Effect of exposure to SO2 and H2O during the carbonation stage of fluidised bed calcium looping on the performance of sorbents of different nature

Abstract

The aim of this study is to investigate the effect of the concentration of sulphur dioxide and steam, in the flue gas subjected to CO2 capture by calcium looping, on the performance of two limestone-based sorbents. The experimental calcium looping tests were carried out in a purposely-designed Twin Bed lab-scale apparatus (consisting of two identical interconnected bubbling fluidised bed reactors), able to reproduce a realistic particle thermal/chemical/mechanical history. The two natural limestones (particle size range 0.4–0.6 mm) were selected in view of their remarkably “high” and “poor” reactivity to CO2, respectively. Calcium looping tests consisted of ten calcination/carbonation cycles (plus an eleventh calcination). Carbonation was carried out at 650 °C in an atmosphere containing 15% CO2 to simulate a typical combustion flue gas, while calcination was operated at 940 °C at 70% CO2 to simulate oxy-combustion conditions. Six different operating conditions for carbonation were tested to study the effect of SO2 and/or H2O, where steam (when present) was fed at 10%, and SO2 (when present) either at 75 ppm (a typical concentration in a pre-desulphurised combustion flue gas) or 1500 ppm (raw flue gas). The CO2 capture capacity was calculated for each carbonation stage, and the attrition rate was obtained by collection of elutriated fines from both reactors. At the end of the test, sorbent particles were further analysed for the determination of: degree of calcium sulphation, particle size distribution, porosimetric and microscopic properties. Relationships among sorbent type, operating conditions and experimental results are here discussed in detail, with the aim of originally outlining general trends arising from the comparison of the performance of the two limestones, which are characterised by very different reactivity.