Chemistry

It doesn't take much experience in fluid mechanics, heat transfer, or thermodynamics to see that significant simplifications from the "state of the art" in these subjects are made for the topics used in fire science. However, the topic which is perhaps most simplified, or even glossed over, is fire chemistry. This is because the chemistry of combustion reactions is extremely complicated. Reactions which we are used to thinking of in global terms are actually composed of a large number of elemental reactions. For example, a simple example is the reaction of hydrogen and bromide to form HBr. This process has the global reaction:

H2 + Br2 2 HBr

The full reaction mechanism for hydrogen bromide formation actually includes 5 steps:

M+Br2 2 Br + M
Br + H2 HBr + H
H + Br2 HBr + Br
H + HBr H2 + Br
M + Br + Br Br + M

In the formalism of elemental chemical reactions, M stands for any atom molecule which participates in a reaction simply by providing kinetic energy through collision to get the reaction going. Its chemical makeup is not effected by reaction, therefore it is not necessary to specify its chemical makeup. In this case, it could be H, Br, H2, or Br2, and any or all of these are probably involved at some point. Each step in the above process has a reaction rate associated with it, a speed at which it progresses. Some of the reactions are faster than others. The reactions compete for some of the same reactant species, so the progress of each reaction is affected by the progress of other reactions.

Looking at reactions that are more relevant to fires, the simplest combustion reaction is the combination of hydrogen with pure oxygen to form water. The full reaction mechanism for that process has 8 steps. As the atoms and molecules involved in the global reaction become more complex, so does the reaction mechanism. The reaction mechanism for the combustion of the simplest hydrocarbon fuel, methane (CH4), in air has over 80 steps! Few of the full reaction mechanisms for the heavier hydrocarbons are even completely known.

Because of this complexity, the full extent of reaction chemistry is often not modeled, even in relatively fundamental combustion studies. Reduced reaction mechanisms, which reasonably predict the rates of formation and destruction of species of concern, such as carbon monoxide and nitric oxide, are often used. In many cases, chemistry is not even modeled at all, but is assumed to instantaneously proceed from the reactants to some set of products, which may include non-ideal products.

Compounding the intractability of chemistry modeling in fire is the complexity of the fuels involved. The diagram below shows a cutaway view of a modern sofa.

There are various natural and synthetic materials. The synthetics are generally very long polymer hydrocarbon chains. The leather and wood are each composed of a number of complex molecules, any of which may be contributing to the fire at any given time. Furthermore, all of these molecules are solids that must pyrolize, or react to give off the gaseous species which actually burn as open flames. The pyrolysis reactions describing the off gas are often quite complex.

A good starting reference for combustion chemistry is Kuo, K. K. (1986) Principles of Combustion, John Wiley and Sons, New York. Probably, the most important issues related to combustion/fire chemistry for a fire scientist are summarized in the fire triangle:

The point is that it takes a fuel, oxidizer and heat source to cause a fire. What do fire simulations do, then, in the absence of a complete understanding of chemistry?

Usually, fire modeling does not actually model the fire itself. Rather, fire modeling focuses on the behavior of the heat and product gases liberated by the fire. How does a fire in a room heat up the floor above? How long does it take until the layer of hot gases collecting at the ceiling builds down head height, or floor level? How long until a person cannot see far enough to find the door to a room engulfed in fire? All of these topics and more can be addressed without a full understanding of fire chemistry. The fire is treated as an input, using data from actual experiments. Fire data is primarily composed of the profile of heat release from a burning object as a function of time. This data is developed in labs, primarily using cone calorimeters (link to experimental methods - cone calorimeter). The calorimeters may also be instrumented to provide additional information besides the rate of heat release, such as the rate of release of various toxic gases like carbon monoxide and hydrogen cyanide.

Tenability Limits

This raises one final chemistry related point - the biochemistry of fires. Fire produces a number of effects which can be hazardous to human life beside simply high temperatures. It depletes the oxygen in the room where it's burning. It releases carbon dioxide, which humans can't breathe, and carbon monoxide, which will bond to hemoglobin in human blood and prevent it from carrying oxygen. Polymer fuels will also produce toxic compounds such as hydrogen cyanide (HCN). For all of these reasons, it is often the smoke, rather than the heat, from a fire that kills.

The table below lists the tenability limits for an average human male. These are the concentrations of various gases that can lead to human incapacitation or death in the times listed (5 or 30 minutes). Obviously, these limits will be reduced for smaller and weaker individuals, such as infants and the elderly.