WANG Jinxiang ¹, ZOU Nieyan ¹, SUN Qichao ¹, XU Hao ¹, YANG Rui ², PAN Mengyao ³

(1. School of Material Science and Engineering,  Anhui university of Science and Technology, Huainan 232001, Anhui, China;

2. Civil Aviation Flight University of China, Guanghan 618300, Sichuan, China;

3. Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China,

Chengdu 611731, Sichuan, China)

Extended abstract:[Background and purposes] Lithium orthosilicate (Li₄SiO₄) adsorbents have received extensive attention in the field of high-temperature CO₂ capture, due to their various advantages, such as large adsorption capacity, high adsorption reaction temperature and good cyclic stability. However, conventional Li₄SiO₄ adsorbents were usually synthesized using pure SiO₂, where dense structures and high cost of pure SiO₂ restrict the availability and practical application of these adsorbents. Rice husk ash has high content of amorphous SiO2, which is suitable for synthesizing Li₄SiO₄ adsorbents. However, the Li₄SiO₄ adsorbents were mostly synthesized with acid treated rice husk ash, which required much acid reagent and may produce environmental problems. Herein, rice husk ash without pretreatment was employed as silicon source to prepare a novel Li₄SiO₄ adsorbent using solid state method. To meet the practical application in industrial circulating fluidized bed and prevent Li₄SiO₄ powder from being elutriated out of the looping system, Li₄SiO₄ powder was further granulated into pellets.[Methods] Li₄SiO₄ adsorbents were prepared by solid-state method. Li₂CO₃ and rice husk ash were added together with n(Li₂CO₃)∶n(SiO₂)=2∶1, which were then continuously ground to ensure uniformity. After that, the mixture was calcined at 700 ℃ for 6 h to synthesize Li₄SiO₄. Finally, after grinding and sieving, Li₄SiO₄ adsorbent with particle size less than 100 μm was obtained. The produced adsorbent from rice husk ash without pretreatment was named as CA-Li₄SiO₄, while the adsorbent from acid-treated rice husk ash was named as A-CA-Li₄SiO₄. CA-Li₄SiO₄ powder was further granulated into pellets (named as CA-Li₄SiO₄-P) by using ball rolling method. Then, the pellets were dried at 80 ℃ for 12 h and calcined at 700 ℃ for 1 h to obtain CA-Li₄SiO₄-P pellets. Chemical compositions of the rice husk ash were determined by using X-ray fluorescence (XRF, Shimadzu). The phase composition of the synthesized adsorbents was detected by using X-ray diffraction (XRD, SmartLab SE). Morphology properties of Li₄SiO₄ adsorbents were detected by using scanning electron microscopy (SEM, TESCAN MIRA LMS). Specific surface area, pore volume and pore size distribution of Li₄SiO₄ adsorbents were determined by using an automatic specific surface area and pore analyzer (Micromeritics APSP 2460). Attrition resistance of pellets was evaluated by using a friability tester (CS-2). In addition, a thermogravimetric analyzer (TGA, SETARAM LABSYS) was used to analyze thermal stability of the CA-Li₄SiO₄-P pellets. CO₂ adsorption characteristic of the Li₄SiO₄ adsorbents and all TGA experiments were performed in a gas flow of 60 mL·min⁻¹.[Results] CO₂ adsorption capacity of the A-CA-Li₄SiO₄ increased from 12.9% to 19.9%, in the temperature range of 550–650 ℃. However, its adsorption was not saturated with 120 min. Compared with A-CA-Li₄SiO₄, CA-Li₄SiO₄ demonstrated a noticeably enhanced CO₂ uptake with faster reaction rate and higher adsorption capacity. The adsorption capacity of CA-Li₄SiO₄ reached 31.9% within 20 min at 650 ℃. The crystallite size was calculated from the XRD patterns via the Scherrer formula. The average crystallite size of CA-Li₄SiO₄ (49.8 nm) was smaller than that of A-CA-Li₄SiO₄ (60.9 nm). This may be related to the fact that the metal impurities in CA-Li₄SiO₄ inhibit its grain growth. During the CO₂ adsorption process, the smaller size of crystalline was beneficial for the occurrence of gas-solid reaction, thus enhancing the adsorption performance of CA-Li₄SiO₄. The specific surface area of A-CA-Li₄SiO₄ is 6.0 m²·g⁻¹. In comparison, CA-Li₄SiO₄ has a larger specific surface area of 7.4 m²·g⁻¹. In addition, CA-Li₄SiO₄ has a larger pore volume (0.047 cm³·g⁻¹) and abundant pores with diameters of 3–4 nm. This increased surface area and pore volume offers more active sites for CO₂, which could promote the CO₂ adsorption reaction of the adsorbent. Besides, the higher porosity is also beneficial for CO₂ to reach the inner part of the adsorbent to interact with Li₄SiO₄. CA-Li₄SiO₄-P pellets with diameters of 2–4 mm showed good structural stability and excellent anti-attrition performance, thus improving the wear resistance and reducing the loss of adsorbents in the looping system. Moreover, Li₄SiO₄ pellets showed relatively stable cycling performance and high adsorption capacity in the adsorption/desorption cycle test. In conclusion, Li₄SiO₄ adsorbents in powder form or in pellets form prepared by rice husk ash without pretreatment have promising prospects for CO₂ capture.[Conclusions] In this work, an economical and environmentally friendly Li₄SiO₄-based adsorbent was successfully synthesized from rice husk ash without pretreatment.(1) Isothermal experimental results showed that the CA-Li₄SiO₄ demonstrated a noticeably enhanced CO₂ uptake with faster reaction rate. The adsorption capacity of CA-Li₄SiO₄ reached 31.9% within 20 min at 650 ℃. Kinetic analysis revealed that the CA-Li₄SiO₄ had a faster chemical adsorption process and a quicker lithium diffusion stage, explaining its higher CO₂ capture performance.(2) XRD, SEM and N₂ isothermal adsorption-desorption tests showed that the alkali metal elements presented in rice husk ash inhibited the grain growth of Li₄SiO₄. Compared with A-CA-Li₄SiO₄, the CA-Li₄SiO₄ adsorbent exhibited a higher specific surface area and better pore structure, which is conducive to increase the CO₂ adsorption capacity of the adsorbent.(3) CA-Li₄SiO₄-P pellets showed high structural stability, excellent wear resistance and cyclic CO₂ adsorption stability, being promising for practical application in industrial circulating fluidized bed.

Key words: rice husk ash; Li₄SiO₄; CO₂ capture; ball rolling method; cyclic adsorption; attrition resistance