1
Q
Liquids and flames
A
- Flames are gaseous reactions
- Gases can be flammable
- Liquids themselves are not flammable, but can release vapours. These vapours do burn with flames.
- Once the flame begins, the heat of the flame then releases more vapours.
- Only gases produce flammable combustion
2
Q
Vapour pressure
A
- Vapour pressure: IUPAC definition: “The pressure exerted by a pure substance (at a given temperature) in a system containing only the vapour and condensed phase (liquid or solid) of the substance.”
- Partial atmospheric pressure exerted by the vapours of a liquid (how much pressure comes from the evaporating gases)
- When vapour pressure = atmospheric pressure, this is the definition of boiling point.
- The more vapour (i.e the more volatile), the more there is to burn (more energy for combustion).
3
Q
Explosive limit in a closed system
A
In a closed system, the explosive limit will be the same as the flammability limit.
4
Q
Flammbility limit in open systems
A
- In an open system, other factors (such as turbulence) can play a role.
- Temperature alters the vapour pressure (and as a result, the likelihood of fire).
5
Q
What is another word for flammable limit?
A
Combustion limit
6
Q
Flammability limits
Vapour
A
- The amount of vapour we have is really important in combustion
- Because our combustion reactions need to be in the right stoichiometry, combustion is only possible within certain ranges.
- We need the correct ratio
- If vapour is too low there isnt enough fuel to get sustainable combustion
- If oxygen is too high the reaction gets suffocated by oxygen
- Turbulent air can disturb your flame and play a factor in the observed flammable limits (upper and lower)
- The likelihood of a fire will be temperature dependant .
7
Q
Further away from the fuel surface, the more time the fuel has…
A
Further away from the fuel surface, the more time the fuel has to spread omnidrectionally
8
Q
Fuel vapours
A
- Fuel vapours are capable of burning in air (21% oxygen) only within a certain range of concentrations.
- If the concentration of flammable gas is too low, it cannot burn. The enthalpy of combustion does not generate enough heat to sustain the flame.
9
Q
Lower flammability limit
A
- The lowest concentration of flammable gas in air that can sustain combustion is called the “lower flammability limit” or “lower explosion limit”.
- Below this is “fuel lean = oxygen rich
- for methane, the lower flammability limit in air is 5.3% by volume experimentally calculated
10
Q
If the concentration of flammable gas is too high
A
If the concentration of flammable gas is too high, it cannot burn. There is not enough oxygen to sustain the reaction.
11
Q
Upper flammability limit
A
- The highest concentration of flammable gas in air that can sustain combustion is called the “upper flammability limit” or “upper explosion limit”.
- Above this is “fuel rich”; e.g. a flooded petrol engine = Oxygen deficient
- for methane, the upper flammability limit in air is 14% by volume
12
Q
Stoichastic concentrations / 2
A
- Low flammable limits tend to be half the amount of stoichastic concentrations as a general rule
- Calculations are indicators but don’t always translate to real world situations
13
Q
What do flammability limits apply to?
A
- The flammability limits in the table apply to fuels in air (21% oxygen) at 1 atmosphere pressure.
- Changing the pressure will change the combustion properties (e.g. aircraft at high altitude – 2000 m↑sea level).
14
Q
Whites rule of thumb
A
The lower flammability limit for a gaseous hydrocarbon is about half the stoichiometric concentration
15
Q
What happens if a flammable vapour is pre-mixed with air wihtin its limits of flammability?
A
If a flammable vapour is pre-mixed with air within its limits of flammability and then ignited, an explosion will result.
16
Q
When do the most powrful explosions occur?
A
The most powerful explosions occur when the vapour is mixed with air in its stoichiometric ratio.
17
Q
A mixture with a low vapour ratio
A
A mixture with a low ratio of vapour to air will cause a “lean” explosion. This can be very powerful, but brief, and may not result in a subsequent fire.
18
Q
A mixture with a high ratio of vapour
A
A mixture with a high ratio of vapour to air will cause a flaming, rolling explosion often followed by a fierce fire.
19
Q
Vapour explosions
A
These vapour phase explosions are not as powerful as condensed phase explosions
20
Q
What are flammability limits?
A
Flammability limits are the range of concentrations of fuel vapour capable of burning in air (21% oxygen).
21
Q
What happens if oxygen concentration falls below 15%?
A
- If the oxygen concentration falls below about 15% (e.g. when a fire is in a closed room) the rate of combustion will decrease.
- At low enough oxygen concentrations, the flames will die out and be replaced by smouldering (or glowing combustion) at the surface of solids.
- This oxygen concentration depends on the flammability limits of the fuel and the temperature of the combustion gases.
- If you don’t provide oxygen at the same rate it is being used up then the oxygen levels will drop.
22
Q
Flashpoint definition
A
- “The minimum temperature at which the vapour produced by a liquid can be ignited momentarily in air”
- The flash point is the temperature at which the vapour concentration reaches the lower flammability limit. Then it can be ignited.
- The ignition source is external, it supplies the initial activation energy and is then quickly removed.
- The resultant flame does not self-sustain at this temperature, after the ignition source is removed.
23
Q
When does vapour pressure of a liquid increase?
A
The vapour pressure of a liquid increases with temperature.
24
Q
Ignition source relating to flashpoint
A
- It is not embedded wihtin the fuel
- Supplies activation energy required to overcome LFL
- Powerful momentary delivery of energy
- Quickly removed after this
25
What influences the sustainability of a resultant flame in a flashpoint?
- The heat generated from enthalpy of combustion
- The heat capacity of the combustion products
- The rate of heat loss from the flame by radiation
- The kinetic rate of production of more vapour.
26
Fire point definition
* “The minimum temperature at which sufficient vapour is produced by a liquid to sustain combustion after ignition in air”
* A few degrees higher than the flash point, not always the case.
* After the ignition source is removed, heat produced by combustion must balance heat loss from the flame, so that the temperature does not drop
* Vapour must also be produced at a rate sufficient to maintain concentration above the lower flammability limit. This depends on the volatility of the liquid.
27
What does a low flashpoint correlate with?
- A low flash point temperature correlates with high vapour pressure, i.e. high volatility
- Some volatile liquids have flash/fire points below ambient temperature. They represent significant fire hazards if ignited.
- From the arsonist’s viewpoint, the best accelerants have low flash / fire points
- E.g petrol
28
Flashpoints above ambient temperature
* Other liquids have flash points significantly above ambient temperature. They cannot be ignited in bulk unless pre-heated to a temperature above their fire point
* e.g. diesel, ethylene glycol
29
Measurement of flashpoint an fire point
* The test fuel is placed in a container. The liquid fuel is slowly and uniformly heated with its temperature monitored.
* Periodically as the temperature is increased, an ignition source is inserted into the container.
* The lowest temperature where a flash occurs is defined as the flash point temperature.
* This is technically quite difficult to do accurately; the vapour must be at the measured temperature. Also requires the presence of air.
* It is difficult to achieve defined experimental parameters.
30
What are flashpoint and fire point largerly determined by?
Volatility
31
What can an absorbent material act like?
An absorbent material can act as a wick. It immobilizes the liquid phase in the vicinity of the flame, so that it can be heated locally to a high temperature.
32
What real world systems are more complex?
* mixtures
* waste fuels
* used lubricating oils
* mixtures of petroleum liquids with solids
* organic-solvent based paints, which form surface films
33
Ignition temperature
* Also called auto-ignition or spontaneous ignition temperature
* The temperature at which the fuel will ignite without any additional source (flame, spark)
* Reflects activation energy more than volatility
* Fuel oil and decane have low ignition temperature
34
Heats of combustion
- Oftern quoated as kL / kg-1
- Many fuels are not pure substances but a mixture of different chemicals e.g petrol
- Investigators can identify chemicals
- Many fules and building materials have complex compositions that cannot be defined on a molecular or molar basis e.g wood, coal, polymers so we have to work in mass
35
Aliphatic
* Aliphatic basically just means ‘non-aromatic’
* Aliphatic can mean cyclic e.g. cyclohexane
* Aliphatic species can have conjugation
36
Aromatic
* Aromatic describes delocalised conjugated rings
* 4n + 2 π-electrons, where n = 0, 1, 2, 3, etc
37
Petrol composition
* ΔHc = 43.7 MJ/kg
* A “distillate fuel”, fractionated over a temperature range.
* Mainly aliphatic hydrocarbons (branched and cyclic aliphatics) with some aromatics and alkylated aromatics.
* Typically C7-C12.
* Boiling point range 320C - 1900C.
* Modern petrol also contains lower molecular weight oxygenates such as alcohols (including ethanol) and ethers.
38
Octane rating
Combustion properties of the material you are looking at are X % similar to iso-octane
39
Paraffin
| Kerosene
* Mainly aliphatic hydrocarbons (straight chain and branched) with some alkenes.
* C10 - C16, usually centred around C12.
* Boiling point range 1750C - 3000C.
* Used as heating oils, jet fuels.
* ΔHc = 46.1 MJ/kg
40
Diesel
* Aliphatic hydrocarbons (straight chain and branched).
* C12 - C25 and above, usually centred around C18.
* Boiling point range 2000C - 3500C.
* Used in diesel engines and as heating oils.
* ΔHc = 42-46 MJ/kg
41
Petroleum ether
* Mixture of aliphatic hydrocarbons - NOT ETHERS.
* Mainly C5 and C6.
* Typical boiling point 300C - 600C.
* Chemical and spectroscopic solvent.
42
Benzine
- An aliphatic fraction - NOT BENZENE.
- Also called ligroin.
- Used as a solvent.
43
White spirit
- A mixture of saturated aliphatic and alicyclic
- C7 to C12 hydro-carbons with a maximum content of 25% of C7 to C12 alkylated aromatics.
- Boiling point range 1300C - 230 0C.
- Used in paints and varnishes, in cleaning products and as a degreasing solvent.
- Wasy to get in bulk
44
Tuepentine or turps
* A natural olefinic (alkene) oil obtained by distillation of pine resin.
* Composition quoted as C10H16.
* Boiling point 1550C - 165 0C.
* Used as a solvent and in thinning paints.
* “Turpentine substitute” is petroleum-based and similar to white spirit.
* Not as readily available
45
Methylated spirits
* ETHANOL containing enough methanol to stop (most) people drinking it.
* Used as a cleaning solvent and to thin paints.
* ΔHc = 29.7 MJ/mol
46
Thinners / paint thinners
* Any organic solvent used to dilute paints. Should be volatile enough to evaporate away as the paint dries.
* Modern paints are increasingly aqueous-based for environmental reasons.
* All these hydrocarbon mixtures are used as accelerants by arsonists.
* They can be bought in large quantities (some in ordinary retail stores) without arousing suspicion.
* It is not illegal to possess them.
* From the arsonist’s viewpoint, less volatile accelerants are best, especially if a delayed timing ignition device is to be used to start the fire.
47
Viscous liquids
Viscous liquids (e.g. diesel) spread less far (deeper pool) than those which flow more easily (e.g. methanol)
48
Porous surfaces in a fire
* Porous surfaces (raw wood, concrete, carpet) absorb liquids and make smaller pools, but evaporation is faster due to wick effect.
* Porous surfaces can provide samples for lab analysis
* Different fuels have different profiles they leave on the ground
* The cooling characteristics can help you identify them
* Some fuels can easily absorb into the substrate and as a result they are easy to collect underneath the surface.
* Methanol is shallow and can easily seap into surfaces.
49
Latent heat of evaporation
- Latent heat of evaporation protects the area under the pool from damage to some extent
- Some of the fuel is protected by the liquid on the surface
- In concrete you may find its going into an area with high surface area and more evaporation so theres less fuel to sample.
- Right underneath where the fuel was poured is the best area to perform sampling as you get localised scorching but the area is mostly protected.
50
Gas lines
* Gas lines can be a source of readily ignitable fuel
* Closed lines are safe because there is no oxygen (too rich)
* Escape can occur due to poor joints, corroded pipes, mechanical or thermal failure
51
Different types of gas
- Methane is a natural gas
- Liquid petroleum gas (LPG): mixture of propane and butane
- Aerosol products (lubricants, paints, insecticides, air fresheners, hair spray, etc.) are in pressurized environments with dimethyl ether or mixtures of butane, propane, isobutane and carry the ability to be flammable.
52
The density of vapours/gas
* The density of the vapour/gas relative to air will determine how it fills a room in the absence of significant air movement
* This determines which levels will be inclined to explode
* Vapour density is the ratio of the (average) molecular mass of the gas/vapour to the average molecular mass of air (29)
* More dense vapours sit lower to the ground and lighter ones sit closer to the ceiling
* Really heavy gases can travel downstairs
* Lower number = lighter
* Above 1 = heavier than air
* In explode zone we get idealised zone of combustion.
