Working principle of the hottest medium speed pulv

2022-08-06
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Working principle of medium speed pulverizer and fan pulverizer

medium speed pulverizer

at present, there are four types of medium speed pulverizers used in China: roller disc medium speed mill, also known as flat disc mill; Roll bowl medium speed mill, also known as bowl mill or RP mill, ball ring medium speed mill, also known as medium speed ball mill or e-mill; Roll ring medium speed mill, also known as MPs mill. The working speed of these coal mills is 50 ~ 300r/min, so they are called medium speed coal mills. See Figure 1 for the above four medium speed mill structures

medium speed mills share a common working principle. Both of them have two groups of relatively moving grinding parts. Under the action of spring force, hydraulic pressure or other external forces, the grinding parts squeeze and grind the raw coal between them, and finally crush it into pulverized coal. The crushed pulverized coal is thrown into the air ring chamber by the rotation of the milling parts. The hot air flow through the air ring chamber brings these pulverized coal to the pulverized coal separator at the upper part of the medium speed mill. The coarse pulverized coal is separated and reground. In this process, the hot air is accompanied by the drying of pulverized coal. During the coal grinding process, a small amount of stones, ironware and other sundries carried in the raw coal are thrown into the air ring chamber at the same time, which finally fall into the glove box and are discharged regularly

Fig. 1 a) flat plate mill

1 - reduction gearbox 2 - grinding plate 3 - grinding roller 4 - pressure spring 5 - coal drop pipe 6 - separator 7 - gas powder mixture outlet 8 - air ring

Fig. 1a is a flat plate mill. Its grinding components are composed of 2 ~ 3 conical rollers and circular flat plates, and the axis of the roller forms an included angle of 15 ° with the flat plate. In order to prevent raw coal from being thrown into the air ring chamber without grinding on the rotating pan, there is no retaining ring at the outer edge of the pan, which also keeps the pan with an appropriate coal seam thickness to improve the grinding effect

Fig. 1b) bowl mill

1 - reduction gearbox 2 - shallow grinding bowl 3 - air ring 4 - pressurizing cylinder 5 - gas powder mixture outlet 6 - raw coal inlet 8 - separator 9 - grinding roller 10 - hot air inlet 11 - sundry scraper 12 - sundry discharge pipe

Fig. 1b is a bowl mill, and its grinding components are roller and bowl shaped grinding plate. In the early days, the steel bowls for bowl mills were deep. With the increase of output, now shallow bowl or inclined plate steel bowls are mostly used

Figure 1 c) medium speed ball milling

1 - guide block 2 - hold down ring 3 - upper grinding ring 4 - steel ball 5 - lower grinding ring 6 - yoke 7 - pebble coal box 8 - valve 9 - hold down spring 10 - hot air inlet 11 - pulverized coal outlet 12 - raw coal inlet

figure 1C shows medium speed ball milling. This coal mill is like a large thrust bearing without cage. About ten steel balls are clamped between the upper and lower grinding rings. The cross-sectional figure of their upper and lower matching is like the letter "e", so it is also called E-type grinding. While the steel ball rotates driven by the grinding ring, it also constantly changes its own rotation axis and rolls. Therefore, the steel ball will always keep a round shape during the whole working life

Fig. 1 D) MPs mill

1 - spring hold down ring 2 - spring 3 - press ring 4 - roller 5 - pressure block 6 - roller 7 - grinding ring 8 - grinding disc 9 - nozzle ring 10 - tension wire rope

Fig. 1D is MPs mill. It is a new type of medium speed mill. The grinding part consists of three convex rollers and a grinding ring with a concave channel. The size of the roller is large, and the edge is nearly spherical; The roller axis is fixed, which makes the coal mill output higher than other medium speed mills. In addition, the grinding pressure of MPS mill is directly transmitted to the foundation through the spring and three impact root tension steel wire rope, so high pressure can be applied to the grinding parts under the condition of light casing. These unique features make MPs mill easier to be large-scale. The Federal Republic of Germany has manufactured MPs mills with coal grinding output of 65t/h

the main characteristics of bowl mill, medium speed ball mill and MPs mill adopted by Xi'an Thermal Power Research Institute were compared. It can be seen from table 1 that medium speed ball milling is suitable for coal with large wear index, and the service life of grinding parts is long, but the operation power consumption is large; Due to its large diameter, its large-scale development is limited by the fact that wangenge, an academician of the Chinese Academy of Sciences, said that it is not recommended to be used in the external wall insulation system. The power consumption of MPS mill and bowl mill is lower than that of medium speed ball mill, while the service life of MPS mill is relatively short. It should be pointed out here that when the wear index kms of the milled coal is ≤ 1.0, no matter which medium speed mill is selected, the service life of the grinding parts is high. At this time, if the bowl mill is used, it can also enjoy the advantages of low power consumption and convenient maintenance. Table 1 Comparison of characteristics of bowl mill, medium speed ball mill and MPs mill

medium speed mill is less adaptable to coal types than low-speed ball mill. It is generally only applicable to bituminous coal and lean coal. The grindability coefficient of coal is kkmha ≥ 50, and the moisture content of raw coal cannot be too high. However, the medium speed mill has the advantages of light weight, small land occupation, simple pipeline of pulverizing system and low investment; It also has the advantages of low power consumption and low noise during operation. Therefore, this medium speed coal mill has been widely used in large capacity units

fan coal pulverizer

fan coal pulverizer is the most widely used high-speed coal pulverizer in the power station. Its structure is shown in Figure 2. The structure of fan mill is similar to that of fan. The impeller with 8 ~ 10 blades rotates at a high speed of 750 ~ 1500r/min. The difference is that these blades are made of manganese steel, which is also called impact plate. The casing is equipped with removable wear-resistant protective plate, also known as armor, which is made of Mn13. During the operation of the fan mill, the raw coal enters the coal mill with the desiccant, and is crushed by the impact plate Lubrizol, which belongs to Berkshire Hathaway, and the impeller frame. The coal particles are further crushed on the armor of the casing. The dried pulverized coal is brought out by the desiccant through the separator, and the coarse pulverized coal falls back to the fan mill for re grinding. The whole coal is mainly crushed by impact

Figure 2 fan coal mill

1 - casing 2 - impact plate 3 - blade 4 - fuel inlet 5 - outlet 6 - shaft 7 - bearing housing 8 - coupling

as a rotating machine, the fan mill has simple structure, convenient manufacture, less floor space and metal consumption, so the initial investment is low. The fan mill also has the advantages of simple pulverizing system and fast equipment. In addition, the fan mill integrates three functions of drying, crushing and conveying, so one fan can be used less. Most of the coal particles in the fan mill are suspended, and the ventilation and drying are very strong; The desiccant used can be composed of hot furnace smoke, cold furnace smoke and hot air. During operation, the proportion of these three media can be adjusted according to the moisture content of coal, which is convenient and flexible to control, and makes the desiccant have good explosion-proof effect

the impact plate of fan mill is worn heavily and its service life is short. For example, the service life of impact plate of large S-type fan mill is only about 1200h. Experience has proved that fan mill is most suitable for grinding high moisture lignite, and can also be used for grinding some softer bituminous coal. In addition, there is a serious air leakage problem in the fan pulverizing system

various types of coal mills have their own advantages and disadvantages and the scope of adaptation to coal types. The type selection of coal pulverizer for coal-fired boiler in power plant design is very important. First, it must be considered according to the type of coal fired and the type of pulverizing system adopted, and the reliability and economy of equipment operation should be considered. When measuring the operation economy of a coal mill, it is necessary to comprehensively analyze three indicators, namely, power consumption for pulverizing, consumption of metal materials for pulverizing and labor consumption for overhaul and maintenance. In addition, the investment, arrangement and installation conditions of coal mills and specific practical problems in operation should be taken into account. (end)

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