Stages in Fire Behaviour I

 

Introduction

Knowledge of basic fire behavior provides a 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 (Bush Firefighter), 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.

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 vapour. 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 vapour products.

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 vapour) phase in order for combustion to occur. This is simple when the fuel is already in the gaseous state (i.e. methane). Liquids must be vaporised before combustion can occur. Some liquids vaporise sufficiently to burn at normal temperatures (i.e. petrol), others require additional heat in order to release sufficient vapour 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.

Pyrolysis

Ignition requires that fuel vapour 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 vapour is being released and mixed with air it can be ignited. As stated above, a fuel can only burn when it is in the vapour or gaseous state.

Pyrolysis is the chemical decomposition of a material due to the application of heat.

As fuels undergo pyrolysis, they release flammable (and toxic) gases. It is these gases that burn.

Products of Combustion

Like the basic model of combustion provided by the fire triangle, description products of combustion as heat, smoke and sometimes light is deceptively simple. Of these three general types of products, heat and smoke are generally of the most interest to firefighters.

Heat refers to the total amount of energy in a substance. Temperature on the other hand refers to a scale of "hotness". A critical aspect of temperature is that heat will flow from substances having higher temperature to those having lower temperature. This is particularly important in understanding both fire spread and fire control tactics.

Smoke is an aerosol comprised of gases, vapour, and solid particulates. Gaseous products such as carbon monoxide are generally colourless, while vapour and particulates provide smoke its varied colours.

Most components of smoke are toxic and present a significant threat to human life. Often smoke is perceived as less of a threat than visible flames. This is not always a correct perception.

Leaking fuel gases such as methane and propane are generally treated with a great deal of respect. However, carbon monoxide, likely the most common fire gas has both a lower ignition temperature and considerably wider flammable range than either of the two most common fuel gases (methane and propane).

The diagram left illustrates the combustibility of smoke from a burning “dolls house”. This often unrecognised hazard presents a significant threat to firefighters if not mitigated by effective fire control and ventilation tactics.

Heat Transfer

In a way, our discussion has jumped ahead of itself. In order for pyrolysis to begin and ignition to occur, heat must be transferred to the fuel. Heat transfer is also important to understanding both fire spread and fire control tactics. Heat is transferred from materials having a higher temperature to those having lower temperature through three principle mechanisms:

Radiation, convection, and conduction.

Conduction: Heat transfer through conduction requires direct contact between a hot object and objects of lower temperature. This is the predominant method of heat transfer in the initial stages of fire development. In addition, direct contact between hot gases and cooler fuel results in heat transfer through conduction.

Convection: When a fluid medium (such as air) is heated it becomes less dense, expands, and rises. Hot products of combustion and pyrolysis products spread through convection and heat other materials on contact (as well as through radiation).

Radiation: Heat energy moves away from a hot object in all directions. Radiant heat is particularly important to fire development in a compartment in that it serves as one of the primary mechanisms for fire spread within the compartment. While normally we think of radiation from flames, any hot object radiates heat energy. Hot gases and in particular particulates (such as carbon) in smoke can radiate significant heat.

Diffusion flame vs Pre–mixed flame

When fuel vapour must mix with air in the combustion zone, the resulting flame is called a diffusion flame (the fuel vapour must diffuse to reach the flammable range in air). In a diffusion flame, fuel vapour diffuses in the air to form a reaction zone containing fuel, air and heat in the correct proportion to support combustion. When fuel vapour is mixed prior to combustion, this is called a pre-mixed flame. Lean and rich (as applied to the entire mixture of fuel gas or vapour and air) only apply to pre-mix flames.