Environmental Engineering Reference
In-Depth Information
Table 3. Calculated elemental composition and moisture content
of the considered waste streams
MSW
Combustible fraction
Humid fraction
Carbon
26,10%
30,84%
16,56%
Hydrogen
1,76%
2,14%
2,45%
Oxygen
0,00%
0,00%
11,57%
Nitrogen
1,32%
1,58%
0,81%
Sulphur
0,27%
0,34%
0,13%
Inert
20,74%
17,91%
24,23%
Moisture
49,81%
47,91%
44,24%
MSW direct combustion could be of particular interest when a well developed separate
collection system is applied. This means that the waste fractions that can be recovered are
separated up-stream of the waste collection, and the remaining non differentiated residual
MSW are characterised by relatively high low heating values (since the humid fraction and
inert fractions are preliminarily eliminated).
The simulation of the WtE process was carried out using an in-house developed
mathematical code, using the Engineering Equation Solver software (F-Chart Software). The
code requires as input the mass flow rate of waste, its elemental composition, the combustion
temperature, the steam cycle maximum pressure, maximum temperature and condenser
pressure, the temperature levels in the heat recovery steam generator (HRSG) and the
estimation of the internal consumption in term of percentage of the produced gross power.
The code calculates the energy and mass balance in the combustion chamber and the
volumetric percentage of oxygen in the combustion gases, estimates the thermal energy
losses, the bottom and fly ash production, and evaluates the steam production in the HRSG,
the steam turbine power output, the net power production and the net efficiency.
The main operating parameters assumed for the simulations are reported in table 4. The
WtE process was simulated for both MSW and combustible fraction, considering as input the
mass flow rates, previously reported in table 1, and elemental compositions, previously
reported in table 3.
Figure 2 shows the simplified schematic of the considered WtE process. The recovery of
the heat released in the combustion is assumed to start within the combustion chamber itself,
placing evaporator pipes in the wall of the combustion and post-combustion zone. This
integrated boiler in the combustion zone represents the most recent technology for WtE and it
is able to improve the energy recovery in this kind of plant, with respect to the previous
technology which was based on adiabatic (unless the unavoidable thermal losses)
combustion. From a modelling point of view, the combustion temperature is imposed, while
the amount of heat recovered in the integrated boiler is subtracted - from energy balance in
the combustion chamber - to such an extent that the excess combustion air is enough to
assure a minimum level of about six percent of oxygen volume in the exhausts.
The main output results from the WtE simulations, are reported in table 5, with reference
to the two different feedings of MWS and combustible fraction.
