The role of the optical properties of solids in solar direct absorption process

Solar Energy Materials and Solar Cells 31 (1993) 61-73 North-Holland

Solar Energy Materials and Solar Cells

The role of the optical properties of solids in solar direct absorption process Ch. Sasse and G. Ingel Deutsche Forschungsanstalt fiir Luft- und Raumfahrt (DLR), Institut fiir Technische Thermodynamik, Pfaffenwaldring 38-40, W-7000 Stuttgart 80, Germany Received 7 December 1992; in revised form 25 March 1993 Solid particles are considered to be good candidates for absorbing solar energy in an endothermic reaction, due to both their ability to absorb solar radiation directly (as opposed to heat transfer mechanism) and their high ratio of absorbed radiation to mass. Only a limited number of studies have been devoted so far to convert solar energy to chemical energy by means of direct absorption. The major advantages of this concept are the inherent high efficiency of the light absorption, and the short time required to reach the desired reaction temperature. In the work described, an attempt to evaluate the effect of the optical properties of the particles and their temperature dependence, upon the solar driven chemical reaction was carried through. The particles investigated were oil shale particles. Their optical properties were measured in the optical lab at the DLR, Stuttgart, whereas the solar gasification experiments of the oil shale, were conducted in the central receiver of the Weizmann Institute. The results show that temperature profiles for the gasification experiments of oil shales can be predicted with good accuracy by assuming isotropic scatter and a particle albedo of 0.5. This is due to the relatively limited role of direct absorption in low expanding fluidized beds. Calculations of a set-up however, where direct absorption is dominant, show resulting temperature profiles to be sensitive to optical properties. Temperature differences arising from calculations based on assumed optical properties compared to those based on measured optical properties are as high as 300°C.

1. Introduction I t is b e l i e v e d t h a t t h e u s e o f s o l a r r a d i a t i o n c a n b e m o r e a t t r a c t i v e e c o n o m i c a l l y if t h e r a d i a t i o n is d i r e c t l y a b s o r b e d in e n d o t h e r m i c r e a c t i o n s in w h i c h a s o l i d is one of the reactants, because solids have a high energy density, and high absorptivity. F o r e x a m p l e , t h e v o l u m e e n e r g y d e n s i t y o f t h e s o l i d p h a s e r e a c t i o n C a C O 3 = C a O + CO 2 is 1139 K W h / m

3, w h e r e a s f o r t h e g a s p h a s e r e a c t i o n

CH 4 + CO 2 = 2CO + 2H 2 it is 0.8 K W h / m 3 ( a t S T P ) [1]. T h e r e f o r e , a s m a l l v o l u m e o f a s o l i d r e a c t a n t is s u f f i c i e n t f o r a b s o r b i n g a l a r g e a m o u n t o f e n e r g y . S o l a r e n e r g y c a n b e u s e d as a suitable source of energy for driving the endothermic reaction, which usually Elsevier Science Publishers B.V.

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Ch. Sasse, G. lngel / Solids in solar direct absorption proce,~s

(depending upon the reaction chosen) requires high heat fluxes delivered at elevated temperatures ( > 700°C). To obtain that, the use of materials with the highest energy density, namely solids undergoing endothermic chemical reaction, seems to be very promising. Specifically, carbonaceous fuels (such as coal, oil shale, bio-mass, organic wastes, etc.) in solar gasification processes. These solids are good absorbers of solar radiation. Therefore, they can absorb directly the energy, without passing through heat transfer mechanism, which decreases the thermal efficiency. The combination of reacting solids (which may absorb large amounts of energy) with their ability to absorb the solar radiation directly, ensures the delivery of solar energy to the site where required, the particle surface. Another approach for obtaining thermal energy from the sun was investigated by several scientists, Hunt et al. [2], Wang and Yuen [3], Rightley et al. [4], and others. Their idea was to heat inert solid particles, which absorb directly the solar radiation, up to elevated temperatures, in order to store solar energy as a sensible heat. Others such as Barber and Fletcher [5], or Flamant et al. [6], were interested in the open loop idea. Barber and Fletcher extracted oil from oil shales by solar retorting, whereas Flamant et al. decomposed CaCO 3 to get CaO. In the experimental work carried out at the Weizmann Institute, by Ingel et al. [7,8], the concept of using solid particles for direct absorption of solar radiation in a chemical reaction, was tested. The aim of this research was to pyrolize oil shales by using concentrated solar energy for producing synthesis gas. From this work, it appears that direct absorption affects the mechanism of heat transfer, and consequently the design of such a system [9]. Comparison of some results in an inconel and quartz reactors indicates that although in both cases the degree of decomposition of Israeli oil shales increases with the temperature of the bed, in the quartz reactor this decomposition was somewhat higher than achieved in the an opaque reactor. But the important fact is that the time required to achieve that final decomposition while using the quartz reactor, was about half of the time required while using the opaque reactor, at the same average bed temperature. To study the reasons for the difference between the two concepts of introducing the required energy into the reacting system, one needs prior knowledge of the optical properties of the absorbing particles. These optical properties can then be used in calculating the absorptance and transmittance of a gas-particle mixture exposed to concentrated solar radiation [10]. The two input parameters for modeling direct absorption in fluidized beds are the phase function and the albedo. For large absorbing particles, where transmission of radiation through the particle is negligible, scattering may be divided into surface reflection and diffraction. If one assumes that the diffraction contribution is confined to a very small acceptance angle in the forward direction of scatter, then this contribution may be neglected [11,12]. The phase function for large particles is then equal to the Fresnel surface reflection as a function of the polar angle. Since the particle rotates arbitrarily in a fluidized bed, the phase function must be azimuthally averaged. The albedo of a large particle is, neglecting diffraction, equal to the total surface reflection of a single particle. For spherical particles with smooth surfaces, phase function and albedo can be calculated from

Ch. Sasse, G. Ingel / Solids in solar direct absorption process

63

[ Particle Selection [

Measurement 1: Particle Nephelometer

/

Measurement 2: Reflectance of Thick Fixed Bed

'o--r,e I

2-Flux Model

[Phase Function ] [ Albedo

I

Input: [ Refractive Index

IGeom. 'eT eor, ]--X Optics I Phase Function, Albedo I

Y

Radiative Transfer Model

I

Absorption and Scattering of Fluidized Bed Fig. 1. Flow diagram of the experimental-numerical produre for determining radiative transfer in absorbing media consisting of large particles.

Mie theory, while for large particles from geometric optics, assuming that the complex refractive index is known. Typical particles utilized in fluidized beds however have highly irregular shape and surface properties and their refractive index can only be estimated with uncertainties. D a t a in the literature are only available for a limited variety of materials. Calculations thus tend to be inaccurate resulting in discrepancies between absorptance measurements in gas-particle mixtures and theoretical predictions. For this reason experimental apparatus and m e a s u r e m e n t procedures were developed to determine optical parameters without having to know the complex refractive index. A model to determine the optical properties of large, absorbing particles will now be described. A flow diagram is shown in fig. 1. The common approach is to assume that particles are spherical in shape (see right branch). The complex refractive index must be known. From Mie theory or geometric optics, the phase function and the albedo can be calculated directly [13,14]. These two optical p a r a m e t e r s are sufficient to solve the radiative transfer equation for arbitrary

64

Ch. Sasse, G. lngel / Solids in solar direct absorption process

optical depths. However, in most cases, the particles .are non-spherical with arbitrary surface structure. Since the complex refractive index is unknown, the phase function and the albedo have to be determined. A combined experimentalnumerical approach [15] was developed for this reason. Two measurements are necessary for determining the two parameters. In "measurement 1", the phase function is determined by electrically suspending a single rotating particle in a fixed position and measuring the scattered light with a nephelometer. The albedo is not measured directly, but determined indirectly by solving the inverse two-flux model for a fixed bed of large optical depths with negligible transmittance. The reflectance of this bed is measured with an integrating sphere, "measurement 2", and is used, together with the measured phase function, as input parameters for the inverse model. The two-flux model is an excellent approximation of the radiative transfer model in the case of large absorbing particles, as shown by Brewster [11] and Sasse [15]. Brewster also showed that the assumption of independent scattering (no influence of scattering by neighboring particles) is valid for large particles, even in the case of dense packing, as long as c/A >10.3, where c is the interparticle clearance and A the wavelength of light. The applicability of the theory of independent scattering was verified by Yamada et al. [16] using the criterion c/A >10.5 with a deviation of 5%, and by Drolen and Tien [17]. Measurements on spherical carbon spheres by Sasse [15] confirmed this. The albedo is calculated as the solution from the inverse model. Together with the phase function, these parameters for the single particle act as input values for the radiative transfer equation, and thus absorption and scattering properties of the fluidized bed can be predicted. For solids whose optical properties are unknown, radiative properties are generally calculated by assuming an isotropic phase function, and assuming a particle albedo of 0.5. The aim of this work was to compare results obtained by this assumption to those obtained by experimental determination of both phase function and albedo. In the case of direct absorption in oil shales, the mentioned assumptions resulted in temperature profiles considerably different to temperature profiles calculated from known optical properties.

2. The experimental work

2.1. The solar experimental set-up The experimental set-up is shown in fig. 2. The experimental procedure was as follows: The solar radiation reflected from the heliostat field was further concentrated prior to its entrance to the system. The concentrated radiation reached a tubular fluidized bed reactor made of quartz, in which the oil shale decomposition took place. Before allowing the solar radiation to reach the set-up, a weighed amount of shale had been placed in the reactor, and an inert fluidizing gas was introduced at a predetermined flow rate, controlled by a mass flow controller (Tylan FC-280). The pyrolysis process started with the exposure of the aperture to

Ch. Sasse, G. Ingel / Solids in solar direct absorption process

65

Fig. 2. Cross section of the 2-D CPC and reactor. The acceptance angle is 10°. the solar flux. During the experiment, different temperatures were measured by thermocouples throughout the system. These temperatures were collected and stored in a personal computer connected to the system. The gases leaving the reactor (being the fluidizing gas and the gases formed during gasification), were cooled and collected in storage vessels for analysis by gas chromatography. After cooling down the reactor, the spent shale was collected, weighed and analyzed for the remaining volatiles by using a thermogravimetric analyzer. 2.2. Measuring optical properties o f oil shale partides For the optical measurements, two types of measurements were conducted: the phase function (scattered light as a function of the polar angle) and the albedo (surface reflectivity). These measurements were done on single particles, and were averaged for measurements made on a number of particles. The scattering properties were measured in an electrodynamic chamber (EDC) (fig. 3). In the EDC a single particle is suspended by electrodynamic forces at a fixed point in space. These electrodynamic forces are created by a hyperbolic ring electrode to which an AC potential is applied, and two endcap electrodes which are hyperboloids of revolution across which a DC potential is applied. The field of the endcap electrodes compensates the gravitational force of the particle. The A C

Ch. Sasse, G. lngel / Solids' in solar direct absorption process

66

PMT

AC-SOURCE~

C A M ~ ELECTRODYNAMIC BALANCE

HALF WAVE PLATE

AR-LASER

I I

BEAM SPLITTER

POLARIZER

PINHOLE

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~-

Fig. 3. Schematic of the electrodynamic chamber for optical measurements on single particles. field of the ring electrode has a minimum in the center and provides a restoring force towards the center in both the axial and radial directions, proportional to the distance the particle moves away from the center. A 5 W argon laser, whose polarization can be changed from vertical to horizontal by using a half wave plate, . serves as a radiation source. Thus it is possible to measure the phase function only at A = 514.5 nm. However, the forward and backward scatter fractions of the phase function for other wavelengths in the solar spectrum can be obtained by taking the spectral hemispherical reflectance measurements into consideration [15]. The b e a m illuminates the particle, and the scattered light of the particle is collected by a lens and is focused onto a pinhole. The light penetrating the pinhole is distributed by a diverging lens onto the sensitive area of a R928 H a m a m a t s u photomultiplier tube mounted on an arm. A stepping motor moves the arm radially around the particle and the scattered radiation is measured between 20 ° and 160 °. The detector signals are digitalized and fed into a microcomputer. The albedo of a single particle was determined in an integrating sphere in the following way. About 7 g of shale were placed in an integrating sphere. The hemispherical spectral reflectance of that particle ensemble is measured by irradiating the particles with a 450 W xenon lamp and collecting the reflected light inside the sphere. The results of the measurements are independent of the lamp source used, since the data were calibrated with a set of reflectance standards (Oriel Corp. # 70497). A solar weighted reflectance can be calculated by weighting the data with the solar spectrum [15]. Since the wall of the sphere reflects diffusely, a detector looking at a section of the wall measures a signal proportional to the reflectance of the sample. A spectrometer with a silicon photodiode is used to

Ch. Sasse, G. Ingel / Solids in solar direct absorption process

67

1"101 Spent

........

all

shale;

temp.

650

°C

Spent oli shale; temp. 950 °C

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.

.

.

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100 Angle in degrees

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Fig. 4. Averaged phase functions of oil shale particles.

cover the range of 450 nm to 1050 nm and a set of filters with a PbS detector scan the I R up to 2000 nm. The results were stored in a computer and served as input data for the radiative transfer model [18]. Since the average phase function of the particles had been pre-determined as described above, the albedo was derived by varying its value until it matched the measured reflectance of the ensemble. Since all of the optical measurements were done without heating the particles, an indirect way of measuring the changes in these properties was used. Raw shale, as well as a number of samples of spent shale obtained at different reaction temperatures were tested. The assumption was that the m e a s u r e m e n t procedure presents an instantaneous snap shot in the heating process. The phase functions of the different samples is shown in fig. 4, and reflectance is plotted in fig. 5. As can be seen in fig. 5, the reflectance of the sample that was decomposed at 950°C is much closer to the raw shale than to the spent shale that was decomposed at 650°C. The reason is that at a high enough temperature only negligible amounts of organic material can be found, whereas at 600-700°C the uncompleted pyrolysis yields samples containing unreacted organic matter and black char with low reflectance. The reflectance of two samples was compared. The first was prepared at the solar tower, by subjecting oil shales to solar radiation. The average bed temperature of 650°C was maintained for 5 minutes before stopping the irradiation. The second sample was p r e p a r e d by heating raw shales electrically to 650°C at a much slower rate. Once the sample reached this temperature, the heating was stopped. The results of the optical measurements on these samples are shown in fig. 6. The similarity is quite striking. It indicates that, within reasonable limits, the rate of the

Ch. Sasse, G. lngel / Solids in solar direct absorption process

68 0.4

raw oil shale

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o~ 0.3 ~6 e-

$pent oil shale; temp. 950 °C

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I

500

I

I

,

i

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,

i

,

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I

1000 1500 Wavelength in nm

2000

Fig. 5. Reflectance of oil shale samples. pyrolysis reactions is not critical for the reflectance value of the spent shales, as long as the final temperature reached is the same.

3. Modelling of the direct absorption pr6cess The theoretical model of the experimental solar system was shown in [19]. The purpose of this present paper is to enlighten the importance of knowing the optical properties of the absorbing solids, for understanding and modelling of such a process. The model takes the change in optical properties of oil shale during irradiation into account. After subjecting the fluidized bed to solar radiation, at any given moment, only the outer layer of particles absorbs the radiation, while the rest of the particles are heated only by the hot particles, indirectly. The thickness of the absorbing layer, is determined by the optical properties of the particles. Based upon the reflectivity and scattering of a single particle, a two-flux model predicting the radial profile of the bed absorptance was developed for irregularly shaped particles, by Sasse [15]. Given the hemispherical reflectance of an ensemble of particles, and the scattering function of a single particle, the model produces the bed absorptance abe d, as a function of the optical depth 7 and the optical properties of the particle, which may vary with temperature as in the case of oil shales. ~-, defined as the dimensionless optical path length from the reactor wall is

69

Ch. Sasse, G. Ingel / Solids in solar direct absorption process

0.4 electric heating solar heating

........ tt~

03

*5

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O

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0,0

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i

i

i

i

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500

i

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1000

1500

2000

Wavelength in nm

Fig. 6. Reflectance of oil shale samples, prepared by solar and electric heating.

where Vbeo is the bed volume, y the radial distance from the outer wall, dp the average particle diameter and No the total n u m b e r of particles present. Based on the solar model it was concluded that experimental results can be predicted reasonably well, using the following way of calculating the direct absorption mechanism. O n c e the relationship between ~- and abe d is established, one obtains a series of curves for each set of optical properties. Such an example is shown in fig. 7 for the 950 ° oil shales, where a set of curves describing the

i.o|

,

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T=20

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Fig. 7. Direct absorption of solar radiation in a fluidized bed vs. the dimensionless optical depth.

fTh. Sasse, G. lngel / Solids in solar direct absorption process

70

absorptance of a fluidized bed is given for a combination of different distances from the reactor wall and various solid concentrations in the bed. The bed volume is divided into several concentric layers, each containing the same number of particles as the other. Assuming that the solids' concentration throughout the bed is constant, it is possible to calculate what part of the incoming radiation is absorbed by each layer. For each layer, r is calculated (knowing the layer's distance from the reactor wail), and using the particles' temperature, ~bed iS obtained. This in turn is used to calculate the net heat input for a single particle Qp depending on its location in the reactor at a given time step: Qp(/) =

Qnetabed(l)/Up.

(2)

Qnet is the net heat in the bed, and the term (1) defines a certain annulus according to the concentric layers division. Repeating this calculation for each one of the layers, the temperature distribution in the bed is obtained. Since good mixing of the bed has been assumed, after a short interval all the particles are mixed, and the bed bulk t e m p e r a t u r e is obtained by averaging over the temperatures of all the particles. The following step was to assume that no information concerning the optical properties of the particles and their temperature dependence, is known. Therefore, one must assume some average optical properties of the particles, and simulate the solar process while using these assumed values. Having done that, it is possible to estimates the possible error introduced to the calculation, while no optical properties information is known.

4. Results and discussion

Fig. 8 is a comparison of theoretical and experimental results for temperature versus reaction time. In the direct absorption model a substantial temperature difference between particles at the boundary layer and in the bulk was found. Model predictions of both temperatures are plotted. At steady state, the thermocouple measurements should correspond to the particles' weighted average temperature. This is indeed the case at sufficiently long reaction times (of the order of 90 seconds). Comparison of theory and experiment at shorter reaction times is not meaningful due to the markedly different heating rates of small solid particles as opposed to the time delay in the thermocouple response at these high heating fluxes, as well as due to the environment which influenced the thermocouple measurement. Fig. 9 presents the final bed temperatures as a representative measurable quantity of the system heat transfer mechanism, at different solar radiation inputs to the secondary concentrator. The theoretical results obtained by using assumed values based on isotropic scatter, are compared with those obtained by using the measured optical properties as well as with the experimental ones. As can be seen in the figure, except at the lowest radiation range, the theoretical final temperatures obtained in the fluidized bed correspond reasonably well with the experimen-

Ch. Sasse, G. Ingel / Solids in solar direct absorption process 800

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700

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Fig. 8. Comparison of experimental and calculated transient temperature profiles in the quartz reactor. upper dashed curve = theoretical for the outer layer; lower dashed curve = theoretical bulk temperature; solid curve = experimental results.

tal ones. T h e i m p o r t a n t o b s e r v a t i o n that may be d r a w n from this figure is that the two theoretical curves do not differ substantially from o n e a n o t h e r , t h e r e f o r e i n d i c a t i n g that the same p r e d i c t i o n of t e m p e r a t u r e profile can be o b t a i n e d assuming isotropic scatter a n d a particle albedo of 0.5 for the low e x p a n d i n g fluidized bed. T h e albedos of the oil shales derived t h r o u g h optical m e a s u r e m e n t s for the raw, 650°C a n d 950 ° samples are 0.54, 0.23 a n d 0.65 respectively.

1000

0

o r-

800

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suits /~ using optical properties of all shale 9 using Isotroplc scattering

/

600 4000

i

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i

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6000 Input radiation to the s y s t e m in K W

Fig. 9. Temperature profiles in the direct absorption process.

72

Ch. Sasse, G, lngel / Sofids in solar direct absorption process 1500 -

?

-

using oll shole optlcol properties

1000

Ct.

E

500

0

~ -

0

I

1

2

3

4

Time in seconds

Fig. 10. Calculated transient temperature profiles in the direct absorption process. T h e a b o v e m a t c h e s result f r o m the l i m i t e d role o f d i r e c t a b s o r p t i o n in t h e e x p a n d i n g f l u i d i z e d b e d e x p e r i m e n t s . T h e d o m i n a n t h e a t t r a n s f e r m e c h a n i s m s in t h e b e d a r e o f convective a n d c o n d u c t i v e n a t u r e . R a d i a t i v e t r a n s f e r is d o m i n a n t in t h e first layers o f particles. I n o r d e r to investigate t h e role o f o p t i c a l p r o p e r t i e s in the first layers, c a l c u l a t i o n s w e r e c a r r i e d o u t t a k i n g only a small q u a n t i t y (0.5 g) o f oil shales thus d e m o n s t r a t i n g t h e p h e n o m e n a o c c u r r i n g in f l u i d i z e d b e d s having a small n u m b e r o f p a r t i c l e s p e r b e d unit v o l u m e , as in circulating beds. R e s u l t s are shown in fig. 10. A s can b e seen, t h e curve using m e a s u r e d o p t i c a l values differs c o n s i d e r a b l y f r o m a s s u m e d o p t i c a l values, e m p h a s i z i n g t h e role o f d i r e c t a b s o r p tion a n d its sensitivity to optical p r o p e r t i e s .

References [1] G.I. Baurerle, H. Pearlman, J.K. Rosemary and T.H. Springer, Solar energy storage by reversible chemical processes, Rockwell Int. Corp. Canoga Park California, final report, Sandia contract FAO 92-7671 (Jan., 1979). [2] A.J. Hunt, J. Ayer, P. Hull, R, McLaughlin, F. Miller, J.E. Noring, R, Russo and W. Yuen, Solar radiant heating of gas-particle mixtures, Lawrence Berkley Laboratory, University of California, Report submitted to the DOE under contract DE-AC03-76SF00098 (June, 1986). [3] K.Y. Wang and W.W. Yuen, J. Solar Energy Eng. 109 (1987) 143. [4] M.J. Rightley, L.K. Matthews and G.P. Mulhoiland, Solar Energy 48 (1992) 363. [5] R. Barber and E.A. Fletcher, Energy 13 (1988) 13. [6] G. Flamant, D. Hernandez, C. Bonet and J.P. Traverse, Solar Energy 24 (1980) 385. [7] G. Ingel, M. Levy and J.M. Gordon, Sol. Energy Mater. 24 (1991) 478. [8] G. Ingel, M. Levy and J.M. Gordon, Oil-Shale Gasification by Concentrated Sunlight: an OpenLoop Solar Chemical Heat Pipe, Energy (1992), in press. [9] G. lngel, Solar Gasification of Oil Shale, Ph.D. thesis, Weizmann Institute of Science, 1992.

Ch. Sasse, G. Ingel / Solids in solar direct absorption process

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[10] R. Siegel and J.R. Howell, Thermal Radiation Heat Transfer, 2nd ed. (Hemisphere Publishing Company, Belmont, CA, 1981) ch. 13. [11] M.Q. Brewster, Radiative transfer in packed and fluidized beds, Dissertation in Engineering at the University of California, Berkeley, 1981. [12] K. Kamiuto, J. Quant. Spectrosc. Rad. Transfer 40 (1988) 21. [13] C.F. Bohren and D.R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley, New York, 1983). [14] H. Hottel and A. Sarofim, Radiative Transfer (McGraw-Hill, New York, 1967). [15] C. Sasse, Determination of the Optical Properties of particles for Solar Heated Fluidized Beds, Ph.D. thesis, University of Karlsruhe, 1992, in German. [16] Y. Yamada, J.D. Carigny and C.L. Tien, J. Heat Transfer 108 (1986) 614. [17] B.L. Drolen and C.L. Tien, J. Thermophys. Heat Transfer 1 (1987) 63. [18] C. Sasse, Sol. Energy Mater. 247 (1991) 409. [19] G. Ingel and M. Levy, Computer Modeling of Solar Gasification of Oil Shale. Comparison with Experiments, Energy (1992), submitted.