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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
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