Quantification of the Existence Ratio of Non-Adhesion Grain Boundaries and Factors Governing the Strength of Coke Containing Low-Quality Coal

Abstract

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.

Development of the Coke Model with the Non-adhesion Grain Boundary and Its Fracture Analysis

“…The model predicts that the coke strength decreases linearly with an increase in the blending ratio of the lowquality coal. The coke strength measured by Kanai et al 6) also shows the same pattern as the one calculated in this study. Figure 10 shows the calculated fracture strength together with the experimental one.…”

“…However, the coke model had only one nonadhesion grain boundary, and the position of a non-adhesion grain boundary was fixed. The investigation in consideration of existence ratio of the non-adhesion grain boundaries such as Kanai et al 6) were not conducted. 7) Saito et al 8) performed fracture analysis for a model coke assuming a diametral compression test, and the springs of the nonadhesion grain boundaries with low fracture strength and low elastic modulus were randomly-placed.…”

Image Recognition Method for Defect on Coke with Low-quality Coal

“…Boundaries In the previous study, 8) it has been known that there are the pores, fractured coke matrices and non-adhesion grain boundaries on the fracture surface of coke from low-quality coal. Hayashizaki et al performed an image analysis for the polished surface of coke and the pores were extracted according to the difference in brightness because the pore had depth and its brightness was dark.…”

“…For the reasons mentioned above, nonadhesion grain boundaries would decrease the coke strength, and the necessity of quantification of non-adhesion grain boundaries has grown. Kanai et al 8) observed the fracture cross-section of coke by a diametral-compression test and extracted non-adhesion grain boundaries by visual assessment and then investigated the correlation between coke strength and non-adhesion grain boundaries. However, the marking method requires a huge effort, and the result of the method would be dependent on an observer.…”

“…8) Coke from caking coal blended with low-quality coal is fractured by a diametral-compression test, and the fracture surfaces are observed by a scanning electron microscopy (SEM) and a 3D laser scanning microscope (LSM). For SEM images, the marking method is used to extract non-adhesion grain boundaries with visual examination.…”