Environmental Engineering Reference
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Table 11. Pyrolysis and energy recovery cycle synthetic results
Combustible fraction
Waste mass flow rate [kg/h]
LHV [kJ/kg]
Entering thermal power [kW]
Net Power output [kW]
Net efficiency [%]
CO 2 mass flow rate output [kg/h]
Anaerobic Digestion
Anaerobic digestion is a sequence of biological processes in which different types of
bacteria break down biodegradable material in the absence of oxygen (Mata-Alvarez , 2002).
Thus the humid fraction mechanically sorted from MSW (table 1), which contains a large
amount of biodegradable fraction, is a good candidate to be processed in such a way. The
main product of anaerobic digestion is a biogas with good LHV, that can be supplied to a
reciprocating engine to produce electricity, as illustrated in Figure 5.
Figure 5. Simplified schematic of the anaerobic digestion process with energy recovery.
The anaerobic biological process could be simulated in a complete way reproducing the
microbial reaction kinetics by mathematical models (Mata-Alvarez , 2002), but this can be
particularly onerous especially if we are dealing with solid waste. For this reason a simplified
approach was followed in this application, which is based on some simplified assumptions,
aimed to evaluate biogas production from anaerobic degradation of humid fraction. Starting
from the material composition of the humid fraction and the moisture content assumed in
table 2, the amount of Total Solids (TS) was evaluated. From TS, assuming the inert content
of table 2, recalculated on dry basis (table 12), the Volatile Solids (VS) content was
estimated. Then, assuming a biodegradability coefficient (BC), coherent with literature data
(Tchobanoglous et al.,1993), for each material fraction, and a VS removal efficiency (RE),
that can be obtained in the anaerobic digestion (Mata-Alvarez , 2002), the amount of the
biodegraded material was evaluated. This amount of biodegraded material was characterised
- for each material fraction - in term of brutal chemical formula of the type C a H b O c S d N e , on
the basis of elemental composition (table 2). Then the general anaerobic stoichiometric
reaction (1) was considered:
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