Lecture 18 Flashcards
what can change the ignition temperature of different paper types
paper is a long chain polymeric material so different properties such as secondary structure and bonding can affect the ignition temperature
how does a smouldering combustion go to a flaming combustion
when gases begin the evaporate from the solid fuel
these gases make up the flame
what is the equation for the combustion of carbon
is this a complete or incomplete combustion - how can we tell?
where does this combustion occur in materials
C(s) + O2(g) –> CO2(g)
generally an incomplete combustion shown by the blue flame due to carbon monoxide production
occurs on the surface of a material
what is the simplest solid fuel
carbon
coal, charcoal, wood/paper is primarily carbon
what is the name of the combustion where no flame occurs
a smouldering combustion
what is pyrolysis
the thermal decomposition of an organic substance to produce lower molecular mass compounds without the involvement of oxygen
involving the breaking of chemical bonds and depolymerisation
do smouldering or flaming combustions do more damage
flaming combustions
what are the ways a solid fuel and produce vapour that can lead to flames (5)
- sublimation
- melting then evaporation = candle wax
- melting then decomposition and evaporation
- decomposition and melting then decomposition and evaporation = plastics
- decomposition an evaporation
why are pyrolysis product residues often hard to analyses to find the origin
as real substances are complex mixtures so give complex mixtures of pyrolysis products
what are two characteristics of pyrolysis products
what two other things can they be
they are volatile and flammable
but can be toxic and corrosive
where can pyrolysis products be carried
in the fire plume and be ignited elsewhere
particularly if the material is lightweight
when will a pyrolysis product burn at the surface of the solid they came from
if their concentration is within their flammability range
what does the solid left behind after pyrolysis products have been carried away become closer to
carbon (char) so hard to find the source of the fuel
what are the main two components of wood and how do their structures differ
cellulose = a long chain polymer
lignin = a branched polymer
what can charring of a surface mis lead to an investigator
the surface temperature of the fuel
what types of wood are easy to ignite
what types are harder
resinous woods e.g pine
hard woods - but can cause hotter and more protraced fires
what three things are manufactured wood products also likely to have that impact its ignition temperature
adhesives holding layers together
surface coatings
varnishes
what are the 5 layers observed when wood is burnt/burning
char layer
char base
pyrolysis zone
pyrolysis zone base
normal wood = high heat capacity
the reaction propagates along this until there is not fuel left
what does the char layer act as in the burning of wood
the smouldering combustion
what does the pyrolysis zone act as in the burning of wood
creates volatiles for the flaming combustion
what is paper made of
what makes paper easier to burn than wood
cellulose
paper has a lower heat capacity and is much thinner, better surface area exposed and air flow
why is a large stack of paper hard to burn
there is no air flow
how is paper often used as an arson set
why
a flammable liquid poured into paper
this is easily ignitable, the paper absorbs the liquid = high surface area of evaporation of gases to be ignited
can can a fire including paper spread easily
paper is lightweight so can be carried away from the original fire in convection currents and start a fore elsewhere
