关键词: 过热蒸汽干燥, 常规干燥, 柳杉锯材

Abstract: [Objective] In order to provide basis for the high value-added utilization, reducing energy consumption and improving production efficiency of Chinese cedar, superheated steam drying and conventional drying of Chinese cedar (Cryptomeria fortunei)lumber were investigated in this study. Drying quality, microstructure and mechanical properties were compared between two kinds of drying wood. The applicability of superheated steam drying on Chinese cedar lumber was also discussed.[Method] The drying quality and mechanical properties of dried lumber were analyzed by national standards. Furthermore, microstructure of Chinese cedar wood under two kinds of drying methods were observed by scanning electron microscope.[Result] As for 50 mm thickness lumber, the drying time and drying rate was 110 h and 1.18%·h^-1 in superheated steam drying, and 193 h and 0.64%·h^-1 in conventional drying, respectively. Final moisture content (MC), MC deviation in thickness and residual drying stress of lumber with superheated steam drying met the requirements of the 1^st grade of national standard for lumber drying quality. The corresponding quality index obtained by conventional drying met the 1^st grade, but the final MC was in 2^nd grade. There was no significant difference in MC distribution and residual drying stress between the two drying methods. As for drying defect, the crook, cup and twist of lumber after superheated steam drying met the requirements of the 1^st grade, however, warp index only met the requirements of the 2^nd grade. All defects' index of conventional drying lumber met the requirements of the 1^st grade. As for mechanical properties, the average MOE value of superheated steam drying and conventional drying lumber was 5 508.37 MPa and 5 237.52 MPa, respectively. However, the average MOR value of superheated steam drying and conventional drying lumber was 32.35 MPa and 34.13 MPa, respectively. The observation of cell wall showed that the extent and number of splits in pits membrane after superheated steam drying was greater than that after conventional drying. Thus, the moisture was easier to transfer and the permeability of wood was improved, and led to the increased drying rate. [Conclusion] Drying rate of Chinese cedar lumber was extremely affected by drying methods. Compared with the conventional drying, the drying time was shortened by 43% and drying rate was improved by 84% in superheated steam drying. There was no significant difference in drying quality and mechanical properties between the two drying methods, except for the final MC and warp. Porosity in wood after superheated steam drying was greater than that of conventional drying, and this is one of the reasons for improving the wood drying rate. All of these results indicated that the application of superheated steam drying for Chinese cedar lumber would be reasonable, and the drying quality could meet the requirements of wood products.

Key words: superheated steam drying, conventional drying, chinese Cedar lumber