Study On The Formation Mechanism Of Hydrochar In The Recycling Process Of Biomass Hydrothermal Carbonization Wastewater

Hydrothermal carbonation is a thermochemical conversion technology used to upgrade biomass for high-grade fuel.However,this technology yields large amounts of organic process wastewater,which must be properly disposed or reused.In this paper,process wastewater was mixed with biomass and reused for hydrothermal carbonization experiments,i.e.,organic process wastewater was recycled.The mechanism of hydrochar formation during process wastewater recycling was studied.Cornstalk was selected as a representative of the biomass,and the process wastewater obtained by hydrothermal carbonization was recycled.The effects of process wastewater recycling on the fuel characteristics of hydrochar were studied by measuring the chemical composition,equilibrium moisture content,chemical structure and functional groups,distribution of carbon-containing functional groups,and microscopic morphology.It was found that the fuel quality of hydrochar was significantly improved after four times of process wastewater recycling.Compared with the hydrochar obtained without wastewater recycling,the higher heating value generation of hydrochar increased from 22.67 MJ/kg to23.31 MJ/kg,the mass yield increased from 55.88%to 62.16%,the equilibrium moisture content decreased from 76.45%to 71.11%,and the CH_X,C=C,C-C functional groups contents increased significantly.The microscopic morphology showed that the hydrochar was composed of matrix and microspheres.The matrix was formed from unhydrolyzed biomass through solid-solid conversion(intramolecular condensation,dehydration and decarboxylation),and microspheres were formed from biomass through hydrolysis,dehydration,decarboxylation,polymerization and aromatization.Based on the changes in the types and relative contents of organic matter in the process wastewater,the chemical reaction pathways of cornstalk during hydrothermal carbonization was established,and the effect of process wastewater recycling on the chemical reaction pathways was analyzed.It was found that organic acids in wastewater promoted the hydrolysis and solid-solid conversion of cornstalk.Microsphere,as an important part of hydrochar,has higher heating value and energy density.The process wastewater was subjected to hydrothermal carbonization experiments to study the growth mechanism of coke microspheres formed by the polymerization of dissolved components in the aqueous phase.During the initial 2 h of hydrothermal carbonization,the microspheres grew rapidly and the average particle size of the microspheres reached 5.97μm.The formation of microspheres by organic matter in the process wastewater included two stages.The first stage was the formation of micronuclei:dissolved components in the process wastewater(e.g.furfural,5-hydroxymethylfurfural,furans,aldehydes)generated aromatized molecules through condensation reactions,and these molecules formed aromatic clusters through aromatization.When the concentration of aromatic clusters reaches a critical value,they were deeply polymerized to form solid-phase micronuclei.The second stage was the growth stage of the solid-phase micronuclei:the organics(furans,phenols,ketones)in the wastewater were connected to the reactive oxygen-containing groups(hydroxyl,carbonyl,and carboxyl)on the surface of the micronuclei,and the micronuclei gradually grew into microspheres by this diffusion connection method.The hydroxyl and carboxyl groups inside the micronuclei increased their carbonization level through intra/intermolecular dehydration and decarboxylation.The study in this paper established a theoretical basis for revealing the effects of process wastewater recycling on the fuel characteristics and formation mechanism of hydrochar.