Grinding of coal

A high, moisture content necessitates finer grinding of coal for rapid water release. For such coals, the classifier in the pulverizer is set to return more of the oversize material for further grinding. 

This effect is well illustrated by a series of experiments on the grinding of coal in a small mill, carried out by Heywood(1). The results are shown in Figure 2.1, in which the distribution of particle size in the product is shown as a function of the number of... 

The Szego mill has been used for dry as well as wet grinding of coal, in both water and oil, for the preparation of coal-slurry fuels. To grind coal in water to the standard boiler size (80 per cent < 74 im) takes about 7 MJ/Mg (20 kWh/tonne). For micronized grinding, to a 15 xm median size, 2-3 passes through the mill are required and the specific energy... 

Several bituninous coals have been treated with molten mixtures of NaOH plus KOH at 350-370 C in a 1 10 coal-to-caustic ratio for 1 hour and then washed with water, dilute acid, and finally with water again. By this procedure, which is similar to that of the TRW Gravi-melt Process, 80-90% of the ash and 70-80% of the total sulfur have been removed with a recovery of 80-90% of the coal on a moisture- and ash-free basis. The desulfurization could be improved by using a finer grind of coal, better washing procedures, or a recleaning step. 

Aerodynamics can play a role in the ash deposition process in all furnaces regardless of the type of firing recent interest in micro-fine grinding of coal is testimony to this fact. It has been postulated that smaller ash particles will follow gas streamlines and be less likely to strike heat transfer surfaces. This is a logical hypothesis for those ash particles that cause deposition due to an impact mechanism. In addition to particle size, particle density and shape also affect aerodynamic behavior. Due to the difference in drag forces, molten, spherical particles will be less likely to follow gas streamlines than angular or irregular particles of the same mass. 

Coal ash shells on clinkers and nests at boundary zones Nonuniform combustion of fuel and partial reaction on clinker surface inadequate grinding of coal and distribution (Kramer, 1960)... 

Comminution Breaking, crushing, or grinding of coal, ore, or rock. 

Preparation (coke making) Fine grinding of coal to ensure adequate fusion of the particles. Usual practice is to grind the coal so that 65%-85% will pass through a 1/8 in. screen opening. 

Disadvantages of the process are the need for fine grinding of coal, operation at low pressures (1-3 atm), and higher oxygen consumption than other coal gasification processes. 

Fine grinding of coal is necessary to assure complete combustion of the carbon for maximum efficiency. The required pulverized fuel fineness is expressed as the percentage of the product passing through various sizes of sieves. Coal classification by rank and end use of product determine the fineness to which coal should be ground (Table 6.15). 

Because woody biomass must be reduced to <6.35 mm (particle sizes, several strategies have been developed. Some demonstrations have ground all wood to this particle size, while others have purchased sawdust and ground only the oversized particles. In all PC cofiring cases, considerable attention has to be paid to fuel preparation—and keeping the grinding of biomass separate from the grinding of coal. 

Since the 1970s, in Russia and other coimtries, on a relatively small scale, coal and oil in power plants and furnaces are replaced by water-coal fuel (WCF), which is also used even as a fuel for turbine and diesel power plants [305]. In addition to being a useful method for utilization of waste coal, the WCF makes it possible to transport coal in railway tanks and through pipelines. A lot of experience in this area has already been acquired in Russia, the U.S., and China. The preparation WCF includes the grinding of coal (up to a size of... 

---