Chemical composition of biomass - Biomass as Energy Source: Resources, Systems and Applications - page 49

Figure 2.6. A van Krevelen diagram for un-treated biomass (open circles); Norway spruce wood chips
torrefied at 260-300 ◦ C for 8 to 25 minutes and commercial charcoal from softwood produced at
about 450 ◦ C (filled circles); and biochar produced at 600 ◦ C (Lehmann et al ., 2011) (diamonds).
The material with the highest average ratios is leaves from switch grass and sugarcane. Data on
biomass and torrefied wood were kindly provided by Tao et al . (2012a) and Nordwaeger et al .
(2013 a and b), respectively.
Biomass is a visco-elastic and fibrous material and its bulk properties are generally difficult
to predict because of its irregular shapes, wide particle size distributions, low bulk densities, and
often high moisture contents. Further, biomass can have particles with very inconvenient shapes
in which one or two dimensions are very much smaller than the third (e.g, straw, fibers and flakes);
this causes phenomena such as nesting. There are various mechanical problems associatedwith the
bulk handling and flow characteristics of biomass, including bridging, compaction, and unwanted
separation in silos and transporters. All of these are exacerbated by the presence of fine particles
and dust, which also increase the risk of dust explosions.
The chemical composition of the material also affects its flow ability and processing. Samuels-
son et al . (2009) and Nielsen et al . (2009) have shown that biomass samples with high
concentrations of extractives in biomass will generate comparatively low wall friction at high
pressures in systems such as pellet presses. High levels of organic acids and inorganic compo-
nents in biomass will increase the wear on steel in transport canals, transporters, mills, presses,
and so on.
2.6.5 Heat treatment of biomass
Pyrolysis is thermal treatment under anaerobic conditions, usually at temperatures of 450-550 ◦ C
or more. Torrefaction is a mild form of pyrolysis conducted at about 250 to 330 ◦ C. In general,
compared to raw and absolutely dried biomass, torrefaction produces material that has lost more of
its mass (as a percentage) than its energy content. The oxygen-containing constituents of biomass
have lower energy contents than other constituents. Consequently, the gases released during
torrefaction typically have higher oxygen contents than the remaining biomass, as demonstrated
by analyses of C, H and O levels in torrefied materials. A good overview of the torrefaction
process can be obtained by constructing van Krevelen diagrams (van Krevelen, 1950) with the
atomic O/C ratio on the horizontal axis and the H/C ratio on the vertical axis. A linear trend
was observed in a series of experiments conducted using different torrefaction temperatures and
treatment times (see Fig. 2.6).
Van Krevelen diagrams for other materials such as coal and anthracite have steeply sloping
H/C trend lines when the O/C ratio is lower than 0.1, as shown by van der Stelt (2011). As such, it
may be hard to obtain oxygen-free materials that still contain hydrogen by pyrolysis of biomass.