Properties of Dense Ceramic Membranes for Energy Conversion Processes

Properties of Dense Ceramic Membranes for Energy Conversion Processes Michael Schroeder, Young Chang Byun Institute of Physical Chemistry CCT 009, 18-0 May 009, Dresden

Coworkers: Young Chang Byun Jianxin Ji Nicolas Gauquelin Funding: Light House Project (finished) Toolbox - Ceramic Membranes for Catalysis 5 academic partners, industrial partners Financial support: German Federal Ministry of Education and Research T H A N K Joint Project MEM-Oxycoal Oxygen Permeable Ceramic Membranes for Coal-fired Power Plants 10 academic partners Financial support: German Federal Ministry of Economics and Industry Y O U HGF-Alliance MEM-BRAIN Gas Separation Membranes for Zero-Emission Power Plants 13 partners Financial support: German Helmholtz Foundation

Outline Introduction why dense ceramic membranes? Types of membranes and membrane materials BCFZ - Planar Membranes Oxygen transport: experimental results Modelling of bulk transport and surface exchange BCFZ - Tubular Membranes Oxygen transport: experimental results Modelling of bulk transport and surface exchange Conclusion

Dense vs. porous membranes Large flux and good selectivity! Air O Sweep gas Porous Membrane Carbon membranes Polymer membranes T < 100 o C BaFe 1-x-y Co x Zr y O 3-δ Polymer Membranes upper limit Carbon Molsieve Air O - e - membrane Dense MIEC O Oxide membranes 700 < T < 1000 o C Sweep gas Data for polymer membranes: Budd et al., J. Membr. Sci. 35 (008) 851 ; for CMS membranes: Kim et al., J. Membr. Sci. 55 (005) 65

Membrane permeation pre-requisites for high fluxes oxygen rich gas phase p oxygen lean gas phase p large driving force high electronic partial conductivity high oxide ion partial conductivity <-> high oxygen vacancy concentration J b ( O )= AT nl σ 0 ion 0 σ ion () a n a g g () n { } = z ion F u V c V 0 partial pressure p dimensionless chemical activities a p g ' h O O O V L p g ''

Membrane permeation pre-requisites for high fluxes oxygen rich gas phase p oxygen lean gas phase p large driving force high electronic partial conductivity high oxide ion partial conductivity -> high oxygen vacancy concentration fast surface kinetics for reduction/incorporation, oxidation/release a g ' a S ' a S '' h a g '' high pressure side: J S ( O )= k 0 eff a g () m a s { ( ) m } 1 O V O h + + O O O reduction/incorporation V

Membrane permeation pre-requisites for high fluxes oxygen rich gas phase p oxygen lean gas phase p large driving force high electronic partial conductivity high oxide ion partial conductivity <-> high oxygen vacancy concentration fast surface kinetics for reduction/incorporation, oxidation/release fast transport of gaseous oxygen to/from membrane surface -> large gas flow rates particularly on the feed side 1 a g ' O V O h + + a S ' a S '' h O O O reduction/incorporation V a g ''

Membrane materials perovskite MIEC oxide/metal composites doped ion conductors J(O ) / m 3 m - h -1 =8 10 5 J(O ) / mol cm - s -1 oxide/oxide composite Sunarso et al., J. Mem. Sci. (008)

McIntosh et al., Solid State Ionics (006) Deviation from stoichiometry Ba 0.5 Sr 0.5 Co 0.8 Fe 0. O 3-δ BaCo 1-x Fe x Zr y O 3-δ Y.C. Byun, PhD Thesis (009) T / K oxygen vacancy fraction > 0% depends on temperature and p(o )

Ba(Co x Fe y Zr z )O 3-δ - Oxygen Permeation - feed flow rate: 300 ml/min, p' g = 0.1 bar - sweep flow rate: 40-50 ml/min, p'' g variabel Defect and transport modelling yields - oxide ion partial conductivity σ 0 ion - surface exchange coefficent k 0 eff assumption: no gas phase transport limitation 173 K

Variation of the membrane thickness L critical thickness L c may be calculated from ratio of σ ion and k eff L c 0,004 cm (40 μm) thick membrane: L >> L c j(o ) L -1 -> bulk transport limited permeation thin membrane: L << L c j(o ) f(l) -> surface exchange limited permeation

Shaping of ceramic hollow fibers Green fiber Polymer solution Oxide powder Slurry Sintered fiber Green fiber Ceramic hollow fiber Spinneret Sintering Membranes prepared by: Fraunhofer Institut Grenzflächen und Bioverfahrenstechnik (Stuttgart, Germany)

1 J(O ) /J(O ) ref 0.5 without pores with pores 0 Sweep gas flow rate cm 3 /min Ar O - Air V s,i a s, j b,i (O ) j b1,i (O ) i a bubble,i additional gas/solid interfaces i-1 i i+1 V f,i a f,i

Local oxygen activities along the tubular membrane - co-current mode 0.5 0.0. V(Ar) = 100 cm 3 /min a a g ( O ) S Oxygen activity 0.15 0.10 a S a g argon x a ( ) g ( O ) as O 0.05 a S a g air 0.00 0 5 10 15 0 5 x / cm x

Local oxygen activities along the tubular membrane - counter-current mode a a g ( O ) S Oxygen activity 0.5 0.0 0.15 0.10 0.05 a S a g. V(Ar) = 100 cm 3 /min a S a g argon x a g ( O ) as O air ( ) 0.00 0 5 10 15 0 5 x / cm x

Comparison of co-current and counter-current mode 1.51 1.. V(Ar) / cm 3 min -1 100 00 400 co-current. V(Ar) / cm 3 min -1 100 00 400 counter-current 15.0 1 1.0 J(O ) / cm 3 min -1 J(O ) /J(O ) ref 0.9 0.6 0.3 0.0 0 0 5 10 15 0 5 Longitudinal position x x /cm / cm J(O ) / cm 3 min -1 J(O ) /J(O ) ref 9.0 6.0 3.0 1073K (co) 1073K (counter) 113K (co) 113K (counter) 1173K (co) 1173K (counter) 0.0 0 0 100 00 300 400 Sweep Ar flow gas flow rate rate cm 3 /min cm 3-1 /min

Conclusions Oxygen transport through thick planar and thin hollow fiber membranes of Ba(Co x Fe y Zr z )O 3-δ was investigated by permeation experiments. The oxygen permeation fluxes of planar membranes were sucessfully modelled by a defect and transport model. The modelling results indicate that: -in thicker planar membranes, oxygen transport is predominantly bulk limited. -in thin tubular membranes, oxygen transport is substantially limited by the surface reactions. Microstructure (e.g. pores) affects the permeation flux Sluggish internal interfaces diminish the flux Tubular membrane in counter-current mode yields slightly higher permeation fluxes when compared to co-current mode.

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Flux performance Reactivity Thermal expansion Compatibility mechanical Surface poisoning Chemical expansion process Stability chemical thermal cycling