Self-Activation Mechanism Investigations on Large K₂CO₃-Doped Li₄SiO₄ Sorbent Particles

Abstract

This work investigated the self-activation behavior of large K 2 CO 3 -doped Li 4 SiO 4 sorbent particles. In this selfactivation mechanism, the sorption ability increased as the number of cycles increased. After the sorption−desorption cycles occurred, the sorption ability of the K 2 CO 3 -doped Li 4 SiO 4 sorbent was remarkably enhanced from approximately 2.0 mmol CO 2 /g sorbent to approximately 5.0 mmol CO 2 /g sorbent at 565°C in 10 vol % CO 2 atmosphere. The fresh and used sorbents were then characterized through N 2 adsorption and SEM methods. Results showed that the average pore size increased from 7 to 32 nm and the surface microstructure changed from dense to porous, because the molten eutectic mixture formed by Li 2 CO 3 and Li 2 SiO 3 can facilitate CO 2 diffusion. The formed CO 2 diffusion channel can provide more CO 2 accessibility; this channel can also reduce the CO 2 diffusion resistance through the product layer. Therefore, the sorption ability of the sorbent is enhanced. Meanwhile, the effects of the self-activation temperature were also investigated and the results revealed that the optimal selfactivation temperature is 615°C. Furthermore, under critical conditions, the self-activated sorbent performed more efficiently than the fresh sample. At 450°C under 10 vol % CO 2 atmosphere, the sorption capacity of the self-activated sorbent was approximately 20 times higher than that of the fresh sample. Finally, a pore−core model was also proposed to illustrate the K 2 CO 3 -doped Li 4 SiO 4 self-activation mechanism. INTRODUCTIONHydrogen is used in various applications, such as ammonia and methanol syntheses, hydrocracking and hydro processing in refineries, and fuel cell production. Currently, hydrogen is mainly generated using the steam methane reforming (SMR) system. However, this traditional method of hydrogen production requires high operation temperatures (800−900°C ), water−gas shift reactors, and purification units for CO 2 removal. 1,2 To overcome the thermodynamic limitations of the SMR system, Carvill 3 proposed a sorption-enhanced SMR (SE-SMR) system, which is a promising technology to replace the conventional SMR system. Compared with the SMR system, the SE-SMR has many advantages, such as relatively low operation temperatures, increased CH 4 conversion and H 2 production, simplified process, and reduced capital cost. The key point of this system is to develop a satisfactory CO 2 solid sorbent. At present, several high-temperature absorbent materials have been proposed as the CO 2 acceptor, such as CaO-based sorbents, 4−7 hydrotalcite-like materials, 8,9 and lithium-based sorbents. 10−12 Among the absorbent materials for CO 2 sorption, lithium orthosilicate (Li 4 SiO 4 ) is one of the most promising because of its large CO 2 capacity, excellent stability, and fast absorption kinetics. The CO 2 sorption process is based on the following reversible reaction:

Environmental Benign Synthesis of Lithium Silicates and Mg-Al Layered Double Hydroxide from Vermiculite Mineral for CO2 Capture

“…This is because more elements were leached out into the acidic wastewater, which was consequently precipitated into LDHs by reacting with Na 2 CO 3 and NaOH. This result clearly indicates that the concentration of leaching acid greatly influences the final products, which may eventually influence the CO 2 capture performance of the final Li 4 SiO 4 and LDH-derived sorbents. The synthesis process was monitored using various material characterization tools.…”

“…The environmentally benign synthesis scheme for obtaining Li 4 SiO 4 and Mg-Al-CO 3 LDH from the vermiculite mineral is illustrated in Figure 1.The raw vermiculite mineral was first thermally pretreated at 900 • C to produce expanded vermiculite (EXVMT) with a higher specific surface area. The chemical composition of the samples was determined by X-ray fluorescence (XRF) spectroscopy (Panalytical Axios-mAX), which included SiO 2 (40.8 wt %), MgO (21.7 wt %), Al 2 O 3 (20.8 wt %), K 2 O (6.5 wt %), Fe 2 O 3 (5.0 wt %), CaO (2.9 wt %), Na 2 O (1.4 wt %), and TiO 2 (0.9 wt %).…”

“…Then, the obtained EXVMT was leached in acidic solution at 80 • C, by which SiO 2 powder and an acidic wastewater containing Mg 2+ , Al 3+ , and other metals, were formed ( Figure 1a). Following this, Li 4 SiO 4 could be synthesized using a solid-state method by reacting the above obtained SiO 2 powder with lithium nitrate (Figure 1b), and Mg-Al-CO 3 LDH could be prepared using a co-precipitation method from the above obtained acidic wastewater (Figure 1c). During the whole synthesis procedure, both the obtained SiO 2 and the acidic wastewater were utilized, representing a promising and environmentally benign route for converting vermiculite minerals into high-value added products.…”

“…Fossil fuel combustion is a major CO 2 source, which accounts for >40% of the total manmade CO 2 among all of the different types of CO 2 emissions [3]. In recent years, pre-combustion CO 2 capture from sorption enhanced hydrogen production (SEHP) processes has attracted considerable attention for capturing CO 2 from flue gas [4][5][6][7][8][9][10][11]. For the SEHP technique, one of the key challenges is the selection of a suitable solid CO 2 sorbent [12,13].…”

“…So far, there are a few reports on the synthesis of Li 4 SiO 4 from waste silicon-containing materials including fly ash [26], rice husk ashes [27], and halloysite [28]. The Li 4 SiO 4 which was synthesized with rice husk ashes achieved a 30.5 wt % maximum sorption at 680 • C and the Li 4 SiO 4 which was synthesized with halloysite reached a maximum adsorption capacity of 34.75 wt % at 700 • C. However, there are almost no reports on the synthesis of LDH-derived CO 2 sorbents from either industrial solid wastes or minerals.…”