| Food waste(FW)requires pretreatment to meet the feed requirements in the disposal process such as incineration or anaerobic digestion,due to the characteristics of high moisture content,high organic matter(such as carbohydrates,proteins and lipids),and complex and changeable composition.Hydrothermal pretreatment has the advantages of skipping pre-drying and strong adaptability to complex feedstocks,which can efficiently realize the reduction and homogenization of food waste,and obtain high value-added products.The carbohydrates,proteins and lipids in food waste are the key to determining the properties of hydrothermal products and affect the application potential of hydrochar.Based on current research,this article conducts research from the three aspects of hydrothermal product characteristics,reaction process and hydrochar utilization.In response to scientific issues(1)The correlation mechanism between the typical components of food waste and the properties of hydrothermal products is not clear;(2)How to reveal the hydrothermal reaction path and optimizate the reaction kinetic model;(3)The influence of feedstock components on the hydrochar utilization performance is still unclear,the following works were carried out.Three types of representative food wastes(9 kinds),i.e.,high carbohydrates FW,high protein FW and high lipids FW,were used for hydrothermal experiments.the correlation between the composition and the properties of the product was revealed,by analyzing the chemical structure of hydrochar and liquid composition.The research results showed that when the carbohydrates(Carbs)content of FW was less than 20%,no hydrochar would be produced.When the Carbs content was greater than 50%,the yield of hydrochar was 20-35%,the aqueous products were mainly furans such as 5-HMF and furfural which accounted for more than 47%.Protein(Pro)improved the yield of hydrochar,and added oxygen and nitrogen heterocyclic structure to hydrochar.When the Pro/Carbs value of FW was 0.3-2,the N content of hydrochar(240 ℃)was 5-8%;and the N content increased with the increase of the Pro/Carbs value.When the Pro content is greater than 40%,the aqueous products were mainly piperazine dione derivatives(26-67%).Lipids(Lip)increased the structure of aliphatic CH of hydrochar.the yield of hydrochar increases as the Lip/Carbs value decreases.When the Lip content is greater than 50%,the fatty acid and amide content in the hydrothermal oil phase product accounts for about 70%,and the concentration increases as the temperature rises.Through the hydrothermal experiment of the model compounds and the calculation of reaction kinetics,the interactive reaction path and reaction rate of the food waste components were clarified.The results showed that carbohydrates were hydrolyzed and dehydrated to produce furans,and then aldol condenses to form polyfuran hydrochar aromatic skeleton.During the hydrothermal constant temperature stage,the increase of hydrochar was mainly due to the Maillard reaction between amino acids and reducing sugars.Amino compounds and reducing sugars undergo nitrogen substitution reactions to generate aminodeoxyketoses,followed by deamination to generate 5-HMF,which increases the concentration of 5-HMF.Amino compounds and furfural formed aldimines and ketimines,and the piperazine dione derivatives produced by the condensation of amino acids were combined with aromatic skeletons to increase the pyrazine and pyridine structures of hydrochar.The fatty acid produced by lipids undergoes amidation reaction with amino compounds,which competes with the Maillard reaction,and the reaction rate constant is 1.4 times of Maillard reaction.Fatty acids and amide substances will adhere to the hydrchar nucleus,inhibit the formation of O/N heterocyclic structure of hydrochar,and enhance its aliphatic CH structure and hydrophobicity.The influences of the components on the utilization performance of hydrothermal carbon were discussed through analyzing the combustion,pyrolysis,and adsorption performance of hydrochar.The research results showed that when the Carbs content of FW was greater than 50%,i.e.,high-carbohydrate FW hydrochar,the ignition temperature(about265 ℃)was lower than that of high-protein and lipids FW hydrochar;and it had a higher calorific value(27-28 MJ/kg),higher activation energy in the first stage of combustion(78-147 k J/mol)and lower burn-out temperature.Therefore,high-carbohydrate FW hydrochar was more suitable as a solid fuel.The lipid content of feedstock increases,the hydrocarbon gas produced by pyrolysis of hydrochar increased.Therefore,High-lipid hydrochar is more suitable for pyrolysis utilization.The the adsorption performance test results of hydrochar-based porous carbon showed that the high-carbohydrate FW hydrochar-porous carbon had a richer microporous structure,with a microporous pore volume accounting for about 80%,which had a strong adsorption capacity for organic pollutants.The high-protein FW hydrochar had a higher nitrogen element content,which promoted the subsequent activation and expansion process,so that the specific surface area was increased up to 2656 m~2/g,and the mesoporous pore volume increased by 2-8 times.the increase of lipid content would slightly reduce the micropore content of hydrochar-porous carbon.The high-carbohydrate and protein FW hydrochar is more suitable as the precursor of porous carbon material.In summary,this article clearly clarified the reaction mechanism of hydrothermal pretreatment based on the influence of feedstock constitution on product performance.This study is of great significance to the hydrothermal pretreatment of food waste.It provided theoretical guidance for the performance optimization of hydrothermal products. |
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