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CHAPTER 9

Fire Characteristics: Solid Combustibles

OBJECTIVES

After studying this chapter, you should be able to:

•   List the three significant differences between the burning of a solid fuel and the burning of gaseous and liquid fuels.

•   Describe the thermal and chemical processes that result in the ignition and burning of a solid.

•   Describe how char formation and melting occur and how they affect the burning rate.

•   List the types of combustible solids.

•   Describe the types of polymers and explain how they gasify.

•   Describe at least four classes of mechanisms by which fire retardant additives act to modify the ignition and burning of solids.

•   Discuss the use of calorimetry to measure the heat-release rates of materials and products.

Introduction

Unexpectedly vigorous burning of solid combustibles has been at the core of some of the pivotal fires in our lifetimes. These include the 1944 Hartford circus fire, which involved a tent that was waterproofed with paraffin wax; the 1967 Apollo 1 fire, where the capsule environment consisted of 100 percent oxygen; the 1986 Dupont Plaza Hotel fire, which was fed by stacked unused furniture; The Station and Kiss nightclub fires (2003 and 2013, respectively), both of which were fed by foam insulation on the walls and ceilings; and the 2007 Sofa Super Store fire. These fires, and millions of less spectacular fires in the United States, underscore the importance of understanding solid fuels, the means by which they burn, and the hazards these fires present.

Fire Stages and Metrics

Solids versus Gases and Liquids

The presentation of fire characteristics in this text began with the simplest case, gaseous fuels. Given that flaming combustion is a gas-phase process, a fuel that starts out as a gas needs only a pathway to ignition. Furthermore, the fuel composition remains identical to the composition of the initial gas mixture throughout the fire, simplifying the combustion chemistry. A gas-phase flame spreads by the chemical action of the chain that propagates the necessary atoms and free radicals. Liquid fuels have a similar degree of simplicity in their gas-phase fuel chemistry, which most commonly involves the vapor from the liquid. As with a gaseous fuel, there is often no change in the fuel chemistry over time. The energetics of vaporization and, for liquid mixtures, the preferential evaporation and burning of the lighter component(s), however, require consideration, as they affect the rates of burning and surface flame spread.

The burning of a solid fuel has three significant and consequential differences from the burning of gaseous and liquid fuels:

•   Significant chemical change generally occurs within the solid during burning. This change results in (1) the fuel becoming non-uniform and (2) this lack of uniformity varying with the extent of the burning and, therefore, over time.

•   The emitted volatiles may not have the same chemistry as the virgin solid.

•   The heat transfer to, from, and within the solid requires consideration of both the changes in the fuel surface and the chemical changes that have occurred below the surface.

Materials and Products

At this point, two important terms must be clarified:

•   A material is a single substance. The simplest material is made of a single chemical component, such as a sheet of a pure plastic. Some materials, such as particleboard, are mixtures of chemicals—in this case, ground wood and a binder. Still other materials, such as a fiber-reinforced composite, are nonhomogeneous.

•   A product is (or is similar to) an item that is available commercially, and is alternatively referred to as a commercial product or a finished product. Examples include an electric cable, an upholstered chair, and a carpet. Such items are composed of one or more materials and are typically not chemically homogeneous.

Real combustible items are products. Large-scale tests are used to measure the burning behavior of products, whereas the fire properties of materials are often determined in bench-scale tests. Some bench-scale tests have been used to characterize small mock-ups of products. The relationship between the fire properties of a product and the fire properties of its component materials remains a subject of research. The use of reduced-scale mock-ups of large combustibles (e.g., an upholstered chair) to characterize ease of ignition has achieved some success, but the use of mock-ups to characterize mass burning rate and flame spread rate requires care in design of the mock-up and interpretation of the results.

Pyrolysis

The involvement of a solid fuel in a fire generally begins with radiative or conductive heat decomposing the solid into sufficiently small fragments that the fragments are able to escape the solid surface and become a gas-phase fuel. Radiant heat, for example, might come from a nearby space heater or the flames from an already burning item; conductive heat might come from an overheated electrical component. Convection can contribute to these modes of fire evolution, but generally convective flow temperatures are not hot enough to volatilize a solid. An important exception to this statement is the hot upper layer in a room near or post flashover.

The decomposition process for a solid fuel is called pyrolysis. If no oxygen is present, the process is termed anaerobic pyrolysis. Most commonly, pyrolysis occurs in air, in which case it is called oxidative pyrolysis. Anaerobic pyrolysis is endothermic; that is, heat must be supplied from somewhere for the decomposition reactions to occur. Oxidative pyrolysis is usually endothermic or thermally neutral. Pyrolysis typically stops when the heat source is removed or turned off.

As discussed in the Physical and Chemical Change chapter, pyrolyzing a solid requires raising the material’s temperature to the point where chemical bonds begin breaking, overcoming any phase change that might occur during this heating process, and releasing volatile compounds or molecular fragments. The heat input required to accomplish this feat is the heat of gasification of a material (which has units of kJ/g). It is an important measure of the ease of ignition of a solid and the flammability of a solid, once ignited. If the chemistry of the remaining fuel changes over time, so will the heat of gasification.

If it is important to understand (or reconstruct) a particular segment of a fire (perhaps the spread of a fire from one combustible item to another), it is necessary to know the heat of gasification for a product during that time interval. If the task is to determine the threat posed by a large fire to the structural integrity of the building, it may be sufficient to use a heat of gasification averaged over the burn life of the combustible.

The minimum condition for igniting a solid is the heating of its exterior surface to a high enough temperature that the pyrolysis gases are produced rapidly enough to exceed their lower flammability limit in the space above the surface. Unlike the vapor from a pure liquid, the pyrolyzate is commonly a mixture of many decomposition products. Its composition depends on the chemistry of the solid fuel, the rate of pyrolysis, and the availability of oxygen. As a result, no tables of flammability limits for solid fuels can be developed. Instead, the gasification rate for ignition is experimentally determined for a particular heating scenario; as indicated in the next paragraphs, it is not a unique property of the fuel in the same way that a heat of vaporization is unique to a liquid.

If the solid is being heated by conduction, the heat generally is supplied at a location away from the fuel’s outer surface. For example, an overloaded electrical conductor heats the wire insulation from the inside, with the heat then being transferred to the inside of the cable jacket. The surface temperature, then, may be the lowest temperature in the solid. In this case, the apparent heat of gasification will be higher than for the case where radiant heat is applied to the exposed surface of the cable jacket.

If the solid is being heated by radiation, and if the radiation is entirely absorbed by the top surface of the solid (i.e., the solid is optically thick), the top layer of the solid will decompose first, followed by further decomposition in depth. Some solids, however, are somewhat transparent to infrared radiation. An example is an acrylic window panel. In this case, heating also takes place below the surface. When decomposition occurs below the surface, subsurface bubbles of pyrolyzate form, pass through the hotter (lower-density) outer material, reach the surface, and burst, spurting volatiles into the air. This process can be a more efficient mode of gasification than surface heating.

Whether the incident heat involves radiation or conduction, an increase in the heating rate is likely to increase the pyrolysis rate. Conductive heat is important in some ignition modes, but is rarely the principal contributor to fire spread and burning intensity. Chemical kinetic principles, as discussed in the Physical and Chemical Change chapter, reveal that the pyrolysis reaction rate increases rapidly with increasing temperature. Under normal indoor ventilation conditions, gasification rates on the order of just a few grams per square meter per second are needed to achieve an ignitable mixture in air.

Ignition to Flaming Combustion

For most organic solids, a temperature between 520 °F and 750 °F (270 °C and 400 °C) is necessary for piloted ignition. As with gaseous and liquid fuels, unpiloted ignition or autoignition is possible if the surface reaches a sufficiently high temperature. For example, when wood is heated radiatively, piloted ignition (initiated by a flame maintained near the surface) occurs when the wood surface reaches a temperature of 570 °F to 750 °F (300 °C to 400 °C), while the same surface must be heated to about 1100 °F (600 °C) to induce autoignition.

The minimum radiative flux that must impinge on a solid to make it ignitable by a pilot flame has been measured for many materials [1]. These values range from 10 kW/m² to 40 kW/m² depending on the nature of the material, including its chemical constituents, reflectivity, size, and orientation with respect to the radiative source. For fluxes in excess of the minimum value, the time to ignition decreases as the flux increases ( Figure 9-1 ).

In most serious fires, more than one combustible product is involved. The first product might be ignited by, for example, a candle. The flames from this product grow and generate far more heat than the original ignition source provided. As a result, there are two distinct modes for the ignition of a second combustible product:

•   The first mode is piloted ignition. If the second product is close to the already burning item, the radiation from the flames pyrolyzes the surface material(s) of the second item. The combination of pyrolysis gases from the two items creates a flammable fuel–air mixture in the space between the two products. The flames extend along this flammable mixture and become attached to the second item.

•   If the second product is farther away from the burning item, it can still become involved by unpiloted radiative ignition. As in the first mode, the thermal radiation from the flames from the burning item pyrolyzes the surface material(s) of the second item. Continuing irradiance leads to increasingly higher surface temperatures, and the pyrolyzate autoignites. This process can be enhanced by the pyrolyzate from the second item absorbing some of the flame radiation from the first item. This heat absorption adds to the temperature rise of the pyrolyzate and shortens the time required to generate a sufficient concentration of flame-propagating free radicals.

During the piloted and unpiloted ignition of solid fuels, potential surface heat losses can affect whether and when ignition will occur. These losses are analogous to the difference between the flash point and the fire point for liquid fuels:

1.  The surface can lose heat by conduction into the interior of the solid. Such a loss will occur if the solid is heated quickly at the surface. If the solid starts to flame, but the heat source is removed, the high temperature at the surface is dissipated by conduction into the depth of the solid. The heat feedback from the flame is insufficient to maintain the surface temperature, and the flame goes out. By contrast, if a thick solid is heated very gradually or from within, when its surface reaches the ignition temperature, its interior is already quite hot and will not drain heat very fast from the surface. Only a little heat feedback from the flame is needed to sustain a flammable concentration of pyrolyzate. Thus, the preignition heating rate and mode are important in achieving sustained ignition of thick solids.

Figure 9-1 Effect of radiative flux intensity on time to achieve piloted ignition [1].

Data from: Babrauskas, V., Ignition Handbook, Fire Science Publishers, Issaquah, WA, 2003.

2.  The surface can lose heat by radiating it away to cooler surroundings. This is best demonstrated by an example. If you place a small burner between two large pieces of wood whose surfaces are facing each other ( Figure 9-2 ), the surface will ignite when the temperature of the two wood surfaces approaches 750 °F (400 °C). The surfaces, which quickly become charred and black, radiate energy to each other with an intensity characteristic of a 750 °F (400 °C) black body. When the burner is removed, this reciprocal radiation/absorption continues. There is no net radiative heat loss from either surface, and the flaming can continue.

If the same test is repeated with a single piece of wood, as in the second panel of Figure 9-2, either the surface will not ignite or it will take a larger burner to cause the ignition. In this scenario, there is no incident thermal radiation from a facing surface, as in the two-piece case. When the burner is removed, the heat continues to be radiated to the surroundings, and the surface cools. Soon the surface is not hot enough to generate a flammable concentration of pyrolyzate.

Ignition to Nonflaming Combustion

As presented in the Combustion Fire and Flammability chapter, nonflaming combustion, also called smoldering or glowing combustion, can occur in a material with the following properties:

•   The material is initially porous—that is, it has a large interior surface area as well as numerous internal “tunnels” that enable the diffusion of oxygen to those surfaces.

•   The material’s interior surfaces support exothermic reaction with oxygen, producing or maintaining a self-supporting, porous, carbonaceous char.

•   The material is a good insulator—that is, heat generated by the surface reaction accumulates within the material and is not efficiently lost to the surroundings.

Figure 9-2 Effect of radiative enhancement on sustained ignition. The flames in the left panel are sustained when the burner is removed; the flames in the right panel are not.

Because of the importance of oxygen diffusion, we might expect the smoldering rate to be dependent on the ambient oxygen volume percentage, and smoldering combustion does occur more readily in oxygen-enriched air than in normal air. Data from Reference [2] show a fourfold increase in the smoldering rate of cellulose rods when the oxygen was increased from 21 percent by volume to 96 percent by volume.

Smoldering can be started by a nonflaming ignition source such as a lit cigarette dropped onto an easy chair or an overheated electrical cable passing through a wood stud. When such an ignition source is applied to the material, the local temperature increases by as much as hundreds of kelvins above room temperature, and the reactivity of oxygen with the material surface begins at that temperature. If the smoldering is to self-sustain or progress, the rate at which heat is generated must exceed the rate at which heat is diffused away. When the ignition source is located in the interior of the material, significant insulation surrounds the hot spot and the early reaction may proceed at a fairly slow rate. In contrast, if the ignition occurs at the surface of the material, a higher heat-loss rate must be overcome, and the initial smoldering reaction must be faster.

The thermal process of spontaneous ignition followed by self-heating is similar, albeit with one major difference: the starting temperature. The classic example is the self-ignition of a haystack [3]. Very little happens unless the moisture content of the hay is greater than approximately 25 percent, which could result from rain or relative humidity of the air near 90 percent. Under humid conditions, aerobic fungi and bacteria grow in the hay at normal outdoor temperatures, generating heat biologically. If the haystack is thermally thick (a few meters across) and heat losses are small, then the biologically generated heat will increase the interior temperature of the haystack to about 165 °F (75 °C), perhaps taking several days or weeks. Above this temperature, the organisms are no longer active. At 165 °F (75 °C), however, the rate at which oxygen reacts with the decomposition products of the hay (which were formed during the biological heating) is significant. This chemical heat raises the temperature of the haystack’s interior. As the temperature continues to rise, however slowly, the temperature-dependent oxidation rate accelerates, resulting in  . After several more weeks, the temperature could rise to the point where flaming ignition occurs spontaneously. This outcome could also happen if the pile were disturbed (e.g., with a pitchfork), bringing fresh air in contact with the glowing region in the interior or if the glowing zone progressed from the interior to the surface.

The key factor in this scenario is that the initial rate of heat generation can be very slow if the insulation is effective at trapping the heat ( Figure 9-3 ). The slope of the green curve in Figure 9-3 depicting the rate of heat generation is initially small but increases progressively with increasing temperature, as is characteristic of chemical reactions. The two heat-loss curves, for a faster cooling rate (red) and a slower cooling rate (blue), are approximately linear (constant slope) because the rate of convective cooling is directly proportional to the difference between the temperature of the warm object and the temperature of the surroundings.

At the initial temperature, the rate of heat generation is positive, but the rate of cooling is zero because the object is initially at the same temperature as the surroundings. As a result, the temperature of the material must rise, and must continue to rise until the cooling curve intersects the heating curve. This intersection point, marked by a black dot in Figure 9-3, corresponds to a balance of heating and cooling, such that no further temperature increase occurs. At the rapid heat-loss rate, the green and red curves intersect, and the heating ceases; that is, the green curve would stop at this temperature. The slower, blue heat-loss curve never intersects the heat-generation curve, and the material proceeds toward sustained burning, as shown in Figure 9-3.

Few materials burst into flame spontaneously. In fact, most common solid materials react so slowly with oxygen at normal temperatures that the self-heating, if measurable at all, usually amounts to a temperature increase of no more than one or two kelvins. Table A-10 in the Fire Protection Handbook [4], however, lists 75 substances that are capable of hazardous spontaneous heating. Among the most dangerous are rags or other fibrous materials in contact with corn oil, fish oils (e.g., cod liver oil), linseed oil, pine oil, soybean oil, tung oil, or any unsaturated oil. Such oils are reactive with oxygen at room temperature. In addition, the rags or fibrous material provide an extensive surface area for the oil–oxygen reaction to take place, and they confine the heat, permitting the temperature to rise. By contrast, saturated oils, such as petroleum-derived oils (common lubricating oil or heating oil) do not cause spontaneous heating. Common materials that are prone to spontaneous ignition, if stored in bulk, include charcoal briquettes, low-grade coal, and some types of animal feed.

Figure 9-3 Relative heat-generation and heat-loss rates, which may sometimes lead to runaway heating and self-ignition.

Char Formation and Melting

After a solid has been ignited and the flame has begun to spread across its surface, two distinct categories of burning behavior are apparent. One class of materials, including woods and certain plastics, burns with the formation of a growing surface char layer. The other class of materials, which includes many of the more common plastics (polyethylenes, polystyrenes, and acrylics1), burns with either no char or a small amount of surface char that blackens the fuel surface but never builds up to a thick layer. The importance of char formation is seen in  Figure 9-4 , which illustrates heat-release rate versus time for a char-forming material (particleboard) and a non-char-forming material (polymethylmethacrylate [PMMA]).

Figure 9-4 Heat release rates versus time for particleboard and acrylic (polymethylmethacrylate) samples under imposed radiative heat fluxes of 25 kW/m² and 50 kW/m².

Note

In terms of its chemical and physical behavior during burning, PMMA is one of the simplest materials. First, during pyrolysis, nearly all of the mass that is volatilized is in the form of methylmethacrylate (MMA). Second, until it reaches its melting point, PMMA does not melt or drip; it sublimes. For these reasons, PMMA is one of the most convenient materials for developing simple combustion models. Very few solid materials exhibit such ideal behavior, so combustion modeling for realistic materials constitutes to be an active field of research.

Texture: Eky Studio/ShutterStock, Inc.; Steel: © Sharpshot/Dreamstime.com

The char formed when the particleboard is heated has a structure similar to that of graphite (pencil lead), a very stable form of carbon. In graphite, the carbon atoms are connected in adjacent six-membered rings (polycyclic structure), forming the orderly structure shown in  Figure 9-5 . In a fire-generated char, irregularities are present in the array of carbon atoms, and one hydrogen atom is typically bonded to one out of every five or six carbon atoms.

Most of the readily pyrolyzed and flammable material is gone at the point that char has formed. The char is brittle, with a very porous, cellular structure, with thin walls and a large fraction of open space. (Think of a household sponge that is rigid, rather than flexible.)

The char is not a good conductor of heat and protects the subsurface material from the heat of the flames. As the char becomes thicker, it progressively slows the rate of conductive heat transfer from the flames above the surface to the virgin material below the surface. This insulation factor reduces the endothermic pyrolysis of this material to form combustible gases and, therefore, slows the rate of burning. This can be seen as the decreasing heat-release rates for the particleboard specimens in Figure 9-4. (The final small increase in the curves near the time of flameout is artificial. The test specimens were mounted on an insulated sheet, which became heated during the test. The last few millimeters of the particleboard specimen were heated both from the front and from the back, so they burned out more quickly.)

The particleboard curves have been smoothed. In a test, as the char gets thicker, it develops cracks and fissures that can provide narrow pathways to the underlying material, releasing brief spurts of flammable vapor before the pathways char over. This mirrors the charring in an actual fire.

The rate of char formation of woods has been reported to be proportional to the radiant heat flux impinging on the surface [5]. For a typical radiant heat flux of 30 kW/m², which might exist just under a flame, the average charring rate would be approximately 0.025 in./min (0.01 mm/s).

Noncharring combustibles generally melt while burning, so there is no insulating layer to provide thermal protection for the subsurface material. In some cases, the melt is very viscous, and little flowing occurs. In other cases (e.g., with some polyethylenes, polypropylenes, and polystyrenes), the melt has a watery consistency. Such materials tend to burn at a high rate throughout the burning period until the fuel is consumed, as exemplified by the acrylic samples in Figure 9-4. This high burning rate can be enhanced if burning drops of molten plastic fall or flow downward, providing a means of spreading the fire. Noncharring combustibles are generally more hazardous than charring combustibles.

Figure 9-5 Ball and stick portrayal of the structure of graphite. The black balls are all carbon atoms.

Mass Burning and Flame Spread

Mass Burning Rate

Once a solid combustible is ignited, its contribution to a fire’s intensity is determined by three related parameters:

•   The rate at which a unit area of the burning surface is consumed

•   The rate at which flames spread over the fuel surface, increasing the burning surface area

•   The combustible’s ability to ignite other combustible items in the vicinity

The first of these, the rate of mass consumption for a defined segment of a burning combustible, is the subject of this section.

The rate of burning of a material or product is expressed as a mass burning rate (g/m2•s) or heat-release rate (kW/m2). If the incident radiative flux to the specimen is increased, both charring and noncharring materials burn more rapidly.

When a material is burning steadily, there is a heat balance at the surface: The net heat to the surface just balances the heat needed to keep supplying fuel to the flame. The net heat to the surface is the heat flux from the flame to the surface minus the rate at which heat is lost by reradiation from the hot surface to the cold surroundings, with both terms expressed in kW/m2 (which is equivalent to kJ/m2•s). The rate of heat absorption per unit surface area (kJ/m2•s) required to sustain a flow of combustible pyrolyzate is the product of the mass rate of gasification per unit surface area (g/m2•s) and the heat of gasification (kJ/g).

Early in the combustion of the first item burning in a compartment, the heat input is derived solely from its own flames. As the burning surface increases, the radiative feedback to the surface at the edge of the flames is approximately 30 kW/m2. As the fire reaches approximately 250 kW in intensity in a room of normal residential dimensions, the hot fire gases in the upper layer of the room and the flame-heated walls reach temperatures where their black body radiation to the product surface is appreciable. Recalling Equation 5-5, and assuming that the soot is radiating as a black body (emissivity = 1), the heat flux per unit surface area (kW/m2) is given by  Equation 9-1 :

For an upper layer that has reached 500K, the radiant flux to the burning product will be approximately 3.5 kW/m2, which is small compared to the flame radiation to the surface. At 800 K, a temperature indicative of imminent flashover, the radiant flux will be approximately 23 kW/m2. Post flashover, the upper-layer temperature can reach approximately 1100K, which emits a radiant flux of 83kW/m2.

Table 9-1  presents some values for heats of gasification of a number of combustible solids. Three observations are worth noting:

1.  The range of the values is quite wide, from 1.19 kJ/g to 3.74 kJ/g.

2.  The chemical composition within a generic type of material may vary considerably, so it comes as no surprise that the data show variation in heats of gasification of samples in each of the rows where there are multiple samples from different sources.

3.  If these heats of gasification (1.19 kJ/g to 3.74 kJ/g) are compared with the heats of combustion of the same materials (15 kJ/g to 44 kJ/g), it is apparent that only a small portion of the heat released by burning must return to the pyrolyzing solid to maintain a continuing supply of combustible vapor to the flame.

Table 9-1 Heat of Gasification for Selected Solids [6]

Material Type

Heat of Gasification (kJ/g)

Number of Materials Tested

acrylonitrile-butadiene-styrene (ABS)

3.23

1

corrugated paper

2.21

1

Douglas fir

1.82

1

nylon 6/61

2.35

1

phenolic plastic

1.64

1

polyesters (PETs) with glass fibers

1.39 to 1.75

2

polyethylenes (PEs)

1.75 to 2.32

2

polyisocyanurate (PIC) foams

1.52 to 3.74

2

polymethylmethacrylate (PMMA)

1.63

1

polyoxymethylene (POM)

2.43

1

polystyrene (PS), granular

1.70

1

polystyrene foams

1.31 to 1.94

5

polyurethane (PU) foams

1.19 …