Jul-Sept 2002 The Second Solar Lime Experimental Campaign

Oct-Dec 2002 Improving the Reactor Numerical Model

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The PSI's expertise in the field of high temperature solar technology is combined with the lime production expertise of QualiCal to develop a reactor that produces lime at a desired production rate and quality while minimizing the projected cost of the reactor and the optical configuration system.

The main objective of this project is indeed to develop a solar reactor concept that can produce high grade CaO with perhaps various degrees of reactivity (high, medium, and low). Particular attention will be given to develop a reactor leading to a cost effective solar calcination plant.

In general, a high reactor thermal efficiency is crucial for a low cost plant. The size of the solar concentrating system is a direct function of the thermal efficiency, and the solar concentrating system can reach up to 50% of the capital cost for the plant. But the reactor concept and the optical system are not independent of each other: the combination of reactor efficiency for a given optical system and that system's cost per unit of heliostat mirror surface area ultimately will determine the overall capital cost of the plant. Thus, early in the project, an assessment has been made to determine if a traditional heliostat field with a solar tower, a traditional heliostat field with a tower reflector and beam down capability, or a sun tracking large parabolic dish would be best suited for the calcination application.

At the beginning of the project, both a critical review of the traditional calcination process technology and an overview of the state-of-the-art of modern calcination plants were also made. The major features of the calcination process were presented and served afterward as a base for the development of the solar lime plant. Because the cost for preparing the CaO must be kept to a minimum, the project also included exploring the relationship between the particle size of calcium carbonate to the reactor's thermal performance, the product quality, and the projected cost of a plant. The goal was to establish a particle size that does not require expensive and energy-intensive grinding equipment and gives a high quality product emerging from a reactor with high thermal performance.

Thus, the second phase of the project included the development of potential solar reactor concepts and their peripheral components for the solar optical systems under consideration.

The following three solar concentrating systems were initially considered for an industrial 0.5 MW hypothetical plant:

· Parabolic dish system;
· Tower system (heliostat field, reactor on top of a tower);
· Tower reflector system (heliostat field, tower reflector, reactor on ground).

The parabolic dish technology was not considered further for two major reasons

· The three-dimensional movement of the reactor has negative implications on the feeding/extraction of particles/gases as well as on the reactor operation;
· The biggest parabolic dish currently available only yields some 300 kW and the use of dispatched dish systems with several reactors is not acceptable for economic reasons.

The optical efficiency of traditional solar tower systems (reactor on top of a tower) was then compared with that of a tower reflector or beam down configuration (reactor on ground), using the available information from literature. Although this information is somewhat contradictory, we could draw the following conclusions:

· For a given reactor input power, the required heliostat area is about 10-20% bigger for the tower reflector system (beam down configuration).
· In addition, the beam down configuration needs a hyperbolic reflector on top of a tower.
· However, a secondary concentrator is needed for both configurations in order to achieve the required temperatures.

In contrast to the optical disadvantages, the tower reflector system has tremendous advantages for the reactor design and operation:

· Operating the reactor on the ground is much easier and reduces costs (no complicated material feeding systems onto the tower).
· The beam down configuration allows a symmetric reactor using direct solar irradiation coming from the top and this might guarantee a more uniform calcination.

On the other hand, the project designing team went through the different existing calcining technologies to find the most adequate system to be matched with a solar optical configuration.

Starting from the reaction's needs, the heating process in a typical kiln is one where the carbonate first decomposes on its outside surface. As the decomposition spreads inward, carbon dioxide must diffuse through the pores. The reaction can be rate limited by this diffusion process. Industry has learned that an adequate flow of gases through all parts of the kiln is essential for achieving good carbon dioxide removal. The gas flow is also essential for good heat transfer leading to good kiln thermal efficiencies.

After a survey of several calcination concepts (rotating hearth, grate kiln or "CID" kiln, fluidized bed, cyclone) two major lime burning furnace types remained for further evaluation:

· Shaft kilns (heat transfer mainly via conduction)
· Rotary kilns (heat transfer mainly via radiation).

The thermal efficiency of a kiln is defined as the theoretical energy required for driving the calcination reaction divided by the actual energy supplied to the kiln. Efficiencies range from 32% for a standard rotary kiln, 47% for a rotary kiln with heat recovery, 55% for a single shaft kiln with counter current heat exchange, and up to as high as 85% for a modern twin shaft parallel flow regenerative lime kiln. The heat recovery systems leading to good efficiencies are quite varied, but they all include a continuous production process in which the carbonate charge flows downwards in the kiln while the gases flow upwards.

Modern twin shaft furnaces are very effective. The total heat requirement is about 850 kcal/kg of CaO (about 1 MWh/ton of CaO), the theoretical need for the calcination (without heating up the limestone) being 760 kcal/kg of CaO. Rotary kilns are less efficient by up to a factor of two.

For our project, the goal for the heat consumption is set between 1 and 2 MWh/ton of CaO. Hence, in a 0.5 MW solar lime plant, the production rate should be about 250-500 kg of CaO per hour.

A few weeks have been dedicated to the definition of the final layout of the first Solar Lime Pilot Reactor; after exploring various and different designing ideas, some new and fascinating, others more conservative but less risky, we decided to go for a solar rotary kiln powered by solar energy to be tested in PSI's solar facilities.
The great advantage of the chosen layout is that, for testing in small scale and even up to 1 MW by using bigger existing solar facilities, no solar secondary concentrators as CPCs are needed.

The first Solar Lime Pilot Reactor will be an open reactor with an horizontal axis; during experiments it should be placed directly on the focus of a fixed parabolic dish reflecting the sunlight coming from the heliostat field. Temperatures inside the reactor are estimated to stay below 1600 °C.