Terminology(2), Spontaneous Combustion, Investigating the Scene

Question 1

What is Pyrolysis?

Answer 1

• Thermal decomposition of an organic substance to produce lower molecular mass compounds, without involving oxygen
• Pyrolysis products can be volatile and flammable. If their concentration is within their flammability range, they may burn at the surface of the solid.
• Solid left behind becomes closer to carbon.
• Real substances give complex mixtures of pyrolysis products
• Pyrolysis products of plastics can be highly toxic.

Question 2

What is smoldering combustion?

Answer 2

• Smoulder - to burn and smoke without flame; it is a form of flameless combustion.
• Occurs in substances that can form a solid char by pyrolysis e.g. wood, fabrics, packaging etc.
• Takes place at surface of material: fuel-air interface; not in gas phase like flaming combustion.
• Can occur at very low O₂ concentrations, especially if the molecular structure contains oxygen.

Question 3

What is spontaneous combustion?

Answer 3

• Some substances undergo natural exothermic reactions
• Heat is lost from the external surface of the material
• Temperatures rise internally. The reaction accelerates, but heat is lost more rapidly, so an equilibrium temperature is reached.
• In a large quantity of material, the surface area is relatively small and the equilibrium temperature is higher. If the temperature exceeds auto-ignition, a fire can start within the bulk of the material.

Question 4

What is characteristic of spontaneous combustion?

Answer 4

• Common in sawdust, coal dust, hay, straw etc.
• Happens in unsaturated oils soaked into rags etc.
• May resemble an arson attack
• Highly reactive chemicals can also ignite spontaneously in air.

Question 5

What is explosive combustion?

Answer 5

• Occurs when vapours, gases or dusts are premixed with an appropriate amount of air (stoichiometric) and ignited.
• Premixing allows entire combustion to occur in very rapid time
• Mechanical effects are recognised as an explosion.

Question 6

What are dust explosions?

Answer 6

• Occurs when finely divided solid fuel is mixed with air and ignited.
• High risk comes from coal dust, sawdust, flour and chemical dusts.
• Flammability/explosion limits are defined in terms of mass
• Finer dusts have a lower explosion limit

Question 7

What is wood?

Answer 7

• Wood is made up of cellulose (~50%), hemicellulose (~25%) and lignin (~25%)
• Cellulose is a polymer (C6-H10-O5) of about 1500 chains.
• Wood differs in density, water content, voltatile resin content etc.
• Ignition and combustion depends on formation of combustible pyrolysis products and their mixing with air.
• These products can be ignited by a flame at flash point temperature or ignite spontaneously at ignition temperature.

Question 8

How easily ignitable is wood?

Answer 8

• Ignition temperatures as low as 230^oC
• Resinous woods such as pine are easily ignited.
• Hard woods are more difficult to ignite, but produce a hotter and more protracted fire
• Many manufactured wood products contain inorganic adhesives, holding layers or pieces of wood together
• May also have surface coatings or varnishes

Question 9

What are plastics?

Answer 9

• Plastics are polymers containing hydrocarbon chainsand other structural units.
• Thermoplastics: have structural linear chains. Undergo reversible melting without appreciable chemical decomposition e.g. polyethylene, polystyrene. Tm < ignition temperature
• Thermosetting plastics: cross-linked structures. Do not melt but decompose chemically and leave a solid char.

Question 10

What are the different decomposition mechanisms of thermosetting plastics?

Answer 10

• end-chain scission: removal of monomers from end of polymer backbone. Produces monomers
• Random scission: main chain bonds broken at random locations until the sections small enough to volatise. Produces range of oligomeric molecules.
• Chain stripping: polymer backbone intact, but molecular species not part of bb break away.
• Cross linking: some undergo further cross-linking during pyrolysis, generating further char.

Question 11

What are the pyrolysis products of cellulosic polymers, polyolefins and polyesters?

Answer 11

Cellulosic polymers: Main pyrolysis products are water

Polyolefins: Hydrocarbons such as polyethylene and polypropylene. Products are oligomers of the chain and hydrogen.

Polyesters: pyrolysis products include carbon monoxide and water.

Question 12

What are the pyrolysis products of polyurethane and nylon?

Answer 12

Polyurethanes are polymers of isocyanates and alcohols. Pyrolysis products include carbon monoxide, hydrogen cyanide and isocyanates.

Nylons: polymers of acids and amines. Pyrolysis products include carbon monoxide and hydrogen cyanide.

Question 13

What are the pyrolysis products of polymethylmethacrylates and PVC?

Answer 13

• Polymethylmethacrylates are polymers of methacrylic acid. Pyrolysis products include the monomer, with methanol and formaldehyde.
• PVC produces vinyl chloride (CH2=CHCl) and HCl. Highly toxic but less flammable than other pyrolysis gases. HCl acts to inhibit combustion. HCl is produced by chain stripping, leaving a char.

Question 14

What are the pyrolysis products of polystyrene?

Answer 14

• Main pyrolysis product is the styrene monomer, which is flammable and highly toxic.
• This is a C₈ molecule which can be confused with residues from petrol

Question 15

How is heat propagated by conduction?

Answer 15

• Conduction is the heat transfer through a material by direct atomic or molecular contact.
• Heat energy in form of atomic vibrations.
• Most important in solids where atoms are in direct contact.
• A fire spreads through a solid fuel by conduction of the activation energy into adjacent unreacted areas of the fuel.
• Materials with low thermal conductivity heat up most rapidly and can soon reach temperatures for pyrolysis and ignition.

Question 16

How is heat propagated by convection?

Answer 16

• Convection is transfer of heat in gas or liquid by circulation of molecules, caused by temperature differences.
• Regions of high temperature are less dense and rise.
• In burning fires, this sets up air currents which draw in oxygen at the base of the fire, increasing ventilation.
• Walls and ceilings are dried and heated by hot gases rising from a fire. Even if the flames do not reach them directly, they can auto-ignite if hot enough.

Question 17

How is heat propagated by radiation?

Answer 17

• Radiation is emission of heat energy as electromagnetic radiation (infra-red energy)
• Intensity is proportional to the fourth power of the absolute temperature; Stefan-Boltzmann Law
• Radiation energy falls of as inverse square of the distance from the source.
• Radiation from flames plays a major role in spread of fire.

Question 18

How does ‘flameover’ lead to radiation-induced flashover?

Answer 18

• If a fire plume cannot escape, it will spread a layer of hot gases underneath the ceiling.
• Flames can spread horizontally at great speed; flameover
• Enormous amounts of heat is radiated downwards.
• At these heats, all flammable gases and fuels in the room begin to decompose. Within seconds they reach ignition temperatures and catch fire.
• This is radiation induced flashover and represents a downward spread of fire.

Question 19

What evidence and information may be obtained at a fire scene?

Answer 19

• Where did the fire start? How did it start? How did it spread?
• Physical evidence includes smoke records, temperature indications, debris layer sequences and remains of ignition devices.
• Chemical evidence include chemical analysis of trace residues and presence of accelerants.
• Circumstantial evidence: evidence of forced entry, witness statements etc.

Question 20

Before the forensic investigation can begin, what must be completed?

Answer 20

• The fire scene must be made safe.
  
  • Risk of structural collapse
  • Live electrical cables
  • Broken sewers/drainage - biohazard
  • dust hazards etc.
• Risk assments must be dynamic.

Question 21

How is the point of ignition located?

Answer 21

• Fire burns upwards. The lowest point of burning generally indicates the seat of the fire.
• Rapid upward circulation of fire gives a V shaped smoke patter on walls, pointing downwards to the seat.
• Directional indicators: typically, the face of the material that is directly exposed to the fire will show greatest damage.
• Glass and plastics melt towards the fire

Question 22

What should the forensic scientist look for after finding the seat of the fire?

Answer 22

• Trails of streamers from accelerants, trails of paper etc.
• Containers capable of holding accelerants
• Ignition devices: matches, lighters, timers etc.
• Signs of forced entry.
• May need to excavate debris layers and reconstruct scene.
• Notes and sketches are highly important.
• If a fire has more than one seat, that is evidence of arson.