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

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 HO 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 HO 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 HO to form Ca (OH), and an attrition-resistant spherical 3A-zeolite, which physically adsorbs HO. 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 CO. By contrast, the zeolite had a more stable behavior in all conditions. The presence of a high CO 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 CO competes with HO 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.