Chapter 4 Chemistry and Physics of Fire

1. Chapter 4Chemistry and Physics of Fire

2. 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

3. 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

4. Learning Objective 1 Fire Triangle and Fire Tetrahedron • FIRE TRIANGLE • Originally based on three elements • Fuel • Air • Heat • FIRE TETRAHEDRON • Fourth component called chemical chain reaction • Produces free radicals

5. 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

6. Learning Objective 2 Describe What Constitutes an Oxidizer • 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

7. 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

8. Learning Objective 3 Describe What Constitutes a Fuel • 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

9. 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

10. 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

11. Learning Objective 5 Explain the Process of Pyrolysis • 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

12. Learning Objective 6 Properties Affecting Solid Fuels • FACTORS AFFECTING SOLID FUEL PYROLIZATION • Mass • Arrangement • Continuity • Moisture content

13. Learning Objective 6 Properties Affecting Solid Fuels • 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

14. 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

15. Learning Objective 7 Properties Affecting Liquid Fuels • OTHER IMPORTANT PROPERTIES OF LIQUIDS • Specific gravity • Volatility • Vapor pressure • Boiling point • Vapor density • Flash point • Miscibility

16. 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

17. 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)

18. Learning Objective 9 Differentiate Heat and Temperature • MEASUREMENTS OF TEMPERATURE • The measure of the hotness or coldness of an object • Fahrenheit • Celsius • Kelvin • Rankine

19. Learning Objective 10 Illustrate the Four Methods of Heat Transfer • ENERGY TRANSFERS FROM HEAT SOURCE TO FUEL • Conduction • Convection • Radiation • Direct flame impingement • or autoexposure

20. Learning Objective 10 Illustrate the Four Methods of Heat Transfer • 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

21. 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

22. Learning Objective 12 Describe the Four Stages of Fire • 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 radiation • Cont.

23. 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

24. 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

25. 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

26. 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