Abstract
Na-doped lithium metatitanate (Na-doped Li2TiO3) absorbent was doped with Na2CO3 and lithium metatitanate (Li2TiO3) was prepared by a solid-state reaction method from mixture of TiO2 and Li2CO3. The Na-doped lithium metatitanate was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and surface area. Carbon dioxide absorption on Na-doped lithium metatitanate was investigated using TG-DTA. The results reveal an increase of the CO2 absorption capacity of the Na-doped materials with respect to pure Li2TiO3. XRD patterns of the doped samples suggest a limited substitution of Li by Na atoms within the Li2TiO3 structure. The results of experimental and modeling work were summarized to better understand the relationship between the sorbent microstructure and carbon dioxide absorption kinetics.
1. Introduction
Carbon dioxide is the largest contributor among greenhouse gases (GHS) in regard to its amount in the atmosphere. The explosive increase in energy consumption of fossil-fuels by the rapid increase of population resulted in an accumulation of anthropogenic carbon dioxide in the atmosphere. By the year 2100, the atmosphere may contain up around 570 ppmv carbon dioxide against 270 ppmv carbon dioxide before the industrial revolution. Carbon dioxide is causing a rise of mean global temperature of around 1.9°C and an increase of mean sea level of 38 m [1]. Fossil-fuel power plants are responsible for roughly 40 percent of total carbon dioxide emissions; hence it is important to capture carbon dioxide from coal-fired power plants in order to maintain the threshold value limit in the atmosphere and avoid any catastrophic effects.
Many researchers are working on the reduction of carbon dioxide emissions using various approaches since the Kyoto protocol was adopted at COP 3 in Japan [2–14]. Current technologies being considered for carbon dioxide capture include absorption, adsorption, membrane process, and disposal of carbon dioxide in deep oceans. If absorption is considered to be an economically viable technology for the carbonation reaction efficiency from combustion of flue gas, the solid absorbent properties are very important [2, 3]. Lithium salts have emerged as an excellent and regenerable absorbent for the capture of carbon dioxide. Lithium salts with higher capacity and selectivity for carbon dioxide were synthesized, characterized, and tested for capturing carbon dioxide either at postcombustion or precombustion temperature [4–10]. Nakagawa and Ohashi reported that the absorption reaction rate is accelerated when lithium zirconate is doped with sodium carbonate and/or potassium carbonate. The doping produces a eutectic of molten carbonate that reduces carbon dioxide diffusion resistance [11]. Xiong et al. have studied the absorption kinetics of carbon dioxide on K-doped and found that the reaction rate accelerated as the absorbent particle size reduced. The temperature effect is complex, and the appropriate temperature is about 550–590°C [12]. Wang et al. employed Na-doped , prepared by high-temperature solid-state reaction, as carbon dioxide absorbent from 500 to 750°C [6]. The influence of sodium doping on the absorption capacity and cycle performance was studied. The results showed that sodium doping could improve the carbon dioxide absorption ability of lithium silicate [13–15]. Satisfactory heat tolerance, high carbon dioxide absorption capacity, and fast kinetics in multicycle operation are seldom provided with currently available absorbents.
Lithium plays an important role in capturing carbon dioxide by forming lithium carbonate. Lithium acts as promoter, titanium acts as stabilizer, and sodium acts as doping agent in the process of capturing carbon dioxide over absorbent. As carbon dioxide is acidic by nature, it is felt that incorporation of basicity in the parent matrix can increase the capacity and selectivity of absorbent to a higher value. Titanium dioxide has a synergy with parent matrix for higher carbon dioxide absorption capacity. Sodium doping causes an important increase of the absorption kinetic. In present study, Na-doped was introduced as a suitable absorbent for carbon dioxide, and the influence of various values on the absorption capacity and cycle performance was discussed through different preliminary experiments.
2. Materials and Methods
2.1. Synthesis of Na-Doped
The high temperature solid-phase process was used to prepare Na-doped in this work. Starting materials were (AR, Sinopharm Chemical Reagent Co., Ltd), TiO2 (AR, Sinopharm Chemical Reagent Co., Ltd), and (AR, Sinopharm Chemical Reagent Co., Ltd). The molar ratio of the starting materials ( : TiO2 : ) was . In general, the composition of absorbent can be given as . The and mole ratios were varied during the synthesis of these absorbents. The six samples were named LiNa-0, LiNa-1, LiNa-3, LiNa-5, LiNa-7, and LiNa-10, respectively, according to the following values of sodium content in % mole. TiO2 were weighed, mixed with a suitable amount of ethanol, and dispersed 15 mins by the ultrasonic power. After and were added, the mixtures were stirred by magnetic stirrer for 2 h and dried at 80°C. The precursor was calcined at 800°C for 20 h. The calcined Na-doped was quenched in air and ground in an agate mortar before testing.
2.2. Characterization
The crystallinity of Na-doped was examined using X-ray powdered diffractometer ARL XTRA X-ray with Cu radiation. The morphology of absorbent was observed by SEM (FEI Nano SEM430). The surface area of absorbent was measured by Micromeritics ASAP 2010 instrument. The N2 BET surface area of the absorbent was observed to be 2.39 m2/g. Since the capture of carbon dioxide over absorbent depends mainly on the chemical reactions, there is no direct correlation between the surface area and the capture of carbon dioxide.
2.3. Evaluation of Absorbents
Carbon dioxide absorption on the Na-doped was performed using a HCT-1 thermo gravimetric analyzer (Beijing Henven Scientific Instrument Factory). About 10 mg Na-doped powders were placed in the sample pan. The heating rate is 10°C/min, absorption temperature is 600°C, absorption time span is 180 min, and flow rate of pure carbon dioxide is 80 mL/min. Prior to absorption measurement, pure dried carbon dioxide was passed over the absorbent through thermo gravimetric analyzer in order to remove inner air.
3. Results and Discussion
3.1. Characterization of Na-Doped
The X-ray diffraction patterns of Na-doped and nondoped absorbents shown in Figure 1 reveal that is only monoclinic crystalline structure as the sodium doping is below 5% Na. However, the characteristic peaks of Na2CO3 and TiO2 appear as the Na amounts are increased through Na doping from sample containing 5% up to 10% Na (samples LiNa-5, LiNa-7, and LiNa-10). XRD results did not show peaks associated with a structure clearly. It implies that a few Li are substituted by Na and a crystallographic phase is formed in crystals as the amount of Na is increased through doping. Furthermore, the crystal defects may increase absorption reactivity due to Na doping.
The SEM images of Na-doped at the magnification of 12000x are presented in Figure 2. It is observed that a greater degree of agglomeration with more irregular particle sizes appears as the Na amount is decreased from sample containing 10% to 1% Na. The morphology of sample LiNa-5 exhibits a more regular uniform crystalline solid material. Moreover, the performance improvement might be attributed to the creation of lattice defects in the crystals by Na doping. In addition, the grain growth of absorbent has been accelerated by Na doping.
| (a) = 0.01. |
| (b) = 0.03. |
| (c) = 0.05. |
| (d) = 0.07. |
| (e) = 0.10. |
3.2. The Influence of Sodium Doping on CO2 Absorption Capacity
CO2 absorption on various Na-doped at 600°C is presented in Figure 3. It is found that sample LiNa-3 possesses the best performance and absorbs as much as 30% (wt) of CO2. Greater than or less than Na amount of sample LiNa-3 will consequently be traduced in a decrease of CO2 absorption capacity. The capture of CO2 over lithium silicate (Li/Si = 4) is 1.6 mmol/g at 525°C [16]. As much as ()% (wt) of CO2 was absorbed by lithium zirconate [17].
3.3. The Influence of Sodium Doping on CO2 Absorption Cycle Performance
CO2 absorption cycle performances of pure nondoped and sample LiNa-3 are presented in Figure 4. As shown, CO2 absorption capacity of two samples both decrease, cycle performance of Na-doped falls 14.28% after 5 times cycle, but that of pure non doped falls 67.03% after the same times cycle. So the cycle performance increases obviously by Na doping. This behavior can be explained in terms of a newly formed structure after Na doping process [18].
3.4. The Double-Shell Mechanism of CO2 Absorption on Na-Doped
Xiao et al. investigated the double-shell mechanism of CO2 absorption on at high temperature [15]. The absorption process of CO2 on Na-doped particles could properly be described by double-shell mechanism. Although eutectic molten carbonate composed of and on the outer layer cannot react with CO2, molten carbonate facilitates the transfer of gaseous CO2 into the inner unreacted layer. Meantime, and released by reach inner layer through TiO2 layer. Then , , and CO2 encounter and react, forming Li2CO3 among inner layer. As this process proceeds, TiO2 layer and molten salt layer are thickened, diffusion time of , , and CO2 is lengthened, and absorption process is slowed down. The relative permeability of and CO2 in absorption process at 500–600°C was listed in Table 1 [19].
From Table 1, the rate-determining steps of CO2 absorption on pure and Na-doped are CO2 diffusion rate and diffusion rate, respectively. diffusion rate is faster than CO2. So absorption rate of CO2 on Na-doped is faster. CO2 absorption process on Na-doped was qualitatively described well by double-shell mechanism.
4. Conclusions
(1)CO2 capacity on Na-doped can be improved by sodium doping. There is the maximum capacity when .(2)XRD patterns show that sodium doping led to crystal defect that could make absorption reactivity increase. From SEM images, it was speculated that sodium doping does not change morphology, but it makes particles looser and fracture phenomenon occurred.(3)CO2 absorption cycle performance decay dropped to 14.28%. Compared with the 67.03% of pure , it has been greatly increased.(4)The double-shell mechanism described CO2 absorption process on Na-doped ; sodium presence changing rate-determining step of the process is the main reason of performance improved.
Conflict of Interests
The authors declare that they have no conflict of interests regarding the publication of this paper.
Acknowledgments
This study was financially supported by Natural Science Foundation of China (Grant: 11375043) and Jiangxi Province Science and Technology Support Program (Grant: 20112BBG70006).