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Research Papers

# Splitting Water and Carbon Dioxide via the Heterogeneous Oxidation of Zinc Vapor: Thermodynamic Considerations

[+] Author and Article Information
Luke J. Venstrom1

Department of Mechanical Engineering, University of Minnesota—Twin Cities, 111 Church Street Southeast, Minneapolis, MN 55455lvenstro@me.umn.edu

Jane H. Davidson

Department of Mechanical Engineering, University of Minnesota—Twin Cities, 111 Church Street Southeast, Minneapolis, MN 55455lvenstro@me.umn.edu

1

Corresponding author.

J. Sol. Energy Eng 133(1), 011017 (Feb 14, 2011) (8 pages) doi:10.1115/1.4003417 History: Received April 06, 2010; Revised December 21, 2010; Published February 14, 2011; Online February 14, 2011

## Abstract

The heterogeneous oxidation of zinc vapor is proposed as a promising reaction path for the exothermic step in the two-step Zn/ZnO solar thermochemical water and carbon dioxide splitting cycles. This approach circumvents mass transfer limitations encountered in the oxidation of solid or liquid zinc, promising rapid hydrogen and carbon monoxide production rates concurrent with a complete conversion of zinc to zinc oxide. In this paper, a parametric thermodynamic analysis is presented to quantify the benefit of achieving a rapid and complete conversion of zinc via the heterogeneous oxidation of zinc vapor. The conversion of zinc in polydisperse aerosol reactors has been limited to 20% for reaction times on the order of a minute, resulting in a cycle efficiency of $∼6%$. The benefit of completely converting zinc via the heterogeneous oxidation of zinc vapor is an increase in efficiency to 27% and 31% for water and carbon dioxide splitting, respectively. The cycle efficiency could be higher if heat recuperation is implemented.

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## Figures

Figure 1

Change in the enthalpy and Gibb’s free energy for the oxidation of zinc by (a) H2O and (b) CO2 with reacting species in their most stable physical states at 1 atm

Figure 2

Comparison of measured heterogeneous zinc vapor hydrolysis rates where H2O:Zn ratios varied between 10 and 60 with the empirical expression of Clarke and Fray (25) where H2O:Zn ratios were unity

Figure 3

Material flow (black arrows) and energy flow (gray arrows) in the two-step Zn/ZnO solar thermochemical cycle for water or carbon dioxide splitting. All material flows are assumed to be at 1 atm, and pumping work is neglected.

Figure 4

Impact of the oxidation reaction temperature on cycle efficiency for (a) water and (b) carbon dioxide splitting. Tsolar=2300 K, C=10,000, and α=β=1.

Figure 5

Temperature of ZnO flow and Zn+0.5O2 flows as they exchange energy with one another in a heat exchanger without heat transfer limitations in the case where there is no recombination of zinc and oxygen (β=1)

Figure 6

Temperature of the Zn/H2O (solid line) and Zn/CO2 (dashed line) reactant streams as energy is added to the streams in the case of complete conversion of zinc (α=1). At an oxidation temperature of 1185 K, 221 kJ is released when H2O is split and 188 kJ is released when CO2 is split.

Figure 7

Impact of zinc conversion on cycle efficiency for three oxidation temperatures and three zinc yields from the solar reactor quench in the (a) water and (b) carbon dioxide splitting cycles. Tsolar=2300 K, C=10,000, and no heat recuperation.

## Errata

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