Calcium looping process simulation based on an advanced thermodynamic model combined with CFD analysis

Abstract The current study presents a new methodology for the simulation of the Calcium Looping (CaL) process based on the coupling of CFD and advanced thermodynamic models. As a first step, CFD models for the two reactors, i.e. the carbonator and the calciner, of a pilot scale Dual Fluidized Bed system are developed and validated by comparing the numerical predictions with corresponding experimental data for pressure distribution, carbonator capture efficiency and sorbents regeneration in the calciner. For the carbonator modeling, the Two-Fluid-Model (TFM) approach is combined with the advanced EMMS scheme in order to provide results with high accuracy, even for the difficult to model dense bottom zone of the riser. A similar approach is adopted for the calciner; numerical results indicate that CO2 follows an almost linear trend along the bubbling bed height, while the bubbling formations might result in a reduced efficiency for the calcination reaction due to the entrapment of CO2 bubbles inside the emulsion phase. Numerical results related mostly to the hydrodynamics of the reactors, such as the solids distribution and residence time are then used as input parameters in a kinetics-based process algorithm. Process modeling simulations reveal the importance of splitting the carbonator riser into two distinct sections, i.e. the bottom zone with dense solid phase and the upper one (freeboard) with a more dilute solid concentration. The heat balance calculation for these two regions demonstrates a big gap between the heat flux density for the bottom zone (19.26 kW/m2) and the freeboard (0.46 kW/m2), which should be taken into account for the design of an effective heat removal system for scaled-up reactors. As a final step, a sensitivity analysis is performed for the optimization of the parameters governing the operation of the whole carbonation–calcination cycle. Efficient sorbent regeneration and high looping ratio enhances the CO2 capture efficiency in the carbonator, whilst low CO2 concentration in the calciner is suggested for more effective lime regeneration.

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