NATURE OF FIRE

NATURE OF FIRE

One generally accepted the definition of fire as a process involving rapid oxidation at elevated temperatures accompanied by the evolution of heated gaseous products of combustion, and the emission of visible and invisible radiation (Abdullah, 2001). The combustion process is a chemical reaction between the oxidation of a fuel in the presence of oxygen with the emission of heat and light. The concept of fire can be symbolized by the Triangle of Fire, which is represented by fuel, heat, and oxygen (Tharmarajan 2007). If the fire is in a fire grate or furnace, this process can be referred to as a controlled fire, and if it is a building on fire, this process is referred to as an uncontrolled fire. The removal of any one of these factors usually will result in the fire being extinguished.

1.    Pyrolysis

Fire is a combustion and oxidation process when fuel material undergoes pyrolysis (Tharmarajan 2007). It is in effect a chemical process when oxygen reacts with the material’s fuel components to liberate stored energy into thermal energy with high temperatures. Fuels that predominantly consist of carbon and hydrogen elements, however, may contain small amounts of sulphur, lead, zinc, etc., and non-combustibles like mineral matter (ash), water and inert gases.

With the exception of hydrogen itself, all common fuels are organic compounds, whose energy is ultimately sourced from the sun through the process of photosynthesis in green plants. The atmosphere contains 21% of oxygen, 78% nitrogen and 1% of other elements. All of these would enter the combustion process where in the case of oxygen; only the amount required for the combustion process is utilized while the excess of it exits in its original oxygen (O2) state. Nitrogen, that requires very high temperatures for oxidation, is inert as far as the combustion process is concerned. Nevertheless, it acts as a moderator or cooling agent in that it absorbs some of the heat of combustion thus assisting in limiting the maximum temperature reached (Abdullah, 2001).

A stoichiometric mixture of air and fuel is one that contains sufficient air (oxygen) for complete fuel combustion. A weak mixture is one that has excess of air (oxygen) and hence favourable for combustion. A rich mixture (excess of fuel) is that of one having deficiency of air (oxygen) and thus considered unfavourable for complete combustion.

2.    Combustion

Complete combustion occurs when the conditions are favourable. The products of complete combustion are carbon dioxide in gaseous form, water in vapour form and heat energy (Tharmarajan 2007). However, small quantities of carbon monoxide and partial flume gas components may form. The amount of energy released in the burning of a substance is called its heat of combustion or combustion enthalpy.

Incomplete combustion occurs when the fuel element is not completely oxidized in the combustion process. Fire would occur in a less vigorous and smouldering state emitting thick smoke containing toxic gases and contaminants. Factors that give rise to incomplete combustion on a burning fuel material are as follows (Abdullah, 2001):

1. Insufficient air to the fuel material (causing local fuel-rich and fuel-lean zones);
2. Insufficient air supply to the flame (providing less than the required quantity of oxygen

• Insufficient reactant residence time in the flame (preventing completion of combustion reactions;

1. Flame impingement on a cold surface (quenching combustion reactions); and
2. Too low flame temperature (slowing combustion reactions).

When there is incomplete combustion to a burning fuel material, the formation and subsequent release of partially oxidized compounds such as carbon monoxide, aldehydes and ketones occurs (Abdullah, 2001). Should such adverse toxic smoke situation predominate in building fires, this would be the main contributing factor to the ill-favoured consequence of high occupant-fatality rate not to mention the lifelong agony and suffering of the survivors.

3.    Ignition

There should be sufficient heat to set a fuel material into combustion and this is called ignition. Ignition occurs in any one or combination of the following forms (Abdullah, 2001):

1. Induced Form (as in internal combustion engines or lighting a fire);
2. Spontaneous Form (when the fuel material ignites itself through electric sparks, friction, grinding or rubbing effect, magnified sunlight, etc.);
3.  Contact Form (through flame contact); and
4. Pilot Form (transmission of heat radiation through a stream of hot gases, volatile and flying brands)