PhD Thesis - Energy Systems Research Unit - University of Strathclyde

As the lower heating value of the pyrolysis oil produced from the pyrolysis of
plastic and tyres is significantly higher than that of the oil produced from wood,
the option is available to change the pyrolysis oil type to one of the ‘other’
categories for the occasion where these two different pyrolysis feedstock types
are desired.
6.2 Anaerobic Digestion
Existing models are available which predict the output of anaerobic digestion
systems. These, however, either require substantial amounts of complex,
proprietary, and not easily available information, or look only at the overall
production for a year, not taking into account possible changes in feedstock type
and amount [4]. Therefore, a more generic and flexible approach has again been
taken, which is based on basic process parameters and typical feedstock
performances. An example of the anaerobic digestion definition window is
given in Appendix 1, Figure A1.15. A graphical representation of the algorithm
described in this section can be seen in Figure 6.4, where each timestep value is
input in turn.
6.2.1 Process Characteristics
The anaerobic digestion process is, by its nature, a continuous process, where
feedstock is introduced periodically (generally daily) and resides in the digester
for 10 to 40 days while the digestion process takes place. Over this retention
time, biogas is given off at a steady rate, which is defined by the volume of
biogas given off during this period per tonne of feedstock. The required
retention time varies with the operating temperature of the digester, but the
overall amount of gas that is produced per tonne of feedstock will remain the
same, though this will vary with different feedstock types [5].
As the digester should remain active where possible, and as different types and
amounts of feed may be available at different times of the year, up to five
different feedstock supplies may be specified. These are defined by how much
feedstock is available (tonnes per day), how much biogas the feedstock will
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