238 Environmental Biotechnology
Figure 10.1 The biomass and bioenergy cycle
one form or another, of which around 1× 1011 tonnes are carbon. (Twidell and
Weir 1994b).
This relationship of energy and matter within the biospheric system, shown
schematically in Figure 10.1, is of fundamental importance to understanding the
whole question of biomass and biofuels. Before moving on to examine how inte-
grated technologies themselves combine, it is worth remembering that the crux of
this particular debate ultimately centres on issues of greenhouse gases and global
warming. Increasingly the view of biomass as little more than a useful long-term
carbon sink has been superseded by an understanding of the tremendous potential
resource it represents as a renewable energy. Able to substitute for fossil fuels,
bioenergy simply releases the carbon it took up during its own growth. Thus,
only ‘modern’ carbon is returned, avoiding any unwanted additional atmospheric
contributions of ancient carbon dioxide.
Derived Biofuels
Methane biogas
Biogas is a methane-rich gas resulting from the activities of anaerobic bacteria,
responsible for the breakdown of complex organic molecules. It is combustible,
with an energy value typically in the range of 21–28 MJ/m^3. The general pro-
cesses of anaerobic digestion and the biochemistry of methanogenesis have been
discussed in earlier sections of this book, so they will not be restated here. As
mentioned previously, the main route for methane production involves acetic
acid/acetate and accounts for around 75% of gas produced. The remainder is
made up via methanol or carbon dioxide and hydrogen, as shown in Figure 10.2.
At various times a number of models have been put forward to aid the pre-
diction of biogas production, ranging from the simplistic to the sophisticated.