Characterization of sorbents
CaO–Al2O3 sorbents were synthesized using the above-mentioned citrate preparation route, and the synthetic sorbents were activated with a four-step heating mode (170 °C/340 °C/440 °C/720 °C, as shown in Fig. 2b). After activation at high temperature, CaO–Al2O3 sorbents are labeled as CA-91 with 91% CaO content, CA-82 with 82% CaO content, and untreated CaO without Al2O3 additive, respectively. The morphologies of these sorbents were characterized by scanning electron microscopy (SEM), as shown in Fig. 4. From the SEM image of untreated CaO, it is found that CaO grains are packed densely with each other and severe sintering of CaO particles exists. CA-82 sorbent has a dense layer structure, and the layers are filled with many nano-grains. From the SEM image of CA-91 sorbent, the particle structure is more porous and sponge-like, and there are many nano-grains distributed on the porous surface. Comparing three SEM images, it is found that the CaO grain diameters of two synthetic CaO–Al2O3 sorbents are far smaller than that of untreated CaO.