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FIR 2303, Fire Behavior and Combustion 1
Course Learning Outcomes for Unit V Upon completion of this unit, students should be able to:
4. Describe the process of burning. 4.1 Describe the three significant differences between the burning of solid fuel and the burning of
gaseous and liquid fuels. 4.2 Illustrate the hazards and harm of smoke exposure to individuals from the burning process. 4.3 Summarize flash point, fire point, and autoignition temperatures.
5. Define the concepts associated with the chemistry of fire.
5.1 List the hazards to firefighters and others from a fire. 5.2 Indicate acute effects and chronic effects from smoke.
6. Discuss various materials considered fuel for fires.
6.1 Evaluate how char formation and melting occur and how they affect the burning rate.
Course/Unit Learning Outcomes
Learning Activity
4.1 Unit Lesson Chapter 8 Unit V Essay
4.2 Unit Lesson Chapter 11 Unit V Essay
4.3 Unit Lesson Chapter 8 Unit V Essay
5.1 Unit Lesson Chapter 11 Unit V Essay
5.2 Unit Lesson Chapter 11 Unit V Essay
6.1 Unit Lesson Chapter 9 Unit V Essay
Required Unit Resources Chapter 8: Fire Characteristics: Liquid Combustibles Chapter 9: Fire Characteristics: Solid Combustibles Chapter 11: Smoke and Heat Hazards
UNIT V STUDY GUIDE
Process of Burning
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In order to access the following resources, click the links below. The following videos show footage of fires with no narration; therefore, no transcripts are needed. Skidgell, M. (2011, April 22). 1944 Hartford Circus fire the big top collapse [Video]. YouTube.
https://www.youtube.com/watch?v=yXywn35PpvI jarhead96. (2010, December 17). New vs old room fire final UL [Video]. YouTube.
https://www.youtube.com/watch?v=aDNPhq5ggoE
Unit Lesson Recap In the previous unit, we evaluated the physical and chemical properties of fire. We covered diffusion flame and the turbulent flame phenomena. We saw where turbulent flame projects heat and flaming across the combustible ceiling, and if this process continues, it could lead to a flashover. We found out the neutral plane forms between the hot layer and cool layer forming a horizontal plane, which conforms to the lower layer of the smoke that is not a straight line as described by some authors. Process of Burning of Legacy and Modern Fire instructors in decades past taught fire behavior and combustion based on what they knew about building materials and the burning characteristics of furnishings. Today, many instructors have coined the phrase “legacy versus modern fires” where they contrast the differences in modern fires versus those previously seen. Legacy fires are considered pre-1949 where building materials primarily consisted of all-natural wood and furnishings were made from organic fibers. Modern fires are considered to consist of current pre- engineered wood products, as well as lightweight structural components and furnishings made with synthetic materials or polymers. Dixon (2017) discusses that fire itself does not behave any differently today than it did in the past. What does this statement really mean? Is every fire the same? There is only one fire triangle or tetrahedron. We have been taught that if you remove any of the elements, the fire goes out. So, what is the difference between legacy and modern fires? Madrzykowski (2013) suggests every fire is different; however, the exothermic reactions that produce heat and light require the same three components to sustain chemical reaction today. These polymers are commonplace today, and the chemical makeup of polymers will change continuously, making fires even more dangerous in the future due to their volatility and flame spread rate. Pre-1949 wood furniture was mainly solid wood with minimal glue. Today, wood furnishings are mainly pressboard laminates with plastic finishes that appear like wood. As seen in previous units, the gasification of these products is different. The pyrolysis of these materials is more volatile. Gann and Friedman (2015) suggest the ignition of the pyrolzate from these synthetic polymers is very similar to the ignition of gases and vapors. They also suggest the difference is the heat loss to the surrounding materials. They surmise these solids (polymers) go through different modes of decompositions during the pyrolysis process than wood furnishings pre-1949. In addition, the layout of pre-1949 residential structures was compartmentalized with more windows and doors kept open for cooling the structure with airflow. Today, with large open areas and HVAC systems, fewer windows and doors are used. Kerber (2012) stated experiments were conducted comparing the failure time of building materials, the impact of windows and doors in compartments, and the impact on fire growth between modern and legacy structures. From these experiments, the author suggests any one change alone may not have been significant; however, the effect of modern furnishings and these design changes effect residential fire behavior. There is no doubt that modern furnishings, made from more flammable synthetic materials (plastics and textiles), play a major role in fire dynamics and the impact on fire tactics. In addition, more open floor plans with taller ceiling heights increases the volume of the room, requiring more water for extinguishment.
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Solid Combustibles Over the centuries, some of the worst fire disasters in history have occurred from the vigorous burning of solid combustibles (Gann & Friedman, 2015). In our textbook, the authors discuss the 1944 Hartford Circus fire that occurred during an afternoon performance of the Ringling Brothers and Barnum & Bailey Circus. Investigators are still not sure what started the fire. However, the cause was listed as the carelessness of an unidentified smoker discarding a lit cigarette in dried grass. Research since then has disproved this theory, suggesting other conditions may have occurred with the fire starting in the vicinity of the men’s toilet enclosure attached to the big top wall (Skidgell, 2015). Why did the fire spread so quickly when the materials were considered to be legacy? Is there really a difference between legacy and modern fires?
From what we understand about fire behavior and combustion, let’s speculate on the fire behavior of the Harford Circus fire (Figure 1). The big top was water proofed with paraffin wax and the walls were untreated canvas (Skidgell, 2015). The investigation suggested a small fire involving the big top wall reached the paraffin wax-coated canvas top, quickly spreading across the top. Most likely, as the paraffin wax- coated canvas burned, it continued producing toxic by- products of combustion with intermittent velocity of the plume
or fluctuation of the flame just ahead of the dense smoke plume. The three major components seen in the dense smoke—such as the unburned solid particles, vapors, and gases—were lifted in the thermal column and accumulated in the top of the tent. The heat of the thermal column, most likely, was sufficient to cause water and paraffin wax droplets to become part of the smoke. The droplets may have even accumulated enough to drop down adding to the flaming process or even burning victims who were trying to escape. The heat from a fire may have caused skin burns and weakened the supports of the tent walls to the point of collapse. Oxygen-rich air was entrained through the openings just below the canvas top where the canvas walls of the big top were lowered during the event to allow airflow. The lowering of the walls probably allowed turbulent diffusion flames to develop in the upper levels of the big top, forming a neutral plane staying at the top of the lowered walls allowing some occupants to escape with clear visibility. Wood support poles held the big top in place with ropes securing the poles. During the fire, the ropes were burned and little to no damage was noted to the support poles. For wood surfaces, the temperature must be 570 ˚F to 750 ˚F, and autoignition occurs at 1100 ºF. The 2012 Material Safety Data Sheet (MSDS, 2010) for paraffin wax indicates that it ignites at 390.2 ˚F, and the autoignition temperature is 473 ˚F. The flash point temperature of paraffin wax is 390.2 ˚F, at which piloted ignition occurs but is not sustained. At the slightly higher fire point, flaming is sustained. Paraffin wax is a solid, and at 99 ˚F it begins to melt into a liquid for application on the canvas fabric and solidifies back into a solid once application has occurred. As you look at the Hartford Circus fire, this would explain why, in the images of the aftermath, the wood support poles were still intact with little to minimal charring. The heat of gasification for the paraffin wax occurred at a much lower temperature than the wood, rapidly breaking the chemical bonds and leading to autoignition of the canvas fabric across the top of the big top. Gann and Friedman (2015) suggest the heat of gasification will change as the chemistry of the fuel changes.
Figure 1
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In the Hartford Circus fire, according to Skidgell (2015), the toilets were on the outside of the big top and attached to the canvas wall, and he speculates that a lit cigarette was discarded between the folds of the canvas walls of the toilet area (Figure 2). This created two planes of canvas material, the wall of the big top and the wall of the toilets. This would have created the effect of radiative enhancement on continued ignition, allowing the fire to progress rapidly up the wall (Gann & Friedman, 2015). As oxygen-rich air was pulled into the flame, the flame temperature increased the soot formation rate, producing thick black plumes of smoke. Pyrolysis Gann and Friedman (2015) describe pyrolysis as the decomposition process for a solid fuel that requires radiative or conductive heat to begin the process. During this process, the chemical bonds begin to break as the temperature increases. The authors propose that the material’s temperature must be raised high enough for chemical bonds to break. They also suggest for most organic solids, the temperature must be approximately 520 ˚F and 750 ˚F.
In Figure 3, and as seen previously in Unit II, the cigarette between the chair seat and back cushion in the smoldering state begins to form heat of the exterior surfaces confined in the crack, allowing the temperature to increase, breaking chemical bonds until flaming occurs (Reference Point 1). As flaming occurs, it is controlled by the rate of oxygen-rich air entrained (Reference Point 2). Heating continues below the surface of the cushion, releasing volatiles into the air with some of them being entrained into the flaming process (Reference Point 3). This process continues with volatiles and solid decomposition occurring simultaneously. After flaming occurs across the surface, the unburned fabric continues pyrolysis, leaving a char layer (Reference Point 4). Conductive heat continues, although it is not as important as the radiation and conduction (Reference Point 5). As flame radiation, convection, and conduction occurs, significant radiative heat is fed back to the fuel surface as thermal feedback (Reference Point 6). As this continues, enhanced radiation from larger flames causes increased mass transfer rates and a significant decrease in convective heat transfer (Gann & Friedman, 2015).
Synthetic Polymer Pyrolysis Gorbett and Pharr (2011) suggest when synthetic polymers undergo pyrolysis, a carbonaceous residue is left behind, and thermoplastics melt when exposed to heat and vaporize. Quintiere (2006) suggests tars and gases are left behind during pyrolysis of solid synthetic materials. Why is it important to understand pyrolysis of solids? Previously, we covered that solids must gasify before ignition can occur as seen in pyrolysis, and
Figure 2
Figure 3
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some materials gasify faster than others. Each of the particles from gasification is suspended in the thermal layer of smoke and can contribute to rollover, flashover, or backdraft. Gorbett and Pharr (2011) suggest these suspended particles are a major fuel constituent that support these phenomena. This incomplete solid pyrolysis in smoke layers is in contradiction to what many authors described to be the cause of flashovers or backdrafts a decade ago. Decades ago, several texts listed carbon monoxide (CO) as the fuel behind the backdraft phenomenon (Crosby et al., 1935). Even more recently, several authors suggested carbon monoxide being suspended in the thermal layers is the cause of backdrafts (Instructor’s Guide to Accompany, 2000; International Fire Service Training Association, 2013). Gorbett and Pharr (2011) state that studies by Bryner, Johnson, and Pitts in 1992; Icove and DeHaan, 2009; Babrauskas, 2003; and Gottuk, 1999 maintain that compartment fires rarely have CO mixtures above 7%. There is no scientific support found in these experimental studies to indicate the volume of CO found in compartment fires reaches the volume needed for the backdraft phenomenon.
Points to Ponder In the scenario, what is the principle contributor of the fire spread? Is it the polymers from the synthetic fabric? Is pyrolysis increasing from the temperature of the products burning? Can autoignition occur? Will the gasification rate contribute to a flashover or possible backdraft? Was smoke inhalation a factor in the firefighter down?
Conclusion Pyrolysis of solids forms gaseous fragments that suspend in the thermal plume (soot). During pyrolysis, the chemical change that occurs is different from the solid itself. The burning rate depends on the radiant heat to the surface of the material as seen in the Hartford Circus fire involving the canvas fabric. Early in the big top fire, the radiant heat was from the flames in the toilet area; later, most of the radiant heat was derived from the surrounding surfaces of the paraffin wax-covered canvas. At first, the flame spread was upward (fastest); then as the flames moved to the side or downward, they became slower. The radiative preheating from the thermal plume, the oxygen-rich air, and the direction of wind also controls the spread of flames.
Building on the Scenario Because of the intense heat, Engine 5 crew backed out of the fire room next to the bedroom. As they opened the door, firefighters noticed a synthetic (polyester) fabric was draped across the ceiling in the bedroom creating a canopy effect. On the main wall, the synthetic fabric was draped from the ceiling to the floor. Engine 5 noted an increase in the heat signature when the wall fabric appeared to shrink as the smoke plume began to reach it. Firefighters then began to notice some type of substance dripping on their arms, gloves, and helmet and realized the ceiling canopy was melting. No fire was visible in the bedroom, and the window appeared to be intact. The door was open, and black smoke began pouring into the bedroom. Firefighters began noticing blackened soot was adhering to their SCBA facemask and bunker gear. When trying to wipe their face masks clean, the blackened soot just smeared limiting visibility. Conditions inside were super-heated from the radiation. At the same time, Engine 2 was still initiating a fast attack in apartment 2-B. They continued to encounter heavy smoke and heat. During the fire attack, Engine 2 knocked over a 2- gallon container of cooking oil and noticed the flame spread rate appeared to increase as the temperature in the room increased. They then entered a small bedroom to look for any victims and fire. After just a few minutes enduring the high heat and smoke conditions, they exited to the hallway thinking everyone was together. Once in the hallway, they met up with Tower 2’s crew and realized they were missing a firefighter. Within seconds, they heard the PASS alarm and found the firefighter unconscious under a mattress in the bedroom from which they had just left. They immediately notified Command and removed the firefighter down the hallway.
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References Crosby, E. U., Fiske, H. A., & Forster, H. W. (1935). NFPA handbook of fire protection (8th ed.). National Fire
Protection Association. Dixon, J. (2017, April 22). Legacy vs. modern fire environment. FireHouse.
https://instructorjohndixon.com/published-articles/entry/legacy-vs-modern-fire-dynamics Gann, R. G., & Friedman, R. (2015). Principles of fire behavior and combustion (4th ed.). Jones & Bartlett
Learning. Gorbett, G. E., & Pharr, J. L. (2011). Fire dynamics. Pearson. Kerber, S. (2012). Analysis of changing residential fire dynamics and its implications on firefighter operational
timeframes. https://newscience.ul.com/wp- content/uploads/2014/04/Analysis_of_Changing_Residential_Fire_Dynamics_and_Its_Implications_o n_Firefighter_Operational_Timeframes.pdf
Instructor’s guide to accompany: Firefighter’s handbook essentials of firefighting and emergency response.
(2000). Delmar Thomas Learning. International Fire Service Training Association. (2013). Essentials of firefighting (6th ed.). Fire Protection
Publications. Skidgell, M. (2015, July 1). The cause and origin of the Hartford Circus fire. Circus Fire 1994.
http://www.circusfire1944.com/uploads/1/3/8/6/13863860/cause_and_origin_final_copy.pdf Madrzykowski, D. (2013). Fire dynamics: The science of firefighting. International Fire Service Journal of
Leadership and Management, 7(2013), 7–15. Material safety data sheet: Paraffin wax. (2010). http://www.united-wax.com/wd/pi/20161024-
120125_2_msds_paraffin_wax_uw.pdf Quintiere, J. G. (2006). Fundamentals of fire phenomena. Wiley.
Suggested Unit Resources In order to access the following resources, click the links below. The following videos contain footage of fires with no narration; therefore, no transcripts are needed. In these videos, observe the fire in relationship to Chapter 9 and the pyrolysis of solids to form gaseous fragments. Jones, K. (2014, March 25). Fire near AIG campus [Video]. YouTube.
https://www.youtube.com/watch?v=Cg9PWSHL4Vg Roberts, R. (2014, February 26). Bowling Green Ave dwelling fire 2/26/14 Morrisville, PA [Video]. YouTube.
https://www.youtube.com/watch?v=l-PF5igB1AE Observe the fire in relationship to the smoke and heat hazards to the firefighters as related to narcotic gases and irritant gases. tornadochaser66. (2014, February 14). Paterson fire 3rd alarm heavy fire throwback to summer 2000 161 &
163 Hamilton Ave [Video]. YouTube. https://www.youtube.com/watch?v=SvlHPj0DZJQ
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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 a reflection paper. Reflect on the concepts you have learned during your readings. In the image of the frying pan and cooking oil, for the cooking oil to start burning, how does the rate of flame spread over the surface of a liquid depend on the flash point? This is not a summary. A reflection paper is an opportunity for you to express your thoughts about the material you are studying by writing about it. Reflection writing 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.