Question Combustion And Oxygen

May 26, 2022
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Hello! I have a question about combustion and low oxygen. I’m writing an article and I want it to be scientifically accurate. The hypothetical scenario in question is a worldwide event where world wide oxygen is reduced to about 11% to 12%. This event will last 4 days. Just before this event there are cataclysmic fires that are put out by the sudden reduction in oxygen. My first question is whether smouldering would still be possible. Obviously fire wouldn’t be possible but would there still be smouldering leftover from the giant fires. Would the smouldering wood still emit a glow? If you blew or channeled air continuously onto the smouldering wood could you still generate a flame even briefly? Also, could people survive a full four days at this level of oxygen? Could they survive at 9% for a full four days without serious health effects. Lastly, if this event was caused by a super volcano would the amount of C02 that displaced the oxygen be lethal over a full four days if the co2 was somehow contained at ground level? Thank you, any help is greatly appreciated!
 
Last edited:
Jan 27, 2020
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Bunny7, you might find the following to be of interest.

The quantitative effect of varying O2 on fire ignition and spread has been studied extensively in the laboratory (e.g., Tewarson, 2000; Babrauskas, 2002), but only on commercial materials. Little attention has been paid to the effects of varying O2 on the burning of forest fuels. One such study examined the ignition of paper strips at varying O2 levels and moisture contents (Watson, 1978). On the basis of Watson's results, it was concluded that forests would burn excessively at concentrations of 25% O2 or higher even when saturated with moisture (Watson et al., 1978). Because plant fossils indicate that forests have persisted over the past 375 million years, Watson et al. (1978) concluded that O2 levels have never exceeded 25% over this time.

The conclusions of the widely cited Watson et al. (1978) paper (e.g., see Lenton, 2001) have been criticized by Robinson (1989). Robinson notes that (1) paper is thermally thin and not representative of naturally thick plant material. This thinness maximizes exposure to oxygen. (2) Charring was not considered. Paper is very low in lignin and does not char as readily as natural lignin-rich plant materials. Charring acts as a protective shield to protect plants against further burning (Nelson, 2001). (3) Moisture contents far less than found in natural vegetation were studied. (4) Translation of O2 results to fuel moisture equivalents and an index of probability of ignition was done using a table that has since been replaced by the U.S. Forest Service because it proved unreliable (Deeming et al., 1977).

Watson's (1978) experiments, and those of Rashbash and Langford (1968), deduced a lower limit for forest burning of about 15% O2. In other words, forests could not have been ignited if the level of O2 in the geological past had ever dropped below about 15%. Forest fires are documented by the occurrence of charcoal (as fusain) in the fossil record.

On the basis of fire spreading rate, Wildman et al. (2004) concluded that the sensitivity to changes in moisture is much larger than the sensitivity to changes in oxygen level. The lack of burning at 35% O2, for both pine wood and pine needles containing 61% H2O, refutes the idea that forest materials will burn above 25% O2 regardless of moisture content (Watson et al., 1978). (The water content of live tree trunks normally exceeds 100% by dry weight and saturated pine needles contain 190% water.) Starting the fire with dry wood or needles crudely simulates a brush fire that spreads to moister fuels, so these experiments with natural materials are more realistic than igniting paper strips. Based on the burning experiments of Wildman et al. (2004), it is quite possible that oxygen levels of the Permo-Carboniferous exceeded 30% without the destruction of all terrestrial life.

The results of the burning experiments of Watson (1978) and Wildman et al. (2004) both point to increased burning at higher O2 levels, suggesting increased fire frequency during the Permo-Carboniferous high-O2 period and a natural selection in favor of fire-resistant plants at that time.

The slow rise of O2 through the Devonian and Carboniferous predicted by modeling (Berner and Canfield, 1989; Berner, 2001) would allow enough time for the evolution of widespread fire defenses. The common occurrence of thick, barklike corky layers on the outside of Carboniferous plants, such as lycopsids and Calamites (Jeffry, 1925, cited in Komrek, 1972) were probably developed as a defense against fire. Also, besides bark structure and composition, the spatial organization, energy partitioning, and reproductive strategies of Carboniferous ecosystems appear to be consistent with those of forests subject to severe fire regimes (Robinson 1989, 1991). Furthermore, present-day species that are probable relics of Carboniferous plants are associated with areas of frequent forest fires (Komrek, 1972).

Late Carboniferous coal beds probably arose from raised megathermal peat bogs analogous to the present-day swamps of Indonesia and Malaysia (Robinson, 1989). However, the Carboniferous coals are unusually rich in fusain, fossil charcoal, compared to their relatively charcoal-free modern analogues. As charcoal is a product of fires, this suggests a greater swamp fire frequency during the Late Carboniferous than at present, due to presumably higher O2 levels. Also, promotion of char formation is an effective way to create fire resistance (Nelson, 2001). This suggests that the high fusain (fossil charcoal) content of the Carboniferous coals could indicate an abundance of plants with increased fire resistance. In addition, the formation and burial of charcoal, which is resistant to biological oxidation, is a positive feedback mechanism for the enhancement of atmospheric O2 level (Berner et al., 2003).

The reflectivity of charcoal is directly proportional to the temperature at which the charcoal formed (Scott, 2000). Jones and Chaloner (1991) have shown that the cell wall morphology of fusainized woods also reflects the temperature of formation. If fire temperature can be correlated with O2 level, then fusain reflectivity and/or cell wall morphology might be a guide to ancient atmospheric O2 concentration. However, many factors may affect fire temperature other than atmospheric O2 level, and, in fact, it is difficult to reconstruct fire regimes from the nature of charcoal even in modern sediments (Clark et al., 1997).

More work on the properties of fossil charcoal is needed before anything more definitive can be stated concerning the usefulness of charcoal as a paleo-O2 indicator.

In structural firefighting one of the primary hazards of entering confined spaces is oxygen deficiency. Oxygen can also be present in concentrations that are too high. Oxygen in concentrations greater than 23% is too oxygen rich and can cause combustible materials to ignite very quickly.

Every year, numerous employees are seriously injured or die in diverse workplaces due to oxygen deficiency. Oxygen-deficient locations are often unpleasant places such as sewers, sewage pump stations, and manure pits. But even pleasant-sounding worksites, such as vessels for producing molasses used on pancakes, can have deadly oxygen-deficient atmospheres. A person can survive for only three minutes without oxygen.

To clarify your concerns aboutCO2, it's toxic. The lung’s ability to exchange CO2 for O2 depends on a CO2 concentration in the alveoli of about 650 ppm. Less than this, from say, hyerventilation, and we get light-headed. More than this, and we begin to see the following list of symptoms at PPM, or parts per million:

Screen Shot 2022-05-26 at 11.39.26 PM.png

On the Apollo 13 mission, the crew was in danger of CO2 poisoning when the limestone canister system became damaged. This device removes CO2 from the circulated air. Adding more O2 to the cabin would not have kept the astronauts from death.

Any material / compound / element that will burn in oxygen, will decay at a particular rate in a fire. So for example, highly combustible materials such as gasoline, or hydrogen, will combust rapidly in a relatively low percentage of oxygen. In the case of hydrogen, H2, the percentage of oxygen required to use up all the H2 present will be 50%, since the compound produced during combustion is H2O. If there is this much oxygen present, the result will be quite explosive. For a controlled fire or reaction, you would control the oxygen input, and / or the hydrogen input.

For an ignition to occur the concentration of the combustible material (gas, vapour, or powder) in the mixture must lie between an upper and a lower flammability limit. A plot of the ignition energy against the fuel concentration in a fuel-air mixture is typically a U-shaped curve on which the lowest point denotes the MIE of the mixture. This is shown in the Figure below:

Screen Shot 2022-05-26 at 11.14.38 PM.png

Oxygen is the key ingredient in the fire triangle and there is a minimum oxygen concentration required to propagate a flame. This is also known as the limiting oxygen concentration. It is the concentration below which combustion, usually in air diluted with an inert gas such as nitrogen or carbon dioxide, does not propagate in a mixture of gases or vapors.

This is an especially useful result, because explosions and fires are preventable by reducing the oxygen concentration regardless of the concentration of the fuel. This is the basis of the prevention technique of inerting.

The MOC has units of % oxygen in air plus fuel. Below the MOC, the reaction cannot generate enough energy to heat the entire mixture of gases (including the inerts) to the extent required for the self-propagation of the flame. The MOC for several chemicals are shown in the Table below:

Screen Shot 2022-05-26 at 11.19.48 PM.png

Typically, when the oxygen concentration within most fuel mixtures fall below about 10 volume % no combustion can occur. As this is only an indicative value, when reliable inerting is required the exact value of the MOC has to be determined. This is measured for an optimum fuel concentration using a very strong ignition source which supplies spark discharge energies in the range of 2 - 10 kJ, depending on the volume of the test apparatus. The MOC is related to the Flammability Limits and the Minimum Ignition Energy.

The minimum ignition energy is the minimum energy input required to initiate combustion. It is the smallest amount of energy stored in a capacitor, that is just sufficient, when discharged across a spark gap, to ignite the most ignitable explosive mixture. All flammables (including dusts) have MIEs. The MIE depends on the specific chemical or mixture, the concentration, pressure, and temperature. A few MIEs are given in the Table below:

Screen Shot 2022-05-26 at 11.23.18 PM.png

As can be seen from the Table, many hydrocarbons have MIEs of about 0.25 mJ. This is low when compared to sources of ignition. For example, a static discharge of 22 mJ is initiated by walking across a rug, and an ordinary spark plug has a discharge energy of 25 mJ. Electrostatic discharges, as a result of fluid flow, also have energy levels exceeding the MIEs of flammables and can provide a ignition source, and thus possibly contributing to plant explosions.

As far as human breathing and oxygen goes, we know humans need oxygen to live, but not as much as you might think. The minimum oxygen concentration in the air required for human breathing is 19.5 percent. The human body takes the oxygen breathed in from the lungs and transports it to the other parts of the body via the body's red blood cells. Each cell uses and requires oxygen to thrive. Most of the time, the air in the atmosphere contains the proper amount of oxygen for safe breathing. But at times, the level of oxygen can drop due to other toxic gases reacting with it and the amount and density of oxygen varies with altitude.

The normal air in our environment consists of a few different gases. Approximately 78 percent of the air is nitrogen gas while only about 20.9 percent is oxygen. The remaining fraction is made up of primarily argon gas, but trace amounts of carbon dioxide, neon and helium are also present.

For humans and many animals to sustain normal functions, the percentage of oxygen required to sustain life falls within a small range. The Occupational Safety and Health Administration, OSHA, determined the optimal range of oxygen in the air for humans runs between 19.5 and 23.5 percent.

For humans and many animals to sustain normal functions, the percentage of oxygen required to sustain life falls within a small range. The Occupational Safety and Health Administration, OSHA, determined the optimal range of oxygen in the air for humans runs between 19.5 and 23.5 percent.

See: https://www.briangwilliams.us/carbon-cycle-2/fires-and-oxygen.html

See: https://sciencing.com/minimum-oxygen-concentration-human-breathing-15546.html

See: https://www.answers.com/chemistry/What_is_the_percentage_of_oxygen_a_fire_needs_to_start

See: http://www.processoperations.com/FireExplode/FE_Chp01.htm

See: https://www.quora.com/What-concentration-of-CO2-is-lethal?share=1

It appears, concerning the questions raised by Bunny7, that there are a large number of critical factors involved with a fire's ignition and its continuation of combustion, especially in view of the varying oxygen percentages for the different fuels , varying ignition temperatures, the presence and % of other gases, humidity, as well as the type of the material to be combusted.

It goes without saying that exterior and woodland fires are constrained by oxygen content as well, however humans would not be able to notice these fires if the atmospheric oxygen content fell below the acceptable level of 19-23.5%.

Regarding human respiration, we find that our lung’s ability to exchange CO2 for O2, which yields CO2 and water, depends on a CO2 concentration in the alveoli of the lungs of about 650 ppm.

As far as a CO2 blanket remaining in a specified location, the primary variables are wind, atmospheric gas percentages, barometric pressure, the maintenance of the CO2 source which is pumping the CO2 gas into the area, ambient temperature in the area, humidity and the topography.
Hartmann352
 
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May 26, 2022
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Bunny7, you might find the following to be of interest.

The quantitative effect of varying O2 on fire ignition and spread has been studied extensively in the laboratory (e.g., Tewarson, 2000; Babrauskas, 2002), but only on commercial materials. Little attention has been paid to the effects of varying O2 on the burning of forest fuels. One such study examined the ignition of paper strips at varying O2 levels and moisture contents (Watson, 1978). On the basis of Watson's results, it was concluded that forests would burn excessively at concentrations of 25% O2 or higher even when saturated with moisture (Watson et al., 1978). Because plant fossils indicate that forests have persisted over the past 375 million years, Watson et al. (1978) concluded that O2 levels have never exceeded 25% over this time.

The conclusions of the widely cited Watson et al. (1978) paper (e.g., see Lenton, 2001) have been criticized by Robinson (1989). Robinson notes that (1) paper is thermally thin and not representative of naturally thick plant material. This thinness maximizes exposure to oxygen. (2) Charring was not considered. Paper is very low in lignin and does not char as readily as natural lignin-rich plant materials. Charring acts as a protective shield to protect plants against further burning (Nelson, 2001). (3) Moisture contents far less than found in natural vegetation were studied. (4) Translation of O2 results to fuel moisture equivalents and an index of probability of ignition was done using a table that has since been replaced by the U.S. Forest Service because it proved unreliable (Deeming et al., 1977).

Watson's (1978) experiments, and those of Rashbash and Langford (1968), deduced a lower limit for forest burning of about 15% O2. In other words, forests could not have been ignited if the level of O2 in the geological past had ever dropped below about 15%. Forest fires are documented by the occurrence of charcoal (as fusain) in the fossil record.

On the basis of fire spreading rate, Wildman et al. (2004) concluded that the sensitivity to changes in moisture is much larger than the sensitivity to changes in oxygen level. The lack of burning at 35% O2, for both pine wood and pine needles containing 61% H2O, refutes the idea that forest materials will burn above 25% O2 regardless of moisture content (Watson et al., 1978). (The water content of live tree trunks normally exceeds 100% by dry weight and saturated pine needles contain 190% water.) Starting the fire with dry wood or needles crudely simulates a brush fire that spreads to moister fuels, so these experiments with natural materials are more realistic than igniting paper strips. Based on the burning experiments of Wildman et al. (2004), it is quite possible that oxygen levels of the Permo-Carboniferous exceeded 30% without the destruction of all terrestrial life.

The results of the burning experiments of Watson (1978) and Wildman et al. (2004) both point to increased burning at higher O2 levels, suggesting increased fire frequency during the Permo-Carboniferous high-O2 period and a natural selection in favor of fire-resistant plants at that time.

The slow rise of O2 through the Devonian and Carboniferous predicted by modeling (Berner and Canfield, 1989; Berner, 2001) would allow enough time for the evolution of widespread fire defenses. The common occurrence of thick, barklike corky layers on the outside of Carboniferous plants, such as lycopsids and Calamites (Jeffry, 1925, cited in Komrek, 1972) were probably developed as a defense against fire. Also, besides bark structure and composition, the spatial organization, energy partitioning, and reproductive strategies of Carboniferous ecosystems appear to be consistent with those of forests subject to severe fire regimes (Robinson 1989, 1991). Furthermore, present-day species that are probable relics of Carboniferous plants are associated with areas of frequent forest fires (Komrek, 1972).

Late Carboniferous coal beds probably arose from raised megathermal peat bogs analogous to the present-day swamps of Indonesia and Malaysia (Robinson, 1989). However, the Carboniferous coals are unusually rich in fusain, fossil charcoal, compared to their relatively charcoal-free modern analogues. As charcoal is a product of fires, this suggests a greater swamp fire frequency during the Late Carboniferous than at present, due to presumably higher O2 levels. Also, promotion of char formation is an effective way to create fire resistance (Nelson, 2001). This suggests that the high fusain (fossil charcoal) content of the Carboniferous coals could indicate an abundance of plants with increased fire resistance. In addition, the formation and burial of charcoal, which is resistant to biological oxidation, is a positive feedback mechanism for the enhancement of atmospheric O2 level (Berner et al., 2003).

The reflectivity of charcoal is directly proportional to the temperature at which the charcoal formed (Scott, 2000). Jones and Chaloner (1991) have shown that the cell wall morphology of fusainized woods also reflects the temperature of formation. If fire temperature can be correlated with O2 level, then fusain reflectivity and/or cell wall morphology might be a guide to ancient atmospheric O2 concentration. However, many factors may affect fire temperature other than atmospheric O2 level, and, in fact, it is difficult to reconstruct fire regimes from the nature of charcoal even in modern sediments (Clark et al., 1997).

More work on the properties of fossil charcoal is needed before anything more definitive can be stated concerning the usefulness of charcoal as a paleo-O2 indicator.

In structural firefighting one of the primary hazards of entering confined spaces is oxygen deficiency. Oxygen can also be present in concentrations that are too high. Oxygen in concentrations greater than 23% is too oxygen rich and can cause combustible materials to ignite very quickly.

Every year, numerous employees are seriously injured or die in diverse workplaces due to oxygen deficiency. Oxygen-deficient locations are often unpleasant places such as sewers, sewage pump stations, and manure pits. But even pleasant-sounding worksites, such as vessels for producing molasses used on pancakes, can have deadly oxygen-deficient atmospheres. A person can survive for only three minutes without oxygen.

To clarify your concerns aboutCO2, it's toxic. The lung’s ability to exchange CO2 for O2 depends on a CO2 concentration in the alveoli of about 650 ppm. Less than this, from say, hyerventilation, and we get light-headed. More than this, and we begin to see the following list of symptoms at PPM, or parts per million:

View attachment 1973

On the Apollo 13 mission, the crew was in danger of CO2 poisoning when the limestone canister system became damaged. This device removes CO2 from the circulated air. Adding more O2 to the cabin would not have kept the astronauts from death.

Any material / compound / element that will burn in oxygen, will decay at a particular rate in a fire. So for example, highly combustible materials such as gasoline, or hydrogen, will combust rapidly in a relatively low percentage of oxygen. In the case of hydrogen, H2, the percentage of oxygen required to use up all the H2 present will be 50%, since the compound produced during combustion is H2O. If there is this much oxygen present, the result will be quite explosive. For a controlled fire or reaction, you would control the oxygen input, and / or the hydrogen input.

For an ignition to occur the concentration of the combustible material (gas, vapour, or powder) in the mixture must lie between an upper and a lower flammability limit. A plot of the ignition energy against the fuel concentration in a fuel-air mixture is typically a U-shaped curve on which the lowest point denotes the MIE of the mixture. This is shown in the Figure below:

View attachment 1970

Oxygen is the key ingredient in the fire triangle and there is a minimum oxygen concentration required to propagate a flame. This is also known as the limiting oxygen concentration. It is the concentration below which combustion, usually in air diluted with an inert gas such as nitrogen or carbon dioxide, does not propagate in a mixture of gases or vapors.

This is an especially useful result, because explosions and fires are preventable by reducing the oxygen concentration regardless of the concentration of the fuel. This is the basis of the prevention technique of inerting.

The MOC has units of % oxygen in air plus fuel. Below the MOC, the reaction cannot generate enough energy to heat the entire mixture of gases (including the inerts) to the extent required for the self-propagation of the flame. The MOC for several chemicals are shown in the Table below:

View attachment 1971

Typically, when the oxygen concentration within most fuel mixtures fall below about 10 volume % no combustion can occur. As this is only an indicative value, when reliable inerting is required the exact value of the MOC has to be determined. This is measured for an optimum fuel concentration using a very strong ignition source which supplies spark discharge energies in the range of 2 - 10 kJ, depending on the volume of the test apparatus. The MOC is related to the Flammability Limits and the Minimum Ignition Energy.

The minimum ignition energy is the minimum energy input required to initiate combustion. It is the smallest amount of energy stored in a capacitor, that is just sufficient, when discharged across a spark gap, to ignite the most ignitable explosive mixture. All flammables (including dusts) have MIEs. The MIE depends on the specific chemical or mixture, the concentration, pressure, and temperature. A few MIEs are given in the Table below:

View attachment 1972

As can be seen from the Table, many hydrocarbons have MIEs of about 0.25 mJ. This is low when compared to sources of ignition. For example, a static discharge of 22 mJ is initiated by walking across a rug, and an ordinary spark plug has a discharge energy of 25 mJ. Electrostatic discharges, as a result of fluid flow, also have energy levels exceeding the MIEs of flammables and can provide a ignition source, and thus possibly contributing to plant explosions.

As far as human breathing and oxygen goes, we know humans need oxygen to live, but not as much as you might think. The minimum oxygen concentration in the air required for human breathing is 19.5 percent. The human body takes the oxygen breathed in from the lungs and transports it to the other parts of the body via the body's red blood cells. Each cell uses and requires oxygen to thrive. Most of the time, the air in the atmosphere contains the proper amount of oxygen for safe breathing. But at times, the level of oxygen can drop due to other toxic gases reacting with it and the amount and density of oxygen varies with altitude.

The normal air in our environment consists of a few different gases. Approximately 78 percent of the air is nitrogen gas while only about 20.9 percent is oxygen. The remaining fraction is made up of primarily argon gas, but trace amounts of carbon dioxide, neon and helium are also present.

For humans and many animals to sustain normal functions, the percentage of oxygen required to sustain life falls within a small range. The Occupational Safety and Health Administration, OSHA, determined the optimal range of oxygen in the air for humans runs between 19.5 and 23.5 percent.

For humans and many animals to sustain normal functions, the percentage of oxygen required to sustain life falls within a small range. The Occupational Safety and Health Administration, OSHA, determined the optimal range of oxygen in the air for humans runs between 19.5 and 23.5 percent.

See: https://www.briangwilliams.us/carbon-cycle-2/fires-and-oxygen.html

See: https://sciencing.com/minimum-oxygen-concentration-human-breathing-15546.html

See: https://www.answers.com/chemistry/What_is_the_percentage_of_oxygen_a_fire_needs_to_start

See: http://www.processoperations.com/FireExplode/FE_Chp01.htm

See: https://www.quora.com/What-concentration-of-CO2-is-lethal?share=1

It appears, concerning the questions raised by Bunny7, that there are a large number of critical factors involved with a fire's ignition and its continuation of combustion, especially in view of the varying oxygen percentages for the different fuels , varying ignition temperatures, the presence and % of other gases, humidity, as well as the type of the material to be combusted.

It goes without saying that exterior and woodland fires are constrained by oxygen content as well, however humans would not be able to notice these fires if the atmospheric oxygen content fell below the acceptable level of 19-23.5%.

Regarding human respiration, we find that our lung’s ability to exchange CO2 for O2, which yields CO2 and water, depends on a CO2 concentration in the alveoli of the lungs of about 650 ppm.

As far as a CO2 blanket remaining in a specified location, the primary variables are wind, atmospheric gas percentages, barometric pressure, the maintenance of the CO2 source which is pumping the CO2 gas into the area, ambient temperature in the area, humidity and the topography.
Hartmann352
Thank you Hartmann352! That’s a lot of good information. For my article I am supposing that the timber they have is extremely dry. I have heard about oxygen reduction and fighting fires. I read somewhere that they can stop flammable combustion but they can’t stop smouldering combustion. Since flames would be the only way they could generate light at night (I’m assuming this happened in the 1700s for my article) I was also wondering if there would be any way for them to burn something to emit light given these conditions (such as using smouldering wood). One of the articles you posted cited a research paper by wildmsn et al where they demonstrated that wood would burn at 12% oxygen if extremely dry. Could this be a reliable source of combustion for the people in the hypothetical scenario if they had very dry wood?
 
Last edited:
Jan 27, 2020
415
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Bunny7, here is a bit more on the combustion of wood, heat needed for combustion and the various factors affecting ignition:

The burning behaviour of wood is complex. The processes behind pyrolysis, ignition, combustion, and extinction are generally well understood, with good agreement in the fire science literature over a wide range of experimental conditions for key parameters such as critical heat flux for ignition (12 kW/m2 ± 2 kW/m2) and heat of combustion (17.5 MJ/kg {Mega Joules*/2.2 lbs.) ± 2.5 MJ/kg). These parameters are key for evaluating the risks posed by using timber as a construction material.

Conversely, extinction conditions are less well defined and understood, with critical mass loss rates for extinction varying from 2.5 g/m2s to 5 g/m2s. A detailed meta-analysis of the fire resistance literature has shown that the rate of burning as characterised by charring rate averaged over the full test duration is observed to vary with material properties, in particular density and moisture content which induce a maximum 18% variability over the ranges expected in design. System properties are also shown to be important, with stochastic phenomena such as delamination and encapsulation failure resulting in changes to the charring rate that cannot be easily predicted. Finally, the fire exposure as defined by incident heat flux has by far the largest effect on charring rates over typical heat fluxes experienced in compartment fires. Current fire design guidance for engineered timber products is largely prescriptive, relying on fixed “charring rates” and “zero-strength layers” for structural analyses, and typically prescribing gypsum encapsulation to prevent or delay the involvement of timber in a fire. However, it is clear that the large body of scientific knowledge that exists can be used to explicitly address the fire safety issues that the use of timber introduces. However the application of this science in real buildings is identified as a key knowledge gap which if explored, will enable improved efficiencies and innovations in design.

To allow fire safe use of exposed structural timber elements, the processes driving the pyrolysis, ignition, and subsequent combustion of timber must be understood.

Pyrolysis is the process by which materials decompose upon exposure to heat. This process effects chemical and physical changes, and thus understanding and quantifying the processes is fundamental to the ignition, burning, and extinction behaviour of a material. To burn, polymers must first decompose into smaller molecules that can exist in the gas phase at ambient conditions. To create a self-sustaining reaction, the combustion of these gases must generate sufficient heat to perpetuate the production of volatiles. Upon heating, the constituent natural polymers present in timber will degrade, producing inert and combustible gases (the nature and composition of which will depend on the char yield), liquid tars, a solid carbonaceous char (typically around 20% the density of virgin wood) and inorganic ash. This can occur before dehydration is completed if the heating rate is fast enough, but will be faster after the sample has dried. Under sustained heating conditions, these pyrolysis products can then undergo further pyrolysis themselves. This process is further complicated due to charring and material variability, and the chemical processes occurring are numerous and interdependent. It is also necessary to distinguish between pyrolysis and combustion. Pyrolysis refers to the thermal decomposition of a substance, is endothermic, and can occur without an oxidiser.

In most scenarios relevant to timber construction it can be assumed that pyrolysis occurs over a relatively narrow zone perpendicular to the exposed face of the material. Wood typically undergoes three main stages of pyrolysis due to its relatively low thermal conductivity and density and relatively high specific heat: dehydration and very slow pyrolysis below 200°C, onset of pyrolysis up to 300°C, and rapid pyrolysis above 300°C.

Detailed pyrolysis reviews are available elsewhere, however some of the key aspects relevant to timber construction, in particular moisture transport, charring, and production of flammable gases are discussed below.

Upon heating, and prior to the onset of pyrolysis, free water begins to evaporate as temperatures within a timber member approach 100°C. Some water vapour will migrate deeper into the sample (away from the source of heat), and re-condense. This increases the local moisture content. This then creates three zones—a dry zone closest to the exposed face (in which pyrolysis occurs); a dehydrating zone; and a wet zone, as illustrated in Fig. 1. It is assumed that most of the water vapour, however, leaves from the surface.

At low heat fluxes, dehydration and pyrolysis will take place independently; at higher heat fluxes they will occur simultaneously. Where dehydration and pyrolysis occur simultaneously, moisture slows the temperature rise, typically until reaching 115°C, due to the energy supplied being used for evaporation rather than heating, and cools the pyrolysis zone through convective mass flow of water vapour. Bound water is typically freed later, at temperatures around 240°C of the water vapor, however, leaves from the surface.

heat flux.png
Chemical and physical processes within a burning timber sample; 𝑞˙′′𝑙,𝑐q˙l,c′′ is the surface heat losses by convection, 𝑞˙″𝑙,𝑟q˙l,r″ is the surface heat losses by radiation, 𝑞˙′′𝑒𝑥𝑡,𝑟q˙ext,r′′ is the external heat flux, 𝑞˙′′𝑟q˙r′′ is in-depth radiation, 𝑞˙′′𝑐𝑜𝑛𝑑q˙cond′′ is conduction into the sample, and 𝑞˙′′𝑐q˙c′′ is convective heat transfer through cracks in the sample.

Mass loss due to pyrolysis*** occurs slowly at temperatures below 200°C, with pyrolyzate consisting mostly of non-combustible volatiles such as carbon dioxide, formic acids, and acetic acids. However prolonged heating at low temperatures can convert hemicellulose (and lignin) into a carbonaceous char at temperatures as low as 95°C or 120°C leaving cellulose largely unreacted. Cellulosic materials have no fluid state, but can soften before breaking down into vapours. In so doing, they may undergo a glass transition, altering their structure and becoming softer and more rubbery. For lignin, this occurs at temperatures around 55°C to 170°C; permanent reductions in strength of timber have been observed at temperatures as low as 65°C. When the temperature exceeds 200°C, the pyrolysate remains mainly non-combustible, however visible discolouration will begin or become more intense, with prolonged exposure to these temperatures causing slow charring.

Uncharred wood remains at moderate temperatures even in long fires due to the high heat losses from the char layer; at a depth of 6 mm below the charline, the temperature is typically reduced to around 180°C; with a total thickness of approximately 35 mm below the char layer being heated. The resulting temperature profile below the char line can be expressed as an exponential or power term for thermally thick wood, or alternatively as a quadratic function. The main pyrolysis reactions then typically begin from 225°C to 275°C.

Hemicellulose is typically the first component of wood to undergo thermal decomposition. The temperatures at which this reaction starts are reported over a wide range from 120°C to 180°C, 200°C to 260°C, 220°C to 315°C, or 200°C to 300°C. This temperature range is dependent on the heating rate, species, density, or moisture content.

Cellulose is typically the next compound to decompose, with decomposition temperatures (which likely have the same dependencies as those for hemicellulose) quoted as 240°C to 350°C, 250°C to 350°C, 315°C to 400°C, 280°C to 400°C or 300°C to 350°C. Cellulose may decompose via two main processes: the first by breaking a link in the carbon ring, cross-linking to produce char alongside carbon monoxide, carbon dioxide, and H2O; the second is chain scission when a link in the polymer chain is broken and levoglucosan molecules can break away, typically at temperatures around 250°C to 300°C. Levoglucosan is a tar which will break down further into combustible gases, or alternatively repolymerise to form char. Low heating rates tend to favour char formation alongside largely non-combustible vapours, thus releasing energy. High heating rates favour the production of levoglucosan, yielding flammable vapours and little or no char, thus consuming energy.

Lignin** usually undergoes pyrolysis at temperatures quoted as 110°C to 400°C, 280°C to 500°C or 225°C to 450°C. Schaffer found that lignin began melting around 160°C, followed by re-hardening from 160°C to 210°C, with only 10% of its weight loss having occurred by 280°C. Lignin produces aromatic products on pyrolysis, and yields more char than cellulose—upon heating to 400°C to 450°C, approximately half of lignin remains as char, contributing significantly to the char yield. Since softwoods have higher lignin contents than hardwoods, they consequently give higher char yields ; this has implications for the burning rates of softwoods compared to hardwoods.

At temperatures between 300°C and 500°C, pyrolysis rates increase rapidly and are accompanied by additional exothermic reactions which cause the temperatures to increase rapidly unless evolved heat can be dissipated. The pyrolyzate now contains flammable gases, and as such flaming ignition will usually have occurred by the time the surface reaches these temperatures. These gases also carry drops of highly flammable tars appearing as smoke; this favours the production of levoglucosan. This rapid decomposition results in a residual char, which is less easily volatilised than the virgin wood.

Overall, there is strong agreement that temperatures around 300°C represent the onset of rapid pyrolysis and char formation although under certain conditions, e.g. extended heating durations, this can occur at significantly lower temperatures. There is reasonable agreement between authors on the order in which constituent polymers react, their chemical processes, and their char yields, but there is wide scatter in the literature regarding important properties such as decomposition temperatures. These differences may be partially attributed to differences in species, heating rate, and testing methods. It is unlikely that an engineering design will account for these factors however, it is necessary that the designers are aware of the chemical processes that will ultimately determine the thermal and mechanical properties of timber when exposed to fire. These data are however essential in developing detailed models of timber pyrolysis that will form the basis of simplified design calculations. It is likely that in engineering design other parameters will bring additional uncertainty and the relative uncertainties must be assessed to ensure robust designs.

Once pyrolysis is underway, in the presence of oxygen, the products of pyrolysis may then undergo a rapid, exothermic combustion reaction. However, the process of the onset of combustion, i.e. ignition, demands its own discussion. Ignition can lead to either smoldering or flaming combustion. Furthermore, ignition can either be piloted, in which a spark or flame energises the gaseous species, or unpiloted, where the volatiles must achieve the necessary energy for ignition through heating alone.
Critical Heat Flux (Kw/m2)
Piloted__________________________________Unpiloted__________________________________
14.6 (Western Red Cedar)
14.6 (American Whitewood)
15.1 (Freijo)
12.6 (African Mahogany)
15.1 (Aak)
15.1 (Iroko)
26.8 (Western Red Cedar)
25.5 (American Whitewood) Lawson & Simms
26.4 (Freijo)
23.8 (African Mahogany)
27.6 (oak)
Ignition properties will vary with test setup, sample orientation, ambient temperature, and heat transfer mode; auto-ignition temperature can vary by more than 150°C for the same material depending on external factors, however introduction of a pilot can reduce the effects of environmental variables. In addition, density, moisture content, thickness, arrangement of wood pieces, and time are all important for the amount of heat necessary for ignition, with wood pieces above 10 mm thick not being easily ignitable. Spontaneous ignition can be aided by exothermic char oxidation, which causes an increase in surface temperature, which can then raise the gas temperature to that required for ignition. Sustained smouldering ignition has been found to occur around heat fluxes of 5 to 10 kW/m2 typically at surface temperatures around 200°C.

See: https://link.springer.com/article/10.1007/s10694-018-0787-y

* Joule - A joule is equal to the kinetic energy of a kilogram massmoving at the speed of one meter per second (one joule is a kg⋅m2⋅s−2). Alternatively, it is the amount of work done on an object when a force of one newton acts in the direction of the object's motion over a distance of one meter (1 joule equal 1 newton meter or N⋅m).

See:https://www.thoughtco.com/definition-of-joule-604543

** lignin - is a complex oxygen-containing organic polymer that, with cellulose, forms the chief constituent of wood. It is second to cellulose as the most abundant organic material on Earth, though relatively few industrial uses other than as a fuel have been found. A secondary metabolite, lignin is concentrated in the cell walls of wood and makes up 24–35 percent of the oven-dry weight of softwoods and 17–25 percent of hardwoods.

Lignin is a phenolic compound (having a —OH group attached to an aromatic ring) and is a mixture of three complex polymeric compounds. The relative amount of each of the three monomers depends on whether the lignin is from gymnosperms, woody angiosperms, or grasses. The lignin adds compressive strength and stiffness to the plant cell wall and is believed to have played a role in the evolution of terrestrial plants by helping them withstand the compressive forces of gravity. Lignin also waterproofs the cell wall, aiding the upward transport of water in xylem tissues. Finally, lignin has antifungal properties and is often rapidly deposited in response to injury by fungi, protecting the plant body from the diffusion of fungal enzymes and toxins.

See: https://www.britannica.com/science/lignin

*** pyrolysis - is the chemical decomposition of organic (carbon-based) materials through the application of heat. Pyrolysis, which is also the first step in gasification and combustion, occurs in the absence or near absence of oxygen, and it is thus distinct from combustion (burning), which can take place only if sufficient oxygen is present. The rate of pyrolysis increases with temperature.

See: https://www.britannica.com/science/pyrolysis

Concerning your question, "Could this be a reliable source of combustion for the people in the hypothetical scenario if they had very dry wood?"

It appears that in order for wood to reach combustion, whether smoldering or burning with visible flames, there are numerous factors involved: orientation of the sample, the existing ambient temperature, the heat transfer mode, the sample's density, moisture content, thickness, arrangement and the time needed for the particular conditions to result in combustion.

As a former firefighter, I can attest to seeing the suspended highly flammable tar particles in the superheated smoke and outgassing from burning wood and other materials. These suspended tar particles, as well as steam, are what coat the outside of the clear masks on Self Contained Breathing Apparatus (SCBA) when a firefighter enters a smoky (Occupied) Structural Working Fire (OSW).
Hartmann352
 
Last edited:
May 26, 2022
4
0
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Bunny7, here is a bit more on the combustion of wood, heat needed for combustion and the various factors affecting ignition:

The burning behaviour of wood is complex. The processes behind pyrolysis, ignition, combustion, and extinction are generally well understood, with good agreement in the fire science literature over a wide range of experimental conditions for key parameters such as critical heat flux for ignition (12 kW/m2 ± 2 kW/m2) and heat of combustion (17.5 MJ/kg {Mega Joules*/2.2 lbs.) ± 2.5 MJ/kg). These parameters are key for evaluating the risks posed by using timber as a construction material.

Conversely, extinction conditions are less well defined and understood, with critical mass loss rates for extinction varying from 2.5 g/m2s to 5 g/m2s. A detailed meta-analysis of the fire resistance literature has shown that the rate of burning as characterised by charring rate averaged over the full test duration is observed to vary with material properties, in particular density and moisture content which induce a maximum 18% variability over the ranges expected in design. System properties are also shown to be important, with stochastic phenomena such as delamination and encapsulation failure resulting in changes to the charring rate that cannot be easily predicted. Finally, the fire exposure as defined by incident heat flux has by far the largest effect on charring rates over typical heat fluxes experienced in compartment fires. Current fire design guidance for engineered timber products is largely prescriptive, relying on fixed “charring rates” and “zero-strength layers” for structural analyses, and typically prescribing gypsum encapsulation to prevent or delay the involvement of timber in a fire. However, it is clear that the large body of scientific knowledge that exists can be used to explicitly address the fire safety issues that the use of timber introduces. However the application of this science in real buildings is identified as a key knowledge gap which if explored, will enable improved efficiencies and innovations in design.

To allow fire safe use of exposed structural timber elements, the processes driving the pyrolysis, ignition, and subsequent combustion of timber must be understood.

Pyrolysis is the process by which materials decompose upon exposure to heat. This process effects chemical and physical changes, and thus understanding and quantifying the processes is fundamental to the ignition, burning, and extinction behaviour of a material. To burn, polymers must first decompose into smaller molecules that can exist in the gas phase at ambient conditions. To create a self-sustaining reaction, the combustion of these gases must generate sufficient heat to perpetuate the production of volatiles. Upon heating, the constituent natural polymers present in timber will degrade, producing inert and combustible gases (the nature and composition of which will depend on the char yield), liquid tars, a solid carbonaceous char (typically around 20% the density of virgin wood) and inorganic ash. This can occur before dehydration is completed if the heating rate is fast enough, but will be faster after the sample has dried. Under sustained heating conditions, these pyrolysis products can then undergo further pyrolysis themselves. This process is further complicated due to charring and material variability, and the chemical processes occurring are numerous and interdependent. It is also necessary to distinguish between pyrolysis and combustion. Pyrolysis refers to the thermal decomposition of a substance, is endothermic, and can occur without an oxidiser.

In most scenarios relevant to timber construction it can be assumed that pyrolysis occurs over a relatively narrow zone perpendicular to the exposed face of the material. Wood typically undergoes three main stages of pyrolysis due to its relatively low thermal conductivity and density and relatively high specific heat: dehydration and very slow pyrolysis below 200°C, onset of pyrolysis up to 300°C, and rapid pyrolysis above 300°C.

Detailed pyrolysis reviews are available elsewhere, however some of the key aspects relevant to timber construction, in particular moisture transport, charring, and production of flammable gases are discussed below.

Upon heating, and prior to the onset of pyrolysis, free water begins to evaporate as temperatures within a timber member approach 100°C. Some water vapour will migrate deeper into the sample (away from the source of heat), and re-condense. This increases the local moisture content. This then creates three zones—a dry zone closest to the exposed face (in which pyrolysis occurs); a dehydrating zone; and a wet zone, as illustrated in Fig. 1. It is assumed that most of the water vapour, however, leaves from the surface.

At low heat fluxes, dehydration and pyrolysis will take place independently; at higher heat fluxes they will occur simultaneously. Where dehydration and pyrolysis occur simultaneously, moisture slows the temperature rise, typically until reaching 115°C, due to the energy supplied being used for evaporation rather than heating, and cools the pyrolysis zone through convective mass flow of water vapour. Bound water is typically freed later, at temperatures around 240°C of the water vapor, however, leaves from the surface.

View attachment 1979
Chemical and physical processes within a burning timber sample; 𝑞˙′′𝑙,𝑐q˙l,c′′ is the surface heat losses by convection, 𝑞˙″𝑙,𝑟q˙l,r″ is the surface heat losses by radiation, 𝑞˙′′𝑒𝑥𝑡,𝑟q˙ext,r′′ is the external heat flux, 𝑞˙′′𝑟q˙r′′ is in-depth radiation, 𝑞˙′′𝑐𝑜𝑛𝑑q˙cond′′ is conduction into the sample, and 𝑞˙′′𝑐q˙c′′ is convective heat transfer through cracks in the sample.

Mass loss due to pyrolysis*** occurs slowly at temperatures below 200°C, with pyrolyzate consisting mostly of non-combustible volatiles such as carbon dioxide, formic acids, and acetic acids. However prolonged heating at low temperatures can convert hemicellulose (and lignin) into a carbonaceous char at temperatures as low as 95°C or 120°C leaving cellulose largely unreacted. Cellulosic materials have no fluid state, but can soften before breaking down into vapours. In so doing, they may undergo a glass transition, altering their structure and becoming softer and more rubbery. For lignin, this occurs at temperatures around 55°C to 170°C; permanent reductions in strength of timber have been observed at temperatures as low as 65°C. When the temperature exceeds 200°C, the pyrolysate remains mainly non-combustible, however visible discolouration will begin or become more intense, with prolonged exposure to these temperatures causing slow charring.

Uncharred wood remains at moderate temperatures even in long fires due to the high heat losses from the char layer; at a depth of 6 mm below the charline, the temperature is typically reduced to around 180°C; with a total thickness of approximately 35 mm below the char layer being heated. The resulting temperature profile below the char line can be expressed as an exponential or power term for thermally thick wood, or alternatively as a quadratic function. The main pyrolysis reactions then typically begin from 225°C to 275°C.

Hemicellulose is typically the first component of wood to undergo thermal decomposition. The temperatures at which this reaction starts are reported over a wide range from 120°C to 180°C, 200°C to 260°C, 220°C to 315°C, or 200°C to 300°C. This temperature range is dependent on the heating rate, species, density, or moisture content.

Cellulose is typically the next compound to decompose, with decomposition temperatures (which likely have the same dependencies as those for hemicellulose) quoted as 240°C to 350°C, 250°C to 350°C, 315°C to 400°C, 280°C to 400°C or 300°C to 350°C. Cellulose may decompose via two main processes: the first by breaking a link in the carbon ring, cross-linking to produce char alongside carbon monoxide, carbon dioxide, and H2O; the second is chain scission when a link in the polymer chain is broken and levoglucosan molecules can break away, typically at temperatures around 250°C to 300°C. Levoglucosan is a tar which will break down further into combustible gases, or alternatively repolymerise to form char. Low heating rates tend to favour char formation alongside largely non-combustible vapours, thus releasing energy. High heating rates favour the production of levoglucosan, yielding flammable vapours and little or no char, thus consuming energy.

Lignin** usually undergoes pyrolysis at temperatures quoted as 110°C to 400°C, 280°C to 500°C or 225°C to 450°C. Schaffer found that lignin began melting around 160°C, followed by re-hardening from 160°C to 210°C, with only 10% of its weight loss having occurred by 280°C. Lignin produces aromatic products on pyrolysis, and yields more char than cellulose—upon heating to 400°C to 450°C, approximately half of lignin remains as char, contributing significantly to the char yield. Since softwoods have higher lignin contents than hardwoods, they consequently give higher char yields ; this has implications for the burning rates of softwoods compared to hardwoods.

At temperatures between 300°C and 500°C, pyrolysis rates increase rapidly and are accompanied by additional exothermic reactions which cause the temperatures to increase rapidly unless evolved heat can be dissipated. The pyrolyzate now contains flammable gases, and as such flaming ignition will usually have occurred by the time the surface reaches these temperatures. These gases also carry drops of highly flammable tars appearing as smoke; this favours the production of levoglucosan. This rapid decomposition results in a residual char, which is less easily volatilised than the virgin wood.

Overall, there is strong agreement that temperatures around 300°C represent the onset of rapid pyrolysis and char formation although under certain conditions, e.g. extended heating durations, this can occur at significantly lower temperatures. There is reasonable agreement between authors on the order in which constituent polymers react, their chemical processes, and their char yields, but there is wide scatter in the literature regarding important properties such as decomposition temperatures. These differences may be partially attributed to differences in species, heating rate, and testing methods. It is unlikely that an engineering design will account for these factors however, it is necessary that the designers are aware of the chemical processes that will ultimately determine the thermal and mechanical properties of timber when exposed to fire. These data are however essential in developing detailed models of timber pyrolysis that will form the basis of simplified design calculations. It is likely that in engineering design other parameters will bring additional uncertainty and the relative uncertainties must be assessed to ensure robust designs.

Once pyrolysis is underway, in the presence of oxygen, the products of pyrolysis may then undergo a rapid, exothermic combustion reaction. However, the process of the onset of combustion, i.e. ignition, demands its own discussion. Ignition can lead to either smoldering or flaming combustion. Furthermore, ignition can either be piloted, in which a spark or flame energises the gaseous species, or unpiloted, where the volatiles must achieve the necessary energy for ignition through heating alone.
Critical Heat Flux (Kw/m2)
Piloted__________________________________Unpiloted__________________________________
14.6 (Western Red Cedar)
14.6 (American Whitewood)
15.1 (Freijo)
12.6 (African Mahogany)
15.1 (Aak)
15.1 (Iroko)
26.8 (Western Red Cedar)
25.5 (American Whitewood) Lawson & Simms
26.4 (Freijo)
23.8 (African Mahogany)
27.6 (oak)
Ignition properties will vary with test setup, sample orientation, ambient temperature, and heat transfer mode; auto-ignition temperature can vary by more than 150°C for the same material depending on external factors, however introduction of a pilot can reduce the effects of environmental variables. In addition, density, moisture content, thickness, arrangement of wood pieces, and time are all important for the amount of heat necessary for ignition, with wood pieces above 10 mm thick not being easily ignitable. Spontaneous ignition can be aided by exothermic char oxidation, which causes an increase in surface temperature, which can then raise the gas temperature to that required for ignition. Sustained smouldering ignition has been found to occur around heat fluxes of 5 to 10 kW/m2 typically at surface temperatures around 200°C.

See: https://link.springer.com/article/10.1007/s10694-018-0787-y

* Joule - A joule is equal to the kinetic energy of a kilogram massmoving at the speed of one meter per second (one joule is a kg⋅m2⋅s−2). Alternatively, it is the amount of work done on an object when a force of one newton acts in the direction of the object's motion over a distance of one meter (1 joule equal 1 newton meter or N⋅m).

See:https://www.thoughtco.com/definition-of-joule-604543

** lignin - is a complex oxygen-containing organic polymer that, with cellulose, forms the chief constituent of wood. It is second to cellulose as the most abundant organic material on Earth, though relatively few industrial uses other than as a fuel have been found. A secondary metabolite, lignin is concentrated in the cell walls of wood and makes up 24–35 percent of the oven-dry weight of softwoods and 17–25 percent of hardwoods.

Lignin is a phenolic compound (having a —OH group attached to an aromatic ring) and is a mixture of three complex polymeric compounds. The relative amount of each of the three monomers depends on whether the lignin is from gymnosperms, woody angiosperms, or grasses. The lignin adds compressive strength and stiffness to the plant cell wall and is believed to have played a role in the evolution of terrestrial plants by helping them withstand the compressive forces of gravity. Lignin also waterproofs the cell wall, aiding the upward transport of water in xylem tissues. Finally, lignin has antifungal properties and is often rapidly deposited in response to injury by fungi, protecting the plant body from the diffusion of fungal enzymes and toxins.

See: https://www.britannica.com/science/lignin

*** pyrolysis - is the chemical decomposition of organic (carbon-based) materials through the application of heat. Pyrolysis, which is also the first step in gasification and combustion, occurs in the absence or near absence of oxygen, and it is thus distinct from combustion (burning), which can take place only if sufficient oxygen is present. The rate of pyrolysis increases with temperature.

See: https://www.britannica.com/science/pyrolysis

Concerning your question, "Could this be a reliable source of combustion for the people in the hypothetical scenario if they had very dry wood?"

It appears that in order for wood to reach combustion, whether smoldering or burning with visible flames, there are numerous factors involved: orientation of the sample, the existing ambient temperature, the heat transfer mode, the sample's density, moisture content, thickness, arrangement and the time needed for the particular conditions to result in combustion.

As a former firefighter, I can attest to seeing the suspended highly flammable tar particles in the superheated smoke and outgassing from burning wood and other materials. These suspended tar particles, as well as steam, are what coat the outside of the clear masks on Self Contained Breathing Apparatus (SCBA) when a firefighter enters a smoky (Occupied) Structural Working Fire (OSW).
Hartmann352
Thank you very much Hartmann352! This all helps me very much. In many of the studies they do emphasize that the arrangement of the wood was important. You have most certainly answered a lot of questions for me. The only question I have remaining is if you were to blow or pump air on heated wood in a low oxygen environment, would it reignite despite the low oxygen environment surrounding it? I’ve heard that when firefighter seal a room with a fire to deplete it of oxygen, that if you were to resupply oxygen that the fire could start again. So I was wondering if the human breath is enough to maintain a low grade fire. Thanks once again.
 
Jan 27, 2020
415
110
1,880
First off, firefighters don't seal rooms to put out fires. When you reopen that room where the fire has burned down to embers due to oxygen deprivation, you'd probably have a flashover.

A flashover or “full-room involvement” is the leading cause of firefighter injuries and deaths. However, sometimes we forget to train on the basics of fire behavior, specifically, remembering when, where, why, and how a flashover will occur.

I’ve heard firefighters state that a flashover is “the rapid fire development followed by full-room involvement and finally thermal collapse,” and “the sudden full-room involvement in flame.” These definitions are good to know. Remember that flashover is a “heat-driven phenomenon.” It’s that simple. If the phenomenon is heat driven, it must be your primary and only concern that you constantly monitor and recognize conditions.

Although the obvious signs of a flashover are superheated, uninfluenced gases and heat, there are many other factors that will significantly increase your chance of being caught in one. One of the most prevalent factors that influences impending flashover is building construction with illegal modifications. These include, most notably, concealed spaces, energy efficient / hurricane windows, room size and/or ceiling height, and illegal partitions inside occupancies. You must be aware of these components of building construction. Some of these factors might be known prior to entering the building. However, you must discover others if heat levels continue to rise regardless if cooling or venting has taken place.

Other factors also influence a flashover way before the fire starts. For example, cell phones have aided in ensuring we are arriving faster to the fire scene than we did 15 years ago. Almost everyone has and carries a cell phone and knows how to dial 911 when there is an emergency, specifically a fire. We are being made aware of fires faster, especially those in their incipient phases. In return, we are getting on the trucks and to the scene faster than those who did this job 10 to 15 years ago. That being said, you must not allow your adrenalin to take over your ability to perform your job correctly on arrival.

When the bells go off, your natural instincts are to race to the truck, rush getting dressed, and rush to the scene. These factors, once added up, create the potential to make a bad decision or miss a key sign the building may be telling you because you rushed. Unfortunately, the lack of live fire training coupled with fewer fires has altered the amount of experience the newer generation firefighter receives. The fire service becomes complacent because it is not getting the repetition of fires and experience. However, do not use this concern as an excuse for why you are losing firefighters each year to flashovers.

Today’s modern turn out gear allows you to push deeper and farther than ever before. This amazing technology is ultimately a double-edged sword; it acts as a protecting bubble, providing a false sense of security to the firefighter wearing the gear. The vapor, water, and thermal barriers of most bunker gear do protect you but only for a limited time when things go wrong.

According to IFTSA Firefighter Principles & Tactics, the bunker gear of today will protect an individual for no more than 15 seconds if succumbed to flashover conditions. Yet, we tend to ignore this fact. More often than not, we find ourselves deeper inside buildings and increasing our chances of being involved in fatal flashover conditions.

Today, fires are much more dangerous than 30 to 40 years ago. The synthetics, plastics, and other “methy-ethel bad stuff” being used to build the furniture and products that typically combust are burning hotter and faster, thereby increasing the intensity and the speed for which the fire travels through the stages.
Now that you know the definition of a flashover, what causes it, and what are the factors that influence it? Start by developing a plan to avoid it. To be successful, you must not be blind to the signs of an impending flashover. To me, being blind (or stubborn) typically involves your will to “keep pushing” until you find the fire and ultimately extinguish it. This is not your fault; it’s simply the fabric from which you are cut when you swore the oath to becoming a firefighter. We all want to be the first one on scene and the first one to stretch the line into the fire and extinguish it. It’s these “bragging rights” that define us as firefighters. We want to succeed, and succeeding means getting to the fire and putting it out.

Early on in my career, I always believed fire was the danger, and that fire would be the single most dangerous element I faced. I have since changed this thought process. I have learned over time and through experience that the true danger is not being able to find the fire when inside a structure because of zero visibility. I’ve learned how important reading smoke and identifying conditions are prior to entering a building. These conditions will forecast what my crews and I will face once inside.

An example of why zero visibility and high heat without visible fire is dangerous would be when you and your crew advance toward where you believe is the location of the fire. If heat levels are doubling and tripling for every 10 to 15 feet, advance the hoseline, and you cannot find the fire while you are being pushed to the floor because of the incredible amount of heat. Alarm bells should be ringing that a flashover is imminent. If not corrected quickly, this is a recipe for disaster and could result in serious injury or a line-of-duty death.

Years ago, many of us were taught to never apply water to smoke. Today, we have completely changed our tune on that outdated tactic. The above scenario clearly dictates that the situation has to be cooled, specifically, overhead. If water does not “rain” down on you after hitting the ceiling (or what is above), then you know the temperature above is well above 1,000 F°.

So, how do you limit and survive a flashover? First, you must remember that not every fire progresses to a flashover, but every fire can. Second, you must not get tunnel vision and miss the signs of flashover. Communication between interior crews is paramount, and you must always have an exit strategy. Regardless, if you are inside the building 10 feet or 100 feet, have a plan and consistently communicate that plan to each other. Search and attack areas must be limited, and interior crews have to be cognizant of how far they have advanced. Use a search rope and be part of everyday search tactics, especially in large occupancies or commercial structures. Finally, always be prepared to “dive or die.” Surviving a flashover means you have approximately 15 seconds of bunker gear protection. Realistically, used gear that has been exposed to heat and elements offers closer to six to eight seconds once you have realized the room is about to flash and your self-contained breathing apparatus (SCBA) mask is still intact.

Limiting your chances for encountering flashover relies on three important factors. First, know and sense the flashover signs and notice the increased heat levels with a thermal imaging camera or your senses. Second, understand the heat-driven phenomenon and the fact that this heat has to be cooled, and quickly. Third, if cooled, the fire needs to be ventilated. Finally, once you have attempted ventilation or cooling and conditions are not changing, put your pride aside and vacate the building. Vacating does not mean you are “going defensive”; it just means you are figuring out what is going on, potentially outside of your view and control. “Regrouping” is a better term that could reveal where the fire is and where another tactic is needed to successfully and safely put out the fire. Your job is to confine and extinguish. If you cannot make this happen and conditions deteriorate, take a step back, reevaluate your tactics, and possibly go another route.
Remember, anyone inside a fire building not wearing a SCBA is most likely dead, so exercise an aggressive risk vs. reward assessment. A successful tour at the firehouse is when everyone goes home to his family. To accomplish that, be aggressive, chose the right nozzle and line for the job, aggressively sweep the floor and ceiling during advancement, perform vigorous nozzle work, and communicate so the attack and ventilation is coordinated. When you have constantly monitored smoke and heat conditions, aggressively applied the right amount of water, and reduced the chances of flashover, your training and experiences have kept you out of harm’s way for yet another tour. This will keep you alive and give you one more chance to leave the firehouse and go home safe!

See: https://www.fireengineering.com/firefighting/understanding-and-avoiding-a-flashover/

Remember, on Jan. 27, 1967, a fire erupted inside the Apollo command module during a preflight rehearsal test, killing three astronauts who were trapped inside. Astronauts Virgil Grissom, Edward White and Roger Chaffee lost their lives when a fire swept through the command module, or CM.

National Aeronautics and Space Administration (NASA) had launched 16 manned space flights in its Mercury and Gemini programs without a single casualty. “Success had become almost routine for us,” NASA flight director Gene Kranz wrote in his book “Failure Is Not an Option.” “The country had gotten complacent.” Perhaps NASA had gotten complacent as well. In spite of orders from Joseph Shea, Apollo Program Manager, the flammable materials were never removed from the Apollo 1 command module.

With 25 days left before the scheduled launch, the crew of Apollo 1 climbed out of a NASA van into sparkling Florida sunshine on January 27, 1967, and ascended the tower of launch pad 34 for a routine simulated launch test. Clad in their spacesuits and carrying their portable air conditioning packs like office workers toting briefcases, the astronauts crossed the 218-foot-high catwalk with vistas of the blue Atlantic waters washing up on the white beaches of Cape Canaveral before climbing inside their command module perched atop a massive booster rocket.

The “plugs-out” test, during which the module was disconnected from the launch pad’s electrical systems and operating under its own power, was classified as non-hazardous since the rocket was unfueled. To make the countdown rehearsal as realistic as possible, the launch pad team sealed the hatch after the astronauts were strapped into their seats inside a cabin pressurized with pure oxygen.

Throughout the afternoon, minor glitches and communication issues between the spacecraft and Mission Control in Houston caused repeated delays. Hours behind schedule, early evening darkness settled around the launch pad as the simulated countdown reached a hold with 10 minutes left as attempts continued to resolve the radio issues. “How are we gonna get to the moon if we can’t talk between two or three buildings?” quipped a frustrated Grissom at 6:30 p.m.

Less than a minute later, the engineers watching the capsule cabin on a closed-circuit television screen were startled by a flash. A spark that likely came from faulty electrical wiring behind a panel door below Grissom’s feet suddenly ignited in the capsule. Fed by the cabin’s pure oxygen, the spark took only seconds to morph into an inferno that tore through the flammable nylon netting and Velcro surrounding the astronauts.

“Hey! We’ve got a fire in the cockpit!” yelled one of the astronauts. Horrified engineers watched on their screens as smoke filled the cabin while White desperately attempted to release the cumbersome hatch. “We have a bad fire! We’re burning up!” came another screaming transmission from the cockpit.

Then silence.

Pad safety workers grabbed extinguishers and rushed to the capsule, but the dense smoke reduced visibility to nearly zero. Even rescuers wearing smoke masks were overcome by toxic fumes, and the tremendous heat burned through their gloves.

It took more than five minutes for the pad crew to open the complicated latch system on the hatch. By that point, it was far too late. The astronauts had virtually no time to unstrap themselves from their seats, let alone escape the flash fire. Burning at hotter than 1,000 degrees Fahrenheit, the blaze melted the astronauts’ space suits and oxygen tubes. The crew likely lost consciousness and died from asphyxiation from inhaling toxic gases. The process of removing the men from the charred capsule couldn’t begin until six hours after the fire, and it took 90 minutes to extricate their bodies, which were fused to the nylon of the cabin interior.

“We did not do our job! We were rolling the dice, hoping that things would come together by launch day, when in our hearts we knew it would take a miracle,” an emotional Kranz told his flight control team three days after the tragedy. “We were too ‘gung-ho’ about the schedule, and we blocked out all of the problems we saw each day in our work. Every element of the program was in trouble, and so were we.”

NASA indeed pressed ahead with the Apollo program, but more than 20 months elapsed before American astronauts returned to the skies. During that time, NASA made thousands of changes to the Apollo spacecraft, including redesigning the hatch, altering the cabin atmosphere to include nitrogen and replacing flammable materials from the interior.

“It was perhaps the defining moment in our race to get to the moon,” Kranz wrote of the fire aboard Apollo 1. “The ultimate success of Apollo was made possible by the sacrifices of Grissom, White and Chaffee. The accident profoundly affected everyone in the program. There was an unspoken promise on everyone’s part to the three astronauts that their deaths would not be in vain.”

Out of the ashes of the Apollo 1 tragedy came crucial safety and performance improvements that allowed NASA to fulfill Kennedy’s pledge in July 1969 by landing Neil Armstrong and Edwin “Buzz” Aldrin on the moon and returning them safely to Earth. Before departing the lunar surface, the Apollo 11 astronauts left behind a reminder of their fallen colleagues—a commemorative medallion bearing the names of Grissom, White and Chaffee.

See: https://www.history.com/news/remembering-the-apollo-1-tragedy

As far human breath rekindling a fire in low oxygen circumstances, that is sort of a non-starter as far as I can see.

If it is in an enclosed space, you risk a flash over by opening a means of ingress into the oxygen deprived space.

In a low oxygen open area, you must remember that humans an only exist within certain oxygen parameters, ca. 19-23% oxygen.

As a result, I'm not sure in what scenario that might require human intervention where breath would be required to rekindle a fire. In a fireplace you use bellows to force atmospheric air into a fire quelled to embers to reignite it. Blowing into a fireplace is inherently dangerous including burns, inhaling soot, making a mess.

Note the above, a pure oxygen atmosphere is highly dangerous while a low oxygen atmosphere is equally hazardous.
Hartmann352
 
May 26, 2022
4
0
10
First off, firefighters don't seal rooms to put out fires. When you reopen that room where the fire has burned down to embers due to oxygen deprivation, you'd probably have a flashover.

A flashover or “full-room involvement” is the leading cause of firefighter injuries and deaths. However, sometimes we forget to train on the basics of fire behavior, specifically, remembering when, where, why, and how a flashover will occur.

I’ve heard firefighters state that a flashover is “the rapid fire development followed by full-room involvement and finally thermal collapse,” and “the sudden full-room involvement in flame.” These definitions are good to know. Remember that flashover is a “heat-driven phenomenon.” It’s that simple. If the phenomenon is heat driven, it must be your primary and only concern that you constantly monitor and recognize conditions.

Although the obvious signs of a flashover are superheated, uninfluenced gases and heat, there are many other factors that will significantly increase your chance of being caught in one. One of the most prevalent factors that influences impending flashover is building construction with illegal modifications. These include, most notably, concealed spaces, energy efficient / hurricane windows, room size and/or ceiling height, and illegal partitions inside occupancies. You must be aware of these components of building construction. Some of these factors might be known prior to entering the building. However, you must discover others if heat levels continue to rise regardless if cooling or venting has taken place.

Other factors also influence a flashover way before the fire starts. For example, cell phones have aided in ensuring we are arriving faster to the fire scene than we did 15 years ago. Almost everyone has and carries a cell phone and knows how to dial 911 when there is an emergency, specifically a fire. We are being made aware of fires faster, especially those in their incipient phases. In return, we are getting on the trucks and to the scene faster than those who did this job 10 to 15 years ago. That being said, you must not allow your adrenalin to take over your ability to perform your job correctly on arrival.

When the bells go off, your natural instincts are to race to the truck, rush getting dressed, and rush to the scene. These factors, once added up, create the potential to make a bad decision or miss a key sign the building may be telling you because you rushed. Unfortunately, the lack of live fire training coupled with fewer fires has altered the amount of experience the newer generation firefighter receives. The fire service becomes complacent because it is not getting the repetition of fires and experience. However, do not use this concern as an excuse for why you are losing firefighters each year to flashovers.

Today’s modern turn out gear allows you to push deeper and farther than ever before. This amazing technology is ultimately a double-edged sword; it acts as a protecting bubble, providing a false sense of security to the firefighter wearing the gear. The vapor, water, and thermal barriers of most bunker gear do protect you but only for a limited time when things go wrong.

According to IFTSA Firefighter Principles & Tactics, the bunker gear of today will protect an individual for no more than 15 seconds if succumbed to flashover conditions. Yet, we tend to ignore this fact. More often than not, we find ourselves deeper inside buildings and increasing our chances of being involved in fatal flashover conditions.

Today, fires are much more dangerous than 30 to 40 years ago. The synthetics, plastics, and other “methy-ethel bad stuff” being used to build the furniture and products that typically combust are burning hotter and faster, thereby increasing the intensity and the speed for which the fire travels through the stages.
Now that you know the definition of a flashover, what causes it, and what are the factors that influence it? Start by developing a plan to avoid it. To be successful, you must not be blind to the signs of an impending flashover. To me, being blind (or stubborn) typically involves your will to “keep pushing” until you find the fire and ultimately extinguish it. This is not your fault; it’s simply the fabric from which you are cut when you swore the oath to becoming a firefighter. We all want to be the first one on scene and the first one to stretch the line into the fire and extinguish it. It’s these “bragging rights” that define us as firefighters. We want to succeed, and succeeding means getting to the fire and putting it out.

Early on in my career, I always believed fire was the danger, and that fire would be the single most dangerous element I faced. I have since changed this thought process. I have learned over time and through experience that the true danger is not being able to find the fire when inside a structure because of zero visibility. I’ve learned how important reading smoke and identifying conditions are prior to entering a building. These conditions will forecast what my crews and I will face once inside.

An example of why zero visibility and high heat without visible fire is dangerous would be when you and your crew advance toward where you believe is the location of the fire. If heat levels are doubling and tripling for every 10 to 15 feet, advance the hoseline, and you cannot find the fire while you are being pushed to the floor because of the incredible amount of heat. Alarm bells should be ringing that a flashover is imminent. If not corrected quickly, this is a recipe for disaster and could result in serious injury or a line-of-duty death.

Years ago, many of us were taught to never apply water to smoke. Today, we have completely changed our tune on that outdated tactic. The above scenario clearly dictates that the situation has to be cooled, specifically, overhead. If water does not “rain” down on you after hitting the ceiling (or what is above), then you know the temperature above is well above 1,000 F°.

So, how do you limit and survive a flashover? First, you must remember that not every fire progresses to a flashover, but every fire can. Second, you must not get tunnel vision and miss the signs of flashover. Communication between interior crews is paramount, and you must always have an exit strategy. Regardless, if you are inside the building 10 feet or 100 feet, have a plan and consistently communicate that plan to each other. Search and attack areas must be limited, and interior crews have to be cognizant of how far they have advanced. Use a search rope and be part of everyday search tactics, especially in large occupancies or commercial structures. Finally, always be prepared to “dive or die.” Surviving a flashover means you have approximately 15 seconds of bunker gear protection. Realistically, used gear that has been exposed to heat and elements offers closer to six to eight seconds once you have realized the room is about to flash and your self-contained breathing apparatus (SCBA) mask is still intact.

Limiting your chances for encountering flashover relies on three important factors. First, know and sense the flashover signs and notice the increased heat levels with a thermal imaging camera or your senses. Second, understand the heat-driven phenomenon and the fact that this heat has to be cooled, and quickly. Third, if cooled, the fire needs to be ventilated. Finally, once you have attempted ventilation or cooling and conditions are not changing, put your pride aside and vacate the building. Vacating does not mean you are “going defensive”; it just means you are figuring out what is going on, potentially outside of your view and control. “Regrouping” is a better term that could reveal where the fire is and where another tactic is needed to successfully and safely put out the fire. Your job is to confine and extinguish. If you cannot make this happen and conditions deteriorate, take a step back, reevaluate your tactics, and possibly go another route.
Remember, anyone inside a fire building not wearing a SCBA is most likely dead, so exercise an aggressive risk vs. reward assessment. A successful tour at the firehouse is when everyone goes home to his family. To accomplish that, be aggressive, chose the right nozzle and line for the job, aggressively sweep the floor and ceiling during advancement, perform vigorous nozzle work, and communicate so the attack and ventilation is coordinated. When you have constantly monitored smoke and heat conditions, aggressively applied the right amount of water, and reduced the chances of flashover, your training and experiences have kept you out of harm’s way for yet another tour. This will keep you alive and give you one more chance to leave the firehouse and go home safe!

See: https://www.fireengineering.com/firefighting/understanding-and-avoiding-a-flashover/

Remember, on Jan. 27, 1967, a fire erupted inside the Apollo command module during a preflight rehearsal test, killing three astronauts who were trapped inside. Astronauts Virgil Grissom, Edward White and Roger Chaffee lost their lives when a fire swept through the command module, or CM.

National Aeronautics and Space Administration (NASA) had launched 16 manned space flights in its Mercury and Gemini programs without a single casualty. “Success had become almost routine for us,” NASA flight director Gene Kranz wrote in his book “Failure Is Not an Option.” “The country had gotten complacent.” Perhaps NASA had gotten complacent as well. In spite of orders from Joseph Shea, Apollo Program Manager, the flammable materials were never removed from the Apollo 1 command module.

With 25 days left before the scheduled launch, the crew of Apollo 1 climbed out of a NASA van into sparkling Florida sunshine on January 27, 1967, and ascended the tower of launch pad 34 for a routine simulated launch test. Clad in their spacesuits and carrying their portable air conditioning packs like office workers toting briefcases, the astronauts crossed the 218-foot-high catwalk with vistas of the blue Atlantic waters washing up on the white beaches of Cape Canaveral before climbing inside their command module perched atop a massive booster rocket.

The “plugs-out” test, during which the module was disconnected from the launch pad’s electrical systems and operating under its own power, was classified as non-hazardous since the rocket was unfueled. To make the countdown rehearsal as realistic as possible, the launch pad team sealed the hatch after the astronauts were strapped into their seats inside a cabin pressurized with pure oxygen.

Throughout the afternoon, minor glitches and communication issues between the spacecraft and Mission Control in Houston caused repeated delays. Hours behind schedule, early evening darkness settled around the launch pad as the simulated countdown reached a hold with 10 minutes left as attempts continued to resolve the radio issues. “How are we gonna get to the moon if we can’t talk between two or three buildings?” quipped a frustrated Grissom at 6:30 p.m.

Less than a minute later, the engineers watching the capsule cabin on a closed-circuit television screen were startled by a flash. A spark that likely came from faulty electrical wiring behind a panel door below Grissom’s feet suddenly ignited in the capsule. Fed by the cabin’s pure oxygen, the spark took only seconds to morph into an inferno that tore through the flammable nylon netting and Velcro surrounding the astronauts.

“Hey! We’ve got a fire in the cockpit!” yelled one of the astronauts. Horrified engineers watched on their screens as smoke filled the cabin while White desperately attempted to release the cumbersome hatch. “We have a bad fire! We’re burning up!” came another screaming transmission from the cockpit.

Then silence.

Pad safety workers grabbed extinguishers and rushed to the capsule, but the dense smoke reduced visibility to nearly zero. Even rescuers wearing smoke masks were overcome by toxic fumes, and the tremendous heat burned through their gloves.

It took more than five minutes for the pad crew to open the complicated latch system on the hatch. By that point, it was far too late. The astronauts had virtually no time to unstrap themselves from their seats, let alone escape the flash fire. Burning at hotter than 1,000 degrees Fahrenheit, the blaze melted the astronauts’ space suits and oxygen tubes. The crew likely lost consciousness and died from asphyxiation from inhaling toxic gases. The process of removing the men from the charred capsule couldn’t begin until six hours after the fire, and it took 90 minutes to extricate their bodies, which were fused to the nylon of the cabin interior.

“We did not do our job! We were rolling the dice, hoping that things would come together by launch day, when in our hearts we knew it would take a miracle,” an emotional Kranz told his flight control team three days after the tragedy. “We were too ‘gung-ho’ about the schedule, and we blocked out all of the problems we saw each day in our work. Every element of the program was in trouble, and so were we.”

NASA indeed pressed ahead with the Apollo program, but more than 20 months elapsed before American astronauts returned to the skies. During that time, NASA made thousands of changes to the Apollo spacecraft, including redesigning the hatch, altering the cabin atmosphere to include nitrogen and replacing flammable materials from the interior.

“It was perhaps the defining moment in our race to get to the moon,” Kranz wrote of the fire aboard Apollo 1. “The ultimate success of Apollo was made possible by the sacrifices of Grissom, White and Chaffee. The accident profoundly affected everyone in the program. There was an unspoken promise on everyone’s part to the three astronauts that their deaths would not be in vain.”

Out of the ashes of the Apollo 1 tragedy came crucial safety and performance improvements that allowed NASA to fulfill Kennedy’s pledge in July 1969 by landing Neil Armstrong and Edwin “Buzz” Aldrin on the moon and returning them safely to Earth. Before departing the lunar surface, the Apollo 11 astronauts left behind a reminder of their fallen colleagues—a commemorative medallion bearing the names of Grissom, White and Chaffee.

See: https://www.history.com/news/remembering-the-apollo-1-tragedy

As far human breath rekindling a fire in low oxygen circumstances, that is sort of a non-starter as far as I can see.

If it is in an enclosed space, you risk a flash over by opening a means of ingress into the oxygen deprived space.

In a low oxygen open area, you must remember that humans an only exist within certain oxygen parameters, ca. 19-23% oxygen.

As a result, I'm not sure in what scenario that might require human intervention where breath would be required to rekindle a fire. In a fireplace you use bellows to force atmospheric air into a fire quelled to embers to reignite it. Blowing into a fireplace is inherently dangerous including burns, inhaling soot, making a mess.

Note the above, a pure oxygen atmosphere is highly dangerous while a low oxygen atmosphere is equally hazardous.
Hartmann352
Thank you so much! You’ve helped me very much!
 
Jan 27, 2020
415
110
1,880
Bunny7 -

Glad I could assist you in your endeavors.

I began reading Fire Engineering when I was 12 and I was a firefighter at FS 37 in Broome County, NY, Fire and EMS Services when I was a senior in HS at 17.
 

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