Coke graphitization and degradation across the tuyere regions in a blast furnace

Highlights

• Coke graphitization can estimate the tuyere region temperatures.

• Coke reactivity correlated with graphitization degree and adsorbed potassium.

• At tuyere level, feed coke experienced more than fifty percent size reduction.

• Coke fines generation correlated with coke graphitization and anisotropy.

Abstract

Changes in particle size distribution and in the degree of graphitization of coke samples from various locations across the tuyere entrance of a blast furnace were examined. X-ray diffraction and a fixed bed reactor were used to measure carbon structure and reaction rate of the coke. The feed coke size was observed to decrease by more than 50% in the descent to the tuyere in the majority of the tuyere-level locations. About 60 wt.% of the core matter from the “bosh” and “raceway” locations consisted of less than half-size feed coke particles. The percentage of −3 mm and −0.45 mm size fractions observed in the recovered drill core material were less than 10 wt.% and 3 wt.% respectively particularly around the raceway locations. The graphitization degree of half-size coke particles is shown to be a suitable indicator of the temperature profile of tuyere-level regions of a blast furnace. The degree of graphitization of tuyere-level coke samples showed an inverse relationship with amount of potassium adsorbed as well as with the apparent reaction rate. The carbon structure as well as anisotropic texture of coke fines indicated higher ordering of carbon compared to the half-size cokes from the similar locations. The −0.45 mm size fraction of fines displayed a large proportion of graphite crystals. The study has confirmed the contribution of surface graphitization of cokes on the fines generation particularly in the “raceway” and “birds nest” regions. The study has implications for using the bench-scale assessment of coke performance and modeling of coke behavior to provide an understanding as to what is likely to occur in an industrial 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.