In its most simple form, the combustion of organic material is an exothermic chemical reaction involving the oxidation of hydrocarbons to produce water vapour and carbon dioxide. For example the chemical reaction for the complete (stoichiometric) combustion of propane is given by:
C H3 8 5O2 3CO2 4H O2 (3.6)
This is a simplification of the chemistry. There are many chemical processes involved, depending on temperatures, pressures and the availability of the materials. Intermediate reactions involve a large number of atoms and free radicals. In many fire situations there will be incomplete combustion, leading to the production of carbon monoxide gas (CO) or solid carbon (C) as soot particles in the flames or smoke. The chemistry changes continually throughout the combustion process.
At room temperatures, some fuels are gases but most are solids or liquids. Gases can mix with air to burn directly without any phase change, but all solid and liquid fuels must be con- verted to the gaseous phase before they can burn. For most liquids, the transition from liquid to the gaseous phase under the application of heat is by evaporation. For some polymers the process is by thermal decomposition into new volatile products. Many solid fuels melt when heated, producing a liquid which can then evaporate or thermally decompose into a gas. Some other fuels, including most wood products, thermally decompose with a transition directly from solid to gaseous phase. This thermal decomposition of wood is known as pyrolysis.
The combustion process for any material requires the availability of oxygen for the oxidation reaction to occur. The most efficient combustion is premixed burning where the gaseous fuel is mixed with oxygen or air containing oxygen before ignition (as in a Bunsen burner).
Combustion will be very rapid if the gases are mixed in the right proportions (e.g. in an internal combustion engine). Combustion will not occur if the mixture has too much or too little oxygen for the given conditions of temperature and pressure. The limiting conditions are called the limits of flammability. In most building fires there is no premixed burning, and the rate of combustion depends on the rate of mixing of air with the gaseous fuels as they become available. The combustion takes place in the region where the gases mix. The mixing is usually driven by buoyancy and turbulence resulting from the convective movement of the flame and combustion products in the plume above the fire.
The maximum temperature that can be reached in a flame is known as the adiabatic flame temperature. This is the theoretical maximum temperature that can be reached when the combustion products are heated from their initial temperature by the heat released in the combustion reaction, with no losses. In flames from a typical burning object, the adiabatic flame temperature may be reached in a small region in the centre of the flame, but the average temperature of the flame will be considerably less.
For an object to be first ignited, there must be an external source of heat to raise the temper- ature of the object to its ignition temperature. If the fire grows after ignition, it may reach established burning after which the flames are large enough to sustain the combustion reac- tion with no assistance from any external source of heat. The burning is driven by heat from the flames which heats the remaining fuel to a sufficient temperature for the production of volatile combustible gases which burn in a dynamic process, producing more volatiles and more flames.
Smouldering is the term given to flameless combustion such as in a cigarette. Smouldering combustion is much slower than flaming combustion, and temperatures are also lower (Ohlemiller, 2008). Smouldering combustion is a particular hazard in residential buildings, because insufficient heat or noise is generated to wake sleeping occupants who can be overcome by the smoke and toxic combustion products. The smoke from smouldering combustion will activate smoke detectors, but it usually has insufficient temperature to activate heat detectors or automatic sprinkler systems. Smouldering combustion does not produce temperatures sufficient to affect structures, so is not considered further in this book.
Ignition occurs when a combustible mixture of gases is heated to temperatures that will trigger the exothermic oxidation reaction of combustion. Ignition almost always requires the input of heat from an external source. The few cases where self‐heating within solid materials can cause spontaneous combustion is a special subject that is not covered in this book. There are numerous possible heat sources that cause building fires to ignite. These include flaming sources (matches, candles, gas heaters, and open fires), smouldering sources (cigarettes), electrical sources ( arcing, overheating), radiant sources (sunlight, hot items, heaters, fires), also hot surfaces, friction, lightning and others. War and terrorism have also been the cause of many fires in buildings, as evidenced by the World Trade Center collapses in 2001 (Shyam‐Sunder, 2005).
Many sources of potential ignition can be reduced or controlled by fire prevention strategies, but some unwanted fires will always occur.
The amount of heat and temperature required to cause ignition depends on the material properties of the fuel, the size and shape of the ignited object, and the time of exposure to heat.
The time to ignition of materials depends on the thermal inertia of the material itself. Thermal inertia is defined later in this chapter as the product of thermal conductivity, density and specific heat. When exposed to the same heat source, the surface of materials with low thermal inertia (e.g. polystyrene foam) will heat more rapidly than materials with higher thermal inertia (e.g. wood) leading to much more rapid ignition.
3.3.2 Pilot Ignition and Auto‐ignition
It is useful to distinguish between pilot ignition which occurs in the presence of a flame or spark and auto‐ignition which is the spontaneous ignition of volatile gases from a fuel source in the absence of any flame or spark. Auto‐ignition requires the gases to be at a higher temper- ature than for pilot ignition. For surfaces exposed to radiant heat flux, the heat flux intensity required to cause auto‐ignition is higher than that required for pilot ignition.
3.3.3 Flame Spread
After ignition has occurred somewhere in a building, fire safety depends greatly on the rate of fire spread. Initial fire spread is caused by spread of flame on the burning object or adjacent combustible materials. The main factor affecting flame spread is the rate of heating of the fuel
ahead of the flame. This in turn depends on the size and location of the flame (causing radiant heating), the air flow direction (causing convective heating), the thermal properties of the fuel (affecting the rate of temperature rise), and the flammability of the fuel (Drysdale, 2011).
Heating ahead of the flame will be more rapid if there are heat sources in addition to the flame itself, such as radiation from a layer of hot gases under the ceiling. Air movement is very important. Flame spread will be much more rapid with air flow in the direction of spread (‘wind aided’) than air flow in the other direction (‘wind opposed’). Upward flame spread is always rapid because the flame can rapidly preheat the material ahead of the burning region.
Flames tend to spread most rapidly on surfaces which have a high rate of temperature increase on exposure to heat flux. These are materials with low thermal inertia which are also more susceptible to ignition. Materials such as low density plastic foams experience rapid flame spread and fire growth for this reason.