chemical energy conversion

Energy 25 (2000) 71–79 www.elsevier.com/locate/energy

Coal gasification with CO2 in molten salt for solar thermal/chemical energy conversion J. Matsunami a, S. Yoshida a, Y. Oku a, O. Yokota a, Y. Tamaura M. Kitamura b

a,*

,

a

b

Research Center for Carbon Recycling and Utilization, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan Department of Materials Science, School of Engineering, The University of Shiga Prefecture, 2500 Hassaka-cho, Hikone City, Shiga 522-8533, Japan Received 7 September 1998

Abstract Coal gasification with CO2 in Na2CO3–K2CO3 molten salt that was used as thermal storage for gas/solid heterogeneous reaction was studied to apply this system for solar thermal/chemical energy conversion. The reactions were performed at 1173 K under various CO2 flow rates, weights of the molten salt and Na2CO3/K2CO3 ratios. The CO2 gas consumption rate increased with increasing CO2 flow rate, however, the conversion efficiency of CO2 to CO was decreased. As the weight of the molten salt increased, the rate of the gasification reaction was decreased. The maximum conversion efficiency of CO2 to CO under the experimental conditions reached 71% at the CO2 flow rate of 310 µmol/s. Thus 37 J of the solar thermal energy can be converted into chemical energy per second by the endothermic process of the Boudouard reaction (C+CO2=2CO⫺169.16 kJ (at 1150 K); coal=10 g), when this gasification reaction would be performed by using concentrated solar heat.  2000 Elsevier Science Ltd. All rights reserved.

1. Introduction Conversion of solar energy into chemical energy by introducing concentrated solar heat into an endothermic reaction is currently of interest for utilization of solar energy, which is promising for clean and renewable energy resource in the next century. If the solar energy could be converted into chemical energy at the Sunbelt, it could be then transported to remote energy consuming sites [1]. Several endothermic reactions such as metal oxide reduction reaction for the water * Corresponding author: Tel.: +81-3-5734-3292; fax: +81-3-5734-3436. E-mail address: [email protected] (Y. Tamaura)

0360-5442/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 5 4 4 2 ( 9 9 ) 0 0 0 5 8 - 4

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splitting reaction [2], methane reforming [3–8] and coal gasification with H2O [9,10] have been studied for solar thermochemical process. Our attention has been directed to coal gasification with CO2 (Boudouard reaction) which is expressed by the following equation C⫹CO2⫽2CO ⌬H°⫽169.16 kJ/mol (1150 K)

(1)

Compared to coal gasification with H2O, given in the following equation, this reaction has the larger formation heat per 1 mole of coal C⫹H2O⫽CO⫹H2 ⌬H°⫽135.75 kJ/mol (1150 K)

(2)

Furthermore, CO2, which is one of the most contributing gases for global warming, can be consumed to convert into CO in this reaction, and no latent heat for water vaporization is needed. Since these coal gasification reactions are heterogeneous systems between gas and solid phase, vast thermal energy is needed for these reactions. Moreover, the solar chemical reactor in which the concentrated solar beam is radiated requires thermal uniformity for stable operation. Using alkali metal molten salt as thermal storage is one solution to these problems because of its high heat capacity [10]. With a molten salt for coal gasification systems, heat conduction to gas would be increased and extra thermal energy could be cut down. Furthermore, alkali metal salt is well known as a catalyst for coal gasification with both H2O and CO2 [11–16]. However, previous studies for catalytic coal gasification were conducted at the ratio of alkali metal salt/coal less than 0.5, and it is known that the catalytic effect became maximum at the ratio around 0.15–0.25 [15]. Therefore, the catalytic effect of alkali metal salt under the condition where it was used as thermal storage was unknown. We have studied the coal gasification with CO2 in molten salt to apply this system to the solar thermochemical plant at the Sunbelt. It has already been reported that the gasification reaction rate in the presence of the molten salt was 3.3 times higher than that without the molten salt, but the conversion efficiency of CO2 to CO was still lower [17]. Since contact between CO2 and coal was considered to be insufficient, the reactor was improved to yield high conversion efficiency. In this paper, the effects of CO2 flow rate, Na2CO3/K2CO3 ratio on the rate of the coal gasification with CO2 using the modified reactor, and weight of the molten salt (eutectic mixture of Na2CO3 and K2CO3) added were studied. Also efficiency of the solar thermal/chemical energy conversion system was evaluated on the basis of the results obtained from the gasification experiments.

2. Experimental Newlands coal (Idemitsu Coal Laboratory) was used in this study; the proximate and ultimate analysis is given in Table 1. Na2CO3 and K2CO3 (Kanto Chemical Co. Inc.) were used as thermal storage because these salts do not evolve hazardous gases such as Cl2 or NO2 in a CO2 atmosphere. Basically, these salts were used as a eutectic mixture (weight ratio=1) because of its low melting point. The experimental apparatus is shown in Fig. 1. A narrow-bore cylindrical stainless reactor (SUS-310S, 34 mm O.D., 2 mm thickness, 150 mm long) was used to enhance contact between CO2 and coal, and a pin hole (苲1 mm diameter), from which CO2 feed gas was introduced as

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Table 1 Proximate and ultimate analysis of coal Proximate analysis (wt%) Moisture Asha

Ultimate analysis (wt%, daf) C H O

S

N

1.7

85.14

0.39

1.71

a

13.1

5.19

7.57

Dry basis.

Fig. 1. Experimental apparatus for coal gasification with CO2 using molten salt: 1, flow controller; 2, pin hole (⭋ 1 mm) to introduce small CO2 gas bubbles into the reaction vessel; 3, reaction vessel made of stainless; 4, cold trap to condense volatile matters; 5, ice bath; 6, glass wool trap to remove coal tar; 7, infrared furnace; 8, thermocouple.

bubbles into the molten salt, was put in at the bottom of the reactor for enlargement of a surface area of the bubble. CO2 feed gas through the stainless tube (3 mm ⭋) was preheated before introduction to the reactor. The cold trap with an ice bath and the glass tube filled with glass wool were connected behind the reactor to capture volatile matters such as steam and tar components of coal. The reactor consisted of three parts (cylinder, top and bottom cover) and the stainless tube (5 mm ⭋) fitted to the top cover reached the bottom of the reactor to measure a reaction temperature through a thermocouple inside. Coal (10 g) and molten salt (20–40 g) were placed in the reaction vessel and were heated by using the infrared furnace (Shinku-Riko RHL-P65CP). CO2 flow rate was first adjusted to 35 µmol/s with a flow controller until the reaction temperature reached 1173 K to avoid clogging the pinhole of the reactor with powder samples, and then increased to 65–310 µmol/s for the

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experiments. The evolved gases were analyzed by GC (Shimadzu GC-8A) with Porapak Q or Molecular sieve 13X. As can be seen from the experimental data (Figs. 2, 4 and 5), the CO2 consumption rate calculated on the basis of gas analysis has some errors. Since CO2 feed gas was introduced into the coal–molten salt mixture, which had large density and was fairly viscous, from the pinhole at the bottom of the reactor, the CO2 flow rate could hardly keep constant. In particular, substantial error was observed when the CO2 flow rate was low, or a small amount of molten salt was used. 3. Results and discussion 3.1. Coal gasification with CO2 in molten salt Fig. 2 shows the time variation of the CO2 gas consumption rate (µmol/s) on the coal gasification as a function of CO2 flow rate in Na2CO3–K2CO3 molten salt (weight ratio=1). The amounts of coal and molten salt were 10 g and 30 g, respectively. The measurement was started after the reaction temperature was reached at 1173 K. During this time (20 min), thermal decomposition of poly-aromatic compounds in the coal into char (coal charring) was almost completed since H2 gas evolution disappeared in this time. Coal charring is represented by the following equation: Coal⫽H2⫹CmHn⫹C

(3)

Therefore, CO2 gas consumption and CO gas evolution would only come from the endothermic process of coke (coal) gasification with CO2 (Boudouard reaction) given by Eq. (1).

Fig. 2. Time variation of the CO2 consumption rate of the coal gasification with CO2 at different CO2 flow rates: CO2 flow rate is (쐌) 65 µmol/s; (왖) 155 µmol/s; (䊏) 310 µmol/s. Reaction condition is: coal=10 g; molten salt (Na2CO3/K2CO3=1/1)=10 g; reaction temperature=1173 K.

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As can be seen in Fig. 2, the CO2 gas consumption rate increased with increasing CO2 flow rate; the CO2 flow rate increased by 2.4 and 4.8 times. At the initial stage of the reaction, however, the CO2 consumption rate was increased 2.2 and 2.9 fold, respectively. The decrease in the increasing ratio of the CO2 consumption rate against that of the CO2 flow rate is explained by a decrease in contact time between the coal and the CO2 in the reactor, because both reacted and unreacted CO2 flowed out of the reactor. Fig. 3 shows the time variation of the CO2 conversion efficiency for Eq. (1) as a function of the CO2 flow rate. As contrasted with the CO2 consumption rate, CO2 conversion efficiency decreased with an increase in the CO2 flow rate. In our previous work, the conversion efficiency reached 25% at the CO2 flow rate of 60 µmol/s (4 g of coal sample) [17]. The conversion efficiency at the CO2 flow rate of 155 µmol/s (10 g of coal sample) in this paper was approximately 3.2 times higher. This result is due to an enlargement of the surface area of CO2 feed gas in the molten salt. As mentioned above, the nozzle for introduction of CO2 bubbles into the reactor enlarges a surface area of bubbles. Therefore, contact between CO2 and coal is important for acceleration of this gasification reaction. Fig. 4 shows the relationship between the amount of the molten salt and the CO2 consumption rate at 1173 K. The weight ratio of Na2CO3 vs K2CO3 used in this study was 1:1 and the CO2 flow rate was kept at 155 µmol/s. It is shown that the CO2 consumption rate increased with the decrease in the addition of the molten salt. This may have resulted from the higher concentration of coal in the molten salt which accelerates this gasification reaction and the molten salt is supposed to act as a thermal storage even at a lower catalyst loading. Another possible explanation is that a degradation of the catalytic effect of alkali metal carbonate salt appears in this addition range. The catalytic effect of alkali metal salt is discussed below.

Fig. 3. Time variation of the CO2 conversion efficiency of the coal gasification with CO2 at different CO2 flow rates: CO2 flow rate is (쐌) 65 µmol/s; (왖) 155 µmol/s; (䊏) 310 µmol/s. Reaction condition is: coal=10 g; molten salt (Na2CO3/K2CO3=1/1)=10 g; reaction temperature=1173 K.

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Fig. 4. Relationship between the weight of the molten salt (Na2CO3/K2CO3=1/1) and the CO2 consumption rate of the coal gasification with CO2: weight of the molten salt is (쐌) 20 g; (왖) 30 g; (䊏) 40 g. Reaction condition is: coal=10 g; CO2 flow rate=155 µmol/s; reaction temperature=1173 K.

The effect of alkali metal cation of the molten salt (total amount: 30 g) on the CO2 consumption rate is shown in Fig. 5. The CO2 flow rate was kept at 310 µmol/s. It is known that a potassium salt is a more effective catalyst than a sodium salt for coal gasification with CO2 [14,15]. However, there is no remarkable enhancement by addition of potassium salt as shown in Fig. 5. This may have been caused by the considerably higher ratio of the molten salt vs coal in these experiments than that in the experiments reported earlier. The catalytic effect of alkali metal carbonate salt on coal gasification with CO2 has been proposed on the basis of the vapor cycle mechanism represented by the following reactions [14,16]: M2CO3⫹2C(s)⫽2M(g)⫹3CO(g) 2M(g)⫹2CO2(g)⫽M2CO3⫹CO(g)

(4) M⫽K, Na

(5)

It has also been reported that a fall in reaction rate at a higher catalyst concentration was observed due to blocking of pores in coal by the catalyst; restricting access of CO2 to a surface of the micropores [14,15]. The experiments in these reports were conducted under lower molten salt/coal ratio (max: 0.35 [15]), whereas the experiments in the present paper were carried out at the ratio of 3. Therefore, the decrease of accessibility of CO2 to coal pores would cause the disappearance of the difference between alkali metal salts. 3.2. Conversion of solar thermal energy into chemical energy The coal gasification system with CO2 using the molten salt presented in this paper can be applied to the solar/chemical energy conversion system, when a concentrated solar heat is used as a thermal source. Since the Boudouard reaction is an endothermic reaction, it can be considered

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Fig. 5. Influence of a variety of the molten salt on the CO2 consumption rate of the coal gasification with CO2: a variety of the molten salt is (쐌) Na2CO3; (왖) Na2CO3/K2CO3=1/1. Reaction condition is: coal=10 g; molten salt=10 g; CO2 flow rate=310 µmol/s; reaction temperature=1173 K.

that solar thermal energy is stored as chemical energy and the product CO gas includes solar energy within itself. Table 2 shows the maximum energy (J) obtained per second by the process of the Boudouard reaction (Eq. (1)) for 10 g of coal sample under various conditions conducted in this paper. The energy listed in Table 2 was estimated on the basis of the CO2 consumption rate and the formation of heat in Eq. (1). The maximum energy obtained among these reaction conditions reached 37 J, thus 1 MW of power could be obtained by using 270 kg of coal. Although numerous experiments have been reported for coal (coke) gasification with CO2, none make use of molten salt for thermal storage. As the molten salt has a high heat capacity, greater amounts Table 2 The maximum energy absorbed per second into the coal gasification with CO2 at 1173 K for 10 g of coal Energya (J)

Reaction condition Na2CO3 (g)

K2CO3 (g)

CO2 flow rate (µmol/s)

15 15 15 10 20 30 0

15 15 15 10 20 0 30

65 155 310 155 155 310 310

a

10 22 30 25 21 37 34

Calculated with the formation heat value of the Boudouard reaction at 1150 K (169.16 kJ/mol).

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of coal can react at the same time than in the absence of the molten salt. Therefore, enlargement of the reaction scale would be readily available. Furthermore, the CO gas product can be converted to methanol according to the shift reaction and methanol synthesis reaction represented by the following equations: CO⫹H2O⫽H2⫹CO2

(6)

CO⫹2H2⫽CH3OH

(7)

This methanol can be transported to remote energy consuming sites and the solar/chemical energy conversion system mentioned above would be a feasible system in the next century for its cleanness and renewability.

4. Conclusion Using Na2CO3–K2CO3 molten salt and a modified reactor, which was designed to make contact between coal and CO2 efficiently in the molten salt, coal gasification with CO2 proceeded successfully. The maximum CO2 consumption rate reached 221 µmol/s at the CO2 flow rate of 310 µmol/s for a 10 g coal sample. It is also shown that the higher CO2 consumption rate was observed with lower amounts of molten salt added. This result would be favorable for solar energy use because a solar beam can be concentrated in a small area. But several problems still remain. While thermal storage and heat conduction were enhanced by using molten salt, contact between coal and CO2 decreased because the molten salt covers the surface of the coal. For solar energy utilization by solar thermochemical process, optimization of the reaction conditions for coal gasification with CO2 using molten salt and improvement of this reaction system for the duration of high reactivity are now in progress.

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