Integrated Environmental Biotechnology - Environmental Biotechnology: Theory and Application - page 235

incineration and, although this route will always be relevant for some unwanted
materials, the situation is less than ideal. For one thing, burning denies the
bridge discussed above, by allowing little or no opportunity for reclamation. If
we extend this to larger environmental issues, like reducing CO 2 production
and the usage of fossil fuels, biomass, and hence environmental biotechnology,
comes to occupy a pivotal position in the sustainability debate.

Bioenergy

The concept of obtaining energy from biomass material was mentioned earlier, in
respect of the biological waste treatment methods involving anaerobic digestion
and fermentation, and represents nothing particularly novel in itself. Methane
and ethanol have been long established as fuels in many parts of the world,
their production and utilisation being well documented. Both of these may be
described as derived fuels, biochemically obtained from the original biomass.
However, to many people around the globe, the most familiar forms of biofuel
are far more directly utilised, commonly via direct combustion and, increasingly,
pyrolysis. Around half the world's population relies on wood or some other form
of biomass to meet daily domestic needs, chiefly cooking. Estimates put the
average daily consumption of such fuels at between 0.5 and 1.0 kg per person
(Twidell and Weir, 1994a). This equates to around 150W which is an apparently
high figure, but one largely explained by the typical 5% thermal efficiency of the
open fire method most commonly encountered.
The energy of all biofuels derives ultimately from the sun, when incident
solar radiation is captured during photosynthesis, as discussed in Chapter 2.
This process collects around 2 10 13 W, each year,

throughout the biosphere as a whole. During biomass combustion, as well as
in various metabolic processes described elsewhere, organic carbon reacts with
oxygen, releasing the energy once more, principally as heat. The residual matter
itself feeds back into natural cycles for reuse. It has been calculated that a yearly
total of some 2.5

10 21 J of energy, or 7 × ×

10 11 tonnes of dry matter circulates around the biosphere, in
one form or another, of which around 1 × 10 11

×

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,
bio-energy 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. However, the way in which some of