Fire Development in a Compartment - Part I

Knowledge of basic fire behavior provides foundation for understanding fire development in a compartment, fire spread throughout a structure, and firefighting strategy and tactics. While for most readers, this information is a review, there are also likely to be a few new concepts or ways of looking at fire behavior phenomenon that will be useful in extending your understanding. A series of study and discussion questions are located at the end of this article. These questions are not a "quiz" on the content, but provide a way for you connect this look at basic fire behavior to structural firefighting. Use the questions as a starting point for a discussion at the kitchen table or informal company drill.

The Basics

If you examine common fire service texts there are a variety of definitions of combustion, but all describe the same phenomenon: A heat producing (exothermic) chemical reaction (oxidation) in which a fuel combines with oxygen. In its simplest form hydrogen and oxygen combine, resulting in the production of heat and water vapor. However, most of the time this process is considerably more complex. In a typical structure fire the wide variety of fuels and limited ventilation produce a complex, toxic, and flammable mixture of solid, gas, and vapor products of combustion are produced by the oxidation reaction (more on this in a bit).

One familiar way of representing the key components of combustion is using the fire triangle. The fire triangle does not provide a complete explanation of the physical and chemical processes involved in the combustion process. However, it will work for the problem at hand, developing a good working knowledge of compartment fire behavior.

Combustion requires fuel and oxygen in the correct proportion as well as sufficient heat energy to start the reaction. Fuel must be in the gas (or vapor) phase in order for combustion to occur. This is simple when the fuel is already in the gaseous state (i.e. methane) as the fuel is already in this state. Liquids must be vaporized before combustion can occur. Some liquids vaporize sufficiently to burn at normal temperatures (i.e. gasoline), others require additional heat in order to release sufficient vapor to support combustion (i.e. fuel oil). However, when dealing with a compartment fire, the fuel is commonly a solid fuel such as wood, paper, or plastic.

Take this a step further and consider how wood fuel burns. As illustrated in Figure 2 when wood is first heated water vapor is driven off as the wood dries. As heating continues, the wood begins to pyrolyse and is decomposed into its volatile components and carbon.

Ignition requires that fuel vapor and oxygen in adequate concentration be heated to their ignition temperature. Note that this does not require the solid wood fuel to be heated to its ignition temperature. If adequate fuel vapor is being released and mixed with air it can be ignited. Fuel vapor and carbon burn separately. Visible flames involve combustion of fuel vapor. On the other hand, oxidation of carbon may take place at the surface of this solid material (such as with glowing coals).

Pyrolysis begins at a considerably lower temperature (below 400o F) than is required for ignition of volatile pyrolysis products (which ranges roughly from 1000o F - 1300o F). Table 1 outlines the pyrolysis effects within different temperature zones (Browne as cited in Pitts, Johnsson, & Bryner) and ignition temperatures of carbon and common volatile components evolved from pyrolysis of wood.

Table 1. Pyrolysis Zones and Ignition Temperatures