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Lignite has special characteristics that distinguish it from other coals. This makes the burning lignite different . The boiler design are different.
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High Moisture Content .
In a Boiler firing bituminous coal the maximum moisture content is in the range of 20 % . For drying this moisture around 30 % of air heated in the air heater to around 320 ° C is used. Lignites with around 45 % to 55 % moisture require much more heat to remove the moisture. Lignite boilers use recirculated Flue gas from the top of the furnace to heat and remove the moisture. The temperature of the flue gas will be in the order of 800 °C and above. This gives sufficient heat for the high moisture removal.
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High Volatile Matter.
The volatile matter content in Lignites are very high, in the range 45 % -55 % on an ash and moisture free basis. This makes it easily ignitable. In the raw form the high amount moisture prevents this happening. However, the moment the lignite is dried , it has the tendency to ignite spontaneously . Using Flue gas as the drying medium helps in preventing this.
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Lignite Fired boiler use special pulverisers . These fall into the category of hammer mills that grind soft coals. These are fan type paddle wheels. These pulverisers do the dual role of recirculating the flue gas and ginding the coal.
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Because of the prescence of high amounts of Sulphur in the form of Ferrous Sulifde, the ash fusion temperatures are low. This makes the lignite a very high slagging potential fuel. Large numbers of wall deslaggers and frequent usage of the wall deslaggers are required to prevent the slagging. Unlike wall blower in bituminous coal fired units wall deslaggers use water as the medium instead of steam. Higher excess air operation, proper distribution of air and larger furnace sizes are also required to reduce the slagging potential. In some case even programmed high pressure water lances are in service to remove the slag. The ash handling system , especially the bottom ash handling has to take care of this high quantity of slag.
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Because of the slagging potential the boilers are generally the tower type design instead of the conventional two pass design. The heat transfer coils are placed above the furnace area.
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These points make the boiler different . The pressure part or the water and steam circuits remain the same.
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In recent times Circulating Fluidized bed boilers are used for burning lignite especially on smaller capacities units. The lower furnace temperatures help in preventing the slagging . Fluidized bed boiler can dry the Lignite in the furnace during the combustion process. This eliminates the costly flue gas recirculation. A CFB can fire lignite along with other coal types giving a high fuel flexibility. Problems in feeding the wet coal are the major issues faced.
Advanced technologies in development include the use of fluidized drier, which dries the lignite first, and vapour heat recovery takes place in the power plant cycle. This method recovers the heat lost due to the vaporisation of the water.