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Abstract
Coke performance in an operating blast furnace is often empirically related to popular bench-scale tests, which are performed at relative much lower temperatures. Due to difficulties of sampling, there is a limited understanding of the tuyere-level coke characteristics. An experimental study was performed to characterise the coke properties at tuyere level of a blast furnace as a function of different levels of supplementary oil injection. Coke samples were obtained through tuyere drilling and analysed using a range of analytical tools including XRD, SEM and high temperature reactors with emphasis on mineralogy, carbon structure and reactivity.
Carbon structure was found to be a suitable indicator to assess tuyere-level temperature. Quartz and mullite were found to be significantly reduced in tuyere-level cokes while SiC and ferrosilicon alloys were the most notable Si bearing minerals formed. Recirculating alkalis were found to have most significant impact on coke apparent reactivity. Graphitization was found to contribute in -0.45 mm coke fines generation.
The injection rate was found to have most distinct effect on the coke samples from the bosh and raceway regions. High injection rate cokes were distinguished by a greater degree of graphitization, less amount of SiC and complete absence of mullite phase. Ferrosilicon phases were not influenced. The apparent reaction rates of high injection rate cokes was shown to be marginally lower, which is attributed to higher degree of graphitization and less amount of adsorbed potassium. The study suggested that the effect of injection rate on the modification of coke properties is mainly attributed to changes in temperature profile of tuyere-level regions.
The ferrosilicon alloys were found to catalyse coke graphitization intensity. The hot metal was shown to have a stronger effect on the ferrosilicon formation compare to in situ formation. Quartz particle size does not seem to be a critical factor in SiC formation in tuyere-level cokes due to such long reaction time at high temperatures. Both SiC and ferrosilicon phases did not increase the coke reactivity. The study established that conventional means of coke testing may not have any significant bearing on the coke properties as it descends into tuyere-level regions.