The combined effect of H2O and SO2 on CO2 uptake and sorbent attrition during fluidised bed calcium looping

The effect of steam and sulphur dioxide on CO2 capture by limestone during calcium looping was studied in a novel lab-scale twin fluidised bed device (Twin Beds – TB). The apparatus consists of two interconnected batch fluidised bed reactors which are connected to each other by a duct permitting a rapid and complete pneumatic transport of the sorbent (limestone) between the reactors. Tests were carried out under typical calcium looping operating conditions with or without the presence of H2O and/or SO2 during the carbonation stage. Carbonation was carried out at 650°C in presence of 15% CO2, 10% steam (when present) and by investigating two SO2 levels, representative of either raw (1500ppm) or pre-desulphurised (75ppm) typical flue gas derived from coal combustion. The sorbent used was a reactive German limestone. Its performance was evaluated in terms of CO2 capture capacity, sulphur uptake, attrition and fragmentation. Results demonstrated the beneficial effect of H2O and the detrimental effect of SO2 on the CO2 capture capacity. When both species were simultaneously present in the gas, steam was still able to enhance the CO2 capture capacity even outweighing the negative effect of SO2 at low SO2 concentrations. A clear relationship between degrees of Ca carbonation and sulphation was observed. As regards the mechanical properties of the sorbent, both H2O and SO2 hardened the particle surface inducing a decrease of the measured attrition rate, that was indeed always very low. Conversely, the fragmentation tendency increased in presence of H2O and SO2 most likely due to the augmented internal stresses within the particles. Clear bimodal particle size distributions for in-bed sorbent fragments were observed. Microstructural scanning electron microscope and porosimetric characterisations aided in explaining the observed trends.