagram of the medium outlet temperature control loop. The specific steps are:

First, in the process of medium temperature control, cascade control is used.

Secondly, the outer loop adopts the medium outlet temperature control loop as the main control loop, and the inner loop adopts the fuel flow control loop as the secondary control loop.

Finally, when the temperature of the medium increases, the amount of fuel is decreased, and when the temperature of the medium decreases, the amount of fuel is increased. Aiming at the large hysteresis characteristic of the medium outlet temperature, the anti-hysteresis MFA control strategy is adopted, and the disturbance-free switching of the control is realized through the state judgment condition.

Referring to Figure 2, it is a block diagram of the temperature control loop of flue gas oxygen content, and the specific steps are:

First of all, in the process of controlling the oxygen content of flue gas, single-loop control is adopted.

Secondly, through the deviation between the oxygen setting value and the process value, enter the controller to carry out PID control operation, and obtain the air supply baffle command;

Finally, the introduction of a gas pressure (flow) compensator as a control feedforward can avoid the situation where the fuel changes due to sudden load changes, but becomes unfavorable due to slow changes in the intake air volume, and improves the fast response of the air supply system characteristic.

Referring to Figure 3, it is a block diagram of the furnace negative pressure control loop, and the specific steps are:

First, in the furnace negative pressure control process, single-loop control is adopted.

Secondly, through the deviation between the furnace negative pressure setting value and the process value, enter the controller to carry out PID control operation, and obtain the baffle control command;

Finally, since the negative pressure value is easily affected by the air supply volume, the feedforward control strategy is adopted for predictable disturbances, and the air supply flow feedforward is introduced to eliminate the air supply disturbance in advance, so that the furnace negative pressure can be quickly stabilized.

Referring to Figure 4, it is a block diagram of the Anti-Delay MFA system, and the specific steps are:

First, a dynamic feedback signal yc(t) is generated as a feedback signal through the lag predictor, and an e(t) is generated for the controller;

Secondly, the MFA anti-lag controller does not require an accurate mathematical model, but only needs a lag time for lag prediction, and combines the strong adaptive ability and robustness of MFA to control the process;

Finally, in order to quickly adapt to the influence of the change of the fuel quantity on the outlet temperature of the medium, the feedforward control of the fuel quantity is adopted to realize the stable and accurate control of the loop.

Referring to Figure 5, it is a structural block diagram of a MIMO MFA control system, and the specific steps are:

First, a MIMO MFA system is formed, which includes two main controllers C ₁₁ , C ₂₂ and two compensation controllers C ₂₁ and C ₁₂ , and the process object includes four sub-processes G ₁₁ , G ₂₁ , G ₁₂ and G ₂₂ ;

Secondly, the process detection variable flue gas oxygen content y ₁ and furnace negative pressure y ₂ are used as feedback signals of the two main loops to compare with the oxygen content set value r ₁ and the negative pressure set value r ₂ to generate deviation signals e ₁ and e ₂ is respectively input to the two controllers, and the outputs of the two controllers are combined with the output of the other compensator to produce the control signals u ₁ and u _2. It can be seen from the nature of the 2×2 process that the input u of the process ₁ and u ₂ mutually affect the outputs y ₁ and y ₂ , and a change in one input will change both outputs at the same time.

Again, the control objective of the 2×2 MFA control system is to generate output control signals u ₁ (t) and u ₂ (t) to force the process variables y ₁ (t) and y ₂ (t) to track their respective setpoints r ₁ ( t) and r ₂ (t) to minimize the deviation signals e ₁ (t) and e ₂ (t):

Referring to Figure 6, it is a logic diagram of _O2 and CO switching control algorithm, and the specific steps are:

First, a new laser analyzer is added to monitor the online measurement of O2 and CO/CH4 in the combustion process in real time. Based on laser CO online measurement, it can be used to measure incomplete combustion, and at the same time, it can make up for the insensitivity of O2 measurement in low oxygen state, and provide reliable measurement basis for low oxygen combustion;

Secondly, improve the DCS control logic, realize the switching control of oxygen and CO, adopt the CO control strategy in the low oxygen state, and adopt the oxygen control method in the set CO control point. The two methods are automatically switched by judging the combustion state. Finally achieve the control target of low oxygen combustion;

Finally, through the cross-control logic in the control scheme, the fuel amount is limited by the air amount when the load is changed, and the air amount is always greater than the fuel amount in the combustion state to ensure the stability of combustion.

Referring to Figure 7, for the self-optimizing combustion optimization algorithm logic, the optimization parameter is the oxygen setting value as an example, and the specific steps are:

1) Parameter initialization to obtain the basic value and boundary value of the optimized parameter;

2) Calculate the optimization objective function value J, and perform mean filtering processing on it in unit time;

3) Judging whether it is within the scope of the optimization dead zone, if it is within the range of D, do not perform the optimization operation, otherwise continue to the next step;

4) Judging whether the combustion system is stable, and whether the operator allows the combustion optimization, if the conditions are met, proceed to the next step, otherwise no operation;

5) Determine whether it is the first time to enter the optimization program, if so, go to step (6), otherwise jump to step (11)

6) Increase the increment of the oxygen setting value (positive direction);

7) Call the optimization algorithm, calculate the J under the boundary conditions and the optimal oxygen setting, and perform a comparison operation on the J and J;

8) Determine whether J is < J, if not, jump to (9), and if so, jump to (10);

9) Reset and optimize the positive direction flag, and continue to judge stability;

10) Set the optimization positive direction flag (reverse optimization), and continue to judge stability;

11) Determine whether it is a forward operation process, if so, jump to (13), if not, jump to (12);

12) Decrease the step size of the oxygen setting value;

13) Increase the step size of the oxygen setting value;

14) Call the optimization algorithm to calculate J under the boundary conditions and the optimal oxygen setting;

15) Carry out a comparison operation on J and J;

16) When the single optimization is over, the end state is set, and the first optimization and other related states are reset;

17) End this operation.

Claims (3)

1. A heating furnace control and combustion optimization method is characterized in that:

1) the conventional control of the medium outlet temperature, the oxygen content of the flue gas and the negative pressure of the hearth is realized by adopting a control strategy combining the conventional control and the advanced control, so that the control stability of the heating furnace is ensured; advanced control of medium outlet temperature, oxygen content of flue gas and negative pressure of a hearth is realized, and quick response and safety of the system under variable load and variable working conditions are ensured;

the switching between the conventional control and the advanced control is realized by state judgment and selection, and when the conventional control is in an automatic state and an operator selects to put in, the conventional control is switched to the advanced control;

aiming at the medium outlet temperature control with large lag, the system can still ensure the stability and the quick response characteristic under variable load and variable working conditions by adopting Anti-Delay MFA (Model-Free Adaptive) control with large lag, and the process is as follows:

1.1) a dynamic feedback signal yc (t) is generated through a hysteresis predictor and is used as a feedback signal, an e (t) is generated for a controller, and aiming at the fact that the oxygen content of smoke and the negative pressure of a hearth belong to a multivariable and strong coupling system, multivariable Model-Free Adaptive (MIMO MFA) control is adopted, so that the system can still ensure the stability and the quick response characteristic under variable load and variable working conditions, and the process is as follows:

1.1.1) the MIMO MFA system consists of 2 × 2MFA controllers, including two master controllers C11, C22 and two compensation controllers C21 and C12, and a process object including four sub-processes G11, G21, G12 and G22;

1.1.2) process detection variables of flue gas oxygen content y1 and furnace negative pressure y2 are used as feedback signals of two main loops, the feedback signals are compared with a flue gas oxygen content set value r1 and a furnace negative pressure set value r2 to generate deviation signals e1 and e2, the deviation signals are respectively input into two controllers, the outputs of the two controllers are respectively combined with the output of the compensator on the other side to generate control signals u1 and u2, the essence of a 2 x 2 process shows that the inputs u1 and u2 of the process mutually influence the outputs y1 and y2, and the two outputs can be simultaneously changed when one input is changed;

1.1.3) the control objective of the 2 × 2MFA control system is to generate output control signals u1(t) and u2(t) to force process variables y1(t) and y2(t) to track their respective set points r1(t) and r2(t) to achieve minimum deviation signals e1(t) and e2 (t);

1.2) the MFA anti-lag controller does not need an accurate mathematical model, only needs a lag time to carry out lag estimation, and controls the process by combining the strong adaptive capacity and robustness of the MFA;

1.3) in order to quickly adapt to the influence of the change of the fuel quantity on the outlet temperature of the medium, the fuel quantity feedforward control is adopted to realize the stable and accurate control of a loop;

2) aiming at the combustion characteristic that most of heating furnaces are in oxygen-enriched combustion, O is adopted₂CO switching control is performed, low-oxygen combustion is realized, pollutant emission is reduced, and the safe service life of equipment is prolonged;

the process is as follows:

2.1) adding a laser analyzer to monitor O in the combustion process in real time₂And CO/CH₄On-line measurement, based on laser CO on-line measurement, for measuring incomplete combustion, and making up O in low oxygen state₂The measurement insensitivity provides reliable measurement basis for the low-oxygen combustion;

2.2) secondly, adopting a CO control strategy in a low-oxygen state, adopting an oxygen control mode in a set CO control point in an oxygen-enriched state, and finally achieving a control target of low-oxygen combustion by judging and switching a combustion state;

2.3) finally, realizing the combustion state that the fuel quantity is limited by the air quantity when the load is changed and the air quantity is always larger than the fuel quantity through a cross control logic in the control scheme, and ensuring the combustion stability;

3) a national standard simplified formula is adopted, a heating furnace efficiency optimization function is constructed, and the oxygen content of the flue gas in the boiler combustion process is optimized in real time through a combustion efficiency self-optimization algorithm, so that the system is ensured to operate in an optimal combustion area.

2. The furnace control and combustion optimization method of claim 1, wherein: in the process of controlling the medium temperature in the step 1), cascade control is adopted, an outer ring adopts a medium outlet temperature control loop as a main control loop, an inner ring adopts a fuel flow control loop as a secondary control loop, the fuel quantity is reduced when the medium temperature is increased, and the fuel quantity is increased when the medium temperature is reduced;

in the process of controlling the oxygen content of the flue gas, single-loop control is adopted, the flue gas enters a controller for PID control operation through the deviation of an oxygen content set value and a process value to obtain an air supply baffle instruction, and meanwhile, a gas pressure compensator is introduced to be used for controlling feed-forward quantity, so that the condition that the combustion condition of fuel is changed due to sudden load change but the combustion condition is poor due to slow change of air inlet quantity is avoided, and the quick response characteristic of an air supply system is improved;

in furnace negative pressure control process, adopt single loop control, through furnace negative pressure set value and process value's deviation, carry out PID control operation in the entering controller, obtain baffle control command, because the negative pressure value easily receives the influence of air supply volume, therefore adopt the feedforward control strategy to the disturbance of foreseeable, introduce air supply flow feedforward, eliminate air supply interference in advance, make the stability that furnace negative pressure can be quick get off.

3. The furnace control and combustion optimization method of claim 1, wherein: the step 3) comprises the following steps:

1) judging the stability of a combustion system, and recording the optimized objective function value of the current boiler and the oxygen content of the flue gas when the combustion system is stable;

2) the given oxygen content of the flue gas is increased by a preset step value of 0.2-0.5 of the oxygen content of the flue gas, and after the system is stable, the following two conditions are adopted:

in the first situation, if the objective function value is increased, the next step is to continue to increase a step value of the oxygen content of the flue gas until the objective function value is reduced, which indicates that the boiler system is in the optimal combustion area at the moment, the optimization is finished, and the optimization is performed sub-optimally when the working condition is changed;

in the second case, if the objective function value is decreased, the optimization step length is reversed, one smoke oxygen content step length is decreased, and after the system is stabilized, if the objective function value is increased, the next step is continued to decrease one smoke oxygen content step length value; and when the objective function value is reduced, indicating that the boiler system is in the optimal combustion area, ending the optimization, and waiting for the suboptimization under the condition of changing the working condition.

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