scenario-based case study

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; Köylü, & 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

Köylü, Ü. Ö., & Faeth, G. M. (1994). Optical properties of soot in buoyant laminar diffusion flames. Journal of

Heat Transfer, 116(4), 971–979. Köylü, Ü. Ӧ., 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.

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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.

https://www.youtube.com/watch?v=OxnxhewgFL8

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.