Chapter 4

Chemistry and Physics of Fire

Introduction

•To appreciate how fire is controlled, we must first understand the chemical and physical properties of fire itself

•Information can be used to predict what the fire will do with the available fuel and where it is headed

•With this knowledge, we are able to choose the proper extinguishing agent and its method of application

Learning Objective 1

Fire Triangle and Fire Tetrahedron

FIRE DEFINED

• Rapid, self-sustaining oxidation process accompanied by the evolution of heat and light in varying intensity

• Combustion described as a chemical reaction that releases energy as heat and usually as light

RUSTING OF IRON

• Not considered combustion

• Proceeds at a slow rate

FIRE TRIANGLE

• Originally based on three elements Fuel Air Heat

FIRE TETRAHEDRON

• Fourth component called chemical chain reaction

• Produces free radicals

Learning Objective 1

Fire Triangle and Fire Tetrahedron

Learning Objective 2

Describe What Constitutes an Oxidizer

CHEMISTRY OF FIRE

• Two basic necessary components Oxidizer Fuel

OXIDIZER

• Evolves or generates oxygen

• Oxygen is most common oxidizer

• Most fires are burning at 21% oxygen

FLOURINE AND CHLORINE

• Listed under halogen family on periodic table

• Both naturally occur as gases

• Flourine is a much stronger oxidizer than oxygen

• Fire in a flourine atmosphere will burn more rapidly than one occurring in air

Learning Objective 2

Describe What Constitutes an Oxidizer

Learning Objective 3

Describe What Constitutes a Fuel

FUEL

• Described as anything that will burn Carbon and hydrogen are two most common

• Hydrocarbons Main element in organic fuels, fuels that were at

one time living things

• Other elements as fuels, including metals Sodium, aluminum, magnesium

FIRES INVOLVING CARBON, HYDROGEN, AND OXYGEN

• Two byproducts of complete combustion Water vapor Carbon dioxide

• Byproducts of incomplete combustion Smoke Carbon monoxide (CO) Carbon dioxide (CO²) Other fire gases

Learning Objective 3

Describe What Constitutes a Fuel

Learning Objective 4

Illustrate the States of Matter

FUEL MAY OCCUR IN THREE STATES OF MATTER

• Solid• Liquid• Gas

MOLECULES AND TEMPERATURE• Molecules in liquids vibrate faster than those in solids,

and gas molecules vibrate the fastest of the three• As this vibration increases, solids become liquids and

liquids become gases

Learning Objective 5

Explain the Process of Pyrolysis

PYROLYSIS

• Chemical decomposition of matter through heat

• Combustion occurs when fuel has been converted to vapor or gas

• Oxidizer and fuel must be gaseous for recombination

• Solid and liquid fuels are converted to gaseous state by the application of energy

IGNITION TEMPERATURE

• Combustion continues without

external input of heat

FREE RADICALS

• Longer molecules break into shorter molecules

• Byproduct of the fuel that directly combines with the oxidizer

Learning Objective 5

Explain the Process of Pyrolysis

Learning Objective 6

Properties Affecting Solid Fuels

FACTORS AFFECTING SOLID FUEL PYROLIZATION

• Mass

• Arrangement

• Continuity

• Moisture content

MEASURING FLAME SPREAD

• Steiner tunnel

Test consists of a 25’ vented tunnel

A fan draws the flame across the surface of the material being tested

Flame spread is determined visually through windows built into the tunnel

Learning Objective 6

Properties Affecting Solid Fuels

Learning Objective 7

Properties Affecting Liquid Fuels

LIQUID FUELS

• Molecules flow freely but do not readily separate

• Will assume the shape of their container

• Physical properties make them difficult to extinguish

• They increase the hazards to persons and property

• If spilled, will flow and increase in size

OTHER IMPORTANT PROPERTIES OF LIQUIDS

• Specific gravity

• Volatility

• Vapor pressure

• Boiling point

• Vapor density

• Flash point

• Miscibility

Learning Objective 7

Properties Affecting Liquid Fuels

Learning Objective 8

Properties Affecting Gas Fuels

GAS AND VAPOR FUEL CHARACTERISTICS

• Defined as a fluid that has no shape or volume and tends to expand indefinitely

• Always fill the container in which they are stored

FLAMMABLE RANGES

• Upper flammable limit• Lower flammable limit • Flammable range

Learning Objective 9

Differentiate Heat and Temperature

SOURCES OF HEAT (FORM OF ENERGY)

• Chemical• Mechanical• Electrical• Nuclear

MEASUREMENTS OF HEAT

• British thermal unit (BTU)• Calorie (metric term)• Joule (international system of units)

MEASUREMENTS OF TEMPERATURE

• The measure of the hotness or coldness of an object Fahrenheit

Celsius

Kelvin

Rankine

Learning Objective 9

Differentiate Heat and Temperature

Learning Objective 10Illustrate the Four Methods of Heat

Transfer

ENERGY TRANSFERS FROM HEAT SOURCE TO FUEL

• Conduction

• Convection

• Radiation

• Direct flame impingement

or autoexposure

STRUCTURE FIRE AND HEAT TRANSFER METHODS

• Fire starts in a stack of boxes in a warehouse, convection catches the ceiling on fire

• Fire spreads up the stack due to direct flame impingement

• Radiated heat spreads the fire to piled stock several feet away

• A pipe running through the wall heats to the point that stock on the other side of the wall ignites

Learning Objective 10Illustrate the Four Methods of Heat

Transfer

Learning Objective 11

Illustrate the Five Classifications of Fire

FIRE HAS BEEN DIVIDED INTO BASIC CLASSIFICATIONS

• Class A: Ordinary combustibles• Class B: Flammable liquids• Class C: Energized electrical• Class D: Combustible metals• Class K: Cooking materials

FOUR STAGES OF FIRE DEVELOPMENT

• Ignition Fire ignites and reaches a point where it no

longer needs input heat from outside sources to keep burning

• Growth Fire releases heat, bringing more fuel to its

ignition temperature Heat is transferred to nearby surfaces though

radiationCont.

Learning Objective 12

Describe the Four Stages of Fire

FOUR STAGES OF FIRE DEVELOPMENT

• Fully developed All available fuels in the fire’s perimeter are

burning

• Decay Occurs when the fire has run out of available fuel Suppression action has reduced the fire to

smoldering embers

Learning Objective 12

Describe the Four Stages of Fire

FLASHOVER

• The contents in the room are brought to their ignition temperature and if sufficient oxygen is present, flashover can occur

• If a flashover were to take place, temperatures, even at floor level, would rise dramatically

• Firefighters cannot survive in a room with a flashover, even wearing full PPE and SCBA

Learning Objective 12

Describe the Four Stages of Fire

BACK DRAFT

• Flame may die out and glowing combustion takes place

• Pyrolysis continues to occur, with amounts of combustible gases produced

• Room is now superheated and charged with smoke and combustible fire gases

• Fire gases and smoke are alternately forced out and sucked back into the structure

Learning Objective 12

Describe the Four Stages of Fire

Summary

•This chapter introduces the chemical and physical properties of the elements found in fires and what actually occurs during the combustion reaction

•A solid understanding of the combustion process and the stages of fire must be in place before we discuss the selection, application, and tactics of extinguishing agents