Solar Facilities for the European Research Area
Solar Thermochemical Reactors Solar Reduction 2
Gaël LEVEQUE, PROMES-CNRS
SFERA II 2014-2017, Solar Reduction 2, 2014/06/27, Odeillo, France
Introduction
Definition of volatile oxide cycles
Theoretical efficiency
Thermal reduction
Carbothermal reduction
Products reactivity
Solar reactor technologies
Rotating cavity type
Beam-down and other vertical configurations
Carboreduction specialized
Conclusions
SUMMARY
2
Introduction
Definition of volatile oxide cycles
Theoretical efficiency
Thermal reduction
Carbothermal reduction
Products reactivity
Solar reactor technologies
Rotating cavity type
Beam-down and other vertical configurations
Carboreduction specialized
Conclusions 3
INTRODUCTION
Volatile oxide cycle: - Cycle based on an oxide which reduction temperature is higher than the boiling temperature of its reduced specie
𝑀𝑂𝑥 = 𝑀𝑂𝑥−𝑦 𝒈 + 𝑦2 𝑂2
Examples of purely thermal volatile oxide cycles:
Oxide/Reduced
specie
Reduction
Temperature
Boiling point
of the product
Productivity
mmolH2/goxide
ZnO/Zn 2070°C 907°C 12 Most advanced couple
GeO2/GeO 1830°C 1000°C ? 10 Transparent in infrared, melts
at 1115°C
CdO/Cd 1590°C 767°C Poor reactivity with water
In2O3/In 2206°C In2O3: 1913°C
In: 2072°C
Impractical
SnO2/SnO 2060°C 1527°C 7 Similar to ZnO/Zn
4
INTRODUCTION
5
Theoretical efficiencies
ZnO/Zn(g) SnO2/SnO(g)
TΔG=0 (°C) >2070 >2060
habsorption =1 −σT4
IC 0.66 0.66
hexergy, max= ηabsorption ∗
ηcarnot 0.57 0.58
Qreactor net (kW) =
𝑛 ΔH + 𝑛 cpdT 560 710
Qsolar (kW)= 𝑄𝑟𝑒𝑎𝑐𝑡𝑜𝑟 𝑛𝑒𝑡
ηabsorption 848 1076
Qquench (kW) 209 413
WFC,α=0(kW) 237 237
hexergy= 𝑊𝐹𝐶
𝑄𝑠𝑜𝑙𝑎𝑟 0.28 0.22
C = 5000, I = 1 kW/m², 1 mol/s of oxide
INTRODUCTION
6
Theoretical efficiencies
Improve the theoretical efficiency:
- Reduce the temperature (hsolar/fuel=0.3 for SnO2 at 1600°C)
Low pressure, high dilution: Cost ?
- Reduce the heat sinks (heat recovery).
Limited by the quench of the products
ZnO equilirium calculations
INTRODUCTION
Main issue: Handling 𝑀𝑂𝑥−𝑦 𝒈 + 𝑦
2 𝑂2
- Reduced specie preferably obtained in the form of a nanopowder by condensation
(high specific surface area) Cooling of the gases and gas-solid separation
But
Reaction inversion, products recombine as the temperature is lowered (heterogeneous reaction from nucleation sites, gases metastables)
To obtain high purity reduced specie:
- Quenching of the gases (temperature drop faster than reaction)
- Dilution of the products to limit recombination
- Oxygen separation ? (oxygen exchange membrane, reducing agent…)
Carbothermal reduction
7
INTRODUCTION
Carbothermal reduction (CR)
MOx + aC = MOx-y + (2a-y)CO + (y-a)CO2
Carbon source: methane, coal…
Reduction rate and purity drastically improved:
- C and CO are reducers metal oxide reduced at lower temperature
- CO2 is less a good oxidizer than O2 recombination limited
Drawbacks:
- Consumption of a carbonaceous feedstock (pseudo-cycle), CO2 emission
- Carbon excess
Oxide/Reduced
specie
CR
Temperature
Boiling point of
the product
ZnO/Zn 950°C 907°C Most advanced couple
MgO/Mg 1850°C 1090°C
8
INTRODUCTION
Carbothermal reduction - Choice of the reducing agent
Solid carbon:
Reduction reaction mainly drove by CO:
MOx + yCO = MOx-y + yCO2
yCO2 + yC = 2yCO limiting step
Beech charcoal generally preferred: good specific surface area, non-fossil sourced, mineral content
Easy to mix with ZnO as powders, good absorber
Methane:
Possibility of combination of carbothermal reduction with CH4 reforming:
MOx + aCH4 = MOx-y + (2a-y)CO + (y-a)CO2 + 2H2
Mixing more difficult, harder to heat
9
INTRODUCTION
Products reactivity
Typical characteristics:
- BET surface in the range 20-40 m²/g for both SnO and Zn
- Mean particle size up to 10 µm, important volume of nano-sized pore
- Micro-sized conglomerate of nano entities (disk-shaped for SnO, needle-shaped ZnO)
- Dependent of the conditions
10
INTRODUCTION
Products reactivity
Highly reactive toward CO2 and H2O (more than any commercial powder)
Morphology
- High specific surface area
- Small particles
Presence of reoxidized specie
- Limits sintering
- Support for Zn(g) oxidation
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Limits surface passivation (~20nm)
Comparative reoxidation at 600°C with H2O
INTRODUCTION - CONCLUSION
Particular features of the volatile oxide cycles: - High temperature (really) - Importance of the recombination reaction for the final purity Low O2 partial pressure: low pressure/high neutral gaz flow Temperature issues: quenching of the gases, cold points and deposits - Particles generally retrieved (filtered) out of the solar reactor Allows continuous operation of the solar reactor Solar reactor designed for only one step - Reduced particles nano-sized (obtained by condensation) Specific surface area regenerated at each cycle High reactivity
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Introduction
Definition of volatile oxide cycles
Theoretical efficiency
Thermal reduction
Carbothermal reduction
Products reactivity
Solar reactor technologies
Rotating cavity type
Beam-down and other vertical configurations
Carboreduction specialized
Conclusions
SUMMARY
13
SOLAR REACTOR TECHNOLOGIES
Roca reactor: 10kW, 1999, PSI-ETH 1
Designed for ZnO thermal reduction
3- Quartz window protected from particles deposition by a gas flow (8)
4- CPC
1- Rotating conical cavity receiver
7- ZnO layer
6- Screw powder feeder
9- Gas outlet
10- Quenching device
14
1 Haueter, P., S. Moeller, R. Palumbo, and A. Steinfeld. “The Production of Zinc by Thermal Dissociation of Zinc Oxide—solar Chemical Reactor Design.” Solar Energy 67, no. 1–3 (July 1999)
SOLAR REACTOR TECHNOLOGIES
Roca reactor: 10kW, 1999, PSI-ETH Objectives:
- Low thermal inertia, thermal shocks resistance Allowing fast start/stop to follow the availability of the solar resource
- Respect the chemistry of the reaction 1850 K < T < 2250 K (ZnO melting point) Quenching of gases
Why a rotating cavity ? Centrifugal force evenly disperses ZnO into a thick layer (2.5 cm) and holds it: used as absorbant, reactive material and insulation. Thick layer ablative regime for improved exergy efficiency Results - Dilution with N2 ~20-25 - Low conversion (35% of zinc)
Unsufficient quenching of the gases
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SOLAR REACTOR TECHNOLOGIES
16
1kW, 2007, PROMES-CNRS2
2 Abanades, Stéphane, Patrice Charvin, and Gilles Flamant. “Design and Simulation of a Solar Chemical Reactor for the Thermal Reduction of Metal Oxides: Case Study of Zinc Oxide Dissociation.” Chemical Engineering Science 62, no. 22 (November 2007)
SOLAR REACTOR TECHNOLOGIES
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1kW, 2007, PROMES-CNRS
- Similar design
- Reduced pressure (18kPa)
- Up to 1700K
- ~70 mg/min ZnO
- Limited ZnO conversion
Air-tightness issues, difficult to monitor the reaction (O2 and temperature measurement)
SOLAR REACTOR TECHNOLOGIES
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Zirrus reactor: 10kW, 2008, PSI-ETH 3
Designed for ZnO thermal reduction
3 Schunk, L. O., P. Haeberling, S. Wepf, D. Wuillemin, A. Meier, and A. Steinfeld. “A Receiver-Reactor for the Solar Thermal Dissociation of Zinc Oxide.” Journal of Solar Energy Engineering 130, no. 2 (2008)
Various configurations tested
Similar structure
Improved mechanical stability (ZnO compressed tile)
Straight cavity, dynamic feeder to spread evenly the reacting powder
Window/cavity distance reduced, opening simplified
SOLAR REACTOR TECHNOLOGIES
20
Zirrus reactor: 10kW, 2008, PSI-ETH4
Design of an efficient quenching apparatus
4Gstoehl, D., A. Brambilla, L. O. Schunk, and A. Steinfeld. “A Quenching Apparatus for the Gaseous Products of the Solar Thermal Dissociation of ZnO.” Journal of Materials Science 43, no. 14 (July 2008).
Objective:
Prevent the metastable Zn(g)/O2 mix from condensing and recombine on the walls
A- « hot » zone, temperature above ZnO dissociation (TZnO/Zn)
B- Transitory zone. T< TZnO/Zn, metastable gas mix sheathed by an argon flow (T>870 K)
C- Argon cold flux (298 K)
Cooling rate up to 117 000 K/s, 94% Zn purity, for 100NL/s of argon per g/s of oxide reduced
SOLAR REACTOR TECHNOLOGIES
21
Zirrus reactor: 10kW, 2008, PSI-ETH5
- Reactor window efficiently protected from particles deposition
- Up to 90% of conversion to Zn with the quenching apparatus (less in
“normal” conditions)
- Kinetic analysis of the reaction
- Development and experimental validation of heat transfer models:
- Solar-to-chemical efficiency ~3%. Major losses from water cooling and gaz
quench (46.7%)
- Up-scaling: potential of 50% and 56% of solar-to-chemical conversion for
respectively 100 kWth and 1 MWth (no water cooling, higher temperature
thus reaction rates, losses reduced by geometry optimization)
5 Schunk, L.O., W. Lipiński, and A. Steinfeld. “Heat Transfer Model of a Solar Receiver-Reactor for the Thermal Dissociation of ZnO—Experimental Validation at 10kW and Scale-up to 1MW.” Chemical Engineering Journal 150, no. 2–3 (August 1, 2009)
SOLAR REACTOR TECHNOLOGIES
22
Scaled-up prototype 100 kW, 2013, PSI-ETH6
6 Villasmil, W., M. Brkic, D. Wuillemin, A. Meier, and A. Steinfeld. “Pilot Scale Demonstration of a 100-kWth Solar Thermochemical Plant for the Thermal Dissociation of ZnO.” Journal of Solar Energy Engineering 136, no. 1 (November 8, 2013)
SOLAR REACTOR TECHNOLOGIES
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Scaled-up prototype 100 kW, 2013, PSI-ETH
Multi-material self-supporting insulation
High thermal inertia, good thermal shock resistance, convection losses reduced
Thermal expansion issues treated
Slightly over-pressured, P~86 kPa
Over 1 000 NL/min of argon in the quenching zone
SOLAR REACTOR TECHNOLOGIES
25
Scaled-up prototype 100 kW, 2013, PSI-ETH
Much steeper relation between dilution and purity than for previous reactors:
50% purity obtained for 0.3 g/min of ZnO and 784 NL/min of argon
Introduction
Definition of volatile oxide cycles
Theoretical efficiency
Thermal reduction
Carbothermal reduction
Products reactivity
Solar reactor technologies
Rotating cavity type
Beam-down and other vertical configurations
Carboreduction-specialized reactors
Conclusions
SUMMARY
26
SOLAR REACTOR TECHNOLOGIES
27
1kW, 2011, PROMES-CNRS7
Designed for ZnO and SnO2 thermal reduction
Non-rotating cavity to improve air-tightness
Insulated refractory cavity (30mm) to reduce thermal losses
Short straight path for the hot gases products
Reacting pellet are fed upward
7 Chambon, Marc, Stéphane Abanades, and Gilles Flamant. “Thermal Dissociation of Compressed ZnO and SnO2 Powders in a Moving-Front Solar Thermochemical Reactor.” AIChE Journal 57, no. 8 (August 2011)
SOLAR REACTOR TECHNOLOGIES
30
1kW, 2011, PROMES-CNRS
- Successfully operated under reduced pressure (as low as 15 kPa) for ZnO and SnO2
reduction
- Significant yields obtained for temperature below 1900K (reactions start at 1663K)
- Half of the products deposited in the exit tube, 10% of reduced species nucleation
- Lower Zn compared to SnO
- Reliable reaction monitoring via gaz analysis / temperature measurement of the pellet surface
SOLAR REACTOR TECHNOLOGIES
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GRAFSTRR, 2012, University of Delaware-PSI-ETH8
8Koepf, Erik, Suresh G. Advani, Aldo Steinfeld, and Ajay K. Prasad. “A Novel Beam-Down, Gravity-Fed, Solar Thermochemical Receiver/reactor for Direct Solid Particle Decomposition: Design, Modeling, and Experimentation.” International Journal of Hydrogen Energy 37, no. 22 (November 2012)
Gravity-Fed Solar-Thermochemical Receiver/Reactor
SOLAR REACTOR TECHNOLOGIES
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GRAFSTRR, 2012, University of Delaware-PSI-ETH
Main features: moving bed feeding and stabilized vortex flow-pattern
Introduction
Definition of volatile oxide cycles
Theoretical efficiency
Thermal reduction
Carbothermal reduction
Products reactivity
Solar reactor technologies
Rotating cavity type
Beam-down and other vertical configurations
Carboreduction-specialized reactors
Conclusions
SUMMARY
33
SOLAR REACTOR TECHNOLOGIES
34
Synmet reactor: 5kW, 2003, PSI-ETH9
Combined ZnO reduction / CH4 reforming
9 Kraupl, Stefan, and Aldo Steinfeld. “Operational Performance of a 5-kW Solar Chemical Reactor for the Co-Production of Zinc and Syngas.” Journal of Solar Energy Engineering 125, no. 1 (2003)
ZnO + CH4 = Zn + 2H2 + CO, ΔH1300 K = 446 kJ/mol
High-quality syngas + Zn at limited temperature
ZnO fed axially (~15g/min), CH4 tangentially (0.8 – 3.2 NL/min, pulsed)
Helical gas-particles stream
SOLAR REACTOR TECHNOLOGIES
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Synmet reactor: 5kW, 2003, PSI-ETH
- Complete conversion of ZnO from 1380 K
- Up to 96% of methane conversion at 1676 K (49% at 1380K)
- Maximum thermal/exergy efficiencies 22% / 5.6%
- CO/CO2 ratio in the range 0.08-0.25
Problematic deposition of unreacted ZnO in the reactor
SOLAR REACTOR TECHNOLOGIES
37
Solzinc: 300kW, 2007, SolZinc Eu-project10
10 Wieckert, C., U. Frommherz, S. Kraupl, E. Guillot, G. Olalde, M. Epstein, S. Santen, T. Osinga, and A. Steinfeld. “A 300 kW Solar Chemical Pilot Plant for the Carbothermic Production of Zinc.” Journal of Solar Energy Engineering 129, no. 2 (2007)
SOLAR REACTOR TECHNOLOGIES
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Solzinc: 300kW, 2007, SolZinc Eu-project
Two-cavity reactor (receiver/reactor)
Separated by graphite/SiC
Batch operation
Up to 500 kg of reacting mix ZnO/0.8C
Industrial beech charcoal
SOLAR REACTOR TECHNOLOGIES
43
Solzinc: 300kW, 2007, SolZinc Eu-project
-About 50 kg/h of Zn dust at 1150°C
-Contains 95% Zn, 5% ZnO, below 10µm
-Low CO2 proportion in the off-gas
-Thermal efficiency 30%
Introduction
Definition of volatile oxide cycles
Theoretical efficiency
Thermal reduction
Carbothermal reduction
Products reactivity
Solar reactor technologies
Rotating cavity type
Beam-down and other vertical configurations
Carboreduction-specialized reactors
Conclusions
SUMMARY
44
Solar reactor technologies development status:
- Technical challenges
Severe temperature conditions (levels and gradient)
Air tightness / pressure
Gas quenching / particles collections
- Active research field in the past decade
- Numerous realization, capitalized experience
- Significant prototypes (>100 kWth)
Issues:
- Improving solar-to-chemical efficiency (chemical conversion, thermal efficiency)
- Neutral gas consumption
New cycles? Innovative design? Quenching solutions? Handling of the products ?
CONCLUSIONS
45
Solar Facilities for the European Research Area
Thank You for your attention
Gaël LEVEQUE [email protected]
SFERA II 2014-2017, Solar Reduction 2, 2014/06/27, Odeillo, France