Study On Product Formation And Biomass Hydrothermal Process Mechanism

As the shortages of energy resources and the increasing pollution to environment, the utilization of biomass has catched singnificant attention. Hydrothermal technology provides a new way for material recycling with high value utilization. In this thesis agricultural wastes (rice straw, cotton stalk, and wheat straw), forestry waste (sawdust), aquatic plant (water hyacinth) and model compounds (cellulose, hemicellulose, lignin) were investigated as raw materials. Heavy oil and biochar were produced in a high-pressure and high-temperature reactor. The mechanism and process of biomass hydrothermal degradation and the benefit factors was studied. This thesis for the usage of biomass hydrothermal conversion and provide some guidance for its industrial application.Firstly of all, the characteristics of products from hydrothermal treatments of cellulose in an autoclave at various temperatures (200℃to400℃) and residence time (5min to2h) were investigated. The gas products mainly consisted of CO2, CO, CH4, and H2from250℃to400℃at the residence time of30min. Heavy oil mainly contained furans, phenols, carboxylic acids, aldehydes, ketones and high molecular compounds at all the studied temperatures. Aldehydes, phenols, ketones, acid groups and sugars in the aqueous phase were determined by gas chromatography-mass spectrometry (GC-MS) and Fourier transform infrared (FTIR). The solid residues were analyzed by elemental analyzers, scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), FTIR and thermogravimetric analyer. The results showed that the residues had core-shell structure and better physicochemical characterization at lower temperature and longer residence time.Then. the interactions among three major biomass components (hemicellulose, cellulose and lignin)were studied based on the extreme vertex design with the upper and lower constraint conditions. The influence of the components on the yield of the hydrothermal products were studied. The quadratic polynomial regression model was proposed and validated. The simulation results agreed very well with the experimental results.The influences of the hydrothermal temperature, pressure, catalyst and raw materials were studied using the orthogonal experimental design to optimize biomass hydrothermal process without the addition of catalyst, the optimum reaction condition to obtain heavy oil was determined. The addition of catalyst not only improve the yield heavy oil, but also restrict the generation of solid residue. Especially, when K2CO3was used as the catalyst, the yield of heavy oil was improved significantly. The liquid product was analyzed by GC-MS, which contained acids, esters, aldehydes, ketones, phenols and their derivatives. The addition of catalyst improved both the oil yield and the quality of oil.The characteristics of hydrochar are significantly different for different raw materials. The yield and heating value of hydrochar are high for wood and straw samples. Even though the yield of hydrochar from water hyacinth is little bit low, the morphology is the best among all the biomass samples stuided. Hydrochar derived from water hyacinth can be used as a new kind of carbon material, which can improve the utilization of biomass resources. The effect of reaction temperature and residence time on hydrothermal carbonation are significant, especially, the effect of temperature on the chemical peoperty of hydrochar is obvious, while, that of the residence time on the physical properties is very significant.Hydrothermal treatments of glucose and water hyacinth were experimentally conducted in the range of30min to24h at240℃, and the chemical and structural properties of hydrochar products were investigated. Hydrochar yield, oxygen/carbon ratio, and hydrogen/carbon ratio in all hydrochar products were1.33wt.%to1.90wt.%,0.19to0.45, and0.94to1.51, respectively. Higher heating value of hydrochar products was16.83MJ/kg to20.63MJ/kg. Char was characterized using thermogravimetric analysis. FTIR, XRD, and XPS techniques. Residence time had little effect on the chemical properties of hydrochar samples. Hydrochar developed better structural characterization as time increased. The formation mechanisms of microspheres on the surface of hydrochar were discussed, and the transformation of cellulose and hemicellulose may contribute more to the formation of microsphere.