Coal Mill

Pulverization is currently the favored method of preparing coal for burning. Mechanically pulverizing coal fragments into fine powders enables coal to burn like gases, thus achieving more efficient combustion. Coal mill comes in handy to do that.

1. How to choose the right coal mill?

In practice, you can select the type of coal mill based on the type of coal (bituminous coal, anthracite, lignite), and its hardness, grindability index, temperature, and type of desiccant, and choose the capacity based on the expected grinding volume.

2. How many types of coal mills?

According to the different rotating speeds of the coal mill, it can be roughly divided into the following three types:

Low-speed coal mill: Its rotating speed range is 15–25 r/min. The low-speed coal mills include cylindrical steel ball mill, steel ball mill with two symmetrical grinding circuits, etc.

Medium-speed coal mill: Its rotating speed range is 50–300 r/min. Flat plate mills, bowl mills, and MPS mills are common medium-speed coal mills.

High-speed coal mill: Its rotating speed range is 500-1,500 r/min. Commonly used high-speed coal mills include fan coal mills and hammer coal mills.

Among all coal mills, cylinder-type steel ball coal mill is widely favored because of its good adaptability to all kinds of coal, strong drying ability, simple structure, high reliability, and easy maintenance. It is an ideal machine for pulverized coal. FTM Machinery adheres to the spirit of innovation and develops high-quality air-swept coal mills (horizontal type) to meet the market demand.

3. What is the structure of the air-swept coal mill?

The air-swept coal mill is mainly composed of a feeding device, main bearing, rotating part, transmission device, discharge device, high-pressure starting device, and lubrication system.

3.1 The feeding device of the coal mill

The feeding device adopts the louver structure that can feed air and materials at an inclined angle, which makes the feeding smoother. A sealing device is provided at the contact point between the feeding device and the rotating part to prevent cold wind from entering from here. In addition, gaskets can be padded between the feed pipe and the air inlet pipe, and the bracket to adjust the center height of the feed device if necessary.

3.2 The rotating part of the coal mill

The main body of the coal mill is the rotating part, in which the raw coal pieces are dried and ground. The rotating part is composed of a hollow shaft, a cylinder, a lifting plate, a compartment plate, lining plates, and the grinding medium (usually steel balls).

3.3 The transmission device of the coal mill

It adopts an edge transmission system with an auxiliary transmission device. The motor slowly drives the mill to rotate after decelerating through the reducer and gears. The pinion and gear shaft are connected by a taper lock, which is convenient for installation and maintenance, helps avoid tooth folding and eccentricity and slippage of the pinion.

3.4 The discharge device of the coal mill

The discharge device adopts a curved pipe that is installed with lining plates inside to prevent wear. It is also equipped with a spiral cylinder through which the unqualified coarse powder will be pushed back to the grinding bin for regrinding. The adoption of spiral cylinder effectively avoids slag and dust accumulation of coal mill, ensures safe production, reduces environmental pollution, and lightens labor intensity of workers.

4. How does the air-swept coal mill work?

When the rotating part of the coal mill rotates slowly, the steel balls in it move together with the help of the ribs on the ribbed lining plates. The movements include the rotation of the steel balls, the relative revolution among the steel balls, and the approximate revolution of the steel balls around the central axis of the cylinder caused by the centrifugal force generated by the rotation of the cylinder.

The rotation of the steel balls and the relative revolution among the steel balls will produce a grinding effect, so the coal filled between the steel balls will be crushed and ground.

In addition, when the steel balls in the cylinder are brought to a certain height, they fall under the action of gravity, resulting in a mutual collision with the steel balls at the lower part, which will also crush the coal between the steel balls. Besides, the collision between the steel balls and the lining plates will also crush the coal.

5. Why and how does FTM Machinery improve the coal mill?

An air-swept coal mill is the core machine for pulverizing pulverized coal. However, according to our investigation, there are some problems with coal mills produced by many manufacturers. Therefore, our company has carried out a technical transformation on the air-swept coal mill to improve production efficiency and save energy.

5.1 Main problems of air-swept coal mill before transformation

Insufficient drying capacity

The original insulation layer of the hot air duct from the outlet of the high-temperature fan to the inlet of the coal mill is thin and mostly damaged so that the heat loss is as high as 100℃. The actual hot air temperature into the mill is only about 190℃, weakening drying effect.

If the moisture content of raw coal is high, it will be more difficult to achieve the expected drying effect. Therefore, the moisture content of pulverized coal exceeds the standard.

Insufficient grinding capacity

The grinding ability of the air swept coal mill itself is lacking, especially for inferior coal. If you want to grind inferior coal more finer at the high level of output, ordinary coal mill is not a smart choice.

Manual screening of steel balls

Steel balls are the important grinding medium of the coal mill, and their performance directly affects the grinding efficiency and output of the coal ball mill. The steel balls will be worn in the grinding process, minimizing the grinding efficiency. So worn steel balls need to be screened and replaced with new ones regularly. The existing mills cannot automatically screen steel balls, so manual screening is required. However, this method requires high labor intensity and long waste of time, which seriously lowers production.

5.2 Improving measures

Strengthen thermal insulation of hot air pipeline

Replace the new pipe insulation layer material that can strengthen the insulation, and add an airlock device at the head of the coal mill to reduce the leakage of cold air. After the transformation, on-site measurement shows that under the same ventilation, the temperature of hot air entering the mill is increased from 190℃ to 220℃–240℃, which provides enough heat to improve the drying efficiency.

Adjust the length ratio of the air-swept coal mill

The length ratio of the drying bin and the grinding bin of most coal mills on the market is 1:1.67. FTM Machinery’s coal mill has been improved. The drying bin of the FTM Machinery coal mill has been shortened from the original 3 m to 2.1 m, and the grinding silo is extended by 0.9 m, so that the length ratio between the drying bin and the grinding bin can reach 1:2.81. Such a change can grind coal pieces more fully, and can also grind inferior coal finer in high quantity.

Please note that after extending the grinding bin, steel balls will be added correspondingly, and a higher-power motor will be replaced.

Automatic screening and unloading device

Three 31*130 mm rectangular screen holes are cast on the grooved liners at the discharge end, and a ball cavity used to discharge steel balls is left between the liners with screen holes and the cover at the outlet end. There is a ball outlet at the bottom of the ball cavity.

The discharge spiral tubes are divided into the inner spiral tube and the outer spiral tube. When the coal mill is working, the steel balls enter the ball cavity through the screen holes, and the other side of the liners drives the balls to rotate. Then the steel balls fall directly from the ball outlet into the outer spiral tube due to their weight. Most of the steel balls fall into the grooved ball collector through a long steel pipe, and a few fall into the inner spiral pipe and return to the ball cavity to continuously screen out the unqualified steel balls.

In this way, unqualified steel balls can be quickly screened out without stopping the machine.

5.3 Improvement effect

After the transformation, the output of the coal mill increases from 18 t/h to 20 t/h. The drying capacity of it is enhanced due to the increase of the air temperature, reducing the moisture content of pulverized coal to ≤1.4%. On the whole, the performance of the coal mill system after the transformation has been greatly improved.

6. Which industries can apply the air-swept coal mill?

Air swept coal mill is widely used in cement industry, thermal power generation industry, metallurgical industry, chemical industry, sugar industry and other industries.

6.1 Air-swept coal mill in cement plants

At present, most cement plants use coal as the main fuel in the clinker production process. Calcining high-quality cement clinker needs a stable coal powder supply. Therefore, coal powder preparation has become an important part of cement production, and coal mill has also become a crucial machine in cement plants.

6.2 Air-swept coal mill in thermal power plants

Coal powder combustion is often used in modern large and medium-sized thermal power plant boilers. Coal powder combustion means that coal fragments are sent to the coal mill through the coal hopper and coal feeder, ground in it, and then stored in the coal powder bin or directly sent to the boiler for combustion. In thermal power plants, the system of making raw coal into qualified pulverized coal with a coal mill as the core is called pulverizing system.

The pulverizing system as an important auxiliary system of coal-fired power plants is to grind qualified pulverized coal to ensure the needs of boiler combustion. The running condition of it will directly affect the safety and economy of the boiler.

7. How does the air-swept coal mill solve the problem of the moisture content in raw coal?

The final moisture content of pulverized coal has a great impact on the continuity and uniformity of pulverized coal, the economy of combustion, the output and the safety of the pulverizing system.

If the moisture content is too high, it is easy to cause caking and blockage. If too low, it is prone to natural explosion and higher drying energy consumption. Therefore, the FTM Machinery air-swept coal mill adopts the following measures to enhance the adaptability of the air-swept coal mill to the moisture of raw coal.

7.1 Reasonably configure drying devices

A drying bin and a lifting plate are installed in the cylinder. After the high-temperature hot air enters through the feeder, the lifting plate in the drying bin will scoop up the coal pieces and throw them up high to fully dry the coal. After drying, the coal pieces enter the grinding bin through the double-layer partition board, where they are crushed and ground.

7.2 Scientifically adjust the hot air temperature and air volume

The drying of raw coal is not only related to the heat exchange effect in the drying bin but also the temperature and air volume of hot air. Therefore, it is also necessary to control and adjust the hot air temperature into the mill scientifically. For example, increase the air volume within a certain range to evaporate the water in the coal mill.

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