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I.

INTRODUCTION

O N E of the most important process parameters in steelmaking processes is the postcombustion ratio (PCR) obtained in the furnace. The postcombustion ratio is defined as PCR =

pct CO2 + pct H20

[ 1]

pct CO + pct C02 + pct H 2 + pct H20 Higher postcombustion ratios imply that more heat is generated in the vessel; this heat is necessary to heat scrap and carry out the steelmaking reactions. Unfortunately, not all the heat generated by the postcombustion reactions is actually transferred to the bath; thus, a heat-transfer efficiency (HTE) term is required. The conventionally defined HTE is HTE =

excess heat in off-gas 1 - hea---tavailab-~lefro----mP-C/ x 100 pct

[2]

With this definition, the HTE will be 100 pct if the gas leaves the vessel at the same temperature as the bath. However, as will be discussed in Section V, this definition is not a very suitable indicator of the heat actually recovered by the bath. In recent years, there has been a trend in conventional oxygen steelmaking to increase the degree of postcombustion in the furnace gases and to efficiently transfer the heat back to the bath. The primary purpose is to increase the amount of scrap that can be melted. There have been a number of different processes developed with one or more of the following features: (a) special lances with secondary oxygen ports; H. GOU, Research Associate, G.A. IRONS, Professor and Chairman, and W.-K. LU, Professor, are with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7. Manuscript submitted April 1, 1992. METALLURGICAL TRANSACTIONS B

(b) separate injection ports from the side of the vessel; and (c) special soft-blowing practices. Generally, it has been found that as the PCR is increased, the HTE falls; tl,21 thus, the net amount of heat transferred to the bath is limited. Additionally, at high PCRs, the off-gas may be so hot that it damages the refractory in the vessel cone or the off-gas system. It is apparent that there is a complex interplay between fluid flow, heat transfer, and combustion in such systems. Most of the achievements in postcombustion have been reached through plant trials, on a more or less empirical basis. Noting the apparent limitations mentioned earlier, there is some need to develop mathematical models to determine if, indeed, these are fundamental limits, ff they are not, modeling may aid in optimization of process parameters. There have been a very few detailed studies on postcombustion and heat-transfer mechanisms in steelmaking processes reported in the literature. Huin et al. measured oxygen jet composition and postcombustion degree in a 6 tonne converter, t31 Based on this information, they were able to develop an integrated jet model to calculate the composition and temperature of the jet and the PCR in the vessel, t41 By using results from a 250 tonne converter, an O2-CO combustion furnace, and a 10 tonne ladle, Hirai e t a l . I'l extracted some empirical relations between the operating conditions and the PCRs. Kat