The image recognition method was proposed to quantify non-adhesion grain boundaries which were considered as a factor of coke strength besides pores, and the correlation between coke strength and the amount of defects evaluated by the method was investigated in comparison with the one by the marking method. Coke with low-quality coal was fractured by a diametral-compression test, and the fracture crosssections were observed by a scanning electron microscopy (SEM) and a 3D laser scanning microscope (LSM). The marking method and image recognition method were applied to SEM and LSM images, respectively. As a result, the fracture strength measured by the diametral-compression test was linearly decreased with an increase in blending ratio of low-quality coal. In the marking method, most non-adhesion grain boundaries were not detected up to 50% in the blending ratio, and the boundaries increased sharply from 50 to 100% in the blending ratio. On the other hand, in the recognition method, the defects which were composed of both pores and non-adhesion grain boundaries, increased linearly with the blending ratio, and the amount of defects corresponded to coke strength. Therefore, the image recognition method is expected as the quantification technique of defects decreasing coke strength.

In order to investigate coke degradation behavior due to CO 2 gasification reaction in the blast furnace, mass transfer analyses with the reaction and stress analyses for coke considering its structure after the reaction were performed. Using the finite element method, CO 2 gas diffusion in a coke lump and consumption of coke matrices owing to the gasification reaction were considered for the coke model in which the actual coke structure was reproduced. The rate-controlling step was also evaluated calculating the Thiele modulus and the effectiveness factor of catalyst obtained from CO 2 concentration distribution in a coke lump. Further, stress analyses assuming a uniaxial tensile test were carried out for the coke model after CO 2 gasification reaction, and the effect of the gasification reaction on a stress state in a coke lump was investigated. As a result, the reaction progressed mainly in the vicinity of the external surface with reaction temperature of 1 673 K while it did uniformly in the whole coke lump with 1 273 and 1 473 K. Thus, the rate-controlling step shifted from the reaction-controlling step to the diffusion-controlling step with an increase in a reaction temperature, and the Thiele modulus and the effectiveness factor of catalyst also showed the same trend. From the stress analysis, coke strength decreased uniformly in the whole coke lump in case of the reaction-controlling step whereas it did mainly in the vicinity of the external surface in case of diffusion-controlling step.

The strength of the coke with the low-quality coals is related to the non-adhesion grain boundaries. Therefore, the effect of the boundaries on the coke fracture was numerically investigated. A coke model reproducing the actual boundaries was developed by the random arrangement of the coal particle polygons and expansion of the polygons based on experimental results. Then, the fracture behavior and strength of the coke model were analyzed using the Rigid Bodies-Spring Model (RBSM) method. The boundaries were generated around the low-quality coals in the model and the predicted amount of the boundaries corresponded with the experimental results. Therefore, the present coke model reproduced the generation of the actual boundaries. Furthermore, the size and complexity of the boundaries increased with an increase in the low-quality coals. In the model, springs at the gap or edge of the boundaries were fractured. The boundaries themselves were found to concentrate stress, and the arrangement and shape of the boundaries were supposed to affect the coke fracture. Moreover, the concentrated boundaries were thought to decrease the coke strength. Stress-strain curves showed that the coke with the larger blending ratio of the low-quality coal fractured with the weaker strength because of the increment in the size and complexity of the boundaries. The calculated fracture strength showed the same pattern as the experimental one in the higher blending ratio of the low-quality coals. Above all, the present method can predict the coke strength with the non-adhesion grain boundaries on the basis of the blending ratio of the low-quality coal.

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