Coal is reduced in size by crushing and pulverizing Pre crushed coal can be

Coal is reduced in size by crushing and pulverizing

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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 4mm. The devices most commonly used for crushing are the rotary breaker, the roll crusher and the hammer mill. 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: 1. Incorporation of a screen to separate fines and small particles to avoid extra fine generation in crushing. 2. Incorporation of a magnetic separator to separate iron pieces in coal, which may damage the crusher. 1. Fuels and Combustion 10 Bureau of Energy Efficiency
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The Table 1.8 gives the proper size of coal for various types of firing systems (b) Conditioning of Coal The fines in coal present problems in combustion on account of segregation effects. Segregation of fines from larger coal pieces can be reduced to a great extent by conditioning coal with water. Water helps fine particles to stick to the bigger lumps due to surface tension of the moisture, thus stopping fines from falling through grate bars or being carried away by the furnace draft. While tempering the coal, care should be taken to ensure that moisture addition is uniform and preferably done in a moving or falling stream of coal. If the percentage of fines in the coal is very high, wetting of coal can decrease the percentage of unburnt carbon and the excess air level required to be supplied for combustion. Table 1.9 shows the extent of wetting, depending on the percentage of fines in coal. (c) Blending of Coal In case of coal lots having excessive fines, it is advisable to blend the predominantly lumped coal with lots containing excessive fines. Coal blending may thus help to limit the extent of fines in coal being fired to not more than 25%. Blending of different qualities of coal may also help to supply a uniform coal feed to the boiler. 1. Fuels and Combustion 11 Bureau of Energy Efficiency TABLE 1.8 PROPER SIZE OF COAL FOR VARIOUS TYPES OF FIRING SYSTEM S. No. Types of Firing System Size (in mm) 1. Hand Firing (a) Natural draft 25–75 (b) Forced draft 25–40 2. Stoker Firing (a) Chain grate i) Natural draft 25–40 ii) Forced draft 15–25 (b) Spreader Stoker 15–25 3. Pulverized Fuel Fired 75% below 75 micron * 4. Fluidized bed boiler < 10 mm *1 Micron = 1/1000 mm TABLE 1.9 EXTENT OF WETTING: FINES VS SURFACE MOISTURE IN COAL Fines (%) Surface Moisture (%) 10 – 15 4 – 5 15 – 20 5 – 6 20 – 25 6 – 7 25 – 30 7 – 8
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The proximate and ultimate analysis of various coals are given in Table 1.10 and 1.11. 1. Fuels and Combustion 12 Bureau of Energy Efficiency TABLE 1.11 ULTIMATE ANALYSIS OF VARIOUS COALS Bituminous Bituminous Indonesian Coal Coal (Sample I) Coal (Sample II) Moisture (%) 5.98 4.39 9.43 Mineral matter (%) 38.63 47.86 13.99 Carbon (%) 42.11 36.22 58.96 Hydrogen (%) 2.76 2.64 4.16 Nitrogen (%) 1.22 1.09 1.02 Sulphur (%) 0.41 0.55 0.56 Oxygen (%) 9.89 7.25 11.88 GCV (kCal/kg) 4000 3500 5500 1.4 Properties of Gaseous Fuels
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  • Spring '17
  • Jerome Ramos

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