Fire Engineering - Chapter 01. Combustible Matter

1. Types of Combustible 

 A. Combustible Solids:-

Woods, grass, fibers, cloths, paper, rubber, hydrocarbon and many plastics.

C. Combustible Liquids:-

Petrol, diesel, kerosene, oils and oil paints.

D. Combustible Gases:-

Methane, propane, butane and LPG.

2. Properties of Combustible

A. Properties of Combustible Solids

Wood

      It is one of the most important materials which is liable to catch fire and burn freely after a certain temperature is reached. Wood is a complex of many substance. Cellulose many amount to about 50% of the weight of dry wood with lingo-cellulose and lignin. There are bodies allied to cellulose.  

Rubber (C₅H₈)

       Commercial rubber consists of caoutchouc, a polymerization product of isoprene, resin like substances, nitrogenous substances, inorganic matter and carbohydrates.Natural rubber (C5H8) is an unsaturated hydrocarbon with heat of decomposition of approximately 9425 kcal/kg and it is chemical akil to turpentine (C10H16), though the differ in ignition temperatures rubber burns at 285⁰C.

Naphthalene (C₁₀H₁₈)

        It is a crystalline solid hydrocarbon with characteristic smell, having a high vapour pressure at ordinary temperature. Its melting points is 79⁰C  and boiling point of the liquid is 218⁰C . the specific gravity of the liquid is 0.97 at the temperature of the melting point of the solid . naphthalene is insoluble in water. It is, however soluble in most of the ordinary organic solvents, particularly in the flaky state.

B. Properties of Combustible Liquids.

    
     Petrol

        Petroleum vapours are all heavier than air and tend to collect at the ground level. Calorific value is quite high and with most petroleum fules it is within the region 10080 to 1100 kcal/kg while for coal –tar fuels, it is below 9520 kcal/kg, as against good quality anthracite coal having calorific value of 842 kcal/kg. The flash point its very low (below 24.4⁰C).

      Kerosene

         Kerosene burns producing a very high temperature with smoky orange – coloured flame and with liberation of soot. The heat of decomposition is to the extent of 11037.6 to 11138.4kcal/kg. It is not so hazardous because its flash points is much above petrol and it is not as volatile as petrol.  

      Diesel

         This is the fraction collected after gasoline and kerosene in the fractionating column and it is used for diesel engines.

C. Properties of Combustible Gases.

      

      LPG

        It is produced during the refining of the crude oil. They are mostly butane (C₄H₁₀) and propane (C3H8) and are stored in the cylinders in the form of liquids. The vapour pressure of these gases are quite high. Commercial butane has a vapour pressure of 2.5 bar and propane exerts 13.2 bar pressure at 37.8⁰C. A mixture of 60% butane and 40% propane gas exerts a pressure of 7.8 bar at 37.8⁰C. The boiling point of propane is about -42⁰C and the same for butane is -5⁰C. 

3. Flash Point

        The flash point of an inflammable substance may be defined as the lowest temperature at which it gives off sufficient vapour so as to form a momentary with air on application of a small flame in the prescribed manner in the flash point apparatus. The ignition temperature of substance is the temperature at which spontaneous ignition can take place. The self ignition point (Sometimes called the auto ignition point or the combustion point).

4. Fire Point

        The lowest temperature at which the heat from combustion of a burning vapour is capable of producing sufficient vapour to enable combustion to continue is known as fire point. It is generally above the flash point.

5. Spontaneous combustion

      Spontaneous combustion occurs as a result of the heat generated by the reacting substance themselves. The substance soon begins to fume and afterwards they burst into flame. The absorption of oxygen by certain vegetable oils, such as by un-boiled linseed oil soaked in cotton , or by certain animal oils and fats, as also the ‘absorption’ of oxygen by certain substances. Heat and light and which come under the definition of combustion though oxygen does not take part in these reactions, e.g. burning of powdered metals like iron, magnesium, aluminium, or hydrogen in an atmosphere of chlorine gas.

6. Auto ignition temperature (A.I.T.)

       As the temperature increases the number of molecules that possess sufficient energy to form an activated complex increase rapidly. There is a critical temperature above which enough molecules possess sufficient energy for the process to become self-propagating, leading to flammable combustion. This critical temperature is called the Auto Ignition Temperature. 

7. Range of flammability and flash point

In the case of flammable vapours, there is a minimum and a maximum concentration of vapour in air, below or above which propagation of flame does not occur. These limiting mixtures of vapour with air are known as the lower and higher limits of flammability and are usually expressed in terms of percentage by volume of vapour and air. Within the limits of flammability lies a narrow range, confined by the explosive limits, which mark the minimum and maximum concentration of vapour in air for the propagation of flame with a speed and violence sufficient to constitute explosion. The concentration at which a gas or vapour may be explosive is in most cases, and roughly, the mean of its upper and lower explosive limits, i.e. at the concentration at which an oxygen balance is achieved for complete combustion. It is interesting to consider mixture of petrol vapour with air as example of the significance of these ranges.

    

7. Successive Explosions

An explosion of a flammable gas or vapour in a vessel does not results in complete combustion and the vessel thereafter does not become safe contrary to the common belief.

A vessel containing a gas or vapour can suffer successive explosion without addition of fresh gas, if the first explosion occurs with a gas concentration close to the upper explosion limit. All the gases, it may by noted, are not necessarily burnt in the first explosion; what remains can often be in concentration above the lower explosion limit, i.e. within the explosive range. 

                                                                                                                              Source - a hand book fire technology