| Under the influence of energy security,sustainable use of resources,and the environmental impact of fossil fuels,human demand for energy is increasingly turning to clean and sustainable resources.Pyrolysis of biomass provides a means for continuous production of energy through products such as bio-oil and pyrolysis gas.Additionally,biochar obtained from pyrolysis of biomass also has many uses,such as the adsorption of heavy metals in the soil.With the development of heating technology,microwave pyrolysis is an effective means to provide the energy required for biomass pyrolysis owing to good penetration of microwaves,low volume selectivity of materials,and selective heating of microwaves.Also,the utilization rate of energy is further improved in microwave heating.The purpose of this paper is to design and manufacture a microwave pyrolysis system that can operate safely and efficiently on a laboratory scale,and to provide a reliable reference for future design.Initially,the design requirements and process flow of the microwave pyrolysis system at the laboratory scale were determined,and the overall direction of the design,experiment,simulation,and programming was considered.The structural design of the microwave pyrolysis equipment was then completed,after which the required power was calculated,and the protective gas input system and pyrolysis gas collection system were designed.According to the international standard of microwave operating frequency and the operating characteristics of related frequencies,2450 MHz was selected as the operating frequency of the magnetron for this equipment,and the matching rectangular waveguide was selected as the national standard BJ22 type.For uniformity of heating and effective use of energy,the final size of the resonant cavity was determined to be 500mm*450mm*440mm according to the design principles and methods.Microwave pyrolysis occurs in a high-temperature working environment,and therefore quartz glass was selected as the container.The size of the quartz glass was determined according to the density properties of the biomass material:radius of 60 mm,and a height of 130 mm.Microwave pyrolysis of biomass produces a large amount of pyrolysis gas and bio-oil.To discharge the gas in time and prevent explosion,the gas needs to be exported through the pipe fittings,so the complete sealing of the resonant cavity could not be achieved.To ensure a safe working environment for the researcher,according to the cut-off wavelength theory,a circular waveguide with a radius of 15 mm was selected to be installed at the connection of the unsealed pipe fittings Similarly,anti-leakage design of copper mesh was added around the furnace door of the container that needs to be opened and closed frequently on one side.In the calculation of power required for microwave pyrolysis of biomass,the DSC curves of wheat straw and pine wood heated up to 800°C were obtained using a synchronous thermal analyzer,to determine the total required heat and specific heat capacity change curve.Considering the high-temperature stage of microwave pyrolysis,the heat dissipation was also estimated,and finally combined with the requirement of an average heating rate of 3°C/s,whereby the required power was calculated to be 0.89 k W.The protective gas input system consisted of nitrogen as the protective gas a coupled with a pressure reducing valve and a flow meter.The liquid gas product collection system adopted a two-stage condensation method to collect liquid phase products in stages.When the gas needed to be collected for detection,the gas collection pump was started,and the evolved pyrolysis gas passed through two gas scrubbers to wash away the mixed liquid products.The gases were then passed through a dry filter cartridge,and finally through the gas collection pump into the gas bag for storage for later detection.Secondly,the number and positions of waveguides and magnetrons to be installed were finally determined through software simulation.The electromagnetic thermal coupling physical field in Comsol Multiphysics was used for simulation,and the position was scanned in stages with the "parametric scanning" function in the software.The absorption power of the material and the temperature gradient of the material were used as the basis for selecting the position.When only one waveguide was installed on the right side of the cavity,the best position was selected as(450,70,250),whereby the absorbed power at this position was959.1 W,and the temperature gradient at third second was 4.78°C/cm.Based on this position,a second port was added to the rear,lower,and left sides for research.It was found that the temperature gradient would be lower at similar absorbed power.Lastly,in the combined work of the two ports,the second selection coordinate was(390.8,0,0),and the temperature gradient in the third second was 3.90°C/cm.The specific heat capacity data obtained from the previous experiment was transferred to the model,and the estimated heat dissipation parameters were added.The simulated heating curve was closer to reality and was compared with the actual heating curve in Chapter 5.The control system of the microwave pyrolysis system was then designed.For the safe operation of the microwave pyrolysis system,a temperature control switch was selected to monitor the operating temperature of the magnetron,and a travel switch was selected to monitor whether the furnace door was tightly closed in real time.S7-200 SMART was programmed by STEP 7-Micro WIN SMART programming software,which identified the microwave pyrolysis equipment startup program,temperature collection subprogram,working time setting subprogram,and constant temperature control subprogram.Win CC flexible SMART V3 was used to design the HMI human interface,which can realize the display of the heating curve and control multiple parameter variables such as power and working time.Finally,performance testing of the microwave pyrolysis system was undertaken.The microwave leakage was lower than the national standard,the condensing effect was good,the pyrolysis gas exhibited low impurities content,and the drying effect was good.Also,the heating effect of the biomass pyrolysis met the design requirements. |
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