gas cleaning utilities. This configuration reduces the fresh water
consumption of about 45% (from 508 m3
/d to 278 m3
/d, on
average).
The flow rates fed to the utilities where steam is generated are
slightly higher than the actual requests, in order to compensate the
higher steam losses due to the increase of water temperature. It
was verified that the temperature of the reclaimed flows are suitable
for the reuse, as shown in Table 2, assuming that the increase
of temperature in the various utilities in the present configuration
can be maintained equal in the cascade configuration. In fact, since
water remains in the liquid phase, the relationship between the
heat exchanged (h) and the difference of temperature (DT) is
linear:
h ¼ c Q q DT ð2Þ
where h is the heat exchanged [J d1
], c is the specific heat of water
[4,186 J kg1 C1
], Q is the water flow rate [m3 d1
], q is the density
of water [1,000 kg m3
] and DT is the difference of temperature
[C]. Therefore, the temperature of used process water will be
around 75 C at the outlet of the scrubbers and 90 C at the end of
the bottom ash quenching. These values were considered acceptable
by the WtE plant managers as they do not imply any malfunctioning
of the existing structures.
However, an increase of temperature can affect steam generation.
The steam loss from the bottom ash quenching can be considered
rather unaffected, because the steam generation is caused by
the direct contact between bottom ashes and water, and the equilibrium
temperature is below 100 C. Nevertheless, as a safety factor,
the value measured in the existing configuration was doubled
in the cascade configuration evaluation. On the contrary, the
vapour losses in the two scrubbers depend on the moisture content
of treated flue gas, which is a function of temperature: considering
that the atmosphere inside the scrubbers is saturated, it is possible
to evaluate a steam loss of 60 m3 d1 at a temperature of 75 C by
means of a Mollier diagram.
The specific water consumption in the cascade configuration is
0.99 m3 tMSW
1 . The discharged flow rate is 196 m3 d1
, which is
higher than the minimum admissible value (180 m3 d1
) previously
defined. Thus, the maximum expected chloride and sulfate
concentrations in the effluent are 1,033 mg L1 and 704 mg L1
respectively, both under the discharge limit.
No operational issues should occur to manage the new concentrations
of metals and suspended solids, which are approximately
double compared with the current ones, because the halving of
the flow rates implies the doubling of the hydraulic retention time
of the reactors. It is however necessary to verify if, due to the doubling
of the concentrations of pollutants and the modification of
45 m3
d-1
Fresh