JPH0260934B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a combustion control method, and particularly to a combustion control method suitable for improving combustion efficiency and reducing NOx generation in combustion using pulverized coal as a fuel whose fuel properties change from time to time. [Background of the Invention] Conventional combustion control mainly controls the air-fuel ratio (the ratio of the air flow rate to the total fuel flow rate injected into the combustion chamber) to a specified value, thereby reducing exhaust gas loss and achieving high combustion efficiency. It consisted of technology to reduce NOx concentration in the exhaust gas to below a limit value by controlling the amount of air injected into the end of the flame. However, the conventional method described above does not directly observe the combustion state inside the combustion chamber, so fuel properties such as pulverized coal fuel change from time to time (content rate and properties such as fixed carbon content, volatile content, ash content, and moisture content). It has been impossible to maintain optimal combustion conditions at all times under various operating conditions (combustion load, environmental constraints, air temperature, etc.). [Object of the Invention] The object of the present invention is to determine and maintain the optimum flame shape within the combustion chamber in order to always observe the operating limit conditions and obtain maximum combustion efficiency under constantly changing fuel properties and operating conditions. The purpose of the present invention is to provide a method for controlling combustion. [Summary of the Invention] The present invention focuses on the fact that combustion efficiency has a strong correlation with CO concentration, and that CO concentration and NOx concentration greatly depend on the flame shape, that is, the method of introducing fuel and air into the combustion chamber. be. The problems of high combustion efficiency and low NOx, which are the objectives of the present invention, are as follows:
It was formulated as a nonlinear optimization control problem with NOx concentration as a constraint and combustion efficiency as an evaluation function. The function of this control is characterized by being broadly divided into two functions: an optimal target flame shape search function and a flame shape control function for maintaining this optimal target flame shape. The optimal target flame shape search function changes the manipulated variables of the fuel supply system and air supply system to determine the flame shape when the NOx concentration is below the operating limit value and the combustion efficiency is maximum as the optimal target flame shape. . However, the flame shape is measured using the flame shape measurement function. The flame shape control function adjusts the manipulated variable so that the measured flame shape becomes the optimal target flame shape. If the fuel properties or operating conditions change during flame shape control, the optimum target flame shape is searched again, the optimum target flame shape is determined under the changed conditions, and the optimum target flame shape is determined in spite of the change in fuel properties. maintain optimal combustion conditions. [Embodiments of the Invention] A case where the present invention is applied to boiler combustion control of a thermal power plant will be described below. Coal, which has been increasingly used as boiler fuel, reduces harmful substances such as nitrogen oxides (NOx), sulfur oxides (SOx), and carbon monoxide (CO) in exhaust gas compared to oil or natural gas. is said to be difficult. Nitrogen contained in coal (hereinafter referred to as "Fuel")
- written as N) is relatively large, so NOx
is said to be difficult to reduce. NOx generated by combustion is caused by the oxidation of Fuel-N (hereinafter referred to as Fuel-NOx) and the oxidation of nitrogen in the air in high-temperature areas (hereinafter referred to as Fuel-NOx).
Thermal - written as NOx). Most of the NOx generated when oil or gas is used as fuel is thermal-NOx, and when coal is used as fuel, most of it is fuel-NOx. Therefore,
Conventional combustion methods that have been developed for oil and gas focus on combustion temperature.
Although effective in reducing thermal NOx, the problem of fuel NOx still remains when applied to coal combustion. On the other hand, a power generation boiler generally has a plurality of burners and has a complicated structure arranged in multiple stages in the direction of flow of combustion gas. Therefore, conventionally, many operating terminals have been operated in accordance with operating standards established during test runs or periodic inspections. Because the properties of coal vary greatly depending on the region where it is produced and the name of the coal, plants that use a variety of fuels must have a sufficient margin in the operating standards mentioned above, and are not always operated to maximize combustion efficiency. In order to achieve constant low NOx and high efficiency operation, skilled operators with advanced and complex decision-making abilities are required. However, as the structures of new boilers become more complex and diversified, it is not only difficult to train skilled operators, but even experienced operators find it difficult to immediately respond to changes in fuel properties and operating conditions over time. It is. Combustion control whose basic goal is low NOx and high efficiency,
It can be formulated as a nonlinear optimization problem as follows. Evaluation function η(x)→Max...(1) Equality constraint g(x)=0...(2) Inequality constraint h(x)<h ₀ ...(3) Here, x(∈R ⁿ ) is the plant state value (vector), η is the combustion efficiency (scalar function), g (∈R ^k ) is the operating condition, h (∈R ^l ) is the operation limiting factor (vector function), h ₀ (∈R ^l ) is the operating limit value (vector). The equality constraint (Equation (2)) relates to operating conditions that are controlled by a control function higher than the combustion control function, such as plant output, steam temperature, and pressure. If you are healthy, you will always be satisfied. Driving restriction factor h
are the NOx concentration in the exhaust gas, the CO concentration, and the manipulated variable, and h ₀ is its limit value. Therefore, in effect
The manipulated variable x (∈R ^m , m=l−
2) is required. Figure 1 shows the burner arrangement in the boiler furnace 20,
A fuel supply system 30 and an air/gas system 40 are shown. FIG. 2 shows the structure of burner 80. The air taken in by the forced draft fan (FDF) 41 is sent to the air preheater (SAH 42 and YAH 43) and is preheated by the heat retained in the exhaust gas. This preheated air is called secondary air 44. The secondary air 44 is mixed with a mixing gas 45 (part of the recirculated gas 46 and extracted from the exit of the dust collector (HEP) 47) and directed to the wind box 79. The wind box 79 is located between the front side 21 and the rear side 22 of the furnace 20.
Each burner stage A to F (23 _A to 23 _F
and NO ports 24 and 25. The flow rate of the secondary air and the mixing gas 48 flowing into each window box is determined by the opening degree of each damper 49 provided at the inlet. Each burner stage is composed of a plurality of (eight in this embodiment) pulverized coal burners 80. On the other hand, the recirculating gas 46 is the primary gas 5
0, Furnace hopper gas 51 and the mixing gas 45
It is divided into three parts. The primary gas 50 is injected into the furnace 20 from a primary gas port 52 located on the outer periphery of the burner 80, as shown in FIG. Further, the furnace hopper gas 51 is introduced into a hopper portion at the bottom of the furnace. The mixing gas 45 is mixed with the secondary air 44 and injected into the furnace 20 from the outer periphery of the primary gas port. The flow rate ratio of these three gases is determined by the opening degrees of the primary gas damper (PGD) 53, the furnace hot gas damper (FHGD) 54, and the mixing damper (GMD) 55. On the other hand, six fuel supply systems 30 consisting of a coal feeder 31 and a pulverized coal mill 32 are installed, A to F, corresponding to each burner stage. FDF
The primary air 56 extracted from the outlet is preheated by a primary air preheater (PAH) 57 and then sent to the pulverized coal mill 3.
2 and used for drying and transporting pulverized coal.
One part of the primary air 58 is a primary air fan (PAF)
After leaving 59, PAH is bypassed. By adjusting the amount of bypass, the temperature of the pulverized coal and the primary air 33 at the pulverized coal mill outlet can be kept constant. Although not shown in the figure, a parallel damper for controlling the temperature of reheated steam is installed at the outlet of the economizer, and by adjusting the opening degree of this damper,
The gas amount distribution flowing to the secondary superheater and primary reheater sides can be changed, and the reheated steam temperature can be controlled to be constant. Figure 3 shows the flame structure in the furnace and the air ratio to each burner stage (ratio of the actual amount of air used to the theoretical amount of air).
Indicates the allocation method. For each burner stage: (1) A region 91 is formed in the lower stage burners (stages A and D) 23 _A and 23 _D to actively generate a cyclic substance. (2) Middle stage burners (B, E stages) 23 _B and 23 _E have
A region 92 is formed in which Fuel-N is actively released. (3) The flame tips of the lower burner and the middle burner are merged to form a NOx reduction region 93. (4) A region 94 is formed in the upper stage burners (stages C and F) in which unburned content from the lower and middle stage burners is completely combusted. In order to realize the above functions, the following measures are taken. Regarding the lower stage burner, we focused on the fact that the residence time of the injected fuel in the furnace is longer than that in other stages, and set the air ratio (denoted as λ ₁ ) to be less than 1.
The method involves burning at a relatively low temperature over a long period of time. In other words, the rate at which nitrogen in the volatile components released when pulverized coal is thermally decomposed in a low-oxygen atmosphere is converted into cyclic substances such as NH ₃ and HCN is higher than that in a high-oxygen atmosphere. Take advantage of that. Therefore, the flame of the lower burner has a low NOx concentration and is rich in reducing substances. However, since the combustion is performed at λ ₁ <1, there is a relatively large amount of unburned matter, and Fuel-N is present in this unburned matter. On the other hand, for the middle stage burner, the air ratio (denoted as λ ₂ ) is set slightly larger than 1, and combustion is performed at a relatively high temperature in a short time. That is, combustion in a high oxygen atmosphere generates a large amount of thermal NOx, but utilizes the fact that fuel-N is actively released. Therefore, although the flame of the middle burner has a high NOx concentration, there is almost no Fuel-N in the unburned content. Further, regarding the upper stage burner, the air ratio (denoted as λ ₃ ) is made larger than λ ₂ to completely burn the unburned content mainly generated from the lower stage burner. By distributing the functions described above to each stage, a NOx reduction region is formed at the flame tip of the upper stage burner, and NOx mainly generated in the middle stage burner is reduced to _NH3 mainly generated in the lower stage burner.
NOx is reduced by reducing substances such as and HCN.
reduction is possible. In the combustion control system, the above air ratio distribution (λ ₁ <
A further optimal air ratio distribution is determined within the range that satisfies the following conditions: 1, λ ₂ > 1, λ ₃ ≫ 1). Low NOx and high efficiency combustion can be achieved by solving the nonlinear optimization problem formulated by equations (1) to (3). Here, we will describe the basic concept for improving combustion efficiency and define combustion efficiency η as an evaluation function. Combustion efficiency is 100% when pulverized coal is completely combusted and the theoretical calorific value is obtained. However, in actual boilers, unburned gas remains due to disturbances in gas flow within the furnace, causing a reduction in combustion efficiency. The end combustion mainly consists of solid carbon C and carbon monoxide CO. In order to increase combustion efficiency, the excess oxygen ratio (ratio of excess oxygen amount to theoretical oxygen amount: hereinafter referred to as Z ₀₂ ) can be increased, but if it is increased too much, the exhaust gas loss of the boiler will increase and the boiler efficiency will decrease. descend. Therefore, in actual boilers, combustion efficiency must be viewed in a broad sense and exhaust gas loss must also be considered. In the following, combustion efficiency will be used in a broad sense. Among the unburned components, the C component is a powder and is captured in the furnace hopper and dust collector, so online measurement is impossible with current measurement technology. On the other hand, the CO content can be measured online because it is a gas, and the CO content relative to Z ₀₂ of the entire boiler can be measured.
Concentration and combustion efficiency generally have the relationship shown by the solid lines in FIG. 4A and B. That is, under the condition that the CO concentration Z _CO is less than or equal to Z _CO,U, the smaller Z ₀₂ is, the higher the combustion efficiency is. When Z ₀₂ =a, the combustion efficiency η is maximum (η _a ). The characteristics shown by the solid line are those for conventional non-optimal operation, but the characteristics shown by the broken line can be expected for optimal operation to which the present invention is applied. In other words, good combustion can be expected even under the same Z ₀₂ , and the CO concentration decreases, resulting in improved combustion efficiency. In this case as well, Z _CO =Z _CO,U . Z ₀₂
It can be expected that the combustion efficiency will be close to the maximum value (ηb) under the condition of =b. In other words, NOx concentration
If Z _NOX and its limit value are Z _ZOX,U , then Z _CO <Z _CO,U
and Z _NOX <Z _ZOX,U , and the parallel damper opening A _PD for reheating steam temperature control is furthermore A PD,L with respect to the lower limit values A A _PD,U , A _{PD,L .} The manipulated variable that satisfies <A _PD,U and minimizes Z ₀₂ is the optimal manipulated variable.
Therefore, since there is no need to know the absolute amount of combustion efficiency, it can be equivalently defined using the following equation. η=100−Z ₀₂ ...(4) The functional configuration of the combustion control system is shown in Figure 5.
Further, the basic processing procedure for control is shown in FIG.
The control system starts operating in response to a command from an operator 97, and first determines an optimal target flame shape (y^ ^* ) 101 and an optimal manipulated variable (x^ ^* ) 102 by an optimal target flame shape search function 200. Here, y^ ^* is the operation vector that satisfies the optimization problem mentioned above,
y^ ^* is the shape vector related to the flame in the furnace at this time. The operation vector is defined as follows.

[Table] Although there are various ways to select the flame shape vector, we will define it here as shown in FIG. 7, which is convenient for representing the arrangement of each flame in the furnace. This shape is determined by computer processing (image processing) of flame images captured by an ITV camera. However, as defined in equation (5), the burners placed at the front and rear of the furnace operate in the same way, so stage A and stage D, stage B and stage E,
The flame shapes of stage C and stage F are symmetrical. Further, the shape vector is defined as the distance from the point on the center line of each flame where the brightness attenuates to a specified value to the furnace wall and the center of the burner. The flame shape control function 300 is for maintaining a stable flame shape even if slight changes in fuel properties, atmospheric temperature, etc. cause disturbances to combustion characteristics. The target value for flame shape control is the optimal target flame shape y ^* , which has already been determined by the optimal target flame shape search function. In this case, a quasi-optimal manipulated variable x^103 that minimizes |y−y^ ^* | is determined. Here, x^
The reason why is called a sub-optimal manipulated variable is that combustion efficiency and NOx concentration are not evaluated when determining x^. In other words, when the disturbance is small, the flame shape
If the control is performed to follow y^ ^* , ‖x^−x^ ^* ‖＜εx ...(6) is satisfied, and the combustion characteristics can be expected to be maintained near the optimum value. However, x^ and x^ ^* in equation (6) are normalized values. Although it is difficult to actually measure fluctuations in fuel properties due to disturbances, it can be said that the magnitude of ‖x^−x^ ^* ‖ represents the magnitude of the disturbance. Focusing on this, if the following equation is satisfied, it is determined that the optimal target flame shape should be changed, and the optimal target flame shape search function 200 is operated again to find a new y ^* . ‖x^−x^ ^* ‖εx ...(7) The optimum target flame shape search function 200 also operates when the load change is completed. This is because if the operating load level changes, the optimal flame shape will naturally change as well. Control cycle management 2 to maintain the manipulated variable during load fluctuations or outside the control cycle
10 and operation amount holding 220 are provided. The operation amount management function 230 is for distinguishing between x^ ^* or x^ as the control operation amount 235 and a search operation amount 237 for finding these by search, and outputting them to the operating device 240. Measurement management function 250
This controls the timing of measuring the flame shape and exhaust gas concentration in response to trial operations. That is, after a trial operation, the flame shape measurement function 260 or the exhaust gas concentration measurement function 270 is activated after waiting until steady combustion is reached. CRT
The interface function 280 is for receiving system operation commands from the operator 97 and for displaying various combustion control states on the screen of the CRT 290 in response to requests from the operator. The boiler combustion process is complex and difficult to formulate for a model that can be used for online control. Here, in order to solve the nonlinear optimization problem mentioned above, we decided to apply the complex method, which is a type of nonlinear programming method, to perform trial operations on an actual plant and evaluate the actual values to determine the optimal target flame. A method is used to determine the shape. Now, if we define the CO concentration (Z _CO ), NOx concentration (Z _NOX ), and parallel damper opening degree ₍ A _PD ) as x ₁₀ = Z _CO x ₁₁ = _{Z NOX} ) The constraint function (vector) h(x) and its control value (vector) h ₀ can be defined by the following equation.

〔Effect of the invention〕

According to the present invention, it is possible to operate while maintaining maximum efficiency while observing environmental regulation values such as NOx concentration and CO concentration contained in exhaust gas.

[Brief explanation of the drawing]

FIG. 1 is for explaining the burner arrangement, fuel supply system, and air/gas system arrangement of a boiler furnace in an embodiment of the present invention. FIG. 2 shows the structure of a burner in an embodiment of the present invention. FIG. 3 is for explaining the flame structure in the furnace and the air ratio distribution method to each burner stage in the embodiment of the present invention. FIG. 4 is for explaining the expected effect on combustion characteristics by applying the present invention in an example of the present invention. FIG. 5 is for explaining the functional configuration of a combustion control system in an embodiment of the present invention. FIG. 6 is for explaining the basic processing means for control in the embodiment of the present invention. FIG. 7 shows the definition of flame shape in an embodiment of the present invention. FIG. 8 is for explaining the basic algorithm for searching for an optimal target flame shape in an embodiment of the present invention. FIG. 9 shows diagrams for explaining the basic algorithm of flame shape control in the embodiment of the present invention. 20... Boiler furnace, 30... Fuel supply system, 31...
Coal feeder, 40...Air gas system, 41...Forced draft fan (FDF), 50...Primary gas, 200...Optimum target flame shape search, 300...Flame shape control, 9
7...Operator, 101...Optimum target flame shape y^ ^* , 1
02...optimal operation amount x^ ^* , 103...semi-optimal operation amount x^,
210...Control cycle management, 220...Manipulated amount retention, 2
30...Manipulation amount management, 235...Control operation amount, 237
...Search operation amount, 240...Control device, 400...Plant, 250...Measurement management, 260...Flame shape measurement,
270...Exhaust gas concentration measurement, 280...CRT interface, 290...CRT, 500...Central power dispatch center, 600...Load fluctuation monitoring.

Claims (1)

[Scope of Claims] 1. A plurality of burners that supply fuel and air to perform combustion are provided in multiple stages in the direction of flow of combustion gas in a furnace, and the flow rate of fuel and/or air supplied to the burners in each stage is controlled. In the combustion control method, the plurality of burners consisting of a lower stage burner, a middle stage burner, and an upper stage burner are provided in the flow direction of the combustion gas of the furnace, and the air ratio of the lower stage burner is made smaller than 1 to generate reducing substances. The air ratio of the middle stage burner is made larger than 1 to perform combustion that generates NOx, and the air ratio of the upper stage burner is made even larger than the air ratio of the middle stage burner.
Combustion is performed to form a NOx reduction region and reduce NOx generated in the middle stage burner with a reducing substance generated in the lower stage burner, and the flow rate of fuel and/or air supplied to each stage burner is changed multiple times. Perform the trial operation of
Measuring and storing the flame shapes of the lower burner, middle burner, and upper burner during each trial, detecting the CO concentration and NOx concentration during each trial, and estimating the in-furnace combustion efficiency based on the CO concentration; The flame shape when the NOx concentration is below a predetermined value and the maximum estimated combustion efficiency is obtained during the plurality of trial operations is set as the target flame shape, and the flame shape by the burners at each stage is set as the target flame shape. A combustion control method comprising controlling the flow rate of fuel or air supplied to the burners in each stage so as to form a flame shape.