Synthesis of a hollow microtubular Ca/Al sorbent with high CO2 uptake by hard templating

A novel hollow microtubular Ca/Al CO2 sorbent was synthesized through a hard templating pathway using absorbent cotton as a template, limestone as a CaO precursor and aluminum nitrate as an Al2O3 precursor. The synthetic Ca/Al sorbent consists of hollow porous microtubes with diameters ranging from 0.5 to 5 μm and replicates the biomorph of the absorbent cotton template well. When the mass ratio of CaO/Al2O3 is 90:10, the hollow microtubular Ca/Al sorbent exhibits the highest CO2 capture capacities during the cycles. The CO2 capture capacities of the novel Ca/Al sorbent after 50 cycles under a mild calcination condition (850 °C in pure N2) and a severe calcination condition (920 °C in 70% CO2/30% N2) are respectively 0.55 and 0.38 g/g, which are 7.91 and 8.93 times greater than those of limestone. The main compositions of the walls of the microtubes in the novel Ca/Al sorbent are CaO and Ca12Al14O33. Ca12Al14O33, as a good supporter, maintains the structural stability of the microtubes during the CO2 capture cycles. CO2 diffuses into the porous wall of the microtube in both directions (simultaneously into the outer and inner surfaces), so the CO2 diffusion resistance in the microtubes is low. The calcium looping system using the novel Ca/Al sorbent shows much lower energy consumption of the calciner and lower cost, compared with using limestone. Thus, Ca-Al-T appears promising in the calcium looping.