Abstract
Chemical looping syngas production is a two-step redox cycle with oxygen carriers (metal oxides) circulating between two interconnected reactors. In this paper, the performance of pure CeO2/Ce2O3 redox pair was investigated for low-temperature syngas production via methane reduction together with identification of optimal ideal operating conditions. Comprehensive thermodynamic analysis for methane reduction and water and CO2 splitting was performed through process simulation by Gibbs free energy minimization in ASPEN Plus®. The reduction reactor was studied by varying the CH4/CeO2 molar ratio between 0.4 and 4 along with the temperature from 500 to 1000 °C. In the oxidation reactor, steam and carbon dioxide mixture oxidized the reduced metal back to CeO2, while producing simultaneous streams of CO and H2 respectively. Within the oxidation reactor, the flow and composition of the mixture gas were varied, together with reactor temperature between 500 and 1000 °C. The results indicate that the maximum CH4 conversion in the reduction reactor is achieved between 900 and 950 °C with CH4/CeO2 ratio of 0.7–0.8, while, for the oxidation reactor, the optimal condition can vary between 600 and 900 °C based on the requirement of the final product output (H2/CO). The system efficiency was around 62% for isothermal operations at 900 °C and complete redox reaction of the metal oxide.
Highlights
Synthesis of non-fossil fuels through carbon dioxide (CO2) recycling via thermochemical or electrochemical pathways has received significant research interest in recent years as a complementary option to mitigate carbon emissions [2, 6, 13, 30]
At lower temperatures (500–600 °C) and for a lower CH4/CeO2 feed ratio, the availability of oxygen and temperature is limited to drive the reaction towards the production of syngas (CO + H2), resulting in the metal oxide to be poorly active for reaction (3)
The performance of the CeO2/Ce2O3 redox pair was evaluated for chemical looping syngas production through methane reduction and carbon dioxide and water splitting using thermodynamic analysis
Summary
Synthesis of non-fossil fuels through carbon dioxide (CO2) recycling via thermochemical or electrochemical pathways has received significant research interest in recent years as a complementary option to mitigate carbon emissions [2, 6, 13, 30]. Thermochemical redox cycles driven by concentrated solar power (CSP) have been widely studied for simultaneous splitting of H2O and CO2 for syngas The oxygen released during reduction depends on the metal cation and its corresponding valence state and on the possible reduction extent. Highest possible dissociations are sought in principle to maximize the oxygen release and uptake during reduction and oxidation, respectively, leading to higher H 2 and CO yields per mass of redox material used in the cycle [3, 33, 41, 46, 58]. Other metals oxides tested for chemical looping include ferrites with different valences, Co3O4, Nb2O5, WO3, SiO2, In2O3, CdO to name few [18, 21, 22, 25, 48] (Fig. 1)
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