Abstract"Non-adhesion grain boundaries" are formed when low-quality coal grains do not adhere to other grains in the carbonization process because of the low dilation of coke. To better understand the effects of non-adhesion grain boundaries on coke strength, the relationship between the existence ratio of non-adhesion grain boundaries and coke strength was investigated quantitatively. The existence ratio of non-adhesion grain boundaries were measured quantitatively by observing the fracture cross-section of coke using scanning electron microscopy (SEM). Coke strength was measured with a diametral-compression test and an I-shape drum index test. As a result, non-adhesion grain boundaries increased with an increase in the blending ratio of low-quality coal. In particular, non-adhesion grain boundaries increased rapidly when the blending ratio of low-quality coal was over 50%. When the ratio was less than 50%, low-quality coals adhered to other caking coal. However, not many low-quality coals adhered to other caking coals when the ratio was over 50%. The tensile strength of coke was not affected by the porosity of coke. However, the tensile strength and the drum index were affected by the existence ratio of non-adhesion grain boundaries. Tensile strength decreased rapidly even for a few non-adhesion grain boundaries because significant defects caused a fracture in the diametral-compression test. However, the I-shape drum index decreased linearly with the existence ratio of the non-adhesion grain boundaries because many fractures occurred during 600 rotations in the drum. The strength of coke containing low-quality coal is governed by the existence ratio of non-adhesion grain boundaries rather than mean values such as the porosity of coke.

The relationship between the strength and the microstructure of ferro-coke with hyper-coal (HPC) addition is investigated. In particular, we focused on the adhesiveness of coal particles. The strength of ferrocoke was evaluated by a tensile strength test and coke microstructure with increasing the amount of HPC addition was observed. In the observation of the microstructure, absolute maximum length and roundness of pores were measured. The result indicated that the strength of ferro-coke increases with an increase in the amount of HPC, because voids between coal particles decrease and adhesiveness of coal particles improve. A pore roundness of less than 0.2 intends coke strength, and the index decreases with HPC addition. We found that a pore roundness of less than 0.05 is included in voids between coal particles. It is suggested that mutual adhesiveness of coal particles is one of the factors affecting coke strength, and this factor can be evaluated by pores of roundness less than 0.05.

This study investigates the dominant factors affecting the strength of ferro-coke, which is produced by blending iron oxide with coal particles, with the addition of hyper-coal (HPC), to produce a high reactivity and strong coke. A diametral compression test for ferro-coke with and without HPC addition is performed. A three-dimensional ferro-coke model is then developed using micro X-ray computed tomography, and the relative proportions of pore, pore wall, iron, and pore space surrounding the iron particles, termed here "defect", are quantified using this model. Moreover, a stress analysis is performed for the ferro-coke model. The diametral compression tests indicate that the strength of ferro-coke increases with the increasing blending ratio of HPC. The image-based modeling indicates that the wall thickness increases and stress concentration is relaxed with increasing addition of HPC due to enhancement of the adhesiveness of coal particles. On the other hand, the relative proportion of the "defect" is independent of HPC addition. Therefore, ferro-coke strength is found to be determined not by the "defect" around iron oxide but by the wall thickness.

The objective of this paper are to study the formation process of coke pore structure in relation to the swelling of coke during the thermoplastic stage and the pore structure at the initial carbonization. The swelling ratio, changes in pore structure and growth behavior of pore which originally present in the coal were evaluated. Two kinds of caking coals were carbonized in nitrogen-atmosphere with an infrared furnace. The swelling ratios were measured, and the cross-sectional images of coal particles at different heating temperatures were observed with an optical microscope. Before carbonization, all pores in both coals were a diameter of less than 100 μm. When the temperature increased up to the level at which the swelling started, the pores with greater than 100 μm-diameters were formed, and they had also simultaneously coalesced. However, each coal had a different temperature for starting swelling and observing the generation and coalescence of pores with a diameter of more than 100 μm. At this point, the formation of pores were expressed with Laplace equations. The results showed the volatile matter inflow affected the change of pore structure. Furthermore, the pore structures within coal particles before and after carbonization were compared by micro-X-ray Computed Tomography. Carbonization up to the temperature for the softening stage increased pore diameters, which provided evidence that the proportion of pores existing within post-carbonized coal had grown based on the pores already existing in pre-carbonized coal.

A finite element model of coke was generated using micro X-ray CT, and coke strength was numerically evaluated using its microstructure. First, three-dimensional (3-D) and two-dimensional (2-D) stress analyses of coke were compared. Next, 3-D stress analyses of cokes produced from caking coal and slightly caking coal were conducted in order to investigate the effect of the 3-D microstructure on coke strength. Furthermore, the strength anisotropy of coke was evaluated by stress analyses assuming a uniaxial tensile test in three directions. We found that the maximum principal stress magnitude and stress concentration area are smaller in the 3-D stress analysis than in the 2-D stress analysis. Moreover, the stress concentration area and strength anisotropy of coke produced from slightly caking coal are larger than those of coke produced from caking coal. The results indicate that stress analysis based on micro X-ray CT images is very useful for investigating the relationship between coke microstructure and strength.