Method for real-time control of coal powder concentration in medium-speed coal mill by speed regulation of rare earth motor

Abstract

The present invention relates to a method for real-time control of the concentration of coal powder in a medium-speed coal mill by speed-changing regulation of a rare earth motor, comprising the steps of: conducting an operation test of the coal mill at different rotation speeds and different primary air volumes at the inlet of the coal mill under the condition of the same coal type and the same coal feed amount; optimizing the original air-coal ratio curve of the pulverizing system; and obtaining the air-coal ratio curve of the coal mill at different rotation speeds. The beneficial effects of the present invention are: for the different rotation speed states of the speed-changing pulverizer, the optimal air-coal ratio curve corresponding to the pulverizer at each rotation speed is proposed, the real-time control of the concentration of coal powder in the medium-speed coal mill is realized, and the stability of the ignition and combustion of coal powder at each rotation speed is improved; the problem of poor combustion stability and burnout performance of coal powder when burning low calorific value and high ash content coal is effectively solved, and the combustion stability and burnout performance of coal powder can also be improved, and the safety, stability and economy of the unit operation are further improved.

Description

Technical Field

[0001] The invention belongs to the field of coal mill control, and in particular relates to a method for real-time control of coal powder concentration of a medium-speed coal mill by speed regulation of a rare earth motor. Background Art

[0002] In recent years, domestic power plants have begun to burn a large amount of low calorific value, high ash content coal. The use of the above-mentioned coal will inevitably cause the pulverized coal combustion stability and burnout performance to deteriorate, which in turn leads to poor boiler combustion efficiency. Under low load conditions, unstable pulverized coal combustion will also affect the safe and stable operation of the unit.

[0003] One of the means to improve the combustion stability of boilers is to increase the fineness of coal powder and the concentration of primary air powder. In response to the above problems, power plant boiler technicians have made a lot of efforts and work, such as increasing the fineness of coal powder by adjusting the separator at the outlet of the coal mill, but the increase in coal powder fineness will cause problems such as reduced output of the coal mill and increased power consumption; the coal powder burnout rate is increased by reducing the primary air volume at the inlet of the coal mill, but the reduction in the primary air volume at the inlet of the coal mill will cause problems such as increased stone coal emissions and accumulation of coal powder in the coal powder pipe.

[0004] The above technical measures cannot fundamentally solve the problem of poor combustion stability and burnout performance of pulverized coal when burning low calorific value and high ash coal. Therefore, a new real-time control method for pulverized coal concentration in a medium-speed coal mill is urgently needed. Summary of the invention

[0005] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a method for real-time control of coal powder concentration in a medium-speed coal mill by speed regulation of a rare earth motor.

[0006] The method for real-time control of coal powder concentration of a medium-speed coal mill by speed regulation of a rare earth motor comprises the following steps:

[0007] Step 1: Under the same coal type and the same coal feed rate, the coal mill is tested for operation at different 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, the time-average values ​​of the primary air speed and coal powder fineness within a certain period of time t are measured through the coal powder sampling hole, and the time-average value of the pebble coal discharge within a certain period of time t is measured through the pebble coal discharge port. The operating parameters of the pulverizing system during this period of time are recorded through the DCS or PI system, and the coal powder concentration is calculated; the design speed of the coal mill is n, and the design maximum output coal feed rate is Q Max , the rated current of the variable speed coal mill rare earth motor inverter is I;

[0008] Step 2: Based on the test results of coal powder concentration and coal powder fineness obtained in step 1, the original air-coal ratio curve of the pulverizing system is optimized; and the air-coal ratio curve of the coal mill at different rotation speeds is obtained.

[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 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 1: the primary air velocity and the 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.

[0015] Preferably, in step 1, five working conditions are tested respectively, and the settings of the rotation speed and the primary air volume at the coal mill inlet in the five working conditions are respectively:

[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, the calculation formula of 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 increasing the pulverized coal concentration in the outlet pulverized coal pipe of a coal mill based on the variable speed regulation of a rare earth motor. According to the different speed states of the variable speed pulverizer, the optimal air-coal ratio curve corresponding to the coal mill at each speed is proposed, thereby realizing the real-time control of the pulverized coal concentration of the medium speed pulverizer and improving the stability of the ignition and combustion of the pulverized coal at each speed.

[0028] 2) The present invention can not only effectively solve the problems of poor combustion stability and burnout performance of pulverized coal when burning low calorific value and high ash coal, but also improve the combustion stability and burnout performance of pulverized coal when burning conventional coal, thereby further improving the safety, stability and economy of unit operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a schematic diagram of the structure of the flour making system;

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

[0031] Figure 3 is the original air-coal ratio curve of the pulverizing system;

[0032] Figure 4 It is a line chart showing the changes in coal powder concentration and stone coal emission under different working conditions;

[0033] Figure 5 This is the optimized air-coal ratio curve of the variable speed coal mill pulverizing system.

[0034] Explanation of the reference numerals: coal bunker 1, coal feeder 2, coal mill 3, variable speed 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, stone coal discharge port 12. DETAILED DESCRIPTION

[0035] 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.

[0036] Embodiment 1

[0037] As an example, Figure 1 As 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; a stone coal discharge port 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.

[0038] 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.

[0039] The method for controlling the coal powder concentration of the medium-speed coal mill in real time by speed-changing regulation of the rare earth motor in the pulverizing system comprises the following steps:

[0040] Step 1: Under the same coal type and the same coal feed rate, the coal mill 3 is tested for five times at different rotation speeds and different primary air volumes at the coal mill inlet. The coal powder fineness standard is controlled to R90 by the damper opening 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. The primary air volume and outlet temperature of 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 rate and the original air-coal ratio curve;

[0041] 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 qt / h; when the coal feed rate is [a, b] t / h, the reference value of the primary air volume at the coal mill inlet is

[0042] Q=q+K×(ma)

[0043] 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.

[0044] The settings of the rotation speed and the primary air volume at the coal mill inlet in the five working conditions are

[0045] 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;

[0046] 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;

[0047] 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;

[0048] 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;

[0049] 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.

[0050] After the pulverizing system has been running stably for a certain period of 30 minutes, the time average of the primary wind speed and the coal powder fineness within a certain period of at least two hours is measured through the coal powder sampling hole 11, and the time average of the pebble coal discharge within a certain period of t is measured through the pebble coal discharge port 12. The operating parameters of the pulverizing system during this period are recorded through the DCS or PI system, and the coal powder concentration is calculated; the design speed of the pulverizer 3 is n, and the design maximum output coal supply is Q Max , the rated current of the variable speed coal mill rare earth motor inverter is I;

[0051] The primary air velocity and coal powder fineness are calculated by weighted average of the measurement results of several coal powder sampling holes 11 on the coal mill outlet powder pipes 9.

[0052] The calculation formula of coal powder concentration c is:

[0053]

[0054] 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.

[0055] Step 2: Combined with the test results of coal powder concentration and coal powder fineness obtained in step 1, the original air-coal ratio curve of the pulverizing system is optimized. Specifically, as the rotation speed increases, the coefficient K of the air-coal ratio curve of the pulverizer 3 is gradually reduced to obtain the air-coal ratio curve of the pulverizer 3 at different rotation speeds.

[0056] Embodiment 2

[0057] On the basis of Example 1, Example 2 of the present application provides an application of the method for increasing the pulverized coal concentration in the pulverized coal pipe at the pulverizer outlet based on the variable speed pulverizer in Example 1 to a pulverizer in a power plant:

[0058] 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:

[0059]

[0060] The parameters of the variable speed coal mill motor are shown in the following table:

[0061]

[0062] In order to obtain a method for increasing the pulverized coal concentration at the outlet pulverized coal pipe of a variable speed coal mill, for a coal mill with a design speed of n and a design output coal quantity of Q, the pulverized coal concentration and pulverized coal fineness tests were conducted at different speeds for the same type of coal and the same coal quantity, with a total of 4 test conditions.

[0063] 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 at 45t / h.

[0064] The coal mill outlet temperature and the inlet primary air volume are controlled by the cold primary air adjustment door 7 and the hot primary air adjustment door 8 at the coal mill inlet, wherein the cold primary air adjustment door 7 controls the coal mill outlet temperature by the size of the opening, and the cold primary air adjustment door 7 is opened wider to reduce the temperature; the hot primary air adjustment door 8 controls the coal mill inlet primary air volume by the size of the air door damper opening; the coal mill inlet cold air adjustment door adjusts the temperature, and the hot air door adjusts the primary air volume. In this embodiment, the coal mill inlet primary air volume is adjusted by changing the air door damper opening by setting an offset on the coal mill inlet hot air adjustment door control panel based on the air-coal ratio control logic, and the coal mill inlet primary air volume is automatically adjusted by the opening of the cold and hot air door dampers at the coal mill inlet to maintain the coal mill outlet temperature unchanged at 75°C.

[0065] The coal powder fineness standard is adjusted by the opening size of the baffle of the static separator 6 at the coal mill outlet. Among them, the adjustment range of the opening of the baffle of the static separator 6 is 10° to 80°. The larger the opening of the baffle of the static separator 6, the larger the coal powder fineness standard, and the smaller the opening of the baffle of the static separator 6, the smaller the coal powder fineness standard. Among them, a large fineness standard refers to a large particle size of the coal powder particles, and a small fineness standard refers to a small particle size of the coal powder particles. In this embodiment, the opening of the static separator at the coal mill outlet is maintained at 55° unchanged.

[0066] 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.

[0067] The variable speed coal mill 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. The reference value of the primary air volume at the coal mill inlet is determined according to the coal feed rate and the original air-coal ratio curve; in this embodiment, the coal feed rate is selected to be 45t / h. Since the maximum output of the coal mill cannot reach 45t / h when the speed is 0.75n, only four working conditions are tested: the speed is 0.9n, the primary air volume at the coal mill inlet is biased to +2t / h; the speed is n, the primary air volume at the coal mill inlet is biased to 0t / h; the speed is 1.1n, the primary air volume at the coal mill inlet is biased to -2t / h; the speed is 1.25n, and the primary air volume at the coal mill inlet is biased to -5t / h.

[0068] Under the boundary conditions of steps 102, 103, and 104, after the operating parameters are stable for 30 minutes, the pulverizing system is tested for primary wind speed, coal powder fineness, and stone coal emission. At the same time, the DCS and PI systems are used to record relevant operating parameters during the test, including coal feed rate, pulverizer current, primary air volume, and the time-averaged value of pulverizer outlet temperature. The test time after stabilization is not less than 2 hours.

[0069] 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 are tested 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. Pebble coal sampling is carried out at the pebble coal discharge port at the bottom of the coal mill, and the time average value is calculated.

[0070] The calculation process of coal powder concentration is as follows:

[0071]

[0072] Where: c is the coal powder concentration, kg / kg;

[0073] M is the average value of coal feeding amount, t / h;

[0074] m is the hourly average value of stone coal emission, kg / h;

[0075] q is the average value of the primary air volume at the coal mill inlet, t / h.

[0076] 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 106 in this embodiment:

[0077]

[0078] The following table shows the coal powder concentration and coal powder fineness at the outlet of the coal mill at different speeds. Figure 4 The line graph shows the changes in coal powder concentration and stone coal emissions under different operating conditions.

[0079]

[0080] From the above table, we can see that: when the rare earth motor variable speed pulverizer feeds 45t / h, as the speed increases, the primary air volume at the pulverizer inlet is appropriately reduced, that is, the air-coal ratio is appropriately reduced, the wind speed of each powder pipe at the pulverizer outlet does not change much, and is all greater than 18m / s; the pulverized coal fineness R90 at the pulverizer outlet does not change much and is within the design range; the pulverizer stone coal emission does not change significantly, while the pulverized coal concentration at the pulverizer outlet increases.

[0081] Therefore, it is found that as the speed increases, appropriately reducing the primary air volume at the pulverizer inlet has the effect of increasing the coal powder concentration at the pulverizer outlet. Based on this conclusion, the variable speed pulverizer air-to-coal ratio curve is optimized, mainly in that when the speed of the variable speed pulverizer increases, the coefficient of the primary air volume at the pulverizer inlet changing with the coal feed volume is appropriately reduced, that is, the air-to-coal ratio is reduced.

[0082] Get as Figure 5 The schematic diagram of the optimized air-coal ratio curve is shown as follows:

[0083] The coal mill speed is 18r / 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+0.847*(m-15.18).

[0084] The coal mill speed is 21r / 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+0.782*(m-15.18).

[0085] The pulverizer speed is 24r / min, and the air-coal ratio curve is consistent with the original design air-coal ratio curve, that is, when the coal feed rate is [0, 15.18) t / h, the primary air volume at the pulverizer inlet is 72.16 t / h; when the coal feed rate is [15.18, 61.5] t / h, the primary air volume at the pulverizer inlet is q=72.16+0.739*(m-15.18).

[0086] 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).

[0087] 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).

[0088] Where q represents the primary air volume at the coal mill inlet, in t / h; m represents the coal feed rate, in t / h.

[0089] Through the above optimization, the problem of poor combustion stability and burnout performance of pulverized coal when burning low calorific value and high ash coal can be effectively solved. For conventional coal, the combustion stability and burnout performance of pulverized coal can also be improved, further improving the safety, stability and economy of unit operation. At the same time, it is also possible to consider replacing 2 to 3 fixed-speed coal mills with rare earth motor variable speed adjustment coal mills for each unit.

Claims

1. A method for real-time control of coal powder concentration in a medium-speed coal mill by speed regulation of a rare earth motor, characterized in that: The following steps are involved: Step 1: Under the condition of the same coal type and the same coal feed rate, the coal mill (3) is subjected to an operation test at different rotation speeds and different coal mill inlet primary air volumes, and the coal mill inlet primary air volume and outlet temperature are controlled by a cold primary air adjustment door (7) and a hot primary air adjustment door (8); the reference value of the coal mill inlet primary air volume is determined according to the coal feed rate and the original air-to-coal ratio curve; 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 qt / h; when the coal feed 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; After the pulverizing system has been running stably for a certain period of time t0, the time average values ​​of the primary air velocity and the coal powder fineness within a certain period of time t are measured through the coal powder sampling hole (11), and the time average value of the pebble coal discharge within a certain period of time t is measured through the pebble coal discharge port (12). The operating parameters of the pulverizing system during this period of time are recorded through the DCS or PI system, and the coal powder concentration is calculated; the calculation formula of the 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; The design speed of the coal mill (3) is n, and the design maximum output coal feed is Q Max , the rated current of the variable speed coal mill rare earth motor inverter is I; Step 2: In combination with the test results of coal powder concentration and coal powder fineness obtained in step 1, the original air-coal ratio curve of the pulverizing system is optimized; 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.

2. The method for real-time control of coal powder concentration of a medium-speed coal mill by speed regulation of a rare earth motor according to claim 1 is 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 real-time control of coal powder concentration of medium-speed coal mill by speed regulation of rare earth motor according to claim 1 is characterized in that: In step 1: the primary air velocity and the 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).

4. The method for real-time control of coal powder concentration of a medium-speed coal mill by speed regulation of a rare earth motor according to claim 1 is characterized in that: In step 1, five working conditions are tested respectively, and the settings of the rotation 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.

5. The method for real-time control of coal powder concentration of medium-speed coal mill by speed regulation of rare earth motor according to claim 1 is characterized in that: In step 1: t0 is 30 min, and t is greater than or equal to 2 h.

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