Kinetics of ferrites for solar thermochemical fuel production

Maria Syrigou, Dimitris Dimitrakis, Souzana Lorentzou, Margaritis Kostoglou and Athanasios G. Konstandopoulos

Aerosol & Particle Technology Laboratory, APTL/CERTH

International Workshop on Solar Thermochemistry

12-14 September, Jülich, Germany

Outline

Developing the kinetic model for thermochemical water splitting and carbon

dioxide splitting

Model evaluation (model – experiments comparison)

Parametric analysis for product yield optimization

Conclusions

Thermochemical Water / Carbon Dioxide Splitting

Solar thermochemical Water Splitting / Carbon Dioxide Splitting (WS/CDS) is a very interesting option for a sustainable energy future.

Current solar pathways are based on a 2-step cycle employing redox materials

1st step: Redox material at high temperature releases oxygen from the surface

2nd step: Τhe redox material captures oxygen from CO2\H2O streams flowing through

Reduction: NiFe2O4 NiFe2O4−δ +δ

2O2

Oxidation: NiFe2O4−δ + δH2O NiFe2O4 + δH2

NiFe2O4−δ + δCO2 NiFe2O4 + δCO

Example of an off-stoichiometric two-step cycle:

CO2 CO

Thermochemical WS/CDS kinetic analysis

This study builds on previous WS kinetic model with Nickel ferrite proposed by Kostoglou et.al., 20141

Carbon monoxide, for the case of CDS, follows a similar path

16 consecutive cycles

redox powder monolithic body

The advance of this method over the previous model

• capable of describing multicycle operations: consecutive cycles over an operational period

• considers different formulations of the redox material: powder and monolithic structures

1.Kostoglou, Margaritis, Souzana Lorentzou, and Athanasios G. Konstandopoulos. "Improved kinetic model for water splitting thermochemical cycles using Nickel Ferrite." International Journal of Hydrogen Energy 39.12 (2014): 6317-6327.

Two regions particle

The nickel ferrite particle is considered to consist of two distinct regions: the outer region (surface) and the inner region (bulk)

RTR

½ O2

bulk RT

RT

RWS

Oxidation Reduction

H2

RCDS

CO2

φ

CO

O2

H2O

ψ

Surface oxygen atoms, φ (mol/g)

Oxygen empty sites (vacancies)

Inner oxygen atoms, ψ (mol/g)

Powder grain

Model equations2

Two mechanisms are considered to take place in the ferrite during WS/CDS: gas-solid reaction and diffusion

Reaction Diffusion

𝑅𝐻2 = 𝑘𝑊𝑆 · 𝑥𝑤𝑛1 · 𝜑𝑡𝑜𝑡 − 𝜑

𝑅𝐶𝑂 = 𝑘𝐶𝐷𝑆 · 𝑥𝐶𝑂𝑛2 · 𝜑𝑡𝑜𝑡 − 𝜑

𝑅𝑂2 = 𝑘𝑇𝑅 · 𝜑

𝑅𝑇 = ±𝑘𝑚 · (𝐾𝜓 − 𝜑)

𝑑𝜑

𝑑𝑡 = 𝑅𝐻2,CO + 𝑅𝑇

𝑑𝜑

𝑑𝑡 = −2𝑅𝑂2 + 𝑅𝑇

𝑑𝜓

𝑑𝑡 = −𝑅𝑇

RH2, RCO, RO2 : production rates

RT : diffusion rate

kWS, kCDS, kTR : reaction coefficients

km : diffusion coefficient

xw, xCO : molar fractions

K : partition coefficient

Thermochemical WS/CDS kinetic analysis

φ, ψ

μm

ole

s/g

redox

Evolution of variables φ, ψ

time, min

𝜓𝑡𝑜𝑡 𝜑𝑡𝑜𝑡

1st cycle

2.Dimitrakis, Dimitrios, Syrigou, Maria, Lorentzou, Souzana, Kostoglou, Margaritis and Konstandopoulos, Athanasios. "On kinetic modelling for solar redox thermochemical H2O and CO2 splitting over NiFe2O4 for H2, CO and syngas production.", Physical Chemistry Chemical Physics (2017): under review

Variable φtot

Reduction factor (δ) is the maximum number of oxygen atoms that can be released from the ferrite at a given temperature.

δ : reduction factor

Mrredox : molecular weight

KWS : partition coefficient at WS temperature

The partition coefficient K, controls the balance between the two regions of the particle

𝑎𝛿 = 𝑚 𝜑𝑡𝑜𝑡 + 𝛫 𝜑𝑡𝑜𝑡 𝜑𝑡𝑜𝑡 =𝛿

2 𝑀𝑟𝑟𝑒𝑑𝑜𝑥∙𝐾𝑊𝑆

𝐾𝑊𝑆 + 1

The variable φtot is the sum of oxygen atoms that have been released from the surface (maximum value of variable φ) once the reduction reaction is completed.

𝐾 =𝜑𝑡𝑜𝑡𝜓𝑡𝑜𝑡

moles of NiFe2O4

Data analysis

Using an explicit Euler method and fitting the differential equations to the smoothed experimental data, the values of the kinetic parameters are obtained2

2.Dimitrakis, Dimitrios, Syrigou, Maria, Lorentzou, Souzana, Kostoglou, Margaritis and Konstandopoulos, Athanasios. "On kinetic modelling for solar redox thermochemical H2O and CO2 splitting over NiFe2O4 for H2, CO and syngas production.", Physical Chemistry Chemical Physics (2017): under review

Kinetic constants of WS as a function of temperature (oC)

𝑘WS = 3 ∙ 10−8 𝑇2 − 7 ∙ 10−5 𝑇 + 0.0435

𝑘𝑚𝑠𝑝𝑙 = 1.8 ∙ 10−3 ∙ 𝑒−2484/𝑇

𝐾𝑇𝑅 = 6.5 ∙ 10−31 ∙ 𝑇9.969

Kinetic constants of TR as a function of temperature (oC)

𝑘𝑇𝑅 = 1626 ∙ 𝑒−2181/𝑇

𝑘𝑚𝑇𝑅 = 8.8 ∙ 108 ∙ 𝑒40910/𝑇

𝐾𝑇𝑅 = 3.5 ∙ 10−7 ∙ 𝑒0.0098𝑇

Kinetic constants of CDS as a function of temperature (oC)

𝑘CDS = 10−8 𝑇2 − 2 ∙ 10−5 𝑇 + 0.014

𝐾CDS = 3.16 ∙ 10−34 ∙ 𝑇10.931

Model evaluation

Experiments performed at: Splitting 1100o C Reduction 1400o C

Water Splitting Carbon Dioxide Splitting mean deviation 15% mean deviation 9%

10g NiFe2O4 synthesized by SHS3

3. Agrafiotis, et al. "Solar water splitting for hydrogen production with monolithic reactors." Solar Energy 79.4 (2005): 409-421.

Experiments performed at: Splitting 1000o C Reduction 1350o C

xCO = 1 xw = 0.32

Model evaluation for the case of co-feeding and monolithic structures

The developed model is also capable of describing WS/CDS performed with extruded NiFe2O4 monoliths of various cells per square inch (cpsi).

The evolution of φ is much faster compared to single-gas feeding

Consecutive WS cycles over two operational

days, employing a 200cpsi monolith

mean deviation 7% mean deviation 11%

Inlet stream: 32% H2O

16% CO2

72% N2

xw =0.64

Co-feeding

𝑑𝜑

𝑑𝑡 = 𝑅𝐻2 + 𝑅𝐶𝑂 + 𝑅𝑇

Structured reactor component

Time optimization

For a given operational period: Short duration of WS step --> more cycles are conducted --> larger amounts of hydrogen are produced Longer duration of WS step --> time for the reaction to be completed --> more hydrogen is being produced per cycle These competitive components constitute the time optimization relationship

This analysis allows to calculate: i. the production yields of given masses of redox material ii. the required ferrite mass by assuming targeted H2 productions

80kg NiFe2O4 for 1kg H2/week (production target of Hydrosol PLANT)

Optimum time steps over an 8h on-sun shift:

Splitting Step 17 min

Reduction Step 65 min (including 20 min heat-up and 20 min cool-down)

Non-isothermal oxidation

Test cases under various operational conditions are simulated, exploring their impact on products yield.

Increased hydrogen production

Isothermal splitting step

TRED = 1400o C

TWS = [1300-1100-1300o C]

Non-isothermal splitting step

higher splitting temperature range compared to the constant WS step

shorter ‘dead’ periods/ longer oxidation time per cycle

TRED = 1400o C

TWS = 1100o C

Conclusions

• The overall mean deviation of the model lies under 15%

The kinetic model will be incorporated in the reactor model.

This can lead to:

Next Step

This kinetics derivation methodology can be adopted for other redox materials used in thermochemical WS and CDS following an off-stoichiometry two-step cycle

• Non-isothermal oxidation could be applied for increasing hydrogen/carbon monoxide productivity

• The optimum time steps for a given scenario have been identified and maximum hydrogen production rate (102 μmoleH2/g/h) is achieved

• The kinetic model is capable of simulating multicycle WS/CDS over NiFe2O4 powder and monolithic structures with adequate accuracy

optimized reactor design

enhanced performance

maximum efficiency of the system

Thank you for your attention !

Acknowledgements

This work has been supported by: • European Research Council (ERC) Advanced Grant Project ARMOS (ERC-2010-AdG 268049- ARMOS)

• FCH-JU project Hydrosol PLANT (FCH-JU-2012- 1)