Method for controlling boiler cold start by variable speed regulation of medium speed coal mill using rare earth motor

Abstract

The present invention relates to a method for controlling the cold start of a boiler by speed regulation of a medium-speed coal mill of a rare earth motor, comprising the steps of: starting the boiler in a cold state, warming the coal mill through a primary air heater until the coal mill start-up conditions are met, confirming that the micro-oil or plasma ignition fire detection is normal, starting the coal mill and the coal feeder; adjusting the coal feed correction coefficient k and the coal mill speed coefficient x according to the boiler heating rate and pressure increase rate. The beneficial effects of the present invention are: by reducing the coal mill speed, the minimum coal feed limit of the coal mill is effectively reduced, and the boiler cold start-up is achieved. Smooth temperature and pressure increase, avoiding the boiler cold start-up when adding coal powder and heating and pressure increase too fast; ensuring that the heating rate and pressure increase rate are both within the specified range, avoiding the local overheating of the heating surface, resulting in a large amount of oxide scale and the occurrence of boiler pipe burst accidents.

Description

Technical Field

[0001] The invention belongs to the field of coal mill control, and in particular relates to a method for controlling cold start of a boiler by speed-changing regulation of a medium-speed coal mill of a rare earth motor. Background Art

[0002] In recent years, due to the need for energy conservation and consumption reduction, the cold start of large coal-fired power plant boilers has adopted the method of micro-oil ignition or plasma ignition to start the medium-speed coal mill and add coal powder, replacing the previous method of using large oil guns to start the boiler. This startup method can save a lot of fuel and has significant economic benefits. However, in order to prevent the violent vibration caused by low coal operation and affect the safety of the equipment, there is a minimum coal limit during the operation of the medium-speed coal mill. This minimum allowable coal amount will cause the boiler to input too much heat at the initial stage of the cold start of the boiler, that is, from the start of boiler ignition to the start of boiler pressure to the stage where the turbine start-up parameters are reached. This will cause the boiler to heat up too quickly, causing the boiler to heat up and increase pressure too quickly, affecting the operating life and safety of the boiler heating surface, and affecting the matching of the turbine start-up parameters.

[0003] In response to the above problems, power plant boiler technicians have made some efforts and work, such as reducing the loading force of the medium-speed coal mill to reduce the minimum coal quantity limit of the coal mill, or using coal with lower calorific value and higher volatile content; and controlling the steam temperature and pressure to match the turbine start-up needs through the exhaust valve or bypass at the initial start-up. However, due to the existence of the minimum coal quantity limit of the coal mill, these technical measures have achieved certain effects, but cannot fundamentally solve the problem of excessive temperature and pressure increase in the initial cold start of the boiler. Summary of the invention

[0004] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a method for controlling the cold start of a boiler by means of variable speed regulation of a medium speed coal mill using a rare earth motor.

[0005] The method for controlling the cold start of a boiler by speed regulation of a medium-speed coal mill using a rare earth motor comprises the following steps:

[0006] Step 1: Start the boiler in cold state, warm up the coal mill through the primary air heater until the coal mill start-up conditions are met, confirm that the micro-oil or plasma ignition fire detection is normal, and start the coal mill and coal feeder;

[0007] Step 2: The speed of the coal mill is x*n, where n is the design speed, x is the coal mill speed coefficient, 0.75≤x≤1.25; the coal feed rate is k*M, where M is the design minimum output limit coal feed rate, k is the coal feed rate correction coefficient, 0.7≤k≤2.0, and the initial values ​​of x and k are both set to 1;

[0008] Step 3: Adjust the coal feed correction factor k and the coal mill speed factor x according to the boiler heating rate and pressure increase rate.

[0009] When the low value of heating rate < heating rate ≤ high value of heating rate, and the low value of pressure rise rate < pressure rise rate ≤ high value of pressure rise rate, keep k = 1;

[0010] When the heating rate is ≤ the low value of the heating rate or the pressure increase rate is ≤ the low value of the pressure increase rate, k>1, until the low value of the heating rate is < the heating rate ≤ the high value of the heating rate or the low value of the heating rate is < the pressure increase rate ≤ the high value of the heating rate;

[0011] When heating rate>high heating rate or pressure rise rate>high pressure rise rate, x=0.75, k<1, until low heating rate<heating rate≤high heating rate and low pressure rise rate<pressure rise rate≤high heating rate.

[0012] Preferably, in step one: the primary air inlet of the coal mill is equipped with a heater coal mill, and the burner corresponding to the coal mill outlet is equipped with a micro-oil ignition device or a plasma ignition device.

[0013] Preferably, in step one: the conditions for starting the coal mill are that the primary air volume at the coal mill inlet and the outlet air temperature both reach set values; the set value of the primary air volume at the coal mill inlet is obtained based on the air-coal ratio curve of the coal mill, and the set value of the outlet air temperature is >60°C.

[0014] Preferably, in step three: the low value of the heating rate, the high value of the heating rate, the low value of the pressure increase rate and the high value of the pressure increase rate are all set according to the curve provided by the boiler manufacturer.

[0015] Preferably, in step three: the low value of the heating rate is 1.5°C / min, the high value of the heating rate is 1.85°C / min, the low value of the pressure increase rate is 0.05MPa / min, and the high value of the pressure increase rate is 0.11MPa / min.

[0016] Preferably, in step three: the coal feed correction coefficient k is changed in batches, and the single change amplitude is less than or equal to 0.2.

[0017] The beneficial effects of the present invention are:

[0018] 1) The method of controlling the cold start of a boiler by means of variable speed regulation of a medium speed coal mill using a rare earth motor proposed in the present invention can effectively reduce the minimum coal feed limit of the coal mill by reducing the rotation speed of the coal mill, thereby achieving smooth temperature and pressure increase during the cold start of the boiler and avoiding excessive temperature and pressure increase during the cold start of the boiler by adding pulverized coal.

[0019] 2) The method of controlling the cold start of a boiler by means of variable speed regulation of a rare earth motor medium speed coal mill proposed in the present invention adjusts the coal feed rate by detecting the heating rate and the pressure increase rate to ensure that the heating rate and the pressure increase rate are within the specified range, thereby avoiding local overheating of the heating surface leading to the massive generation of oxide scale and the occurrence of boiler tube burst accidents. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the operation of the milling system;

[0021] Figure 2 This is a schematic diagram of sampling the powder pipe at the outlet of the coal mill;

[0022] Figure 3 This is the corresponding relationship diagram between coal quantity and speed change of coal mill;

[0023] Figure 4 This is the logic diagram of the boiler's cold start temperature and pressure increase control.

[0024] Explanation of the reference numerals: coal bunker 1, coal feeder 2, coal mill 3, rare earth motor 4, frequency conversion control unit 5, static separator 6, cold primary air adjustment door 7, hot primary air adjustment door 8, coal mill outlet powder pipe 9, boiler 10, coal mill outlet powder pipe coal powder sampling hole 11. DETAILED DESCRIPTION

[0025] The present invention is further described below in conjunction with embodiments. The description of the following embodiments is only used to help understand the present invention. It should be noted that for ordinary persons in the art, without departing from the principle of the present invention, the present invention can also be modified in some ways, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

[0026] Embodiment 1

[0027] As an embodiment, a method for controlling the cold start of a boiler by speed regulation of a rare earth motor medium-speed coal mill is provided, and its pulverizing system includes: a coal bunker 1, a coal feeder 2, a coal mill 3, a rare earth motor 4, a frequency conversion control unit 5, a static separator 6, a cold primary air adjustment door 7, a hot primary air adjustment door 8, a coal mill outlet powder pipe 9 and a boiler 10; the coal mill outlet powder pipe 9 is provided with a coal mill outlet powder pipe coal powder sampling hole 11; the frequency conversion control unit 5 is electrically connected to the rare earth motor 4, and the rare earth motor 4 is electrically connected to the coal mill 3; the coal bunker 1 is connected to the coal feeder 2, and the coal feeder 2 is connected to the coal mill 3; a static separator 6 is provided at the outlet of the coal mill 3, the static separator 6 is connected to the coal mill outlet powder pipe 9, and the coal mill outlet powder pipe 9 is connected to the boiler 10; the coal mill 3 is also provided with a cold primary air adjustment door 7 and a hot primary air adjustment door 8.

[0028] The coal mill 3 is equipped with a frequency converter, which is provided with a remote control signal interface; the frequency conversion control unit 5 controls the frequency converter, which drives the rare earth motor 4, and the rare earth motor 4 drives the coal mill 3 to rotate. The coal mill adjusts the coal feeding amount and changes the coal mill speed according to the boiler cold start temperature and pressure rise curve through the rare earth motor frequency conversion control unit 5, and the coal mill 3 inlet is equipped with a heater. The design speed of the coal mill 3 is n, the design minimum output limit coal feeding amount is M, and the rated current of the rare earth motor frequency converter is I; the parameters of the coal feeding amount are obtained through the operating parameters of the coal feeder 2.

[0029] The primary air inlet of the coal mill is equipped with a heater, and the corresponding burner at the outlet is equipped with a micro-oil ignition device or a plasma ignition device; the coal mill is equipped with a frequency converter, and the frequency converter is provided with a remote control signal interface; the frequency conversion control unit 5 controls the frequency converter, and the frequency converter drives the rare earth motor 4, and the rare earth motor 4 drives the coal mill 3 to rotate.

[0030] The method for controlling the cold start of the boiler by speed regulation of the medium-speed coal mill of the rare earth motor of the pulverizing system specifically comprises the following steps:

[0031] Step 1: The boiler is started in cold state. The coal mill is warmed by the primary air heater until the primary air volume at the coal mill inlet and the outlet air temperature reach the set values. The set value of the primary air volume at the coal mill inlet is obtained according to the air-coal ratio curve of the coal mill 3. If the set value of the outlet air temperature is greater than 60°C, it is considered that the start-up conditions of the coal mill 3 are met. Confirm that the micro-oil or plasma ignition fire detection is normal, and start the coal mill 3 and the coal feeder 2.

[0032] Step 2: The speed of the coal mill is x*n, where n is the design speed, x is the coal mill speed coefficient, 0.75≤x≤1.25; the coal feed rate is k*M, where M is the design minimum output limit coal feed rate, k is the coal feed rate correction coefficient, 0.7≤k≤2.0, and the initial values ​​of x and k are both set to 1;

[0033] Step 3: Adjust the coal feed correction factor k and the coal mill speed factor x according to the boiler heating rate and pressure increase rate.

[0034] When 1.5℃ / min<heating rate≤1.85℃ / min, and 0.05MPa / min<pressure increasing rate≤0.11MPa / min, keep k=1;

[0035] When the heating rate is ≤1.5℃ / min or the pressure increase rate is ≤0.05MPa / min, increase k in steps until 1.5℃ / min<heating rate ≤1.85℃ / min or 0.05MPa / min<pressure increase rate ≤0.11MPa / min;

[0036] When the heating rate is greater than 1.85°C / min or the pressure increase rate is greater than 0.11 MPa / min, x=0.75, and k is reduced in steps until 1.5°C / min<heating rate≤1.85°C / min and 0.05 MPa / min<pressure increase rate≤0.11 MPa / min.

[0037] Among them, the single change range of the coal feed correction coefficient k is less than or equal to 0.2.

[0038] The test of speed adjustment of coal mill to control cold start of boiler was carried out to verify the effect of the present invention:

[0039] The design speed of a coal mill 3 is n = 24r / min, and the design output coal is 61.5t / h; the specific parameters are shown in the table below:

[0040]

[0041] The parameters of the rare earth permanent magnet motor are shown in the following table:

[0042]

[0043] For the coal mill 3 with a design speed of n and a design minimum output coal quantity of M, the minimum coal quantity test was carried out at different speeds for the same coal type, with a total of three test conditions: 0.75n, 0.9n, and 1.0n.

[0044] The parameters of the coal feeding amount are obtained through the operating parameters of the coal feeder. The coal feeder can control the coal feeding amount through the belt speed, and the coal feeding amount is calibrated regularly; if a larger coal feeding amount is to be obtained, a faster belt speed is required. In this embodiment, the coal feeding amount is selected to be tested at the minimum coal feeding amount of the pulverizing system at different speeds.

[0045] The outlet temperature and inlet primary air volume of the coal mill are controlled by the cold primary air adjustment door 7 and the hot primary air adjustment door 8. The hot primary air adjustment door 8 controls the inlet primary air volume of the coal mill through the opening of the damper; the cold primary air adjustment door 7 adjusts the temperature. In this embodiment, the inlet primary air volume of the coal mill is obtained according to the air-coal ratio curve of the coal mill 3, and no offset is set, and the outlet temperature of the coal mill is adjusted to maintain 75°C.

[0046] The fineness of the coal powder is adjusted by the opening of the baffle of the static separator 6 at the outlet of the coal mill. The opening adjustment range of the static separator baffle is 10° to 80°. The larger the opening of the static separator baffle, the larger the coal powder fineness standard, and the smaller the opening of the static separator baffle, the smaller the coal powder fineness standard. A large fineness standard refers to a large coal powder particle size, and a small fineness standard refers to a small coal powder particle size. In this embodiment, the opening of the static separator at the outlet of the coal mill is maintained at 55°.

[0047] The hydraulic loading force of the coal mill roller is 4Mpa when the coal feed rate is [0, 20]t / h.

[0048] The speed of the rare earth motor of the coal mill is controlled by the frequency conversion control unit 5, thereby driving the grinding roller of the coal mill to operate at different speeds. In this embodiment, tests are carried out at three speeds of 0.75n, 0.9n and 1.0n. The minimum coal feeding amount of the pulverizing system is the amount of coal before the coal mill vibrates abnormally or makes abnormal sounds.

[0049] Under each operating condition, after the pulverizer reaches the minimum coal feed rate and the operating parameters are stable for 30 minutes, the coal powder fineness test is carried out. At the same time, the relevant operating parameters during the test are recorded by the DCS and PI systems, including the coal feed rate, pulverizer current, primary air volume, pulverizer outlet temperature, and the time-averaged value of stone coal emission. The test time after stabilization is not less than 2 hours.

[0050] like Figure 2 The figure shows a schematic diagram of sampling the powder pipe at the coal mill outlet in this embodiment. Under different working conditions, the coal powder fineness test is carried out in the six powder pipes at the coal mill outlet in turn, and the weighted average is used to obtain the coal powder fineness R90 under the working condition.

[0051] The following table shows the unit consumption of coal pulverizing under different working conditions when the coal powder fineness at the outlet of the pulverizer is within the required range and does not change much. Figure 3 The line chart shows the minimum coal feed rate of the coal mill under different operating conditions.

[0052]

[0053] The results of this embodiment show that: 1) compared with the original design speed of 24r / min, when the speed is reduced to 21r / min and 18r / min, the minimum coal feed rate of the coal mill is significantly reduced, among which the minimum coal feed rate is reduced by about 26.28% when the speed is 18r / min compared with 24r / min; 2) compared with 21r / min and 18r / min, when the coal mill allows the minimum coal feed rate, the coal powder fineness R90 does not change much and is within the design range.

[0054] Combined with the results of this embodiment, at the initial start-up of the unit, the frequency can be controlled to 743 Hz by the frequency conversion control unit 5, corresponding to a coal mill speed of 18 r / min, and the minimum allowable coal feed rate can be reduced to about 10.63 t / h, ensuring that the boiler temperature and pressure increase rate is within the specified range, which can improve the adjustment flexibility and ensure the safety of the boiler cold start-up.

[0055] Embodiment 2

[0056] On the basis of Example 1, this example provides a method for controlling the cold start of a boiler by speed adjustment of a medium-speed coal mill of a rare earth motor, which is applied to a coal mill of a certain power plant. The coal mill has a design speed of 24r / min and a design output of 61.5t / h of coal. The specific coal mill parameters are shown in the following table:

[0057]

[0058] The parameters of the rare earth permanent magnet motor are shown in the following table:

[0059]

[0060] For a coal mill with a design speed of n and a design minimum output coal quantity M, and targeting a medium-speed coal mill with variable speed regulation of a rare earth motor, temperature and pressure increase application tests were carried out under three operating conditions: minimum coal quantity at design speed n, minimum coal quantity at 0.9n speed, and minimum coal quantity at 0.75n speed.

[0061] Start the boiler in cold state, and put the induced draft fan and the forced draft fan into operation; adjust the furnace negative pressure to -100Pa, put the induced draft fan into automatic operation, adjust the air supply volume to more than 30% of the total air volume, put the air supply into automatic operation, blow the furnace for 5 minutes, and reset the boiler MFT signal; start the primary fan, adjust the primary air main pipe pressure to 5kPa, and put the primary fan into automatic operation; open the outlet and inlet dampers of the medium-speed coal mill E with rare earth motor speed regulation, and adjust the primary air flow of coal mill 3 to 72t / h; turn on the inlet primary air heater of coal mill 3, and coal mill 3 starts warm grinding; put into operation the micro-oil ignition device corresponding to the 6 burners of coal mill 3, and confirm that the fire inspection is normal;

[0062] When the outlet temperature of coal mill 3 meets the start-up conditions, start coal mill 3 and start coal feeder E, and set the coal feed rate to about the minimum allowable coal feed rate of coal mill design, which is 16t / h;

[0063] In this embodiment, the primary air volume at the inlet of the test coal mill is set to 0 t / h according to the air-coal ratio control logic, and the outlet temperature of the coal mill is automatically adjusted to be between 65 and 85°C by the opening of the cold and hot air door dampers at the inlet of the coal mill; the opening of the static separator at the outlet of the coal mill is maintained at 55°; the hydraulic loading force of the coal mill roller is 4 MPa when the coal feed rate is [0, 20] t / h; when the coal feed rate is [20, 60] t / h, the hydraulic loading force of the coal mill roller is y=0.3x-2 (MPa); when the coal feed rate is [60, 70] t / h, the hydraulic loading force of the coal mill roller is 16 MPa.

[0064] Keep the pulverizer 3 running at the designed speed of 24r / min and the coal feed of 18t / h. After 5 minutes, the boiler starts to increase the temperature and pressure, and observe the temperature and pressure increase response: with the combustion of coal powder and the increase of furnace temperature, the combustion of coal powder is enhanced, and the temperature and pressure increase of the boiler will gradually accelerate; adjust the coal feed according to the temperature and pressure increase rate, and record the relevant parameters.

[0065] During other cold start-up of the boiler, keep the speed of pulverizer 3 at 21r / min and 18r / min, and the coal feed at 18t / h, and observe the temperature and pressure increase response: when the temperature and pressure increase rate of the boiler reaches the manufacturer's specified value (generally ≯1.85℃ / min, 0.1MPa / min), reduce the coal feed of pulverizer 3 appropriately, and observe the temperature and pressure increase of the boiler: if the temperature and pressure increase is still relatively fast, continue to reduce it to 13t / h and 10t / h; otherwise, increase the coal feed until the temperature and pressure increase rate reaches the ideal state.

[0066] The following table is the record results of the temperature and pressure control rate test of the initial cold start of the boiler at three speeds under three cold start conditions of the boiler in this embodiment:

[0067]

[0068] The results of this embodiment show that: 1) During the cold start of the boiler, due to the minimum coal feed limit at the design speed of the coal mill of 24r / min, the temperature and pressure increase rate after the boiler is pressurized is significantly higher. 2) At 21r / min, because the minimum coal feed can be as low as 13t / h, the temperature and pressure increase rate from the boiler pressure start to the start-up stage is more controllable; 3) At 18r / min, because the minimum coal feed can be as low as 10t / h, the adjustable range of coal volume is wider, which can fully meet the temperature and pressure increase rate requirements of the boiler cold start. During the test, the coal mill operated normally, indicating that a lower coal mill speed is conducive to the safe cold start of the boiler.

[0069] Combined with the results of this embodiment, at the initial stage of unit startup, the variable speed control unit 5 controls the speed adjustment of the medium-speed coal mill to control the cold start of the boiler: the frequency of the variable speed control unit 5 is set to 743Hz, corresponding to the coal mill speed of 18r / min, and the minimum coal feed rate is reduced to about 10.63t / h, ensuring that the boiler temperature and pressure increase rate is within the specified range, avoiding local overheating of the heating surface, resulting in a large amount of oxide scale generation and boiler tube burst accidents.

Claims

1. A method for controlling the cold start of a boiler by speed regulation of a medium-speed coal mill using a rare earth motor, characterized in that: The following steps are involved: Step 1: The boiler is started in cold state. The coal mill is warmed up by a primary air heater until the coal mill start-up conditions are met. After confirming that the micro-oil or plasma ignition fire detection is normal, the coal mill (3) and the coal feeder (2) are started; Step 2: The speed of the coal mill is x*n, where n is the design speed, x is the coal mill speed coefficient, 0.75≤x≤1.25; the coal feed rate is k*M, where M is the design minimum output limit coal feed rate, k is the coal feed rate correction coefficient, 0.7≤k≤2.0, and the initial values ​​of x and k are both set to 1; Step 3: Adjust the coal feed correction factor k and the coal mill speed factor x according to the boiler heating rate and pressure increase rate. When the low value of heating rate < heating rate ≤ high value of heating rate, and the low value of pressure rise rate < pressure rise rate ≤ high value of pressure rise rate, keep k = 1; When the heating rate is ≤ the low value of the heating rate or the pressure increase rate is ≤ the low value of the pressure increase rate, k>1, until the low value of the heating rate is < the heating rate ≤ the high value of the heating rate or the low value of the heating rate is < the pressure increase rate ≤ the high value of the heating rate; When the heating rate is greater than the high value of the heating rate or the pressure rise rate is greater than the high value of the pressure rise rate, x=0.75, k<1, until the low value of the heating rate is less than the heating rate ≤ the high value of the heating rate and the low value of the pressure rise rate is less than the pressure rise rate ≤ the high value of the pressure rise rate; the low value of the heating rate, the high value of the heating rate, the low value of the pressure rise rate and the high value of the pressure rise rate are all set according to the curve provided by the boiler manufacturer.

2. The method for controlling the cold start of a boiler by speed regulation of a rare earth motor medium speed coal mill according to claim 1 is characterized in that: In step 1: the primary air inlet of the coal mill (3) is equipped with an air heater, and the burner corresponding to the outlet of the coal mill (3) is equipped with a micro-oil ignition device or a plasma ignition device.

3. The method for controlling the cold start of a boiler by speed regulation of a rare earth motor medium speed coal mill according to claim 1 is characterized in that: In step 1: the conditions for starting the coal mill are that the primary air volume at the coal mill inlet and the outlet air temperature both reach the set values; the set value of the primary air volume at the coal mill inlet is obtained according to the air-coal ratio curve of the coal mill (3), and the set value of the outlet air temperature is greater than 60°C.

4. The method for controlling the cold start of a boiler by speed regulation of a rare earth motor medium speed coal mill according to claim 1 is characterized in that: In step three: the low value of the heating rate is 1.5°C / min, the high value of the heating rate is 1.85°C / min, the low value of the pressure increase rate is 0.05MPa / min, and the high value of the pressure increase rate is 0.11MPa / min.

5. The method for controlling the cold start of a boiler by speed regulation of a rare earth motor medium speed coal mill according to claim 1 is characterized in that: In step three: the coal feed correction coefficient k is changed in batches, and the single change range is less than or equal to 0.2.

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