Dynamic modeling of a dual fluidized-bed system with the circulation of dry sorbent for CO2 capture

In CO2 capture processes with the circulation of dry sorbents, the regeneration energy as well as the capture efficiency are the key factors determining the overall energy efficiency of the CO2 capture. In an aspect of repeated circulation and regeneration of a sorbent, a dynamic model for a dual fluidized-bed system was developed, which includes a fast fluidized-bed carbonator and a bubbling fluidized-bed regenerator. A potassium carbonate-based sorbent for CO2 capture was applied in the fluidized-bed system and rigorous kinetic models for the carbonation and regeneration reactions were adopted. The validity of the developed model was confirmed by accurately predicting the experimental results from the dual fluidized-bed system at various operating conditions. The CO2 removal performance was found to slightly deteriorate from 52.8 to 51.9% during continuous cyclic operation when the regeneration was carried out under a nitrogen atmosphere at 150 degrees C. However, when CO2 gas was used for the regeneration under the same conditions, the capture performance dropped to 18.6% owing to partial regeneration of the sorbent. A case study for the regeneration condition was conducted using a CO2-rich gas to find the effective regeneration condition. The regeneration conversion under CO2 atmosphere could be improved by increasing the regeneration gas velocity and regeneration temperature. At a regeneration temperature of 160 degrees C, the capture performance was found to be 73.2%, with the energy required to capture one mole of CO2 being 234.8 kJ/mol-CO2. To reduce the energy requirement to less than 200 kJ/mol-CO2 in the dual fluidized-bed system, a granulated sorbent, satisfying the physical and chemical stability for fluidized-bed operation, should be developed for the regeneration below 145 degrees C with the same working capacity (0.46 mol/kg-solid). Alternatively, the working capacity should be improved by 30% at the regeneration temperature of 160 degrees C. The developed model can be further used for improving capture performance and energy efficiency.