Pollutant Emission Control Method for Thermal Power Units Based on Variable Coal Quantity Regulation

Abstract

The present invention relates to a method for controlling pollutant emissions of a thermal power unit based on variable-speed regulation of coal quantity, including: controlling nitrogen oxide emissions for upper-layer coal mills, middle-layer coal mills, lower-layer coal mills, and middle-upper pulverizing systems, and controlling SO2 or dust emissions. The beneficial effects of the present invention are as follows: By utilizing the speed regulation characteristics of variable-speed coal mills, according to the influence of different pulverizing systems and their coal types on pollutant generation, rapid coal quantity distribution and adjustment of the output of the pulverizing system are carried out during the rapid load change process, reducing or suppressing pollutant generation from the source side, alleviating the pressure on the output adjustment of environmental protection equipment under rapid load change conditions, and ensuring the compliance of the unit's emission indicators.

Description

Technical Field

[0001] The present invention relates to the technical field of thermal power generation, and more specifically, it relates to a method for controlling pollutant emissions of a thermal power unit based on variable-speed regulation of coal quantity. Background Art

[0002] With the construction of a new power system, grid dispatching agencies conduct assessments on thermal power generating units operating in parallel with automatic generation control (AGC), and the requirements for indicators such as the response rate, response time, and control accuracy of thermal power units are getting higher and higher. To shorten the response time of thermal power units and improve the AGC regulation rate and accuracy, some units have started to select pulverizing systems with variable-speed coal mills. By changing the speed of the coal mill, the variable load rate and accuracy of thermal power units can be significantly improved, which can effectively improve the stability level of the power grid and enhance the AGC performance of the units and their competitiveness in the power auxiliary service market.

[0003] Each system of the thermal power unit is under real-time control by a distributed control system (DCS). The DCS performs amplitude limiting and speed limiting processing on the received AGC load command to generate a unit load command and a main steam pressure set value, and then compares them with the real-time power and real-time pressure of the unit to adjust the boiler fuel quantity, air supply volume, and feed water flow rate, thereby changing the output of the boiler to meet the work demand of the steam turbine; at the same time, the DCS controls the opening of the steam turbine governor valves to change the output of the steam turbine, and then adjusts the power generation of the unit. The DCS realizes the response to the AGC command by coordinating the energy supply and demand balance between the boiler and the steam turbine and adjusting the unit output in real time. When changing the load, the generator control system will control the coal feeding quantity and air supply volume to respond to the unit power command, and at the same time make corresponding adjustments to the speed of the coal mill to achieve the purpose of the unit quickly responding to the command.

[0004] To ensure that the emission indicators of the unit meet the environmental protection requirements, corresponding environmental protection equipment is installed in today's thermal power units. According to the different pollutants removed, the treatment systems for flue gas pollutants in thermal power units are mainly divided into: desulfurization, denitrification, and dust removal systems. Among them, the desulfurization system uses limestone slurry or seawater to remove sulfur in the flue gas, and adjusts the limestone slurry system according to the set value, measured value of the net flue gas SO2 concentration, and power command. The denitrification system uses ammonia generated by the decomposition of liquid ammonia or urea to remove nitrogen oxides in the flue gas, and adjusts the ammonia injection amount and the output of the ammonia production system according to the set value, measured value of the net flue gas NOx, and power command to ensure that the NOx emission of the unit does not exceed the standard. The dust removal system uses an electric field to adsorb dust in the flue gas to achieve the purpose of flue gas dust removal. The generation of NOx in the original flue gas is mainly related to the flue gas temperature and the oxygen content in the flue gas. The higher the temperature and the oxygen content in the flue gas, the more NOx is generated in the original flue gas. SO2 and dust mainly come from the fuel.

[0005] At present, the treatment of pollutants in thermal power units is generally carried out on the flue gas side. That is, pollutants in the flue gas generated by coal combustion are removed through environmental protection equipment, which is a passive post-treatment method. However, from the overall process of boiler combustion and the flue gas side, the generation and final treatment of pollutants often have a large inertia and delay. When external influencing factors such as equipment failures and operating condition changes occur, the output adjustment of environmental protection equipment is likely to be untimely, so it cannot respond in a timely manner during the rapid load change process, resulting in the over-standard emission of flue gas pollutants from the unit. At present, the research on pollutant control methods mainly focuses on flue gas treatment, and the research on pollution control at the source mainly focuses on low-nitrogen combustion, and there are no effective control means for the source control of other pollutants. Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a method for controlling pollutant emissions in thermal power units based on variable-speed regulation of coal quantity.

[0007] In the first aspect, a method for controlling pollutant emissions in thermal power units based on variable-speed regulation of coal quantity is provided, which is executed by a control system for pollutant emissions in thermal power units based on variable-speed regulation of coal quantity, and includes:

[0008] S1. Control the emission of nitrogen oxides, including:

[0009] S101. For the upper-layer coal mills and the middle-layer coal mills, when the load changes and the NOx in the original flue gas tends to rise, slow down the rate of increase in coal quantity and the rate of increase in rotational speed during load increase, and accelerate the rate of decrease in coal quantity and the rate of decrease in rotational speed during load decrease;

[0010] S102. For the lower-layer coal mills, without considering the impact of the NOx in the raw flue gas, the lower-layer coal pulverizing system still receives the fuel master control command according to the normal control strategy. The balance of the fuel quantity is controlled by the fuel master PID, and the speed control is carried out according to the variable load control requirements.

[0011] S103. For the middle and upper-layer coal pulverizing systems, when the unit undergoes a load change and the NOx concentration in the raw flue gas rises, the coal quantity command for the coal pulverizing system will be readjusted, including: during the load increase stage, reducing the speed of increasing the output of the middle and upper-layer coal pulverizing systems; during the load decrease stage, accelerating the speed of decreasing the output of the middle and upper-layer coal pulverizing systems.

[0012] S2. Control the SO2 or dust emissions, including:

[0013] S201. When feeding coal, convey the high-sulfur or high-ash coal to the coal layer of the variable-speed coal pulverizing system.

[0014] S202. The unit adjusts the output of each coal pulverizing system and the speed of the coal mills according to the power generation power command, and the desulfurization system or the dust removal system controls the system output according to the set value of the SO2 concentration or the dust content in the clean flue gas.

[0015] S203. When the SO2 or dust concentration in the clean flue gas continues to rise and approaches the limit value, a part of the negative coal quantity offset is superimposed to reduce the coal quantity, and the subtracted coal quantity is related to the rising speed of the SO2 concentration or the dust concentration in the clean flue gas.

[0016] S204. When the emissions concentration rises, the speed control loop of the coal mill will reduce the speed of the coal mill, and use the change in speed to dynamically reduce part of the high-sulfur or high-ash coal entering the furnace.

[0017] Preferably, in S103, the variable-speed adjustment of the coal quantity is achieved through the rate adjustment coefficient of the coal pulverizing system. This coefficient is a function f(x) related to the NOx concentration in the raw flue gas, and is adaptively adjusted considering different operating conditions of load increase and decrease. Assume the load increase rate adjustment coefficient is y1, the NOx concentration in the raw flue gas is x1, the load decrease rate adjustment coefficient is y2, y1 = θ1x1 + ω1, y2 = θ2x1 + ω2; θ1, θ2, ω1, ω2 are all real numbers, which are set respectively according to the characteristics of the unit's load increase and decrease. The values of θ1, θ2, ω1, ω2 are calculated through the estimated (x1, y1) and (x1, y2), and are continuously adjusted according to the actual operating conditions.

[0018] Preferably, in S103, when triggering the slowdown of the output increase rate of the coal pulverizing system during the load increase process, the speed of the coal mill will remain unchanged; when triggering the acceleration of the output decrease rate of the coal pulverizing system during the load decrease process, the speed of the coal mill will be reduced; while the speed value is reduced, it is necessary to ensure that the maximum load-carrying capacity is not lower than the current coal quantity.

[0019] Preferably, in S1, different weights are assigned to the influence of the change in the coal quantity of the pulverizing systems at different positions on the NOx content in the raw flue gas.

[0020] Preferably, in S1, override control of the ammonia injection quantity or urea supply quantity is performed according to the variable load coal quantity feedforward of the unit and the inertial characteristics of the boiler air and flue gas system.

[0021] Preferably, in S1, auxiliary control of NOx is achieved by coal quantity distribution and mill speed control when the unit operating conditions are stable.

[0022] Preferably, in S1, the net flue gas NOx content is auxiliarily adjusted by the air volume.

[0023] Preferably, in S203, the process of coal quantity decrease is progressive. After subtracting a certain amount of coal, wait for the change in the emission concentration. If the coal quantity still needs to be reduced after a period of time, continue to reduce the coal quantity of this mill group; otherwise, gradually restore the coal quantity.

[0024] Preferably, in S204, when the unit load is high, the coal quantity is not reduced and the speed is not adjusted.

[0025] Second, a pollutant emission control system for a thermal power unit based on variable-speed regulation of coal quantity as described in the first aspect is provided, including a boiler, a pulverizing system configured with variable-speed mills, a flue gas denitration device and an ammonia production device, an electrostatic precipitator, a desulfurization device, a distributed control system, and related regulating mechanisms and measuring devices; all or part of the pulverizing systems of the boiler are selected to be configured with variable-speed mills.

[0026] The beneficial effects of the present invention are as follows: By utilizing the speed regulation characteristics of the variable-speed mills and according to the influence of different pulverizing systems and the coal types they burn on pollutant generation, rapid coal quantity distribution and adjustment of the pulverizing system output are performed during the rapid variable load process, reducing or suppressing pollutant generation from the source side, alleviating the pressure on the output adjustment of environmental protection equipment under rapid variable load conditions, and ensuring the compliance of the unit emission indicators. In addition, when the operating conditions fluctuate and the output of the environmental protection system is abnormal, the present invention can perform auxiliary control of pollutant generation by adjusting the coal quantity distribution, the output of the pulverizing system, and the mill speed, reducing or avoiding situations such as excessive emissions and load reduction. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a schematic structural diagram of a pollutant emission control system for a thermal power unit based on variable-speed regulation of coal quantity;

[0028] Figure 2 is a control logic diagram of variable-speed regulation of coal quantity under variable load conditions;

[0029] Figure 3It is a circuit diagram for calculating the rate adjustment coefficient of the pulverizing system;

[0030] Figure 4 It is a circuit diagram for controlling the speed of the upper and middle pulverizers under variable load conditions;

[0031] Figure 5 It is a circuit diagram for calculating the urea demand based on coal quantity distribution;

[0032] Figure 6 It is a circuit diagram for calculating the correction coefficient of coal quantity distribution;

[0033] Figure 7 It is an auxiliary control circuit diagram for the coal quantity distribution of the upper pulverizer under steady-state conditions;

[0034] Figure 8 It is a circuit diagram for controlling the air volume with NOX correction;

[0035] Figure 9 It is a schematic diagram of the control strategy for SO2 or soot emissions in the auxiliary control of coal quantity distribution;

[0036] Figure 10 It is a control curve diagram for the coal quantity and speed during the variable load from 500MW to 550MW of a certain unit;

[0037] Figure 11 It is a control curve diagram for NOx in the auxiliary control of coal quantity distribution of the unit. Specific implementation mode

[0038] The present invention will be further described below in conjunction with embodiments. The description of the following embodiments is only for helping to understand the present invention. It should be noted that for those of ordinary skill in the technical field, without departing from the principle of the present invention, several modifications can still be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0039] Embodiment 1:

[0040] A pollutant emission control system for a thermal power unit based on variable-speed regulation of coal quantity, as Figure 1 shown, includes a boiler, a pulverizing system equipped with variable-speed pulverizers, a soot denitration device and an ammonia production device, an electrostatic precipitator, a desulfurization device, a distributed control system, and related regulating mechanisms and measuring equipment; considering factors such as investment costs, all or part of the pulverizing system of the boiler is selected to be equipped with variable-speed pulverizers. The distributed control system receives the unit power command to adjust the output of the pulverizing system, and at the same time, monitors the emission indexes of the unit in real time, and adjusts the output of the environmental protection system and distributes the coal quantity of the pulverizing system in combination with the judgment result of the unit operation condition.

[0041] Embodiment 2:

[0042] At present, the treatment of pollutants in thermal power units is generally carried out on the flue gas side. However, from the generation to the final treatment of pollutants, there is often a large inertia and time delay. When external influencing factors such as equipment failures and changes in operating conditions occur, the output adjustment of environmental protection equipment is likely to be untimely, thus unable to respond in a timely manner during the rapid load change process, resulting in the over-standard emission of flue gas pollutants from the unit. In order to ensure that the environmental protection indicators of the unit are qualified, especially the adaptability of environmental protection facilities under the condition of a significant increase in the load change rate of the unit, the present invention proposes a method for controlling the pollutant emissions of thermal power units based on variable-speed regulation of coal quantity. This control method utilizes the speed regulation characteristics of variable-speed coal mills and, according to the influence of different coal pulverizing systems and the coal types they burn on pollutant generation, quickly distributes the coal quantity and adjusts the output of the coal pulverizing system during the rapid load change process, reducing or suppressing pollutant generation from the source side, alleviating the pressure on the output adjustment of environmental protection equipment under the rapid load change condition, and ensuring the qualification of the unit's emission indicators.

[0043] According to the different types of pollutants, the present invention is mainly divided into a nitrogen oxide (NOx) control part and a desulfurization and dust control part. Specifically, the method for controlling the pollutant emissions of thermal power units based on variable-speed regulation of coal quantity provided by the present invention includes the following steps:

[0044] S1. Control the nitrogen oxide emissions. To ensure that the NOx emissions of the unit do not exceed the standard, under the load change condition, different coal quantity and speed controls will be carried out according to the influence of coal pulverizing systems at different positions on NOx, including:

[0045] S101. For the upper-layer coal mill, when the coal quantity increases and the speed is increased, the increase in the coal quantity of the upper-layer coal mill will significantly increase the flame temperature, and the increase in the flue gas temperature will cause the NOx content in the raw flue gas at the furnace outlet to rise rapidly, which is not conducive to the control of NOx. Therefore, the control strategy for its coal quantity distribution is "slow increase and fast decrease". When the load changes and the NOx in the raw flue gas tends to rise, the rate of coal quantity increase and speed increase is slowed down during load increase, and the rate of coal quantity decrease and speed decrease is accelerated during load decrease; for the middle-layer coal mill, the influence on the flue gas temperature is slightly less than that of the upper-layer coal mill, and the same "slow increase and fast decrease" adjustment effect is achieved, and the same "slow increase and fast decrease" adjustment effect is also achieved.

[0046] S102. For the lower-layer coal mill, when the coal quantity and speed change, the influence on the flue gas temperature is relatively small, and its influence on the NOx content in the raw flue gas is relatively small. To take into account the load change ability of the unit, the lower-layer coal pulverizing system normally responds to the fuel master control instruction, and the speed of the coal mill is increased / decreased according to different load increase / decrease conditions. Therefore, without considering the influence of the NOx in the raw flue gas, the lower-layer coal pulverizing system still receives the fuel master control instruction according to the normal control strategy, and the balance of the fuel quantity is controlled by the fuel master PID, and the speed control is carried out according to the load change control requirements.

[0047] S103. The coal quantity regulation strategy for the medium and upper pulverizing systems is as follows: Figure 2 , Figure 3 As shown in the figure. The coal feeding command for the pulverizing system is divided into two parts: a conventional loop and a variable speed adjustment loop. The part within the box is the variable speed adjustment loop. When both loops are not in operation, they are in a tracking state to ensure a seamless switch of the coal quantity command when the control state changes. When the unit undergoes a load change and the NOx concentration in the raw flue gas increases (mainly for the medium-layer coal mills), the coal quantity command for the pulverizing system will be readjusted, including: during the load increase stage, reducing the lifting speed of the output of the medium and upper pulverizing systems; during the load decrease stage, accelerating the decrease speed of the output of the medium and upper pulverizing systems to ensure that the NOx during the load change process does not exceed the standard.

[0048] S2. Control the SO2 or dust emissions, including:

[0049] S201. When loading coal, convey high-sulfur or high-ash coal to the coal bed of the variable speed pulverizing system;

[0050] S202. The unit adjusts the output of each pulverizing system and the rotational speed of the coal mills according to the power generation power command, and the desulfurization system or the dust removal system controls the system output according to the set value of the SO2 concentration or the dust content in the clean flue gas;

[0051] S203. SO2 (dust) mainly comes from the sulfur content (ash content) in coal. The pulverizing systems at different positions have little impact on the emission concentration. Mainly consider reducing the coal quantity of high-sulfur coal (or high-ash coal) when the pollutant concentration in the clean flue gas approaches the limit value. Therefore, when the SO2 or dust concentration in the clean flue gas continues to rise and approaches the limit value, a part of the negative coal quantity offset is superimposed to reduce the coal quantity. The subtracted coal quantity is related to the rising speed of the SO2 concentration or the dust concentration in the clean flue gas. If the rise is faster, the subtracted coal quantity is more; otherwise, it is less. If the pollutant concentration no longer rises or shows a downward trend, the coal quantity is no longer subtracted;

[0052] S204. When the emission concentration rises, the rotational speed control loop of the coal mill will reduce the rotational speed of the coal mill, and use the change in rotational speed to dynamically reduce part of the high-sulfur or high-ash coal entering the furnace.

[0053] In S103, the variable-speed regulation of the coal quantity is achieved through the rate adjustment coefficient of the coal pulverizing system. This coefficient is a function f(x) related to the NOx concentration in the original flue gas, and adaptive adjustment is carried out considering different operating conditions of load increase and decrease. Assume the load increase rate adjustment coefficient is y1, the NOx concentration in the original flue gas is x1, the load decrease rate adjustment coefficient is y2, y1 = θ1x1 + ω1, y2 = θ2x1 + ω2; θ1, θ2, ω1, and ω2 are all real numbers, which are set respectively according to the characteristics of the unit's load increase and decrease. The values of θ1, θ2, ω1, and ω2 are calculated through the estimated (x1, y1) and (x1, y2), and are continuously adjusted according to the actual operating conditions.

[0054] In S103, to match the change in the coal quantity, the rotational speed of the coal mill will also be adjusted accordingly. When the rate of increase in the output of the coal pulverizing system is triggered to slow down during the load increase process, the rotational speed of the coal mill will remain unchanged; when the rate of decrease in the output of the coal pulverizing system is triggered to accelerate during the load decrease process, the rotational speed of the coal mill will decrease; while the rotational speed value decreases, it is necessary to ensure that the maximum load-bearing capacity is not lower than the current coal quantity to prevent coal mill blockage. The rotational speed control logic is as Figure 4 shown.

[0055] In S1, during the rapid load change process, the NOx control of the denitration system is corrected by combining signals such as the operating information of the coal mill group and the feedforward of the coal quantity during load change. Different weights are assigned to the influence of the change in the coal quantity of the coal pulverizing systems at different positions on the NOx content in the original flue gas. The influence weights can be determined according to the operating data or tests. The upper coal mill has a greater influence, and the lower coal mill has a smaller influence. Then, according to the real-time coal quantity distribution result, the influence of the current overall operating condition of the coal pulverizing system on NOx is calculated, and the ammonia injection amount (or urea supply amount) is corrected in real time. In addition, during the rapid load change process, since the feedforward value of the coal quantity is relatively large, the traditional denitration control using the unit load command as its feedforward control strategy is likely to cause the problem of insufficient ammonia injection amount (or urea supply amount). In response to this, the overrun control of the ammonia injection amount (or urea supply amount) will be carried out according to the feedforward of the coal quantity during the unit load change and the inertial characteristics of the boiler flue gas system to ensure the NOx emission index during the rapid load increase and decrease process. The calculation generation logic and the schematic diagram of the coal quantity distribution correction coefficient are as Figure 5 , Figure 6 shown, where the calculation of the theoretical oxygen demand in the boxed part is the prior art, and the coal quantity distribution correction coefficient is calculated from the unit load and the total value of each coal mill coal quantity.

[0056] In S1, the auxiliary control of NOx is achieved through coal quantity distribution and coal mill rotational speed control when the unit operating condition is stable. When the unit operating condition is stable, if the NOx abnormally increases due to factors such as external disturbances or denitration system failures, the output of the upper coal pulverizing system is overridden and reduced, and the rotational speed of the coal mill is synchronously reduced to assist in stabilizing the NOx. The specific judgment and control logic are as Figure 7As shown. This logic is mainly used for the upper-level coal pulverizing system. When the unit is under non-variable load conditions and this coal pulverizing system is operating, if the NOx content in the net flue gas approaches the limit value, this logic subtracts a certain amount of coal from the current coal feeding amount command. The subtracted coal amount is related to the rising speed of the NOx content in the net flue gas. If the rise is relatively fast, the subtracted coal amount is more; otherwise, it is less. Since there is a certain lag in the impact of coal amount change on the NOx content, the process of coal amount reduction is progressive. That is, after subtracting a certain amount of coal, wait for the change of NOx. If the action condition is still met after a period of time, continue to reduce the coal amount; otherwise, gradually restore the coal amount. The accumulated subtracted coal amount passes through the upper and lower limits and is superimposed on the coal amount command of the coal pulverizing system sent by the fuel master controller, and finally forms the coal amount command for the coal feeder. At the same time, when there is an increase in NOx, reduce the speed of the coal mill, and use the change in speed to dynamically reduce part of the coal amount entering the furnace. To avoid possible blockage caused by speed change, a high limit is set before the coal amount command finally acts on the coal feeder.

[0057] In S1, under steady-state conditions, use the air volume to assist in adjusting the NOx content in the net flue gas. On the premise of ensuring that the air-coal ratio is within a reasonable range, inhibit the rise of NOx by appropriately reducing the air volume. The air volume setting generation circuit adds a bias of the NOx content in the net flue gas to the air volume setting on the basis of the traditional oxygen content correction. By comparing the actual value and the set value of the NOx content in the net flue gas and considering the unit load factor, correct the air volume setting value of the unit to assist in stabilizing the NOx emission index of the boiler. The control strategy is as Figure 8 shown.

[0058] In S203, since the change in the coal amount of high-sulfur coal (or high-ash coal) has a certain lag in the impact on the emission concentration, the process of coal amount reduction is progressive. After subtracting a certain amount of coal, wait for the change of the emission concentration. If it is still necessary to reduce the coal amount after a period of time, continue to reduce the coal amount of this grinding group; otherwise, gradually restore the coal amount. The accumulated subtracted coal amount passes through the upper and lower limits and is superimposed on the coal amount command generated by the combustion master controller, and finally forms the coal amount command for the coal feeder of this grinding group. The balance between the actual total coal amount and the coal amount command is adjusted by the fuel master controller. The control strategy is as Figure 9 shown.

[0059] In S204, since the change in speed will cause a change in the load-carrying capacity of the coal pulverizing system, before the coal feeding amount command finally acts on the coal feeder, it is necessary to limit its high limit to avoid the occurrence of grinding blockage. In addition, when the unit load is relatively high, each coal mill may be close to the output upper limit, and the coal amount and speed are not reduced when the unit load is relatively high.

[0060] Example 3:

[0061] A 660MW supercritical thermal power unit is equipped with 6 coal pulverizing systems. Among them, the upper coal pulverizing system A and the lower coal pulverizing system E are equipped with variable-speed coal mills, and the other 4 coal pulverizing systems are fixed-speed coal mills. One day, the unit operates at a load of 500MW, and a total of 5 coal pulverizing systems, namely A, B, C, D, and E, are in operation. The total coal feeding amount of the unit is 196t / h, and the output of each coal pulverizing system is 39.4t / h. The conventional speed of the variable-speed coal mill is 24r / min, the maximum speed is 31r / min, and the minimum speed is 18r / min; the upper limit of the output of each coal pulverizing system is 52t / h, the output of the variable-speed coal pulverizing system at the highest speed gear is 60t / h, and the output at the lowest speed gear is 40t / h. At a certain moment, the unit receives an AGC instruction to change from 500MW to 600MW, and the load change rate is 25MW / min. The DCS uses the control strategy proposed in the present invention to control the coal amount and the speed of the coal mill. During the load change process, the unit load instruction, the total coal amount instruction, and the coal amount instructions of each coal pulverizing system change as Figure 10 shown. Among them, curve 1 represents the target load instruction, curve 2 represents the load instruction, curve 3 represents the total coal amount, curve 4 represents the coal amount of mill E, curve 5 represents the coal amount of the conventional coal mill, curve 6 represents the coal amount of mill A, curve 7 represents the speed of mill E, and curve 8 represents the speed of mill A.

[0062] As Figure 10 shown, when the unit starts to change the load rapidly, the DCS generates the unit load instruction according to the target load instruction, and the total coal amount increases as the unit load instruction rises. At the same time, to meet the load change rate requirement, an additional part of the feed-forward coal amount is added. After the load change starts, to suppress the generation of NOx, different rates of control are carried out for the coal amounts of the upper variable-speed coal mill A and the lower variable-speed coal mill E: Since mill A is located in the upper layer, the rapid increase in its coal amount will exacerbate the generation of NOx. Therefore, during the load change process, a relatively slow change rate is adopted, mainly following the change of the total coal amount, and the speed is not adjusted, as shown in curve 6 and curve 8; Mill E is located in the bottom layer, and the rapid increase in its coal amount has relatively little impact on the generation of NOx. To balance the unit load change rate, the speed of this coal mill is increased (curve 7) and the adjustment rate is accelerated (curve 4); Other conventional coal mills mainly undertake the task of keeping the total coal amount the same as the instruction during the load change process, and the adjustment mainly receives the instruction of the fuel master controller (curve 5). When the load change process ends, the coal amount adjustment of each mill group will receive the fuel master controller instruction again, so the output of each coal mill will tend to be balanced. After the coal amount returns to normal, the speed of the coal mill will also return.

[0063] In addition, under steady-state operating conditions, the present invention has the function of assisting in stabilizing the emission indicators of the unit, mainly by monitoring the unit's emission indicators to adjust the coal quantity distribution and speed command. The working principle will be described below with reference to the diagrams. The unit equipment configuration is as described in the above example. At a certain moment, the unit is operating stably at a load of 500 MW. After 1 minute, due to a malfunction in the denitration system, the NOx emissions of the unit increase rapidly. To stabilize the unit's emission indicators, in addition to adjusting the output of the denitration system, the DCS control logic further distributes the coal quantity. Specifically as Figure 11 shown. Among them, curve 1 represents the NOx emission concentration of the unit, curve 2 represents the total coal quantity, curve 3 represents the coal quantity of mill A, curve 4 represents the coal quantity of other mills, curve 5 represents the speed of mill E, and curve 6 represents the speed of mill A.

[0064] As Figure 11 shown, when the unit is stable, due to external disturbances and other reasons, the NOx emission concentration of the unit suddenly increases in about 1 minute. To stabilize the NOx emissions of the unit, the control logic reduces the coal feed quantity of the upper mill A by 3 t / h, with an interval of 30 s (curve 4); at the same time, under the condition that the coal feed quantity permits, the speed of the coal mill is synchronously set to the low gear (curve 6) to reduce the generation of NOx. During this process, to ensure the balance of the total coal quantity, the coal quantities of other coal mills will be adjusted correspondingly to increase their respective outputs (curve 3). When the NOx emissions start to decline, the reduction amplitude of the coal feed quantity of mill A is adjusted to 1.5 t / h to prevent excessive decline. Subsequently, when the NOx emission concentration is qualified, the coal feed quantity of mill A starts to slowly recover, and other mills also make corresponding adjustments, and finally the coal feed quantities of each mill return to the same level. During this process, when the coal feed quantity of mill A rebounds, the speed also synchronously rebounds to the normal speed.

Claims

1. A method for controlling pollutant emissions of a thermal power unit based on variable-speed regulation of coal quantity, characterized in that, Executed by the unit pollutant emission control system based on coal quantity distribution during rapid load change, including: S1. Control the nitrogen oxide emission, including: S101. For the upper-layer coal mills and middle-layer coal mills, when the load changes and the NOx in the raw flue gas tends to rise, slow down the rate of coal quantity increase and speed increase during load increase, and accelerate the rate of coal quantity decrease and speed decrease during load decrease; S102. For the lower-layer coal mills, regardless of the influence of the NOx in the raw flue gas, the lower-layer coal pulverizing system still receives the fuel master control instruction according to the normal control strategy, the balance of the fuel quantity is controlled by the fuel master PID, and the speed control is carried out according to the load change control requirements; S103. For the middle- and upper-layer coal pulverizing systems, when the unit load changes and the NOx concentration in the raw flue gas rises, the coal quantity command of the coal pulverizing system will be readjusted, including: during the load increase stage, reduce the lifting speed of the output of the middle- and upper-layer coal pulverizing systems; during the load decrease stage, accelerate the decreasing speed of the output of the middle- and upper-layer coal pulverizing systems; S2. Control the SO2 or dust emission, including: S201. When feeding coal, convey the high-sulfur or high-ash coal to the coal layer of the variable-speed coal pulverizing system; S202. The unit adjusts the output of each coal pulverizing system and the speed of the coal mills according to the power generation power command, and the desulfurization system or dust removal system controls the system output according to the set value of the SO2 concentration or dust content in the clean flue gas; S203. When the SO2 or dust concentration in the clean flue gas continues to rise and approaches the limit value, superimpose a part of the negative coal quantity offset to reduce the coal quantity, and the subtracted coal quantity is related to the rising speed of the SO2 concentration or dust concentration in the clean flue gas; S204. When the emission concentration rises, the speed control loop of the coal mill will reduce the speed of the coal mill, and use the change of the speed to dynamically reduce part of the high-sulfur or high-ash coal entering the furnace.

2. The method for controlling pollutant emissions of a thermal power unit based on variable coal quantity regulation according to claim 1, wherein In S103, the variable-speed adjustment of the coal quantity is achieved through the rate adjustment coefficient of the coal pulverizing system. This coefficient is a function f(x) related to the NOx concentration in the raw flue gas, and is adaptively adjusted considering different operating conditions of load increase and decrease. Assume the load increase rate adjustment coefficient is y1, the NOx concentration in the raw flue gas is x1, the load decrease rate adjustment coefficient is y2, y1 = θ1x1 + ω1, y2 = θ2x1 + ω2; θ1, θ2, ω1, ω2 are all real numbers, which are set respectively according to the characteristics of the unit load increase and decrease. The values of θ1, θ2, ω1, ω2 are calculated through the estimated (x1, y1) and (x1, y2), and are continuously adjusted according to the actual operation conditions.

3. The method for controlling pollutant emissions of a thermal power unit based on variable-speed regulation of coal quantity according to claim 2, wherein In S103, when triggering to slow down the rising rate of the output of the coal pulverizing system during the load increase process, the speed of the coal mill will remain unchanged; when triggering to accelerate the decreasing rate of the output of the coal pulverizing system during the load decrease process, the speed of the coal mill will be reduced; while the speed value is reduced, it is necessary to ensure that the maximum load-carrying capacity is not lower than the current coal quantity.

4. The method for controlling pollutant emissions of a thermal power unit based on variable-speed regulation of coal quantity according to claim 3, characterized in that In S1, different weights are given to the influence of the coal quantity change of the coal pulverizing systems at different positions on the NOx content in the raw flue gas.

5. The method for controlling pollutant emissions of a thermal power unit based on variable-speed regulation of coal quantity according to claim 4, characterized in that, In S1, the overrun control of the ammonia injection amount or urea supply amount is carried out according to the load change coal quantity feedforward of the unit and the inertial characteristics of the boiler air and flue gas system.

6. The method for controlling pollutant emissions of a thermal power unit based on variable-speed regulation of coal quantity according to claim 5, characterized in that, In S1, when the unit operating conditions are stable, auxiliary control of NOx is achieved through coal quantity distribution and coal mill speed control.

7. The method for controlling pollutant emissions of a thermal power unit based on variable-speed regulation of coal quantity according to claim 6, wherein In S1, the air volume is used to assist in adjusting the NOx content in the clean flue gas.

8. The method for controlling pollutant emissions of a thermal power unit based on variable-speed regulation of coal quantity according to claim 1, wherein In S203, the process of coal quantity reduction is progressive. After subtracting a certain amount of coal, wait for the change in the emission concentration. If coal quantity still needs to be reduced after a period of time, continue to reduce the coal quantity of this mill group; otherwise, gradually restore the coal quantity.

9. The method for controlling pollutant emissions of a thermal power unit based on variable-speed regulation of coal quantity according to claim 8, characterized in that In S204, when the unit load is high, do not reduce the coal quantity and adjust the speed.

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