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RESEARCH PAPERS

Simulation of Thermal and Chemical Processes in Annular Layer of ZnO–C Mixtures

[+] Author and Article Information
Irina Vishnevetsky1

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

Michael Epstein2

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

Rahamim Rubin3

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

1

Tel: +972-8-9342284; Fax: +972-8-9344117

2

Tel: +972-8-9343804; Fax: +972-8-9344117

3

Tel: +972-8-9342282; Fax: +972-8-9344117

J. Sol. Energy Eng 127(3), 401-412 (Sep 14, 2004) (12 pages) doi:10.1115/1.1877473 History: Received April 28, 2004; Revised September 14, 2004

A special setup, electrically heated, enabling the simulation of the process conditions encountered in a solar chemical reactor, is described. The setup allows us to study the thermal and chemical processes in different solid (powder or granules) reactant layers from the beginning of the heating until the reaction is completed, in a heating condition typical for indirectly, externally heated solar reactors. The particular case of the ZnO carboreduction process is analyzed in this paper as an example. Tests were executed using different powder mixtures of ZnO–C to demonstrate the layer-wise nature of the process. The results show that the reactivity and the behavior of mixtures strongly depend on their components structures, impurities, and stoichiometry. This method can be generally applied for studying endothermic chemical reactions involving other solid reactants.

Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 1

Principle scheme of the electrical setup. (1) Water-cooled current feeder; (2) graphite packing; (3) SiC heater; (4) hermetic frame; (5) vacuum and inert gas inlet tubes; (6) electrical contacts; (7) radial thermocouples; (8) gas outlet tubes; (9) Zn condenser; (10) high-density alumino-silicate (Duraboard) rings; (11) water-cooling tubes; (12) reaction powder; (13) axial thermocouples; (14) insolated blanket; (15) current clips

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Figure 2

Typical axial temperature distributions of the heater obtained at different times during the heating process (a) and principal scheme of the axial thermal losses; thin line—outer heater surface, thick line—inner heater surface (b) (for ΔQradt6=t7=20°C, t4=t5=100–300°C, t1,t2,t3 are averaged in the areas 1, 2, 3 using axial temperature curve fitting, ε=0.9)

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Figure 3

Axial thermal losses as a function of heater temperature for low sintering powder (a), high sintering powder (b), high reactivity mixture (c); 1−ΔQcond, 2−ΔQrad, 3−ΔQ=ΔQcond+ΔQrad

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Figure 4

An example of the main experimental parameters vs time for carboreduction of ZnO: Radial temperatures of the reaction bed (1:r=0.0205m, 2:r=0.027m, 3:r=0.0335m, 4:r=0.038m), off gas flow rate (5), electrical power input (IΔU) (6)

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Figure 5

Radial temperature distribution at different times during temperature rising, (a) empty reactor; (b) filled with ZnO powder only, (c) filled with high reactivity ZnO–C mixture

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Figure 6

Apparent thermal conductivity radial distribution for low sintered powder (1), high reactivity mixture at the reaction beginning (2) and empty reactor (3)

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Figure 7

Apparent thermal conductivity as a function of time and radial coordinates from the beginning to the end of reaction for high reactivity mixture with bulk density 1.1∙103kg∕m3

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Figure 8

Temperature dependence of Kapp for high reactivity mixture with bulk density 0.52∙103kg∕m3, 1—no reaction; 2—reaction begins; 3, 4—reaction in progress; 5—empty reactor

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Figure 9

Apparent thermal conductivity as a function of temperature for different ZnO powders (Table 1) with the same heating rate as for mixtures (total heating time 80–90 min), 1—Zinc Oxide Active, 2—Zinc Oxide Brown FG, 3—Zinc Oxide Brown

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Figure 10

Time (a) and averaged temperature (b) dependence of the average thermal conductivity coefficient, 1—high reactivity mixture (with Beech charcoal), 2—low reactivity mixture (with graphite particles), 3—high sintered ZnO powder, 4—low sintered ZnO powder

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Figure 11

The main experimental parameters vs time, in carboreduction of ZnO, for highly reactive mixture: Temperature at different distances from the heater (1-4, radial coordinate, see in Fig. 4), CO-flow (5), resistance of the central part of the heater (6) (Circles indicate reaction completion at different depths of the annular reaction volume)

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Figure 12

Reaction time for the annular reactor as a function of its outer radius expressed as the ratio of the outer radius to the radius of the heater for 120kW∕m2 power input flux, high reactivity mixture (ρmix≈0.5g∕cc)

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Figure 13

Chemical energy and chemical flux related to heater central part outer surface vs coordinate (a) and time (b), 1−δEchem,2−Fchem.

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Figure 14

Flow rates of typical off gas components vs time: 1—CO, 2—H2,3—CO2,4—CH4

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Figure 15

Correlation between oxidation temperature with air and the temperature of the beginning of gasification in the reactor volume for different kinds of carbon

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Figure 16

Reactivity parameters for different carbon materials in mixture with the same ZnO powder (molar ratio of 1:0.8) as a function of their gasification beginning temperature (tCO-out); (a) parameters of reaction rate related to appropriate parameter of most reactive Beach Charcoal: R1B.Char.=0.29[W∕m∙K∙min], R2B.Char.=8[cm∕hour], R3B.Char.=3.4[g∕cm2hour], R5B.Char.=0.06[mol∕mol∙min]; (b) parameters of reaction quality

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Figure 17

Kapp-aver as a function of mixture bulk density before the reaction starts (t=500–600°C)

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Figure 18

Reactivity characteristics of different mixtures of ZnO with Beech Charcoal at molar ratio close to 1, (a) Kapp-aver rising rate, (b) reaction time

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Figure 19

Quality parameters for under stoichiometric mixtures of different ZnO powders with Beech Charcoal

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Figure 20

CO output flow rate vs. time for mixtures with surplus of carbon, 1—activated charcoal (ZnO:C 1:1.5), 2—graphite particles (ZnO:C 1:3)

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Figure 21

Apparent thermal conductivity coefficient—local (a) and averaged (b) for mixture with activated charcoal with stoichiometric ratio ZnO:C of 1:1.5

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Figure 22

Back reaction losses of mixtures with C:ZnO molar ratio less and more than 1 comprising graphite particles (ZnO:C of 1:0.8; 1:3) and activated charcoal (ZnO:C of 1:0.7; 1:1.5)

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