Transcript NH 3 BH 3 TGA Comparison

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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Objectives
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.
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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?
Punctured / Ruptured Tank
Penetration
Storage
Vessel
Temperature
Humidity
Water presence
Media geometry
Spilled Media
Media Temperature Depends on
Ta, Ti, dH/dt, keff, cpeff, …
Heat Generated by
Chemical Reaction Volume
H2
Ambient Atmosphere at Temperature
Contains O2, N2, CO2 & H2O(l), H2O(g)
Possible Water Film
Liquid
Water
y
t
Surface
x
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Groundwork - Ammonia Borane
United
Nations
UN Test
Result
Pyrophoricity
Pass
Self-Heat
Fail
Burn Rate
Fail
Water Drop
Pass
Surface
Contact
Fail
Water
Immersion
Pass
4
NH3BH3 TGA Experimental Results
 TGA experiments were
conducted in an Argon
atmosphere.
 First and second
dehydrogenation
reactions occurred
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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
 Ea
Reaction Kinetics:
R T
 Reaction 1-2:
 Ea
= 128 [kJ/mol]
 A0
= 3.836x10-11 [1/s]
 c
= 0.1573 [1/K]
 mol% = 14% borazine*
R  Ae
Argon
Gas
Phase
5 mm
A  A0  ecT
Sample
1 mm
1 mm
 Reaction 3-4:
 Ea
= 76 [kJ/mol]
 A0
= 106 [1/s]
 c
=0
 mol% = 41% borazine*
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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
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
Air
Phase
Sample
Sample
(5-20 mg)
Not to scale
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NH3BH3 Calorimetry in Dry Air
2.5
Experimental Data
Simulation
Normalized Heat Flow (mW/mg)
2
 Furnace ramped to
150ºC
1.5
 Additional exothermic
heat flow during the
temperature ramping
1
 Endothermic dip due
to foaming and
melting of the material
for T > 110 oC
0.5
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Time(h)
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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
Top Surface
1.5cm
t
Bottom Surface
20 cm
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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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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
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