Solution Manual for Kirk's Fire Investigation, 8th Edition

2 The Basic Science and Dynamics of Fire ESTIMATED LESSON TIME: 8 HOURS CHAPTER OBJECTIVES After reading this chapter, the student should be able to: ■ Describe the basis of how elements can combine with other atoms to form molecules by chemical interaction. ■ Explain simple oxidation reactions involving carbon, carbon monoxide, or sulfur with oxygen. ■ Describe four items formed during the thermal decomposition of wood (pyrolysis). ■ List and describe three physical states of fuel. ■ Describe different modes by which fuel vapors are generated from a solid fuel. ■ Demonstrate a clear understanding of the combustion properties of wood, paper, plastics, paint, metals, and coal. ■ Explain the importance of dust particle size and concentration in dust explosions. ■ Explain how the flammability limits of a vapor/air mixture are affected as a function of initial temperature. ■ Describe how the heat of combustion of a fuel is of importance to the fire investigator. ■ Describe the behavior of liquid pools on porous and nonporous surfaces. ■ Describe the distribution and effects of radiant heat from a burning pool of an ignitable liquid. BACKGROUND INFORMATION Fire is a chemical reaction that produces physical effects. As a result, the fire investigator should have some familiarity with the simpler chemical and physical properties involved. Because fire consists of a number of chemical reactions occurring simultaneously, it is important to understand first what a chemical reaction is and how it is involved in a fire. The body of fire dynamics knowledge as it applies to fire scene reconstruction and analysis is derived from the combined disciplines of physics, thermodynamics, chemistry, heat transfer, and fluid mechanics, many of the subjects required under NFPA 1033 (2014, pt. 1.3.7). Accurate determination of a fire’s origin, intensity, growth, direction of travel, duration, and ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e extinguishment requires that investigators rely on and correctly apply all principles of fire dynamics. Investigators must realize that variability in fire growth and development is influenced by a number of factors such as available fuel load, ventilation, and physical configuration of the room. There is a large group of very common fuels for which accurate and precise combustion data cannot generally be tabulated—solid fuels. Many of the typical combustion properties may have little or no meaning for this group. For this reason, it is often not possible to list such familiar properties as flash point or explosive range for the most common fuels of all: wood, plastic, paper, and the like. Despite this limitation, the properties of these fuels are of the greatest importance, and to some extent they can be described, either by numerical data or in general terms. The chief reason that any numerical values at all can be attributed to the solid fuels is that when heated, most, if not all of them, undergo heat decomposition or pyrolysis with the production of simpler molecular species that do have definite and known properties. Even so, precise values are not often available, because in some instances the pyrolysis of solid materials has been inadequately studied and because the pyrolysis of a single solid material gives rise to a great many simpler products. The resulting complex mixture has physical and chemical properties unlike those of any pure compound. Now that we have examined the fundamental chemical and physical properties of fire and fuels, we are ready to investigate the combustion mechanisms of particular fuels in greater detail. Unfortunately, there is no subject about which even experienced fire investigators are so likely to err. Most fire investigators are not chemists, but some chemical knowledge is essential to the proper interpretation of fires, as we have seen fires to be strictly chemical reactions. To compound the investigator’s confusion about fuels and the tests used to evaluate them, there is a whole vocabulary of terms that applies to tests devised in the laboratory to define the properties of fuels of investigative significance. It is the purpose of this chapter to discuss some of the concepts that are basic to understanding how materials burn, the conditions and limitations that apply to the combustion of fuels, and the conventional methods of expressing combustion properties in terms of laboratory tests. Fire is an exothermic oxidation reaction that proceeds at such a rate that it generates detectable heat and light. There are four recognized categories of combustion or fire types: (1) diffusion flames, (2) premixed flames, (3) smoldering combustion, and (4) spontaneous combustion (Quintiere 1998). Diffusion flames make up the category of most natural-fuel–controlled flaming fires such as candles, campfires, and fireplaces. These occur as fuel gases or vapors diffuse from the fuel surface into the surrounding air. In the zone where appropriate concentrations of fuel and oxygen are present, flaming combustion will occur. Oxygen in the air will move (diffuse) to the flame reaction zone as it is consumed in the combustion process. It becomes apparent that fire is a complex process subject to many conditions that influence its growth, spread, and development. These conditions include the physical state of the fuel, its ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e mixture with oxygen, and the effects of heat throughout the fire environment. The physical state of the fuel (solid, liquid, or gas) has an important effect on the combustion reaction. LESSON OUTLINE The following outline addresses key topics from Chapter 2. Space is provided for you to write lecture notes directly on the outline. I. Elements, Atoms, and Compounds II. The Oxidation Reaction III. Carbon Compounds IV. Organic Compounds V. Hydrocarbons VI. Petroleum Products A. Petroleum Distillates B. Nondistallates VII. Carbohydrates VIII. State of the Fuel IX. Solid Fuels A. Pyrolysis ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e 1. Nonpyrolyzing Fuels B. Combustion Properties of Wood 1. Components of Wood 2. Ignition and Combustion of Wood 3. Ignition Variables 4. Decomposed Wood 5. Low Temperature Ignition of Wood 6. Degradation to Char 7. Flame Temperature 8. Char Rates 9. Charcoal and Coke C. Wood Products 1. Plywood and Veneer Board 2. Particleboard or Chipboard 3. Exterior Deck Materials 4. Other Cellulosic Building Materials D. Paper E. Plastics ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e 1. General Characteristics 2. Behavior of Plastics 3. Polyvinyl Chloride 4. Nylon 5. Polyurethane and Polystyrene Foams 6. Latex or Natural Rubber 7. Thermoplastics F. Changes in Materials 1. Post-World War II Finishes and Furnishings 2. Changing Contemporary Finishes and Furnishings 3. Paint G. Metals 1. Magnesium 2. Aluminum H. Coal I. Combustion of Solid Fuels 1. Flame Color 2. Smoke Production ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e X. Liquid and Gaseous Fuels A. Physical Properties of Fuels 1. Vapor Pressure 2. Flammability (Explosive) Limits 3. Closed Systems 4. Open Systems 5. Flash Point 6. Flame Point/Fire Point 7. Ignition Temperature 8. Ignition Energy 9. Boiling Points 10. Vapor Density 11. Pyrolysis and Decomposition of Liquids B. Hydrocarbon Fuels 1. Natural Gas 2. Liquefied Petroleum Gas 3. Petroleum ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e 4. Gasoline 5. Kerosene and Other Distillates 6. Diesel Fuel 7. Lubricating Oils 8. Specialty Petroleum Products C. Combustion of Liquid Fuels D. Nonhydrocarbon Liquid Fuels 1. Alcohols, Solvents, and Similar Nonhydrocarbons E. Alternative Fuels or Biofuels 1. Biodiesel 2. Ethanol F. Fuel Gas Sources 1. Gas Lines 2. Natural Gas A. LP Gas ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e 1. Characteristics and Uses 2. Pressurized Containers 3. Leaks 4. Mechanical Fracture 5. Failure from Heat XI. Basic Fire Dynamics A. Basic Units of Measurement B. The Science of Fire 1. Fire Tetrahedron 2. Types of Fires There are four recognized categories of combustion or fire types: • Diffusion flames o These make up most natural fuel–controlled flaming fires. • Premixed flames o Fuel and oxygen are combined prior to ignition. • Smoldering combustion o This is typically a slow-propagating, self-sustained exothermic process characterized by visible charring and absence of flame. • Spontaneous combustion ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e o Self-heating can produce enough heat in a mass of fuel to produce flaming or smoldering combustion at a critical point known as thermal runaway. C. Heat Transfer 1. Conduction 2. Convection 3. Radiation 4. Superposition • Conduction is transfer of thermal energy through a solid from a warmer to a cooler region. • Convection is transfer of heat energy via movement of liquids or gases from a warmer to a cooler region. • Radiation is the emission of heat energy as electromagnetic (infrared) waves from a surface at a temperature above absolute zero (0 K). o Radiant heat travels in straight lines. • The total thermal effect on a material’s target surface may be a combination of all three methods of heat transfer. Superposition • Superposition is the combined effect of two or more fires or heat transfer effects, which may produce confusing fire damage indicators. ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e o Flame impingement involves both radiative and convective heat transfer to a surface as well as conduction through the surface. D. Heat Release Rate 1. Release Rates 2. Heat of Combustion 3. Mass Loss 4. Mass Flux 5. Heat Flux 6. Steady Burning 7. Combustion Efficiency E. Fire Development 1. Phase 1: Incipient Ignition 2. Phase 2: Fire Growth 3. Phase 3: Fully Developed Fire 4. Phase 4: Decay F. Enclosure Fires ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e 1. Minimum Heat Release Rate for Flashover 2. Alternative Methods for Estimating Flashover 3. Importance of Recognizing Flashover in Room Fires 4. Post-flashover Conditions G. Other Enclosure Fire Events 1. Duration 2. Smoke Alarm Activation 3. Sprinkler Head and Heat Detector Activation SELF-TEST QUESTIONS 1. 2. The smallest unit of an element that takes part in a chemical reaction is a(n): A. isotope. B. atom. C. compound. D. molecule. What can result when flammable gases are mixed with air before ignition? A violent explosion will result upon ignition. 3. Which natural element is essential for common combustion? Oxygen ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e 4. Explain the oxidation reaction for the chemical formula 2H2+O2 → 2H2O. Two diatomic molecules of hydrogen combine with one diatomic molecule of oxygen to form two molecules of water. Because water is a more stable compound than the gases that form it, the reaction occurs with great vigor and the output of much heat. 5. What common gases/vapors are formed by basic reactions in a fire? Water, carbon dioxide, carbon monoxide 6. What is the relationship of hydrocarbons to gasoline? Gasoline is not a true petroleum distillate but rather a blended product. It contains a complex mixture of hydrocarbons that evaporate or burn at different rates. 7. What are some differences between carbohydrates and hydrocarbons in the study of combustion processes? They differ in their chemical nature and also in their mode of oxidation, the heat output of the reaction, and in other respects. They also differ in the temperature at which they ignite, the quantities of oxygen used per unit of fuel, the rate at which they will oxidize, and other properties related to their physical state. 8. A(n) ___________ is a substance that cannot be decomposed into simpler substances by chemical or physical treatment. A. atom ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e 9. B. proton C. element D. molecule All matter is composed of elements or combinations of elements called: A. atoms. B. protons. C. neutrons. D. compounds. 10. ___________ are compounds composed solely of carbon and hydrogen. 11. A. Hydrocarbons B. Tetracarbons C. Chlorocarbons D. Carbohydrates Give a brief description of the various phases of a fire’s development. Phase 1: Incipient ignition – Low heat, some smoke, no detectable flame Phase 2: Growth – More heats, a lot of smoke, flame spread Phase 3: Fully developed fire – Massive heat and smoke output, extensive flame Phase 4: Decay – Decreased flame and heat, considerable smoke, with increased smoldering 12. What is meant by lower explosive limit? ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e When a gas is present at a concentration below its lower explosive limit (LEL, or lower flammability limit – LFL), it is considered too lean a mixture to burn and cannot be ignited. 13. List five hydrocarbon fuels and state their properties. • Natural gas—consists chiefly of methane with some ethane and propane • Liquefied petroleum gas (LPG)—a mixture of propane and n-butane with small quantities of ethane, ethylene, propylene, isobutane, and butylene • Petroleum—thick oil varying in color from light brown to black • Gasoline—most important fuel of petroleum origin and is a mixture of volatile, lowboiling, and midrange hydrocarbons • Kerosene—has a high boiling point range with a minimum flash point • Lubricating oils—not readily combustible at ordinary temperature; at elevated temperatures they can add to the fuel load of an established fire • Specialty petroleum products are encountered as household and industrial solvents and can be used as accelerants 14. What is the phenomenon known as pyrolysis? Pyrolysis is a process in which material is decomposed, or broken down, into simpler molecular compounds by the effects of heat alone; it often precedes combustion. DISCUSSION QUESTIONS 1. Define aliphatic and paraffinic. ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e Answer: Aliphatic: Relating to, or being an organic compound (such as alkane) having an openchain structure. (Source: Merriam Webster dictionary. https://www.meriamwebster.com/dictionary/aliphatic. Last retrieved April 26, 2017). Paraffinic: Characteristic of paraffin wax or a paraffin hydrocarbon. 2. Discuss the process of how solid fuels burn. Answer: Solid fuels, as a rule, do not burn as solids. When a solid is burning, a portion of its surface may be smoldering, even glowing. Solids burn by direct combustion of oxygen with their surface (smoldering combustion), as volatized materials that have melted and vaporized, or as complex fuels that pyrolyze to form combustible gases and vapors and leave a noncombustible solid residue. 3. Discuss smoldering. Answer: Smoldering is typically a slow-propagating, self-sustained exothermic process in which oxygen combines directly with the fuel at its surface or within it, if it is porous. If enough heat is being produced, the surface may glow with incandescent reaction zones. Smoldering fires are characterized by visible charring and absence of flame. A discarded or dropped cigarette on the surface of a couch, mattress, or other upholstered furniture can cause a smoldering fire. Although smoldering combustion does not emit a visible glow, it does generate heat detectable by touch. 4. Discuss the types of metals that can serve as fuel in a fire. Answer: • Most metals can be burned, and many metals, when finely divided, are susceptible to combustion in air. • A metal like uranium is extremely hazardous when finely divided because of its tendency to oxidize in air. • Iron in the form of steel wool or fine powder will burn readily and requires only a modest ignition source. • Very fine iron powder is pyrophoric. • Magnesium is not readily ignited unless finely divided. ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e • Aluminum is more difficult to ignite than magnesium because of its tendency to form a fine adherent film of oxide on the surface. 5. Discuss flammability (explosive limits), and open and closed systems. Answer: Mixtures of flammable gases or vapors with air will combust only when they are within particular ranges of concentration. When a gas is present at a concentration below its lower explosive limit (LEL, or lower flammability limit—LFL), it is considered too lean a mixture to burn and cannot be ignited. At concentrations above its upper explosive limit (UEL), the fuel/air mixture is too rich to burn and will not ignite. Within a closed system, if the mixture will not explode, it will not ignite. For this reason, the explosive range and flammability range of a gas or vapor may be thought of as one and the same. The explosive ranges and vapor pressures of possible fuels at a fire scene must be carefully considered, for they can play a significant part in determining accidental or incendiary origin and in ruling out some hypothetical situations. In an open system, other variables control the ignitability of the fuel/air mixture. If a mixture of vapor and air is initially too lean for an explosion, efforts to ignite it may raise the temperature and increase the amount of material volatilized sufficiently to cause ignition. More important, probably, is the too-rich mixture that will not ignite. 6. Discuss various ways that fire and heat are often transferred. Answer: During a fire, heat is usually transferred from the fire plume to surrounding objects, compartment surfaces, and occupants by three fundamental methods: (1) conduction, (2) convection, and (3) radiation. Through the process of conduction, thermal energy passes from a warmer to a cooler area of a solid material. The second process, convection, is the transfer of heat energy via movement of liquids or gases from a warmer to a cooler area, also known as Newton’s law of cooling. The third process, radiation, is the emission of heat energy via electromagnetic waves from a surface at a temperature above absolute zero (0 K). 7. Discuss combustion efficiency. Answer: Combustion efficiency is the ratio of a substance’s effective heat of combustion to the complete heat of combustion and is expressed by the term X, where X = Δheff /Δhc. Thus, for fuels with more complete combustion, values of X approach 1. Fuels with lower combustion ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e efficiencies—characterized by flames containing products of incomplete combustion, which produce sootier and more luminous fires—have values of X between 0.6 and 0.8 (Karlsson and Quintiere 2000). SUGGESTED CLASSROOM EXERCISES 1. Pyrolysis is defined as the decomposition of a material into simpler compounds brought about by heat. Discuss the pyrolysis of wood and devise an experiment to demonstrate this concept. 2. Discuss its flash point and flammable range to establish its role in the fire. 3. Discuss how oxygen in air interacts with pyrolyzing products as they are formed. ©2018 by Pearson Education, Inc. Icove/Haynes/Instructor’s Manual for Kirk’s Fire Investigation, 8/e