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MgO-based sorbents were prepared by impregnation with K2CO3 at different loadings. This study examined the CO2 absorption and regeneration properties of MgO-based sorbents at various pressures. The CO2 capture capacity of the PMI-30 sorbent increased to 204.4 mg CO2/g sorbent with increasing absorption pressure through CO2 absorption by MgO itself and K2CO3 by generating structures, such as MgCO3·3H2O and K2Mg(CO3)2. However, no KHCO3 phase was observed after CO2 absorption at 1, 10, and 20 atm. The CO2 capture capacity of the MgO and PMI-10, 20, 30, and 40 sorbents was the 94.6, 129.9, 156.6, 204.4, and 239.4 mg CO2/g sorbent, respectively. The CO2 capture capacity of MgO in the PMI sorbent was relatively constant regardless of the decreasing MgO content and increasing K2CO3 content. The CO2 absorption ability of MgO was calculated by substracting theoretical CO2 capture capacity of K2CO3 from the total capacity of sorbents. The TPD experiment performed at 1 atm after CO2 absorption at 20 atm showed that the regeneration temperature of the PMI sorbents differed according to the K2CO3 loading. [ABSTRACT FROM AUTHOR]

[Display omitted] • Cyclic tests of CO 2 adsorption are performed at high temperature and pressure. • ETC is proposed as doing promoter with application under harsh sorption condition. • Excellent cyclic stability is achieved by MgO-ETC due to its porous structure. • LiNaK doping is found to lower the oxygen vacancy formation energy. • Deep K incorporation decreases oxygen ion migration energy from 3.68 to 2.04 eV. MgO-based sorbents are promising candidates for CO 2 capture as they are widely available and feasible in thermodynamics, but the development of MgO-based sorbents operating at elevated temperature and pressure is still a challenging task due to sintering. Herein, we selected eutectic carbonates with high melting point as promoters for the application under elevated conditions. Among various eutectic carbonates, MgO with eutectic ternary LiNaK carbonate (ETC) doping exhibited the highest MgO conversion thanks to crystal defects based on the XRD analysis. The highest CO 2 capture capacity of 0.73 g CO2 /g sorbent was obtained with 20 wt% ETC at 400 °C and 2 MPa. With further elevation of operating parameters to 540 °C and 5 MPa, the sample still shows a stable MgO conversion of 0.69 after 30 cycles, where the material exhibits a porous structure that inhibits the sintering. The density function theory calculations reveal that the LiNaK doping lowers the formation energy of surface oxygen vacancy, providing possibility for the subsequent oxygen ion migration. The deep K incorporation is most effective to promote the oxygen ion diffusion with decreased oxygen ion migration energy barrier from 3.68 to 2.04 eV. This study can guide the design of high-performance CO 2 sorbents at wider application conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Na 2 CO 3 addition can enhance the initial sorption rate of MgO-KNO 3 greatly. • CO 3 2– can improve sorption and diffusion rates by providing chemisorption sites. • A stable MgO conversion of 0.86 is achieved for MgO-Na 2 CO 3 -KNO 3 at 400 °C, 2 MPa. • MgO-Na 2 CO 3 -KNO 3 is feasible for actual precombustion CO 2 capture below 480 ℃,2 MPa. MgO-based sorbents are promising candidates for precombustion CO 2 capture performed at elevated temperature and pressure in thermodynamics, where the development of efficient sorbents is of great importance. Herein, we report MgO-Na 2 CO 3 -KNO 3 sorbents with fast sorption kinetics, high capture capacity and good cyclic stability at elevated conditions. The effects of nitrate species, Na 2 CO 3 doping amounts and sorption conditions on the MgO conversion were investigated during cyclic test. In comparison with LiNO 3 and NaNO 3 , the sample with KNO 3 possesses the highest MgO conversion, which increases from 0.78 to 0.86 after 30 cycles with sorption at 400 °C, 2 MPa. For nitrate-promoted MgO, Na 2 CO 3 plays an essential role in the initial fast sorption rate. The sorbent with 60 mol%Na 2 CO 3 possesses the highest MgO conversion of 0.89 after 30 cycles. Moreover, during the operation range of 400 ℃-480 ℃, 2 MPa, the sorbent exhibits an excellent cyclic stability with porous structure. Density functional theory calculations are further conducted to investigate the mechanism of performance improvement brought by Na 2 CO 3. We find dissolved CO 3 2– ions will provide chemisorption sites through the formation of C(CO 2)-O(CO 3 2–) bonds and can serve as the CO 2 carrier in molten nitrate salts, thus improving both sorption and diffusion rates of CO 2. [ABSTRACT FROM AUTHOR]

Regenerable MgO-based sorbent, which was prepared and evaluated in the thermogravimetric analyzer (TGA) in part 1, was also evaluated in high-pressure packed-bed unit in CO 2 /N 2 /H 2 O mixture and simulated pre-combustion syngas environment. In CO 2 /N 2 /H 2 O environment, the CO 2 absorption capacity of the sorbent increases with increasing temperatures from 6.7% at 350 °C to 9.5% 450 °C. The sorbent is capable of achieving over 95% CO 2 capture and 40% conversion in the water gas shift (WGS) reaction, which should be attributed to positive effect of WGS reaction in producing CO 2 during the process. The sorbent reactivity and absorption capacity toward CO 2 , as well as its WGS catalytic activity decreases with increasing temperature. The maximum pre-breakthrough WGS conversion occurs at 350 °C, which diminishes as the sorbent is carbonated. The variable diffusivity shrinking core reaction model coupled with the two-fluid computational fluid dynamics (CFD) model was shown to accurately predict the break-through gas compositions at different operating conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Li 3 PO 4 and CaCO 3 -doped, NaNO 3 -promoted MgO showed high CO 2 adsorption performance. • Li 3 PO 4 enhanced the surface adsorption of CO 2 by providing moderate basic sites. • CaCO 3 accelerated the carbonation of bulk MgO via the formation of CaMg(CO 3) 2. • The optimized sorbent showed good cyclic stability under harsh but realistic conditions. Molten nitrate-promoted MgO-based sorbents represent a potential class of materials for CO 2 capture at intermediate temperatures. However, their poor stability under harsh but realistic regeneration conditions (e.g., at high temperatures in pure CO 2) is a challenge for the application of CO 2 capture. In this work, a series of NaNO 3 and/or Li 3 PO 4 promoted MgO and MgO-CaCO 3 sorbents were prepared, characterized, and evaluated. The results showed that the NaNO 3 -Li 3 PO 4 promoted MgO-CaCO 3 sorbents had high CO 2 adsorption performance in terms of rate, capacity and stability, which was attributed to the promoting effect of Li 3 PO 4 and CaCO 3. Li 3 PO 4 can enhance the surface adsorption of CO 2 by providing some extra moderate basic sites, while CaCO 3 can accelerate the carbonation reaction of bulk MgO via the formation of CaMg(CO 3) 2. In addition, Li 3 PO 4 can hinder the sintering of sorbent particles, and consequently improve the stability of the sorbent. The optimized sorbent had a stable CO 2 uptake of 0.12 (or 0.26) g CO 2 /g sorbent when 30% (or pure) CO 2 was used at the adsorption stage in long-term cyclic operation (desorption at 500 °C in pure CO 2 for both cases), showing promising applications in intermediate-temperature CO 2 capture and thermochemical energy storage. [ABSTRACT FROM AUTHOR]

Abstract: The variable diffusivity model was extended to include physically expanding product layer and two reacting zones, consisting of a high-reactivity outer shell and a low-reactivity inner core, to describe the gas/solid carbonation reaction involving carbon dioxide and a potassium-promoted half-calcined dolomite sorbent. The model provides an excellent fit to the TGA experimental data on sorbent carbonation at various operating conditions. The parameters in the model which include an initial product layer diffusivity and diffusivity decay rate can correctly capture the trends on the effects of operating conditions on the physical properties of the sorbent observed in the experiments. [Copyright &y& Elsevier]

Abstract: Highly reactive and mechanically strong low-cost regenerable MgO-based sorbents were prepared by modification of dolomite which involved partial calcinations followed by impregnation with a potassium-based salt. The sorbents are capable of removing CO2 from gasification-based processes such as Integrated Gasification Combined Cycle (IGCC). The sorbents have high reactivity and good capacity toward CO2 absorption in the temperature range of 300–450°C at 20atm. and can be easily regenerated at 500°C. The reaction appears to be first order with respect to CO2 concentration with an activation energy of 44kJ/mol. The reactivity and the absorption capacity of the sorbents increase with increasing temperature, as long as the partial pressure of CO2 is above the equilibrium value for sorbent carbonation. The reactivity of the sorbents appears to improve in the presence of steam, which is likely due to the increase in the BET surface area and the porosity of the sorbent. A two-zone expanding grain model, consisting of a high-reactivity outer shell and a low-reactivity inner core is shown to provide an excellent fit to the TGA experimental data on sorbent carbonation at various operating conditions. [Copyright &y& Elsevier]

[Display omitted] • Cyclic tests of CO 2 adsorption are conducted at high temperature and high pressure. • MgO-Na 2 CO 3 prepared via ball-milling method exhibits a strong skeleton structure. • Excellent cyclic stability is achieved at the sorption condition of 500 °C, 3 MPa. • Na-doped defect is found to enhance CO 2 adsorption with increased electron transfer. • Na 2 CO 3 is found to benefit CO 2 adsorption through electrostatic interactions. MgO-based mixture is a promising candidate for CO 2 capture due to its high theoritical capture capacity, non-corrosiveness, low cost and wide availability. In this work, the effects of preparation methods and sorption conditions on the characteristics of MgO-Na 2 CO 3 are studied. A stable MgO conversion of 0.32 for CO 2 sorption is obtained during the 30-cycle tests at 400 °C, 2 MPa, where the material exhibits a strong skeleton structure with increased specific surface area when prepared by the ball-milling method. A severe sorption condition of 500 °C, 3 MPa is also tested, where the material shows a stable MgO conversion of 0.3 after 20 cycles. With the increase of Na 2 CO 3 content in the mixture, a higher MgO conversion can be achieved, but the cyclic stability will be impaired. In addition to the experiments, density function theory calculations are conducted to explore the sorption mechanism of MgO-Na 2 CO 3 , where CO 2 adsorptions on the pure, the Na-doped, and the Na 2 CO 3 -MgO co-adsorption surfaces are compared. The defect is found to help induce electrons from the Na and Mg atoms to the CO 2 molecule, thus benefiting the CO 2 adsorption process. The presence of Na 2 CO 3 also benefits the adsorption process of MgO due to its electrostatic interaction with CO 2 molecules. This work can serve as the guideline for the design of MgO-based CO 2 absorbent especially for extreme operating conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mineral MgO promoted with CaCO 3 and molten salts shows promise for carbonate looping. • Faster sorption rate is enabled with elevated molten salt and lower CaCO 3 contents. • Higher molten salt loadings lead to more severe sintering and cyclic activity loss. • Extended carbonation conversion enhances cyclic operation and counteracts sintering. • Long sorption duration or low temperature with 30 % CO 2 flow improves stability. This study evaluated the CO 2 capture performance of MgO obtained from mineral magnesite and doped with limestone and molten Li, Na and K nitrates under consecutive sorption/desorption cycles via thermogravimetric analysis. Increasing the molten promoter loading resulted in a higher CO 2 sorption rate, while elevated CaCO 3 amounts impeded CO 2 diffusion. When exposed to a 30 % CO 2 flow at 300 °C for 30 min, the sorbent with alkali salts and CaCO 3 to MgO molar ratios of 0.20 and 0.05 respectively attained a capture of 7.2 mol CO 2 /kg of sorbent and a negligible activity loss (∼6%) after 50 cycles. This performance was promising, considering the mineral nature of precursors. In-situ X-ray diffraction revealed the growth of the MgO crystal after each desorption, proving the gradual MgO sintering. However, the morphological transformations occurring during cyclic operation and especially if high conversions of MgO are reached, trigger an alkali salt redistribution that grants high stability. [ABSTRACT FROM AUTHOR]

[Display omitted] • CO 2 capture with magnesite-derived MgO promoted with limestone and alkali nitrates. • In-situ XRD elucidated the effect of operating conditions on sorption mechanism. • Higher alkali salt contents accelerated the MgCO 3 and CaMg(CO 3) 2 product generation. • CaMg(CO 3) 2 nuclei formation was faster compared to MgCO 3 , but absent below 300 °C. • CaMg(CO 3) 2 higher calcination temperatures than MgCO 3 can reduce sorption activity. Carbonate looping using MgO-based sorbents has recently aroused the scientific interest as an auspicious technology for intermediate temperature (200−400 °C) CO 2 capture, with the main hindrance being the slow sorption kinetics of MgO. This study investigated the preparation of MgO-based sorbents derived from mineral magnesite and promoted with a mixture of Li, Na and K nitrates, which by shifting into a molten state can reinforce the CO 2 capture kinetics. Mineral limestone was also tested as promoter, which enabled the formation of CaMg(CO 3) 2 as another carbonate product except MgCO 3. In-situ X-ray diffraction was employed to interpret the sorption mechanism of materials with different promoter contents and under various operating conditions. Even though CaMg(CO 3) 2 required a minimum temperature of 300 °C for formation in contrast to MgCO 3 , it generally displayed a faster nucleation rate, which was profitable mainly when applying a low CaCO 3 to MgO molar ratio. However, the CaMg(CO 3) 2 decomposition demanded higher temperatures than MgCO 3 , which were associated with more pronounced sintering of the sorbent and elevated energy requirements of the process. The increase of the alkali nitrate content led to a deterioration of the pore network, but also enabled a faster generation and growth of carbonate products. Lowering the CO 2 concentration of gas feedstock for more realistic carbonate looping applications required a slightly milder operating temperature for efficient CO 2 sorption to enhance both CO 2 solubility and carbonation kinetic driving force. The results proved the feasibility of mineral ores as sustainable low-cost precursors for intermediate-temperature CO 2 capture. [ABSTRACT FROM AUTHOR]