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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
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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 fundamental thermodynamics & 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
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Modeling and Risk Mitigation
Punctured / Ruptured Tank
StorageVessel
Penetration
Possible Water Film
Ambient Atmosphere at TemperatureContains O2, N2, CO2 & H2O(l), H2O(g)
Heat Generated byChemical Reaction Volume
Media Temperature Depends onTa, Ti, dH/dt, keff, cpeff, …
Surface
LiquidWater
y
x
t
H2
Spilled Media
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?
TemperatureHumidityWater presenceMedia geometry
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United
Nations
Groundwork - Ammonia Borane
UN Test Result
Pyrophoricity Pass
Self-Heat Fail
Burn Rate Fail
Water Drop Pass
Surface Contact
Fail
Water Immersion
Pass
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NH3BH3 TGA Experimental Results
TGA experiments were conducted in an Argon atmosphere.
First and second dehydrogenation reactions occurred
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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*
NH3BH3 TGA Numerical Simulation
Tc0
TR
E
eAA
eARa
1 mmSample
Argon Gas
Phase
1 mm
5 mm
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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)
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NH3BH3 Calorimetry Simulation
Sample
AirPhase
Sample (5-20 mg)Not to scale
Setaram C-80 Calorimeter options :-Dry Air/Argon-Air/Argon with water vapor-Temperature
Wall temperatures were ramped at 0.5 ºC/min
Atmosphere: Dry Air
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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
0
0.5
1
1.5
2
2.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Time(h)
Nor
mal
ized
Hea
t F
low
(m
W/m
g)
Experimental Data
Simulation
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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
Bottom Surface
1.5cm
20 cm
t
Top Surface
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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.
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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).
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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.
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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
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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.
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Special Thanks to the following people:
SRNL
Bruce Hardy Stephen Garrison Josh Gray Kyle Brinkman Joe Wheeler
Department of Energy
Ned Stetson, Program Manager
Acknowledgements
THIS WORK WAS FUNDED UNDER THE U.S. DEPARTMENT OF ENERGY (DOE) HYDROGEN STORAGE PROGRAM MANAGED BY DR. NED STETSON