Working principle of medium speed coal mill and fan coal mill

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Medium-speed coal grinding machine Currently, there are four types of medium-speed coal grinding machines used in China:: Roller-disc medium speed grinding mill, also known as flat disc grinding mill ； Roller-bowl medium-speed mill, also known as bowl-type mill or RP-type mill, ball-ring medium-speed mill, also known as medium-speed ball mill or E-type mill ； Roller-ring medium speed grinding mill, also known as MPS grinding mill. The working speed of these coal mills is 50-300r/min, so they are called medium-speed coal mills. The above four medium-speed grinding structures can be seen in Figure 1. Medium speed grinders have a common working principle. They all have two sets of relatively moving grinding parts. The grinding parts squeeze and grind the raw coal between them under the action of spring force, hydraulic pressure or other external forces, and finally break them into coal powder. Through the rotation of the grinding parts, the broken coal powder is thrown to the air ring chamber. The hot air flow flowing through the air ring chamber brings the coal powder to the coal powder separator on the upper part of the medium speed mill. The excessively coarse coal powder is separated and reground. In this process, hot air is also accompanied by drying of coal powder. During the coal grinding process, a small amount of stones, ironware and other debris entrained in the raw coal are also thrown into the air ring chamber. They finally fall into the debris box and are regularly discharged. Figure 1 a) Flat disc mill 1 - Reduction gearbox 2 - Grinding disc 3 - Grinding roller 4 - Pressure spring 5 - Coal drop pipe 6 - Separator 7 - Gas powder mixture outlet 8 - Air ring Figure 1a shows a flat disc mill. Its grinding components are composed of 2 to 3 tapered rollers and a circular flat disc. The axis of the roller forms an angle of 15° with the flat disc. In order to prevent the raw coal from being thrown into the air ring chamber without being ground on the rotating flat plate, there is no retaining ring on the outer edge of the flat disc. The retaining ring also maintains an appropriate thickness of the coal seam on the flat disc to improve the grinding effect. Figure 1b) Bowl mill 1 - Reduction box 2 - Shallow edge grinding bowl 3 - Air ring 4 - Pressurized cylinder 5 - Gas powder mixture outlet 6 - Raw coal inlet 8 - Separator 9 - Grinding roller 10 - Hot air inlet 11 - Debris scraper 12 - Debris discharge pipe Figure 1b is a bowl mill, and its grinding parts are rollers and bowl-shaped grinding discs. In the early days of bowl mill manufacturing, the steel bowl was deeper. With the increase in output, shallow bowl-shaped or inclined plate-shaped steel bowls are now used. Figure 1 c) Medium speed ball mill 1 - guide block 2 - compression ring 3 - upper grinding ring 4 - steel ball 5 - lower grinding ring 6 - yoke 7 - gravel coal box 8 - valve 9 - compression spring 10 - hot air inlet 11 - pulverized coal outlet 12 - raw coal inlet Figure 1c is a medium speed ball mill. This coal mill is like a large thrust bearing without a cage. About ten steel balls are sandwiched between the upper and lower grinding rings. Their upper and lower matching cross-section patterns are like the letter "E", so it is also called E-type grinding. While the steel ball rotates driven by the grinding ring, it also continuously changes its axis of rotation and rolls. Therefore, the steel ball maintains its spherical shape throughout its working life. Figure 1 d) MPS mill 1 - Spring compression ring 2 - Spring 3 - Pressure ring 4 - Roller 5 - Pressure block 6 - Roller 7 - Grinding ring 8 - Grinding disc 9 - Nozzle ring 10 - Tightened wire rope Figure 1d shows the MPS mill. It is a new type of medium speed grinder. The grinding components are three convex rollers and a grinding ring with concave grooves. The roller is large in size and has an almost spherical edge ； The roller axis is fixed, which makes the coal grinding output higher than other medium-speed grinding mills. In addition, the grinding pressure of the MPS mill is directly transmitted to the foundation through springs and three tensioned steel wire ropes, so high pressure can be applied to the grinding parts under the condition of a lightweight casing. These unique features make MPS mills easier to scale up. The Federal Republic of Germany has manufactured an MPS mill with a coal grinding output of 65t/h. Xi'an Thermal Engineering Research Institute compared the main features of bowl mills, medium-speed ball mills and MPS mills. It can be seen from Table 1 that medium-speed ball mill is suitable for coal types with larger wear index, and the life of the grinding parts is longer, but the operating power consumption is large. ； Due to its large diameter, its development to large size is restricted. The power consumption of MPS mill and bowl mill is lower than that of medium-speed ball mill, and the life of the grinding parts of MPS mill is relatively short. It should be pointed out here that when the wear index kms of the type of coal being ground is ≤ 1.0, no matter which medium-speed mill is selected, the service life of the grinding parts will be longer. At this time, if a bowl-type mill is used, it can also enjoy the advantages of low operating power consumption and convenient maintenance. Table 1 Comparison of the characteristics of bowl mill, medium speed ball mill and MPS mill. The coal type adaptability of medium speed mill is not as wide as that of low speed ball mill. It is generally only suitable for bituminous coal and lean coal, and the grindability coefficient of coal kkmHa≥50, and the moisture content of raw coal cannot be too high. However, the medium-speed mill is light in weight, occupies a small area, has simple pulverizing system pipelines, and saves investment. ； It also has the advantages of low power consumption and low noise during operation. Therefore, this kind of medium-speed coal mill has been increasingly widely used in large-capacity units. Fan coal mill Fan coal mill is the most commonly used high-speed coal mill in power stations. Its structure is shown in Figure 2. The structure of the fan mill is similar to that of a fan. The impeller with 8 to 10 blades rotates at a high speed of 750 to 1500r/min. The difference is that these blades are made of manganese steel, which is also called impact plate. The casing is equipped with a removable wear-resistant plate, also known as armor, which is made of Mn13. When the fan mill is running, after the raw coal enters the coal mill with the desiccant, it is crushed by the impact plate and the impeller frame. The coal particles are further crushed by the armor of the casing. The dry coal powder is taken out by the desiccant through the separator, and the excessively coarse coal powder falls back into the fan mill and is ground again. The entire coal is broken mainly by impact. Figure 2 Fan coal mill 1 - casing 2 - impact plate 3 - Yellen 4 - fuel inlet 5 - outlet 6 - shaft 7 - bearing box 8 - coupling As a rotating machine, fan mill has a simple structure, is easy to manufacture, requires less floor space and less metal consumption, so the initial investment is low. The fan mill also has the advantages of simple powder making system and fast equipment installation. In addition, the fan mill integrates the three functions of drying, crushing and conveying, so one less fan can be used. Most of the coal particles in the fan mill are in a suspended state, and ventilation and drying are very strong. ； The desiccant used can be composed of a mixture of hot furnace smoke, cold furnace smoke and hot air. During operation, the proportions of these three media can be adjusted according to the moisture content of the coal, making the control convenient and flexible, and making the desiccant have a good explosion-proof effect. The impact plate of the fan mill wears out greatly during operation and has a short service life. For example, the impact plate life of a large S-type fan mill is only about 1200 hours. Experience has proven that fan mills are most suitable for grinding high-moisture lignite, and can also be used to grind some softer bituminous coals. In addition, the fan coal grinding system has serious air leakage problems. Various types of coal mills have their own advantages and disadvantages and their adaptability to coal types. The selection of coal mill for coal-fired boilers in power plants is very important. First of all, it must be considered based on the type of coal being burned and the type of pulverizing system used. The reliability and economy of equipment operation must also be taken into consideration. When measuring the economical operation of a coal mill, it is necessary to comprehensively analyze three indicators: power consumption for pulverizing, metal material consumption for pulverizing, and labor consumption for maintenance and repair. In addition, investment, layout and installation conditions of the coal mill, and specific practical problems in operation must also be taken into consideration.