Calcium-looping reforming of methane realizes in situ CO2 utilization with improved energy efficiency

Abstract

Closing the anthropogenic carbon cycle is one important strategy to combat climate change, and requires the chemistry to effectively combine CO₂ capture with its conversion. Here, we propose a novel in situ CO₂ utilization concept, calcium-looping reforming of methane, to realize the capture and conversion of CO₂ in one integrated chemical process. This process couples the calcium-looping CO₂ capture and the CH₄ dry reforming reactions in the CaO-Ni bifunctional sorbent-catalyst, where the CO₂ captured by CaO is reduced in situ by CH₄ to CO, a reaction catalyzed by catalyzed by the adjacent metallic Ni. The process coupling scheme exhibits excellent decarbonation kinetics by exploiting Le Chatelier's principle to shift reaction equilibrium through continuous conversion of CO₂, and results in an energy consumption 22% lower than that of conventional CH₄ dry reforming for CO₂ utilization. The proposed CO₂ utilization concept offers a promising option to recycle carbon directly at large CO₂ stationary sources in an energy-efficient manner.

Fig. 1. Illustration of the proposed in situ CO₂ utilization process.

(A) Process schematic of the catalytic CaL reforming of methane, and the (B) micrometer-scale morphology and (C and D) nanoparticle information of the CaO-Ni bifunctional sorbent-catalyst (freshly reduced CaO/Ni_9) driving the proposed process.

Fig. 2. Metal-support interaction in the material.

(A) XPS spectra corresponding to Ni 2p_3/2 and (B) H₂-TPR profiles of the prepared CaO-Ni bifunctional sorbent-catalysts. a.u., arbitrary units.

Fig. 3. Reaction identification on the CaO-Ni interface.

(A to C) Temperature-programmed reactions on the surface of freshly reduced CaO/Ni_9 to identify the coupling reactions involved in the CaL methane reforming process. (D) Gibbs free energy of related reactions as a function of temperature.

Fig. 4. Reaction studies of the CaL methane reforming process.

(A) Syngas yield and H₂-to-CO molar ratio and (B) conversion efficiency of CO₂ and CH₄ along with their consumption molar ratio as a function of the cycle number using CaO/Ni_9. (C) Specific yield of H₂ and CO (average yield per gram of the loaded Ni) along with their molar ratio and (D) average conversion efficiency of CO₂ and CH₄ along with their average consumption molar ratio during the 10 cycles as a function of the Ni/(Ca + Ni) molar ratio in the sorbent-catalyst.

Fig. 5. Comparison of decarbonation kinetics between the CaL methane reforming and separate CaL processes.

In situ XRD characterization for isothermal decarbonation of (A) CaCO₃ in a N₂ atmosphere, (B) CaCO₃ in a CH₄ atmosphere, and (C) carbonated CaO/Ni_9 in a CH₄ atmosphere at 1073 K. (D) Decarbonation rate as a function of time at 1073 K.

Fig. 6. Energetics of the CaL methane reforming process.

(A) Energy consumption for CO₂ utilization (${\mathit{E}}_{{\text{CO}}_2}$, kilojoules per mole of CO₂ converted) and (B) fuel requirement for syngas production (${\mathit{X}}_{{\text{CH}}_4}$, moles of CH₄ consumed per mole of syngas produced) as a function of carbonation and calcination temperatures. The black curve indicates the value corresponding to the application of conventional MDR using the CO₂ supplied by conventional CaL. The solid red circle indicates the value corresponding to the operating temperatures of the CaL methane reforming process investigated in this study.