Reaction characteristics of catalyzed Chlorella hydrothermal carbonization to co-produce functional carbon materials
Journal Title: Energy Environmental Protection - Year 2024, Vol 38, Issue 2
Abstract
Microalgae carbonization technology has attracted wide attention because of no requirement for drying high moisture feedstock, green reaction solvent, and relatively mild reaction conditions, which can co-produce functionalized carbon materials such as carbon dots (CDs) and hydrochar (HC). It is necessary to add suitable catalysts in the hydrothermal process, as the carbon materials prepared by microalgae hydrothermal carbonization currently show low yield and mediocre performance. Therefore, the effects of H_2SO_4, CH_3COOH, NaOH, and Na_2CO_3 as catalysts on the preparation of CDs and HC from the hydrothermal carbonization of Chlorella pyrenoidosa (CP) were explored, and a kinetic model for the generation of multiphase products during hydrothermal carbonization was established by lumped parameter method. The results showed that all four catalysts could increase the CDs yield, and the addition of NaOH could increase the CDs yield by about 73.3%. The CDs size became smaller, and the average particle size was only 1 nm with the addition of an acid catalyst, while the CDs size did not change significantly, and the particle size was concentrated at 3.0~4.0 nm under the condition of a basic catalyst. The CH_3COOH catalyzed system produced CDs with a fluorescence quantum yield of 7.88% when CP was hydrothermally treated at 230 ℃ for 100 min. The strongest emission fluorescence with a wavelength of 420 nm was emitted under the excitation of 340 nm. Elemental analysis showed that all four catalysts decreased the N content in CDs, and all of them increased the HHV (High Heating Value) of hydrochars. The improvement effect of HHV from strong to weak was H_2SO_4>CH_3COOH>NaOH>Na_2CO_3. Thermogravimetric analysis showed that the conversion of microalgal carbohydrates was faster than that of proteins during the hydrothermal carbonization process. The addition of CH_3COOH promoted the conversion of microalgal proteins, while the alkaline catalyst was not conducive to the conversion of carbohydrates, and these four catalysts had no noticeable effect on the conversion of microalgal lipids. The kinetic analysis showed that the main reactions in the CH_3COOH catalyzed microalgae hydrothermal carbonization process included the production of hydrochars via CDs polymerization, the precipitation of bio-oil via microalgae degradation, and the generating of other phase products via microalgae degradation. The microalgal CDs were mainly generated by the polymerization of small molecules in other phase products and the direct cracking of macromolecules in microalgae. The hydrolysis of CDs was challenging to occur, and it was difficult to reverse the hydrolyzed microalgal bio-oil at 230 ℃ in the presence of CH_3COOH.
Authors and Affiliations
ZHOU Qingming|Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, China, Institute of Engineering Thermophysics, Chongqing University, China, ZHANG Jingmiao*|Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, China, Institute of Engineering Thermophysics, Chongqing University, China, ZHU Xianqing|Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, China, Institute of Engineering Thermophysics, Chongqing University, China, HUANG Yun|Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, China, Institute of Engineering Thermophysics, Chongqing University, China, XIA Ao*|Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, China, Institute of Engineering Thermophysics, Chongqing University, China, ZHU Xun|Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, China, Institute of Engineering Thermophysics, Chongqing University, China, LIAO Qiang|Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, China, Institute of Engineering Thermophysics, Chongqing University, China
Keywords
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- EP ID EP737169
- DOI 10.20078/j.eep.20240310
- Views 7
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