Infiltration of carbon in pores within coke and charcoal by methane cracking
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INTRODUCTION
IN the blast furnace for iron making, coke has the role of providing bed permeability to gas as well as serving as a reducing agent and energy source. Therefore, the coke must have the strength to sustain the weight of the burden. However, coke strength is decreased by the reaction C + C O 2 ~-- 2CO. t~l This reaction is desired because it produces CO which reduces Fe304 and FeO. However, it results in degradation of coke due to oxidation. This degradation leads to fine coke production and this fine coke hinders the passage of gas through the burden. This is an undesirable situation for the operation of the blast furnace. The traditional method of avoiding this problem has been to use high grade coke resistant to oxidation by CO2. However, stocks of high grade coking coal are more limited and more expensive than ordinary coal. The purpose of the present investigation is to find an alternative solution to this problem, one which does not require high grade coking c o a l - - t h a t is, a way to convert low grade coke to a higher grade by enhancing its resistance to oxidation with CO2. The oxidation with CO2 mainly occurs in the lower part of the shaft of the blast furnace where the temperature is 1173 to 1773 K. t21 The rate limitation of oxidation (for normal sizes of coke) in the blast furnace is mixed control by chemical reaction and gas diffusion through pores within the coke. ~31In this temperature regime, oxidation occurs on the internal pore surface. The small pores (30 n m < r < 0.3/xm) participate in oxidation to a great extent due to their larger (by a hundred times or more) surface area compared to that of large pores (r > 10 /xm). Consequently, there is a possibility of reducing the oxidation rate if these small pores can be infiltrated by some means. Methane cracking was selected for testing as one means of infiltration. The methane diffuses into small pores by Knudsen diffusion because the mean free path of C H 4 is 0.3 /zm at 1273 K and atmospheric pressure. It decomposes into carbon and hydrogen, and the carbon produced is expected to deposit in the small pores if appropriate conditions are chosen. Y. SHIGENO, formerly Visiting Scholar, Department of Materials Science and Mineral Engineering, University of California, is Research Associate, Research Institute of Mineral Dressing and Metallurgy, Tohoku University, Sendai, Japan. J.W. EVANS, Professor of Metallurgy, is with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA, 94720. Manuscript submitted September 30, 1991. METALLURGICAL TRANSACTIONS B
A few trials for modification of coke to raise its resistance to oxidation by CO2 have previously been made. Ogawa et al.fa] examined the infiltration of carbon in pores within metallurgical coke by thermal cracking of tar. However, much soot evolved, preventing application of this technique to industrial use. This method may be categorized as chemical vapor infiltration (CVI), first proposed by Pfeifer et al.tS3 Those investigators inten
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