PhD Thesis - Energy Systems Research Unit - University of Strathclyde

met by process integration and the use of some of the fuel produced. Many
available systems provide a straight route from feedstock, through pyrolysis or
gasification, to CHP generation via gas or steam turbines. With this, a high
degree of efficiency through process integration is possible, using the waste heat
from the turbines to provide process heat for the pyrolysis or gasification stage
[62].
2.5.2 Anaerobic Digestion
Anaerobic digestion is the natural decomposition of organic matter, by bacteria,
when no oxygen is present, which produces a mixture of methane and other byproducts.
Any mixture of organic inputs may be used (e.g. animal and human
sewage, crop residues, newspaper, abattoir waste, food processing and
agricultural waste), as long as the dry matter content of the feedstock mixture
does not exceed 15%, making this a suitable option for wet wastes. The outputs
of this process are a medium heating value biogas mixture (typically containing
60% methane and 40% carbon dioxide), and a solid and liquid fertiliser mix.
Anaerobic digestion is carbon dioxide neutral as the making of the biogas and its
subsequent use in any type of plant does not produce any more carbon dioxide
than would be produced through the natural decomposition of the waste [63].
The inputs and outputs of this process are shown in Figure 2.5.
The digestion process may take place in a variety of different designs of
digester, all based around the same, main process characteristics. The waste is
pumped periodically (usually once per day) into the digester, which consists of
an airtight container, with an expandable cover to allow the build up of biogas
before siphoning. The waste remains in the digester for between ten to forty
days (retention time), and, as new waste is introduced, an equal amount of
fertiliser output is displaced. Some designs require the mechanical mixing of
the feed in the digester, others circulate biogas through the mixture to achieve
this, and others do not require mixing. Other electrical requirements may
include pre-processing of the waste, and pumping as necessary [52,64].
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