Kinetics of ferrites for solar thermochemical fuel production

Transcription

1 International Workshop on Solar Thermochemistry 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 September, Jülich, Germany

2 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

3 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 Example of an off-stoichiometric two-step cycle: 1 st step: Redox material at high temperature releases oxygen from the surface Reduction: NiFe 2 O 4 NiFe 2 O 4 δ + δ 2 O 2 2 nd step: Τhe redox material captures oxygen from CO 2 \H 2 O streams flowing through Oxidation: NiFe 2 O 4 δ + δh 2 O NiFe 2 O 4 + δh 2 NiFe 2 O 4 δ + δco 2 NiFe 2 O 4 + δco CO CO 2

4 Thermochemical WS/CDS kinetic analysis This study builds on previous WS kinetic model with Nickel ferrite proposed by Kostoglou et.al., Carbon monoxide, for the case of CDS, follows a similar path The advance of this method over the previous model capable of describing multicycle operations: consecutive cycles over an operational period 16 consecutive cycles considers different formulations of the redox material: powder and monolithic structures redox powder monolithic body 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 (2014):

5 Two regions particle The nickel ferrite particle is considered to consist of two distinct regions: the outer region (surface) and the inner region (bulk) Surface oxygen atoms, φ (mol/g) Inner oxygen atoms, ψ (mol/g) Oxygen empty sites (vacancies) Oxidation Reduction R CDS CO CO 2 R T R TR O 2 bulk R T R WS H 2 ψ H 2 O ½ O 2 φ Powder grain

6 Thermochemical WS/CDS kinetic analysis Two mechanisms are considered to take place in the ferrite during WS/CDS: gas-solid reaction and diffusion Reaction R H2 = k WS x w n1 φ tot φ n2 R CO = k CDS x CO φ tot φ R O2 = k TR φ R H2, R CO, R O2 : production rates Model equations 2 Diffusion R T = ±k m (Kψ φ) dφ dt = R H2,CO + R T dφ dt = 2R O2 + R T dψ dt = R T φ, ψ μmoles/g redox φ tot Evolution of variables φ, ψ ψ tot R T k WS, k CDS, k TR k m : diffusion rate : reaction coefficients : diffusion coefficient 1 st cycle time, min x w, x CO K : molar fractions : partition coefficient 2.Dimitrakis, Dimitrios, Syrigou, Maria, Lorentzou, Souzana, Kostoglou, Margaritis and Konstandopoulos, Athanasios. "On kinetic modelling for solar redox thermochemical H 2 O and CO 2 splitting over NiFe 2 O 4 for H 2, CO and syngas production.", Physical Chemistry Chemical Physics (2017): under review

7 Variable φ tot The partition coefficient K, controls the balance between the two regions of the particle K = φ tot ψ tot Reduction factor (δ) is the maximum number of oxygen atoms that can be released from the ferrite at a given temperature. moles of NiFe 2 O 4 aδ = m φ tot + Κ φ tot φ tot = δ 2 Mr redox K WS K WS + 1 The variable φ tot is the sum of oxygen atoms that have been released from the surface (maximum value of variable φ) once the reduction reaction is completed. δ Mr redox K WS : reduction factor : molecular weight : partition coefficient at WS temperature

8 Data analysis 2.Dimitrakis, Dimitrios, Syrigou, Maria, Lorentzou, Souzana, Kostoglou, Margaritis and Konstandopoulos, Athanasios. "On kinetic modelling for solar redox thermochemical H 2 O and CO 2 splitting over NiFe 2 O 4 for H 2, CO and syngas production.", Physical Chemistry Chemical Physics (2017): under review Using an explicit Euler method and fitting the differential equations to the smoothed experimental data, the values of the kinetic parameters are obtained 2 Kinetic constants of WS as a function of temperature ( o C) k WS = T T k mspl = e 2484/T K TR = T Kinetic constants of CDS as a function of temperature ( o C) k CDS = 10 8 T T K CDS = T Kinetic constants of TR as a function of temperature ( o C) k TR k mtr K TR = 1626 e 2181/T = e 40910/T = e T

9 Model evaluation Water Splitting mean deviation 9% Carbon Dioxide Splitting mean deviation 15% Experiments performed at: Splitting 1000 o C Reduction 1350 o C 10g NiFe 2 O 4 synthesized by SHS 3 x w = 0.32 x CO = 1 Experiments performed at: Splitting 1100 o C Reduction 1400 o C 3. Agrafiotis, et al. "Solar water splitting for hydrogen production with monolithic reactors." Solar Energy 79.4 (2005):

10 Model evaluation for the case of co-feeding and monolithic structures The evolution of φ is much faster compared to singlegas feeding mean deviation 11% Co-feeding dφ dt = R H 2 + R CO + R T Inlet stream: 32% H 2 O 16% CO 2 72% N 2 Structured reactor component The developed model is also capable of describing WS/CDS performed with extruded NiFe 2 O 4 monoliths of various cells per square inch (cpsi). x w =0.64 mean deviation 7% Consecutive WS cycles over two operational days, employing a 200cpsi monolith

11 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 Optimum time steps over an 8h on-sun shift: Splitting Step Reduction Step 17 min 65 min (including 20 min heat-up and 20 min cool-down) This analysis allows to calculate: i. the production yields of given masses of redox material ii. the required ferrite mass by assuming targeted H 2 productions 80kg NiFe 2 O 4 for 1kg H 2 /week (production target of Hydrosol PLANT)

12 Non-isothermal oxidation Test cases under various operational conditions are simulated, exploring their impact on products yield. Isothermal splitting step Non-isothermal splitting step T RED = 1400 o C T WS = 1100 o C T RED = 1400 o C T WS = [ o C] higher splitting temperature range compared to the constant WS step shorter dead periods/ longer oxidation time per cycle Increased hydrogen production

13 Conclusions The kinetic model is capable of simulating multicycle WS/CDS over NiFe 2 O 4 powder and monolithic structures with adequate accuracy The overall mean deviation of the model lies under 15% The optimum time steps for a given scenario have been identified and maximum hydrogen production rate (102 μmoleh 2 /g/h) is achieved Non-isothermal oxidation could be applied for increasing hydrogen/carbon monoxide productivity Next Step The kinetic model will be incorporated in the reactor model. This can lead to: optimized reactor design enhanced performance This kinetics derivation methodology can be adopted for other redox materials used in thermochemical WS and CDS following an off-stoichiometry two-step cycle maximum efficiency of the system

14 Acknowledgements This work has been supported by: European Research Council (ERC) Advanced Grant Project ARMOS (ERC-2010-AdG ARMOS) FCH-JU project Hydrosol PLANT (FCH-JU ) Thank you for your attention!