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Re: Pyrophoric Carbonization
Posted by: dcarpenter (IP Logged)
Date: January 22, 2018 06:59PM
The phenomenon is related to the self-heating of wood.
It is well known that wood can self-heat. It is also well known that it can self-heat to the critical condition of thermal runaway.
The theory of self-heating to thermal runaway provides that for a specific pile configuration and size, there is a critical exposure temperature to cause self-heating to thermal runaway. Similarly, for a specific exposure temperature, there is a critical pile size. That is why there is no single ignition temperature. This theory also holds for the ignition of fuels at higher temperature where the induction time (or delay time) approaches zero with increasing temperature.
All of the research to-date shows no change in the thermal properties of wood with "long-term" thermal exposure at temperature nominally below 200 degrees Celsius.
We also have antidotal evidence that this phenomenon may happen at exposure temperatures below 200 degrees Celsius. So the traditional hypothesis has been disproven for a range of exposure temperatures. Thus, if this potentially observed phenomenon of "low temperature" ignition can happen is does not seem to be explained by the conventional hypothesis of a change in chemical composition that results in a change in thermal properties that "lowers the ignition temperature. Thus, a new hypothesis needs to be formulated.
Based on my review of the significant research that is related to this phenomenon, there are some relevent conditions that need to be taken into account to formulate a new hypothesis. The first is that the theory and experiments, and testing are based on a constant exposure temperature. That is, there is no feedback between the heat losses at the surface of the pile, through convection, and the internal pile surfaces where the exothermic oxidation reactions are occurring. Once this thermal energy is transferred to the surface of the pile it is lost to an infinite heat sink. That is, the surrounding and exposing air is not being heated with an increase in temperature as a result.
Self-heating is a "related rates problem." The rate of heat generated vs. the rate of heat loss through the pile to the surface of the pile. If the rate of heat generated within the pile is greater than the rate of heat loss from the pile. The internal temperature of the pile will increase. The opposite is also true. If the rates are the same, the internal temperature of the pile will remain constant.
If one were to insulate the surface of the pile, the theory of self-heating would produce a reduction in the critical temperature required for thermal runaway. That is one way to "lower the ignition temperature" of the wood under a specific set of conditions. But is this the only way?
Think of enclosure or void space filled with a finite amount of air with very little leakage paths. For example, a stud space in a wall or a platform under a water heater. If the internal surfaces are made of wood, then, under the right conditions, the wood could self-heat. The heat generated at the surface of the wood would be lost to the air inside the void space. The amount of air is finite and is not transported across the boundaries of the enclosure or void space. Thus, as thermal energy is added to the air, the temperature of the air increases. That is, the exposing temperature that is providing thermal energy for the oxidation reaction is not constant, but is increasing with time. With the traditional hypothesis, this thermal energy is lost to the surroundings. Think wood box versus an 2 by 4 in open air.
While the theory of self-heating to thermal runaway provides for this specific boundary condition, I do not believe this boundary condition has been tested. an experimental series to test this hypothesis is not trivial, but it can be done. Higher temperatures will take the shortest time to thermal runaway, if it can happen. Lower temperatures will take more time and may show that other hypotheses may be required to explain the phenomenon over a relatively large range of exposure temperatures.
Bottom line. There are a specific set of boundary conditions that have been disproven. So, if your scenario is substantially similar to those, you can reach a scientifically reliable determination based on the available evidence. In other scenarios, there is evidence, based on the theory of self-heating to thermal runaway that it cannot be disproven. There is no current model that will allow prediction of the critical exposure temperature for a specific pile size.
That is the best that can be done to-date.
Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com
It is well known that wood can self-heat. It is also well known that it can self-heat to the critical condition of thermal runaway.
The theory of self-heating to thermal runaway provides that for a specific pile configuration and size, there is a critical exposure temperature to cause self-heating to thermal runaway. Similarly, for a specific exposure temperature, there is a critical pile size. That is why there is no single ignition temperature. This theory also holds for the ignition of fuels at higher temperature where the induction time (or delay time) approaches zero with increasing temperature.
All of the research to-date shows no change in the thermal properties of wood with "long-term" thermal exposure at temperature nominally below 200 degrees Celsius.
We also have antidotal evidence that this phenomenon may happen at exposure temperatures below 200 degrees Celsius. So the traditional hypothesis has been disproven for a range of exposure temperatures. Thus, if this potentially observed phenomenon of "low temperature" ignition can happen is does not seem to be explained by the conventional hypothesis of a change in chemical composition that results in a change in thermal properties that "lowers the ignition temperature. Thus, a new hypothesis needs to be formulated.
Based on my review of the significant research that is related to this phenomenon, there are some relevent conditions that need to be taken into account to formulate a new hypothesis. The first is that the theory and experiments, and testing are based on a constant exposure temperature. That is, there is no feedback between the heat losses at the surface of the pile, through convection, and the internal pile surfaces where the exothermic oxidation reactions are occurring. Once this thermal energy is transferred to the surface of the pile it is lost to an infinite heat sink. That is, the surrounding and exposing air is not being heated with an increase in temperature as a result.
Self-heating is a "related rates problem." The rate of heat generated vs. the rate of heat loss through the pile to the surface of the pile. If the rate of heat generated within the pile is greater than the rate of heat loss from the pile. The internal temperature of the pile will increase. The opposite is also true. If the rates are the same, the internal temperature of the pile will remain constant.
If one were to insulate the surface of the pile, the theory of self-heating would produce a reduction in the critical temperature required for thermal runaway. That is one way to "lower the ignition temperature" of the wood under a specific set of conditions. But is this the only way?
Think of enclosure or void space filled with a finite amount of air with very little leakage paths. For example, a stud space in a wall or a platform under a water heater. If the internal surfaces are made of wood, then, under the right conditions, the wood could self-heat. The heat generated at the surface of the wood would be lost to the air inside the void space. The amount of air is finite and is not transported across the boundaries of the enclosure or void space. Thus, as thermal energy is added to the air, the temperature of the air increases. That is, the exposing temperature that is providing thermal energy for the oxidation reaction is not constant, but is increasing with time. With the traditional hypothesis, this thermal energy is lost to the surroundings. Think wood box versus an 2 by 4 in open air.
While the theory of self-heating to thermal runaway provides for this specific boundary condition, I do not believe this boundary condition has been tested. an experimental series to test this hypothesis is not trivial, but it can be done. Higher temperatures will take the shortest time to thermal runaway, if it can happen. Lower temperatures will take more time and may show that other hypotheses may be required to explain the phenomenon over a relatively large range of exposure temperatures.
Bottom line. There are a specific set of boundary conditions that have been disproven. So, if your scenario is substantially similar to those, you can reach a scientifically reliable determination based on the available evidence. In other scenarios, there is evidence, based on the theory of self-heating to thermal runaway that it cannot be disproven. There is no current model that will allow prediction of the critical exposure temperature for a specific pile size.
That is the best that can be done to-date.
Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com
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