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5.4 (a) Vertical and (b) horizontal dry continuous processes (adapted
from Nizami and Murphy, 2010).
5.5.4 Dry continuous reactors
Dry continuous reactors tend to be plug flow systems. In a plug flow system
the substrate is introduced at one end and flows through the reactor as a
plug without mixing. The fresh feedstock is inoculated by returning some
effluent. A substrate concentration gradient exists between the inflow and
outflow ends of the reactor and, theoretically, if the tube is long enough all
the VS will be degraded on reaching the outlet. Thus, in theory, the effluent
should contain lower concentrations of VFAs and yield higher removal
efficiencies than from a completely mixed reactor operating at the same
HRT. However in practice, friction from the walls, convection currents from
heating systems and gas production cause mixing. These systems may be
vertical or horizontal (Fig. 5.4).
5.6 Parasitic energy demand of process
The reactor configuration has a significant impact on the energy balance of
the system. The next section continues the analysis from Box 5.1 and Box
5.2 on digestion of OFMSW. A dry continuous process is chosen with data
provided by the technology provider, as described by Murphy and
McCarthy (2005).
5.6.1 Thermal parasitic energy demand
The thermal parasitic demand for digestion of OFMSW is evaluated at
2.9% of the energy content of the biogas in Box 5.4. The higher the moisture
content the higher the parasitic demand. If an external heat source is
provided then the efficiency of combustion of that source will have an effect
on the exact parasitic demand. If, for example, natural gas is combusted at
85% thermal efficiency, then the parasitic thermal demands rises by the
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