Relationship between Internal Pressure and Coke Strength and Implications for Semi Soft Coking Coals in Blends

The development of new coking coal resources around the world and the changing coal quality from existing resources will require steel producers to examine new blend options for the feed to their coke ovens to maintain coke quality. The value of a coal depends on the contribution of that coal to the quality of the coke produced from that blend, knowing this value assists in the marketing of the coal. For coal producers to investigate washing options, mine site blending or port blending options to control coke strength for their customers they are limited to costly testing in pilot scale ovens. The expense of this type of testing limits the number of blends that are evaluated. Alternatively, the producers must rely on potentially dubious correlations between laboratory tests and coke quality.

Current work at ALS Coal and other researchers has demonstrated that the contraction of the plastic layer is related to the properties of the coal, the void space into which the coal can expand (bulk density) and the pressure exerted on the plastic layer (normally by the generation of internal coking pressure). All these factors control the porous structure of the coke and therefore influence the coke strength. The hypothesis investigated in this project was; Can the contraction as measured in the Sapozhnikov Test or Sole Heated Oven (SHO) be used to obtain a better estimate of the impact of a non-hard coking coal on the coke structure of a blend?

In this project the ASTM SHO test was adapted to measure the contraction due to the pressure exerted on the plastic layer during coking of a coal charge. The design of the modified SHO aims to measure the contraction of the plastic layer under controlled heating conditions and different loads. The method of applying the load to the charge has the capability of being altered during a test giving measurements of contraction at different loads. The application of different loads during a modified SHO test enabled data to be collected on how contraction changes with applied force giving a better insight into how component coals may interact in a coke charge. A penetrometer was used to determine the height of the coke-coal interface during the test. Knowing the coke height allows the change in plastic layer thickness under different loads to be estimated.

This work has provided a number of key insights into the factors influencing the behaviour of coals during coke-making. It is known that porosity plays a key role in influencing coke strength. This study has shown how both the viscoelastic properties and the applied load on the plastic layer influence the degree that bubble growth and coalescence occurs, which in-turn influences the porosity. Controlling the viscoelastic properties and/or the applied force could enable desired control of pore structure development and hence coke strength. The SHO is a useful tool for assessing the influence of applied force on bubble coalescence and degree of contraction (shrinkage) of the charge, both of which control pore structure development and, hence, coke strength. It is important to note that applied force influences the degree of pore coalescence taking place but will also influence the degree of shrinkage of the pore network structure. Under constant force, the degree of shrinkage is thought to be dominated by the rate of volatile release. It follows that the SHO could be used to screen blending scenarios and identify blends for which improved strength is expected to be achieved under realistic applied forces. Rheometry is a useful tool for assessing whether interactions are occurring during blending, and the influence that this has on bubble growth and coalescence behaviour.

This work also showed how the viscoelastic path plays an important role in bubble growth/coalescence behaviour whereby one coal studied (coal Y) tracked back through the bubble growth region due to it retaining much of its elastic character. The consequence of this is two periods where bubble growth dominates. This behaviour is expected to have an influence on the final coke strength achieved.