Fires 7 - Solid Fuels Flashcards
Why is it hard to define ignition temperature of paper?
as lots of different types of paper
What are compounds that are primarily elemental carbon?
coal, charcoal, wood/paper char
What does combustion of solid carbon look like?
C (s) + O2 (g) –> CO2 (g)
- it is usually incomplete
- this is process that is happening on the surface of the material (which is a limited area)
- no flame occurs so this is smouldering combustion
- get incandescently hot regions on the surface where atoms on the surface of the fuel are interacting with the oxygen in the air - gives blue flames caused by carbon monoxide vapours combusting
How does solid make flames?
- solids do not make flaming combustions happen
- solid cannot directly produce the flame as flames happen in the gas phase so we need a vapour
How many pathways are there for solids to become a vapour?
sublimation: transition from solid phase straight to vapour phase without becoming a liquid but it is unlikely (gas to solid = deposition)
melt the solid then evaporate the liquid to get a gas e.g. a candle
solid melts to give liquid then the liquid decomposes producing flammable vapours e.g. pyrolysis
solid might first decompose to give liquid then liquid can further decompose to give vapour (this is what tends to happen with plastics)
direct decomposition and evaporation of solid to vapour phase
define pyrolysis
what does pyrolysis involve?
thermal decomposition of an organic substance to produce lower molecular mass compounds without involving oxygen
involves breaking of chemical bonds, depolymerisation (polymers –> monomers) frequently done by radical processes
why is pyrolysis favourable?
the smaller molecules made are easier to promote into vapour phase so products are often volatile (lower boiling point) and more likely to be gases
What are the 2 properties of pyrolysis products?
What happens to the products?
volatile
flammable
- if their concentration is within their flammability range they may burn at the surface of the solid
- if not they can also be carried in the fire plume and ignite elsewhere
- they could also be carried elsewhere settled down and used for trace analysis
What else is formed during pyrolysis?
not everything in solid forms these volatile compounds, they are normally left with something behind and in the absence of oxygen at the surface of fuel this tends to be reduced to simple carbon or char
What are pyrolysis residues useful for?
useful for analysis but can be hard to find origin regarding pyrolysis products
what is wood made up of?
describe each of these
cellulose (50 %), hemicellulose (25 %) and lignin (25 %)
- cellulose is a long chain polymer (C6H10O5)n with n about 1500
- left side is sugar rings - makes it a carbohydrate and it is linked via an ether group
- this is a repeating pattern which will go on throughout and gives wood a lot of its strength
- it is highly oxygenated - helps it burn
- hemicellulose is also a carbohydrate polymer but its structure is much less regular than this
- lignin has branching all over the place
- it is a cross linked polymer - so harder to burn and much harder to break down the bonds and free the volatile species but it can still be burnt
- aromatic units with an oxygenated kind of aliphatic chain
Why is the scientific literature on ignition temperatures of types of wood is confusing?
depends on the type of wood for wood products but also on the way the experiment has been done:
- is this for ignition by a flame (piloted ignition) where you’ve got a section which is already at a very high temperature
- or was it by autoignition so they just raised the whole thing to that temperature
- was it flaming combustion or smouldering/glowing combustion
- was surface temperature accurately measured? (this is important because charring can take place at relatively low temperatures)
What is the process by which wood can be cooked and then become more ignitable?
e.g. if have a fireplace and the dusting which is taking out the gases is made of metal
- next to the metal is wood which will slowly pyrolysis and in doing that it can release ignitable vapours in itself will turn into chemical
- eventually the charcoal becomes much more easily to ignite than the wood itself
- fires can start this way
What are woods that are hardest and easiest to ignite?
easiest: resinous woods e.g. pine (flammable in nature compared to wood)
hardest: hard woods but they are dense so can cause a hotter and more protracted fire
what do many manufactured wood products contain?
- organic adhesives - to hold layers or pieces of wood together
- surface coatings or varnishes which are going to contain bits of plastic
- these will all have their own flammability properties
What are the sections of pyrolysis and burning of wood?
What are the pros and cons of woods heat capacity?
char layer: where char forms and this is where pyrolysis has been happening and in the char there is smouldering combustion
char base
pyrolysis zone: where volatiles created for flaming combustion. carbon is here now reacting directly with oxygen.
pyrolysis zone base: where process is actively occurring
normal wood: wood has a pretty high heat capacity so it takes quite a massive amount of energy to spread the fire through the wood which keeps everything localised
although bits can be saved (even very close to the fire), but its the high heat capacity that means you get the char layer and the char base which results in the fire in the first place
What is paper made from?
Compare sheet of paper to stack of paper
cellulose - same as wood
a free sheet of paper can be lit easily as much lower heat capacity
a large stack of paper has no air flow so it is hard to burn
- it is not uncommon to find stacks of paper in fires which have not really been ignited beyond some charring around the outside
- in stacks, the sheets are very densely packed together and it is hard for the oxygen to get in and set them alight
- can use this to put out small fire using newspaper - smother the fire by restricting the oxygen
How can paper spread fire rapidly?
depending on distribution, can spread fire rapidly
- if papers are scattered around the room and they’re all free they can burn really quickly and that can be used to spready the fire onto other things
- but if they burn too quickly then they are going to burn out before they have had a chance to ignite other things
What is a common way of starting a fire with paper?
pouring flammable liquid on paper
- a common arson set - look for unburnt traces of accelerant
- paper is used as a wick in this case rather than as a fuel
- the paper soaks up the fuel, higher surface area of liquid fuel in contact with air so higher vapour pressure
what are some natural fabrics?
what are these composed of?
give three/four properties?
cotton, linen
- cellulose, like wood, but as with paper they have been reformatted into a different large structure
1 - easy to burn with flame
2 - smoky smouldering for extended time (even if not enough oxygen to make volatiles for flame, it will still smoulder for a prolonged period)
3 - large surface area:volume ratio
- more like a single sheet of paper, they have been turned into these very fine threads and there will be a certain degree of air circulation going between these threads which means that its much easier to get the right ratio of volatiles to oxygen to get a flame
wool, silk (real silk)
- composed of proteins (any poly-amino acid structure), predominantly keratin
- get amide linkages and side chains
1 - high ignition temp - difficult to burn
2 - low heat of combustion - when they do ignite they do not release much energy
3 - self extinguishes (once got it to burn because it is not sending enough energy it tends to go out by itself because heat becomes limiting factor)
4 - gives off HCN when it does burn (as amides are burnt)
what are synthetic fabrics?
give an example
plastics with high surface area:volume ratios because they have been reformatted
e.g. silk is different to synthetic silk which is made by silkworms and is a protein
define polymer
a large molecule composed of many subunits (monomers) joined together e.g. plastic
define oligomer
a small portion of a polymeric chain (i.e. a couple of subunits or monomers long)
