Investigations at a 10 kWth calcium looping dual fluidized bed facility: Limestone calcination and CO2 capture under high CO2 and water vapor atmosphere

Abstract

Calcium looping post-combustion CO2 capture process takes advantage of the reversible carbonation of CaO, during which CO2 contained in flue gas is absorbed by CaO forming CaCO3 and then is released when the reverse step of the reaction, named calcination, takes place. In the scope of the present paper, the performance of the process will be presented as recorded from experiments performed at University of Stuttgart, in the 10kWth Dual Fluidized Bed calcium looping facility under conditions closer to those expected industrially: wet flue gas in the carbonator reactor and atmospheres rich in CO2 and H2O in the regenerator reactor. The decomposition of limestone was realized under CO2 presence up to 75vol.% d.b., water vapor presence up to 35vol.%, balanced with N2 while the carbonation of CaO was carried out under 10–16vol.% CO2 and up to 10vol.% water vapor. Various temperatures were tested, i.e. between 620 and 680°C for the carbonator and 870 up to 930°C for the regenerator. Results showed that the demands in Ca to be circulated between the reactors is an increasing function of the CO2 vol.% in the regenerator and decreasing function of the presence of water during both sorbent carbonation and calcination. The carbonator CO2 capture efficiency decreases with increasing carbonator temperature and decreasing regenerator temperature. Efficiencies for both carbonator and regenerator of more than 90% were recorded for looping ratios (molCa/molCO2) less than 10 under oxyfired conditions when water vapor was present. With regard to chemical properties, the sorbent exhibited a residual activity (also referred to as average maximum carbonation conversion) almost double (∼20%) in the case where water vapor was present in the carbonator and regenerator, as opposed to when water vapor was absent. However, sorbent friability was enhanced resulting in material loss up to 0.095molCa/molCO2 to be captured or 4.75wt.%/h based on the total system inventory.