Improved performance of modified CaO-Al2O3 based pellets for CO2 capture under realistic Ca-looping conditions

Abstract

Calcium looping (CaL) technology implementation under realistic conditions is limited due to some problems related with sorbents' deactivation at high temperatures, being the incorporation of inert additives or supports identified as a promising alternative to overcome these issues. In this work, alumina was incorporated to CaO-based pellets containing limestone and waste marble powder (WMP) as Ca source by extrusion followed by calcination at 800 °C and granulation, using different CaO/Al2O3 ratios (50/50, 60/40 and 70/30) and Methocell™ as pore forming agent. Sorbents were tested in a laboratory scale fixed and fluidized bed reactor. The carbonation was carried out with 25% of CO2 at 700 °C, while the calcination was performed under realistic industrial conditions (70 – 80% of CO2 at 930 °C), foreseeing the subsequent CO2 utilization. During the CaL tests calcium aluminates (CaAl2O4 and Ca3Al2O6) were formed, reducing the amount of CaO available for capture (31–42% of free CaO) but improving the sorbents’ CO2 capture performance due to the role of these inert compounds as structural supports and the increase of sorbents’ surface area. The high amount of calcium aluminate support (58–69%) improved the mechanical resistance of the sorbents, with reduced fragmentation and abrasion problems during the fluidization. For both reactors, the highest CaO conversion was achieved for 60/40 CaO/Al2O3 based pellets. Considering the pure WMP sorbent as reference, the total amount of CO2 captured after 10 cycles by 60/40 CaO/Al2O3 based pellets (g CO2/g CaO) increased 93% and 135%, in the fixed and fluidized bed reactors, respectively.