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The
Solar Lime Experimental Campaign has been conducted daily in the
PSI Small Solar Furnace between July and September 2001.
The investigations concerning the solar thermal decomposition of
limestone (i.e. the reaction ) included two phases. Phase 1 was
dedicated to pre-testing the reactor performance in hot (solar)
conditions. All major solar reactor components (feeding system,
rotating drum, front plate, product outlet), the flux measurement
and data acquisition systems, and the methods to determine the calcination
degree and the product reactivity, were tested and approved. Phase
2 was a systematic study of the solar calcination reactor performance.
During our initial solar experimental campaign the solar lime
reactor operated reliably for more than 100 hours during a total
of 24 sunny days, withstanding most of the thermal shocks that
occurs in solar applications.
The raw material used for testing the 10 kW solar reactor for the
solar calcination process was extremely pure Carrara marble (CaCO3
content close to 98%). We examined different particle size fractions
in the range of 1-5 mm that cannot be treated by current industrial
technologies for calcination, since they either use grain sizes
below 1 mm (flash calciners or fluidized bed reactors), or above
about 10 mm (rotary kilns), or even above 40 mm (vertical shaft
kilns).
In addition, some experimental time was dedicated to investigate
the solar thermal decomposition of magnesium carbonate (reaction
). The raw material for these latter tests was delivered by the
Canadian Baymag.
A large amount of solar experiments were conducted to thoroughly
understand the behavior of the solar lime reactor and to relate
the quicklime quality, the quicklime reactivity and the reactor
efficiency to the experimental variables (basically the drum speed
of rotation and feeding rate).
 The
Main Results of the Solar Lime Experimental Campaign 2001 are
summarized as follows:
1. Complete calcination (nearly 100% degree of calcination) was
achieved for temperatures between 1050°C and 1250°C.
2. The maximum production rate was 1.3 kgCaO/h with an acceptable
degree of calcination (97.68%) and a burning temperature in the
solar lime reactor of 1150°C.
3. Quicklime was produced in a very wide range of reactivity: T60
ranged between few seconds (below 15) and several minutes (above
30), where T60 indicates the time needed for quicklime to be heated
from 20°C to 60°C when reacting with water. As expected,
the highest reactivity was reached at lower temperatures (1050°C)
and for lower production rates.
4. The reactor's efficiency, defined as the enthalpy of the calcination
reaction at a specified temperature divided by the solar energy
input, reached 20% for this non-optimized reactor.
 The
obtained results show the potential of solar technology for the
calcination of limestone.
Aside the test data evaluation, a numerical model aimed at the reactor's
thermal performance estimation is actually being validated: A Ph.D.
thesis is addressed to this important task. So far, the following
numerical modeling activities have been accomplished:
- Parts of a modular numerical model including (1) Monte-Carlo ray-tracing
simulation of the solar concentrating system (wavelength-depending
emitter of the solar radiation, parabolic dish) and of the solar
reactor; distribution of the solar radiation on the reactor front
and within the reaction chamber; (2) Coupling between the radiation
and the heat conduction in the solar reactor using the finite volume
method for the heat conduction processes; (3) Kinetic model of the
process (needs improvements and verification).
- Simplified model for the energy balance of the horizontal rotary
reactor (conical and cylindrical reaction chamber); within this
context, a "Semesterarbeit" at ETH Zurich has been accomplished.
- Monte-Carlo simulation of heat transfer processes for a new solar
reactor concept under consideration.
It
is expected that the numerical model will be available for validation
with experimental data in spring 2002.
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