NFPA 921 Sections 3-1 through 3-3.3
Chemistry of Combustion
[interFIRE VR Note: Tables and Figures have not been reproduced.]
3-1. Chemistry of Combustion. The fire investigator should have
a basic understanding of combustion principles and be able to use them to
help in interpretation of evidence at the fire scene and in the development
of conclusions regarding the origin and cause of the fire.
The body of knowledge associated with combustion and fire would easily
fill several textbooks. The discussion presented in this section should
be considered as introductory. The user of this guide is urged to consult
the technical literature for additional details.
3-1.1. Fire Tetrahedron. The combustion reaction can be characterized
by four components: the fuel, the oxidizing agent, heat, and an uninhibited
chemical chain reaction. These four components have been classically symbolized
by a four-sided solid geometric form called a tetrahedron (see Figure 3-1.1).
Fires can be prevented or suppressed by controlling or removing one or more
of the sides of the tetrahedron.
A fuel is any substance that can undergo combustion. The majority of
fuels encountered are organic and contain carbon and combinations of hydrogen
and oxygen in varying ratios. In some cases, nitrogen will be present; examples
include wood, plastics, gasoline, alcohol, and natural gas. Inorganic fuels
contain no carbon and include combustible metals, such as magnesium or sodium.
All matter can exist in one of three phases: solid, liquid, or gas. The
phase of a given material depends on the temperature and pressure and can
change as conditions vary. If cold enough, carbon dioxide, for example,
can exist as a solid (dry ice). The normal phase of a material is that which
exists at standard conditions of temperature [21°C (70°F)] and pressure
[14.7 psi (101.6 kPa) or 1 atmosphere at sea level].
Combustion of a solid or liquid fuel takes place above the fuel surface
in a region of vapors created by heating the fuel surface. The heat can
come from the ambient conditions, from the presence of an ignition source,
or from exposure to an existing fire. The application of heat causes vapors
or pyrolysis products to be released into the atmosphere where they can
burn if in the proper mixture with air and if a competent ignition source
is present. Ignition is discussed in Section 3-3.
Some solid materials can undergo a charring reaction where oxygen reacts
directly with solid material. Charring can be the initial or the final stage
of burning. Sometimes charring combustion breaks into flame; on other occasions
charring continues through the total course of events.
Gaseous fuels do not require vaporization or pyrolysis before combustion
can occur. Only the proper mixture with air and an ignition source are needed.
The form of a solid or liquid fuel is an important factor in its ignition
and burning rate. For example, a fine wood dust ignites easier and burns
faster than a block of wood. Some flammable liquids, such as diesel oil,
are difficult to ignite in a pool but can ignite readily and burn rapidly
when in the form of a fine spray or mist.
For the purposes of the following discussion, the term fuel is used to
describe vapors and gases rather than solids.
3-1.1.2.* Oxidizing Agent. In most fire situations, the oxidizing
agent is the oxygen in the earth's atmosphere. Fire can occur in the absence
of atmospheric oxygen when fuels are mixed with chemical oxidizers. Many
chemical oxidizers contain readily released oxygen. Ammonium nitrate fertilizer
(NH4NO3), potassium nitrate (KNO3), and hydrogen peroxide (H2O2) are examples.
Normal air contains 21 percent oxygen. In oxygen-enriched atmospheres,
such as in areas where medical oxygen is in use or in high-pressure diving
or medical chambers, combustion is greatly accelerated. Materials that resist
ignition or burn slowly in air can burn vigorously when additional oxygen
is present. Combustion can be initiated in atmospheres containing very low
percentages of oxygen, depending on the fuel involved. As the temperature
of the environment increases, the oxygen requirements are further reduced.
While flaming combustion can occur at concentrations as low as 14 to 16
percent oxygen in air at room temperatures of 70°F (21°C), flaming
combustion can continue at close to 0 percent oxygen under post-flashover
temperature conditions. Also, smoldering combustion once initiated can continue
in a low-oxygen environment even when the surrounding environment is at
a relatively low temperature. The hotter the environment, the less oxygen
is required. This later condition is why wood and other materials can continue
to be consumed even though the fire is in a closed compartment with low
oxygen content. Fuels that are enveloped in a layer of hot, oxygen-depleted
combustion products in the upper portion of a room can also be consumed.
It should be noted that certain gases can form flammable mixtures in
atmospheres other than air or oxygen. One example is a mixture of hydrogen
and chlorine gas.
For combustion to take place, the fuel vapor or gas and the oxidizer
should be mixed in the correct ratio. In the case of solids and liquids,
the pyrolysis products or vapors disperse from the fuel surface and mix
with the air. As the distance from the fuel source increases, the concentration
of the vapors and pyrolysis products decreases. The same process acts to
reduce the concentration of a gas as the distance from the source increases.
Fuel burns only when the fuel/air ratio is within certain limits known
as the flammable (explosive) limits. In cases where fuels can form flammable
mixtures with air, there is a minimum concentration of vapor in air below
which propagation of flame does not occur. This is called the lower flammable
limit. There is also a maximum concentration above which flame will not
propagate called the upper flammable limit. These limits are generally expressed
in terms of percentage by volume of vapor or gas in air.
The flammable limits reported are usually corrected to a temperature
of 32°F (0°C) and 1 atmosphere. Increases in temperature and pressure
result in reduced lower flammable limits possibly below 1 percent and increased
upper flammable limits. Upper limits for some fuels can approach 100 percent
at high temperatures. A decrease in temperature and pressure will have the
opposite effect. Caution should be exercised when using the values for flammability
limits found in the literature. The reported values are often based on a
single experimental apparatus that does not necessarily account for conditions
found in practice.
The range of mixtures between the lower and upper limits is called the
flammable (explosive) range. For example, the lower limit of flammability
of gasoline at ordinary temperatures and pressures is 1.4 percent, and the
upper limit is 7.6 percent. All concentrations by volume falling between
1.4 and 7.6 percent will be in the flammable (explosive) range. All other
factors being equal, the wider the flammable range, the greater the likelihood
of the mixture coming in contact with an ignition source and thus the greater
the hazard of the fuel. Acetylene, with a flammable range between 2.5 and
100 percent, and hydrogen, with a range from 4 to 75 percent, are considered
very dangerous and very likely to be ignited when released.
Every fuel/air mixture has an optimum ratio at which point the combustion
will be most efficient. This occurs at or near the mixture known by chemists
as the stoichiometric ratio. When the amount of air is in balance with the
amount of fuel (i.e., after burning there is neither unused fuel nor unused
air), the burning is referred to as stoichiometric. This condition rarely
occurs in fires except in certain types of gas fires. (See Chapter 13.)
Fires usually have either an excess of air or an excess of fuel. When
there is an excess of air, the fire is considered to be fuel controlled.
When there is more fuel present than air, a condition that occurs frequently
in well-developed room or compartment fires, the fire is considered to be
In a fuel-controlled compartment fire, all the burning will take place
within the compartment and the products of combustion will be much the same
as burning the same material in the open. In a ventilation-controlled compartment
fire, the combustion inside the compartment will be incomplete. The burning
rate will be limited by the amount of air entering the compartment. This
condition will result in unburned fuel and other products of incomplete
combustion leaving the compartment and spreading to adjacent spaces. Ventilation-controlled
fires can produce massive amounts of carbon monoxide.
If the gases immediately vent out a window or into an area where sufficient
oxygen is present, they will ignite and burn when the gases are above their
ignition temperatures. If the venting is into an area where the fire has
caused the atmosphere to be deficient in oxygen, such as a thick layer of
smoke in an adjacent room, it is likely that flame extension in that direction
will cease, although the gases can be hot enough to cause charring and extensive
3-1.1.3. Heat. The heat component of the tetrahedron represents
heat energy above the minimum level necessary to release fuel vapors and
cause ignition. Heat is commonly defined in terms of intensity or heating
rate (Btu/sec or kilowatts) or as the total heat energy received over time
(Btu or kilojoules). In a fire, heat produces fuel vapors, causes ignition,
and promotes fire growth and flame spread by maintaining a continuous cycle
of fuel production and ignition.
3-1.1.4. Uninhibited Chemical Chain Reaction. Combustion is a
complex set of chemical reactions that results in the rapid oxidation of
a fuel producing heat, light, and a variety of chemical by-products. Slow
oxidation, such as rust or the yellowing of newspaper, produces heat so
slowly that combustion does not occur. Self-sustained combustion occurs
when sufficient excess heat from the exothermic reaction radiates back to
the fuel to produce vapors and cause ignition in the absence of the original
ignition source. For a detailed discussion of ignition, see Section 3-3.
Combustion of solids can occur by two mechanisms: flaming and smoldering.
Flaming combustion takes place in the gas or vapor phase of a fuel. With
solid and liquid fuels, this is above the surface. Smoldering is a surface-burning
phenomenon with solid fuels and involves a lower rate of heat release and
no visible flame. Smoldering fires frequently make a transition to flaming
after sufficient total energy has been produced or when airflow is present
to speed up the combustion rate.
3-2. Heat Transfer. The transfer of heat is a major factor in
fires and has an effect on ignition, growth, spread, decay (reduction in
energy output), and extinction. Heat transfer is also responsible for much
of the physical evidence used by investigators in attempting to establish
a fire's origin and cause.
It is important to distinguish between heat and temperature. Temperature
is a measure that expresses the degree of molecular activity of a material
compared to a reference point such as the freezing point of water. Heat
is the energy that is needed to maintain or change the temperature of an
object. When heat energy is transferred to an object, the temperature increases.
When heat is transferred away, the temperature decreases.
In a fire situation, heat is always transferred from the high-temperature
mass to the low-temperature mass. Heat transfer is measured in terms of
energy flow per unit of time (Btu/sec or kilowatts). The greater the temperature
difference between the objects, the more energy is transferred per unit
of time and the higher the heat transfer rate is. Temperature can be compared
to the pressure in a fire hose and heat or energy transfer to the waterflow
in gallons per minute.
Heat transfer is accomplished by three mechanisms: conduction, convection,
and radiation. All three play a role in the investigation of a fire, and
an understanding of each is necessary.
3-2.1. Conduction. Conduction is the form of heat transfer that
takes place within solids when one portion of an object is heated. Energy
is transferred from the heated area to the unheated area at a rate dependent
on the difference in temperature and the physical properties of the material.
The properties are the thermal conductivity (k), the density (p),
and the heat capacity (c). The heat capacity (specific heat) of a
material is a measure of the amount of heat necessary to raise its temperature
(Btu/lb/degree of temperature rise).
If thermal conductivity (k) is high, the rate of heat transfer
through the material is high. Metals have high thermal conductivities (k),
while plastics or glass have low thermal conductivity (k) values.
Other properties (k and c) being equal, high-density (p)
materials conduct heat faster than low-density materials. This is why low-density
materials make good insulators. Similarly, materials with a high heat capacity
(c) require more energy to raise the temperature than materials with
low heat capacity values.
Generally, conduction heat transfer is considered between two points
with the energy source at a constant temperature. The other point will increase
to some steady temperature lower than that of the source. This condition
is known as steady state. Once steady state is reached, thermal conductivity
(k) is the dominant heat transfer property. In the growing stages
of a fire, temperatures are continuously changing, resulting in changing
rates of heat transfer. During this period, all three properties thermal
conductivity (k), density (p), and heat capacity (c)
play a role. Taken together, these properties are commonly called the thermal
inertia of a material and are expressed in terms of k, p,
c. Table 3-2.1 provides data for some common materials.
The impact of the thermal inertia on the rise in temperature in a space
or on the material in it is not constant through the duration of a fire.
Eventually, as the materials involved reach a constant temperature, the
effects of density (p) and heat capacity (c) become insignificant
relative to thermal conductivity. Therefore, thermal inertia of a material
is most important at the initiation and early stages of a fire (pre-flashover).
Conduction of heat into a material as it affects its surface temperature
is an important aspect of ignition. Thermal inertia is an important factor
in how fast the surface temperature will rise. The lower the thermal inertia
of the material, the faster the surface temperature will rise. Conduction
is also a mechanism of fire spread. Heat conducted through a metal wall
or along a pipe or metal beam can cause ignition of combustibles in contact
with the heated metals. Conduction through metal fasteners such as nails,
nail plates, or bolts can result in fire spread or structural failure.
3-2.2. Convection. Convection is the transfer of heat energy by
the movement of heated liquids or gases from the source of heat to a cooler
part of the environment.
Heat is transferred by convection to a solid when hot gases pass over
cooler surfaces. The rate of heat transfer to the solid is a function of
the temperature difference, the surface area exposed to the hot gas, and
the velocity of the hot gas. The higher the velocity of the gas, the greater
the rate of convective transfer.
In the early history of a fire, convection plays a major role in moving
the hot gases from the fire to the upper portions of the room of origin
and throughout the building. As the room temperatures rise with the approach
of flashover, convection continues, but the role of radiation increases
rapidly and becomes the dominant heat transfer mechanism. See 3-5.3.2 for
a discussion of the development of flashover. Even after flashover, convection
can be an important mechanism in the spread of smoke, hot gases, and unburned
fuels throughout a building. This can spread the fire or toxic or damaging
products of combustion to remote areas.
3-2.3. Radiation. Radiation is the transfer of heat energy from
a hot surface to a cooler surface by electromagnetic waves without an intervening
medium. For example, the heat energy from the sun is radiated to earth through
the vacuum of space. Radiant energy can be transferred only by line-of-sight
and will be reduced or blocked by intervening materials. Intervening materials
do not necessarily block all radiant heat. For example, radiant heat is
reduced on the order of 50 percent by some glazing materials.
The rate of radiant heat transfer is strongly related to a difference
in the fourth power of the absolute temperature of the radiator and the
target. At high temperatures, small increases in the temperature difference
result in a massive increase in the radiant energy transfer. Doubling the
absolute temperature of the hotter item without changing the temperature
of the colder item results in a 16-fold increase in radiation between the
two objects. (See Figure 3-2.3.)
The rate of heat transfer is also strongly affected by the distance between
the radiator and the target. As the distance increases, the amount of energy
falling on a unit of area falls off in a manner that is related to both
the size of the radiating source and the distance to the target.
3-3.* Ignition. In order for most materials to be ignited they
should be in a gaseous or vapor state. A few materials may burn directly
in a solid state or glowing form of combustion including some forms of carbon
and magnesium. These gases or vapors should then be present in the atmosphere
in sufficient quantity to form a flammable mixture. Liquids with flash points
below ambient temperatures do not require additional heat to produce a flammable
mixture. The fuel vapors produced should then be raised to their ignition
temperature. The time and energy required for ignition to occur is a function
of the energy of the ignition source, the thermal inertia (k, p,
c) of the fuel, and the minimum ignition energy required by that
fuel and the geometry of the fuel. If the fuel is to reach its ignition
temperature, the rate of heat transfer to the fuel should be greater than
the conduction of heat into or through the fuel and the losses due to radiation
and convection. Table 3-3 shows the temperature of selected ignition sources.
A few materials, such as cigarettes, upholstered furniture, sawdust, and
cellulosic insulation, are permeable and readily allow air infiltration.
These materials can burn as solid phase combustion, known as smoldering.
This is a flameless form of combustion whose principal heat source is char
oxidation. Smoldering is hazardous, as it produces more toxic compounds
than flaming combustion per unit mass burned, and it provides a chance for
flaming combustion from a heat source too weak to directly produce flame.
The term smoldering is sometimes inappropriately used to describe a nonflaming
response of a solid fuel to an external heat flux. Solid fuels, such as
wood, when subjected to a sufficient heat flux, will degrade, gasify, and
release vapors. There usually is little or no oxidation involved in this
gasification process, and thus it is endothermic. This is more appropriately
referred to as forced pyrolysis, and not smoldering.
3-3.1. Ignition of Solid Fuels. For solid fuels to burn with a
flame, the substance should either be melted and vaporized (like thermoplastics)
or be pyrolyzed into gases or vapors (i.e., wood or thermoset plastic).
In both examples, heat must be supplied to the fuel to generate the vapors.
High-density materials of the same generic type (woods, plastics) conduct
energy away from the area of the ignition source more rapidly than low-density
materials, which act as insulators and allow the energy to remain at the
surface. For example, given the same ignition source, oak takes longer to
ignite than a soft pine. Low-density foam plastic, on the other hand, ignites
more quickly than high-density plastic.
The amount of surface area for a given mass (surface area to mass ratio)
also affects the quantity of energy necessary for ignition. It is relatively
easy to ignite one pound of thin pine shavings with a match, while ignition
of a one-pound solid block of wood with the same match is very unlikely.
Because of the higher surface area to mass ratio, corners of combustible
materials are more easily burned than flat surfaces. Table 3-3.1 shows the
time for pilot ignition of wood exposed to varying temperatures.
Caution is needed in using Table 3-3.1, as the times and temperatures
given are for ignition with a pilot flame. These are good estimates for
ignition of wood by an existing fire. These temperatures are not to be used
to estimate the temperature necessary for the first item to ignite. The
absence of the pilot flame requires that the fuel vapors of the first item
ignited be heated to their autoignition temperature. In An Introduction
to Fire Dynamics, Dougal Drysdale reports two temperatures for wood
to autoignite or spontaneously ignite. These are heating by radiation, 600°C
(1112°F), and heating by conduction, 490°C (914°F).
For spontaneous ignition to occur as a result of radiative heat transfer,
the volatiles released from the surface should be hot enough to produce
a flammable mixture above its autoignition temperature when it mixes with
unheated air. With convective heating on the other hand, the air is already
at a high temperature and the volatiles need not be as hot.
Figure 3-3.1(a) illustrates the relationship between ignition energy
and time to ignition for thin and thick materials. When exposed to their
ignition temperature, thin materials ignite faster than thick materials
(e.g., paper vs. plywood). [See Figure 3-3.1(b).]
3-3.2. Ignition of Liquids. In order for the vapors of a liquid
to form an ignitible mixture, the liquid should be at or above its flash
point. The flash point of a liquid is the lowest temperature at which it
gives off sufficient vapor to support a momentary flame across its surface
based on an appropriate ASTM test method. The value of the flash point may
vary depending on the type of test used. Even though most of a liquid may
be slightly below its flash point, an ignition source can create a locally
heated area sufficient to result in ignition.
Atomized liquids or mists (those having a high surface area to mass ratio)
can be more easily ignited than the same liquid in the bulk form. In the
case of sprays, ignition can often occur at ambient temperatures below the
published flash point of the bulk liquid provided the liquid is heated above
its flash point and ignition temperature at the heat source.
3-3.3. Ignition of Gases. Combustible substances in the gaseous
state have extremely low mass and require the least amount of energy for
*A-3-1.1.2 For more information on flammability limits,
see USBM Flammability Characteristics of Combustible Gases and Vapors.
* A-3-3 For additional information, see Ohlemiller, Smoldering Combustion.
For more information, contact:
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Taken from NFPA 921Guide for Fire and Explosion Investigations
1998 Edition, copyright © National Fire Protection Association,
1998. This material is not the complete and official position of the NFPA
on the referenced subject, which is represented only by the standard in
Used by permission.