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In this work, the SO2 removal performances of the aluminum- and the magnesium-modified carbide slags fabricated by the combustion synthesis from CO2 capture cycles at the calcium looping conditions were investigated in a thermogravimetric analyzer and a dual fixed-bed reactor. The effects of sulfation temperature, number of CO2 capture cycles and calcination condition on the SO2 removal performances of Al- and Mg-modified carbide slags experienced the multiple CO2 capture cycles were studied. The sulfation temperature in the range of 800–950 °C shows a little effect on SO2 removal capacities of Al- and Mg-modified carbide slags experienced the multiple CO2 capture cycles. As the number of CO2 capture cycles increases, the sulfation conversions of the original and modified carbide slags decrease rapidly. However, Al- and Mg-modified carbide slags possess obviously higher SO2 removal capacity and cyclic stability than original carbide slag due to the good supports Ca3Al2O6 and MgO, respectively. And the larger surface areas and volumes of pores in 5–20 nm in diameter of Al- and Mg-modified carbide slags promote the higher SO2 removal capacity than original carbide slag. The modified carbide slag shows higher sintering resistance in high concentration of steam calcination condition during the CO2 capture cycles, compared with high concentration of CO2 calcination condition. Therefore, Al- and Mg-modified carbide slags from CO2 capture cycles based on calcium looping calcined under high steam concentration achieve SO2 removal capacities. [ABSTRACT FROM AUTHOR]

• The adsorption of CO 2 and H 2 O by CaO/Ca 12 Al 14 O 33 was studied by DFT calculations. • The chemical adsorption of CO 2 and H 2 O on Ca 12 Al 14 O 33 -supported CaO is competitive. • The adsorption of CO 2 on Ca 12 Al 14 O 33 -supported CaO is stronger than that of H 2 O. • The adsorbed H 2 O activates adjacent O atoms of CaO, which benefits CO 2 adsorption. • The DFT can predict the CO 2 adsorption performance of different CaO-based sorbents. The synthetic CaO/Ca 12 Al 14 O 33 sorbent has been regarded as an efficient CO 2 sorbent during the calcium looping cycles. In this work, the CO 2 adsorption performance of the synthetic CaO/Ca 12 Al 14 O 33 sorbent in the presence of H 2 O was investigated by density functional theory (DFT) calculations. DFT was used to analyze the adsorption of CO 2 and H 2 O on the synthetic sorbent. Both structural parameters (atomic layout and electronic configuration) and the adsorption parameters (energy, bond lengths, angles, bond population and charge transfer) of CO 2 and H 2 O on the synthetic sorbent were determined. To confirm the feasibility of DFT analysis, the CO 2 capture performance of the synthetic sorbent in the presence of steam was examined in a dual fixed bed reactor. The results indicate strong interactions between the C atom and the O atom in CaO and interactions between the H atom and the O atom in CaO. The higher adsorption energy of CO 2 than H 2 O on pristine CaO and Ca 12 Al 14 O 33 -supported CaO suggests that the adsorptions of CO 2 and H 2 O are competitive and that the adsorption of CO 2 is stronger than that of H 2 O. The adsorption of H 2 O leads to the activation of adjacent O atoms of CaO and thus stronger CO 2 adsorption on the O atom with H 2 O adsorbed. The electrons in the p orbital of the O atom near the Fermi level play important roles in CO 2 adsorption by CaO. The presence of Ca 12 Al 14 O 33 hinders CO 2 adsorption by CaO. A low Ca 12 Al 14 O 33 content in synthetic sorbents should be maintained to ensure high CO 2 capture capacity and sintering resistance of synthetic sorbents. DFT calculations can predict the CO 2 adsorption performance of CaO-based sorbents with additives under different reaction atmospheres. [ABSTRACT FROM AUTHOR]

Highlights • Synthetic CaO-based CO 2 sorbent was prepared using paper fibre as a biotemplate. • Synthetic sorbent is mainly composed of CaO and Ca 12 Al 14 O 33. • Synthetic sorbents possess microtube-like structure with superior CO 2 uptake. • Synthetic sorbent has improved sintering resistance in severe calcination condition. Abstract Calcium looping is regarded as an effective and viable way to address CO 2 emissions. To overcome the loss-in-capacity problems of calcium-based sorbents with the number of calcium looping cycles, a novel CaO/Ca 12 Al 14 O 33 sorbent with a microtube-like structure was prepared from carbide slag and Al(NO 3) 3 ·9H 2 O using paper fibre as a biotemplate. The CO 2 capture performance and microstructure of the novel synthetic sorbent under calcium looping conditions were investigated. The results show that the utilization of the biotemplate is good to retain the high cyclic CO 2 capture reactivity of the synthetic sorbent. Due to the unique hollow porous structure, the CO 2 capture capacity of the synthetic sorbent containing 7.5 wt% Al 2 O 3 retains 0.56 and 0.33 g/g after 30 cycles under mild and severe calcination conditions, respectively, which are 2.56 and 2.11 times higher than those of carbide slag under the same respective calcination conditions. With the presence of 10% steam in the carbonation atmosphere, the CO 2 capture capacity of the synthetic sorbent retains 0.55 g/g under the severe calcination conditions after 10 cycles. The native hierarchical biostructure of paper fibre is preserved in the synthetic sorbent. CaO and Ca 12 Al 14 O 33 are uniformly distributed in the synthetic sorbent, resulting in a high sintering resistance during multiple CO 2 capture cycles. CO 2 can penetrate through the microtube-like structure of the synthetic sorbent from two directions, i.e., from the outer surface and inner surface. This phenomenon effectively enlarges the contact area between CO 2 and CaO. The CaO/Ca 12 Al 14 O 33 sorbent with a hollow porous structure by means of a biotemplate appears promising in the calcium looping technology for CO 2 capture. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A bifunctional system integrating with CO 2 capture and heat storage is proposed. • Original carbide slag can be used in CaO/Ca(OH) 2 thermochemical heat storage process. • High Cl content results in low cyclic CO 2 capture capacity of carbide slag. • Hydration performance of carbide slag with Cl is improved after CO 2 capture cycles. • CaO after CO 2 capture cycles is promising to be used for heat storage. Abstract Carbide slag is an industrial waste generated from ethylene gas production in chlor-alkali plants. Here, a novel system coupling calcium looping and CaO/Ca(OH) 2 thermochemical heat storage using carbide slag were proposed to simultaneously capture CO 2 and store heat. For CaO/Ca(OH) 2 thermochemical heat storage, the hydration/dehydration performance of original carbide slag and carbide slag that experienced calcium looping cycles for CO 2 capture was investigated. The performances of the two types of carbide slag with and without chlorine were compared. The dehydration conversions of carbide slag improved with the increase of dehydration temperature. The chlorine content has no apparent effect on the hydration/dehydration performance of original carbide slag. However, for CO 2 capture, carbide slag with high chlorine content shows lower carbonation conversion than that of carbide slag without chlorine. The hydration/dehydration conversions of carbide slag that experienced CO 2 capture cycles are lower than those of original carbide slag. For carbide slag with chlorine, the hydration conversion can be improved by more than one CO 2 capture cycle. Therefore, carbide slag that experienced various CO 2 capture cycles is still suitable to be used in CaO/Ca(OH) 2 thermochemical heat storage although calcium looping has an adverse effect on the hydration/dehydration performance of carbide slag. [ABSTRACT FROM AUTHOR]

• In-situ conversion mechanism of captured CO 2 by CaO in H 2 was studied by DFT theory. • Detailed reaction paths of direct reduction of CaCO 3 to form CO and CaO by H 2. • The presence of H 2 reduces a 0.35 eV in the energy barrier for CaO generation. • H 2 addition leads to reduced C O orbital overlap and electron cloud disturbances. In-situ conversion of the captured CO 2 by CaO to CO is realized by using renewable H 2 in the calcination stage of calcium looping process, which effectively reduces the decomposition temperature of CaCO 3 and alleviates the sintering of CaO. However, the mechanism for this reaction is unclear which is difficult to determine just by experimental methods. In this work, the reaction mechanism for in-situ conversion of captured CO 2 by CaO to CO in presence of H 2 during calcium looping process was investigated by density function theory (DFT) analysis. The CaCO 3 direct decomposition was compared to clarify the effect of H 2 on CaO regeneration. The electron differential densities (EDD) and partial density of states (PDOS) for HCO 3 * and CO 3 * models were also compared. The results show the detailed reaction pathway for in-situ conversion of the capture CO 2 by CaO in presence of H 2 in the calcination stage of CaCO 3. H 2 is adsorbed on the CaCO 3 surface to form OH*, CO 2 *and HCO 3 *. CO 2 * further is decomposed into CO and O*. HCO 3 * is decomposed into CO 2 and OH*. Then, two OH* are attracted to each other to form H 2 O and O*. The CO 3 * hydrogenation is the rate control step in this reaction with energy barrier of 3.12 eV. The presence of H 2 causes a 0.35 eV reduction in the energy barrier for CaO generation on CaCO 3 surface. The overlap of C O orbitals and the disturbance of electron clouds around O atom all confirm that the addition of H 2 makes C and O atoms in HCO 3 * have small interaction energy. The DFT calculations reinforce the possible mechanism of in-situ conversion of the captured CO 2 by CaO in presence of H 2 during calcium looping process. [ABSTRACT FROM AUTHOR]

• Novel SiC/Mn co-doped CaO pellets are prepared for calcium looping heat storage. • Co-doped pellets show good heat storage capacity in multiple heat storage cycles. • Optical and thermal capacities of pellets rise by 16.6 and 2.2 times, respectively. • Co-doped pellets exhibit high crushing strength and attrition resistance in cycles. To achieve high heat storage capacity, excellent mechanical, optical and thermal properties in calcium looping thermochemical heat storage, the novel SiC/Mn co-doped CaO pellets were fabricated by the extrusion-spheronization method. The heat storage performances of the novel CaO pellets were studied under harsh calcination (pure CO 2 , 950 °C) and pressurized carbonation (pure CO 2 , 13 bar, 800 °C) conditions in a dual fixed-bed reactor system. The additions of SiC and Mn enhance the cyclic stability of CaO pellets during the multiple heat storage cycles. When the mass ratio of CaO:SiC:MnO 2 is 100:5:5, the co-doped CaO pellets exhibit an excellent heat storage performance. The effective conversion and heat storage density of the co-doped CaO pellets are 41.5% higher than those of original CaO pellets, respectively. Meanwhile, the co-doped CaO pellets possess a good optical absorption capacity and a high thermal conductivity, which are 17.6 and 3.2 times as high as those of original CaO pellets, respectively. In addition, the co-doped CaO pellets show much higher crushing strength and lower weight loss during the attrition test. The stronger interaction between SiC and CaO cluster mitigates the movement of CaO, which is determined by density functional theory simulation. The adsorption of CO 2 onto the CaO surface is enhanced by the addition of Mn. Therefore, SiC/Mn co-doped CaO pellets used in the calcium looping thermochemical heat storage process appear promising. [ABSTRACT FROM AUTHOR]

A Ca 3 Al 2 O 6 -stabilized carbide slag from calcium looping cycles for CO 2 capture was proposed as a H 2 S sorbent in zero emission carbon process. The H 2 S removal performance of Ca 3 Al 2 O 6 -stabilized carbide slag experienced different calcium looping cycles was investigated in a muti-fixed-bed reactor. The fabricated sorbent containing the mass ratio of CaO to Ca 3 Al 2 O 6 = 73:27 shows higher CO 2 capture capacity. The sulfidation temperature and H 2 S concentration exhibit the important effects on H 2 S removal by the fabricated sorbent from CO 2 capture cycles. The number of CO 2 capture cycles has a little effect on H 2 S removal by the fabricated sorbent. The sulfidation conversions of the fabricated sorbent after 100 CO 2 capture cycles at 60 and 120 min can reach 1.3 and 1.6 times as high as those of carbide slag, respectively. Ca 3 Al 2 O 6 restrains the rapid growth of CaO grains in the fabricated sorbent in CO 2 capture cycles. Compared with carbide slag, the fabricated sorbent possesses larger surface area and more stable structure during CO 2 capture cycles because of the support of Ca 3 Al 2 O 6 , so it exhibits higher sulfidation conversion. H 2 S removal using Ca 3 Al 2 O 6 -stabilized carbide slag from calcium looping cycles seems promising in zero emission carbon process. • C3A-modified carbide slag has high CO 2 uptake and stability in multiple CaL cycles. • The modified carbide slag after CO 2 capture cycles shows excellent H 2 S uptake. • C3A improves CO 2 and H 2 S removal capacities of carbide slag from CaL cycles. • CO 2 capture and H 2 S removal by modified carbide slag can be realized in ZEC process. [ABSTRACT FROM AUTHOR]

Thermochemical energy storage of CaO/CaCO 3 system is a rapidly growing technology for application in concentrated solar power plant. In this work, the energy storage reactivity and attrition performance of the limestone during the energy storage cycles were investigated in a fluidized bed reactor. The effects of CO 2 concentration, reaction temperature, fluidization velocity, particle size and number of cycles were discussed. With increasing CO 2 concentration from 80% to 100%, the energy storage capacity and attrition rate of the limestone increase by 11% and 9%, respectively. The feasible carbonation and calcination temperatures are 850–870 °C and 800–850 °C, respectively. The energy storage capacity of the limestone improves with increasing fluidization velocity in carbonation stage. As the fluidization velocity increases from 0.04 to 0.06 m/s, the attrition rate of the limestone after 5 cycles increases by 96%. Smaller particles show higher energy storage and attrition resistance during the cycles. Further, the cyclic stability of the limestone carbonated at higher fluidization velocity is higher than that carbonated at static (solid-like) state. The limestone operated at the fluidization state exhibits a higher cyclic energy storage capacity than that at the static (solid-like) state. Higher fluidization velocity significantly mitigates the pore-plugging and sintering of the limestone. • Energy storage and attrition behaviors of limestone are studied in a fluidized bed. • Limestone achieves high cyclic energy storage capacity under fluidization. • Limestone presents a good attrition resistance in energy storage under fluidization. • High fluidization velocity mitigates pore-plugging of limestone for energy storage. [ABSTRACT FROM AUTHOR]

• Al 2 O 3 /CeO 2 doped CaO-based material has high energy storage capacity under 1.3 MPa. • Al 2 O 3 /CeO 2 doped CaO-based material has high sintering resistance under 1.3 MPa. • CeO 2 doping facilitates carbonation reaction of CaO-based material. • Al 2 O 3 /CeO 2 doped CaO-based material possesses porous and stable structure. The calcium looping energy storage is a promising technique for thermochemical energy storage in concentrated solar power plants. Nevertheless, natural CaO-based materials, such as limestone, have an obvious decline in energy storage capacity during cyclic CaO/CaCO 3 energy storage. In this work, a novel Al 2 O 3 /CeO 2 co-doped CaO-based material for energy storage is synthesized by a wet-mixing method. And the thermochemical energy storage performance of the Al 2 O 3 /CeO 2 co-doped CaO-based material under high carbonation pressure is studied. Additionally, the influences of the doping amount of Al 2 O 3 /CeO 2 , the carbonation pressure and the temperature on the energy storage performance of the synthetic material are discussed. The main compositions of the synthetic material are CaO, Ca 12 Al 14 O 33 and CeO 2. When 5 wt% Al 2 O 3 and 5 wt% CeO 2 are doped on CaO, the synthetic material shows the highest and most stable energy storage capacity under the carbonation pressure of 1.3 MPa during 30 cycles. The Ce3+ ions existing on the surface of the synthetic material can create oxygen vacancies on the surface of CaO and increase the amount of surface adsorbed oxygen, which facilitates the carbonation of CaO. In addition, the synthetic material possesses strong basicity and provides a large surface area and pore volume during the multicycle energy storage. The high energy storage performance of the synthetic material is attributed to the high pressure in the carbonation process, the good support of Ca 12 Al 14 O 33 and the catalytic function of CeO 2. The synthetic material can reduce the overall cost in concentrated solar power plants, thus it appears promising. [ABSTRACT FROM AUTHOR]