Heterogeneous oxidation of zinc vapor by steam and mixtures of steam and carbon dioxide

The kinetics of the heterogeneous oxidation of zinc vapor by water vapor were measured in a tube flow reactor for temperatures from 800 to 1100 K, zinc vapor partial pressures up to 0.39 atm, and water vapor partial pressures up to 1.0 atm. The results extend the prior data for oxidation of zinc by water vapor from zinc partial pressures on the order of 0.01 atm to higher values appropriate for fuel production via the Zn/ZnO thermochemical cycle. Measured oxidation rates span 10−7-10−5 mol cm−2 s−1. A second order, reversible reaction rate expression r″=kZn-H2OpZn(g)pH2O-pH2Keq is developed from regression of the data and a numerical model of advective and diffusive mass transfer. The kinetic parameter kZn-H2O is a non-monotonic function of temperature with a negative activation energy for temperatures between 800 and 1050 K, consistent with prior studies. In a second set of experiments, the rate of the heterogeneous oxidation of zinc vapor by mixtures of water vapor and carbon dioxide was measured. The product gas is hydrogen rich due to faster surface reaction kinetics for oxidation with water vapor than with carbon dioxide. We conclude that it is preferable to split water and carbon dioxide in separate reactors rather than co-produce H2 and CO in a single reactor for production of synthesis gas in the Zn/ZnO solar thermochemical redox cycle.