International Workshop on Solar Thermochemistry September 12-14, 2017, Ju lich, Germany

Transcription

1 International Workshop on Solar Thermochemistry September 12-14, 2017, Ju lich, Germany The challenge of optimum combination between reactivity and cyclic structural stability of materials for thermochemical storage systems and a strategy towards this goal G. Karagiannakis, C. Pagkoura, K. Sakellariou, N. Tsongidis, S. Lorentzou, A. G. Konstandopoulos 1 Aerosol & Particle Technology Laboratory, CPERI/CERTH, Thessaloniki, Greece 2 Department of Chem. Eng., Aristotle University, Thessaloniki, Greece

2 Outline Overview of thermal/thermochemical storage systems for CSP Main aspects of structured materials for 2 indicative TCS schemes Structured materials development per scheme Examples of structured materials in TCS reactors under near realistic conditions Conclusions & future perspectives

3 The importance of energy storage in CSP plants Thermal Energy Storage (TES) integration potential: Main advantage of CSP c.f. other renewable energy technologies On-sun conditions: surplus heat diverted to a storage system Off-sun operation: stored heat to power block cycle prolonged power production Heat transfer Fluid (HTF) Heat Storage Medium (HSM) TES general categories: i) sensible heat; ii) latent heat; iii) thermochemical heat

4 Thermochemical Energy Storage (TCS) for solar-thermal processes The main principle of TCS Exploitation of reversible chemical reactions for the controlled storage (endothermic step) & release (exothermic step) of available (solar or other) energy Examples: Co 3 O 4 /CoO, CaO/Ca(OH) 2, CaO/CaCO 3, Mn 2 O 3 /Mn 3 O 4, Typically gas-solid reactions Main advantages Potential for very high energy density: up to 3 MJ/kg Heat released at a relatively constant temperature (reaction equilibrium) Potential for longer term energy storage cf. sensible heat storage Main challenges Potential irreversible effects or cyclic loss of performance The need to store/recycle gaseous reactants Cyclic phase changes potentially detrimental to structural/thermo-mechanical stability

5 The building blocks of future TCS reactors Examples of thermo-mechanical stability loss Co 3 O 4 /CoO (fresh) Co 3 O 4 /CoO (after 10 redox cycles) CaO/Ca(OH) 2 (fresh) CaO/Ca(OH) 2 (after 10 cycles) CaO/Ca(OH) 2 (fresh) CaO/Ca(OH) 2 (after 2 cycles) Why structural materials? To overcome the adverse effects of multi-cyclic phase changes, one needs to stabilize the material via use/formation of a strong binder/inert matrix

6 The case of redox thermochemical schemes: the example of Co 3 O 4 /CoO couple Co 3 O 4 3CoO + 0.5O 2(g), ΔH = 205 kj/mol react, T eq. = 894 o C

7 Honeycomb structures Lab-scale structures prepared by extrusion & slurry coating: 30 mm x 30 mm Screening of various additives to improve cyclic thermo-mechanical stability Ce 2 O 3, YSZ, MgO, SiC, Fe 2 O 3, Al 2 O 3, Mn 3 O 4, SiO 2 Two approaches qualified: 10 cycles 10 cycles Co 3 O 4 Al 2 O 3 extruded honeycomb Coated cordierite honeycomb

8 Visualization of redox multi-cycling effect on structural stability (I) Extruded honeycombs - SEM Co 3 O 4 fresh Co 3 O 4 after 52 cycles Co 3 O 4 Al 2 O 3 fresh Co 3 O 4 Al 2 O 3 after 104 cycles Formation of cobalt aluminate (CoAl 2 O 4 ) in combination with adequate wall thickness mitigated structural deformation

9 Visualization of redox multi-cycling effect on structural stability (II) Coated honeycombs - SEM Fresh coated honeycomb Coated honeycomb after 116 cycles Coating layer swelling was profound but: No appreciable detachment detected No cycle-to-cycle significant increase in pressure drop by gas flow

10 What about redox performance? Multi-cyclic evaluation w.r.t. O 2 released/consumed under air flow ( o C) Co 3 O 4 extruded Co 3 O 4 Al 2 O 3 extruded μmol O 2 /g Co 3 O cycle number Co 3 O 4 coated cordierite

11 Scaling-up for further testing Scaled-up extrusion Extrusion in progress Green bodies after drying After calcination Fall-back option: coating of cordierite honeycombs Calcination of coated segments channels close-up photo

12 Semi-pilot reactor at a solar platform (DLR, Juelich Solar Tower) Tescari, S. et al, 2017, Experimental evaluation of a pilot-scale thermochemical storage system for a concentrated solar power plant, Applied Energy 189, Karagiannakis, G. et al, 2016, Thermochemical storage for CSP via redox structured reactors/heat exchangers: The RESTRUCTURE project, AIP Conf. Proceedings 1850, (2017); doi:

13 The case of CaO/Ca(OH) 2 couple Ca(OH) 2 CaO + H 2 O (g), ΔH = 100 kj/mol react, T eq. = 518 o C

14 Structured composite particles Small pellets (< 2mm) for fluidized bed reactors Solid (or slurry) mixing of CaO precursor & additive wetting rotary forces for shaping slow drying calcination Additives tested: Al 2 O 3, SiO 2, silicates of alkali/alkaline earth metals, clays, Qualified approach: CaO-kaolinitic clay composite pellets

15 Structural stability performance Crushing strength measurements of particles before & after hydration/dehydration cycles Basic principles of stabilization mechanism 3CaCO 3(s) + Al 2 (Si 2 O 5 )(OH) 4 CaO (s) + Ca 2 Al 2 SiO 7(s) + SiO 2(s) + 2H 2 O (g) + 3CO 2(g) 10μm

16 Hydration/dehydration performance Performance determined by TGA under high steam flow ( o C) Performance up to 50% of the one corresponding to pure CaO Kinetics comparable to pure CaO

17 Scaled-up production & testing Approx. 2 kg from the best performing composite particles Particle size: 500 μm 2 mm 15 cycles in a fluidized bed reactor at CEA-Ines (France): 425 o C, hydration in steam, dehydration in air Measurement of hydration/dehydration performance + crushing strength of particles (every 5 cycles) Hydration/dehydration performance similar to TGA results Severe fragmentation of particles limited to < 1.5 wt% of initial mass Particles after 15 cycles

18 Conclusions & future improvements Approach of structured composite particles via formation/utilization of ceramic binders and/or supporting matrix Effective measure for mitigation of thermo-mechanical stresses caused by cyclic phase changes Higher stability at the expense of performance but decrease is not detrimental Main aspects of involved mechanisms elaborated Scaled-up production achieved to a certain degree & promising results were obtained Further pre-pilot/pilot multi-cyclic studies under realistic conditions important to validate the concept & provide feedback for additional improvement & optimization

19 Acknowledgments The European Commission for partial funding of this work through the FP7 projects RESTRUCTURE (GA No: ) and StoRRe (GA No: ) Our partners in the above projects for the fruitful collaboration Thank you for your attention!