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Numerical Investigation on Hot Metal Flow in Blast Furnace Hearth through CFD
Chen-En HUANG, Shan-Wen DU and Wen-Tung CHENG
(Department of Chemical Engineering, National Chung Hsing University, Taiwan.)
(May 26, 2008)
The behavior of hot metal flow in the hearth of a blast furnace is always considered as one of the key factors for determining blast furnace campaign life. To provide a useful insight into the hearth of No. 2 blast furnace at China Steel Corporation (CSC), a numerical model has been developed to analyze the flow and heat transfer under various cooling and operational conditions. The model solves three dimensional Navier–Stoke equations with conjugate heat transfer and Darcy’s law for hot metal flowing through the deadman with porous structure by computational fluid dynamics (CFD). The calculated results indicate that the circulatory hot metal is enhanced when the deadman becomes sitting with gutter coke-free space. As a result, the temperature at the hearth corner increases. This suggests the existence of gutter coke-free space may cause elephant foot type erosion. With drainage of hot metal, the heat flux of tap hole significantly increases.
For the sake of safety, it may be needed to individually monitor the cooling water temperature flowing through the copper staves, as well as to install thermocouples around the tap holes. The prediction shows that the heat flux of the hearth is insensitive to the temperature of cooling water before the refractories are eroded. It implies that the performance of the water chiller may be limited in the beginning of the
blast furnace campaign.
To prolong the life span of a blast furnace for maximizing its total hot metal production is one of the operation targets of steel works worldwide. Washed by high temperature molten iron and slag, the hearth becomes the decisive factor for the campaign life of the blast furnace. Notably, in response to the high demand in the steel market, most blast furnaces are being intensively operated for higher productivity.
This may cause premature hearth lining failures. To abate the damage to the hearth, it is necessary to gain an understanding of the hot metal flow and heat transfer in the hearth, as these strongly affect the erosion of a blast furnace hearth. Due to the hostile conditions in the hearth, only indirect measurements, such as the installation of thermocouples within the hearth lining, can be made. Instead, mathematical simulation models can provide a brief insight into hot flow patterns and temperature distributions of the hot metal in the hearth.
A two-dimensional simulation model has been developed by Yosihkawa and Szekely to predict the wear rate of a hearth lining. In the model, only the natural convection of the hot metal was considered. As a result, the wearing rate was accelerated by higher cooling rate of the hearth. This conclusion was different from the study published by Paschkis and Mirsepassi. By means of the calculated results, Tomita and Tanaka concluded that the solidified shell formed in the hearth lining surface could be a useful countermeasure to protect the hearth from high heat load. A similar suggestion has been made by Cheng. It is known that the hearth is mainly filled with coke lumps.
Molten iron and slag are collected in the empty spaces between the lumps, and tapped out through the tap holes at certain intervals. Therefore, the hot metal flow and heat transfer in the hearth are strongly influenced by the structure of the deadman. Shibata employed Darcy’s law for estimating the momentum loss of hot metal flowing through the coke bed. The calculated results showed that a strong peripheral flow was induced when the deadman became semi-sitting in shape, and heat load in the circular corner of hearth bottom increased significantly. From model experiments and numerical calculations, Shimizu proved that the size of the deadman coke and the volume of fine coke in the deadman strongly influenced the gas and liquid flow in the blast furnace. A three-dimensional model based on a computational fluid dynamics
(CFD) was developed by Panjkovic to analyze the temperature distribution in the hot metal and in the refractories.
The model was validated using the measured temperatures from BHP Steel No.5 blast furnace. As shown in the CFD based calculation results by Cheng the location of the peak value of shear stress coincided with the location of higher temperature measured within the hearth wall. It suggests that the refractory may be under threat of erosion when the measured temperatures nearby increase significantly.
After 12 years and 10 months in operation, No.2 blast furnace of China Steel Corporation (CSC) was blown down in October 2005 for revamping, and its 3rd campaign has begun since 26th January 2006. During the revamping, the furnace hearth was equipped with stave cooing instead of shell water spray cooling. This is the first time one of CSC’s blast furnaces has used stave cooling in the hearth. Since cooling and deadman type play essential roles in the heat transfer and hot metal flow in hearth, in this study, a three dimensional CFD based model has been established specially for No.2 blast furnace to estimate the hot metal behaviors in the hearth.
The model was validated using the measured refractory temperatures. A variety of operating and cooling parameters, composed of cooling water temperature and deadman type, as well as casting operation, were taken into consideration to account for their impacts on the hot metal flow pattern and heat flux in the hearth.
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