Chemistry of combustion

1. Chemistry of combustion
1
Dr. Zayed Al-Hamamre
The University of Jordan, School of Engineering,
Chemical Engineering Department, 11942
Amman, Jordan
Email Address: z.hamamre@ju.edu.jo

2. • Definitions
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3. What is combustion?
• Combustion is the scientific word for burning.
• It is the chemical chain reaction that takes place when a
substance burns and reacts with oxygen to produce heat and
light energy.

4. 4
For fire to exist, three things must be
present at the same time
o Enough oxygen to to sustain combustion,
o Enough heat to raise the material
temperature to its ignition temperature,
o Fuel or combustible substance which
produces high exothermic reaction to
propagate heat to not-yet- burnt material
nearby

5. 5
Fire Triangle

6. 6
Fire Tetrahedron
The fire tetrahedron
or fire pyramid adds
a fourth component—
chemical chain
reaction—as a
necessity in the
prevention and
control of fires.

7. Fire Tetrahedron
The free radicals formed during combustion
are important intermediates in the
initiation and propagation of the combustion
reaction.
Fire suppression materials scavenge these
free radicals
Moreover, if one of the three components
is eliminated, then there will not be a fire
(or explosion)
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8. Oxygen Heat
Fuel
OXYGEN SOURCE
HEAT SOURCES
• Approx. 16% Required
• Normal air contains 21% of
oxygen
• Some Fuels contains its own
oxygen supply
GASES
To Reach Ignition Temp.:
Open Flame, the Sun, Hot
Surface, Sparks & Arcs,
Friction, Chemical Action,
Elec. Energy & Gas
Compression
LIQUIDS SOLIDS
Natural Gas,
Propane, CO,
Butane, Hydrogen,
Acetylene,
Gasoline, Kerosene,
Turpentine, Alcohol,
Paint, Varnish, Olive
oil, Lacquer
Coal, Wood, Paper,
Cloth, Wax, Grease,
Leather, Plastic,
Sugar, Grain, Hay,

9. 9
Oxygen combines with other substances to
produce new products – oxidation
CH4 + 2O2  CO2 + 2H2O
Stored chemical energy converted to
energy in the form of heat and light.
Energy in chemical reaction comes from
the breaking and formation of bonds.

10. When substances combust, the new substances
formed are called ‘oxides’.
Examples:
1. Coal (made from carbon) burns and forms
carbon dioxide.
carbon oxygen carbon dioxide
+ 
C O2 CO2
+ 

11. 2. Hydrogen burns and forms dihydrogen oxide (i.e water!).
hydrogen oxygen water
+ 
2H2 O2 2H2O
+ 
3. Methane burns and forms carbon dioxide and water.
methane oxygen carbon dioxide
+  water
+
CH4 2O2 CO2
+  2H2O
+

14. 14

15. Fuel
Material consumed by the combustion
process
Fuel must be present in certain
concentrations.
Typical cases where fuel occur are if there
is a leak, during filling operations, transfer
operations, or excessive dusts.
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16. Fuel
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 States of matter: Solid materials have dense arrangements
of molecules whereas gaseous molecules are more free-
flowing.

17. Organic – C, O, H and sometimes N
– Wood is 40-50% cellulose and hemicellulose
• (5 and 6 carbon sugars)

18. Wood tends to produce oxygenated combustion products

19. Other fuels
Inorganic fuels
– Mg, Al, S, Zn, etc
– Note surface area is important
• Wood dust ignites easily
• Diesel ignites in a spray, but is difficult to light
in a pool
• Metals used in pyrotechnic devices are finely
powdered

20.  A liquid burns when the temperature is high enough to
vaporize it (flash point).
 Once the Flash Point is reached, a light or spark can cause
the gas/vapor to reach the ignition point and combustion to
occur. Accelerant Flashpoint (oC)
Acetone -20
Gasoline -46
Kerosene 52-96
Mineral Spirits 40-43
Turpentine 32-46
Liquid Fuels

21.  Flash Point
– Lowest temperature at which a flammable liquid gives off
enough vapor to form an ignitable mixture with air
 Flammable Liquids (NFPA)
– Liquids with a flash point < 100°F
 Combustible Liquids (NFPA)
– Liquids with a flash point > 100°F

22. Solid Fuel
 If there is not enough heat to pyrolyze the
solid fuel to produce flames, the solid fuel
does not combust through-and-through, only on
the surface.
 A solid must be hot enough to decompose
into gaseous products (pyrolysis).
 The chemical breakdown of solids into
gas is known as pyrolysis.
 This produces a glowing effect – Examples: embers, burning
cigarette, etc.

23.  Wood for example, cannot generate a vapor easily.
 It has to first be exposed to enough heat to breakdown
some of the solid organic material into gases that are
combustible.

24.  Generally only gases (vapors) will burn. Solids and liquids
will not burn
 Sufficient heat is required to vaporize some of the fuel
and break oxygen molecules apart into oxygen atoms
 Fire will produce enough heat to continue vaporization and
keep fire going
A fuel will produce a flame only
when it is in the gaseous state.

25. Oxidizing Agents
Usually oxygen from air
Temperature greatly affects a fires need for oxygen
– @25 oC need 14-16% O2
– When O2 is limited, flaming combustion may diminish;
combustion will continue in surface or smoldering
mode.
– At high ambient temperatures, flaming combustion
may continue at much lower oxygen concentrations.
– @900-1100 oC flashover conditions- (spontaneous
ignition of entire room) nearly 0% oxygen is needed

26. 3–26
Oxygen Concentrations
When O2 concentration is higher than normal,
materials have different burning characteristics.
Fires in oxygen-enriched atmospheres are difficult to
extinguish and present a potential safety hazard.
Flammable explosive range — Range of concentrations
of fuel vapor and air
This is the effect of the flammable limit, the upper and
lower concentrations of a flammable gas and air
expressed in % fuel that can be ignited at a specific
temperature and pressure

27. Oxygen Concentrations
For oxygen, we often use “inerting” with nitrogen,
helium blankets over flammable materials to reduce
O2 content below that where you can have
combustion.

28. Ignition Sources
Heat is a common ignition source.
Although we can eliminate ignition sources, it is
almost inevitable that an ignition source will be
available if there is a large release of flammable
material that cannot be diluted quickly.
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29. Types of Fire
Flaming combustion
– Common open flame fires like
gas burners
– Gas to gas reaction, fuel must
be in gaseous state
– Liquids and solids don’t burn
in an open flame
– these must undergo chemical
or phase change first
– Oxygen must be above 10%
– Flames and smoke are visible
Flaming fire

30. Smoldering Combustion:
Glowing slow, low-temperature, combustion
occurs without the generation of flames
sustained by the heat evolved.
– It is a solid to gas reaction
– Surface of solid reacts directly with
oxidizer (oxygen directly attacks the
surface of a condensed-phase fuel)
– Often due to a deficiency of oxidizer
– Less than 10% oxygen is needed
– Can burst into explosive flame if oxygen
is suddenly supplied
Smoldering fire

31. Partial or incomplete combustion
Stoichiometric ratios are rarely involved in combustion
– Oxidation reactions often don’t go to completion
– This is called incomplete combustion and results in
formation of carbon monoxide (Oxidation reactions
often don’t go to completion)
Complete vs incomplet combustion in a gas pilot light

32. Effect of Fuel on a Fire
Fires have either excess air or excess fuel

33. Effect of venting
Venting a fire has important effects
– Gases inside a room may be oxygen
starved
– Gases venting to outside may ignite
– Gases venting into an enclosed room will
not spread a fire
– Opening doors and windows may cause a
smoldering fire to reignite

34. Heat
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Sufficient heat is required to produce a transition
from solid to liquid to vapor phase – only vapors burn
Additional energy is also required to initiate the
chemical reaction
Once initiated such reactions are exothermic with a
large increase in entropy

35. Initiation of Fire
The heat required to initiate a fire is a
critical step
– Matchespaperstickswood
– Each step is critical and the underlying
process is to get the wood hot enough
that it produces volatile gases that burn.

36. Types of ignition
Spontaneous ignition
– Chemical or biological processes that
create sufficient heat to ignite the
reacting material
– Basically heat is produced faster than it
can be dissipated.
– Common with vegetable oils, hay
Spontaneous combustion
of hay in a barn

37. Auto-ignition
Ignition of a material in the absence of
flame or spark (non-piloted ignition)
All combustible materials must reach their
autoignition temperature to burn
– Thus one could light paper two ways:
• Use a match to heat a small section to
ignition
• Heat the entire piece of paper in an
oven.

38. Flash Point
There is a temperature above which a fuel
will flash when presented with a flame –
this is the flash point
@ 10-20 degrees above the flash point
there is sufficient vapor pressure to
sustain a flame
The auto-ignition temperature is the
temperature of spontaneous ignition
– For kerosene, the flash point is 100F and
the ignition temperature is 410F.

39. Chemical Chain Reactions
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Reactions become self sustaining when sufficient heat
from exothermic reactions radiates back to cause
ignition away from source
The burning process involves pyrolysis, the breakdown
of solids to produce gases and free radicals

40. Removal of Free Radicals
Halon fire extinguishers work by shutting
down the propagation of radicals
Bromine and chlorine quickly shut down free
radicals

41. Principles of Fire Extinction
Starvation
Removal of un-burnt material from fire area
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42. Smothering
Cutting off the supply of oxygen from fire area
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43. Cooling
Removal of heat from the burning material/fire
area
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44. Chemical Flame Inhibition: Breaking of chain
reaction.
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Extinguishing mechanism of multipurpose mono-ammonium phosphate dry chemical

45. Products of Combustion
Combustion produces smoke and other
substances.
Specific products depend on:
– Fuel
– Temperature
– Amount of oxygen available
Few fires consume all available fuel.

46. Smoke
Airborne products
of combustion
Consists of:
– Ashes
– Gases
– Aerosols
Inhalation of smoke
can cause severe
injuries.

47. Smoke Contents
Particles
– Solid matter consisting of unburned,
partially, or completely burned
substances
– Can be hot and/or toxic
Vapors
– Small droplets of liquids suspended in air
– Oils from the fuel or water from
suppression efforts

48. Smoke Contents
Gases
– Most gases produced by fire are toxic.
– Common gases include:
• Carbon monoxide
• Hydrogen cyanide
• Phosgene