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FIR 2303, Fire Behavior and Combustion 1

Course Learning Outcomes for Unit VI

Upon completion of this unit, students should be able to:

2. Categorize the components of fire.
2.1 Restate the two main types of smoke aerosols.

4. Describe the process of burning.

4.1 Demonstrate the principal combustion products formed in fires.
4.2 Explain how soot forms.

Course/Unit
Learning Outcomes

Learning Activity

2.1
Unit Lesson
Chapter 10, pp. 175–194
Unit VI Essay

4.1
Unit Lesson
Chapter 10, pp. 175–194
Unit VI Essay

4.2
Unit Lesson
Chapter 10, pp. 175–194
Unit VI Essay

Required Unit Resources

Chapter 10: Combustion Products, pp. 175–194

In order to access the following resource, click the link below.

Pandey, P., & Pundir, B. P. (2015). Role of fluid-dynamics in soot formation and microstructure in acetylene-

air laminar diffusion flames. International Journal of Spray and Combustion Dynamics, 7(1), 25–38.
https://journals.sagepub.com/doi/pdf/10.1260/1756-8277.7.1.25

Unit Lesson

Recap

In the previous unit, we covered pyrolysis of solids forming gaseous fragments suspended in the thermal
plume as soot. Pyrolysis of solids undergoes chemical changes, and once the flaming is independent of the
ignition source, the burning rate is dependent on the radiant heat preheating combustibles. The speed,
magnitude, and direction of flame spread is affected by the airflow and the availability of oxygen-rich air.

Combustion Products and Smoke

Many movies and TV series show firefighters crawling down high-rise corridors or into rooms on fire with
raging flames splitting their helmets or flames skipping across the ceiling. The flames glow orange-yellow as
they dance across the ceiling, and there is no visible smoke. In the TV series Chicago Fire , many scenes
show the actor’s face without wearing a self-contained breathing apparatus (SCBA) face mask and many
times wearing oversized SCBA face masks in heavy fire conditions without any smoke or by-products visible
(Gilvary & Dale, 2014). Where is the smoke? In previous units, we learned that incandescent orange-yellow

UNIT VI STUDY GUIDE

Various Materials and Their
Relationship to Fire as Fuel

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flaming produces smoke or fire effluents. Even though it is a television show, are we teaching the wrong
information regarding combustion products? Is oversimplification of these scenes dangerous?

Fire Smoke

Several decades ago, many firefighters were proud to be known as smoke eaters. Firefighters willingly
entered the lethal smoke to search for victims gasping for breaths of air whenever possible, using rags or
even beards to try and filter the smoke. In more recent times, smoke eaters are seasoned firefighters that
earned the title as a rite of passage, wearing their badge of courage of soot around their nose and mouth with
hoarse voices. This was the era of wearing long bunker coats, ¾ boots with no SCBA; or if you had SCBAs
on the apparatus, you were told they were only for real emergencies. In a typical structure fire, it was believed
that the smoke contained particulates consisting of wood, wool, and paper burning, and it was not realized
how lethal the smoke was. Today we understand the gasification of fuel and the combustion rate of
carbonaceous solid particles and aerosol mist in smoke are extremely toxic and flammable. At present,
smoke from a residential structure fire may contain fragments from plastics, foams, fabrics, carpets, wood
products, and synthetic materials. Each of these materials undergoes pyrolysis in a fire and become
deposited in the smoke. Nonetheless, exposures to any of these products result in respiratory hazards. As a
result, many fire departments developed air management policies and procedures for wearing SCBAs. The
trend today is to require firefighter air replenishment systems (FARS) installed in large area buildings, high
rises, as well as other areas considered difficult to replenish air for firefighter safety.

General Nature of Smoke

The general makeup of smoke is carbonaceous solid particles
and aerosol mist (Gann & Friedman, 2015). Several authors
suggest soot is the result of unburned carbon particles from
incomplete combustion resulting in aggregates of soot
(Gorbett & Pharr, 2011; Ko¨ylu¨, & Faeth, 1994; Köylü et al.,
1995). Gann and Friedman (2015) described turbulent fires as
producing large soot particles that stick to other particles
forming larger aggregates called coagulation, which is
dangerous and extremely flammable. What causes these
larger aggregates of smoke? As seen previously, diffusion
flames are seen as less-localized and tend to burn slower,
producing more soot as oxygen is diffused into the flames.
This can be seen in fires where synthetic material, such as in
a chair, burns and then self-extinguishes leaving large
amounts of black soot adhered to the walls and ceiling above
the neutral plane. Radiative heat from the soot in flames and the gasification of the synthetic material caused
visual obscuration known as aerosols (Gann & Friedman, 2015). These same aerosols are what residential
smoke detectors detect during a fire. Smoke is mainly gaseous products made up of H2O and CO2. In
addition, when incomplete combustion occurs, CO and other organic molecules are generated (Figure 1).

Smoke Color

On the fire ground, when descriptions of the
smoke color are given, it paints a picture for all
incoming units to know if there is a working fire
(Figure 2). However, many company officers have
different definitions on the color of smoke. For
some company officers, light-colored smoke is
dark and others dark-colored smoke is light. This
is based many times on the experience of the
company officer. Nonetheless, we know that
carbonaceous solid particles that generate the
incandescent orange-yellow flame have black
smoke originating from the flame (Gann &
Friedman, 2015). Light-colored smoke is aerosol
mist that cools and condenses and is normally

Figure 1

Figure 2

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seen in the early stages of flaming or when being extinguished. In addition, well-ventilated to under-ventilated
conditions affect the color of smoke. Low-volume, light-colored smoke that is slow moving may be the
beginning stage of a fire or it could be a well-developed fire where the thermal column is starting to cool.
Light-colored smoke could even be carbonaceous solid particles producing black smoke; however, if it is a
dark rainy day the smoke looks light as compared to the background color of the storm. For decades, many
fire instructors taught that smoke color indicates what is burning. Maybe that is correct if items burning were
isolated. In legacy and modern fires, there are multiple items burning made from multiple chemical products
producing different particulates that continuously combine with other particulates changing the color of the
smoke.

Soot Formation

Soot is a complex mixture of different gases and particulates resulting from incomplete combustion producing
mainly carbon and hydrogen. The soot formation process occurs with diffusion and fuel-rich premixed flames
when under-ventilated burning occurs (Gann & Friedman, 2015). This process is also affected by the amount
of moisture or humidity in the air. Moisture or humidity in the air affects the temperature of thermal
fragmentation of fuel molecules producing soot residue that can adhere to walls and the ceiling. As
firefighters, you may have been in a fire where the soot is almost sticky in nature and adheres to your face
mask and bunker gear. If you recall, most likely this fire was a smoldering fire where temperatures were not
extreme. Soot from oxygen-rich fires burns very fast with high temperatures and leaves less residue. Soot
particulate matter diameters range from 0.01 to 1 µm in size (Gann & Friedman, 2015). Items that are
statically charged will also cause soot to adhere to it more readily than matter that is not electrostatically
charged. During the flaming process, soot particles collide and merge to form larger molecules and deposit on
the surfaces from the neutral plane up. However, some particles unite with other particles growing larger in
size until the mass is large enough that gravity causes the soot to fall below the neutral plane. Gann and
Friedman (2015) describe coagulation as the bond of particles to form larger particles and the more turbulent
the flaming, the larger the particle. In addition, fuels with hydrogen carbon ratios higher than saturated
hydrocarbons produce sootier flames.

Optical Density

Many times, firefighters experience the
obscuration of a light beam traveling
through the smoke as they crawl
through a structure. The visibility
reduction of the light beam in smoke is
dependent on ventilation and the
particles of incomplete combustion
(Figure 3). Smoke is laden with droplets
of condensation from fire gases and
soot particles making visibility difficult
even in small fires (Gorbett & Pharr,
2015). A pot on the stove with fewer
smoke particles allows the light beam to
pass through the smoke due to less
light absorption. However, dense, thick,
black smoke reduces visibility because
only a fraction of the light can pass
through due to light absorption (Ingason
& Persson, 2006).

Points to Ponder

In the scenario below, do the smoke aerosols present risk to the firefighters? Why did the smoke conditions
change from being able to cut the particles and aerosols with light to being completely obscured? Was it the
fire-generated soot and aerosols that reduced the visibility? Does the heavy soot exceed the tenability limit of
the firefighters?

Figure 3

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Conclusion

Smoke or fire effluent is sometimes black and sometimes white in appearance, depending on the aerosols
and gases. Soot is mainly carbon particles from under-ventilated flaming. In the flame, carbonaceous solid
particles produce the incandescent orange-yellow glow and black smoke emanating from the flame. Both soot
and smoke reduce the ability of firefighters to be able to see in a fire. Smoke conditions can change radically
as the fire consumes more and more combustible materials, producing gaseous products of H2O and CO2.
Incomplete combustion generates CO, as well as other organic molecules.

References

Gann, R. G., & Friedman, R. (2015). Principles of fire behavior and combustion (4th ed.). Jones & Bartlett

Learning.

Gilvary, M. (Writer), & Dale, H. (Director). (2014, March 4). Keep your mouth shut (Season 2, Episode 15) [TV

series episode]. In D. Wolf (Executive Producer). Chicago Fire. Wolf Films, Universal Television.

Gorbett, G. E., & Pharr, J. L. (2011). Fire dynamics. Pearson.

Ingason, H., & Persson, B. (2006). Prediction of optical density using CFD. Fire Safety Science—Proceedings

of the Sixth International Symposium, 817–828. http://iafss.org/publications/fss/6/817/view/fss_6-
817.pdf

Ko¨ylu¨, U¨. O¨., & Faeth, G. M. (1994). Optical properties of soot in buoyant laminar diffusion flames. Journal of

Heat Transfer, 116(4), 971–979.

Ko¨ylu¨, U¨. ?., Faeth, G. M., Farias, T. L., & Carvalho, M. G. (1995). Fractal and projected structure properties

of soot aggregates. Combustion and Flame, 100(4), 621–633.
https://www.sciencedirect.com/science/article/abs/pii/001021809400147K

Building on the Scenario

Engine 12 (second alarm) made entrance to the stairwell from Side “A” and began to push
towards the area of the fire pulling a 1 ¾” handline. They noticed the stairwell was smoke-logged.
At first, they were able to cut through the smoke with just flashlights and the nearby exit sign on
the second floor was still visible. As they pushed deeper down the hallway, the light was cutting
the smoke less and less.

Finally, the dark smoke was completely concealing everything, making it more and more difficult
to search with the flashlights only penetrating a few inches. The soot particles in the air seemed
to soak up every ounce of the light as they bumped into Tower 2 dragging the unresponsive
firefighter from Engine 5 down the hallway away from the fire. Engine 12 continued towards the
fire, noting the conditions were changing radically and the tenability was taking a toll on them.
Engine 12’s crew became disorientated from the dense black smoke and heat.

FIR 2303, Fire Behavior and Combustion 5

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Suggested Unit Resources

In order to access the following resource, click the link below.

In the video, you will see aerosol droplets resulting from condensation of gases that are beginning to cool as
they leave the vicinity of the flames inside of the container. However, as seen in the video, the aerosol
droplets will ignite when the fire is placed in the right mixture as it is leaving the container. There are two main
types of smoke aerosols distinguished by the color of the smoke. This is fire footage with no narration, so no
transcript is needed.

VentEnterSearch. (2011, April 30). Smoke combustion [Video]. YouTube.

Learning Activities (Nongraded)

Nongraded Learning Activities are provided to aid students in their course of study. You do not have to submit
them. If you have questions, contact your instructor for further guidance and information.

For this activity, you are asked to prepare flash cards. Our textbook suggests that both soot and aerosols
reduce the ability to see in a fire. Smoke is sometimes black and sometimes white in appearance. Using the
PowerPoint presentation linked here, explain what each concept/term means in one or two sentences on back
of the flash card. You may use various sources, including your textbook or other scholarly material; however,
the point of flash cards is for you to actively and skillfully conceptualize, apply, analyze, and evaluate the
differences in soot and aerosols from what you have you experienced in live fires and read in this lesson.
What has been your observation, experience, reflection, or reasoning for the differences in color? Is there a
difference? Why, or why not?

This is an opportunity for you to express your thoughts about the material you are studying by writing about it.
Conceptual thinking is a great way to study because it gives you a chance to process what you have learned
and increases your ability to remember it.

If you have any questions or do not understand a concept, contact your professor for clarification.