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Lecture 17. Hydrogen Storage

Lecture 17. Hydrogen Storage. Metal Hydrides H 2 Storage with other Chemical Methods. H 2 Storage in Metal Hydrides. M + H 2  MH 2 Some metals, particular alloys of titanium, iron, manganese, nickel, chromium and other can react with H 2 in a reversible reaction;

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Lecture 17. Hydrogen Storage

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  1. Lecture 17. Hydrogen Storage • Metal Hydrides • H2 Storage with other Chemical Methods

  2. H2 Storage in Metal Hydrides • M + H2 MH2 • Some metals, particular alloys of titanium, iron, manganese, nickel, chromium and other can react with H2 in a reversible reaction; • The forward reaction is slightly exothermic. To release H2 a small amount of heat must be supplied; • It is possible to have reversible reactions at ambient temperatures; • H2 is stored at a modest pressure and release at ambient pressure.

  3. H2 Storage in Metal Hydrides • Application of Metal Hydride in Fuel Cells

  4. H2 Storage in Metal Hydrides • Application of Metal Hydride in Portable Applications

  5. H2 Storage in Metal Hydrides • Comparison between compressed H2 and hydride

  6. H2 Storage in Metal Hydrides • Thermodynamics and Pressure-Temperature-Composition Properties • The reversible capacity is the plateau width • Plateau pressure increases with increasing T • There is a hysteresis in LnP vs H/M plot

  7. H2 Storage in Metal Hydrides • Other Important Properties of Metal Hydrides • Activation: for hydriding/dedydriding and kinetics; • Kinetics: most of hydrides have fast kinetics at ambient T; • Gaseous impurities resistance: poisoning, retardation, reaction or innocuous • Cyclic stability; • Safety: pyrophoricity -- combustion when exposed to air • Alloy cost

  8. H2 Storage in Metal Hydrides • Available Metal Hydrides

  9. Hydrogen Storage in Chemicals • Chemical Methods • H2 is stored in H2 containing chemicals; • The chemicals used must very easily release H2; • The manufacturing process must be simple and use little energy; • The chemicals must be safe to handle; • Chemicals used: methanol, alkali metal hydrides, sodium borohydride, ammonia and others

  10. Hydrogen Storage in Chemicals

  11. Hydrogen Storage in Chemicals • Methanol • CH3OH + H2O  CO2 + 3H2 (Reforming) • CH3OH + O2 2CO2 + 3H2 (Partial Oxidation) • Reforming reaction generates 0.188 kg of H2 per kg of methanol; • Partial oxidation generates 0.125 kg of H2 per kg of methanol; • Autothermal reaction generates 0.125-0.188 kg of H2 per kg of methanol • Full utilization of methanol is impossible; • Ethanol can basically used the same way, but more expansive.

  12. Hydrogen Storage in Chemicals • Methanol • Reforming reaction can occur at 200 C with proper catalysts; • Attempt to use methanol in cars failed, but mixed fuel survived; • The success of direct methanol fuel cell makes methanol an attractive fuel; • Direct methanol fuel cell (DMFC) has several issues; • Need methanol distribution infrastructure for large commercial use;

  13. Hydrogen Storage in Chemicals • Methanol (Mixed POX/SR reactor scheme)

  14. Hydrogen Storage in Chemicals • Methanol

  15. Hydrogen Storage in Chemicals • Methanol (Typical PEMFC Efficiencies) • Mixed POX/SR of methanol has shorter startup time, suitable for automotive applications.

  16. Hydrogen Storage in Chemicals • Alkali metal hydride • CaH2 + 2H2O  Ca(OH)2 + 2H2 • NaH + H2O  NaOH + H2 • Energy density from these metal hydrides are quite high; • Na and Ca are abundant and their hydrides are not easy to obtain; • Need to dispose the caustic hydroxide and water; • Large quantities of water are consumed; • The energy required to make and transport these hydrides is greater than that release from fuel cell

  17. Hydrogen Storage in Chemicals • Alkali metal hydride

  18. Hydrogen Storage in Chemicals • Sodium Borohydride • NaBH4 + 2H2O  NaBO2 + 4H2 (H=-218 kJ/mol) • The reaction doesn’t proceed spontaneously, it is highly controllable; • It is exothermic, at rate of 54.5 kJ per mole of H2 produced; • The products is a mixture of H2 and water vapor, desirable for PMEFC; • Catalyst is needed to promote the reaction; • Solid NaBH4 is flammable; • NaBH4 solution is stable and nonflammable (in 3% NaOH solution).

  19. Hydrogen Storage in Chemicals • H2 Production from Sodium Borohydride

  20. Hydrogen Storage in Chemicals • Sodium Borohydride

  21. Hydrogen Storage in Chemicals • Advantages of Sodium Borohydride • The solution is safe than most of the liquids used for H2 production; • Generates pure H2; • The reactor needed to release H2 doesn’t consume energy and can operate at ambient pressure and temperatures • The H2 production rate can be simply controllable; • H2 production reactor is very simple and low cost; • If desired the product H2 can contain large quantities of water vapor, which is highly desirable for PEMFC.

  22. Hydrogen Storage in Chemicals • Sodium Borohydride: Application in Fuel Cell

  23. Hydrogen Storage in Chemicals • Sodium Borohydride: Application in Fuel Cell

  24. Hydrogen Storage in Chemicals • Ammonia • NH3  ½ N2 + 3/2 H2 (H=+46.4 kJ/mol) • It is only used for mobile/portable application, not for stationary since methane steam reforming process is used in ammonia synthesis; • This reaction happens at >900 C with a suitable catalyst; • Large amounts of energy needed for vaporization and reaction; • Theoretical efficiency of this process is 77% (upper limit); • Mixture of H2 and N2 needs separation before feed into a fuel cell; • Ammonia reformer has serious issue in the startup period.

  25. Hydrogen Storage in Chemicals • Comparison between Ammonia and Methanol • Similar cost and production methods as methanol; • The product H2 per liter of carrier is slightly higher than methanol; • Ammonia is much more difficult to store, transport and handle; • Ammonia is more dangerous and toxic; • Ammonia cracking is much more complex; • Ammonia cracking requires higher temperatures; • The product H2 is not pure and difficult to use. • Conclusion: Use of ammonia as H2 carrier is going to be confined to only most unusual circumstances

  26. Comparison of Hydrogen Storage

  27. Overview: Chemical Hydrogen Storage Energy Source Hydrogen Stored Energy Point-of-use H2 storage Hydrogen • Attractive Features: • Liquid or solid fuel infrastructure • Potential for no H2 handling by consumer • Diversity of options • Off-board or on-board regeneration Cost, Energy Efficiency Thermodynamics Regeneration Kinetics Chemical hydrogen storage

  28. Chemical Hydrogen Storage It’s the right combination of a material and a reaction Dehydrogenation: Hydrolysis: XHn + n H2O = n H2 + X(OH)n HnX---YHn = n H2 + XY (e.g. NaBH4, LiH) (e.g. decalin -> naphthalene) Dehydrocoupling: … and families of reactions yet to be developed XHn + YHn = n H2 + XY (e.g. NH3 + BH3) Each reaction family has numerous opportunities

  29. Overall Center Approach Potential Candidates for Chemical Hydrogen Storage Material Wt % (>6.5%) Theoretical Max. Efficiency for Regeneration (70%) Exp’tl Demonstration of Hydrogen Removal Eng Assessment of H2 Removal Exp’tl Demo of Regeneration Eng. Assessment of Regeneration Systems Engineering Assessment Viable Chemical Hydrogen Storage Systems

  30. Amine-Borane Dehydrogenation/Regeneration [-H2N-BH2-]n [-HN=BH-]n H3N-BH3 - H2 - H2 - H2

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