A comparative analysis of integrating thermochemical oxygen pumping in water-splitting redox cycles for hydrogen production

Solar thermochemical hydrogen (STCH) is a promising route for renewable fuel production, but efficiency and cost concerns must be addressed before it can contribute meaningfully to decarbonization efforts. The oxygen removal (reduction) step of two-step STCH redox cycles needs high temperature (1300–1500 °C) and low oxygen partial pressure (10-5-10-3 bar). This consumes significant energy by way of redox reheating, re-radiation losses and pumping work. Thermochemical oxygen pumping (TcOP) has been shown to be more efficient than mechanical pumps in the medium vacuum range, enabling higher heat-to-fuel efficiency and potentially lower reduction temperatures. We have previously proposed a novel Reactor Train System for STCH and reported significant efficiency and productivity improvements by replacing mechanical pumps with TcOP. The current work is a comparative analysis of different implementations of TcOP and its hybridization with conventional oxygen removal schemes like mechanical vacuum pumps and inert gas sweep. The integration of these hybrid TcOP systems with different STCH reactor systems using redox monoliths or particles is analyzed in terms of energy use along with mass and heat transfer considerations. Our results show that direct oxygen transfer between STCH and TcOP reactors results in the best performance for reduction oxygen partial pressures below 10 Pa. Furthermore, we show that achieving reduction pressures of 1 Pa and lower is challenging with existing reactor systems due to a combination of low gas density and relatively high molar flowrates. This challenge also applies to other oxygen removal systems like vacuum pumps and inert sweep.