CaO Grains Research Articles

Lime reactivity and overburning: the case of limestones belonging to Tuscan Nappe sequence (NW Tuscany, Italy)

This study examines limestone properties and calcination process to enhance product quality. Limestone burning produces lime (CaO, calcium oxide) and carbon dioxide (CO2). Lime is a substance highly reactive and turns into slaked lime (Ca(OH)2, calcium hydroxide) when exposed to water. Six limestone samples from Tuscan Nappe sedimentary sequence, outcropping in the Monti d’Oltre Serchio area (NW Tuscany, Italy), were selected and calcined at different temperatures (800, 900, 1000 and 1100 °C). The obtained lime was slaked, and chemical, mineralogical and petrographic analyses were conducted to study its reactivity during slaking process. Key factors influencing lime reactivity were identified: calcination temperature/time and limestone characteristics (chemical and mineralogical composition). The lime reactivity was measured through the rate of lime hydration reaction. Results showed that higher reactivity in lime, lower calcination temperature. The increase in temperature and time leads to an increase of CaO grain size and, consequently, to a decrease in reactivity. Temperature increase has a more significant effect on the increasing of grain size and reactivity than time. The optimal calcination temperature was found to be 900 °C, like that of ancient limekilns. The study emphasized the close link between lime reactivity and chemistry/mineralogy of limestone. Overall, the research provides insights for improving limestone calcination processes and obtaining superior products.

Infusion of Fly Ash/MgO in CaO-based sorbent for high-temperature CO2 capture: Precursor selection and its effect on uptake kinetics

Calcium Looping Technology for high-temperature CO2 capture suffers from a gradual decline of uptake capacity over repeated cycles caused by the sintering-induced agglomeration of CaO particles. The latter is typically inhibited by infusing inert into the sorbent to disperse CaO grains. In this respect, current work investigates the infusion of inerts like Fly ash (FA) and Magnesium salt in CaO-based sorbent through sol-gel combustion, assessing sorbent performance under harsh conditions: carbonation at 650 °C for 30 min and calcination at 950 °C for 15 min under a CO2 (100%) atmosphere. The study includes (i) in-depth characterization of various sorbents: both unsupported and modified with inert, synthesized using inorganic (InOP) and organometallic (OMP) precursors for different fuel-to-metal oxide molar ratios (MR = 2 and 3), (ii) thermogravimetric analysis for evaluating uptake kinetics based on carbonation conversion (XCBN) and cyclic stability and (iii) kinetic modeling to evaluate rate parameters. Kinetic analysis after a single cycle revealed the highest XCBN (78%) for unsupported InOP (MR = 2 and 3) and OMP (MR = 2) among thermally pre-treated sorbents. MgO-modified InOP (MR = 2) performed better (XCBN = 66%) than its OMP counterpart, while both MgO-modified sorbents (MR = 3) performed similarly (XCBN = 66%). Cyclic stability studies conducted for 30 cycles established higher stability of FA-modified OMP with lower deterioration by 13.47% (XCBN = 22.60%) than unsupported OMP by 62.99% (XCBN = 15.06%). Overall, the results showcase FA infusion in OMP as a viable route to develop stable modified CaO-sorbents with considerable carbonation conversion.

Self-Regenerative Ni-Doped CaTiO3/CaO for Integrated CO2 Capture and Dry Reforming of Methane.

In this work, a new type of multifunctional materials (MFMs) called self-regenerative Ni-doped CaTiO3/CaO is introduced for the integrated CO2 capture and dry reforming of methane (ICCDRM). These materials consist of a catalytically active Ni-doped CaTiO3 and a CO2 sorbent, CaO. The article proposes a concept where the Ni catalyst can be regenerated in situ, which is crucial for ICCDRM. Exsolved Ni nanoparticles are evenly distributed on the surface of CaTiO3 under H2 or CH4, and are re-dispersed back into the CaTiO3 lattice under CO2. The Ni-doped CaTiO3/CaO MFMs show stable CO2 capture capacity and syngas productivity for 30 cycles of ICCDRM. The presence of CaTiO3 between CaO grains prevents CaO/CaCO3 thermal sintering during carbonation and decarbonation. Moreover, the strong interaction of CaTiO3 with exsolved Ni mitigates severe accumulation of coke deposition. This concept can be useful for developing MFMs with improved properties that can advance integrated carbon capture and conversion.

TiO2/MnFe2O4 co-modified alkaline papermaking waste for CaO-CaCO3 thermochemical energy storage

CaO-CaCO3 thermochemical energy storage is a promising technology for solar energy utilization and storage. Alkaline papermaking waste (APW) from paper mills, which is mainly composed of CaCO3. Herein, TiO2/MnFe2O4 co-modified APW was synthesized. The energy storage capacity of TiO2/MnFe2O4 co-modified APW was studied during CaO-CaCO3 thermochemical energy storage cycles in a duel fixed bed reactor. TiO2/MnFe2O4 co-modified APW mainly consists of CaO, MgO, CaTiO3, CaMnO3, and Ca2Fe2O5. The introductions of TiO2 and MnFe2O4 significantly improve the cyclic stability and optical absorption property of APW in 30 cycles. The co-modified APW (the mass ratio for CaO:TiO2:MnFe2O4 = 100:5:5) possesses high sintering resistance, energy storage, and optical absorption capacities in 30 cycles. The energy storage conversion and capacity of the co-modified APW are 0.72 and 2288.57 kJ/kg after 30 cycles, respectively. The optical absorptance of the co-modified APW reaches 49%, which is 9.25 times as high as that of APW. CaTiO3 and CaMnO3 effectively mitigate the sintering of CaO grains. CaMnO3 and Ca2Fe2O5 significantly increase the optical absorptance of APW. The co-modified APW possesses stronger basicity and more oxygen vacancies, which is favourable for the carbonation reaction. Thus, TiO2/MnFe2O4 co-modified APW is a good energy storage material.

Integrated experimental and theoretical studies for unravelling CO2 capture of dual function CeO2-CaO bio-based sorbents

Solid CaO-based sorbents for CO2 capture represent a promising alternative technology compared to traditional amine scrubbing methods. However, CaO-based sorbents are rapidly sinter and easily decrease CO2 uptake capacity with regenerated carbonation–calcination cycle. Thus, in this study, doping CaO with CeO2 make it an excellent candidate for improving stability and facilitating CO2 capture efficiency. The dual function of CaO bio-based sorbents with varying ratios of cerium (Ce) were derived from naturally occurring blood clam using the gel-combustion method. Their nanostructures and physicochemical properties were characterized through the specific methods, including in situ XAS, CO2 chemisorption and physisorption, XPS, XRD, and SEM. An in-depth understanding of CO2 adsorption mechanism was further emphasized using a density functional theory (DFT) simulation. The results demonstrate that nanosized CeO2-CaO, with a mole ratio of 1:9, exhibited the highest performance for CO2 capture (6.44 mmol CO2/g sorbent). It is postulated that Ce serves as a physical barrier, effectively partitioning adjacent CaO grains and thereby diminishing the sintering rate. CaO is identified as the primary adsorption site, in contrast to CeO2. Mixing the Ce-Ca dual oxide initiates the electron-rich O at the O’ site. The electron localization at the O’ active site improves the activity of O’, making the performance CeCa(200) for capturing CO2 capture dominant. This discovery suggests that CeO2-CaO holds promise as a sustainable alternative sorbent for future CO2 capture applications.

Structural reconfiguration of Al/CaO adsorbent by Ni doping to improve sintering resistance and arsenic removal performance

CaO-based adsorbents have poor adsorption performance due to high temperature sintering. In this study, a trace amount of Ni was added to an Al/CaO adsorbent using the Pechini hydrothermal method and anhydrous grinding method. The modified adsorbent was then used to remove As2O3, and both experimental and theoretical approaches were employed. The experimental results showed that the addition of Ni transformed the surface of the adsorbent from a two-dimensional network structure to a three-dimensional cluster structure, significantly increasing its specific surface area and improving its adsorption capacity for As2O3. The adsorption capacity reached 933.1 mg/kg at 900 °C, which was 3.79 times higher than that of pure CaO and 8.2 times higher than that of Al2O3. Additionally, the presence of nano-scale NiO particles prevented the sintering of CaO grains, allowing the adsorbent to maintain its pore structure even after the reaction. Density functional theory (DFT) calculations showed that NiO was the most active site for As2O3 adsorption on the surface of Al/Ni-CaO. Ni acted as an electron transport bridge, enhancing electron transfer on the material's surface and increasing adsorption energy. These findings are important for future research on high-temperature removal of As2O3 with doped CaO-based adsorbents.

A g-C3N4 nanotube assisted and (Ce-Mg) co-doped limestone-derived sorbent: Bidirectional diffusion channels and double vacancy mechanism constructed for CO2 capture

A novel (Ce-Mg) co-promoted CaO sorbent with multi-dimensional through-hole structure was fabricated by a hydrothermal template method based on natural limestone, to address the evident decline of CO2 adsorption performance during circulation. A trend of first increasing and then stabilizing was achieved for CO2 long-term durability of the as-prepared sorbent by the hydrothermal recrystallization process. The CO2 absorption capacity of the (Ce-Mg) co-promoted tubular sorbents exceeded that of the reference sample without hydrothermal template treatment by ∼ 400% after 30 cycles. 97% of the initial CO2 capacity can be maintained at the 30th cycle. This significant CO2 uptake capacity can be attributed to the multiple synergistic interactions yielded by incorporating g-C3N4 nanotube template with (Ce-Mg) co-doping. A bidirectional gas diffusion channel was constructed based on the inner-outer multi-dimensional connection structure of g-C3N4, which allowed the sorbent to maintain the prominent specific surface area and pore volume during cycles. After 30 cycles, the specific surface area of the limestone-derived sorbents can still remain at 16.31 m2/g. Furthermore, a double vacancy mechanism that can facilitate the gas transport through both the product layer and the surface layer of CaO grains caused by the co-doping of (Ce-Mg) was proposed, which can also promote the electron transport between CaO and CO2. This synergistic strategy proved useful for improving the CO2 uptake performance and cycle durability of CaO sorbents in practical applications.

Fabrication of high-strength and anti-hydration water-soluble calcia-based ceramic core modified with nano-ZrO2 via direct ink writing method

An additive manufacturing method, named direct ink writing (DIW), was adopted to fabricate high-strength and anti-hydration water-soluble calcia-based ceramic cores in this paper. The green bodies were prepared using calcium carbonate as precursor material, polyethylene glycol aqueous solution as binder and nano-ZrO2 as modifier, and then calcia-based ceramic cores were obtained by sintering at 1400 °C. The effect of different nano-ZrO2 contents on the properties and microstructure of the calcia-based ceramic cores was investigated. The results indicated that when the nano-ZrO2 content increased, the shrinkage rate of the ceramic cores in the X, Y and Z directions gradually decreased, the bending strength increased significantly and the hydration resistance enhanced, while the water-soluble rate decreased. The incorporation of nano-ZrO2 led to an augmentation in the CaZrO3 phase content within the CaO matrix of the calcia-based ceramic core. The CaZrO3 phase was interconnected and uniformly distributed around the CaO grain boundaries, which was the reason for increasing the bulk density of the ceramic core and increasing the sintering neck, reducing linear shrinkage rate and improving bending strength, while decreasing the water-soluble rate. The high-strength and anti-hydration water-soluble calcia-based ceramic core is achieved by adding 10 wt% nano-ZrO2, with the bending strength of 11.68 MPa, the shrinkage rate of 16.72%, 48h moisture absorption rate of 5.88%, and the water-soluble rate of 4.21 g/(s·m2) in 60 °C water, which is suitable for the rapid casting of the complex internal castings.

Electrospun Zr-Doped CaO Sorbent for CO2 Capture.

A Zr-doped CaO sorbent for high-temperature CO2 capture was fabricated using electrospinning. The nanofiber sorbent with an average filament diameter of about 160 nm is characterized by an initial CO2 uptake capacity of 12.1 mmol/g, a specific surface area of 79 m2/g, an indentation Young's modulus of 520 MPa, and a hardness of 1.6 MPa. After 50 carbonation/decarbonation cycles, the sorbent has a decent CO2 uptake capacity of 9.7 mmol/g due to the uniform distribution of CaZrO3 in the CaO nanofibers to prevent CaO grain growth caused by CaCO3 sintering. It is revealed that the sorbent CO2 uptake capacity decreases both with an increase in the decarbonation temperature and with an increase in the CO2 concentration in the gas flow upon carbonation, where the sorbent CO2 uptake capacity is more sensitive to the decarbonation temperature than to the CO2 concentration in the gaseous stream during carbonation. It is assumed that the electrospun regenerable Zr-doped CaO sorbent is effective for removing CO2 from flue gases.

Understanding the synergistic effect of Ni/Al2O3 on CO2 adsorption and sintering properties of CaO in sorption-enhanced steam reforming process

Sorption-enhanced steam reforming with in-situ CO2 removal by high temperature solid sorbents is an environmentally friendly and sustainable approach for hydrogen production. Ca-Ni-Al bifunctional materials with both catalytic activity and sorption activity have received extensive attention recently. However, the synergistic effect of Ni/Al2O3 on the CO2 adsorption and sintering properties of CaO sorbents have not been studied, and the atomistic mechanisms behind them still remain unclear. Here, we investigated the synergistic effect of Ni/Al2O3 on the carbonation properties and sintering resistance of CaO sorbents by experiments combined with density functional theory (DFT) calculations. Results show that the phase transformation for reduction of NiO and NiAl2O4 to Ni promotes the reaction between CaO and Al2O3, which is beneficial to the formation of the inert framework Ca12Al14O33. After adding 5 % and 15 % Ni, the BET specific surface area decreases from 11.85 to 7.59 and 4.51 m2/g, respectively. The catalytic component Ni is dispersed on the CaO surface, which blocks the pore structure and eventually leads to a decrease in CO2 capture capacity. Under high temperature calcination, Ni aggravates the sintering of CaO, which is not conducive to cyclic adsorption. The Al2O3 support can effectively improve the pore structure and maintain cycle stability of CaO sorbents. DFT calculations suggest that the catalytic component Ni, the Al2O3 support and the Ni-Al2O3 bi-component have little effect on the CO2 adsorption activity. The Ni significantly weakens the adsorption energy of Ca4O4 cluster on CaO surface from −2.58 eV to −1.57 eV, resulting in the growth of CaO grains. The strong interaction between Ca4O4 cluster and Al2O3 surface with the adsorption energy of −7.34 eV makes Ca4O4 cluster migration difficult, thereby inhibiting the agglomeration and sintering of CaO particles. Meanwhile, the addition of Ni component does not significantly reduce the anti-sintering ability of Al2O3 support on CaO sorbents. For Ni-Ca-Al bifunctional materials, the strong interaction between Ni-Ca is not conducive to the carbonation performance of CaO sorbents and the strong interaction between Ni-Al should be appropriately avoided to weaken the efficacy of Al2O3 support, which was confirmed by experiments.

Influences of solid content of slurry on the microstructure of CaO crucible prepared by slip casting

AbstractCaO crucible was prepared from Ca(OH)2 slurry via a slip‐casting method in order to avoid hydration problem caused by free CaO during preparation process. The effect of solid content of Ca(OH)2 slurry on microstructure of CaO crucible was investigated. It was found that the rheological property of the slurry was significantly improved when the solid content of the slurry within a certain range. The change of solid content of slurry affects the microstructure of the final products. When the solid content of the slurry was 71 wt%, the manufactured CaO crucible was of smallest mean pore size (2.58 μm) after 1600°C heating treatment. The heated CaO sample was of lower apparent porosity (4.1%) and higher bulk density (2.93 g/cm3). And the size distributions of most CaO grains are between 10 and 40 μm.

Highly robust ZrO2-stabilized CaO nanoadsorbent prepared via a facile one-pot MWCNT-template method for CO2 capture under realistic calcium looping conditions

This research assessed the textural and structural characterizations and CO2 capture activity of novel and highly thermal-resistance ZrO2-stabilized adsorbents templated with MWCNT, prepared via a facile one-pot preparation approach. Various MWCNT contents, 2.5, 5, and 10 wt%, were incorporated into the CaO adsorbent containing 12.8 wt% ZrO2 species. The conducted structural properties revealed that the CaO grain size, surface area, and pore volume of untemplated ZrO2-supported CaO improved by 33.25%, 185%, and 141% through merging with 10 wt% MWCNT, conformed with FESEM images that showed the highly porous structure. Moreover, the TGA analyses under the severe calcium looping conditions, carbonation under 15 vol% CO2 balanced with N2 at 650 °C for 10 min, and calcination under 100 vol% CO2 at 930 °C for 10 min, demonstrated the incorporation of 10 wt% MWCNT into the ZrO2-stabilized CaO adsorbent increased cyclic durability and the ultimate CO2 capture capacity from 29.5% and 0.03 g CO2/g adsorbent to 61.12% and 0.1 g CO2/g adsorbent, indicating 107% and 233.3% enhancement, respectively. In addition to the significant reduction in CaO grain size and the formation of more high-volume pores, the influence of the MWCNT on calcium zirconate distribution into the CaO structure, mitigating CaO sintering and the agglomeration of CaO grains is another potential reason for the discussed multicyclic and textural improvements. The acquired findings indicated the effectiveness of MWCNT-template preparation method on textural, structural, and multicyclic properties of nano-scale ZrO2-promoted CaO adsorbents.

CaO/CaCO3 thermochemical energy storage performance of high-alumina granule stabilized papermaking soda residue

CaO/CaCO3 thermochemical energy storage has been considered as a promising technology in the concentrated solar power plants. In this work, the high-alumina granule stabilized soda residue, which contains CaO, MgO, Ca12Al14O33, and Ca2SiO4, was manufactured by wet-mixing method, and explored for thermochemical energy storage via CaO/CaCO3 cycles. The effects of the addition amount of high-alumina granule, carbonation pressure, and number of cycles on the energy storage capacity of the stabilized soda residue were investigated. MgO, Ca12Al14O33, and Ca2SiO4 inhibit the growth and agglomeration of CaO grains and enhance the sintering resistance of soda residue; besides, the addition of high-alumina granule increases the basicity of soda residue. Moreover, the increase of carbonation pressure improves the energy storage performance of the stabilized soda residue. Hence, the stabilized soda residue with the mass ratio of high-alumina granule to CaO = 10:100 exhibits the excellent energy storage capacity and cyclic stability in the CaO/CaCO3 cycles. The energy storage density and effective conversion of the stabilized soda residue are approximately 2394 kJ·kg−1 and 75% after 30 cycles at 1.1 MPa. The stabilized soda residue is an efficient material for CaO/CaCO3 energy storage system.

Stability of CaO-based Sorbents under Realistic Calcination Conditions: Effect of Metal Oxide Supports

Solid metal oxides with high Tammann temperatures have been investigated as stabilizers to remedy the sintering of CaO sorbents during the calcium looping process. A methodical comparison between different metal stabilizers must take into account two factors: (1) equal molar ratio of metal stabilizer to active sorbent and (2) homogenous dispersion of the stabilizer in the sorbent. The negligence of one of these two factors has caused a discrepancy among the current work in the literature with respect to the optimum metal stabilizer for CaO sorbents. In this work, seven metal promoters (Mg, Y, Ce, Nd, Zr, La, and Al) were studied to assess their stabilization effect on CaO sorbents used in cyclic carbonation and calcination experiments. Scanning electron microscopy/energy dispersive X-ray analysis elemental mapping was performed on all sorbents to ensure uniform dispersion of stabilizers in the sorbents. Results indicate that among the metal supports tested in this study, Mg-stabilized sorbents are most efficient in maintaining a high uptake capacity through cycles, under both mild and harsh calcination conditions. This is partly attributed to the high Tammann temperature of MgO stabilizer in comparison to other promoters, resulting in efficient separation of CaO grains and inhibition of sintering and agglomeration. Mg-stabilized sorbents (Mg/Ca = 1:14) showed a first-cycle uptake capacity of 0.68 and 0.63 g/g under mild and harsh calcination conditions, respectively. Sorbents with higher stabilizer ratio (as high as Mg/Ca = 1:2) were tested in 100 cycles and showed a residual uptake capacity of ∼0.15 g/g. In contrast to previously reported results in the literature, the stabilization effect of Mg promoter was observed even at Mg/Ca molar ratios as low as 1:14.

Exsolution-like synthesis of Ni/(Nd2O3,CaO) nanocomposites from Nd2-xCaxNiO4 precursors for catalytic applications

A series of K2NiF4-like complex oxides Nd2-xCaxNiO4 (x ​= ​0–0.4) was obtained by the freeze drying synthesis method. Thermal decomposition of these precursors in H2 at 900 ​°C resulted in the formation of Ni/(Nd2O3,CaO) composites with different Nd2O3/CaO ratios. Their microstructure was similar to the products of the redox exsolution of perovskites: spherical 30 ​nm particles of Ni metal were allocated at the surface of dense agglomerates of Nd2O3 and CaO grains. The as-obtained composites demonstrated significant catalytic activity, selectivity, and stability in the reactions of dry reforming and partial oxidation of methane (DRM and POM, respectively) at T ​≥ ​800 ​°C.

Na2CO3 promoted CaO-based heat carrier for thermochemical energy storage in concentrated solar power plants

Calcium looping (CaL) is a promising thermochemical energy storage (TCES) technology to convert solar energy to power in CO2 Brayton cycle. However, the energy storage density (ESD) of the CaO-based heat carries decays drastically over the CaL cycles, and the energy storage performance of the CaO-based materials in a close-loop CaL-TCES system is still unclear. In this work, a novel Na2CO3 modified method was proposed to enhance the ESD of the CaO-based materials over the close-loop CaL-TCES process, and the enhancing mechanism was investigated by the simultaneous thermal analyzer, X-ray diffraction, electron microscopy, N2 physisorption, X-ray photoelectron spectroscopy. The “10Na2CO3/CaO” showed more promising ESD over the cycles, even though it suffered sintering. After 60 cycles, the “10Na2CO3/CaO” still held an ESD of 594 kJ/kg, which was 2.27 times that of “Neat CaO” when cycled at “850 °C–900 °C” mode. Increase the carbonation temperature from 650 °C to 850 °C, the cyclic energy storage densities of the “10Na2CO3/CaO” ascended rapidly, which was opposite to the “Neat CaO”. The optimum operating condition for the “10Na2CO3/CaO” is “850 °C–900 °C” when operated in the close-loop CaL-TCES system. The Na+ species that enriched on the surface of the CaO grains were in amorphous or melting state, and they would react with CaO to form Na-Ca solid solution over the cycles. The “10Na2CO3/CaO” has a tendency of sintering over the cycles, but Na2CO3 improves reaction kinetics. One suggested mechanism is that the Na+ species may break through the mass transfer barriers to absorb CO2 during the carbonation process. Therefore, although the Na2CO3 doped CaO materials had poorer surface topography, they held much higher ESD over the repeated cycles.

Preparation of CeO2/CaO with Anti-sintering for Efficient Capture of As2O3 from Flue Gas at a High Temperature

Arsenic emissions from coal-fired power plants are highly toxic and volatile, posing a serious threat to human health and the natural environment. The removal of arsenic from coal-fired flue gases has attracted widespread attention worldwide. Calcium-based sorbents can capture arsenic, thus solving the environmental problem of heavy metal arsenic pollution from coal-fired power plants, but its sintering and low capture efficiency impede its application. Doping CaO with CeO2 is an efficient technique to improve resistance to sintering and facilitate capture efficiency. In this study, the CeO2/CaO sorbent was synthesized by the sol–gel method and then carried out in the flue gas at high temperatures from 600 to 900 °C to capture gaseous arsenic. The introduction of CeO2 not only reduces the grain size of CaO, but uniformly dispersed CeO2 forms a barrier layer that effectively prevents the growth and sintering of CaO grains. As a result, the sintering resistance of the sorbent is enhanced, and the sorbent exhibits excellent thermal stability. On the basis of characterization analysis, lattice oxygen provided by cerium oxide greatly promotes the reaction process of As(III) oxidation to As(V). The experimental results demonstrate that CeO2/CaO possesses the highest arsenic capture capacity compared to untreated CaO and CeO2. With the increase of the temperature, the capture efficiency of CeO2/CaO increases and reaches a maximum of 1.90 mg/g at 900 °C. Additionally, the density functional theory (DFT) was further used to study the adsorption energy of untreated CaO and CeO2/CaO. The results exhibited that CeO2/CaO has stronger adsorption capacity for As2O3. Finally, the adsorption mechanism of As2O3 on CeO2/CaO was proposed. As a result of the excellent anti-sintering property and high capture capacity, CeO2/CaO is expected to be an excellent sorbent for arsenic removal from coal combustion flue gas.

CO2 capture activity of a novel CaO adsorbent stabilized with (ZrO2+Al2O3+CeO2)-based additive under mild and realistic calcium looping conditions

The present investigation concentrated on the configuration of a novel synthetic adsorbent via the sol-gel auto-combustion method through incorporating ZrO2, Al2O3, and CeO2 simultaneously during the development of CaO adsorbent. The impacts of disparate calcium precursors and molar ratios of calcium precursors to promoters were also studied. The adsorbent developed from soluble calcium nitrate precursor showed the higher surface area and smaller CaO grain size accompanied by a more enhanced capture capacity and CaO conversion at the mild and realistic process conditions during the carbonation-calcination cycles. Among synthesized adsorbents with disparate the molar ratio of Ca/(Ze + Al + Ce) (calcium precursor: promoters), the prepared sample with the molar ratio of 20/1 (CN/20) indicated the most promising performance with the best morphological properties, higher sorption capacity, and CaO conversion in comparison with 15/1 (CN/15) and 30/1 (CN/30) during 18 carbonation-calcination multiple cycles under the realistic condition. The most highlighted result of this work was that cyclic performance of modified adsorbents is linearly proportional to their morphological properties, in that CN/20 with the largest free surface area of 31.3 m2/g captured 4.23 g CO2/gr adsorbent, 9.58 % and 11.6 % higher than that captured by CN/15 and CN/30, respectively, during 18 realistic carbonation/calcination cycles.

NO removal performance of CO in carbonation stage of calcium looping for CO2 capture

Calcium looping realizes CO 2 capture via the cyclic calcination/carbonation of CaO. The combustion of fuel supplies energy for the calciner. It is unavoidable that some unburned char in the calciner flows into the carbonator, generating CO due to the hypoxic atmosphere in the carbonator. CO can reduce NO in the flue gases from coal-fired power plants. In this work, NO removal performance of CO in the carbonation stage of calcium looping for CO 2 capture was investigated in a bubbling fluidized bed reactor. The effects of carbonation temperature, CO concentration, CO 2 capture, type of CaO, number of CO 2 capture cycles and presence of char on NO removal by CO in carbonation stage of calcium looping were discussed. CaO possesses an efficient catalytic effect on NO removal by CO. High temperature and high CO concentration lead to high NO removal efficiency of CO in the presence of CaO. Taking account of better NO removal and CO 2 capture, the optimal carbonation temperature is 650 °C. The carbonation of CaO reduces the catalytic activity of CaO for NO removal by CO due to the formation of CaCO 3 . Besides, the catalytic performance of CaO on NO removal by CO gradually decreases with the number of CO 2 capture cycles. This is because the sintering of CaO leads to the fusion of CaO grains and blockage of pores in CaO, hindering the diffusion of NO and CO. The high CaO content and porous structure of calcium-based sorbents are beneficial for NO removal by CO. The presence of char promotes NO removal by CO in the carbonator. CO 2 /NO removal efficiencies can reach above 90%. The efficient simultaneous NO and CO 2 removal by CO and CaO in the carbonation step of the calcium looping seems promising.

Improved hydration resistance of CaO granules via sol‐processed metal oxide protective layers

AbstractWell‐sintered CaO granules with hydration resistance are very important for the manufacturing and application of CaO‐containing refractories. CaO granules with sol‐processed metal oxide protective layers were prepared by granulation and surface treatment. The obtained results indicated that the usage of nano sol greatly improved the hydration resistance of CaO granules, especially when the CaO granules were treated by alumina sol. The promoting effect of nano sols in the sintering processes improved the density of CaO granules. Nanoparticles reacted with lime at lower temperature and a new phase was formed uniformly in the treated layer, which promoted densification of the treated layer. The grain size of CaO granules in the treated layer was increased obviously with the promotion of sintering. Moreover, nanoparticles were favorable for the formation of direct bonding between CaO grains by decreasing the dihedral angle.