Lecture 9 Fires & Explosions F21 PDF

INTRODUCTION The role of the forensic scientist in regards to scientific fire investigations: determination of the cause of the fire; detection of arson accelerants, collection and analysis of physical evidence from arson scenes. Fire & Explosions PROFESSOR LINDA ROURKE 1 2 Chemistry Of Fires A...

INTRODUCTION The role of the forensic scientist in regards to scientific fire investigations: determination of the cause of the fire; detection of arson accelerants, collection and analysis of physical evidence from arson scenes. Fire & Explosions PROFESSOR LINDA ROURKE 1 2 Chemistry Of Fires And Fuels Chemistry of Combustion Chemistry of Combustion ■ Fire or Combustion →a chemical reaction involving the oxidation of a fuel; the fuel and the substance causing the oxidation (the oxidant) combine to release energy in the form of heat and light - an exothermic reaction. 3 ■ 4 Chemistry of Combustion Chemistry of Combustion Since most fuels involved in fire investigation are organic compounds, water and carbon dioxide are the major products of combustion. (ethanol) C2H6O + 3O2 ® 2CO2 + 3H2O (glucose) C6H12O6 + 6O2 ® 6CO2 + 6H2O 5 The nature of the chemical products of combustion depend on the makeup of the fuel and the conditions of combustion; most fuels are organic hydrocarbons which upon burning result in the oxidized forms of hydrogen and carbon: 2H2 (gas) + O2 (gas) ® 2H2O (gas) C (solid) + O2 (gas) ® CO2 (gas) ■ Most liquid fuels, like gasoline, kerosene and fuel oil, are mixtures of many different hydrocarbons; composition not fixed. 6 1 Types of Combustion Flaming Combustion ■ Flaming Combustion ■ Glowing Combustion ■ ■ ■ 7 8 Flaming Combustion Flaming Combustion Liquid fuels: flaming combustion occurs when some of the liquid is vaporized to gas before it burns. ■ Resulting heat is fed back to the liquid producing more gaseous fuel to be combusted. ■ Stopping the continual volatilization of fuel will extinguish the combustion. Solid fuels, such as wood, require heating the solid until irreversible chemical changes take place resulting in the formation of volatile gaseous products. ■ This is thermal decomposition = destructive distillation = pyrolysis. ■ 9 ■ 10 Glowing Combustion Glowing Combustion Gas-solid surface reaction between oxygen in air + solid fuel. ■ Occurs for solids which cannot be pyrolyzed to produce sufficient amounts of flammable gas to undergo flaming combustion. Carbon, which cannot be volatized at reasonable temperatures, can be heated enough to initiate an exothermic oxidation rxn ® glowing combustion. ■ Combustion @ surface consumes that layer and combustion continues on newly exposed layer until fuel is consumed. ■ 11 Occurs only when fuels are in gaseous state. If both fuel and oxygen are mixed in the right proportion prior to ignition, an explosion would occur upon ignition; if rate of fuel supply is limited, the rate of combustion would be controlled and a flame would be observed. Gas explosion and torch flames are both gas phase reactions. ■ 12 2 Glowing Combustion ■ ■ Glowing Combustion Reaction rate of glowing combustion limited by the surface area or the surface-tovolume ratio of the fuel and the supply of oxygen; extremely rapid reactions occur with finely divided particles mixed with air, hence charcoal dust explosions. With low surface to volume ratios, can increase reaction rate by providing better access to air - send air current over fuel. 13 Pyrolyzable fuels such as cloth, wood, and paper may undergo glowing combustion when conditions are insufficient to support flaming combustion: limited access to air ■ Combustion at surface is called smoldering. ■ 14 Conditions for Combustion Fire Pentagon Ignition The fire pentagon is a model which symbolizes the five primary factors required to initiate and sustain a fire: Heat Heat Fuel Oxygen Free Radical Chain Reactions Ignition Free Radical Chain Reactions free radicals are unstable molecular fragments which serve as intermediates in the combustion reactions. Removal of any of these five factors will prevent/extinguish the fire. Fuel Oxygen 15 16 Free Radical Chain Reactions Ignition Fire Pentagon Heat Fire Pentagon Fuel Oxygen Deprive oxygen to extinguish fire place heavy cloth or layer of foam over fire. Water is an effective extinguishing agent because of its ability to remove heat; absorbs heat from fire ■ It takes more heat to raise water temperature than for any other common substance -- specific heat of water is high; ■ Considerably more heat is absorbed if the water is vaporized. ■ Free Radical Chain Reactions Ignition Heat Fuel Oxygen http://www.deneefe.co.nz/zoom,149.sm 17 18 3 Fire Pentagon Fire Extinguishers Free Radical Chain Reactions Ignition Heat ■ Fuel Oxygen http://en.wikipedia.org/wi ki/Fire_extinguisher Extinguishing agents like halogenated hydrocarbons extinguish fire by interfering with the free radical chain reactions. 19 http://www.ggfire.com/images/Fire-ExtinguisherTypes.gif 20 Combustion Properties of Fuels Types of Fuels Four major classes: ■ Gases ■ Liquids ■ Pyrolyzable solids ■ Nonpyrolyzable Ignition Temperature - temperature necessary to initiate combustion reactions for gas, liquid vapor or nonpyrolyzable solid; for fuels in pyrolyzable solid class, it is the temperature necessary to decompose fuel to yield ignitable vapor. Self-ignition (auto-ignition) Temperature - for pyrolyzable solid, temp at which vapors will self ignite; @ higher temp than ignition temp. ■ solids 21 ■ 22 Combustion Properties of Fuels ■ 23 Water, water based foams, dry powder chemicals, CO2, halons Combustion Properties of Fuels Flashpoint - property of liquid fuels; it is lowest temp of liquid fuel at which enough volatilization occurs to produce an ignitable vapor at its surface; at room temperature, kerosene is below flashpoint ® lighted matches dropped into a pool of kerosene will not ignite; gasoline has a low flashpoint ® at room temp it will always have ignitable vapor. ■ Fire point - a few degrees above flashpoint; temp at which a spark will ignite the fuel and produce enough heat to sustain flaming combustion for at least 5 seconds; ignition of fuel @ flashpoint will result in brief flash which consumes the entire vapor, then dies out. 24 4 Fire Patterns Fire Patterns ■ Path taken by fire depends on fuel source. ■ Homogeneous flammable mixture has spherical shape. ■ Most fires conform to the shape of the available fuel sources. 25 ■ ■ 26 Fire-Scene Investigation Fire-Scene Investigation Primary concern in an arson investigation is to determine the cause of the fire; involves: 1. Determination of the point of origin of the fire. 2. Examination of electrical/mechanical equipment to evaluate the possibility of the fire having been caused by its failure. 3. Examination of burning patterns. 4. Recognition and collection of relevant physical evidence. 5. Analysis and reconstruction of the scene. 6. Determination of the cause of the fire. 27 ■ ■ ■ ■ ■ Begin investigation prior to cleanup or overhaul operations Investigator first examines building’s exterior and surrounding area; anything unusual is documented, collected, sent to lab. Forced entries: examine toolmarks or other procedures used at burglary scene. Examine broken glass, if any; thermal vs. mechanical glass fractures. Glass chips, latent prints, blood and tissue. 28 Collection of Arson-Related Physical Evidence Laboratory Examination Of Arson Evidence ■ Arson accelerant, ignition devices (matchbook, Molotov cocktail, candle, electrical appliances). ■ Collect any item suspected of containing flammable liquid in sealed, clean, unused, air tight container; label appropriately. 29 Structural fire patterns - must take into account that fuel source is usually pyrolyzable solids which must first be heated to pyrolyzation Greatest pyrolysis occurs where heat is most intense; since hot gases rise, this will be above the fire and thus further combustion occurs above the initial starting point of the fire in an inverted cone or V pattern. ■ Identify ignition devices; identify and characterize flammable liquids; examine other associated physical evidence. 30 5 Laboratory Examination Of Arson Evidence Methods of Analyzing Accelerants Common Accelerants Gasoline is most common accelerant, then kerosene, charcoal lighter fluid, paint thinner, and lacquer solvent. 31 ■ Recovery of accelerants from arson debris - use Gas Chromatography most often 32 Methods of Analyzing Accelerants Accelerant Recovery: Vapor Concentration Sample preparation: methods include 1. head-space 2. solvent extraction 3. vapor concentration ■ Solid Phase Microextraction (SPME) ■ adsorption onto charcoal 33 34 Identification of Accelerants Gasoline Can be accomplished by GC ■ analyze peak patterns ■ relies on the number and relative sizes of the peaks obtained. http://www.sisweb.com/referenc/applnote/ap2-a.htm 35 36 6 Explosives And Explosions Explosion: ■ Physically, a violent outburst accompanied by a shock wave, intense heat, and loud noise. ■ Chemically: an extremely rapid, exothermic chemical reaction. 37 38 Explosives And Explosions Explosives And Explosions ■ Diffuse explosions - fuel air mixtures; damage is widespread but uniform ■ Concentrated explosions - occur with true explosives - greatest damage near source of explosives Explosive train: Igniter ® primer or detonator ® main charge ® explosion ■ Igniter produces small initial amt of energy - electrical or mechanical ■A booster charge may be intermediate between primer and main charge. 39 40 Explosives And Explosions Types of Explosives Explosive train: ■ Primer/detonator energy sufficient to initiate main charge. ■ Main charge produces large amounts of energy in a confined space - explosion results. ■ Release of high pressure and high heat causes destruction. 41 ■ ■ ■ Explosives are generally chemical compounds that are not stable in their ordinary form; Heat, shock or striking it may cause rapid decomposition resulting in an explosion liberating large amounts of heat and gas. Speed with which compound decomposes determines its type and the intensity of destruction. 42 7 Types of Explosives ■ Low – Explosives → propellants burning explosives, easy to initiate Used to propel bullets, project rockets: ■ Detonation velocity <1000 meters/second ■ High – Low Explosives ■ ■ Explosives – Detonation velocity >1000 meters/second – – 43 44 Primary High Explosives Secondary High Explosives → primer or detonator used in blasting caps and firearms cartridge primers ■ extremely sensitive to heat, shock or friction: relatively insensitive to heat, shock or friction ■ requires intense shock wave from primary explosive to detonate it ■ ■ ■ – – – – – lead azide lead styphnate mercury fulminate. 45 – – 2,4,6 trinitrotoluene (TNT) ammonium nitrate nitroglycerin dynamite 46 Collection and Preservation of Explosion Evidence Laboratory Analysis Of Explosive Residues ■ Analysis directed toward ID of explosive, determination of manufacturer, tracing of source. ■ Microscopical Examination ■ Chemical Color Tests ■ Microcrystal Tests ■ Thin-Layer Chromatography ■ Instrumental Analysis ■ Carefully and systematically search scene for traces of detonating device. ■ Locate origin of blast: crater present. ■ Collect all debris and material in that area. ■ Analyze in lab. 47 black powder (charcoal, sulfur and potassium nitrate), smokeless powder (nitrocellulose with nitroglycerin; explode under confinement), solid fuel rocket propellants. 48 8 Current Issues in Fire Investigation Current Issues in Fire Investigation Cameron Todd Willingham Case ■ Texas death penalty case ■ Alleged arson; his 3 young daughters died in the fire ■ Fire investigators testified that more than 20 indicators of arson were found at the scene Cameron Todd Willingham Case ■ Fire investigators trained as apprentices; not required to have a science degree. ■ Some of the traditional indicators of arson have been subsequently found to be consistent with non-arson related fires by scientific experimentation. 49 50 Current Issues in Fire Investigation Current Issues in Fire Investigation Cameron Todd Willingham Case ■ Evidence that debunked the testimony of the original fire investigators was submitted to a Texas review board weeks before his scheduled execution. ■ The board was not required to meet to decide on the new evidence and denied his petition for clemency. Cameron Todd Willingham Case ■ Willingham proclaimed his innocence before, during and after trial. ■ He was put to death by lethal injection in 2004. ■ http://www.newyorker.com/reporting/20 09/09/07/090907fa_fact_grann 51 52 Current Issues in Fire Investigation Texas Forensic Science Commission ■ Commissioned a study to investigate the evidence. ■ Several experts have denounced the original evidence that was used to convict Willingham. 53 9