The impact fragmentation tendency of limestone particles in calcium looping systems: effect of steam and sulphur dioxide

The Calcium Looping (CaL) process carried out in dual interconnected Fluidized Bed (FB) systems is a technique able to capture the CO2 contained in the flue gases produced from power plants. It is based on the alternated temperature-swing CO2 uptake by a calcium-based sorbent, with CO2 capture taking place in a carbonator operated at around 650–700°C followed by the release of concentrated CO2 in a calciner operated at around 900–950°C, according to the reversible carbonation reaction CaO(s)+CO2(g)=CaCO3(s). The utilization of fluidized bed reactors implies the unavoidable occurrence of attrition and fragmentation phenomena, with consequent changes in the particle size (and residence time) distribution of the sorbent, which in turn may influence its CO2 capture capacity. Moreover, the possible presence, in the combustion flue gas entering the carbonator, of SO2 and/or steam would have relevant effects on the sorbent reactivity. In the recent past, the effect of the presence of SO2/H2O in the carbonator atmosphere on the CO2 capture capacity and on the abrasion tendency of the sorbent particles has been investigated. However, the effect of sulphur dioxide and steam on the impact fragmentation tendency of the sorbent in CaL systems has been so far neglected. Accordingly, the aim of this work was to fill this gap. Calcium looping experiments have been performed on a reference high-Ca limestone in a batch lab-scale twin fluidised bed apparatus, purposely designed for the study of chemical looping processes in conditions as close as possible to reality, in terms of cycling of temperatures and of reaction atmospheres. Six different operating conditions for carbonation were tested to study both the single effect of SO2 and H2O and the combined effect of them. The CO2 capture capacity was calculated for each carbonation stage and, at the end of the test, sorbent particles were further analysed for the determination of the degree of Ca sulphation. After the CaL tests, the exhausted sorbent particles were fed to an ex situ impact test apparatus, which is based on the well-established concept of entraining particles in a gas stream at controlled velocity, and impacting them against a target. The analysis of data showed that samples obtained in the presence of SO2 resulted to be harder than in absence of it, as higher SO2 concentrations in the carbonator determined thicker CaSO4-based shells around the particles. The presence of steam in the carbonator, orientating the reactivity of CaO towards CO2 rather than SO2 (when present), on the one hand determined particles less resistant than those obtained in the absence of H2O and in the presence of SO2. On the other hand, as steam indeed favours the carbonation reaction of CaO, we have observed particles with a larger CaCO3 fraction and therefore more resistant than those obtained in conditions where both steam and SO2 were absent.