242 Environmental Biotechnology
Figure 10.4 Energy from biogas utilisation
other uses. A representative energy flow for gas-engine generators is shown in
Figure 10.4.
Ethanol fermentation
Fermentative processes have been described earlier, both in the general wider
metabolic context and specifically in regard to their potential use in the treatment
of biowaste. Fermentation produces a solution of ethanol in water, which can be
further treated to produce fuel-grade ethanol by subsequent simple distillation,
to 95% ethanol, or to the anhydrous form by azeotropic codistillation using
a solvent.
The relative ease with which liquid fuels can be transported and handled,
coupled with their straightforward delivery to, and inherent controllability of
combustion in, engines makes them of considerable importance. Ethanol is a
prime example in this respect, since it can be used either as a direct replacement
for petrol, or as a co-constituent in a mix. Though at 24 G J/m^3 ,ithasalower
calorific value than petrol (39 G J/m^3 ), in practice any performance discrepancy
is largely offset by its better combustion properties.
There are thriving ethanol industries in many countries of the world, generally
using specifically energy-farmed biomass in the form of primary crop plants, like
corn in the USA and sugarcane in Brazil. In another example of the importance
of local conditions, the production costs of ethanol and the market price realised
by the final fuel depend on many factors external to the technology itself. Hence
the indigenous economy, employment and transport costs, government policy,
taxation instruments and fiscal incentives all contribute to the overall commercial
viability of the operation.
Brazil, where ethanol/petrol mixing has been routine since the 1970s is an
excellent example. Although the country’s use of ethanol partial substitution has
a relatively long history, dating back to the 1930s, the real upsurge of acceptance
of ‘gasohol’ lay in an unusual combination of events, partly driven by the energy
crisis of the mid-1970s. Rising oil prices, which increased by over 25% in less