Determination of the Optimum Size of Skip Coke

Abstract

The unit consumption of coke in blast furnaces is decreasing significantly thanks to the implementation of various measures, including introduction of the practice of injecting pulverized-coal fuel (PCF). However, the decrease in the volume fraction of coke in the charge has appreciably increased the mechanical, thermal, and chemical loads on this material. For example, the introduction of PCF injection technology has increased the coke-based ore burden from 2.5‐3.5 to 4‐5 and lengthened the time these materials are in the furnace from 6‐8 to 9‐10 h. Moreover, the increase in the amount of time that coke is in the furnace is being accompanied by proportionate increases in the quantities of pig iron, slag, hearth gases, carbon dioxide, and alkalis that pass through it. This situation has quickly made it more urgent to improve the hot-strength index of coke while also improving its preparation and optimizing its fractional composition. The optimum fractional composition of the skip coke that is charged into a blast furnace has been a topic of discussion since the last century. In the course of attempting to optimize this characteristic of skip coke, a number of researchers have proposed using the fraction 25(30)‐60 mm to bring its coarseness closer to that of the ore-bearing charge materials and reduce the value of the ratio of the maximum size of the coke lumps to their minimum size to two or less. The reasoning behind these conclusions is that the “...best gas permeability is obtained during the initial part of the charging operation, when the finer fractions do not fill the interstices between the coarse fractions” [1]. Trial heats conducted with graded coke of the above-indicated fraction showed an improvement in the main technical-economic indices [2‐4]. Another group of researchers has recommended using coke of the 40‐80 mm fraction, especially to increase the smelting rate and the quantity of liquid smelting products obtained in large blast furnaces. To illustrate, the content of the 40‐25 mm fraction of skip coke rose to 25% at the Cherepovets Metallurgical Combine when the efficiency of its coke screens declined. This development seriously complicated the operation of the combine’s 2700-m 3 blast furnace [5].