| Wood drying is an important process for the use of wood.Drying quality and efficiency have become key factors to limit the wood recovery and quality of wood products.For the lower permeability of wood,there are many problems during drying process,such as long drying times,high energy consumption,and easily occurrence of drying defects,which seriously restrict its high efficient utilization.Microwave(MW)treatment is an effective method to increase wood permeability,which could provide favorable conditions for efficient and high quality drying of refractory wood.In this study,the heartwood of Chinese fir(Cunninghamia lanceolata)was studied in this paper,and the optimum MW process parameters for high-efficent drying was determined.The effects of variation of microstructure and pore structure on wood permeability were analyzed.The influence mechanism of temperature and pressure changes inside the wood,microstructure and pore structure variation on wood permeability and drying characteristics were revealed.Major conclusions under current experimental conditions(Rated power of MW equipment:20 k W;Thickness of Chinese fir lumber:50 mm;Initial moisture content range from 20%to 60%)were summarized as follows:(1)The effects of MW-treatment on drying of Chinese fir were evaluated.The optimum MW process parameters for rapid drying of Chinese fir were as follows:initial moisture content(MC)of 55%and applied MW energy of 43 k Wh/m~3.After MW treatment,the drying rate and moisture diffusion coefficient were effectively increased,the drying rate of quarter-sawn and flat-sawn samples with MW-treatment increased by 8.66%and 16.56%,respectively,and the drying rate in early stage was greatly increased.During the later stage,the moisture diffusion coefficient of quarter-sawn and flat-sawn samples with MW-treatment increased by 10.27%and 12.40%,respectively.Meanwhile,the MC deviation in the thickness and residual stress of MW-treated quarter-sawn samples decreased by 48.06%and 42.92%,respectively,in comparison with the control samples.Those indices for MW-treated flat-sawn samples decreased by 31.79%and 48.59%,the appearance quality of wood samples was not significantly influenced by MW treatment.(2)The effect of MW intensity on microstructural changs of Chinese fir were evaluated.Scanning electron microscope(SEM)micrographs demonstrate that after low-intensity microwave treatment,parts of the bordered pit membranes in tracheids of Chinese fir lumber were damaged;micro-fibrils on the margo were ruptured while the torus basically remained intact.Micro-cracks were evident at both ends of the cross-field pit apertures,propagating to the cell walls of tracheids.The middle lamella between ray parenchyma cells and longitudinal tracheids also cracked,resulting in the two components separated with a crack width generally ranging between 1?m?25?m.After high-intensity microwave treatment,damage to the wood microstructure was more severe compared to low-intensity microwave treatment,with macro-cracks having a width range of 100?m?130?m being generated.In addition,on the fracture surface of macro-cracks,the bordered pit membranes in tracheids fell off,cross-field pit membranes disappeared and the ray parenchyma cells were seriously damaged,exhibiting cracking of the tracheid walls.The results by high resolution X-ray tomography showed that the cross section morphology of tracheid gradually became round or elliptic by low-intensity MW treatment.But for the higher-intensity MW treatment wood,the deformation of the tracheid was more obvious,and part of the tracheid wall was broken.The damage was mainly concentrated in the central region of tracheid wall,while the cell corner remained basically intact.(3)The effect of MW intensity on pore structure of Chinese fir were evaluated.Mercury intrusion test showed that the median pore diameter,average pore diameter and total mercury intrusion volume were increased after MW treatment.Both low-intensity and high-intensity MW treatment can increase the pore diameter corresponding to the pit membrane openings(Peak value increased from 674.7 nm to 831.8 nm and 1047.6 nm,respectively).The number of pores in the tracheid lumen diameter range also significantly increased.Nitrogen adsorption results showed that the number of pores in the pore diameter range of 1.2 nm?3.7 nm increased significantly for the low-intensity MW-treated wood.High-intensity MW treatment can also increase the pore size of some of the pores and create some new pores,resulting in an increase in the number of pores in the range of 2.2 nm to 10 nm.The results of NMR cryoporpmetry also showed that the pore number within the pore diameter range of 1.6 nm?2.7 nm increased in the saturated cell wall of the low-intensity MW-treated wood.High-intensity MW treatment can also increase the pore size of the pores,resulting in an increase in the number of pores in the range of 2.7 nm to 8 nm.The porosity of cell wall increases with the increase of MW intensities.(4)Temperature and pressure changes inside the wood under different MW process conditions were investigated and the correspondence between temperature and pressure was found.The results also showed that two types of temperature-changing characteristics were found during MW treatment.Type one is composed of two periods:a rapid temperature-increasing period and a rapid temperature-decreasing period.Type two consists of three periods:a rapid warming-up period,a slow warming-up period(or constant temperature period)and a slow cooling period.Under the same MW treatment condition,when the initial MCwas 35%,the peak temperature and pressure were the highest.The higher the microwave energy density,the higher the peak temperature and pressure inside the wood were.The peak temperature and pressure reached 110.3°C and 144.6 k Pa under the optimum MW treatment condition.(5)The relationship between the temperature and pressure changes inside the wood,changes of microstructure and pore structure were analyzed to reveal the mechanisim of MW pretreatment.The higher steam pressure could enhance the damage degree of the wood microstructure and pore structure,resulting in the permeability increasing.However,when the initial MC of wood was low or the MW energy density was high,the steam pressure generated inside wood is high,and the internal crack was easy to occur.Under the optimum MW treatment condition,parts of the bordered pit membranes in tracheids of Chinese fir lumber was damaged,micro-cracks were evident at both ends of the cross-field pit apertures,propagating to the cell walls of tracheids.The middle lamella between ray parenchyma cells and longitudinal tracheids also cracked,resulting in the two components separated to form micro-cracks.The changes of wood microstructure caused the pore diameter corresponding to the pit membrane openings to become larger,the peak value increased from 674.7 nm to 831.8nm.In addition,the number of pores in the tracheid lumen diameter range also significantly increased,which resulted in 42.9%(quarter-sawn)and 33.3%(flat-sawn)increase in air permeability in the thickness direction of Chinese fir lumbers.Consequently,the drying characteristics of Chinese fir were improved without generating internal checks. |
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