Cyclic CO2 Capture Performance Research Articles

Coal fly ash-bound limestone-derived sorbent pellets for high-temperature CO2 capture

Poor CO2 capture stability and serious elutriation are the issues commonly occur when limestone circulating in the fluidized bed-based Calcium Looping (CaL) system. Hence, three types of coal fly ash-bound (CFA-bound) limestone-derived sorbents were synthesized via mixing the limestone, calcined lime and hydrated lime with coal fly ash, respectively. The generated ternary Ca-Al-O and Ca-Si-O phases offer structural stability for CFA-bound limestone-derived sorbents and improve their cyclic CO2 capture performance. Moreover, extrusion-spheronization method was adopted to produce CFA-bound limestone and hydrated lime sorbent pellets to synergistically enhance cyclic CO2 capture capability and mechanical properties. It is found that hydrated lime is superior to limestone as the calcium precursor to prepare CaO-based sorbent pellets due to its desirable pore structure and high binding force. Therefore, the CFA-bound hydrated lime pellets containing 15 wt % of fly ash (HL/CFA15-P) exhibits a high carbonation conversion of 40 % after 17 cycles, which is nearly 1.4 times that of limestone pellets. The discrepancy on carbonation conversion between the two sorbent pellets is enlarged to ∼1.6 times after 100 cycles, which is calculated using a semi-empirical equation. Moreover, HL/CFA15-P exhibits outstanding mechanical property, whose compressive strength reaches 6±1.7 MPa, which is comparable to the cement-bound CaO-based sorbent pellets.

Improvement in cyclic CO2 capture performance and fluidization behavior of eggshell-derived CaCO3 particles modified with acetic acid used in calcium looping process

Although calcium-based materials are the most promising adsorbents used in calcium looping process for carbon dioxide removing, their CO2 capture capacity decaying besides poor fluidization, still are the important challenges. In the present investigation, eggshell as a cheap, easily available and unpolluted source of calcium carbonate was used for CO2 capturing in calcium looping process. Eggshell particles were treated with various volume concentrations of acetic acid to improve its sorption capacity. According to the TGA results after 20 carbonation/calcination cycles, the effective carbonation conversion of modified eggshell with 5%, 20%, 30% and 40%. v/v acetic acid was 21.33%, 24.26%, 25.97% and 28.97%, respectively, which is considerable compared to 20.54% for untreated eggshell. The effect of initial eggshell particle size on the adsorption behavior of final adsorbent was also investigated by using two different sizes including dp< 45 µm and dp> 320 µm. The results showed that the effective conversion of the adsorbent containing 40%. v/v acetic acid derived from small particle size eggshells was 9.32% higher than that from larger particle size eggshells. In terms of fluidization behavior, surprisingly the addition of acetic acid to the eggshell particles also increased the bed expansion ratio as 8% and 36.2% at gas velocities of 0.27 and 6.67 cm/s, respectively. Further improvement in the fluidity of eggshell modified with 40% acid was performed by manually mixing of SiO2 nanoparticles at different weight percentages. According to the results, adding 7.5 wt% SiO2 leaded to the homogeneous and agglomerate particulate fluidization.

Decay on Cyclic CO2 Capture Performance of Calcium-Based Sorbents Derived from Wasted Precursors in Multicycles

In order to obtain the cheap waste calcium-based sorbent, three wasted CaCO3 precursors, namely carbide slag, chicken eggshells, and analytical reagent-grade calcium carbonate, were selected and prepared at 700 °C to form calcium-based sorbents for CO2 capture. TGA was used to test the CO2 uptake performance of each calcium-based sorbent in 20 cycles. To identify the decay mechanism of CO2 uptake with an increasing number of cycles, all calcium-based sorbents were characterized by using XRF, XRD, and N2 adsorption. The specific surface area of calcium-based sorbents was used to redefine the formula of cyclic carbonation reactivity decay. The carbonation conversion rate of three calcium-based sorbents exhibited a decreasing trend as the cycle number increased. Chicken eggshells exhibited the most significant decrease rate (over 50% compared with Cycle 1), while carbide slag and analytical reagent-grade calcium carbonate showed a flat linear decline trend. The specific surface area of the samples was used to calculate carbonation conversion for an infinite number of cycles. The carbonation conversion rates of three calcium-based sorbents were estimated to decrease to 0.2898, 0.1455, and 0.3438 mol/mol, respectively, after 100 cycles.

Improvement in Cyclic Co2 Capture Performance and Fluidization Behavior of Eggshell-Derived Caco3 Particles in Calcium Looping Process

Improvement in Cyclic Co2 Capture Performance and Fluidization Behavior of Eggshell-Derived Caco3 Particles in Calcium Looping Process

Effect of calcination temperature and extent on the multi-cycle CO2 carrying capacity of lime-based sorbents

This paper investigates the effect of calcination temperature and duration on the cyclic CO2 capture performance of natural lime-based sorbents. Tests showed that increasing calcination temperature considerably reduced the sorbent reactivity over the first few calcination-carbonation cycles. No significant variation in performance was observed when a sorbent remained at the calcination temperature after completing limestone decomposition. On the other hand, incomplete limestone calcination significantly altered the sorbent cyclic utilization and CO2 carrying capacity. A semi-empirical method is proposed to estimate the fast reaction-controlled carbonation extent at different reaction cycles, calcination extents and temperatures. This method is shown to work well for two different naturally-derived limestones exposed to different calcination conditions.

Microemulsion-derived, nanostructured CaO/CuO composites with controllable particle grain size to enhance cyclic CO2 capture performance for combined Ca/Cu looping process

Abstract Combined Ca/Cu looping process is a very most promising CO2 capture technology, in which chemical looping combustion provides the heat for calcining CaCO3 in calcium looping by employing CaO/CuO composites. However, such CaO/CuO composites demonstrated a fast decay in CO2 uptake capacity during cyclic operation. In order to solve this problem, nanostructured CaO/CuO composites was synthesized using a microemulsion method in this work. The synthetic parameters, including the mole ratio of water to surfactant and mole concentration of precursors, were investigated systematically for the microemulsion system. A fixed-bed reactor was used to assess the redox characteristics and CO2 capture performance. The results showed that the particle grain size of the nanostructured CaO/CuO composites can be controlled by the variation of the mole ratio of water to surfactant. With the increase in the mole ratio of water to surfactant from 9 to 23, the mean particle grain size of the CaO/CuO composites increased from 0.19 to 0.57 μm. A high mole concentration of precursors also resulted in a much enhanced CO2 capture performance. Compared to the successive fast CO2 uptake decay for the reference CaO/CuO composites developed by a conventional co-precipitation method, the CaO/CuO composites fabricated by the microemulsion method demonstrated an increasing CO2 uptake in the initial cycles followed by a relatively slow decline in the subsequent cycles, exceeding uptake of the reference material by 104% after 19 cycles. The kinetic analysis showed that the CaO/CuO composites fabricated by the microemulsion method had much lower activation energy than the reference material.

Reactivation mode investigation of spent CaO-based sorbent subjected to CO2 looping cycles or sulfation

Rapid loss-in-capacity of naturally occurring limestone for CO2 capture is the most prominent issue in the calcium looping process. The synthesis of highly efficient sintering-resisting sorbents is a common solution to mitigate capacity decay. However, the long-term CO2 looping cycles, particularly in the presence of SO2, still cause the generation of large amounts of spent CaO-based sorbents. Comparatively, the reactivation of spent sorbent is more economical than the replenishing of fresh sorbent. In this work, three types of reactivation modes (i.e., hydration, simultaneous hydration/ impregnation and acidification) were applied to recover the CO2 capture capacities of the spent, synthetic CaO-based sorbents (CaO/MgO = 75 wt%/25 wt%). It is found that hydration is effective to recover the cyclic CO2 capture performance of the spent sorbent due to the formation of more cracks and channels that extend to the interior of sorbent particles. Therefore, the sorbent reactivated via sole hydration exhibits a high capacity of 0.390 g/g after 40 cycles. Moreover, NaCl impregnation combined with hydration produce lager amounts of macropores within the reactivated sorbents after initial sever sintering that are responsible for their desirable CO2 capture stability. The introduction of filtration during acidification reactivation process can effectively enhance the initial CO2 capture capacity (an improvement percentage of ~14.7%) owing to the removal of the irreversible CaSO4.

A Carbide Slag-Based, Ca12Al14O33-Stabilized Sorbent Prepared by the Hydrothermal Template Method Enabling Efficient CO2 Capture

Calcium looping is a promising technology to capture CO2 from the process of coal-fired power generation and gasification of coal/biomass for hydrogen production. The decay of CO2 capture activities of calcium-based sorbents is one of the main problems holding back the development of the technology. Taking carbide slag as a main raw material and Ca12Al14O33 as a support, highly active CO2 sorbents were prepared using the hydrothermal template method in this work. The effects of support ratio, cycle number, and reaction conditions were evaluated. The results show that Ca12Al14O33 generated effectively improves the cyclic stability of CO2 capture by synthetic sorbents. When the Al2O3 addition is 5%, or the Ca12Al14O33 content is 10%, the synthetic sorbent possesses the highest cyclic CO2 capture performance. Under harsh calcination conditions, the CO2 capture capacity of the synthetic sorbent after 30 cycles is 0.29 g/g, which is 80% higher than that of carbide slag. The superiority of the synthetic sorbent on the CO2 capture kinetics mainly reflects at the diffusion-controlled stage. The cumulative pore volume of the synthetic sorbent within the range of 10–100 nm is 2.4 times as high as that of calcined carbide slag. The structure of the synthetic sorbent reduces the CO2 diffusion resistance, and thus leads to better CO2 capture performance and reaction rate.

Stabilized Performance of Al‐Decorated and Al/Mg Co‐Decorated Spray‐Dried CaO‐Based CO2 Sorbents

AbstractThe sharp loss‐in‐capacity in CO2 capture as a result of sintering is a major drawback for CaO‐based sorbents used in the calcium looping process. The decoration of inert supports effectively stabilizes the cyclic CO2 capture performance of CaO‐based sorbents via sintering mitigation. A range of Al‐decorated and Al/Mg co‐decorated CaO‐based sorbents were synthesized via an easily scaled‐up spray‐drying route. The decoration of Al‐based and Al/Mg‐based supports efficiently enhanced the cyclic CO2 capture capability of CaO‐based sorbents under severe testing conditions. The CO2 capture capacity losses of Al‐decorated and Al/Mg co‐decorated CaO‐based sorbents were alleviated, representing more stable CO2 capture performance. The stabilized CO2 capture performance is mainly attributed to the formation of Ca12Al14O33, MgAl2O4, and MgO that act as the skeleton structures to mitigate the sintering of CaCO3 during carbonation/calcination cycles.

Plastic/rubber waste-templated carbide slag pellets for regenerable CO2 capture at elevated temperature

Plastic/rubber waste-templated carbide slag pellets were prepared via an extrusion-spheronization method for high-temperature CO2 capture. Four types of common plastic and rubber wastes (i.e. waste plastic bottle composed of polyethylene terephthalate, disposable plastic cup composed of polypropylene, waste plastic pipe composed of polyvinyl chloride and scrap tire composed of rubber, sulfur and carbon black) were selected as the templating materials. It is found that the addition of waste plastic pipe can cause inferior CO2 capture performance for carbide slag pellets (merely 0.123 g CO2/g sorbent after 25 cycles). It is mainly attributed to the deactivation of carbide slag pellets as a result of the interaction between CaO and the chlorine released from waste plastic pipe. The other chlorine-free plastic and rubber wastes are effective to improve the cyclic CO2 capture performance of carbide slag pellets via altering their porosities. Particularly, the carbide slag pellets doped with 5 wt% of waste plastic bottle exhibit the highest CO2 capture capacity of 0.277 g CO2/g sorbent after 25 cycles, compared to 0.218 g CO2/g sorbent for carbide slag pellets. Moreover, the pre-calcination atmosphere plays an important role on the cyclic CO2 uptake of plastic/rubber waste-templated pellets. The CO2 capture capability of plastic/rubber waste-templated pellets pre-calcined in air is inferior to that pre-calcined in N2. It is mainly due to the heat released from the combustion of plastic or rubber wastes in air accelerates severe sintering for the plastic/rubber waste-templated carbide slag pellets.

Influence of water vapor on cyclic CO2 capture performance in both carbonation and decarbonation stages for Ca-Al mixed oxide

Little research was carried out to study the effect of water vapor with different contents on the CO2 adsorption performance during extending cycles in both carbonation and decarbonation stages. This study described the CO2 adsorption performance of Ca-Al mixed oxides under different water vapor concentrations. The results revealed that the reactivity and durability of both fresh and spent sorbents were remarkably enhanced by adding water vapor. The initial CO2 carbonation capacity increased from 56 wt% without water to 70.7 wt% with 1.6% water vapor at the adsorption temperature of 600 °C. The stability of Ca-Al mixed oxides was also investigated during 220 cycles with 1% steam vapor. The CO2 adsorption capacity retained 55.6 wt% after 220 cycles, indicating an excellent long-term durability. The excellent CO2 adsorption capacity and stability of the mixed oxides in the existence of water vapor may be attributed to the following reasons: water vapor facilitated CO2 diffusion in the CaCO3 product layer during the carbonation stage as well as the decomposition rate of CaCO3 during the calcination stage; water vapor facilated the formation of the loose porous structure, which was favorable for the subsequent carbonation of CaO.

CO2 capture using mesocellular siliceous foam (MCF)-supported CaO

CaO is a promising material as an alternative CO2 capture material which can be used at high temperature. However, CaO sinters to form large particles under long-term high temperature conditions, resulting in a rapid decrease of its surface area and the capacity of CO2 capture. Incorporating CaO into inert materials is a promising strategy to enhance the performance of CO2 capture. This work investigated a novel composite material called mesocellular siliceous foam (MCF)-supported CaO to enhance the stability and capacity of CaO-based materials for CO2 capture. The crystal structure, surface morphology and porosity property of the developed composite materials were investigated. Thermogravimetric measurements were carried out to study the cyclic CO2 capture performance of the MCF-supported CaO composites. The results showed that a part of CaO reacted with the silica wall, and the formation of Ca2SiO4 within the MCF framework limited the presence of CaO in the mesopores, thus inhibited the sintering of CaO. The sample of MCF-3CaO exhibited a better performance of CO2 capture and long-term stability, compared with the materials prepared with lower CaO loading. This work contributes to the development of high temperature CO2 capture adsorbents, which can be applied for decarbonizing major industrials e.g. power plant.

Porous spherical calcium-based sorbents prepared by a bamboo templating method for cyclic CO2 capture

Calcium looping is regarded as one of the most promising technologies to capture CO2; however, calcium based sorbents experienced rapid decay in CO2 capture capacities and serious attrition during cyclic high temperature reactions; thus, it is critical to develop new calcium based sorbent with high CO2 capture capacity and high attrition resistance. This paper presents a new modeling approach with bamboo, a pore-creating agent templating, to enhance the CO2 capture and attrition resistance performance of the calcium based sorbents simultaneously. The cyclic CO2 capture performance were tested by a simultaneous thermal analyzer (STA), and the pore structure were analyzed by field emission scanning electron microscope (FSEM) and N2 absorption/desorption technologies. The bamboo powder was analyzed by pyrolysis, proximate analysis and elemental analysis method. A friability tester (FT) was used to investigate the mechanical strength of these modeled sorbents. The results show that all the bamboo templated sorbents behaved enhanced CO2 capture performances, especially the molded sorbents with 2 wt% mass ratio of bamboo templated (CaS-B2) showed the highest capture capacity of 0.262 g/g after 25 cycles, which was 33.7% higher than that of molded sorbents without bamboo templating (CaS). The modeling method is beneficial to prepare sorbents with favorable mechanical strength, and the optimum templating mass ratio is 2 wt% (lost a minority of 0.166 wt% during 20,000 turns, which was almost negligible comparing to 3.64 wt% of limestone).

The effect of HCl and steam on cyclic CO2 capture performance in calcium looping systems

Calcium looping is CO2 capture technology that is considered to be technically feasible at an industrial scale using a variety of fuels such as natural gas, coals, biomass, refuse derived fuels, and biofuels. Unfortunately, many of these fuels contain significant quantities of chlorine which principally converts to gaseous HCl during combustion or gasification. To date, very few studies have examined the effect of HCl on sorbent CO2 capture performance using calcium-based sorbents under realistic carbonation and calcination conditions. In this work, experiments were conducted using thermogravimetric analysis and fixed bed reactor testing to determine the effect of HCl addition during carbonation and calcination over repeated cycles using a Canadian limestone. The presence of HCl was found to increase sorbent reactivity towards CO2 capture when steam was injected during calcination. The resulting decomposition of CaCl2 to CaO during calcination caused changes in the particle morphology, which in turn decreased the CO2 diffusional resistance during carbonation. Fixed bed test results provided confirmation of full sorbent dechlorination under typical oxy-fuel calcination conditions. It was shown that both particle surface area and pore volume were higher during tests where HCl was present during carbonation and that greater than 99% HCl capture could be achieved without adversely affecting sorbent CO2 capture performance when steam was present during both carbonation and calcination.

Enhanced Cyclic Stability and CO2 Capture Performance of MgO-Al2O3 Sorbent Decorated with Eutectic Mixture

Recently, alkali nitrate activated MgO has been proposed as an alternative solid sorbent for high temperature CO2 capture due to the facile solvation of MgO in the activation step. However, it tends to be unstable under multi-cyclic sorption-regeneration process due to continuous deformation. In this work, it is demonstrated that MgO-Al2O3 composite sorbent decorated with eutectic mixture (KNO3-LiNO3) can be a feasible candidate for high temperature CO2 capture without losing cyclic performance. It is noteworthy that the eutectic mixture could activate both MgO and MgO-Al2O3 composite, while NaNO3 can only activate MgO. Also, the inactive MgO-Al2O3 is activated at high temperature sorption with the help of the eutectic mixture. Further, it is demonstrated that the EM-MgO-Al2O3 could overcome aggregation and shows stable CO2 sorption performance for 10 cycles corresponding to ∼8.7 wt.%.

Core‐shell structured CaO‐based pellets protected by mesoporous ceramics shells for high‐temperature CO2 capture

AbstractIn this study, a series of core‐shell structured spheriform CO2 sorbents were synthesized by using CaO‐based pellets as cores and different mesoporous metal oxides (e.g. alumina, ceria, and yttrium‐stabilized zirconia) as shells through a repeated wet impregnation coating process. Cyclic CO2 capture performance of the obtained sorbents was investigated using a thermogravimetric analyzer. Among all the core/shell sorbents under study, the pellets coated with a layer of alumina exhibit the best performance in the retention of CO2 uptake over 20 cycles with the lowest activity loss of only 30.4 %, attributed to the existence of the thermal stable porous alumina shell which prevents the sintering and the aggregation of the CaO grains. Moreover, the attrition study using an air‐jet apparatus and a standard test method reveals that such sorbents exhibit enhanced attrition resistance due to the protection of the porous shell providing them with a great potential for application in fluidized bed conditions.

Pelletization of MgO-based sorbents for intermediate temperature CO2 capture

The pelletization of MgO-based sorbents is necessary before implementing intermediate temperature CO2 capture for industrial applications. In the current work, MgO-based sorbent pellets were prepared using an extrusion–spheronization method, with the aim of achieving an effective granulation method for MgO sorbents. Another objective of this investigation is attempting to find out the differences between the sorbents in pelleted and powdered forms. The compression and friability tests have proved that the prepared pellets have outstanding mechanical properties, which are of significant importance for the fluidization in CFBC for the practical applications. N2 adsorption-desorption results have shown obvious variations in the surface areas and pore size distributions of the sorbents before and after granulation. These variations further result in different carbonation/regeneration rates and cyclic CO2 capture performance for the powdered and pelleted sorbents. All these differences in the physicochemical properties between the sorbents pellets and corresponding powders indicate that the results derived from the powdered sorbents in previous studies should be re-examined before the practical application for the realization of highly-efficient CO2 capture.

Synthesis of Efficient CaO Sorbents for CO2 Capture Using a Simple Organometallic Calcium-Based Carbon Template Route

A facile carbon template method was employed to modify the microstructure of calcined organometallic calcium compounds (i.e., calcium acetate, calcium citrate, and calcium gluconate) for high-temperature CO2 capture. The effects of the decomposition atmosphere, carbon source, and pyrolysis temperature on the physical and chemical properties as well as the cyclic CO2 capture performance were determined using various morphological characterization techniques and detailed thermogravimetric analysis. During pyrolysis in an inert atmosphere, in situ carbon formed from organometallic calcium compounds, which served as a template for controlling the sintering of CaCO3, and the subsequent carbon burnoff in air promoted the fabrication of porous CaO. Among the three organometallic calcium compounds, the pyrolysis process of calcium gluconate exhibited the earliest carbonization and the template with the highest organic carbon content and the slowest decomposition of CaCO3. Therefore, the most favorable structure (...

High Temperature CO2 Capture on Novel Yb2O3-Supported CaO-Based Sorbents

Incorporation of CaO in inert solid support has been identified as an effective approach to improve the cyclic CO2 capture performance for CaO-based sorbents. In this work, Yb2O3-supported CaO-based sorbents were fabricated via the wet-mixing technique. Elemental dispersion observed by FSEM-EDS showed that the ultrafine active species of CaO particles was finely separated by Yb2O3. Different amounts of Yb2O3 were introduced and 10−15 wt% was found to be the optimal content range for the support to function as the metal skeleton. In comparison with pure CaO, the improved CO2 capture performance was observed for CaYb10 over prolonged carbonation-calcination cycles under a severe test condition. This improvement could be ascribed to the enriched macropores within 50–100 nm, which was identified by the N2 adsorption–desorption analysis. In addition, the microstructure of the synthetic sorbent (analyzed through TEM images) indicated that CaO particles of ∼160 nm were formed with the Yb2O3 nanocrystallines (∼15...

Mechanical Modification of Naturally Occurring Limestone for High-Temperature CO2 Capture

The rapid decrease of CO2 capture capacity is one of the most challenging problems hindering the use of naturally occurring limestone in the calcium looping process. In this work, the mechanical modification method (dry planetary ball milling) was used to improve the cyclic CO2 capture performance of naturally occurring limestone. Low-cost Bayer aluminum hydroxide sourced from the industrial-scale production of alumina from bauxite ore was used as the precursor of the inert support to enhance the CO2 sorption stability of the ball-milled sorbents. It was found that the CO2 uptake of the milled sorbents could be further improved by increasing the ball-milling time because this generated more amounts of fine particles. Moreover, the pellets produced from ball-milled limestone powder possessed a relatively high CO2 capture capacity of 0.252 g/g in the 25th cycle, which is nearly 1.3 times the capture capacity of naturally occurring limestone powder. This indicates that the combination of mechanical modificat...