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Aluminium-stabilized calcium-based materials are currently an excellent solution for capturing CO 2 directly from industrial flue gas. However, there is still no report on the technology of low-cost and large-scale assembly of Ca 3 Al 2 O 6 and CaO. In this study, we innovatively put forward a silicon-assisted strategy to facilely synthesize high-purity Ca 3 Al 2 O 6 and evaluate its cyclic performance for stabilizing uptake CO 2 after assembly. With regard to the Si/Al ratio of 0.2, Ca 3 Al 2 O 6 was prepared selectively among 850–900 °C, with only 5.2% CaO residually. The introduction of silicon inhibited the formation of Ca 5 Al 6 O 14 by-products and the mechanism of silicon at the calcium-aluminum interface was also explained by systematic characterization. The Ca 3 Al 2 O 6 modified sorbent with simple assembly showed extremely high cycle stability improvement compared with the pure precipitated calcium carbonate derived CaO. These works provide technical guidance for the industrial scale-up of the rational design of Al-stabilized calcium-based adsorption materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • An innovative manner was used in the co-pyrolysis for more CO 2 consumption. • CP CO2 evaluation was innovatively constructed and realized in the CO 2 -looping system. • More CO 2 entered into pyrolysis products by Ca-based additives for improving CP CO2. • CaO not only improved CP CO2 by 21.2% but also reduced the total HMs at 500 ℃. • Mechanism for CO 2 assisted Ca-based on CP CO2 with temperature was proposed. In this study, the CO 2 -looping system was firstly attempted to build for exploring the effect of calcium-based (Ca-based) additives on pyrolytic characteristics and products in co-pyrolysis of sewage sludge (SS) and biomass. Then, an innovation method for the consumption of CO 2 (CP CO2) evaluation was proposed and carried out in the CO 2 -looping system, in which it was evaluated based on the syngas volume and CO 2 proportion. The results revealed that the addition of CaO and Ca(OH) 2 could significantly improve CP CO2 by 21.2% in the co-pyrolysis of 500 ℃, while the CaCO 3 addition decreased the CP CO2 by 1.0%. Besides, the co-pyrolysis temperature more than 700 ℃ would play more significant role on the CP CO2 than the addition of Ca-based additives. Then the effect of Ca-based types on biochar characteristics was also identified accompanied by the bio-oil composition. Results showed that the CaO and Ca(OH) 2 additives not only increased biochar mass and pH, but also enhanced the dissolved organic carbon content and the variety of dissolved organic matter, and reduced the total HMs content. Meanwhile, it was 15 types of components that was identified in bio-oil by NIST mass spectral library, found that the intensity of heptadecanenitrile and estra-1,3,5(10)-trien-17 beta-ol was influenced by Ca-based additives. Moreover, the CaO and Ca(OH) 2 played important roles for improving the CP CO2 in the CO 2 -looping system, which was further verified by Fact Sage. Finally, the mechanism for the Ca-based additives on CP CO2 was proposed at different co-pyrolysis temperatures. These findings can widen paths for biochar application and development of CO 2 capture materials. [ABSTRACT FROM AUTHOR]

Amine-functionalized adsorbents have become a hot topic in research on CO 2 separation from biogas. However, developing superior adsorbents with high adsorption capacity and stable cyclic stability under a CO 2 regeneration atmosphere remains difficult. In this study, azeotropic distillation, a green, recyclable method, was used to expand the pores of the nano-Al 2 O 3 support. The pore volume of pore-expanded nano-Al 2 O 3 reached 1.62 cm3 g−1, and the derived amine-functionalized nano-Al 2 O 3 adsorbent ('60%PEI@AD-Al 2 O 3 ') possessed a superior CO 2 uptake of 199.4 mg·g−1 adsorbent with rapid adsorption kinetics. '60%PEI@AD-Al 2 O 3 ' showed favourable cyclic stability under CO 2 regeneration atmosphere with a final CO 2 uptake of 133.8 mg·g−1 adsorbent after 50 cycles, which possessed a competitive CO 2 uptake compared with the reported amine-functionalized adsorbents with anti-urea stability. This superior performance is attributed to the large pore volume of the pore-expanded nano-Al 2 O 3 support and the anti-urea stability for '60%PEI@AD-Al 2 O 3 ', which does not require active amine modification and avoids diluting the active amine content. Furthermore, the effect of water vapour in biogas on adsorption performance was studied. Water vapour could significantly promote CO 2 adsorption at low temperatures (<90 °C), and it was accompanied by strong water vapour adsorption, which would potentially increase the regeneration energy consumption. Additionally, water vapour could effectively inhibit the formation of urea compounds, further improving the cyclic stability. Because of the facile, scalable preparation method, the synthesised amine-functionalized nano-Al 2 O 3 adsorbent has broad application prospects for CO 2 separation. [Display omitted] • Nano-Al 2 O 3 was pore-expanded via n-butanol solvent to a pore volume of 1.62 cm3 g−1. • Polyethylenimine was impregnated on nano-Al 2 O 3 for CO 2 separation from biogas. • '60%PEI@AD-Al 2 O 3 ' adsorbent shows superior CO 2 uptake of 199.4 mg·g−1 adsorbent. • '60%PEI@AD-Al 2 O 3 ' adsorbent possesses favourable cyclic and anti-urea stability. • Water vapour can inhibit the urea formation and improve the cyclic stability. [ABSTRACT FROM AUTHOR]

• Polyethylenimine is impregnated on porous nano-Al 2 O 3 derived from coal fly ash. • The PEI@Al 2 O 3 adsorbent is efficient for biogas upgrading via adsorbing CO 2. • The PEI@Al 2 O 3 adsorbent is extremely stable under realistic regeneration condition. • Nano-Al 2 O 3 support enhances cyclic stability via resisting urea linkages formation. • PEI impregnation process is not restricted by the pore size of nano-Al 2 O 3 support. Cyclic CO 2 adsorption via amine-functional adsorbents is widely accepted as an efficient route for biogas upgrading and CO 2 mitigation. However, the fast deactivation and insufficient CO 2 uptake are the major drawbacks of this technology. Herein, we developed a green and facile approach, including the recycling of porous nano-Al 2 O 3 support from coal fly ash (CFA) and the impregnation of PEI onto the nano-Al 2 O 3 support with large pore volume, to enhance both the CO 2 adsorption capacity and the cyclic stability of amine-functional adsorbents. The as-synthesized 50%PEI@Al 2 O 3 adsorbent possessed a CO 2 adsorption capacity of 136 mg g adsorbent −1 and showed superior cyclic stability with merely a negligible decay of <0.3% after 10 cycles. Even under the realistic regeneration conditions (at 165 °C in pure CO 2), it still maintained extremely stable adsorption capacity with a final CO 2 uptake of 111 mg g adsorbent −1, markedly increased by 5.5-fold than that of traditional 40%PEI@SiO 2 adsorbent. It was concluded that the nano-Al 2 O 3 support was benefical for the resistance of urea linkages formation during the regeneration stage, which would inactivate the functional amidogen. This strategy significantly enhanced the cyclic CO 2 uptakes of amine-functional adsorbents through the recycling of CFA-derived nano-Al 2 O 3 support, and is thus a green technology for practical biogas upgrading via CO 2 adsorption. [ABSTRACT FROM AUTHOR]