Mechanisms of direct and in-direct sulfation of limestone

• SO2 absorption area of direct sulfation was different from the indirect sulfation.
• The grain model can be applied to the indirect sulfation.
• The shrinking unreacted core model can be applied to the direct sulfation.
• The kinetic parameters of the reaction models were obtained from experimental data.
• The diffusion mechanism was considered both gas phase and solid-state diffusion.

The general desulfurization process referred to as “indirect sulfation” takes place via a two-step process. Calcination makes limestone decompose to calcium oxide (CaO), and then SO2 molecules are adsorbed onto the calcined limestone. However, if the CO2 partial pressure in the system is higher than the equilibrium CO2 pressure over the limestone, an adsorption reaction between limestone and SO2 molecules can take place directly in the uncalcined state. This one-step process, called “direct sulfation,” can apply to desulfurization in pressurized fluid-bed combustion (PFBC) or an oxy–fuel combustion system capturing CO2. Limestone samples were reacted in a TGA apparatus, and the degrees of sulfation were measured under 100% air conditions for indirect sulfation and 80% CO2/20% O2 conditions for direct sulfation to compare the mechanisms and kinetics of the two sulfation processes. SEM images of sulfated particles and EDS analyses showed that the gas diffusion of indirect sulfation takes place readily through macro-pores between the grains at a low conversion rate, and SO2 adsorption occurs over the interior of the limestone particles, whereas SO2 adsorption in direct sulfation takes place at the reaction interface, and the product (CaSO4) layer permeates into the interior from the surface of the limestone particles as direct sulfation proceeds. Thus, the grain model and the shrinking unreacted core model are applicable to describing the kinetics of indirect and direct sulfation, respectively. These model predictions agreed with experimental data, and the kinetic parameters obtained from experiments are consistent with the mechanisms. Furthermore, the high activation energy of the diffusion of direct sulfation and indirect sulfation with high conversion indicate that the diffusion mechanisms in the particles in direct sulfation and indirect sulfation at high conversion are both gas diffusion in pores and solid-state diffusion. These results may contribute to understanding why the characteristics of direct sulfation differ from those of indirect sulfation under certain conditions.