ON THE KINETICS OF ALH3 DECOMPOSITION AND THE SUBSEQUENT AL OXIDATION

Transcription

1 Fraunhofer ICT: ES - Energetic Systems ON THE KINETICS OF LH DECOMPOSITION ND THE SUBSEQUENT L OXIDTION Eisenreich N., Keßler., Koleczko., Weiser V. Fraunhofer-Institut fuer Chemische Technologie (ICT), J.-v.-Fraunhoferstr. 7, D-767 Pfinztal, Germany, ne@ict.fraunhofer.de

2 ON THE KINETICS OF LH DECOMPOSITION ND THE SUBSEQUENT L OXIDTION Hydrogen storage: High pressure tanks 5 7 bar or higher in composites, mechanical stability Liquid hydrogen tanks: boil-off, cooling/isolation, safety Solid hydrides Heavy metals: low gravimetric storage Light metals: adsorption in nano structures Light metals: chem.bonded hydrides

3 Hydrogen Storage Systems - General Survey 18% lh as model substance for storage of hydrogene hydride 17% 16% 15% 14% 1% L H (cryo.) morphous light metal powders are about 5% Hydrogen Storage Capacity (%) 1% 11% 1% 9% 8% 7% 6% 5% MgH lh LiNH MOF MgTiH NaBH4 4% % % 1% comp. H (7 bar) % Year

4 α-lh polyhedra as cubes, cubic octahedron and hexagonal prisms For comparison: lex: electro-exploded l surface 1.8 m /g Resulting l is not passivated!!! On dehydrogenation => nano-porous l structures emerge with surface 15 to m /g from lh of.69 m /g surface area

5 ON THE KINETICS OF LH DECOMPOSITION ND THE SUBSEQUENT L OXIDTION Risks arising from dehydrogenated hydrides * : The solids are highly porous or nano-structured to expose high surface areas to easy hydrogen access but also to easy air access on accidents. The metals => pure state, highly sensitive to oxidation if contact to air. The temperatures are elevated on operation in relation to oxidation reactions of metals, even they are considered moderate. In addition gaseous hydrogen is present Passivation reactions: thin protecting oxide layers (for l ~ 4 nm) => 1-% of metals might be oxidised at high reaction rates: Thermal explosion of container and the subsequent distribution Explosion of released metal of nano-size in air Even a Deflagration Detonation Transfer might be possible if well distributed nano-particles in air ignite. Investigation of this effect in more detail!!!

6 l from dehydrogenated lh behaves like l nano or ultra-fine particles weight temperature K TG-curve heated ( K/min) in an inert r-atmosphere till 48K initiating dehydrogenation, then cooled down to 5 K with air entrainment causing passivation and subsequent heating to 1 K (5 K/min) with two steps of oxidation at 85 K and 11 K

7 Modelling the reactions => kinetics for simulation of thermal explosions 1.6 TG in ir, 5, 1K/min Methods of Thermal DSC in ir, 1 K/min nalysis: DSC, TG, X-Ray 5 4 weight temperature K Dehydrogenation and formation of l O -4nm passivation layer chemical controlled 1 nm thick particle oxidation diffusion controlled particle oxidation weight temperature K Dehydrogenation and formation of l O -4 nm passivation layer Chemical Controlled 1 nm thick particle oxidation

8 Reaction at 44 K: lh => l => l-crystallites + O => l-crystallites with lumina-layer (higher T, further oxidation) d [ lh ] dt [ l] d[ lh ] d dt d [ l O ] dt = k = n dt k = 1, oxid ( Z, E, T ) [ lh ]( ln[ lh ]) k, cryst ( Z, E, T ) ( Z, E, T ) f ([ l] ) f cryst n 1 n k lane dehydration = vrami- Erofeev-Mechanism ( Z, E, T ), oxid ([ l] ) f ([ l] ) l passivation = 1st order oxidation-mechanism Solution: [ l] = Z S T [ l] = k ( u) [ l] cryst T [ l O ] = k ( u) [ l] ne 4 T ( E T ) Z1 k1( u) ( u) du ( u) du S 1 ( E 1, u) n 1 e Z S ( E k( Z, E, T ) = Ze Z1, u) ( S E RT 1 ( E u)) 1 [ lh ] ; S( E, T ) = n du = e ( Z S E E N RT RT e dt = Te n= 1 ( E, T )) n n!( 1) n 1 RT E n

9 Kinetic model related to TG and DSC curves TG + DSC curves are constructed from the set of equations solving the reaction scheme by: TG(T) = W alane W hyd + p W oxy ; W alane ~ [lh ], W oxy ~[l O ], W hyd ~[l] and the DSC curve by: DSC(T)= Q 1 d[lh ]/dt + p Q d[l O ]/dt Least squares fit of TG(T) and DSC(T) to achieve E and Z the different reaction chieve kinetic parameters of alane dehydrogenation in a separate experiment with total exclusion of air TG(T) = W = weight of TG curve DSC(T) = DSC-curve Q1 and Q = heat of reactions, p partial oxidation of l in passivation reaction

10 Modelling of lane dehydrogenation and passivation (at 45 K) fit of TG / DSC curves at heating rates.5-1 K/min TG/DSC of dehydr. lh and l passivation concentration DSC K/min 1 K/min K/min K/min temperature / K temperaturek Recovered TG/conc. of dehydr. lh TG concentration weight temperaturek Fit of TG: dehydr. lh and l passivation temperaturek TG of dehydrogenation lh

11 mw heat DSC temperature / K 1 K/min 1 K/min K/min K/min temperaturek TG DSC-curve of dehydrogenation in rgon subtracted from DSC-curve in air mwmg temperaturek Fit of DSC: dehydrogenation of lh and l passivation at 1K/min DSC fit of dehydrogenation lh at 4 heating rates

12 rel. crystal size temperature K Crystal growths estimated from weight increase on passivation depending on the heating rate, 5, 1 and K/min t higher temperatures of dehydrogenation, less surface area is available for passivation and therefore for the oxidation reaction, which might lead to hazardous thermal explosion Results of kinetic parameters / discussion of kinetic compensations effect in the paper Method: evaluated Dehydrogenation Dehydrogenation simultaneously TG-DSC TG-DSC (fitted n) Oxidation TG- DSC ctivation energy [J] Pre-expon. Log Z vrami-erofeev order n.58