Energy & Fuels 1999, 13, 961-964
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Coal/CO2 Gasification System Using Molten Carbonate Salt for Solar/Fossil Energy Hybridization Shinya Yoshida, Jun Matsunami, Yukitoshi Hosokawa, Osamu Yokota, and Yutaka Tamaura* Research Center for Carbon Recycling and Utilization, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
Mitsunobu Kitamura Department of Materials Science, School of Engineering, The University of Shiga Prefecture, 2500 Hassaka-cho, Hikone City, Shiga 522-8533, Japan Received June 25, 1998
The gasification of active carbon and coal with CO2 (the Boudouard reaction: C + CO2 ) 2CO) was studied using a molten salt (the mixture of K2CO3 and Na2CO3) at 1123 K to apply this system to solar energy conversion into chemical energy. On the solar/chemical energy hybridization system to obtain CH3OH as a solar fuel, the coal with molten salt can directly gasify by the Boudouard reaction. The gasification reaction rate of active carbon and coal with CO2 into CO was enhanced by 1.5 and 3.3 times in the presence of the molten salt, respectively, compared with the absence of the molten salt. The coal gasification using molten salt was suggested to have two steps: the first step is coke production by the coal pyrolysis, and the second step is the Boudouard reaction by the catalytic effect of the alkali metal cations (K+ and Na+).
Introduction Solar energy can be used by transforming it to electricity (via photovoltaic generation) and chemical energy. A strong solar insolation at the sunbelt will be the most promising renewable energy resource in the 21st century. If the solar energy at the sunbelt can be converted into chemical energy such as syn-fuels (CH3OH, (CH3)2O, etc.), it can be transported from the sunbelt to remote energy-consuming sites. Solar energy can be converted into syn-fuels by a STC (solar thermochemical) process,1-3 where concentrated solar heat is absorbed in endothermic reactions. The solar/chemical energy conversion efficiency is 75-80% for the concentrated solar heat (1000 sun).4 Several endothermic reactions such as metal oxide reductions for watersplitting reaction5 or coal gasification,6-13 coal liquefaction, methane reforming, and coal gasification with * Author to whom correspondence should be addressed. Tel: +813-5734-3292. Fax: +81-3-5734-3436. E-mail:
[email protected]. ac.jp. (1) Tamaura, Y. International Conference on Technologies for AIJ, Vancouver, Canada, May 6-29, 1997. (2) Levy, R.; Levitan, M.; Meirovich, E.; Segal, A.; Rosin, H.; Rubin, R. Solar Energy 1992, 48, 395-402. (3) Arthur, I. Sol. Energy Mater. 1991, 24, 733-741. (4) Kesserlring, P. High Flux Dish-Solar Reactor, Deutsche Forschungsanstalt fur Lu und Aumfahrt, June, 1994. (5) Ehrensberger, K.; Kuhn, P.; Shklover, V.; Oswald, H. R. Solid State Ionics 1996, 90, 75-81. (6) Palumbo, R. D.; Campbell, M. B.; Grafe, T. H. Energy-Int. J. 1992, 17, 179-190. (7) Tsuji, M.; Sano, T.; Tabata, M.; Tamaura, Y. Energy-Int. J. 1995, 20, 869-876. (8) Tsuji, M.; Wada, Y.; Tamaura, Y.; Steinfeld, A.; Kuhn, P.; Palumbo, R. Energy Convers. Mgmt. 1996, 37, 1315-1320.
H2O14 have been studied for the STC process. We have proposed a global solar energy delivery system, where carbon is used as a solar energy carrier in STC processes and the syn-fuels containing solar energy are transported from the sunbelt of the world to the remote energy-consuming sites. Natural gas (methane), coal, and CO2 have been studied as solar energy carriers in STC processes. For example, the methane reforming reactions and the carbon gasification reactions (water gas reaction and the Boudouard reaction) are given by the following equations:
Methane reforming CH4 + H2O ) CO + 3H2 ∆H° ) 225.73 kJ/mol (1000K) (1a) CH4 + CO2 ) 2CO + 2H2 ∆H° ) 260.51 kJ/mol (1000K) (1b) Carbon gasification (water gas reaction) C + H2O ) CO + H2 ∆H° ) 135.79 kJ/mol (1000K) (2) (Boudouard reaction) C + CO2 ) 2CO ∆H° ) 169.25 kJ/mol (1125K) (3) At high-temperature STC processes, CO2 is a good energy carrier, since it can be transferred as a gaseous carrier by pipeline. Therefore, the STC process with CO2
10.1021/ef980144n CCC: $18.00 © 1999 American Chemical Society Published on Web 07/02/1999
962 Energy & Fuels, Vol. 13, No. 5, 1999
Yoshida et al.
Table 1. Proximate and Ultimate Analysis and Density of Active Carbon and Coal ultimate analysis (wt %, daf) sample
C
H
active carbon coal
97.9 90.32
0.4 4.14
a
O 3.68
proximate analysis
S
N
asha
0.36
1.7 1.50
22.3 17.70
density (g/cm3)
moisture
packed
true
0.90
0.43-0.48 0.75-0.85b
1.9-2.2 1.3-1.8b
b
Dry basis. Measured after pyrolysis.
carrier at high temperatures has been studied extensively by a number of researchers. By using molten carbonate salt in the coal gasification, we expected thermal storage,14 thermal uniformity,15 and a catalytic effect.16-18 In the present paper, we have studied the gasification of active carbon and coal with CO2 using a molten carbonate salt (the mixture of K2CO3 and Na2CO3) at 1123 K to apply this system to solar energy conversion into chemical energy. This molten carbonate salt system is expected to be as a thermal storage, which is capable of stable operation under the fluctuation of insolation and thermal uniformity inside the solar reactor under a concentrated solar energy radiation in STC processes. Experimental Section The samples were active carbon and Shan-Xi coal supplied from Takeda Chemical Ind. Ltd. and Idemitsu Coal Lab., respectively. Particle sizes of each sample were 100-400 µm. Proximate and ultimate analysis of them were made, and the results are shown in Table 1. Their packed and true densities are also shown. Experimental apparatus is shown in Figure 1. The reaction vessel made of stainless steel (58-85 mm diameter, 100 mm height) was separable into two cells (upper and lower). The CO2 feed gas was made to flow onto the molten salt surface. The inner lid (copper plate) was fit with the lower cell to make CO2 stream along the molten salt surface, for the increase of the contact time of CO2 feed gas with the molten salt. The active carbon or coal sample (0.25 or 0.27 g, respectively) was packed in the sample feeder, which was made of vinyl chloride tubing, equipped with the inlet stainless steel tube. The mixture of Na2CO3 and K2CO3 (weight ratio ) 1) was used as molten salt because of its low melting point (983 K).19 The molten salt (10 g) was placed in the reaction vessel and was heated by using an electric furnace, where the CO2 flow rate was 12 µmol/s. The reaction was conducted at 1123 K, and the temperature was measured through the thermocouple fit with the reactor bottom. When the temperature of the reactor reached the desired temperature, active carbon or coal sample was (9) Mimori, K.; Togawa, T.; Hasegawa, N.; Tsuji, M.; Tamaura, Y. Energy-Int. J. 1994, 19, 771-778. (10) Tsuji, M.; Wada, Y.; Tamaura, Y. Energy Fuels 1996, 10, 225228. (11) Steinfeld, A.; Fletcher, E. A. Energy-Int. J. 1991, 16, 10111019. (12) Zhang, Z.-G.; Scott, D. S.; Silveston, P. L. Energy Fuels 1994, 8, 637-642. (13) Zhang, Z.-G.; Scott, D. S.; Silveston, P. L. Energy Fuels 1995, 9, 479-483. (14) Epstein, M. International Workshop on High-Temperature Solar Chemistry, PSI, Switzerland, August 17, 1995. (15) Valenti, M. Mech. Eng. 1995, 117, 72-75. (16) Ruan, X.-Q.; Wu, Y.-Q.; Liu, Z.-L.; Li, S.-F. Fuel 1987, 66, 568571. (17) Hauseran, W. B. Int. J. Hydrogen Energy 1994, 19, 413-419. (18) Alam, M.; Debroy, T. Carbon 1987, 25, 279-288. (19) Reisman, A. J. Am. Chem. Soc. 1959, 81, 807-811.
Figure 1. Experimental setup for coal/CO2 gasification using the molten salt.
Figure 2. Time variation of the CO evolution rate for the gasification of active carbon with CO2 at 1123 K. (b, without the molten salt; 2, with the molten salt).
dropped into the reaction vessel and the measurement of the evolved gas composition by ND-IR tester (Shimadzu CGT-10-2A) was started. Results and Discussion Enhancement of CO Evolution. Figure 2 shows the time variation of the CO evolution rate (µmol/s) in the case of using the active carbon sample substituted for coke (b: without the molten salt; 2, with the molten salt). The maximum CO evolution rate with the molten salt was increased by 1.5 times (12 to 18 µmol/s) compared with that without the molten salt. The gasification rate acceleration of active carbon with CO2 is considered to come from the catalytic effect of the molten salt for the Boudouard reaction. The catalytic effects of the alkaline metal cations on the steam gasification (eq 2) or the Boudouard reaction have been
Solar/Fossil Energy Hybridization
Energy & Fuels, Vol. 13, No. 5, 1999 963
Figure 3. Time variation of the CO evolution rate for the gasification of coal with CO2 at 1123K (b, without the molten salt; 2, with the molten salt). Table 2. Components of Evolved Gas in the Pyrolysis of Coal at 1173 K in the Absence of the Molten Salt (cm3 coal 1 g) H2
CH4
CO
CO2
180
54
39
11
reported,16-18 and the effect of K2CO3 and Na2CO3 is explained on the basis of the vapor cycle mechanism represented by following reactions:18 Catalytic reaction of K2CO3 and Na2CO3
1/2M2CO3 + C ) M(g) + 3/2CO
(4a)
M(g) + CO2 ) 1/2M2CO3 + 1/2CO
(4b)
(Total) C + CO2 ) 2CO
(4c)
where M is either K or Na. This mechanism would be applicable for the system using the molten salt from the calculation of the equilibrium pressures of K(g) and Na(g)18 under the experimental condition in the present system. Figure 3 and Table 2 show the time variation of the CO evolution rate (µmol/s) using the coal sample (b, without the molten salt; 2, with the molten salt), and the components in the evolved gas when the coal sample was pyrolized in the He atmosphere at 1173 K, respectively. As can be seen in Table 2, the main product gases other than CO and CO2 were H2 and CH4, which would hardly react with CO2 without a metal catalyst under this experimental condition. Therefore, the almost CO evolution in this system can be considered to come from the Boudouard reaction and coal pyrolysis. As can be seen in Figure 3, the maximum CO evolution rate with the molten salt was increased by 3.3 times compared with that without the molten salt. The CO evolution rate of coal gasification was smaller than that of active carbon in each case. However, the increase in the maximum CO evolution rate for coal with the molten salt (3.3 times) was larger than that for active carbon (1.5 times). When coal sample was used, the acceleration of coal pyrolysis in the molten salt would lead to enhancement of the CO evolution other than the catalytic effect of the molten salt. The poly aromatic compound in the coal sample can hardly react with CO2 in a solid/gas-phase reaction as shown in Figure 3. The coal pyrolysis with the molten salt became a liquid/solid-
Figure 4. Reacted carbon yield versus time (b, coal without the molten salt; 2, coal with the molten salt; [, active carbon without the molten salt; and ×, active carbon with the molten salt).
phase reaction where the coal surface and the molten salt ease the heat flow (from the molten salt to the coal). These results suggest that the coal gasification in the presence of the molten salt will be a two-step reaction: the first-step reaction is coal pyrolysis, and the secondstep reaction is coke gasification (the Boudouard reaction). Figure 4 shows a set of results for reacted carbon yield versus time at different cases of sample gasification with CO2 (b, coal without the molten salt; 2, coal with the molten salt; [, active carbon without the molten salt; and ×, active carbon with the molten salt). As can be seen in Figure 4, the carbon conversion into CO did not attain 100% in all series. Especially, the reacted carbon yield with the molten salt decreased more rapidly than that without the molten salt. These reactions will be prevented by the sedimentation of the coke particles into the molten salt. Since the true density (2.2 g/cm3) of coke was larger than that of the molten salt (about 1.8 g/cm3), coke particles would be sedimented into the molten salt and cannot be in contact with the CO2 flowing on the molten salt surface. When a stirring system which can disperse coke particle in the molten salt is equipped in the reactor, the Boudouard reaction will continuously take place on the molten salt/CO2 interface, and the CO evolution rate will be kept constant at the maximum value obtained in this study. Solar/Fossil Energy Hybrid Fuel Plant. The coal gasification system with CO2 in the presence of the molten salt (CG-CO2/molten salt system) studied in the present paper is available for the solar/chemical energy conversion system using concentrated solar heat (STC process). Figure 5 shows one of the realizable systems for the production of solar hybrid fuels (methanol), production, which consists of two sections [H2 production from CH4 reforming and coke (coal) gasification]. Solar methanol is one of the most reasonable syn-fuels, since methanol can be transported from the sunbelt of the world to the remote energy-consuming sites. In addition, a huge amount of solar methanol can be transported using a conventional oil tanker. The solar methanol can be substituted for petroleum, and it will be used in the places where fossil fuels are being consumed for the existing fossil-fuel thermal engines such as fossil power plant.
964 Energy & Fuels, Vol. 13, No. 5, 1999
Yoshida et al. Table 3. Heat Capacity, Melting Point and Density of Molten Salts
Figure 5. Solar/fossil hybrid fuels (methanol) production system.
As can be seen in Figure 5, the endothermic processes of the CH4 reforming reaction (eq 1a or 1b) can be applied to H2 production, and the Boudouard reaction (eq 3) can be applied to coke gasification. The product CO gas can be converted to methanol using a shift reaction (CO + H2O ) H2 + CO2) and a methanol synthesis reaction (CO + 2H2 ) CH3OH). We call this methanol “solar hybrid fuel”, since it is produced from fossil fuels which include the carbon as solar energy carrier, and the solar energy. Molten salt can be introduced to the reactor of coke gasification section in Figure 5 for the stable operation and the uniform heat supply for the solar reactor. In addition, the coal can be directly gasified with CO2 in the presence of the molten salt, which enhances the coal pyrolysis, therefore the coke gasification section can be replaced with coal gasification (CG-CO2/molten salt system). However, in the coal gasification with CO2 using the molten salt, the ash problems remain. The main ash components of the coal were Al2O3, SiO2, and CaO. Al2O3 and SiO2 were suspected of inactivating the molten salt, but CaO was expected to be an effective catalyst of the coke gasification. Al2O3 and SiO2 would irreversibly react with the molten salt, and would produce K2O.Al2O3.xSiO2 and Na2O.Al2O3.xSiO2 compounds.20 This production will decrease the mass of the molten salt and reduce the catalytic effect on the coal gasification. On the other hand, CaO has a catalytic effect represented by the following reactions:19
CaO + CO2 ) CaO.O + CO
(5a)
CaO.O + C ) CaO + C(O)
(5b)
C(O) ) CO
(5c)
where CaO.O is surface peroxide, and the rate-determining step is the release of the CO from the carbon surface. By increasing the partial pressure of the CO2, the rate of gasification increased, indicating that the oxide is the active species. This catalytic reaction has (20) Sams, D. A.; Shadman, F. Fuel 1983, 62, 880-882. (21) Janz, G. J.; Lorenz, M. R. J. Electrochem. Soc. 1961, 108, 10521058. (22) Janz, G. J.; McIntyre, J. D. E. J. Electrochem. Soc. 1962, 109, 842-849. (23) van Artsalen, E. R.; Yaffe, I. S. J. Phys. Chem. 1955, 59, 118127. (24) Jaeger, F. M. Z. Anorg. Chem. 1917, 101, 1-214. (25) Kayembe, N.; Pulsifer, A. H. Fuel 1976, 55, 211-216.
molten salt
heat capacity (J K-1 g-1)
melting point (K)
density (g/cm3)
KOH NaOH KCl NaCl K2CO3 Na2CO3 Li2CO3 Rb2CO3 Cs2CO3
1.48 2.08 0.99 1.19 1.51 1.79 2.51 0.89 0.63
679 596 1044 1073 1174 1131 1003 1146 1066
1.86 (at 800 K) 1.73 (at 700 K) 1.48 (at 1123 K) 1.53 (at 1123 K) 1.88 (at 1200 K) 1.94 (at 1200 K) 1.79 (at 1100 K)
been reported to proceed even if CaO is in molten salt. Moreover, the sulfur in the coal would be trapped by the reaction with CaCO3 to form CaS in the molten salt.14 Limestone is calcined to form lime (CaO), which reacts with sulfur compounds according to the following reactions:
CaCO3 ) CaO + CO2
(6a)
CaO + SO2 + 3CO ) CaS + 3CO2
(6b)
CaO + H2S ) CaS + H2O
(6c)
However, CaO would irreversibly react with SiO2 to produce CaO.xSiO2 which has no catalytic effect on the coal gasification in this CG-CO2/molten salt system. Table 3 shows the heat capacity and the density of various molten salts. The suitability of each molten salt for the STC process has been discussed on the basis of those data.19,21-24 The heat capacities of K2CO3 (1.51 J K-1 g-1) and Na2CO3 (1.79 J K-1 g-1) are higher than those of the other potassium or sodium salts and the other alkaline carbonate salts (except for NaOH (2.08 J K-1 g-1) and Li2CO3 (2.51 J K-1 g-1)). If the potassium, sodium chloride, or nitrate salts are used in the Boudouard reaction, they will react with CO2 to form the molten carbonate salts at 1123 K, and gases such as Cl2 and NO2 will be evolved. The carbonates other than those of potassium and sodium are decomposed to CO2 and metal oxides in the temperature range of 10731173K. Compared with K2CO3 or Na2CO3, Li2CO3 has a larger heat capacity, but Li2CO3 was reported to have less catalytic effect on the coal gasification.25 Hence, K2CO3 and Na2CO3 are suitable for thermal storage of the STC process. Conclusion On the solar/chemical energy hybridization system to obtain CH3OH as a solar fuel, the coal with molten salt can gasify by the Boudouard reaction without the coke production on another step. The gasification of active carbon and coal with CO2 in the presence of the molten salt was found to enhance the gasification rates, compared with those in the absence of the molten salt. The maximum gasification rate of active carbon was enhanced by 1.5 times and that of coal by 3.3 times. The process of the coal gasification in this system appeared to contain two steps: the coal pyrolysis and the catalytic gasification. The acceleration of the former step is due to the easy heat flow of the molten salt, and the latter step is due to the catalytic effect of alkaline metal cations. We proposed the solar/fossil energy hybrid fuel production system using molten salt for the STC process. EF980144N
