Coal fly ashes (COFA) are readily available and reactive materials suitable for COsub.2 sequestration due to their substantial alkali components. Therefore, the onsite collaborative technology of ...COFA disposal and COsub.2 sequestration in coal-fired power plants appears to have potential. This work provides an overview of the state-of-the-art research studies in the literature on COsub.2 sequestration via the mineralization of COFA. The various COsub.2 sequestration routes of COFA are summarized, mainly including direct and indirect wet carbonation, the synthesis of porous COsub.2 adsorbents derived from COFA, and the development of COFA-derived inert supports for gas-solid adsorbents. The direct and indirect wet carbonation of COFA is the most concerned research technology route, which can obtain valued Ca-based by-products while achieving COsub.2 sequestration. Moreover, the Al and Si components rich in fly ash can be adapted to produce zeolite, hierarchical porous nano-silica, and nano-silicon/aluminum aerogels for producing highly efficient COsub.2 adsorbents. The prospects of COsub.2 sequestration technologies using COFA are also discussed. The objective of this work is to help researchers from academia and industry keep abreast of the latest progress in the study of COsub.2 sequestration by COFA.
The effects of CaO content and post-heat treatment were investigated on the phase stability and mechanical and thermal properties of Ca-PSZ. ZrOsub.2 specimens with 5–10 mol% CaO were sintered, and ...post-heat treatment was performed at 1200 °C for 100 h. Subsequently, to test and analyze the crystal structure and the microstructure, the mechanical and thermal properties of the specimens were evaluated. All specimens were partially stabilized by 5–10 mol% CaO (5CSZ–10CSZ) in a mixed monoclinic and tetragonal phase; however, peaks of the secondary phase of CaZrOsub.3 were observed in 10CSZ. The ratio of the monoclinic phase decreased from 62.50% (5CSZ) to 21.02% (10CSZ) as the CaO content increased. Additionally, the monoclinic phase ratio decreased from 59.38% (5CSZ) to 19.57% (9CSZ) after the post-heat treatment; an increase to 24.84% was observed for 10CSZ. An increase in Vickers hardness from 676.02 to 1256.25 HV and flexural strength from 437.7 to 842.7 MPa was observed with increasing CaO content. The post-heat treatment resulted in further increases in these values as the CaO content increased from 5CSZ to 9CSZ; however, the Vickers hardness and flexural strength of 10CSZ decreased by approximately 8% and 9%, respectively. The thermal expansion coefficient exhibited the same tendency as the mechanical properties. This coefficient increased from 8.229 × 10sup.−6 to 9.448 × 10sup.−6 Ksup.−1 with increasing CaO content and was enhanced after the post-heat treatment in 5CSZ to 9CSZ; however, the thermal expansion coefficient of 10CSZ decreased by approximately 4% after the post-heat treatment. The mechanically and thermally stable tetragonal phase increased, and the monoclinic phase decreased as the doped Ca replaced the Zr sites, as was confirmed by the X-ray diffraction (XRD) analysis. The post-heat treatment and the increased Ca addition further facilitated the replacement of Zr sites by Ca. However, at high Ca concentrations of 10CSZ, an equilibrium phase of CaZrOsub.3 was formed as a secondary phase at the post-heat treatment temperature, resulting in low performance.
Gamma dicalcium silicate (γ-2CaO∙SiOsub.2, abbreviated as γ-Csub.2S) is considered a potential candidate as a construction material owing to its high carbonation reactivity and consequent COsub.2 ...absorption. This study investigates the diffusion of COsub.2, a physical process, into hardened cement paste and the resulting carbonation, a chemical process. COsub.2 diffuses from a region of high concentration to one of a lower concentration, which is the inner core of the hardened cement. This study aimed to examine whether the diffusion of COsub.2 into the ordinary Portland cement (OPC)/γ-Csub.2S composite paste followed the conventional laws of diffusion. We also studied the diffusion of CaCOsub.3 to determine if carbonation products were formed in the pores and examined the capture of COsub.2. The paste specimens were prepared and subjected to COsub.2 in the carbonation chambers for varying periods. The results showed that the CaCOsub.3 deposited in the pores affected the rate of diffusion of COsub.2 in the mortars and pastes, resulting in the densification of such bodies and a decreased rate of diffusion, leading to the shutdown of diffusion. The diffusion of COsub.2 in hardened cement pastes made from OPC and γ-Csub.2S follows Fick's second law, wherein there is a change in the concentration of COsub.2 diffusing at a particular distance with time.
The effect of the addition of Bsub.2Osub.3 and the CaO/Alsub.2Osub.3 ratios on the CaO–Alsub.2Osub.3–Bsub.2Osub.3 melts’ structure was fully analyzed by molecular dynamics (MD) simulations. The ...results show that, with the increase in CaO/Alsub.2Osub.3 ratios, the charge compensation for AlOsub.4sup.5− tetrahedron changed from the oxygen tricluster structure to the charge provided by Casup.2+, and the structure of AlOsub.4sup.5− tetrahedron was stabilized. The addition of Bsub.2Osub.3 reduced the degree of polymerization of the aluminate melt, which, in turn, lowered the melt viscosity; in the systems with CaO/Alsub.2Osub.3 mole ratio less than 1, the added Bsub.2Osub.3 mainly existed in the form of BOsub.3sup.3−, which reduced the symmetry and the strength of the melt network structures; in the higher CaO/Alsub.2Osub.3 ratio systems, the depolymerization ability of the addition of Bsub.2Osub.3 on aluminate network structures was enhanced with the increase in CaO/Alsub.2Osub.3 ratios, and the overall degree of polymerization of the melt decreased. The viscosity measurement shows that Bsub.2Osub.3 lowers the viscosity of CaO–Alsub.2Osub.3 melts, which was consistent with the results predicted by MD simulations.
A Zr-doped CaO sorbent for high-temperature COsub.2 capture was fabricated using electrospinning. The nanofiber sorbent with an average filament diameter of about 160 nm is characterized by an ...initial COsub.2 uptake capacity of 12.1 mmol/g, a specific surface area of 79 msup.2/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 COsub.2 uptake capacity of 9.7 mmol/g due to the uniform distribution of CaZrOsub.3 in the CaO nanofibers to prevent CaO grain growth caused by CaCOsub.3 sintering. It is revealed that the sorbent COsub.2 uptake capacity decreases both with an increase in the decarbonation temperature and with an increase in the COsub.2 concentration in the gas flow upon carbonation, where the sorbent COsub.2 uptake capacity is more sensitive to the decarbonation temperature than to the COsub.2 concentration in the gaseous stream during carbonation. It is assumed that the electrospun regenerable Zr-doped CaO sorbent is effective for removing COsub.2 from flue gases.
To elucidate the behavior of fluoride evaporation in an electroslag remelting process, the non-isothermal evaporation of the low-fluoride CaFsub.2-CaO-Alsub.2Osub.3-MgO-TiOsub.2-(Nasub.2O-Ksub.2O) ...slag is studied using thermogravimetric analysis. The evaporation law of the melted slag is further verified using thermodynamic calculations. Fourier transformation infrared (FTIR) spectroscopy is used to evaluate the change in slag structure. It is discovered that the principal evaporating substances are CaFsub.2, KF, and NaF, while the evaporation of MgFsub.2, AlFsub.3, and AlOF is less. KF evaporates absolutely in the early stage of the reaction, and CaFsub.2 evaporates in a large proportion during the late reaction period. At 1500 °C, the order of vapor pressure is KF > CaFsub.2. When Ksub.2O and Nasub.2O are added to the residue sample at the same time, the evaporation ability of KF is stronger than that of CaFsub.2 and NaF. As the Ksub.2O content increases from 0 to 8.3 wt%, evaporation increases from 0.76% to 1.21%. The evaporation rates of samples containing more Ksub.2O and those containing more Nasub.2O are 1.48% and 1.32%, respectively. Under the same conditions, Ksub.2O has a greater effect on evaporation than Nasub.2O. FTIR results show that the addition of Ksub.2O depolymerizes the network structure and that Ksub.2O can depolymerize the network structure better than Nasub.2O.
The vaporization and crystallization of CaF.sub.2-CaO-Al.sub.2O.sub.3 slags with different MgO contents for electroslag remelting were investigated by various analytical methods. The results show ...that the main volatiles from investigated slag with 8.8 mass% MgO content at 1723 K are CaF.sub.2 and AlF.sub.3 and contain trace MgF.sub.2. The mass loss of CaF.sub.2-CaO-Al.sub.2O.sub.3-(MgO) slag decreases with the increase of MgO content. The addition of MgO within 8.8 mass% has little effect on the viscosity of investigated slag, implying that the viscosity change is not the reason to inhibit the fluoride vaporization. At 1723 K, the vapor pressures of CaF.sub.2 and AlF.sub.3 decrease with the increase of MgO content; thus, fluoride vaporization becomes weak. The fluoride vaporization of slag with MgO addition under vacuum at 1823 K is also inhibited. The MgO addition can decrease the crystallization temperature of 11CaO·7Al.sub.2O.sub.3·CaF.sub.2, but excessive MgO addition can lead to the precipitation of MgO.
In this study, solidified films of CaO-Alsub.2Osub.3-BaO-CaFsub.2-Lisub.2O-based mold fluxes with different contents of Alsub.2Osub.3 addition were acquired by immersing an improved water-cooled ...copper probe in bulk molten slags. This probe can obtain films with representative structures. Different slag temperatures and probe immersion times were employed to investigate the crystallization process. The crystals in the solidified films were identified using X-ray diffraction, the morphologies of the crystals were observed using optical microscopy and scanning electron microscopy, and the kinetic conditions, especially the activation energy of devitrified crystallization in glassy slags, were calculated and discussed based on the differential scanning calorimetry. The results indicated that after adding extra Alsub.2Osub.3, the growing speed and thickness of the solidified films increased, and more time was required for the film thickness to reach a steady state. In addition, fine spinel (MgAlsub.2Osub.4) precipitated in the films at the early stage of solidification after adding 10 wt% of extra Alsub.2Osub.3. Together with LiAlOsub.2, spinel (MgAlsub.2Osub.4) acted as nuclei for the precipitation of BaAlsub.2Osub.4. The apparent activation energy of initial devitrified crystallization decreased from 314.16 KJ/mol (original slag) to 297.32 KJ/mol (5 wt% Alsub.2Osub.3 added) and 269.46 KJ/mol (10 wt% Alsub.2Osub.3 added). The crystallization ratio of the films also increased after adding extra Alsub.2Osub.3.
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