As a promising bioenergy pre-treatment technology, torrefaction has the potential to make a major contribution to the thermal modification of biomass. (BATIDZIRAI et al., 2013) showed detailed insights into state of the art prospects of the commercial utilization of torrefaction technology over time identifying process performance characteristics such as thermal efficiency and mass yield and discussing their determining factors through analysis of mass and energy balances. The majority of the torrefaction technologies being developed are based on already existing reactor concepts designed for other purposes such as drying or pyrolysis (KIEL, 2011) and thus only require technical upgrading for torrefaction applications. The reactors being developed are in most cases established technologies that companies are familiar with and have been optimizing for torrefaction applications. Currently, no single technique is fundamentally superior to the others as all of them have their advantages and disadvantages (BATIDZIRAI et al., 2013). Proper selection of reactor is important as each reactor has unique characteristics and is well suited to handle specific types of biomass. Therefore, for given biomass properties and application, the proper technology can be selected.
In recent years, many companies have invested in the development of roasting processes. The main technologies known to date and advantages and disadvantages of each technologies are presented in Table 3.
Chemistry and kinetics
Biomass pyrolysis chemistry is complex due to the wide variety of chemical species generated, variabilities in feedstock characteristics, and the wide range of temperature, pressure, and heating rate conditions which must be considered. Moreover, it is technically difficult to separate the effects of secondary reactions and the catalyzing effects of mineral components. Chemistry research received a strong push after the oil embargo of the late 1970’s and many seminal papers were published in the early 1980’s. Similar economic motivations combined with recent technical advancements in instrumentation have caused a resurgence of this field.
Experimental methods for pyrolysis and torrefaction chemistry and kinetics are studied with a variety of experimental devices and some commonly used techniques include are TGA, DTG, Fourier transform infrared spectroscopy (FTIR), high performance liquid chromatography (HPLC), gas chromatography (GC), and bomb calorimetry. For a more comprehensive review on experimental analysis see (BAHNG et al., 2009). TGA analysis allows precise measurement of mass loss under controlled temperature profiles and is therefore used to validate and measure kinetic models and parameters. DTG shows the rate at which products are formed, and can be used to compare the pyrolysis and combustion profiles of raw and torrefied feedstocks (BRIDGEMAN et al., 2010). FTIR allows the real-time analysis of volatiles released during pyrolysis and torrefaction (CHEN et al, 2012a) (LV et al,, 2015). HPLC and GC can be used in tandem with TGA during batch experiments to perform mass balance and volatile composition analysis.
Based on the chemical formulas of the three constituents, the atomic O/C ratios in cellulose, hemicellulose and lignin are found to be 0.83, 0.80 and 0.47-0.36, respectively, and their atomic H/C ratios are 1.67, 1.6 and 1.19-1.53, respectively. In view of their distinct compositions and structures, cellulose, hemicellulose and lignin possess different thermal decomposition characteristics. Generally speaking, the thermal decomposition temperature (TDT) of hemicellulose is the lowest among the three constituents at the range of 220 and 315°C. Cellulose decomposes at temperatures between 315 and 400°C. Lignin is featured by gradual decomposition for the temperature ranging from 160 to 900°C (LU et al., 2012) Figure 10a and b show the typical thermogravimetric (TGA) and derivative thermo-gravimetric (DTG) curves of the standard samples of cellulose (Alfa Aesar, A17730), hemicellulose (SIGMA, X-4252), lignin (Tokyo Chemical Industrial Co., L0045), xylose (SIGMA, X-1500) and glucose (Panreac Quimica SA, 131341). In some biomass samples, the decomposition peaks of cellulose and hemicellulose from DTG can be identified clearly (CHEN et al., 2010). Whereas the two peaks overlap in some biomass samples so that it is hard to be distinguished. (CHEN et al., 2015).
Hemicelluloses are branched polysaccharides composed of 5-carbon sugars such as xylan (the majority in hardwoods), and 6-carbohydrates, such as glucose and mannose (the majority in conifers) (TRIBOULOT et al., 2001) that plays a primordial role in the cell wall cohesion. Indeed, it allows the bonding between the cellulose fibers and lignin. Hemicellulose chains are amorphous and contain many hydroxyl groups which make it the most hydrophilic biomass compound. It is therefore considered to be the main responsible for the affinity of wood with water (COLIN, 2014).
Hemicelluloses are the most highly degraded polymers at torrefaction temperatures (NOCQUET et al., 2014; CHEN et al., 2011b). The main reactions involved in hemicelluloses torrefaction are dihydroxylation, deacetylation and depolymerization (WEILAND and GUYONNET, 2003) . Under the most severe treatment conditions, almost all hemicellulose is degraded (CHEN et al., 2012). Finally, xylan is more sensitive to temperature than glucomannans, so deciduous trees have a greater loss of mass than conifers under identical treatment conditions (PRINS et al., 2006).
Cellulose is the major biomass component. Cellulose chain is formed from 5,000 to 10,000 units of glucose (ROUSSET, 2004). These chains are assembled in the form of microfibrils which themselves form fibrils. It should be noted that some portions of the microfibrils are disordered (amorphous cellulose) while others are ordered (crystalline cellulose). It is possible to define a crystallinity index (crystalline cellulose / total cellulose ratio) which is generally between 0.6 and 0.7 for raw wood (TRIBOULOT et al., 2001).
Cellulose thermal degradation has been the subject of several studies TRIBOULOT et al., 2001; NOCQUET et al., 2014; CHEN et al., 2011b). It appears that cellulose has significant mass losses for temperatures above 250°C (NOCQUET et al., 2014; CHEN et al., 2011b). After product solid and the volatiles released analysis, it has been shown that at these temperatures the main degradation mechanism is dehydroxylation (SARVARAMINI et al., 2013). The loss of -OH groups would thus lead to the formation of a less hydrophilic cellulose containing unsaturated pyranoses. The molecules produced by these reactions are mainly water molecules, but also levoglucosan, CO and CO2 at the highest temperatures (280-300°C).
Another trend often observed is the increase in the cellulose crystallinity index at low temperatures (120-180°C) (AKGÜL et al., 2006). This increase is mainly due to the preferential degradation of amorphous cellulose, which increases the proportion of crystalline cellulose (WIKBERG, 2004). However, other authors explain this evolution by a change in molecular organization that would transform amorphous cellulose into crystalline cellulose (MELKIOR et al., 2012; SINGH et al., 2013). It is therefore not impossible that these two phenomena occur simultaneously to lead to an increase in the crystallinity index. This increase has a direct impact on the properties of torrefied wood because the cellulose crystalline configuration limits the water penetration into the fibers, which makes the material less hygroscopic (TRIBOULOT et al., 2001; SINGH et al., 2013). However, it has been observed that for the treatments at the highest temperatures, the crystalline cellulose is degraded in turn, which can promote the return of water (HILL et al., 2013).
Lignin are amorphous compounds that rigidify the cell wall and allow cohesion between the different cells. These alcohols form polymers (mainly hydroxyphenyl, gaiacyl and syringyl units) which themselves form lignin whose composition differs according to the biomass considered. Lignin torrefaction studies have shown that degradation starts at lower temperatures than cellulose (about 150°C) (SARVARAMINI et al., 2013). However, mass loss is limited (less than 15wt%) for temperatures below 250°C (NOCQUET et al., 2014).
Indeed, if certain volatile materials are released (mainly water, CO, CO2 and formaldehyde), in particular as a result of demethoxylation reactions, the main reactions occurring in the treatments temperature ranges are reactions of condensation (WINDEISEN et al., 2007; ROUSSET et al., 2009). These lead to the formation of crosslinked compounds derived from lignin. Depolymerization reactions are then carried out at temperatures above 250°C, whereas degradation of the monomers produced would only occur from 300°C (MELKIOR et al., 2012).
Produced volatile matter consists of condensable gases and permanent gases (non-condensable). The relative proportions of these two types of compounds depend on the biomass, the duration and treatment temperature (PRINS et al., 2006b). However, the condensable gases mass yield (ratio of the produced condensable mass to dry biomass initial mass) is always higher than non-condensable gases.
Condensable gases are mainly composed of water, acetic acid, formic acid, methanol, lactic acid and furfural. Water and acetic acid (markers of hemicelluloses degradation) are largely in the majority regardless of the treatment conditions (PRINS et al., 2006b; BATES and GHONIEM, 2012). The production of carbon monoxide seems to be favored by the high temperatures. Heavy condensable species (mainly aromatic compounds which can be subjected to material recovery), present in small quantities, have also been identified (CHEN et al., 2011b). Finally, recently, Nocquet et al., (2014) highlighted the importance of formaldehyde production as the second condensed species produced after water, as shown in Figure 11.
The incondensable gases commonly observed are CO2 and CO, being CO2 the majority Perhaps, CO/CO2 ratio increases with the roasting temperature (PRINS et al., 2006b). Small amounts of CH4 are also observed at higher processing temperatures, particularly when roasting agricultural residues (DENG et al., 2009).
Interaction between different biomass constituents
As previously discussed, the individual behavior of biomass main components subjected to torrefaction has been widely studied. However, their common evolution within the material remains little known. Indeed, there is still considerable uncertainty about the synergetic effect of cellulose, hemicelluloses and lignin degradation, but also on the role of ash in raw biomass.
Despite the lack of data on these phenomena, certain hypotheses have been put forward:
• Acetic acid released during the degradation of hemicelluloses acts as a catalyst for depolymerization of cellulose (WIKBERG et al., 2004; WINDEISEN et al., 2007) or even lignin (MELKIOR et al., 2012);
• Radical compounds formed by hemicelluloses degradation could react with the phenolic compounds of lignin (ROUSSET et al., 2009).
• Alkali metals (mainly potassium) in the ash would act as catalysts for roasting (SALEH et al., 2013; SALEH et al., 2013b; SADDAWI et al., 2012).
These hypotheses lead us to believe that the behavior of biomass can not be assimilated to the sum of the behaviors of its constituents (NOCQUET et al., 2014). They have compared the change in the mass yield of beech during torrefaction with the predicted evolution by additivity of the behavior of its various constituents: for temperatures above 250°C, the additivity law does not work. Moreover, the observation of the loss of mass of mixtures of the various pure components made it possible to demonstrate that the main interactions concern the cellulose-lignin and cellulose-hemicellulose mixtures (COLIN, 2014).
Thermo-acoustic torrefaction lab-scale reactor conception
The motivation for this section arises from the potential coupling of an acoustic system to a torrefaction reactor to improve the wood heat treatment.
In torrefaction analysis references no work was found coupling torrefaction with an acoustic field over pyrolysis or oxidative conditions. The assumption is that an acoustic field within a torrefaction reactor modifies the pressure and particles velocities around the wood sample. The combined effect of heat and acoustics could modify the interaction between reactor gaseous environment and wood sample, modifying degradation processes development (SILVEIRA et al., 2017).
To that end, an acoustic system was applied inside an existing torrefaction reactor (ROUSSET et al., 2012) and subsequently characterized. Three different methodologies were used in terms of time and frequency domains. This characterization allowed the measurement of the flow rate and acoustic intensity at the exact spot where the sample was in the reactor. These acoustic results were analyzed and used to predict which acoustic frequency and intensity produced the ideal conditions for obtaining higher particles velocities around the wood sample. The acoustic system coupled to the existing torrefaction reactor (ROUSSET et al., 2012) is illustrated Figure 14.
The acoustic experiment was performed with a humidity of 50%, an average temperature of 24°C, speed of sound 𝑐=345 m.s−1 and an air density of 𝜌=1.23 kg.m−3. Within the experimental acoustic system, the desired frequencies were produced by an HP 33120A wave generator with a broadband frequency of 20Hz – 20 kHz. The acoustic wave was delivered by a Selenium D220TI 8 speaker connected by a flexible duct (ROSSETO, 2001) to the reactor cavity measuring 41×32×40 cm. Different acoustic frequencies produce different excitations of the reactor’s cavity, hence a different pressure field. Frequencies were explored within a range of 0-3000Hz.
Thermogravimetric analysis TGA
Thermogravimetric analysis of a micro-particle sample of Eucalyptus grandis was performed to get information on solid mass evolution versus time and temperature. This analysis allowed the characterization of thermodegradation in micro-scale, providing information on the mass loss and volatile release dynamics (identification of functional groups throughout the treatment by a FTIR equipment connected in line with the TGA). These data were used for the discussions of thermoacoustic torrefaction and degradation kinetic model.
The thermal behaviors of the samples (about 15 mg of milled wood per run in ceramic crucibles with a 60 mesh) were investigated using a SDT Q600 TA which provides instantaneous measurement of mass variation (TGA). The samples were heated at a linear heating rate of 20°C.min−1 until 105°C and kept for 30 minutes to assure dry condition. After drying, a heating rate of 5°C.min−1 was imposed until the desired temperature of 210, 230, 250, 270 and 290 °C. Thereafter, they were torrefied for 60 minutes. Nitrogen was used as purging gas at a flow rate of 50 mL.min-1. The torrefaction treatment parameters are listed in Table 6.
Table of contents :
1. RESEARCH OBJECTIVES AND MOTIVATION
1.1 Research Outline
2. STATE OF ART
2.1 Energy context
2.2.2 Thermochemical conversion pathway
2.2.3 Torrefaction process
2.2.4 Torrefaction technologies
2.2.5 Chemistry and kinetics
2.3 Biomass thermal decomposition numerical models
2.3.1 Kinetic Model
2.3.2 Composition Model
2.4.1 Frequency domain
2.4.2 Time domain
3. CASE OF STUDY
3.1 Thermo-acoustic torrefaction lab-scale reactor conception
3.2 Acoustic characterization techniques
3.2.1 Acoustic velocity/pressure formulation
3.3 Biomass thermodegradation
3.3.2 Biomass torrefaction
3.3.3 Biomass thermo-acoustic torrefaction
3.3.4 Torrefied solid product analysis
3.4 Biomass torrefaction model
3.4.1 Wood kinetics model formulation
3.4.2 Biomass solid and volatile composition model
4.1 Reactor acoustics characterization
4.2 Biomass torrefaction results
4.2.1 Thermogravimetric (TGA): Micro-samples results
4.2.2 FTIR results
4.2.3 Torrefied solid product pyrolysis results
4.2.4 Thermogravimetric (TGA): Macro-samples results
4.3 Biomass thermoacoustic torrefaction results
4.3.1 Temperature and solid yield dynamics
4.3.2 Chemical analysis
4.3.3 Optimum frequencies
4.4 Biomass numerical model
4.4.1 Biomass kinetic model validation
4.4.2 Eucalyptus Kinetics
4.4.3 Composition Model
5. CONCLUSIONS AND PERPSECTIVES