As a kind of special machinery equipment of coal pulverizing,
coal mill is an important accessory equipment in pulverized coal furnace. During the process of pulverizing coal, the superficial area of the coal pulverized keeps growing. To increase the superficial area, the intermolecular joint force must be overcome, so energy will be consumed. There are three approaches of getting coal into pulverized coal, namely, crushing, shredding and tripsis. Among the above approaches, tripsis costs the most energy while crushing costs the least.
Various kinds of coal mills are offered in the market. According to the revolving speed of the workpiece in coal mill, we can divide coal mill into three types, low-speed, medium-speed and high-speed ones.
Low-speed
coal mill is best represented by ball mill of drum-type, which is usually called steel ball mill or ball mill for short. It is a totating cylinderical rotary drum. In some cases, the top and the bottom of the rotary drum are conical. With steel balls in it, the revolving speed of the rotary drum is 15~25 r/min. When working, the steel balls keeps hitting and crushing the coal and makes it into pulverized coal. Hot air in the rotary drum will dry the coal and then send it out of the rotary drum. After separated by separator, pulverized coal with certain particle size will be sent to bunker or directly burned.
Usually, a ball mill is heavy and huge with high power consumption and big noise. But it is suitable for different types of coal and especially suitable for hard anthracite.
Medium-speed coal mill has a revolving speed of 50~300 r/min. It usually consists of two milling parts in relative motions. Coal is crushed intopulverized coal when it goes through the space between the two parts. Similar to that with low-speed coal mill, pulverized coal is then dried and sent to separator. The coarse grained coal will be sent to the mill to be pulverized again.
Medium-speed coal mill has the advantages of being structure-compact and space-saving. What's more, its power consumption is only 50~75% of that of low-speed coal mill. But with more complex structure and productive process, its maintenance costs are higher. Besides, medium-speed coal mill is not suitable for hard coal.
High-speed coal mill of course has higher revolving speed, which reaches 50~1500 r/min. It is mainly made up with a high-speed rotor and a part called mill shell. Fan mill and hammering mill are the most common seen. With fan mill, coal will be impacted by the high-speed rotor, crash with the mill shell or the coal itself and then become pulverized coal.
For this type of coal mill, the mill and the separator are integrated to become one compact system with simple structure. It is especially suitable for lignite with a high water content and anthracite which is easy to volatilize. Since Fan mill is easy to be abraded, its continuous operation time is much shorter than other types' mills and it is not suitable to crush hard coal.
