(499d) Counter-Current Regenerative Redox Chemical Looping for Syngas Production: Packed Bed Reactor Design, Evaluation, and Feasibility Study

Solar radiation provides a semi-infinite amount of energy to Earth, enough to meet annual global demands in just 1 hour utilizing minimal surface area [1]. Even with the limited efficiency of solar conversion technologies, solar energy may still be one of the most promising sources of clean energy, necessary to decarbonize the energy sector. The diurnal and proximal variation of renewable resources in general, however, make it necessary that cost-effective, long-term storage solutions be investigated. Current efforts in solar energy storage systems span thermal, mechanical, chemical, electrochemical, and hybrid systems [2].

Of those, chemical storage as fuels is attractive due to its easy transportation and compatibility with current energy infrastructure. Recent solar-to-fuel research has focused largely on thermochemical conversion of carbon dioxide and steam to syngas, a mixture of carbon monoxide and hydrogen, which can be further upgraded to liquified hydrocarbon fuels via downstream chemical processes, such as Fischer-Tropsch synthesis. One feasible mechanism for this transformation is regenerative redox chemical looping utilizing nonstoichiometric metal oxides as an oxygen exchanger. In this process, metal oxide is first exposed to a reducing environment deficient in oxygen at high temperatures, allowing oxygen to be released thus creating oxygen vacancies in the lattice. Following reduction, the metal oxide is exposed to an oxidizing environment containing carbon dioxide and/or steam, allowing the metal oxide to return to its initial state while producing carbon monoxide or hydrogen [3]. There are ongoing efforts on improving the reactor efficiency which to date has been limited to 5%, with various proposed designs aimed at recovering the sensible heat losses between reduction and oxidation and minimizing losses. A significant part of these efforts includes the development of new redox materials that can operate at lower reduction temperatures. One aspect that is not widely pursued is the feedstock conversion, which is inherently low in these systems, ranging from less than 0.1% up to only 20% for some materials/reactors. Low conversion of CO₂ and/or H₂O results in large and energy/capital intensive separation processes, jeopardizing the commercial viability of the process.

Recently, the use of nonstoichiometric oxides in a redox reactor with alternating flow directions during reduction and oxidation has been proposed, achieving similar performance to counter-current membrane systems due to the gradient of chemical potential stored in the oxide. This has been demonstrated for the water-gas shift (WGS) [4] and reverse water-gas shift (RWGS) [5] reactions using a packed bed reactor. Such a system overcomes the design, fabrication, and operating limitations of high-temperature membrane reactors, with demonstrated conversion of over 90% for these thermodynamically-limited reactions. It also negates the need to design complex reactors with moving oxide materials at high temperatures. These findings call for further investigation of the counter-current configuration, optimizing reactor sizing and operating conditions, along with an updated system-level technoeconomic analysis.

Therefore, this study aims to evaluate the economic feasibility of the newly proposed counter-current thermochemical regenerative redox chemical looping reactor utilizing nonstoichiometric metal oxides, evaluate reactor productivity via performance indicators, and conduct a system-level technoeconomic analysis on the process.

Physics-based models have been developed using the commercially available finite element software COMSOL to simulate the fluid flow through porous media, mass transfer, and kinetics of the system. 1D models have been developed for both co-current and counter-current configurations for comparison and validation against available data, allowing for a quick parametric study to establish the dependencies between the main parameters and reactor performance. A 2D axisymmetric model will consequently be developed for the counter-current configuration for a more detailed analysis. Key model assumptions include: (1) the oxidizer is in equilibrium with its corresponding thermolysis reaction at the entrance of the reactor, (2) fluid properties are evaluated for the bulk inlet species, and (3) reaction kinetics are limited by the metal oxide and rapidly approach equilibrium.

Reactor performance is evaluated over a wide range of reactor dimensions and operating conditions via performance indicators used in the field of solar thermochemistry [6]. Because both the oxidation and thermolysis reactions produce the same products, selectivity and yield are not needed to characterize the reactor performance. Instead, only cycle conversion and solar-to-fuel efficiency are required.

Cycle conversion is defined as the ratio of total unreacted reactant to total available reactant in the feedstock. Solar-to-fuel efficiency η_{solar-to-fuel} represents the fraction of input energy that can be recovered by product combustion over the different energy input terms: the sensible heat required to increase the reactor temperature from the oxidation to reduction temperature, the endothermic heat required to drive the reduction reaction, the work required to pump sweep gas during reduction and oxidizer during oxidation, and the work required to separate released oxygen contaminant from inert gas. In practice, η_{solar-to-fuel} is the reactor efficiency, neglecting losses due to solar collection and downstream processes such as reactant/product separation and gas-to-liquid synthesis.

Simultaneously optimizing cycle conversion and efficiency is a challenge, often trading off between each other. Achieving high conversion requires long cycle times and diminished kinetic rates near complete conversion, thus decreasing solar-to-fuel efficiency. Contrarily, infinitesimally small cycle times maximizes fuel production rates, but inefficiently utilizes reactants thus decreasing conversion [7]. However, when considering the effects of reducing large downstream separation units, a different optimal operation regime is expected compared to previous works to date.

The developed models will demonstrate the improved performance of a counter-current regenerative redox reactor, maximizing the difference between oxidizer concentration and metal oxide vacancies across the reactor length, resulting in increased conversion with minor losses in efficiency due to increased reduction energy demands and downstream separation work. Preliminary results from the 1D model confirmed this, showing achievable conversions up to 90% using the counter-current configuration. Utilizing shorter time cycles will increase the reactor efficiency while obtaining much higher conversion than demonstrated values to date. It is not expected that reactor diameter will have a dramatic effect on reactor performance but increasing it may allow for higher oxidizer flowrates while maintaining high conversion and decreasing the required pumping work.

By performing rigorous numerical investigation, the conversion-efficiency tradeoff will be evaluated in detail, informing further efforts with guidance on reactor sizing, optimal operating conditions, and expected results for both experimental testing and computational validation. The subsequent system-level technoeconomic analysis will determine the system feasibility and identify technological gaps and recommended directions for future research efforts to advance the field of solar thermochemical hydrogen and fuel production.

References:

[1] Steinfeld A, Meier A. Solar fuels and materials. in: Cleveland C (ed) Encyclopedia of energy, vol 5. Elsevier, Amsterdam, (2004) pp 623–637. https://doi.org/10.1016/B0-12-176480-X/00322-3.

[2] J. Mitali, S. Dhinakaran, A.A. Mohamad, Energy storage systems: a review, in: Energy Storage and Saving, Volume 1, Issue 3 (2022) pp 166-216. https://doi.org/10.1016/j.enss.2022.07.002.

[3] A. Meier, A. Steinfeld, Solar Energy in Thermochemical Processing, in: C. Richter, D. Lincot, C.A. Gueymard (Eds.), Sol. Energy, Springer New York, New York, NY (2013) pp. 521–552. https://doi.org/10.1007/978-1-4614-5806-7_689.

[4] Metcalfe, I.S., Ray, B., Dejoie, C. et al. Overcoming chemical equilibrium limitations using a thermodynamically reversible chemical reactor. Nat. Chem. vol 11, (2019) pp 638–643. https://doi.org/10.1038/s41557-019-0273-2.

[5] B. Bulfin, M. Zuber, O. Gräub, A. Steinfeld, Intensification of the reverse water–gas shift process using a countercurrent chemical looping regenerative reactor, Chem. Eng. J. 461 (2023). https://doi.org/10.1016/j.cej.2023.141896.

[6] B. Bulfin, M. Miranda, A. Steinfeld, Performance Indicators for Benchmarking Solar Thermochemical Fuel Processes and Reactors, Frontiers in Energy Research (2021). https://doi.org/10.3389/fenrg.2021.677980.

[7] Timothy C. Davenport, Chih-Kai Yang, Christopher J. Kucharczyk, Michael J. Ignatowich, Sossina M. Haile, Maximizing fuel production rates in isothermal solar thermochemical fuel production, Applied Energy (2016). https://doi.org/10.1016/j.apenergy.2016.09.012.