Chemical composition of biomass - Biomass as Energy Source: Resources, Systems and Applications - page 50

Figure 2.7. Changes in relative biomass atomic (molar) ratios after losses of C, H, O, COCO 2 ,H 2 OandCH 4
in processes such as fermentation (biogas formation), torrefaction, pyrolysis, and gasification.
Furthermore, as indicated by the somewhat anomalous average value for bamboo (see Fig. 2.6),
the starting atomic ratios in the untreated biomass may influence the product O/C and H/C ratios
after treatment.
Charcoal, which is also known as biochar, is a carbon-rich product produced by pyrolysis of
plant biomass at about 350-600 ◦ C. The storage of biochar in soil has been suggested as a method
for sequestering carbon from CO 2 in the atmosphere and thus mitigating climate change. The
attractiveness of this idea is increased by the fact that biochar enhances plant growth in many soil
types, facilitating bioenergy production and increasing crop yields. Woolf et al . (2010) studied
the technical potential for using pyrolysis on a global scale to produce biochar for storage in
soils. They estimate that the maximum potential offset is 12% of current anthropogenic CO 2 -
C-equivalent emissions without endangering soil conservation habitat or food security. That is
to say, a sustainable global implementation of biochar has the potential to negate approximately
1.8 Pg CO 2 -C e of the 15.4 Pg CO 2 -C e emitted annually.
In gasification, all of the hydrogen in the substrate can potentially be extracted for syngas
production and for forming more complex products such as dimethylether (DME), which has the
empirical formula CH 3 OCH 3 , an H/C ratio of 3 and an O/C ratio of 0.5. If hydrogen is trapped
in the residual carbon after gasification, this directly reduces the efficiency of the process. This
is especially true if the raw material has lower H/C ratios than the gasification products, which
is the case for polymers (bio-plastics) consisting of C n H (1 . 5-2) n and other elements. As such,
pretreatments such as torrefaction that are used to make biomass more suitable for gasification
could reduce its H/C ratio. This in turn would reduce the product's potential value. On the
other hand, torrefaction could increase the scope for producing more suitable substrates that
leave smaller quantities of carbon-containing residues during gasification. New pretreatments
and approaches to gasification should therefore be developed to keep as much of the substrate's
hydrogen as possible in a reduced state and minimize the presence of hydrogen-containing biochar
residues.
The results presented in Figure 2.6 and 2.7 can be combined to estimate the net effects of
different processes on the basis of C, H and O analysis of samples. The dashed line in Figure 2.7
shows the linear relationship for torrefied samples presented in Figure 2.6. For these samples,
both the H/C and O/C ratios decrease at higher temperatures and with prolonged treatment. The
direction of this linear relationship indicates that torrefaction under anaerobic conditions induces
a major overall reaction whereby the oxygen in biomass reacts with its hydrogen to produce water.
The torrefaction reaction is described in more detail in Chapter 7 of this topic and was recently
reviewed by van der Stelt (2011). Likewise it can be postulated from Figure 2.7 that the remaining
material after biogas production, during which the main species lost from the biomass is CH 4 ,
will have a lower H/C ratio but probably an elevated O/C ratio, especially if the losses of CO,