Thermodynamics of CeO₂ Thermochemical Fuel Production

Abstract

In this work the thermodynamics of thermochemical fuel production using a CeO 2 redox cycle are studied. The need to reduce the oxygen partial pressure in order to improve efficiency is investigated, with both sweep gas and vacuum pumping considered as methods of achieving this. At ambient pressure the cycles can be maximized with respect to the temperature swing, the minimum oxygen partial pressure, and the extent of the oxidation reaction. For reduction at 1500°C the maximum efficiency was found to be 4.5%, which is significantly lower than the values found in previous studies. In addition isothermal operation had very low efficiency (less than 2%) under all of the conditions considered. If the system is operated at lower than ambient pressure, the pumping efficiency will depend on the pressure. From an investigation of commercially available pumps the pressure dependence was given an analytical expression. The results showed the cycles have an optimal operating pressure and that using sweep gas, as well as pumping, only reduced the overall efficiency. The efficiency was maximized with respect to the temperature swing, the reduction pressure, and the extent of oxidation, giving a peak efficiency of 7.5% for a reduction temperature of 1500°C. Reducing the pressure during reduction could also be beneficial due to improved reaction kinetics at lower pressure and an increased yield due to lower oxygen partial pressures. Recovering heat from both the high temperature ceria and the oxidation reaction, and using it as process heat, was also considered. With 60% of this heat being recovered, the peak efficiency for the 1500°C pumped cycle increased to 11%. Finally the practicality of the cycles, in terms of the quantity of ceria required to maintain continuous operation, are considered, and some suggestions for improving the cycle are given.

Annual Performance of a Solar-Thermochemical Hydrogen Production Plant Based on CeO2 Redox Cycle

“…This result is in line with previous works (Bader et al, 2013;Ermanoski, 2015;Ermanoski et al, 2013;Bulfin et al, 2015). Figure 4 shows the simulation details of 5 days (from 28 August to 1 September).…”

“…Furthermore, it is assumed that steam lines are pre-heated up to 200°C in order to prevent condensation. Finally, a solid-solid heat exchanger is placed between both reactors (receiver and oxidiser) to recover the sensible heat of CeO 2 particles exiting the receiver as proposed in previous works (Bader et al, 2013;Ermanoski, 2015;Ermanoski et al, 2013;Bulfin et al, 2015).…”

“…In addition, the medium includes a function for knowing the minimum required amount of water to achieve the oxidation based on the water equilibrium and heat of reaction explicit in δ (Bulfin et al, 2015).…”

“…The thermochemical efficiency was largely explored in previous works (Bader et al, 2013;Ermanoski, 2015;Ermanoski et al, 2013;Bulfin et al, 2015). In this context, a maximum of 68% could be obtained if all the CeO 2 is reduced to Ce 2 O 3 at 2200°C with the sun as only heat source.…”

Principles of doping ceria for the solar thermochemical redox splitting of H2O and CO2

“…However, ceria requires high temperatures (>1400 C) and low oxygen partial pressures (<10 À4 bar) in order to obtain a reasonable reduction extent (d). 3,[19][20][21][22] Because of this, the specic H 2 yield per unit mass of ceria is limited.…”