Study On Microwave-induced Emission Of VOCs In Medium Density Fiberboard Based On Fractal Reconstruction

Good indoor air quality is an important prerequisite for ensuring daily comfort and physical health,and volatile organic compounds(Volatile Organic Compounds,VOCs)emitted by various wood-based panels(WBP)are an important source of pollution that reduces indoor air quality.At present,China’s WBP industry is developing rapidly,which lead the risk of residents’ exposure to VOCs continues to increase.Therefore,studying the emission and control of VOCs in WBP is not only of great significance to further improve the quality of wood-based panels and promote the sustainable development of China’s WBP industry,but also is of great significance to further improve indoor ambient air quality,reduce the risk of residents exposing in VOCs,and reduce the harm of VOCs to residents’ physical and mental health.Based on the characteristics of rapidity and penetration of microwave heating,a method of using microwave to induce VOCs desorption in the WBP is proposed in this paper,so as to quickly remove the VOCs gas inside the board to reduce the initial VOCs concentration inside the board.And the heat transfer and mass transfer process of the VOCs involved in the method in the board were studied,taking Medium Density Fiberboard(MDF)as the research object and formaldehyde as the VOCs model gas.In the present paper,the microstructure of MDF before and after heat treatment was first observed by scanning electron microscope,and then the results were analyzed by computer program to obtain the characteristic parameters such as porosity and pore size distribution of the MDF,so as to understand the effect of microwave induced process on microstructure of MDF,and the observed characteristic parameters were used for reconstructed the pore structure of MDF based on fractal theory to explore the effect of MDF’s microstructure on its heat and mass transfer processes.The results show that the fiber structure inside of MDF makes the mass transfer parameters anisotropic,and the mass transfer capacity perpendicular to the thickness direction of the MDF is about 10%better than that in the thickness direction,but the heat transfer parameters of MDF are isotropic,which is not affected by the fiber structure.Secondly,the thermal stability of MDF and the density,moisture content,heat transfer parameters and electromagnetic parameters that affect the temperature rise of MDF under microwave induction were determined experimentally,which can provide accurate parameter basis for studying the heat transfer and mass transfer process in the microwave-induced desorption of VOCs in MDF.According to the results,the thermal diffusivity of the sheet decreases from 1.322 × 10-7 m2/s to 1.115 × 10-7 m2/s,and the imaginary part of the complex relative permittivity of the sheet increases from about 0.1425 to about 0.2138,When the temperature increases from 25℃ to 180 ℃.Thirdly,a sealed environment chamber equipment was built to measure the change curve of formaldehyde emission with time in the sealed environment of MDF samples of different forms,and the influence of temperature on the strength of desorption and release of VOCs and the diffusion coefficient of VOCs in the internal pores of the MDF were studied.The experimental results reveal that the saturated emission concentration of formaldehyde from the pores of the MDF increased from around 184.7 × 10-3 mg/m3 to about 519.6 × 10-3 mg/m3 in an exponential trend,as the emission temperature increased from 25℃ to 40℃.And the diffusion coefficient of formaldehyde increased from 5.666×10-10 m2/s to 13.39×10-10 m2/s.Finally,a heat and mass transfer model of MDF in a microwave oven was established.Using formaldehyde as the model gas,the diffusion behavior of VOCs in the pores of MDF was studied by numerical simulation when nitrogen was purged to flow over the MDF,and the effects of microwave power and gas flow rate on the microwave-induced desorption of VOCs in MDF sheets were also determined.The simulation results show that,when using microwaves to induce the emission of VOCs from MDF,the nitrogen flow rate mainly affects the change of the formaldehyde concentration inside MDF while the microwave power mainly affects the temperature change,since the thermal diffusivity of MDF is only on the order of 10-7.The microwave power has no gain in promoting the emission of formaldehyde in the board,but will cause the local temperature to be higher than the thermally stable temperature of MDF.When the microwave power is 500 W,increasing the nitrogen flow rate from 0.001 m/s to 5 m/s can reduce the average concentration of residual formaldehyde in MDF from about 2 mg/m3 to about 0.3 mg/m~3 after microwave-induced desorption for 90 s.