Solar carbothermic production of zinc: Christian Wieckert (1.4 - SFERA

SFERA Winter School 

Solar Fuels & Materials Page 251 

Solar Carbothermic Production of Zinc 

Christian Wieckert 

Solar Technology Laboratory, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland 

SFERA Winter School Zurich March 24,25 2011 

Solar Carbothermic Production of Zinc 

� Introduction 

Outline 

� Thermodynamic equilibrium and kinetic information 

� Overview Solzinc-project 

� Laboratory scale solar reactor design and testing 

� Pilot plant for carbothermic solar Zn production 

� Conceptual design demonstration plant 

� Conclusions 

SFERA Winter School Zurich March 24,25 2011/ Solar Carbothermic Production of Zinc 

2



C 

(Coke, Coal, 

Biomass,..) 

Carbothermic ZnO reduction for solar energy storage 

Solar Zn Reactor 

1200ºC 

ZnO + C 

� 

Zn + CO 

Reactants 

CO 2 

e.g. gas motor 

Offgas 

(CO) 

Zn 

ZnO Recycling 

Electric Power 

Syngas 

H 2+CO 


Zinc/air fuel cell 

Zn + ½ O 2 � ZnO 

Electric Power 

Thermodynamic equilibrium composition 


Hydrogen 

Zn + H 2O � ZnO + H 2 

Zn + CO 2 � ZnO + CO 

1 mol ZnO + 1 mol C 1 mol ZnO + 0.9 mol C 

Zn(g), CO 

3 

4



Temperature C 

Carbothermic reduction of ZnO: 

Tmin for “no ZnO” in thermodynamic equilibrium 

2000 

1800 

1600 

1400 

1200 

1000 

800 

ZnO -> Zn + ½ O 2 

ZnO + ½ C -> Zn + ½ CO 2 

ZnO 

0.0 0.2 0.4 0.6 0.8 1.0 

�=mol C/mol ZnO 

No ZnO 


C 

ZnO + C -> 

Zn + CO 

For comparison: pyrometallurgical Zn-production: � � 4 (“Imperial Smelting Process”) 

Thermogravimetric analyses (dynamic runs with 10°C/min): 

ZnO + Coal/Coke (stoichiometry: C fix :ZnO=0.8) 


ASME J. Solar Energy Engineering 124 (2002), 55-62 

Carboreduction of ZnO: kinetics depends strongly on coal/coke-type 

ZnO + C Zn +CO 

�H = 350 kJ/mol 

Reduction mainly via 

solid-gas reactions: 

ZnO + CO Zn + CO2 C + CO2 2 CO 

Petcokes 


5 

6



Major investigation of carbothermic ZnO reduction: EU-project SOLZINC 

Key project data 

6 project partners from 5 countries 

3 million € for 4 years 

250 person months 

SOLZINC project phases 

Kick-off: Select solar process and solar reactor concept 

2002/3: Lab scale (5 kW solar input): Reactor and process optimization 

Lab-scale testing of Zn production from offgas 

2003/4: Design, realisation, commissioning of pilot plant (300 kW solar input) 

2005/6: Pilot plant test operation 

Conceptual design of demonstration plant (5-8 MW solar input) 

Scenario and cost studies 


Solar Reactor Concepts for ZnO+C 

Two reactor designs tested experimentally: 

� “Two cavity reactor” (indirect heating, with window, batch, PSI) 

� “Annular reactor” (also tested without window, 

Weizmann Institute of Science) 

Solar Beam 

Quartz 

Window 

Water-cooled 

Flanges 

Thermocouple 

Ceramic Wall 

Condenser- 

Separator 

CPC 

Reactants: 

ZnO+C 

Outlet Tube 

Solid Zinc 

Filter 

Gas-inlet Tube 

for Window 

Flushing 

Inner Heating Wall 

CO+CO 2 

Chemistry in Britain 37 (2001), Nr. 5, pp. 30-32 


Concentrated solar radiation 

Upper cavity 

Quartz window 

Separation 

wall 

Lower cavity 

(reaction 

chamber) 

~1200ºC 

ZnO + C 


Two cavity design selected for upscaling 

in EU-SOLZINC project 

7 

Packed bed 

8



Tests with 

ZnO/C: 

„2-cavity“ solar reactor: design evolution 

0 15cm 

8/2001 9-12/2001 4-6/2002 

Similar reaction rate in continuous and in batch operation -> focus on batch 


Two cavity laboratory batch reactor (from 8/2002) 

Zn-dust 


Product gas 

(CO, CO 2 , N 2 , H 2 ) 

9 

10



Temperature [°C] 

R 

1600 

solarin 

1400 

1200 

1000 

800 

600 

400 

200 

0 

Two-cavity laboratory batch reactor 


Typical laboratory test with 500 g ZnO/ beech charcoal mixture 

(0.9 mol C fix per mol ZnO) 

Production rate 0.6 kg Zn/h 

HP( 

T) 

�H 

( 298K) 

�thermal 

� 

� 20% 

Q 

[kW] 

10 

8 

6 

4 

2 

0 

Q solarin 

CO 2 

00:00 00:15 00:30 00:45 01:00 01:15 01:30 

Time [hh:mm] 

T separation wall 

T wall above packed bed 


CO 

T bottom of packed bed 

H 2 

Q solarin 

0.2 

0.18 

0.16 

0.14 

0.12 

0.1 

0.08 

0.06 

0.04 

0.02 

0 

~1200ºC 

ZnO + C 

Gas flow rate [mol/min] 

11 


12



Reaction rate in mol/m2/s 

0.175 

0.15 

0.125 

0.1 

0.075 

0.05 

0.025 

0 

Solar reactor modeling 

Effective reaction rate per bed surface area 

ZnO + different carbon-sources (stoichiometry C/ZnO=0.8) 

1100 1150 1200 1250 1300 1350 

Wall temperature above bed in °C 

Beech charcoal 

Kalzinat (Petcoke) 


• Solving unsteady energy equation 

• Coupling conduction/radiation heat 

transfer to chemical kinetics 

• Shrinking packed bed 

ZnO/C bed 

Doctoral theses Thomas Osinga, ETHZ 16180 (2005) 


Activated charcoal, solar furnace PSI 

Activated charcoal, solar simulator ETH 

Linear (Beech charcoal ) 


Fixed bed 

top temperature 

Zn production rate 

13 

14



Conclusions laboratory tests 

� Solar Zn-production realized with different coals/cokes 

• reaction rate depends on C-material 

• industrial beech charcoal selected as “standard” 

� Stoechiometry in two-cavity reactor 

• C/ZnO (molar) = 0.8-1: complete reaction (no C or ZnO left); CO 2 /CO about 0.05-0.3 

• For C/ZnO (molar)



The Solzinc pilot plant - optical system 

Solar facilities at Weizmann Institute of Science/Israel Secondary concentrator 

Upper cavity above separation plates 

Upper part 

(stationary) 

Lower part 

Carrier gas inlet 

Lower cavity 

(reaction chamber) 


SOLZINC solar pilot furnace/reactor 

ZnO/C batch 

� 300 KW solar power input 

140 cm 

Quartz window 

Offgas pipe 

with heater 

Zn(g)+CO 


17 

Principle: 

- “2-cavity” reactor 

- fixed bed of ZnO/C-mixture 

- 1 batch per day, adapted to 

the availability of the sun 

Features: 

P solarin � 300 kW 

D reaction chamber inside = 1.4 m 

H bed � 0.5 m 

Capacity � 500 kg ZnO/C 

Standard C: 

industrial beech charcoal 

Lining: SiC plates 

Insulation: Al 2 O 3 -SiO 2 

Separation plates: 

graphite, SiC on graphite 

Lower part easy to lift down 

for refilling 

18



2nd 

Chimney 

N 2 

Solzinc Pilot Reactor open 


SOLZINC pilot plant: Major components 

Solar irradiation 

Quartz window 

Solar reactor 

Quench/Cooler 

50 Nm3 /h 

(Zn, CO, CO2 , H2 , N2 ) 

Recycled offgas to reactor (outer loop) 

Water 

10 Nm 

lock 

(emergency exit) 

3 /h 

Inner recycling loop 

350 Nm3 /h 

Cyclone 

Zn dust 


Fan 

Bag filter 

Zn fine dust 

19 

Chimney/ 

burner 

25 Nm 3 /h 

(CO, CO 2 , H 2 , N 2 ) 

20



Solzinc pilot plant in operation 


Typical pilot test with 100kg ZnO (Grillo 2011) and 16 kg beech charcoal (gravel, Profagus) 

°C 

1400 

1200 

1000 

800 

600 

400 

200 

0 

Temperature separation plates 

Temperature reaction 

chamber side wall 

11.02 h 12.14 h 13.26 h 14.38 h 15.50 h 17.02 h 

CO 2 

CO 

Bed bottom temperature 

kg Zn/h 


60 

50 

40 

30 

20 

10 

0 

kg/h ; Vol-% 

21 

22



volume-% 

100 

80 

60 

40 

20 

Overall Zn production rate as function of temperature above bed 

0 

kmol Zn/h 

0.7 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0.0 

TLP1 K 

1493 

0.8 

1449 1408 1370 1333 1299 

50 

0.67 0.69 0.71 0.73 0.75 0.77 

1000/TLP1 1/K 

�� 


�� 

�� 

d um 


Linear (a=0.9) 

Main products: Zn-dusts (� 95% Zn + 5% ZnO) 

Apparent size distribution (volume in particles < d) 

bag filter dust 

d �m 

cyclone dust 

0.1 1 10 100 

40 

30 

20 

10 

0 

kg Zn/h 

23 

24



Conclusions Pilot Plant 

� Successful operation processing ZnO mixed with beech charcoal 

(0.8-1 mol Cfix per mol ZnO) 

� Production rate at about 1150 °C: about 50 kg/h Zn-dust. 

� Thermal efficiency � thermal = Q sink/Q solarin � 30% 

� Main product: Zn-dust containing typically 95% Zn, 5% ZnO. 

Size in the range



2.16 t/h 

ZnO 

Batch 

mixer 

0.32 t/h 

C 

Key input data: 

C fix :ZnO=0.8 

beech charcoal 

� thermal = 55% 

� gas motor = 40% 

m 

Y Axis Title 

200 

150 

100 

Option: Offgas system for Zn-dust production 

Indicative mass and energy flows 

5.3 MW 

Solar irradiation 

Aperture 

Solar reactor 

Quench/Cooler 

1.7 t/h Zn(g) 

Recycled offgas to reactor (outer loop) 

50 

Conceptual Design Solzinc Demonstration Plant 

0 

50 m 2 

Inner recycling loop 

Cyclone 

1.6 t/h Zn dust 



Layout heliostat field for Rehovot/Israel 

N 

-100 -50 0 50 100 

X Axis Title 


m 

0.5 MW 

Electric 

power 

Fan 

Bag filter 

0.1 t/h Zn 

fine dust 

Chimney 

Gas 

motor 

95% Zn, 5% ZnO 

27 

186 heliostats à 54m 2 

Total reflective area: 10063 m 2 

ASME J. Solar Energy Engineering 130 (2008), 014505 

28



5 m 


Optical system with several secondary concentrators (CPC’s) 

Demo-plant: 

surface area 285 m 2 

height 59 m 

Second focal point 

Heliostat field 

Hyperboloidal mirror 

Solar Receiver reactor 

7 Secondary 

concentrators 



Heliostat field 



252 cm height 

5 MW solar reactor – design proposal 

One reactor with 7 quartz windows, 

each below a secondary concentrator (“CPC”) 

Separation wall 

Graphite/SiC plates on SiC-rods 


29 

30



Industrial plant for carbothermic ZnO reduction 

Estimates for 30 MW plant 

Approximate plant data: 

Area heliostats: 60000 m 2 

Area land: 0.15 km 2 

Solar power in reactor 30 MW 

Coke (0.8 stoichiometry) 2 t/h (14 MW) 

Zinc production: 11 t/h (16 MW) 

25000 t/a 

Energy content offgas: 8 MW 

Total investment cost: �18 M€ 

Building 

Tower Reflector 

Tower 

Secondary 

Concentrator 

Reactor 

If cyclic process to electricity: 

Power from Zn: 11 MW el (2300 h/a) 


Heliostat Field 

Power from offgas: 3.2 MW el and 3.2 MW low-T heat 

(gas engine) 

Conclusions 

� Solar carbothermic reduction of ZnO has been successfully 

demonstrated on laboratory and pilot scale, opening interesting options for 

- production of Zn from ZnO with very low CO2-emissions - production of Zn as energy carrier for the flexible storage of solar 

energy, suited for on-demand production of 

- electricity in Zn-air fuel cells/batteries or 

- hydrogen via reaction of the Zn with steam 

- syngas via reaction of the Zn with steam and CO2-mixtures � Zn-dust production was demonstrated. Production of bulk-Zn or of Znpowder 

is possible, as well 

� Beech charcoal has been used as reductant. Other coals/coke might be 

used, as well 

� A 5 MW demonstration plant has been conceptually designed 


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Solar carbothermic production of zinc: Christian Wieckert (1.4 - SFERA

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