In the coal gas DRI process, carbon deposition can be fully avoided by using a minute quantity of H_2S in the gas phase. The maximum quantity of H_2S for low volatile coal gas at about 550 deg C when the rate of soot formation is maximum is 0.5 percent without influencing the reduction rate. The rate of carbon deposition follows a binomial pattern with respect to all parameters influencing the carbon deposition, such as T, a_c and P_(H2S), with maxima of about 550 deg C. For low volatile coal gas with zero post combustion, the rate values are zero at 400 and 900 deg C. The rate of carbon deposition follows an equation of the type shown in Equation (5) according to Arrhenius' law. For temperatures above or below about 550 deg C, the reaction rate is dominated by thermodynamics and kinetics, respectively. the powerful effect of H_2S on suppressing soot on a DRI surface is entirely due to limitations introduced by S on kinetics of soot formation at high and low temperatures. for a specific temperature and gas mixture, there is a minimum P_(H2S) corresponding to complete elimination of carbon (P_(H2S)~0). For any specific gas composition and temperature as long as P_(H2S) < P_(H2S)~0, catastrophic swelling and whisker growth are expected with severe degradation of the ore.
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