Optimization Strategies for Oxygen Partial Pressure Reduction in High-Temperature Solar Membrane Reactors

Abstract

The EU-funded SOMMER project focuses on developing high-temperature ceramic membrane reactors for the sustainable production of solar fuels and chemical commodities. These reactors directly convert water (H₂O) and carbon dioxide (CO₂) into hydrogen (H₂) and carbon monoxide (CO), the primary components of syngas. Using tubular membranes, the reactors separate the reaction zone from the oxygen release zone, enabling continuous operation, enhanced heat recovery, and improved overall energy efficiency. A major challenge in these systems is maintaining low oxygen partial pressure on the permeate side, which is critical for redox activity and conversion efficiency. A zero-dimensional Python-based model employing the Cantera library was developed to simulate reactor behavior, dividing the system into retentate, membrane, and permeate regions. Two operational scenarios were considered: on-sun operation, where the reactor is directly heated by concentrated sunlight up to 1500°C, and off-sun operation, where a heat transfer fluid maintains high temperatures up to 900°C, with methane introduced as a reducing agent on the permeate side. The model investigated the influence of temperature, sweep gas flow rate, pressure, and oxygen extraction methods on the conversion of H₂O and CO₂, as well as the effects of oxygen impurities and pressure gradients. Results indicated that high sweep gas flow rates are crucial for significant conversion in on-sun operation, with water splitting requiring a high nitrogen-to-feed molar ratio at 1500°C, while CO₂ conversion could be achieved at lower temperatures or reduced flow ratios. Lowering the absolute pressure on the permeate side effectively reduces sweep gas demand, whereas elevated pressures have minimal impact on conversion. In off-sun operation, low molar flow ratios suffice for high conversions, although temperatures above 900°C offer diminishing returns, and the introduction of reducing agents like methane enhances the oxygen partial pressure gradient, improving overall conversion. Oxygen removal strategies were also evaluated, showing that thermochemical oxygen pumps, while highly effective, remain in the developmental stage, whereas electrochemical pumps are commercially available but require external energy. Combining sweep gas with reducing agents proved effective in maintaining low oxygen partial pressure, and integrating advanced oxygen removal technologies has the potential to further optimize reactor performance. Overall, the study provides a detailed analysis of key operating parameters and strategies to enhance the efficiency and feasibility of high-temperature solar membrane reactors for syngas production.