Theoretical insight into the competitive effect of CO2 and additive H2O in coke gasification

Abstract

To reduce CO2 emissions, hydrogen-rich fuels are widely used in ironmaking, which significantly increases H2O amounts in blast furnace. Recent works have mostly focused on the coke gasification with CO2 or H2O alone, but lacked the interaction between two gases and the microscopic mechanism. This work combines particulate coke reaction experiments and theoretical calculations to explore the competitive effects of CO2 and additive H2O during coke gasification from a microscopic perspective. Experimental results indicate that the coke gasification rate with H2O is dramatically higher than that with CO2, and that H2O inhibits the reaction between CO2 and coke. Both the orderly stacking of carbon layers and the proportion of graphitic carbon are more obviously increased upon the addition of H2O. Reactive force fields simulations reproduce the experimental phenomena and provide the reaction network from elementary reactions·H2O molecules react with carbon layers in the form of OH radicals. Based on density functional theory, OH-oxidation reactions at all reactive sites of the carbon layer (wrinkled sites, edges, and defects) are confirmed to have relatively low activation energies, and some are even barrierless processes. The oxidation rate by OH radical is much higher than that by CO2, thus significantly inhibiting the oxidation by CO2. In contrast, OH radicals are also inactivated by CO2 in gas phase. Furthermore, atoms at edges and defects of carbon layers are hydrogenated by H2O, promoting the reconstruction and orderly stacking of carbon layers. This work explains the coke gasification mechanism in a mixed CO2-H2O atmosphere and competitive effects between CO2 and the additive H2O in blast furnace, and would provide theoretical support for determination and optimization of hydrogen-rich blast furnace technology.