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

The objective of this project was to explore the relationships between the expansion and contraction behaviour of coals and blends of coals during carbonisation and then relate this behaviour to the properties of the coke produced.  Two type of coke are recognised in this project:

· Cokes where the coke strength is strongly controlled by the bond between fused coal particles due to either a deficient in fusible components or low fluidity of the fusible components.  These are identified as coke with adhesion controlled breakage;

· Cokes where the coke strength is strongly controlled by the pore size and pore wall thickness of the fused material.  These are identified as cokes with porosity controlled breakage.

Samples of three widely-traded Australian coking coals were tested.  Coal A represents a coal that would have strong adhesion control breakage producing poor coke quality if coked at the pilot scale or small scale (7kg).  Coal B is a hard coking coal that produces some oven wall force when coked in a movable wall coke oven. Coal C is a typical Australian soft coking coal.

Testing was conducted in ALS's modified Sole Heated Oven (SHO) on a range of blends made from combinations of these coals at different applied loads and different coal bulk densities.  

This project has clearly demonstrated that the plastic layer can be assumed to be a viscoelastic solid and the associated mechanical properties of that plastic layer can be determined in the ALS SHO.  The Strength Modulus, defined as stress/deformation (MPa/m), was determined from the SHO results.  

These results were found to agree with simple theories of how the viscoelastic properties may change with blend composition if each blend component is treated as an individual foam. These findings have begun to identify of the mechanisms that lead to coke strength, notably when coke strength is non-additive with blend composition.

At a constant load an increase in bulk density will result in a greater expansion of the coke in the SHO.  Due to this expansion there is no change in the porous structure of the coke thus there is little variation in coke strength.  That is, the applied force is controlling the coke strength.

The SHO testing of Coal A with both Coals B and C demonstrates how the addition of binder, Coals B or C, assists in improving the coke strength of an adhesion controlled breakage coal.  The porosity varies linearly with blend composition while the I₆₀₀ does not.  In a blend series, the point where the I₆₀₀ starts to decrease rapidly with decreasing percent of binding coal indicates the maximum carrying capacity of the binder coal. Coal B has a higher carrying capacity than coal C for SHO tests at a load of 5kPa.

For blends of Coal A with Coal B or Coal C there are strong linear relationships between the blend composition with Strength Modulus and coke porosity.  For these blends the plastic layer of the individual coals are behaving as individual foams.

The rheometry and SHO results for expansion, Strength Modulus, coke porosity and I₆₀₀ on the blends of Coal B with Coal C showed non-linear behaviour with blend composition. The strong linear link between the SHO determined Strength Modulus and an estimate of the theoretical coke strength for these blends demonstrates that the mechanical properties of the plastic layer are controlling the coke strength for these blends.

A possible mechanism for this non-linear behaviour is that Coal C physically flows into the inter-particle voids of Coal B that are normally not filled when Coal B is coked alone.  This mechanism was tested with a simple model that assumes the porous structure of Coals C and B are determined just by the applied load and Coal C filled the inter-particle voids of Coal B.  The predicted blend porosity of this simple model matched the measured porosity of the blends at different applied loads and bulk densities supporting this hypothesis.

This project demonstrates the reduction in expansion, as measured in the SHO, of Coal B (a high OWP coal) by the addition of Coal C (a low OWP coal).  This reduction in expansion is not linear with blend composition. Several authors have linked expansion measured in the ASTM SHO test with the OWP.  The mechanism for the reduction in expansion is explained by the mechanism controlling porosity discussed above.

Future SHO work should further examine the non-linear behaviour of blends similar to blends of Coals B and C, but at more blend compositions and at least three SHO loads starting at below 3kPa and up to 10kPa. This expanded dataset together with the modelling of the SHO temperature profile should allow the testing of theories developed in polymer science for blending of viscoelastic solids.