Method of adjusting medium-speed coal mill by motor speed change to improve boiler's low-load stable combustion capability

Abstract

The present invention relates to a method for improving the low-load stable combustion capability of a boiler by adjusting a medium-speed coal mill with a motor with variable speed, including the following steps: conducting a low-load operation test on the coal mill with the same type of coal, the same coal feed, different rotation speeds, and different primary air volumes at the inlet of the coal mill under the condition of 40% BMCR or below; optimizing the coal feed and speed control mode of the pulverizing system under low-load conditions to obtain the coal feed and speed control mode when the boiler is running at low load; optimizing the air-coal ratio curve, coal feed and speed control mode of the pulverizing system during the low-load operation of the boiler. The beneficial effects of the present invention are: when the unit's deep peak-shaving load is at or below the BMCR condition of 40% BMCR, the speed of the coal mill is adjusted by the rare earth motor with variable speed to improve the fineness of the coal powder at the outlet of the coal mill; improving the coal powder concentration in the powder pipe at the outlet of the coal mill, which can effectively solve the problems of deep peak-shaving, low furnace temperature, and poor combustion stability of the burner when the boiler is running at low load.

Description

Technical Field

[0001] The invention belongs to the field of coal mill control, and in particular relates to a method for adjusting a medium-speed coal mill by means of motor speed change to improve the low-load stable combustion capability of a boiler. Background Art

[0002] With the adjustment of energy structure, new energy sources such as photovoltaic and wind power have developed rapidly in recent years. In order to alleviate the problem of new energy consumption, further tapping the deep peak-shaving capacity of traditional coal-fired units, which account for the majority of power generation, is the most convenient, fast and effective means to alleviate the current difficulties in new energy consumption. The relatively low furnace temperature of the boiler during low-load operation of thermal power units and the poor combustion stability of the boiler pulverized coal burner without oil are difficult problems that need to be solved urgently during deep peak-shaving of the units.

[0003] In response to the problem of poor combustion stability of boiler pulverized coal burners during low-load operation of the unit, domestic researchers have made a lot of efforts and work, such as fine-tuning the combustion adjustment at low load, optimizing and modifying the pre-combustion chamber burner, dynamic separator, pulverized coal distributor and other components in the pulverizing system, modifying the dual-fuel bin for low-load burning of high-quality and high-calorific value bituminous coal, and modifying the combustion monitoring device. 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 adjusting a medium-speed coal mill by means of a motor variable speed to improve the low-load stable combustion capability of a boiler.

[0005] The method for adjusting the medium-speed coal mill by motor speed change to improve the low-load stable combustion capability of the boiler comprises the following steps:

[0006] Step 1: Conduct a low-load operation test on the coal mill at 40% BMCR or below with the same type of coal, the same coal feed rate, different rotation speeds, and different primary air volumes at the coal mill inlet; after the pulverizing system has been running stably for a certain period of time t0, measure the time-average value of the primary air velocity and the fineness of the coal powder through the coal powder sampling hole within a certain period of time t, measure the time-average value of the pebble coal discharge volume through the pebble coal discharge port, measure the time-average value of the furnace outlet temperature through the furnace outlet temperature test hole, and record the operating parameters of the pulverizing system and the intensity of the burner outlet fire detection through the DCS or PI system;

[0007] Step 2: Combine the test results in step 1 to perform calculation and analysis, optimize the coal feed rate and speed control method of the pulverizing system under low load conditions, and obtain the coal feed rate and speed control method when the boiler is running at low load; optimize the air-coal ratio curve, coal feed rate and speed control method of the pulverizing system during low load operation of the boiler.

[0008] Preferably, in step 1: during low-load operation of the unit, the unit is withdrawn from the AGC and operated under the CCS coordinated control mode, and 2 to 3 coal mills are kept in operation, some of which are variable-speed coal mills.

[0009] As a preferred embodiment, in step 1: the air volume and outlet temperature of the primary air at the inlet of the coal mill are controlled by the cold primary air adjustment door and the hot primary air adjustment door; the reference value of the primary air volume at the inlet of the coal mill is determined according to the coal feed rate and the original air-coal ratio curve;

[0010] The original air-coal ratio curve is as follows: when the coal feed rate is [0, a) t / h, the primary air volume at the coal mill inlet is Q = qt / h; when the coal feed rate is [a, b] t / h, the primary air volume at the coal mill inlet is

[0011] Q=q+K×(ma)

[0012] Among them, Q is the primary air volume at the coal mill inlet, in t / h; q is the minimum ventilation volume designed for the coal mill, in t / h; m represents the coal feed rate, in t / h; a is the minimum coal feed rate of the coal mill, in t / h; b is the maximum output designed for the coal mill, in t / h; K is a constant provided by the coal mill manufacturer.

[0013] Preferably, in step one: the coal powder fineness standard is controlled to be R90 by the baffle opening of the static separator; and the coal mill loading force is determined by the original loading curve of the coal mill.

[0014] Preferably, in step one: the primary air velocity and the coal powder fineness are calculated by weighted average of the measurement results of the coal powder sampling holes on several coal mill outlet powder pipes.

[0015] As a preferred method, in step 1: the base speed of the coal mill is n, and the designed maximum output coal feed is Q Max , the rated current of the variable speed coal mill rare earth motor inverter is I; five working conditions are tested respectively, and the speed and the primary air volume of the coal mill inlet are set as follows:

[0016] The speed of the coal mill is 0.75n, and the deviation of the primary air volume at the coal mill inlet relative to the reference value is +5t / h;

[0017] The speed is 0.9n, and the deviation of the primary air volume at the coal mill inlet relative to the reference value is +2t / h;

[0018] The speed is n, and the offset of the primary air volume at the coal mill inlet relative to the reference value is 0t / h;

[0019] The speed is 1.1n, and the deviation of the primary air volume at the coal mill inlet relative to the reference value is -2t / h;

[0020] The rotation speed is 1.25n, and the deviation of the primary air volume at the coal mill inlet relative to the reference value is -5t / h.

[0021] Preferably, in step 1: t0 is 30 min, and t is greater than or equal to 2 h.

[0022] As a preferred method, in step 1: the calculation formula for the coal powder concentration c is:

[0023]

[0024] Wherein, c is the coal powder concentration, kg / kg; M is the hourly average value of coal feed, t / h; m is the hourly average value of stone coal discharge, kg / h; q is the hourly average value of primary air volume at the pulverizer inlet, t / h.

[0025] Preferably, in step 2: as the rotation speed increases, the coefficient K of the air-coal ratio curve of the coal mill is gradually reduced to obtain the air-coal ratio curve of the coal mill at different rotation speeds.

[0026] The beneficial effects of the present invention are:

[0027] 1) The present invention proposes a method for improving the low-load stable combustion capability of a boiler based on the variable speed regulation of a medium-speed coal mill by a rare earth motor. When the deep peak load of the unit is at or below 40% BMCR, the speed of the coal mill is regulated by the variable speed regulation of the rare earth motor to improve the fineness of the coal powder at the outlet of the coal mill.

[0028] 2) The present invention optimizes the air-coal ratio curve under different rotation speeds, increases the coal powder concentration in the pulverizer outlet powder pipe, and can effectively solve the problems of deep peak regulation, low furnace temperature when the boiler is running at low load, and poor combustion stability of the burner. While improving the operating safety of the unit, it can also further widen the deep peak regulation capacity of the unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a schematic diagram of the operation of the milling system;

[0030] Figure 2 This is a schematic diagram of sampling the powder pipe at the coal mill outlet and sampling the pebble coal at the bottom of the coal mill;

[0031] Figure 3 This is a schematic diagram of the furnace outlet point temperature test hole;

[0032] Figure 4 Air-coal ratio curve of pulverizing system;

[0033] Figure 5 It is a line chart showing the changes of coal powder concentration and coal powder fineness under different working conditions;

[0034] Figure 6 It is a line chart showing the changes in burner fire detection intensity and furnace outlet temperature under different working conditions;

[0035] Figure 7 This is the optimized air-coal ratio curve of the variable speed coal mill pulverizing system.

[0036] Explanation of the reference numerals: coal bunker 1, coal feeder 2, coal mill 3, variable speed coal mill 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, coal mill bottom stone coal discharge port 12, furnace outlet temperature test hole 13. DETAILED DESCRIPTION

[0037] 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.

[0038] Embodiment 1

[0039] As an example, Figures 1 to 3 As shown, a method for adjusting the medium-speed coal mill by motor speed change to improve the low-load stable combustion capability of the boiler is:

[0040] Step 1: Design the base speed of coal mill 3 to be n, design the maximum output coal quantity to be Q, and the rated current of the variable speed coal mill motor inverter to be I;

[0041] During the low-load operation of the boiler, i.e. 40% BMCR condition or below, the unit is withdrawn from the AGC and operated under the CCS coordinated control mode, keeping 2 to 3 coal mills in operation, of which N are variable-speed coal mills, N ≤ 3; under the same boundary conditions, the pulverizing system is tested for pulverized coal concentration, fineness, stone coal emission and furnace outlet temperature at different speeds and different primary air volumes for the same type of coal and the same coal volume, and the operating parameters of the pulverizing system during this period and the average value of the burner outlet fire detection intensity are recorded through the DCS or PI system;

[0042] For a coal mill with a design speed of n, a design coal volume of Q, a variable speed coal mill motor inverter rated current of I, a loading mode of hydraulic loading, and coal powder fineness adjusted by a static separator damper, the coal mill outlet temperature is controlled by the cold primary air adjustment door 7 at the coal mill inlet and is set at 80°C; the coal mill inlet primary air volume is controlled by the coal mill hot primary air adjustment door 8, and the coal mill inlet primary air volume refers to the coal-pulverizing system air-coal ratio curve, and the coal mill inlet primary air volume is adjusted by the hot primary air adjustment door 8 air volume offset; the coal mill coal fineness is controlled according to the coal mill hydraulic loading force and the damper of the coal mill outlet static separator 6, and the coal powder fineness standard is R90, and the coal mill hydraulic loading force refers to the mill hydraulic loading curve, and the damper of the coal mill outlet static separator 6 is set to a fixed opening;

[0043] For the same type of coal, at the same coal feed rate,

[0044] The coal mill speed is 0.75n, and the primary air volume bias at the coal mill inlet is +5t / h;

[0045] The speed is 0.9n, and the primary air volume bias at the coal mill inlet is +2t / h;

[0046] The speed is n, and the primary air volume offset at the coal mill inlet is 0t / h;

[0047] The speed is 1.1n, and the primary air volume bias at the coal mill inlet is -2t / h;

[0048] The speed is 1.25n, and the primary air volume offset at the coal mill inlet is -5t / h

[0049] Under five working conditions, the pulverizing system was tested for no less than 2 hours after running stably for 30 minutes, where n is the design speed of the coal mill, and the offset refers to the offset relative to the reference value of the primary air volume at the coal mill inlet;

[0050] like Figure 2 The figure shows a schematic diagram of sampling the powder pipe at the coal mill outlet and sampling the pebble coal at the bottom of the coal mill in this embodiment. Under different working conditions, the wind speed and coal powder fineness test are carried out in turn at the 6 powder pipes at the coal mill outlet, and the weighted average is used to obtain the coal powder fineness R90 under the working condition. The pebble coal is sampled at the pebble coal discharge port at the bottom of the coal mill, and the time average value is calculated.

[0051] like Figure 3 The figure shows a schematic diagram of the furnace outlet point temperature test hole. The boiler furnace outlet point temperature is tested using an infrared high-temperature thermometer. The test is conducted once every 0.5 hours under each operating condition. The average value is recorded during the temperature measurement and finally weighted to obtain the furnace outlet temperature under that condition.

[0052] The reference value of the primary air volume at the coal mill inlet is based on the air-coal ratio curve of the coal milling system: when the coal feed rate is [0, a] t / h, the primary air volume at the coal mill inlet is [q] t / h; when the coal feed rate is [a, b] t / h, the primary air volume at the coal mill inlet is

[0053] Q=q+K*(ma)

[0054] Where Q represents the primary air volume at the coal mill inlet, in t / h; q is the coal mill design minimum ventilation volume, in t / h; m represents the coal feed rate, in t / h; a is the coal mill minimum coal feed rate, in t / h; b is the coal mill design maximum output, in t / h; k is a constant provided by the coal mill manufacturer.

[0055] Carry out a wind speed and coal powder fineness test at the coal powder sampling hole 11 of the coal mill outlet powder pipe, count the pebble coal discharge at the pebble coal discharge port 12 at the bottom of the coal mill, carry out a point temperature test at the boiler furnace outlet 13, and record the operating parameters of the pulverizing system and the average value of the burner fire detection intensity during this period through the DCS or PI system.

[0056] The calculation process of coal powder concentration is as follows:

[0057]

[0058] Where: c is the coal powder concentration, kg / kg;

[0059] M is the average value of coal feeding amount, t / h;

[0060] m is the hourly average value of stone coal emission, kg / h;

[0061] q is the average value of the primary air volume at the coal mill inlet, t / h.

[0062] Step 2. Combined with the test results of coal powder concentration, coal powder fineness, burner outlet fire detection intensity, furnace outlet temperature, etc. in step 1, calculation analysis and research are carried out to optimize the air-coal ratio curve, coal feed rate and speed control method of the pulverizing system during low-load operation of boiler 10, and obtain a method based on rare earth motor variable speed regulation of medium-speed coal mill to improve the low-load stable combustion capability of the boiler.

[0063] According to the test results, the coal feeding amount and speed control mode of the pulverizing system under low load conditions are optimized, and the reasonable coal feeding amount and speed control mode when the boiler is running at low load are obtained; according to the test results, the original air-coal ratio curve of the pulverizing system under low load conditions is optimized, and the air-coal ratio curve of the coal mill at different speeds is obtained.

[0064] Among them, Figure 1As shown, the pulverizing system includes: a coal bunker 1, a coal feeder 2, a coal mill 3, a variable speed coal mill 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 variable speed coal mill motor 4, and the variable speed coal mill 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, and 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, and the outlet of the boiler 10 is equipped with a test hole 13 for point temperature; a stone coal discharge port 12 is provided at the bottom of the coal mill 3; the coal mill 3 is also provided with a cold primary air adjustment door 7 and a hot primary air adjustment door 8. The pulverizing system is equipped with a frequency converter, which is provided with a remote control signal interface; the frequency converter control unit 5 controls the frequency converter, which drives the variable speed coal mill motor 4, and the variable speed coal mill motor 4 drives the coal mill 3 to rotate.

[0065] When the pulverizing system is in operation, the raw coal enters the coal feeder 2 from the coal bunker 1, and enters the coal mill 3 through the coal feeder downcomer. The coal mill 3 is driven to rotate by the variable speed coal mill motor 4 and the frequency conversion control unit 5. After the raw coal particles are ground into coal powder, they are separated by the folding baffle of the static separator 6. The coal powder with suitable fineness enters the boiler 10, and the coal powder with slightly coarser fineness re-enters the coal mill 3 for grinding. The power for the coal powder to enter the boiler 10 mainly comes from the primary air at the coal mill inlet. In addition to allowing the coal powder to enter the boiler 10, the primary air also heats the coal powder, which helps the ignition and combustion of the coal powder after entering the boiler 10.

[0066] Embodiment 2

[0067] Based on Example 1, Example 2 of the present application provides an application of the method for improving the low-load stable combustion capability of a boiler by adjusting the medium-speed coal mill based on the speed change of a rare earth motor in Example 1 to a coal mill in a power plant:

[0068] The design speed of the coal mill is 24r / min, and the design minimum coal output is 15.18t / h. The specific coal mill parameters are shown in the table below:

[0069]

[0070] The parameters of the variable speed coal mill motor are shown in the following table:

[0071]

[0072] In order to obtain a method based on the variable speed regulation of the medium-speed coal mill by the rare earth motor to improve the low-load stable combustion ability of the boiler, for the coal mill with the design speed n and the design output coal quantity Q, during the low-load operation of the boiler, the primary air velocity, coal powder concentration and coal powder fineness of the coal mill outlet powder pipe were tested at different speeds under the same coal type and the same coal quantity. At the same time, the temperature test was carried out at the furnace outlet, and the time average of the burner fire detection intensity during the test was recorded by DCS, for a total of 5 test conditions.

[0073] In this embodiment, the AGC is removed from the unit, and the unit is operated in CCS coordination mode. The unit load is operated under 30% BMCR conditions, and the total fuel volume of the boiler is about 90 t / h. Under this condition, a total of 3 coal mills are operated on the middle and lower layers of the front wall and the lower layers of the rear wall, of which the variable speed mill is located in the lower layer of the front wall.

[0074] The parameters of the coal feed rate are obtained through the operating parameters of the coal feeder. The coal feeder can control the coal feed rate through the belt speed, and the coal feed rate is calibrated regularly; if a larger coal feed rate is to be obtained, a faster belt speed is required. The coal feed rate of the variable speed coal mill is withdrawn and the manual coal feed rate is stabilized at 30t / h, and the coal feed rate of the other two pulverizing systems is operated in an automatic state. The outlet temperature of the coal mill is automatically adjusted by the opening of the cold and hot air damper damper at the coal mill inlet to maintain it at 80℃.

[0075] The fineness of the coal powder is adjusted by the opening of the static separator baffle at the coal mill outlet. 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 coal mill outlet is maintained at 55°.

[0076] The hydraulic loading force of the coal mill roller is 4MPa when the coal feeding rate is [0, 20]t / h; y=0.3x-2(MPa) when the coal feeding rate is (20, 60]t / h; 16MPa when the coal feeding rate is (60, 70]t / h.

[0077] The variable speed motor speed of the coal mill is controlled by the frequency conversion control unit 5, thereby driving the coal mill roller to operate at different speeds. This embodiment is tested under the following five working conditions:

[0078] The speed is 0.75n, and the primary air volume bias at the coal mill inlet is +5t / h;

[0079] The speed is 0.9n, and the primary air volume bias at the coal mill inlet is +2t / h;

[0080] The speed is n, and the primary air volume offset at the coal mill inlet is 0t / h;

[0081] The speed is 1.1n, and the primary air volume bias at the coal mill inlet is -2t / h;

[0082] The speed is 1.25n, and the primary air volume offset at the coal mill inlet is -5t / h

[0083] According to the measurement method and coal powder concentration calculation formula given in Example 1, relevant data are obtained. The following table shows the calibration results of the primary air velocity of the powder pipe at the outlet of the variable speed coal mill under different working conditions measured according to step 107 in this embodiment:

[0084]

[0085] The following table shows the results of parameters such as coal powder concentration at the pulverizer outlet, coal powder fineness, burner fire detection intensity and furnace outlet temperature under different working conditions. Figure 4 The line graphs showing changes in pulverized coal concentration and pulverized coal fineness under different operating conditions and the line graphs showing changes in burner fire detection intensity and furnace outlet temperature under different operating conditions are shown.

[0086]

[0087] The results of this embodiment show that: 1) When the rare earth motor variable speed coal mill feeds 30 t / h, as the speed increases, the primary air volume at the coal mill inlet is appropriately reduced, that is, the air-coal ratio is appropriately reduced, and the wind speed of each powder pipe at the coal mill outlet does not change much, and is greater than 18 m / s; 2) When the rare earth motor variable speed coal mill feeds 30 t / h, the speed increases, and the air-coal ratio is appropriately reduced, the coal powder fineness R90 changes significantly and gradually becomes finer, which is conducive to the ignition and burnout of coal powder; 3) When the rare earth motor variable speed coal mill feeds 30 t / h, the speed increases, and the air-coal ratio is appropriately reduced. When it is appropriately reduced, the emission of stone coal is small and the change is not obvious; 4) When the rare earth motor variable speed pulverizer feeds 30t / h, the speed increases, and the air-coal ratio is appropriately reduced, the coal powder concentration gradually increases, which is beneficial to the enhancement of the combustion stability of the coal powder burner; 5) When the rare earth motor variable speed pulverizer feeds 30t / h, the speed increases, and the air-coal ratio is appropriately reduced, the fire detection intensity of the coal powder burner shows a trend of gradual strengthening; 6) When the rare earth motor variable speed pulverizer feeds 30t / h, the speed increases, and the air-coal ratio is appropriately reduced, the furnace outlet temperature shows an upward trend.

[0088] Combined with the results of this embodiment, it is recommended that the rare earth motor variable speed coal mill should be operated at a high speed of 27r / min or 30r / min when the unit is running at low load with deep peak regulation. At the same time, when the rare earth motor variable speed coal mill is running at high speed, the air-coal ratio curve can be optimized to obtain the following Figure 7 The schematic diagram of the optimized air-coal ratio curve is shown below:

[0089] The coal mill speed is 27r / min. When the coal feed rate is [0, 15.18) t / h, the primary air volume at the coal mill inlet is 72.16 t / h; when the coal feed rate is [15.18, 61.5] t / h, the primary air volume at the coal mill inlet is q=72.16+0.696*(m-15.18).

[0090] The coal mill speed is 30r / min. When the coal feed rate is [0, 15.18)t / h, the primary air volume at the coal mill inlet is 72.16t / h; when the coal feed rate is [15.18, 61.5]t / h, the primary air volume at the coal mill inlet is q=72.16+0.631*(m-15.18).

[0091] Where q represents the primary air volume at the coal mill inlet, in t / h; m represents the coal feed rate, in t / h.

[0092] By changing the speed control mode of the rare earth motor variable speed coal mill and optimizing the wind-coal ratio curve, the problems of low furnace temperature and poor burner combustion stability during deep peak regulation and low load operation of the unit can be effectively solved, and the deep peak regulation capacity of the unit can be expanded while improving the safety of the unit operation. At the same time, it is also possible to consider replacing 2 to 3 fixed speed coal mills of each unit with rare earth motor variable speed regulating coal mills, further improving the safety and stability of the boiler during low load operation and expanding the deep peak regulation capacity of the unit.

Claims

1. A method for improving the low-load stable combustion capability of a boiler by adjusting the speed of a medium-speed coal mill by means of a motor, characterized in that: The following steps are involved: Step 1: Conduct a low-load operation test on the coal mill (3) at 40% BMCR or less with the same type of coal, the same coal feed rate, different rotation speeds, and different primary air volumes at the coal mill inlet; during the low-load operation of the unit, withdraw the unit from the AGC and operate it under the CCS coordinated control mode, keeping 2 to 3 coal mills (3) in operation, of which some of the coal mills (3) are variable-speed coal mills; under the same boundary conditions, for the same type of coal and the same coal volume, conduct coal powder concentration, fineness test, stone coal discharge and furnace outlet temperature test at different rotation speeds and different primary air volumes, and record the operating parameters of the pulverizing system during this period and the time average of the burner outlet fire detection intensity through the DCS or PI system; The calculation formula of coal powder concentration c is: Wherein, c is the coal powder concentration, kg / kg; M is the hourly average value of coal feeding, t / h; m is the hourly average value of stone coal discharge, kg / h; q is the hourly average value of primary air volume at the coal mill inlet, t / h; After the pulverizing system has been running stably for a certain period of time t0, the time average value of the primary air velocity and the coal powder fineness is measured through the coal powder sampling hole (11) within the certain period of time t, t0 is 30 minutes, and t is greater than or equal to 2 hours; the time average value of the pebble coal discharge amount is measured through the pebble coal discharge port (12), and the time average value of the furnace outlet temperature is measured by performing a point temperature test through the furnace outlet temperature test hole (13); the primary air volume and outlet temperature at the coal mill inlet are controlled by the cold primary air adjustment door (7) and the hot primary air adjustment door (8); the reference value of the primary air volume at the coal mill inlet is determined according to the coal feed amount and the original air-to-coal ratio curve; The original air-coal ratio curve is specifically that when the coal feed rate is [0, a) t / h, the primary air volume at the coal mill inlet is Q = qt / h; When the coal feeding rate is [a, b] t / h, the primary air volume at the coal mill inlet is, Q=q+K×(ma) Among them, Q is the primary air volume at the coal mill inlet, in t / h; q is the minimum ventilation volume designed for the coal mill, in t / h; m represents the coal feed rate, in t / h; a is the minimum coal feed rate of the coal mill, in t / h; b is the maximum output of the coal mill, in t / h; K is a constant provided by the coal mill manufacturer; The primary air velocity and coal powder fineness are calculated by weighted average of the measurement results of the coal powder sampling holes (11) on several coal mill outlet powder pipes (9); the boiler furnace outlet temperature is tested by an infrared high temperature thermometer, and the test is performed once every 0.5 hours under each working condition. The average value is recorded during the temperature measurement and finally weighted to obtain the time average value of the furnace outlet temperature; Step 2: Combine the test results in step 1 to perform calculation and analysis, optimize the coal feed rate and speed control method of the pulverizing system under low load conditions, and obtain the coal feed rate and speed control method during low load operation; optimize the air-coal ratio curve, coal feed rate and speed control method of the pulverizing system during low load operation of the boiler.

2. The method for improving the low-load stable combustion capability of a boiler by regulating a medium-speed coal mill by means of a motor variable speed according to claim 1, characterized in that: In step 1: the coal powder fineness standard is controlled to R90 by the opening of the baffle of the static separator (6); the loading force of the coal mill (3) is determined by the original loading curve of the coal mill (3).

3. The method for improving the low-load stable combustion capability of a boiler by regulating a medium-speed coal mill by means of a motor variable speed according to claim 1, characterized in that: In step 1, the base speed of the coal mill (3) is n, and the designed maximum output coal feed is Q Max , the rated current of the variable speed coal mill rare earth motor inverter is I; five working conditions are tested respectively, and the settings of the speed and the primary air volume at the coal mill inlet in the five working conditions are: The speed of the coal mill is 0.75n, and the deviation of the primary air volume at the coal mill inlet relative to the reference value is +5t / h; The speed is 0.9n, and the deviation of the primary air volume at the coal mill inlet relative to the reference value is +2t / h; The speed is n, and the offset of the primary air volume at the coal mill inlet relative to the reference value is 0t / h; The speed is 1.1n, and the deviation of the primary air volume at the coal mill inlet relative to the reference value is -2t / h; The rotation speed is 1.25n, and the deviation of the primary air volume at the coal mill inlet relative to the reference value is -5t / h.

4. The method for improving the low-load stable combustion capability of a boiler by regulating a medium-speed coal mill by means of a motor variable speed according to claim 1, characterized in that: In step 2: as the rotation speed increases, the coefficient K of the air-coal ratio curve of the coal mill (3) is gradually reduced to obtain the air-coal ratio curve of the coal mill (3) at different rotation speeds.

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