COAL SIZING AND DRYING
6.3.1 Coal Sizing
Coal preparation before gasification always involves some form of sizing of the coal. For moving-bed gasifiers this could be restricted to crushing of the coal followed by sieving out the lumps required for gasification. A separate drying step is not required for moving-bed gasifiers as the drying takes place in the gasifier itself, using the lowest level sensible heat in the product gas.
For the finer-sized coals or petcoke it is essential for dry-coal feed entrained-flow gasifiers that the feed is dry before entering the ring-roller mill that is usually used for the grinding. The drier is integrated with the mill in one recycle loop that is directly heated with a gas burner, as used in conventional pulverized coal (PC) boilers. For details, the reader is referred to the literature in this field (Perry and Chilton 1973, pp. 8-16; Stultz 1992). The gas that dries the coal is part of the same loop that also classifies the coal particles leaving the mill. For fluid-bed gasifiers, drying is not always essential, and more diverse grinding machines are used that depend on the feedstock and the particle size required.
For coal-water slurry gasifiers, driers are not required, and rod mills are mostly used for size reduction.