Evolution of the Pore Structure in Coke and Implications on Coke Strength

This project has focused on the evolution of the pore structure in coke during resolidification as pore structure plays a key role in the strength properties of the coke.  It is known that the porosity of the plastic layer can be very high and that soon after the porosity plummets as the process of resolidification takes place.  The mechanism by which the porosity decreases is vital for providing high strength and yet the mechanism is not fully understood; the reason(s) why the porosity decreases remain unknown.  Earlier project C23048 set out to determine whether pore coalescence was the reason why porosity decreased and found that whilst pore coalescence does cause a decrease in both porosity and mean pore size the extent of the decrease is small.  This project set out to determine the role of confinement and applied force.  In project C23048 signs were noticed that the pore structure might become channel-like during resolidification.  A channel-like pore structure makes sense if there is a preferred pathway for volatile release.  If this is occurring, then it might give rise to an anisotropic pore structure and consequently anisotropic strength properties.  In this project aims to determine whether there are preferred pathways in the developing coke structure and whether inertinite plays a role in pore structure development and seeing the changes that occur as coke is heated to higher temperatures up to 900°C.  

By modelling permeability and tortuosity in the digitised 3D coke structure obtained from X-ray micro CT analysis, preferred pathways were found that do exist in real coke oven samples.  The preferred pathway was found to be perpendicular to the direction of applied force, i.e. parallel to the oven walls despite this being the longest path out.  The tortuosity was approximately doubled when moving from a direction of parallel to perpendicular with the oven wall and the modelled permeability halved.  It is hypothesised that applied force could squeeze the pore structure cutting off the openings in the direction toward the wall.  

Furthermore, it was also found that the Young's modulus decreased by almost 25% going from the parallel to perpendicular direction.  This result indicates that anisotropy in coke's mechanical properties can occur.  This anisotropy could have implications for coke strength.  The coke might be more prone to breaking/shearing along particular planes when in the blast furnace.  Furthermore, variable permeability suggests that reactivity toward CO₂ may also be anisotropic, and this could also have implications for strength deterioration behaviour in the blast furnace.

It may also follow that within a coal blend, high coking pressure coals (that generate high internal gas pressures) could play a role in the blend by assisting in the contraction of pores generated by other coals within the blend.  

Slight decreases were measured in both the porosity and mean pore size as coke is heated to high temperatures of 900°C.  This shows that whilst coke has resolidified it is still able to deform due to molecular realignment and continued loss of volatiles (hydrogen).

Results regarding inertinite were inconclusive and require further examination.  It appears that pores in the vitrinite derived carbon are smaller during the early softening/expansion phase when inertinite is present which may be due to the inertinite aiding volatile release.  Then pores appear to be larger at minimum viscosity, which may be due the inertinite acting as a buttress against applied load.  During resolidification the pores appear to be very similar with and without inertinite.