In situ CO2 capture using CaO/γ-Al₂O₃ washcoated monoliths for sorption enhanced water gas shift reaction

In situ capture of CO₂ allows the thermodynamically constrained water gas shift (WGS) process to operate at higher temperatures (i.e., 350 C) where reaction kinetics are more favorable. Dispersed CaO/γ-Al₂O₃ was investigated as a sorbent for in situ CO₂ capture for an enhanced water gas shift application. The CO₂ adsorbent (CaO/γ-Al₂O₃) and WGS catalyst (Pt/γ-Al₂O₃) were integrated as multiple layers of washcoats on a monolith structure. CO₂ capture experiments were performed using thermal gravimetric analysis (TGA) and a bench scale flow through reactor. Enhancement of the water gas shift (EWGS) reaction was demonstrated using monoliths (400 cells/in.2) washcoated with separate layers of dispersed CaO/γ-Al₂O₃ and Pt/γ-Al₂O₃ in a flow reactor. Capture experiments in a reactor using monoliths coated with CaO/γ-Al₂O₃ indicated that increased concentrations of steam in the reactant mixture increase the capture capacity of the CO₂ adsorbent as well as the extent of regeneration. A maximum capture capacity of 0.63 mol of CO₂/kg of sorbent (for 8.4% CaO on γ-Al₂O₃ washcoated with a loading of 3.45 g/in.3 on monolith) was observed at 350 C for a reactant mixture consisting of 10% CO₂, 28% steam, and balance N₂. Hydrogen production was enhanced in the presence of monoliths coated with a layer of 1% Pt/γ-Al2O₃ and a separate layer of 9.4% CaO/γ-Al₂O₃. A greater volume of hydrogen compared to the baseline WGS case was produced over a fixed amount of time for multiple cycles of EWGS. The CO conversion was enhanced beyond equilibrium during the period of rapid CO₂ capture by the nanodispersed adsorbent. Following saturation of the adsorbent, the monoliths were regenerated (CO₂ was released) in situ, at temperatures far below the temperature required for decomposition of bulk CaCO₃. It was demonstrated that the water gas shift reaction could be enhanced for at least nine cycles with in situ regeneration of adsorbent between cycles. Isothermal regeneration with only steam was shown to be a feasible method for developing a process