In Situ Synthesis of a Hydrochar-Functionalized CaO Composite for High-Temperature CO₂ Capture

The calcium looping (CaL) process is an emerging technology for the capture of CO2 from flue gases. However, the rapid loss of the CO2 capture capacity in CaO-based sorbents limits their industrial application. Herein, we presented a novel in situ hydrochar functionalization strategy that integrates cellulose hydrothermal liquefaction and in situ modification of CaO-based sorbents to improve the CO2 capture capacity. The in situ-functionalized eggshell-based sorbent demonstrates a CO2 capture capacity of 0.253 g/g after 33 cycles under the realistic application environment (harsh cycles), which is 377% higher than that of the raw counterparts and significantly superior to the sorbents synthesized by the traditional torrefaction condensate modification routes. Moreover, 8 MPa and 275 °C might be the optimum conditions in this in situ hydrochar functionalization strategy to synthesize the effective sorbent. During hydrothermal liquefaction, cellulose generates abundant organic acids, which react and chelate with CaO, altering the structure of the sorbents. As a result of the neutralization reaction between CaO and organic acids, the reaction favors liquefaction over carbonization in accordance with Le Chatelier's principle. This process leads to the formation of mesopores during the combustion of the organic complex, improving the surface area of the sorbent. The in situ hydrochar functionalization strategy not only increases the porosity and specific surface area of the materials but also alters the number of alkaline sites, thus enhancing their CO2 capture capacity. The in situ hydrochar functionalization presents a promising approach for synthesizing effective sorbents for CO2 capture.