One apparent change in a torrefied biomass is the colour. Studies have shown that the colour changed to a more intense brown with increased torrefaction condition whether it is increase in temperature and longer residence time (Bridgeman et al, 2010; Phanphanich and Mani, 2011). Figure 3.8 illustrates the changes in colour for woody biomass fuels at various conditions.
Figure 3.8. Images of willow: a) untreated; b) low temperature, short residence time; c) low
temperature, long residence time; d) high temperature; short residence time; e) high temperature, long residence time (Bridgeman et al., 2010).
3.3.2 Particle size and shape
The other observable transformation is the difference in particle size and shape. Arias et al (2008) looked into these changes using an optical microscope to get a deeper insight of the structural modification of eucalyptus that was subjected to torrefaction (at 240°C and 280°C with a residence time of 3 h). Figure 3.9 shows that the raw biomass fuel started off as being highly fibrous in nature and it became more spherical and less fibrous when torrefied. Particle sizes also decreased with increased conditions (temperature and residence time). When a sieving process was conducted, a large number of small particles were able to pass through the sieves, which indicate the reduction in size for torrefied biomass and was assumed to have become more spherical.
Figure 3.9. Images of raw eucalyptus (RE), torrefied eucalyptus (TRE) at 240°C and 280°C
respectively (Arias et al., 2008).
These changes can be explained by the transformation in the structure of hemicellulose. Repellin et al (2010) stated that torrefaction leads to the shrinkage of the lignocellulosic material, thus, creates stress in the wood fibres and favours cracks. Almeida et al (2010) examined raw and torrefied Eucalyptus Saligna (E.Saligna) via Scanning Electron Microscopy (SEM) and the results can be observed in Figure 3.10. Here, damage to the structure with several fractures appeared in the most fragile tissues, while the raw E.Saligna was seen to be strong and intact.
Figure 3.10. SEM images of raw (left) and torrefied (right) E.Saligna at 280ºC with a
residence time of 5 hrs (Almeida et al., 2010).
3.3.3 Mass loss
Figure 3.11 illustrates the reduction of the percentage mass of reed canary grass (RCG) with increasing temperature, from 230 to 290°C (Bridgeman et al., 2008). This change was not only primarily due to the loss of moisture via evaporation during the drying stage but also because of the further release of reaction water vapour and the production of volatiles from the degradation of hemicellulose and minor decomposition of cellulose throughout the treatment process.
3.3.4 Moisture, ash, volatile and fixed carbon content
Apart from the colour, particle size and shape, physicochemical properties of interest in a biomass also include the contents of moisture, ash, volatile matter and fixed carbon. This can be determined from proximate analysis. It is one of the two fundamental standard fuel analysis that provides an indication for the suitability of a biomass fuel to be utilised for energy purposes. The changes obtained from proximate analysis of torrefied biomass fuels have been widely studied in Bourgois and Guyonnet (1988), Pentananunt et al (1990), Felfli
et al (2005); Blagini et al (2006), Bridgeman et al (2008; 2010), Sadaka and Negi (2009),
Yan et al (2009), Almeida et al (2010), Pimchuai et al (2010), Wannapeera et al (2011), Medic et al (2012) and Pirraglia et al (2012), to name a few. In general, with increased severity of torrefaction condition, the moisture content and volatiles are reduced, while those of ash and fixed carbon increased. Figure 3.12 illustrates the changes in these contents for torrefied rice husks, where the moisture content and volatiles have reduced from 4.0 to 2.5% and 55 to 30% respectively, while the ash content has increased from 20 to 32% when the temperature increased from 250 to 300ºC with a residence time of an hour. In this experiment, temperature is seen to have a more significant influence to the results than
residence time. With a longer residence time, the moisture content continued to decrease, but minor changes were observed in volatile and ash contents.
Figure 3.11. Mass loss during torrefaction of reed canary grass at different temperatures
(Bridgeman et al., 2008).
Figure 3.12. The change in the moisture, volatile and ash contents of torrefied rice husks
3.3.5 Carbon, nitrogen, hydrogen and oxygen contents, Higher heating value (HHV) and Van Krevelen plot
Bridgeman et al (2008) stated that the ultimate analysis of a biomass gives an indication to any changes that occur in its chemical composition when it is exposed to torrefaction. Reed canary grass, willow and wheat straw respectively, were torrefied at four final temperatures (230, 250, 270 and 290ºC). Figure 3.13 illustrates the influence of temperature on the carbon, hydrogen and oxygen contents of those three biomass fuels. The carbon content increased, while contents of the other two elements decreased with increased temperature. These values provide useful information for the determination of higher heating value (HHV). The results showed that the HHV increased with increased temperature. For example, willow that was torrefied at 290ºC has a calorific value of 21900 kJ kg-1, while that torrefied at 250ºC was 20200 kJ kg-1 and when it was untreated, the calorific value was 20000 kJ kg-1.
Figure 3.13. Changes in a) carbon, b) hydrogen and c) oxygen upon torrefaction at increasing
Another useful information that can be obtained from this analysis is the indication as to how close the torrefied biomass is to low rank coals in terms of chemical composition, and in turn, higher heating value, by plotting a Van Krevelen diagram (Figure 3.14). Many authors have used this technique such as in Bridgeman et al (2008), Wu et al ( 2012), van der Stelt et al (2011), Rousset et al (2011), Prins et al (2006), and Phanphanich and Mani (2011). As can be seen in Figure 3.14, those that were torrefied at higher temperatures tend to have similar characteristics, as they are approaching closer to those of low rank coals such as lignite. Therefore, the more severe the torrefaction condition is, the closer the biomass’ characteristics are to those of coals. However, one cannot just torrefy the biomass at any high temperature because there is one crucial parameter that needs to be taken into account and that is energy yield. Energy yield has been found to decrease with increasing temperature (Bergman et al., 2005a; Bridgeman et al., 2010). However, the energy yield can be controlled by carefully adjusting the temperature and residence time during the torrefaction (Wannapeera et al., 2011).
Figure 3.14. Van Krevelen diagram for coals, raw and torrefied biomass fuels, where A-D
represents the severity of torrefaction conditions. Table 3.2 provides the description for such conditions, where A as the least severe and D as the most severe (Bridgeman et al., 2008).