CFD Analysis of the CaO-CO₂ Reaction in a Thermo- Gravimetric Apparatus

Abstract

Thermo-gravimetric analysis (TGA) is a useful technique to study the kinetics of gas-solid reactions and specifically it has been widely used to investigate the carbonation reaction between carbon dioxide and calcium oxide based solid sorbents, which is the basis of a promising CCS technology. Typical conversion rates of the carbonation reaction initial fast stage measured by TGA are lower than about 0.06 s-1. However, TGA results can be significantly affected by the external mass transfer when a fast gas-solid reaction, such as the carbonation reaction, is considered, namely, when the intrinsic conversion rates are higher than 0.2 s-1. In this case the conversion measurement using the thermo-gravimetric analysis may lead to inaccurate results (apparent reaction rate) if the mass transfer of the gaseous reactant is not properly accounted for. In this work a non-stationary computational fluid dynamics (CFD) study of the carbonation reaction was performed considering a horizontal TGA type, using the CFD commercial code ANSYS FLUENT® 15.0, neglecting inter-particle and intra-particle mass diffusion. A surface reaction model was assumed at the crucible surface and the conversion-time profiles were calculated varying the reactant gas flow rate. The velocity field around and inside the crucible were computed and analyzed as well as the reactant concentration profiles, in order to evaluate the effect of the operative conditions on the reaction rate measurements. It was found that the calculated apparent conversion rate is about 5-6 times slower than the intrinsic conversion rate due to the non-stationary carbon dioxide concentration profile established from the bulk to the crucible surface.