Kinetics of the Carbonation Reaction of Lithium Orthosilicate Using a Typical CO2 Concentration of Combustion Gases

The aim of this work was to investigate the carbonation kinetics of lithium orthosilicate (Li4SiO4) by thermogravimetry and via thermodynamic simulations, using CO2 concentrations of 15 vol.% (typical of combustion gases) and 100 vol.%. Tests were performed in a thermogravimetric analyzer, in two sequential steps: (1) pre-treatment at 750 oC with N2 and (2) thermal analysis, non-isothermal (at 10 oC min-1 up to 1000 oC) or isothermal (at 550 oC, 600 oC and 650 oC). According to the nonisothermal results, the carbonation of Li4SiO4 occurs in the range of 450-746 oC and the decarbonation above it. Also, it was possible to capture up to 24.9 wt.%CO2. The isothermal kinetics showed that an increase in temperature promotes an increase in the reaction rate. Yet, the adsorption capacity is limited by the thermodynamics at higher temperatures and the kinetics is slow at low CO2 concentrations.


Introduction
In the medium term, carbon capture and storage (CCS) is an essential technology for reducing CO 2 emissions through its capture in fossil fuel-fired power plants 1,2 . Several technologies are applied to remove CO 2 from exhaust gases; however, many still have economic and operating limitations. The main restricting factor is the high temperature at which the CO 2 is generated 3 . Hence, the development of technologies capable of removing this gaseous product at high temperatures, without the need of cooling the gaseous stream, is extremely desirable.
The technique applied to capture CO 2 from an exhaust of a power station includes the use of a calcium based sorbent (usually derived from natural limestone), which is repeatedly carbonated and calcined during the process [4][5][6][7] . Calcium oxide (CaO) owns a high CO 2 capture capacity at temperatures of 600-700 o C; however, it presents low stability during repeated carbonation and decarbonation cycles, and requires high energy for its complete regeneration at 950 o C: problems that must be surpassed in order to improve the process efficiency [8][9][10][11] . The sintering of CaO sorbent during calcination leads to a drastic reduction in the surface area, which can affect the adsorption reaction rates 1 . The carbonation reaction for CaO is given by Eq. 1 1,4,12 .

CaO CO CaCO
Ceramics of alkali metals (Li, Na, K, etc.) are included in another group of CO 2 sorbents suitable for high temperatures and greatly studied in recent years 10,13,14 . Among these, lithium containing materials such as lithium zirconate (Li 2 ZrO 3 ) and lithium orthosilicate (Li 4 SiO 4 ) seem to be promising CO 2 acceptors in the temperature range of 450-700 o C, being the Li 4 SiO 4 the most reactive [15][16][17][18][19] . The CO 2 uptake on Li 4 SiO 4 is almost 50 wt.% greater than the weight change for Li 2 ZrO 3 13,20 . The reaction of Li 4 SiO 4 is attributed to the mechanism by which lithium oxide (Li 2 O) within the crystalline structure of Li 4 SiO 4 reacts reversibly with CO 2 according to the reaction given by Eq. 2 3,13,14,20 . The lithium orthosilicate can theoretically adsorb up to 36.7 wt.% of its own weight.

Li SiO CO
Li SiO Li CO Advantages of using lithium orthosilicate in high temperature processes include the high sorption rate and capacity, excellent cyclability properties and raw materials of low cost 13,14,[20][21][22][23][24] . It also requires a much lower temperature to be recovered when compared to the CaO. According to Amorim et al. (2016) 24 , the advantages of Li 4 SiO 4 also include the fast carbonation and decarbonation kinetics, the use in 48-repeated reaction cycles, as well as the good mechanical properties. In general, a higher concentration of CO 2 is beneficial for the CO 2 sorption of Li 4 SiO 4 sorbents 17,25,26 . Still, the sorption performance of Li 4 SiO 4 sorbents decays with the decrease of CO 2 concentration and the suitable temperature to the desorption process depends on the CO 2 concentration 16 17,29 . It is known that the CO 2 concentration in flue gases depends on the fuel such as coal (12-15 vol.%CO 2 ) and natural gas (3-4 vol.%CO 2 ), and the effect of CO 2 concentration in the sorption performance of Li 4 SiO 4 should be deeply studied.
In this work, the carbonation kinetics of lithium orthosilicate was studied in non-isothermal and isothermal tests by thermogravimetry, and also, via thermodynamic simulations, in order to evaluate the carbon capture at concentrations of 100 vol.% and 15 vol.%CO 2 (typical of flue gases).

Material preparation and characterization
Lithium orthosilicate (Li 4 SiO 4 ) was supplied by Chemetall Company (Frankfurt, Germany) with purity of 97.5% and is presented as a crystalline powder with a particle diameter up to 350 µm and density of 2.4 g cm -3 3 . The specific surface area of Li 4 SiO 4 was calculated using the Brunauer-Emmett-Teller model 30 , being the isotherm of adsorption/desorption obtained in a New 2200E by Quantachrome. The particles morphology was determined by scanning electron microscopy (SEM) using a JEOL JSM-6390LV microscope. Crystalline phases of the solid were found by X-ray diffraction (XRD) analysis, conducted in an X'Pert diffractometer by Philips, with a scan of 0,038/s and Cu Kα radiation.
Gases used in the thermogravimetric tests were nitrogen (purity 99.996 vol.%) and industrial carbon dioxide, both supplied by White Martins company (Santa Catarina, Brazil).

Thermogravimetric measurements
Thermogravimetry is still the most widely used technique to determine the adsorption kinetics and provides accurate real-time data under well-controlled conditions. Accordingly, in this work, carbonation tests with CO 2 were carried out in a thermogravimetric analyzer model DTG-60 (Shimadzu, Japan). Reactions were conducted in two sequential steps 24,31 : (1) pre-treatment and (2) thermal analysis ((i) non-isothermal and (ii) isothermal conditions): (1) The pre-treatment was applied for all procedures, by heating the sorbent (10 mg, placed as a thin layer into a platinum pan) from room temperature up to 750 o C, under N 2 atmosphere. The aim of this stage was to eliminate impurities on the solid surface; (2) The thermal analysis was carried out as follows: (ii) Non-isothermal analysis were conducted immediately after cooling the solid down to room temperature, under N 2 , and then by heating at a rate of 10 o C min -1 up to 1000 o C, with 15 vol.%CO 2 (0.15 atm) or 100 vol.%CO 2 (1 atm). The aim of this experimental set was to identify the operating temperature range for Li 4 SiO 4 carbonation; (ii) Isothermal analysis were conducted after cooling the solid down to the final carbonation temperature of 550, 600 or 650 o C, under N 2 , and then by replacing the inert gas to the CO 2 gas, also with 15 or 100 vol.%CO 2 .
Reactions occurred until there was no more weight variation. Both N 2 and CO 2 flow rates were set to 100 mL min -1 during all experiments. The CO 2 partial pressure of 0.15 bar was selected to simulate a typical concentration of flue gases (combustion gases).

Thermodynamic data
Thermodynamic equilibrium reactions between Li 4 SiO 4 and CO 2 at different temperatures were theoretically determined using the FactSage® 6.3 program (FACT -Facility for the Analysis of Chemical Thermodynamics). This software has a thermodynamic database that allows calculating the conditions for multicomponent equilibrium by minimizing the Gibbs free energy.
The equilibrium partial pressure of CO 2 (for the carbonation reaction with Li 4 SiO 4 , Eq. 2) was calculated at different temperatures. Then, the changes in Gibbs free energy (ΔG) were evaluated for the carbonation reactions of both CaO (for comparison) and Li 4 SiO 4 (Eq. 1 and 2, respectively) as function of temperature for CO 2 partial pressures of 1 atm and 0.15 atm. According to the thermodynamics, the reaction proceeds when ΔG < 0. In the case of adsorption of CaO and Li 4 SiO 4 , ΔG depends on both temperature and CO 2 concentration, and becomes larger with the increase of these two parameters. The temperature at which the carbonation and decarbonation processes are "balanced" is called the equilibrium temperature, and is the temperature when ΔG equals zero.

Characterization
Porous structure analysis of Li 4 SiO 4 generated an N 2 adsorption-desorption isotherm of type II, corresponding to a non-porous or macroporous sorbent. The specific surface area determined by the BET method was 0.56 m 2 /g which is a characteristic value found for other compounds as Li 2 ZrO 3 and CaO 32-34 .
The low surface area can be a limiting feature to the carbonation reaction, once the CO 2 has no access to the entire active area of the solid, thus requiring the gas diffusion through the product layer 35 . SEM analysis for Li 4 SiO 4 with different approximations are shown in Fig. 1. The solid is basically composed of agglomerates of granular particles, mostly in polyhedral shape 23,24 . In addition, the solid surface possess a smooth feature, confirming the low surface area found by the porous structure analysis.
The XRD pattern presented in Fig. 2

Simulation results
The thermodynamic equilibrium reactions between Li 4 SiO 4 and CO 2 at 1.0 atm of CO 2 are presented in Tab. 1. It is verified that the sorbent reacts generating different products, depending on the temperature range. Furthermore, it is observed that the stoichiometric ratio CO 2 :Li 4 SiO 4 decreases with increasing temperature, which is a negative aspect from the viewpoint of CO 2 capture kinetics.
The variation of the equilibrium partial pressure of CO 2 as function of temperature is shown in Fig. 3. It can be seen that the amount of adsorbed CO 2 increases with temperature and, e.g., at a CO 2 pressure of 1.0 atm the equilibrium temperature of Li 4 SiO 4 is equal to 723 o C. It is important to note that the adsorption process (for both Li 4 SiO 4 and CaO) is strongly dependent on CO 2 partial pressure in the product flow, in a specific reaction temperature.   ? + + 724-1000 Decarbonation Figure 3. Equilibrium partial pressure of CO 2 as a function of temperature for Li 4 SiO 4 sorbent.
The changes in the Gibbs free energy (∆G) to the carbonation reactions of Li 4 SiO 4 and CaO are shown in Fig. 4 as a function of temperature. Negative values indicate the direct order of the reaction. The equilibrium temperature difference between the two sorbents was found to be equal to 170 °C (723 °C to Li 4 SiO 4 and 893 °C to CaO) at 1.0 atm of CO 2 (Fig. 1a). The Li 4 SiO 4 requires a considerably lower regeneration temperature when compared to the CaO, and, thus, the reaction between CO 2 and Li 4 SiO 4 is more easily reversible. Also, according to Fig. 4 (b), for 0.15 atm of CO 2 , the sorbents Li 4 SiO 4 e CaO have equilibrium temperatures of 596 o C and 777 o C, respectively, indicating that the CO 2 capture is expected to occur at temperatures lower than these.

Thermal decomposition
The thermogravimetric profile for the thermal decomposition of Li 4 SiO 4 under N 2 atmosphere is shown in Fig. 5. It is possible to identify two main steps of mass loss: (1) from room temperature up to 300 o C, attributed to the elimination of the water present on the solid surface (dehydration) and to the dehydroxylation process of Li 4 SiO 4 ; (2) from 400 o C up to 750 o C, related to the decarbonation process, once the sorbent is capable of capture CO 2 even at room temperature 13,20,23,37 . A total weight loss of 2.93 wt.% was verified, which is very close to the impurity content provided by the supplier, of 2.5 wt.%.

Non-isothermal carbonation
Thermogravimetric results for the non-isothermal analysis of the pretreated Li 4 SiO 4 are shown in Fig. 6. According to Fig. 6 (a), at 1.0 atm of CO 2 , the carbonation reaction occurs in the temperature range of 500-746 o C, with a CO 2 uptake of 24.9 wt.%, which is lower than the maximum theoretical capacity for this solid (36.7 wt%, 8.34 mmol CO 2 /g Li 4 SiO 4 ). Here, the maximum experimental temperature for carbonation reaction was higher than the theoretical value of 723 o C. It is important to emphasize that both the temperature at which the maximum CO 2 capture occurs and the adsorption capacity may depend on the experimental conditions of the analysis.  Thought, the experimental decarbonation temperature is much lower than that found for CaO, as previously described.
According to Fig. 6 (b), at 0.15 atm of CO 2 , the carbonation reaction occurs in the temperature range of 450-640 o C, with a CO 2 uptake of 2.8 wt.%. Thus, the isothermal capture experiments should be performed at temperatures lower than 640 o C. In addition, the temperature of 750 o C is adequate to guarantee the decarbonation reaction of the solid, independent of the CO 2 concentration in the gas stream 24 .
The abrupt weight decrease at the temperatures of 723 o C and 640 o C, respectively for 1.0 atm and 0.15 atm of CO 2 , is attributed to the decarbonation reaction. The reduction of initial decarbonation temperature (at 0.15 atm of CO 2 ) can be related to both thermodynamic (Fig. 4) and kinetic aspects, as discussed below. The kinetics of the carbonation reaction is faster at the beginning due to the reaction of CO 2 on the exposed surface of Li 4 SiO 4 , which is practically pure. As the reaction proceeds, the carbonation rate decreases, possibly due to diffusive limitations. At 1.0 atm of CO 2 (Fig. 7a), the carbonation reaction rate increases with temperature due to the decrease of the influence of the diffusive process 24 (kinetics is favored). The same behavior is observed at 0.15 atm of CO 2 (Fig. 7b) when increasing the temperature from 550 o C to 600 o C. However, the carbonation process is thermodynamically unfavored (∆G > 0) at temperatures higher than 600 o C (Fig. 4b); thus, the CO 2 adsorbed at 0.15 atm of CO 2 and 650 o C is almost negligible.

Isothermal carbonation
The mechanism for capturing CO 2 in lithium compounds appears to occur in two steps. First, the reaction of CO 2 on the solid surface occurs until the complete formation of the product layer, mainly composed by lithium carbonate (Li 2 CO 3 ). In the second step, the reaction is controlled by diffusive processes, either by the diffusion of lithium in the reaction products or the diffusion of CO 2 in the Li 2 CO 3 layer 38 .

Conclusion
It was found that the lithium orthosilicate is a non-porous sorbent, with low surface area, which may be a limiting factor for the carbonation reaction. According to the non-isothermal results, the carbonation of Li 4 SiO 4 occurs in the range of 450-746 o C and the decarbonation is favorable above it. Also, it was possible to capture up to 24.9 wt.%CO 2 , which is lower than the maximum theoretical capacity of 36.7 wt.%CO 2 . The isothermal kinetics showed that an increase in temperature promotes an increase in the reaction rate. According to the experimental results and the thermodynamic simulations, the lower is the CO 2 gas concentration, the lower is the amount captured and also the equilibrium temperature of adsorption, thus limiting the carbon capture in exhaust gases.

Acknowledgments
This research was supported by the Coordination for the Improvement of Higher Education Personnel (CAPES-Brazil) and the National Council of Technological and Scientific Development (CNPq-Brazil). The authors would like to thank LCME-UFSC for the technical support provided in the electron microscopy analysis.