Thermochemical CO2 Gasification of Coal Using a Reactive Coal

202

Energy & Fuels 2000, 14, 202-211

Thermochemical CO2 Gasification of Coal Using a Reactive Coal-In2O3 System T. Kodama,* A. Aoki, H. Ohtake, A. Funatoh, T. Shimizu, and Y. Kitayama Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan Received June 23, 1999. Revised Manuscript Received September 17, 1999

A number of coal-metal oxide systems were examined for a thermochemical CO2 gasification of coal in a two-step cyclic redox mode, as well as in a normal single-step mode for the purpose of utilizing solar high-temperature heat below 1173 K. In the two-step cyclic redox mode, metal oxide was reacted with coal powder as an oxidant in the absence of CO2 at 1173 K to produce CO, H2, and the component metal which was reoxidized with CO2 to generate CO at lower temperatures in a separate step. In2O3 was found to be the most reactive metal oxide among the thermodynamically promising metal oxides. The two-step cyclic CO2 gasification could be repeated using the In2O3/In redox system by resupplying the consumed coal to the system. About 80% of the coal supplied to the system was gasified at the temperature range of 1023-1173 K. The In2O3 phase was completely regenerated without the formation of other solid phases after repeating the two-step gasification. In a normal single-step, continuous feeding mode of CO2 at a constant temperature of 1173 K, the coal-In2O3 system also showed the most reactivity for the Boudouard reaction of C + CO2 f 2CO in the CO2 gasification of coal at the metal content of 17 wt % in the coal and metal oxide mixture. Depending on the indium content in the mixture, the initial coal-conversion rates with the coal-In2O3 system were 2.5-4 times as fast as that in the coal-CO2 reaction without any catalysts. The separation of used In2O3 from the remaining coal ash by In2O evaporation was also proposed.

Introduction The conversion of solar energy to chemical fuels enables solar energy storage and transportation.1-4 Direct thermochemical conversion of solar high-temperature heat to chemical fuels is desired. The goal is an industrially important endothermic process that can be driven by high-temperature heat. Gasification of coal is one of the most attractive candidates for the solar thermochemical conversion process and in recent years solar coal gasification processes have been studied extensively.5-13 The conversion of coal to synthesis gas * 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) Fletcher, E. A. Energy 1979, 4, 61-66. (2) Sizmann, R. CHIMIA 1989, 43, 202-206. (3) Tamaura, Y. Int. Conference on “Technologies for AIJ”; Riemer, P. W. F., Smith, A. Y., Thambimuthu, K. V., Eds.; Vancouver, Canada, 1997; pp 481-486. (4) Palumbo, R. D. J. Phys. IV France, Proceedings of the 9th SolarPACES International Symposium on Solar Thermal Concentrating Technologies; 1999, 9, Pr3-35-Pr3-40. (5) Gregg, D. W.; Taylor, R. W.; Campbell, J. H.; Taylor, J. R.; Cotton, A. Solar Energy 1980, 25, 353-364. (6) Berjoan, R.; Coutures, J. P. Solar Energy 1983, 31 (2), 137-143. (7) Flechsenhar, M.; Sasse, C. Energy 1995, 20 (8), 803-810. (8) Tsuji, M.; Wada, Y.; Tamaura, Y.; Steinfeld, A.; Kuhn, P.; Palumbo, R. Energy Fuels 1996, 10, 225-228. (9) Tsuji, M.; Wada, Y.; Tamaura, Y.; Steinfeld, A.; Kuhn, P.; Palumbo, R. Energy Conserv. Mgmt. 1996, 37 (6-8), 1315-1320. (10) Kodama, T.; Watanabe, Y.; Miura, M.; Sato, M.; Kitayama, Y. Energy 1996, 21 (12), 1147-1156. (11) Tamaura, Y.; Wada, Y.; Yoshida, T.; Tsuji, M. Energy 1997, 22 (2/3), 337-342. (12) Kodama, T.; Miura, S.; Shimizu, T.; Kitayama, Y. Energy 1997, 22 (11), 1019-1027.

(syngas) provides a chemical pathway for the production of synthetic liquid fuels such as methanol. Steam or CO2 gasification of coal is highly endothermic, being a strongly high-temperature-dependent and energy-dependent process. Gasification of coal has been widely studied and is presently practiced at an industrial scale,14-16 but 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 solar coal-gasification processes, about 26-30% of CO2 emission can be ideally reduced in comparison to the conventional coal gasification because the process heat is supplied from a clean renewable solar energy.13 Optimal operating temperatures for converting concentrated solar radiation into chemical-free energy range from 800 to 1500 K for a blackbody solar cavityreceiver under peak solar flux intensities between 1000 and 12000 kW m-2.17-19 It is thermodynamically and kinetically advantageous to conduct the endothermic process at a higher temperature in the range of 8001500 K. However, the drawbacks are related to the requirement for better optics for concentrating solar (13) Kodama, T.; Aoki, A.; Shimizu, T.; Kitayama, Y. Energy Fuels 1998, 12 (4), 775-781. (14) Keller, J. Fuel Process. Technol. 1990, 24, 247-268. (15) Harig, H.-D. VDI Ber. 1992, 984, 169-194. (16) Wison, J. S.; Halow, J.; Ghate, M. R. CHEMTECH 1988, 123128. (17) Fletcher, E. A.; Roger, L. M. Science 1977, 197, 1050-1056. (18) Fletcher, E. A. J. Minn. Acad. Sci. 1983/84, 2, 30-34. (19) Steinfeld, A.; Schubnell, M. Solar Energy 1993, 50 (1), 19-25.

10.1021/ef990135u CCC: $19.00 © 2000 American Chemical Society Published on Web 11/20/1999

Thermochemical CO2 Gasification of Coal

energy and the requirement for high-temperature reactor materials of construction. These technical requirements usually translate in expensive components. It can help alleviate these problems to conduct the process at temperatures below about 1273 K. Furthermore, if the proposed solar process realizes a highly efficient conversion below 1273 K, it may be applied for worldwide solar concentrating facilities where direct solar radiation or area for collecting solar radiation is very restricted. For use of other high-temperature sources such as nuclear heat, it will be necessary to reduce the operating temperature below 1273 K at the highest. Recently, two-step thermochemical processes for coal gasification or methane reforming using metal oxide redox systems have been proposed and experimentally demonstrated utilizing concentrated solar energy.8-13,20-23 In this proposed two-step cyclic redox mode, metal oxide is reacted with coal or methane at high temperatures to produce a CO/H2 mixture (syngas) and the component metal which is reoxidized with H2O or CO2 to generate H2 or CO, at lower temperatures in a separate step. Solar-processed metal, H2, CO and further processed methanol are considered for the chemical storage and transportation of solar energy. Tamaura et al. demonstrated the two-step steam gasification by a coal-Fe3O4 system using a high-flux solar furnace around 1473 K.11 Tsuji et al. also examined the two-step steam gasification using a coal-ZnO system above 1173 K in laboratory experiments.8 We studied the reactivities of ironbased 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.12,13 Steinfeld et al. conduced the combined ZnO-reduction and CH4-reforming processes in a 5kW prototype reactor that yielded 90% conversion of ZnO to Zn on a material basis.22,23 A direct or single-step coal-gasification process with steam or CO2 using solar energy has also been experimentally investigated around 1175-1425 K using a 23 kW solar furnace.5 Steam or CO2 was passed through the coal bed on which the sunlight was focused directly. More than 40% of the sunlight arriving at the focus external to the reactor was chemically stored as fuel value in the product gas. The reactive metal oxide redox systems for the two-step cyclic process or the active catalysts for the single-step process may improve the kinetics and chemical conversion, and reduce the operating temperature requirements for the solar coal-gasification processes,24,25 but their use is subjected to the feasibility of recovering catalysts from the remaining coal ash. In the present work, we studied the reactivity of a number of coal-transition metal oxide systems for the thermochemical CO2 gasification of coal below 1173 K (20) Steinfeld, A. C&E Int. Symp. on CO2 Fixation & Efficient Utilization of Energy, Tokyo, Nov 29-Dec 1, 1993; Tokyo Institute of Technology: Tokyo, 1993; pp 123-132. (21) Steinfeld, A.; Kuhn, P.; Karni, J. Energy 1993, 18 (3), 239249. (22) Steinfeld, A.; Frei, A.; Kuhn, P.; Wuillemin, D. Int. J. Hydrogen Energy 1995, 20 (10), 793-804. (23) Steinfeld, A.; Brack, M.; Meier, A.; Weidenkaff, A.; Wuillemin, D.; Energy 1998, 23 (10), 803-814. (24) Takarada, T.; Nabatame, T.; Ohtsuka, Y.; Tomita, A. Coal Science and Technology 11, Int. Con. on Coal Science, Elsevier: New York, 1987; pp 547-550. (25) Meijer, R.; Muhlen, H.-J.; Kapteijn, F.; Moulijn, J. A. International Conference on Coal Science, Proceedings; Tokyo, Japan, 1989; Vol. 1, pp 337-340.

Energy & Fuels, Vol. 14, No. 1, 2000 203

in two different reaction modes: one is the two-step cyclic CO2 gasification by alternate feeding of N2 and CO2 with temperature swing below 1173 K. Another is the normal single-step CO2 gasification by continuous feeding of CO2 at a constant temperature of 1173 K. We also indicated the possibility that the reactive metal oxide In2O3 may be separated and recovered from the remaining coal ash by the evaporation of In2O above 1373 K. Thermodynamic Analysis The basic reaction of CO2 gasification of coal is the Boudouard reaction:

C(graphite) + CO2 f 2CO ∆H°298 K ) 171 kJ (1) In the two-step thermochemical process using a coalmetal oxide system, metal oxide reacts with coal in the first high-temperature step. The following reactions between carbon and metal oxides to produce CO and the metals are thermodynamically possible around 1173 K.

ZnO + C f Zn + CO ∆H°298 K ) 238 kJ

(2)

/5V2O5 + C f 2/5V + CO ∆H°298 K ) 200 kJ

(3)

/3In2O3 + C f 2/3In + CO ∆H°298 K ) 198 kJ

(4)

/2MoO2+ C f 1/2Mo + CO ∆H°298 K ) 184 kJ

(5)

1

1

1

1

/3WO3 + C f 1/3W + CO ∆H°298 K ) 170 kJ

(6)

/4Fe3O4 + C f 3/4Fe + CO ∆H°298 K ) 169 kJ

(7)

1

(8)

1

/2SnO2 + C f 1/2Sn + CO ∆H°298 K ) 178 kJ

The variations of ∆G° for the reactions as a function of temperature were shown in Figure 1a. The metals formed by the reaction can thermodynamically decompose CO2 to CO at relatively low temperatures as shown by Figure 1b. The Boudouard reaction can thermodynamically proceed at 1173 K. There is, however, no indication that it kinetically proceeds with high efficiencies. Our two-step process may catalytically provide a kinetic pathway through the coal-metal oxide reaction. We previously studied the reactivities of ZnO/Zn and Fe3O4/Fe redox systems below 1173 K.12,13 One of the objects of the present work is to study the reactivities of In2O3/In, SnO2/Sn, V2O5/V, MoO3/Mo, and WO3/W redox systems for the two-step CO2 gasification below 1173 K. There is also the possibility that the reactive coalmetal oxide system catalyzes the Boudouard reaction of CO2 gasification of coal in the normal single-step, continuous feeding mode of CO2 at a constant temperature below 1173 K. In comparison to the two-step process, the normal single-step process has many industrial advantages, eliminating the needs for alternate feeding of reactants and temperature swing. The coalmetal oxide systems were also examined for the CO2

204 Energy & Fuels, Vol. 14, No. 1, 2000

Kodama et al.

Figure 1. Variations of ∆G° of the reactions (a) C + 1/yMxOy f CO + x/yM, and (b) CO2 + x/yM f 1/yMxOy + CO with temperature for several candidates of redox pairs of metal oxides. 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. represents fixed carbon.

gasification in the normal continuous feeding mode of CO2 at a constant temperature of 1173 K. Experimental Section Preparation of Materials. SnO2, In2O3, and WO3 were prepared by thermal decomposition of the hydroxides or H2WO4 in air at 573-673 K; the hydroxides were prepared by hydrolysis of the metal chloride solutions. MoO2 was synthesized by H2-reduction of MoO3 at 923 K; MoO3 was prepared by the thermal decomposition of (NH4)6Mo7O24 in air at 623 K. V2O5 was purchased from Kanto Chemical Co., Inc. The metal oxides thus prepared were identified by X-ray diffractometry (XRD) with Cu KR radiation (Rigaku, RAD-γA diffractometer). The metal oxides thus prepared was mixed with pulverized coal. 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 27000 kJ/kg. The grain size of the coal powder was smaller than 300 µm. Mode of Two-Step Cyclic Gasification by a CoalMetal Oxide System. Metal oxide was mixed with coal (0.2 g or 10.6 of mmol C) at a molar ratio of oxygen in metal oxide to carbon in coal (O/C molar ratio) equal to 1.2. The coal/metal oxide mixture was packed in the reactor of a quartz tube with a diameter of 8 mm and a length of 240 mm. The experimental setup is illustrated in Figure 2. The reactivities for the following reactions were examined for the metal oxides:

conversion to j ) Aj/[amount of carbon in the coal used (mmol)] (11) Aj is an amount of product j of interest (mmol) in the effluent collected during a coal-gasification step. CO2 conversion in the CO2-reduction step was estimated by

CO2 conversion ) BCO/[FCO2,in × tCO2]

metal oxide + CHn (coal) f metal + xCO + (1 - x)CO2 + n/2H2 (9) metal + (2 - x)CO2 f metal oxide + (2 - x)CO

to 1173 K (heating rate ) 60 K min-1) in an infrared furnace (ULVAC, E45) while passing N2 gas at a flow rate of 0.41 mmol min-1 and the coal-gasification step was carried out for 30 min at 1173 K in the N2 gas stream. After the coal-gasification step, the reactor was cooled to a desired temperature for the CO2reduction step while passing N2 gas. The CO2-reduction step was then performed at the desired constant temperature. The reduced metal oxide reacted with CO2 when feeding CO2 at a flow rate of 0.41 mmol min-1 through 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 gas products evolved during the coal-gasification and CO2reduction steps, the effluent was collected in a bottle to replace water. The volume of the collected effluent was measured and the contents of the products were determined by gas chromatography. Coal conversion to a product of interest such as CO, CO2, or CH4 was determined on a carbon basis according to the relation,

(10)

Hereafter, the first step (eq 9) is referred to as the coalgasification step and the second step (eq 10) as the CO2reduction step. The coal-gasification step was first performed. The coal/metal oxide mixture in the reactor was rapidly heated

(12)

BCO is an amount of CO (mmol) in the effluent collected during the CO2-reduction step. FCO2,in is the flow rate of CO2 (mmol min-1) for the inlet of the reactor. tCO2 represents the reaction time (min) for the CO2-reduction step. Mode of Single-Step Gasification by a Coal-Metal Oxide System. Metal oxide was mixed with coal (0.2 g or 10.6 mmol of C) at 1.7-71 wt % of the metal in the coal and metal oxide mixture (the O/C molar ratios ) 1.6 × 10-3 - 1.2). The



Figure 2. Schematic of the experimental setup.

Figure 3. Time variations of partial pressures of the evolved CO (b), CO2(0), and H2(2) (a) in the coal-gasification step for 30 min at 1173 K and at an N2 flow rate of 0.41 mmol min-1, and (b) in the CO2-reduction step for 60 min at 1023 K and at a CO2 flow rate of 0.41 mmol min-1 for the coal-In2O3 system. 0.2 g of coal (10.6 mmol of C) was mixed with 1.2 g of In2O3 (4.3 mmol) at the O/C molar ratio ) 1.2. coal/metal oxide mixture was packed in the same reactor used for the two-step reaction system. The mixture in the reactor was rapidly heated in an infrared furnace (ULVAC, E45) to 1173 K (heating rate ) 60 K min-1) while passing CO2 at a flow rate of 0.41 mmol min-1. The reactivity for the reaction,

CHn(coal) + CO2 f 2CO + n/2H2

(13)

was tested on the coal/metal oxide mixture in the CO2 gas stream at a constant temperature of 1173 K. Changes in the partial pressures of the gaseous products evolved were measured by using gas chromatography with TCD. The effluent was collected in a bottle to replace water to determine the total amounts of gas products evolved. Coal conversion to CO was estimated by

coal conversion to CO ) [CCO/2]/ [amount of carbon in the coal used (mmol)] (14) CCO is an amount of CO (mmol) in the effluent collected during the reaction. Separation of In2O3 from Remaining Coal Ash. The solid phase after the two-step cyclic CO2 gasification was heated at 1373 K when passing a CH4/CO2 mixture (PCH4 ) 0.1) at a flow rate of 0.41 mmol min-1 for 90 min (indium separation process). Parts of the quartz tube of the reactor

close to the outlet was ice-cooled to collect the evaporated indium oxide (the cooling trap). After this indium separation process, a significant amount of yellow solid was deposited on the inside wall of the cooling trap. Taking a part of yellow solid out of the cooling trap, it was subject to the XRD analyses. After the single-step CO2 gasification using the coal-In2O3 system (17 wt % of indium in the mixture), the solid phase containing the In2O3 and remaining coal ash was also subject to the indium separation process at 1373 K. The indium amounts in the solid phases before and after the indium separation process were determined by the X-ray fluorescence analysis with an accelerating voltage of 50 kV and a current of 30 mA by the working curve method using the lithium borate matrix (Rigaku Geigerfelx SX-3063P).

Results and Discussion Two-Step Cyclic CO2 Gasification by the CoalMetal Oxide System below 1173 K. A. Coal-Gasification Step. The mixture of coal and metal oxide was rapidly heated to 1173 K and the coal-gasification step was carried out for 30 min. Figure 3a shows a typical evolution profile of the products for the coal-In2O3 system. The conversions of coal and selectivities were

206

Energy & Fuels, Vol. 14, No. 1, 2000

Kodama et al.

Table 2. Coal Conversions and Selectivities for Products in the Coal-Gasification Step for 30 min at 1173 K coal conversion to/% materiala

CO

CO2

Fe3O4b ZnOb V2O5 WO3 SnO2 MoO2 In2O3 CO2c

9.5 17.9 1.5 15.4 20.9-24.9 12.2 35.1-45.3 13.1

9.3 2.3 4.1 6.0 32.7-37.1 3.3 22.4-33.5

CH4 2.6 4.4 3.1-3.8 3.8 2.9-3.8 0.4

selectivity/% total

CO

CO2

21.4 24.6 5.6 21.4 50.7-61.8 19.3 61.3-81.7 13.5

44.4 73.0 26.8 72.0 29.4-33.9 63.2 55.4-57.3 97.0

43.3 9.2 73.2 28.0 60.0-64.5 17.1 36.5-41.0

CH4 12.3 17.8 6.1-6.2 19.7 3.6-6.2 3.0

a Metal oxide was mixed with coal (0.2 g or 10.6 mmol of C) at a molar ratio of oxygen in metal oxide to carbon in coal (O/C molar ratio) equal to 1.2. b Results are taken from ref 12. c Only coal (0.2 g or 10.6 mmol of C) was allowed to react directly with a CO2(coal-CO2 reaction). CO2 was fed at a rate of 0.41 mmol min-1 and at 1173 K for 30 min.

listed in Table 2. Only coal (0.2 g or 10.6 mmol C) was reacted directly with CO2 under similar reaction conditions to carry out the direct single-step CO2 gasification without any catalysts (the coal-CO2 reaction). The coal conversion of the coal-CO2 reaction was only 14%. The coal conversions for the coal-V2O5, -MoO2, and -WO3 reactions were less than 21%. However, In2O3 and SnO2 showed much higher coal conversions than the coalCO2 reaction. For these reactive metal oxides, the coalgasification step was performed three times to check the reproducibility. 61-82% and 51-62% of the coal conversions were obtained for the coal-In2O3 and coal-SnO2 systems, respectively. We previously showed the reactivities of ZnO and Fe3O4 for coal under the same reaction conditions.12 The reactivities were much lower than those of the In2O3 and SnO2 as shown in Table 2. The XRD patterns of the solid phase after the coalgasification step showed that In2O3 and SnO2 were almost reduced to the metallic phases of In and β-Sn, respectively (Figure 4). The CO selectivities in the coalIn2O3 and coal-SnO2 systems were 55-57% and 2934%, respectively. In the proposed two-step process, a high selectivity for CO is not strongly required in the first coal-gasification step because the net reaction of eqs 9 and 10 gives CHn (coal) + CO2 f 2CO + n/2H2 and CO2 is not emitted in total. But a high CO selectivity is preferable, eliminating subsequent chemical separation. From both of the coal conversion and the CO selectivity, the coal-In2O3 system is the most promising for the two-step cyclic process. The MALT226 was used to compute the equilibrium compositions of the systems, C + CO2, C + 1/3In2O3, and C + 1/2SnO2, at 1 atm and over the range of temperature of interest (Figure 5). In the all cases, the temperatures above 1173 K are thermodynamically enough to obtain the high conversion of carbon to CO over 90%. However, the experimental results of the coal conversion to CO (Table 2) were very far from the equilibrium conversions, especially for the coal-CO2 reaction. This will come from the fact that the heat and mass transfer conditions of the packed-bed reactor used were not enough to attain the equilibrium conversions. These reaction conditions may be improved if one uses a fluidized-bed reactor instead of a packed-bed reactor. In the coal-gasification steps using In2O3 and SnO2, we could estimate the material balance of oxygen between solid-phase and gas products because in this reaction the metal oxides were almost reduced to the metallic phases and an amount of oxygen released from (26) Yamauchi, S.; Netsu Sokutei 1985, 12 (3), 142-144.

Figure 4. XRD patterns of the solid phases of (a) In2O3, and (b) SnO2 after use of the coal-gasification step for 30 min at 1173 K while passing N2 gas at a flow rate of 0.41 mmol min-1. 0.2 g of coal (10.6 mmol of C) and 1.0-1.2 g of metal oxide were used. The O/C molar ratio was 1.2.

the metal oxides could be calculated. Oxygen in the metal oxides is released as gas products of CO, CO2, and H2O. We could not measure the evolved amount of H2O in the present work. The amounts of oxygen recovered as CO and CO2 was 90% (the ratio of total amount of oxygen in evolved CO and CO2 to that in the used In2O3) for the coal-In2O3 reaction. In this case, H2O evolution was assumed to scarcely occur in the coal-gasification step. For the coal-SnO2 reaction, the material balance of oxygen was only 79% (the ratio of total amount of oxygen in evolved CO and CO2 to that in the used SnO2) although the SnO2 was completely reduced to the metallic β-Sn (Figure 4b). This may be because about 20% of oxygen of SnO2 was converted to H2O in the coalgasification step. To estimate the material balance of carbon in the coal-gasification step, the used mixture of coal and metal oxide was subject to combustion in an O2 flow at 1173 K; the effluent was collected in a



Figure 6. XRD pattern of the solid phase of In2O3 after performing the CO2-reduction step for 60 min at 1023 K while passing CO2 gas at a flow rate of 0.41 mmol min-1 after the coal-gasification step for 30 min at 1173 K and at an N2 flow rate of 0.41 mmol min-1. 0.2 g of coal (10.6 mmol of C) and 1.2 g of In2O3 (4.3 mmol) were used. The O/C molar ratio was 1.2.

Figure 5. Equilibrium composition of major components of the systems, (a) C + CO2, (b) C + 1/3In2O3, and (c) C + 1/2SnO2, at 1 atm as a function of temperature. The MALT 26 was used to compute the equilibrium compositions of the systems.

bottle to replace water and total amount of evolved COx (mainly CO2) was determined by gas chromatography to determine the amount of unreacted and remaining carbon (DCOx; mmol) after the coal-gasification step. The material balance of carbon was estimated by the following equation:

[amount of carbon in the coal used (mmol)] (15)

Figure 7. Conversions of coal and CO2 when repeating the two-step process six times using the coal-In2O3 system. 0.2 g of coal (10.6 mmol of C) was initially mixed with 1.2 g of In2O3 (4.3 mmol), and the O/C molar ratio was 1.2. The coalgasification steps were performed for 30 min at 1173 K and at an N2 flow rate of 0.41 mmol min-1. The CO2-reduction steps were carried out for 60 min at 1023 K and at a CO2 flow rate of 0.41 mmol min-1. a 0.2 g of coal (10.6 mmol of C) was newly added to the used coal/In2O3 mixture before the third and fifth runs. The coal conversions for the third-sixth runs refer to the newly added coal.

More than 90% of the material balance of carbon was obtained for the coal-In2O3 and coal-SnO2 reactions. B. CO2-Reduction Step. The metallic In in the solid phase after the coal-gasification step was reacted with a CO2 feed below 1173 K. First, to determine the reaction temperature for the CO2-reduction step, the solid phase after the coal-gasification step was slowly heated from 673 to 1123 K at 3.3 K min-1 while passing CO2 at a flow rate of 0.41 mmol min-1 through it. The maximal CO evolution was observed at 1023 K. Figure 3b shows a CO-evolution profile of the CO2-reduction step by the coal-In2O3 system at a constant temperature of 1023 K after performing the coal-gasification step. A significant amount of CO was evolved. The XRD analysis of the solid phase after the CO2-reduction step showed that metallic In was completely oxidized to In2O3 (Figure 6). C. Repetition of Two-Step Cyclic Reaction. The twostep cyclic CO2 gasification of coal using the In2O3/In

redox system was demonstrated (Figure 7 and Table 3). We alternately repeated the coal-gasification step for 30 min at 1173 K and the CO2-reduction step for 60 min at 1023 K. First, the two-step process was repeated twice (the first and second runs). The total conversion of coal in the first and second runs reached 80%. Then, a 0.2-g portion of coal (10.6 mmol C) was added to the used mixture to carry out the third and fourth runs successively. In the third and fourth runs, 77% of the newly added coal was gasified in total. Then 0.2 g of coal (10.6 mmol C) was further added to the used mixture, and the fifth and sixth runs were performed with the result that 77% of the added coal was gasified. In every CO2-reduction step, the significant amount of CO continued to be evolved, showing the CO2 conversions from 10% to 52% (Figure 7). In the XRD pattern of the In2O3 after the sixth run, only strong peaks of In2O3 appeared, indicating that the In2O3 was regenerated after the two-step CO2 gasification.

material balance of carbon ) [ACO2 + ACO + ACH4 + DCOx]/

208


Kodama et al.

Table 3. Results of Repetition of the Two-Step Process by the In2O3/In Redox Systema coal-gasification stepb evolved amount/mmol run 1st 2nd coal 0.2 gc f 3rd 4th coal 0.2 gc f 5th 6th

CO2-reduction stepb selectivity/%

evolved amount/mmol

temp/K

CO

CO2

CH4

H2

CO

CO2

CH4

1173 1023 1173 1023

4.70

2.50

0.39

2.00

61.9

32.9

5.2

0.30

0.59

0.81

33.7

66.3

1173 1023 1173 1023

4.31

2.49

0.74

0.57

1173 1023 1173 1023

3.84

1.86

1.46

0.71

0.06

0.31

0.06

62.8

36.3

0.38

56.5

43.5

0.31

63.9

30.9

0.07

67.3

32.7

CO

H2

7.28

1.28

1.43

0.52

7.14

0.08

3.50

0.36

5.80

0.07

4.59

0.06

0.9

5.2

In2O3 (1.2 g or 4.3 mmol) was mixed with coal (0.2 g or 10.6 mmol of C) at the O/C molar ratio ) 1.2. The coal-gasification step was carried out for 30 min at 1173 K and at an N2 flow rate of 0.41 mmol min-1. The CO2-reduction step was performed for 60 min at 1023 K and at a CO2 flow rate of 0.41 mmol min-1. c 0.2 g of coal (10.6 mmol of C) was added to the used mixture of In2O3 and coal. a

In our previous report, the In(III)-ferrite was found to be a very reactive working material for the proposed two-step process.13 Repetition test of the two-step process was also carried out for the coal-In(III)-ferrite system under reaction conditions similar to those used in the present work. The total coal conversion in the first and second runs reached 90%, and the CO selectivity was more than 68%. However, the conversion decreased to 62% by repeating the cyclic reaction six times in the similar way to Figure 7. To continue the two-step process with a high conversion, the coal-In2O3 system studied here is superior to the coal-In(III)ferrite system. Another problem in using the coalIn(III)-ferrite system is to recover the used ferrite from the remaining coal ash. Single-Step CO2 Gasification by the Coal-Metal Oxide System at 1173 K. The coal-metal oxide systems were examined for the CO2 gasification of coal in the normal continuous feeding mode of CO2 at a constant temperature of 1173 K. Metal oxide was mixed with coal at 17 wt % of the metal content in the coal/ metal oxide mixture: the O/C molar ratio was about 0.05. Figure 8 shows the CO evolution profiles when passing CO2 through the mixtures at a flow rate of 0.41 mmol min-1. Much larger CO evolutions were observed for the coal-In2O3 and coal-SnO2 systems. The coal conversions after 60 min reaction were given in Table 4. Even at the low O/C molar ratios of about 0.05, the coal conversions to CO reached 98% and 93% for the coal-In2O3 and coal-SnO2 systems, respectively, although that by the coal-CO2 reaction in the absence of metal oxides was only 41%. Especially, the partial pressure of CO evolved in the single-step gasification initially attained 90% with the coal-In2O3 system (Figure 8), indicating the most reactivity of the coalIn2O3 system for the Boudouard reaction in the CO2 gasification of coal. In the XRD pattern of the solid phase after the gasification, only the strong peaks of In2O3 appeared. These results indicate that the In2O3 can be also the very active catalyst for the CO2 gasification of coal in a normal single-step, continuous feeding mode of CO2 at a constant temperature of 1173 K. Figure 9 shows the relation between the coal conversion and the reaction time with various indium contents

b

Figure 8. Time variations of the partial pressure of the evolved CO during the single-step CO2 gasification by the coal-metal oxide system at 1173 K and at CO2 flow rate of 0.41 mmol min-1. Coal (0.2 g or 10.6 mmol of C) was mixed with the metal oxide (0.05-0.08 g) at 17 wt % of the metal content in the mixture. Symbols: In2O3(9), SnO2(b), V2O5(O), MoO2(4), ZnO(2), WO3(0), Fe3O4(]), CO2(3). a Only coal (0.2 g or 10.6 mmol of C) was allowed to directly react with CO2 at a CO2 flow rate of 0.41 mmol min-1 and at 1173 K.

in the coal/In2O3 mixture. Our experimental setup could not monitor the coal conversion continuously during the coal gasification. Thus, in Figure 9, the time variation of coal conversion was deduced by repeating the same runs several times with interruption after different reaction times. With this approach, discrete relationships between coal conversion and reaction time were available. As shown in Figure 9, the initial coalconversion rates with the coal-In2O3 system (reaction time < 30 min) were 2.5-4 times as fast as that in the coal-CO2 reaction without any oxidant of metal oxide. The indium content of 17 wt % (the O/C molar ratio ) 0.05) was enough to catalytically enhance the coal gasification. This saves the required amount of In2O3



Table 4. Results of the Single-Step CO2 Gasification by Coal-Metal Oxide System at 1173 K coal conversion to/%

evolved amount/mmol

materiala

CO

CH4

total

CO

CH4

H2

ZnO V2O5 In2O3 MoO2 WO3 Fe3O4 SnO2 CO2b

47.6 60.7 97.8 56.4 41.8 46.5 92.8 40.6

2.4 4.0 2.2 4.6 11.2 3.9 4.5 3.3

50.0 64.7 100 61.0 53.0 50.4 97.3 43.9

10.32 12.90 20.98 11.99 8.88 9.92 20.05 8.61

0.26 0.42 0.32 0.49 0.54 0.42 0.49 0.36

1.34 1.43 1.24 1.25 1.19 1.25 1.10 1.21

a Metal oxide(0.05-0.08 g) was mixed with coal (0.2 g or 10.6 mmol of C) at a 17 wt % of the metal content in the coal/metal oxide mixture. b Only coal (0.2 g or 10.6 mmol of C) was allowed to react directly with CO2 (coal-CO2 reaction). CO2 was fed at a rate of 0.41 mmol min-1 and at 1173 K for 60 min.

Figure 10. Enthalpy-temperature diagrams of (a) the twostep cyclic CO2 gasification and (b) the single-step CO2 gasification by the coal-In2O3 system. In2O3 is assumed to be recycled at ambient temperature. For the two-step cyclic CO2 gasification, the system of 1 mol of carbon + 1 mol of CO2 + 1/ In O is used for the calculation. For the single-step CO 3 2 3 2 gasification, the system of 1 mol of carbon + 1 mol of CO2 + 0.017 mol of In2O3 is used.

Figure 9. Time variations of coal conversions in the singlestep CO2 gasification of coal by the coal-In2O3 system with various indium contents. 0.2 g (10.6 mmol of C) of coal was used. Symbols: 1.7(1), 4.1(4), 17(O), and 71(9) wt % of indium contents in the coal-In2O3 mixtures. CO2 was fed at a 0.41 mmol min-1 and at 1173 K. a Only coal (0.2 g or 10.6 mmol of C) was allowed to react with CO2 at CO2 flow rate of 0.41 mmol min-1 at 1173 K.

for the CO2 gasification of coal in comparison to the twostep cyclic process with the coal-In2O3 system in which the O/C molar ratio ) 1 is theoretically required at least. Model and Thermal Efficiency. To estimate the idealized thermal efficiency for the proposed two-step cyclic gasification process by the coal-In2O3 system, the enthalpy-temperature diagram is displayed in Figure 10a. Here we assumed that all of the used In2O3 can be recycled at ambient temperature (path 7 f 1). Path 1 f 2 is the process of heating 1 mol of carbon, 1 mol of CO2 and 1/3 mol of In2O3 at standard conditions to 1023 K. Path 2 f 3 is the CO2-reduction step of CO2 + 2/3In(l) f 1/3In2O3 + CO at 1023 K by which 24.7 kJ of heat is produced. Path 3 f 4 is the process of heating 1 mol of carbon and 1/3 mol of In2O3 to 1173 K. Path 4 f 5 is the coal-gasification step of eq 4 at 1173 K, which requires 193.3 kJ of heat. Paths 5 f 6 and 6 f 7 are the cooling processes of 1 mol of CO + 2/3 mol of In(l) from 1173 to 1023 K, and 2 mol of CO + 1/3 mol of In2O3 from 1023 to 298 K, respectively. These cooling processes produce 82.2 kJ of heat. Of this heat, heat

produced by cooling of the CO effluent from the solar reactor can be easily utilized to heat the CO2 feed in a heat exchanger. Heat produced by cooling of the CO effluent (1 mol at 1173 K and 1 mol at 1023 K are released from the solar reactor) to ambient temperature is 50.0 kJ, which is much larger than heat required for heating the CO2 feed (1 mol) from ambient temperature to 1023 K (34.7 kJ). Thus, heat required for path 1 f 5 is 254.6 kJ but, of this required heat, 34.7 kJ can be supplied by the heat exchange. Therefore, 219.9 kJ is necessary to be supplied by absorbing the solar radiation (Qabs). In2O3 (1/3 mol) is cooled to ambient temperature and mixed again with newly added carbon (1 mol). The schematic diagram of this system is illustrated in Figure 11. In practice, pumping work is required to supply CO2 and the mixture of carbon and In2O3 to the reactor but it was ideally neglected here. The useful work produced as chemicals by this system is the exergy change of eq 1 at standard conditions (∆0 ) 120.1 kJ). Thus, we define the idealized thermal efficiency of the system as follows:

ηth ) ∆0/Qabs

(16)

The idealized overall system efficiency can be given by

ηoverall ) Qabs/Qsolar × ηth

(17)

where Qsolar is total solar energy coming from the concentrator. The ηth was estimated to be 0.546 for the two-step cyclic process by the coal-In2O3 system. For the proposed single-step gasification process by the coal-In2O3 system, the enthalpy-temperature dia-

210


Kodama et al.

Figure 12. Temperature variations of the vapor pressures of In(g) and In2O(g).

volatile oxide of In2O(g) is formed by a process corresponding to the following reaction:27

In2O3(c) + 4In(l) f 3In2O(g) ∆H°298 ) 784 kJ (18) Figure 11. A schematic system model of the proposed solar thermochemical process for the two-step cyclic or single-step CO2 gasification by the coal-In2O3 system. Symbols used are defined as follows. Qabs: heat absorbed from the solar radiation; Qw: waste heat which is produced by the processes of cooling In2O3 or In(l) in the solar reactor to ambient temperature; Qw′: waste heat which is produced by cooling CO from the heat exchanger to ambient temperature; and T0: ambient temperature.

gram is displayed in Figure 10b. Here we assumed that the required amount of the In2O3 catalyst is 0.017 mol for 1 mol of carbon: this amount of In2O3 was determined from 17 wt % of indium in the mixture of coal and In2O3. It was also assumed that all of the used In2O3 can be recycled at ambient temperature (path 4 f 1). Path 1 f 2 is the process of heating 1 mol of carbon, 1 mol of CO2, and 0.017 mol of In2O3 at standard conditions to 1173 K. Path 2 f 3 is the coal gasification with CO2 in eq 1 at 1173 K, which requires 168.9 kJ of heat. Path 3 f 4 is the process of cooling 2 mol of CO and 0.017 mol of In2O3. In2O3 (0.017 mol) is cooled to ambient temperature and mixed again with newly added carbon (1 mol). Heat required for path 1 f 3 is 229.3 kJ but, of this required heat, 42.9 kJ (heat required for heating 1 mol of the CO2 feed from ambient temperature to 1173 K) can be supplied by the heat exchange. Hence, Qabs is 186.4 kJ. The idealized thermal efficiency is 0.644 for the single-step process by the coal-In2O3 system, which is higher than that for the two-step cyclic process. Separation of In2O3 from Remaining Coal Ash. The separation and recovery of used metal oxides from the remaining coal ash are needed for the CO2 gasification using the coal-metal oxide systems. Indium oxide may be separated from the ashes by In2O evaporation. The vapor pressure of In2O(g) is very high at high temperatures and exceeds 1 atm above 1373 K as shown in Figure 12. It is reported that at high temperatures a

This reaction will be utilized for the separation and recovery of indium oxide from the ash as follows. After the CO2 gasification of coal using the coal-In2O3 system in the two-step cyclic or single-step process below 1173 K, the used coal/In2O3 mixture is reduced with CH4 above 1373 K to convert In2O3 to In2O(g) by the reaction of eq 18. The formed In2O vapor may be collected in a cooling trap connected to the outlet of the reactor where it will be converted to In2O3 and metallic In according to eq 18. The formed metallic In can be readily airoxidized to In2O3. A small amount of In2O(g) may form during the CO2 gasification processes performed below 1173 K, but this indium oxide vapor also will be collected in the outlet cooling trap when performing the CO2 gasification. This indium separation process in the coal-In2O3 system was roughly demonstrated for the solid phases after the two-step cyclic and single-step CO2 gasification processes. The solid phase used for the CO2 gasification was heated at 1373 K when passing a CH4/CO2 mixture. After the indium separation process, a significant amount of yellow solid was deposited on the inside wall of the outlet cooling trap. The yellow solid deposited from the solid phase used for the two-step cyclic CO2 gasification was identified to be In2O3 by XRD analysis. The metallic In formed by the reverse reaction of eq 18 is considered to be oxidized to In2O3 by the CO2 feed. We could not determine the recovery of In2O3 because it was difficult to collect all of the yellow solid deposited on the wall of the cooling trap; most of the yellow solid stuck tightly to the wall of the quartz tube. For the solid phase used for the single-step CO2 gasification, the amount of the yellow solid deposited in the cooling trap was too small to perform the XRD analysis. This is because the amount of In2O3 initially used (0.05 g or 0.18 mmol of In2O3) for the preparation of the mixture (27) Jenko, M.; Erjavec, B.; Pracek, B. Vacuum 1990, 40 (1-2), 7780.



Table 5. Results of the Indium Separation Process at 1373 K for the Coal/In2O3 Mixture (indium content ) 17 wt %)

recovery of In2O3 from the ash and improve the efficiency of recovering In2O3.

weight of the coal/In2O3 mixture/g

Conclusions

solid phase before the single-step CO2 gasification after the single-step CO2 gasification after the indium separation process

indium amount in the coal/In2O3 mixture/g

0.25

0.041

0.052

0.034a

0.022

0.0022a

a The indium amount was determined by X-ray fluorescence analysis.

was very small. Thus, in this case, the indium amount in the solid phase before and after the indium separation process was determined by the X-ray fluorescence analyses. The change of the indium amount in the coal/ In2O3 mixture was summarized in Table 5. A 0.041-g portion of indium was contained in the coal/In2O3 mixture before the single-step CO2 gasification, which decreased to 0.034 g after the CO2 gasification. The loss of the indium was about 17%. This loss would be mainly due to the loss of taking the solid phase out of the reactor; we could not collect all of the used mixture from the reactor to subject it to the X-ray fluorescence analysis because a small portion of the mixture stuck on the reactor wall. After the indium separation process, the indium amount decreased to 0.0022 g, indicating that 95% of indium was evaporated in a form of In2O(g), and moved from the remaining coal ash, probably to the cooling trap. Further detailed investigations using a well-designed cooling trap system are required for the quantitative indium separation process to determine the

The coal-In2O3 system was found to be one of the most reactive systems for the thermochemical CO2 gasification of coal both in the two-step cyclic and the normal single-step reaction processes. The CO2 gasification of coal using the coal-In2O3 system is superior in efficiency to the direct CO2 coal-conversion without using any metal oxides as oxidants below 1173 K. Especially, the single-step CO2 gasification with the coal-In2O3 system has many industrial advantages in comparison to the two-step cyclic CO2 gasification. It may be possible to separate and recover the used In2O3 from the remaining coal ash by In2O evaporation at a proper reducing atmosphere above 1373 K. Our findings show that the thermochemical coal-gasification process using the reactive coal-metal oxide system may realize an efficient CO2 gasification of coal utilizing solar heat as an energy source below 1173 K. In practice, the reaction conditions of the solar reactor, especially heat and mass transfer conditions, have a strong influence in the reaction rates and degree of chemical conversion. A fluidized-bed reactor may be favorable for improved reaction rates and degree of chemical conversion in comparison to a packed-bed reactor. Our coal-In2O3 system may be applied for a fluidized-bed reactor to improve the reaction efficiencies. Further investigation is now in progress for a fluidized-bed reactor using the coal-In2O3 system. EF990135U

Thermochemical CO2 Gasification of Coal Using a Reactive Coal

Recommend Documents