Hypothetical Accident Scenario Modeling for Condensed Hydrogen Storage Materials

1. Hypothetical Accident Scenario Modeling for Condensed Hydrogen Storage Materials Charles W. James Jr, Matthew R. Kesterson, David A. Tamburello, Jose A. Cortes-Concepcion, and Donald L. Anton Savannah River National Laboratory September 14, 2011

2. The objective of this study are to understand the safety issues regarding solid state hydrogen storage systems through: Development & implementation of internationally recognized standard testing techniques to quantitatively evaluate both materials and systems. Determine the fundamentalthermodynamics & chemical kinetics of environmental reactivity of hydrides. Build a predictive capability to determine probable outcomes of hypothetical accident events. Develop amelioration methods and systems to mitigate the risks of using these systems to acceptable levels. Objectives

3. Media Temperature Depends on Ta, Ti, dH/dt, keff, cpeff, … Ambient Atmosphere at Temperature Contains O2, N2, CO2 & H2O(l), H2O(g) H2 Heat Generated by Chemical Reaction Volume y Possible Water Film Liquid Water t Surface x Modeling and Risk Mitigation • Accident Scenario (from UTRC risk assessment): Storage system ruptured and media expelled to environment in either dry, humid or rain conditions. • Risk: Under what conditions will there be an ignition event? What are the precursors to the ignition event? • Temperature • Humidity • Water presence • Media geometry Punctured / Ruptured Tank Penetration Storage Vessel Spilled Media

4. Groundwork - Ammonia Borane United Nations

5. NH3BH3 TGA Experimental Results • TGA experiments were conducted in an Argon atmosphere. • First and second dehydrogenation reactions occurred

6. Argon Gas Phase 5 mm Sample 1 mm 1 mm NH3BH3 TGA Numerical Simulation • COMSOL model: • 2-D, axisymmetric • Conduction, Convection, & Radiation Heat Transfer • Weakly Compressible Navier-Stokes Equations • Maxwell-Stefan Species Convection and Diffusion Reaction Kinetics: • Reaction 1-2: • Ea = 128 [kJ/mol] • A0 = 3.836x10-11 [1/s] • c = 0.1573 [1/K] • mol% = 14% borazine* • Reaction 3-4: • Ea = 76 [kJ/mol] • A0 = 106 [1/s] • c = 0 • mol% = 41% borazine*

7. NH3BH3 TGA Comparison • Theoretical curve only takes into account H2 reaction (no other products) • Additional 14 mol-% and 41 mol-% material loss during reaction (for simplicity, all losses assumed borazine)

8. Sample (5-20 mg) Not to scale Air Phase Sample NH3BH3 Calorimetry Simulation • Wall temperatures were ramped at 0.5 ºC/min • Atmosphere: Dry Air • Setaram C-80 Calorimeter options : • -Dry Air/Argon • -Air/Argon with water vapor • -Temperature

9. NH3BH3 Calorimetry in Dry Air • Furnace ramped to 150ºC • Additional exothermic heat flow during the temperature ramping • Endothermic dip due to foaming and melting of the material for T > 110 oC

10. 1.5cm Accident Scenarios • 50 grams of NH3BH3 was assumed to collect on the ground following a Gaussian distribution. • Mesh consisted of over 9,000 triangular elements • Scenario 1 • A heat source (ex. Car muffler) sits 4 inches above the NH3BH3. • Multiple iterations of Scenario 1 were simulated modifying the heat source temperature from 225ºC to 300ºC • Scenario 2 • The NH3BH3 falls onto a heated surface • Multiple iterations of Scenario 2 were simulated modifying the heat source temperature from 100ºC to 125ºC Top Surface t Bottom Surface 20 cm

11. Results – Overhead Heating • Reactions 1 and 2 went to completion • Reactions 3 and 4 started, but the reaction rate was slow. • Highest overhead temperature was 300ºC. • Simulations were initiated at higher temperatures, but the timestep needed by the solver was too small for the simulation to conclude in a reasonable timeframe.

12. Results – Overhead Heating Continued • Above 250ºC, the first reaction goes to completion under 1 hour. • At 300ºC, the first reaction is completed within 11 minutes • Below 250ºC, the second dehydrogenation does not start within the simulation time. • At 300ºC, the second dehydrogenation reaction is progressing (slowly).

13. Results – Ground Heating • Ground temperatures above 125ºC were not modeled due to the high rate of hydrogen release and the resulting decrease in simulation timestep. • Initial release of hydrogen occurs at the outer rim of the NH3BH3 mound. • The maximum mound temperature progresses inward toward the center axis, at which point high pressure spikes due to hydrogen release were observed.

14. Results – Ground Heating • At 125ºC, the first dehydrogenation reaction proceeds quickly. • First reaction goes to completion within 2 minutes. • Second dehydrogenation reaction starts, but proceeds very slowly due to the ground temperature being held at 125ºC

15. Conclusions • COMSOL Multiphysics models successfully modeled dehydrogenation of Ammonia Borane as seen in the TGA and Calorimetry experimental comparisons. • Additional models were developed to simulate the release of hydrogen in postulated accident scenarios. • Temperatures above 125ºC (below heat) and 300ºC (above heat) yielded extremely fast hydrogen release rates. • High pressure spikes were observed during the hydrogen release which could be a precursor to the foaming seen experimentally.

16. Acknowledgements Special Thanks to the following people: • SRNL • Bruce Hardy • Stephen Garrison • Josh Gray • Kyle Brinkman • Joe Wheeler • Department of Energy • Ned Stetson, Program Manager THIS WORK WAS FUNDED UNDER THE U.S. DEPARTMENT OF ENERGY (DOE) HYDROGEN STORAGE PROGRAM MANAGED BY DR. NED STETSON