AU2018294134B2

AU2018294134B2 - Vertical Roller Mill and Method of Operating the Same

Info

Publication number: AU2018294134B2
Application number: AU2018294134A
Authority: AU
Inventors: Fuminori Ando; Shigeru Hotta; Shinichi Miyagawa
Original assignee: Kawasaki Heavy Industries Ltd; Kawasaki Jukogyo KK
Current assignee: Kawasaki Motors Ltd
Priority date: 2017-06-30
Filing date: 2018-06-22
Publication date: 2021-06-10
Anticipated expiration: 2038-06-22
Also published as: AU2018294134A1; CN110753583A; WO2019004096A1; JP2019010620A

Abstract

By supplying hot gas to a mill inlet in a vertical roller mill, a prescribed pre-heating time is applied and pre-heating occurs so as to achieve a prescribed supply start inlet temperature at which the discharge gas temperature at the mill outlet is at least 65°C and the hot gas temperature at the mill inlet is 150–250°C. After pre-heating, high moisture content coal supply to a rotating table is started and at least either the flow rate or the temperature of the hot gas supplied to the mill inlet is adjusted such that the temperature of the discharge gas at the mill outlet is maintained at 65–100°C.

Description

DESCRIPTION

Title of Invention: VERTICAL ROLLER MILL AND METHOD OF OPERATING THE SAME Technical Field

[0001] The present invention relates to a vertical roller mill configured to grind low-rank, high-moisture coal and a method of operating the vertical roller mill. Background Art

[0002] Low-rank coal such as brown coal is considered to account for half of the world's coal reserve and is attracting attention as a promising energy resource. However, low-rank coal may have a high moisture content, because of which its use as fuel results in low energy efficiency. In recent years, low-rank coal with a moisture content of around 35 to 40% has been used without reforming; however, it is desired for low-rank, high-moisture coal with a moisture content of 50% or more (hereinafter simply referred to as "high-moisture coal") to be subjected to reforming such as moisture content reduction or grinding in order to improve the energy efficiency and transportation efficiency. Therefore, there is a demand for a technique that enables efficient drying and grinding of high-moisture coal.

[0003] Vertical roller mills have been conventionally used to dry and grind coal. However, most of conventional vertical roller mills are intended to grind high-rank coal such as bituminous coal having a high degree of coalification, and it is difficult to efficiently dry and grind high moisture coal using an existing vertical roller mill for high-rank coal under conventional operation conditions. Patent Literature 1 proposes a technique for drying and grinding high moisture coal using a vertical roller mill.

[0004] The vertical roller mill of Patent Literature 1 includes a rotary table rotatable in a mill housing and a grinding roller pressed against a surface of the rotary table. The material to be ground is fed to the central portion of the rotary table and bitten in between the grinding roller and the rotary table, thereby being ground. The ground material is moved to the peripheral portion of the rotary table under the action of a centrifugal force generated by rotation of the rotary table. The peripheral portion of the rotary table is spaced from the mill housing to provide a gap between the mill housing and the rotary table, and hot gas blows upward through this gap. The ground material moved to the peripheral portion of the rotary table is dried and carried upward by the blowing hot gas and discharged from an upper portion of the mill housing.

[0005] In the vertical roller mill of Patent Literature 1, the temperature of the hot gas to be supplied is set to about 300°C for high-rank coal, while for low-rank coal the temperature is increased to 400 to 600°C. Thus, in order for a shaft supporting the rotary table to avoid being excessively heated by the high-temperature hot gas, the vertical roller mill is equipped with a heat shield plate that blocks the hot gas from being applied to the shaft.

[0005a] It is effective, as in Patent Literature 1, to dry high-moisture coal by sufficiently increasing the temperature of the hot gas supplied into the mill housing. However, increasing the temperature of the hot gas to 400 to 600°C may cause ignition of the coal. Additionally, since the interior of the vertical roller mill is subjected to high temperature, heat-resistant components capable of withstanding high temperature need to be used, and this increases the initial cost of the vertical roller mill.

[0005b] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a vertical roller mill able to keep operating stably in an economical manner when grinding high-moisture coal and a method of operating the vertical roller mill. Citation List Patent Literature

[0006] Patent Literature 1: Japanese Laid-Open Patent Application Publication No. 2015 116525 Summary of Invention

[0007] In one aspect of the invention there is provided a method of operating a vertical roller mill configured to grind high-moisture coal, the vertical roller mill including a rotary table, a plurality of grinding rollers, and a mill casing having a mill inlet provided below the rotary table and a mill outlet provided above the rotary table, the method including: preheating the vertical roller mill by supplying hot gas to the mill inlet for a predetermined preheating time so that the temperature of exhaust gas from the mill outlet reaches 100°C and so that the temperature of the hot gas at the mill inlet reaches a predetermined feed start inlet temperature, the predetermined feed start inlet temperature being 150°C or higher and 250°C or lower; and after the preheating, starting feed of the high-moisture coal onto the rotary table and regulating at least one of the flow rate and the temperature of the hot gas supplied to the mill inlet so that the temperature of the exhaust gas from the mill outlet is maintained at a temperature of 65°C or higher and 100°C or lower.

[0008] In another aspect of the invention there is provided a vertical roller mill including: a rotary table rotatable about a rotation axis extending in a vertical direction; a plurality of grinding rollers pressed against an upper surface of the rotary table; a mill casing enclosing the rotary table and the plurality of grinding rollers, the mill casing including a mill inlet provided below the rotary table, a mill outlet provided above the rotary table, and a hot gas blow nozzle through which hot gas introduced from the mill inlet blows around the rotary table; a feeder that feeds a material to be ground onto the rotary table, the material being high-moisture coal; a hot gas flow rate regulator that regulates the flow rate of the hot gas supplied to the mill inlet; a hot gas temperature regulator that regulates the temperature of the hot gas supplied to the mill inlet; a mill inlet temperature sensor that detects a mill inlet temperature, the mill inlet temperature being the temperature of the hot gas at the mill inlet; a mill outlet temperature sensor that detects a mill outlet temperature, the mill outlet temperature being the temperature of exhaust gas containing the hot gas at the mill outlet and the ground material blown upward by the hot gas; and a controller that controls operations of the feeder, the hot gas temperature regulator, and the hot gas flow rate regulator based on values detected by the mill inlet temperature sensor and the mill outlet temperature sensor, wherein the controller causes the hot gas temperature regulator and the hot gas flow rate regulator to operate to preheat the vertical roller mill by supplying the hot gas to the mill inlet for a predetermined preheating time so that the mill outlet temperature reaches 100°C and so that the mill inlet temperature reaches a predetermined feed start inlet temperature, the predetermined feed start inlet temperature being 150C or higher and 250C or lower, and after the preheating, the controller causes the feeder to operate so that the material to be ground is fed onto the rotary table at a predetermined feed rate and causes at least one of the hot gas temperature regulator and the hot gas flow rate regulator to operate so that the mill outlet temperature is maintained at a temperature of 65C or higher and 100C or lower.

[0009] Disclosed herein is a method of operating a vertical roller mill configured to grind high-moisture coal, the vertical roller mill including a rotary table, a plurality of grinding rollers, and a mill casing having a mill inlet provided below the rotary table and a mill outlet provided above the rotary table, the method including: preheating the vertical roller mill by supplying hot gas to the mill inlet for a predetermined preheating time so that the temperature of exhaust gas from the mill outlet reaches or exceeds 65C and so that the temperature of the hot gas at the mill inlet reaches a predetermined feed start inlet temperature, the predetermined feed start inlet temperature being 150°C or higher and 250°C or lower; and after the preheating, starting feed of the high-moisture coal onto the rotary table and regulating at least one of the flow rate and the temperature of the hot gas supplied to the mill inlet so that the temperature of the exhaust gas from the mill outlet is maintained at a temperature of 65°C or higher and 100°C or lower.

[0010] Also disclosed herein is a vertical roller mill including: a rotary table rotatable about a rotation axis extending in a vertical direction; a plurality of grinding rollers pressed against an upper surface of the rotary table; a mill casing enclosing the rotary table and the plurality of grinding rollers, the mill casing including a mill inlet provided below the rotary table, a mill outlet provided above the rotary table, and a hot gas blow nozzle through which hot gas introduced from the mill inlet blows around the rotary table; a feeder that feeds a material to be ground onto the rotary table, the material being high-moisture coal; a hot gas flow rate regulator that regulates the flow rate of the hot gas supplied to the mill inlet; a hot gas temperature regulator that regulates the temperature of the hot gas supplied to the mill inlet; a mill inlet temperature sensor that detects a mill inlet temperature, the mill inlet temperature being the temperature of the hot gas at the mill inlet; a mill outlet temperature sensor that detects a mill outlet temperature, the mill outlet temperature being the temperature of exhaust gas containing the hot gas at the mill outlet and the ground material blown upward by the hot gas; and a controller that controls operations of the feeder, the hot gas temperature regulator, and the hot gas flow rate regulator based on values detected by the mill inlet temperature sensor and the mill outlet temperature sensor. The controller causes the hot gas temperature regulator and the hot gas flow rate regulator to operate to preheat the vertical roller mill by supplying the hot gas to the mill inlet for a predetermined preheating time so that the mill outlet temperature reaches or exceeds 65°C and so that the mill inlet temperature reaches a predetermined feed start inlet temperature, the predetermined feed start inlet temperature being 150°C or higher and 250°C or lower, and

4a

after the preheating, the controller causes the feeder to operate so that the material to be ground is fed onto the rotary table at a predetermined feed rate and causes at least one of the hot gas temperature regulator and the hot gas flow rate regulator to operate so that the mill outlet temperature is maintained at a temperature of 65°C or higher and 100°C or lower.

[0011] With the vertical roller mill and the operation method described above, the moisture content of the high-moisture coal fed onto the rotary table is decreased, and the decrease in moisture content is accompanied by a reduction in the resistance imposed on the grinding rollers and the rotary table from the material to be bitten in between the rollers and the table. Thus, the to-be-ground material is successfully bitten in between the rotary table and the grinding rollers soon after the start of feed of the high-moisture coal onto the rotary table, and the thickness of the layer of the material between the rotary table and the grinding rollers is maintained within a suitable range, so that the fluctuation in the load imposed on the rotary table is reduced. This allows the vertical roller mill to keep operating stably. Additionally, the temperature of the hot gas supplied to the vertical roller mill is sufficiently lower than the temperature as specified in Patent Literature 1, and the rotary table, for which load fluctuation is reduced, can be rotationally driven with reduced energy consumption. This enables economical operation.

[0012] Further disclosed herein is a method of operating a vertical roller mill configured to grind high-moisture coal, the vertical roller mill including a rotary table, a plurality of grinding rollers, and a mill casing having a mill inlet provided below the rotary table and a mill outlet provided above the rotary table, the method including: feeding the high-moisture coal onto the rotary table continuously or intermittently while regulating at least one of the flow rate and the temperature of hot gas supplied to the mill inlet so that the temperature of the hot gas at the mill inlet is maintained at a temperature of 150°C or higher and 320°C or lower and so that the temperature of exhaust gas from the mill outlet is maintained at a temperature of 65°C or higher and 100°C or lower.

[0013] Still further disclosed herein is a vertical roller mill including: a rotary table rotatable about a rotation axis extending in a vertical direction; a plurality of grinding rollers pressed against an upper surface of the rotary table; a mill casing enclosing the rotary table and the plurality of grinding rollers, the mill casing including a mill inlet provided below the rotary table, a mill outlet provided above the rotary table, and a hot gas blow nozzle through which hot gas introduced from the mill inlet blows around the rotary table; a feeder that feeds a material to be ground onto the rotary table, the material being

4b

high-moisture coal; a hot gas flow rate regulator that regulates the flow rate of the hot gas supplied to the mill inlet; a hot gas temperature regulator that regulates the temperature of the hot gas supplied to the mill inlet; a mill inlet temperature sensor that detects a mill inlet temperature, the mill inlet temperature being the temperature of the hot gas at the mill inlet; a mill outlet temperature sensor that detects a mill outlet temperature, the mill outlet temperature being the temperature of exhaust gas containing the hot gas at the mill outlet and the ground material blown upward by the hot gas; and a controller that controls operations of the feeder, the hot gas temperature regulator, and the hot gas flow rate regulator based on values detected by the mill inlet temperature sensor and the mill outlet temperature sensor. The controller causes the feeder to operate so that the material to be ground is fed onto the rotary table at a predetermined feed rate, and the controller causes at least one of the hot gas temperature regulator and the hot gas flow rate regulator to operate so that the temperature of the hot gas at the mill inlet is maintained at a temperature of 150°C or higher and 320°C or lower and so that the temperature of the exhaust gas from the mill outlet is maintained at a temperature of 65°C or higher and 100°C or lower.

[0014] With the vertical roller mill and the operation method described above, the to-be ground material is successfully bitten in between the rotary table and the grinding rollers, and the thickness of the layer of the material between the rotary table and the grinding rollers is maintained within a suitable range, so that the fluctuation in the load imposed on the rotary table is reduced. This allows the vertical roller mill to keep operating stably. Additionally, the temperature of the hot gas supplied to the vertical roller mill is sufficiently lower than the temperature as specified in Patent Literature 1, and the rotary table, for which load fluctuation is reduced, can be rotationally driven with reduced energy consumption. This enables economical operation. Advantageous Effects of Invention

[0015] The present invention enables a vertical roller mill to keep operating stably in an economical manner when grinding high-moisture coal. Brief Description of Drawings

[0016] FIG. 1 shows the schematic configuration of a vertical roller mill to which an operation method according to an embodiment of the present invention is applied. FIG. 2 shows the configuration of a control system for the vertical roller mill. FIG. 3 shows the flow of the operation process of the vertical roller mill. FIG. 4A is a diagram related to a result obtained in Experimental Example 1 and shows the fluctuation over time in the torque value of a mill motor. FIG. 4B is a diagram related to a result obtained in Experimental Example 1 and shows the fluctuation over time in the thickness of a layer of a material between a rotary table and grinding rollers.

FIG. 5A is a diagram related to a result obtained in Comparative Experimental Example 1 and shows the fluctuation over time in the torque value of a mill motor. FIG. 5B is a diagram related to a result obtained in Comparative Experimental Example 1 and shows the fluctuation over time in the thickness of a layer of a material between a rotary table and grinding rollers. FIG. 6 is a diagram related to a result obtained in Experimental Example 2. Description of Embodiments

[0017] Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows the schematic configuration of a vertical roller mill 1 to which an operation method according to an embodiment of the present invention is applied. It should be noted that the present invention is not limited to the configuration of the vertical roller mill 1. The following describes a typical configuration of the vertical roller mill 1.

[0018] [Schematic Configuration of Vertical Roller Mill 1] As shown in FIG. 1, the vertical roller mill 1 includes: a rotary table 2 having an upper surface to the central portion of which a material to be ground is fed; and a plurality of grinding rollers 3 configured to bite in the to-be-ground material between them and the rotary table 2 and grind the material. The rotary table 2 and the grinding rollers 3 are enclosed by a mill casing 7.

[0019] The rotary table 2 is driven by a table actuator 5 to rotate about a vertical rotation axis passing through the center of the rotary table 2. The table actuator 5 is constituted by, for example, a mill motor 51 and a reduction gear 52 that amplifies the rotation torque output from the mill motor 51 and that transmits the amplified rotation torque to the rotary table 2.

[0020] The grinding rollers 3 are arranged at regular angular intervals on the circumference of a circle centered on the rotation axis of the rotary table 2. FIG. 1 shows one of the grinding rollers 3 as an example. Each of the grinding rollers 3 is resiliently pressed toward the rotary table 2 by a roller pressing device 4 including a drive source such as a hydraulic cylinder. To the roller pressing device 4 is attached a layer thickness detector 25 that detects the thickness of a layer of the to-be-ground material (and the ground material) between the grinding roller 3 and the rotary table 2.

[0021] A funnel-shaped inner cone 11 is provided above the rotary table 2. The inner cone 11 has an outlet located above the central portion of the rotary table 2. A separator 9 is provided in an upper portion of the interior of the mill casing 7, and the separator 9 is located above the inner cone 11. A feed chute 75 for delivering the to-be-ground material into the inner cone 11 penetrates the mill casing 7. A feeder 14 feeds the to-be-ground material to the inlet of the feed chute 75.

[0022] The separator 9 is constituted by, for example, a classifying rotor 91, a guide vane 92 located closer to the periphery of the separator 9 than is the classifying rotor 91, and a separator actuator 8 that drives the classifying rotor 91 into rotation. The separator actuator 8 is constituted by, for example, a separator motor 81 and a transmission system 82 that transmits rotational output from the separator motor 81 to the classifying rotor 91.

[0023] In the mill casing 7, a mill outlet 71 is provided above the rotary table 2 and the separator 9. To the mill outlet 71 is connected an exhaust path 31, and gas exhausted from the mill flows into the exhaust path 31. At the mill outlet 71 or the inlet of the exhaust path 31 there is provided a mill outlet temperature sensor 22 that detects the temperature of hot gas exhausted from the mill outlet 71 (this hot gas will hereinafter be referred to as "mill exhaust gas").

[0024] In the exhaust path 31 there is provided a collector 33 that collects the ground material entrained in the mill exhaust gas. The collector 33 may be, for example, a bag filter or a cyclone. A flow rate regulator 34 is also provided in the exhaust path 31, and a flow rate sensor 23 is provided between the collector 33 and the flow rate regulator 34 in the exhaust path 31. The flow rate regulator 34 is, for example, an exhaust fan that forcibly expels gas from the mill casing 7. The flow rate of the mill exhaust gas exhausted from the mill outlet 71 can be regulated by changing the rotational speed of the exhaust fan. The flow rate of the mill exhaust gas corresponds to the flow rate of the hot gas supplied to the mill inlet 72, and the value of the mill exhaust gas flow rate detected by the flow rate sensor 23 (or a value resulting from processing of the detected value) can be used as the value of the flow rate of the hot gas supplied to the mill inlet 72. In the vertical roller mill 1 according to the present embodiment, as described above, the amount of gas exhausted from the mill casing 7 is controlled by the flow rate regulator 34 provided in the exhaust path 31, and thereby the flow rate of the hot gas supplied to the mill inlet 72 is controlled. Alternatively, a gas delivery fan (not shown) for delivering the hot gas to the mill inlet 72 may be provided instead of the flow rate regulator 34, and the amount of gas delivery by the gas delivery fan may be controlled to regulate the flow rate of the hot gas supplied to the mill inlet 72.

[0025] A circulation path 32 is connected to the exhaust path 31 at a location downstream of the flow rate regulator 34. The exhaust path 31 and the circulation path 32 form a circulation route that allows part (or all) of the mill exhaust gas flowing through the exhaust path 31 to return to the mill inlet 72. In the circulation path 32 there is provided a circulating gas flow rate regulator 351 that regulates the flow rate of the circulating gas to be returned to the mill inlet 72.

The circulating gas flow rate regulator 351 may include, for example, at least one flow rate regulating means such as a valve or a damper.

[0026] Ahot gas blow nozzle 73 in the shape of a ring or hot gas blow nozzles 73 arranged in a ring are provided between the periphery of the rotary table 2 and the mill casing 7. Ahot gas source 13 is connected to the mill inlet 72 of the mill casing 7 which is provided below the rotary table 2. This connection is made, for example, via a pipe. The hot gas source 13 may be any means that can generate hot gas, such as a cement firing furnace, a hot air generator, an incinerator, or a boiler. The hot gas supplied to the mill inlet 72 from the hot gas source 13 blows upward through the hot gas blow nozzle(s) 73.

[0027] For the hot gas source 13, a heat source flow rate regulator 352 that regulates the flow rate of hot gas exhausted from the hot gas source 13 is provided. The heat source flow rate regulator 352 may be, for example, a combustion fan that delivers combustible air to the hot gas source 13 or a dilution fan that delivers diluent gas to be mixed with gas generated by the hot gas source 13. A heat source temperature regulator 353 that regulates the temperature of the hot gas exhausted from the hot gas source 13 is also provided for the hot gas source 13. The heat source temperature regulator 353 may be, for example, a fuel supply device that supplies fuel to the hot gas source 13. The above-described heat source flow rate regulator 352, heat source temperature regulator 353, and circulating gas flow rate regulator 351 constitute a hot gas temperature regulator 35 that regulates the temperature of the hot gas supplied to the mill inlet 72.

[0028] At or in the vicinity of the mill inlet 72 there is provided a mill inlet temperature sensor 21 that detects the temperature of the hot gas supplied to the mill casing 7. The hot gas exhausted from the hot gas source 13 is mixed with the circulating gas returned toward the mill inlet 72 through the circulation path 32, and the mixed gas is supplied to the mill inlet 72. The temperature of the hot gas supplied to the mill inlet 72 can be regulated to a desired temperature by the hot gas temperature regulator 35 (i.e., the heat source flow rate regulator 352, the heat source temperature regulator 353, and the circulating gas flow rate regulator 351) operating to regulate the temperature of the hot gas exhausted from the hot gas source 13, the flow rate of the hot gas exhausted from the hot gas source 13, and the flow rate of the circulating gas to be mixed with the hot gas. A controller 15 performs feedback control of the operation of the hot gas temperature regulator 35 based on the temperature detected by the mill inlet temperature sensor 21 so that the temperature of the hot gas at the mill inlet 72 is regulated to a control target temperature.

[0029] In the vertical roller mill 1 configured as described above, once the material to be ground (in this embodiment, high-moisture coal) is delivered to the feed chute 75 by the feeder 14, the to-be-ground material is fed through the feed chute 75 and the inner cone 11 to that portion of the rotary table 2 which is around the rotational center of the rotary table 2. The to be-ground material fed onto the rotary table 2 is moved radially outward by a centrifugal force resulting from the rotational motion of the rotary table 2 and is then bitten in between the rotary table 2 and the grinding rollers 3 operating in conjunction with the rotation of the rotary table 2, thereby being ground. The rotary table 2 and its vicinity are heated by the hot gas blowing through the hot gas blow nozzle(s) 73 surrounding the rotary table 2, and the resulting heat gradually decreases the moisture content of the to-be-ground material fed onto the rotary table 2 and the moisture content of the material being ground on the rotary table 2.

[0030] The ground material is moved to the periphery of the rotary table 2 by the centrifugal force and is dried and carried upward by the stream of the hot gas blowing upward around the rotary table 2. The ground material, and spillage including gravel and bits of metal, which are not entrained in the stream of the hot gas, are dropped from the periphery of the rotary table 2 due to the centrifugal force and are collected into a collection box 76 disposed below and radially outward of the rotary table 2.

[0031] The ground material carried upward by the hot gas passes between fixed blades of the guide vane 92 and between rotary blades of the classifying rotor 91, thereby being classified into coarse particles having a particle size (particle diameter) larger than a desired particle size and fine particles having a smaller particle size than the coarse particles. The particle size of the end product can be adjusted by changing the rotational speed of the classifying rotor 91.

[0032] The fine particles classified through the separator 9 are entrained in the gas to be exhausted from the mill casing 7 and are discharged outside the mill through the mill outlet 71. The mill exhaust gas flowing into the exhaust path 31 from the mill outlet 71 passes through the collector 33, and thus particularly fine particles (refined particles) are separated. The refined particles separated are collected as the end product. The coarse particles classified through the separator 9 slide down on the inner cone 11 and return onto the rotary table 2. The coarse particles are subjected to grinding again together with the to-be-ground material fed through the feed chute 75.

[0033] In the vertical roller mill 1 configured as described above, the separator 9 is provided in the interior of the mill casing 7. The separator 9 may be provided in the exhaust path 31 connected to the mill outlet 71 of the mill casing 7.

[0034] [Configuration of Control System for Vertical Roller Mill 1] FIG. 2 shows the configuration of a control system for the vertical roller mill 1.

The operation of the vertical roller mill 1 according to the present embodiment is controlled by the controller 15. The controller 15 controls the operations of the table actuator 5, the separator actuator 8, the feeder 14, the flow rate regulator 34, and the hot gas temperature regulator 35 (i.e., the circulating gas flow rate regulator 351, heat source flow rate regulator 352, and heat source temperature regulator 353) based on values detected by the mill inlet temperature sensor 21, the mill outlet temperature sensor 22, the flow rate sensor 23, and the layer thickness detector 25. The table actuator 5, the separator actuator 8, the feeder 14, the flow rate regulator 34 for hot gas, and the hot gas temperature regulator 35 may be manually operated by an operator based on values detected by the mill outlet temperature sensor 22 and flow rate sensor 23.

[0035] The controller 15 is a so-called computer and includes a processor, a memory, and a communication interface (all of which are not shown). The memory may be embodied by any of various devices such as a RAM, a ROM, a flash memory, and a hard disk. The memory stores an OS and various control programs which are executed by the processor and also stores various data retrieved by the processor. The communication interface is controlled by the processor and uses wireless or wired communication means to exchange data with the table actuator 5, the separator actuator 8, the feeder 14, the flow rate regulator 34, and the hot gas temperature regulator 35 (i.e., the circulating gas flow rate regulator 351, heat source flow rate regulator 352, and heat source temperature regulator 353) and receive detection signals from the mill inlet temperature sensor 21, mill outlet temperature sensor 22, flow rate sensor 23, and layer thickness detector 25.

[0036] The processor performs various processing operations by executing various programs stored in the memory. In other words, the processing operations in the controller 15 are implemented by hardware and software executed by the processor. Such software is stored in advance in the memory or any other storage medium.

[0037] [Method of Operating Vertical Roller Mill 1] Hereinafter, a method of operating the above-described vertical roller mill 1 to grind high-moisture coal will be described with reference to FIG. 3. The term "high-moisture coal" refers to low-rank coal such as brown coal which has a moisture content of 50% or more. FIG. 3 shows the flow of the operation process of the vertical roller mill.

[0038] The controller 15 monitors the temperature of hot gas at the mill inlet 72 (this temperature will hereinafter be referred to as "mill inlet temperature") based on detection signals from the mill inlet temperature sensor 21. The controller 15 also monitors the temperature of exhaust gas from the mill outlet 71 (this temperature will hereinafter be referred to as "mill outlet temperature") based on detection signals from the mill outlet temperature sensor 22.

[0039] First, the controller 15 brings the table actuator 5 into operation to rotate the rotary table 2 and brings the separator actuator 8 into operation to rotate the classifying rotor 91 (step Si).

[0040] Next, the controller 15 starts preheating of the vertical roller mill 1 (step S2). Specifically, the controller 15 controls the operations of the flow rate regulator 34 for hot gas and the hot gas temperature regulator 35 to feed the hot gas at a feed rate such that the vertical roller mill 1 is heated in accordance with a predetermined preheating schedule. The controller 15 may regulate the temperature and the flow rate of the hot gas supplied to the mill inlet 72. The controller 15 may regulate the flow rate while keeping the temperature constant or may regulate the temperature while keeping the flow rate constant. That is, the controller 15 may cause at least one of the flow rate regulator 34 for hot gas and the hot gas temperature regulator 35 to operate to regulate at least one of the temperature and the flow rate of the hot gas supplied to the mill inlet 72.

[0041] The predetermined schedule is an operation schedule arranged so that the mill outlet temperature reaches or exceeds a predetermined feed start outlet temperature and the mill inlet temperature reaches a predetermined feed start inlet temperature in a predetermined preheating time after the start of supply of the hot gas. In the preheating step, the mill inlet temperature may be a main control target, and the mill outlet temperature may exceed the feed start outlet temperature.

[0042] In the above step, the feed start inlet temperature is 150°C or higher and 250°C or lower. The feed start outlet temperature is 65°C or higher. The preheating time may be any period of time which is 1 hour or more and equal to or less than an economically acceptable limit.

[0043] Upon detecting that the mill outlet temperature has reached or exceeded the predetermined feed start outlet temperature and that the mill inlet temperature has reached the predetermined feed start inlet temperature (YES in step S3), the controller 15 ends the preheating (step S4).

[0044] Subsequently, the controller 15 causes the feeder 14 to operate to feed the to-be ground material at a predetermined feed rate (step S5). After the start of feed of the to-be ground material, the controller 15 controls the operations of the flow rate regulator 34 for hot gas and the hot gas temperature regulator 35 to regulate at least one of the flow rate and the temperature of the hot gas supplied to the mill inlet 72 so that the mill outlet temperature is maintained at a predetermined operating outlet temperature (step S6).

[0045] In the above step, the operating outlet temperature is 65°C or higher and 100°C or lower. The control target value of the temperature of the hot gas supplied to the mill inlet 72 during operation is 150°C or higher and 320°C or lower. If the temperature of the hot gas at the mill inlet 72 is lower than 150°C, the to-be-ground material cannot be successfully bitten in between the grinding rollers 3 and the rotary table 2. If the temperature of the hot gas at the mill inlet 72 is higher than 320°C, the to-be-ground material may ignite.

[0046] To end the grinding of high-moisture coal in the vertical roller mill 1, the controller 15 stops the feeder 14 (step S7) and subsequently stops the table actuator 5 and the separator actuator 8 (step S8).

[0047] A decrease in the moisture content of the high-moisture coal present on the rotary table 2 is accompanied by a reduction in the resistance imposed on the grinding rollers 3 and rotary table 2 between which the coal is to be bitten in. When the to-be-ground material is suitably bitten in between the grinding rollers 3 and the rotary table 2, the thickness of the layer of the material being ground between the grinding rollers 3 and the rotary table 2 is kept within a suitable range, and the pressing force applied by the grinding rollers 3 to the rotary table 2 is reliably applied as a pressing grinding force to the material layer. Additionally, a relative sliding force generated due to the difference in peripheral velocity between each grinding roller 3 and the rotary table 2 is efficiently applied to the material layer. When the vertical roller mill 1 is operated according to the above-described operation method to dry and grind high-moisture coal, fluctuation in rotational torque of the mill motor 51, i.e., fluctuation in rotational load on the rotary table 2, is reduced as clearly demonstrated by the results of experiments described below. Thus, the vertical roller mill can keep operating stably.

[0048] The reduced fluctuation in rotational load on the rotary table 2 is expected to ensure that the to-be-ground material is successfully bitten in between the grinding rollers 3 and the rotary table 2 and that the thickness of the layer of the material between the grinding rollers 3 and the rotary table 2 is appropriately controlled to prevent insufficient grinding.

[0049] Additionally, the temperature of the hot gas supplied to the vertical roller mill 1 is sufficiently lower than the temperature as specified in Patent Literature 1 (400 to 600°C), and the possibility of ignition of the to-be-ground material is reduced. Furthermore, the rotary table 2, for which load fluctuation is reduced, can be rotationally driven with reduced energy consumption, and thus economical operation can be achieved. The use of the operation method as described above therefore allows efficient grinding and drying of high-moisture coal along with reduction in electricity consumed by the table actuator 5 and makes it possible to obtain a ground material having a desired product quality.

[0050] [Experimental Examples]

Hereinafter, Experimental Examples 1 and 2 of the present invention and Comparative Experimental Example 1 will be described. Table 1 presented below shows operation conditions of the vertical roller mill 1 in Experimental Example 1 and Comparative Experimental Example 1, in particular the feed start inlet temperature, the feed start outlet temperature, and the operating outlet temperature (the listed values are control target values). The experiments were conducted using a test vertical roller mill which is smaller in scale than vertical roller mills for actual use.

[0051] [Table 1] Mill outlet Feed start inlet temperature at start of Operating outlet temperature feed of brown coal temperature

Experimental Example 70 0 C 1 Comparative Experimental Example 115 0 C 75 0 C 600 C 1

[0052] [Experimental Example 1] In Experimental Example 1, hot gas was supplied to the mill inlet 72 of the test vertical roller mill at a constant flow rate, and the vertical roller mill 1 was preheated for a preheating time of 1 hour to allow the mill inlet temperature to reach 1500 C (control target value). The mill outlet temperature was approximately 100 0C at the moment when the mill inlet temperature reached 150 0C, and this mill outlet temperature was sufficiently higher than 65 0C corresponding to the feed start outlet temperature. After the vertical roller mill 1 was preheated in this manner, the mill motor 51 and the separator motor 81 were rotated, and feed of brown coal having a moisture content of 60 to 64% was started. After the start of feed of the brown coal, in order to maintain the mill outlet temperature at 700 C (control target value), the temperature of the hot gas supplied to the mill inlet 72 was regulated between 150 to 3200 C, and at the same time the flow rate of mill exhaust gas exhausted from the mill outlet 71 was regulated. The flow rate of the mill exhaust gas corresponds to the flow rate of the hot gas supplied to the mill inlet 72. After the start of feed of the brown coal, the torque value of the mill motor 51 and the thickness of the layer of the material between the rotary table 2 and the grinding rollers 3 were measured for about 1 hour. During the experiment, the feed rate of the brown coal was kept constant, the rotational speed of the mill motor 51 was kept constant, the rotational speed of the separator motor 81 was regulated within plus/minus about 10% of the rated rotational speed, and the flow rate of the hot gas blowing through the blow nozzle(s) 73 was kept constant.

[0053] FIG. 4A shows the fluctuation over time in the torque value of the mill motor 51 in Experimental Example 1. FIG. 4A also shows the mill inlet temperature, the mill outlet temperature, and the feed rate of the to-be-ground material. FIG. 4B shows the fluctuation over time in the thickness of the layer of the material between the rotary table 2 and the grinding rollers 3 in Experimental Example 1. The time axes of FIG. 4A and FIG. 4B are the same.

[0054] It is seen from FIG. 4A that the torque value of the mill motor 51 in Experimental Example 1 began to increase immediately after the start of feed of the brown coal and that after a lapse of about 10 minutes from the start of feed of the brown coal, the torque value fluctuated with an approximately constant amplitude and the central value of the fluctuation range was approximately constant. The torque value of the mill motor 51 serves as a measure of the rotational load imposed on the rotary table 2. It is seen from FIG. 4B that the thickness of the layer of the to-be-ground material layer in Experimental Example 1 began to increase about 5 minutes after the start of feed of the brown coal and that after a lapse of about 10 minutes from the start of feed of the brown coal, the layer thickness fluctuated with an approximately constant amplitude and the central value of the fluctuation range was approximately constant.

[0055] The above result demonstrates that in Experimental Example 1 the to-be-ground material began to be bitten in between the rotary table 2 and the grinding rollers 3 about 10 minutes after the start of feed of the brown coal and that afterwards the to-be-ground material was successfully bitten in between the rotary table 2 and the grinding rollers 3 and thus the thickness of the layer of the material between the rotary table 2 and the grinding rollers 3 was maintained within a suitable range. This leads to the conclusion that in Experimental Example 1 the moisture content of the brown coal fed onto the rotary table 2 decreased relatively quickly after feed of the brown coal onto the rotary table 2, and consequently the resistance imposed on the grinding rollers 3 and the rotary table 2 from the to-be-ground material decreased enough to allow the material to be successfully bitten in between the grinding rollers 3 and the rotary table 2.

[0056] The torque value of the mill motor 51 was stable after the to-be-ground material began to be bitten in, and this indicates that the fluctuation in the load imposed on the rotary table 2 was reduced. The temperature of the hot gas supplied to the mill inlet 72 was sufficiently lower than the temperature as specified in Patent Literature 1. It is therefore concluded that the vertical roller mill 1 operated in an economical manner in Experimental

Example 1.

[0057] In Experimental Example 1, the mill inlet temperature fluctuated between 150°C and 200°C after the start of feed of the brown coal. The fact that the mill kept operating stably while the mill inlet temperature was in this temperature range indicates that the mill inlet temperature may be 150°C or higher during steady operation. The mill inlet temperature required for maintaining the mill outlet temperature at a certain temperature depends on the feed rate and the moisture content of the to-be-ground material. In Experimental Example 1, where a test vertical roller mill smaller in scale than vertical roller mills for actual use was used and the operation time was relatively short (about 1 hour), the mill inlet temperature increased only up to about 200°C. In the case of continuous operation of a vertical roller mill for actual use, the mill inlet temperature is regulated to a temperature of 320°C or lower at which the to-be-ground material cannot ignite.

[0058] [Comparative Experimental Example 1] In Comparative Experimental Example 1, the vertical roller mill 1 was operated under the same conditions as in Experimental Example 1 described above, except that the feed start inlet temperature was 115°C and the operating outlet temperature was 60°C (both of the temperature values are control target values), and the torque value of the mill motor 51 and the thickness of the layer of the material between the rotary table 2 and the grinding rollers 3 were measured for about 1 hour. In the preheating of the vertical roller mill 1, the mill outlet temperature was about 75°C at the moment when the mill inlet temperature reached 115°C, and this mill outlet temperature was sufficiently higher than 65°C corresponding to the feed start outlet temperature.

[0059] FIG. 5A shows the fluctuation over time in the torque value of the mill motor 51 in Comparative Experimental Example 1. FIG. 5A also shows the mill inlet temperature, the mill outlet temperature, and the feed rate of the to-be-ground material. FIG. 5B shows the fluctuation over time in the thickness of the layer of the material between the rotary table 2 and the grinding rollers 3 in Comparative Experimental Example 1. The time axes of FIG. 5A and FIG. 5B are the same.

[0060] It is seen from FIG. 5A that the torque value of the mill motor 51 in Comparative Experimental Example 1 began to gradually increase immediately after the start of feed of the brown coal and that after a lapse of about 10 minutes from the start of feed of the brown coal, the torque value irregularly fluctuated in a high-value range as compared to the torque value of the mill motor 51 in Experimental Example 1. It is seen from FIG. 5B that the thickness of the to-be-ground material layer in Comparative Experimental Example 1 began to increase about 10 minutes after the start of feed of the brown coal and that afterwards the layer thickness irregularly fluctuated both in a manner in which the value is high and the fluctuation amplitude is small as compared to those of the thickness of the to-be-ground material layer in Experimental Example 1 and in a manner in which the value is low and the fluctuation amplitude is small as compared to those of the thickness of the to-be-ground material layer in Experimental Example 1.

[0061] The above result leads to the inference that in Comparative Experimental Example 1, the to-be-ground material began to be bitten in between the rotary table 2 and the grinding rollers 3 about 10 minutes after the start of feed of the brown coal but there occurred a phenomenon in which the to-be-ground material was not successfully bitten in because of the thickness of the to be-ground material layer being greater than a suitable value and a phenomenon in which the pressing force of the grinding rollers 3 fails to be appropriately applied to the to-be-ground material layer because of the layer thickness being smaller than the suitable value. Additionally, the torque value of the mill motor 51 fluctuated in association with the above phenomena. The torque value of the mill motor 51 was higher than in Experimental Example 1, which means that the electricity consumption by the mill motor 51 was also higher and the operation was uneconomical.

[0062] [Experimental Example 2] In Experimental Example 2, in order to determine the suitable range of the feed start inlet temperature, the measurement of the torque value of the mill motor 51 was conducted by varying the mill inlet temperature at which the to-be-ground material is fed while maintaining the mill outlet temperature at 60°C (control target value). The operation conditions of the vertical roller mill 1 in Experimental Example 2 were as follows: the flow rate of the hot gas supplied was constant, the rotational speed of the mill motor 51 was constant, the rotational speed of the separator motor 81 was regulated within plus/minus about 10% of the rated rotational speed, and the feed rate of the brown coal was constant.

[0063] FIG. 6 shows the fluctuation over time in the torque value of the mill motor 51 and the fluctuation over time in the mill inlet temperature in Experimental Example 2. It is seen from FIG. 6 that the torque value of the mill motor 51 in Experimental Example 2 irregularly fluctuated when the mill inlet temperature was lower than 150°C and that when the mill inlet temperature was 150°C or higher, the torque value was stable in a relatively low-value range. This leads to the conclusion that the feed start inlet temperature is preferably 150°C or higher. Reference Signs List

[0064] 1 Vertical roller mill

2 Rotary table 3 Grinding roller 4 Roller pressing device Table actuator 51 Mill motor 52 Reduction gear 7 Mill casing 71 Mill outlet 72 Mill inlet 73 Hot gas blow nozzle 75 Feed chute 8 Separator actuator 81 Separator motor 82 Transmission system 9 Separator 91 Classifying rotor 92 Guide vane 11 Innercone 13 Hot gas source 14 Feeder 15 Controller 21 Mill inlet temperature sensor 22 Mill outlet temperature sensor 23 Flow rate sensor 25 Layer thickness detector 31 Exhaust path 32 Circulation path 33 Collector 34 Flow rate regulator (hot gas flow rate regulator) 35 Hot gas temperature regulator 351 Circulating gas flow rate regulator 352 Heat source flow rate regulator 353 Heat source temperature regulator

Claims

1. A method of operating a vertical roller mill configured to grind high-moisture coal, the vertical roller mill including a rotary table, a plurality of grinding rollers, and a mill casing having a mill inlet provided below the rotary table and a mill outlet provided above the rotary table, the method including: preheating the vertical roller mill by supplying hot gas to the mill inlet for a predetermined preheating time so that the temperature of exhaust gas from the mill outlet reaches 100C and so that the temperature of the hot gas at the mill inlet reaches a predetermined feed start inlet temperature, the predetermined feed start inlet temperature being 150°C or higher and 250°C or lower; and after the preheating, starting feed of the high-moisture coal onto the rotary table and regulating at least one of the flow rate and the temperature of the hot gas supplied to the mill inlet so that the temperature of the exhaust gas from the mill outlet is maintained at a temperature of 65°C or higher and 100°C or lower.

2. The method according to claim 1, wherein after starting the feed of the high-moisture coal onto the rotary table, the hot gas is supplied to the mill inlet so that the temperature of the hot gas at the mill inlet is maintained at a temperature of 150°C or higher and 320°C or lower.

3. A vertical roller mill including: a rotary table rotatable about a rotation axis extending in a vertical direction; a plurality of grinding rollers pressed against an upper surface of the rotary table; a mill casing enclosing the rotary table and the plurality of grinding rollers, the mill casing including a mill inlet provided below the rotary table, a mill outlet provided above the rotary table, and a hot gas blow nozzle through which hot gas introduced from the mill inlet blows around the rotary table; a feeder that feeds a material to be ground onto the rotary table, the material being high-moisture coal; a hot gas flow rate regulator that regulates the flow rate of the hot gas supplied to the mill inlet; a hot gas temperature regulator that regulates the temperature of the hot gas supplied to the mill inlet; a mill inlet temperature sensor that detects a mill inlet temperature, the mill inlet temperature being the temperature of the hot gas at the mill inlet; a mill outlet temperature sensor that detects a mill outlet temperature, the mill outlet temperature being the temperature of exhaust gas containing the hot gas at the mill outlet and the ground material blown upward by the hot gas; and a controller that controls operations of the feeder, the hot gas temperature regulator, and the hot gas flow rate regulator based on values detected by the mill inlet temperature sensor and the mill outlet temperature sensor, wherein the controller causes the hot gas temperature regulator and the hot gas flow rate regulator to operate to preheat the vertical roller mill by supplying the hot gas to the mill inlet for a predetermined preheating time so that the mill outlet temperature reaches 100°C and so that the mill inlet temperature reaches a predetermined feed start inlet temperature, the predetermined feed start inlet temperature being 150°C or higher and 250°C or lower, and after the preheating, the controller causes the feeder to operate so that the material to be ground is fed onto the rotary table at a predetermined feed rate and causes at least one of the hot gas temperature regulator and the hot gas flow rate regulator to operate so that the mill outlet temperature is maintained at a temperature of 65°C or higher and 100°C or lower.

4. The vertical roller mill according to claim 3, wherein after the material to be ground is fed onto the rotary table, the controller causes at least one of the hot gas temperature regulator and the hot gas flow rate regulator to operate so that the temperature of the hot gas at the mill inlet is maintained at a temperature of 150°C or higher and 320°C or lower.