| Biomass,a wide range of sources and a large amount of production,is regarded as a renewable carbon neutralization resource.Hydrothermal,is a method that has the advantages of low energy consumption and a high product conversion rate in the production of biochar from waste biomass.However,the low yield of solid phase products(hydrochar)and the high treatment cost of aqueous phase products(hydrothermal aqueous phase)in traditional hydrothermal carbonization make the actual operation of the hydrothermal carbonization process not ideal.For this study,two processes,named “aqueous phase recirculation” and “co-carbonization”,were selected to improve the traditional hydrothermal carbonization process.Aqueous phase circulation refers to reusing the aqueous phase product from the last hydrothermal reaction as the next hydrothermal reaction solvent.This process can significantly enhance the value of hydrothermal carbonization wastewater and reduce the cost of the hydrothermal carbonization process.Co-carbonization refers to the use of two or more raw materials for hydrothermal carbonization.Co-carbonization can utilize the synergistic effect of different raw materials in the hydrothermal process,thereby significantly promoting the formation of hydrothermal carbon.However,in the current research,the mechanism of action of the two on hydrothermal carbonization is unclear,and there is a lack of research on improving hydrothermal carbonization by combining the two processes.Therefore,this thesis aims to preliminarily evaluate the impact and feasibility of the combination of the two processes on hydrochar production,and study the mechanism of aqueous phase cycling and co-carbonization on hydrothermal carbon formation.In this study,three model compounds,namely wheat straw cellulose(cellulose,Ce),xylan(hemicellulose,He),and soya-protein(protein,Pr)were processed in four sets of experiments with different cycle times,different mixing combinations,different residence times,and different aqueous phase crossvalidation.The physicochemical properties of hydrochar were mainly discussed by elemental composition analysis,X-ray photoelectron spectroscopy(XPS)analysis,Fourier transform infrared spectroscopy(FTIR)analysis,and Pyrolysis gas chromatography-mass spectrometry(Py-GC-MS)analysis,The physicochemical properties of the aqueous phase are mainly analyzed by water quality analysis(including p H,chemical oxygen demand,total organic carbon,total nitrogen,and ammonia nitrogen)and silane derivatization GC-MS.The elemental analysis results show that both co-carbonization and aqueous phase cycling promote the formation of hydrothermal carbon and the removal of oxygen,and the combination of the two enhances these effects.In addition,XPS and PyGC-MS results show that the hydrothermal carbon of cellulose/hemicellulose is mainly composed of oxygen-containing compounds,such as furan and furfural;The hydrochar of protein is mainly composed of nitrogen-containing compounds,such as amino acids and nitrogen heterocycles.Subsequently,based on the results of aqueous phase GC-MS analysis and the above characterization results,the formation mechanism of hydrochar was proposed.In the process of cocarbonization and aqueous phase recycling,the main mechanisms that promote the formation of hydrochar are the Maillard reaction between amino acids and sugars,the catalytic action of acids in the recycled aqueous phase(mainly for the Maillard reaction),and the co-carbonization of compounds accumulated in the aqueous phase through polymerization or condensation reactions.In terms of the overall results,co-carbonization combined with aqueous phase recirculation has significant effects on improving the yield and quality of hydrochar for traditional carbonization processes,which means strong feasibility and potential for further development. |
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