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© 2007 SRI International Self-Ignition of Hydrogen Jet Fires by Electrostatic Discharge Induced by Entrained Particulates Erik Merilo, Mark Groethe, Richard Adamo SRI International Robert Schefer, William Houf, Daniel Dedrick Sandia National Laboratories 4th International Conference on Hydrogen Safety (ICHS) San Francisco, California September 12-14, 2011
© 2011 SRI International 2 Outline Spontaneous Ignition of Large Hydrogen Releases Introduction and Theory Objective and Approach Experimental Setup Static Charge Buildup Results Attempted Self-Ignition Results Summary
© 2011 SRI International 3 Groethe, M., Merilo, E., Colton, J., Chiba, S., Sato, Y. and Iwabuchi, H., Large-scale Hydrogen Deflagrations and Detonations, International Journal of Hydrogen Energy, 32(13), 2007, pp. 2125-2133.. Spontaneous Ignition: Example High-Speed Video Study performed for NEDO and IAE in Japan Ignition occurred 100% of time for release pressures above 24 atm and leak diameters of 42 mm Ignition location occurred near equipment support struts The ignition point is 6 m above the jet exit and in subsonic flow. Thus, shock heating is not the ignition source.
© 2011 SRI International Spontaneous Ignition of Hydrogen Astbury and Hawksworth (2007) performed a review of spontaneous ignition incidents and of postulated mechanisms –For 86% of incidents the source of ignition was not identified –Discussed four potential mechanisms of spontaneous ignition: 1.Reverse Joule-Thomson effect 2.Electrostatic ignition 3.Diffusion ignition 4.Hot surface ignition Diffusion ignition has been the primary focus of subsequent research Very limited research has been performed to investigate electrostatic ignition 4 Astbury, G.R., & Hawksworth, S.J. (2007). Spontaneous ignition of hydrogen leaks: Review of postulated mechanisms. International Journal of Hydrogen Energy, 32, 2178–2185.
© 2011 SRI International Conditions Required for an Electrostatic Discharge Ignition Ignition of a flammable mixture is not caused by charge buildup alone A number of stages must occur for the charge to ignite a mixture (ISSA, 1996; Hearn, 2002): Charge separation (generation of electrostatic charge) Charge accumulation Charge removal –Charge removal by dissipation → no ignition –Charge removal by electrostatic discharge → possible ignition Presence of a flammable mixture Discharge energy greater than the minimum ignition energy 5 ISSA. (1996). Static electricity: Ignition hazards and protection measures. ISSA: Heidelberg, Germany. Hearn, G.L. (2002). Static electricity: Guidance for plant engineers. http://www.wolfsonelectrostatics.com/info_pdfs/guidanceforplantengineers-staticelectricity.pdf
© 2011 SRI International Proposed Mechanisms for Electrostatic Discharge Ignition of Hydrogen Release Charge separation (generation of electrostatic charge) – Potential for solid particles to be present in hydrogen systems – When iron oxide particles move through pipes, interaction between the particles and the pipe wall can lead to charge separation by triboelectric charging. Triboelectric charging is a type of contact charging that takes place when two different materials are rubbed against each other Charge accumulation – Occurs when the rate of charge separation exceeds the charge dissipation rate – Charge can accumulate on entrained particles Generates an electric field Electric field can charge conductors in close proximity by induction – Impact charging by particles can cause charge accumulation to occur on objects in the release 6
© 2011 SRI International Proposed Mechanisms for Electrostatic Discharge Ignition of Hydrogen Release Charge removal by electrostatic discharge – Spark discharge between isolated conductors – Brush discharge – Corona discharge Presence of a flammable mixture – Wide flammability range of hydrogen means that a release could produce a sizeable volume of flammable mixture Discharge energy greater than the minimum ignition energy – Hydrogen has a very low spark discharge energy required for ignition For a near stoichiometric mixture, the minimum ignition energy of hydrogen and air is 0.017 mJ (Ono & Oda, 2008) Near the flammability limits, the spark ignition energy required to ignite a hydrogen-air mixture is only about 6 mJ 7 Ono, R., & Oda, T. (2008). Spark ignition of hydrogen-air mixture. Journal of Physics: Conference Series, 142, 012003.
© 2011 SRI International Static Charge Buildup: Objective & Approach Objective –Determine if static charge accumulation on iron oxide particles entrained in a hydrogen jet release could lead to a spark discharge ignition or a corona discharge ignition. Approach –Initial baseline tests with only hydrogen –Ignition tests with energy input from an external power supply –Entrained particulate electrification characterization tests –Self ignition by entrained electrified particulate 8
© 2011 SRI International Release Facility 9
© 2011 SRI International Ignition Tests with Energy Input from an External Power Supply 10
© 2011 SRI International Ignition Tests with External Power Supply 110-mJ spark was used to show the release could be ignited at the selected location Four tests were conducted with a 10 kV-17kV AC corona generator connected to a copper probe –No ignition events occurred 11 AC corona
© 2011 SRI International Entrained Particulate Electrification Characterization Tests 12
© 2011 SRI International Entrained Particulate Electrification Characterization Tests Nozzle Ring Charged Plate Detector Nozzle Charge accumulation caused by iron oxide particles in the flow was evaluated by measuring voltage on detectors surrounding the release jet Static level monitoring system was used to make measurements
© 2011 SRI International Release Characterization Tests Electrostatic potential measurement on the ring charged-plate detector for release tests with no particles added 14 Ring Charged- Plate Detector
© 2011 SRI International Iron Oxide Particles 200x Four iron oxide samples were tested: three iron (III) oxide and one iron (II) oxide. All four particles were tested in external particle entrainment tests
© 2011 SRI International Voltage Induced on the Charge Plate All four iron oxide particles induced a negative charge on the detector –Electrons were stripped, giving particles a positive charge –Of the four samples, Sample B produced the highest charge –Sample B was selected for the internal entrainment tests Charge increased with increasing particle mass. Variation of Particle SampleVariation of Total Particle Mass
© 2011 SRI International Self Ignition by Entrained Electrified Particulate 17
© 2011 SRI International Self Ignition by Entrained Electrified Particulate Ignition experiments focused on two phenomena associated with electrostatic discharge ignition of hydrogen jets: 1.Spark discharges from isolated conductors 2.Corona discharges Three types of ignition events were observed: 1. Floating plate with grounded probe ignition 2. Ungrounded plate ignition 3. Nozzle charged plate detector ignition
© 2011 SRI International Floating Plate with Grounded Probe Ignition A series of ignition tests were performed with a circular ungrounded plate in close proximity to a grounded probe In this configuration six ignitions occurred Ignition occurred in three out of four tests with only 0.1 g of iron (III) oxide particles present Results show that entrained particulates can be a source of spontaneous ignition Ungrounded Plate Grounded Probe
© 2011 SRI International High Speed Video: Floating Plate with Grounded Probe Ignition 20
© 2011 SRI International Floating Plate with Grounded Probe Ignition 10.80 ms18.80 ms 0.80 ms1.60 ms Ungrounded Plate Ignition Nozzle Ignition Ignition occurred in 6 of 8 tests Available spark discharge energies between 0.094 and 0.358 mJ
© 2011 SRI International Ungrounded Plate Ignition 22 Ungrounded Plate Ungrounded copper plate was used to investigate the potential for charged particles causing a spontaneous ignition event by a corona discharge 13 tests were conducted with the ungrounded plate resulting in two ignition events Two ignition mechanisms appear possible: – Electrostatic spark discharge – Corona discharge Possible electrostatic discharge between ungrounded plate and ungrounded cable housing Difficult to force a corona discharge ignition to occur with this geometry Ignition
© 2011 SRI International High Speed Video: Ungrounded Plate Ignition 23
© 2011 SRI International Nozzle Charged Plate Detector Ignition Four ignition events occurred in close proximity to an ungrounded metal tube surrounding the jet next to the release nozzle No ignitions occurred when a nozzle charged plate detector was not present No ignitions of this type occurred without particles entrained in the flow. More research is required to determine the cause of these ignitions
© 2011 SRI International High Speed Video: Nozzle Charged Plate Detector Ignition 25
© 2011 SRI International Nozzle Charged Plate Detector Ignition: Standard and IR Video ~ -33 ms~ 0 ms~ 33 ms ~ -33 ms~ 0 ms~ 33 ms Standard and IR video frames show that the iron oxide particulate had already exited the nozzle and that the hydrogen jet extended between 0.3 and 0.9 m away from the nozzle before ignition occurred This indicates that the ignition events were not related to diffusion ignition Ignition H 2 Jet
© 2011 SRI International Static Charge Buildup: Summary Iron oxide particles had positive charge in all tests –Electrons were removed –Iron (III) oxide produced higher charge than iron (II) oxide when a comparable mass of particulates was used Experiments showed that entrained particulates can be a source of spontaneous ignition –Ungrounded plate in close proximity to a grounded probe caused ignition to occur in 6 out of 8 tests All ignition events observed in this study occurred in close proximity to ungrounded metal objects –No ignition events were observed in the presence of grounded metal alone Ungrounded metal plates were charged as high as -41.5 kV with no ignition occurring –Result suggests that inducing a corona discharge with electrified particulate is unlikely for the geometries studied
© 2011 SRI International Acknowledgment This work was supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen, Fuel Cells and Infrastructure Technologies Program under the Codes and Standards subprogram element managed by Antonio Ruiz. 28
© 2011 SRI International 29 Questions?
© 2011 SRI International Discharge Mechanisms: Spark Discharge Occurs when isolated conductors in close proximity are charged to different electrostatic potentials –Electric field is formed –If the field strength exceeds the breakdown strength of the surrounding atmosphere, a spark discharge can result Breakdown strength about is 30 kV/cm under normal atmospheric conditions A spark discharge is a discrete discharge where a single plasma channel is formed across the gap between the conductors 30 Charged Conductor
© 2011 SRI International Discharge Mechanisms: Brush Discharge Can occur when a conductive electrode is brought into an electric field of sufficient strength –Electrode radius of curvature is greater than 3 to 5 mm (Luttgens & Glor, 1989). –Can take place regardless of the field’s origin (Glor, 2003). The presence of the electrode distorts the field –Dielectric strength of the surrounding gas can be exceeded locally Several separate plasma channels can form on the surface of the electrode. 31 Glor, M. (2003). Ignition hazard due to static electricity in particulate processes. Powder Technology, 135–136, 223– 233. Luttgens, G., & Glor, M. (1989). Understanding and controlling static electricity. Expert Verlag. Charged Object ++++++++
© 2011 SRI International Discharge Mechanisms: Corona Discharge The conditions required are similar to those that create a brush discharge Generated in areas of high field strength –Can develop around sharp points Occurs when the field strength exceeds the breakdown field strength of the surrounding medium –Ionizes and becomes conductive Ionization of the surrounding fluid is limited to the region around the conductor where the field strength is exceeded –Current flows The critical voltage at which a corona discharge will occur is influenced by: –Geometry of the point –Distance to ground –Properties of the surrounding mixture 32 Charged Object ++++++ + + +
© 2011 SRI International Discharge Mechanisms The type of discharge that can occur is influenced by the conductivity of the materials used and the geometric configuration When objects are charged, an electric field is formed around the objects If the charge is high enough, there can be locations where the electric field exceeds the dielectric strength of the surrounding gas, and a discharge by ionization takes place The dielectric strength of a gas depends on the ionization energy of the molecules and the mean free path of electrons, and is therefore dependent on gas composition and pressure 33
© 2011 SRI International Discharge Incendivity Incendivity is the ability of a discharge to ignite a flammable mixture The incendivity of a discharge is dependent on: –Total energy released –Time and spatial distribution of energy Can be affected by humidity and temperature Total energy of a discharge can be used to estimate its incendivity Spark discharges are the most incendive discharges (Glor, 2003). Brush discharges are more incendive than corona discharges Objects charged to a negative potential are significantly more incendive than objects charged to a positive potential (Luttgens & Glor, 1989). 34 Glor, M. (2003). Ignition hazard due to static electricity in particulate processes. Powder Technology, 135–136, 223– 233. Luttgens, G., & Glor, M. (1989). Understanding and controlling static electricity. Expert Verlag.
© 2011 SRI International Discharge Incendivity While calculating the discharge energy associated with a spark discharge is straightforward, for other types of discharge calculations are highly complex The best way to determine the incendivity of a discharge and to approximate its energy is through a phenomenological approach –In doing so, the equivalent energy of a discharge can be established –Determined by matching the ignition threshold for the discharge to the required spark discharge energy for a flammable mixture 35 Discharge Maximum Equivalent Energy Spark dischargeVirtually unlimited Propagating Brush Discharge (PBD)≈ 100–1000 mJ Brush discharge (positive)≈ 10 mJ Brush discharge (negative)≈ 1.0 mJ Corona discharge (positive)≈ 0.1 mJ Maximum equivalent energy of discharge (Britton, 1999) Britton, L.G. (1999). Avoiding static ignition hazards in chemical operations: A CCPS concept book. American Institute of Chemical Engineers: New York, NY.
© 2011 SRI International 36 Probe Configurations H 2 Jet Charged Particles Probe + + ++++++ + + + + ++++++ + + + + + Measure charge buildup on probe Perform tests with no particles to show that shock initiation is not occurring Grounded Probe (2008 test) Charged Probe + + +++++ + + +++++ Grounded Probe with Floating Charge Collection Plate Plate builds up charge and creates a corona or spark that could ignite the gas A A - + V Ungrounded
© 2011 SRI International Particle Entrainment Locations Nozzle Iron Oxide External Entrainment Connector Internal Entrainment Connector Particle Entrainment Tube (Sealed for Test) (Open for Test) Nozzle Valve External EntrainmentInternal Entrainment 37
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