(378f) Thermodynamic Studies of a Novel Water Splitting Thermochemical Cycle

Authors: 
Olmos, F., UCLA
Manousiouthakis, V., University of California Los Angeles, Los Angeles

A theoretical study of the constitutive reactions from a novel water splitting thermochemical cycle is presented, which aims to find their  respective operating conditions as part of the cycle. The cycle is categorized as a three-step cycle that is based on the thermal decomposition of sodium carbonate Na2CO3 at temperatures higher than 1131.25 K (melting point of sodium carbonate). The chemical species involved in the cycle are: water H2O, oxygen O2, hydrogen H2, sodium Na, sodium carbonate Na2CO3, sodium hydroxide NaOH, and carbon dioxide CO2. The chemical reactions of the cycle are:

1) 2Na2CO3 ↔ 4Na + 2CO2 + O2

2) 4Na + 4H2O ↔ 4NaOH + 2H2

3) 4NaOH + 2CO2 ↔ 2Na2CO3 + 2H2O

with lead to the overall water decomposition reaction:

4) 2H2O ↔ 2H2 + O2

Reaction 1) is endothermic, and thus, heat needs to be provided in order for the decomposition of sodium carbonate to be carried out, typically at temperatures higher than its melting point. Reactions 2) and 3) are exothermic.

In this work, each individual reaction involved in the cycle is analyzed thermodynamically in order to find the temperature, pressure, sweeping gases, and flowrate/dilution conditions ranges for which each reaction can be carried out. This is done through the Gibbs minimization method which allows to find condition ranges due to its flexibility on changing parameters. This work provides fundamental information regarding the cycle's reactions species at high temperature and low pressure, not currently available in the literature. Additionally, the results from this work pave the way for the design of potential flowsheets that can be subjected to optimization studies and even coupled with a Rankine power cycle in order to co-generate electricity.