Sn Carbothermic Cycle for Splitting Water

The SnO2 / Sn Carbothermic Cycle for Splitting Water and Production of Hydrogen The carboreduction in SnO2 to produce Sn and its hydrolysis with steam to generate hydrogen were studied. The SnO2 / C / Sn system has several advantages compared with the most advanced cycle considered so far, which is the ZnO/C/Zn system. The most significant one is the lower reduction temperatures (850– 900° C for the SnO2 versus 1100– 1150° C for the ZnO). The rate of carbothermal reduction was studied experimentally. SnO2 powder (300 mesh, 99.9% purity) was reduced with beech charcoal and graphite using a thermogravimetric analysis apparatus and fixed bed flow reactor at a temperature range of 800– 1000° C. Optimal temperature range for the reduction with beech charcoal is 875– 900° C. The reaction time needed to reach conversion of SnO2 close to 100% is 5–10 min in this temperature range. The transmission electron microscopy results show that after cooling, the product of carboreduction contains mainly metallic Sn with a particle size of 1 – 3 ␮m. The hydrolysis step is crucial to the success of the entire cycle. Reactions between the steam and solid tin having as powder structure similar to the reduced one were performed at a temperature range of 350– 600° C. Results of both the reduction and hydrolysis reactions are presented in addition to thermodynamic analysis of this cycle. 关DOI: 10.1115/1.4001403兴

Michael Epstein e-mail: [email protected]

Irina Vishnevetsky Alexander Berman Solar Research Facilities Unit, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel

Keywords: carboreduction of metal oxide, metal hydrolysis, tin dioxide thermochemical cycle

1

SnO2 + C → Sn共l兲 + CO2,

Introduction

Solar hydrogen production via two-step water splitting in a thermochemical cyclic process based on metal oxide redox reactions have been extensively reported in the literature during the last decade 关1兴. The first step in the cycle, which is the dissociation of the metal oxide, is endothermic, and solar energy can be used for its execution. The second step is usually exothermic. Steam is used to hydrolyze the metal to produce hydrogen and recover the metal oxide 关2,3兴. The use of different carbonaceous materials as reducing agents in the first step to lower the reaction temperature has also been reported 关4,5兴 for the MgO/Mg and ZnO/Zn systems. Carbonaceous materials such as charcoal, graphite, and methane have been experimented and reported 关6兴. When using wood charcoal or biogas, as presented hereafter, the net global contribution of CO2 to the atmosphere is zero, and the system is fully benign from an ecological point of view. The metal oxide carbothermal and methanothermal reduction processes can also be considered as carbon gasification 关7兴 and methane reforming, having higher solar contribution 共higher solar energy content in the calorific value of the products兲 and providing chemical storage solution in the form of the metal. The metal can be hydrolyzed to generate hydrogen decoupled from the solar step in time and location. This step constitutes, therefore, efficient chemical storage of solar energy supporting the gasification step. This paper presents an experimental study done on the SnO2 / C / Sn/ H2O system. Theoretical analysis of the system is presented in Ref. 关8兴. The carboreduction step is performed according to SnO2 + 2C → Sn共l兲 + 2CO共g兲,

⌬H650°C = 357.5 kJ

共1兲

or Contributed by the Solar Energy Division of ASME for publication in the JOURSOLAR ENERGY ENGINEERING. Manuscript received October 2, 2008; final manuscript revised May 14, 2009; published online June 14, 2010. Assoc. Editor: Mark Mehos.

NAL OF

Journal of Solar Energy Engineering

⌬H650°C = 177 kJ

共2兲

and the hydrolysis step is Sn + 2H2O共g兲 → SnO2 + 2H2共g兲,

⌬H500°C = − 87.8 kJ 共3兲

In the case of reaction 共1兲, the CO product can react with water to generate 2 moles of H2 and CO2. The advantages of this system compared with the most advanced cycle studied so far, which is the ZnO/C/Zn system 关5兴, are as follows: 共a兲

共b兲

The oxide reducing temperatures are lower by 200– 300° C 共850– 900° C for the SnO2 / C / Sn system compared with 1100– 1200° C for the ZnO/C/Zn兲. This fact substantially alleviates the difficulties encountered in the reactor materials of construction, simplifies the operation, and increases the process thermal efficiency. In the case of the ZnO/C/Zn system, the product Zn is vaporized at the reaction temperature and has to be cooled very fast 共quenched兲 at the exit of the reactor to avoid back reaction and to separate it from the off-gases stream. In the case of the SnO2 / C / Sn system, the Sn remains in the reactor and can be separated directly from there. The hot product gases, comprising mainly of CO and CO2, can be cooled in a conventional heat exchanger 共since no quenching is required兲 and their sensible heat can be efficiently exploited.

These advantages are more substantial when comparing to the MgO/C/Mg system, where the reduction temperature is above 1750° C. The reduction in SnO2 can also be performed with methane according to SnO2 + 2CH4 → Sn共l兲 + 2CO + 4H2,

⌬H650°C = 534 kJ 共4兲

The rate of reduction in SnO2 共Cassiterite兲 with coconut charcoal and graphite has already been reported by Padilla and Sohn 关9兴 at

Copyright © 2010 by ASME

AUGUST 2010, Vol. 132 / 031007-1

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Fig. 1 Lowest temperatures of full reduction in SnO2 and ZnO for different carbon to metal oxide molar ratios

a temperature range of 1073–1273 K. Their results indicate that SnO2 is reduced directly to Sn through gaseous intermediates of CO and CO2, and the overall rate of reduction is controlled by the oxidation of the carbon by CO2 共Boudouard reaction: C + CO2 → 2CO兲. They have calculated an energy of activation of 220.9 kJ/mole for the reduction with coconut charcoal at a temperature range of 1073–1273 K, and 323.8 kJ/mole for the graphite at 1198–1273 K. Comparing the rate of oxidation of coconut charcoal in CO2 − CO mixtures with the rate of reduction in SnO2 with the same charcoal, they concluded that there is a catalytic action of the tin formed during the reaction. The catalytic activity of metal oxides on Boudouard reaction, which, in general, limits the overall reduction rate of metal oxides at a temperature range of 800– 1000° C, is a well known phenomenon 关10,11兴 studied this effect in mixtures of coal with the metal oxides ZnO, Fe2O3, and In2O3. The experimental results with and without the oxides clearly show that the Boudouard reaction is catalyzed by these oxides, where the In2O3 has the largest effect. Regarding the tin oxide, contradicting results have also been published. Unlike in Padilla’s work 关9兴, Mitchell and Parker 关12兴 claimed that the presence of tin oxide has only a slight effect on the rate of oxidation of graphite in CO2. One of the purposes of this paper is to verify this issue in view of the potential superiority of the carboreduction in tin oxide over the zinc oxide as the endothermic step in a solar redox cycle for water splitting and production of hydrogen. The SnO2 / SnO/ Sn+ C / CH4 systems were theoretically investigated as an option for storage of solar energy 关8兴. It is shown there that instead of direct oxidation of CH4 or carbon with O2, their oxidation with SnO2 can finally produce hydrogen, which has a higher calorific value since this is an endothermic process. This addition of calorific value to the products can be contributed by solar energy. In the case of the cycle expressed by reactions 共4兲 and 共3兲 the upgrading is 18%. In the case of reactions 共1兲 and 共3兲 the upgrading is 23%. In addition, the product tin is stable at ambient conditions and can be stored as a solid or a liquid 共above 232° C兲. The hydrolysis reaction 共3兲 can be carried out separately from the solar operation. There is, however, very little information in the literature, particularly experimental, on this reaction, and one of the purposes of this paper is to shed light on this process. The solar carbothermal reduction in SnO2 can, therefore, be considered as a process for solar gasification of carbon at relatively low temperatures and for obtaining hydrogen, which has a higher calorific value and means to store solar energy in a chemical form. Additionally, the H2 and CO products are obtained separately, and since the tin is not evaporated, there is no need for quenching, handling, and separation of the solid powder from the effluent stream as in the case of the ZnO/C/Zn system.

2

Thermodynamics

Figure 1 shows the lowest equilibrium temperatures at which full conversion is reached for both SnO2 and ZnO at various ratios of carbon to metal oxide 共C / MOx兲. One can see that for a stoichi031007-2 / Vol. 132, AUGUST 2010

Fig. 2 Temperature when the back reaction begins due to H2 generation for the hydrolysis of Sn and Zn at different water to metal ratios

ometric composition with a molar ratio of 2, the tin oxide is fully converted to tin at around 600° C. This is compared with the ZnO/C system ZnO + C → Zn + CO,

⌬H950°C = 356.4 kJ

共5兲

It can be seen from Fig. 1 that full conversion of ZnO is reached at only 920° C for a stoichiometric molar ratio of 1, given by reaction 共5兲. The reduction in tin oxide, therefore, occurs at about 300° C lower than the ZnO. This promising thermodynamic data have been validated also in terms of the rate of reaction as shown hereafter. As the molar ratio of the C / SnO2 is made smaller, the reduction temperature in SnO2 is increased, e.g., for C / SnO2 = 1.5/ 1 the full conversion is shifted to 650° C, while tin monoxide is formed in addition to the tin metal. In the case of tin dioxide reduction with CH4 共reaction 共4兲兲 the oxide is practically reduced to the metal above 650° C, and a mixture of H2, CO, and CO2 is obtained for atmospheric pressure and molar ratio of SnO2 / CH4 = 1 / 1. Biogas can also be used to reduce the oxide. As an example, for the feed mixture of SnO2 / CH4 / CO2 = 1 / 1 / 0.67, the oxide is fully converted to the metal tin above 675° C. The hydrolysis of tin to generate the hydrogen and recover the oxide 共reaction 共3兲兲 is a slightly exothermic process. If steam is used at 500° C, ⌬H = −87.8 kJ. Thermodynamically, at a stoichiometric ratio of H2O / Sn= 2 / 1, tin monoxide is formed already above 100° C, and as the temperature is increased, the product hydrogen reacts back with the SnO2 to form an increasing amount of the monoxide. Above 600° C the oxide is reduced back to tin. To detain this back reaction, excess of water has to be used. Figure 2 shows the equilibrium temperature at which the back reaction of Eq. 共3兲 starts for different H2O / Sn ratios. As this ratio is increased, the back reaction starts at higher temperature. A comparison is made with the Zn hydrolysis, and it can be seen from Fig. 2 that the back reaction of the zinc oxide with the hydrogen is much less problematic. The above equilibrium calculations have been done with the aid of the HSC CHEMISTRY 关13兴 code version 6.0. Energy balance comparison between the ZnO/Zn and SnO2 / Sn carbothermic cycles is shown in Table 1. This energy balance shows that the amount of hydrogen produced in either cycle is similar, both per gram metal and for the total cycle 共partly because of the higher valency of the Sn兲, assuming converting the CO in shift reaction as follows: CO + H2O → CO2 + H2,

⌬H450°C = − 37.7 kJ/mole

共6兲

The solar contribution indicated in this table is the ratio of the solar input into the process 共including the heat of reaction and the sensible heat required to bring the reactants to the reaction temperature兲 divided by the total energy output 共via oxidation of the final hydrogen product兲. Also it can be seen that the differences in the reaction enthalpy for the minimum temperature required for Transactions of the ASME


Table 1 Energy balance comparison between the ZnO/Zn and SnO2 / Sn carbothermic cycles. H2 total includes hydrogen produced from the CO via water shift reaction. ⌬H 共kJ/mole兲

Reaction SnO2 + 2C → Sn共l兲 + 2CO Sn+ 2H2O → SnO2 + 2H2 ZnO+ C → Zn+ CO Zn+ H2O → ZnO+ H2

⌬H 共kJ/kg兲

H2 total 共liters/gr兲

Solar contribution 共%兲

357.5 共650° C兲 2369/kg SnO2 349.8 共900° C兲 ⫺92.5 共450° C兲 ⫺780 kJ/kg Sn 0.38/gr Sn 0.59/gr SnO2 4378 kJ/kg ZnO 356.4 共950° C兲 352 共1150° C兲 ⫺110.8 共450° C兲 ⫺1694.2 kJ/kg Zn 0.34/gr Zn 0.55/gr ZnO

full reduction of the metal oxides and the values at the practical temperatures are small.

3

H2 共liters/gr兲

Carboreduction of SnO2

3.1 Experimental. Carbothermal reduction experiments were performed on samples of SnO2 powder 共300 mesh, 99.9% purity兲 with beech charcoal and graphite using thermogravimetric analysis 共TGA兲 method in a temperature range of 800– 1000° C. The samples were reacted in small cylindrical crucibles 共D = 6 mm and h = 6 mm兲 made of alumina. The weight of the samples was 4–20 mg. The C/Sn molar ratio was 3. The weight loss measurements were monitored by means of Q-600 TA instrument. Beech charcoal was provided by Chemviron Carbon 共Lancashire, UK兲 with the following properties: particle size of ⬍200 ␮m, fixed carbon content of 83%, volatiles of ⬍13%, and ash of 4%. The composition of the ash of the beech charcoal is given in Table 2. Graphite powder 共by Sigma-Aldrich, Munich, Germany兲 with a particle size of 1 – 2 ␮m, and 99% carbon was also experimented as a reducing agent. Argon was used as a carrier gas. In addition, a flow system was used to study both the phenomena occurring in a fixed bed and the effect of the morphology of the participating solids on the reduction in SnO2. Experiments with beech charcoal and graphite were carried out in this system with analysis of the product gases 共CO, CO2, and CH4兲 using infra-red 共IR兲 analyzers. Figure 3 shows the setup for the reduction in SnO2 in the flow system. The ceramic reactor 共alumina tube Din = 17 mm, and L = 1100 mm兲 was placed in an electrical furnace 共Carbolite, Hope-Valley, UK, model STF 15/450, maximum working temperature of 1500° C兲. The reaction mixture 共0.5–4 g兲 was placed in a crucible made of alumina 共Dout = 16 mm, Din = 13 mm, and L = 70 mm兲. The temperature inside of the crucible was measured by a moving thermocouple. The crucible can be moved with the thermocouple from low to high temperature zone inside the furnace to quickly reach the desirable temperatures needed for the reaction. Argon was used as a carrier gas through the mass flow meter, the reactor, and the IR analyzers. The flow rate of the argon was 0.5–0.6 l/min. Figure 4 shows typical results. The total amounts of CO and CO2 共also C and O2兲, and the conversion of carbon and oxygen were calculated using the flow rate measurements and the concentrations of CO and CO2. 3.2 Results and Discussion of the Carboreduction Tests. The results of the TGA experiments on the reduction in the SnO2 with beech charcoal are shown in Fig. 5. It was found that at temperatures below 800° C the rate of the reaction is low. The optimal temperature for the process with beech charcoal is 875– 900° C. One can see that the reaction time needed to reach

36

48

the conversion of SnO2 close to 100% is 5–10 min in this temperature region. Figure 6 demonstrates the results of the TGA experiments on carboreduction in SnO2 with graphite. The rate of the reduction in the SnO2 with graphite is significantly lower than that with beech charcoal 共see Figs. 5 and 6兲. It is known that impurities in the charcoal can be active centers for carboreduction. High activity of beech charcoal can be explained by the presence of FeO, MnO, Na2O, and K2O. Most of the studies on the reduction in metal oxides with solid carbon consider the following two-stage mechanism 关1,4,9兴: SnO2共s兲 + 2CO共g兲 = Sn共s,l兲 + 2CO2共g兲

共7兲

CO2共g兲 + C共s兲 = 2CO共g兲

共8兲

Reaction 共8兲 proceeds at a slower rate and, therefore, it is the rate controlling step in the overall reduction process 关9兴. Since the overall process is controlled by the oxidation of carbon with CO2, the CO2 / CO ratio should be determined by the equilibrium of reaction 共7兲. The experiments, therefore, show that the flow rate of the carrier gas only slightly affects the kinetics of the reduction. The calculated equilibrium molar ratio CO2 / CO is about 2.5, and the weight ratio is about 4 in the temperature range of 800– 900° C. The experimental values of the weight ratios CO2 / CO was found to be 1.5–2 at the above temperatures. This means that there is some contribution of the reduction of the SnO2 with CO to the overall process. Conversions of carbon in the SnO2 reduction process were calculated using the TGA data 共see Fig. 5兲 and the experimental values of the CO2 / CO ratios obtained in the flow system. Figure 7 shows the carbon conversions 共relative to the stoichiometric amount of carbon兲 versus time at different temperatures. The first order reaction rates were calculated using the initial part of the curves in Fig. 7 and are given in Table 3 together with the values of Padilla 关9兴 for SnO2 reduction with coconut charcoal. The rate R is defined from the equation: ln共1 − conversion兲 = −Rt, where t is the time. One can see that beech charcoal is slightly more active than the coconut used by Padilla. The activation energy was calculated using the rate values presented in Table 3. The result is 235.4 kJ/mole compared with 220 kJ/mole reported by Padilla for the coconut. The flow reactor with fixed bed comprising SnO2 with beech charcoal and graphite was used to study the process and effect of the morphology of the solid products. Results from transmission electron microscopy 共TEM兲 show that after the cooling of the solid products at the end of the carboreduction, they contained mainly coal and metallic Sn with a particle size of 1 – 3 ␮m. Figure 8 shows the TEM image for the products of the carboreduction in SnO2. Conversion of the SnO2 in the flow system was

Table 2 Composition of the ash of the beech charcoal „main components… Component

SiO2

CaO

MgO

Al2O3

FeO

Na2O

K 2O

MnO

TiO2

SO3

Wt %

20.0

44.8

14.4

2.4

3.3

6.9

4.4

3.0

1.3

3.3


AUGUST 2010, Vol. 132 / 031007-3


Fig. 3 A scheme of the experimental setup for the study of the carboreduction in SnO2. Fig. 6 TGA results: kinetics of the reduction in SnO2 with graphite

about 95%, and only traces of SnO2 were observed in the x-ray defraction 共XRD兲 spectra. Experiments on the kinetics of oxidation of charcoal 共without SnO2兲 were also conducted as a reference to compare with the rate of SnO2 carboreduction. Figure 9 shows the TGA results of oxidation of charcoal with CO2. The flow rate of N2 was 49 ml/min, and the flow rate of CO2 was 22 ml/min 共concentration of CO2 about 31%兲. The experiments show that the carbothermal reduction rate is much faster than the oxidation rate. This proves that tin catalyzes the C + CO2 reaction. To study the catalytic effect of the tin 共in the form of powder, with a particle size ⬍10 ␮m兲, it was added to the beech charcoal. The experiments were conducted with x = Sn/ C molar ratio of 0.025, 0.05, 0.1, 0.2, and 0.37. Figure 10 shows the significant catalytic effect of tin even at a value as low as 0.025. The rate of reaction was increased almost proportionally to x for x ⬍ 0.2 and to a lesser extent for x ⬎ 0.2. As mentioned above, there is a controversy in the literature about the

catalysis of carbon oxidation by tin 共or tin oxides兲. Our experimental results prove that tin is indeed a catalyst for reaction of carbon oxidation with CO2.

4

Hydrolysis of Tin and Hydrogen Production

The product of the carboreduction in SnO2 is tin powder composed of particles of less than 1 – 3 ␮m with residual presence of unreacted carbon that can, in principle, be gasified by purging a CO and CO2 mixture, which is produced in the process. In the presence of Sn in the reactor and due to its catalytic effect, the remaining carbon can be gasified at 875° C with this mixture via a recycling stream. The results of hydrolysis of different tin powders were discussed previously 关14兴. This study reveals that the main parameter influencing the productivity is the morphology and the size of the particles. The structure of tin powder provided by Sigma-Aldrich, St. Louis, MO, with 99% tin content 共product number 520373, ⬍10 ␮m兲 is practically identical with the reduced tin dioxide powder 共compare structures shown in Figs. 8 and 11兲. This powder was selected as an appropriate stimulant to assure reproducibility of the results and since small amount of impurities in the tin 共⬍1%兲 had no influence on the hydrolysis results 关14兴 and practically, tin dioxide can be fully reduced to its metal 共as shown above兲. The main hydrolysis results obtained

Fig. 4 Carboreduction in SnO2 with charcoal in flow system at 830° C; concentrations of CO, CO2, and CH4 versus time

Fig. 7 Carbon conversion calculated from TGA data versus time Table 3 Reaction rate for the oxidation of charcoal in the process of reduction in SnO2

Fig. 5 TGA results: kinetics of the reduction in SnO2 with beech charcoal

031007-4 / Vol. 132, AUGUST 2010

Temperature 共°C兲

Beech charcoal 共l/min兲

Coconut charcoal 关8兴共l/min兲

800 850 875 900

0.033 0.07 0.167 0.33

0.0055 0.031 0.0667 0.124

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Fig. 8 TEM image of the product after the SnO2 reduction with graphite: typical particles size of 1 – 3 ␮m Fig. 11 Experimental setup for hydrolysis experiments „the photo on the left shows the structure of the powder in the tests…

Fig. 9 Kinetics of the oxidation of charcoal by CO2

Fig. 10 Catalytic effect of Sn on the oxidation of beech charcoal at 875° C for six different Sn to C ratios in the range of 0–0.37

with this powder and a comparison with the results of solar zinc powder having similar structure are presented here. 4.1 Experimental. The powder hydrolysis was done in an experimental setup similar to the one used for studying the hydrolysis of zinc 关3兴 and described there in detail. The method is based on measuring the hydrogen flow rate 共the main indication of the reaction rate兲 released from a batch reactor when feeding with steam. The study includes selection of the operation parameters and their variation with time, aiming at better process controlling and maximization of the conversion. The main experimental parameters of the setup 共see Fig. 11兲 are as follows: sample volume of up to 20 cm3, reactor temperature of up to 650° C, steam mass Journal of Solar Energy Engineering

flow rate of 0.01–10 g/min, and initial steam partial pressure of up to a few bars. The stainless steel sample holder having holes in its bottom is lined with zirconia felt and partly filled with tin powder 共see Fig. 11兲. It is placed in an electrically heated reactor made of stainless steel tube of 1⬙ in diameter and 0.5 m long half-filled with Pyrex beads and exposed to steam and nitrogen mixture under controllable flow, pressure, and temperature. The outer diameter of the sample holder matches the inner diameter of the reactor tube. The powder layer is also covered with zirconia felt to avoid possible blowing losses. K-type thermocouples 共TCs兲 are used for measuring the temperatures in the reactor and the reaction zone 共see Fig. 11兲. These temperatures can be different because of the exothermic nature of the reaction. Pressure is controlled by pressure transmitters 共Sensor Technik Simach AG, Zurich, Switzerland兲. The feeding block is installed at the inlet of the reactor and is built to feed the desirable superheat steam-nitrogen mixture. It comprises a nitrogen cylinder 共99.99% purity兲 with mass flow controller, piston micrometering pump with a resolution of 0.01 gH2O / min delivering water to a spiral boiler and associated valves. Carrier nitrogen gas is flowing permanently and can be mixed with the preset superheat steam at a certain moment by switching-over the corresponding valves. A purifying block is installed at the output of the reactor. It comprises water cooling condenser, dry ice trap, and filter. Separated from water vapors, hydrogen-nitrogen mixture enters the measurement block. It contains mass flow meters of different ranges and multirange hydrogen analyzer Colomat 6 共by Siemens AG, Karlsruhe, Germany兲. All test parameters were recorded every 10 s. 4.2 Results and Discussion of Tin Hydrolysis Tests. The conversion versus time following the starting of the steam flow for different temperatures in the reactor is shown in Fig. 12. They were calculated based on the hydrogen flow yield divided by the theoretical hydrogen productivity of the loaded powder 关3,14兴. It can be seen from this figure that increasing the reactor temperature from 350° C to 525° C results in 70% conversion after 2 h. Further temperature increase does not lead to higher conversion. On the other hand, starting the steam flow during the preheating period at lower temperature 共360– 620° C兲 ensures 85% conversion after 2 h. The results shown here were obtained in a series of tests conducted with the same sample mass of tin powder 共9.5 g兲, the same carrier gas 共N2兲 flow rate of 100 cm3 / min, the same AUGUST 2010, Vol. 132 / 031007-5


Fig. 15 Mass fractions of Sn, SnO, and SnO2 in powder after 3 h tests at different constant temperatures in the reactor Fig. 12 Tin powder conversions versus time at different reactor temperatures

water mass flow rate of 0.55 g/min, and at the same initial steam partial pressure of 0.93 bar. Comparing the data presented in Figs. 12 and 13 共for Sn and Zn, respectively兲 leads to the conclusion that tin powder hydrolysis differs substantially from zinc powder hydrolysis despite the similar structure, the same sample mass, and the same test conditions. With Zn powder, increasing the reactor temperature to 540– 550° C leads to 85% Zn conversion after the first 6–7 min. These results can be predicted from the data presented in Table 1 and Fig. 2, and can be explained by 共1兲 lower exothermic heat release in the case of the tin and 共2兲 the back reaction between the product H2 and the oxide, which occurs at lower temperatures in the case of tin. It can be seen in Fig. 14 that in the case of zinc powder hydrolysis, the heat released during the exothermic hydrolysis leads to a maximal temperature rise of 150– 180° C in the reaction zone, while in the case of the tin powder, the temperature in the reaction zone remains practically the same as in the reactor. The kinetics of the back reaction at the temperatures of interest was analyzed experimentally. The reduction rate of SnO2 with hydrogen was measured using a SDTQ600 Gravimeter. It was found that switching from a pure N2 flow to a mixture of N2 and H2 with 0.25 bar absolute partial pressure of H2 at 600° C leads to mass losses in

the SnO2 sample at a relative rate of 3.4% / min of the initial oxygen weight in the sample. Thermodynamics anticipate that under equilibrium at 350° C and stoichiometric composition of the reactants Sn and H2O, both SnO2 and SnO exist 共roughly 75% and 25%, respectively in the solid兲, and in the case of surplus of H2O, SnO does not exist at this temperature. Results of the X-ray analysis 共Fig. 15兲 indicate that in contrast to thermodynamic predictions, only Sn and SnO were observed remaining in the powder after the test at a temperature of 350° C. At higher temperatures, the mass fraction of the tin monoxide decreases and the mass fraction of the tin dioxide increases. At 600° C, after 3 h, the sample holder contained only SnO2 and a minor amount of unreacted Sn. These results suggest that three reactions with different kinetics take part in the hydrolysis step Sn共l兲 + H2O共g兲 = SnO共s兲 + H2共g兲, − 38.7 kJ,

Fig. 14 Peak temperatures in the reaction zone as a function of the temperature in the reactor

031007-6 / Vol. 132, AUGUST 2010

共9兲

⌬H500°C =

K620°C ⬇ 1

Sn共l兲 + 2H2O共g兲 = SnO2共s兲 + 2H2共g兲, − 87.8 kJ,

Fig. 13 Zinc powder conversions versus time at different reactor temperatures

K480°C ⬇ 1

SnO共s兲 + H2O共g兲 = SnO2共s兲 + H2共g兲, − 49.1 kJ,

⌬H500°C =

共10兲

⌬H500°C =

K550°C ⬇ 1

共11兲

If the steam flow starts at lower temperatures during the preheating period 共heating rate of about 3 ° C per min兲, tin monoxide is accumulated in the sample holder according to reaction 共9兲. When reaching 600° C, reaction 共10兲 provides additional hydrogen compensating for the hydrogen losses due to the back reaction of reaction 共11兲. This can explain the behavior of the 360– 620° C curves in Fig. 12. The relative reaction rate can be estimated as the rate of conversion increase 共dconv/ dt兲 during the first few minutes of the isothermal tests 共see the first four curves in Fig. 12兲 conducted with the same sample mass and steam partial pressure. A logarithmic plot of these data is presented in Fig. 16. Simple point interpolation estimates the apparent activation energy as 42 kJ/mol.

Fig. 16 Logarithmical plot for the initial relative hydrolysis rate calculated from the results of the tests conducted with the same sample mass and the same steam partial pressure

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This value closely agrees with the value presented earlier 关14兴, where the reaction rate was estimated using the initial hydrogen flow rate.

5

Summary and Conclusions

A comprehensive study on the carboreduction in tin dioxide and the hydrolysis of the product solid tin was conducted. The experiments show that the carbothermal reduction rate is much faster than the oxidation rate. This proves that the tin catalyzes the C + CO2 reaction. The optimal temperature for the reduction in SnO2 using beech charcoal is 875– 900° C. Full conversion is reached with reasonable rate. The tin oxide reducing temperatures are lower by 200– 300° C compared with the carboreduction in ZnO 共1100– 1200° C兲. This fact substantially alleviates the difficulties encountered in the carboreduction in zinc oxide with respect to the reactor materials of construction, simplifies the operation, and increases the process thermal efficiency. Starting the carboreduction process with SnO2 powder of few microns size mixed with charcoal of similar size results in tin powder product rather than agglomerates of solidified bulk liquid tin. This form of tin powder was hydrolyzed with steam. Its rate is lower compared with the hydrolysis of zinc mainly due to the fact that lower amount of heat is released and the back reaction starts at lower temperatures. This, however, enables better controllability of the process. Over 80% conversion was demonstrated when steam flow starts already at a temperature of 360° C during the preheating up to 600° C.

Acknowledgment The authors would like to express their gratitude to Mr. Adi Arnon for his invaluable assistance in preparing and carrying out the experiments, and to Dr. Ronit Popovitz-Biro and Dr. Yishay Feldman for performing the TEM and XRD analyses.


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AUGUST 2010, Vol. 132 / 031007-7
