1

SOLAR THERMO-

Thermochemical cycles based

on the ZnO/Zn or SnO2/SnO redox couples :

Kinetic characterizations and study of solar reactors

March, the 25th, 2011, ETH Zürich

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

Marc CHAMBON, Stéphane ABANADES*, Gilles FLAMANTPROMES laboratory (France)

Post-doc on the ZIRRUS Scale-up Project at PSI

HYDROLYSIS

CONCLUSIONS & OUTLOOK

1*stephane.abanades@promes.cnrs.fr

SOLAR THERMO-

WATER-SPLITTING

Requirement : E = W + Q

• Work (W) : Dissociation via water electrolysis

• Heat (Q) : Dissociation via thermal agitation

H2O(l) H2 + ½ O2

At t t P ΔG ΔH TΔS

ΔH0 = 286 kJ/molΔS0 = 163 J/mol/K

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

At constant P : ΔG = ΔH - T.ΔS

If ΔS > 0 : work requirement ↓ if T ↑

HYDROLYSIS

CONCLUSIONS & OUTLOOK

2

Thermolysis ( 2400K < )

Drawbacks :

• Refractories resistance• O2 and H2 blended

SFERA Winter School Solar Fuels & Materials Page 238

2

SOLAR THERMO-

THERMOCHEMICAL CYCLESSeveral chemical steps (i) equivalent to water-splittingEndothermal reactions with ΔS > 0 favored if T ↑The opposite for exothermal ones

ΔSH2OH2 + ½ O2 = ∑ΔSpositive + ∑ΔSnegativei i

Thermodynamic system

P ibl ifCHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

ΔS limited (< 100 J/mol/K )

TC ↓ ΔSmin ↑ Required number of steps ↑

4 5 6 t / h b idi d Thermolysis2 steps 3 steps

Process possible if :

∑ΔSpositive ≥ ΔSmin

with ΔSmin = ΔG0H2OH2 + ½ O2 / (TC – T0)i

HYDROLYSIS

CONCLUSIONS & OUTLOOK

3

4, 5, 6 steps / hybridized

500 1000 1500 20003000K

Concentrated solar reactorsNuclear reactors

Thermolysisp

SOLAR THERMO-

2-STEP THERMOCHEMICAL CYCLES

Solar step : oxide dissociation

ZnO Zn(g) + ½ O2

SnO2 SnO(g) + ½ O2

ΔS = 196 to 227 J/mol/KΔH = 456 to 530 kJ/mol

TC = 2000 K ΔSmin = 140 J/mol/K

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS ½ O

Water-splitting step : H2 generation via reduced species hydrolysis

Zn + H2O(g) ZnO + H2

SnO + H2O(g) SnO2 + H2

ΔS = -55 J/mol/KΔH = -110 to -48 kJ/mol

HYDROLYSIS

CONCLUSIONS & OUTLOOK

4

recycling

½ O2DISSOCIATION(1500-2300K)

MxOy = MxOy-1 + 1/2 O2

H2O

MxOy

H2

HYDROLYSIS(500-1000K)

MxOy-1 + H2O = MxOy + H2

MxOy-1

SFERA Winter School Solar Fuels & Materials Page 239

3

SOLAR THERMO-

½ O2DISSOCIATION(1500-2300K)

MxOy = MxOy-1(g) + 1/2 O2 MxOy-1(g)

« VOLATILE » OXIDES THERMOCHEMICAL CYCLES

(1-f)recombination

MxOy

fcondensation

MxOy-1(s)

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS• Refractories : mechanical resistance & chemical stability

HYDROLYSIS(500-1000K)

MxOy-1 + H2O = MxOy + H2

condensation

HYDROLYSIS

CONCLUSIONS & OUTLOOK

5

• Recombination solar step efficiency ↓Solution : quenching and dilution (neutral gas) of the product gases

dilution ratio = neutral gas flow-rate / MxOy-1 flow-rate

• Condensation particles with high specific areas fast hydrolysis

SOLAR THERMO-

MOTIVATIONS AND TASKS

Ultimate goal : technico-economical study :1) Behavior of these cycles in reality2) Solar reactors with commercially affordable materials

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

1) reactivity of the various steps :experiments and modeling to determine kinetic parameters

2) dissociation at slightly lower T allowing these materials :building and operating solar reactors operating under reduced pressure

HYDROLYSIS

CONCLUSIONS & OUTLOOK

6

SFERA Winter School Solar Fuels & Materials Page 240

4

SOLAR THERMO-

REACTOR FOR STUDYING THE RECOMBINATIONMxOy-1 + ½ O2 MxOy

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS0.5

0.6

0.7

g.m

in-1)

SnO at Patm SnO at P = 0.1 bar SnO at P = 0.01 bar ZnO at Patm ZnO at P = 0.01 bar

Pellet T not measured Estimated via the kinetics

HYDROLYSIS

CONCLUSIONS & OUTLOOK

7

• Parabola : 1 kW, C = 15 000• ZnO or SnO2 pellet : 1 g• N2 : 0.5-4 NL/min• Pressure : 3-85 kPa• Duration : ≈ 30 min 1200 1250 1300 1350 1400 1450 1500 1550

0.0

0.1

0.2

0.3

0.4

Ma

ss lo

ss (

mg

Temperature (°C)Source: Charvin et al., 2008

• Weight variation + duration mean dissociation rate : r (mol/s)• Collected powder analyzed reduced species (MxOy-1) fraction : fmol

SOLAR THERMO-

REDUCED SPECIES FRACTIONS

fmol = nMxOy-1 / (nMxOy-1 + nMxOy)

n : moles

• XRD analysis + calibration curves

Methods

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

• XRD analysis + calibration curves

SnO 110

29.9°

26.6°

SnO2 110

HYDROLYSIS

CONCLUSIONS & OUTLOOK

8

• O2 online analysis at the solar reactor output + mole balance (□)

• Full oxidation via thermogravimetry (○)

• Chemical method (destructive one) (▲) : Zn + 2 HCl(aq) ZnCl2(aq) + H2

SFERA Winter School Solar Fuels & Materials Page 241

5

SOLAR THERMO-

MODELING THE RECOMBINATION REACTION

zdnO2 = -(S.dz).k.CO2

x.CMxOy-1y with k = A0

.exp(-Ea/RT)

C : concentrations x, y : kinetic ordersA0, Ea : kinetic parameters

dnO2 = Ftot.dYO2 Ftot : total flow-rate

O2 balance on a reactor slab (section S)

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

Assumption : plug-flow reactor

Y : mole fraction C = Y.(P/RT)YO2 = YMxOy-1/2

dYO2/dz = - A.YO2n with A = (S/Ftot).2y.k.(P/RT)n

n = x + y : global order

∫YO2,i

YO2,o

Integration : dYO2/YO2n = -A.z with YO2,i = r/(2.Ftot) and YO2,o = fmol

.YO2,i

fmol = [1+(n-1).B.rn-1]1/(1-n) Approximation : fmol ≈ 1 – B.rn-1

HYDROLYSIS

CONCLUSIONS & OUTLOOK

9r0.4

Maximum correlations (R2) for n ≈ 1.1 (Zn) and n ≈ 1.4 (SnO)

SOLAR THERMO-

ESTIMATING THE ACTIVATION ENERGIES

z

T

T

Previous experiments

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

Temperature profile function of Tr, Tr depending on r

Tr

To

Assumption : T(z) = 1 / (1/Tr + K.z)

Source : Badie et al., 2005

g(Tr) = ln(∂f(Tr)/∂Tr) + (2+n).ln(Tr) = ln(-C) - Ea/(RTr)

Intermediary mapping f : Tr [1 – To/Tr]/To.r1-n.[fmol

1-n – 1] = C. T-(2+n).exp(-Ea/RT).dT∫Tr

TO

HYDROLYSIS

CONCLUSIONS & OUTLOOK

10

2 parameters for characterizing the recombination

421.4SnO

321.1Zn

Ea (kJ/mol)n

SFERA Winter School Solar Fuels & Materials Page 242

6

SOLAR THERMO-

ROTARY CAVITY SOLAR REACTORZnO Zn(g) + ½ O2 TC ≈ 2000K

Goals : • Assessment of a solar reactor concept (1 kW)• Continuous injection of the reactant (micronic powder)• Controlled atmosphere (reduced P, neutral gas)

• ZnO : ≈ 70 mg/minCHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

Source : Charvin et al., 2007

ZnO : 70 mg/min• N2 : 5 NL/min• Pressure : 18 kPa <• Duration : ≈ 30 min

HYDROLYSIS

CONCLUSIONS & OUTLOOK

11

Reduced P : ↓ dissociation T 2 advantages :• Radiative losses T4 (-10% if TC ↓ 50K (-2,5%))• Standard refractories can be used

SOLAR THERMO-

SCREENING OF REFRACTORING CAVITIES

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

Chosen cavity

tests

Profiles of the investigated cavities

HYDROLYSIS

CONCLUSIONS & OUTLOOK

12

• Alumina tube (Tmax = 2223K) + alumino-silicate insulation (Tmax = 1873K)• No significant mechanical damages nor chemical interactions for sintered alumina

(≠ thermodynamic previsions)

ZirconiaAluminaAlumino-silicate (fibers)

SFERA Winter School Solar Fuels & Materials Page 243

7

SOLAR THERMO-

ZnO DISSOCIATION RUNS

Oxide temperature during the injection

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

• ≈ 0.5 g of product collected• Recovery yield (filter) : 5-20%

Tequilibrium ≈ 1700K, Ttube ≈ 1100K

HYDROLYSIS

CONCLUSIONS & OUTLOOK

13

Limitations

• Low precision of the mesures (temperatures & O2 concentration)• Very low production

Recovery yield (filter) : 5-20%• Dilution ratio (FN2/FZn) : 300-500

SOLAR THERMO-

Cavity and insulation parts

MOVING-FRONT SOLAR REACTOR

• Characterizing the reacting zone :- reactant injected as stacked pellets (8-mm cylinders)- T measured at several places and O2 fraction analysis at the output

• Producing enough powder for the next steps

Goals :

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSISHYDROLYSIS

CONCLUSIONS & OUTLOOK

14

• Neutral gas : 4-5 NL/min• Pressure : 15-20 kPa• Duration : ≈ 30 min

SFERA Winter School Solar Fuels & Materials Page 244

8

SOLAR THERMO-

THERMAL SIMULATIONS

Temperature profile Thermal losses distribution

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSISHYDROLYSIS

CONCLUSIONS & OUTLOOK

15

Tcavity ≈ 2000K Tcavity/insulation ≈ 1800KTinsulation ≈ 1000K

SOLAR THERMO-

EXPERIENCES

Tcavity(pyro) ≈ 1900K Tcavity/insulation ≈ 1700K

Thermal equilibrium

ZnO dissociation

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

cavity/insulation

Tinsulation ≈ 1100K

Dissociations start at Tpyro ≈ 1700K SnO2 dissociation

HYDROLYSIS

CONCLUSIONS & OUTLOOK

16

Measures sufficient for estimating the dissociation kinetics?

SFERA Winter School Solar Fuels & Materials Page 245

9

SOLAR THERMO-

KINETIC STUDIES FOR THE DISSOCIATION REACTION I

Assumption : negligible recombination at the beginning

Reactivity characterized by the O2 fraction dynamic measure (while heating)

ZnO : ablative mode

FZn = Sirr.k with k = A0

.exp(-Ea/RT)

F : molar flow-rate Sirr : ZnO irradiated surface A0, Ea : kinetic parametersCHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

F : molar flow rate Sirr : ZnO irradiated surface A0, Ea : kinetic parameters

ln(YO2) = ln(Sirr.A0/Ftot) - Ea/RT Y : mole fraction

HYDROLYSIS

CONCLUSIONS & OUTLOOK

17

Ea = 380±16 kJ/mol et A0 = (2±1)x107 kg.m-2.s-1

Agreement with other studies (PSI/ETH)

SOLAR THERMO-A time t : T profile known from Tpyro and 1-D model

SnO2 (volume reaction) : 1rst order assumption dmi / dt = k.mi

KINETIC STUDIES FOR THE DISSOCIATION REACTION II

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

- Profile discretization (20K) i slabs (mi and Ti)

- Calculation :

- Minimizing the measure/model values Ea and A0

( 1673K –> dissociation begins)

FO2 = ½ ∑(mi/MSnO2).A0.exp(-Ea/RTi) (mol/s)

i

HYDROLYSIS

CONCLUSIONS & OUTLOOK

18Ea = 385±15 kJ/mol et A0 ≈ 2x106 s-1

SFERA Winter School Solar Fuels & Materials Page 246

10

SOLAR THERMO-

CHARACTERIZING THE PRODUCTS

3 types of deposits

Mass of collected deposits

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

Powder fractions (B & C) : 1/3-1/2

Several grams in ≈ 30 min

Reduced species fractionsZn : ≈ 40%SnO : ≈ 70%

Respective reduced species fractions

HYDROLYSIS

CONCLUSIONS & OUTLOOK

19

Dilution ratio: 50-100

Thermochemical efficiency :(FMxOy-1

. ∆Hheating+diss/Qsolar) = 2-3%

Improvements // 1rst reactor

ZnO SnO2

SOLAR THERMO-

MORPHOLOGICAL STUDIES

Macro Micro Nano

SnO

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

Zn

HYDROLYSIS

CONCLUSIONS & OUTLOOK

20

Specific area (BET) :Zn : 15-20 m2/g 40-55 nmSnO : 40-60 m2/g 15-25 nm

Nanoparticles agglomerates (~10 microns)

High reactivity expected for the hydrolysis step

SFERA Winter School Solar Fuels & Materials Page 247

11

SOLAR THERMO-

FIXED-BED HYDROLYSIS REACTORMxOy-1 + H2O MxOy + H2

• Fixed bed : ≈ 0.1 g of powder• Regulation : isoT, heating rates• Ar : 0.18 NL/min, Patm

• H2O : 0.1-0.05 g/min (≈ 25 %mol)CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

2 g ( )

Source : Charvin et al., 2007

Zn SnO

HYDROLYSIS

CONCLUSIONS & OUTLOOK

21

I : injection time

• Quasi-total reactions, high reactivities for Zn• Fast mode followed by a slow one

SOLAR THERMO-

KINETIC MODEL

Oxide layer growing over the reduced species particles diffusion processes hindered Reactivity is getting slower

Previous experiment

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

Source: Funke et al., 2008

hindered Reactivity is getting slower

Specific area after hydrolysis :Zn : ≈ 10 m2/g

HYDROLYSIS

CONCLUSIONS & OUTLOOK

22

SnO : ≈ 30 m2/g

+ No significant morphological variations at the nano-scale (ESEM)

Limited sintering (none between aggregates)

SFERA Winter School Solar Fuels & Materials Page 248

12

SOLAR THERMO-

DETERMINING THE HYDROLYSIS KINETICSdα / dt = k.CH2O

x.f(α) with k = A0.exp(-Ea/RT)

Empirical kinetic model : f(α) = (1-α)n

With heating rates

TI : injection temperatureβ : heating rate (K/s)

Zn

isoT conditionsln(dαI/dt) = cst - Ea/RT

CHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

SnO[1/(n-1)].[(1 – α)1-n - (1 – αI)1-n] = k. (tα-tI)

Zn : Ea ≈ 81 kJ/mol ; SnO : Ea ≈ 124 kJ/mol

HYDROLYSIS

CONCLUSIONS & OUTLOOK

23Zn : n ≈ 2.0 ; A0 ≈ 1.6x105 s-1

SnO : n ≈ 1.6 ; A0 ≈ 2.0x104 s-1

Zn : n ≈ 1.8 ; A0 ≈ 2.0x105 s-1

SnO : n ≈ 1.5 ; A0 ≈ 2.2x104 s-1

SOLAR THERMO-

CONCLUSIONS

• Kinetic study of the recombination via an inverse method

• 2 solar reactor built and operated with “common” refractories for continuous dissociation of volatile oxides under reduced pressure :- Fast way to estimate the kinetic parameters for the dissociation reaction- Powders with reduced species signficantly produced- Reactor properties evaluatedCHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

• Kinetic study of the hydrolysis reaction with these powders

- Reactor properties evaluated

OUTLOOK

• Hydrolysis reactivity is a double-edged sword :fast hydrolysis for Zn but significant recombination during the solar step

HYDROLYSIS

CONCLUSIONS & OUTLOOK

24

• Implementing and studying quenching devices

• Influence of the morphological properties on the hydrolysis kinetics

• …

SFERA Winter School Solar Fuels & Materials Page 249

13

SOLAR THERMO-

ACKNOWLEDGEMENT

• Financing : CNRS and Languedoc-Roussillon council

• Technical support : Roger GARCIACHEMISTRY

MOTIVATIONS & TASKS

RECOMBINATION

DISSOCIATION

HYDROLYSIS

pp g

• Characterizations : Dimitri GORAND, Eric BECHE (PROMES)Christine ROLLAND, Ange NZIHOU (EMAC) Anne JULBE, Julius MOTUZAS (IEM)

• Assistance : Alexis SOLIGOFederico GUTIERREZ-CORIA

HYDROLYSIS

CONCLUSIONS & OUTLOOK

25

And YOU for your attention!

SFERA Winter School Solar Fuels & Materials Page 250