Abstract
Solar fuel generation from thermochemical H2O or CO2 splitting is a promising and attractive approach for harvesting fuel without CO2 emissions. Yet, low conversion and high reaction temperature restrict its application. One method of increasing conversion at a lower temperature is to implement oxygen permeable membranes (OPM) into a membrane reactor configuration. This allows for the selective separation of generated oxygen and causes a forward shift in the equilibrium of H2O or CO2 splitting reactions. In this research, solar-driven fuel production via H2O or CO2 splitting with an OPM reactor is modeled in isothermal operation, with an emphasis on the calculation of the theoretical thermodynamic efficiency of the system. In addition to the energy required for the high temperature of the reaction, the energy required for maintaining low oxygen permeate pressure for oxygen removal has a large influence on the overall thermodynamic efficiency. The theoretical first-law thermodynamic efficiency is calculated using separation exergy, an electrochemical O2 pump, and a vacuum pump, which shows a maximum efficiency of 63.8%, 61.7%, and 8.00% for H2O splitting, respectively, and 63.6%, 61.5%, and 16.7% for CO2 splitting, respectively, in a temperature range of 800 °C to 2000 °C. The theoretical second-law thermodynamic efficiency is 55.7% and 65.7% for both H2O splitting and CO2 splitting at 2000 °C. An efficient O2 separation method is extremely crucial to achieve high thermodynamic efficiency, especially in the separation efficiency range of 0–20% and in relatively low reaction temperatures. This research is also applicable in other isothermal H2O or CO2 splitting systems (e.g., chemical cycling) due to similar thermodynamics.
Highlights
The world faces environmental issues induced by large amounts of fossil fuel utilization that produce large quantities of COx, NOx, SOx, and ash emissions
Qenthalpy is the heat consumed by the enthalpy change of H2 O or CO2 splitting reaction; W P is the energy consumed to maintain a low pressure for oxygen separation and expressed as: WP = nO2 · RT0 ln P0 /PO2 /ηp where nO2,out is the molar amount of separated O2 ; R is the universal gas constant, taken as
This is due to the mole increase for both H2O and CO2 splitting reactions, which results in volume expansion and a subsequent backward shift in equilibrium at higher reaction pressures
Summary
The world faces environmental issues (e.g., global warming, haze) induced by large amounts of fossil fuel utilization that produce large quantities of COx , NOx , SOx , and ash emissions. Chemical cycle systems [11,12,13,14,15,16], both of which require a low permeate oxygen partial pressure to separate oxygen and shift the equilibrium toward high conversion at relatively low temperature As they are thermodynamically similar, this research focuses on H2 O or CO2 splitting in oxygen-permeable membrane (OPM) reactors. Li et al [20] studied the effects of gas heat recovery and reactor flow configurations on thermodynamic performance, and maximum efficiencies of 1.3% and 3.2% were obtained in H2 O and CO2 splitting, respectively Steinfeld and his collaborators [6,7] established the high-temperature solar thermochemical membrane reactor, and experimentally validated stable H2 and CO production by OPM (CeO2 membrane) reactor. The energy efficiency variation is systematically discussed under various temperatures and pressures, and the thermodynamic performance limits for H2 or CO generation are calculated and discussed
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