Evaluation of two sorbents for the sorption-enhanced methanation in a dual fluidized bed system

Abstract

A novel process configuration for sorption-enhanced methanation (SEM) based on the technology of dual interconnected fluidized beds was recently proposed. The idea consists of a chemical looping system where in one reactor (the methanator), catalytic methanation occurs simultaneously with H₂O capture by a suitable sorbent to drive the equilibrium towards methane formation, while in another reactor (dehydrator), the regeneration of the sorbent takes place. In this work, two possible H₂O sorbents for such a SEM process were tested under simulated SEM conditions. The sorbents were a granular CaO obtained from natural limestone, which reacts with H₂O to form Ca (OH)₂, and an attrition-resistant spherical 3A-zeolite, which physically adsorbs H₂O. Each test consisted of 10 complete hydration/dehydration cycles in a lab-scale twin fluidized bed device. The temperature range investigated for hydration was 200–300 °C, while that for dehydration was 350–450 °C. CaO showed a decay of the steam capture capacity with the number of cycles, induced by the irreversible carbonation of the sorbent with CO₂. By contrast, the zeolite had a more stable behavior in all conditions. The presence of a high CO₂ concentration reduced the steam capture capacity of both sorbents, but this effect was more evident for CaO, because of the abovementioned carbonation reaction. For the zeolite, results suggest that CO₂ competes with H₂O for adsorption in the sorbent. In-bed particle fragmentation was limited for CaO and absent for the zeolite. On the whole, results indicate that both sorbents can be used for the SEM process. The zeolite shows a better asymptotic capture capacity than CaO and it is not affected by deactivation during the cycles, though it is a more expensive material.