Module 2: Fundamentals of Fire
and Combustion
date
Section 2.1
Fire
Learning Outcomes
1. Define fire
2. Identify the parts of the fire tetrahedron
3. Define heat of combustion and combustion
efficiency
4. Compare the four types of flames
5. Define the terms, flame plume and flame spread
Definition of Fire
• A rapid oxidation
process, which is a
chemical reaction
resulting in the
evolution of light and
heat in varying
intensities.
• {Insert Picture}
Fire suppression personnel.
Source: Photo by Katie Steenken.
Fire Chemistry
The study of chemical processes that occur in fires,
including changes of state, decomposition, and
combustion
Fire Tetrahedron
• Four Components
–
–
–
–
The Oxidizing Agent
The Fuel
The Heat
The Uninhibited
Chemical Reaction
The fire tetrahedron.
What is burning?
Fire Tetrahedron
The Fuel
• Any substance that can undergo combustion under
specified environmental conditions
liquid
solid
gas
Fire Tetrahedron
The Oxidizing Agent
– Usually the oxygen in the air (approximately 21%)
Can also be
– Fire gases that contain unreacted oxygen
– Chemical oxidizers {insert picture – see additional slide
appended to end of file for ideas}
Fire Tetrahedron
• The Heat
Surface heating and the evolution of pyrolysis gases.
Source: Adapted from Bengtsson, L., Enclosure Fires, 2001, Raddnings Verket.
Fire Tetrahedron
• Uninhibited Chemical Chain Reaction
– {insert picture}
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Fire as an Oxidation Reaction
Ignition Energy
Fuel Vapour + Oxidiser
Products
Reaction Energy
Various Types of ‘Combustion’
12
Importance of Oxygen
• Oxygen enriched
– combustion intensity may increase
– fire spread may be accelerated, particularly when new
oxygen is added to an oxygen depleted fire environment
• Oxygen depleted
For possible graphics see
next slide
– often the case in real fires
– fire spread is decelerated
– can undergo transition to smouldering
Reduced Oxygen – real fire
10 – 15% oxygen (in nitrogen)
Combustion Efficiency
Smoke – incomplete reaction
GASES
SMOKE IS FUEL!!
AEROSOLS
AIR
PARTICLES
SMOKE TRANSFERS HEAT!!
Non-flaming Combustion
Smouldering (nonflaming) Combustion
• oxidation reaction
and heat release
occur on solid
surface of fuel
• oxidizer is gas, fuel is
solid – porous and
forms char
Flaming Combustion
• Flames are the visible,
luminous indication of
where the oxidation
(combustion) reactions
are taking place.
• reaction and heat
release occur in gas
phase surrounding fuel
• both oxidizer and fuel
Diffusion versus premixed flame: (left) diffusion flame;
are gaseous
(right) premixed flame.
Premixed and Diffusion Flames
Fire Plume
Fire plume. Source: Photo by UW Fire lab
we may have a better one
Flame Spread
Section 2.2
Gaseous Combustion
Learning Outcomes
1. Explain the flammable limits of a gaseous fuel.
2. Explain the term vapor density and how it
affects fire hazards associated with gaseous
fuels.
Vapour Mixes with Air
Flammability limits
Flammability Limits: Acetylene
LFL
UFL
Flammable range
-- mixture burns
0% fuel
100% air
(21% oxygen)
2.5% fuel
97.5% air
(21% oxygen)
100% fuel
0% air
(21% oxygen)
80% fuel
20% air
(21% oxygen)
Volume % corrected to 0°C and 1 atmosphere pressure, 21% oxygen in air
Flammability Limits: Acetylene
Ideal (stoichiometric mixture)
7.7% fuel
Maximum flame temperature (theory)
92.3% air
Maximum flame speed (theory)
LFL
UFL
burns
0% fuel
100% air
(21% oxygen)
2.5% fuel
97.5% air
(21% oxygen)
Optimum %
Peak T, flame speed
Fuel rich ~9% fuel
100% fuel
0% air
(21% oxygen)
82% fuel
20% air
(21% oxygen)
Volume % corrected to 0°C and 1 atmosphere pressure, 21% oxygen in air
Flammability Limits and Fire
Too rich . . .
Too lean . . .
Just right . . .
Flammability Limits
LFL
UFL
Flammable range
limits are functions of pressure, temperature and
oxygen concentration
• many fires → atmospheric pressure
• temperature dependence → often small
• flammable range is bigger with more oxygen
Flammable range and Fires
How does flammable range factor in?
Vapour Density
SG fuel > 1, fuel heavier than air → sinks, collect
{Video of vapour density boxes – see Peter Ehler on
details}
SG = 1.59
Fuel Gas Explosions
Unconfined Vapour Cloud (UVCE)
BLEVE
Boiling
Liquid
Expanding
Vapour
Explosion
BLEVE
Section 2.3
Ignitable Liquids
Learning Outcomes
1. List and describe factors that differentiate
flammable and combustible liquids.
2. Explain the similarities and differences in an
ignitable liquid’s flash point and fire point.
Flammable Liquids
93.4 C
60 C
37.8 C
22.8 C
Highly Flammable
Flashpoint < 32 C
Flashpoint
The lowest temperature of a liquid, as determined
by specific laboratory tests, at which the liquid gives
off vapors at a sufficient rate to support a
momentary flame across its surface.
Open cup
Closed cup
NFPA 321, 921
Fire Point
The lowest temperature at which the flame
becomes self-sustained and continues burning in
the vapour formed above the liquid
Usually a few degrees higher than the flash point
Flammable Liquids
Fuel
Flashpoint
Gasoline
-45
Naptha
50
Methyl/ethyl alcohol
~55
Turpentine
95
Fuel Oil
100
Mineral Spirits
104
Motor Oil
Peanut Oil
3-450
540
Easier to ignite
Liquids
Section 2.4
Fundamentals of Fire and Combustion
Learning Outcomes
1. Explain thermal decomposition in solids.
2. Explain and describe pyrolysis.
3. Describe different types of solid fuels and their fuel
characteristics.
4. Describe factors affecting solid combustion.
Fuel Types
• Cellulosic Fuels
• Upholstered Furniture
• Polymer Fuels
• Combustible Metals
Legacy Vs Modern
Pyrolysis (wood)
HCs
tars
CH4
H2
heat
Pyrolysis, thermal decomposition
ash
char
char
pyrolysis zone
unburned wood
Pyrolysis
Fuel
Hc
(kJ/g)
Lv
(kJ/g)
Hc/ Lv
Heptane
Octane
Styrene
MMA
41.2
41.0
27.8
25.0
0.55
0.60
0.64
0.52
74.9
68.3
43.4
48.1
Polyethylene
Polystyrene
PMMA
Wood
38.4
27.0
24.2
12.4
2.30
1.76
1.62
1.80
16.7
15.3
14.9
6.9
(kJ/g)
burning
rate
Pyrolysis
Key Factors in Fires
• Fuel Orientation and
Geometry
• Surface Area to Mass Ratio
• Density
• Moisture Content
• Thermal Inertia
• Fire Retardants
Fuel Orientation
Fuel Geometry
• Images of flame effects
on concave and convex
surfaces
Surface Area to Mass Ratio
Surface Area & Mass Ratio
Thermal Inertia, kρcp
thermal
conductivity
increases
Properties of Solid Fuel
• Thermally thin
Thermally thick
Fuel Density (alternate)
Fire… a complex challenge
Fuel
Oxidizer
Pyrolysis, Ignition
Critical heat - chemical reaction
Critical
Temp
Chemical
Chain
Reaction
Reducing
Agent
Real fires  Interactions
Oxidizing
Agent