Study On Hydrothermal Carbonization Of Food Waste And Derived Product Utilization

With the acceleration of urbanization and rapid population growth,China’s municipal solid waste（MSW）production is increasing dramatically.As an important component of MSW,food waste plays a major role in increasing its production.Food waste is rich in organic matter and nutrients,and has the dual attributes of pollutants and resources.If it can be effectively treated,it will have huge energy and nutrient recovery potential.In recent years,hydrothermal carbonization technology has great application potential in the treatment of food waste because of its advantages such as converting wet biomass into solid fuels and carbon-containing materials.This study addresses the characteristics of high moisture content,high organic matter content,and high nutrient content in China’s food waste,as well as the difficulty of processing food waste and low energy recovery in traditional treatment processes.Based on the summary of previous research and pre-tests,the research on the characteristics of the hydrothermal treatment of food waste and derived product utilization based on the response surface analysis method.By analyzing the distribution and physicochemical characteristics of hydrothermal carbonization products,the mechanism of the influence of process conditions on the distribution,composition and properties of hydrothermal carbonization products of food waste is clarified,and the distribution of nutrient elements and the law of energy conversion are explored and revealed.Finally,obtaining the best process conditions for energy and nutrient element recovery through response surface model fitting optimization.The main research work has the following aspects:The Box-Behnken（BBD）response surface design experiment was used to investigate the effects of reaction temperature,residence time and moisture content on the distribution and physicochemical characteristics of hydrothermal carbonization products of food waste.The results show that the hydrothermal carbonization products of food waste are mainly distributed in the solid（44.0～56.5%）and the liquid phase（30.4～55.7%）,and there are fewer gas phase products（3.5～16.5%）.The reaction conditions have a significant effect on the distribution of hydrothermal products.With the increase of the reaction temperature,the liquid phase products gradually transform into the solid and the gas phase.The influence of residence time is more significant at low temperature,the prolonged residence time is conducive to the formation of hydrochars when at the 180℃.The increase of moisture content leads to the decrease of hydrochars yield,but this effect will be weakened with the increase of reaction temperature.When the moisture content increases from 80%to 90%,the hydrochars yield at 180℃,220℃and 260℃will decrease by 20.0%,9.6% and 6.1% respectively.After the hydrothermal reaction of food waste,the concentration of carbon,nitrogen and phosphorus in the solid phase product are significantly increased,the H/C and O/C atomic ratios are significantly reduced,the heating value and energy density are significantly improved,and the combustion performance of food waste is effectively improved,its product hydrochars can be used as a clean fuel for energy application.Liquid phase products are rich in a large amount of organic acids and nutrients such as SCOD,TN and TP with high concentration,which have a high potential for nutrient recovery.CO₂ is the main gas component of the hydrothermal carbonization（>75%）,and due to the high concentration（679.3～2178.0ppm）of H₂S gas generated during the hydrothermal carbonization,the subsequent processing steps of gas recovery need to be considered in practical applications.Based on the relationship between mass balance and energy conservation,the distribution of three kinds of nutrient elements carbon,nitrogen and phosphorus in different carbonization conditions and energy balance of the hydrothermal carbonization process were analyzed.The results shows that in evaluating all the carbonization conditions,most of the carbon in the raw material retain in the solid phase（57.2～86.5%）,only a small amount transfer to the gas phase（1.7～9.4%）and the liquid phase（5.4～28.4%）.Similar to the carbon distribution,nitrogen in the raw material was mainly retained in the solid phase（60.0～88.4%）,and only 19.5～43.2%of the nitrogen was transferred to the liquid phase.Different to the distribution of carbon and nitrogen,about 36.0～70.4%of the phosphorus in the raw material was transferred to the solid phase,and 29.1～78.0%to the liquid phase.The results of energy balance analysis showed that hydrothermal carbonization is helpful for energy recovery,except for the test group with the moisture content of 90%,all the test group achieved positive net energy balance.And when the reaction temperature was 220℃,the retention time was4h,and the moisture content was 80%,the hydrothermal carbonization of food waste could achieve the maximum net energy recovery.Using Design Expert software to analyze the response surface test results,the results show that the regression models fitted by the four response values are extremely significant（P<0.01）,and there is no misfit（P>0.05）,and the correlation coefficient R²>0.97,indicating that these models can better reflect the influence of test parameters on the energy yield（EY）,carbon fixation rate（CF）,nitrogen fixation rate（NF）and phosphorus fixation rate（PF）of hydrochars during the process of hydrothermal carbonization of food waste.The response surface and contour analysis results show that the reaction temperature and moisture content have a significant antagonistic effect on hydrochars energy,carbon and nitrogen recovery;while the reaction temperature and residence time have a clear synergistic effect on phosphorus fixation.Based on model analysis and experimental verification,the optimal process conditions for obtaining energy and nutrient recovery in hydrochars are:reaction temperature 220℃,retention time 3.8h,and moisture content 80%.Under these conditions,the maximum EY,CF,NF and PF values can be obtained,91.3%,90.9%,90.3% and 68.1% respectively.