Effect of Fly Ash as an Additive on the Limestone Dissolution Rate Constant

Abstract

Limestone dissolution is a very important factor in flue gas desulfurization systems because it determines its reactivity toward SO 2 . Fly ash, a siliceous material, has been reported to improve sorbent reactivity. This study investigates the effect of adding fly ash to limestone on its dissolution rate constant. The experiments were carried out using a pH stat apparatus where the effects of the reaction variables, fly ash/limestone ratio, slurry pH, reaction temperature, and concentration of acid, used were investigated. The central composite design (CCD) of the experiment was used to develop a model that correlates the dissolution rate constant and the reaction variables. It was found that fly ash had a positive effect on the dissolution rate constant of limestone, with the pH having the most significant effect. The dissolution rate constant was found to increase with an increase in the temperature and acid concentration. X-ray diffraction (XRD) analysis showed products of hydration formed, which are mainly calcium silicate hydrates, on the samples. This led to an increase in the specific surface area, as observed in the Brunauer− Emmett−Teller (BET) analysis.

Dissolution Reactivity and Kinetics of Low-Grade Limestone for Wet Flue Gas Desulfurization

“…Reaction temperature has a significant role in the limestone dissolution process and is closely related to the dissolution rate-limiting step. The reaction rate in a fluid–solid system is primarily controlled by one of the following steps: diffusion through the fluid film, diffusion through the ash, or a chemical reaction between the fluid reactant and solid particles at the surface of the unreacted limestone sample . This paper treats the reaction process as a noncatalytic heterogeneous reaction system and the limestone particle as nonporous with no structural change as the dissolution progresses.…”

“…The reaction rate in a fluid−solid system is primarily controlled by one of the following steps: diffusion through the fluid film, diffusion through the ash, or a chemical reaction between the fluid reactant and solid particles at the surface of the unreacted limestone sample. 34 This paper treats the reaction process as a noncatalytic heterogeneous reaction system and the limestone particle as nonporous with no structural change as the dissolution progresses. As such, the shrinking core model 35 is used to describe the dissolution kinetics, and it considers the first reaction to occur at the particle surface before moving toward the core.…”