Coke graphitization and degradation across the tuyere regions in a blast furnace
Introduction
Coke is one of the most critical and probably the most expensive raw material used for steelmaking via the blast furnace route. Therefore, efficient utilization of coke must be considered as a major priority of iron producers. Consequently, coke replacement by supplementary fuel injection by means of pulverized coal specifically, but also oil, natural gas and some waste synthetic polymer materials, has become a major driver of cost reduction and up to 50% replacement has been successfully achieved. Coke plays multiple roles in a blast furnace by providing reducing gases, carburization source, permeable medium and also burden support. At high injection rates, the coke quality becomes more pertinent as less coke is available to fulfill these roles.
Ideally, coke is desired to gasify without any degradation during the descent from top to bottom in a working blast furnace. However, coke size decreases during the descent in a blast furnace and generates some fine material and the extent to which this occurs is related to its inherent strength. A high reactivity as measured for coke in the commonly employed Nippon Steel reactivity test, is frequently attributed to a high degree of degradation in a blast furnace [1], [2]. In this widely used bench-scale test, about 200 g of 19 mm −20 mm sized coke particles are reacted with 100% CO2 at 1100 °C for 2 h. The weight loss of coke after completion of the test provides the so-called Coke Reaction Index (CRI), and the percentage of coke particles retained on +10 mm sieve after tumbling for 600 revolutions in a standard drum configuration provides the measure of strength known as the Coke Strength after Reaction (CSR) value of coke. The CSR test conditions are considered by many as harsher than the actual blast furnace reaction environment [3], [4].
The current coke quality parameters are empirically based and as such are constrained by their inability to simulate some of the critical high temperature phenomena of a working blast furnace, such as graphitization and coke reactions that occur in the very active zone adjacent to the tuyere-level. Due to the complexity of blast furnace, such as large scale of operation, high temperatures and pressures, etc., it is often not possible to extract representative coke samples from operating blast furnaces. However, in recent times, a tuyere drilling technique has enabled technologists to effectively extract coke samples from an operating blast furnace thereby providing a source of potentially useful information about various important phenomena that occur [5], [6], [7], [8], [9]. Despite this innovation, understanding of the modification of coke behavior as it reaches across tuyere regions is still limited. Therefore, there is continuing interest to further clarify the impact of blast furnace conditions particularly those in tuyere-level regions on the transformation of coke properties. In this paper, tuyere core samples obtained from a medium size European blast furnace have been examined to characterize coke size degradation and graphitization behavior at various locations around tuyere-level. The implications of coke graphitization on the modification of reactivity and fines generation are also discussed.
Section snippets
Tuyere drilling
In this study, coke samples were obtained by tuyere drilling of blast furnace (#1) of Ruukki Oyj, Rahee. The drilling for this study was carried out on 7 June 2007 [10]. Blast Furnace configuration and operating data during the drilling period are summarized in Table 1. The hearth diameter and the effective volume of the blast furnace was 8 m and 1086 m3 respectively, and was equipped with 21 tuyeres for injecting preheated heavy distillation residue oil. In this campaign, a typical high
Coke size distribution across tuyere regions
Table 2 provides the proportion of different size particles in various tuyere level zones. Coke samples from D2 to D4 sections represent the raceway zone cokes. Section D5 and D6 provide the birds nest coke samples while the samples from sections D7 to D10 represent the deadman cokes. Fig. 4 shows the images of cokes particles from four core sections. Bright spots on the surface of few raceway coke particles indicate the solidified hot metal (Fig. 4b).
At tuyere level, the raceway and the birds
Conclusions
Tuyere core samples were examined to characterize the degree of graphitization and changes in size distribution of different types of coke particles in the tuyere-level regions of a working blast furnace. The following conclusions can be drawn from the results obtained and observations made.
- (1)
After descent to tuyere level, coke particles displayed more than a 50% size reduction such that about 60 wt.% of core in the raceway and birdsnest regions consisted of less than half the size of feed coke
Acknowledgements
This study was carried with the financial support from Australian Coal Association Research Program. Authors acknowledge the support provided by Rutaruukki Oyj staff for tuyere drilling, and Mr Allah Wasaya and Dr Byong-chul Kim for the XRD measurements. We also express our sincere thanks to Dr Dave Osborne, Mr Stephen Brant, Mr Sauli Kallio and the reviewers for their critical comments and insight.
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