Types of Coal Grinding Mill

The mills used for coal grinding and drying are either tumbling mills or roller mills.

Tumbling Mills

The tube mill or ball mill is especially suitable for the indirect firing system, i.e., where there is no direct connection between mill and kiln and where the pulverized and dried coal is stored in an intermediate bin of ample capacity. Thus the mill can be operated economically at a constant optimum rate of throughput, independently of the demands of the burners fed by it.

The ball mill is insensitive to foreign bodies in the feed material, and the wear of the grinding media can be compensated without any great effort or cost. The relatively long residence time of the coal in the mill has the effect of equalizing any short-term variations in the quality of the mill feed, thanks to the blending action of the system. Also, harder constituents such as quartz and pyrite are effectively pulverized.

Ball mills for coal grinding are almost invariably operated as air-swept mills. As a rule, in order to cope with the relatively high moisture content of the raw coal, the mill is preceded by a drying compartment. The mill is mounted in trunnion bearing, usually at both ends. An advantageous alternative system of mounting that enables larger quantities of gas to be introduced into the mill is the sliding shoe bearing (Fig. 1.22)

Air-swept tube mill with drying compartment and sliding shoe bearing at inlet end (Krupp-Polysius)

Fig. 1.22 Air-swept tube mill with drying compartment and sliding shoe bearing at inlet end (Krupp-Polysius)

Roller Mills

Ring-ball mill for coal grinding; standard type, designed to resist pressure surge (Claudius Peters)

Fig. 1.23 Ring-ball mill for coal grinding; standard type, designed to resist pressure surge (Claudius Peters)

As already noted, the designation “roller mill” is often used as a generic one, comprising mills in which the grinding elements may not only be various types of roller, but may alternatively consist of balls. An advantageous feature for direct firing systems is the short residence time of the material in these mill, so that mill operation can be quickly adjusted to suit the firing requirements in any given time. Economically advantageous is moreover the fact that the power consumption of a roller mill drive is more closely dependent on the rate of material tluoughput than that of a tumbling mill. The throughput control ratio is about 1:2 in all types of roller mill.

Quartz and pyrite are frequently present in coal. They cause a higher rate of wear of the grinding elements, so that more frequent renewal of these parts is necessary.

And the operational availability of the plant is correspondingly diminished. This is obviously a drawback in direct firing with close interconnection of mill and kiln.

Roller mill for coal grinding (Krupp-Polysius)

Fig. 1.24: Roller mill for coal grinding (Krupp-Polysius)

The Claudius Peters direct-firing mill is a ring-ball mill which is available in two versions for operation under inert internal atmosphere and designed to an explosion- resistant specification so that it can withstand pressure surges of 3.5 bar or 50 psi (Fig. 1.23)

The Krupp-Polysius RMK roller mill can be supplied with a housing designed to resist pressure surges of upto 8 bar. This range of coal grinding mills comprises throirghputs from 2.3 to 62 t/hour for a Hardgrave grind ability index of 55 and a product fineness corresponding to 12% retained on the 0.09 nun sieve (Fig. 1.24)

The Atox coal grinding mill is developed F.L. Smidth (Fig. 1.25). It has a flat- topped grinding table, and the three grinding rollers are each mounted on a shaft which is attached to a central yoke. The mill is designed to grend Coal.

Atox roller mill for coal grinding (F.L. Smidth)

Fig. 1.25 Atox roller mill for coal grinding (F.L. Smidth)

Three-roller direct firing mill LM 26.30 D, of modular design (Loesche GmbH)

Fig. 1.26 Three-roller direct firing mill LM 26.30 D, of modular design (Loesche GmbH)

MPS Roller Mill (Gebr. Pfeiffer)

Fig. 1.27 MPS Roller Mill (Gebr. Pfeiffer)

Type “E” ring-ball mill (Fives-Cail Babcock)

Fig. 1.28 Type “E” ring-ball mill (Fives-Cail Babcock)

Emissions from Coal Usage

The main emissions from coal combustion at thermal power plants are Carbon dioxide (CO,), Nitrogen oxide (NOx), Sulphur oxides (SOx), Chlorofluorocarbons (CFCs), carbonaceous material (soot), and air-bom particles such as fly ash, also know as suspended particulate matter (SPM) and other trace gas: Carbon dioxide, nitrous oxide, and chlorofluorocarbons are greenhouse gases. Evidence accumul Inter-government Panel on Climate Changes (IPCC) suggests that emissions of these greenhouse gas might be responsible for climate change, a global concern. Possible consequences projected by II.

  • • a rise in sea levels
  • • a more vigorous hydrological cycle that may increase the severity of floods and drought and may cause more extreme climatic events; and
  • • ecological change that could threaten agr icultural productivity.

Oxides of nitrogen and sulphur, also play an important role in atmospheric chemistry and are largely responsible for atmospheric acidity. Particulates and black carbon (soot) are of concern in the recitative 10 forci earth. They also have a significant negative impact on human health cairsing lung tissue irritation linked to cancer and other serious diseases.

The pollutants emitted from thermal power plants depend largely irpon the fuel birmed, the furnace excess air and any addrtional devices used to reduce the emission. At present, the only control of SPM in thermal power plants in India is electrostatic precipitator to control the emission of fly ash SPM nitric oxides (NO), soot, and SPM emissions from each of the thermal (coal-fired) power plants in it has been cornpirted using basic principles of combustion. These calculations are based on a theoretic the input data, such as chemical composition of the coal used in the power plants, coal used per excess air used during combirstion and the power generation from each plant. This inpirt data has collected from the published information. The present method to estimate the emissions is one of available methods for emissions inventory process. The other methods used in different countries the gurdelrnes recommended by IPCC, and may require large resources. Emissions from combustion.

Product

Mass/kg coal

Mol. wt

Kmoles/kg coal

% volun

co2

1.38

44

0.03136

15.39

so2

0.016

64

0.00025

0.12

o2

0.24

32

0.0075

3.68

n2

4.61

28

0.1646

80.77

Total

0.2038

 
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