PhD Thesis - Energy Systems Research Unit - University of Strathclyde

Larger scale biodiesel production can take place through a continuous process
that goes through the same stages as the batch process and is, therefore, not
easily stopped and started [72]. The energy required for either process is around
2% of the electricity and 3% of the heat that would be obtained from a typical
suitable CHP plant using the fuel produced [73].
A substantial amount of alcohol is required for this process, and methanol is the
easier alcohol to use, though it is more difficult to make in a sustainable manner.
Suitable processes are beginning to be developed using ethanol, which can be
made by fermentation, as described in the next section. The batch
transesterification process can be time consuming, and safety precautions must
be taken at all stages due to the chemicals being used and the reactions taking
place.
2.5.4 Fermentation
Fermentation is the biological conversion of sugar, starch or cellulose, to ethanol
and carbon dioxide, through anaerobic respiration by yeast or other organisms.
Roughly half of the weight of sugars is converted to ethanol, and half to carbon
dioxide, and the ethanol produced has roughly 90% of the energy content of the
sugars used, or roughly 96% of the heat of combustion of the cellulose used.
The carbon dioxide produced is pure, and can be sold to industry [17].
Possible feedstocks for fermentation include sugar cane, sugar beet, fruit waste,
wheat, cereals, potatoes, wood, newspapers, and municipal waste. Of these
feedstocks, grains can be stored without degradation, but tubers, fruits and beet
crops rot rapidly once harvested, and should be used as soon as possible. Any
crop residues remaining after the harvest of energy crops, can be burned directly
to produce heat and/or electricity, or put into an anaerobic digester or gasifier,
depending on the residue type, and energy needs [74].
Pre-treatment of the feedstock is required to produce simple, fermentable sugars
(glucose and fructose), and the type of pre-treatment required depends on the
54