High Performance of Fly Ash Derived Li ₄SiO₄-Based Sorbents for High Temperature CO₂ Capture

Shenggen Wu, Xiaoli Liang, Qi Zhang^*, Guiru Zhang

Department of Chemical Engineering, East China University of Science and Technology, Shanghai, China.

DOI: 10.4236/jeas.2021.111001                

Abstract

It is urgent to develop excellent solid CO₂ sorbents with higher sorption capacity, simpler synthetic process, better thermal stability and lower costs of synthesis in CO₂ capture and storage technologies. In this work, a number of Li₄SiO₄-based sorbents synthesized by lithium carbonate with three different kinds of fly ashes in various molar ratios were developed. The results indicate that the Li₂CO₃:SiO₂ mole ratio used in the sorbents synthesis significantly affects the CO₂ absorption properties. The sorption capacity increased with the excess of Li₂CO₃ first and then decreased when the excessive quantity was beyond a certain amount. The experiments found that FA-Li₄SiO₄_0.6, CFA-Li₄SiO₄_0.4, HCl/CFA-Li₄SiO₄_0.3 presented the best sorption ability among these fly ash derived Li₄SiO₄ samples, and the corresponding weight gain was 28.2 wt%, 25.1 wt% and 32.5 wt%, respectively. The three sorbents with the optimal molar ratio were characterized using various morphological characterization techniques and evaluated by thermogravimetric analysis for their capacity to chemisorb CO₂ at 450°C - 650°C, diluted CO₂ (10%, 20%) and in presence of water vapor (12%). The adsorption curve of FA- Li₄SiO₄_0.6 at different temperatures was simulated with the Jander-Zhang model to explore the influence of carbon dioxide diffusion on adsorption reaction. Further experiments showed that the adsorbent had a good sorption capacity in a lower partial pressure of CO₂ and the presence of steam enhanced the mobility of Li⁺. What’s more, FA-Li₄SiO₄_0.6, CFA-Li₄SiO₄_0.4 and HCl/CFA-Li₄SiO₄_0.3 particles showed satisfactory sorption capacity in fixed-bed reactor and excellent cyclic sorption stability during 10 sorption/ desorption cycles.

Keywords

Fly Ash, Lithium Orthosilicates, Molar Ratios, CO₂ Capture

1. Introduction

Carbon dioxide (CO₂) is considered to be the main cause of greenhouse effect. The CO₂ atmospheric concentration has reached an unprecedented high level of 410.79 ppm in 2018 due to the massive emission of carbon dioxide derived from the large-scale combustion of fossil fuel [1] [2] [3]. Carbon capture and storage (CCS) has been proposed as one of the most promising technologies to mitigate CO₂ emissions from flue gases of coal-fired plants and industrial sites [2] [4] [5]. However, it is a big challenge to develop excellent CO₂ sorbents with high sorption capacity, simple synthetic process, high thermal stability and low costs of synthesis. The use of solid sorbents at high temperature is receiving increasing attention due to low energy and material consumption compared to low temperature capture system. High-temperature solid sorbents used in post-combustion system can avoid high regeneration requirements, equipment corrosion and high solvent costs problems, and can effectively use the waste heat from power or industrial sites [6]. The typical solid sorbents are hydrotalcite-like sorbents, CaO-based sorbents and Li₄SiO₄-based sorbents. Hydrotalcite-like sorbents absorb CO₂ in the temperature range of 200˚C - 400˚C and can be regenerated at a relatively low temperature. However, its moderate CO₂ sorption capacity has quite limited its practical application as a CO₂ sorbent. CaO-based sorbents have competitive advantages because of high theoretical sorption capacity and low cost [7]. But the rapid loss of reaction activity due to sintering, attrition and elutriation [8] and high desorption temperature during absorption/desorption cycles challenges its practical application [9].

Li₄SiO₄ is a promising sorbent because of its high CO₂ capture capacity, rapid sorption rates and mild regeneration temperature (<750˚C) compared with the regeneration temperature of CaO (>900˚C) [10] [11] [12]. With the increasing demand of solid CO₂ sorbents, reducing the cost of synthesis will undoubtedly enhance the edge of Li₄SiO₄-based sorbents for CO₂ capture in flue gas applications [13] [14]. Low cost materials like fly ash, rice husk ash, diatomite as SiO₂ precursors used to synthesize the Li₄SiO₄-based sorbents has attracted considerable attention in recent years due to their high availability and low cost [15] [16] [17]. Fly ash is a mineral residue resulting from the combustion of coal in power plants, and it contains considerable silica (SiO₂), alumina (Al₂O₃), iron oxides (Fe₂O₃), calcium oxides (CaO) [18]. Relevant research reported that Li₄SiO₄ doping with Al, Fe, Ca could enhance its sorption ability and Li⁺ mobility [19] [20] [21]. Therefore, the preparation of Li₄SiO₄-based sorbents from fly ash is expected. This practice not only can reduce the cost of CO₂ sorbents, but also can solve the problem of fly ash disposal to minimize significant economic and environmental impacts.

Olivares-Marín et al. [22] firstly reported novel Li₄SiO₄-based sorbents from fly ash. The sorbent derived from fly ash in the presence of 40 mol% K₂CO₃ presented a 10.7 wt% sorption capacity at 600˚C in 100% CO₂ atmosphere and reached the plateau of maximum capture capacity in less than 15 mins. Izquierdo et al. [23] synthesized fly ash derived Li₄SiO₄-based sorbents and found that the sorption capacity of the sorbents from fly ash and Li₂SiO₃ or LiOH via solid state method were 7.3 wt% and 11.3 wt% at 600˚C, respectively. In our previous work [24], Li₄SiO₄-based sorbents from fly-ash(FA) subjected to different pretreatments were synthesized for CO₂ capture at high temperatures, finding that different preconditioned FA had different components and significantly affected the morphology, the porosity and the adsorption performance of the sorbents. However, the Li₂CO₃:SiO₂ mole ratio used to synthesize Li₄SiO₄-based sorbents remains to be studied.

In this work, three starting fly ashes (original fly ash, pre-calcined fly ash, further acid leached and pre-calcined fly ash) were utilized to develop fly ash derived Li₄SiO₄-based sorbents. The effects of the mole ratios of Li₂CO₃:SiO₂ (fly ash) on sorption capacity, the sorbents compositions and particle size were investigated. The effects of experiment conditions, including the temperature, CO₂ partial pressures and moisture were also studied. The fixed-bed reactor has been used to analyze the sorption performance of particle sorbents.

2. Experimental Section

2.1. Preparation of Sorbents

Different Li₄SiO₄-based sorbents were synthesized by using three kinds of fly ashes (FA, CFA, HCl/CFA) and excessive lithium carbonate (AR, Sinopharm Chemical Reagent Co. Ltd) as starting materials, and the Li₂CO₃:SiO₂ molar ratio was (2 + x):1, where x varied in the range of 0.1 to 0.7. The CFA was obtained by calcining the original FA at 900˚C for 10 h in air, and the HCl/CFA was obtained as follows: 1) immersing the CFA samples in 10% conc. (weight) HCl aqueous solution at 60˚C for 2 h; 2) washing several times with deionized water until no acid was detected in the filtrate; 3) drying the resultant at 100˚C for 12 h. The quantitative composition of the three fly ashes was analyzed by X-Ray Fluorescence Spectrometer and reported in our previous paper [24]. The mixed powder samples were calcined at 750˚C for 6 h. After calcination, the obtained samples were grounded by using a mortar and were subsequently screened into 20 - 40 mesh large particles, and the rest were further homogenized into powder. The obtained samples from original fly ash, calcined fly ash, further acid leached and pre-calcined fly ash were named as FA-Li₄SiO₄_x, CFA-Li₄SiO₄_x and HCl/CFA- Li₄SiO₄_x, respectively.

2.2. Characterization of Sorbents

The phase compositions of the developed Li₄SiO₄-based sorbents were analyzed by X-ray diffraction with a diffractometer (RIGAKU D/MAX 2550 VB/PC, Japan) coupled to a copper anode X-ray tube and XRD peaks were identified using Jade 6.0 software. The surface morphologies of the sorbents were observed by scanning electron microscope (SEM, JSM-6360LV). The specific surface area, pore volume and pore size distribution of the synthesized sorbents were determined using a Micromeritics 3H-2720PS4 instrument.

The CO₂ sorption isotherms of the synthesized Li₄SiO₄-based sorbents were tested using a WRT-3P TG equipment. Before CO₂ sorption experiments, the sorbents were first heated from room temperature to 700˚C at a heating rate of 20˚C/min in N₂ flow (100 mL/min) to remove potential CO₂ and moisture until the weight stayed stable. Then, the temperature was changed to the desired sorption temperature, and the flow gas (100 mL/min) was switched to pure CO₂ flow or mixture gas (N₂ and 10 vol% or 20 vol% CO₂) to start the reaction for 150 min. Steam was also introduced into the reaction system, and the concentration of steam was controlled by changing the temperature of the water bath (12 vol% of H₂O concentration, vapor pressure of H₂O at 50˚C = 0.12 atm). The line from the water bath to the TG was wrapped with thermostatic bandage. Cycling tests were carried out using TGA. Sorption was conducted at 600˚C in pure CO₂ flow of 100 mL/min for 30 mins, and desorption was carried out at 700˚C in pure N₂ flow of 100 mL/min for 60 mins.

The particles of three fly ash derived Li₄SiO₄-based sorbents were tested in a fixed-bed reactor at various temperatures under pure CO₂ atmosphere. The adsorption section (60 mm long) was filled with 2 g of 20 - 40 mesh sorbent particles mixed with 20 - 40 mesh Rasching ring as a diluting agent. For the breakthrough experiments, the sorbent was heated to 700˚C in N₂ flow (100 mL/min) to remove potential CO₂ and moisture, and then switched the temperature to desired temperatures (450˚C - 600˚C) to adsorb CO₂ in pure CO₂ (30 mL/min). The exhaust was analyzed using an online M3000 micro gas chromatograph equipped with a TCD detector and a Porapak Q column. The equilibrium capacity was equal to the quantity of the absorbed CO₂ at equilibrium. The calculating equations were as follows [25]:

$S_{\text{abs}}=S_{\text{blank}}-S_{{\text{CO}}_{\text{2}}}$

$Q_{\text{abs}}=\frac{F\times S_{\text{abs}}\times M_{{\text{CO}}_{\text{2}}}}{24.5\times m}$

Among them, S_blank was the integral value of CO₂ concentration with time under the blank test condition; $S_{{\text{CO}}_{\text{2}}}$ was the integral value of CO₂ concentration with time in the outlet gas after the CO₂ adsorption reaction; F was the inlet gas flow, in ml/min; 24.5 was the molar volume constant of gas at room temperature; m was the mass of adsorbent added during the reaction process in g; $M_{{\text{CO}}_{\text{2}}}$ was the molar mass of carbon dioxide.

3. Results and Discussion

3.1. Optimization and Characterization of Adsorbents

The CO₂ sorption isotherms of three kinds of sorbents from different pretreatments with different Li₂CO₃:SiO₂ molar ratios at 600˚C are shown in Figure 1. It can be seen that the Li₂CO₃:SiO₂ molar ratio had a significant influence on the synthesized sorbents. The overall weight gain of three kinds of sorbents increased

Figure 1. CO₂ sorption isotherms of three kinds of sorbents from different pretreatments with different Li₂CO₃:SiO₂ molar ratios at 600˚C: (a) FA-Li₄SiO₄ sorbents; (b) CFA- Li₄SiO₄ sorbents; (c) HCl/CFA-Li₄SiO₄ sorbents.

with the excess of Li₂CO₃ first, then decreased when the excessive quantity was beyond a certain amount. Among these FA-Li₄SiO₄ samples, as shown in Figure 1(a), FA-Li₄SiO₄_0.6 sorbents presented the best sorption ability, whose weight gain was 28.2 wt%. While for CFA-Li₄SiO₄ and HCl/CFA-Li₄SiO₄ sorbents, as shown in the Figure 1(b) and Figure 1(c), the optimal samples were CFA- Li₄SiO₄_0.4 and HCl/CFA-Li₄SiO₄_0.3 respectively, and the corresponding sorption capacity was 25.1 wt% and 32.5 wt% respectively. According to the calculation chemical Equation (1), (2), (3) of facts age 6.4, the lithium carbonate can react with the alkali oxides contained in three kinds of FA at 750˚C, and therefore the lithium carbonate was insufficient to produce enough lithium orthosilicate before the content of Li₂CO₃ reached the optimal level. The by-product Li₂SiO₃ that has no adsorption capacity at a temperature higher than 250˚C was easy to generate due to the insufficient content of Li, resulting in low adsorption capacity of the synthesized sorbents [25]. However, excessive lithium carbonate doping led to a decrease in the content of the active component lithium orthosilicate in the adsorbent, and thus the adsorption performance decreased.

${\text{Li}}_{\text{2}}{\text{CO}}_{\text{3}}+{\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}={\text{2LiFeO}}_{\text{2}}+{\text{CO}}_{\text{2}}\Delta \text{G}=-\text{31}.\text{1kJ}$ (1)

${\text{Li}}_{\text{2}}{\text{CO}}_{\text{3}}+{\text{Al}}_{\text{2}}{\text{O}}_{\text{3}}={\text{2LiAlO}}_{\text{2}}+{\text{CO}}_{\text{2}}\Delta \text{G}=-\text{53}.\text{16kJ}$ (2)

${\text{Li}}_{\text{2}}{\text{CO}}_{\text{3}}+\text{CaO}+{\text{SiO}}_{\text{2}}={\text{Li}}_{\text{2}}{\text{CaSiO}}_{\text{4}}+{\text{CO}}_{\text{2}}\Delta \text{G}=-\text{82}.\text{45kJ}$ (3)

Three optimal Li₄SiO₄-based sorbents were further characterized by XRD, and the results are shown in Figure 2. The main components of the three sorbents were Li₄SiO₄, Li₂CaSiO₄, LiFeO₂ and LiAlO₂, and a small amount of Ca₂SiO₄, Li₂SiO₃, Li₂CO₃ and Ca(OH)₂. The existence of a small amount of Li₂CO₃ indicates that Li₂CO₃ was added in excess, which could avoid the formation of Li₂SiO₃ due to insufficient Li₂CO₃ and made sure the full reaction of SiO₂ and other metal elements. The FA-Li₄SiO₄_0.6 sorbent had a broad hump in the 2θ region between 18˚ and 35˚ which showed the presence of amorphous glassy phase [26]. This was because the calcination temperature was not enough high to release all amounts of amorphous glassy phase. Compared with FA-Li₄SiO₄_0.6 and CFA-Li₄SiO₄_0.4, HCl/CFA-Li₄SiO₄_0.3 had no sharp peak of Li₂CaSiO₄, and the Ca species mainly existed in the form of Ca(OH)₂. The decomposition product of Ca(OH)₂ was CaO, which could improve the CO₂ sorption capability of the sorbent by the occurrence of Li-Ca-CO₂ interactions ( ${\text{Li}}_{\text{4}}{\text{SiO}}_{\text{4}}+\text{CaO}+{\text{CO}}_{\text{2}}\to {\text{Li}}_{\text{2}}{\text{CaSiO}}_{\text{4}}+{\text{Li}}_{\text{2}}{\text{CO}}_{\text{3}}$ ) [13] [21]. Moreover, the peak of LiAlO₂ of HCl/CFA-Li₄SiO₄_0.3 sorbents increased more remarkably. Relevant literature [20] has reported that LiAlO₂ can promote the diffusion of Li⁺ which is beneficial to the increase of the sorption capacity of these three sorbents.

Figure 3 shows the SEM morphologies of the three optimal fly ash Li₄SiO₄- based sorbents. It can be seen that CFA-Li₄SiO₄_0.4 was mainly composed of irregular particle sizes, and its average particle sizes were as large as 9 μm.

Figure 2. XRD patterns of FA-Li₄SiO₄_0.6, CFA-Li₄SiO₄_0.4 and HCl/CFA-Li₄SiO₄_0.3 sorbents.

Figure 3. SEM images of the three optimal sorbents.

Moreover, there were grain agglomerates between particles. This kind of surface morphology is mainly related to the high-temperature solid phase synthesis method because sorbent particles are easy to migrate and aggregate together at high temperatures [27]. While the surface morphologies of FA-Li₄SiO₄_0.6 and HCl/CFA-Li₄SiO₄_0.3 were obviously different from that of CFA-Li₄SiO₄_0.4. Their particles were relatively looser and the average particle size was smaller (about 3 μm and 5 μm, respectively). And there were also fewer agglomerations between particles. FA-Li₄SiO₄_0.6 exhibited the smallest particle size of the three sorbents. It was reported that that alkali metal, alkaline earth metal, Mg, Al and other elements can inhibit the growth of sorbent grains and reduce the sintering effect of sorbent particles [28] [29]. Since the FA was not calcined and contained a large amount of hetero elements such as an alkali metal, the FA derived FA-Li₄SiO₄_0.6 had smaller particle size.

Figure 4 compares the N₂ adsorption/desorption isotherms of three optimal Li₄SiO₄-based sorbents. These three sorbents presented similar adsorption/de- sorption isotherms belonging to “type II” isotherm [30], indicating that the resulting sorbent samples had non-porous structure and aggregates of plate-like particles. According to the IUPAC classification, the three sorbents exhibited narrow “H3” hysteresis loops, indicating that the sorbents contained irregular stacked pores or slit-like pores.

Table 1 summarizes the BET surface area, average pore diameter and pore volume for three samples. As can be seen from the table, FA-Li₄SiO₄_0.6 and

Figure 4. N₂ adsorption/desorption isotherms of three optimal Li₄SiO₄-based sorbents.

Table 1. Physicochemical properties of three optimal Li₄SiO₄-based sorbents.

HCl/CFA-Li₄SiO₄_0.3 had higher specific surface areas (1.97 and 1.94 m²/g respectively) than CFA-Li₄SiO₄_0.4 (1.0087 m²/g). This result was consistent with the particle size distribution of SEM images. The adsorbent with smaller particles had larger specific surface area. The average pore diameters of three Li₄SiO₄-based sorbents were between 2 - 50 nm, which belonged to mesoporous materials.

3.2. CO₂ Capture Studies

3.2.1. Effect of Temperature

Figure 5 shows the CO₂ uptake isotherms of the three sorbents at pure CO₂ at different sorption temperatures to evaluate the effect of temperature on CO₂ uptake. The CO₂ adsorption capacity of these three adsorbents first increased with an increasing temperature and then decreased when the experimental temperature exceeded 600˚C, which showed a similar sorption trend to previous literature [22] [24] [29]. This phenomenon could be mainly attributed to the disappearance of the mesopore on the external shell of the Li₄SiO₄ at high temperature [29]. FA-Li₄SiO₄_0.6 obtained its maximum CO₂ sorption capacity of 28.2 wt% at 600˚C. Furthermore, when the sorption temperature was 600˚C or 650˚C, the sorption rate of FA-Li₄SiO₄_0.6 was exactly rapid to obtain the maximum capacity

Figure 5. CO₂ sorption isotherms of FA-Li₄SiO₄_0.6, CFA-Li₄SiO₄_0.4 and HCl/CFA-Li₄SiO₄_0.3 under pure CO₂ at different temperatures.

within 10 mins, which was faster than other reported low-cost ashes derived from Li₄SiO₄-based sorbents [17] [22] [27] [31] [32] [33]. For CFA-Li₄SiO₄_0.4, the sorption capacity was 25.1 wt% at 600˚C. However, its sorption performance at 550˚C reached 26 wt%, beyond that of 600˚C with the sorption time prolonging enough, which is probably attributed to the relatively high content of LiFeO₂ in CFA-Li₄SiO₄_0.4 sorbent. The thermodynamic equilibrium temperature of LiFeO₂ was about 550˚C via FactSage 6.4 software analysis [24], indicating LiFeO₂ can adsorb CO₂ at the temperature of 550˚C to a certain extent while it cannot capture CO₂ at 600˚C. HCl/CFA-Li₄SiO₄_0.3 showed the most excellent sorption capacity among these three sorbents at 450,500,550,600 and 650˚C due to its relatively high surface area and content of LiAlO₂. The maximum CO₂ sorption capacity for HCl/CFA-Li₄SiO₄_0.3 was 32.5 wt% at 600˚C. When the temperature was 650˚C, the CO₂ uptake of the sorbent showed a decreasing trend at the later stage of reaction. This is because that too high temperature will lead to an increase in the CO₂ equilibrium concentration of the sorbent, while the concentration of CO₂ in the gas phase does not change. Therefore, an increase in temperature will lead to a decrease in the driving force of CO₂ diffusion to adsorbent particles, which is not conducive to the adsorption reaction, and even leads to the occurrence of a desorption reaction.

In order to understand the reaction mechanism of fly ash derived Li₄SiO₄-based sorbents, FA-Li₄SiO₄_0.6 adsorbents in pure CO₂ atmosphere at different temperatures were fitted to the Jander-Zhang model. Compared with other simulation methods, the Jander-Zhang model can well explain the influence of the diffusion of CO₂ in the shell on the CO₂ adsorption characteristics. The Jander-Zhang model assumes that the adsorption reaction is kinetically controlled by the diffusion of CO₂, and the effects of the CO₂ surface adsorption reaction rate of the adsorbent are not considered. The Jander-Zhang model can be described by the following formula:

${\left[1-{\left(1-Z\alpha \right)}^{1/3}\right]}^3=k{\left(P_{{\text{CO}}_{\text{2}}}\right)}^{n}t$

where k is the reaction rate constant. Zα represents the proportion of Li₂CO₃ in the products layer (Li₂SiO₃ and Li₂CO₃), and $\left(1-Z\alpha \right)$ represents the proportion of unreacted Li₄SiO₄ and generated Li₂SiO₃. Because the ratio of density to molar mass of Li₂CO₃ is similar to Li₂SiO₃, theoretically Z is supposed to be around 0.5. According to previous literature reports, the products of Li₄SiO₄ exist in the form of double shells, and the CO₂ concentration is nonlinearly distributed in the product layer. During the adsorption process of the Li₄SiO₄ sorbent, Li⁺ and O²⁻ are generated on the surface of the unreacted Li₄SiO₄ and diffuse to the outer surface of Li₂SiO₃; CO₂ diffuses through the solid Li₂CO₃ layer and reacts with Li⁺ and O²⁻ on the outer surface of Li₂SiO₃. Since the size of Li⁺ and O²⁻ are smaller than CO₂ molecules, the diffusion of CO₂ in the solid Li₂CO₃ layer is expected to be much slower than the diffusion of Li⁺ and O²⁻ in Li₂SiO₃. Therefore, CO₂ diffusion is more likely to be a reaction speed control step [31]. The fitting results of FA-Li₄SiO₄_0.6 adsorbents in pure CO₂ atmosphere at different temperatures are shown in Figure 6. The Jander-Zhang model can fit the experimental results well at 450˚C, 475˚C, 500˚C, 525˚C, 550˚C, which means that the diffusion rate of CO₂ plays a decisive role in the whole CO₂ adsorption process of the adsorbents at a lower temperature. The fact is in agreement with the fitting results of double exponential model reported in previous literature

Figure 6. Fit of CO₂ sorption on FA-Li₄SiO₄_0.6 sorbent with Jander-Zhang model.

[32] [33]. However, when the reaction temperature is more than 600˚C, the Jander-Zhang model cannot fit the initial stage of CO₂ adsorption well, which illustrates that at high temperatures, the reaction rate is determined by the surface chemisorption reaction rate in the initial stage of the reaction and by the diffusion rate of CO₂ in the later stage of the reaction.

3.2.2. Effect of Diluted CO₂ and Moisture on Sorption

The effects of various CO₂ concentration and humidity atmosphere on the sorption behavior of the three sorbents were investigated. In this experiment, moisture content of 12 vol% and different CO₂ partial pressures (from 10 vol% to 100 vol%) at 600˚C were used to evaluate the influences of humidity and various CO₂ partial pressures on the developed Li₄SiO₄-based sorbents. As can be seen from Figure 7, due to the limitation of sorption equilibrium of Li₄SiO₄, the weight gain of the three sorbents in the presence and absence of steam decreased when the CO₂ partial pressure reduced from 100 vol% to 10 vol% [31]. The adsorption capacity of the three adsorbents in the presence of 20 vol% CO₂ can achieve about 70% - 80% of that of the three adsorbents under pure carbon dioxide, indicating that the Li₄SiO₄-based sorbents have a good practicability at a lower partial pressure of CO₂. And the presence of 12 vol% steam enhanced the sorption uptake due to the enhancement of the mobility of Li⁺ in the diffusion control stage according to double-shell mechanism [34]. As a result, the absorption capability and sorption rate of Li₄SiO₄-based sorbents were improved.

3.2.3. Fixed-Bed Reactor Test and Stability Test

Besides, a fixed-bed reactor was used to investigate CO₂ sorption on the prepared fly ash derived Li₄SiO₄-based sorbents. Compared to thermogravimetric

Figure 7. CO₂ sorption isotherms of (a) FA-Li₄SiO₄_0.6 and (b) CFA-Li₄SiO₄_0.4 and (c) HCl/CFA-Li₄SiO₄_0.3 under different CO₂ pressure with 12 vol% H₂O.

analysis (TGA), fixed-bed reactors can provide more reliable sorption capacity in formation in practical application. Figure 8 shows the CO₂ sorption performance of FA-Li₄SiO₄_0.6, CFA-Li₄SiO₄_0.4 and HCl/CFA-Li₄SiO₄_0.3 sorbent particles (20 - 40 mesh) in the temperature range from 450˚C to 650˚C in pure CO₂ flow in a fixed-bed reactor. It can be seen that the three sorbents had a similar tendency of sorption performance. The equilibrium and breakthrough capacities of three sorbents initially increased and then decreased with the increase of temperature. All of the three sorbents reached the highest sorption capacity at 600˚C, and the maximum sorption capacities of FA-Li₄SiO₄_0.6, CFA-Li₄SiO_{4_}0.4 and HCl/CFA-Li₄SiO₄_0.3 were 0.213 g CO₂/g sorbent, 0.153 g CO₂/g sorbent and 0.238 g CO₂/g sorbent respectively. The experimental results

Figure 8. CO₂ capture breakthrough curves of (a) FA-Li₄SiO₄_0.6 and (b) CFA-Li₄SiO₄_0.4 (c) HCl/CFA-Li₄SiO₄_0.3 in the fixed-bed reactor at different temperatures in 100 vol% CO₂ atmosphere.

in the fixed-bed reactor were consistent with the sorption trends of the powder sorbents tested via TG. Compared with the sorption capacity of the powdered sorbents, the sorption capacity of FA-Li₄SiO₄_0.6, CFA-Li₄SiO_{4_}0.4 and HCl/CFA- Li₄SiO₄_0.3 particles was lower than that of the powdered sorbents, which was attributed to the larger diffusion resistance of CO₂ in the particulate sorbents than that in the powdered sorbent.

HCl/CFA-Li₄SiO₄_0.3 is the best sorbent in terms of sorption capacity, while FA-Li₄SiO₄_0.6 sorbent has the advantage of simple synthetic process and relatively

Figure 9. Stability test of CO₂ sorption-desorption cycles on FA-Li₄SiO₄_0.6 sorbent (absorption: 600˚C, 100 vol% CO₂; desorption: 700˚C, 100 vol% N₂).

good adsorption performance, the industrial prospect is promising. Our previous literature [24] has reported the HCl/CFA-Li₄SiO₄ sorbent had an excellent cyclic performance. In order to investigate the cyclic stability of FA-Li₄SiO₄_0.6, ten sorption and desorption cycles were carried out in a pure CO₂ flow at 600˚C. The results are shown in Figure 9. It can be seen that FA-Li₄SiO₄_0.6 presented an excellent cycling stability. Its CO₂ sorption capacity reduced slightly from 26 wt% to 24 wt% after 10 cycling processes. However, it should be pointed out that during the desorption process, the desorption rate was fast in the early stage, but became slower in the later stage, which resulted in the extension of the whole desorption time. Longer desorption time may be due to the lack of loose porous surface of the sorbent, which leads to a slower desorption rate at a later stage.

4. Conclusions

Li₂CO₃ and three kinds of fly ash from different pretreatments were used to synthesize Li₄SiO₄-based sorbents in different Li₂CO₃:SiO₂ molar ratios. The results indicate that the weight gain of three kinds of sorbents first increased with the excess of Li₂CO₃, and then decreased when the excess exceeded a certain amount. The optimal fly ash derived Li₄SiO₄-based sorbents with the best sorption ability among these samples are FA-Li₄SiO₄_0.6, CFA-Li₄SiO₄_0.4 and HCl/CFA-Li₄SiO₄_0.3, whose weight gain was 28.2 wt%, 25.1 wt% and 32.5 wt%, respectively. SEM and BET analyses indicate that FA-Li₄SiO₄_0.6 and HCl/CFA- Li₄SiO₄_0.3 had smaller particle sizes and higher specific surface areas than that of CFA-Li₄SiO₄_0.4. And XRD analyses indicate that HCl/CFA-Li₄SiO₄_0.3 had a relatively high content of Ca(OH)₂ and LiAlO₂, which were beneficial to the improvement of the sorption capacity.

The three kinds of optimal sorbents were tested in the temperature range of 450˚C - 650˚C, diluted CO₂ (10%, 20%) and in the presence of water vapor (12%). The optimal sorbents showed the best adsorption performance at 600˚C. When the CO₂ partial pressure reduced from 100% to 10%, 20%, the CO₂ sorption capacity decreased due to the limitation of sorption equilibrium of Li₄SiO₄. And the adsorption capacity of sorbents at 20 vol% CO₂ can reach 70% - 80% of that of the sorbents at 100 vol% CO₂. The presence of water vapor enhanced CO₂ absorption capacity at 600˚C due to enhancement of the mobility of Li⁺ in the diffusion control stage according to double-mechanism.

The experimental results indicate that FA-Li₄SiO₄_0.6 and HCl/CFA-Li₄SiO₄_0.3 showed excellent sorption capacity at 600˚C in fixed-bed reactor. Compared with the sorption capacity of the powdered sorbents, the sorption capacity of particles was lower than that of the powdered sorbents. The excellent cyclic sorption stability of FA-Li₄SiO₄ and HCl/CFA-Li₄SiO₄ during 10 sorption/desorption cycles were confirmed by previous literature and work in this article respectively.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Shanghai [Grant No. 16ZR1408200], and the Fundamental Research Founds for the Central University [Grant No. 222201817013].

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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