Carbon structure of coke at high temperatures and its influence on coke fines in blast furnace dust
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. INTRODUCTION
GREENHOUSE gas emissions are the biggest challenge for the steel industry worldwide, particularly for the sustainability of ironmaking via the blast furnace (BF) route. Even though several new technologies for steelmaking are emerging, the BF will continue to be the most dominant process for ironmaking from ore.[1,2] Increasing environmental concerns associated with cokemaking are influencing the economy of the BF route for steelmaking. In order to remain competitive, the steel industry is looking for opportunities to reduce the reliance on coke consumption in a BF. Pulverized coal injection (PCI) is the most accepted and widely used strategy to replace coke and hence improve the economic and environmental efficiency of the BF process. For example, the United States steel industry is expecting significant reduction in coke rates by targeting a coke rate consumption of 250 kg/t-HM, over the next 15 years.[3] Coke performs many roles in a BF, e.g., a thermal role to provide heat energy, a chemical role to act as a reducing agent, and a mechanical role to maintain the permeability for upward flowing gases. At lower coke rates, the thickness of the coke layer in the stack and cohesive zone decreases, while higher coal injection rates lead to increased residence SUSHIL GUPTA, Research Associate, and VEENA SAHAJWALLA, Professor, are with the School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia. Contact e-mail: [email protected] PINAKIN CHAUBAL, Director of Technology, is with Research & Development, Ispat Inland Inc., East Chicago, IN 46312. TED YOUMANS, Manager, is with Operations Iron Producing, International Steel Group Inc., Sparrows Point, MD 21219. Manuscript submitted June 9, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
time of coke in the lower part of a BF.[4] Coal injection primarily supplements the thermal and the chemical role of coke. In addition to reactivity, mechanical properties of coke such as high-temperature strength becomes increasingly important, as less coke is available to perform the necessary mechanical and chemical roles in a BF. Coke quality at high PCI rates has been extensively investigated in the past.[1,5–8] At high PCI rates, coke is believed to display an accelerated degree of degradation and fines generation.[8,9] Often, due to inferior coke quality, excessive coke fines generation and accumulation occurs in different parts of a BF, causing many operational problems, e.g., reduced permeability, undesirable gas and temperature distribution, and the possible hanging of the burden.[8] Many factors influence coke degradation, e.g., the mechanical stress, the solution loss reaction, the alkaline attack, the thermal gradient, and the shock impact by high-speed blast.[9] Fines generation is a complex phenomenon as several mechanisms are responsible for coke fines generation in different parts of a BF. While most of the coke fines are consumed in the lower part of a BF, a small proportion of coke fines are disc
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