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Arson and Explosives

Arson and Explosives. The Chemistry of Combustion. Arson Statistics. Arson is the leading cause of fire in the US 50% of arson arrests are juveniles Arson peaks on 1/1, 7/4 and 10/31 Site distribution 50% of arson fires occur outdoors 30% in structures 20% in vehicles

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Arson and Explosives

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  1. Arson and Explosives The Chemistry of Combustion

  2. Arson Statistics • Arson is the leading cause of fire in the US • 50% of arson arrests are juveniles • Arson peaks on 1/1, 7/4 and 10/31 • Site distribution • 50% of arson fires occur outdoors • 30% in structures • 20% in vehicles • Poorest areas have 14X higher arson rates http://www.usfa.fema.gov/statistics/arson/

  3. Arson Statistics http://www.usfa.fema.gov/statistics/arson/

  4. What is fire? • Rapid oxidation of a gaseous fuel • Rapid release of energy (heat) from the exothermic reaction • Gases are heated to high temperature • Gases and soot emit light and heat

  5. Fire Triangle Energy • Vaporization • Ignition • Increased T increases reaction rate • Heat from exothermic reaction provides more energy Fuel Oxidizer • Mixing • Composition within flammability range

  6. Exothermic Reactions • Energy stored in product bonds is less than energy stored in reactant bonds • Extra energy is released as heat of reaction • Energy barrier between reactants and products Activated Complex Sum of reactant bond energies Activation Energy Heat of Reaction (Released) Sum of product bond energies http://www.ucalgary.ca/~kmuldrew/cryo_course/figures/fig11_2.gif

  7. Exothermic Reactions • Molecules only react during collisions • Energy to get over the activation barrier comes from the kinetic energy of the molecules in the collision • Products contain less energy than reactants so exothermic reactions have negative energies

  8. Exothermic Reactions • The higher the temperature, the more collision pairs can get over the activation barrier and react http://mediaserv.sus.mcgill.ca/content/2004-Winter/120(FDA)/Kinetics-Th16-FDA/images/slide0085.jpg

  9. Temperature Dependence of Reaction Rate • Temperature is a measure of average kinetic energy of molecules • Molecules collide with enough energy to exceed barrier height (activation energy) • At higher T more reactants exceed barrier so reaction rate increases

  10. Reaction Rates • Every 10o C increase in Tincreases rate by a factor of 2-3X

  11. A Typical Combustion Reaction: Propane C3H8 +5O2 3CO2 + 4H2O + H Break 8 C-H bonds 2 C-C bonds 5 O=O bonds Make 6 C=O bonds 8 O-H bonds Look up table of bond energies (kJ/mol) H  8*413 + 2*347+ 5*489 – 6*805 – 8*464 -2054 kJ/mol Heat released!

  12. A Typical Combustion Reaction: Propane • 6 molecules do not collide at once! • Reaction is stepwise and complicated • Too little O2 incomplete combustion C3H8 +4O2 CO2 + 2CO + 4H2O + H’

  13. Flash Point • Lowest temperature at which there is sufficient vapor pressure to cause a flammable mixture if ignited by spark or flame

  14. Fire Point • Just above flash point • Lowest temperature at which a substance continues to burn

  15. Ignition Temperature • Minimum temperature at which fuel will spontaneously ignite • No spark or flame needed! • Still must have oxygen

  16. Ignition Temperatures http://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html

  17. Flammable Range • Fuel/air mix must be within certain limits for flame propagation • Lean mix • Too little fuel • Not enough heat produced to sustain fire • Rich mix • Too little oxygen

  18. Flammable Range Gasoline Where’s your carburetor set? 1.4% 7.6% http://www.osha.gov/SLTC/smallbusiness/sec8.html

  19. Backdraft http://www.firetactics.com/BACKDRAFT-STAFFS.jpg

  20. Backdraft

  21. Backdraft • Above flammable range • Fire is starved of oxygen • Flames die off, gases still hot • Window or door opens introducing more oxygen • Fire reignites with possible explosion

  22. Detecting Flammables Hot filament detectors • Non-specific (any flammable works) • Flammable gas is burnt by hot filament • Filament is heated further by heat of combustion • Filament resistance increases • Measure change in resistance • Real time

  23. Detecting Flammables Vapor concentrators • Strips impregnated with activated charcoal • Place in headspace • Heat can gently to desorb volatiles • Volatiles absorb on charcoal • Charcoal is removed and extracted to concentrate residues • Not real time

  24. Detecting Flammables Ion Mobility Spectrometers • Atmospheric pressure mass spectrometer or gas phase electrophoresis • Ions formed at atmospheric pressure • Time of flight, but ion must diffuse through atmospheric pressure gas • Flight time depends on mass, shape of molecule, charge on ion, voltage

  25. Ion Mobility Spectrometer http://www.sensir.com/Smiths/InLabSystems/IonScan/IMS---image.jpg

  26. Ion Mobility Spectrometer http://www.sandia.gov/mstc/images/ims2.jpg

  27. Ion Mobility Spectrometer Hand held spectrometer http://www.scdhec.com/lwm/html/images/equipment/APD2000.jpg

  28. IMS v. GC/MS • IMS is more sensitive (ng or 0.1 L) • IMS is faster • 20 s v. 20 min • IMS is portable (handheld) • IMS is cheaper • GC/MS is definitive, IMS is presumptive

  29. Explosions The Chemistry of Extremely Rapid Combustion

  30. Explosions • A chemical or mechanical action resulting in the rapid expansion of gases • High temperatures • Violent shock wave • Loud noise

  31. Mechanical Explosions • No chemical reaction • Heating sealed container • PV= nRT • Pressure increases • Mechanical integrity of container fails • Example: Pressure cooker explodes

  32. Chemical Explosions • Rapid chemical conversion of solid or liquid to gas • Volume increase of 10,000-15,000X are possible • Temperatures of 3000-4000 oC • Pressures of >>100,000 tons/in2 • Expansion velocity 5 mi/s (7000 mi/hr) • Process takes only microseconds • In high explosives the oxidizer is part of the explosive —no need for oxygen from air

  33. Chemical Explosions • Classified by reaction rate • Low explosives deflagrate, produce subsonic (<1000 m/s) pressure waves • Oxidizer and combustible are mixed mechanically • Black powder, smokeless powder, gasoline in internal combustion engine • Used as propellants in ammunition • High explosives detonate, produce supersonic (>1000 m/s) pressure waves • Oxidizer and combustible are in same molecule • RDX, TNT, dynamite • Shatter targets

  34. Black Powder • KNO3, Charcoal, Sulfur • 10 KNO3 + 8 C + 3 S → 2 K2CO3 + 3 K2SO4 + 6 CO2 + 5 N2 • Volume expansion 5100 X • Simply burns if unconfined • Reaction travels from grain to grain • Used as safety fuse • Black powder in fabric or plastic burns slowly • Used for detonation of high explosives

  35. NitrocelluloseSmokeless Powder • HNO3 reacts with cellulose • OH groups replaced by nitrate groups • “Gun cotton”—primary component of gunpowder

  36. Nitrocellulose

  37. Bullets and Cartridges • Shock from firing pin deflagrates primer which deflagrates smokeless powder • Gases propel bullet from crimp • Empty cartridge is ejected http://www.hevanet.com/ranstead/Cartrige.jpg

  38. Shotgun Shells • Instead of a bullet, tiny metal beads (shot) are propelled outward http://www.hevanet.com/ranstead/FirearmID.htm

  39. High Explosives • Primary explosive (primer) • Extremely sensitive to heat, friction, and shock • Detonates rather than burns in open • Lead azide, lead styphanate, diazodinitrophenol • Blasting caps

  40. High Explosives • Secondary explosive • Insensitive to heat, friction, and shock • Burns rather than detonates in open • Requires primer to detonate • Dynamite, TNT, PETN, RDX

  41. But……. • Detonation is extremely rapid • A blast initiated at one end of a 5 mi long garden hose filled with RDX will arrive at the other end in 1 second!

  42. Secondary Explosives http://www.spie.org/web/oer/april/apr98/images/freds_fig1.gif

  43. Reaction of TNT • 6 CO + 3/2 N2 + 5/2 H2 + C Unimolecular reaction—not limited by collisions, gas mixing NO atmospheric oxygen required http://www.fas.org/man/dod-101/navy/docs/es310/chemstry/chemstry.htm

  44. Priority of Reactions • C + O  CO • 2 H + O  H2O • CO + O  CO2 • Remaining H, N form H2, N2 • Remaining C forms soot

  45. Effects of Explosion • Blast Pressure Effect • Fragmentation Effect • Incendiary Effect

  46. Blast Pressure Effect • Reaction is over in 100 s • At detonation point pressure is 1400000 lbs/sq in (105 atm) • Blast moves at 7,000 mi/hr • Initial blast is positive pressure • 2nd phase is suction pressure • Blast energy dissipates in a few hundred feet

  47. Blast Pressure Effect • Blast is a wave • Reflects • Focuses • Refracts • Blasts in enclosed structures can do more damage because of reflections and focusing

  48. Fragmentation Effect • Bomb casing expands to 1 ½ times its diameter before fragmenting • Fragments and shrapnel reach velocities of 2700 ft/s (speed of military bullet) • 50% of energy released goes into destroying casing and fragmentation

  49. Incendiary Effect • Low explosives burn (flash) for longer • High explosives burn hotter • Incendiary effects are usually minor compared to fragmentation and blast effects unless normal combustion of another fuel is ignited by the blast

  50. Detection and Analysis • Search for undetonated residues in crater and debris • Search for detonating mechanism or parts • Ion mobility spectrometer for screening • Samples wiped down • Gently heated inlet vaporizes molecules

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