Technical Market Support » Metallurgical Coal
The key objective of this project was to develop a bench-scale methodology for the evaluation of coke quality under the blast furnace (BF) ironmaking conditions. To achieve this objective, BF conditions were simulated by the gasification of coke with typical BF gas-temperature (CO2-CO-H2-N2) profiles up to 1400 °C and annealing of the gasified coke at 2000 °C in N2 atmosphere. The degradation of coke subjected to simulated BF conditions was characterised using specific material characterisation techniques. Six cokes made from coals over a range of coal ranks were studied in this project, five of them had similar CSR value. The cokes were subjected to BF-like gasification and annealing conditions to understand the impact of parent coal rank on the behaviours of coke and mechanism of coke degradation under the simulating conditions within BF.
The cokes showed significant differences in the microtexture of their fused components. The reactive maceral derived components (RMDC) of coke produced from low rank coal was dominated by the fine mosaic with minor very fine mosaics and rare statistical isotropic microtextural types; the fused components in the coke made from high rank coal were predominately coarse mosaic and foliate microtextures. Gasification with subsequent annealing at 2000 °C caused significant reduction in the reflectance of both types of RMDC microtexture. However, the reflectance of inert maceral derived components (IMDC) was almost unchanged. The textural variations in the prominent RMDC components in each coke were different. Conspicuous pock marks presented on the RMDC of coke from low rank coal after annealing at high temperature; while, the delamination took place on the foliate formed from high rank coal and left sinuous voids between the lamellae of foliate. Upon the high temperature annealing, the promotion of graphitisation (Lc) of the coke made from low rank was much lower than that of coke produced from high rank coal; correspondingly, its microstructure was less changed from the initial cross-linked structure. The degradation of its microstrength was also less than that which occurred in the coke from the parent coal with high rank.
The porosity of all cokes experienced significant enlargement after gasification and annealing under the BF-like conditions; the development of coke porosity is attributed to the Boudouard reaction, which takes place in the high temperature thermal reserve zone and cohesive zone, and the mineral reactions of coke experiences in the high temperature region of BF. The impact of carbon-mineral reactions and further devolatilization of coke upon high temperature on its porosity enlargement needs to be further detailed studied.
Due to the different behaviours in the coke porosity development and microstrength degradation upon the treatments under the simulated BF gas and thermal conditions, the cokes made from different parent coals showed significant differences in their macrostrength after gasification with subsequent annealing at 2000 °C; this, despite five of them having similar performance in the standard NSC test. In this study, the cokes produced from higher rank coals had more severe degradation upon simulated BF conditions. The correlation between coke macrostrength, microstrength and porosity indicated that the degradation of coke macrostrength was attributed to the deterioration of the coke microstrength and the development of coke porosity caused by the gasification and high temperature annealing experienced by the cokes. The porosity development of cokes in this study was limited in a small range; the greater strength degradation of cokes made from higher rank coals was attributed to the more significant degradation in their microstrength upon BF-like gasification and annealing.
This study has shown a strong relationship between coke performance under the simulated BF conditions and the properties of its parent coal, and the performance of coke closely relates to its response toward the high temperature annealing (2000°C).