1. Funded by FCH JU (Grant agreement No. 256823) 1 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 1

2. Funded by FCH JU (Grant agreement No. 256823) 2 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 2  Astbury and Hawksworth (2007) have performed an analysis of hydrogen ignition mechanisms, and stated that over the last century, there have been reports of high pressure hydrogen leaks igniting for no apparent reason.  They underlined that although many leaks have ignited, there are also reported leaks where no ignition has occurred.  Investigations of ignition often simply do not stand up to rigorous scientific analysis.  Clearly there are gaps in the knowledge of the exact ignition mechanism for releases of hydrogen.  Some potential mechanisms will be considered in this lecture: electrostatic discharge, mechanical ignition, hot surface ignition, adiabatic compression, spontaneous ignition of sudden release (diffusion ignition), etc.

3. Funded by FCH JU (Grant agreement No. 256823) 3 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 3  The search using the Major Hazard Incident Database Service of the Health and Safety Executive (UK) revealed 81 incidents involving releases of hydrogen with ignition.  Of those, a delay between release and ignition was reported for only 4 releases.  In 11 cases, the source of ignition was identified, but in the remainder, i.e. 86.3% of incidents, the source was not identified.  This contrasts with data on the non-hydrogen releases, where 1.5% did not ignite, and only 65.5% of ignition sources were not identified.  It is worth noting that since this is a Major Hazard Incident Database, releases of hydrogen which simply dispersed and did not involve fire, explosion, are not recorded, so the non-ignition being reported as zero is not necessarily an indication that all hydrogen releases ignited.

4. Funded by FCH JU (Grant agreement No. 256823) 4 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 4  Electric sparks (static charges, short circuiting, fuse tripping, contactors)  Adiabatic compression (pressure increase)  Mechanical sparks (grinding, impact)  Ionizing radiation (radioactivity)  Electromagnetic radiation  Ultrasonic radiation  Light (laser / flash)  Chemical reactions  Metallic particles  Hot surfaces  Explosives  Flames

5. Funded by FCH JU (Grant agreement No. 256823) 5 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 5  Minimum ignition energy (MIE) of flammable gases and vapours is the minimum value of the electric energy, stored in the discharge circuit with as small a loss in the leads as possible, which (upon discharge across a spark gap) just ignites the quiescent mixture in the most ignitable composition.  For a given mixture composition the following parameters of the discharge circuit must be varied to get the optimum conditions : capacitance, inductivity, charging voltage, as well as shape and dimensions of the electrodes and the distance between electrodes.  The minimum ignition energy of hydrogen in air is low as 0.017 mJ, and more than order of magnitude is lower for hydrogen in oxygen – only 0.0012 mJ (1.2 micro-Joule!).

6. Funded by FCH JU (Grant agreement No. 256823) 6 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 6  The minimum ignition energy of hydrogen is smaller than for other fuels:  Hydrogen 0.017 mJ  Methane0.28 mJ  Propane0.25 mJ  Gasoline0.23-0.46 mJ  Methanol0.14 mJ  A weak spark caused by the discharge of static electricity from a human body may ignite these fuels in air.  The ignition energy is a function of hydrogen concentration in flammable mixture.

7. Funded by FCH JU (Grant agreement No. 256823) 7 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 7  Ignition energy is a function of hydrogen concentration in air (or any other oxidiser).  Hydrogen has such low minimum ignition energy that it is often difficult to determine the exact mechanism and the cause of an ignition when it occurs.  Ignition source with energy 0.020 mJ ignites mixtures 15%-35% ; source with energy 1 mJ energy – about 6%-64% Ignition energy, Micro-Joules Hydrogen concentration in air, % by volume U. Schmidtchen, 3 rd ISCARW on Hydrogen Safety, Belfast, May 2009.

8. Funded by FCH JU (Grant agreement No. 256823) 8 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 8 The ignition energy at the Lower Flammability Limit (LFL) that is of interest for hydrogen safety, is somewhat similar for these three gases (quite high compared to the minimum ignition energy yet many sources would provide this level of energy). Source with ignition energy 0.24 mJ will not ignite methane or propane yet will ignite hydrogen-air mixtures in the range (green dash lines): about 6.5%-58%

9. Funded by FCH JU (Grant agreement No. 256823) 9 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 9 ZoneDescriptionFrequency 0 An area where an explosive gas atmosphere is present continuously or for long periods >1000 h/a 1 An area where an explosive gas atmosphere is likely to occur in normal operation >10 h/a but <1000 h/a 2 An area where an explosive gas atmosphere is not likely to occur during normal operation, or if it occurs will only exist for a short period <10 h/a No zoneSafe areaNo Ex-Atmosphere present at any time  Equipment used in a zone 0 area has to be intrinsically safe. The equipment has to be certified by a notified body in order to get it marking.  Regular maintenance will also be required to ensure that the equipment remains intrinsically safe.

10. Funded by FCH JU (Grant agreement No. 256823) 10 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 10  Three main types of electrostatic discharge: spark, brush, corona.  Spark discharge is single plasma channel between the high potential conductor and an earthed conductor.  Brush discharge is a discharge between a charged insulator and a conducting earthed point. They are characterised by many separate plasma channels, combining at the conductor. As the charged surface is a non- conductor, a capacitance and hence energy cannot be determined.  Corona discharges are silent, usually continuous discharges with a current but no plasma channel. A corona discharge is able to ignite a hydrogen–air mixture without there being a discrete spark. The voltage required for a corona depends on tip-radius. The prevention of ignition of hydrogen deliberately vented to atmosphere has been applied before by using a polished toroidal ring at the end of the vent (tip radius is large). In practice, corrosion and dirt deposits are effectively small radius protrusions…

11. Funded by FCH JU (Grant agreement No. 256823) 11 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 11  The energy of a spark discharge is E=CV 2 /2, where C is the capacitance of the item, and V is the potential (voltage).  A person has a capacitance of about 100 pF. Taking a typical MIE of 0.29 mJ for a hydrocarbon-air mixtures, then the voltage required to produce a spark of sufficient energy to ignite is V= (2 E / C ) 1/2, i.e. 2408 V.  For air, the dielectric strength is about 30 kV/cm. So, the gap required between the charged conductor and the earthed point for breakdown to occur would be 2400 / 30= 0. 08 cm or 0.8 mm.  People typically cannot feel an electrostatic shock of less than about 1 mJ, so would be unaware of the potential to ignite a hydrocarbon–air mixture (and hydrogen-air mixtures – see next slide).

12. Funded by FCH JU (Grant agreement No. 256823) 12 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 12  If hydrogen is now considered, the corresponding voltages and gaps are much reduced. The dielectric strength of hydrogen atmospheres is only 17.5 kV/cm. Its quenching gap is only 0.64 mm (not 2-3 mm as for hydrocarbons). The variation of dielectric strength with concentration of hydrogen in air mixtures is unknown, but as a simple case, it could be assumed to be linear within a specified narrow concentration range, so the theoretical dielectric strength of an about stoichiometric mixture of 30% of hydrogen in air would be (0.3x17.5 + 0.7x30) = 26.25 kV.  With a quenching distance of 0.64 mm and a dielectric strength of 26.25 kV/cm the breakdown potential would correspond to a voltage of 26.25x0.064= 1.68 kV ( 2 kV can be generated easily without people being aware of it), which corresponds to a stored energy of 0.5x[100x10 −12 ]x[1.68x10 3 ] 2, i.e. 0.141 mJ. This is more than sufficient to ignite the stoichiometric hydrogen–air mixture ( requires 0.017 mJ ).

13. Funded by FCH JU (Grant agreement No. 256823) 13 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 13 Zone Maximum area, cm Group IIAGroup IIBGroup IIC 00.3 0.1 13.0 2.0 2No limit Zone Maximum area, cm 2 Group IIAGroup IIBGroup IIC 050254 1100 20 2No limit  Hydrogen belongs to the flammable gas Group IIC.  CENELEC (2003) restrictions on widths of narrow materials depending on zones and gas categories ( left ).  CENELEC (2003) restrictions on chargeable surface depending on zones and gas categories ( right ) – restricts areas of insulating materials that may become charged, and limits the maximum charge that can be transferred from the surface in the form of a brush discharge (maximum tolerable charge transferred for hydrogen is 10 nC ).

14. Funded by FCH JU (Grant agreement No. 256823) 14 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 14  The key properties of burning metal particles or sparks that are relevant to their ability to cause ignition of a flammable atmospheres are:  Size  Material  Velocity  Temperature  Number  Combustion rate and time  There is a metal-to-metal contact pressure and relative velocity threshold for spark production during impact, rubbing or grinding. Above the threshold metal particles are lost from the weaker of the two materials. Generally, particles are only produced when the relative velocity between the two surfaces exceeds 1 m/s.

15. Funded by FCH JU (Grant agreement No. 256823) 15 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 15  The lowest temperatures for ignition being associated with large volumes and surface areas.  Hot surface ignition temperature (°C) is a function of characteristic size of hot surface (mm).  Hydrogen is represented by the curve “Group IIC”.  Catalytic surface, e.g. platinum, has a dramatic effect on the ignition temperature: ignitions are reported at temperatures as low as 70°C (standard AIT is 510°C ).

16. Funded by FCH JU (Grant agreement No. 256823) 16 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 16  The ideal gas, when compressing it at constant entropy, would increase the pressure due to the compression in accordance with the relationship PV  = const.  It can be shown with use of the equation of state for the ideal gas that as well TV  -1 = const.  For example, for compression ratio V 1 / V 2 =28 at the initial temperature of 293.15 K (  =1.39) would increase to T 2 = T 1 ( V 1 / V 2 )  - 1 = 1075.2 K, i.e. the temperature rise is 782 K.  In experiments conducted by Pan et al. (1995), the actual measured temperature realised by a compression ratio of 28 times was only 149 K.  Based on this, Astbury and Hawksworth (2007) concluded that isentropic compression ignition is unlikely to occur in practice.

17. Funded by FCH JU (Grant agreement No. 256823) 17 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 17  Hydrogen “ignition” pressure as a function of pipe length in series of tests with flat burst disks : Pinto - hydrogen was compressed just before the burst disk rupture by a piston; Mogi - aqueous Na 2 CO 3 solution to visualise flame.  Lowest pressure 2.04 MPa observed by Dryer et al. (2007). H2Air Heated air Shock frontCold H2 Contact surface Ignition Rupture disk location Flame front

18. Funded by FCH JU (Grant agreement No. 256823) 18 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 18 Mechanism: rupture disk separates high pressure hydrogen and air in a pipe (both at ambient temperature). After disk ruptures a shock wave heats air). Hot air mixes with cold hydrogen at contact surface and ignition happens. TemperatureHydrogen H2Air Heated air Shock frontCold H2 Contact surface Ignition Rupture disk location (x=0) Flame front

19. Funded by FCH JU (Grant agreement No. 256823) 19 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 19 Instantaneous Rupture disk (32  s)  Time of rupture disk opening affects the ignition process.  Use of valves (“slow” opening even for fast valves) sometimes eliminates ignition at pressures when rupture disk activation ignition (this is thought due to “killing” the strong shock heating, etc.).

20. Funded by FCH JU (Grant agreement No. 256823) 20 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 20 CFD predictive capability as hydrogen safety engineering tool. Experiments (Golub et al., 2010):  1 – 210 mm long (D=16 mm) tube;  2 – 280 mm long (D=10 mm) tube;  3 – T-shape PRD;  4 – burst disk (13.5-29 bar)  Pressure gauges  Light sensors Results:  No ignition – 13.5 bar  Ignition – 29 bar (24 bar?!) Spontaneous ignition in T-shape PRDSpontaneous ignition in T-shape PRD

21. Funded by FCH JU (Grant agreement No. 256823) 21 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 21  Yellow – heated by shock wave air.  Red – heated by reflected shock wave air.  Light blue in the pipe – flowing hydrogen.  Dark blue – cooled during expansion to atmosphere hydrogen. Temperature scale: 0-1100 K

22. Funded by FCH JU (Grant agreement No. 256823) 22 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 22 Temperature scale: 0-2400 K

23. Funded by FCH JU (Grant agreement No. 256823) 23 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 23 OH fraction scale: 0.010-0.001

24. Funded by FCH JU (Grant agreement No. 256823) 24 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 24 Ignition – YES, sustained fire – NO. Temperature scale: 0-2400 K

25. Funded by FCH JU (Grant agreement No. 256823) 25 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 25 Ignition – YES, sustained fire – NO. OH fraction scale: 0.010-0.001

26. Funded by FCH JU (Grant agreement No. 256823) 26 © HyFacts Project 2012/13 CONFIDENTIAL – NOT FOR PUBLIC USE 26  Using electrical equipment and instrumentation classified for the zone in which it is located. New mechanical equipment will need to be selected in the same way;  Earthing of all plant/equipment;  Elimination of surfaces above auto-ignition temperatures of flammable materials being handled/stored (see above);  Provision of lightning protection;  Correct selection of vehicles/internal combustion engines that have to work in the zoned areas;  Correct selection of equipment to avoid high intensity electromagnetic radiation sources, e.g. limitations on the power input to fibre optic systems, avoidance of high intensity lasers or sources of infrared radiation;  Prohibition of smoking/use of matches/lighters;  Controls over the use of normal vehicles;  Controls over activities that create intermittent hazardous areas, e.g. tanker loading/unloading;  Control of maintenance activities that may cause sparks/hot surfaces/naked flames through a Permit to Work System;  Precautions to control the risk from pyrophoric scale, usually associated with formation of ferrous sulphide inside process equipment. What is not covered in the list above is to do with human factors, training and supervision of personnel.