Solar-Thermal Splitting of Water Using a ZnO/Zn Thermochemical Cycle
The hydrolysis of zinc powders with water vapor to form hydrogen and zinc oxide was
studied by thermogravimetric analysis (TGA) and with an aerosol flow tube reactor. Two
zinc particle sizes, <10 µm and 158 nm, were used. Temperatures in the flow reactor
ranged from 380 oC to 540 oC, and water concentrations between 0.5 and 3% at ambient pressure were used. The highest conversion observed in the flow system was about 24% for the nano-sized powder at 540 oC and a gas residence time of ~0.5 s. Experiments with the TGA indicated that complete conversion of zinc to zinc oxide could be achieved for longer residence times. Non-isothermal TGA measurements were used to develop the
empirical kinetic rate law for the 158 nm zinc particles:
dá/dt = 2·106exp(-1.3·104/T) (1-α)2
where á is fractional conversion, t is time in minutes, and T is temperature in K. Reaction rates were higher in the aerosol flow reactor, likely due to lower mass and heat transfer resistances.
Key Words: water splitting, zinc powder, hydrogen, aerosol reaction, thermogravimetric
analysis, hydrolysis