Fractal-like random pore model applied to CO2 capture by CaO sorbent

CO2 capture from industrial flue gas by CaO in fluidised bed calcium looping processes is one of the means to reduce anthropogenic greenhouse gas emissions. The heterogeneous carbonation reaction has been widely investigated in literature, but in almost all cases the product layer (CaCO3-rich) CO2 diffusivity has been considered as a constant parameter. In this work, the application of fractal-like dynamics concepts to the present case has led to the formulation of an expression for the product layer diffusivity that is function of the carbonation time. This expression has been used in the canonical random pore model, to propose an original fractal model, whose superior accuracy has been demonstrated by validation with experimental data obtained under realistic operating conditions (dual interconnected fluidised beds system, use of a CaCO3-rich limestone sorbent, carbonation carried out at 650 °C treating a flue gas with 15% CO2 and in absence/presence of steam - 10% when present - and SO2 - 1500 ppmv when present-). Main model parameters were the following: fractal heterogeneity parameter, average value for product layer diffusivity, characteristic time for CO2 diffusion into the sorbent pores, intrinsic kinetic constant, average value for a modified Biot number. Their values were discussed in the light of the reaction mechanism, and of the effect of the absence/presence of steam and SO2 in the flue gas to be treated. It has been observed that the presence of steam in the carbonator favours both the intrinsic carbonation kinetics and the CO2 product layer diffusion, while the presence of SO2 limits the product layer diffusion.