Chapter 03-Combustion Processes

• Explain the theories underlying combustion processes
• Describe how fire researchers have identified combustion processes using a variety of different classifications
• Provide a description of the stages and events of fire as it progresses from the initial stage to its final stage
• Explain the causes of flame over, flashover, and backdraft and review the procedures to prevent and protect against such events
• Describe the various methods by which heat and unburned gases move in a confined environment
• Define the five classes of fires and explain how they are classified

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Combustion Processes
Chapter 3

2. © 2009 Delmar, Cengage Learning
Objectives
•Explain the theories underlying combustion
processes
•Describe how fire researchers have identified
combustion processes using a variety of
different classifications
•Provide a description of the stages and events
of fire as it progresses from the initial stage to its
final stage

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Objectives (cont’d.)
•Explain the causes of flame over, flashover, and
backdraft and review the procedures to prevent
and protect against such events
•Describe the various methods by which heat and
unburned gases move in a confined environment
•Define the five classes of fires and explain how
they are classified

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Introduction
•This chapter:
–Considers physical and chemical process involved in
fire combustion and relates them to procedures of fire
services to confine, control, and extinguish
uncontrolled fires
–Emphasizes combustion processes as fires progress
–Reviews fire classification methods, fire extinguishing
agents, and their advantages and disadvantages

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What is Combustion?
•Planned and controlled, self-sustaining chemical
reaction between fuel and oxygen with evolution
of heat and light
•Differs from fire
•Represented by fire tetrahedron
–Heat
–Fuel
–Oxygen
–Chemical reaction

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What is Combustion (cont’d.)
Figure 3-2 The new fire tetrahedron

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Spontaneous Combustion
•Does not require independent ignition source
•Material heats to piloted ignition temperature
•After ignition, flames spread
•Coal is an example of a porous solid material
that when heated, eventually reaches ignition
temperature and combustion begins

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Methods of Fire Classification
•Type of combustion
•Rate of fire growth
•Available ventilation
•Type of materials that are burning
•Stages or phases of a fire

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Types of Combustion
•Three stages
–Pre-combustion
•Fuel heated to ignition point
•Particulates released
•Entrainment gathers additional oxygen
•Heat energy radiated back into fuel
–Smoldering combustion
–Flaming combustion

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Smoldering Combustion
•Absence of flame
•Presence of hot materials on surface where
oxygen diffuses into fuel
•Two phases
–Solid
–Gas
•Incompleteness creates very high levels of
carbon monoxide

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Flaming Combustion
•Encountered in most emergency incidents
•Presence of flames
•Gas or vapor has to be burning
•Two categories
–Gaseous fuel premixed with air before ignition
–Diffusive flaming
•Flames are generally yellow due to incomplete burning
process
•Light and heat also emitted

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Fire Classification by Type of
Substance Burning
•Class A
–Fires involving combustion of ordinary cellulosic
materials
•Class B
–Fires involving flammable liquids
•Class C
–Fires involving energized electrical equipment or
wires

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Fire Classification by Type of
Substance Burning (cont’d.)
•Class D
–Fires involving combustible metals
•Class K
–Fires involving cooking oils
•Saponification: process of chemically converting the
fatty acid contained in a cooking medium (oil or grease)
to soap or foam

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Fire Classification by Stages and
Events
•Fire stages:
–Ignition stage
–Growth stage
–Fully developed stage
–Decay stage
•Fire events:
–Flameover or rollover
–Flash over
–Backdraft

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Figure 3-7 Temperatures associated with the stages of fire
and the unique fire events

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Flame Over
•Flames travel through or across unburned
gases in upper portions of confined area during
fire development
Figure 3-8 Flame over/rollover

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Flashover
•When heating is enough to bring other
materials in room to ignition temperature,
igniting all fuel materials in the room into
flaming combustion
Figure 3-9 Flashover

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Backdraft
•Additional oxygen entering the compartment is
heated and expands
•Increased pressure inside room
•Windows, walls, and weak points in the building
suddenly pushed outward
•Firefighters caught in the sudden, explosive rush
of fire can be killed instantly

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Building Construction and Fire
Spread
•Efficiency declines if fires move vertically
through buildings or bypass horizontal
construction barriers
–Pre-WWII concrete construction inhibited vertical
movement
•Post-WWII drywall spreads fires to other areas
of building quickly
•Compartmentation is safe areas in high-rises

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Fire Rating of Materials
•Building’s ability to withstand a fire differ
because of:
–Variations in workmanship
–Methods of installation
–Different sets of test methods
–Sizes of test specimens
•Rated fire resistance of construction has some
but not a substantial impact on the spread of fire

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Weather Conditions
•Impact the burning characteristics of inside
building fires and outside fires
•Stack effect: temperature difference between
the outside temperature of building and
temperature inside the building
•Windy conditions outside can impact horizontal
ventilation activities

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Relative Humidity
•Moisture in the form of water vapor
•Always present
•Affects amount of moisture in fuel
•Impacts direction of fire gas movement

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Mass/Drying Time
•Impacts how long it will take source of ignition to
raise material to ignition temperature
•Thicker or heavier mass will take longer to raise
the temperature of the material
•Law of latent heat of vaporization: heat
absorbed when 1 gram of liquid transformed into
vapor at boiling point under 1 atmosphere of
pressure
–Result in BTUs per pound or calories per gram

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Heat Measurement
•Heat always flows from higher temperature
materials to lower temperature materials
•Four temperature scales
–Kelvin
–Rankin
–Celsius
–Fahrenheit

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Figure 3-11 Relationship among temperature scales

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Heat Transfer
•Important in all aspects of combustion process
•Responsible for continuance of combustion
process
•Four methods of transfer:
–Conduction
–Convection
–Radiation
–Direct flame impingement

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Conduction
•Transfer of heat energy from hot to cold side of
medium by means of energy transfer from
molecule to adjacent molecule or atom to atom
Figure 3-12
Conduction is the
transfer of heat energy
from a material by
direct contact between
the movements of
molecules of another
higher energy material

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Convection
•Movement of heat
energy by agitation of
air molecules
–Reduces density of
molecules, making
heated air lighter than
cooler air
Figure 3-13
Convection involves
the transfer of heat by
circulating currents

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Radiation
Figure 3-14 Radiation is energy that travels across a space and does not
need an intervening medium, such as a solid or a fluid

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Direct Flame Impingement
Figure 3-15 Flames
directly impinging upon
the materials transfer
the heat, raising their
temperature to the
point where combustion
occurs

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Summary
•Combustion process defined by type, rate of fire
growth, amount of ventilation, and type of
substance that burns
•Classifications of fires: Class A, B, C, D, and K
•Physical and chemical properties of fuels
feeding fires affect how a fire will burn, spread,
and quickness of burning rate