Effect of Fly Ash as an Additive on the Limestone Dissolution Rate Constant Lawrence Koech,* ,† Hilary Rutto, † Hein Neomagus, ‡ Ray Everson, ‡ and Letsabisa Lerotholi † † Department of Chemical Engineering, Vaal University of Technology, Vanderbijlpark Campus, Private Bag X021, Vanderbijlpark 1900, South Africa ‡ School of Chemical and Minerals Engineering, North-West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom 2520, South Africa 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. 1. INTRODUCTION Flue gas desulfurization (FGD) is the most relevant technology used in large-scale power utilities, such as coal-fired power plants, to remove sulfur dioxide from flue gas. To improve the SO 2 removal efficiency and sorbent utilization in the FGD system, the reactivity of sorbents can be improved by the addition of siliceous materials. Siliceous compounds can be found in fly ash, which mainly contains SiO 2 , Al 2 O 3 , Fe 2 O 3 , and CaO. The use of fly ash in FGD systems offers economic advantages and also reduces environmental pollution because it is a waste product from coal-fired power plants. The abundance of SiO 2 and Al 2 O 3 in fly ash makes it considered as a pozzolanic material. SiO 2 and Al 2 O 3 react with limestone in the presence of water to form calcium silicate hydrates, calcium aluminate hydrates, or calcium aluminosili- cate hydrates. 1-3 The pozzolanic reaction products become deposited on the surface of fly ash, and its surface area increases with increasing hydration time and temperatures. 4 The surface area is an important parameter because sorbent reactivity with SO 2 strongly depends upon it. The pozzolanic products increase sorbent reactivity by making calcium ions more accessible during chemosorption reaction. 5 Recent studies on the use of fly ash in FGD systems have indicated an improved sorbent utilization when it is used during sorbent preparation. Ogenga et al. 1 studied the influence of hydration variables on South African calcium/siliceous-based material. An increase in the specific surface area of the sorbent prepared from fly ash was observed from 8.8 to 23.6 m 2 /g. It was also found out that the sorbent had an increased porous structure compared to fly ash or CaO alone. Chiung et al. 2 also studied the kinetics of reaction of Ca(OH) 2 /fly ash sorbent with SO 2 . The sorbent prepared from Ca(OH) 2 and fly ash showed increased reactivity toward SO 2 , as compared to pure Ca(OH) 2 with a high degree of Ca utilization observed. An increase in the conversion of SO 2 was also observed with an increase in the specific surface area in the prepared sorbent. Shi and Xu 6 found out that lime reactivity improves significantly when fly ash is added because fly ash increases lime dispersion and extends its effective surface area. Lee et al. 5 developed regression models to correlate the significance of variables to the surface areas in different sorbents. Their results showed that sorbents prepared from fly ash and oil palm exhibited the highest SO 2 removal capacity, which was attributed to the difference in microstructural properties of the sorbents. Their X-ray diffraction (XRD) analysis indicated the presence of calcium aluminosilicate hydrate as a product of hydration, which contributed to an increased surface area. The effect of fly ash on the reactivity of limestone is investigated in this study. This is performed using a pH stat apparatus by analyzing calcium ions in solution, which provides a direct measure of limestone reactivity in FGD systems. The experimental results were correlated to the shrinking core model during dissolution of limestone in a wet FGD process. A central composite design (CCD) is used to investigate the effects of the reaction variables (i.e., slurry pH, reaction temperature, fly ash/limestone ratio, and acid concentration) on the dissolution rate constant and also develop a predictive quadratic model, which relates the rate constant and dissolution variables. Received: January 31, 2015 Revised: April 21, 2015 Article pubs.acs.org/EF © XXXX American Chemical Society A DOI: 10.1021/acs.energyfuels.5b00243 Energy Fuels XXXX, XXX, XXX-XXX