CaO Particles Research Articles

Sintering mechanism of calcium oxide/calcium carbonate during thermochemical heat storage process

Thermochemical heat storage based on CaO/CaCO3 cycles suffers from an obvious decline in heat storage capacity attributed to the sintering of particles. Reactive molecular dynamic simulations are carried out to investigate the sintering mechanism of CaO and CaCO3 particles. Lower mass center distance of the CaO particles, larger degree of lattice disordering and higher diffusivities illustrate that the carbonation stage plays a dominating role in the sintering process. The calculated diffusion activation energy decreases to 49.9% and pre-exponential factor increases to 2.98 times of those in the absence of CO2, resulting in the accelerated agglomeration of particles. The activation energy is the dominating factor to enhance the diffusion at lower CO2 concentrations, whereas the pre-exponential factor dominates at higher CO2 concentrations. The lower CO2 concentrations raise the importance of CO2 molecules in bond breakage and formation. On the other hand, CaO particles have more opportunities to react with CO2 molecules at higher CO2 concentrations. High surface energy is the driving force towards sintering, and therefore, the decline of the surface potential energy and specific surface area through sintering are thermodynamically favored. The CO2 absorptivity of sintered CaO particles declines due to the reduced specific surface area. Finally, thermochemical heat storage experiments and microstructure characterizations show the increased degree of lattice disordering, which verifies the molecular dynamics simulation results before. The calculated quantitative descriptors and sintering mechanism will provide guidance for material modification, especially by increasing the atomic diffusion resistance and improving the initial skeleton structure of CaO materials.

Effects of iron/silicon/magnesium/aluminum on CaO carbonation of CO2 in steel slag-based building materials during carbonation curing

Carbonation curing can enhance the mechanical properties of steel slag-based building materials and consume large amounts of CO2, making it a widely process. Due to the complexity of steel slag, in addition to CaO, other concomitant metallic/nonmetallic elements, such as Fe, Si, Mg and Al also have a certain influence on carbonation reaction. In this study, Fe2O3, Fe3O4, SiO2, MgO and Al2O3 was added to CaO to form a binary system according to the chemical composition of real steel slag. The influences of these oxides on the carbonation rate and CaO carbonation efficiency were investigated, respectively. The results showed that compared with pure CaO, the rates of carbonation reaction of CaO + Fe2O3, CaO + Fe3O4, CaO + SiO2, CaO + MgO and CaO + Al2O3 were increased by 5.5%, 13.2%, 13.2%, 6.9% and 8.5%, respectively. The carbonation efficiency of CaO in CaO + Fe2O3, CaO + Fe3O4, CaO + MgO, CaO + Al2O3 increased by 3.6%, 4.3%, 10.1%, and 1.2%, respectively. Synergistic effect was recognized between CaO and Fe2O3, Fe3O4, MgO and Al2O3, respectively. The carbonation efficiency of CaO in CaO + SiO2 decreased by 2.4%. One reason should be the carbonation products, which exhibited ectopic dispersion and covered the surfaces of Fe2O3, Fe3O4, MgO and Al2O3, which effectively helped to avoid in-situ aggregation and thus covered the surfaces of the CaO particles. This improved the carbonation reaction rate and efficiency. Another reason could be the alkaline oxides, such as Fe2O3, Fe3O4 and MgO, and amphoteric oxides, such as Al2O3, stimulated the carbonation reaction of CaO and CO2, while acid oxides, such as SiO2, inhibited the carbonation reaction.

Mn-promoted CaO-based adsorbents with enhanced CO2 uptake performance

The adsorption of CO2 over CaO-based adsorbents has been an effective way to capture CO2 at high temperature, but the adsorption capacity was hampered and the cycle durability was poor. To address the said problems, this paper reported the effect of Mn doping on the structure, morphology and CO2 adsorption performances of CaO-based adsorbents synthesized by a sol-gel method. The characterization results showed that the addition of Mn was helpful to form abundant micropores and mesopores in the doped CaO-based adsorbent, and Mn can effectively prevent the sintering and agglomeration of CaO particles. Moreover, electron transfer occurred on the modified CaO-based absorbents. These findings suggested that Mn species induced both structural and electronic promotion effect, resulting in smaller particle, higher dispersion and specific surface area of CaO-based adsorbents and formation of some electron-rich oxygen and Ca species, ultimately boosting CO2 adsorption performance. Especially, with the molar ratio of Ca/Mn being 20, the adsorbent Ca/Mn-20 presented the best CO2 uptake and excellent cycle stability (up to 10 cycles). In addition, it can be concluded that the adsorption process of CO2 was more obedient to the pseudo-second-order kinetic model, indicating the adsorption process was dominated by chemical adsorption.

Effects of ambient temperature and powder gas ratio on jet characteristics of O2 + CO2 and CaO particles injected by a swirl-type oxygen lance nozzle

The dephosphorization of converters is greatly influenced by the amount of lime added and method used. At present, the oxygen jet mixing of a certain proportion of CO2 to participate in converter smelting has been successfully applied; however, there is little research on the jet characteristics and the relationship between gas and solid particles in the process of oxygen jet mixing injection with a swirl oxygen lance.In this study, the effects of ambient temperature and powder gas ratio on the jet characteristics and particle distribution of four hole swirl oxygen lance nozzle were simulated.The results show that with an increase in the ambient temperature, the velocities of the carrier gas jet and CaO particles increase, and the distribution of CaO particles is more concentrated.

Evolution of physicochemical properties of quick lime at converter-smelting temperature

Abstract The volume stability caused by the hydration of f-CaO is one of the main obstacles to the comprehensive utilization of steel-making slag. In view of the f-CaO produced by incomplete dissolution of lime, it is necessary to strengthen the dissolution behavior of lime in the converter process. The reactivity of lime determines the dissolution efficiency and is closely related to its microstructure. The experimental results show that the reactivity and porosity of quick lime decrease and the average diameter of pore increases with an increase in temperature. The CaO crystals gradually grow up under the action of grain boundary migration. When the temperature increased from 1,350 to 1,600°C, the lime reactivity decreased from 237.60 to 40.60 mL, the porosity decreased from 30.55 to 15.91%, the average pore diameter increased from 159.10 to 1471.80 nm, and the average CaO particle size increased from 0.33 to 9.61 µm. The results indicate that reactivity is decreased because of the deformation and growth of CaO crystals and the decrease in porosity in reactive lime. This will cause an obstacle to the dissolution of lime and is not conducive to the control of f-CaO in slag.

Hydration reactivity enhancement of calcium oxide–based media for thermochemical energy storage

AbstractThe calcium oxide/water/calcium hydroxide system is a promising material system for thermochemical energy storage (TCES). Its high reactivity under various experimental conditions during the cyclic operation of heat storage‐and‐release is a key challenge for practical applications. In this study, we developed a new type of highly durable TCES medium with hydration reactivity enhancement. The composite materials were prepared by heat treatment of a mixture of calcium carbonate and additives, consisting of tetraethoxysilane (TEOS) and a silane coupling agent, bis(3‐triehoxysilylpropyl) pertetrasulfide (SCA). The kinetic performances of the composites and CaO were compared. It was observed that the additives promoted hydration reactivity at 450°C and 70 kPa, and 0.6 wt% composite exhibited the highest thermal output. Furthermore, it was found that the enhancement in hydration reactivity was affected by the experimental conditions as significant enhancements were observed when the experimental conditions were closer to equilibrium, while almost no enhancement was observed when the conditions were far from equilibrium or at a high vapor partial pressure. Sample characterization via X‐ray diffractometry and scanning electron microscopy revealed that the agglomeration of CaO particle grains was alleviated by pinning effects, and the component that enhanced the hydration reactivity was identified as Ca2SiO4 nanoparticles.

Reverse-phase extraction and separation of minerals in cement clinker and the sulfur fixation mechanism for CaO, Al2O3, Fe2O3 and CaSO4 particles

Reverse-phase extraction and separation of minerals in cement clinker and the sulfur fixation mechanism for CaO, Al2O3, Fe2O3 and CaSO4 particles

Transesterification Using Ultrasonic Spray of Triolein Containing CaO Particles into Methanol Vapor in a 3-Phase Reactor

Ultrasonic spraying was used in a three-phase reactor to produce small droplets of triolein mixed with CaO as a solid catalyst at temperatures above the boiling point of methanol for enhancement of the transesterification of triolein. Droplets fell in the methanol countercurrent flow and were collected at the bottom of the reactor, followed by circulation to the ultrasonic spray system. The experimental parameters included triolein flow rates of 2.5–9.0 mL/min, reaction temperatures of 70–100 °C, and catalyst contents of 1.0–7.0 wt%. The methanol feed rate was set to be constant. The results suggested that the enhancement was successful after using the three-phase reactor by generating a high contact surface area for the droplets, which was a key factor for determining the performance. Comparing the results with conventional transesterification in the liquid phase using the same CaO at 60 °C, the three-phase reactor produced a methyl ester yield 2–5% higher during the 60 min trial period. However, the yield became lower after 60 min because the mass transfer of methanol to the droplets was limited. The transesterification kinetics were estimated based on the experimental data—assuming a first-order reaction—and the results indicated a range of the rate constant, an apparent activation energy, and a pre-exponential factor of 1.21–3.70 × 10−2 min−1, 36.1 kJ mol−1, and 64.9 min−1, respectively, suggesting that the three-phase reactor was effective for fast transesterification at the initial stage.

Viscosity of Slag Suspensions with a Polar Liquid Matrix

Under the operating temperatures employed in steelmaking, most slags and fluxes often contain solids, such as undissolved CaO and its reaction products; thus, they are more viscous than their fully liquid states. However, few studies have considered the dielectric interactions of solid particles with the liquid matrix in such systems. In the present study, the viscosity of suspensions of dispersed particles consisting of polyethylene beads in a matrix of silicone oil or aqueous glycerol at room temperature was measured. Then, empirical models for estimating the viscosity based on the Einstein–Roscoe equation were proposed. Furthermore, the viscosity of suspensions of CaO and MgO particles dispersed in a matrix of CaO–Al2O3–SiO2–MgO slag at 1773 K was measured, and the feasibility of the proposed viscosity equations was investigated. As expected, the viscosities of the suspensions of polyethylene beads dispersed in silicone oil and glycerol increased with an increasing bead volume fraction. Under comparable measurement conditions, the viscosities of the glycerol suspensions were higher than those of the silicone oil suspensions. The proposed viscosity models based on the Einstein–Roscoe equation and the capillary number reproduced the viscosity of the silicone oil suspensions but underestimated that of the glycerol suspensions. The trend of increasing viscosity of the molten slag suspensions with dispersed CaO and MgO particles was similar to that of the room-temperature suspensions, exhibiting Bingham non-Newtonian behavior. The viscosity model composed with the results from the glycerol aqueous suspensions underestimated the slag viscosity, which can be attributed to the repulsive forces in the high-polarity liquid matrix.

Calcium catalytic behavior during in-situ gasification of different aged coal chars in CO2 and steam

Exchangeable calcium (Ca) in coals has significant catalysis during gasification. In previous work, coals were usually pyrolyzed for different time periods before char gasification. The extent of char aging during pyrolysis may change the state of Ca, and affect its catalytic behavior during gasification. This speculation is investigated by characterizing different aged chars and their in-situ gasification in a thermogravimetric reactor. Calcium-exchanged coal samples were first isothermally pyrolyzed at different pyrolysis time and then in-situ gasified in CO2 or steam at different temperatures. The pyrolyzed and partially gasified chars were collected and analyzed. The results show that char aging significantly changes the state and dispersion of Ca. While the nascent chars (product of 1-min pyrolysis) show no visible Ca-containing particles on the surfaces, the aged chars (product of 20-min pyrolysis) show evident CaO particles ranging from a few to tens of nanometers. During gasification, the extent of char aging does not change the catalytic action of Ca in either CO2 (surface recession mode) or steam (channeling mode). Increasing temperature favors the gasification reactivity of both nascent and aged chars in CO2, and that of aged chars in steam. However, the gasification reactivity of the nascent chars in steam does not seem to be considerably affected by increasing temperature. The increase of catalytic activity may be mostly offset by the decrease of Ca dispersion.

Bifunctional Ni-Ca based material for integrated CO2 capture and conversion via calcium-looping dry reforming

Effective integration of CO2 capture and its conversion is an attractive strategy to reduce the anthropogenic CO2 emissions meanwhile achieve the potential revenue of CO2 molecule. Calcium-looping dry reforming of methane (CaLDRM) has emerged as such a promising process, implemented over a bifunctional reactor combining a CaO-based sorbent and a Ni-based catalyst, to achieve CO2 capture and in-situ conversion with CH4 into syngas. Herein, we synthesize a bifunctional Ni-Ca based material, i.e. Ni and CeO2 nanoparticles co-loaded on ZrO2-coated CaCO3, which enables isothermal capture and release of CO2 at the temperature favorable for DRM reaction, allowing to operate CaLDRM process in a single reactor by simple gas switching. Thanks to the stabilization effect of the ZrO2 layer, both CaO and Ni particles are exempted from severe sintering, maintaining the activity of capture and catalysis. The addition of CeO2 contributes not only to combat the accumulation of inactive carbon during long-term DRM but also to activate CO2 and CH4, accordingly enhancing syngas production, during cyclic CaLDRM. Importantly, the bifunctional material can succesfully drive CaLDRM cycles, converting over 40 % of CH4 and CO2, under 5 vol% CO2 feed concentration (as low as real flue gas) at 720 °C, which is a thermodynamically extremely unfavorable condition (equilibrium conversion below 3 %).

Calcium particle films promote a photoprotection on sweet potato crops and increase its productivity

Environmental conditions, such as high air temperature, can limit sweet potato cultivation in tropical areas. The calcium particle film application has been used to minimize the effects of adverse climate conditions on plants, with the potential to increase crop yield. This study aimed to evaluate calcium oxide particle films (CaO, particle size ≤ 2 µm, reflectance > 90%) on photoprotection and productivity of sweet potato crops in the Brazilian Northeast region. Three concentrations of CaO (5, 10, and 15%) and water (control) were applied. CaO films were daily monitored through a colorimeter and reapplied whenever the brightness was reduced to maintain their reflectance status. In the pre-harvest period, the chlorophyll content (a and b), leaf temperature, and chlorophyll a fluorescence was measured. At 10 and 15%, CaO films seem to promote an effect similar to an artificial shading on the sweet potato crop, increasing the content of chlorophylls a, b, and total, reducing the chlorophyll a/b ratio and leaf temperature. Both concentrations also promoted an efficient control of the energy absorption and electron transport flux into photosystems and electron transport chains. As a result of CaO film application, the productivity of sweet potato increased by 2%, 62%, and 43% to CaO 5, 10, and 15%. Therefore, CaO particle films at 10 or 15% seems to promote some photoprotection on sweet potato crops, leading to a yield increase. Thus, this tool may improve sweet potato cultivation in regions with adverse weather conditions.

Facile reactivation of used CaO-based CO2 sorbent via physical treatment: Critical relationship between particle size and CO2 sorption performance

Herein we report a new and facile mechanical approach for maximizing the CO2 sorption performance of CaO sorbents, and also for recovering the sorption performance of the used (or deactivated) sorbent. We compared the physicochemical properties of CaO-based sorbents, which could change depending on their preparation methods, and determined that the CaO particle size was the most critical factor that affected their CO2 sorption performance. The CO2 sorption performance of CaO-based sorbents could be maximized via the simple physical reduction of their particle size, and this was experimentally demonstrated using ball-milling. The CO2 sorption uptake of CaO prepared using the conventional solid-state method was significantly increased to the highest ever reported level after ball-milling, and it reached 98.0% of the theoretical maximum CO2 sorption capacity. The most important application of the particle-size-dependency of CaO-based sorbents on their CO2 sorption uptake is the reactivation of used sorbents via reducing the size of the aggregated or sintered bulky particles that form after cyclic usage. The CO2 sorption uptake of the used CaO (~32.9 wt% after 10 alternative sorption–regeneration cycles) was successfully recovered (almost doubled) to ~64.7 wt% after ball-milling. This is the first time a facile mechanical-grinding-based reactivation method, which appeared to be highly efficient and directly applicable to the continuous calcium looping technology, has been developed and used.

Calcium-Modified Ni-SDC Anodes in Solid Oxide Fuel Cells for Direct Dry Reforming of Methane

Direct internal reforming is one of the promising methods of utilizing hydrocarbon fuels in solid oxide fuel cells (SOFCs). Among direct internal reforming technologies, dry reforming of methane (DRM) is attracting attention as an alternative to steam reforming of methane. In this study, a cermet material composed of nickel and samarium-doped ceria (Ni-SDC) was investigated as an SOFC anode for the direct internal DRM operation. Compared to a conventional nickel-yttria-stabilized zirconia (Ni-YSZ) anode, the Ni-SDC anode showed superior current-voltage characteristics. However, poor carbon balances were recorded in anode outlet gas analysis, indicating coke formation on the Ni-SDC anode. The addition of calcium to the Ni-SDC anode was also tested. Measured anode outlet gas compositions suggested that the calcium addition suppressed coke formation, while the power generation performance of the pristine Ni-SDC anode was maintained. Microscopic observations showed that a certain part of the calcium additive existed as CaO particles on the anode surface, which could be responsible for the improved coking resistance.

Simulation and analysis of O2–CaO jet behavior with different shrouding fuel mediums in electric arc furnace steelmaking

The O2–CaO jet with shrouding combustion flame demonstrates some advantages in quick melting and effective slag foaming by delivering lime powder deeply into the molten bath. Combining the discrete particle model and the eddy dissipation concept model with the detailed chemical kinetic mechanisms (GRI-Mech 3.0), computational fluid dynamics models of the O2–CaO jet with different shrouding fuel mediums injection were developed. Firstly, the results of the numerical simulation were validated by the measured data of cold test for O2–CaO injection. Then, the interaction between the particles and the gas jet of the O2–CaO jet and the effect of shrouding fuel medium species on the fluid flow characteristics of the O2–CaO jet were analyzed. Results show that the CaO particles can be effectively heated by the shrouding high-temperature combustion flame and compared with the O2–CaO jet without shrouding gas, the potential core length was increased about 2.5, 3.3 and 4.3 times by the shrouding flame generated from CO, CH4 and C3H8. And with shrouding CH4 and C3H8 injection, the CaO particles can be clustered together in a long distance, which would be helpful to improve the utilization efficiency of CaO particles.

Atomistic mechanisms study of the carbonation reaction of CaO for high-temperature CO2 capture

Ca-Looping is a promising technology for capturing CO2. Here, ReaxFF molecular dynamics simulations combined with thermogravimetric analysis (TGA) were employed to provide atomic-level insights into the carbonation reaction of CaO with CO2. Results showed that two fast kinetic stages appear when the temperature increases. To understand this phenomenon, the diffusion, structural evolution as well as the carbonates accumulation were analyzed. It was found that the recovery of the initial fast reaction rate largely stems from the formations of pyrocarbonate anion. On CO2-CaCO3 interface, the solid product molecules are continuously propagated and joined in the existing C2O52− phase with multiple instances of formation and dissociation. Furthermore, surface carbonates can combine with crystal oxygen in the form of metastable corner-linked CO44− tetrahedron to diffuse inward the CaO particle. The increase of temperature greatly accelerates the formation of outer polymeric phases of CO2-CO32− and diffusion of inner CO32−, causing more CO2 molecules to be adsorbed and the carbonation reaction rate rises sharply.

K2O Doped Dolomite as Heterogeneous Catalyst for Fatty Acid Methyl Ester Production from Palm Oil

Biodiesel obtained from palm oil over an environmentally friendly catalyst is highlydesirable. For that matter, dolomite, a natural material was used as a catalyst in this work, and this included potassium oxide (K2O)-doped dolomite, 5 wt% K/D, 10 wt% K/D, 15 wt% K/D, and 20 wt% K/D. X-ray diffraction analysis of dolomite revealed the CaO and MgO phases with high crystallinity, in which intensity reduced after doped with varying concentrations of K2O. When the catalysts were evaluated, the K2O-doped dolomite exhibited a better catalytic activity for palm oil transesterification. In the presence of K2O, the methyl ester reached 98.7%, with the highest being displayed by 15 wt% K/D as compared to 87% over dolomite at reaction temperature of 60 °C, 12:1 methanol to palm oil ratio, 1 wt% catalyst amount and 1 h reaction time. SEM revealed that as more K2O was doped on dolomite, the particles became more agglomerated, with a reduced BET surface area of 1.3 m2/g in 20 wt% K/D as opposed to homogeneously small-sized MgO and CaO particles in dolomite with a high BET surface area of 19.0 m2/g. However, the high activity of the doped catalyst was dictated by the high amount of basic site, as evidenced in TPD-CO2 which showed an increase in the capacity of the basic site with an increased amount of K2O. The catalyst was also reusable up to six times with a negligible decrease in activity due to K+ leaching.

Na,Ca-based catalysts supported on activated carbon for synthesis of biodiesel from soybean oil

Different catalysts containing CaO particles dispersed in commercial activated carbon were prepared by wet impregnation followed by heat treatments. The catalytic efficiency of the prepared materials was investigated by evaluating their performance in biodiesel production by the transesterification reaction of soybean oil with methanol. The catalysts were prepared by three different routes; the first two routes involved the use of co-precipitation, with Ca(NO3)2·4H2O as the source of calcium and either NaOH (route 1) or NH4OH (route 2) as the precipitating agent, which were mixed with the activated carbon in aqueous suspension; in the third method (route 3), NaOH and CaO were directly mixed with the activated carbon in aqueous suspension. In all cases, the as-prepared materials were heat-treated at 800 °C for 3 h under argon flow. The best catalytic efficiency was achieved for the Na,Ca-based catalyst prepared by route 3, which was mainly composed of a mixture of CaO and Na2CO3 crystals dispersed in the porous carbon network. Catalytic recycling tests were also performed to evaluate the possibility of catalyst reuse, which revealed a drop in efficiency after 3 cycles; however the reused catalysts still showed significant activity, attributed to the permanence of Na+ ions within the porous carbon structure. Analysis of the Ca and Na contents in biodiesel samples synthesized in the three reaction cycles revealed low concentrations of metals; in the case of Ca, the concentration leached into the biodiesel after the 3rd cycle was 9 times lower than that of a biodiesel sample synthesized using pure CaO. The results obtained showed that the chosen methodology is a promising route for the development of heterogeneous carbon-supported Na,Ca-based catalysts with considerable surface area and good catalytic efficiency in the transesterification reaction for biodiesel production.

Simultaneous NO/SO2 removal by coconut shell char/CaO from calcium looping in a fluidized bed reactor

A simultaneous NOx/SO2 removal system using bio-char and CaO combined with calcium looping process for CO2 capture was proposed. The simultaneous NO/SO2 removal performance of coconut shell char/CaO experienced CO2 capture cycles was investigated in a fluidized bed reactor. The effects of reaction temperature, mass ratio of CaO to coconut shell coke, CaO particle size and number of CO2 capture cycles from calcium looping process were discussed. The NO removal efficiency of char is improved under the catalysis of CaO. The reaction temperature plays an important role in the simultaneous NO/SO2 removal. Coconut shell char/CaO achieve the highest NO and SO2 removal efficiencies at 825 oC, which are 98% and 100%, respectively. The mass ratio of CaO to coconut shell char of 60: 100 is a good choice for the simultaneous NO/SO2 removal. Smaller CaO particle size contributes to higher NO and SO2 removal efficiencies of coconut shell char/CaO. The NO and SO2 removal efficiencies of coconut shell char and cycled CaO from calcium looping declined slightly with the number of CO2 capture cycles. In addition, the Ca-based materials balance in process of simultaneous NOx/SO2 removal combined with calcium looping is given. The novel simultaneous NO/SO2 removal method using bio-char and cycled CaO from calcium looping process appears promising.

Multiscale model for steam enhancement effect on the carbonation of CaO particle

A multiscale model including scales at the surface, grain, and particle was developed to understand the steam enhancement mechanism on the carbonation of CaO particle. The interaction of H2O with CaO occurring at the surface scale exhibited an enhancement effect on the product layer diffusion at the grain scale, while the grain growth due to the product layer diffusion at the grain scale exhibited an important effect on the pore structure and intraparticle gas diffusion at the particle scale. The developed multiscale model was validated with the experimental data, and the effect of steam addition on carbonation performance was discussed. The effect of steam on CaO carbonation at higher temperatures (600 ~ 700 °C) was much less significant than that at lower temperatures (400 ~ 500 °C), due to the pore plugging phenomenon. In addition, the controlling step of the carbonation in the presence of steam was analyzed. Calculated results show that the external gas diffusion resistance is small enough during the reaction process. The controlling step of the carbonation at 400 °C is the chemical reaction step, while the controlling step of the carbonation at 700 °C is the intraparticle gas diffusion step.