Research On Microwave Dielectric Properties Measurement Technology Of Biomass Materials At Variable Temperature

Biomass materials as the main raw material of biomass-based renewable energy,its dielectric properties parameters,namely the dielectric constant and dielectric loss,whether the appropriate deployment of the two will directly affect the energy conversion rate and efficiency of the material in the microwave pyrolysis process.Therefore,it is necessary to regulate the dielectric properties of the material by adding a reasonable catalyst or microwave absorber during microwave pyrolysis.And the choice of the type and addition amount of additives depends to a certain extent on the accurate measurement of the complex permittivity of biomass materials under variable temperature environment.Some of the existing test methods are well applicable to the testing of permittivity of biomass materials at room temperature,but there is a significant lack of effective variable temperature test methods and systems.The main causes for this problem are the peculiarities presented by biomass materials at variable temperatures: on the one hand,the materials are highly volatile,easily deformed and appear in various forms such as solid,liquid and gas in the pyrolysis reaction;on the other hand,the complex permittivity of the material vary with temperature over a large span(order of magnitude variation),and the frequency dependence of dielectric properties is high,etc.The above factors require the dielectric properties test methods and systems to take into account both the wide frequency band,high accuracy and wide measurement range,and to achieve in-situ variable temperature testing,while preventing the sample from contaminating the system,making it difficult to apply some of the existing high-temperature test methods.These requirements make most of the existing high-temperature test means hard to apply.To address the above challenges,this dissertation has carried out the research on microwave dielectric properties measurement technology of biomass materials at variable temperature from four aspects: theoretical research,test fixture design,test modeling and system development.Based on the analysis of the existing test methods and systems,a gradual open-circuit coaxial cavity with separate high-temperature and low-temperature parts is proposed as a test sensor for the variable-temperature dielectric property testing of biomass materials,which solves the problem of difficult to achieve high test sensitivity and in-situ testing,and significantly improves the cavity quality factor and hightemperature stability,and also eliminates the problem of volatile contamination.The complex permittivity test model of the open-circuit coaxial cavity method is established using the mode matching method and the high-order vector finite element fast numerical analysis method for the body of revolution cavity.According to the requirements of variable temperature testing,a high-temperature vacuum protection system is developed,and automated testing is realized through system integration and writing test software.The main research content and innovative contributions of this dissertation are summarized as follows:1.The significance of the research on microwave dielectric properties measurement technology of biomass materials at variable temperature is clearly explained,and the problems and difficulties of the existing test methods and devices are summarized by carefully reviewing and analyzing the relevant studies on the measurement of dielectric properties of biomass materials at home and abroad.New structures and methods are proposed in this dissertation,to realize the in-situ testing of the dielectric properties of biomass materials during variable temperature processes.2.For the theoretical analysis of the eigenvalue problem of resonant cavity,the modes and field distributions of circular waveguides and coaxial lines are firstly briefly introduced,and the mode-matching algorithm is used to derive the reentrant coaxial cavity.Secondly,in order to deal with the eigenvalue solution problem of complex structure resonant cavity,the principle of variational method and finite element analysis process are introduced and presented.Finally,the advantages and disadvantages of the two theoretical analysis methods are compared,which provide the theoretical basis and calculation method for the establishment of the subsequent test model.3.According to the temperature-dependent characteristics of biomass materials,an open coaxial cavity opened to the circular waveguide below cut-off is adopted as the test sensor to achieve high sensitivity in situ testing of samples.With a comprehensive consideration of the requirements of sample heating,cavity quality factor and sample size,the gradient structure and spurious mode suppression structure are introduced,and the broadband gradient type open coaxial cavity is proposed.To further improve the working performance of the sensor at high temperature,a split design of the resonant cavity is proposed,the novel high-temperature coaxial cavity with high quality factor,high stability,fast heating rate and long service life is developed.4.For the conventional open-circuit coaxial cavity structure,a hierarchical material testing model with high accuracy and fast solution speed is established by mode matching matching technique.For the complex structure of the gradient open-circuit coaxial cavity,based on the characteristics of the body of revolution,a method of calculating region segmentation and separation based on the rotary body model,and a test physical model is established for the rotationally symmetric complex structure of cavity,which solves the problem of high precision and rapid solution of dielectric properties.The comparison and validation of different methods in terms of solution accuracy and solution speed are analyzed in detail for several typical cases.In order to realize the high accuracy test under variable temperature,the theory of variable temperature test is proposed from two aspects of cavity calibration and specimen size correction,and the model of the test from normal temperature to variable temperature is completed.5.In order to establish a variable temperature test system,a vacuum insulation system is developed to improve the working performance and service life of the test sensor;an ultrasonic electromagnetic induction heating equipment and a homemade induction coil are used to achieve rapid heating of the cavity localization;the vacuum system and the cavity are protected by a water-cooling circulation system,and the automatic control of the temperature in the sample area is achieved by combining the heating system and the temperature control and acquisition system.Through the integration of the system,the determination of the test procedure and the writing of the automatic test software,the automatic test of the dielectric properties of the material under variable temperature is realized.6.The reliability and stability of the test system are evaluated through extensive experiments,and the accuracy of the test system is verified by testing several commonly used reference samples and comparing the test results with data in published literature.Two typical biomass materials are tested and the results are analyzed.Through the principle analysis and theoretical calculation method,the main system error sources and their uncertainties are clarified,and the test errors are calculated and synthesized to complete the evaluation of the overall system performance.The test system established in this dissertation realizes in situ variable temperature testing of dielectric properties of various materials including typical biomass at 0.3?3GHz and room temperature?1000°C.