By use of coal fly ash as an additive in pulverized biomass fuel boilers, harmful alkali species can be bound in alkali alumina silicates that are less harmful. In this study, potassium scavenging by coal fly ash (CFA) at conditions of pulverized-fuel (PF) boilers was modeled. Under the investigated conditions, evaporated potassium salts were captured with suspended CFA particles. Two modeling approaches were investigated, shrinking core model (SCM) and uniform conversion model (UCM). Both approaches simulated the impacts of chemical kinetics, diffusion of gaseous salts around the additive particles, and thermodynamic equilibrium on potassium conversion. Moreover, the SCM included the diffusion resistance in a product layer around the particle. The models have been evaluated against entrained flow-reactor (EFR) measurements from the literature for capturing KOH, KCl, K2CO3, and K2SO4 by CFA. Chemical kinetic rate coefficients for the reaction between the potassium salts and CFA have been derived from the EFR data measured at relatively lower temperatures of 800–900 °C. The porosity properties of the reacted CFA were also estimated in the present work. The effects of temperature, salt concentration, and CFA particle size on the model prediction have been examined and evaluated against the experimental data. The results indicated that in most conditions, the SCM prediction is more reliable, probably due to the inclusion of diffusion resistance of a product layer around the particle. Comparing the SCM with experimental data shows that the model can reasonably predict the reaction of CFA with potassium salts at conditions investigated here: 800–1450 °C, salt to additive ratios of 0.05–0.96, and for CFA particles of 6–34 μm.