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• Monometallic and bimetallic (Fe, Co, Ni, Cu, Zn) doped CaO-Ca 12 Al 14 O 33 was prepared. • Doped materials were tested for sorption-enhanced water-gas-shift to produce H 2. • Fe/Ni synergistically promotes CaO-Ca 12 Al 14 O 33 in CO 2 capture and H 2 production. • Fe/Ni interaction improves electron transfer to enhance catalytic redox activity. • Fe/Ni interaction boosts oxygen vacancy to promote CO 2 capture and H 2 production. Herein, a series of monometallic and bimetallic (Fe, Co, Ni, Cu, Zn) doped CaO-Ca 12 Al 14 O 33 materials were prepared and used for H 2 production from sorption-enhanced water–gas-shift (SEWGS). The synergetic effect of bi- transition metal elements doping on CaO-Ca 12 Al 14 O 33 for H 2 production via SEWGS was investigated through the experimental and density functional theory calculation. The results indicate that Fe/Ni-doped CaO-Ca 12 Al 14 O 33 exhibits the highest H 2 production activity among these bifunctional materials. The synergetic promoting effect of the Fe/Ni doping on CaO/Ca 12 Al 14 O 33 contributes to the excellent and stable H 2 production performance. The initial CO conversion and H 2 concentration are as high as 98.8% and 93.4% for the optimal Fe/Ni-doped CaO-Ca 12 Al 14 O 33 at the Fe/Ni molar ratio of 1:1, respectively, which retain 95.7% and 89.6% after 20 cycles. The mechanism of Fe/Ni-doped CaO/Ca 12 Al 14 O 33 for H 2 production via SEWGS is determined. The Fe/Ni interaction boosts the electron transfer from Fe to Ni species, thereby promoting the reductivity of the Fe/Ni-doped CaO/Ca 12 Al 14 O 33. In addition, the Fe/Ni interaction facilitates the O–H bond cleavage to generate H 2 and inhibits the undesirable CH 4 by-product formation. Moreover, the interaction between Fe and Ni species is beneficial for the formation of oxygen vacancy. Therefore, the catalytic activity and in-situ CO 2 removal capacity are enhanced by using Fe/Ni-doped CaO-Ca 12 Al 14 O 33 to produce more H 2. These significantly enhance the potential of the Fe/Ni-doped CaO-Ca 12 Al 14 O 33 for efficient H 2 production via SEWGS. [ABSTRACT FROM AUTHOR]

CaO/Ca(OH)2 thermochemical heat storage (THS) technology is considered to be one of the most promising technologies for large‐scale solar energy storage. However, the THS performance of raw CaO‐based materials decreases during multiple cycles. In this work, CaO derived from calcium acetate (Ac‐CaO) is prepared and applied to a coupled system that achieved simultaneous CaO/Ca(OH)2 THS and CO2 capture. The CO2 capture and THS performances of Ac‐CaO are always higher than those of calcined limestone owing to the preferable pore structure, whereas Ac‐CaO exhibits decreasing CO2 capture and THS performance resulting from sintering and the formation of CaCO3 from CaO or Ca(OH)2 with ambient CO2 during air cooling, respectively. In the coupled CaO/Ca(OH)2 THS and CO2 capture system, Ac‐CaO is subjected to 10 CO2 capture cycles, 30 THS cycles, 1 CO2 capture cycle, 10 THS cycles, 1 CO2 capture cycle, and 10 THS cycles sequentially. The hydration and dehydration conversions of Ac‐CaO in the 31st THS cycle reach 91.7 and 93.6%, respectively, which are 1.6 and 1.6 times higher than those recorded prior to the 11th CO2 capture cycle owing to the decomposition of CaCO3 during calcination. The carbonation conversion of Ac‐CaO achieves 89.9% in the 11th CO2 capture cycle, which is 22.3% higher than that recorded prior to the 10 THS cycles owing to reactivation from the hydration process during THS. The CO2 capture and CaO/Ca(OH)2 THS processes are enhanced in the coupled system using Ac‐CaO; therefore, the coupled system appears promising for CaO/Ca(OH)2 THS and CO2 capture. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

• Mn/Mg-copromoted carbide slag is produced using dolomite and trace Mn(NO 3) 2. • MgO acts as skeleton and Mn acts as an electron-transfer promoter for CO 2 capture. • The optimal MnO 2 content in the Mn/Mg-copromoted carbide slag is 0.75 %. • MnO 2 positively affects the diffusion-controlled stage of carbonation. • The optimal Mn/Mg-copromoted carbide slag captures 0.52 g CO 2 /g after 10 cycles. Loss-in-capacity of carbide slag in CO 2 capture restricts the development of industrial wastes in calcium looping technology. In this work, a novel Mn/Mg-copromoted carbide slag was prepared using carbide slag, dolomite and trace Mn(NO 3) 2 additive. Experimental tests were carried out in the fixed-bed reactor to evaluate how the preparation and the reaction conditions influenced the CO 2 capture performance of Mn/Mg-copromoted carbide slag during calcination/carbonation cycles. Results show that MgO diminishes the sintering of synthetic sorbents. The optimal Mn/Mg-copromoted carbide slag (mass ratio of CaO:MgO:MnO 2 = 89:10:1) exhibits the highest CO 2 capture capacity of 0.52 g/g after 10 cycles under the severe calcination condition (100 % CO 2 , 950 °C) and the wet carbonation condition (15 % CO 2 /20 % steam/N 2), which is 1.7 times as high as that of untreated carbide slag. MnO 2 positively affects the slow carbonation stage by enhancing the electron transfer between CaO and CO 2. Observations of the morphology of Mn/Mg-copromoted carbide slag indicate that the stabilized CO 2 capture performance is mainly attributed to porous structure, MgO as the skeleton and MnO 2 as an electron-transfer promoter. [ABSTRACT FROM AUTHOR]

• CO 2 adsorption performance of Mn/Ca 3 Al 2 O 6 co-doped CaO is analyzed by DFT. • Adsorption energy of CO 2 on co-doped CaO is 2.2 times higher than that on CaO. • The robust co-doped CaO structure results from O-Al and O–Mn bond formation. • Mn promotes electron transport and CO 2 adsorption in Mn/Ca 3 Al 2 O 6 co-doped CaO. Calcium looping (CaL) is a promising technology for post-combustion CO 2 capture and concentrated solar energy storage. The transition metal and calcium aluminate co-doped CaO materials have drawn a lot of attention due to the superior CO 2 adsorption capacity. In this work, the co-promotion mechanism of Mn and Ca 3 Al 2 O 6 on the CO 2 adsorption of CaO-based materials in the calcium looping process was investigated by density functional theory. The structural properties and CO 2 adsorption performance of CaO, Ca 3 Al 2 O 6 doped CaO, and Mn/Ca 3 Al 2 O 6 co-doped CaO structures were determined. The formation of O–Al and O–Mn bonds plays a crucial role in preventing the doped CaO from sintering. The adsorption energy of the CaO cluster on the Ca 3 Al 2 O 6 and Mn/Ca 3 Al 2 O 6 surface is −5.66 and −11.15 eV, respectively, which is 2.4 and 4.7 times higher than that on the CaO surface, indicating the enhanced structural stability of the doped CaO. The presence of Ca 3 Al 2 O 6 has less contribution to CO 2 adsorption. The adsorption energy and charge transfer for CO 2 on Ca 3 Al 2 O 6 –CaO (−2.03 eV, −0.73 e) is the same as pure CaO (−1.93 eV, −0.71 e). Introducing Mn remarkably improves CO 2 adsorption performance by enhancing electron transport. The adsorption energy and charge transfer for CO 2 on Mn/Ca 3 Al 2 O 6 –CaO are the highest with −4.45 eV and −0.81 e, respectively. The Mn/Ca 3 Al 2 O 6 co-doped CaO demonstrates superior structural stability and enhanced CO 2 adsorption performance, which seems promising for efficient CO 2 adsorption in the CaL process. [ABSTRACT FROM AUTHOR]

• CH 4 reforming of CaCO 3 and Fischer-Tropsch are integrated with CO 2 capture by CaO. • Thermodynamic simulation of the integrated process is performed by Aspen Plus. • In-situ conversion and utilization of captured CO 2 to synthesize fuels are achieved. • The integrated design reduces CaO demand and promotes Fischer-Tropsch fuels yield. • Energy conversion and exergy efficiencies reach 43.74% and 41.81%, respectively. Calcium-looping is an attractive approach for removing CO 2 in flue gas from the coal-fired power plant to form CaCO 3. The integration of CO 2 capture via calcium-looping and in-situ dry reforming of methane with CaCO 3 allows the simultaneous CaCO 3 decomposition to regenerate CaO and syngas production. The produced syngas is expected for further utilization, e.g. Fischer-Tropsch synthesis to produce valuable fuels. In this work, an integrated process of calcium-looping in-situ dry reforming of methane and Fischer-Tropsch was developed using Aspen Plus. Moreover, two other processes were also simulated as the comparison, consisting of the serial process of calcium-looping CO 2 capture, ex-situ dry reforming of methane with CO 2 and Fischer-Tropsch and the standalone process of calcium-looping CO 2 capture. The results indicate that the reduction in the demand for CaO sorbent and the enhancement in Fischer-Tropsch products yield and carbon conversion efficiency are achieved by integrating in-situ dry reforming of methane with CaCO 3. Additionally, integrating CO 2 capture and in-situ dry reforming of methane with CaCO 3 in the case of the integrated process is efficient for enhancing the system efficiency and energy availability. Compared with the serial process, the integrated process shows a drop in the heat energy requirement of 28.08% and possesses the energy conversion efficiency of 43.74% and exergy efficiency of 41.81%. This work exhibits the prospect of the integrated process of calcium-looping in-situ dry reforming of methane and Fischer-Tropsch in the field of CO 2 capture and utilization for Fischer-Tropsch fuels production. [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]

Novel Mn‐doped CaO was prepared by the combustion method. The CO2 capture performance of Mn‐doped CaO, carbonated in the presence of steam and under severe calcination conditions (950°C and 70% CO2/30% N2) during calcium looping cycles, was investigated in a dual fixed‐bed reactor. The intercoupling effects of Mn and steam on CO2 capture by CaO were also studied. Doping of Mn in CaO by the combustion method greatly improved the CO2 capture capacity of CaO. The carbonation conversions of Mn‐doped CaO increased with increasing steam concentration from 0 to 15%. When the molar ratio of Mn/Ca was 0.75 : 100, Mn‐doped CaO achieved the highest CO2 capture capacity. Under severe calcination conditions, the carbonation conversion of Mn‐doped CaO, where the molar ratio of Mn to Ca = 0.75 : 100 in the presence of 15% steam, was about 0.4 after ten cycles (carbonation for 5 min at 650°C under 15% CO2/15% steam/N2), which was 4.38 times as high as that of the original CaO in the absence of steam. The cyclic CO2 capture capacities of CaO were improved by Mn and steam. Synergistic enhancement effects of Mn and steam on the CO2 capture capacities of CaO were also found. The effect of steam on the carbonation conversion of Mn‐doped CaO was stronger than that of the original CaO. Mn in the presence of steam showed a more positive effect on CO2 capture by CaO. X‐ray photoelectron spectroscopy analysis showed that doping of Mn in CaO enhanced the transport of electrons in the carbonation of CaO, which helped to increase the carbonation rate. When steam was present in the carbonation, Mn‐doped CaO possessed a more porous structure and smaller CaO grains than the original CaO during the cycles. Simulation calculations using periodic density functional theory (DFT) showed that CO2 molecules were easier to absorb on CaO owing to the doping of Mn and the presence of steam. The synergistic enhancement effects of Mn and steam on CO2 captured the performance of CaO. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd. [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]

A novel calcium-based synthetic CO 2 sorbent with hollow core-shell structure was prepared by a carbon microsphere template route where carbide slag, alumina cement and glucose were employed as the low-cost calcium precursor, support and carbon source, respectively. The effects of the alumina cement addition, the pre-calcination temperature during the preparation process, the carbon template addition and calcination conditions on CO 2 capture performances of the calcium-based synthetic sorbents were studied during calcium looping cycles. The synthetic sorbent containing 5 wt.% alumina cement possesses the highest CO 2 capture capacity during calcium looping cycles, which is mainly composed of CaO and Ca 12 Al 14 O 33 . The CO 2 capture capacities of the synthetic sorbent under mild and severe calcination conditions can retain 0.37 and 0.29 g/g after 20 cycles, which are 57% and 99% higher than those of carbide slag under the same conditions, respectively. The synthetic sorbent possesses a hollow micro-sphere morphology with a nano-structured shell and meso-porous structure, which decreases the diffusion resistance of CO 2 . Periodic density functional theory (DFT) calculations are used to explain why Ca 12 Al 14 O 33 can effectively retard both agglomeration and sintering of the synthetic sorbent. The hollow core-shell model is proposed to explain the CO 2 capture mechanism of the synthetic sorbent. For the same CO 2 capture efficiency, the energy consumption in the calciner using the synthetic sorbent is much lower than those using carbide slag and natural limestone. This work designs a good method to prepare the hollow sphere-structured synthetic sorbents with high CO 2 capture capacity and provides a promising way to integrate efficient CO 2 capture with the utilization of industrial waste. [ABSTRACT FROM AUTHOR]

[Display omitted] • The pyrolysis-CO 2 gasification-WGS process of oil sludge based on CaL is proposed. • H 2 production and in-situ CO 2 capture are achieved through this process. • H 2 yield and CO 2 capture efficiency are 0.34 Nm3/kg-oil sludge and 98%, respectively. • Energy conversion efficiency and exergy efficiency are 36% and 32%, respectively. The treatment of oil sludge (OS) from petrochemical industry is a challenge. Traditional OS gasification process has the disadvantages such as low H 2 yield and high CO 2 emission. In this work, a novel pyrolysis-CO 2 gasification-water gas shift process of OS for efficient H 2 production based on calcium looping was proposed, which includes four stages: drying, pyrolysis, char gasification with recycled CO 2 and water gas shift reaction for H 2 production by in-situ CO 2 capture using CaO. The pyrolysis-CO 2 gasification-water gas shift process of OS was thermodynamically simulated by Aspen Plus. The effects of the mass ratio of steam/OS, temperature, mass ratio of CaO/OS, the proportion of CO 2 reused on H 2 generation and efficiencies of CO 2 capture, energy conversion, and exergy were analyzed. The OS pyrolysis and CO 2 gasification of char were also studied by experiment. Compared with direct steam gasification of OS process, H 2 yield (0.32 Nm3/kg-OS) and efficiencies of energy conversion (36%) and exergy (32%) increase by 0.28 Nm3/kg-OS, 13.62%, and 17.16%, respectively. The novel process is promising for energy recovery from OS. [ABSTRACT FROM AUTHOR]

In this work, calcination under high-concentration steam that can be implemented by O 2 /H 2 O combustion was proposed to replace high-concentration CO 2 that usually was implemented by O 2 /CO 2 combustion in calcium looping. CO 2 capture performance of the carbide slag, an industrial solid waste from chlor-alkali plants, was investigated in a dual fixed-bed reactor under high-concentration steam calcination condition during the calcium looping cycles. Calcined under high-concentration steam (95%) atmosphere, the carbide slag can be completely and quickly decomposed at 800 °C, which is 150 °C lower than the calcination temperature under high-concentration CO 2 (100%) atmosphere. The carbonation conversions of the carbide slag calcined under high-concentration steam condition after 1 and 10 cycles are about 42% and 36% higher than those calcined under high-concentration CO 2 condition, respectively. This is because when the carbide slag is calcined under high-concentration steam, the relatively smaller CaO grains and more porous structure are generated, which are beneficial for CO 2 capture by carbide slag. Due to the low calcination temperature and the high CO 2 capture capacity of the carbide slag, the energy requirement in the calciner to capture per mole CO 2 under high-concentration steam condition is lower than that under high-concentration CO 2 condition. High-concentration steam calcination in place of high-concentration CO 2 calcination improves the CO 2 capture efficiency from 0.68 to 0.88 and saves a quarter of the energy consumption in the calciner for capturing per mole CO 2 in the calcium looping system, when the ratio of recycled carbide slag flow rate to CO 2 flow rate is 2 and the sorbent make-up ratio is 0.09. Thus, it is reasonable to consider O 2 /H 2 O combustion as an alternative energy-supply way of O 2 /CO 2 combustion for the calcium looping of carbide slag. [ABSTRACT FROM AUTHOR]

A novel method that CaO was modified with the by-product of biodiesel by the combustion was proposed to improve its CO 2 capture capacity at calcium looping conditions. The CO 2 capture performance of CaO modified with the by-product of biodiesel during the calcium looping cycles was investigated in a twin fixed-bed reactor and a thermogravimetric analyzer. The effects of ratio of by-product of biodiesel to CaO, combustion duration and temperature on the CO 2 capture performance of CaO modified with the by-product of biodiesel were studied. When the ratio of by-product of biodiesel/CaO is 25 mL/g, the modified CaO achieves the highest CO 2 capture capacity during the cycles. The feasible combustion temperature and duration are 800 °C and 60 min, respectively. CO 2 capture capacity of the modified CaO can retain 0.5 g CO 2 /g sorbent after 20 cycles (carbonation at 700 °C for 20 min in 20% CO 2 /80% N 2 , calcination at 850 °C for 10 min in N 2 ), which is higher than that of the modified CaO with the various organic solutions. The modified CaO still shows much higher CO 2 capture capacity than original CaO under the severe calcination condition. The by-product of biodiesel modification greatly improves CO 2 capture rate of CaO in the chemical-controlled stage, but shows a little effect on rate in the diffusion-controlled stage. The cyclic CO 2 capture capacity of the deactivated CaO is significantly reactivated after the by-product of biodiesel modification. The modified CaO exhibits more porous structure and higher sintering resistance during the CO 2 capture cycles. [ABSTRACT FROM AUTHOR]

A novel synthetic sorbent was fabricated from CaO and sepiolite by the hydration and its CO 2 capture performance was investigated during the calcium looping cycles. The effects of the sorbent preparation conditions including preparation method, hydration temperature, hydration duration and sepiolite content on CO 2 capture by synthetic CaO/sepiolite sorbent were examined in a dual fixed-bed reactor. The results showed that CaO/sepiolite possesses higher CO 2 capture capacity than original CaO. CO 2 capture capacity of CaO/sepiolite after 10 cycles is 39% and 56% higher than those of hydrated CaO and original CaO, respectively. The hydration temperature has an important effect on CO 2 capture by CaO/sepiolite during the preparation. When the hydration temperature is 95 °C, the obtained CaO/sepiolite exhibits the highest cyclic CO 2 capture capacity, because the good supports such as MgO and Ca 2 SiO 4 are formed in the calcined CaO/sepiolite at 95 °C, which can improve the sintering resistance of CaO during the cycles. However, these supports are not found in the calcined CaO/sepiolite at the other hydration temperatures. In addition, CaO/sepiolite possesses more porous structure, larger surface area and pore volume, compared to hydrated CaO and original CaO. After the 1st calcination, the volume of pores in 10–100 nm in diameter of CaO/sepiolite is much higher than those of hydrated CaO and original CaO, respectively, which facilitates CO 2 capture of the sorbent. CaO/sepiolite appears promising as an effective and low-cost CO 2 sorbent at calcium looping conditions. [ABSTRACT FROM AUTHOR]

The re-carbonation was proposed to improve the CO 2 capture performance of the carbide slag, an industrial solid waste, calcined under high concentration of steam during the calcium looping process. The effects of re-carbonation conditions, including duration and CO 2 concentration on CO 2 capture by the cycled carbide slag were studied. The results show that the re-carbonation significantly improves the CO 2 capture capacity of the carbide slag. Notably, the re-carbonation dramatically reactivates the carbide slag which has experienced 11 cycles, increasing its carbonation conversion from 0.19 to about 0.6. In addition, the carbide slag shows higher re-carbonation conversions than the limestones reported by other researchers. A porous structure of the carbide slag is formed after the calcination from the extra CaCO 3 generated in the re-carbonation step, which improves its CO 2 capture capacity. In addition, the impacts of the re-carbonation on the CO 2 capture ratio of the calcium looping system and the energy consumption in the calciner were discussed. The re-carbonation improves the CO 2 capture ratio from 0.46 to 0.88 and reduces the energy consumption in the calciner from 321 kJ/mol CO 2 to 261 kJ/mol CO 2 , when the molar ratio of sorbent to CO 2 is 2 and the make-up ratio of the sorbent is 0.16 in the calcium looping system using the carbide slag. Thus, the re-carbonation and high concentration of steam in the calcination is an effective way to improve the CO 2 capture capacity of the carbide slag and reduce the energy consumption in the calciner during the calcium looping process. [ABSTRACT FROM AUTHOR]

Calcium looping technology is considered to be one of the most feasible techniques for CO 2 capture. A low-cost and easy method was proposed to improve the cyclic CO 2 capture capacity of the limestone by discontinuously adding hydrogen chloride (HCl) in the carbonation step in some cycles rather than in the each cycle at the calcium looping conditions. The effects of the discontinuous addition of HCl under the various conditions on the CO 2 capture performance of the limestone at the calcium looping conditions were investigated in a dual fixed-bed reactor. The results show that HCl addition during the initial several cycles leads to the formation of CaClOH and the moderate CaClOH in the carbonation product is favorable to CO 2 capture by the limestone. HCl addition only in the initial 3 cycles changes the effect of carbonation temperature on CO 2 capture by the limestone. The optimum carbonation temperature for the limestone with the addition of HCl is 700 °C in the range of 650–750 °C. Higher CO 2 volume fraction in the carbonation leads to lower CO 2 capture capacity of the limestone with the addition of HCl only in the initial 3 cycles. The discontinuous addition of HCl during the various cycles significantly enhances the CO 2 capture capacity of the limestone. HCl addition in the carbonation stage during only the initial cycles improves the pore structure of the calcined limestone and retards the fusion of CaO grains, which contributes to high CO 2 capture capacity of the sorbent in the multiple cycles. [ABSTRACT FROM AUTHOR]

• Sorption-enhanced biomass gasification is integrated with CH 4 reforming of CaCO 3. • Thermodynamic simulation of the integrated process is carried out by Aspen Plus. • The integrated design promotes CO 2 capture by CaO in gasification to yield more H 2. • Integrated CH 4 reforming achieves in-situ conversion of captured CO 2 into syngas. • Energy conversion exergy and exergy efficiency reach 74.5% and 68.6%, respectively. Sorption-enhanced H 2 production using CaO promotes H 2 generation by capturing CO 2 in the form of CaCO 3. Coupling methane reforming of CaCO 3 in sorption-enhanced H 2 production combines CaO regeneration with in-situ CO 2 utilization. This integrated concept is expected to achieve high H 2 production with CO 2 capture and in-situ conversion into syngas. To evaluate the overall performance of the integrated sorption-enhanced staged-gasification of biomass and in-situ CO 2 utilization by methane reforming process, the thermodynamic simulation was performed using Aspen Plus. The parameter optimization of the integrated process was carried out. The performance of the integrated process was also compared with two other processes, such as the serial sorption-enhanced staged-gasification and methane reforming process and the standalone sorption-enhanced staged-gasification process. The results exhibit that integrated methane reforming enhances H 2 production from sorption-enhanced staged-gasification, because it can lower the CaO regeneration temperature and enhance the CO 2 capture capacity. The integrated process produces a total gas product with H 2 /CO around 2, which is suitable for direct Fischer-Tropsch synthesis. Compared with the serial sorption-enhanced staged-gasification and methane reforming process, the integrated process results in 73.5% less CO 2 emission and 135% more CO 2 utilization. Moreover, the integrated process exhibits the higher energy conversion and exergy efficiencies than two other processes, which are as high as 74.5% and 68.6%, respectively. This work shows that the integrated process has technical and environmental potential for the utilization of biomass and CH 4 to produce H 2 and syngas with CO 2 capture and in-situ utilization. [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]

The effects of reaction temperature, fluidization number, particle size and number of calcination/carbonation cycles on attrition and CO 2 uptake characteristics of carbide slag during the calcium looping cycles were investigated in a fluidized bed reactor. The attrition behaviors of carbide slag and limestone were also compared. Higher calcination temperature in 850–950 °C improves the attrition rate of carbide slag particles. CO 2 uptake capacity of carbide slag increases with fluidization number in carbonation. As fluidization number increases from 7 to 15, the mean diameter of carbide slag particles decreases by 4% and the attrition rate increases by 1.7 times after 10 cycles. Smaller particles show higher attrition resistance during the cycles. The breakage of larger particles of limestone is more severe than that of carbide slag during the cycles. CO 2 uptake capacity of the carbide slag is practically identical to that of limestone in short carbonation duration (e.g. 5 min) during the cycles. The effects of the number of calcination/carbonation cycles on the mean diameter and the attrition rate of carbide slag are less than those of limestone. Carbide slag possesses higher attrition resistance than limestone during the multiple CO 2 capture cycles. [ABSTRACT FROM AUTHOR]

A novel magnesia-stabilized carbide slag (MSCS) was synthesized with carbide slag, magnesium nitrate hydrate and by-product of biodiesel by combustion, which was used as a CO 2 sorbent during the calcium looping process. The effects of preparation condition (combustion temperature, combustion duration, by-product of biodiesel addition and magnesia addition) and CO 2 capture condition (carbonation and calcination atmosphere) on CO 2 capture capacity of MSCS were investigated during the calcium looping cycles. The main compositions of MSCS are CaO and MgO. The addition of by-product of biodiesel in the preparation of the sorbent leads to the uniform mix of MgO and CaO grains in MSCS, which shows an obviously positive effect on its CO 2 capture capacity. Only on the condition of the addition of by-product of biodiesel, MgO derived from magnesium nitrate hydrate improves the cyclic CO 2 capture capacity and durability of MSCS during the multiple cycles. MSCS with a mass ratio of CaO to MgO of 80:20 combusted at 850 °C for 60 min exhibits higher CO 2 capture capacity and greater durability. The CO 2 capture capacity of MSCS can retain 0.42 g/g after 20 cycles, which is 60% higher than that of carbide slag. MSCS calcined under the high concentration of steam displays much higher CO 2 capture capacity and better sintering resistance during the cycles, compared to MSCS calcined under the high concentration of CO 2 . The addition of steam in the carbonation enhances CO 2 capture capacities of MSCS and carbide slag. MSCS consists of CaO–MgO grain groups and the support of MgO sustains the high sintering resistance of the sorbent. MSCS remains much larger surface area and pore volume than carbide slag during the cycles, compared to carbide slag. MSCS appears promising as an effective and low-cost CO 2 sorbent during the calcium looping. [ABSTRACT FROM AUTHOR]

This work investigated the effect of steam concentration in carbonation on CO 2 capture by carbide slag during the calcination/carbonation cycles in a dual-fixed bed reactor. Under severe calcination conditions, the pore structure of calcined carbide slag carbonated in the presence of 20 vol. % steam is better than that in the presence of 60 vol. % steam. In the presence of steam in carbonation, the drop of carbide slag in carbonation conversion under severe calcination conditions (950 °C, pure CO 2 ) is relatively smaller than that without steam. Higher steam concentration results in significantly higher carbonation conversion of carbide slag for short carbonation duration (e.g., 5 min), because higher steam concentration leads to greater accelerating effect on CO 2 diffusion through product layer. Steam exhibits a two-side effect on the carbonation performance of carbide slag, and the effect of high steam concentration on carbonation conversion is higher than that on pore structure. [ABSTRACT FROM AUTHOR]

In carbonation stage of calcium looping process, NO could be reduced by CO using CaO as a catalyst, allowing simultaneous CO 2 and NO removal. However, to achieve efficient NO removal, the required CO/NO molar ratio in the carbonator is very high (up to 30:1), which leads to lots of unreacted CO in the exhaust gas. In this work, to maintain high efficiency of NO removal at low CO concentration in carbonator, Ce-doped CaO was employed to simultaneously enhance NO and CO 2 removal performance. The effects of Ce-doped CaO on simultaneous CO 2 and NO removal in bubbling fluidized bed reactor were investigated during the carbonation stage of calcium looping process. When the molar ratio of CO/NO is about 3.57:1, Ce-doped CaO obtains an 83% CO 2 capture efficiency and a 97% NO removal efficiency in 20 cycles in chemical reaction controlling stage. The average CO concentration in exhaust gas of Ce-CaO was reduced by 79% at the same NO removal efficiency as Mn-doped CaO. The presence of Ce3+ makes the charge imbalance of the material, resulting in more charge transfer from the Ce-CaO surface to the molecules. The density functional theory reveals that Ce doping reduces the energy barriers of the conversion of NO to N 2 and the CO 2 adsorption, promoting the simultaneous NO/CO 2 removal in carbonation stage. Therefore, under a low molar ratio of CO/NO in carbonation stage of calcium looping, Ce considerably improves the simultaneous CO 2 and NO removal action of CO and CaO. [Display omitted] • Ce markedly catalyzes NO and CO 2 removal in carbonation stage of calcium looping. • NO/CO 2 removal efficiencies reach 97% and 83% at low CO concentration, respectively. • CO concentration in exhaust gas using Ce-doped CaO decreases significantly. • Effect of Ce-doped CaO on NO and CO 2 removal in calcium looping is studied by DFT. • Ce reduces the energy barriers of NO and CO 2 removal on CaO from 8.97 to 7.12 eV. [ABSTRACT FROM AUTHOR]

The carbide slag is an industrial waste from chlor-alkali plants, which can be used to capture CO 2 in the calcium looping cycles, i.e. carbonation/calcination cycles. In this work, the cycled carbide slag from the calcium looping cycles for CO 2 capture was proposed to remove HCl in the flue gas from the biomass-fired and RDFs-fired boilers. The effects of chlorination temperature, HCl concentration, particle size, presence of CO 2 , presence of O 2 , cycle number and CO 2 capture conditions in calcium looping cycles on the HCl removal behavior of the carbide slag experienced carbonation/calcination cycles were investigated in a triple fixed-bed reactor. The chlorination product of the cycled carbide slag from the calcium looping after absorbing HCl is not CaCl 2 but CaClOH. The optimum temperature for HCl removal of the cycled carbide slag from the carbonation/calcination cycles is 700 °C. The chlorination conversion of the cycled carbide slag increases with increasing the HCl concentration. The cycled carbide slag with larger particle size exhibits a lower chlorination conversion. The presence of CO 2 decreases the chlorination conversions of the cycled carbide slag and the presence of O 2 has a trifling impact. The chlorination conversion of the carbide slag experienced 1 carbonation/calcination cycle is higher than that of the uncycled calcined sorbent. As the number of carbonation/calcination cycles increases from 1 to 50, the chlorination conversion of carbide slag drops gradually. The high calcination temperature and high CO 2 concentration in the calcination of calcium looping decrease the chlorination conversions of the cycled carbide slag. Increasing the calcination time in the calcium looping is adverse to HCl removal and extending the carbonation time slightly improves the chlorination conversions. The microstructure of the cycled carbide slag shows an important effect on HCl removal capacity. [ABSTRACT FROM AUTHOR]

Papermaking soda residue (PSR) is calcium-rich waste produced by the papermaking industry. In this study, the thermochemical energy storage performances of the original PSR and PSR modified with citric acid were investigated under pressurised carbonation during CaO-CaCO 3 cycles in a twin fixed-bed reactor. The effects of carbonation pressure, reaction temperatures, and number of cycles on the energy storage performance were also discussed. The modified PSR exhibits higher energy storage capacity than the original PSR. The energy storage performances of the two PSRs are improved with increasing carbonation pressure from 0.1 to 1.1 MPa. The energy storage density and effective conversion of the modified PSR at 1.1 MPa are approximately 2210 kJ·kg-1 and 0.7 after 30 cycles, respectively. The energy storage density of the modified PSR at 1.1 MPa is approximately 156% greater than that of the original PSR at 0.1 MPa. Carbonation and calcination temperatures of 850 °C are suitable for the energy storage of both PSRs. The surface area and pore volume of the modified PSR are 1.84 and 1.75 times those of the original PSR, respectively. The modified PSR is a suitable energy storage material. • PSR as a calcium-rich waste from paper mills is proposed as energy storage material. • Modification with citric acid markedly improves the energy storage capacity of PSR. • Modified PSR has high energy storage capacity under 1.1 MPa. • Modified PSR maintains porous and stable structure in energy storage cycles. [ABSTRACT FROM AUTHOR]

In this work, cycled MgO-modified calcium-based waste from calcium looping was used for H 2 S removal. The H 2 S removal performance of MgO-modified carbide slag from calcium looping was examined. The effect mechanism of MgO addition on H 2 S removal by CaO was analyzed using density functional theory calculation. MgO-modified carbide slag from calcium looping possesses the enhanced H 2 S removal performance. The sulfidation temperature as well as H 2 S concentration have large impacts on the sulfidation performance of cycled MgO-modified carbide slag, while experienced multiple calcium looping cycles has slight effect. The sulfidation conversion of MgO-modified carbide slag after 10 calcium looping cycles under the severe and mild calcination conditions are 0.82 and 0.96 mol/mol, respectively. MgO-modified carbide slag from calcium looping keeps the more porous microstructure because MgO mitigates the sintering of CaO. The theoretical calculation results show that the adsorption and dissociation of H 2 S on CaO (0 0 1) surface are promoted by MgO. The sintering resistance and sulfidation activity of CaO are enhanced by MgO addition, contributing to the excellent H 2 S removal by cycled MgO-modified carbide slag from calcium looping. Therefore, cycled MgO-modified carbide slag from calcium looping seems prospective to be used for H 2 S removal. [Display omitted] • MgO-modified carbide slag from calcium looping cycles is proposed for H 2 S removal. • Cycled MgO-modified carbide slag exhibits enhanced sulfidation performance. • MgO addition mitigates growth of CaO grains and pore plugging of carbide slag. • Effect mechanism of MgO on H 2 S removal by CaO is revealed by DFT calculation. • H 2 S adsorption and dissociation on CaO surface are promoted by MgO addition. [ABSTRACT FROM AUTHOR]

• NO is efficiently removed by CO in carbonation of Mn-doped CaO in calcium looping. • MnO in Mn-doped CaO efficiently catalyzes NO removal by CO in carbonation. • Mn promotes CO 2 capture by CaO in fluidization state. • Mn is a bi-function additive for simultaneous NO/CO 2 removal by CO and Mn-doped CaO. • NO removal and CO 2 capture efficiencies reach 99% and 85%, respectively. By utilizing CO as NO reductant and CaO as the catalyst, NO removal by CO can be realized in the carbonation stage of calcium looping for CO 2 capture, but the carbonation of CaO decreases the catalytic effect on NO removal. To realize efficient and stable NO removal in the carbonation stage, CaO was modified by Mn in this work. NO removal performance of CO in the carbonation stage of Mn-doped CaO in the calcium looping process was investigated in a bubbling fluidized bed reactor. In the carbonation stage, NO removal efficiency sharply increases from 0% to 99% and CO 2 capture efficiency increases from 73% to 88% due to the presence of Mn. Compared with CaO, Mn-doped CaO possesses a stronger catalysis on NO removal by CO and higher CO 2 capture capacity. MnO in Mn-doped CaO is an excellent and stable catalyst for NO removal by CO, which greatly mitigates the negative effect of the carbonation and the sintering of CaO on NO removal. The molar ratio of Mn/Ca = 3.5:100 is recommended. To achieve efficient NO removal and CO 2 capture in the carbonation stage of Mn-doped CaO, CO concentration in the range of 3000–4000 ppm and carbonation temperature of 650 °C are recommended. Under severe calcination conditions (950 °C and 80% CO 2), CO 2 capture and NO removal efficiencies reach 85% and 99% in 10 cycles, respectively. The periodic density functional theory calculations show that the catalytic active sites for NO removal by CO are changed from O-tops to Mn-tops in Mn-doped CaO, so the adsorptions of NO and CO 2 are greatly enhanced, promoting the subsequent simultaneous NO removal and CO 2 capture. Thus, Mn is a bi-functional additive to promote CO 2 capture by CaO and catalyze NO removal by CO. Mn-doped CaO seems promising for efficient simultaneous NO removal and CO 2 capture in the carbonation stage of the calcium looping process. [ABSTRACT FROM AUTHOR]

• CaO/Ca(OH) 2 heat storage is integrated with gasification of fuel for H 2 production. • Thermodynamic simulation of the integrated process is performed by Aspen Plus. • Integrated heat storage raises CO 2 capture by CaO to yield more H 2. • Energetic and exergetic performances are improved by integrated heat storage. A novel H 2 production process integrated CaO/Ca(OH) 2 heat storage and sorption enhanced staged gasification of biomass/coal based on calcium looping was proposed. The proposed process consists of four units such as co-pyrolysis of biomass and coal, char gasification with recycled CO 2 , sorption enhanced H 2 production and CaO/Ca(OH) 2 heat storage. CaO is used as CO 2 sorbent to enhance water gas shift reaction for H 2 production. Exhausted CaO experienced the multiple cycles from the sorption enhanced H 2 production unit is sent to the CaO/Ca(OH) 2 heat storage unit for the reactivation. Then the reactivated CaO is fed into the sorption enhanced H 2 production unit again for the next cycle. To evaluate the overall performance of the proposed process, the thermodynamic simulation was performed by using Aspen Plus. The results show that the integrated process is more beneficial for H 2 production compared with the normal sorption enhanced staged gasification process. Integrated CaO/Ca(OH) 2 heat storage unit reduces the amount of required CaO and promotes H 2 production, energy conversion efficiency and exergy efficiency of the normal process. The novel process operated at the optimum conditions achieves 17.4% higher H 2 yield than the normal process. At this condition, the corresponding H 2 yield of 1.08 Nm3/kg-fuel, energy conversion efficiency of 42.1%, exergy efficiency of 39.4% and CO 2 capture efficiency of 96.6% are achieved in the integrated process. Therefore, H 2 production process integrated CaO/Ca(OH) 2 heat storage and sorption enhanced staged gasification based on calcium looping appears promising. [ABSTRACT FROM AUTHOR]

Calcium looping uses CaO carbonation/calcination cycles to capture CO 2 , which is a promising carbon capture technology for coal-fired power plants. However, after CaO absorbs CO 2 to generate CaCO 3 , the catalytic performance of CaO for NO x reduction by CO decreases drastically. In this work, Mn-doped CaO was used to improve NO reduction by CO in carbonation of calcium looping and the mechanism of Mn-doped CaO on NO reduction by CO was determined by the density functional theory. The experimental results show that Mn doping not only improves the CO 2 capture capacity of CaO, but also increases NO reduction by CO. Density functional theory calculation results show that the Mn atom provides new adsorption sites for CO and NO on CaO surface, respectively. After addition of Mn, the values of co-adsorption energy for CO and NO on CaO surface increases by 1.5 and 6.9 times in the carbonation stage, respectively. In addition, the total energy barrier for the reaction of NO reduction by CO under the catalysis of CaO in carbonation stage decreases from 11.08 to 8.66 eV due to the addition of Mn. The negative effect of the carbonation of CaO on NO removal by CO is greatly mitigated by Mn. Therefore, NO removal by CO in the presence of Mn-doped CaO is significantly improved in the carbonation stage of the calcium looping. [Display omitted] • Effect of Mn-doped CaO on NO-CO reaction in calcium looping was studied by DFT. • Mn significantly catalyzes NO removal by CO in carbonation stage of calcium looping. • Mn changes reaction paths and the adsorption sites of NO and CO in presence of CaO. • Mn decreases energy barriers of NO reduction reaction by CO from 11.08 to 8.66 eV. [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]

• Effect of Cu on NO-char reaction in carbonator of calcium looping is studied by DFT. • Density of state result proves that Cu-char keeps stable NO removal ability in CaL. • Cu reduces energy barriers of NO reduction by char and CO. • Cu changes reaction paths and the order of absorbed reactants and released products. • Char shows a higher NO reduction activity than CO in the presence of Cu. Cu can improve NO removal efficiency of char and CO in carbonation stage of calcium looping, but the mechanism of NO reduction by char and CO in the presence of Cu has been rarely reported. In this work, the density functional theory was utilized to investigate the effect of Cu on NO reduction by both char and CO in carbonation stage of calcium looping for CO 2 capture. Density of state result proves that Cu decreases the possibility of char deactivation and retains stable promoting effect for NO reduction. Adsorption energies and structural parameters were used to determine adsorption sites of reactant molecules (CO, NO and O 2) and Cu atom on the basic configuration of zigzag graphite structure with six benzene rings. Adsorption energies of reactant molecules on char surface in the presence of Cu follow the order: CO < NO < O 2. Cu decreases energy barriers for NO reduction by char, NO reduction by CO and CO oxidation by O 2 from 31.73, 61.31, 113.86 kcal/mol to 19.99, 12.51, −11.68 kcal/mol, respectively. Accordingly, Gibbs free energies reduce from 1.14, −7.69, −66.02 kcal/mol to −39.95, −9.21, −68.10 kcal/mol, respectively. Therefore, Cu significantly promotes NO reduction by both char and CO as well as CO oxidization by O 2. Furthermore, char has more significant effects on the reduction of NO than CO in the presence of Cu. Thus, the effect of Cu on reaction paths is determined. [ABSTRACT FROM AUTHOR]

• CaO-based micrometre-sized tubular composite is yielded by template for heat storage. • Energy density of the composite is up to 2924 kJ/kg after 30 heat storage cycles. • The composite maintains porous and hollow structure during multiple cycles. • CaO-based micrometre-sized tubular composite has high sintering resistance. • The micrometre-sized tubular composite has large surface area and pore volume. To ameliorate the decay in heat storage performance of CaO-based materials with the number of CaO/CaCO 3 heat storage cycles, in this study, we used the template method to fabricate CaO-based micrometre-sized tubular composites containing CaO, Al 2 O 3 , and CeO 2 and analysed their performance at a high carbonation pressure. It was found that these composites, when doped with 2.5 wt% Al 2 O 3 and 1 wt% CeO 2 , exhibited an excellent heat storage performance. The heat storage capacity of the novel composites decreased by only 2% over 30 heat storage cycles, and their effective conversion and energy density were as high as 0.92 and 2924 kJ/kg, respectively, after 30 cycles. Morphological analysis of these composites indicated the presence of porous microtubules with diameters in the range of 5–10 μm. Ca 12 Al 14 O 33 and CeO 2 grains in the microtube walls effectively stabilised the structure during cyclic heat storage. Therefore, the novel composites have promising applications in CaO/CaCO 3 -based heat storage processes. [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]

• Simultaneous NO/CO 2 removal by Cu-modified biochar/CaO in calcium looping is proposed. • Cu strongly catalyzes NO removal by biochar and CO in carbonator of calcium looping. • CaO not only catalyzes NO removal by CO but also absorbs CO 2 in carbonator. • 90% NO and 80% CO 2 are removed by Cu-modified biochar/CaO. A novel method to realize the simultaneous NO/CO 2 removal by Cu-modified biochar and CaO in the carbonation stage of calcium looping was proposed. Cu-modified biochar was added in the carbonation stage as a NO reductant, meanwhile CaO was added as a CO 2 sorbent. The simultaneous NO/CO 2 removal performance of Cu-modified coconut shell char/CaO in the carbonation stage of calcium looping was investigated in a bubbling fluidized bed reactor. CO is generated by the oxidation of the coconut shell char in the presence of O 2 in the carbonation. Cu-modified biochar and the generated CO efficiently reduce NO. Cu in Cu-modified biochar shows a strong catalysis on NO reduction by both char and CO. Besides, Cu enhances the conversion of CO to CO 2 , which is absorbed by CaO. CaO also catalyzes NO removal by CO, but the catalytic effect gradually becomes weak as the carbonation proceeds. When Cu content in Cu-modified coconut shell char is 0.77%, mass ratio of Cu-modified biochar to CaO is 1:16 and O 2 concentration is 3%, NO removal and CO 2 capture efficiencies of Cu-modified biochar/CaO in the carbonator are 94% and 80% at 650 °C, respectively. To reach the same NO removal efficiency of 94% in the carbonator, the required addition amount of Cu-modified biochar is only approximately 20% of untreated biochar. [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]

• Simultaneous NO/CO 2 removal system using biochar/CaO in calcium looping is proposed. • Coconut shell char/limestone shows good simultaneous NO/CO 2 removal performance. • CaO shows a strong catalysis effect on NO reduction by CO produced by char oxidation. • Porous structure of biochar enhances NO reduction under calcium looping conditions. • NO removal and CO 2 capture efficiencies are above 97% and 80%, respectively. A novel simultaneous NO/CO 2 removal system using biochar and calcined limestone in the calcium looping process was proposed. Coconut shell char and calcined limestone were added into a carbonator in the calcium looping process as the NO reductant and CO 2 sorbent, respectively. The simultaneous NO/CO 2 removal performance of coconut shell char/calcined limestone in the calcium looping process was investigated in a bubbling fluidized bed reactor. NO and CO 2 in flue gases are effectively and simultaneously removed by coconut shell char/calcined limestone in the presence of O 2. O 2 plays an important role in NO removal by coconut shell char. The calcined limestone shows a strong catalytic effect on NO reduction by CO generated by the reaction of coconut shell char and O 2. The calcined limestone supports active sites for NO reduction by CO. High CO concentrations and high carbonation temperatures have positive effects on NO reduction by CO with calcined limestone catalysis. However, the catalytic effect of calcined limestone is weakened by its carbonation, which is promoted by the high temperature and additional CO 2 produced by the oxidation of char. The simultaneous NO removal and CO 2 capture efficiencies can reach above 97% and 80%, respectively. The porous structure of coconut shell char is an important factor in enhancing NO reduction with calcined limestone catalysis in the presence of O 2. [ABSTRACT FROM AUTHOR]

• A novel hollow microtubular Ca/Al sorbent was prepared by hard templating synthesis. • Absorbent cotton was a good hard template to fabricate efficient CO 2 sorbent. • Novel Ca/Al sorbent possesses excellent CO 2 uptake which is 0.58 g/g over 30 cycles. • Novel Ca/Al sorbent possesses high sintering resistance during multiple cycles. • CO 2 diffusion resistance in microtubes of novel Ca/Al sorbent is low. A novel hollow microtubular Ca/Al CO 2 sorbent was synthesized through a hard templating pathway using absorbent cotton as a template, limestone as a CaO precursor and aluminum nitrate as an Al 2 O 3 precursor. The synthetic Ca/Al sorbent consists of hollow porous microtubes with diameters ranging from 0.5 to 5 μm and replicates the biomorph of the absorbent cotton template well. When the mass ratio of CaO/Al 2 O 3 is 90:10, the hollow microtubular Ca/Al sorbent exhibits the highest CO 2 capture capacities during the cycles. The CO 2 capture capacities of the novel Ca/Al sorbent after 50 cycles under a mild calcination condition (850 °C in pure N 2) and a severe calcination condition (920 °C in 70% CO 2 /30% N 2) are respectively 0.55 and 0.38 g/g, which are 7.91 and 8.93 times greater than those of limestone. The main compositions of the walls of the microtubes in the novel Ca/Al sorbent are CaO and Ca 12 Al 14 O 33. Ca 12 Al 14 O 33 , as a good supporter, maintains the structural stability of the microtubes during the CO 2 capture cycles. CO 2 diffuses into the porous wall of the microtube in both directions (simultaneously into the outer and inner surfaces), so the CO 2 diffusion resistance in the microtubes is low. The calcium looping system using the novel Ca/Al sorbent shows much lower energy consumption of the calciner and lower cost, compared with using limestone. Thus, Ca-Al-T appears promising in the calcium looping. [ABSTRACT FROM AUTHOR]

Calcium looping is a promising technology to capture CO2 from the process of coal-fired power generation and gasification of coal/biomass for hydrogen production. The decay of CO2 capture activities of calcium-based sorbents is one of the main problems holding back the development of the technology. Taking carbide slag as a main raw material and Ca12Al14O33 as a support, highly active CO2 sorbents were prepared using the hydrothermal template method in this work. The effects of support ratio, cycle number, and reaction conditions were evaluated. The results show that Ca12Al14O33 generated effectively improves the cyclic stability of CO2 capture by synthetic sorbents. When the Al2O3 addition is 5%, or the Ca12Al14O33 content is 10%, the synthetic sorbent possesses the highest cyclic CO2 capture performance. Under harsh calcination conditions, the CO2 capture capacity of the synthetic sorbent after 30 cycles is 0.29 g/g, which is 80% higher than that of carbide slag. The superiority of the synthetic sorbent on the CO2 capture kinetics mainly reflects at the diffusion-controlled stage. The cumulative pore volume of the synthetic sorbent within the range of 10–100 nm is 2.4 times as high as that of calcined carbide slag. The structure of the synthetic sorbent reduces the CO2 diffusion resistance, and thus leads to better CO2 capture performance and reaction rate. [ABSTRACT FROM AUTHOR]

[Display omitted] • Reaction mechanism of calcium-looping CH 4 dry reforming was studied by DFT study. • Detailed reaction pathway of CaL-DRM process on Ni-CaCO 3 surface was determined. • The presence of CH 4 reduces a 0.737 eV in the energy barrier for CaCO 3 dissociation. • CH* and C* effectively activate the C-O bond of CO 3 *. The calcium-looping dry reforming of methane (CaL-DRM) process couples CO 2 capture and dry reforming of methane process using Ni/CaO dual-functional material. This process in-situ converts CO 2 captured by CaO through reacting with CH 4 , catalyzed by Ni, to produce syngas. However, researches on reaction mechanism for CaL-DRM are rare. In this work, the fundamental reaction mechanism of calcium-looping dry reforming of methane is elucidated by density functional theory (DFT) analysis. The energy barriers for elementary reactions involved in various potential pathways were investigated to determine the primary reaction pathway. The DFT analysis shows two possible reaction pathways for the CaL-DRM, with the CH* assisted CaCO 3 dissociation pathway being the more favorable one. Along this pathway, CH 4 undergoes three dehydrogenation steps to form CH*. Then, CH* reacts with the CaCO 3 * to produce CHO* and CaCO 2 *, accompanying the cleavage of one C-O bond in the carbonate. Afterwards, CHO* dissociates into CO* and H*. Finally, another C-O bond in CaCO 2 * breaks, generating CO* and CaO. Notably, CH 4 reduces the energy barrier for CaCO 3 dissociation from 3.47 to 2.733 eV. Moreover, the perturbation of electron clouds around O in CaCO 3 * in the presence of CH or C, as evidenced in electron density differential results, highlights the effective activation of C-O bond by CH or C, thereby promoting CaCO 3 decomposition. This work provides further support for the potential mechanism for in-situ CO 2 utilization achieved through CaL-DRM. [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]

[Display omitted] • Cu/Ce-modified CaO greatly enhances NO reduction by CO in CaL CO 2 capture process. • Cu/Ce interaction promotes oxygen vacancies and surface oxygen generation on CaO. • Cu/Ce modification improves the co-adsorption of CO/NO/CO 2 on CaO. • 99 % NO reduction at low molar ratio of CO/NO = 2 in CaL process is achieved. • 99 % CO conversion is achieved during NO reduction process. In the calcium looping CO 2 capture process, using CO in the flue gas to reduce NO at a high molar ratio of CO to NO (e.g., above 10:1) can achieve efficient removal of NO. However, the lots of unreacted CO causes secondary pollution. In this study, a novel Cu/Ce co-modified CaO was proposed for cyclic CO 2 capture and the catalytic NO reduction at a low CO:NO molar ratio (e.g., 2:1). The experimental findings indicate that Cu/Ce modification improves the performance of CaO in both catalytic NO reduction and carbonation. The Cu/Ce co-modified CaO contained the optimal Cu/Ce/Ca molar ratio of 1:5:100 possesses both NO removal and CO conversion efficiencies of above 99 %, along with CO 2 removal efficiency exceeding 88%. Microscopic analysis determines that the electron transfer between Cu and Ce facilitates the conversion of lattice oxygen into surface oxygen, mitigating the competition between CO 2 and CO for surface oxygen. Furthermore, density functional theory calculations demonstrate that CO/NO/CO 2 exhibits non-competitive adsorption behavior on Cu/Ce co-modified CaO. Consequently, Cu/Ce-modified CaO possesses superior catalysis for efficient NO reduction by CO at low CO:NO molar ratio in calcium looping process. [ABSTRACT FROM AUTHOR]