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Characterising the Degradation of Cokes Made from Australian Coals and Subjected to Simulated Blast Furnace Operating Conditions

Technical Market Support » Metallurgical Coal

Published: August 19Project Number: C26043

Get ReportAuthor: Xing Xing, Oleg Ostrovski, Paul Zulli, Harold Rogers | University of New South Wales, University of Wollongong, University of Newcastle

Stage 1

The key objective of the first stage 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 showed 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).

Stage 2

The key objective of the second stage of this project was to further understand the degradation of cokes from coal blends under the blast furnace (BF) conditions and further develop methods for the characterisation of cokes processed under simulated BF conditions.

The specific methodologies included gasification of coke in the BF gas atmosphere (CO-CO2-N2-H2-H2O) from 900 to 1400 °C (corresponding to the conditions from the thermal reserve zone to the cohesive zone) and annealing of coke at 1800 and 2000 °C in the N2-H2-CO atmosphere (corresponding to the conditions within the raceway and the interval between the raceway and the underside of cohesive zone). The treated cokes were characterised using specific materials characterisation techniques to determine the change of coke properties in both micro- and macro-scales. These methodologies were applied to the cokes made from single coals in the first stage of this project, the comparison between cokes gasified in the different gas atmospheres provided an opportunity to understand the effect of gasification atmosphere on the coke performance. These methodologies were also applied to the pilot oven cokes produced from binary/trinary blends of these singles coals. The comparison of the weighted average values of single coal cokes and the measured values of the cokes from blends enhanced the understanding of the effect of coal blending on the coke performance under the simulated BF conditions. A pair of cokes produced in the pilot coke oven and commercial coke battery with same commercial blend were comparatively studied using these methodologies to verify the reproductivity of the pilot oven to the commercial coke battery.

Compared to the gasification conducted in the CO-CO2-N2-H2 atmosphere, the addition of H2O vapor in the current work significantly increased the coke reactivity and caused more remarkable changes in coke pore structure on the lump surface. As a result, the I-drum tumbling strength of cokes gasified in the H2O containing atmosphere showed more evident degradation than those gasified without H2O vapour. However, the extra degradation caused by involving of H2O did not penetrate to the lump core; therefore, the tensile strength measured in the lump core did not show extra degradation in the H2O containing atmosphere.

Blending coals together promoted the capacity of produced coke to develop greater graphitisation upon high temperatures in the BF; such effect was mainly contributed by the greater graphitisation development of the lenticular and ribbon microtextures. Blending coals together also resulted in a reduction of coke reactivity in both simulated BF conditions and standard CSR/CRI test conditions. Correspondingly, the measured macrostrength after BF treatments on both lump surface and lump core were enhanced by the coal blending; similarly, the coke hot strength (CSR) was also improved by blending coal together. The enhanced coke macrostrength was achieved by the reduced porosity due to the coal interactions during carbonisation.

The properties of coke produced in the 400 kg pilot coke oven were slightly different from the same blend coke produced in the industrial coke battery due to the different coking conditions. After gasification and annealing simulating the BF conditions, however, these two cokes showed comparable properties in the terms of graphitisation degree, micro- and macro-strength. The comparison of cokes produced from different coking facilities indicated that the pilot oven could produce coke with comparable properties of those from commercial coke battery.

Coke macrostrength degradation in the BF was caused by the degradation of coke microstrength and the development of coke pore structure during gasification and annealing. The degradation of coke microstrength under the BF conditions was caused by the graphitisation accompanied by the change in its crystallite microstructure. The different behaviours of coke microtextures were attributed to their different propensities for graphitisation upon the high temperatures. Gasification in the BF caused significant development in the pore structure on the lump surface with consumption of both IMDC and RMDC microtextural groups; however, such degradation did not penetrate to the lump core. Therefore, significant macrostrength degradation was only observed on the coke surface. The high temperatures during simulated BF annealing modified the coke pore structure and caused significant microstrength degradation across the entire coke lump; therefore, significant degradation on both lump surface and lump core occurred after annealing.

This study has shown that the simulated BF gasification and annealing had strong effects on both coke micro- and macro-properties. Besides the gasification and annealing to which coke was exposed, the interactions between gasified coke and molten liquids (hot metal and slag) in the regions within and below the BF cohesive zone also could cause significant degradation e.g. coke size reduction and fines generation. In the regions below the cohesive zone, coke continues to physically support the entire furnace burden above. The other functions of cokes in this region include carburisation of hot metal and reduction of oxides in the primary slag. Previous studies concerning coke-hot metal interactions mainly focused on the carbon dissolution from feed cokes. However, as reported in our current study, the simulated BF gasification etc. caused remarkable changes in the properties of feed coke in the terms of pore structure, mineralogy, petrography and morphology. These can significantly affect the interactions between coke and hot metal, thereby influencing the performance of coke in the lower regions of BF. Moreover, the specific impact of “dissolution” on the quality of gasified coke has not been systematically investigated.

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