Steam reactivation of spent CaO-based sorbent for multiple CO2 capture cycles

This study examines steam reactivation of sorbent to improve the reversibility of multiple CaO-CO2 capture cycles. Experiments to obtain spent sorbent were performed in a tube furnace, and reactivation was achieved using steam in a pressurized reactor. Sorbent activity for CO2 capture was then tested in a thermogravimetric analyzer (TGA), in multi-cycle carbonation tests. After reactivation the sorbent had even better characteristics for CO2 capture than that of the natural sorbent. The average carbonation degree over 10 cycles for the reactivated sorbent approached 70%, significantly higher than for the original sorbent (35-40%). This means that the same sorbent may achieve effective CO2 capture over a large number of cycles, in the absence of other phenomena such as attrition. Partially sulfated sorbents may also be reactivated, but hydration itself is also hindered by sulfation.     

Fluidized bed combustion systems integrating CO2 capture with CaO

Capturing CO2 from large-scale power generation combustion systems such as fluidized bed combustors (FBCs) may become important in a CO2-constrained world. Using previous experience in capturing pollutants such as SO2 in these systems, we discuss a range of options that incorporate capture of CO2 with CaO in FBC systems. Natural limestones emerge from this study as suitable high-temperature sorbents for these systems because of their low price and availability. This is despite their limited performance as regenerable sorbents. We have found a range of process options that allow the sorbent utilization to maintain a given level of CO2 separation efficiency, appropriate operating conditions, and sufficiently high power generation efficiencies. A set of reference case examples has been chosen to discuss the critical scientific and technical issues of sorbent performance and reactor design for these novel CO2 capture concepts.     

Development of regenerable MgO-based sorbent promoted with K2CO3 for CO2 capture at low temperatures

To improve their CO2 absorption capacity, alkali-based sorbents prepared by impregnation and wet mixing method of potassium carbonate on supports such as activated carbon and MgO (KACI30, KACP30, KMgI30, and KMgP30), were investigated in a fixed bed reactor (C02 absorption at 50-100 degrees C and regeneration at 150-400 degrees C). Total CO2 capture capacities of KMgI30-500 and KMgP30-500 were 178.6 and 197.6 mg CO2/g sorbent, respectively, in the presence of 11 vol % H2O even at 50 degrees C. The large amount of CO2 capture capacity of KMgP30-500 and KMgI30-500 could be explained by the fact that MgO itself, as well as K2CO3, could absorb CO2 in the presence of water vapor even at low temperatures. In particular, water vapor plays an important role in the CO2 absorption of MgO and KMgI30-500 even at low temperatures below 60 degrees C, in marked contrast to MgO and CaO which can absorb CO2 at high temperatures. The CO2 capture capacity of the KMgI30-300 sorbent, however, was less than that of KMgI30-500 due to the formation of Mg(OH)2 which did not absorb CO2. MgO based-sorbents promoted with K2CO3 after CO2 absorption formed new structures such as K2Mg(CO3)2 and K2Mg(CO3)2 x 4(H2O), unlike KACI30 which showed only the KHCO3 crystal structure. The new Mg-based sorbents promoted with K2CO3 showed excellent characteristics in that it could satisfy a large amount of CO2 absorption at low temperatures, a high CO2 absorption rate, and fast and complete regeneration.     

SO2 retention by reactivated CaO-based sorbent from multiple CO2 capture cycles

This paper examines the reactivation of spent sorbent, produced from multiple CO2 capture cycles, for use in SO2 capture. CaO-based sorbent samples were obtained from Kelly Rock limestone using three particle size ranges, each containing different impurities levels. Using a thermogravimetric analyzer (TGA), the sulfation behavior of partially sulfated and unsulfated samples obtained after multiple calcination-carbonation cycles in a tube furnace (TF), following steam reactivation in a pressurized reactor, is examined. In addition, samples calcined/sintered under different conditions after hydration are also examined. The results show that suitably treated spent sorbent has better sulfation characteristics than that of the original sorbent. Thus for example, after 2 h sulfation, > 80% of the CaO was sulfated. In addition, the sorbent showed significant activity even after 4 h when > 95% CaO was sulfated. The results were confirmed by X-ray diffraction (XRD) analysis, which showed that, by the end of the sulfation process, samples contained CaSO4 with only traces of unreacted CaO. The superior behavior of spent reactivated sorbent appears to be due to swelling of the sorbent particles during steam hydration. This enables the development of a more suitable pore surface area and pore volume distribution for sulfation, and this has been confirmed by N2 adsorption-desorption isotherms and the Barrett-Joyner-Halenda (BJH) method. The surface area morphology of sorbent after reactivation was examined by scanning electron microscopy (SEM). Ca(OH)2 crystals were seen, which displayed their regular shape, and their elemental composition was confirmed by energy-dispersive X-ray (EDX) analysis. The improved characteristics of spent reactivated sorbent in comparison to the original and to the sorbent calcined under different conditions and hydrated indicate the beneficial effect of CO2 cycles on sorbent reactivation and subsequent sulfation. These results allow us to propose a new process for the use of CaO-based sorbent in fluidized bed combustion (FBC) systems, which incorporates CO2 capture, sorbent reactivation, and SO2 retention.     

Highly efficient CO2 sorbents: development of synthetic, calcium-rich dolomites

The reaction of CaO with CO(2) is a promising approach for separating CO(2) from hot flue gases. The main issue associated with the use of naturally occurring CaCO(3), that is, limestone, is the rapid decay of its CO(2) capture capacity over repeated cycles of carbonation and calcination. Interestingly, dolomite, a naturally occurring equimolar mixture of CaCO(3) and MgCO(3), possesses a CO(2) uptake that remains almost constant with cycle number. However, owing to the large quantity of MgCO(3) in dolomite, the total CO(2) uptake is comparatively small. Here, we report the development of a synthetic Ca-rich dolomite using a coprecipitation technique, which shows both a very high and a stable CO(2) uptake over repeated cycles of calcination and carbonation. To obtain such an excellent CO(2) uptake characteristic it was found to be crucial to mix the Ca(2+) and Mg(2+) on a molecular level, that is, within the crystalline lattice. For sorbents which were composed of mixtures of microscopic crystals of CaCO(3) and MgCO(3), a decay behavior similar to natural limestone was observed. After 15 cycles, the CO(2) uptake of the best sorbent was 0.51 g CO(2)/g sorbent exceeding the CO(2) uptake of limestone by almost 100%.     

Novel CaO-SiO2 sorbent and bifunctional Ni/Co-CaO/SiO2 complex for selective H2 synthesis from cellulose

Catalysis- and sorption-enhanced biomass gasification is a promising route to high-purity hydrogen (H(2)); however, most CaO-based sorbents for CO(2) capture have poor surface area and mechanical properties, lose carrying capacity over multiple uses, and have insufficient porosity to accommodate extra catalyst sites. We aimed to develop a high-surface-area CaO-SiO(2) framework onto which catalysts could be grafted. The best CaO-SiO(2) sorbent (n(Ca)/n(Si) = 2:1) maintained a CaO conversion of 65% even after 50 carbonation-decarbonation cycles, better than commercial micrometer-sized CaO or tailored CaO, because of stabilization via Ca-O-Si interactions and an ordered porous structure. Bimetallic catalyst grains (Ni/Co alloy, <20 nm) could be evenly loaded onto this structure by impregnation. The resulting bifunctional complex produced H(2) at nearly the same rate as a mixture of catalyst and commercial CaO while using less total sorbent/catalyst. Furthermore, this complex was much more durable due to its higher coking resistance and stable structure. After 25 carbonation-decarbonation cycles, the new catalyst-sorbent complex enhanced the H(2) yield from cellulose far more than a mixture of catalyst and commercial CaO did following the same treatment.     

Nanostructured sorbents for capture of cadmium species in combustion environments

The pathways of cadmium species to form a submicrometer-sized aerosol in a combustion system exhaust were established. Cadmium oxide was the predominant species formed in the experiments and resulted in particles of a mean size of 26-63 nm with number concentrations in the range of 2-8 x 10(6) cm(-3). Two different nanostructured sorbents, a solid montmorillonite (MMT) and an in situ generated agglomerated silica, were used for capture of the cadmium species. The MMT sorbent was not stable at 1000 degrees C, and structural changes resulted. MMT did not suppress nucleation of cadmium species and partially captured it by weak physisorption as established by the leachability tests. In contrast, the in situ generated silica nanostructured agglomerates had a high surface area, suppressed nucleation of cadmium species vapors, and chemisorbed them effectively resulting in a firm binding, as compared to the MMT sorbent. There is an optimal temperature-time relationship at which the capture process is expected to be most effective. The leaching efficiency under these conditions was less than 3.2%. The nanostructured silica agglomerate size can be tuned for effective capture in existing particle control devices.     

Sequential capture of CO2 and SO2 in a pressurized TGA simulating FBC conditions

Four FBC-based processes were investigated as possible means of sequentially capturing SO2 and CO2. Sorbent performance is the key to their technical feasibility. Two sorbents (a limestone and a dolomite) were tested in a pressurized thermogravimetric analyzer (PTGA). The sorbent behaviors were explained based on complex interaction between carbonation, sulfation, and direct sulfation. The best option involved using limestone or dolomite as a SO2-sorbent in a FBC combustor following cyclic CO2 capture. Highly sintered limestone is a good sorbent for SO2 because of the generation of macropores during calcination/carbonation cycling.     

Influence of high-temperature steam on the reactivity of CaO sorbent for CO₂ capture

Calcium looping is a high-temperature CO(2) capture technology applicable to the postcombustion capture of CO(2) from power station flue gas, or integrated with fuel conversion in precombustion CO(2) capture schemes. The capture technology uses solid CaO sorbent derived from natural limestone and takes advantage of the reversible reaction between CaO and CO(2) to form CaCO(3); that is, to achieve the separation of CO(2) from flue or fuel gas, and produce a pure stream of CO(2) suitable for geological storage. An important characteristic of the sorbent, affecting the cost-efficiency of this technology, is the decay in reactivity of the sorbent over multiple CO(2) capture-and-release cycles. This work reports on the influence of high-temperature steam, which will be present in flue (about 5-10%) and fuel (～20%) gases, on the reactivity of CaO sorbent derived from four natural limestones. A significant increase in the reactivity of these sorbents was found for 30 cycles in the presence of steam (from 1-20%). Steam influences the sorbent reactivity in two ways. Steam present during calcination promotes sintering that produces a sorbent morphology with most of the pore volume associated with larger pores of ～50 nm in diameter, and which appears to be relatively more stable than the pore structure that evolves when no steam is present. The presence of steam during carbonation reduces the diffusion resistance during carbonation. We observed a synergistic effect, i.e., the highest reactivity was observed when steam was present for both calcination and carbonation.     

Influence of the concentration of CO2 and SO2 on the absorption of CO2 by a lithium orthosilicate-based absorbent

A novel, high temperature solid absorbent based on lithium orthosilicate (Li(4)SiO(4)) has shown promise for postcombustion CO(2) capture. Previous studies utilizing a clean, synthetic flue gas have shown that the absorbent has a high CO(2) capacity, >25 wt %, along with high absorption rates, lower heat of absorption and lower regeneration temperature than other solids such as calcium oxide. The current effort was aimed at evaluating the Li(4)SiO(4) based absorbent in the presence of contaminants found in typical flue gas, specifically SO(2), by cyclic exposure to gas mixtures containing CO(2), H(2)O (up to 25 vol. %), and SO(2) (up to 0.95 vol. %). In the absence of SO(2), a stable CO(2) capacity of ～ 25 wt % over 25 cycles at 550 °C was achieved. The presence of SO(2), even at concentrations as low as 0.002 vol. %, resulted in an irreversible reaction with the absorbent and a decrease in CO(2) capacity. Analysis of SO(2)-exposed samples revealed that the absorbent reacted chemically and irreversibly with SO(2) at 550 °C forming Li(2)SO(4). Thus, industrial application would require desulfurization of flue gas prior to contacting the absorbent. Reactivity with SO(2) is not unique to the lithium orthosilicate material, so similar steps would be required for other absorbents that chemically react with SO(2).     

Fabrication of CaO-based sorbents for CO₂ capture by a mixing method

Three types of sorbent were fabricated using various calcium and support precursors via a simple mixing method, in order to develop highly effective, durable, and cheap CaO-based sorbents suitable for CO(2) capture. The sorption performance and morphology of the sorbents were measured in a thermogravimetric analyzer and a scanning electron microscopy, respectively. The experimental results indicate that cement is a promising low-cost support precursor for contributing to the enhancement of cyclic CO(2) sorption capacity, especially when organometallic calcium precursors were used. A sorbent (with 75% CaO content) made from calcium l-lactate hydrate and cement showed the highest CO(2) sorption capacity of 0.36 g of CO(2)/g of sorbent and its capacity decreased only slightly after 70 cycles of carbonation and calcination.     

Integration of calcium and chemical looping combustion using composite CaO/CuO-based materials

Calcium looping cycles (CaL) and chemical looping combustion (CLC) are two new, developing technologies for reduction of CO(2) emissions from plants using fossil fuels for energy production, which are being intensively examined. Calcium looping is a two-stage process, which includes oxy-fuel combustion for sorbent regeneration, i.e., generation of a concentrated CO(2) stream. This paper discuss the development of composite materials which can use copper(II)-oxide (CuO) as an oxygen carrier to provide oxygen for the sorbent regeneration stage of calcium looping. In other words, the work presented here involves integration of calcium looping and chemical looping into a new class of postcombustion CO(2) capture processes designated as integrated CaL and CLC (CaL-CLC or Ca-Cu looping cycles) using composite pellets containing lime (CaO) and CuO together with the addition of calcium aluminate cement as a binder. Their activity was tested in a thermogravimetric analyzer (TGA) during calcination/reduction/oxidation/carbonation cycles. The calcination/reduction typically was performed in methane (CH(4)), and the oxidation/carbonation stage was carried out using a gas mixture containing both CO(2) and O(2). It was confirmed that the material synthesized is suitable for the proposed cycles; with the very favorable finding that reduction/oxidation of the oxygen carrier is complete. Various schemes for the Ca-Cu looping process have been explored here that would be compatible with these new composite materials, along with some different possibilities for flow directions among carbonator, calciner, and air reactor.     

Concurrent Separation of CO(2) and H(2)O from Air by a Temperature-Vacuum Swing Adsorption/Desorption Cycle

A temperature-vacuum swing (TVS) cyclic process is applied to an amine-functionalized nanofibrilated cellulose sorbent to concurrently extract CO(2) and water vapor from ambient air. The promoting effect of the relative humidity on the CO(2) capture capacity and on the amount of coadsorbed water is quantified. The measured specific CO(2) capacities range from 0.32 to 0.65 mmol/g, and the corresponding specific H(2)O capacities range from 0.87 to 4.76 mmol/g for adsorption temperatures varying between 10 and 30 °C and relative humidities varying between 20 and 80%. Desorption of CO(2) is achieved at 95 °C and 50 mbar(abs) without dilution by a purge gas, yielding a purity exceeding 94.4%. Sorbent stability and a closed mass balance for both H(2)O and CO(2) are demonstrated for ten consecutive adsorption-desorption cycles. The specific energy requirements of the TVS process based on the measured H(2)O and CO(2) capacities are estimated to be 12.5 kJ/mol(CO2) of mechanical (pumping) work and between 493 and 640 kJ/mol(CO2) of heat at below 100 °C, depending on the air relative humidity. For a targeted CO(2) capacity of 2 mmol/g, the heat requirement would be reduced to between 272 and 530 kJ/mol(CO2), depending strongly on the amount of coadsorbed water.     

Application of amine-tethered solid sorbents for direct CO2 capture from the ambient air

While current carbon capture and sequestration (CCS) technologies for large point sources can help address the impact of CO(2) buildup on global climate change, these technologies can at best slow the rate of increase of the atmospheric CO(2) concentration. In contrast, the direct CO(2) capture from ambient air offers the potential to be a truly carbon negative technology. We propose here that amine-based solid adsorbents have significant promise as key components of a hypothetical air capture process. Specifically, the CO(2) capture characteristics of hyperbranched aminosilica (HAS) materials are evaluated here using CO(2) mixtures that simulate ambient atmospheric concentrations (400 ppm CO(2) = "air capture") as well as more traditional conditions simulating flue gas (10% CO(2)). The air capture experiments demonstrate that the adsorption capacity of HAS adsorbents are only marginally influenced even with a significant dilution of the CO(2) concentration by a factor of 250, while capturing CO(2) reversibly without significant degradation of performance in multicyclic operation. These results suggest that solid amine-based air capture processes have the potential to be an effective approach to extracting CO(2) from the ambient air.     

A biomass-supported Na2CO3 sorbent for flue gas desulfurization

A novel sorbent for SO2 removal has been investigated. The sorbent is obtained by conventional incipient wetness impregnation of abandoned biomaterials (straw or dried leaves) with an aqueous solution of Na2CO3. A material with the composition 80 wt % Na2CO3/straw shows a desulfurization activity which is both higher and faster than that of the reference sample Na2CO3/gamma-Al2O3. The breakthrough and stoichiometric SO2 adsorption efficiencies for 80 wt % Na2CO3/straw reach 48.9% and 80.6%, respectively, at a temperature of 80 degrees C. The adsorption efficiencies are almost constant in the temperature range 70 to 300 degrees C. According to IR and XPS analysis the main products observed on the spent sorbent are sulfite below 150 degrees C and sulfate at 300 degrees C. The Na2CO3 in 80 wt % Na2CO3/straw can potentially be recycled by the oxidation of the straw with concomitant reduction of the sulfite species to elemental sulfur, making the proposed process CO2 neutral.     

Carbon dioxide capture by functionalized solid amine sorbents with simulated flue gas conditions

A novel solid amine sorbent was prepared using KIT-6-type mesoporous silica modified with tetraethylenepentamine (TEPA). Its adsorption behavior toward CO(2) from simulated flue gases is investigated using an adsorption column. The adsorption capacities at temperatures of 303, 313, 333, 343, and 353 K are 2.10, 2.29, 2.58, 2.85, and 2.71 mmol g(-1), respectively. Experimental adsorption isotherms were obtained, and the average isosteric heat of adsorption was 43.8 kJ/mol. The adsorption capacity increases to 3.2 mmol g(-1) when the relative humidity (RH) of the simulated flue gas reaches 37%. The adsorption capacity is inhibited slightly by the presence of SO(2) at concentrations lower than 300 ppm but is not significantly influenced by NO at concentrations up to 400 ppm. The adsorbent is completely regenerated in 10 min at 393 K and a pressure of 5 KPa, with expected consumption energy of about 1.41 MJ kg(-1) CO(2). The adsorption capacity remains almost the same after 10 cycles of adsorption/regeneration with adsorption conditions of 10 vol % CO(2), 100 ppm SO(2), 200 ppm NO, 100% relative humidity, and a temperature of 393 K. The solid amine sorbent, KIT-6(TEPA), performs excellently for CO(2) capture and its separation from flue gas.     

Modeling CO2 mass transfer in amine mixtures: PZ-AMP and PZ-MDEA

The most common method of carbon dioxide (CO(2)) capture is the absorption of CO(2) into a falling thin film of an aqueous amine solution. Modeling of mass transfer during CO(2) absorption is an important way to gain insight and understanding about the underlying processes that are occurring. In this work a new software tool has been used to model CO(2) absorption into aqueous piperazine (PZ) and binary mixtures of PZ with 2-amino-2-methyl-1-propanol (AMP) or methyldiethanolamine (MDEA). The tool solves partial differential and simultaneous equations describing diffusion and chemical reaction automatically derived from reactions written using chemical notation. It has been demonstrated that by using reactions that are chemically plausible the mass transfer in binary mixtures can be fully described by combining the chemical reactions and their associated parameters determined for single amines. The observed enhanced mass transfer in binary mixtures can be explained through chemical interactions occurring in the mixture without need to resort to using additional reactions or unusual transport phenomena such as the "shuttle mechanism".     

CO2 capture in different carbon materials

In this work, the CO(2) capture capacity of different types of carbon nanofibers (platelet, fishbone, and ribbon) and amorphous carbon have been measured at 26 °C as at different pressures. The results showed that the more graphitic carbon materials adsorbed less CO(2) than more amorphous materials. Then, the aim was to improve the CO(2) adsorption capacity of the carbon materials by increasing the porosity during the chemical activation process. After chemical activation process, the amorphous carbon and platelet CNFs increased the CO(2) adsorption capacity 1.6 times, whereas fishbone and ribbon CNFs increased their CO(2) adsorption capacity 1.1 and 8.2 times, respectively. This increase of CO(2) adsorption capacity after chemical activation was due to an increase of BET surface area and pore volume in all carbon materials. Finally, the CO(2) adsorption isotherms showed that activated amorphous carbon exhibited the best CO(2) capture capacity with 72.0 wt % of CO(2) at 26 °C and 8 bar.     

Cost structure of a postcombustion CO2 capture system using CaO

This paper presents the basic economics of an emerging concept for CO2 capture from flue gases in power plants. The complete system includes three key cost components: a full combustion power plant, a second power plant working as an oxy-fired fluidized bed calciner, and a fluidized bed carbonator interconnected with the calciner and capturing CO2 from the combustion power plant. The simplicity in the economic analysis is possible because the key cost data for the two major first components are well established in the open literature. It is shown that there is clear scope for a breakthrough in capture cost to around 15 $/t of CO2 avoided with this system. This is mainly because the capture system is generating additional power (from the additional coal fed to the calciner) and because the avoided CO2 comes from the capture of the CO2 generated by the coal fed to the calciner and the CO2 captured (as CaCO3) from the flue gases of the existing power plant, that is also released in the calciner.     

Effects of bonding types and functional groups on CO2 capture using novel multiphase systems of liquid-like nanoparticle organic hybrid materials

Novel liquid-like nanoparticle organic hybrid materials (NOHMs) which possess unique features including negligible vapor pressure and a high degree of tunability were synthesized and their physical and chemical properties as well as CO(2) capture capacities were investigated. NOHMs can be classified based on the synthesis methods involving different bonding types, the existence of linkers, and the addition of task-specific functional groups including amines for CO(2) capture. As a canopy of polymeric chains was grafted onto the nanoparticle cores, the thermal stability of the resulting NOHMs was improved. In order to isolate the entropy effect during CO(2) capture, NOHMs were first prepared using polymers that do not contain functional groups with strong chemical affinity toward CO(2). However, it was found that even ether groups on the polymeric canopy contributed to CO(2) capture in NOHMs via Lewis acid-base interactions, although this effect was insignificant compared to the effect of task-specific functional groups such as amine. In all cases, a higher partial pressure of CO(2) was more favorable for CO(2) capture, while a higher temperature caused an adverse effect. Multicyclic CO(2) capture tests confirmed superior recyclability of NOHMs and NOHMs also showed a higher selectivity toward CO(2) over N(2)O, O(2) and N(2).