The effect of steam on CO2 uptake and sorbent attrition in fluidised bed calcium looping: The influence of process conditions and sorbent properties

Calcium looping was investigated with a focus on the effect of exposure to steam during calcination and/or carbonation stages. Experiments were carried out using an Italian limestone in a lab-scale fluidised bed reactor. Calcium looping cycles were performed at temperatures of 940 and 650 degrees C and in atmospheres containing 70 and 15% CO2 for calcination and carbonation stages, respectively. When present, steam accounted for 10% of the fluidising gas. CO2 concentration in the flue gas was continuously monitored for the evaluation of the CO2 capture capacity in each carbonation stage. The rate of fines generation by attrition during iterated carbonation and calcination stages was measured by collection of elutriated fines in filters at the exhaust. The extent of fragmentation was characterised by analysis of the particle size distribution of in-bed sorbent fragments after each calcination and carbonation stage. Porosimetric analyses on selected samples complemented the overall characterisation. Results of this investigation were compared with those obtained in a previous study on a different (more reactive) limestone sorbent with the aim of outlining a general mechanistic frame of the mutual relationships among the exposure to steam, sorbent microstructural features, CO2 capture capacity and attrition. The propensity to attrition and fragmentation was specifically scrutinised, as it has been largely unexplored in the literature under operating conditions relevant to calcium looping.