Role of Ettringite in the Reuse of Hydrated Fly Ash from Fluidized-Bed Combustion as a Sulfur Sorbent:  A Hydration Study

Abstract

Waste from fluidized-bed combustion (FBC) has a low potential for reuse. One possibility for its recycling lies in a hydration process aimed at reactivating the SO 2 sorption ability of the unconverted lime. The formation of ettringite, as well as calcium hydroxide, in the hydrated FBC fly ash to be reinjected into the reactor could be of importance because ettringite is able to play a chemical and physical role in SO 2 capture. The aim of this paper is to investigate the conditions under which ettringite is formed by the liquid-phase hydration of FBC waste. To this end, two industrial FBC fly ashes were hydrated at temperatures of 20 and 70 °C, for curing times ranging from 30 min to 96 h. Ettringite concentrations and hydration levels of up to 50 and 74%, respectively, were measured. The experimental data were also employed to set up a simple kinetic model for ettringite synthesis. With respect to the CaSO 4 concentration, this reaction was of first order for hydration times up to 8 h and of second order thereafter. In this work, the main characterization techniques employed were X-ray fluorescence, X-ray diffraction, and differential thermal and thermogravimetric analyses. Thermogravimetric analysis was also used for quantitative purposes.

Utilization of Fly Ashes from Fluidized Bed Combustion: A Review

“…They found that hydration at 70 • C for 48 h of the raw fly ash yields an ettringite-rich material, and ettringite represents the main source of free lime available for sulfur uptake as the reactivated ash is calcined. Bernardo et al [172] studied ettringite in more detail because it can play a chemical and physical role in capturing SO 2 . They investigated the conditions under which ettringite is formed by the liquid-phase hydration of FBC waste.…”

Design of High Volume CFBC Fly Ash Based Calcium Sulphoaluminate Type Binder in Mixtures with Ordinary Portland Cement

“…Bernardo et al [ 50 ] have suggested that ettringite formation starts immediately during the first hours of hydration, and that the rate of its precipitation is limited only by the availability of the reactants. Ettringite forms at the expense of the dissolved Ca, sulphate and aluminium [ 51 ].…”

Mechanical Activation of the Ca-Rich Circulating Fluidized Bed Combustion Fly Ash: Development of an Alternative Binder System

“…The availability of Ca-sulfate is typically considered as the limiting reactant for ettringite formation in hydrated CFBC ashes [55]. However, the Ca-rich CFBC ash is low in reactive silica and aluminum, but rich in Ca-sulfate, and it can precipitate with ettringite concurrent gypsum [27,56].…”