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Experimental results of reactivity measurements on coal and chars to steam, hydrogen and steamhydrogen mixtures are compared and their application to technical processes is discussed. The change of surface area as a function of burn-off has a minor significance for chars made from coal with high initial accessible surface areas. In this case the reaction rate under constant steam pressure is first order with respect to the amount of carbon being gasified. The reaction rate of hydrogasification markedly decreases with burn-off, since the activation energy increases with burn-off due to the consumption of reactive carbon during the reaction. The reaction of carbon with steam hydrogen mixtures can be described as a superposition of carbon steam reaction and hydrogasification. The overall rate relative to that with hydrogen or steam alone can increase if a high amount of reactive carbon is present and immediately reacts with hydrogen. It can decrease if the inhibition of the steam carbon reaction by hydrogen predominates. Some consequences for the technical performance of coal gasification are: The overall reaction rate in a fluid bed decreases with increasing bed height, since hydrogen-steam ratios and therefore the inhibition by hydrogen increase. The overall reaction rate in a moving bed with countercurrent flow of carbon and reacting gases increases with increasing overall pressure since highly reactive carbon, formed in the pyrolysis zone, can immediately react with high partial pressures of hydrogen. |
