Coke Reactivity and Characterisation

Coke quality in the blast furnace is measured by its resistance to degradation. The roles of coke as a fuel and as a reducing agent are not limiting factors in terms of blast furnace performance. However, the role of the coke as a permeable medium is crucial in economic blast furnace operation. The degradation of the coke varies with the position in the blast furnace but in all cases involves the combination of reaction with CO₂, H₂0 or 0₂ and the abrasion of coke lumps against each other and other components of the burden. In trying to relate coke quality to its ultimate use in the blast furnace, one of the primary attributes of the coke quality is the coke strength.

The Coke Reactivity test is a highly regarded measure of the performance of coal.  This test has two components; the Coke Reactivity Index (CRI) and the Coke Strength after Reaction (CSR).  A coal which, when coked, achieves a low CRI value and a high CSR value is highly regarded in the market, primarily because this test has been related to blast furnace performance, particularly fuel rate and permeability of the burden.

Coal characteristics which have been shown to influence coke reactivity include coal rank, ash %, various components of the ash chemistry and full maceral reflectograms. In most Australian coals these coal characteristics account for the major proportion (~ 70%) of the variation seen in the Coke Reactivity Index (CRI) and the Coke Strength after Reaction (CSR).

It has proved difficult to quantify what controls the remainder of the variation observed in the coke reactivity test.  It is known that ash percentage and ash chemistry control variability to some extent, with some minerals or elements catalysing the gasification reaction.  However, the equations developed using these elements can, at times, be poor predictors of the CRI and CSR values.

This project examined three distinct characteristics of the coke and their impact on coke quality as defined by the NSC Reactivity test.  They were:

• Compressive strength testing of pre-and post-reaction cokes to allow some understanding of how gasification effects coke strength.
• The coke lump shape and surface area to volume ratios provided additional information on the gasification behaviour of coke; whether it is preferentially gasified on the surface of the coke lumps or whether it is a pervasive effect.  
• The structural elements of the coke included porosity and fissure formation.  These elements were considered to impact on coke reactivity because they have some affect on permeability of the coke and thus the ability of reducing gases to penetrate into the coke lumps.  

Together these tests provided a more complete understanding of how the nature of coke influences the coke's inherent reactivity and strength.

The CRI of the cokes produced in this project followed the same relationship with CSR as other cokes in ALS-ACIRL's extensive database of coking test results.  The optimum maximum reflectance range for low CRI and high CSR is 1.3 to 1.5%. General trends, such as decreasing CRI (increasing CSR) was associated with increasing semifusinite and increasing CRI (decreasing CSR) was associated with increasing liptinite and vitrinite, were found in this project.   There is a lack of strong relationships between macerals and coke microtexture; this was also found by Sharma et al. [2005]. There was no evidence that more of one microtexture was consumed due to the greater reaction with CO₂ of the crushed coke compared to the cored coke.  This implies that the more reactive microtextures, such as non porous isotropic derived from inertinite, are consumed early in the test and the remaining less reactive material in consumed at a constant rate. It was shown that the difference in reactivity of the crushed and cored samples is only due to the shape of the particles. Thus, accicular coke, with a larger surface area compared to more equant or blocky coke, should give a higher CRI.  To determine the extent that shape will influence the CRI all that is required is for the equivalent spherical diameter to be calculated for the coke tested. For the coals used in this project, there was minimal difference in shape of the crushed cokes and this could be due to the crushing required to prepare the sample for the reactivity test.

The compression test proved to be a good indicator of the inherent strength of coke.  The simple sample preparation and testing allowed the testing of 40 samples of the pre reactivity cokes and over 30 samples of the post reactivity cokes.  It was shown that the Young's modulus of surface breakage of the cored particle was lower than the Young's modulus of the unconfined compression breakage and therefore surface breakage would be the initial breakage mode of a particle of unreacted coke for the coals used in this project.  This was not the case for coke after reaction with CO₂ where the Young's modulus of surface breakage was similar or in some cases higher than the Young's modulus of the unconfined compression.  The Young's modulus for surface breakage of the pre reactivity coke decreased with porosity and number of pore walls greater than 50 µm.

The relationship between CSR and compressive strength post reactivity followed the trend as that suggested by Andriopoulos et al. [2002] where drum related matters moderate the effect of the bulk material property.  The compression test gave a better differentiation for the higher strength cokes.

Porosity was found to have a major influence on coke strength, which suggests that coke textures play only a minimal role in determining coke strength.  There was only a general trend that indicated that there is a minimum in porosity corresponding to the optimum Ro_max for maximum pre reactivity coke strength.  The vitrinite concentration had a strong influence on the formation of large pores and therefore porosity.

The porosity increases after reaction with CO₂ thus leading to weakening of the coke structure.  This increase in porosity can not be directly related to the rank or macerals of a coal, though the increase in porosity depends on rank of the coal and the microtexture of the coke. There was no evidence that the increase in porosity was limited to the edges of the coke.  The non-porous isotropic microtexture, roughly related to the inertinite macerals other than semi fusinite, does contribute to the loss in strength of the post reactivity coke, but seems to be rank dependent with the lower rank coals suffering a greater reduction in coke strength.  Coals with a Ro_max greater than 1.3% did not show an increase reduction in coke strength post reactivity test with increasing inertinite less semi fusinite.  Coke microtexture analysis was not done on the post reactivity coke.  However, a subjective visual inspection of some coke images suggested a decrease in inertinite-derived material in the post reactivity.  

Not all of the reduction in coke strength, especially for coals with a reflectance below 1.3%, could be attributed to the amount of non-porous isotropic microtexture.  By examining the different grey levels in the images it was found that material with a grey level between 75 (reflectance ~ 7%) and 110 (reflectance ~ 9%), called grey 2 material, was lost from the coke structure for coals with a Ro_max of less than 1.3%.  In this rank range and when the grey 2 material was concentrated in small pore walls then there is a large reduction in coke strength due to the attack of this material by CO₂. For coals with a reflectance less than 1.3%, this grey 2 material correlated with the sum of the fused carbon domains very fine and fine.

These results highlight the need to associate the coke microtexture with the microstructure.  That is, the size and composition of pore walls have a strong influence on the changes that occur within the coke due to reaction with CO₂ and therefore on coke strength after reaction.