Environmental Engineering Reference
In-Depth Information
removing humid and inert fractions and concentrating high energy content materials—as
plastic, wood and paper—in a stream which is commonly called combustible fraction (CF) or
in some cases—if some law requirements are complied with—can be addressed as Refuse
Derived Fuel (RDF).
When MSW mechanical sorting is applied, besides the combustible fraction stream, also a
humid fraction (HF) is obtained, characterised by a high presence of organic biodegradable
fraction. At present the most common fate for this stream is biological aerobic stabilisation, but
another option to push energy recovery also for this stream is biological anaerobic digestion,
which can be applied through different technologies (wet and dry digestion). Through this
process a biogas with elevated content of methane can be produced and supplied to engines for
energy recovery. It is worth noticing that the anaerobic digestion is not an alternative to the
aerobic biostabilisation, since an aerobic post-treatment for the anaerobic digestate is always
required, in order to reach high levels of stabilisation and sanitary risk avoidance.
According to the considerations above, the energy recovery potentials from MSW were
evaluated including not only the thermo-chemical processes but also their integration with
anaerobic digestion, considering for each process the appropriate entering stream.
The analysis is reported in reference to a study case in terms of MSW flow rate and
composition, as it will be illustrated in the next paragraph.
All the considered processes were analysed by means of thermodynamic and chemical
simulation using specific tools, illustrated in the following.
MSW Characteristics and Pre-treatment
In order to perform a comparison among the different possibilities for energy recovery
from MSW, a reference study case was assumed, characterised by the features reported in
table 1 in terms of MSW material composition, mass flow rate and low heating value (LHV).
With respect to this MSW composition a simplified mechanical sorting process was
considered, based on grinding, metals removal by magnetic separator and sieving (figure 1).
The process outputs are mainly a combustible fraction—adequate for feeding a thermal
treatment—and a humid fraction, characterised by a large presence of organic fraction—
appropriate to feed a biological process. The material composition, mass flow rate and LHV
of the output stream are reported in table 1.
Figure 1. Schematic of simplified mechanical sorting line.
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