High-temperature CO2 Sorption Research Articles

High-temperature CO2 sorption over Li4SiO4 synthesized from diatomite: study of sorption heat and isotherm modeling.

The Li4SiO4 seems to be an excellent sorbent for CO2 capture at post-combustion. Our work contributes to understanding the effect of the natural Algerian diatomite as a source of SiO2 in the synthesis of Li4SiO4 for CO2 capture at high temperature. For this purpose, we use various molar % (stoichiometric and excess) of calcined natural diatomite and pure SiO2. To select the best composition, CO2 sorption isotherms at 500°C on the prepared Li4SiO4 are obtained using TGA measurements under various flows of CO2 in N2. The sorbent having 10% molar SiO2 in diatomite (10%ND-LS) exhibits the best CO2 uptake, probably due to various factors such as the content of the different secondary phases. A comparative study was performed at 400 to 500°C on this selected 10%ND-LS and those with stoichiometric composition obtained with diatomite and pure SiO2. The obtained isotherms show the endothermic character of CO2 sorption. In addition, the evolution of isosteric heat highlights the nature of the involved CO2/Li4SiO4 interactions, by considering the double-shell mechanism. Finally, the experimental sorption isotherms are confronted with some well-known adsorption models to explain the phenomenon occurring over our prepared sorbents. Freundlich and Jensen-Seaton models present a better correlation with the experimental results.

High Temperature CO2 Capture Performance and Kinetic Analysis of Novel Potassium Stannate.

For the first time, the use of stannate-based sorbents was investigated as high temperature CO2 sorption to evaluate their potential to contribute towards reducing carbon emissions. The sorption capacity and kinetics of commercial tin oxide, sodium, potassium and calcium stannates and lab synthesised potassium stannates were tested using thermogravimetric analysis. Commercial K2SnO3 was found to possess the largest CO2 uptake capacity (2.77 mmol CO2/g or 12.2 wt%) at 700 °C, which is among the highest for potassium sorbents, but the CO2 desorption was not successful. On the contrary, the in-house synthesised K-stannate (K-B) using facile solid-state synthesis outperformed the other sorbents, resulting in a CO2 uptake of 7.3 wt% after 5 min, an adsorption rate (0.016 mg/s) one order of magnitude higher than the other stannates, and stability after 40 cycles. The XRD and XPS analyses showed that K-B contains a mixture of K2SnO3 (76%) and K4SnO4 (21%), while the Scherrer crystal sizes confirmed good resistance to sintering for the potassium stannates. Among the apparent kinetic model tested, the pseudo-second order model was the most suitable to predict the CO2 sorption process of K-B, indicating that chemical adsorption is dominant, while film-diffusion resistance and intra-particle diffusion resistance governed the sorption process in K-B. In summary, this work shows that solid-state synthesised potassium stannate could be an effective sorbent for high temperature separation, and additional work is required to further elucidate its potential.

Sorption enhanced steam methane reforming in a bubbling fluidized bed reactor: Simulation and analysis by the CPFD method

This work reports the modelling and simulation results of a bubbling fluidized bed reactor using the Computational Particle Fluid Dynamics (CPFD) method of the Barracuda® software. The reactor under investigation is the carbonator installed in the ENEA ZECOMIX research infrastructure, where Steam Methane Reforming (SMR) happens simultaneous with CO2 capture via solid sorbents. In this intensified process, namely Sorption Enhanced Steam Methane Reforming (SE-SMR), steam methane reforming is coupled with high temperature CO2 sorption and calcium looping (CaL) process, in order to increase the H2 yield, beyond thermodynamic limits. Currently, the reactor is operated in batch mode and is used also for sorbent regeneration, by switching the fluidizing gas flow from steam/methane to oxy-burner combustion products. With the aim of studying the process when it is operated as a closed loop, in this paper the reactor is continuously fed by a fresh sorbent flow and a riser/calciner reactor for sorbent regeneration, to be connected with the carbonator, has been sized. The continuous circulation of solid material between the two reactors ensures the maintenance of different operating temperatures and therefore greater operational optimization.The numerical analysis presented in this paper will serve as a valid support for the experimental activities. For this purpose, a sensitivity study on the SE-SMR process has been conducted, by varying the main operating conditions (e.g. sorbent conversion, sorbent/catalyst ratio, fluidizing gas flow), to evaluate the hydrogen purity yield. Two different kinetic mechanisms have been compared for the gas phase reactions. A post-processing routine has been written, in order to analyze bubbles sizes and velocities inside the fluidized environment. The effect of sorbent and catalyst particles segregation has been also investigated. The same modelling approach has been used for the sizing of the fast riser calciner reactor.

Scalable synthesis of Li4SiO4 sorbent from novel low-cost orthoclase mineral for CO2 capture

A novel Li4SiO4 sorbent has been synthesized using low-cost and naturally occurring mineral (orthoclase) as silicon source for high-temperature CO2 sorption. This new CO2 sorbent O-Li4SiO4 exhibited looser microstructures with enhanced surface area and porosity compared to the sorbent using conventional SiO2 powder as raw materials (S-Li4SiO4) perhaps due to the existence of the support oxides and alkali impurities. As a result, the CO2 sorption rate and cyclic capacity of the new sorbent were significantly enhanced. Over 20 cycles under low CO2 sorption conditions (15%), O-Li4SiO4 still showed high sorption capacity of ∼ 0.255 g CO2/g sorbent, nearly 8 times higher than the last-cycle capacity of S-Li4SiO4. The new sorbent powder was further granulated into spherical pellets and it was demonstrated that the mechanical granulation process led to the densification of sorbent structures and, thus, the reduction of sorption capacity. However, the reduced cyclic CO2 sorption performance could be regained by adding microcrystalline cellulose as pore-forming materials. In addition to the good chemisorption performance, the pellets also presented excellent mechanical properties, i.e., high compressive strength and attrition resistance, to satisfy the requirements for realistic application. Thanks to its fast sorption rate, high CO2 capture capacity, good stability, excellent mechanical strength, and characterizations of low cost and scalable synthesis, orthoclase-derived Li4SiO4 sorbent is a promising candidate for realistic CO2 removal.

Development of a High Temperature CO2 Sorbent Based on Hydrotalcite for a H2-Rich Syngas Production

To adapt hydrotalcite-based sorbents (also known as layered double hydroxides—LDHs) to high-temperature CO2 sorption compatible with tar steam reforming, the addition of CaO was investigated, maintaining the LHDs porosity and accessibility but mostly assuring the CO2 sorption stability during sorption/desorption cycles. In co-precipitation synthesis, the investigated parameters are (i) various interlayer anions with different sizes and valences (carbonate, oxalate, and stearate); (ii) various pH values; (iii) different Mg/Ca molar ratios. The characterization of these modified LDHs by TGA, XRD, N2 adsorption, SEM, sorption capacity, and sorption/desorption stability (cyclic TGA) allowed understanding the effect of the various synthesis parameters and highlighted the effect of oxalate use as the interlayer anion. After calcination of sorbent with Mg/Ca/Al ratio = 1/2/1, typical LDH sand roses were formed both with carbonate and oxalate anions: this former exhibited the highest sorption capacity and accessibility of CaO sites at 600 °C, higher than pure CaO. However, the best stability during cycles was obtained with the sorbent from oxalate and Mg/Ca/Al ratio = 1.5/1.5/1 at pH 10 for which comparable sorption results are reached. For these two samples, the observed macro-porosity was associated with the highest specific surface area and pore volume.Graphic Abstract

High temperature CO2 sorption using mixed oxides with different Mg/Al molar ratios and synthesis pH

Scheme of post- and pre-combustion processes for CO 2 capture at high temperature using hydrotalcite-derived mixed oxides as CO 2 sorbents. • Several hydrotalcite-derived mixed oxides were prepared to be used as CO 2 sorbents. • Different Mg/Al molar ratios and synthesis pH were used to prepare the materials. • The CO 2 sorption isotherms were determined at 300 °C for screening purposes. • 8.5HT7 showed a high CO 2 sorption capacity: 1.62 mmol/g @ p CO 2 = 1 bar and 300 °C. • The effect of steam over successive CO 2 sorption–desorption cycles was assessed. Several hydrotalcites were prepared by the co-precipitation method with different Mg/Al molar ratios (from 2 to 20) and synthesis pH (8.5 and 10); upon calcination, the hydrotalcite-derived mixed oxides (HDMOs) were obtained, to be used as CO 2 sorbents at high temperature, for instance in post- or pre-combustion CO 2 capture processes. The sorption equilibrium isotherms were determined for screening purposes of all materials prepared using a magnetic suspension microbalance (static experiments) at 300 °C, along with a detailed physicochemical characterization (TG-DTG, ICP-OES, XRD, FTIR, physical sorption–desorption of N 2 at −196 °C and TPD-CO 2 ). In addition, the most promising HDMOs, in terms of CO 2 sorption capacity, were submitted to successive CO 2 sorption–desorption cycles (dynamic experiments) in a packed column at 300 °C, with and without water in the feed, to assess their potential in a sorption–desorption cyclic operation. The results indicate that a synthesis pH of 8.5 provides samples with better CO 2 sorption capacities and that there is an optimum Mg/Al molar ratio (between 4 and 10). The sample with a synthesis pH of 8.5 and a Mg/Al molar ratio of 7 (called 8.5HT7) provided an outstanding CO 2 sorption capacity (1.62 mmol·g −1 at 1 bar of CO 2 and 300 °C) when compared with the other samples herein prepared, but also when compared with commercial (MG63 and MG70) and with other similar materials found in the literature. Finally, an average working capacity of 1.04 mmol·g −1 and 2.11 mmol·g −1 was obtained after repeated sorption–desorption cycles at 0.4 bar of CO 2 and 300 °C for the 8.5HT7 HDMO under dry and wet conditions (45 vol.% H 2 O), respectively.

Influence of ash composition on high temperature CO2 sorption

To avoid increasing emissions of carbon dioxide to the atmosphere different approaches are being tested. One of them is based on adsorption process where new sorbents are still being developed and researched. Numerous studies consider physical or chemical adsorption promising for carbon dioxide capture from flue gas. Its competitiveness depends on finding a sorbent providing low acquisition cost combined with high stability and sorption capacity. Fly ashes from coal-fired power plants could accomplish these requirements and thus could be applicable for CO2 capture in industrial scale. Within this work different fly ashes were tested for CO2 capture and their physicochemical properties were determined to decide, which material is more suitable for high temperature CO2 chemisorption. It was confirmed that elemental composition, together with BET surface and pore size distribution, are important factors. Also other characterizations are, however, necessary to be measured to get all information potentially affecting the possible application in industry. Samples characterizations were performed by TGA, SEM, pore size analysis and X-ray fluorescence. The collected and evaluated data proved that certain fly ashes offered satisfactory capacities, which could be reached repeatedly after the appropriate thermal regeneration.

Numerical simulation of a bubbling fluidized bed reactor for sorption-enhanced steam methane reforming under industrially relevant conditions: Effect of sorbent (dolomite and CaO-Ca12Al14O33) and operational parameters

Sorption-Enhanced Steam Methane Reforming (SE-SMR) is a promising technology for effective production of hydrogen with simultaneous CO2 capture from conventional fuels (e.g. methane, coal) and alternative fuels (e.g. biomass). SE-SMR combines steam methane reforming reaction and water-gas shift conversion reaction with a high temperature CO2 sorption system using a mixture of solid catalyst and sorbent. In this work, a methodology that combines Taguchi robust Design of Experiments (DoE) with previously validated CFD simulations [19] is presented. The main objective is to carry on a multi-criteria analysis of the effect of different sorbents and pretreatments and operation parameters in the response of the 500 kWth bubbling fluidized bed (BFB) reactor installed in the ZECOMIX (Zero Emissions of CarbOn with MIXed technologies) research infrastructure of ENEA. The effect of different sorbents (i.e. naturally occurring dolomite and synthetic CaO-Ca12Al14O33) and pretreatment seems to be negligible at high scales compared with heat and mass transfer mechanisms. Other parameters (i.e. temperature, pressure, particle diameter, gas velocity, bed height, presence of catalyst and sorbent, treatment of sorbent, syngas composition) have been also evaluated. The present methodology can be a useful approach for a preliminary design and optimization of a full-scale SE-SMR reactors.

Impregnation of hydrotalcite with NaNO3 for enhanced high-temperature CO2 sorption uptake

Impregnation with alkali metals is a convenient and attractive way to enhance the CO2 sorption uptake of hydrotalcite. In this study, NaNO3 was used as a novel alkali metal precursor to increase the basicity of commercial hydrotalcite. Also, the influence of NaNO3 on the structural and textural properties and the CO2 sorption performance were investigated for hydrotalcites having different Mg:Al molar ratios. The change in the pore structure after impregnation with NaNO3 was investigated by N2 adsorption–desorption analysis, and the crystalline structures of hydrotalcites with different Mg:Al molar ratios were compared using X-ray diffraction. Because of the basicity increase caused by introducing NaNO3, enhanced CO2 sorption uptake was obtained for the hydrotalcites, and the hydrotalcite containing 30 wt% NaNO3 in a mixed MgO and Al2O3 structure showed the highest CO2 uptake of 1.15 mol·kg−1 at 200 °C and 1 atm. The degree of enhancement in the CO2 sorption uptake was dependent on the Mg:Al molar ratio in pristine hydrotalcite. Large amounts of Al2O3 appeared to highly destabilize the functional groups on the surface of the hydrotalcite, contributing to the increased basicity and CO2 sorption performance of hydrotalcite.

Effect of Zr and Li on high temperature CO2 sorption characteristics of CaO

In this work, a series of Zr and Li incorporated synthetic CaO based sorbents were prepared by a simple precipitation method for high temperature (600–800 °C) CO2 capture. After impregnation and subsequent calcination, the sorbents were characterized by N2 adsorption isotherm, scanning electron microscopy and X-ray diffraction techniques. The cyclic performance in CO2 processes was evaluated by breakthrough measurements. ZrCa3Li4O7 exhibited the maximum CO2 sorption of 28.4 wt% and it showed excellent stability. The CO2 performance of ZrCa3Li4O7 still remained stable when tested under a high decarbonation temperature of 900 °C.

Simplified synthesis of K2CO3-promoted hydrotalcite based on hydroxide-form precursors: Effect of Mg/Al/K2CO3 ratio on high-temperature CO2 sorption capacity

Hydrotalcite was synthesized from hydroxide-form precursors to prepare a novel high-temperature CO2 sorbent, and the effect of Mg/Al ratio on CO2 sorption was studied. To enhance the CO2 sorption capacity of the sorbent, K2CO3 was coprecipitated during the synthetic reaction. X-ray diffraction analysis indicated that the prepared samples had a well-defined crystalline hydrotalcite structure, and confirmed that K2CO3 was successfully coprecipitated in the samples. The morphology of the hydrotalcite was confirmed by scanning electron microscopy, and N2 adsorption analysis was used to estimate its surface area and pore volume. In addition, thermogravimetric analysis was used to measure its CO2 sorption capacity, and the results revealed that the Mg: Al: K2CO3 ratio used in the preparation has an optimum value for maximum CO2 sorption capacity.

High-temperature CO2 sorption by Ca-doped Li4SiO4 sorbents

Ca-doped Li4SiO4 (LiCa) sorbent was synthesized using a solid-state reaction method. The CO2 sorption capacity of the sorbents was thermogravimetrically analyzed in the presence of a pure CO2 flux. The cyclic performances of the sorbents were investigated in a twin fixed-bed reactor with absorption and desorption temperatures of 700 °C and 800 °C, respectively. The results indicate that the surface area of Li4SiO4 increased from 0.064 m2 g−1 to 0.314 m2 g−1 as the Ca doping content increased to 32 mol%, which enabled the sufficient contact between CO2 and Li4SiO4, and enhanced its CO2 sorption. The transformation of the Ca species from Ca2SiO4 to Li2CaSiO4 during the absorption process was helpful for the transfer of CO2 to Li4SiO4, and the transformation of Li2CaSiO4 to Ca2SiO4 in the desorption process favors CO2 desorption. Due to the special composition and structure, Li4SiO4 doped with 6 mol% of Ca had a maximum CO2 sorption of 35.1 wt% at 700 °C for 1 h and exhibited an excellent cyclic performance for CO2 sorption/desorption.

Citrate Sol–Gel Method for the Preparation of Sodium Zirconate for High-Temperature CO2 Sorption

The use of sorbents capable of capturing high-temperature CO2 increases the overall efficiency of CO2 removal processes, without the need for cooling the flue gases. In this study, Na2ZrO3 for high-temperature CO2 sorption was newly prepared using a citrate sol–gel method. The characteristics and CO2 sorption behavior of the prepared Na2ZrO3 were examined by field-emission scanning electron microscopy, N2 adsorption analysis, X-ray diffraction, and thermogravimetric analysis, and compared with those of Na2ZrO3 synthesized using a conventional solid-state method. Na2ZrO3 could be synthesized at lower temperature by using the citrate sol–gel method compared to the case of solid-state method. Na2ZrO3 prepared using the citrate sol–gel method showed excellent CO2 sorption performance: high sorption capacity (17.5–21.0 wt % at 823 K and 1 bar), fast sorption kinetics, and good cycling stability. Kinetic analysis was also conducted by fitting the CO2 sorption behavior to the double exponential model.

Effect of pH-controlled synthesis on the physical properties and intermediate-temperature CO2 sorption behaviors of K–Mg double salt-based sorbents

High-temperature CO2 sorption has been investigated as a new CO2 capture technology, and the development of high-temperature CO2 sorbents is critically important for the realization of this technology. In this study, K–Mg double salt-based sorbents were synthesized using a new pH-controlled precipitation method and the prepared sorbents were tested for medium high (intermediate) temperature CO2 sorption. In the new method for the synthesis of K–Mg double salt-based sorbents, the pH of the reagent solution was controlled by adding K2CO3 and/or KOH, resulting in varied ratios of different components in the sorbent. The CO2 sorption ability was enhanced in K–Mg double salt-based sorbents synthesized using the new pH-controlled method compared to the samples synthesized by the conventional precipitation method. The newly prepared K–Mg double salt-based sorbents had a high CO2 sorption uptake of up to 2.09molkg−1 at 300°C and 1bar. In addition, they showed fast sorption kinetics and good cyclic stability during multiple sorption/desorption procedures.

Potassium-based sorbents from fly ash for high-temperature CO2 capture.

Potassium-fly ash (K-FA) sorbents were investigated for high-temperature CO2 sorption. K-FAs were synthesised using coal fly ash as source of silica and aluminium. The synthesised materials were also mixed with Li2CO3 and Ca(OH)2 to evaluate their effect on CO2 capture. Temperature strongly affected the performance of the K-FA sorbents, resulting in a CO2 uptake of 1.45mmol CO2/g sorbent for K-FA 1:1 at 700°C. The CO2 sorption was enhanced by the presence of Li2CO3 (10 wt%), with the K-FA 1:1 capturing 2.38mmol CO2/g sorbent at 700°C in 5min. This sorption was found to be similar to previously developed Li-Na-FA (2.54mmol/g) and Li-FA (2.4mmol/g) sorbents. The presence of 10% Li2CO3 also accelerated sorption and desorption. The results suggest that the increased uptake of CO2 and faster reaction rates in presence of K-FA can be ascribed to the formation of K-Li eutectic phase, which favours the diffusion of potassium and CO2 in the material matrix. The cyclic experiments showed that the K-FA materials maintained stable CO2 uptake and reaction rates over 10 cycles.

High Temperature CO2 Sorption on Li2ZrO3 Based Sorbents

In this study, the Li2ZrO3 based sorbents with different compositions were synthesized by the solid-state reaction method from the mixtures of Li2CO3, K2CO3 and ZrO2. CO2 sorption properties of Li2ZrO3 based sorbents were investigated by analyzing the phases and microstructure changes with the help of thermogravimetric analysis, X-ray diffraction and scanning electron microscopy. The thermodynamic calculations were carried out based on the second law of thermodynamics. Li2CO3/K2CO3-doped Li2ZrO3 sorbent with the composition of 36.23 wt % Li2CO3, 55.12 wt % ZrO2 and 8.65 wt % K2CO3 was considered to achieve excellent capability for high temperature CO2 sorption and presented the maximum sorption rate at 525 °C and 0.15 atm of CO2 partial pressure. The sorbent kept rather stable for multicycles sorption and regeneration, and maintained its original capacity during 12 cycle processes. There were three distinct phases in the nonisothermal CO2 sorption process while the main CO2 sorption occurred during the second phase. An improved iterative Coats–Redfern method was used to evaluate nonisothermal kinetics of the CO2 sorption process, and the kinetic parameters were derived by the MATLAB model. The Fn nth-order reaction model predicted accurately the main phases and differences in the activation energies and the frequency factors for different sorbents in the sorption phases corroborated different mechanism integral functions and reaction orders.

Characteristics of Na–Mg double salt for high-temperature CO2 sorption

Abstract A novel high temperature CO 2 sorbent, Na–Mg double salt, was prepared by a precipitation method. Thermogravimetric analysis was used to measure the thermal stability and to investigate the CO 2 sorption behavior at high temperatures. The prepared Na–Mg double salt showed unique CO 2 sorption characteristics in the temperature range of 300–500 °C. A remarkably high CO 2 sorption uptake of 3.48 mol/kg (15.3 wt%) was measured at 375 °C and was maintained during repeated sorption/desorption cycles. Electronic microscopy and X-ray diffraction were used to determine the Na–Mg double salt morphology and structure, respectively. The elemental composition ratio in the Na–Mg double salt was characterized by inductively coupled plasma atomic emission spectroscopy and an elemental analysis. In situ X-ray diffraction analysis revealed that the CO 2 sorption mechanism was based on the reaction of CO 2 with MgO and Na 2 CO 3 in the double salt to form Na 2 Mg(CO 3 ) 2 . In addition to the high CO 2 sorption uptake and good cyclic stability, the Na–Mg double salt showed exceptionally fast sorption and desorption.

High-temperature CO2 sorption on Na2CO3-impregnated layered double hydroxides

Layered double hydroxide (LDH), one of representative high-temperature CO2 sorbents, has many advantages, including stable CO2 sorption, fast sorption kinetics, and low regeneration temperature. However, CO2 sorption uptake on LDH is not high enough for practical use; thus it is usually enhanced by impregnation with alkali metals such as K2CO3. In this study, LDH was impregnated with Na2CO3, and analyses based on scanning electron microscopy, N2 gas physisorption, in situ X-ray diffraction, and Fourier transform infrared spectroscopy were carried out to elucidate the characteristics of sorbents and the mechanism of CO2 sorption. Although the surface area of LDH decreased after Na2CO3 impregnation, CO2 sorption uptake was greatly enhanced by the additional basicity of Na2CO3. The crystal structure of Na2CO3 in the Na2CO3-impregnated LDH changed from monoclinic to hexagonal with increasing temperature, and the sorbed-CO2 was stored in the form of carbonate. Thermogravimetric analysis was used to measure CO2 sorption uptake at 200–600 °C. The sample of Na2CO3: LDH=0.35: 1 weight ratio had the largest CO2 sorption uptake among the tested sorbents, and the CO2 sorption uptake tended to increase even after 400 °C.

High temperature CO2 sorption with gallium-substituted and promoted hydrotalcites

Hydrotalcites (HTC) and gallium-substituted hydrotalcites (HTC-10Ga) were prepared by co-precipitation and modified by impregnation with alkali (K and Cs) and alkaline-earth (Sr) metals. The materials were tested and screened for CO2 sorption at 573K. The results indicate that the modification with potassium greatly enhanced the sorption capacity when compared with the original materials (both HTC and HTC-10Ga). An outstanding sorption capacity (2.01mmolcm−3 at 1.08bar) was obtained for the sample partially substituted with gallium and modified with potassium (HTC-10Ga–20K). Moreover, this material was also submitted to sorption–desorption cycles towards CO2 under low-pressure conditions (<0.0001–0.15bar) to assess its use in cyclic operation. An average working capacity of 0.97mmolcm−3 and 0.25mmolcm−3 was obtained after repeated cycles at 573K and 473K, respectively. The sorption kinetics was assessed through uptake measurements, showing two parallel resistances, which were well described by the proposed model.

Studies of Ca-based high temperature sorbents for CO2 capture

The natural mineral dolomite has been calcined and impregnated with Ti-, Zr-, Al-nanoparticle suspensions with the aim of improving the cyclic stabilities in high temperature CO2 sorption processes. After impregnation and subsequent calcination the materials were characterized by N2 adsorption (BET surface area and micropore volume) and powder X-Ray diffraction while the cyclic performance in CO2 sorption processes was evaluated by breakthrough measurements: 30 cycles with sorption in 10% dry CO2 at 600°C and regeneration at 850°C in N2 were run on each sample.