摘要
【目的】采用过热蒸汽预处理杨木锯材后进行常规干燥,探究过热蒸汽预处理对杨木锯材干燥速率、干燥质量和干燥时间的影响,以期提高杨木锯材干燥速率、缩短干燥周期、降低干燥能耗,为杨木锯材的高附加值利用提供技术依据。【方法】采用温度为110、120和125℃的过热蒸汽对初含水率100%~150%、规格900 mm×120 mm×50 mm(轴向×弦向×径向)的杨木锯材预处理5 h,之后进行常规干燥,分析过热蒸汽预处理对锯材含水率、应力、外观质量以及后期常规干燥速率的影响,依据国家标准对过热蒸汽预处理及未处理材常规干燥后的干燥质量进行评价。【结果】1)过热蒸汽预处理结果表明:预处理温度从110℃增加到125℃时,锯材含水率下降比例从59.26%增加到77.11%,含水率在较短时间内大幅度降低并接近木材纤维饱和点;预处理后锯材的残余应力较大,在2.71%~7.75%之间;锯材顺弯、横弯、翘曲和扭曲指标的干燥质量等级均为一级;锯材皱缩比例在25.00%~36.96%之间。2)常规干燥结果表明:经110、120和125℃过热蒸汽预处理后,锯材常规干燥速率较未处理材分别增大7.97%、16.23%和78.42%;从含水率和应力方面分析,过热蒸汽预处理材常规干燥后的干燥均匀度和厚度上的含水率偏差指标质量等级均达到一级,终含水率指标的质量等级为二级;锯材预处理过程中产生的残余应力在常规干燥过程中被释放,常规干燥后残余应力指标质量等级为二级;从外观干燥质量分析,预处理材常规干燥后的顺弯、横弯、翘曲和扭曲指标质量等级均达到一级;锯材在过热蒸汽预处理过程中产生的皱缩在常规干燥过程中部分得到恢复,常规干燥后锯材皱缩比例分别降低至28.60%、9.09%和15.00%。3)总干燥时间分析结果表明:与未处理材的常规干燥相比,110、120和125℃过热蒸汽预处理材的总干燥时间分别缩短了24.96%、44.22%和67.24%。【结论】过热蒸汽预处理可以显著降低杨木锯材的含水率,并提高杨木锯材常规干燥的干燥速率,缩短干燥时间。综合考虑杨木锯材干燥质量和干燥效率,50 mm厚杨木锯材过热蒸汽预处理温度以120℃较好。
【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 from2. 71% to 7. 75%. Appearance quality index of the SHS pretreated specimens met the requirements of the 1 stdrying 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 and125 ℃ 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 stgrade in accordance with national standard of lumber drying quality. However,the final MC met the2 ndgrade requirements. Additionally,residual drying stress generated in SHS pretreatment was released in the following conventional drying process,and met the 2 ndgrade 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 by24. 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 ℃.
作者
侯俊峰
鲍永泽
周永东
Hou Junfeng;Bao Yongze;Zhou Yongdong(Research Institute of Wood Industry, CAF Beijing 100091)
出处
《林业科学》
EI
CAS
CSCD
北大核心
2018年第2期131-136,共6页
Scientia Silvae Sinicae
基金
林业公益性行业科研专项(201404502)
关键词
杨木锯材
过热蒸汽预处理
常规干燥
干燥质量
poplar lumber
superheated steam pretreatment
conventional drying
drying quality
