| High temperature heat treatment technology mainly used nitrogen, vacuum, steam or vegetable oil as shielded medium at present, while there is lack of study in biomass gas heat treated wood. Compared with traditional industrial heat treatment technology, biomass gas heat treatment has many unique advantages in production efficiency, product quality and pollutant emissions. Considering the increasingly prominent of supply-and-demand in Chinese wood products market, it is important and realistic significance to study the biomass gas high temperature heat treatment technology to improve the utilization of fast-growing forest and value-added forestry products.In this paper, Larix gmelinii (Rupr) Kuzen was used as test material. This paper offers a systematic study of heat treatment:the temperature of heat treatment is 150℃,160℃,170℃, 180℃,190℃,200℃ and 210℃; The duration time of heat treatment is 2 h,4 h,6 h and 8 h, respectively. The study contains physical properties, chemical composition, combustion behavior, aritificial weathering and cell wall micromechanical properties of wood under different heat treatment process. Moreover, the different properties of heat-treated wood by nitrogen and biomass gas were discussed.The main research results contain:1. With increasing temperature and duration time of heat treatment, the moisture expansion and shrinkage rate, hydroscopicity, lightness value of wood decreased obviously. At the same time, the dimensional stability increased gradually. Both temperature and duration time are important factors for the properties of heat-treated wood and temperature has greater influence than duration time. With same heat treatment process, the moisture expansion and shrinkage rate are almost same for heat-treated wood by nitrogen and biomass gas. When temperature reached 190℃ and duration time reached 6 h, the color of Larix spp sapwood and heartwood are basically trend to same, which overcome the inhomogeneous color of sapwood and heartwood. Moreover, the heat-treated wood can partly replaced valueable timber for the closed color.2. The surface of wood turned to be grey and cell wall formed several cracks under artificial weathering (UV radiation and moisture spray). For short term weathering,210℃ heat-treated wood represent better color stability; While for long term weathering, the difference between heat-treated and untreated wood are not obvious.3. CONE was used to characterize the combustion parameters of wood. Because of the chemical transformation and micro-structure changes caused by heat treatment, the mass loss rate and average-heat release rate of 210℃ heat-treated wood decreased 46% and 42%, which shows positive effect to reduce fire hazard.4. By wood chemical analysis, heat treatment casued the content of hemicellulose and cellulose of sapwood reduced from 33.3% and 38.8% to 11.2～23.3% and 33.7～38.3%, while the lignin content increased from 26.4% to 36.7～49.3%; The content of hemicellulose and cellulose of heartwood reduced from 32.7% and 37.9% to 11.2～22.6% and 33.6-37.2%, while the lignin content increased from 26.9% to 37.5～48.7%. As expect, the chemical composition of heat treated wood by nitrogen and biomass gas are basically same.5. During high temperature heat treatment process, acetyl of hemicellulose breaked and formed acetic acid. The acetic acid promotes the hydrolysis reaction of hemicellulose and cellulose in acid environment; The molecular chain breaked; Degree of polymerization decreased; Produced oligosaccharide, disaccharide and monosaccharide. TG-FTIR detected the evaporation gas during wood pyrolysis process, such as water, CO, CO2, formaldehyde, phenolics and methane. After heat treatment, the evolved gas curves of pyrolysed wood has been changed obviously. When pyrolysis temperature is below 250℃, nonconjugated C=O bond is breaking and forming CO2; When pyrolysis temperature reaches 250-340℃, a large amount of (3-0-4 bond is breaking and forming CO2; When pyrolysis temperature is above 260℃, ether bond and diaryl ether bond is breaking and forming CO; Condensation polymerization and demethoxy reactions of lignin diphenylmethane happens during pyrolysis reactions, active points of aromatic rings increase and chemical reactivity of lignin increases. Due to the complicated chemical structure of lignin, phenolic compounds contain guaiacol, dimethoxyphenol and their derivative. TG-GC-MS detected a large amount of pyrolysis productions, such as acids, esters, alcohols, furans and so on. After heat treatment the evolved gas decreased clearly, while the constituents of evolved gas are more complicated.6. The temperature-response mechanism of real-time micromechanical properties of heat-treated wood cell wall has been detected by nanoindentation equipped with thermal-stage. The reduced elastic modulus of S2 layer cell wall of heat-treated Larix spp was decreased from 20.8GPa to 18.8-19.2GPa after heat treatment and the hardness increased from 0.61N/mm2 to 0.69-0.74N/mm2 at ambient environment(20℃). The modulus and hardness of heat-treated wood cell wall represent better stability at high temperature conditions. Burger model J(t)= J0+J1t+J2 [1-exp (-t/τ0B)] has well simulated the creep behavior of wood cell wall. Heat treatment reduced the creep ratio of wood cell wall, which is mainly caused by the recondensation and crosslinking reactions of lignocellulose structure, and better crystallinity of cellulose.7. Comparison the properties of several heat-treated wood by different shielded medium (Nitrogen, Air, biomass gas and vegetable oil). With increasing heat-treated temperature, the degradation reactions of wood performed higher intensity and mass loss rate increased gradually. The cell wall of both heat-treated wood by nitrogen and biomass gas represent a small amount of cracks, while the cell wall of air-heat treated wood show much more deep cracks. The higher deviation of modulus and hardness of oil heat-treated wood cell wall are related to the oil penetration into the wood cell wall. Generally, all these heat treatment have reduced the creep behavior of wood cell wall. Suitable shielded medium, temperature and duration time should be choosen by the finally application of wood products. |
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