Jul-Sept 2002 The Second Solar Lime Experimental Campaign

Oct-Dec 2002 Improving the Reactor Numerical Model

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Solar energy is, by definition, a never ending thermal resource that can be used as an efficient and powerful direct heater thanks to its great radiant power. As a matter of fact, the latest technologies allow to highly concentrate the heat carried by the solar radiation and to directly convey it into reactors where high temperature chemical processes can take place. Experience shows in particular that, in order to reach the highest efficiency, the radiant power is best to be applied directly on the reactants surface while the heat conduction should be conveniently kept to a minimum.

The calcination reaction is, by the other side, a highly endothermic process, requiring temperatures over 900 °C, in which the heat conduction, together with the residence time needed for a complete conversion of calcium carbonate to calcium oxide, increases with the size of the limestone particles that have to be calcined. This because the calcination reaction proceeds slowly from the outer surface to the deeper layers of any stone and, as the heat gets to the core, the carbon dioxide is released. Namely, while the reaction is proceeding to the core, new pores are formed for the carbon dioxide to escape from the deeper layers.

In order to maximize the efficiency of the future reactor, it has therefore been decided to investigate the calcination of small grained limestone particles (1-5 mm) within this project; it seems indeed reasonable to assume that, when calcining those small limestone particles, the reaction should mainly occur because of the direct solar radiation while the heat conduction within each particle should be negligible. Besides, this choice looks like a great challenge since no available technologies today are suited to conveniently calcine such a fine material.

The first step of the Solar Lime Project has been entirely dedicated to the characterization of the raw material that will be used during the solar experiments; the material is pure Carrara Marble in particle sizes ranging between 1 and 5 mm. The goal was to determine the needed time for complete calcination when varying either the temperature or the particle size and to build a simple numerical model to estimate the calcination time based on those two parameters (particle size and temperature).

In a first experimental phase, simple experiments were run using an electric furnace to calcine small samples of limestone material up to 1340 °C. The electric furnace was first heated to the required temperature; the cold sample material, in a quartz crucible, was then introduced into the hot furnace. This procedure was chosen to simulate the conditions in a solar furnace where the burning temperature is reached almost instantaneously. The degree of calcination was finally measured as a function of the temperature, the residence time and the bed height for three of the available particle size fractions (1-1.5 mm, 1.5-2 mm, and 2-3 mm).

For thin particle beds, the heat transfer occurs mainly by direct heating (radiation only) requiring a short residence time and resulting in homogeneous quicklime. For thick particle beds, the heat transfer occurs mainly by indirect heating (external radiation and internal conduction) requiring a long residence time and leading to inhomogeneous quicklime.

To develop a numerical model that should useful to estimate the residence time of a limestone particle (for any known temperature), we then conducted, in the second phase of this experimental campaign, simple thermogravimetric investigations (TG) with the Carrara Marble.

The TG measurements were conducted with some 400 mg samples (releasing about 100 ml CO2 gas) under ideal conditions for different gas compositions (pure air and a mixture of N2 with varying CO2 content). The TG results allow to estimate an "ideal calcination rate" without delay effects due to heat transfer.

The kinetic parameters in the Arrhenius equation were then determined by fitting the data to a contracting geometry rate law and, according to the found numbers, only 4 seconds are required to obtain complete calcination at 1600 °C. As expected, the results change when the carrier gas is rich in CO2. When increasing the CO2 content in the gas phase, the calcination proceeds at significantly higher temperatures. Furthermore, it is known that the sample weight, the shape of the crucibles, the heating rate, etc., influence the kinetic parameters.

These results suggest that a combination of heat transfer (heating up) and mass transfer (CO2 transport from the sample) plays a role for the time needed for the dissociation of the larger samples. Hence the process shall allow an operation with a very thin material layer and with the highest possible temperature (requiring the shortest residence time). The temperature is limited by melting effects and/or by bad reactivity of the product.

This solar simulator provides a rapid external source of intense thermal radiation that approaches the heat transfer conditions of highly concentrating solar systems. The light source is an high-pressure argon arc enclosed in a clear quartz envelope. It is closed-coupled to precision optical reflectors of elliptical shape to produce an intense beam of concentrated radiant energy. It delivers continuous radiant power up to 100 kW with a peak flux intensity exceeding 5000 kW/m2 (equivalent to a solar concentration of 5000 suns; 1 sun = 1 kW/m2) and process temperatures exceeding 3000 K. The limestone bed was directly irradiated, thus simulating the situation relevant for the solar reactor.

To comply with the rectangular spot characteristics of the arc lamp, few grams of calcium carbonate were positioned on a SiC plate in an area of about 10 cm length and 2 cm width. The material height was only a few millimeters, corresponding to 1 or 2 layers of particles.

It was found that a residence time of 6 minutes was sufficient to calcine such a thin layer of 2-3 mm limestone samples completely. It can be estimated that within one hour about 50 kg of calcium carbonate might be calcine.

At the end of this laboratory experimental campaign, the synthesis of the obtained results allows us to be confident that the calcination process using solar energy is somehow feasible.
We then decided that a small scale Solar Lime Pilot Reactor (up to 10 kW) should be designed and built in order to be ready for testing within summer 2001.

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