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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