High temperature CO2 capture performance and kinetic analysis of Na4SiO4 ceramics

Alkali silicate-based ceramics sorbents were regarded as particularly suitable materials for CO₂ capture at high temperatures, however, the CO₂ capture behaviors of Na₄SiO₄ had been seldom investigated. In this work, the Na₄SiO₄ ceramics samples were prepared using the one-step synthesized method, and the CO₂ sorption/desorption performances at high temperatures, the thermal stability, and the cycling stability of Na₄SiO₄ were systematically investigated. It was demonstrated that the maximum CO₂ sorption capacity of SO-3 sample reached 19.4 wt% at 725 °C, and the optimal condition of cycling tests were 750 °C for sorption and 800 °C for desorption based on the sorption/desorption capacity and rate, which exhibited good thermal stability and high cyclic stability. Besides, the kinetic analysis results showed that the diffusion process was the rate-determining step of CO₂ adsorption, which was more dependent on temperature than the chemisorption process. The structure and surface morphology variations were also investigated, it was interesting that there was a special “fish scale” surface structure after the sorption process, revealing that the melting phenomenon happened during the chemisorption reaction process. By comparing with common sorbent Li₄SiO₄, the material and CO₂ capture costs of Na₄SiO₄ were much lower. These results proved that Na₄SiO₄ was expected to be a suitable high temperature CO₂ capture material as a good supplement to alkali silicate-based ceramics sorbents.     

Partial nitridation of Li4SiO4 and ionic conductivity of Li4.1SiO3.9N0.1

The Li content and anion lattice of Li₄SiO₄ were modified to improve ionic conductivity. Li₂CO₃ and Si₃N₄ were mixed in a ratio of Li/Si?=?4.5 and heated in NH₃ at 820?°C, which resulted in the formation of the oxynitride, Li_4.1SiO_3.9N_0.1. Powder X-ray diffraction analyses revealed Li_4.1SiO_3.9N_0.1 and Li₄SiO₄ to be isostructural with a subtle variation in the lattice constants. Diffuse-reflectance absorption spectroscopy, however, showed a significant decrease in the band gap, from 5.6?eV in Li₄SiO₄ to 4.8?eV in Li_4.1SiO_3.9N_0.1. X-ray photoelectron spectra of the Li 1s and Si 2p levels revealed enhanced lattice covalency in Li_4.1SiO_3.9N_0.1 compared to the oxide phase. The ionic conductivity of Li₄SiO₄ and Li_4.1SiO_3.9N_0.1 were measured by ac impedance spectroscopy over the temperature range 100–400?°C. Non-linear fitting analysis of the equivalent circuit revealed that the ionic conductivity of Li_4.1SiO_3.9N_0.1 was approximately one order of magnitude higher than that of Li₄SiO₄.     

High-temperature CO2 adsorption over Li4SiO4 sorbents derived from different lithium sources

Li₄SiO₄ is a promising sorbent for high temperature CO₂ capture. It could be synthesized from three different Li sources (LiNO₃, LiOH, and Li₂CO₃) by using the solid state reaction method. The effects of Li sources on the structure and CO₂ adsorption/desorption properties of Li₄SiO₄ sorbents were analyzed in this work. The results showed that Li₄SiO₄ sorbents could be synthesized at a lower temperature by using LiNO₃ and LiOH as the starting materials, which could reduce the sintering during the synthesis process and increase the surface area of synthesized Li₄SiO₄. During the CO₂ adsorption/desorption cycles, Li₄SiO₄ sorbents derived from LiNO₃ and LiOH presented higher initial CO₂ adsorption capacities than those from Li₂CO₃. After 15 cycles, the adsorption efficiency of Li₄SiO₄ derived from LiNO₃ showed no or slight decrease, while that from LiOH rapidly decreased to 20% of the initial value. This was because Li₄SiO₄ derived from LiNO₃ had high surface area and porosity before CO₂ adsorption/desorption cycles, and its surface area even increased after cycles. However, the surface area of Li₄SiO₄ derived from LiOH decreased greatly due to serious sintering. For Li₄SiO₄ derived from Li₂CO₃, its morphology and surface area were almost unchanged before and after CO₂ adsorption/desorption cycles.     

Economical assessment of competitive enhanced limestones for CO2 capture cycles in power plants

CO₂ capture systems based on the carbonation/calcination loop have gained rapid interest due to promising carbonator CO₂ capture efficiency, low sorbent cost and no flue gases treatment is required before entering the system. These features together result in a competitively low cost CO₂ capture system. Among the key variables that influence the performance of these systems and their integration with power plants, the carbonation conversion of the sorbent and the heat requirement at calciner are the most relevant. Both variables are mainly influenced by CaO/CO₂ ratio and make-up flow of solids. New sorbents are under development to reduce the decay of their carbonation conversion with cycles. The aim of this study is to assess the competitiveness of new limestones with enhanced sorption behaviour applied to carbonation/calcination cycle integrated with a power plant, compared to raw limestone. The existence of an upper limit for the maximum average capture capacity of CaO has been considered. Above this limit, improving sorbent capture capacity does not lead to the corresponding increase in capture efficiency and, thus, reduction of CO₂ avoided cost is not observed. Simulations calculate the maximum price for enhanced sorbents to achieve a reduction in CO₂ removal cost under different process conditions (solid circulation and make-up flow). The present study may be used as an assessment tool of new sorbents to understand what prices would be competitive compare with raw limestone in the CO₂ looping capture systems.     

Analysis of CO2 sorption/desorption kinetic behaviors and reaction mechanisms on Li4SiO4

The CO₂ sorption/desorption kinetic behaviors on Li₄SiO₄ were analyzed. The theoretical compositions of the sorption/desorption reactions were calculated using FactSage. The sorption/desorption process on Li₄SiO₄ was investigated by comparing the shrinking core, double exponential, and Avrami–Erofeev models. The Avrami–Erofeev model fits the kinetic thermogravimetric experimental data well and, together with the double‐shell mechanism, clearly explains the sorption/desorption mechanism. The sorption process is limited by the rate of the formation and growth of the crystals with double‐shell structure consisting of Li₂CO₃ and Li₂SiO₃. The whole desorption process is found to be controlled by the rate of the formation and growth of the Li₄SiO₄ crystals. Furthermore, the influence of steam on the CO₂ sorption process was analyzed. It has been observed that the presence of steam enhance the mobility of Li⁺ and, therefore, the rate of diffusion control stage. © 2012 American Institute of Chemical Engineers AIChE J, 59: 901–911, 2013     

High‐Temperature Capture of CO2 on Lithium‐Based Sorbents Prepared by a Water‐Based Sol‐Gel Technique

A convenient water‐based sol‐gel technique was used to prepare a highly efficient lithium orthosilicate‐based sorbent (Li₄SiO₄‐G) for CO₂ capture at high temperature. The Li₄SiO₄‐G sorbent was systematically studied and compared with the Li₄SiO₄‐S sorbent prepared by solid‐state reaction. Both sorbents were characterized by X‐ray diffraction, scanning electron microscopy, nitrogen adsorption, and thermogravimetry. The CO₂ sorption stability was investigated in a dual fixed‐bed reactor. Li₄SiO₄‐G exhibited a special Li₄SiO₄ structure with smaller crystalline nanoparticles, larger surface area, and higher CO₂ adsorption properties as compared with Li₄SiO₄‐S. The Li₄SiO₄‐G sorbent also maintained higher capacities during multiple cycles.     

Novel Li4SiO4-based sorbents from diatomite for high temperature CO2 capture

Using inexpensive porous diatomite as silicon source, novel Li₄SiO₄-based sorbents for high temperature CO₂ capture were prepared through the solid-state reaction method at lower temperature (700 °C). Effect of different raw material ratios on CO₂ absorption capacity was investigated. The results showed that CO₂ absorption capacity was dependent on the raw material ratio. When the raw material ratio was 2.6:1, the CO₂ absorption capacity reached 30.32 wt% (83% of the theoretical absorption capacity) in the atmosphere (50 mL/min N₂ and 50 mL/min CO₂). Meanwhile, it was found that the as-prepared Li₄SiO₄-based sorbents from diatomite exhibited good absorption–desorption performance.     

Effect of rice husk ash addition on CO2 capture behavior of calcium-based sorbent during calcium looping cycle

Rice husk ash/CaO was proposed as a CO₂ sorbent which was prepared by rice husk ash and CaO hydration together. The CO₂ capture behavior of rice husk ash/CaO sorbent was investigated in a twin fixed bed reactor system, and its apparent morphology, pore structure characteristics and phase variation during cyclic carbonation/calcination reactions were examined by SEM-EDX, N₂ adsorption and XRD, respectively. The optimum preparation conditions for rice husk ash/CaO sorbent are hydration temperature of 75 °C, hydration time of 8 h, and mole ratio of SiO₂ in rice husk ash to CaO of 1.0. The cyclic carbonation performances of rice husk ash/CaO at these preparation conditions were compared with those of hydrated CaO and original CaO. The temperature at 660 °C–710 °C is beneficial to CO₂ absorption of rice husk ash/CaO, and it exhibits higher carbonation conversions than hydrated CaO and original CaO during multiple cycles at the same reaction conditions. Rice husk ash/CaO possesses better anti-sintering behavior than the other sorbents. Rice husk ash exhibits better effect on improving cyclic carbonation conversion of CaO than pure SiO₂ and diatomite. Rice husk ash/CaO maintains higher surface area and more abundant pores after calcination during the multiple cycles; however, the other sorbents show a sharp decay at the same reaction conditions. Ca₂SiO₄ found by XRD detection after calcination of rice husk ash/CaO is possibly a key factor in determining the cyclic CO₂ capture behavior of rice husk ash/CaO.     

Direct synthesis of acetic acid from CH4 and CO2 by a step-wise route over Pd/SiO2 and Rh/SiO2 catalysts

The theoretical and experimental feasibility of direct conversion of CH₄ and CO₂ to acetic acid by an isothermal step-wise route over Pd/SiO₂ and Rh/SiO₂ catalysts was investigated. The methyl radical formation from CH₄ dissociation and CO₂ inserting into the intermediate are regarded as two limiting steps. Preliminary experimental results have shown that the following step-wise route can circumvent the thermodynamic limitation of this direct synthesis at low temperatures. Pd catalysts are more active than Rh catalysts at 170 °C and 200 °C, while formic acid is only produced on Pd catalysts. The optimum contact time of CH₄ and CO₂ with catalysts is 1 min under the experimental conditions. And there is no apparent deactivation resulting from carbon deposition for catalysts during the successive reaction cycles.     

Enhancement of reactivity in surfactant-modified sorbent for CO2 capture in pressurized carbonation

The calcination/carbonation loop of calcium-based (Ca-based) sorbents is considered as a viable technique for CO₂ capture from combustion gases. Recent attempts to improve the CO₂ uptake of Ca-based sorbents by adding calcium lignosulfonate (CLS) with hydration have succeeded in enhancing its effectiveness. The optimum mass ratio of CLS/CaO is 0.5 wt.%. The reduction in particle size and grain size of CaO appeared to be parts of the reasons for increase in CO₂ capture. The primary cause of increase in reactivity of the modified sorbents was the ability of the CLS to retard the sintering rate and thus to remain surface area and pore volume for reaction. The CO₂ uptake of the modified sorbents was also enhanced by elevating the carbonation pressure. Experimental results indicate that the optimal reaction condition of the modified sorbents is at 0.5 MPa and 700 °C and a high conversion of 0.7 is achieved after 10 cycles, by 30% higher than that of original limestone, at the same condition.     

The formation and thermal stability of spurrite,Ca5(SiO4)2CO3

The reversible equilibrium: 2Ca₂SiO₄ + CaO + CO₂ ? Ca₅(SiO₄)₂CO₃ has been studied using F^? and C?^? ions as mineralizers. A pressure-temperature curve is given for the reaction in the range of CO₂ pressures between 0.08 and 1 atmosphere. At these pressures, the decomposition temperatures of spurrite are 790 ± 5°C and 912 ± 5°C respectively. At a given CO₂ pressure the thermal stability of spurrite is greater than that of CaCO₃.     

K-doping effect in the kinetics of CO2 capture at high temperature over lithium silicates obtained from rice husks: In situ/operando techniques

K-doped lithium silicates were synthesized as CO₂ sorbents using rice husks as silica source. Two different doping methods were used to prepare the solids, incipient wetness impregnation and co-impregnation. Noticeable differences were observed between the sorbents obtained by each route. Unlike the co-impregnated sample, a KLi₃SiO₄ phase was detected by XRD in the doped-samples prepared by incipient wetness impregnation. This phase was associated with a porous layer deposition over the sorbent particle surface, which was observed by SEM. The effect of CO₂ partial pressure and sorption temperature was evaluated for the solids. A remarkable improvement of the capture efficiency was observed in the doped-samples, especially at low temperature and CO₂ partial pressure. The kinetics of the samples was studied using the double exponential model and operando Raman and DRIFT spectroscopies. Different reactivity properties were found depending on the synthesis method employed. The complete analysis of the results allowed us to propose a transformation mechanism for the solids during the sorption step, highlighting the improvement of surface reaction and volumetric diffusion processes in the CO₂ capture step.     

烟气中水蒸气对钙基吸收剂碳酸化的影响特性

在循环煅烧/碳酸化反应系统上考察煅烧气氛和碳酸化气氛中水蒸气含量以及CO₂分压对钙基吸收剂成型颗粒碳酸化的影响,通过对钙基吸收剂微观结构分析(扫描电镜、氮吸附分析)以理解水蒸气影响碳酸化特性的机理。结果表明,煅烧气氛和碳酸化气氛中的水蒸气均可提高钙基吸收剂的碳酸化转化率,水蒸气含量分别为10%和5%时,吸收剂的碳酸化性能较好;水蒸气在碳酸化气氛中对高铝水泥改性吸收剂的改善作用较石灰石显著。煅烧气氛中的CO₂分压越高,烧结现象越严重,降低钙基吸收剂的捕集效率;碳酸化气氛CO₂分压提高,有利于提高钙基吸收剂的碳酸化转化率。烟气中水蒸气丰富了吸收剂的微观孔隙,使得吸收剂捕集CO₂性能得到改善。     

Behaviors and kinetic models analysis of Li4SiO4 under various CO2 partial pressures

The absorption behaviors of Li₄SiO₄ sorbent under various CO₂ partial pressures and temperatures were investigated through numerical and experimental methods. It was found that Li₄SiO₄ showed poor absorption capacity at high temperatures (>525°C) under CO₂ partial pressure of 5066 Pa. This phenomenon was explained by the thermodynamic results from FactSage5.5 software. Meanwhile, a modified Jander‐Zhang model based on the double‐shell structure of the Li₄SiO₄ sorbent was developed to describe the absorption kinetic behaviors of CO₂ on Li₄SiO₄. The results showed that the modified Jander‐Zhang model could fit the kinetic experimental data well. Furthermore, the influence of steam on CO₂ absorption was also analyzed by the modified Jander‐Zhang model. The results showed that the activation energy in the absorption process with steam was smaller than that without steam, which indicated that the presence of steam could promote the CO₂ diffusion in product layer, therefore, improving the sorption capacity. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2153–2164, 2017     

Effects of Zn2SiO4 on the phase evolution,thermal expansion and mechanical properties of LiAlSiO4 ceramic

LiAlSiO₄ (abbreviated as LAS) ceramics doped with variable mass percent of Zn₂SiO₄ were prepared by conventional solid-state route. The effects of Zn₂SiO₄ on the phase evolution, microstructure, thermal expansion and mechanical properties have been fully investigated. The results show that Zn₂SiO₄ reacted with LAS matrix to produce Li₂Al₂Si₃O₁₀ and ZnAl₂O₄. Fine-grain ZnAl₂O₄ phase accumulated on the grain boundaries of the main phase, which was helpful to improve the density. Simultaneously, both of the flexure strength and Vickers hardness of the multiphase ceramics were significantly enhanced with the increasing mass percent of Zn₂SiO₄ for the reason of dispersion strengthening effect. In addition, when the content of Zn₂SiO₄ increased from 10?wt% to 22.5?wt%, the coefficient of thermal expansion (CTE) of the composite ceramics increased monotonously from ??5.24?×?10^?6/K to 1.49?×?10^?6/K. Typically, the LAS ceramic doping with 17.5?wt% Zn₂SiO₄ sintered at 1175?°C for 4?h possesses excellent properties: α?=?0.65?×?10^?6/K, H_v =?5.34?GPa, σ_s =?102.6?MPa, which is a promising material in laser gyroscope and precision machining fields.     

Nanostructured Ca-based sorbents with high CO2 uptake efficiency

Calcium-based carbon dioxide sorbents were made in the gas phase by scalable flame spray pyrolysis (FSP) and compared to the ones made by calcination (CAL) of selected calcium precursors. Such flame-made sorbents consisted of nanostructured CaO and CaCO₃ with twice as much specific surface area (40-60 m²/g) as the CAL-made sorbents. All FSP-made sorbents exhibited faster and higher CO₂ uptake capacity than all CAL-made sorbents at intermediate temperatures. CAL of calcium acetate monohydrate resulted in sorbents with the best CO₂ uptake among all CAL-made ones. At higher temperatures both FSP- and CAL-made sorbents (esp. from CaAc₂·H₂O) exhibited very high initial molar conversions (95%) but sintering contributed to grain growth that reduced the molar conversion down to 50%. In multiple carbonation/decarbonation cycles, the nanostructured FSP-made sorbents demonstrated stable, reversible and high CO₂ uptake capacity sustaining maximum molar conversion at about 50% even after 60 such cycles, indicating high potential for CO₂ uptake. The top performance of flame-made sorbents is best attributed to their nanostructure (30-50 nm grain size) that allows operation in the reaction-controlled carbonation regime rather than in the diffusion-controlled one when sorbents made with larger particles are employed.     

High-pressure sorption isotherms and sorption kinetics of CH4 and CO2 on coals

Using a manometric experimental setup, high-pressure sorption measurements with CH₄ and CO₂ were performed on three Chinese coal samples of different rank (VR_r = 0.53%, 1.20%, and 3.86%). The experiments were conducted at 35, 45, and 55 °C with pressures up to 25 MPa on the 0.354-1 mm particle fraction in the dry state. The objective of this study was to explore the accuracy and reproducibility of the manometric method in the pressure and temperature range relevant for potential coalbed methane (CBM) and CO₂-enhanced CBM (CO₂-ECBM) activities (P > 8 MPa, T > 35 °C). Maximum experimental errors were estimated using the Gauss error propagation theorem, and reproducibility tests of the high-pressure sorption measurements for CH₄ and CO₂ were performed. Further, the experimental data presented here was used to explicitly study the CO₂ sorption behaviour of Chinese coal samples in the elevated pressure range (up to 25 MPa) and the effects of temperature on supercritical CO₂ sorption isotherms.The experiments provided characteristic excess sorption isotherms which, in the case of CO₂ exhibit a maximum around the critical pressure and then decline and level out towards a constant value. The results of these manometric tests are consistent with those of previous gravimetric sorption studies and corroborate a crossover of the 35, 45, and 55 °C CO₂ excess sorption isotherms in the high-pressure range. The measurement range could be extended, however, to significantly higher pressures. The excess sorption isotherms tend to converge, indicating that the temperature dependence of CO₂ excess sorption on coals at high-pressures (>20 MPa) becomes marginal. Further, all CO₂ high-pressure isotherms measured in this study were approximated by a three-parameter excess sorption function with special consideration of the density ratio of the “free” phase and the sorbed phase. This function provided a good representation of the experimental data.The maximum excess sorption capacity of the three coal samples for methane ranged from 0.8 to 1.6 mmol/g (dry, ash-free) and increased from medium volatile bituminous to subbituminous to anthracite. The medium volatile bituminous coal also exhibited the lowest overall excess sorption capacity for CO₂. However, the subbituminous coal was found to have the highest CO₂ sorption capacity of the three samples. The mass fraction of adsorbed substance as a function of time recorded during the first pressure step was used to analyze the kinetics of CH₄ and CO₂ sorption on the coal samples. CO₂ sorption proceeds more rapidly than CH₄ sorption on the anthracite and the medium volatile bituminous coal. For the subbituminous coal, methane sorption is initially faster, but during the final stage of the measurement CO₂ sorption approaches the equilibrium value more rapidly than methane.     

Synthesis,kinetic study,and reaction mechanism of Li4SiO4 with CO2 in a slurry bubble column reactor

Abstract

This study was performed to investigate the synthesis, kinetic and reaction mechanism of Li₄SiO₄ with CO₂ in a slurry bubble column reactor. The Li₄SiO₄ powder sample was prepared via a solid-state reaction. The sample was characterized via X-ray diffraction (XRD) analysis and verified as a single phase. The median diameter of the sample was measured using the laser diffraction and scattering method as about 20?μm. The synthesized sample was suspended in binary molten carbonate of Li₂CO₃–K₂CO₃ having a molar ratio of 38:62. The experimental results show that Li₄SiO₄ in the slurry bubble column absorbed approximately a stoichiometric amount of CO₂. The kinetic study shows that the CO₂ reaction behavior on the Li₄SiO₄ surface was fitted to a double exponential model and the limiting step of the reaction was lithium diffusion. The mass transfer coefficient of CO₂ and rate constant of reaction with Li₄SiO₄ were studied to understand the overall absorption mechanism in the reactor. The resistance for the direct reaction of CO₂ on the Li₄SiO₄ was much smaller than the resistance for the mass transfer of CO₂ to the Li₄SiO₄. We can conclude that the direct contact of CO₂ with Li₄SiO₄ was the main path for the reaction.     

Improving Physical Adsorption of CO2 by Ionic Liquids‐Loaded Mesoporous Silica

CO₂ sorption capacities of the neat and silica‐supported 1‐butyl‐3‐methylimidazolium‐based ionic liquids (ILs) were measured under atmospheric pressure. The silica‐supported ILs were synthesized by the impregnation‐vaporization method and charactrized by N₂ adsorption/desorption and thermogravimeteric analysis (TGA). Evaluation of the effects of influential factors on sorption capacity demonstrated that by increase of the temperature, flow rate, and the weight percentage of ILs in sorbents, the sorption capacity decreases. Among the sorbents, [Bmim][TfO] and SiO₂‐[Bmim][BF₄](50) had the highest capacity. By increasing the IL portion in SiO₂‐[Bmim][BF₄], the selectivity for CO₂ to CH₄ could be improved. The CO₂‐rich sorbents could be easily recycled.     

Low temperature solid-state synthesis routine and mechanism for Li3V2(PO4)3 using LiF as lithium precursor

Monoclinic lithium vanadium phosphate, Li₃V₂(PO₄)₃, has been successfully synthesized using LiF as lithium source. The one-step reaction with stoichiometric composition and relative lower sintering temperature (700 °C) has been used in our experimental processes. The solid-state reaction mechanism using LiF as lithium precursor has been studied by X-ray diffraction and Fourier transform infrared spectra. The Rietveld refinement results show that in our product sintered at 700 °C no impurity phases of VPO₄, Li₅V(PO₄)₂F₂, or LiVPO₄F can be detected. The solid-state reaction using Li₂CO₃ as Li-precursor has also been carried out for comparison. X-ray diffraction patterns indicate that impurities as Li₃PO₄ can be found in the product using Li₂CO₃ as Li-precursor unless the sintering temperatures are higher than 850 °C. An abrupt particle growth (about 2 μm) has also been observed by scanning electron microscope for the samples sintered at higher temperatures, which can result in a poor cycle performance. The product obtained using LiF as Li-precursor with the uniform flake-like particles and smaller particle size (about 300 nm) exhibits the better performance. At the 50th cycle, the reversible specific capacities for Li₃V₂(PO₄)₃ measured between 3 and 4.8 V at 1C rate are found to approach 147.1 mAh/g (93.8% of initial capacity). The specific capacity of 123.6 mAh/g can even be hold between 3 and 4.8 V at 5C rate.