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The
second solar experimental campaign has been conducted in the PSI
Small Solar Furnace between July and September 2002.
After
mounting the complete reactor system on the experimental table in
the solar furnace all the system components have been validated
and the data acquisition system programmed to record simultaneously
the thermocouple temperatures (some of them transmitted from the
mini telemetry system), the feeder speed of rotation, the drum speed
of rotation, the shutter position (opening angle) and the direct
normal solar irradiation.
The solar flux distribution on a white target was measured in advance
with a CCD camera and the solar power entering the circular aperture
computed using the calibration data obtained with a Kendall pyrometer.
The raw material feed rate has finally been determined by calibrating
the dozing system for two different grain sizes (1.5-2 mm and 2-3
mm) of very pure Carrara marble (CaCO3 content close to 98%).
The indirect-heated 10-kW reactor prototype has then operated with
two different external power sources, namely:
- With
solar energy for more then 60 hours of operation. The maximum
solar power that can be supplied to the reactor is about 10 kW.
- With
an electric heating element for more then 50 hours of operation.
The electric heating element was inserted into the cavity closing
completely the reactor. The cavity temperature is measured with
a platinum thermocouple and can be set to a maximum of 1200°C
by controlling the power supplied to the SiC heating element.
The maximum electric power that can be supplied to the reactor
is close to 7 kW.
The
Main Results of the Solar Lime Experimental Campaign 2002 are summarized
as follows:
- Using
electric heating, the maximum CaO production rate with an acceptable
degree of calcination (>95%) was about 2.9 kg/hr for 2-3 mm
Carrara marble; using solar energy, the maximum CaO production
rate with an optimum degree of calcination (98%) was about 4 kg/hr
for 2-3 mm Carrara marble. Note that the available electric power
was lower then the available solar power.
- For
electric heating experiments, the thermal efficiency of the solar
lime reactor, defined as the ratio of process heat used for the
chemical reaction to the electric power input, was near 30% with
a maximum of 35% (Fig. 5). Remark: the available electric power
input was not sufficient to completely calcine the limestone material
with an acceptable degree of calcination (>95%) for a CaO production
rate exceeding 2.9 kg/hr.
Similarly, for solar experiments the thermal efficiency was more
than 30% with a maximum near 35% for a CaO production rate of
4 kg/hr, while still maintaining a high degree of calcination
(>95%). Preliminary results from an energy balance yield the
relative energy fractions for a variety of solar experiments performed
at different operating conditions (Fig.6).
- Quicklime
with a wide range of reactivity was produced in the solar lime
reactor. T60 ranged from 15 seconds to 12 minutes, where T60 indicates
the time needed for quicklime to be heated from 20°C to 60°C
when reacting with water. The highest reactivity was reached for
short particle residence time (high drum speed) and, consequently,
higher production rate, as well as at lower temperatures.
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