Thermal behavior of wood heat treated under industrial conditions

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Wood cell wall

The source of wood materials is the plant cell walls, which play important roles in determining the structural integrity of the plant and in defense against pathogens and insects.
A wood cell wall is typically composed of three layers, namely, the primary cell wall, the secondary cell wall, and the middle lamella. Cellulose, hemicelluloses, and lignin have different distributions in these layers. Not all types of cells have secondary cell walls. The mature secondary cell wall accounts for the largest proportion of the lignocellulosic biomass, but its structure and organization begins in the primary cell wall (Davison et al., 2013). The secondary cell wallis the predominant structure in woody biomass, which usually consists of three sublayers; they are Sl, S2 and S3. Nevertheless, the integrated structure of cellulose, hemicellulose, and lignin in plant cell walls is still not fully understood (Cheng et al., 2015). The schematic of wood cell wall is shown in Fig. 2-2.

Chemical composition ofwood

Wood, as the main representative of lignocellulosic biomass, is mainly constituted by cellulose (a polymer glucosan), hemicellu1oses (also called polyose), and lignin (a complex tridimensional phenolic polymer) (Bamdad et al., 2018; Ding et al., 2018; Hernândez et al., 20 17), as shown in Fig. 2-3. In addition to these constituents, wood also con tains extractives and inorganic materials (also called ash) (Chen et al., 2015c). The relative contents of these constituents depend on the nature of biomass. For example, cellulose contents (wt%) in softwood, hardwood, and agricultural biomass are 41-50 %, 39-54 %, and 24-50 %, respectively; hemicellulose contents in these materials are 11-27 %, 15-36 %, and 22-35 %, respectively, and lignin contents are 27-30 %, 17-29 %, and 7-29 %, respectively (Kambo & Dutta, 2015). It follows that the relative contents of these constituents in biomass are generally ranked as cellulose > hemicelluloses > lignin. Table 2-1 is summarized the proportions of compositions (hemicellulose, cellulose, lignin, extractives, and ash) from various biomass (Fuller et al., 2018; Wang et al., 2017).

Characteristics of hemicelluloses, cellulose, and lignin

The structures and components in hemicelluloses, cellulose, and lignin are very different each other. Hemicelluloses, denoted by (C5H8O4)m (m: degree of polymerization, 100-200), is a branched mixture of various polymerized monosaccharides such as xylose, glucose, mannose, galactose, arabinose and glucuronic acid. The reactivity of hemicellulose is also higher than that of cellulose (Bach & Skreiberg, 2016). Cellulose, denoted by (C6H10O5)m (m: degree of polymerization, 7,000-12,000), is a linear homopolysaccharide composed of β-Dglucopyranose units linked together by (1→4)-glycosidic bonds (Balat et al., 2008). Cellulose has high amount of carbon compared to the other lignocellulosic components which leads to a significant proportion of energy content in biomass (Mosier et al., 2005). Cellulose molecules have a strong tendency to form intra- and inter-molecule hydrogen bonds which create crystalline micro-fibrils surrounded by amorphous cellulose (Acharya et al., 2015). On account of this special structure, cellulose is more thermally stable than hemicellulose (Bach & Skreiberg, 2016).
Lignin, denoted by [C9H10O3‧(OCH3)0.9-1.7]m (Chen et al., 2011), is a three-dimensional, highly branched, and polyphenolic substance which consists of an irregular array of variously bonded “hydroxy-” and “methoxy-” substituted phenylpropane units (Chen & Kuo, 2011b). Lignin in lignocellulosic biomass acts a binding element for cellulose and hemicellulose structures (Acharya et al., 2015); it also works as a glue in the densification processes (Chen et al., 2015c). The glass transition temperature of lignin is between approximately 135 and 165 °C (Reza et al., 2012). When the temperature during pelletization is higher than the glass transition temperature and the moisture content is between 10% and 15%, lignin in biomass softens and enhances the inter-particles binding.
Overall, cellulose provides a supporting fibrous mesh which is reinforced by lignin polymers (Simoneit, 2002). Cellulose is mainly responsible for the structural strength of wood. Hemicelluloses molecules are less structured than cellulose and their sugar composition varies widely among different tree species. The lignin biopolymers are the fillers in woody tissue making it a complex substance. On account of these inherent differences in structure and composition, the properties among the three constituents are different each other. For example, the hydrophobicity of cellulose is medium, and those of hemicellulose and lignin are low and high, respectively (Kambo & Dutta, 2015). The thermal decomposition characteristics of cellulose, hemicellulose, and lignin demonstrate different reactivities. The thermal decomposition temperature (TDT) of hemicellulose is between 220 °C and 315 °C. Therefore, when biomass is treated by mild pyrolysis, this thermal treatment generally has a drastic impact on hemicelluloses (Chen & Kuo, 2010). The TDT of cellulose is normally between 315 °C and 400 °C (Lu et al., 2012; Yang et al., 2007). The main part of lignin is thermally sable; therefore, its complete degradation requires relatively high temperatures and sufficient time. Although lignin softens at temperature as low as 135 °C (Ciolkosz & Wallace, 2011), its thermal decomposition temperature is in the range of 160-900 °C (Chen et al., 2015c). Fig. 2- 4 shows the thermal decomposition of standard samples, such as cellulose (AlfaAesar, A17730), hemicellulose (SIGMA, X-4252), lignin (Tokyo Chemical Industrial Co., L0045), xylose (SIGMA, X-1500) and glucose (Panreac Quimica SA, 131341), by the thermogravimetric analysis (TGA) (Chen et al., 2015c).

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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.

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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