FORMATION OF PRODUCT LAYERS IN SOLID-GAS REACTIONS FOR REMOVAL OF ACID GASES

Gas-solid reactions occurring in the dry scrubbing of acidic emissions (e.g. SO2, H2S, and HCl) from flue gases cease at certain ''maximum conversions'', X(m) (often <100%). Based on free energy-work analysis, a crystallisation and fracture model is presented to describe the mechanism for the formation of a product layer (PL). As a special feature of the model, the chemical potential of a reaction is shown to be dependent on mechanical work. A population model is proposed which suggests that the reaction rate depends on the population of nuclei of the critical size (PNC) at the reaction interface and the reaction stops when the PNC approaches zero. The existence of a maximum thickness of FL, h(m), has been shown mathematically; h(m) is demonstrated to be influenced by various parameters. It is proposed that the structure of a PL is characterised by the initial critical radius (r(ki)) of the nuclei. The larger the value of r(ki), the more porous is the PL with a larger coefficient of PL diffusion and less mechanical work. A low conversion of sorbent results from either a small or excessively large r(ki). A high conversion can only be achieved around the optimum value of r(ki). Experimental data available in the literature are interpreted successfully by the present model for reactions of Ca-sorbents with SO2, H2S and HCl.