Investigation on the SO2 Uptake by the Hydration Products of Fluidized Bed Combustion Waste

Fluidized Bed Combustion (FBC) residues contain both fuel-derived ash, characterized by a poor pozzolanic activity, and spent sorbent, originating from the partial conversion of CaO to CaSO4 through in situ reaction with SO2; due to its relatively high content of unreacted lime, the spent sorbent is responsible for exothermal and expansive phenomena upon hydration [1]. Hence FBC waste is generally unsuitable for recycling in traditional fields such as cement and concrete industries [2]. The application of water-reactivated FBC waste as a sorbent for in-furnace SO2 capture has long been considered a viable alternative to other uses. The underlying concept is based on the waste ability to quantitatively yield Ca(OH)2 by hydration of unreacted CaO [3,4]. For the spent sorbent particle, this process leads to the swelling of its core (mostly lime-based) that in turn favours the break-up of its shell (mostly CaSO4-based). When the hydrated sorbent is fed again to the combustor, it loses its chemically-bound water yielding further shell breakage, and the new-formed CaO is characterized by a larger specific surface area. In this way a new access to SO2 is provided upon resulphation of the reactivated sorbent. The hydration-reactivation method is attractive since it enhances the SO2 uptake, reduces the costs of waste disposal and the consumption of natural resources, limits the CO2 emissions related to the calcination of common carbonate-based sorbents.