1
Q
A
2
Q
Flame
A
- The region where chemical interactions between gases occur. The result being the evolution of heat and light.
- it’s something which happens in the gas phase and is accompanied by the evolution of heat and light.
3
Q
Glowing/smouldering combustion
A
- No flame, hot materials on the surface of which combustion (oxidation) is occurring.
- Not a gas phase reaction
- Coal and cigarettes smoulder
- Some things in coal can go on to flame combust but the coat itself does not flame combust.
4
Q
Pyrolysis
pyro (heat) lysis (breaking of)
A
- Decomposition of molecules via heat (oxygen not required!)
- If you apply enough heat to any material, it will break down eventually.
- Thermal decomposition of organic substances to produce compounds of lower molecular mass, without using oxygen.
- Could also be an inorganic substance that makes smaller compounds.
- This decomposition initiates radical chain reactions.
5
Q
Products of pyrolysis
A
- Products formed can be both flammable and volatile. If the concentration of the products is within the flammability range, they can burn at the surface of the solid.
- They can also be carried in the fire plume and then ignite elsewhere.
- Remaining residues become char (closer to carbon).
6
Q
Pyrolysis
This decomposition initiates radical chain reactions.
A
- So it often produces radicals and Heat is good at making radicals, So that can produce the reactive molecules. The products are often volatile, meaning they evaporate and flammable, easily oxidise.
- Small molecules can burn as they evaporate.
- Some products can be very toxic and some of these structures can also resemble accelerants that people might use in arson.
- So you need to be careful if you suspect something is arson because of the accelerants you’ve detected.
- Was it that or was it just some decomposed plastic?
- Plastics mimic arsenic products when they decompose.
7
Q
Pyrolysis key points
A
- Solids and liquids cannot be on fire.
- Only gases produce flames.
8
Q
Does paper burn?
Pyrolysis
A
- The solid paper burns, but is NOT on fire.
- The cellulose in the paper undergoes a pyrolytic decomposition which forms a flammable gas.
- The flammable gas produced can then burn. This produces the flaming combustion
- Nothing in the paper initially is in the flame.
9
Q
Pyrolysis
Ethanol
A
- After ignition ethanol volaties easy
- The volatised ethanol gives off heat when burned
- This heat then evaporates and vapour comes off the liquid ethanol.
- This leaves gaseous ethanol molecules in the air which is what causes flamming combustion and what burns.
10
Q
What process happens when a cigarette burns?
A
- Smouldering combustion and pyrolysis
- The decomposition of paper (cellulose) and part of the tobacco forms things that can go on to flame combust
11
Q
What do we need for a flame to occur?
A
For something to be a flame it must be in the gaseous state
12
Q
Methane equation
Fuel and oxygen in the correct ratio
A
CH4 + 2 O2 –> CO2 + 2 H2O
13
Q
Candle example
A
- Heat generated from the flame heats the wax so it becomes molten
- This pyrolyses the wax so it enters the gaseous phase
- This gaseous phase contributes to the burning of the candle.
- The flame is far away from the candle because the fuel needs oxygen to burn properly.
- Right next to where the fuel is, there isn’t enough oxygen.
- As you move away from the flame you get a zone of combustion where the fuel and oxygen is in the right ratio (more oxygen is available)
- As we increase the distance we have too much oxygen and not enough fuel so you need to be in the optimal range
14
Q
What leads to different shaped flames?
A
Concentration of fuel vapours differ depending on the fuel used, this leads to different shaped flames.
15
Q
Why does the flame protrude above the wax on candles?
A
- It’s not the wax burning it is the vapours which are released which is above the wax.
- As you get further away from that, the amount of oxygen gets higher, but the amount of fuel is and not enough to have the combustion.
16
Q
Solid fuel burning
A
- We get surface pyrolysis because of the heat from the fire which produces the radicals.
- These vapours rise up and when they get far enough away from the fuel, where the ratio of air is correct, then you will get the flames happening and they will gradually get oxidised into carbon monoxide and carbon dioxide.
- As they do this, they rise and as they do that, they’re going to draw in new air at the bottom.
- Once the molecules are fully oxidised they come out of the top becauses theres a convection plume.
- Further away from the surface, all the fuel is being burnt up and all you get the non-flammable products.
17
Q
What gets formed when the fuel to oxygen ratio is optimised (zone of idealised combustion)?
Solid fuel burning
A
- When a fire is at optimum oxidation, hydrogen cyaninde, water and carbon dioxide are formed.
18
Q
What happens when the fuel to oxygen ratio isn’t ideal?
Solid fuel burning
A
Where the fuel to oxygen isn’t idealised we get incomplete combustion.
19
Q
What is a convection plume?
A
CO2 and water and sometimes some CO from incomplete combustion
20
Q
What happens in the centre point of a solid fuel burning?
A
Center point is difficult for oxygen to access so we still get some incomplete combustion.
21
Q
Laminar flame
A
A candle is called a laminar flame because it has a very linear structure.
22
Q
Flaming combustion of candle
Temperature
A
- Solid cant wax melts from flame
- The liquid is taken up through the wick and it evaporates very easily (outer edge of diffusional combustion)
- Once it’s in the gaseous phase it sets on fire and we get a nice, clear delineation of temperatures.
- Closest to the fuel the temperature is lower because energy is being take up by evaporation and there isn’t as much oxygen in the air.
- As you get further away from the wick, the temperature drastically increases and we reach the hydrocarbon cracking region.
- At this point we get a very fast reaction and have plenty of oxygen (the inner edge of combustion)
- Eventually the fuel will get used up and limit the extent of the flame.
- Past this point the flame will die away and the temperatures will go down again (soot formation region)
23
Q
Inside zones on laminar flame
A
Not as hot because heat rises
24
Q
Outwards zones on laminar flames
A
Increased temperature
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Inner edge of diffusional combustion
In the inner edge of diffusional combustion there is idealised combustion so there is little CO formed
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Why is a laminar flame optimal
- Laminar flame is a an optimal flame in terms of its shape.
- It has an area where oxygen can come into the flame and we have good circulation
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Flames in a forensic situation
* Air flow gets drawn in by heat and is replaced by oxygen
* Typically an uneven distribution of oxygen so it is less optimal
* Vapours (from the pyrolysis products in gaseous stage) travel away from vertical fuel and create a wall of vapours
* Zone of idealised combustion is further away than it is in the candle
* Oxygen can never get to the fuel so the area optimised is much further away
* Flames rises up and oxygen comes in and continues to feed the flame
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Turbulent flames
- Where the airflow, which is being so as being drawn in by the heat, which is going up is being drawn in at such a rate that it's no longer even.
- The vapours come away from the fuel, be that solid or liquid.
- Pyrolysis products formed and enter into gaas phase which causes flaming combustion
- And the flame happens some way from its surface, because that's where you've got the optimum ratio of oxygen and fuel.
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Most flames are X flames?
Diffusion flames
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Diffusion flames
- The gases or vapors supporting the flame diffuse upwards or outwards from the surface of the fuel.
- Oxygen diffuses towards the fuel from the surrounding air.
- Whether that's by pyrolysis or by evaporation, the fire is actually cooler underneath where it is compared to the surroundings up here. So the fuel itself can insulate the floor.
- A pool of liquid fuel can insulate and protect the underlying surface from the heat of the fire which can be helpful in investigations.
- There isn't the availability for it to get underneath though.
- Fuel source has provided a barrier and protection the the surrounding area underneath.
- This is a good spot to observe when considering if arson hs occurred.
- The substrate the fuel is on top of is also important e.g concrete absorb a lot of the fuel.
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What is flame colour determined by
* Flame colour is determined by the wavelengths of light emitted, which depend on the flame contents.
* Flame colour can be important to determine as it can identify elemental components of a flame.
* Sometimes the colour of the flame can be an important thing to establish because it will tell you if certain things are present.
* If you have complete combustion. Then the colour is determined by the chemistry of what's inside and particularly the elements which are there.
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Colour of flame for complete combustion of hydrocarbons?
Blue flame
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Why is flame colour important for hydrocabons?
It can determine how hot the fire is
34
Inorganic elements flame colours
For inorganic elements we get different colours based on their relative absorption and emissions
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LiCl flame colour
Red
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Yellow flames
* ‘Classic’ yellow flames are from incomplete combustion.
* Soot, or other liquid/solid products raised to incandescent temperatures.
* Whatever material you have, it will emit light (and heat) if you get it hot enough.
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Dull red (first visible glow)
500 - 600oC
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Dull red flamw
600-800oC
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Bright cherry red flame
800-1200oC
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Orange
1000-1200oC
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Bright yellow
1200-1400
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White
1400-1600
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Typical colour of flames observed in fire investigations and why?
- Incomplete combustion
- Things which are making the light are solid or liquid particles, which are just got super hot and they've got so hot that they are emitting light. This is what we call incandescent.
-
44
Glowing/smouldering combustion
- Surface oxidation
- Absence of flame
- Presence of very hot materials on surface of which combustion is proceeding. e.g. coal fire (blue flames = CO)
- Hot enough to melt metals etc.
- Colour is from incandescence
- Most of the oxygen comes from the matieral itself
45
Where does glowng/smouldering combustion occur?
* Occurs in substances that can form a char by pyrolysis (wood, cellulosic fabrics, packaging materials)
* Takes place at surface of material, not in gas phase.
* Because it doesn’t take place in the gas phase, can occur at very low O2 concentrations, especially if there is already oxygen in the fuel (e.g. carbohydrates like cellulose)
* Char formation can slow a fire by acting as a physical barrier to flaming combustion.
* Flames will often occur if more O2 becomes available: “backdraft” is a major hazard in firefighting.
46
Backdraft / ventilation induced flashover
- When a room runs out of oxygen so the fire dies down to a smouldering combustion but once more oxygen is introduced (for example from opening a door) enormous flames begin to form again.
- Major hazard in firefighting because the flames can rise up very quickly.
47
Characteristics of smouldering combustion
- Smouldering fires have a small associated amount of direct damage associated with them, but can produce large amounts of poisonous CO.
- Smouldering fires are usually the first and/or the last stage of a fire incident.
- A slow smouldering fire can persist for prolonged periods of time before producing flames.
- E.g matches, cigarettes, etc
48
The application of heat
* Every method by which fires are ignited involve application of heat
* Under the right circumstances, the addition of heat is enough to cause a fire to start (via the generation of radicals).
* Heat not only is the driving force of fires, but also accelerates them.
* Heat spreads fires.
* Heat causes damage.
49
In early stages of a fire, what is the limiting factor?
* Heat is the limiting factor in the early stages of a fire – it is lost rapidly to the surroundings
* And we normally only have a small flame to begin with but reaction rates typically double with every additional 10 °C.
* So once it starts going, it accelerates
* Next limiting factor is oxygen: a closed room will deplete oxygen and result in smouldering fire
* Last limiting factor is fuel – until the entire
* structure is destroyed
50
Three routes of heat transfer
1. Conduction
2. Convection
3. Radiation
51
Conduction
* Transfer of heat through a material by direct atomic or molecular contact.
* The heat energy is atomic vibration.
* Conduction is most important in solids, where the atoms are in direct contact with each other.
* The reason fires spread through a solid fuel is because of conduction. The required activation energy is conducted into adjacent unreacted areas of fuel.
* Materials with a low thermal conductivity (insulators) heat up the most quickly. As a result, they quickly reach the required temperature for pyrolysis (and therefore ignition).
* Wood is a good thermal conductor
52
Hihg thermal conductivity
- Materials like metals have a high thermal conductivity (and as a result dissipate heat).
- This dissipation means heat gets conducted away.
- This does have other consequences though:
metals can transfer large quantities of heat energy to other locations which means you could end up with multiple sources of combustion.
- Specfic heat capacity is also important
53
Specific heat capacity
How much energy you need to put into a substance to raise its temperature by one degree.
54
Steel girder
- high thermal conductivity
- Dissipates heat
- Could transfer heat to something whihc is easily set alight and could start a new fire in another location.
55
Convection
* Transfer of heat in a gas or a liquid by the circulation of molecules, which is caused by temperature differences.
* Regions of high temperature are less dense, and therefore rise upwards.
* Atoms have more relative motion, have to spread out.
* In a burning fire, this creates air currents.
* These air currents draw more oxygen to the base of the fire, increasing ventilation.
* Also dissipates heat around the room.
* Walls and ceilings are dried and heated by the hot gases rising (hot air is less dense) from a fire (the “fire plume”).
* There is a movement of gases whihc ses up currents ahd dissipates heat around the room.
* Even if the flames do not reach them directly, they can auto-ignite if hot enough.
56
Radiation
- The emission of heat as electromagnetic radiation (IR region)
- Doesn't involve the transfer of heat through molecules.
- The intensity of the radiation is proportional to the fource power of the absolute temperature (Stefan boltzman law)
- It increases rapidly in intensity as the temperature gets hotter.
- The radiation energy falls of as the inverse square of the distance from the source.
- Radiation from flames plays a major role in the spread of fire.
57
What happens if a fire plume can't escape?
- It will spread to a layer of hot gases underneath the ceiling which may be flammable.
- These flames can spread horizontally (flameover) at fast rates
- It'll radiate an enormous amount of heat energy downward.
- Can produce high levels of radiant heat at floor level.
- With these ehat fluxes, all flammable gases and fuels in the room begin to decompose.
- Within a few seconds they reach their ignition temperature and catch fire (radiation induced flashover)
- it represents downward spread of the fire.
- Flameover always occurs before flashover.
- When there is enough heat trasnfer eventually everythin catches on fire in radiation flashover.
58
Flashover
- Flashover represents the transition from “fire in the room” to “room on fire”
- Flashover can occur even in a compartment that is open on 3 sides
- Radiation induced flashover is one of the quickest ways a fire can spread.
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Direct flame impingment
* Combination of both convective and radiative mechanisms
* Plume of hot gases rises because it is light and expanding.
* This creates convection currents and at the same time the reaction is creating radiation.
* This radiation will start to pyrolyse the new fuel and generates flammable gases, which are ignited by flames of plume.
