Chapter 03-Combustion Processes

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Combustion Processes

Chapter 3

© 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 3-13Convection involvesthe 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 Flamesdirectly impinging upon the materials transfer the heat, raising theirtemperature to thepoint 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