Abstract:

The general desulfurization sorbent, limestone is limited in sulfation temperature range and suffers from rapid loss of reactivity and incomplete utilization. However, plentiful shells are generally abandoned as garbage especially in coastal area, which consist of CaCO₃ similar to limestone. Desulfurization with shell shows a higher calcium utilization and better sulfation property than limestone under various experimental conditions. The desulfurization reaction process of calcium\|based sorbent including four species of shell and a sort of limestone were investigated by thermogravimetric analysis method. The kinetic behavior was expressed by the  modified grain model. The activation energy (E_a) and the pre-exponential factor (k₀) of surface reaction, the activation energy (E_p) and the pre\|exponential factor (D₀) of product layer diffusion reaction were calculated and explained according to the model. It is shown that   the modified grain model can describe the desulfurization course of shells and limestone. Additions of alkali metal salts may improve microstructure and product diffusion of sorbent during high temperature sulfation, and enhance the initial reaction rate and the final CaO conversion of sorbents. The kinetic parameters of shell desulfurization show a compensation effect. There are linear relationships between logarithms of the pre-exponential factor lnk0, lnD₀ and activation energies E_a, E_p respectively. The method based on the isokinetic temperature Ts to estimate sorbent activity is applied to different control stage. Alkali metal salts can enhance the initial reaction rate, at the same time can accelerate the high temperature sintering of sorbent, so there is optimal content in sorbent. The advisable molar ratio of alkali metal cations (mainly Na ⁺ and K ⁺) to Ca²⁺ in the sorbent is best kept to 2% approximately at the desulfurization temperature range of 1 073-1 273 K.

Key words: Desulfurization; Kinetics; Calcium-based sorbent; Compensation effect; Shell