Heat And Mass Transfer And Experimental Study Of Microwave Puffing Process Of Tobacco Stem Biomass

As a renewable and clean resource,the value-added utilization of biomass has important scientific significance and social value for environmental protection,resource conservation,and economic contribution in the field of energy.To enhance the development and utilization of agricultural biomass tobacco stalk resources and to improve the effective utilization of agricultural waste has become a hot spot for research scholars.In order to break through the problems of traditional technology,microwave heating technology has become a new way to recycle waste biomass stems due to high thermal efficiency,simple process,low energy consumption,and low pollution.However,the intrinsic correlation of the microwave multi-physics field coupled thermal effect of biomass stems and the reaction mechanism of its interaction with microwave have not been fully understood.In this study,for the first time,a multiphysical field coupled simulation study of microwave expansion of biomass tobacco stalk was conducted to combine numerical simulation and experiments,using the finite element method to study the microwave energy absorption and conversion,heat and mass transfer and volume expansion,to reveal the multi-physical field coupled mechanism of microwave expansion of tobacco stalk,and to investigate the microwave The effects of waveguide mode,frequency,power,sample moisture,and sample cavity position on the heating uniformity and microwave energy utilization efficiency of microwave expansion of biomass tobacco stems were investigated.The main results are as follows:(1)A three-dimensional geometric model of the microwave radiation system is developed and the assumptions related to the model are presented.Based on the energy conservation equation,heat transfer equation,mass transfer equation,and structural mechanics equation,the heat transfer takes into account the dielectric loss,evaporation,and convective heat transfer of the tobacco stalk sample,and also takes into account the phase change of water and the expansion deformation of the biomass tobacco stalk caused by thermal stress,and constructs a multiphase porous medium numerical model with multi-physics field coupling of the biomass tobacco stalk in the microwave field.The multi-physical fields of electromagnetic field,heat transfer field,mass transfer field,and solid mechanics field are fully coupled.This can provide a theoretical basis and a new perspective for the precise control of microwave expansion of waste biomass tobacco stalk.(2)Using the finite element method to solve the expansion process of the tobacco stalk sample under microwave heating conditions,the biomass tobacco stalk is irradiated by microwave and absorbs microwave energy,the dielectric loss converts microwave energy into heat,and the temperature inside the tobacco stalk increases significantly.As the temperature increases,the hot spot expands to the surrounding area,and the temperature field is gradually homogenized through heat conduction.The reliability of this multi-physics field coupling model was experimentally verified,and the maximum relative errors of temperature verification were all less than 10%.The electric field and standing waves with strong and weak distribution are formed in the cavity,and the heterogeneous electric field distribution characteristics are favorable to the absorption of microwaves and accumulation of heat inside the tobacco stalk sample,but the uneven microwave energy distribution also leads to the uneven heating of the tobacco stalk sample,and the uneven temperature distribution is reflected in the formation of hot spots caused by the interfacial polarization and standing waves,and the huge temperature gradient is the main reason for the formation of hot spots.(3)The microwave energy conversion accelerates the molecular movement of water and evaporation,causing the diffusion and evaporation of water inside the tobacco stalk sample and other mass transfer phenomena.The evaporation of a large amount of heated water inside the tobacco stalk generates pressure,and the vapor pressure acts as a driving force to make the tobacco stalk tend to expand.The mass transfer process of biomass tobacco stalk is that the internal moisture content of the sample is higher than the surface,forming an internal and external moisture gradient,and the internal moisture of the sample keeps diffusing to the low moisture content surface,and finally,the moisture content is reduced to 1.32% for 120 s.The biomass tobacco stalk forms a basic stable structure.When the vapor pressure inside the stem sample was greater than the structural strength of the stem fibers,the biomass stem expanded and deformed.The simulation found that the center temperature and moisture content of the tobacco stalk sample were higher than the surface,and the direction of heat and mass transfer within the tobacco stalk sample was consistent,which was favorable to the microwave expansion of the biomass tobacco stalk.(4)Controlling key parameters can change the internal temperature distribution and energy absorption of biomass tobacco stalks under microwave irradiation by affecting the microwave electric field intensity distribution.The cavity design of double-fed waveguide has better electric field distribution,microwave energy absorption and heating uniformity for biomass tobacco stalk,and the energy efficiency is 98.02% and 21.39% higher than that of single-fed waveguide mode,respectively.The microwave frequency of 2.40 GHz leads to localized overheating of biomass fume tobacco stalks,and reasonable control of the frequency variation can produce a variety of resonant and cavity modes,thus improving microwave energy absorption and ensuring stable and efficient microwave frequency matching.Selecting the appropriate microwave power according to the purpose can improve efficiency and reduce energy consumption.Low microwave power is suitable for better thermal uniformity and energy efficient microwave heating,while high power is suitable for fast differential heating.The moisture content of the biomass tobacco stalk and the cavity position has a significant effect on the electric field distribution.By adjusting the moisture content of the biomass tobacco stalk(30%)and keeping it in the low region of the cavity(0-10cm),the maximum microwave energy utilization can be achieved with an average energy efficiency of 59.15% and a maximum energy efficiency of 87.79%.It provides technical support and new ideas for the preparation of sustainable bioenergy and biomass materials using microwave heating of biomass tobacco stalks.