Fluidized Bed Coal Gasification with CO2 under Direct Irradiation with

Jul 10, 2002 - The light-to-chemical (enthalpy) energy conversion via the solar gasification increased with increasing the energy-flux density of irra...
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Fluidized Bed Coal Gasification with CO2 under Direct Irradiation with Concentrated Visible Light T. Kodama,* Y. Kondoh, T. Tamagawa, A. Funatoh, K-I. Shimizu, and Y. Kitayama Department of Chemistry & Chemical Engineering, Faculty of Engineering, and Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan Received February 28, 2002

Solar CO2 gasification of coal was demonstrated under direct irradiation of the fluidized coal bed with the concentrated visible light from a solar furnace simulator in a small-scale quartz reactor. Pulverized Australian bituminous coal (the average particle size of 140 µm) was tested. The peak energy-flux density, FDpeak of the incident light beam was varied up to 1270 kW m-2. The light-to-chemical (enthalpy) energy conversion via the solar gasification increased with increasing the energy-flux density of irradiation. The maximum energy conversion of 8% was obtained at the optimum gas velocity for fluidization of 0.96 N m min-1.

1. Introduction Steam or CO2 gasification of coal is highly endothermic, being a strongly high-temperature-dependent and energy-dependent process although the utilization of coal through conversion to gaseous fuels embodies a number of different technical approaches. The energy necessary to drive the coal gasification reactions is usually supplied by partial coal combustion, but this releases large amounts of CO2 to the atmosphere. The clean high-temperature energy from concentrated solar radiation in the sun belt may be used to supply the process heat of the coal gasification reactions. The use of high-temperature solar heat to drive the endothermic reactions associated with coal gasification has been suggested and investigated in the last 20 years.1-15 This * Author to whom correspondence should be addressed. Tel: +81-25-262-7335. Fax: +81-25-262-7010. E-mail:tkodama@ eng.niigata-u.ac.jp. (1) Lakshmanan, S.; Manasse, F. K.; Mathur, V. K. Production of fuels from high-temperature solar thermal synthesisseconomic analysis. Fundamentals and Applications of Solar Energy. AICHE Symp. Ser. 1980, 76 (198), 156. (2) Gregg, D. W.; Aiman, W. R.; Otuki, H. H.; Thorsness, C. B. Solar Energy 1980, 24, 313-321. (3) Gregg, D. W.; Taylor, R. W.; Campbell, J. H.; Taylor J. R.; Cotton, A. Solar Energy 1980, 25, 353-364. (4) Aiman, W. R.; Thorsness, C. B.; Gregg, D. W. Solar coal gasification: Plant design and economics. Lawrence Livermore Laboratory, Livermore, California, UCRL Preprint 84610, 1981. (5) Taylor, R. W.; Berjoan, R.; Coutures, J. P. Solar Energy 1983, 30, 513-525. (6) Beattie, W. H.; Berjoan, R.; Coutures, J. P. Solar Energy 1983, 31, 137-143. (7) Epstein, M.; Spiewak, I.; Funken, K. H.; Ortner, J. Proc. Solar Engineering Conf. 1994, 79-91 (8) Flechsenhar, M.; Sasse, C. Solar Energy 1995, 20, 803-810. (9) Tamaura, Y.; Wada, Y.; Yoshida, T.; Tsuji, M.; Ehrensberger, K.; Steinfeld, A. Energy 1997, 22 (2/3), 337-342. (10) Kodama, T.; Aoki, A.; Shimizu, T.; Kitayama, Y. Energy Fuels 1998, 12, 775-781. (11) Funken, K.-H.; Lu¨pfert, E.; Hermes, M.; Bru¨hne, K.; Pohlmann, B. Solar Energy 1999, 65, 15-19. (12) Ono, H.; Yoshida, S.; Nezuka, M.; Sano, T. Energy Fuels 1999, 13, 579-584. (13) Le´de´, J. Solar Energy 1999, 65, 3-13.

process, so-called “solar coal gasification”, gives not only a highly useful and transportable end product, but also results in the storage of a significant fraction of the solar energy in the chemical bonds of the fuel molecules. Thus, solar energy may be transformed into a form that is both storable and transportable, and which can be used in existing equipments. The basic reaction of steam or CO2 gasification is the water gas reaction (eq 1) or the Boudouard reaction (eq 2), which produces syngas:

C + H2O(l) f CO + H2 ∆H°298K ) 175 kJ

(1)

∆H°298K ) 172 kJ

(2)

C + CO2 f 2CO

These endothermic reactions are the basis for upgrading the calorific value of coal, ideally by 44-45%, using solar energy. If the syngas produced by the coal gasification is used to generate electricity, e.g., by a conventional gas turbine, a combined cycle, or a fuel cell, about 30% of CO2 emission can be ideally reduced by the solar coal gasification in comparison to the conventional coal gasification using the partial coal combustion.10,14 In solar coal gasification, coal particles can be heated directly by concentrated solar radiation, resulting in efficient and rapid heating rates. Radiation enters the gas-particle mixture through a quartz window and due to the low albedo of the particles, radiation is absorbed near the surface. Heat losses are thus minimized. Direct absorbing particle receiver-reactors are candidates for conducing high-temperature chemical conversions. Gregg et al.3 first demonstrated the coal-gasification process with steam or CO2 around the 1175-1425 K using direct solar irradiation in a 23-kW solar furnace. The solar radiation was focused directly onto fixed coal bed (14) Aoki, A.; Ohtake, H.; Shimizu, T.; Kitayama, Y.; Kodama, T. Energy 2000, 25, 201-218. (15) Kodama, T.; Funatoh, A.; Simizu, T.; Kitayama, Y. Energy Fuels 2000, 14, 1323-1330.

10.1021/ef020053x CCC: $22.00 © 2002 American Chemical Society Published on Web 07/10/2002

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Fluidized Bed Coal Gasification with CO2

Table 1. Proximate and Ultimate Analysis of Coal proximate analysis wt %

ultimate analysis wt %, daf

coal sample

moisture

ash

V.M.a

F.C.b

C

H

N

S

O

Australian bituminous coal

7.2

13.2

35.5

44.1

80.06

6.14

1.51

0.55

11.74

a

V.M. is volatile material. b F.C. is fixed carbon.

through a quartz window in the gasification reactor. Steam or CO2 was passed through the heated coal bed. More than 40% of the sunlight arriving at the focus external to the reactor was chemically stored as fuel value in the product gas. Tayor et al.5 demonstrated solar gasification of charcoal with steam or CO2 in a packed-bed or fluidized-bed reactor using 2-kW solar furnace. The steam was generated by spraying water directly on to the surface of the charcoal and, at the same time, heating the charcoal at the focus of a solar furnace. Half of the steam reacted with carbon and 30% of the incident solar energy was stored as chemical enthalpy. In their charcoal gasification with CO2, the fraction of the incident solar energy utilized to CO was 40% in the case of the packed-bed reactor and 10% for the fluidized-bed reactor. There also have been a number of reports on solar thermal conversion of oil shale and biomass.16-20 In Japan, a project was recently proposed to develop a solar methanol production system in the sun belt.21 In this project, methanol is to be produced from natural gas (methane) and coal via methane reforming and coal gasification using solar heat as the process heat. Then, the methanol is to be transported oversea to Japan. Australia has many coal deposits and they are located in a region of abundant sunshine or the sun belt. Australia with its advantage in terms of usable dessert area and maritime transport is best suited for the construction of solar fuel production system. In this paper, the fluidized coal-bed gasification with CO2 was examined under direct irradiation with the high-energy-flux visible beam using a solar simulator in a small-scale reactor. The reactivity of Australian bituminous coal was tested. The light-to-chemical energy (enthalpy) conversion via the coal gasification was estimated. 2. Experimental Section 2.1. Materials. Australian bituminous coal was used in the present work. The proximate and ultimate analyses of the coal are given in Table 1. The calorific value of the coal is about 27 000 kJ kg-1. The coal was ground to pass through a -48 mesh screen. The average particle size of the coal powder was 140 µm. 2.2. Mode of Operation for a Fluidized Bed Reactor. The experimental setup is illustrated in Figure 1. The coal powder (5.0 g) was placed on a porous quartz frit of distributor in the quartz tube reactor with an inner diameter of 20 mm. The static bed height was about 20 mm. The quartz tube (16) Antal, M. J.; Hofmann, L.; Moreiro, J. R. Solar Energy 1983, 30, 299-312. (17) Hofmann, L.; Antal, M. J. Solar Energy 1984, 33, 427-440. (18) Berber, R.; Fletcher, E. A. Energy 1988, 13 (1), 12-23. (19) Ingel, G.; Levy, M.; Gordon, J. M. Energy 1992, 17 (12), 11891197. (20) Rustamov, V. R.; Abdullayev, K. M.; Aliyev, F. G.; Kerimov, V. K. Int. J. Hydrogen Energy 1998, 23 (8), 649-652. (21) Tamaura, Y. Solar Thermal 2000, Proc. 10th Solar PACES Int. Symp. Solar Thermal Concentrating Technol. 2000, 189-192.

Figure 1. Schematic of the experimental setup for a fluidized bed reactor. reactor was placed with its axis perpendicular to the axis of Xe-beam concentrator. The reactor was insulated with refractory bricks. An aperture was made at the side of the insulation facing to the Xe-beam concentrator, through which the Xe beam illuminated the coal bed in the quartz reactor. There existed a long (about 45 cm) freeboard region in the reactor. To measure the coal-bed temperatures, R- and K-type thermocouples covered with an inconel were placed at the central position of the irradiated bed surface, and at the position 20mm deep from the center of the irradiated bed surface into the coal bed, respectively; Hereafter these coal bed temperatures measured at the two different positions are designated as T1 and T2, respectively. CO2 was fed to the reactor at a flow rate of 0.05-1.20 N dm3 min-1. The gas velocity U was compared to the minimum fluidization velocity Umf: The Umf was estimated by the following equation by Wen and Yu:22

[

]

d3pFg(Fs - Fg)g UmfdpFg ) (33.7)2 + 0.0408 µ µ2

1/2

- 33.7

(3)

The fluidized coal bed in the reactor was directly irradiated using a concentrated Xe-arc lamp beam to commence the CO2 gasification of coal. The steam in the effluent gases from the reactor was condensed in a cooling trap connected to the exit of the reactor. The dry effluent gases were analyzed by gas chromatography equipment (Simadzu, GC-4C) with a TCD detector to determine the gas composition. The energy flux densities of the incident Xe beam at the positions of the irradiated bed surface were previously measured using a heat flux transducer with a sapphire window attachment (Medherm, 64-100-20/ SW-1C-150). The Xe-arc lamp produces a spectrum similar to that of the sun. This energy distribution is approximated in the visible (22) Wen, C. Y.; Yu, Y. H. A generalized method for predicting the minimum fluidization velocity. AIChE J. 1966, 12 (3), 610-612.

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Kodama et al. gasification were caused only by the Boudouard reaction, the Fout was estimated by the following relation:

Fout )

2Fin yCO + 2yCO2

(5)

where Fin is the flow rate (mol s-1) of the CO2 feed and yCO2 is the mole fraction of CO2 in the effluent. The coal conversion, X, is estimated by

X ) 1/2 ×

Figure 2. The energy-flux distribution of incident visible beam in a 35-mm-diameter circular area under irradiation. region by a blackbody radiating at a temperature of 60007000 K. Two types of the Xe-arc lamp concentrators with different full powers of the lamps were used for irradiation of the coal bed: one had a 3 kW full-power Xe lamp (Ushio U-Tech, 3 kW XEBEX HIBEAM IIIR, Tokyo, Japan) and another had a 5 kW full-power lamp (CINEMECCANICA, 5kW ZX8000H, Milano, Italy). The intensity and distribution of the incident beam flux for irradiation could be varied by changing the focus diameter at the position of the coal bed or by changing the power supply to the Xe-arc lamp. For the 3 kWlamp concentrator, the optimum focus diameters for the solar gasification were first determined: the CO2 gasification of coal was performed at various focus diameters of irradiation using the full power on the Xe lamp. The fastest coal conversion rate was obtained for a 35-mm focus diameter for the 3 kW-lamp concentrator. The 35-mm focus diameter was used for all the gasification experiments for the 3 kW-lamp concentrator. Figure 2 shows the energy flux distribution of the incident visible beam in the 35-mm-diameter circle area under the irradiation when using the full power on the Xe lamp. The area surrounded with wide solid lines in Figure 2 showed the irradiated coal-bed surface area. In this case, the central peak flux density (FDpeak) reached 850 kW m-2 and the total energy of the incident beam onto this irradiated area (Winc) was estimated to be 220 W. To further intensity the energy-flux density of the visible beam for irradiation, the 5 kW-lamp concentrator was used instead of the 3 kW-lamp concentrator. In this case, the focus diameter of irradiation was set to be 20 mm. The FDpeak of the irradiation could be increased up to 1270 kW m-2 when using the full power on the Xe lamp. The total energy of the incident visible beam onto the 20-mm-diameter circle area on the coal-bed surface reached 260 W. The quartz tube reactor with a larger inner diameter of 40 mm was also used for the CO2 gasification of coal in order to increase the bed diameter from 20 to 40 mm. The coal powder (20.0 g) was placed on the distributor in the reactor to set the static coal-bed height to about 20 mm. CO2 was fed to the reactor at a flow rate of 0.10-2.00 N dm3 min-1. Then, the fluidized coal bed was directly irradiated with the Xe-arc lamp beam to commence the CO2 gasification using the 5 kW-lamp concentrator. The CO production rate (RCO) in the coal gasification can be determined by

RCO ) yCO × Fout

(4)

where yCO is the mole fraction of CO in the dry effluent and Fout is the flow rate (mol s-1) of the dry effluent. Assuming that the CO production and CO2 consumption in the coal

∫R t

0

CO

dt/W0

(6)

where t represents the reaction time and W0 is the initial amount of carbon in the coal used. The integral on the righthand side of eq 6 was evaluated graphically as the area under RCO against the t curve. The fraction of the light energy stored by the reaction of CO2 with coal was calculated from the measured light energy and the CO production rate during the gasification. The lightto-chemical energy (enthalpy) conversions (ηchem) were estimated as the following equation:

ηchem )

∆H°298K(CO)[kJ mol-1] × RCO[mol s-1] Winc[kW]

(7)

where ∆H°298K(CO) represents the standard enthalpy change of the Boudouard reaction per mole of CO and Winc is the total energy of the incident visible beam onto the irradiated coalbed surface. The quartz reactor in contact with the coal bed became frosted gradually when the bed temperature of T1 exceeded 1400 K. This would be due to progressive devitrification (recystallization). At temperatures below 1300 K, the quartz reactor was not damaged even after several hours of contact with irradiated coal. Thus, in our experiments, a fresh and nonfrosted surface of the quartz reactor was always used for irradiation of the coal bed.

3. Results and Discussion CO2 gasification of coal was performed for various intensities of the incident visible beam for irradiation. The bed diameter was 20 mm and the superficial gas velocity of CO2 feed was fixed at 1.59 N m min-1. The FDpeak of the incident beam was varied from 420 to 1270 kW m-2. Figure 3 shows the time variations of the coalbed temperatures of T1 and T2. The coal-bed temperature at the irradiated surface (T1) rapidly increased in the initial 2-3 min. The bed temperature at the position 20-mm deep from the surface into the bed (T2), however, increased at a relatively slow rate. In the steady state, the bed temperature rapidly decreased from the irradiated surface into the bed and the large temperature gradient still existed between T1 and T2, indicating that an uniform fluidized coal bed could not be formed. The temperature differences between T1 and T2 in the steady state reached about 300 K. Figure 4 shows the time variations of the mole fraction of CO in the effluent, the CO production rate (RCO) and the coal conversion (X) during the beam irradiation of the coal bed. The chemical gasification efficiency increased with increasing the intensity of the incident visible beam or the FDpeak. The peak or maximum CO production rate was observed within an initial 35 min of the gasification as can be seen in Figure 4b. The maximum CO production rates and the light-to-chemical energy conversions (maximum ηchem), which were estimated from these

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Figure 3. The time variations of the coal-bed temperatures of (a) T1 and (b) T2 for various energy-flux densities of incident visible beam for irradiation. The bed diameter was 20 mm. The gas velocity of CO2 was fixed to be 1.59 N m min-1. Symbols: FDpeak ) (9) 1270, ([)850, (b) 660, (2) 535, and (×) 420 kW m-2.

maximum RCO values (eq 7), are plotted against the FDpeaks of the incident beam in Figure 5. The maximum RCO increased linearly with an increase in the FDpeak value. The maximum ηchem increased also linearly with the increasing energy-flux density of irradiation at the FDpeak values less than 800 kW m-2. However, at the FDpeak values higher than 800 kW m-2, the energy conversion could not be effectively increased by the increased flux density of irradiation. At a higher temperature of the coal bed, the reradiation loss (Qreradiation) is increased, which can be estimated by the following equation:

(8)

Figure 4. The time variations of (a) the mole fractions of CO in the effluent, (b) the CO production rates (RCO), and (c) the coal conversions (X) during coal gasification for various energyflux densities of incident visible beam for irradiation. The coalbed diameter was 20 mm. The gas velocity of CO2 was fixed to be 1.59 N m min-1. Symbols: FDpeak ) (×) 420, (2) 535, (b) 660, (]) 850, and (9) 1270 kW m-2.

where Th is the temperature of the device, FD is the flux density of irradiation, R and  are, respectively, the absorptance for solar radiation and the emittance of the device, and σ is the Stefan-Boltzmann constant. Assuming the absorptance and the emittance of the coal bed to be R )  ) 1, the Qreradiation is estimated to be about 30% when the FD ) 1270 kW m-2 and Th ) 1600 K which could be seen in our experiment (See Figure 3a).

In addition, the heat losses due to the thermal conduction and convection must be very large in such a small reactor used here, which also become larger at a higher temperature of the coal bed or the reactor. These will be the reasons why the light-to-chemical energy conversion was not increased with increased flux density of irradiation at the FDpeak values higher than 800 kW m-2 although the chemical conversion rate was effectively increased. These large heat losses will be circumvented

Qreradiation ) 1 -

RFD - σTh4 FD

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Kodama et al.

Figure 5. The maximum CO production rate (RCO) and the maximum light-to-chemical energy conversions (ηchem) as a function of the peak flux density (FDpeak) of the incident visible beam for irradiation. The coal-bed diameter was 20 mm. The gas velocity of CO2 was fixed to be 1.59 N m min-1.

Figure 7. The time variations of (a) the mole fractions of CO in the effluent, (b) the CO production rates (RCO), and (c) the coal conversions (X) during coal gasification for various gas velocities of CO2. The coal-bed diameter was 20 mm. The peak flux density (FDpeak) of the incident visible beam for irradiation was 1270 kW m-2. Symbols: (b) 0.16, (+) 0.32, ([) 0.64, (9) 0.96, (0) 1.27, (O) 1.59, (2) 2.23, (]) 3.18, and (×) 3.82 N m min-1.

Figure 6. The time variations of the coal-bed temperatures of (a) T1 and (b) T2 for various gas velocities of CO2. The coalbed diameter was 20 mm. The peak flux density (FDpeak) of the incident visible beam for irradiation was 1270 kW m-2. Symbols: (0) 0.16, (O) 0.32, (4) 0.64, (]) 0.96, (1) 1.27, (×) 1.59, (+) 2.23, (2) 3.18, and ([) 3.82 N m min-1.

in a large-scale, industrial solar reactor, resulting in the increased energy conversion. Effect of the gas velocity of fluidization was studied under the beam irradiation with the FDpeak of 1270 kW m-2: The bed diameter of 20 mm was used here. Figure 6 shows the time variations of T1 and T2 for various superficial gas velocities for fluidization. The T1 quickly increased in the initial 1-5 min of the irradiation and exceeded 1300 K. On the other hand, the T2 exceeded

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Figure 9. The maximum CO production rates (RCO) and the maximum light-to-chemical energy conversions (ηchem) as a function of the gas velocity of CO2. The bed diameters were (4) 20 mm and (0) 40 mm. The peak flux density (FDpeak) of the incident visible beam for irradiation was 1270 kW m-2. Figure 8. The coal-bed temperatures of T1 and T2 for various gas velocities of CO2. The coal-bed diameter was 20 mm. The peak flux density (FDpeak) of the incident visible beam for irradiation was 1270 kW m-2.

1073 K in the initial 5-7 min of the irradiation. In the steady state, there still existed large temperature and decreasing gradients from the irradiated surface into the bed for any case, indicating that an uniform fluidized coal bed could not be formed. Figure 7 shows the results of the coal gasification under irradiation. The levels of the CO production rate varied in the range of 4-15 mmol min-1, depending on the gas velocity. The optimum gas velocity for the chemical gasification efficiency was found to be 0.96 N m min-1. In this optimum condition, the CO production rate attained the maximum value in 10-15 min of the irradiation. At this period of the irradiation, the mole fraction of CO in the effluent approached 70% and the coal conversion X was 25-40%. As shown in Figure 7, in most cases, the maximum RCO values were observed in 5-15 min of irradiation. Figure 8 shows the T1 and T2 at 10 min of irradiation as a function of the gas velocity. There existed the temperature differences between T1 and T2 by 200-500 K. T1 seems to be distributed randomly for the gas velocity. On the other hand, it appears that the T2 plots show the maximum peak around the gas-velocity range of 1.0-1.5 N m min-1 and the temperature difference was reduced in this range. This indicates that the best heat transfer in the nonuniform fluidized coal bed occurred in the range of 1.0-1.5 N m min-1. As shown in Figure 9, the relatively high chemical and energy efficiencies were obtained in this gas-velocity range. The optimum gas velocity of 0.96 N m min-1 corresponds to 2.9 Umf at room temperature, but it reached 49 Umf and 37 Umf at T1 and T2, respectively. Thus, the vigorous and nonuniform fluidized coal bed was made because of these high gas velocities and the large difference in the gas velocities between at the irradiated surface and at the inside of the coal bed. Many coal particles were blown up from the coal-bed region at these high gas velocities. In the long freeboard region, the coal particles blown up were cooled and they fell down to the bed again.

Figure 10. The time variations of the CO production rates (RCO) for various gas velocities of CO2. The coal-bed diameter was 40 mm. The peak flux density (FDpeak) of the incident visible beam for irradiation was 1270 kW m-2. Symbols: (×) 0.08, (b) 0.16, ([) 0.64, (9) 0.96, (0) 1.27, and (O) 1.59 N m min-1.

The CO2 gasification under direct irradiation with the visible beam was also demonstrated using the bed diameter of 40 mm. Figure 10 shows the time variations of the CO production rates for various superficial gas velocities when using the bed diameter of 40 mm: The FDpeak of the incident beam for irradiation was 1270 kW m-2. The similar levels of the RCO were observed as to those using the bed diameter of 20 mm under the same reaction conditions (Figure 7b). The optimum gas velocity was also found to be 0.96 N m min-1 here. As shown in Figure 9, the relations between the gas velocity and the maximum RCO observed were similar for the bed diameters of 20 and 40 mm. This would be due to the fact that the solar coal gasification occurred almost only in the quite narrow region in the vicinity of the irradiated bed surface. Since the concentrated visible beam was irradiated onto the lateral side of the coal bed, the maximum RCO would be not influenced by the bed diameter. The gas velocity for fluidization to facilitate the heat transfer in this narrow region may determine the maximum CO production rate in the both cases.

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Figure 9 also shows the maximum ηchem values for the both cases of the bed diameters of 20 and 40 mm as a function of the gas velocity. The maximum ηchem value of about 8% was obtained at the optimum gas velocity of 0.96 N m min-1. In this case, the peak flux density of the incident visible beam reached 1270 kW m-2, but the average flux density of the beam for the coal-bed surface under irradiation was estimated to be about 830 kW m-2. Taylor et al.5 reported 10% of the light-to-chemical energy conversion via solar gasification of charcoal in a fluidized-bed reactor by using the average flux density of about 1200 kW m-2: There was no information on the FDpeak of the incident solar radiation. Thus, there will be the possibility that the light-to-chemical energy conversion via solar CO2 gasification of coal in a fluidized bed reactor system can be still increased by using higher energy flux visible beams than 1200 kW m-2 on the average. The large-scale solar concentrating systems have been developed for about 30 years.23,24 Systematic development of the four types of the solar concentrating system (parabolic trough, power tower, dish, and double concentration) has led to their increasing capability for converting concentrated solar thermal energy into electricity, process heat, and chemical fuels. In a tower system, a field of two-axis tracking mirrors reflects the solar energy onto a receiver that is mounted on top of a centrally located tower. Power tower systems usually achieve concentration ratios of 300-1500. The dish system uses a parabolic dish concentrator to focus the sunlight to a thermal receiver. The concentration ratios usually range from 600 to 2000. The doubleconcentration system consists of a heliostat field, the “reflective tower”, and a ground receiver equipped with a secondary concentrator. The optical path of a reflective tower comprises a heliostat field illuminating a hyperboloidal reflector. The reflector is placed on a tower below the aim point of the field. The upper focal point of the hyperboloid coincides with the aim point of the (23) Tyner, C. E.; Kolb, G. J.; Meinecke, W.; Trieb, F. J. Phys. IV France, Proc. 9th Solar PACES Int. Symp. Solar Thermal Concentrating Technol. 1999, 9, Pr3-17-Pr3-22. (24) Becker, M.; Meinecke, W. J. Phys. IV France, Proc. 9th Solar PACES Int. Symp. Solar Thermal Concentrating Technol. 1999, 9, Pr3-23-Pr3-34.

Kodama et al.

field. The reflector directs the beams downward. On the ground, secondary concentrators of the compound parabolic concentrator (CPC) type are arranged to recover and enhance the concentration of the solar energy. The concentration factor is in the range of 5000 to 10000. In the regions of the so-called “sun belt”, the maximum insolation reaches about 1 kW m-2. Thus, in the sun belt, high-flux-density beams in the range of 1000 to 10000 kW m-2 can be obtained by a tower, dish, and double concentration systems. 4. Conclusion CO2 gasification of the fluidized coal bed exposed to the high-flux visible beam was demonstrated in a smallscale quartz reactor using a solar furnace simulator. The peak energy flux density of the beam for irradiation was varied up to 1270 kW m-2 (the average flux density up to 830 kW m-2). At the maximum energy-flux density used for irradiation, the optimum gas velocity for fluidization for fine Australian bituminous coal (the average particle size of 140 µm) was a high value of 0.96 N m min-1, which corresponded to 37-49 times the minimum fluidization velocity Umf. The fraction of the incident light energy utilized to produce CO (stored) was about 8%. In the small reactor used here, the heat losses due to thermal conduction and convection will be very large, which may be circumvented in a large-scale, industrial solar reactor, resulting in the increased energy conversion. In addition, higher solar flux in the range of 1000 to 10000 kW m-2 will be feasible by the advanced solar concentrating system such as the doubleconcentration system, which may also increase the energy conversion efficiency. Nomenclature dp, mean diameter of coal particle (m) pg, density of gas (kg m-3) ps, density of coal (kg m-3) µ, viscosity of inlet gas (kg m-1 s-1) g, acceleration due to gravity (9.81 m s-2) EF020053X