Three main types of coke are produced from coking coals:

The coking processes may be carried out in either slot-type ovens, beehive ovens, newer slot-type ovens with preheating of the charge, travelling grate ovens, rotary kilns or in formed-coke plants. Only the first two processes have widespread commercial application. Slot ovens are generally located adjacent to or close to the steelworks where valuable by-product gases can be collected and used. Beehive (non- recovery) ovens that produce larger and less reactive cokes for foundry use can be located at any convenient site. Formed-coke processes have been developed but have not been commercially implemented because the qualities of that coke have been proven to be inferior to slot-oven and beehive oven cokes.

Aspects of current coking practice that influence coke properties are heating rate and duration, charge bulk density, final temperature, and degree of preheating the charge.  

The value of by-products from carbonisation fluctuates, depending on the oil price, and is secondary to the value of the coke. Nevertheless, the production of by-products is important to the overall economics of a steelworks. The composition and level of production of the by-products is dependent on the types of coals used in the coking blend.

The steel industry is the largest consumer of coke, using it to reduce iron ore to pig iron in blast furnaces where the coke has three roles:

During the movement of the burden down the furnace, the coke within it is subjected to mechanical degradation and chemical attack. Coke size decreases by:

Increasing trends by the steel industry to use pulverised coal injection (PCI) to reduce coke requirements has placed more stringent requirements on the burden quality, in particular on the coke quality. The industry uses layer charging of different coke size ranges. It is also recognised that larger coke particles with a relatively uniform size distribution are required to reduce blast resistance. It is also important that the coke has high and consistent impact strength and abrasion resistance at the furnace operating temperature, as the presence of fines increases the burden resistance, increases slag viscosity and increases carbon loss through the off-gases. Low and consistent coke moisture content is desirable as water vapour catalyses the oxidation of carbon monoxide to carbon dioxide and reduces the extent of direct reduction of the iron ore. Also, high moisture diminishes the carbon feed rate for a constant gravimetric or volumetric coke input. Lower coke reactivity is also required at high PCI rates.

Reifenstein (2005) reviewed several coke quality prediction models. The introduction of this report backgrounds the importance of coal rank and coal macerals on coke quality. The author then explored most coking indices, based on coal properties, used in coking coal trade.

General relationships exits between coke quality parameters and blast furnace performance, but these relationships are weak due to the complexities caused by

Coking properties are mainly affected by rank, maceral types and inorganic matter of the coal. Rank is best determined by the mean maximum reflectance of vitrinite (Rv,max) in the coal. Volatile matter on a dry mineral matter free basis is also an indicator of coal rank. Measurement of the plastic properties during carbonisation (e.g. Gieseler fluidity) is commonly used in evaluating component coals to be used in a coking blend, though there is some debate on how fluidity influences the important coke properties of coke strength after reaction (CSR) and the size of the stabilised coke.  

The table below lists the main quality parameters commonly used in the evaluation of coking coals.

In blending coals for coke manufacture, the properties of the blend can be estimated as follows: