Abstract
A novel thermochemical dual-membrane reactor is considered with the goal of efficiently converting CO2 to fuels using concentrated solar energy as the process heat source. In contrast to the temperature-swing redox cycle, in this isothermal system the thermolysis of H2O at above 1800 K is assisted by removal of O2 across an oxygen-permeable membrane and of H2 across a hydrogen-permeable membrane. The latter is consumed by a stream of CO2 via the reverse water-gas shift reaction to re-form H2O and continuously generate CO. The net reaction is the splitting of CO2 to CO and ½O2. Because reactions at such high temperature are expected to be thermodynamically controlled, thermodynamic models are developed to calculate the equilibrium limits of the proposed dual-membrane configuration. For comparison, two reference configurations comprising either a single oxygen-permeable membrane or a single hydrogen-permeable membrane are analyzed. At 1800 K, 1 bar total pressure, and (not applicable for the hydrogen-membrane reactor) 10 Pa O2, the equilibrium mole fraction of fuel is 2% with a single oxygen membrane, 4% with a single hydrogen membrane, and 15% in the dual-membrane system. In all cases, total selectivity of CO2 to CO and O2 is obtained. Assuming thermodynamic equilibrium, the solar-to-fuel energy efficiency realistically attainable is 4% with a single oxygen membrane, 8% with a single hydrogen membrane, and 17% in the dual-membrane configuration at the aforementioned conditions. By increasing the pressure of the feed of steam to 100 bar, the dual-membrane model system could theoretically approach full mass conversion of CO2 and reach up to 26% solar-to-fuel energy efficiency. However, developing appropriate and stable ceramic materials for such a system poses a significant challenge.
Highlights
Liquid hydrocarbon fuels are convenient energy carriers because of their high volumetric energy density, storability, and transportability with the existing global infrastructure
The thermodynamic equilibrium models and energy efficiency analysis form the basis of parametric studies to compare the DMR configuration to the single-membrane configurations and identify the operating conditions needed for best performance
We examined the concept of a thermochemical membrane reactor to split CO2 and produce solar fuels from a thermodynamic perspective for three configurations: a single oxygen-permeable membrane (OMR), a single hydrogenpermeable membrane (HMR), and an oxygen-permeable and hydrogen-permeable dual-membrane (DMR) configuration
Summary
Liquid hydrocarbon fuels are convenient energy carriers because of their high volumetric energy density, storability, and transportability with the existing global infrastructure. The cavity absorption efficiency, ηabs, is only applied on the inputs added to the system in the form of concentrated solar heat: reaction enthalpy, Qreaction; sensible heating of gases, Qsensible; and FIGURE 3 | Process diagram of the dual-membrane reactor including process units and heat inputs. Different subsets of these process units are applicable for different membrane configurations and operating conditions. Qseparation,real can already be anticipated to have a large impact on ηsolar-to-fuel
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