How does combustion work in carbon?
- Coal, charcoal, wood/paper char is primarily elemental carbon
- Happens on the surface of the material
- No flame occurs (smouldering combustion)
- Blue flames can be observed due to CO
What are the different ways that solids produce flames?
- Sublimation
- Melting into a liquid and then evaporation into a vapour (wax)
- Melting and then decomposition and evaporation - pyrolysis
- Decoposition and melting then decomposition and evaporation (plastics)
- Decomposition and evaporation straight into a vapour
Define sublimation
Going from a solid straight to a vapour
What is deposition?
Gas to solid
Give an example of melting and evaporation to produce a flame
Heat from the flame melts the solid to give you a little ball of wax, the wax evaporates and bends to continue the process
Define pyrolysis
Thermal decomposition of an organic substance to produce lower molecular mass compounds, without involving oxygen
* Breaking of chemical bonds
* Depolymerisation (polymers are broken into monomers)
* Often done through radical processes
Facts about pyrolysis products
- Can be volatile and flammable (turned big molecule into a small molecule, will have a lower boiling point and more likely to be gases)
- If their conc is within their flammability, they may burn at the surface of the solid
- They can also be carried in the fire plume and ignite elsewhere
- The solid left behind becomes closer to carbon (char)
- Residues are useful for analysis but can be hard to find origin
Pyrolysis and burning of wood
- Charring can take place at relatively low temperatures
- Hard woods are more difficult to ignite but can cause a hotter and more protracted fire
- Char forming from smouldering combustion (pyroylsis is happening)
- Then slightly further in to the wood you get a pyrolysis zone where the process is actively occuring
- After the pyrolysis zone there is the normal wood - wood has a pretty high heat capacity so it takes a lot of energy to spread the fire through the wood so it stays localised
What is produced in the pyrolysis zone of wood?
Volatile species which then combust further (flaming combustion)
Why is the high heat capacity of wood good and bad?
- Good because parts can be saved even those that are close to the fire
- Bad because it is the high heat capacity that means you get a char layer and the char base which results in the fire in the first place
Paper and fire
- A single sheet will be lit easily
- A stack of paper has no air flow and is hard to burn - sheets are densely packed together and its hard for the oxygen to get in and set them alight
- Can use flammable liquid to start a fire - soaks up the fuel, higher surface area of liquid fuel in contact with air, higher vapour pressure
Cotton and linen and fires
- Composed of cellulose like wood
- Large surface area: volume ratio
- Easy to burn with flame
- Much easier to get the right ratio of volatiles to oxygen to get a flame
- Will produce lots of pyrolysis products that will burn too
- Smoky smouldering for extended time
Wool and silk and fires
- Composed of protein
- High ignition temp, low heat of combustion
- Diffcult to burn, and dont release a lot of energy when they do so they self-extinguish
- Gives off HCN when it does burn
Difference between synthetic and real silk in regards to fires
- Real silk is made from proteins
- If we have synthetic plastics, they will have a high surface area to volume ratio because they have been reformatted
Define thermoplastics
Have structures of linear chains, they undergo reversible melting without appreciable chemical decompositions
Examples of thermoplastics
- Polyethylene
- Polystyrene
- Nylon
Define thermosetting plastics
Have cross-linked structures, do not melt, but decompose chemically and leave a solid char
Examples of thermosetting plastics
- Polyesters
- Rigid polyurethane foam
What is the main difference between thermoplastics and thermosets on fire?
- If you’ve got a thermoplastic, then the plastic is going to melt. It may melt while being on fire. So you may have droplets of burning plastic or a pool of molten burning polymer which could flow around.
- Can get hot spots around the main pool
- And this is going to provide another way of spreading fire around the room.
- If you have a thermosetting plastic, it burns more similarly to wood, so you get volatile molecules and a solid char.
- Get more of a transition from a solid phase straight to a gas phase
Is lightly closs-linked polymers more likely to act like thermosets or thermoplastic?
Thermoplastic because the femer links may be able to be broken easily making it resemble a thermoplastic
What are the four mechanisms that thermosetting plastics decompose?
- End-chain scission
- Random scission
- Chain Stripping
- Cross-linking
What is end-chain scission?
Successive removal of monomer units from the end of the polymer backbones
* produces monomers
What is random scission?
Main chain bonds are broken at random location along the polyer backbone
* Smaller chains and oligomers are formed
What is chain stripping?
The polymer backbone remains intact, but molecular species which are not part of main chain break away
* bridges that get removed more easily
