Energy & Fuels 1999, 13, 579-584
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Kinetics and Simulation on a High-Temperature Solar Thermochemical Energy Conversion Process on the Boudouard Reaction Hiroyuki Ono, Shinya Yoshida, Masaki Nezuka, Taizo Sano, Masamichi Tsuji, and Yutaka Tamaura* Tokyo Institute of Technology, Research Center for Carbon Recycling and Utilization, Ookayama, Meguro-ku, Tokyo 152-8552, Japan Received June 24, 1998
The kinetic aspect of a highly endothermic reaction system for solar thermochemical energy conversion was studied using a Xe lamp. The Xe light beam with an energy intensity of 34 W/cm2 was irradiated onto carbon in a flow of CO2. The maximum rate of CO evolution was attained at 1700 s of irradiation, and the solar/chemical energy conversion efficiency was ca. 5%. A novel solar thermochemical energy conversion model based on an energy distributing process was proposed to evaluate the chemical energy conversion efficiency. In the model proposed, the chemical energy conversion efficiency, ak, and efficiency of energy absorption, , were defined. ak varies with the proceeding reaction, since the reaction conditions at each point of the reactor changed with consumption of active carbon by the Boudouard reaction. In the Boudouard reaction, the maximum chemical energy conversion efficiency was estimated to be 16%.
Introduction The theoretical maximum efficiency for solar thermochemical energy conversion increases dramatically with solar concentration and was thermodynamically estimated to be 70% at 1000 sun.1 It offers good opportunities for converting solar energy into chemical energy. The Boudouard reaction, C/CO2 redox system (C + CO2 f 2CO, ∆H° ) 171.54 kJ at 900 K), has been investigated to constitute an efficient solar thermochemical process.2 The energy conversion efficiency attained 40% for the packed-bed reactor. Laboratory experimentation showed that the rate of the Boudouard reaction was dependent on the CO2 partial pressure, heterogeneity of carbon, and temperature.3 Alternatively, one can use metal oxide as the oxygen donor instead of CO2. The solar thermochemical process using metal oxide, e.g., Fe3O4 and ZnO, has been demonstrated to be very effective when it is applied to energyintensive endothermic reactions such as coal gasification.4,5 The conversion efficiency attains 40% for the carbothermal reduction4 of Fe3O4 + C f 3FeO + CO (∆H° )188.74 kJ at 900 K). The reduced oxide, FeO, was used for reaction with CO2 to form CO gas and oxidized phase or Fe3O4. Some metal oxides, such as Fe3O4, MgFe2O4, LiMn2O4, ZnFe2O4, MnFe2O4, Ni0.15Fe2.85O4, were found to be reactive for this cyclic process.6,7 These materials will provide a promising (1) Kesselring, P. International Workshop on High-temperature Solar Chemistry, PSI, Switzerland, August 1995, 17, 21. (2) Taylor, R. W.; Berjoan, R.; Coutures, J. P. Sol. Energy 1983, 30, 513-525. (3) Calo, J. M.; Perkins, M. T. Carbon 1987, 25, 395-407. (4) Tamaura, Y.; Wada, Y.; Yoshida, T.; Tsuji, M.; Ehrensberger, K.; Steinfeld, A. Int. J. Energy 1997, 22, 337-342. (5) Steinfeld, A.; Kuhn, P.; Karni, J. Int. J. Energy 1993, 18, 239249.
pathway for gasification of coal. However, these process have some problems. Using coal or char as a carbon source, ash will remain in the reaction field. Ash remaining is composed of aluminosilicates and may be reacted with a metal oxide. The reaction will deteriorate the activity of metal oxides. Separation of ash is essential but needs additional energy and technical progress for the process. To solve the problems, more investigation is needed. When process energy is directly supplied by irradiating concentrated sunlight at 500-1500 sun onto the specimen surface, a temperature gradient is inevitably generated within the materials.8 The material is highly heated around a focal point. Solar energy is then converted to thermal energy through crystal lattice vibration, and chemical reactions are initiated. The chemical energy conversion process using direct irradiation of concentrated solar radiation for solid reactants is studied for details of the mechanism: e.g., methane reforming with CO2 or H2O using solid catalysts, oneor two-step coal gasification using metal oxides,9 singlestep water splitting, or two-step water splitting using metal oxides. Basic knowledge of the chemical energy conversion processes using concentrated solar radiation is also required for receiver/reactor designs of the solar thermochemical transformation. The present paper deals with modeling of an energy distributing process in the thermochemical conversion (6) Mimori, K.; Togawa, T.; Hasegawa, N.; Tsuji, M.; Tamaura, Y. Int. J. Energy 1994, 19, 771-778. (7) Tsuji, M.; Sano, T.; Tabata, M.; Tamaura, Y. Int. J. Energy 1995, 20, 869-876. (8) Schubnell, M.; Tschudi, H. R. Appl. Phys. 1995, A60, 581-587. (9) Carrasco, F.; Rivera, J.; Utrera, E.; Moreno, C. Fuel 1991, 70, 13-16.
10.1021/ef980140i CCC: $18.00 © 1999 American Chemical Society Published on Web 03/02/1999
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Using eqs 1, 2, and 3, Qk is represented as follows
Qk ) (1 - ak-1)Qk-1 ) ()k-1{(1 - ak-1)(1 - ak-2)(1 - ak-3) ... (1 - a3)(1 - a2)(1 - a1)}Q1 (4) Figure 1. Schematic of the solid reactant shape used for modeling the solar thermochemical energy conversion process.
The total solar energy converted to chemical energy, ΣQck, is then given by
ΣQck ) ΣakQk ) Q1Σ()k-1ak{(1 - ak-1)(1 - ak-2)(1 - ak-3) ... (1 - a3)(1 - a2)(1 - a1)} (5) In the reaction conditions where the sample consumption by the reaction is negligible, ak(k ) 1 - n) may be assumed to be a constant, a. Then, eq 5 can be simplified as follows Figure 2. Schematic of the solar thermochemical energy conversion model.
ΣQck ) aΣQk ) aQ1Σ{(1 - a)}k-1
and estimation of the total energy conversion efficiency on the Boudouard reaction.
The overall chemical energy conversion, η, can be defined by
Solar Thermochemical Energy Conversion Model
η)
A proposed model for a solar thermochemical energy conversion process will be described as follows. Cylindrical solid reactant (carbon for the present) is used for solar thermochemical energy conversion (Figure 1). When concentrated, the light beam is irradiated in a longitudinal direction and a temperature gradient will be generated within the solid reactant. CO and CO2 concentration gradients will take place along with the Boudouard reaction. In the present model, a sample was separated to n parts (Figure 2). Energy received by the kth part (Qk) is distributed as chemical energy in the kth part (Qck), heat loss at the kth part (Qlossk) and heat transferred to the (k+1)th part (Qk+1). The energy balance equation is then given by
1
(6)
n
∑ Qck × 100
Q1 k)1
) a
1 - {(1 - a)}n × 100 (%) 1 - (1 - a)
(7)
If n f ∞, then η shows the maximum value, ηmax
ηmax ) )
1
∞
∑ Qck × 100
Q1 k)1
a × 100 (%) 1 - (1 - a)
(8)
The chemical energy conversion efficiency for the kth part, ak, is defined as follows
ηmax is dependent on a and . ηmax on the Boudouard reaction can be estimated from a and of the reaction. It is not easy to get an exact value of chemical energy conversion efficiency at the kth part experimentally. Thus, the variation of ak was calculated, while the solar thermochemical energy conversion is proceeded by the Boudouard reaction. The details of the calculation will be described in a later section.
ak ) Qck/Qk
Experimental Section
Qk ) Qck + Qlossk + Qk+1
(1)
(2)
ak can be 1 when the reaction heat is larger than the incident energy at the kth part in a unit time. For the purpose of investigating the effect of energy distribution on solar thermochemical energy conversion, the ratio of Qk+1 to (Qlossk + Qk+1) may be defined as an efficiency of energy absorption,
)
Qk+1 Qlossk
+ Qk+1
(3)
This is relevant to the ratio of heat energies which were not converted to the chemical energy. can be 1 when there is no heat loss on the energy absorption step.
Chemicals. The active carbon used was made from coal. All chemicals used in the present study were of analytical grade: Ar (99.9999%), CO2 (99.9%), CO (99.95%), and active carbon (carbon 78.94 wt %, moisture 12.14 wt %, ash 8.91 wt %). Measurement of the Rate of the Boudouard Reaction. The dependence of the rate of the Boudouard reaction on the temperature and partial pressures of CO and CO2 was determined using MAC Science TG-DTA equipment for the simulation on solar thermochemical conversion. The reaction rate was studied at different CO2 partial pressures at given temperatures in the range of 1073-1373 K. Typically, a 11 mg portion of carbon was placed in a platinum pan. Before each run, gases in the system were fully purged with Ar and then heated to the given temperature at a rate of 70 K/min whileAr (35 µmol/s) was passed through the equipment. A
Kinetics and Simulation of the Boudouard Reaction
Energy & Fuels, Vol. 13, No. 3, 1999 581
Figure 3. Schematic of the solar reactor.
T
Figure 5. Arrhenius plot of Ra vs 1/T.
gasified at 180 s, and almost the same quantity of active carbon was gasified at 1800 s in 1073 K. The rate of the Boudouard reaction, Ra, can be determined using the following equation
Ra ) -
Figure 4. Typical profile of the Boudouard reaction (PCO2: 0.96, PAr ) 0.04 (atm)). mixed gas of CO2 (30-95%) and Ar (the rest) was introduced at a flow rate of 83-149 µmol/s to initiate the Boudouard reaction. In addition, the effect of CO on the reaction was studied by mixing known concentrations of CO gas. Thermochemical Energy Conversion Using a Xe Lamp. The chemical energy conversion process was studied using a quartz tube reactor equipped with a Xe lamp. Active carbon (1.00 g) was placed in a quartz tube of 16 mm i.d. supported by an alumina cement furnace (Figure 3). CO2 was introduced to the reactor at a flow rate of 40.1 µmol/s. The temperature of the specimen was monitored using type K thermocouples placed at 2.0 mm intervals from the front to the back (four positions) of the specimen. Evolved CO and unreacted CO2 were continuously determined with a gas analyzer equipped with a nondispersive infrared spectrometer (Shimadzu, model CGT-10-2A). The specimen was irradiated by a Xe lamp beam with an intensity of 34 W/cm2. The energy intensity of the Xe lamp beam was determined as follows. Water (0.56 mol), active carbon (0.70 g), and magnetic stirrer tip (0.12 g) was packed in the reactor. Then the Xe lamp beam was irradiated onto the reactor while the temperature was measured at the center of the reactor. From the increase in the temperature of water, the incident light beam energy was determined.
Results and Discussion Kinetics of the Boudouard Reaction by TG-DTA. Figure 4 shows a typical profile of the Boudouard reaction at temperatures of 1073-1373 K and at PCO2 ) 0.96, PAr ) 0.04 atm. The time when the reactant gases were introduced was taken as the zero time of reaction. At 1373 K, ca. 10 mg of active carbon was
1 dmt × 100 (%/s) mt dt
(9)
where mt denotes the sample mass at time t. Ra when 40% of the carbon was gasified was used as the rate of the Boudouard reaction in this study. The temperature dependence of Ra could be observed in the series of experiments. Arrhenius plots of Ra vs 1/T at PCO2:0.31-0.96 atm are presented in Figure 5. The plots showed essentially parallel straight lines. From the slopes of the lines, the values of the apparent activation energy of the Boudouard reaction were 104-124 kJ/mol. Several authors have reported the activation energy of the Boudouard reaction.10-12 Reported activation energies were in the range of 170-415 kJ/mol using graphite or coal. The activation energy obtained from the present study showed smaller values than the reported values. The activation energy was 415 kJ/mol when graphite was used as the carbon source. Graphite had a large graphite grain structure and was unreactive because of its stability and small surface area. Active carbon had a larger surface area and was reactive for the reaction. The activation energy was in the range of 170-222 kJ/ mol when coal was used as a carbon source. Coal is needed to be pyrolyzed to char, prior to the Boudouard reaction. The charring step may increase the apparent activation energy. When active carbon was used as the carbon source, the charring step was not needed prior to the Boudouard reaction. Thus, the activation energy obtained had a smaller value than the reported values. The dependence of the Boudouard reaction on the partial pressures of CO and CO2 may be understood using the Langmuir-Hinshelwood mechanism for the Boudouard reaction13,14 (10) Hampartsoumian, E.; Murdoch, P.; Pourkasanian, M.; Trangmar, D.; Williams, A. Combust. Sci. Technol. 1993, 92, 105-121. (11) Mirasol, J.; Cordero, T.; Rodriguez, J. Carbon 1993, 31, 5361. (12) Dai, Y.; Raupenstrauch, H.; Posch, M.; Staudinger, G. Fuel 1994, 73, 1624-1627.
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Figure 6. Ra vs the total flow rate of inlet gas in the range of 38-149 µmol/s.
Cf + CO2 T Cf-(O) + CO
(10)
Cf-(O) f CO + Cf
(11)
where Cf is a free active site and Cf-(O) is the reactive intermediate on the carbon surface. Equation 11 is regarded to be an irreversible reaction. Chemisorption of CO2 depends on the partial pressure of CO2 and results in Cf-(O) at the first step of the reaction (eq 10). When a CO molecule exists in the reactant gas mixture, the following reaction will occur
CO + Cf T Cf-(CO)
(12)
where Cf-(CO) denotes CO chemisorbed by the free active site. The CO chemisorbed occupied the free active site of the carbon. The rate of the surface reaction is proportional to the active site number.13 The free active site number is decreased by the chemisorption of CO, and the first step of the reaction (eq 10) is inhibited. Therefore, the presence of CO in the reaction gas will inhibit the Boudouard reaction. The dependence of Ra on the total gas flow rate of the inlet gas was measured (Figure 6). The dependence of Ra on the temperature and partial pressures of CO and CO2 was determined in the total flow rate of inlet gas range 83-149 µmol/s. In this range, Ra was independent of the total flow rate of the inlet gas. In the total flow rate of inlet gas range below 60 µmol/s, Ra showed a smaller value than that at the total flow rate of inlet gas above 60 µmol/s. In the total inlet flow gas rate range below 60 µmol/s, CO evolved inhibited the reaction, since a CO chemisortion reaction (eq 12) might occur. Ra was plotted (Figure 7) at several temperatures and partial pressures of CO and CO2. The dependence of Ra on the temperature and partial pressures of CO and CO2 could be represented by the following equations
Ra ) fT + g
(13)
where (13) Walker, P. L.; Rusinko, F.; Austin, L. G. Gas reaction of Carbon; Academic Press: New York, 1959; pp 133-136 (14) Radovic, L.; Jiang, H.; Lizzio, A. Energy Fuels 1991, 5, 68-74.
Figure 7. Dependence of Ra on the temperature and partial pressures of CO2 and/or CO.
f ) exp(-0.57PCO){(1.63 × 10-5)ln(PCO2) + (3.52 × 10-5)} g ) exp(-0.57PCO){(1.76 × 10-2)ln(PCO2) (3.72 × 10-2)} PCO, PCO2, and T denote the partial pressure of CO and CO2 and the temperature, respectively. Fitting the results reproduced the experimental values well. The rate increased with temperature linearly at any reactant gas composition studied and increased with the partial pressure of CO2. The CO2 partial pressure dependence of the Boudouard reaction was larger at higher temperatures than lower temperatures. Adding CO to the reactant gas decreased the rate of the reaction. The retarding effect of CO on the Boudouard reaction was observed at each temperature. Taking into account the CO and CO2 partial pressures at each part of the cylindrical reactor, eq 13 was used to estimate the energy converted to chemical energy following estimation of the variation of ak. Gasification of Activated Carbon Using a Xe Lamp. The inlet gas was CO2 (40.1 µmol/s) only. In the beginning of irradiation (0-570 s irradiation), the temperature was increased quickly (74 K/s) to ca. 1000 K, where the Boudouard reaction proceeded (Figure 8). The CO evolution rate gradually increased at this time range. After a rapid increase of the temperature to 1000 K, the rate of temperature increase was decreased to 14 K/s, and a nearly constant temperature around 1150 K was reached at around 2000 s of irradiation. The maximum CO evolution rate attained 43 µmol/s at 2000 s of irradiation. Thereafter, the CO evolution rate
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Energy & Fuels, Vol. 13, No. 3, 1999 583
Figure 8. Change in flow rates of reactant gas (CO2) and product gas (CO) and temperature on the Boudouard reaction by irradiation of a Xe lamp beam with an energy intensity of 34 W/cm2.
Figure 9. Calculated and experimental values of the total CO evolution rate.
decreased gradually, though the temperature of the sample was kept at 1150 K. At 6000 s, the CO evolution rate decreased to 8.0 µmol/s. This was primarily due to consumption of active carbon by the Boudouard reaction. The temperature measured by thermocouple 1 was about 350 K higher than that by thermocouple 4 after 3000 s of irradiation. The temperature gradient was calculated from an interval of thermocouples and differences in the temperature. Irradiation of a concentrated Xe lamp beam generated a temperature gradient (580 K/cm) in the active carbon. From the results, it was suggested that solar thermochemical energy conversion was always attended by the reaction condition where the temperature of the solid reactant was heterogeneous. Estimation of the Variation of ak. ak was estimated from Qck and Qk using the following methods. At first, Qck was calculated as follows. In the system studied, chemical energy stored, Qck, may be given by
Qck ) mkRa∆H°
(14)
where mk and ∆H° denote that carbon exists at the kth part (in moles) and the reaction enthalpy of the Boudouard reaction, respectively. Ra in each part of the carbon sample was estimated using the reactant pressure dependence of the Boudouard reaction, eq 13, and the temperature of the carbon sample measured in the Xe lamp experiment shown in Figure 8. Assuming that gas flow was longitudinal without turbulent flow, the effect of the gas composition on the reaction was considered at each part. The total evolution rate of CO gas from the reactor was also calculated, assuming that the rate was the sum of the gas evolution rate at 1-12th part. Figure 9 shows that calculated and experimental value of the total CO evolution rate. Calculated values showed good agreement with experimental values. The calculation using above methods was considered to be able to reproduce the experiment. Next, Qk was estimated using eqs 1 and 3. was assumed to be in three levels (0.6, 0.8, 1). From Qck and Qk, the variation of ak was calculated. Figure 10 showed the variation of ak
Figure 10. Variation of ak vs irradiation time at three levels of .
with irradiation time while the Boudouard reaction proceeded. At ) 0.6, the part which has the highest ak value was given by the seventh part at 5000 s of irradiation, and 13% of the incident energy for seventh part was converted to chemical energy. At ) 0.8, the 2-5th parts had close ak values and the chemical energy conversion efficiency was ca. 3%. At ) 1, the first part has the highest ak value at 1000 s of irradiation and 4.5% of incident energy was converted to chemical energy. A more exact estimation of ak may need direct measurement of . Comparison of Maximum and Empirical Chemical Energy Conversion Efficiency, ηmax and ηexp. The chemical energy conversion efficiency was investi-
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exp
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Figure 11. ηmax vs at a ) 0.01 - 1.
gated. It was assumed that ak was kept constant, a, and the maximum chemical energy conversion efficiency, ηmax, was evaluated in eq 8 at several a and values. Calculated ηmax values are shown in Figure 11. At a ) 1, ηmax increases linearly with the efficiency of energy absorption, . At a < 1, ηmax showed curves with . ηmax at ) 0.6 and a ) 0.05 attained 6%. In contrast, ηmax achieved 43% at ) 0.6 and a ) 0.5. The results showed that a large a value was essential for high chemical energy conversion efficiency. ηmax using the Boudouard reaction was estimated assuming that a was the maximum ak shown in Figure 10. Calculated ηmax was 16% for the Boudouard reaction, where a ) 0.13 and ) 0.6. On the other hand, the empirical chemical energy conversion efficiency, ηexp, can be calculated from eqs 7 and 14 as follows:
ηexp ) Σ(mkRa∆H°)/Q1
(15)
Figure 12 shows the change in the chemical energy conversion efficiency (ηexp) with irradiation time. In the present experimental setup, the empirical chemical energy conversion efficiency was only 5% at 2000 s of irradiation. The calculated ηmax (16%) was 3 times as large as ηexp (5%). If ak is kept constant by the appropriate reactant feed, ηexp will be close to ηmax. In solar thermochemical energy conversion, the efficiency of chemical energy conversion largely depends on a reactor form and setup, reaction enthalpy, and reaction rate. Considering that it is difficult to insulate the solar reactor perfectly, a large a value was required for improvement of conversion efficiency. Such a reaction is a candidate for converting solar energy into chemical energy.
Figure 12. Change in the chemical energy conversion efficiency (ηexp) with irradiation time.
Conclusion The apparent activation energies of the Boudouard reaction were evaluated at PCO2 ) 0.31-0.96 atm. Evaluated activation energies were in the range of 104124 kJ/mol. The results showed that the activation energy was independent of the partial pressure of CO2, and the reactivity of active carbon was higher than that of coal or graphite. The kinetics of the Boudouard reaction were studied using TGA. The dependence of the reaction rate on the temperature and partial pressure of CO and CO2 was determined. A solar thermochemical energy conversion model was proposed. In the model, it was assumed that the energy received by the kth part (Qk) is distributed as chemical energy in the kth part (Qck), heat loss at the kth part (Qlossk), and heat transferred to the (k+1)th part (Qk+1). The chemical energy conversion efficiency at the kth part and the ratio of energy absorbed by the kth part to the energy transferred from (k - 1)th part were defined as ak and . Using the model and dependence of the rate of the Boudouard reaction on the temperature and partial pressures of CO2 and CO, the variation of ak in the kth part was studied at three levels of efficiency of energy absorption, . Assuming that ak was constant, the maximum chemical energy conversion efficiency, ηmax, was evaluated at several a and values. The calculated empirical energy conversion efficiency, ηexp, was 5%. On the other hand, the calculated ηmax, where ) 0.6 and a ) 0.13, was 16%. The variation of the maximum chemical energy conversion efficiency with ak and was predictable from the present study. EF980140I
