Combustion Properties of Liquid and Gaseous Fuels PDF

CHAPTER 4 Combustion Properties of Liquid and Gaseous Fuels Courtesy of Jamie Novak, Novak Investigations and St. Paul Fire Dept. KEY TERMS accelerant, p. 106 flame point, p...

CHAPTER 4 Combustion Properties of Liquid and Gaseous Fuels Courtesy of Jamie Novak, Novak Investigations and St. Paul Fire Dept. KEY TERMS accelerant, p. 106 flame point, p. 94 ignition temperature, p. 94 adiabatic, p. 104 flammable liquid, p. 94 odorant, p. 113 autoignition temperature, p. 94 flammability limits, p. 88 olefinic, p. 107 BLEVE, p. 120 flash point, p. 87 vapor density, p. 99 boiling point, p. 98 ignitable liquids, p. 112 combustible liquid, p. 94 ignition energy, p. 97 OBJECTIVES After reading this chapter, you should be able to: List and describe the three types of fuels. Describe the physical properties of fuels. Explain how the flammability limits of a vapor/air mixture are affected as a function of initial temperature. Explain the relationships of the terms flash point, flame point/fire point, ignition temperature, and ignition energy. Define the terms boiling point and vapor density. Describe how the heat of combustion (defined in Chapter 2) of a fuel is of importance to the fire investigator. Explain the difference between hydrocarbon and nonhydrocarbon fuels. 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. Discuss several failure modes of fuel gas lines. 85  For additional review materials, appendices, and suggested readings, visit www.bradybooks.com and follow the MyBradyKit link to register for book-specific resources. Register for MyFireKit by following directions on the MyFireKit student access card provided with this text. If there is no card, go to www.bradybooks.com and follow the MyFireKit link to Buy Access from there. N ow 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 apply to tests devised in the labo- ratory 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 gaseous and liq- uid fuels burn, the conditions and limitations that apply to the combustion of such fuels, and the conventional methods of expressing combustion properties in terms of laboratory tests. Types of Fuel Combustion properties have different significance, or at times perhaps no significance, when applied to various types of fuel. Thus, the concept of flash point, discussed later, is essential information when studying liquids, is rarely ever used for solids, and would have no meaning if applied to gases. The relationship between the type of fuel and the type of property is clarified later. GASES Flammable materials found in the gaseous form will burn whenever mixed with the proper amount of air and properly ignited. Flash point has no significance with such materials, nor has boiling point. However, the flammable range of mixture with air, the vapor density, and ignition energy and temperature are important properties. LIQUIDS AND THEIR VAPORS Vapors from liquids are the materials that directly support the flame over a liquid fuel. For the liquid/vapor combination, vapor pressure, flash point, and, to a lesser degree, boiling point are all important considerations. Ignition temperature, range of flammable concentrations of vapor/air mixtures, and vapor densities are critical properties of the vapors themselves. Liquids themselves do not undergo combustion, except in exotic cases; instead, it is the vapors emanating from liquids that can ignite and burn as flames. SOLIDS Solids burn by direct combination of oxygen with their surface (smoldering combustion), as volatilized materials that have melted and vaporized, or as complex fuels that pyrolyze 86 Chapter 4 Combustion Properties of Liquid and Gaseous Fuels to form combustible gases and vapors and leave a noncombustible solid residue. Reactive metals such as magnesium, sodium, potassium, or phosphorus, and carbonaceous fuels such as charcoal, burn only at their surface, as a glowing fire. If the vapors being pyrolyzed from common solid fuels are disregarded, most of the properties just mentioned do not apply to solids. Nonpyrolyzable solids have no flash point in the strict sense, no vapor density, and no single ignition temperature. They have a flammable mixture range only when they are finely divided and stirred into air as a combustible dust suspension. The physical properties that control their flammability are such factors as density, con- ductivity, and thermal capacity. Their porosity and melting point will also influence their performance as fuels under some conditions. The chemical properties of a solid fuel will determine the nature of volatile pyrolysis products and the rate at which they are gener- ated. The physical and chemical properties of solid fuels and their ignitability will be examined in greater detail in Chapter 5. Physical Properties of Fuels Physical properties of fuels of particular interest to the fire investigator include vapor pressure, flammability or explosive limits, flash point, flame point/fire point, ignition tem- perature, ignition energy, boiling points, vapor density, and heat of combustion. VAPOR PRESSURE Pressure Any liquid exposed to air will evaporate as molecules escape its surface. If this process occurs in a sealed system, an equilibrium state will be reached in which the vapor reaches saturation, and there is no further evaporation. The partial atmospheric pressure exerted by the vapor at this stage is called the saturated vapor pressure, which is most often meas- Saturated vapor ured in millimeters of mercury (mmHg) (atmospheric pressure is 760 mmHg) or kilopas- cals (kPa), where 1 atmosphere (atm) is 101.32 kPa. The vapor pressure is a direct measure of the volatility of a liquid and is determined by its molecular weight, its chem- Liquid ical structure, and the temperature (see Figure 4-1). The higher the temperature, the more liquid will evaporate and the higher the vapor pressure. Table 4-1 displays several pressure FIGURE 4 -1 Vapor equivalents. pressure is the pressure As seen in Figure 4-2, the relationship between temperature and vapor pressure is not created by a liquid (or solid) linear but is a logarithmic function with controlling constants that are different for each evaporating to saturation liquid. For a given class of compound (alkane, alcohol, branched alkane), the lower the in a sealed vessel. molecular weight, the higher the vapor pressure. The vapor pressure of an aromatic or that of a normal alkane is lower than that of a branched alkane of the same molecular weight at the same temperature, so a branched (iso) alkane will have a lower flash point flash point than an alkane of the same molecular weight. For example, benzene (molecular weight Temperature at which 78): flash point, 11°C; hexane (MW 82): flash point, 22°C; isohexane (MW 82): an ignitable vapor is first produced by a flash point, 29°C. The vapor pressure is a fundamental physical property that has a liquid fuel. great influence on the flammability of liquid fuel. When the vapor pressure of a liquid reaches 760 mmHg, the liquid is at its boiling point, so the temperature at which that TABLE 4-1 Pressure Equivalents kPa mmHg psi atm bar in. w.c. 689.2 5170 100 6.80 7 — 101.32 760 14.7 1 1.013 (1013 mbar) 406.7 6.89 51.7 1 0.068 0.07 (70 mbar) 27.67 0.69 5.2 0.1 0.007 0.007 (7 mbar) 2.77 Chapter 4 Combustion Properties of Liquid and Gaseous Fuels 87 FIGURE 4-2 Calculated 900 saturation vapor pressure (in mmHg) as a function of 800 temperature (°C) for three common hydrocarbons: 700 n-pentane, n-hexane, and n-pentane n-octane. Upper dashed Vapor pressure (mmHg) 600 line is atmospheric pressure (760 mmHg). Middle 500 (long-dashed) line corre- sponds to the upper explo- 400 sive limit (UEL) (8% in air, 61 mmHg) for n-alkanes. 300 The lowest dashed line corresponds to the lower 200 n-hexane explosive limit. Note that at all temperatures above 100 5°C, pentane and hexane n-octane are above their UEL at 0 10 0 10 20 30 40 saturation. Temperature ( C) occurs is recorded as its boiling temperature. When the temperature of a liquid fuel is such that its vapor pressure reaches the percentage of 760 mmHg that is equivalent to the lower flammability limit of the fuel (see next section), the fuel is said to be at its flash point, as discussed later in this section. The vapor pressure is also dependent on the con- tour of the surface of the liquid, being higher for a convex surface than for a flat surface at the same temperature. This partly explains why an aerosol of small droplets or a liquid distributed on the fibers of a porous wick is often easier to ignite than a pool of the same liquid. Also playing a critical role in such ignitions is that the larger total surface area and small size of the aerosol droplets allows them to absorb heat more quickly from their sur- roundings. A thin film of liquid on a fiber of a wick can absorb heat very quickly from applied heat flux as well as from the surrounding air and supporting fiber. The increased temperature causes it to evaporate more quickly. Being a thin film, it has no bulk liquid to “cool” it by convective heat transfer. The capillary action of the porous wick quickly replaces the evaporated fuel to sustain the evaporation or combustion. FLAMMABILITY (EXPLOSIVE) LIMITS 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 flammability limits lower explosive limit (LEL, or lower flammability limit—LFL), it is considered too lean a The lower and upper mixture to burn and cannot be ignited. At concentrations above its upper explosive limit concentrations of an (UEL), the fuel/air mixture is too rich to burn and will not ignite. The explosive range air/gas or air/vapor mixture in which com- between these two limits is illustrated in Figure 4-3 and Tables 4-2 and 4-3. Some mate- bustion or deflagration rials such as hydrogen, carbon monoxide, and carbon disulfide have very wide ranges will be supported. within which flame will propagate to create an explosion. However, most fuels, such as petroleum distillate hydrocarbons, have quite narrow explosive ranges. Fuel/air mixtures are often considered to be in either closed or open systems, as dis- cussed next. Closed Systems 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 acciden- tal or incendiary origin and in ruling out some hypothetical situations. As can be seen 88 Chapter 4 Combustion Properties of Liquid and Gaseous Fuels  100% by volume 0% by volume FIGURE 4-3 Schematic of flammable gas/air mix- tures above and below explosion limits. Based on No combustion possible Brannigan, F. L., R. G. Bright, due to too little oxygen and N. H. Jason, eds., Fire (mixture too strong) Investigation Handbook, NBS Handbook 134. Washington, DC: U.S. Government Printing Office, 1980, p. 139. Increasing fuel content Increasing air content Limited combustion possible around ignition sources Upper explosion limit (UEL) Explosion occurs as soon as ignition is applied Lower explosion limit (LEL) No explosion due to too little gas (mixture too weak) 0% by volume 100% by volume Gas content Air content from Figure 4-4, flammability limits are temperature dependent: the higher the initial tem- perature, the wider the range. Technically, the explosive limits and flammability limits are determined under different conditions—constant pressure versus constant volume—and are therefore very nearly the same. Open Systems In an open system, other variables control the ignitability of the fuel/air mixture. If a mix- ture 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. The application of a brief arc will not be expected to cause a fire; however, prolonged application of a flame may add so much additional heat that the flammability limit is affected or may produce Saturated A FIGURE 4-4 The flam- Upper mability limits of a vapor/ vapor-air limit air mixture as a function mixtures of initial temperature at a Volume % fuel in air constant pressure, show- Autoignition ing the higher the tem- Flammable perature, the wider the Mist mixtures explosion range. From Zabetakis, M. G., “Flammability Characteristics of Combustible C D Lower Gases and Vapors.” U.S. Bureau limit of Mines, U.S. Government B printing Office, 1965, p. 3. TL TU AIT Temperature Chapter 4 Combustion Properties of Liquid and Gaseous Fuels 89  TABLE 4-2 Flammability (Explosive) Limits and Ignition Temperatures of Common Gases EXPLOSIVE IGNITION MINIMUM LIMITS TEMPERATURE IGNITION (IN AIR) (MINIMUM) ENERGY Fuel Lower Upper °C °F mJ Natural gas 4.5 15 482–632 900–1,170 0.25 Propane (commercial) 2.15 9.6 493–604 920–1,120 0.25 Butane (commercial) 1.9 8.5 482–538 900–1,000 0.25 a Acetylene 2.5 81 305 581 0.02 Hydrogen 4 75 500 932 0.01 Ammonia (NH3) 16 25 651 1,204 — Carbon monoxide 12.5 74 609 1,128 — Ethylene 2.7 36 490 914 0.07 Ethylene oxide 3 100 429 804 0.06 Sources: Fire Protection Handbook, 19th ed. (Quincy, MA: National Fire Protection Association, 2003), table 8-9.2; and SFPE Handbook of Fire Protection Engineering, 4th ed. (Quincy, MA: Society of Fire Protection Engineers and the National Fire Protection Association, 2008), table 3-18.1. a Higher concentrations (up to 100) may detonate. so much turbulence as to mix in additional air and thus start a fire. Because of the initial overly rich mixture, this resultant fire is expected to develop into what is known as a rolling fire of some intensity. It must be emphasized that the reason fire is ignited in both cases is that the system was altered by introduction of heat from the ignition source. Such an alteration is not possible in a closed system and will not be effective in producing a fire any more than in producing an explosion. Thus, on a warm day, when the gasoline/air mixture inside a tank of gasoline is richer in gasoline vapor than the upper explosive limit, an arc could safely be produced inside the tank with no ignition. At very low tempera- tures, when fewer vapors are produced and therefore may form a mixture with the air that is within the explosive range, an explosion may result (see Table 4-2). Vapor pressure considerations play a critical role in evaluating incidents where the flame external to a partially full container of flammable liquid is thought to have caused the vapor inside the container to explode by combustion. As we saw in Figure 4-1, the vapor pressure of pentane (the major, most volatile liquid component in automotive gaso- line) is, at saturation, well above its UEL (8 percent) for any reasonable temperature (above -20°C; -4°F).1 This means that if there is a significant quantity of liquid in the container and a reasonable time has elapsed, the concentration of vapors inside the con- tainer will be far too rich to support combustion. A flame can be sustained at the mouth of the vessel if there is enough vapor emanating to mix with the surrounding air, but no explosion will occur. (Heating of a closed vessel containing liquid can cause a mechanical explosion if the heating is sufficient to create a vapor pressure that exceeds the mechani- cal strength of the vessel. See Chapter 12 for a discussion of such mechanical explosions.) To test the theoretical (calculated) concept, empirical testing of plastic fuel containers filled with gasoline was conducted by Novak and by DeHaan in cooperation with the New Zealand Police Service.2 In each case, gasoline vapors emanating from the filler spout were ignited readily. There was no explosion, but there was a small clear flame about 12 cm (5 in.) high in each test, as shown in Figure 4-5a. As the plastic container melted, the opening 90 Chapter 4 Combustion Properties of Liquid and Gaseous Fuels increased in area (Figures 4-5b and c), and heat was absorbed by the gasoline (sometimes to boiling). The flame plume got larger, but there was no propagation into the can to cause an explosion. Eventually, the entire top of the can melted or burned away, exposing the entire horizontal surface of the gasoline, as seen in Figures 4-5d and e. The maximum size of the fire is controlled by the area of the pool exposed multiplied by an experimentally determined kilowatts per area factor. For gasoline, that factor is around 1,800 to 2,000 kW/m2. For a typical 20-L (5-gal) container 38 cm 38 cm (15 in. 15 in.) in size, this means a maximum fire of about 400 kW. Estimates of the HRR of these tests based on plume height revealed a maximum fire of ~150 kW (seen in Figure 4-5c), the reduction being due to the “lip” of plastic around the pool, which reduced entrainment efficiency. Even when the test was conducted with a flat-style 10-L container half full, lying on its side with no cap (Figure 4-6a), there was no explosion. In that test the upper side of the reclining can melted away over a period of minutes (Figures 4-6b–d). Eventually, the plastic containers failed as the gasoline overflowed from expan- sion, and a very large pool fire resulted, but there was no explosion (Figure 4-6e). The melted remains of such containers (Figures 4-5e–g and 4-6f) often contain pockets of unburned fuel and bear critical manufacturing data. For liquid fuels with lower vapor pressures, propagation or ignition can occur within a fuel container. This is now recognized as a problem with ethanol-based motor fuels, where ignition inside fuel storage tanks is possible at normal temperatures.3 Guidry reported an incident in which denatured (ethyl) alcohol (whose vapor pressure curve is much lower than that of pentane and has a much broader explosive range) caused an explosion within a container.4 Under the ambient temperature and use conditions, an ignitable vapor concentration existed inside the container. When a flame ignited the stream of liquid pouring from the container, the flame propagated into the container. This caused an explosion inside the container that split the container and spewed burning alco- hol over the person holding it, inducing serious burns. In cases involving the home storage of gasoline, burn injuries can occur when the can- ister is tipped or knocked over, spilling its contents and dispersing vapors within the area of the container. In a case study by Kennedy and Knapp of 25 burned children less than 6 years old, the research showed that in each case vapors from overturned or spilled gaso- line were ignited by a pilot light from either a water heater or dryer. The study noted that children at this age are totally unaware of the hazards gasoline introduces into their envi- ronment. Other problems cited in the study are that older children and adults who may misuse gasoline as an inhalant, solvent, or fire starter thus also may place themselves in harm’s way by unwittingly exposing the vapors to a competent ignition source.5 The tragic explosion of flight TWA 800 off the coast of New York in July 1996 was caused by a very unusual set of conditions when a nearly empty jet fuel tank was heated (by ambient summertime conditions and the operation of onboard air conditioners). The fuel was then exposed to a reduced atmospheric pressure as the aircraft gained altitude (increas- ing the fuel’s partial vapor pressure). At 4,000 m (13,000 ft) an accidental electrical ignition source within the tank ignited the vapors as they reached their LEL. The resulting internal fuel tank explosion caused massive damage to the fuselage and brought the plane down.6 Automotive fuel tank explosions in movies are created by using high explosives. In real life, automotive fuel tanks very rarely explode, but when a tank does, it is usually as a result of a collision that ruptures it and allows all the liquid to drain out; the concentration of vapor in the tank can then fall below the UFL, and the vapor may explode if ignited. FLASH POINT Characteristics of Flash Point The term flash point is basic to the description of many common liquid fuels, since it can be used to assess the fire hazards of ignitable liquids under various conditions (ignitable Chapter 4 Combustion Properties of Liquid and Gaseous Fuels 91 FIGURE 4-5A Vapors of one gallon of FIGURE 4-5B Burning vapors at 10 minutes. FIGURE 4-5C Gasoline boiling as vapors gasoline in a plastic container ignited at Courtesy of Jamie Novak, Novak Investigations and burn at 20 minutes. Courtesy of Jamie Novak, open spout. Courtesy of Jamie Novak, Novak St. Paul Fire Dept. Novak Investigations and St. Paul Fire Dept. Investigations and St. Paul Fire Dept. FIGURE 4-5D Melted remains of plastic FIGURE 4-5E Melted gasoline container FIGURE 4-5F Underside of melted container can with burning gasoline. Courtesy of Jamie after fire (top). Courtesy of Jamie Novak, Novak bears molded information. Courtesy of Jamie Novak, Novak Investigations and St. Paul Fire Dept. Investigations and St. Paul Fire Dept. Novak, Novak Investigations and St. Paul Fire Dept. FIGURE 4-5G Container cut open showing pockets that often trap residual liquid fuel. Courtesy of Jamie Novak, Novak Investigations and St. Paul Fire Dept. FIGURE 4-5 Ignition test of 3.8-L (1-gal) plastic container filled with gasoline, with open filler spout. 92 Chapter 4 Combustion Properties of Liquid and Gaseous Fuels FIGURE 4-6A Ignition—no explosion FIGURE 4-6B One minute—quiet flame at mouth of filler. FIGURE 4-6C Five minutes—filler neck is melt- FIGURE 4-6D Ten minutes—side of can melting, ing, flames growing longer. exposing more gasoline to air. FIGURE 4-6E After 14 minutes, can melts in a FIGURE 4-6F Melted container after fire. pool fire. FIGURE 4-6 Plastic 10-L gas can lying on its side, half full of gasoline, with no cap. All Courtesy of New Zealand Police Service, Canterbury CIB. liquids being those that fall under the common classifications of flammable or com- bustible liquids). The flash point of a material is the lowest temperature at which it pro- duces a flammable vapor. At its flash point temperature, the vapor pressure of the fuel (as a percentage of 760 mmHg) is equal to that fuel’s lower limit of flammability. For exam- ple, the flash point of n-decane is 46°C (115°F). Its vapor pressure at that temperature is 6 mmHg, which is 0.78 percent of 760 mmHg. This value is very close to the measured lower explosive limit of 0.75 percent. A liquid fuel must be able to generate a vapor in sufficient quantity to reach that lower limit in air before it can burn. This does not mean that the vapor will ignite spontaneously at this temperature but only that it can be ignited by a flame, a small arc, or other local source of heat. For example, the flash point of low- octane automotive gasoline is about 43°C ( 45°F). This flash point value means only Chapter 4 Combustion Properties of Liquid and Gaseous Fuels 93  that gasoline produces a vapor that can be ignited at any temperature above about – 43°C, not that gasoline will spontaneously ignite at such an extremely low temperature. Determining Flash Point The flash point is determined by placing a small sample of the fuel in the cup of a testing apparatus and heating or cooling it to the lowest temperature at which an arc or small pilot flame will cause a little flash to occur over the surface of the liquid. No fire results from such a test other than the little flash of flame, which immediately extinguishes itself. Several recognized flash point testers are used today, including the Pensky-Martens, Cleveland, and Tagliabue (Tag) testers. Each device has a particular temperature range and fuel type for which it is most accurate. The Tag closed-cup tester, for example, is the accepted method (described in ASTM D56) for testing nonviscous liquids with a flash point below 93°C (200°F).7 The Pensky-Martens tester (closed-cup) is best suited for liq- uids with a flash point above 93°C (200°F) and is described in ASTM D93.8 The open- cup tests are used for evaluating hazards of ignitable liquids where they are encountered in open-air situations—during transportation or spills, for instance. The Tag open-cup method is described in ASTM D1310,9 and the Cleveland open-cup method is described in ASTM D92.10 Most tables of flash point data for fire safety purposes list the closed- cup flash point of most materials. As a general rule, the open-cup flash points for the same materials would be a few degrees ( 10 percent) higher than the closed-cup values. The flash point tests used in the industry are described in detail in the NFPA Fire Protection Handbook and will not be repeated here. The same source contains extensive tables of flash point data for all types of flammable liquids. Some representative flash points are listed in Table 4-3 for general reference. NFPA11 classifies ignitable liquids according to their reported flash points: flammable liquid A liquid having a flash Class I: Flashpoint below 38°C (100°F) (flammable liquids) point below 38°C Class II: Flashpoint between 38°C and 60°C (100°F–140°F) (combustible liquids) (100°F). Class III: Flashpoint above 60°C (140°F) (combustible liquids) combustible liquid A liquid having a flash FLAME POINT/FIRE POINT point at or above 38°C (100°F). The flame point or fire point is the lowest temperature at which a liquid produces a vapor that can sustain a continuous flame (rather than the instantaneous flash of the flash flame point point). It is usually just a few degrees above the rated flash point. Although it is not often Temperature at which a flame is sustained by reported, the flame point is probably a more realistic assessment of a fuel’s contribution evaporation or pyrolysis to a fire environment than the flash point. The fire points of a few common fuels are of a fuel. shown in Table 4-3. The exact values of flash point and flame point temperatures of tested fuels depend on the following factors:12 ignition temperature (same as autoignition) the size of the ignition source, The minimum tempera- how long it is held over the source each time, ture to which a sub- the rate of heating of the liquid, and stance must be heated in air to ignite inde- the degree of air movement over the fuel. pendently of the heat- ing source (i.e., in the Note that Glassman and Dryer (see Table 4-3) reported minimum fire points for alcohols absence of any other to be equal to their minimum flash points and observed a significant difference between ignition source; non- values obtained using flame igniters and those using electrical arc ignition, apparently as a piloted ignition). result of the infrared absorption characteristics of alcohol fuels. autoignition tempera- ture The temperature IGNITION TEMPERATURE at which a material will The ignition temperature is important to all considerations of a fire. Sometimes called ignite in the absence of any external pilot source the autoignition temperature (AIT) or spontaneous ignition temperature (SIT), it is the of heat; spontaneous temperature at which a fuel will ignite on its own without any additional source of igni- ignition temperature. tion. For purposes of comparing liquid and gaseous fuels, ignition temperature can be 94 Chapter 4 Combustion Properties of Liquid and Gaseous Fuels  TABLE 4-3 Flash Points and Flame (Fire) Points of Some Ignitable Liquids FLASH POINT FLASH POINT (CLOSED CUP)* (OPEN CUP) FIRE POINT Fuel °C °F °C °F °C °F Gasoline (auto, low octane) 43 45 — — — — Gasoline (100 octane) 38 38 a Petroleum naphtha 29 20 JP-4 ( jet aviation fuel) 23 to 1 10 to 30 Acetone 20 4 Petroleum ether 18 0 Benzene 11 12 Toluene 4 40 Methanol 11 52 1(13.5)† 34(56) 1(13.5) 34(56) Ethanol 12 54 n-Octane 13 56 Turpentine (gum) 35 95 b Fuel oil (kerosene) 38 100 Mineral spirits 40 104 n-Decane 46 115 52† 125 61.5† 143 Fuel oil #2 (diesel) 52 (min) 126 Fuel oil (unspecified) — — 133‡ 271 164‡ 327 Jet A 43–66 110–150 § p-Xylene 25 77§ 29§ 84§ — — JP-5 (Jet aviation fuel) 66 151 Sources: *Fire Protection Guide to Hazardous Materials (Quincy, MA: National Fire Protection Association, 2001), except as noted. † Calculated minimum limit for open-cup tests with flame igniter. Higher values in parentheses are for spark source ignition. Source: I. Glassman and F. L. Dryer, “Flame Spreading across Liquid Fuels,” Fire Safety Journal 3 (1980–1981): 123–38. ‡ SFPE Handbook of Fire Protection Engineering, 4th ed. (Quincy, MA: NFPA, 2008), table 2-8.5. § ScienceLab.com, MSDS p-xylene, 2008. a Generic name for miscellaneous light petroleum fractions used in such consumer products as cigarette lighter fuels and fuels for camping stoves and lanterns. b The flash point of kerosene is set by law in many jurisdictions and may be higher in compliance with local laws. measured by injecting a small quantity of fuel into a hot-air environment (usually a closed container) at various temperatures. In the standard test, ASTM E659, the test chamber is an electrically heated 1-L glass flask (sometimes called a Setchkin apparatus).13 When the container is at or above the fuel’s ignition temperature, the fuel will burst into flame upon injection. For example, low-octane gasoline, with a flash point of -43°C (-45°F), requires a minimum temperature of about 280°C (536°F) to catch fire (100 octane gasoline has an AIT of 456°C). This is the minimum temperature that must be reached by the match, spark, lighter, or other igniting instrument in order for that material to burn. With one or Chapter 4 Combustion Properties of Liquid and Gaseous Fuels 95  two exceptions, such as catalytic action, the temperature of at least a portion of the fuel must be raised to its ignition temperature before fire can result from any fuel. Ignition temperatures of common materials are generally so high as to rule out spontaneous com- bustion, except for a very small category of materials and only under very special condi- tions. One exception, for instance, is carbon disulfide (CS2), which is encountered as an industrial cleaning solvent. Carbon disulfide has various reported AITs of 90°C to 120°C (194° to 240°F). In general, for a given series of hydrocarbons, the longer the carbon backbone, the lower the AIT of the material. Note the sequence of AITs for n-alkanes in Table 4-4, where n-pentane has an AIT of 260°C, while n-octane and higher alkanes have an AIT of around 208°C. Catalytic oxidation of some fuels can occur at temperatures much lower than the AIT, but these reactions are very rare and occur under special conditions. For example, plat- inum in a finely divided state may serve to allow (catalyze) combustion of certain flam- mable gases in air at low initial temperatures. In a fuel cell or a hand warmer this may be important, but it is very rare in general fires. For solids, in which pyrolytic decomposition must precede ignition, there is another value called the piloted ignition temperature, which will be discussed in Chapter 5. TABLE 4-4 Minimum Autoignition Temperatures, Flammable Ranges, and Specific Gravities of Some Common Ignitable Liquids IGNITION TEMPERATURE MINIMUM IGNITION FLAMMABLE RANGE FUEL (°C) (°F) ENERGY (mJ) (% IN AIR @ 20°C) SPECIFIC GRAVITY Acetone 465 869 1.15 2.6–12.8 0.8 Benzene 498 928 0.2 1.4–7.1 0.9 Diethyl ether 160 320 –– 1.9–36.0 0.7 Ethanol (100 percent) 363 685 –– –– 0.8 Ethylene glycol 398 748 –– –– 1.1 Fuel oil #1 (kerosene) 210 410 –– –– Fuel oil #2 257 495 –– –– Gasoline (low octane) 280 536 –– 1.4–7.6 0.8 Gasoline (100 octane) 456 853 –– 1.5–7.6 0.8 Jet fuel (JP-6) 230 446 –– –– Linseed oil (boiled) 206 403 –– –– Methanol 464 867 0.14 6.7–36.0 0.8 n-Pentane 260 500 0.22 1.5–7.8 0.6 n-Hexane 225 437 0.24 1.2–7.5 0.7 n-Heptane 204 399 0.24 –– 0.7 n-Octane 206 403 –– 1.0–7.0 0.7 n-Decane 210 410 –– –– 0.7 Petroleum ether 288 550 –– 1.1–5.9 0.6 Pinene (alpha) 255 491 –– –– Turpentine (spirits) 253 488 –– –– 1 Sources: Data taken from Fire Protection Guide to Hazardous Materials (Quincy, MA: National Fire Protection Association, 2001); C. F. Turner and J. W. McCreery, The Chemistry of Fire and Hazardous Materials (Boston: Allyn and Bacon, 1981). 96 Chapter 4 Combustion Properties of Liquid and Gaseous Fuels  Virtually all fires originate where there is a localized high temperature in a region in which an appropriate fuel/air mixture occurs. The local region may be very small, as in the case of an electrically induced arc, a mechanical spark, or a minute flame. The impor- tant concept is that at this very small point in space, a temperature in excess of the ignition temperature occurs in the presence of appropriate fuel and air (or oxygen), and this energy is transferred to an adequate volume of ignitable fuel. Such circumstances are not unusual, but from the investigative standpoint, it must always be accepted that these circumstances constitute a minimum requirement for any fire whatever to result. That tiny initial flame then propagates through adjoining mixtures in the flammable range. Many times, no source of such relatively high temperatures appears to be present, and the origin of a fire seems very mysterious. In such instances it should be remembered that it is actually not difficult to produce a source of energy in a very small area. Fires can be kindled by rubbing two sticks together, by the flint-and-steel method (which strikes a very small but hot spark), and by other similar means. In the start of a fire, it is only the heat content of the source (as reflected in part by its temperature) that counts—not the area over which the temperature holds. A nail in a shoe heel striking a rock may ignite a fire, not because the shoe is hot but only because an almost infinitesimally small region (the spark) reaches a very high temperature. Except in industrial fires, where pressurized ves- sels and delivery lines make for special cases, it is rare indeed for any large amount of fuel to be heated past its ignition temperature. If even a small portion of it is so heated, and the temperature is above the ignition point, a fire can result. This is why so many fires occur under conditions that do not seem to be hazardous. As long as the fuel gas or vapor concentration is within its flammability range in the vicinity of this small but intensely energetic source, there can be ignition. The fundamental consideration is not the temper- ature of an ignition source compared with the ignition temperature of the fuel but the transfer of heat energy from the source to the fuel. An ignition source may be present that is at a temperature that is technically higher than the AIT listed for that fuel, but there still may not be ignition if enough energy is not transferred to the fuel. It should be noted that the AIT is a minimum environmental temperature measured by a particular laboratory technique (ASTM E659 being the most common) and is dependent on the size, shape, and even surface material and texture of the confinement vessel used in the test apparatus. The real-world ignition sources that a fuel might encounter will usually have different size, shape, and confinement and so will almost always have to be at a tem- perature considerably higher than the listed AIT. AIT data are more for reference and comparison than for a specific determination. Some minimum ignition temperatures of common fuels are listed in Tables 4-2 and 4-3. Because the minimum ignition temperature is dependent on the method used for its determination, that value should be used prima- rily for comparisons between fuels. Testing has revealed that “hot surface” ignition of gasoline in open air, for instance, requires that the temperature of the surface be at least 200°C (360°F) above the listed autoignition temperature [i.e., 500°C 700°C (930°F 1,300°F)]. It is thought that the hot surface produces rapid evaporation, which mini- mizes contact time.14 The material and texture of the hot surface will also affect minimum ignition temperatures. The “grade” of gasoline is also critical: the higher the grade or octane rating, the higher the AIT. (See the discussion in Chapter 9.) IGNITION ENERGY Every fuel gas or vapor has a minimum ignition energy. This is the amount of energy that ignition energy The must be transferred to the fuel to trigger the first oxidation. For most fuels, this is a very quantity of energy that small amount of energy; for hydrocarbon fuels, the minimum ignition energy is on the must be transferred into a fuel/oxidizer order of 0.25 millijoule (mJ) (see Table 4-2). This amount is dependent on the concentra- combination to trigger tion of the vapor and is often at its minimum when the fuel vapor is at its stoichiometric a self-sustaining or ideal concentration (as shown in Figure 4-7). For a vapor/air mixture to ignite, there combustion. Chapter 4 Combustion Properties of Liquid and Gaseous Fuels 97  FIGURE 4-7 Ignitability curve and flammability Limits of limits for methane/air flammability mixtures at atmospheric Spark energy (mJ) pressure and 20°C. From Zabetakis, M. G., Ignitability “Flammability Characteristics limits of Combustible Gases and 1 Vapors,” Washington, DC: U.S. Bureau of Mines, U.S. 0.5 Government Printing Office, 1965, p. 2. 0.2 2 4 6 8 10 12 14 16 18 Volume % methane in air are four required conditions. First, an ignition source must have adequate energy (above the minimum ignition energy for that fuel); second, the fuel must be present at a concentra- tion within its flammability range; third, there must be contact between the source and the fuel while it is in that range; and finally, the contact must be of sufficient duration for enough energy to be transferred from the source to the fuel. Unless all four of these con- ditions are met, there will not be an ignition. Several of the flammability properties are interrelated. For instance, the minimum ignition energy of a fuel is dependent on fuel concentration, as can be seen in Figure 4-7. This means that a very energetic source can ignite mixtures closer to the flammability lim- its for that fuel, while a very weak source may be competent only when the mixture is near its stoichiometric mixture. In addition, the flammability limits themselves are dependent on starting, or initial, temperature, as was shown in Figure 4-4: the warmer the starting (pre-fire) mixture, the wider the limits. Although gaseous-phase explosions are treated in depth in Chapter 12, there are sev- eral observations about fire behavior that are related to concentration to be discussed here. The speed with which a flame front progresses through a fuel/air mixture is depend- ent on concentration. The flame speed reaches a maximum when the mixture is near its stoichiometric or ideal mixture. As a result, overpressures (caused by rapidly expanding gases) are also at or near their maxima. A premixed hydrocarbon fuel/air mixture near its stoichiometric mixture theoretically can produce pressures up to 120 psi (8 bar) of pres- sure under ideal conditions.15 A lean mixture propagates at a slower speed and produces less heat and, therefore, lower pressures. Because all the fuel is consumed in the reaction, there is no subsequent fire or flame. In contrast, a rich mixture results in a less vigorous explosion with less mechanical damage due to lower pressures and slower speeds. The excess fuel produces smoke or soot and a following flaming fire. Thus, the heat effects and blackening are often greater and the blast damage less when the mixture is rich than when it is lean. The explosive limits of some of the more hazardous liquids and gases are discussed in Chapter 13. BOILING POINTS Boiling points are sometimes important in fire investigations, but they are of secondary significance compared with other properties discussed in this chapter. Most liquids may boiling point The be expected to be heated to their boiling point in a fire. However, the very important (pressure-dependent) distinction between a liquid fuel that is reasonably pure and will therefore have a rea- temperature at which sonably definite boiling point and one that is a mixture of many components of variable a liquid changes to its gas phase. boiling points must be grasped, since boiling point ranges are used to characterize many fuels encountered in fires (See a later section of this chapter for examples of boiling point ranges.) 98 Chapter 4 Combustion Properties of Liquid and Gaseous Fuels  A pure, single-component liquid fuel will have a definite boiling point at which an entire sample may be distilled without a change in temperature of the vapors. The gen- eral public has available to it relatively few pure liquid fuels compared with the much larger number of mixtures that have a boiling range rather than a boiling point. Such mix- tures range from gasoline, with more than 200 individual compounds, to a large number of manufactured mixtures for special purposes such as cleaning or industrial solvent action. When heated, such mixtures allow the distillation of their most volatile con- stituents first, followed in turn by constituents of decreasing volatility (a process often called weathering, which will be discussed more fully in Chapter 14 ). In general, it is the most volatile constituent of a mixture that is of most significance as it relates to the fire hazard. When a liquid fuel that has a boiling point range (e.g., a mixture such as gaso- line) burns, the more volatile species evaporate first and fastest from the surface layer. The bulk liquid beneath the surface will retain the lighter species for some time, since they will have to migrate by diffusion (aided by convective flows) through the bulk liquid to reach the surface before they can evaporate.16 A consequence of such fractional distillation is that the residue of a petroleum distillate may have a lower autoignition temperature than the bulk liquid, possibly posing a risk of delayed ignition.17 In initiating a fire, materials of high boiling point will rarely be above their flash point at ambient temperatures, and only the more volatile (low boiling point) will be expected to show special hazard. As a rule, the boiling point and the flash point tend to parallel each other, so the low-boiling material will usually have a low flash point as well. The presence of a mixture does not greatly diminish the fire hazard of the most volatile com- ponent in that mixture. A mixture of gasoline and diesel fuel, for instance, exhibits a flash point indistinguishable from that of gasoline alone until the gasoline represents less than about 5 percent of the mixture. VAPOR DENSITY The fundamental property of a vapor that predicts its behavior when released in air is its vapor density. The vapor density, that is, the density of a vapor relative to that of air, may vapor density The be simply calculated by dividing the molecular weight of the vapor by the mean molecular ratio of the weight of weight of air, which is approximately 29. This relationship is represented by the equation a given volume of gas or vapor to that of an Vapor density (gas or vapor) Molecular wt of gas>Molecular wt of air equal volume of air. For example, the lightest gas, hydrogen, has a molecular weight of only 2; therefore, its vapor density (V.D.) is V.D. 2>29 0.07 Methane, the simplest hydrocarbon fuel gas found in swamp gas and natural gas, has a molecular weight of 16. Its vapor density, then, is roughly 16> 29, or 0.55. Ethane, the other major component of natural gas, has a molecular weight of 30, making it almost exactly the same density as air, and giving it a vapor density of 1.03. The vapor densities of some common fuels are shown in Table 4-5. Gases or vapors with a vapor density greater than 1.0 are heavier than air and tend to settle through the air into which they are released until they encounter an obstruction, such as a floor, after which they tend to spread outward at this level, much in the same manner as if they were liquids. In contrast, vapors lighter than air tend to rise through the air until an obstruction, such as a ceiling, is encountered, after which they spread at the high level. The tendency to spread is less absolute than with liquids because the dif- ference in density as compared with air is much less than compared with any liquid, and air currents produce mixing. Some mixing with the air is inevitable, and the nearer the vapor density is to that of air, the greater the mixing of fuel and air. This effect of greater mixing is due to the process known as diffusion. All gases, regardless of their molecular weight or vapor density, tend to diffuse into each other when the opportunity exists. Chapter 4 Combustion Properties of Liquid and Gaseous Fuels 99  TABLE 4-5 Vapor Densities of Common Fuel Gases and Vapors FUEL VAPOR DENSITY (AT STANDARD TEMPERATURE AND PRESSURE) Hydrogen 0.07 Methane 0.55 Acetylene 0.90 Carbon monoxide 0.97 Ethane 1.03 Propane 1.51 Butane 1.93 Acetone 2.00 a Pentane 2.50 b Hexane 3.00 a Lightest liquid component of gasoline. b Lightest major component of camping fuels. Differences in diffusion rates of gases depend on their vapor density: when a fuel vapor is much heavier than air (V.D. 1.0), its diffusion rate will be much less than when its density is much closer to that of the air into which it is diffusing, that is, when its vapor density is close to 1.0. The diffusion of a vapor is a very slow process. In a closed container, diffusion of a vapor will eventually produce a uniform distribution after some hours. In an open system such as a room, diffusion of vapors from a volatile liquid does not produce a uniform distribution but rather a gradient of concentration from very high (saturation vapor pressure) to very low, as represented in Figure 4-8. Methane, as noted, is lighter than air and thus tends to rise in a room (as long as its temperature is the same as that of the room air). Its rate of diffusion is greater than that for heavier molecules, and so it tends to mix with air and form a gradient of concentra- tion, as illustrated in Figure 4-8a. The same is true for natural gas, which is a mixture of methane and ethane and other light gases, so its vapor density will be between 0.55 and 1.0, depending on the relative proportions of the components. Ethane is rarely encountered by itself, but because its vapor density is close to 1, it will diffuse readily in air and neither rise nor settle in air, there being no gravitational separation. (It should be noted that diffu- sion acts to keep vapors diffused and mixed in air and once mixed in air, vapors will not “settle out” to re-form layers of high concentration.) The same is true for acetylene (C2H2) with a molecular weight of 26 and therefore a vapor density of 0.9, as well as ethene (C2H4) and carbon monoxide (CO), each with a molecular weight of 28 and therefore a vapor density of 28/29, or 0.97. All other commonly encountered fuel gases and vapors have a vapor density significantly greater than 1 and will tend to sink when released in air. For example, propane, the next heavier hydrocarbon, (V.D. 1.51) will mix readily with air, but it will tend to sink as it does so. [It should be noted that a flammable propane/air mixture (2 9.6 percent) will have a vapor density very close to that of air.] Butane (V.D. 1.93) sinks more readily and tends not to diffuse as quickly. All higher hydrocarbons, char- acteristic of most petroleum products, will of course have slower diffusion rates (diffusivity) along with their greater vapor density. Gasoline vapors, for instance, contain a mixture of all the most volatile components of the liquid blend. This mixture, being considerably heavier than air, will sink rapidly to the floor or ground surface. Gasoline vapors will therefore pour into low regions or down drains and will carry their intrinsic hazard to the lowest possible region (see Figures 4-8b and c for a graphic representation). 100 Chapter 4 Combustion Properties of Liquid and Gaseous Fuels  FIGURE 4-8 Typical Rich mixtures of natural gas or flammable fluid vapors in Explode closed rooms. (a) Natural gas (methane) in a room. (b) Gasoline in a room. (c) Gasoline (or butane) in a room with various Lean elevations. (a) Lean Explode Liquid Rich (b) Lean Source Explode Rich (c) It should be remembered that from a fire hazard standpoint, the vapor density and diffusion rate of the lightest (most volatile) component of a mixture is what will deter- mine the spread and ignitability of vapors, not the properties of the bulk liquid. For instance, gasoline has a range of components, some of which, like n-decane, have a very low vapor pressure at room temperature and a correspondingly high flash point. It is the isobutane and n-pentane content of automotive gasoline that generates the bulk of the vapors at ordinary room temperatures and produces the significant ignition risk. Gasoline’s heavier components such as the xylenes do not contribute significantly to the vapors until the gasoline has been evaporating for many minutes, by which time the lighter components will have already been ignited.18 In general, the vapors of fresh auto- motive gasoline will behave exactly like those of n-pentane in their movement. It is apparent that various concentration layers will exist (with a gradient of interme- diate concentrations between the heaviest and lightest layers). Some of these vapor layers will be within their explosive (flammable) limits, while the mixture in other layers will be Chapter 4 Combustion Properties of Liquid and Gaseous Fuels 101  either too rich or too lean to be ignited. These distributions represent release into a sealed room with still air in which there is no physical activity and in which the vapors are at the same temperatures. In assessment of real-world distribution of vapors, the effects of temperature and air currents must not be overlooked. Mechanical activity such as a fan, furnace, operating machinery, or even someone walking will stir vapors into mixtures throughout a room. The draft created by a water heater burner, furnace burner, or open window may be enough to redistribute vapors and may draw them into contact with an open-flame ignition source at the same time. Even butane if warmed sufficiently, will rise to the top of a room much like methane because the vapor density drops rapidly with increased temperatures. Vapors from a flammable liquid cooled by evaporative cooling will be even denser, and their convective flow will enhance their tendency to settle.19 When a gas mixture is formed from gases at different temperatures, the temperature alone may control its movement initially until the temperature differences are equalized. The alert investigator must be aware of all these influences (including timer- or thermostat- controlled equipment that may cycle on with no one in attendance) that can distribute and circulate vapors and possibly provide an ignition source as well. In the United States, gas-fired water heaters have been required since 2005 to be designed so as to make it very difficult for their burners to ignite gasoline vapors. One design places the entire heater in an 18-in.-deep “bucket” to keep vapors out of the burner chamber. The other uses a flash suppressor on the combustion chamber to prevent propagation outward. These designs will be described in more detail in Chapter 6. The concentration gradient established by a heavier-than-air gas (such as LP gas) leaking into still air has been shown to be very steep.20 This means that a person stand- ing in the room may not be able to detect the leak by its odorant while a layer below knee height in the same room is within its explosive range. Recent experiments have shown that thermal currents, even in a small space such as a camper or RV, result in almost com- plete mixing of propane into air, with no detectable gradient.21 One interesting property of dense vapors (V.D. 2.5) is their propensity to flow hori- zontally, like a viscous liquid. Pentane vapor released from a pool at room temperature will diffuse upward only a few centimeters, and then the mass of vapor will slump side- ways and spread outward along the surface at a rate of about 0.05 m/s in still air. This so-called advective flow can spread vapors along a surface much more quickly than diffu- sion, but this flow is easily overcome by even a modest draft.22 Careful consideration of the distribution of fuel gases and vapors must be made when assessing the likelihood of ignition by possible sources in the compartment. If an ignition source like a candle flame is high up in a room, LP gases or gasoline vapors are going to require a long time to diffuse sufficiently to reach their flammable range in the vicinity of the candle flame unless there is some external means of circulation like a fan or strong draft, or the vapors are released under pressure (jet). Similarly, a kerosene heater burning normally at floor level in a room is unlikely to ignite natural gas leaking into the room until the gas can fill the room from the top down in the absence of mechanical circula- tion. The mere presence of an ignition source and a fuel source in the same room is not a guarantee that there will be ignition. The assessment of possible ignition sources for such vapor/air mixtures should take into account all these contributing factors, including the heights of, and protective enclosures around, appliance burners. It was once thought that the distribution of blast effects from a vapor/air deflagration was directly linked to the preignition distribution of vapors. However, experiments by one of the authors have demonstrated that even when a highly localized (floor) layer of n-hexane vapors is ignited, the pressures are equalized throughout a moderate-sized com- partment within 5 milliseconds (ms).23 This means that the walls of the average compart- ment will be exposed to the same pressure at the same time from within and will therefore fail where it is weakest structurally. It is frequently noted in explosions in wood-frame struc- tures that it is the bottom of the wall that moves rather than the top, despite whether the 102 Chapter 4 Combustion Properties of Liquid and Gaseous Fuels  FIGURE 4-9 Pocketed burning of natural gas accumulated under this wooden floor produced significant charring of joists and floor. Courtesy of Jamie Novak, Novak Investigations and St. Paul Fire Dept. deflagration was fueled by natural gas or LP gas. This result can mean that the bottoms of the walls were not as well anchored to the foundation or slab as the tops of the walls were to the structure above. The bottoms of wood-frame walls are also much more likely to be weakened by termites or dry rot. When a fire follows an explosion, however, if the turbu- lence of the explosion has not completely dissipated the layer of fuel vapor, the fire will tend to burn at the interface between the richest layer of fuel and the air (as discussed previ- ously). Thus, a fire following a hexane (or gasoline) vapor explosion will tend to burn atop or within a remaining layer of dense vapor (where the fuel-rich layers will burn longest). The distribution of fire damage low down in the room after a vapor ignition then indicates a heavier-than-air vapor has been involved. The reverse will occasionally occur on the underside of a natural gas layer trapped or pocketed on the underside of a floor or roof structure, as in Figure 4-9. In either case, the location and distribution of postexplosion fire damage may give important clues as to the vapor density of the fuel responsible. The volume of vapor that can be generated by the evaporation of a known volume of a volatile liquid may be important in evaluating the ignition potential of accidental or even intentional spills. If total evaporation can be assumed, the volume of vapor gener- ated (at standard temperature and pressure) by 1 U.S. gallon (3.8 L) of liquid can be cal- culated from the empirical relationship Vapor (in cubic feet, ft3) 111 Specific gravity of liquid* Vapor density of liquid For example, 1 U.S. gallon of diethyl ether, with a specific gravity of 0.7 and a vapor den- sity of 2.55, will produce 111 0.7>2.55 30.4 ft3 of diethyl ether vapor Because diethyl ether has a flammability or explosive range of 2 to 36 percent in air, 30.4 ft3 of vapor, if uniformly distributed, will produce an explosive mixture in a room of 84 to 1,520 ft3 in volume. In metric units the equivalent relationship for 1 L of liquid (0.001 m3) is Vapor (in cubic meters, m3) 0.85 Specific gravity Vapor density *See Table 4-4 for specific gravity values. Chapter 4 Combustion Properties of Liquid and Gaseous Fuels 103  For commercial propane, the liquid/vapor expansion rate is on the order of 272; that is, 1 L of liquid propane will produce about 272 L of vapor at standard temperature and pressure. Commercial butane has an expansion factor of approximately 234.24 The expansion rate for LP gas itself will vary with the composition of the LP gas, since the rates for each component are different. HEAT OF COMBUSTION Another property of fuels that is often of great importance to the investigator is the heat of combustion of the fuels involved, as introduced in Chapter 2. If the approximate amounts of fuels in a room can be estimated by weight, the intensity of burning observed can be compared with the possibilities of various combinations of fuels. As we saw in Chapter 3, the rate at which heat is released is more critical to reconstructing the events of a fire than the total amount of heat, but it is sometimes useful to be able to recognize when the fire damage to a room simply does not tally with the amount of heat that should have been produced by the fuels known to be present. The causes for such differences can then be explored. The scientific method question is: Was there so much damage that it cannot be accounted for by assessment of fuel loads normally present? Was there reduced fuel because stock had been removed prior to the fire? Was there reduced ventilation available, or was there enhanced combustion due to oxygen enrichment? Was the lack of damage due to the incomplete or inefficient combustion of the available fuels? Some of the most com- mon fuels, such as wood, have such variable properties as to give only approximations of the actual output in a specific instance. Table 4-6 summarizes the heat of combustion of a few of the most important fuels from the standpoint of the fire investigation. adiabatic Conditions The heat of combustion is used to calculate a theoretical adiabatic flame temperature of equilibrium of tem- for each fuel. These calculated values are sometimes erroneously used to estimate the perature and pressure. potential effect the flames will have on surrounding materials. Work by Henderson and Lightsey indicates that the actual measured flame temperatures produced by such fuels when burned in air are much lower than previously thought.25 The measured tempera- tures they reported are included in Table 4-6. The roles of the flame temperatures and heat of combustion in fire investigation will be explored in later chapters. It should be noted that the real-world maximum open-flame temperatures of nearly all “normal” fuels burn- ing in turbulent diffusion flames is about 900°C (1,700°F), and this figure is used in most calculations. Some fuels (such as styrene, crude oil, and urethane foam) that burn with very smoky flames, so that the heat produced cannot be readily lost (low emissivity), pro- duce very high temperatures in the flames. Fuels that burn very cleanly with no soot or pyrolysis products (alcohols) also have a low emissivity and high flame temperatures. Hydrocarbon Fuels By far the most important fuels from the standpoint of fire investigation are the hydro- carbons, which range from the light gas methane to much heavier compounds, including oils and asphalts. (For a discussion of the chemistry of hydrocarbons, see Chapter 2.) Because of the importance of hydrocarbons to fire investigation, some of their specific properties will be discussed in order of their molecular weight or chemical classification. NATURAL GAS Natural gas, consisting chiefly of methane, is of obvious significance. Natural gas from different geological formations shows considerable variation in composition and may contain some noncombustible gases. It is typically 70 to 90 percent methane, 10 to 20 percent ethane, and 1 to 3 percent nitrogen, with traces of other gases.26 For most pur- poses, the investigator may consider its properties to be the same as those of methane, without introducing serious error. 104 Chapter 4 Combustion Properties of Liquid and Gaseous Fuels  TABLE 4-6 Heats of Combustion and Flame Temperatures of Some Fuels HEAT OF COMBUSTION FLAME TEMPERATURE (°C) FUEL (MJ/kg) NET ADIABATIC ACTUAL Acetylene 49.9a (55.8MJ/m3) 48.2b 2,325 b 3 c Butane 49.5 (112.4 MJ/m ) 45.7 1,895 3 c Carbon monoxide (11.7 MJ/m ) 10.1 Charcoal 33.7 34.3 2,200d 1,390d Coleman fuel 770e Coal (anthracite) 30.9–34.8 Cotton 16 Ethane 47.5 (60.5 MJ/m3)c 47.4 1,895 Ethyl alcohol 29.7 26.8 840e Fuel oil #1 (kerosene) 46.1 Hexane 48.3 (164.4 MJ/m3)c 43.8 1,948/2,200e Hydrogen 130.8 (12.1 MJ/m3)c Gasoline 43.7 810e Methane 55.5 (34 MJ/m3)c 50 1,875e Propane 50.4 (86.4 MJ/m3)c 46.4 1,925e 970e Polyethylene 43.2 Polypropylene 43.2 Polystyrene 40 Polyurethane 23 Wood 20 16–20 1,590f ~600–1,000f a Fire Protection Handbook, 19th ed. Quincy( MA: National Fire Protection Association, 2003), table A.1. b SFPE Handbook of Fire Protection Engineering, 4th ed. (Quincy, MA: Society of Fire Protection Engineers and the National Fire Protection Association, 2008), tables 1-5.3 and 1-5.6. c R. J. Harris, The Investigation and Control of Gas Explosions (New York: E and FN Spon, 1983), 6–7. d R. W. Henderson and G. R Lightsey, “Theoretical Combustion Temperature of Wood Charcoal,” National Fire and Arson Report 3 (1985): 7. e R. W. Henderson and G. R Lightsey, “The Effective Flame Temperatures of Flammable Liquids,” Fire and Arson Investigator 35 (December 1984): 8. f R. W. Henderson and G. R Lightsey, “Theoretical vs. Observed Flame Temperatures during Combustion of Wood Products,” Fire and Arson Investigator 36 (December 1985). LIQUEFIED PETROLEUM GAS Liquefied petroleum gas (LPG) is commonly used in rural areas where natural gas is not available. It is a mixture of propane and n-butane, with small quantities of ethane, ethyl- ene, propylene, isobutane, and butylene.27 Depending on its source and its intended use, it can be quite variable in its composition, ranging from almost pure propane to a heav- ier mixture largely composed of butane. For LPG sold in the United States, the investiga- tor can consider LPG the same as propane in its physical and combustion properties without serious error (in some other countries, LPG can comprise primarily butane). If butane is known to have been involved (from lighters, torches, or aerosol containers), it is best to use its reference values for calculations. Chapter 4 Combustion Properties of Liquid and Gaseous Fuels 105 FIGURE 4-10 Boiling Jet fuel A Fuel oil #2 point ranges of straight Kerosene petroleum distillates. Diesel MPD Waxes Straight-run Oils Asphalts LPG gasoline 50 0 100 200 300 400 500 600 650 Boiling point ranges (ºC) of straight petroleum distillates PETROLEUM Petroleum in its crude state is a thick oil varying in color from light brown to black. It con- tains a very large number of compounds and different types of compounds, which are sep- arated to a considerable extent in the manufacture of petroleum fuels. Products obtained from the distillation of petroleum include gases, petroleum ether, straight-run gasoline, kerosene, medium distillates including paint thinner–type mixtures and solvents, diesel fuels, heavy lubricating oils, petroleum jelly, paraffin, waxes, asphalt, and coke. The distil- lation curve of petroleum products is represented in Figure 4-10. GASOLINE Gasoline, widely used in vehicles and appliances and perhaps the most important fuel of petroleum origin, is a mixture of volatile, low-boiling, and midrange hydrocarbons. It con- tains hydrocarbon compounds with boiling points between approximately 32°C (90°F) and 205°C (400°F). The average molecular weight of gasoline is sometimes taken to be close to that of n-octane, about 114. The petroleum distillate described as “straight-run gasoline” contains the raw hydrocarbons that distill off between 40°C and 200°C (100°F to 395°F) and is used as camping fuel and sometimes called “white gas” or naphtha. [The range of compounds included is often measured by the “carbon number” or molecular length of n-alkanes included. In this case, this distillate “cut” includes C4 (butane ) to C12 (dodecane).] Modern automotive gasoline contains more than 200 hydrocarbons in a com- plex mixture (which is not just the straight cut or fraction but a blend of many compounds, especially aromatics) whose relative component concentrations vary little from refinery to refinery. This means that it is not generally possible to identify one oil company’s product from another based on a comparison of the basic hydrocarbon “profiles” of various gaso- lines, Recently, forensic chemists have noticed that component ratios are changing and sub- accelerant A fuel stitutions are being made, so laboratory identification of gasoline as an accelerant is no (usually a flammable longer as straightforward as it once was. Before gasoline is sold as a motor fuel, however, liquid) that is used to a variety of additives are added by the refinery. These additives often include compounds initiate or increase the intensity of speed of to improve the burning characteristics of the fuel. Dyes were once added to distinguish spread of a fire. leaded from unleaded motor fuels. Today, nearly all automotive gasolines are unleaded, and dyes are rarely seen except to identify lower-taxed fuels for agricultural use. Oxygenates, especially ethanol and methanol, are very common in automotive gaso- lines today. In many areas they are completely replacing MTBE (methyl t-butyl ether), once commonly used as an oxygenate additive. The additive packages used by one manufacturer are usually different from those used by others and offer some means of distinguishing fresh, unburned, and uncontaminated gasolines. Unfortunately, due to the industry prac- tice of exchanging raw petroleum feedstocks, additive packages, and even finished gasoline depending on market and supply conditions, it is not possible in most cases to identify a particular retail brand of gasoline from a sample of the fuel as it is delivered to the retail customer. However, recent advances in gas chromatography and data interpretation are making it possible to compare unburned fuels against suspected sources with considerable specificity. See Chapter 14 for a further discussion of identification of such products. 106 Chapter 4 Combustion Properties of Liquid and Gaseous Fuels KEROSENE AND OTHER DISTILLATES Kerosene is defined by ASTM D3699 as having a boiling point range from 175°C to 300°C (350°F to 572°F) with a minimum flash point of 38°C (100°F).28 Kerosene and the distillate fuels of higher boiling ranges were considered useful only for illumination until the diesel motor became a common device. Although diesel fuel is similar to kerosene, it spans a wider range of less-volatile components. The modern jet engine and other turbine- driven engines further increased the use and economic significance of kerosene-like fuels. The classification scheme used for laboratory characterization of ignitable liquids (those liquids considered either flammable or combustible) includes an intermediate range called medium petroleum distillates (having a boiling point range between about 125°C and 215°C (250°F to 400°F). This range includes carbon numbers from C9 to C17. Many household and commercial products fall into this class—paint thinners, mineral spirits, some charcoal starters, and even some insect sprays. This classification scheme will be dis- cussed in more detail in Chapter 14. Kerosene and similar petroleum distillates have long been of significance in the set- ting of deliberate fires. Their lower volatility presents a lesser hazard to the user than does gasoline. The liquid evaporates more slowly, so that less haste is required in its ignition, and there is much less danger of explosion. Kerosene-class compounds have very low autoignition temperatures, on the order of 210°C (410°F) (see Table 4-4). Interestingly, despite their lower AITs, their high flash points make them less likely to aid the spread of fire (see Table 4-3). Their compositions involve several series of higher-molecular-weight hydrocarbons. Kerosene (fuel oil #1, Jet Fuel A) generally contains paraffinic and olefinic olefinic Hydrocarbons hydrocarbons in the C10 to C16 range. (This range denotes hydrocarbons having 10 to 16 containing double carbon atoms linked together; see Appendix B.) These have boiling points between 175°C carbon ¬