Model of transport and chemical kinetics in a solar thermochemical reactor to split carbon dioxide

Abstract

A 3-D model of a solar reactor for the isothermal ceria redox cycle is presented. Reaction rate parameters are extracted for the redox reactions at 1773 K. Recovery of 90% of the sensible heat of the gases is achieved. For CO 2 splitting, CO is produced at 3.6×10 -4 mol s -1 for a 4.2 kW solar input. a b s t r a c t Solar thermochemical reactors to carry out the nonstoichiometric reduction and oxidation of cerium dioxide (ceria) to split water and carbon dioxide provide a pathway to store sunlight in a chemical fuel. One of the challenges in the design of these reactors is understanding the complex coupling of heat and mass transfer and redox chemistry. To elucidate this coupling, we present a three-dimensional, transient model of a recently developed prototype solar reactor that implements an isothermal, pressure-swing ceria redox cycle. Radiative transport is modeled by a hybrid Monte Carlo/finite volume approach and paired with the transport and chemical processes within a fixed bed of porous ceria particles. Morphology specific reaction rate coefficients for the gas-solid reactions in ceria are extracted for the first time from global rate measurements in a bench-top reactor at 1773 K. Results demonstrate the interdependent spatial and temporal variations in temperature, species concentration and reaction rates, and provide insight on the effects of optical properties on reactor performance. For a solar input of 4.2 kW, the reactor achieves nearly isothermal cycling at 1791 K with carbon monoxide produced continuously at 3.6 Â 10 À 4 mol s À 1 . At this temperature, global reaction rates are driven by advective mass transport rates and the intrinsic material thermodynamics. Predicted surface temperatures and fuel production rates compare favorably to measured data.

The morphological stability and fuel production of commercial fibrous ceria particles for solar thermochemical redox cycling

“…The isothermal cycle has the same thermodynamic limitations as direct thermolysis of CO 2 or H 2 O, but with the benefit of separating the product gases into two streams. As demonstrated through implementation of a prototype reactor, isothermal cycling reduces thermal stresses on ceramic components, and facilitates continuous fuel production and effective heat recovery (Chandran and Davidson, 2016;Hathaway et al, 2016Hathaway et al, , 2015 . On the other hand, solar-to-fuel efficiency is higher for a temperature-swing cycle (Brendelberger et al, 2015;Bulfin et al, 2015;Ermanoski et al, 2014;Jarrett et al, 2016;Krenzke and Davidson, 2015).…”

“…The encouraging behavior of the electrospun fibers at 1673 K motivated the development and evaluation of mm-sized fibrous ceria particles for use in a fixed bed isothermal solar thermochemical reactor (Bader et al, 2015;Bala Chandran et al, 2015;Chandran and Davidson, 2016;Hathaway et al, 2016Hathaway et al, , 2015Venstrom et al, 2015Venstrom et al, , 2014.…”

Maximizing fuel production rates in isothermal solar thermochemical fuel production

“…Thus, an alternative metric for optimization, which does not suffer from this arbitrary behavior and can thus reveal fundamental performance differences between oxide materials (which appear away from the → limit), is required. In our earlier work, we introduced the nondimensional characteristic hydrogen production rate [31] defined by ∆ (33) where (34) and for completeness we define the quantity (35) As described previously, is the hydrogen production rate normalized to the combined flow rates of the oxidation and reduction gases, ; is independent of both this total flow rate and the total mass of oxide in the reactor.…”

“…To this end, detailed reactor models, which, as proposed here, treat oxygen stoichiometry changes in the reactive oxide to be limited by gas flow, have very recently appeared in the literature. [33,36,37] The thermo-kinetic model presented here can provide guidance for enhancing fuel production rates for a generic reactor design, so long as it obeys the constraints that the material stoichiometry and gas phase composition be spatially uniform in the reaction zone. In particular, we show that although some improvement in rate can be realized by employing reactive materials with improved thermodynamic properties, the production rate is primarily increased by optimizing the gas flow parameters and reactor operation conditions.…”

“…Accordingly, the subsequent analysis of the impact of cycle time focuses on fuel production rates.To aid the discussion, we extend the concept of the characteristic hydrogen production rate, , to a form, , that describes the characteristic hydrogen production rate experienced by the material over an incremental swing in nonstoichiometry. By analogy to Eq (33),. is expressed the rate of hydrogen production for a complete cycle, in which the cycle time 541 includes that required to reduce the material by as well as the time required to 542 oxidize it by -, at a nonstoichiometry (in both half-cycles) of .…”