Thermal behavior of wood heat treated under industrial conditions 

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Wood heat treatment

Among the various techniques of wood treatment mentioned above, wood heat treatment (or thermal modification of wood) is an eco-friendly technology because no chemicals are utilized and added into this process to improve the wood’s durability and dimensional stability, whereas toxic chemicals may be used in other chemical modification methods (Candelier et al., 2016; Esteves & Pereira, 2008). In addition, unlike chemically treated woods impregnated by biocidal active compounds, the heat treated wood can be recycled at the end of life cycle without detrimental impact to the environment (Candelier et al., 2016).

Characterization of wood heat treatment

In general, wood heat treatment is conducted in an inert atmosphere (i.e. N2 atmosphere) where the temperature and heating rate are in the ranges of 180-240 °C and 0.1-2 °C min-1, respectively. The upgraded properties conferred to the wood are the result of thermal degradation of wood cell wall polymers occurring during treatment (Candelier et al., 2016). When wood undergoes heat treatment, hemicelluloses are the most active components compared with cellulose and lignin (Chen et al., 2017). In the study of Chaouch et al. (2010), they examined heat treatment of different softwood and hardwood species at 230 °C under nitrogen. The results revealed that the main modifications occurring during wood heat treatment are due to the degradation of hemicelluloses through depolymerisation reactions to C5 and C6 monosaccharides. In the related study, Chaouch et al.(2013) found that the profiles of mass loss dynamics were also different between softwood and hardwood species. The mass losses in treated hardwood species were higher than treated softwood species, and it was mainly due to the higher acetyl content present in hemicelluloses of hardwood. Meanwhile, the O/C and H/C ratio of treated woods were also lower than untreated woods because of the thermal degradation. Candelier et al. (2011) investigated the volatile products generated during wood heat treatment between 180 and 230 °C for 15 min by a thermodesorption coupled to gas chromatography coupled to mass spectroscopy (TD-GC–MS). The obtained results suggested that the high amounts of acetic acid generated during thermal degradation of strongly acetylated glucuronoxylan in hardwoods were associated to the formation of numerous degradation products, such as furfural, methylfurfural and vanillin (Candelier et al., 2013c).
After heat treatment, some mechanical properties are reduced, because of the degradation of hemicelluloses, which connects cellulose and lignin in the cell wall (Candelier et al., 2013b). It is reported that the surface hardness of heat treated wood is improved, while other mechanical properties, such as bending strength, compression strengths, cleavage strength and shear strength, are considerably weakened according to the treatment intensities (Candelier et al., 2016; Korkut & Hiziroglu, 2009). Dwianto et al (1996) revealed that the degradation of hemicelluloses causes the cross-linking reactions in the lignocellulosic matrix and the crystallization of microfibrils, as well as the relaxation of stresses stored in microfibrils and matrix. Yildiz et al. (2006) examined the effects of heat treatment on compression strength (CS) of spruce wood, and the results indicated that the CS values of treated wood generally exhibit a decrease when increasing the treatment time and the temperature. Tankut et al. (2014) investigated the mechanical properties of different heat-treated woods (black pine, scotch pine, oriental spruce, iroko, and ash). They pointed out that heat treatment clearly decreased the modulus of rupture (MOR) by 19% and the modulus of elasticity (MOE) by 24%. Although the mechanical performance of treated wood is decreased, it implies that the grindability iimproved. The enhancement of grindability in treated wood is a benefit for fuel application (Chen et al., 2015c; Colin et al., 2017).

Heat treatment under steam / oil

Instead of wood thermal treated in nitrogen condition, different inert mediums, such as steam and oil (Hill, 2007), can also perform heat treatment. In the study of Esteves (2006), t the decreases of hygroscopicity and wettability of wood after steam treatment at 190-210 °C were observed. The equilibrium moisture content of treated samples decreased in the range of 46-61 %, and the radial contact angle of a water drop on wood surface increased from 40 ° to around 80 ° after treatment. Cao et al. (2012) examined dimensional stability of Chinese fir by steam-heat treatment at 170-230 °C for 1-5 h. They indicated that the dimensional stability of treated wood was improved remarkably, and the maximum increase rate of ASE (anti-shrink efficiency) was 72.63 % for heartwood and 70.71 % for sapwood. Moreover, they also pointed out that the treatment temperature played an important role on the improvement of dimensional stability.
Li et al. (2015a) studied the structural characterization of steam-heat treated wood (Tectona grandis) from 120 °C to 220 °C at intervals of 20 °C. The results indicated that the changes of chemical structure become more intense with increasing treatment temperature, and the most significant changes occurred at the treatment temperature of 200 °C. The cleavage of the β-O-4 linkages and the splitting of the aliphatic methoxyl chains from the aromatic ring in lignin were also found with increasing treatment temperature. Yin et al. (2017) performed the heat treatment of spruce by compression combined with steam treatment (CS-treatment). They investigated the changes of chemical structure and cellulose crystallinity by Raman microscopy and X-ray diffraction, respectively. The results revealed that the cellulose structure was affected by the treatment and that β-aryl-ether links associated to guaiacyl units of lignin were depolymerized followed by re-condensation reactions. They also reported that the crystallinity index (CrI) and crystallite thickness (D200) of cellulose for CS-treated wood were significantly increased owing to crystallization in the semi-crystalline region of cellulose.
Regarding to oil heat treatment (or oleothermal treatment) of wood, this process can improve the properties of wood through synergetic effect of the oil and heat (Lee et al., 2018). The related studies have pointed out that the mechanical properties (such as MOR, MOE, and bending strength) of oil heat treated wood could be better than that treated under nitrogen or steam, and the performance of oil heat treatment is depending on the type of oil (Lee et al., 2018; Li et al., 2015b; Rapp & Sailer, 2000). In the study of Lyon (2007), they evaluated biological resistance of oil heat treated wood (Japanese cedar and beech) by three different vegetable oils (soybean oil, rapeseed oil, and linseed oil) at 130 °C. The results found that linseed oil was the most effective oil to produce durable samples followed by soybean and rapeseed oil. This observation could be attributed to the high content of polyunsaturated fatty acids in linseed oil, and results in effectively prevented the penetration of water into the wood samples.

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Heat treatment under vacuum

The vacuum process is a novel and promising technology which is suitable for biomass pyrolysis, carbonization, and wood heat treatment (Candelier et al., 2013b; Dewayanto et al., 2014; Ismadji et al., 2005). The applications of the aforementioned thermochemical processes with the vacuum technique are summarized in Table 2-4. In a vacuum process, heat is mainly transferred to the sample through conduction, and a vacuum pump is employed to continuously remove volatile compounds released from biomass, thereby accelerating the thermal degradation of polysaccharides in biomass (Hill, 2007).
As far as wood heat treatment in vacuum is concerned, it is an alternative and novel technology for thermal modification of wood where the oxygen content in a reactor is reduced to avoid wood combustion (Sandak et al., 2015). Allegretti et al. (2012) studied thermal modification of spruce and fir under vacuum (150, 210, and 350 mbar) at the temperature range from 160 to 220 °C. There are four different funguses (P. placenta, C. puteana, T. versicplor, and G. trabeum) were used to examined the durability of heat treated wood. The results indicated that the most aggressive fungus on heat treated wood was brown rot P. placenta, and it caused mass loss in the range from 10 to 20 wt%. Based on the results, they reported that heat treated wood at 220 °C for 2.5 h showed a significant improvement of durability. de Oliveira Araújo et al. (2016) investigated heat treatment of three different common wood species (bracitinga, feroba mica, and cumaru) in South America under nitrogen and vacuum at the temperature range from 180 to 220 °C. The results reported that the mass loss were lower for the treatment under nitrogen than under vacuum for all treatment temperatures and species. The equilibrium moisture content (EMC) of treated wood was significantly reduced, and it was more effective to reduce hygroscopicity of wood under vacuum than nitrogen.
Candelier et al. (Candelier et al., 2013a) carried out the heat treatment of beech under nitrogen and vacuum, as well as made the comparison of chemical composition. All treatments were performed at 220 °C for mass losses resulting from wood thermal degradation of approximately 12 wt%. The results indicated that wood treated under nitrogen present higher Klason lignin and carbon content, lower hemicelluloses and neutral monosaccharides contents comparatively to treated wood under vacuum. Ferrari et al. (2013a, 2013b) investigated heat treatment of Turkey oak under vacuum, steam, and vacuum combined with steam conditions. The temperature for wood treatment was between 100 and 110 °C under steam, as well as 160 °C under vacuum. The results indicated that there was a significant influence on color difference before and after treatment under vacuum. However, the treatment under steam at 110 °C for 24 h can obtain wood products with greater color homogeneity.

Table of contents :

Chapter 1 Introduction
1.1 Background
1.2 Objectives
1.3 Overview
Chapter 2 State of the Art
2.1 Wood material
2.1.1 Wood cell wall
2.1.2 Chemical composition of wood
2.1.2a Characteristics of hemicelluloses, cellulose, and lignin
2.1.2b Extractives and ash
2.1.3. Wood preservation processes
2.2. Wood heat treatment
2.2.1 Characterization of wood heat treatment
2.2.2 Heat treatment under different atmospheres
2.2.2a Heat treatment under steam / oil
2.2.2b Heat treatment under vacuum
2.3 Kinetics of biomass pyrolysis
2.3.1 Non-isothermal pyrolysis
2.3.2 Isothermal pyrolysis
Chapter 3 Methodology
3.1 Material preparation
3.2. Experimental system and procedure
3.3 Analysis of wood samples
3.3.1 Proximate analysis
3.3.2 Elemental analysis
3.3.3 Fiber analysis
3.3.4 Thermogravimetric analysis
3.3.5 Scanning electron microscope
3.3.6 FTIR and XRD analyses
3.3.7 Color measurement
3.3.8 EMC and contact angle examinations
3.4 Numerical modeling
3.4.1 Kinetic model
3.4.2 Elemental composition model
Chapter 4 Results and Discussion
4.1 Thermal behavior of wood heat treated under industrial conditions
4.1.1 Mass loss dynamics during heat treatment
4.1.2 Thermogravimetric analysis of treated wood
4.1.3 SEM of treated wood
4.1.4 Proximate and elemental analyses of treated wood
4.2 Property changes of heat treated wood
4.2.1 Changes of chemical structure
4.2.2 Changes of color
4.2.3 Changes of hygroscopicity and wettability
4.2.4 Correlations between element removals and changes of color and hygroscopicity
4.3 Kinetic modeling of wood heat treatment
4.3.1 Solid yield prediction and kinetic parameters
4.3.2 Characteristics of solid and volatile products
4.3.3 Prediction of elemental composition
4.3.4 Characteristics of devolatilization
Chapter 5 Conclusions and Perspectives
5.1 Conclusions
5.2 Perspectives and suggestions
References

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