Читать книгу Coal-Fired Power Generation Handbook - James Speight G., James G. Speight - Страница 64

The tumbler (tumbling mill) is a grinding and pulverizing machine consisting of a shell or drum rotating on a horizontal axis. The material coal is fed into one end of the tumbler where it comes into contact with grinding material, such as iron balls. As the tumbler rotates, the material and grinding balls tumble against each other, the material being broken chiefly by attrition.

Tumblers are cylindrical size reduction devices and are essentially lined drums supported by hollow trunnions at each end. The units are manufactured with overflow, grate, or peripheral, discharge arrangements.

In summary, proper coal sizing is one of the key measures to ensure efficient combustion (Table 3.5). Proper coal sizing, with specific relevance to the type of firing system, helps towards even burning, reduced ash losses and better combustion efficiency. As described above, coal is reduced in size by crushing and pulverizing. Pre-crushed coal can be economical for smaller units, especially those which are stoker fired. In a coal handling system, crushing is limited to a top size of 6 or 4 mm. The devices most commonly used for crushing are the rotary breaker, the roll crusher, and the hammer mill.

Table 3.5 Approximate size dimensions of coal for combustion in various coal-fired systems.

System	Size, inches (mm)
Hand Firing (a) Natural draft (b) Forced draft	1-3 (25-75) 1-1.6 (25-40)
Stoker Firing (a) Chain grate i) Natural draft ii) Forced draft (b) Spreader Stoker	1-1.6 (25-40) 0.6-1.0 (15-25) 0.6-1.0 (15-25)
Pulverized Fuel Fired	75% below 75 microns*
Fluidized bed boiler	<0.4 (<10 mm)

*1 Micron = 1/1000 mm

It is necessary to screen the coal before crushing, so that only oversized coal is fed to the crusher. This helps to reduce power consumption in the crusher. Recommended practices in coal crushing are (i) incorporation of a screen to separate fines and small particles to avoid extra fine generation in crushing, and (ii) incorporation of a magnetic separator to separate iron pieces in coal, which may damage the crusher.