Fires 5 Flashcards
(37 cards)
Define pyrolysis
Decomposition of molecules via heat (oxygen not required)
* if enough heat is applied to any material it will eventually break down
Define a glowing/smouldering combustion
No flame, hot materials on the surface of which combustion (oxidation) is occuring
* not a gas phase reaction
* Thermal decomposition of organic substances to produce compounds of lower molecular mass, without using oxygen
* Produces radials
* Products formed can be both flammable and volatile ?????
Define a flame
The region where chemical interactions between gases occur, the result being the evolution of heat and light
Key points about pyrolysis
- solids and liquids cannot be on fire
- only gases produce flames
- The solid undergoes a pyrolytic decomposition, this then forms a flammable gas, the flammable gas can then burn
What process(es) are happening when a cigarette is burned?
Smouldering combustion and pyrolysis
* Have glowing embers which are reacting with air - smouldering combustion
* Paper is degraded and changing colour - pyrolysis
What things do we need for a flame?
- Oxygen, heat and fuel
- Need the correct ratio of fuel and oxygen
- Needs to be gaseous
What is the zone called when there is the correct ratio of fuel and oxygen?
Combustion zone
Height in related to flames
- The observed height of the flame is variable
- The ratio of oxygen to fuel usually changes as the distance from the fuel surface changes
- This is why a candle flame tends to be a little bit above the actual wax, it is not the wax which is burning, it is the vapours produced which ae released as you get further away from that
What is happening when a solid fuel is burning?
- Pyrolysis is happening at the surface because of the heat of the fire
- This makes really volatile and flammable small radicals which move upwards
- As these vapours move up they will get far enough away from the fuel, to a point where the ratio of fuel and oxygen is correct and then you will get the flames
- These vapours will then get gradually oxidised into carbon monoxide and carbon dioxide
- As these molecules move up, they will draw in new air at the bottom which feeds the newly formed pyrolysis products
- Will produce fully oxidised (CO2) and partially oxidised (CO, C) products
What is the type of flame that a candle produces?
Laminar flame
* idealised type of flame (optimal shape) because it has a very linear structure
* Got loads of area where oxygen can come in
* Most of the soot will form fully oxidised products because this is an idealised combustion
How does a candle burn (flaming combustion)?
- The heat melts the wax to form a liquid
- This liquid is taken up by the wick and evaporates and while it is still in a gas state, it will set on fire
- Clear delineation of temperatures
- Fuel is used up at the top of the flame (fast reaction, plentiful oxygen supply
Why is the temperature closes to the fuel one of the lower ones in a candle?
The energy is being taken up by evaporation and there is not as much oxygen in the air, as you get further away from the wick the temperature will increase
How do flames persist in a more forensic setting, when the flames wont be tidy like with a candle?
- Will get turbulent flames, this is where the airflow, which is being drawn in by the heat, is going up and its being drawn in at such a rate that it is no longer even
- Creates a wall of vapours
- Only really one area where the oxygen can come in (ratio of surface area to oxygen is less) therefore the distribution of oxygen is uneven (less optimal)
What is a turbulent flame?
Where the airflow, which is being drawn in by the heat, is going up, it is being drawn in at such a rate that it is no longer even
What are diffusion flames?
The gases or vapours supporting the flame diffuse upwards or outwards from the surface of the fuel, oxygen diffuses towards the fuel from the surrounding air
* Most flames are diffusion flames
What is significant about fires that are produced from a pool of liquid fuel?
- Underneath where the fuel was, the floor will be less badly burnt than the surrounding area, this is because the fire is actually cooler underneath compared to the surrounding areas so the fuel can insulate the floor
- Fuel source provides a layer of protection to the area underneath it - can spot where a fire has occurs, depends on what material the fuel is on (concreate will absorb a lot of the fuel compared to a glossy floor)
Why are flames different colours?
Theoretically
- Flame colour is determined by the wavelength of light emitted which depend on the flame contents
- Can be used to identify elemental compositions of a flame
- Hydrocarbons burn blue in plentiful oxygen supply
- Elements have different colours based on emission
Why are flames different colours?
In an actual fire investigation
- Classic yellow flames are from incomplete combustion
- Soot or other liquid/solid products raised to incadescent temperatures
- Temperature detemines the colour of the flame
What is incandescent temperatures?
The things which are making the light in flames are solid or liquid particles which get super hot, and so hot that they emit light
Glowing/smouldering combustion
- Surface oxidation (not in gas phase)
- Absence of flame
- Presence of very hot materials on surface of which combustion is occuring
- Occurs in substances that can form a char by pyrolysis
- Because it doesnt take place in the gas phase, can occur at very low O2 concentration, especially if there is already oxygen in the fuel (carbohydrates like cellulose)
- Char can slow a fire because it acts as a physical barrier to flaming combustion - prevents oxygen from getting to the fuel source
What is a backdraft?
- If a room runs out of oxygen, then the fire will die down to a smouldering combustion, firefighters open the door, they let in a lot of oxygen, and suddenly you get an enormous amount of flames forming
- Also called ventilation induced flashover
- Major hazard in firefighting because the flames can rise up quickly
Characteristics of a smouldering combustion
- Small associated amount of direct damage associated with them
- Can produce large amounts of poisonous CO
- 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
The application of heat in a fire
- 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 (generation of radicals)
- Heat not only is the driving force but also accelerates them
- Heat spreads fire
- Heat causes damage
- First limiting factor of fires
What is the order in which limits a fire?
- Heat - it is lost rapidly to the surroundings
- Oxygen - a closed room will deplete oxygen and result in a smouldering fire
- Fuel - until the entire structure is destroyed
