Numerical Analysis of Raceway Formation and Combustion in a Blast Furnace With Pulverized Coal Injection
Blast furnace is a counter-current flow chemical reactor, where minerals such as iron ore are reduced to iron by coke combustion inside the furnace. The blast furnace is an essential and important equipment in the iron-steel making industry. More than 95% of iron oxides are reduced into hot metal through blast furnace process. Blast furnaces are very likely to coexist with the next-generation method in ironmaking industry due to its cost-effective and high-productive advantages. During the blast furnace process, iron oxides in forms of sinter or pellet and coke are alternatively charged layer by layer from the top of the furnace. While hot blast air with the temperature ranging from 1300K to 1500 K is introduced into the furnace through multiple blowpipes and tuyeres that encircle the lower part of the blast furnace. The blast air at high velocity creates a cavity zone in front of the tuyere, which is referred as raceway surrounded by loose coke bed. Iron oxides in the descending burden are reduced to iron by reducing gases generated by the coke combustion. A number of complex chemical reactions and heat transfer phenomena are involved in this process. In the past two decades, auxiliary fuels such as coal and natural gas are widely utilized in the blast furnace process to replace a certain amount of coke in order to minimize the total energy consumption and environmental impacts. Pulverized coal injection (PCI) is one of the technologies applied to reduce coke consumption, since non-coking coal is about 40% cheaper than metallurgy coke. In the practice, pulverized coal particles are injected into the blast furnace through an injection lance that inserted in the blowpipe, and combust in the raceway with hot blast air. Thus, a high PCI rate is expected to overcome the productivity barrier due to the reliance of coke. However, under the high PCI rate operation, the injected coal particles cannot burnout completely before exiting raceway region. As a result, the unburnt char may accumulate along the boundary of raceway. Such insufficient coal combustion can result in inappropriate distributions of gas flow and temperature, and it has a major impact on furnace productivity and stability. Therefore, it is necessary to understand the essential behavior and the combustion process at the lower part of the furnace regarding to the maximization of PCI rate. It is intended in the present study to numerically analyze the pulverized coal injection, raceway formation, and raceway combustion behaviors in the lower part of the blast furnace. A comprehensive multiphase reacting flow computational fluid dynamics (CFD) model has been developed. Further, a parametric study has been performed to study the effects of PCI rate, wind rate, coal composition, carrier gas type, and oxygen enrichment rate on raceway formation and combustion.
Zhou, Purdue University.
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