Steam Reactivation of Partially Utilized Limestone Sulfur Sorbents

Abstract

A novel dual-environment thermogravimetric reactor was used to evaluate the steam reactivation of three limestones having differing sulfation morphologies. A clear correlation was observed between the morphology of the limestones and the ability of steam to reactivate "fully sulfated" sorbents. Unreacted core particles led to the highest conversion of CaO to Ca(OH)₂ and the highest increase in Ca utilization (40%), followed by network sulfating limestone particles (9%). Virtually no hydration or reactivation was observed for uniformly sulfated particles. Steam hydration of an unreacted core sulfating limestone showed a decrease in reactivation with increasing temperature in the range 250 to 500°C. However, there was no clear correlation between CaO to Ca(OH)₂ conversion and the extent of reactivation, likely due to changes in the rate-limiting resistance. At 500°C, Ca(OH)₂ is unstable, resulting in no transformation of CaO and no reactivation. Reduced levels of hydration and reactivation at 450°C, compared to lower temperatures, is believed to result from the reaction rate exceeding the diffusion rate, leading to a plugged Ca(OH)₂ product layer preventing further hydration, or a combined effect of smaller micropores and lower steam partial pressure inside the particles. Similar levels of hydration, but variations in reactivation, at 250 to 400°C are attributed to the effect of faster evolution of gas when particles are heated more quickly.