Transport and Chemical Phenomena in a Solar Thermochemical Reactor to Split Carbon Dioxide and Water to Produce Synthesis Gas

Abstract

Numerical study of a solar thermochemical reactor for isothermal reduction and oxidation of cerium dioxide is presented with the goal of accomplishing effective gas phase heat recovery and efficient gas utilization — the two most important attributes for achieving high fuel conversion efficiency in the reactor. Models were applied in tandem with and/or compared to experiments to interpret physical processes in the reactor and for validation. To achieve gas phase heat recuperation, a counterflow, ceramic heat exchanger filled with alumina reticulated porous ceramic was designed to operate at 1773 K. The focus of the modeling was selection of the morphology of the reticulated porous ceramic/ foam to balance heat transfer and pressure drop. Foam morphologies of 5–30 pores per inch (PPI) with porosities of 65–90% were considered. Large pore sizes, or equivalently low PPI, augment radiative penetration and reduce pressure drop. Solid phase conduction is the dominant mode of heat transfer over a majority of the heat exchanger length. Consequently, lower porosity improves the overall heat exchange at the penalty of increased pressure drop. The tradeoff in heat transfer performance and pressure drop point to use of higher porosity, 85–90%, and large pore sizes to optimize the solar-to-fuel efficiency. The final design is a 1.4 m long heat exchanger filled with 10 and 5 PPI, 85% porous, alumina foam in the annulus and center tube. This design recuperates more than 90% of the sensible energy of the reactant and product gases with less than 15 kPa pressure drop through the bed of ceria and heat exchanger. Motivated by the need to investigate the influence of transport processes on reactor performance, especially on the temperature distribution and the reaction rates, a transient, three-dimensional computational model of the reactor was developed. A hybrid Monte Carlo/finite volume approach was used to model radiative transport in the reactor surfaces and in the participating media. The chemical kinetics of the cyclic, gas-solid reactions were modeled by obtaining the best fit reaction rate coefficients from global rate data from a bench top reactor at 1773 K. For a solar input of 4.2 kW and gas flow rates of 0.67×10-4 mol s-1 gceria-1, the model predicts nearly isothermal cycling at an average temperature of 1791 K. Results elucidate that spatial variations in temperature, species concentration and reaction rate are interrelated and more pronounced along the gas flow direction. Carbon monoxide is produced continuously at 3.6×10-4 mol s-1, translating to 100 W of stored chemical energy. The overall reaction rates are driven by gas phase advection and the intrinsic material thermodynamics, rather than surface kinetics. The numerical modeling framework developed in this dissertation is robust and conducive to study other thermochemical processes in a high temperature solar reactor.