Solar Thermochemical Conversion of Carbon Dioxide into Fuel via MnFe2O4 based Two-Step Redox Cycle

efficiency analysis of based CO 2 splitting (CDS) cycle is reported. HSC Chemistry software is used for performing the calculations allied with the model developed. By 45 maintaining the reduction nonstoichiometry equal to 0.1, variations in the thermal energy required to drive the cycle ( 𝑄̇ 𝑇𝐶 ) and solar-to-fuel energy conversion efficiency ( 𝜂 𝑠𝑜𝑙𝑎𝑟−𝑡𝑜−𝑓𝑢𝑒𝑙 as a function of the ratio of the molar flow rate of inert sweep gas ( 𝑛̇ 𝑖𝑛𝑒𝑟𝑡 ) to the molar flow rate 2 𝑂 4 ( 𝑛̇ 𝑀𝑛𝐹 ), i.e., 𝑛̇ 𝑖𝑛𝑒𝑟𝑡 𝑀𝑛𝐹 , reduction temperature ( 𝑇 𝑟𝑒𝑑 ), and gas-to-gas heat recovery effectiveness ( 𝜀 𝑔𝑔 ) are studied. The rise in 𝑛̇ 𝑖𝑛𝑒𝑟𝑡 𝑛̇ 𝑀𝑛𝐹 ⁄ is responsible for the decrease in 𝑇 𝑟𝑒𝑑 . At the maximum 𝜂 𝑠𝑜𝑙𝑎𝑟−𝑡𝑜−𝑓𝑢𝑒𝑙 to 17.5%

The developed thermodynamic model is presented in Fig. 1. 123 The equations listed above shows that the reduction and re-oxidation of 2 4 occurs in two 124 separate steps.
Step-1 deals with the release of O2 due to thermal reduction of The inert sweeping gas method is applied to maintain the partial pressure of O2 in the 128 reduction chamber. The entrance of the inert sweep gas in the reduction chamber is located at separator-1 is operated with an assumed efficiency ( −1 ) equal to 15% and as per the process 143 described in published literature (Ehrhart et al. 2016). The heat energy required for the 144 separation of O2 from inert sweep gas is calculated as per the following set of equations: HEX-3 (gas-to-gas heat exchanger) is placed in the model to reduce the temperature of O2 149 separated from the inert sweep gas from −1 to 0 = 298 K. After cooling, O2 is transferred to 150 an ideal CO/O2 fuel cell. The inert sweep gas, separated from the O2, is heated from −1 to 151 by going through a series of three gas-to-gas heat exchangers, namely, HEX-1, HEX-2, and 152 HEX-3. If required, supplementary heat is also provided with the help of an auxiliary heater-1. 153 The energy required to heat the inert sweep gas is estimated as follows: The heat dissipated during CO production vis CDC is computed by using the following equation.
170 ̇− is assumed to be rejected to the ambient. 171 HEX-5 (gas-to-gas heat exchanger) and an auxiliary heater-2 are installed in the model to pre-172 heat CO2 from 0 to . Eq. (14) is used to calculate the heating energy in the case of the CO2.
The resue of CO2 is possible only if it is separated from CO, for which a separator-2 is included. 178 Separator-2 is operated at −2 = 400 K and efficiency ( −2 ) equal to 15% (Carrillo and   179 Scheffe 2019). As the CO2/CO separation is carried out at 400 K, the CO2/CO gas mixture 180 temperature is reduced from to −2 by passing through HEX-5. Following three equations 181 are used for determining ̇− 2 (heat energy needed for the separation).
182  The fuel cell is fed with the O2, which is first separated from the inert sweep gas and then 269 cooled to 298 K by passing through HEX-3. Alternatively, by going through HEX-2, HEX-3, HEX-4, 270 and heater-1, the inert sweep gas is pre-heated from −1 to . this cooling is assumed to be rejected to the ambient. As an essential step to close the cycle, CO 319 is then transported to the fuel cell and reacts with the O2, producing CO2. that a higher percentage of solar energy is absorbed and hence ̇− losses are lower (Fig. 7). 344 On the other hand, as is less than 100%, ̇ is recorded to be higher than ̇ for 345 all ̇⁄ values. For example, ̇ is recorded to be higher than ̇ by 14.0 kW, 13.3 346 kW, 13.1 kW, 13.3 kW, and 13.6 kW at ̇⁄ equal to 10, 30, 50, 70, and 90, respectively. The influence of on ̇ℎ −2 is reported in