Abstract: [Objective] In order to provide basis for improving drying rate,reducing energy consumption and the high value-added utilization of poplar lumber, the effects of pretreatment with superheated steam (SHS)on conventional drying rate, drying quality and drying time of poplar lumber (Populus tomentosa) were studied in this paper.[Method] The SHS pretreatment conditions were following:the temperature was 110, 120 and 125℃ respectively, and the duration was 5 h. The sample dimension was 900 mm×120 mm×50 mm(longitudinal×radial×tangential), and the initial moisture content(MC) was ranged between 100% and 150%. The effects of SHS pretreatment on the MC, stress, appearance quality and drying rate during the conventional drying of poplar lumber were analyzed. Furthermore,the drying quality of poplar lumbers with SHS pretreated and untreated was evaluated respectively according to the Chinese national standard.[Result] 1) The results of SHS pretreatment showed that the MC decline ratio was increased from 59.26% to 77.11% as the SHS pretreatment temperature increasing from 110℃ to 125℃, and MC of lumber was remarkably decreased and close to fiber saturation point (FSP) in a short time by SHS pretreatment. Residual stress in the SHS pretreated lumbers was ranged from 2.71% to 7.75%. Appearance quality index of the SHS pretreated specimens met the requirements of the 1^st drying grade. However, the collapse ratio of SHS pretreated lumbers was ranged from 25.00% to 36.96%. 2)Experimental result of conventional drying revealed that the average drying rate of lumber pretreated by SHS under the temperature of 110, 120 and 125℃ was increased by 7.97%, 16.52% and 78.42% in comparison with conventional drying index of the untreated lumbers. The drying quality index of drying uniformity among different boards and MC deviation in thickness met the requirements of the 1^st grade in accordance with national standard of lumber drying quality. However, the final MC met the 2^nd grade requirements. Additionally,residual drying stress generated in SHS pretreatment was released in the following conventional drying process, and met the 2^nd grade requirements. Appearance quality index of the SHS pretreated lumbers after conventional drying met the requirements of the 1^stdrying grade.The collapse occurred in SHS pretreatment was partly recovered during the following conventional drying process.Compared with SHS pretreated lumbers, collapse ratio of the conventional dried lumbers was decreased to 28.60%, 9.09% and 15.00%, respectively.3)Results of total drying time analysis showed that the total drying time of the lumbers pretreated by SHS under 110,120 and 125℃ was decreased by 24.96%, 44.22% and 67.24% as compared with the conventional dried lumbers.[Conclusion] The MC of poplar lumbers was decreased significantly by SHS pretreatment. The drying rate was improved, and the drying time was shortened in the following conventional drying process. In overall consideration of the drying quality and drying efficiency, the optimized SHS pretreatment temperature of 50 mm-thick poplar lumber is 120℃.

Key words: poplar lumber, superheated steam pretreatment, conventional drying, drying quality