Coking Conditions

In addition to the coal properties listed in the previous section, the carbonization conditions influence the properties of the resultant coke. Carbonization conditions of interest include the particle size of the coal charged to the coke oven, charge density, rate of heating, oven design, and special conditions such as preheating and partial briquetting. It is important to pulverize the coals (measured as the quantity of coal passing a 1/8-in. screen, which is typically 80% but can vary from 50 to 100%) to reduce the inert particles of the coal as well as to reduce the size of coal particles that exhibit low fluidities [2]. Higher levels of pulverization tend to make a more homogenous mixture of the reactive and inert components of the coals blended. The bulk density of the coal charged to the oven is also adjusted in order to produce denser or more homogenous cokes. In addition to varying the coal particle size, this adjustment is accomplished by adding small amounts of water or oil to the blend.

There has been much effort to increase the productivity of coke ovens through improved oven design and operating practices. The rate of heating the coal charge has been shown to be important in coke yield and properties [2]. As coking rate is increased, the coke size, shatter index, and stability decrease while the hardness factor increases. Coke size becomes more uniform with increased heating rate. Faster coking rates and the resultant in situ crushing are advantages to the iron makers who crush coke to produce a uniformly sized burden for the furnace [2]. Preheating high-oxygen coals or marginally coking coals can substantially improve the quality of coke from these coals and reduce the required coking times [2]. Coking times for these coals can be reduced by 30 to 45%, while preheating strengthens the resultant cokes.

In an effort to reduce the cost of blast furnace coke and to extend the range of ranks and types of coals that can be incorporated into blast furnace coke blends, formed coke processes have been developed. Formed coke processes involve carbonizing coal or a blend of coals (that may contain low-rank or highly volatile coals) that have been compressed into shaped briquettes.

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