Carbonization

Carbonization is the process by which coal is heated and volatile products— gaseous and liquid—are driven off, leaving a solid reside called char or coke. Coal carbonization processes are classified into high-temperature operations if they are performed at temperatures greater than 1650°F or low-temperature operations if they are conducted below 1350°F. These temperatures are somewhat arbitrary, as they reflect the pronounced physical changes that coal undergoes at temperatures between 1110 and 1470°F [19]. Carbonization processes reaching into the 1350 to 1650°F range are termed medium-temperature processes.

Coals of a very definite range of rank soften on heating, swell on decomposition, and resolidify on continued degasification [1]. Devolatiliza-tion is a continuous process but a distinction can be made between the primary carbonization stage, in which mainly tar is evolved, and the secondary carbonization stage, in which only gas is split off. The characteristic temperatures and stages of the carbonization process are illustrated in Figure 5-20 [1].

The main purpose of high-temperature carbonization is the production of metallurgical coke for use in blast furnaces and foundries. Some coke is used for the manufacture of calcium carbide and electrode carbons, as reductant in certain ferrous and nonferrous open-hearth operations, and in foundries to produce cast iron; however, more than 90% of the coke produced is used in blast furnaces to smelt iron ore and produce pig iron, and modern coke-making practices are virtually dictated by the coke quality in this market. Low-temperature carbonization has been mainly used to provide town gas for residential and street lighting, tars for use in chemical production, and smokeless fuels for domestic and industrial heating.

Coal Plastic transient state Semi-coke

Pre-softening Pre- Swelling Stiffening Resolidified stage swelling stage

(shrinking)

Primary carbonization stage Secondary carbonization stage

Ts = softening temperature

Tsw = temperature of initial swelling

Tmd = temperature of maximum devolatilization rate

Tr = resolidification temperature

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