| Lignocellulosic biomass is an abundant bio-renewable carbon source and considered as a promising alternative to fossil resource.Development of highly efficient technologies for biomass utilization is of great significance for solving energy crisis and environmental problems.The inherent heterogeneity and recalcitrance of lignocellulosic biomass render the low utilization efficiency with conventional biomass utilization technologies.Microwave heating has the characteristics of fast,volumetric and selective heating,which makes its wide application in biomass thermochemical conversion process.In this dissertation,microwave-assisted organosolv method of lignocellulosic biomass biorefinery with high biomass utilization rate was explored,and the dynamic dielectric property of the mixed reactant was studied.Based on that,pilot study on microwave-assisted organosolv biorefinery of lignocellulosic biomass was carried out.Finally,the heating process of the mixed reactant in the pilot scale microwave reactor was simulated.The results from this dessertation will provide both practical and fundamental knowledge for the development of industrial-scale,high-efficiency and low energy consumption biomass biorefinery technologies.（1）The effects of reaction temperature,time,catalyst dosage and solvent type on the performance of the microwave-assisted organosolv lignocellusic biomass refinery were investigated.Compared with the aqueous solutions of low boiling organic solvents used in conventional biomass refinery process,ethylene glycol/dioxane,ethanol/dioxane and ethylene glycol/water can obtain higher lignin removal.Excessive reaction temperature will not only cause serious degradation of cellulose,but also increase the condensed structure and pseudo-lignin content of lignin products.Acid catalyst is the key factor to destroy the stubborn structure and realize the efficient fractionation of the main components of lignocellulosic biomass.With ethylene glycol/dioxane（1/2,v/v）as solvent,the optimum fractionation conditions were:120℃,10 min and with 1.5%of acid catalyst.The lignin products obtained under optimum condition have the advantages of small molecular weight,less condensation structure and good solubility in conventional solvents.And,the hemicellulose degradation solution is rich in hydroxyl groups and can be used to prepare polyurethane foams.The lignin content of the crude cellulose product is only 1.2%.Under the same reaction condition,the lignin removal rate of poplar and bamboo was close（about 94%）,the cellulose retention rate and lignin recovery rate of bamboo were higher than that of poplar by using ethylene glycol/water as solvet.The cellulose retention rate and lignin recovery rate of poplar were 78.5%and 56.1%respectively,while that of bamboo was 86.7%and 71.5%respectively.（2）Based on the above research,methanol/dioxane was employed for poplar biorefinery under microwave irradiation and conventional heating.The mixing ratio of methanol and dioxane was optimized,and the difference of fractionation efficiency and physicochemical properties of the fractions under microwave and oil bath heating was compared.The results showed that the optimal fractionation efficiency（88%of lignin removal,70%of hemicellulose removal and 85%of cellulose retention）was obtained by the mix ratio of methanol/dioxane are 3/1（v/v）.Combined with the analysis of the properties of methanol/dioxane,such as,the ability of hydrogen bond binding,solubility parameters and the solubility of alkali lignin,it is further confirmed that the solubility to lignin solvent is the key to improve the removal rate of lignin during biomass refinery process.In addition,the thermal response ability of the solvent system under microwave irradiation is also an important factor to promote the fractionation efficiency.The lignin products obtained under this mix ratio have the advantages of small molecular weight and less condensation structure.Crude cellulose products can almost reach the theoretical saccharification rate（more than 99%）under conventional enzymatic hydrolysis conditions.Hemicellulose degradation solution consist mainly monosaccharides and glycosides.In conclusion,the total utilization rate of poplar wood by lignocellulose biomass refinery mode with glucose solution,glycosides and organosolv lignin as products was 74%.（3）The study on the mechanism of microwave action on complex reaction system is mainly focus on the dynamic change of the dielectric properties of the system druing the reaction process.The interaction mechanism between microwave and the reactant during the microwave-assisted organosolv biorefinery process was studied by using the mixture of ethylene glycol,water,solid biomass powder and sulfuric acid as the model reaction system.The results show that the dielectric property of mixed reactant is linearly related to the amount of solid biomass and the water content in the reaction system.Because of the ionic conduction mechanism in the microwave field,the addition of sulfuric acid（exists in the mixed reactants in the form of ions）will increase the dielectric loss of the mixed reactants,but it doesn’t affect its dielectric constant.Temperature mainly affects the dielectric properties of the mixed reactants by affecting the relaxation process of dipoles and ions in the electromagnetic field.Both the dielectric loss caused by dipole（water molecule,ethylene glycol molecule）polarization and ion(H+,SO42-)ions conduction occurs near the frequency 915 MHz.Therefore,the dielectric loss of the mixed reactant with dipole and ion at this frequency shows a complex nonlinear relationship with temperature.The dynamic change of dielectric properties of the mixed reactant can be approximated as a function only related to temperature.Based on the experimental data and related theories,an expression describing the variation of dielectrical properties of the mixed reactants with temperature was proposed.（4）A pilot-scale microwave assisted organosolv lignocellulosic biomass refinery scheme was set up and used for bamboo fractionation.To meet the scale for industrial production,a 1 m3pilot-scale microwave reactor was designed with direct introducing of microwave to reaction chamber.This design aim to ensure efficient microwave feeding and microwave reactor’pressure requirement.And,this microwave reactor adopts a heating strategy of combination of electromagnetic heat and convection heat transfer to overcome the limited microwave penetration depth and ensure the uniformity of the temperature of reactant.The experimental results show that the temperature difference between upper and lower reactant is not more than 4℃,and the reflected microwave power less than 5%.Up to 93%of lignin,85%of hemicellulose removal rate and nearly 90%cellulose can be achieved by using this pilot-scale system in bamboo powder refinery.The fractionation efficiency is better than that of laboratory scale microwave and conventional heating equipment under the same conditions.The lignin products obtained in pilot-scale system have similar properties to those obtained under laboratory conditions with the characteristics of small molecular weight,no sulfur and low ash.The hemicellulose degradation solution is rich in high value-added compounds such as xylose,glucose,formic acid,acetic acid and furfural,and the total utilization rate of raw bamboo feedstock reaches 81.5%.（5）Based on the study of the interaction mechanism between microwave and the reactant,the heating process of mixed reactants in the pilot-scale microwave reactor was numerically simulated.The results show that the stirring system of the pilot-scale microwave reactor can quickly make the mixed reactants form a circulating flow field.The reactant near the stirring shaft moves upward and the reactant near the wall of the reactor flow down.The reflected microwave power and the distribution of electromagnetic fields in mixed reactant are related to the filling coefficient of the reactor.When the filling coefficient is in the range of 0.91～0.93,the relected microwave powder is within the acceptable range（less than 10%）.During the whole heating process,most of the mixed reactant has good temperature uniformity and the temperature difference between the upper and lower reactant is about 7℃.There are also small areas of"hot spots"（about 50℃higher than the average temperature of the mixed reactant）,which are mainly concentrated in the area near the stirring shaft.The“hot spot”is mainly caused by the higher electric field intensity in the region.In general,the numerical simulation results are basically consistent with the experimental data,indicating that the numerical model proposed in this paper can reveal the temperature variation of heating mixed reactant in the pilot-scale microwave reactor,which has guiding significance for the development of large-scale microwave heating equipment. |
