Abstract
This research aimed to investigate the hot strength of coke prepared by briquetting and carbonization of lignite (Coke D) and mechanisms causing the strength, which have not been well understood. The Coke D samples underwent heating in a nitrogen atmosphere and up to 1000 °C. Three different gasification conditions were employed: inert condition (Inert), 6 min CO2 gasification time (React6), and 9 min CO2 gasification time (React9). The samples were imaged by three-dimensional X-ray computed tomography (CT), and their pore structure was evaluated. Coke D-Inert and Coke D-React6 exhibited decreasing porosity due to the expansion of coke matrix at elevated temperatures. Conversely, the porosity of Coke D-React9 increased owing to the significant disappearance of coke matrix by gasification, despite some expansion of coke matrix. Furthermore, material constants (i.e., Young's modulus) were predicted using the finite element method with coke models derived from the CT images. In the numerical simulation, fracture strains obtained from experiments in our previous work were applied as boundary conditions. The estimated Young’s modulus was 30.6 (maximum value 48.1; minimum value 14.9) on the outer side and 24.4 (max. 41.5; min. 12.4) on the inner side in Coke D-Inert, 16.6 (max. 21.1; min. 13.7) on the outer side and 17.1 (max. 20.4; min. 13.7) on the inner side in Coke D-React6, and 29.4 (max. 30.9; min. 26.5) on the outer side and 22.5 (max. 26.0; min. 19.6) on the inner side in Coke D-React9. Comparing Young's modulus of Coke D-React6 and Coke D-React9, Coke D-React9 showed a higher Young's modulus. These results suggested the significant impact of fracture strain on the predicted value of Young's modulus. The fracture strain was likely influenced by the effects of creep and matrix disappearance due to the reaction with CO2. Nano-indentation measurements of Coke D-Inert (average value 23.4; max. 29.4; min. 15.3) and Coke D-React9 (average 25.4; max. 28.3; min. 22.4) microstructures suggested no change in the material constants at room temperature but the potential for alterations at elevated temperatures.
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