CaCO3/CaO materials possess the advantages of low cost, high energy storage density, and working temperature, which offer these materials the potential to be used in thermochemical energy storage systems for concentrated solar power plants. However, CaCO3/CaO materials possess poor antisintering and optical absorption abilities, largely limiting their practicability for direct solar utilization. In this study, binary ion doping of Fe/Mn and Zr-based stabilizer incorporation were simultaneously conducted to improve the cyclic thermal energy storage/release performance of CaCO3/CaO materials. The spectral absorbance of synthetic CaO-based composites (ranging from 77.8% to 84.0%) doped with binary ions of Fe/Mn is greatly increased in comparison to that of pure CaO (similar to 12.2%) due to the generation of black Ca2Fe2O s and Ca4Mn3O10. The cyclic thermal energy storage/release performances of synthetic CaO-based composites were comparatively investigated under two thermal energy storage modes (CSP-N-2 and CSP-CO2). The Zr-doped, CaO-based composites exhibit a cycling stability superior to those of Zr-free CaO-based composites due to the generated inert CaZrO3 with desirable antisintering ability, and the superiority is more prominent under CSP-CO2 mode. After 50 cycles, the CaO-based composite with a molar ratio of Ca:Zr = 100:6.7 exhibits a remarkably stable energy release density of 1.02 MJ/kg under CSP-CO2 mode, retaining 88.9% of its initial energy release density.

Sawdust wastes were used as the low-cost feedstock to prepare porous carbons by KOH activation, and the preparation parameters were optimized via orthogonal experiment. Four main activation parameters (i.e., sawdust type, KOH/sawdust ratio, activation temperature and activation time) were investigated. It is found that activation temperature and KOH/sawdust ratio are the two most important parameters that affect the porous texture properties (specific surface area and micropore volume) of sawdust wastes-derived porous carbons. The optimal sawdust wastes-derived porous carbon possesses a high specific surface area of 1601 m2/g and a micropore volume of 0.75 cm3/g, therefore exhibiting a desirable CO2 adsorption capacity of 4.25 mmol/g at 0 degrees C under ambient pressure. Comparatively, the sawdust wastes-derived porous carbon extrudates exhibit the inferior CO2 adsorption capacity, dropping to 2.85-3.03 mmol/g. It is mainly attributed to the extrusion treatment and binder addition causing the collapse and blockage of the original porous structure, consequently increasing the CO2 diffusion and adsorption resistance. However, the sawdust wastes-derived porous carbon extrudates possess the advantages of null pressure build-up, improved bulk density and high compress strength (7.3-12.3 MPa) considering the real application point of view.

Ca-rich solid waste, carbide slag, was adopted to prepare the CaO-based sorbent for CO2 uptake at elevated temperature. To alleviate the elutriation issue of CaO-based sorbent from the calcium looping system, the granulation of carbide slag was conducted via an extrusion-spheronization method. Calcium aluminate cement possessing excellent refractory property was selected as the binder. A range of cement-bound carbide slag pellets were prepared with the active CaO content varying from 25 to 70 wt %. The incorporation of calcium aluminate cement significantly decreases the CO2 capture capacities of the carbide slag pellets due to the irreversible consumption of active CaO by the silicates or aluminates within the cement. However, the average compression strengths of the fresh (3.29-9.82 MPa) and calcined (0.47-2.46 MPa) cement-bound carbide slag pellets gradually increase with the increase of cement addition. Additionally, the addition of rice husk can improve the CO2 capture performance of the cement-bound carbide slag pellets during the fast CO2 capture stage because of the generation of in stiu pores and cavities within the sorbent pellets. Although the cyclic CO2 capture performance of the cement-bound carbide slag pellets degrades under oxy-fuel calcination condition, to some extent, the low-cost and easily scaled-up preparation route may offsets the increased CO2 avoid cost because of the loss-in-capacity.

Calcium looping is a promising postcombustion CO2 capture technology due to its low cost and widespread applicability. However, CaO-based sorbents are prone to encounter severe sintering and elutriation during practical carbonation/calcination cycles. To overcome the above issues, core-in-shell CaO-based pellets composed of a highly reactive CaO-based core and a hard cement-based outer shell were prepared. The highly reactive core contains 80 wt % Ca(OH)(2) and 20 wt % cellulose, which was prepared via an extrusion-spheronization method. The cement-based outer shells were prepared via an approach of coating, and different amounts of cellulose (varying from 0 to 40 wt %) were added as a pore-forming template. It is found that the mechanical properties of the fresh, core-in-shell, cellulose-templated CaO-based pellets are gradually improved with the increased addition ratio of cellulose in the outer shell. It is mainly attributed to the adequately dispersed cellulose fibers reinforcing the cement-based outer shell. Although the high-temperature calcination causes the internal structure of the CaO-based pellets to become loose, they still exhibit relatively desirable compression strengths (0.95-1.80 MPa). Moreover, the porous outer shell contributes to promoting the accessibility of CO2 to the highly reactive core pellet, consequently obtaining superior CO2 capture performance. After 15 cycles, the core-in-shell, cellulose-templated CaO-based pellets containing 40 wt % of cellulose in the outer shell exhibit the highest CO2 capture capacity of 0.144 g/g, which is nearly 6.8 times that of the core-in-shell pellets with pure cement shell.

Granulation is an effective method to mitigate the issue of CaO-based sorbent elutriation in the calcium looping process. To further enhance CO2 capture performance of CaO-based pellets, pore-forming templates are usually added. In this work, rice straw, wheat straw, and corn stalk were adopted as the pore-forming templates. The effect of different regeneration conditions (mild, moderately severe, and severe calcination conditions) on agricultural waste-templated, CaO-based pellets was comparatively investigated. Severe calcination conditions remarkably cause deactivation of the agricultural waste-templated, CaO-based pellets due to the accelerated sintering. Additionally, the existence of alkaline elements within agricultural wastes results in poor CO2 capture performance for agricultural waste-templated, CaO-based pellets. The pretreatment of water- and acid-washing can remove most of the alkaline elements within raw agricultural wastes. The CaO-based pellets added with 10 wt % of acid-washed rice straw exhibit the highest cumulative CO2 capture capacity of 2.074 g/g during 15 cycles, which is over 2.3 times that of those pellets added with 10 wt % raw rice straw. Therefore, water-washing and acid-washing are potential approaches to mitigate the adverse effect of alkaline elements within agricultural wastes, which consequently improves the CO2 capture performance for the agricultural waste-templated, CaO-based pellets.

Low-cost and easily scaled-up acidification is a potential synthesis approach to produce CaO-based composites derived from steel slag for thermal energy storage/release via multicyclic calcination/carbonation. Acid concentration, acidification temperature, and acidification duration were comparatively investigated. The cyclic thermal energy storage/release properties of such composites are closely related to the acidification parameters, mainly attributed to the differences in the composition of the synthetic composites under different acidification conditions. The synergistic effect of the increased Ca and Mg content as well as the reduced content of Si and Fe within the composites contributes to improve their cyclic thermal energy storage/release performance.