Energy & Fuels 2000, 14, 1323-1330
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Metal-Oxide-Catalyzed CO2 Gasification of Coal Using a Solar Furnace Simulator T. Kodama,* A. Funatoh, T. Shimizu, and Y. Kitayama Department of Chemistry & Chemical Engineering, Faculty of Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan Received July 31, 2000. Revised Manuscript Received September 18, 2000
Metal-oxide-catalyzed CO2 gasification of coal was demonstrated in small packed-bed and fluidized-bed reactors using a solar furnace simulator, for the purpose of converting solar hightemperature heat to chemical fuels. The catalytic activities of In2O3 and ZnO were investigated because used In2O3 or ZnO catalyst may be separated from remaining coal ash by In2O or Zn evaporation at high temperatures and at a reducing atmosphere. Bituminous coal with or without the metal-oxide catalyst in the quartz-tube reactor was directly irradiated by the concentrated Xe-lamp beam and CO2 was fed to the reactor at pCO2 ) 1.0. In the packed-bed reactor, In2O3 and ZnO much improved the chemical coal conversion by about 4-5 and 2-3 times at the catalyst loading of 17 wt %-In and 30 wt %-Zn in the coal-metal-oxide mixture, respectively, at temperatures around 1000-1400 K. In the fluidized-bed reactor at a small catalyst loading (810 wt %-metal in the coal-metal-oxide mixture) and at 1073-1163 K, In2O3 catalytically increased the coal-conversion rate by 3 times but ZnO scarcely showed the catalytic activity. This metalcatalyzed coal gasification process offers the efficient solar production of the syngas calorifically upgraded by solar energy.
Introduction The efficient utilization of high-temperature heat from concentrated solar radiation in the sun belt represents a subject which is of current interest.1-5 The conversion of solar heat to chemical fuels has the advantage of producing energy carriers for storing and transporting solar energy from the sun belt to the remote population centers. Steam or CO2 gasification of coal is highly endothermic, being a strongly high-temperature-dependent and energy-dependent process, and in recent years solar coal-gasification processes have been proposed and demonstrated.6-18 The basic reaction of steam or CO2 gasificaion of coal is the water gas reaction or * Author to whom correspondence should be addressed. Fax: +8125-262-7010. E-mail:
[email protected]. (1) Fletcher, E. A.; Roger, L. M. Science 1977, 197, 1050-1056. (2) Fletcher, E. A. J. Minn. Acad. Sci. 1983/84, 49 ( 2), 30-34. (3) Steinfeld, A.; Schubnell, M. Solar Energy 1993, 50 (1), 19-25. (4) Grasse, W.; Tyner, C. E.; Steinfeld, A. J. Phys. IV France, Proceedings of the 9th SolarPACES International Symposium on Solar Thermal Concentrating Technologies 1999, 9, Pr3-9-Pr3-15. (5) Tamaura, Y. Solar Thermal 2000, Proceedings of the 10th SolarPACES International Symposium on Solar Thermal Concentrating Technologies 2000, pp 189-192. (6) Gregg, D. W.; Aiman, W. R.; Osuki, H. H.; Thorsness, C. B. Solar Energy 1980, 24, 313-321. (7) Gregg, D. W.; Taylor, R. W.; Campbell, J. H.; Taylor, J. R.; Cotton, A. Solar Energy 1980, 25, 353-364. (8) Taylar, R. W.; Berjoan, R.; Coutures, J. P. Solar Energy 1983, 30 (6), 513-525. (9) Flechsenhar, M.; Sasse, C. Energy 1995, 20 (8), 803-810. (10) Tsuji, M.; Wada, Y.; Tamaura, Y.; Steinfeld, A.; Kuhn, P.; Palumbo, R. Energy Fuels 1996, 10, 225-228. (11) Tsuji, M.; Wada, Y.; Tamaura, Y.; Steinfeld, A.; Kuhn, P.; Palumbo, R. Energy Convers. Mgmt. 1996, 37 (6-8), 1315-1320. (12) Tamaura, Y.; Wada, Y.; Yoshida, T.; Tsuji, M. Energy 1997, 22 (2/3), 337-342. (13) Kodama, T.; Miura, S.; Shimizu, T.; Kitayama, Y. Energy 1997, 22 (11), 1019-1027.
the Boudouard reaction, which produces syngas:
C + H2O f CO + H2 ∆H°298K ) 131 kJ
(1)
∆H°298K ) 171 kJ
(2)
C + CO2 f 2CO
These endothermic reactions are the basis for upgrading the calorific value of coal, using solar energy. The calorifically upgraded product of syngas can be stored and transported to be combusted in a conventional gas turbine (GC) or a combined cycle (CC), in order to generate electricity at high conversion efficiency (up to 55% in a modern, large CC). The product syngas can be also readily converted to liquid fuel such as methanol, which can be very easily stored and transported overseas by a conventional oil tanker.5 Another future potential utilization of the product syngas or further processed methanol is to use them for fuel cells with high conversion efficiency, higher than GC and CC. In the conventional coal gasification, coal itself is burned with oxygen or air internally in the gasifier to supply the process heat, which releases large amounts of CO2. In a solar coal-gasification process, about 2730% of CO2 emission can be ideally reduced in compari(14) Kodama, T.; Aoki, A.; Shimizu, T.; Kitayama, Y. Energy Fuels 1998, 12 (4), 775-781. (15) Ono, H.; Yoshida, S.; Nezuka, M.; Sano, T.; Tsuji, M.; Tamaura, Y. Energy Fuels 1999, 13, 579-584. (16) Matsunami, J.; Yoshida, S.; Oku, Y.; Yokota, O.; Tamaura, Y.; Kitamura, M. Energy 2000, 25, 71-79. (17) Kodama, T.; Aoki, A.; Ohtaka, H.; Funatoh, A.; Shimizu, T.; Kitayama, Y. Energy Fuels 2000, 14, 202-211. (18) Aoki, A.; Ohotaka, H.; Shimizu, T.; Kitayama, Y.; Kodama, T. Energy 2000, 25, 201-218.
10.1021/ef000169y CCC: $19.00 © 2000 American Chemical Society Published on Web 10/24/2000
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son to the conventional coal gasification because the process heat is supplied from a clean renewable solar energy.14,18 Optimal operating temperature for converting concentrated solar radiation into chemical-free energy range from 800-1300 K for a blackbody solar cavity-receiver under peak solar flux intensities between 1000 and 12000 kW m-2.1-3 Gregg et al. demonstrated the coal-gasification process with steam or CO2 around 1175-1425 K using direct solar irradiation in a 23-kW solar furnace.7 The solar radiation was focused directly onto fixed coal bed 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. Taylor et al. demonstrated solar gasification of charcoal with steam or CO2 in a packed-bed or fluidized-bed reactor using 2-kW solar furnace.8 In the charcoal gasification with CO2, the fraction of the incident solar energy utilized to CO (stored) was 30% in the case of the packed-bed reactor and 10% for the fluidized-bed reactor. In recent years, a two-step cyclic coal gasification using a redox system of metal-oxide catalyst has been also studied for the purpose of converting solar hightemperature heat to chemical fuel of syngas, with high efficiencies. In the two-step cyclic redox mode, metal oxide is reacted with coal powder as an oxidant in an inert atmosphere to produce CO, H2, and the component metal which is reoxidized with steam or CO2 to generate H2 or CO at lower temperatures in a separate step.10-14,17,18 Tamaura et al. demonstrated the two-step cyclic steam gasification of coal by an Fe3O4/FeO redox system using a high-flux solar furnace around 1473 K.12 Tsuji et al. also examined the two-step cyclic steam gasification using a ZnO/Zn system above 1173 K in laboratory experiments.10.11 We studied the reactivities of iron-based oxides (ferrites) for the coal-metal-oxide reaction and showed that the In(III)-ferrite had much higher reactivity for the two-step cyclic coal gasification than Fe3O4 and ZnO below 1173 K.13,14 The active catalysts effectively improve the kinetics and chemical coal conversion, and reduce the operating temperature requirements for the solar coal-gasification process, but their use is subject to the feasibility of recovering catalysts from the remaining coal ash. Recently, we found that In2O3 is a very active catalyst for CO2 gasification of coal in the normal single-step reaction mode, and also proposed the separation of In2O3 catalyst from remaining coal ash by In2O evaporation at high temperatures above 1373 K.17 It is known that at high temperatures and at a reducing atmosphere volatile oxide of In2O(g) is formed by a process corresponding to the following reaction:19
In2O3(c) + 4In(l) f 3In2O(g) ∆H°298K ) 784 kJ (3) The vapor pressure of In2O(g) exceeds 1 atm at 1373 K and at pO2 ) 10-7.5, as shown by Figure 1. ZnO catalyst will be also separated from the remaining coal ash by Zn evaporation at high temperatures. The boiling point of metallic zinc is about 1200 K. Thus, ZnO can be separated as zinc vapor from the ash by the reduction (19) Jenko, M.; Erjavec, B.; Pracek, B. Vacuum 1990, 40 (1-2), 7780.
Kodama et al.
Figure 1. Variations of partial pressures of In2O(g) and In(g) in the In-O system with the oxygen partial pressure at 1373 K. This figure is based on the data from ref 19.
with gaseous reductant such as CH4 at temperatures above 1200 K:
ZnO + CH4 f Zn(g) + CO + 2H2 ∆H°298K ) 443 kJ (4) Steinfeld et al. already investigated the process similar to eq 4 as the solar thermal production of zinc and syngas via a combined ZnO-reduction and CH4-reforming process, using a solar furnace.20,21 They reported 10-90% of Zn yields at 1200-1600 K in a 5 kW prototype reactor. Metallic zinc recovered can be readily oxidized to ZnO in air. Their solar process and reactor may be utilized for the ZnO-catalyzed coal gasification, in order to separate and recover ZnO catalyst from the coal ash. In present work, solar CO2 gasification of coal with In2O3 or ZnO catalyst in the normal single-step mode was demonstrated by a solar furnace simulator using direct irradiation of Xe-lamp beam. The catalytic effects of the metal oxides in the packed-bed and fluidized-bed reaction systems were examined. Experimental Section Materials. Australian biuminous 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 27000 kJ kg-1. The coal was ground to pass through a -48 mesh screen. The grain size of the coal powder was smaller than 300 µm. In2O3 and ZnO powders were prepared by thermal decomposition of the hydroxides in air at 573 K; the hydroxides were prepared by hydrolysis of the metal chloride solutions. The metal oxides thus prepared were identified by X-ray diffractometry (XRD) with Cu KR radiation (Rigaku, RAD-γA diffractometer). The average grain sizes of ZnO and In2O3 were determined by laser diffraction particle size analyzer (Shimadzu, SALD-3000) to be 4 and 9 µm, respectively. Mode of Operation for a Packed-Bed Reactor. The experimental setup for a packed-bed reaction system is illustrated in Figure 2a. The coal powder (0.1 g) with or without the metal oxide was packed in the reactor of a quartz tube with an inner diameter of 7 mm and a length of 240 mm. The length of the coal bed in the reactor tube was set to 10 mm.
Metal-Oxide-Catalyzed CO2 Gasification of Coal
Energy & Fuels, Vol. 14, No. 6, 2000 1325
Figure 2. Schematic of the experimental setups for (a) a packed-bed reactor and (b) a fluidized-bed reactor. CO2 was fed to the reactor at a flow rate of 10 Ncm3 min-1. The coal bed in the reactor was directly irradiated using concentrated Xe-arc lamp (Ushio U-Tech, 3 kW XEBEX HIBEAM IIIR, Tokyo, Japan) beam for 10-35 min. In the original Xe-arc lamp light, there exist the strong line spectra, which are characteristic of Xe-arc emission, in the wavelength range of 800-1000 nm. Therefore, for solar simulation, about 90% of the concentrated Xe-arc lamp light in the wavelength range of 750-1200 nm was cut by a heat absorbing filter, prior to the irradiation of the coal bed. The circular diameter of the focal area was set to 25 mm to irradiate the coal bed (10 mm) throughout. The energy flux intensity (kW m-2) of the Xe beam at the center of the focal area was previously measured by a heat flux transducer with sapphire window attachment (Medtherm, 64-100-20/SW-1C-150) placed in the center position of the focal area. The temperature of the coal bed was measured using a K-type thermocouple placed at the center of the coal bed in contact with it packed inside the reactor.
Changes in the partial pressures of the gaseous products evolved were measured by using gas chromatography (Shimadzu, GC-4C) with TCD. To determine the total amounts of CO evolved during the CO2 gasification of coal, the effluent was collected in a bottle to replace water. The volume of the collected effluent was measured, and the content of CO was determined by gas chromatography. In the CO2 gasification of coal using the packed-bed and fluidized-bed reactors studied here, the main product was CO and the other products such as H2 and CH4 were trace. Thus, the main reaction of the CO2 gasification of coal was the Boudouard reaction of eq 2. Coal conversion when using a packed-bed reactor was determined on a carbon basis according to the following relation:
coal conversion ) [ACO/2]/W0
(5)
ACO is a total mole amount of CO in the effluent collected by
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Kodama et al.
the water replacement in the bottle during the coal gasification, and W0 is the initial mole amount of carbon in the coal. Mode of Operation for a Fluidized-Bed Reactor. The experimental setup for a fluidized-bed reaction system is illustrated in Figure 2b. The coal powder (5.0 g) with or without the metal oxide of In2O3 or ZnO was placed on a porous quartz frit of distributor in the quartz tube reactor with an inner diameter of 22 mm; In2O3 and ZnO were mixed with coal at 8 wt %-In and 10 wt %-Zn in the coal-metal-oxide mixture, respectively. The static bed height was about 20 mm. The quartz tube reactor was placed with its axis perpendicular to the axis of Xe-beam concentrator at the focal point. CO2 was fed to the reactor at a flow rate of 0.50 N dm3 min-1. The gas velocity U was about 2.3 times the minimum fluidization velocity Umf; the Umf was estimated by the following equation by Wen and Yu:22
[
]
3
dp Fg(Fs - Fg)g UmfdpFg ) (33.7)2 + 0.0408 µ µ2
1/2
- 33.7 (6)
The fluidized coal bed in the reactor was directly irradiated using concentrated Xe-arc lamp beam. The circular diameter of the focal area was set to 20 mm. The energy flux intensity of the Xe beam at the center of the focal area was 735 kW m-2, which was previously measured by the heat flux transducer. The temperature of the coal bed was measured using a K-type thermocouple placed at the center of the coal bed. An electric furnace with a half cylinder type was placed on the side of the quartz tube reactor opposite the Xe-beam irradiation, to preheat the fluidized coal bed; Only by the Xe-beam irradiation, the temperature of the fluidized coal bed could not be raized to above 1073 K. The fluidized coal bed was preheated to 723 K while passing CO2 feed and then irradiated with the Xe beam to commence the CO2 gasification of coal. Changes in the partial pressures of the gaseous products evolved were measured by using gas chromatography. The CO production rate (RCO) in the coal gasification using the fluidized-bed reactor can be determined by
RCO ) pCO × Fout
(7)
where pCO is the partial pressure of CO in the effluent, and Fout is the flow rate (mol s-1) of the effluent from the outlet of the reactor. Assuming that the CO production and CO2 consumption in the coal gasification were caused only by the Boudouard reaction, the Fout was estimated by the following relation:
Fout ) 2Fin/(pCO + 2pCO2)
(8)
where Fin is the flow rate (mol s-1) of the CO2 feed and pCO2 is the partial pressure of CO2 in the effluent. The coal conversion when using the fluidized-bed reactor is estimated using the following equation:
coal conversion ) 1/2
∫R t
0
COdt/W0
(9)
where t represents the reaction time. The integral on the righthand side of eq 9 was evaluated graphically as the area under RCO against t curve.
Results and Discussion Solar-Simulated CO2 Gasification of Coal in a Packed-Bed Reactor. Figure 3 shows the time varia(20) Steinfeld, A.; Frei, A.; Kuhn, P.; Wuillemin, D. Int. J. Hydrogen Energy 1995, 20 (10), 793-804. (21) Steinfeld, A.; Brack, M.; Meier, A.; Weidenkaff, A.; Wuillemin, D. Energy 1998, 23 (10), 803-814. (22) Wen, C. Y.; Yu, Y. H. A. I. Ch. E. J. 1996, 12 (3), 610-612.
Figure 3. Time variations of (a) the temperature of the coal bed and (b) the pCO in the effluent during the coal gasification in the packed-bed reactor. The coal bed with or without the metal oxide of ZnO or In2O3 was irradiated by an Xe beam with the central energy flux intensity of 360 kW m-2 for 35 min. An amount of 1.0 g of coal was used. A flow rate of CO2 feed (pCO2 ) 1.0) was 10 Ncm3 min-1.
tions of the temperature of the coal bed and pCO in the effluent when the coal bed with or without the metaloxide catalyst was irradiated by an Xe beam with the energy flux intensity of 360 kW m-2 for 35 min. The temperature of the coal bed without the metal oxide rapidly increased to 1057 K in 3 min and then gradually increased to 1107 K. The coal beds with the metal oxide also showed similar temperature profiles. In the absence of the metal oxide, the pCO level in the effluent was less than 15% throughout irradiation and the coal conversion was only 19% after 35 min of irradiation (Table 2). However, in the presence of ZnO (30 wt % of Zn in the mixture of coal and metal oxide), the pCO level in the effluent reached 43% in the initial stage of the gasification. The coal conversion attained 55% after 35 min of irradiation (Table 2). With the mixture of In2O3 and coal (17 wt %-In in the mixture), the pCO level reached 68% in the initial stage and 83% of the conversion was obtained after 35 min of irradiation. In the XRD patterns of the solid samples after the coal gasification, only the strong peaks due to ZnO or In2O3 were observed along with the small peaks due to the coal ash (mainly silicon oxide), as shown by Figure 4. ZnO and In2O3 can catalyze the solar-simulated CO2 gasification of coal in the packed-bed reaction system. The catalytic activity of In2O3 is much greater than that of ZnO. The effect of catalyst loading (metal wt % in the mixture of coal and metal oxide) on coal conversion was shown in Figures 5 and 6. For ZnO catalyst, the coal conversion showed the maximal peak at the Zn content ) 30 wt %. The fall in the conversion with increasing catalyst loading would be due to blocking of pores in
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Energy & Fuels, Vol. 14, No. 6, 2000 1327
Table 1. Proximate and Ultimate Analysis of Coal proximate analysis/wt % coal sample
moisture
ash
V.M.a
Australian bituminous coal
7.2
13.2
35.5
a
ultimate analysis/wt %, daf F.C.b
C
H
N
S
O
44.1
80.06
6.14
1.51
0.55
11.74
V.M. is volatile material. b F.C. is fixed carbon.
Table 2. Coal Conversion for the Coal Gasification with or without Metal-Oxide Catalyst by a Packed-Bed Reactor a
catalyst
irradiation time (min)
none
10 35
ZnOd
35
In2O3d
10 35
energy flux intensity of Xe beamb (kW m-2) 260 360 560 360 430 460 560 360 430 460 560 260 360 560 360
coal-bed temperature (K) 3 minc final 986 1123 1242 1057 1056 1102 1113 1044 1072 1107 1194 1015 1074 1212 1088
1017 1151 1263 1107 1103 1139 1146 1198 1251 1279 1414 1075 1220 1360 1266
coal conversion (%) 3.5 ( 0.5 13.1 ( 1.2 20.7 ( 1.7 18.6 ( 1.2 23.1 ( 1.5 32.4 ( 1.7 27.1 ( 1.3 55.2 ( 1.5 58.8 ( 2.5 66.4 ( 2.2 79.7 ( 2.8 15.4 ( 1.5 52.0 ( 3.4 74.7 ( 4.0 83.4 ( 2.7
a
Only coal (0.1 g) or the mixture of coal (0.1 g) and metal oxide was irradiated by an Xe lamp beam while passing CO2 (pCO2 ) 1.0) at the flow rate of 10 Ncm3 min-1. b The central energy flux intensity of the Xe beam: the circular diameter of the focal area of irradiation was 25 mm. c After 3 min of the Xe beam irradiation. d Coal was mixed with ZnO or In O at 30 wt %-Zn or 17 wt %-In 2 3 in the coal-metal-oxide mixture, respectively.
the carbon by the catalyst.23,24 For In2O3 catalyst, the conversion rapidly increased with an increase in the catalyst loading at the In content
