A 45 kW concentrating solar furnace was used to study the thermal decomposition of ZnO at a nominal nitrogen pressure of 1 bar within the temperature range of 1950-2400 K. Flash assisted multi-wavelength pyrometry was used to establish both the hemispherical emissivity of the ZnO and its irradiated surface temperature for a given solar flux. We found that the decomposition rate is described well by the equation, m = 1.4 × 109exp(-328, 500/8.314 T) g m-2 s-1. The uncertainty in the equation depends on temperature, but for temperatures near 2000 K it is ±70% at a 95% confidence interval. The emissivity is 0.9 for temperatures above 1900 K. Furthermore, a one-dimensional unsteady and steady-state heat transfer model that includes the physical processes radiation, conduction, and chemical decomposition was developed using the above expression for the reaction rate. The model predicts the measured steady-state ZnO surface temperatures and the time to reach steady state. For an average solar flux of 170 W cm-2 and after 400 s of transient conditions, the calculated temperature profiles within the ZnO solid as a function of time are within 100 K of measured profiles.
- Decomposition kinetics
- Solar thermal decomposition