| Food waste represents one of typical biowastes,which is rich in organic matter and moisture.However,current research mainly focuses on the anaerobic digestion,ethanol fermentation,and other biological treatment technologies for biogas and bioethanol production,and etc.,and the cost is high in the early stage.In recent years,hydrothermal carbonization technology has been generally applied to the conversion of food waste into solid fuel.The formation mechanism of hydrochar,the clean properties?distribution and transformation of nitrogen,sulfur and chlorine?,and the pelletization methods are critical to the preparation of clean solid fuel,which are not well studied.Based on current research,this study carries out research work following these aspects.Firstly,the composition and structure characteristics of hydrochar prepared from food waste by hydrothermal carbonization were determined.The carbonization mechanism of food waste was investigated by studying the surface functional grou ps and main carbon structure of hydrochar coupled with the components of liquid products.The results showed that the carbonization degree of biochar was close to lignite and bituminous coal.An increase of temperature could improve the carbonization degree of food waste and increased the HHVs of hydrochar to28.73?31.73 MJ/kg.The functional groups of-OH,-NH and C=O in food waste were sharply weakened by dehydration and decarboxylation at high hydrothermal carbonization temperature,while the C=C was enhanced which resulted in the increase of aromatic structure of biochar at high temperature,and poly-furan and aromatic structure were the dominant carbon structures.In addition,hydrochar exhibited cross-linked microspheres structure,and the size and degree of cross-linking were further increased at higher temperature.The results also suggested that the hydrolysis and dehydration of carbohydrates?5-HMF and furfural?were the main intermediates for hydrochar formation,and the heterogeneous pyrolysis-like process was another pathway of hydrochar formation from hydrothermal carbonization of food waste.Secondly,in view of the lack of study on the preparation of clean fuel by hydrothermal carbonization of food waste,the distribution and transformation mechanism of nitrogen in biochar,bio-oil,and aqueous product during hydrothermal carbonization were explored.The results indicated that increasing hydrothermal carbonization temperature reduced the total nitrogen ratio in hydrochar from 52.67%to 35.01%,and the total nitrogen content in bio-oil was gradually increased to approximately 30%.The aqueous product mainly composed of organic nitrogen,and the deamination of protein and amino acids was enhanced due to the conversion into more ammonia nitrogen at high temperature.Amines and N-heterocyclic compounds produced by Maillard reaction or from protein?or amino acids?pyrolysis were the main N-containing compounds in bio-oil,while increasing hydrothermal carbonization temperature could led more amine and pyridine components formation.After hydrothermal carbonization,inorganic nitrogen in biochar was sharply decreased,while pyridine-N was formed in hydrochar for the first time due to the cyclization of amines.Meanwhile,result showed that high tempera ture hydrothermal carbonization could enhance the incorporation of more nitrogen into hydrochar structure as more stable chemical forms,Pyridine-N and Quaternary-N.Thirdly,the removal behavior and mechanism of sulfur and chlorine,two key elements,during hydrothermal carbonization of food waste were studied.Additionally,the combustion characteristics and kinetics of hydrochar were further analyzed,and the release pollutants were analyzed.The results showed that hydrothermal carbonization could increase the removal efficiency of sulfur and chlorine for hydrochar,and the removal efficiency reached the maximum value at 260oC?82.50%and 95.80%,respectively?.Compared with food waste,the content of Thiophene-S in hydrochar decreased by 20.30%,while Sulfoxide-S increased by 10.20%.Increasing hydrothermal carbonization intensity increased the content of Sulfate-S in hydrochar,and organic sulfide was converted into inorganic sulfur.However,the chlorine of hydrochar and food waste were inorganic form.Increasing the carbonization degree of hydrochar led to a decrease of the volatile combustion peak as well as the ignition temperature.The releases of HCl,SO2 and NO were recognized formed mainly in the combustion stage of volatile matter,and the highest release amounts of HCl and SO 2were reduced by about 60%and 55%,respectively.However,the release amount of NO was increased due to the increase of nitrogen content and stable chemical forms of nitrogen in the hydrochar prepared by hydrothermal carbonization temperature above220 oC.Additionally,in order to facilitate the storage,transportation and application of hydrochar fuel,hydrochar was pelletized to improve the physical,chemical,and combustion properties.In this study,molasses was used to assist the pelletization of hydrochar under low pressure.The results showed that molasses hydrochar fuel increased the H/C ratio,while molasses/CaO hydrochar fuel could increase O/C ratio.The“solid bridge”structure was formed due to the recrystallization process of molasses made hydrochar form stable structure.The compressive strength of hydrochar was higher than that of hydrochar pellets by 1 MPa,while the addition of CaO/molasses could increase by 1.5 MPa.Molasses reduced the combustion range of hydrochar and decreased the ignition temperature of the fuel,which was contrary with molasses/CaO derived fuel pellets.Finally,in order to study the preparation of solid fuel by hydrochar pelletization under high pressure,hydrochar from hydrothermal carbonization of food waste and sawdust blend was pelletized under high pressure process since the lignocellulose component provideed mechamical support for the hydrochar pellet.The physicochemical properties,compression energy consumption,storage performance,and combustion performance of the fuel were systematically explored.The results showed that increasing the blend ratio of food waste could increase the carbonization degree of hydrochar and reduce the energy consumption by approximately 16 J?when food waste was added with 75%?.The optimal tensile strength value?1.32?6.80 MPa?of the hydrochar pellets was produced under 220 oC due to the formation of“solid bridge”by lignocellulose of wood sawdust,which made the fuel structure more compact,and the increase of the blend ratio of food waste extended the combustion range and reduced the ignition temperature of the fuel.In summary,this topic focuses on the production of clean solid fuel from hydrothermal carbonization of food waste.The chemical components,hydrochar structure and carbonization mechanism are explored.The conversion mechanism of nitrogen,the removal mechanism of sulfur and sulfur,and the combustion performance were studied in depth.Meanwhile,two pelletization ways,molasses addition and hydrothermal carbonization with wood sawdust blend,were proposed for solid fuel preparation.This study is of great significance for the resource utilization of food waste and provides a theoretical basis for the field of application as biofuel. |
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