Pulverized-Fuel Preparation

the crushing and drying of solid fuel that is to be burned in chamber furnaces. The fineness of the fuel particles after grinding is determined by screen analysis and ranges from 90 to 1,000 microns; coals poor in volatiles, such as anthracite, are ground more finely. In pulverization, nongrindable impurities, such as slivers or metal objects, are first removed from the fuel, which is then precrushed into pieces no larger than 15 mm and finally ground up in pulverizers of the ball or hammer type. In most cases, grinding and predrying are combined in a single pulverizing-drying system.

Drying systems may be of the closed or open type. In closed systems the used drying agent (air or flue gases) is exhausted into the furnace, and in open systems it is discharged into the atmosphere. Individual pulverizing-drying systems supply fuel to separate boiler units, receiving the drying agent from the same boiler unit. In a centralized fuel-preparation system, fuel for a group of boiler units is prepared in a special pulverizing plant.

Individual closed fuel preparation systems of the storage and direct-fired types are most commonly used. Storage systems supply the pulverized fuel (dust) to the burners independently of the operating conditions of the pulverizer, since there is a fuel reserve in the bin. In direct-fired systems the pulverized fuel is fed immediately to the burners, that is, the operation of the boiler unit is rigidly linked to the output of the pulverizing-drying system. However, the direct-fired system is simpler and cheaper and therefore has gained wider use.