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Research Papers

The SnO2/Sn Carbothermic Cycle for Splitting Water and Production of Hydrogen

[+] Author and Article Information
Michael Epstein

Solar Research Facilities Unit, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israelmichael.epstein@weizmann.ac.il

Irina Vishnevetsky, Alexander Berman

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

J. Sol. Energy Eng 132(3), 031007 (Jun 14, 2010) (7 pages) doi:10.1115/1.4001403 History: Received October 02, 2008; Revised May 14, 2009; Published June 14, 2010; Online June 14, 2010

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 (850900°C for the SnO2 versus 11001150°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 8001000°C. Optimal temperature range for the reduction with beech charcoal is 875900°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 13μm. 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 350600°C. Results of both the reduction and hydrolysis reactions are presented in addition to thermodynamic analysis of this cycle.

Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

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

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Figure 2

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

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Figure 3

A scheme of the experimental setup for the study of the carboreduction in SnO2.

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Figure 4

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

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Figure 5

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

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Figure 6

TGA results: kinetics of the reduction in SnO2 with graphite

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Figure 7

Carbon conversion calculated from TGA data versus time

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Figure 8

TEM image of the product after the SnO2 reduction with graphite: typical particles size of 1–3 μm

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Figure 9

Kinetics of the oxidation of charcoal by CO2

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Figure 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

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Figure 11

Experimental setup for hydrolysis experiments (the photo on the left shows the structure of the powder in the tests)

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Figure 12

Tin powder conversions versus time at different reactor temperatures

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Figure 13

Zinc powder conversions versus time at different reactor temperatures

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Figure 14

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

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Figure 15

Mass fractions of Sn, SnO, and SnO2 in powder after 3 h tests at different constant temperatures in the reactor

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Figure 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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