Fire is a process or an instant of combustion in which fuel or combustible gas mixture is ignited and combined with oxygen in giving out light, heat and flame together with various reactant products. It can also be termed as a rapid oxidation of a substance in a chemical process.
Rusting being a slower oxidative process is not covered under this definition.
Flames being glowing hot gases is the visible portion of the fire. The colour of the flame and the fire’s intensity may vary depending on the fuel that is burning as well as on any impurities.
Generally, fire in its most native form often results in destruction; the intensity of which is capable to taking life or causing serious damage to buildings. Various types of fire incidents, such as forest fire, wildlife fire, major building fire incidents, nuclear power station fire/explosion incidents, etc are important scenarios that have impact on ecological systems around the globe. The negative effects of fire include mass destruction of forest; increased soil erosion; contamination of underground water source and a rise in atmospheric pollutants. These destructions are threatening and attributing to hazards to human life.
In the modern building design, understanding fire behaviour is imminent for mega projects or high-rise buildings as the world is progressing from prescriptive code design to performance-based fire engineering design. Increasingly important is the design for life safety as we build larger and higher complexes.
Finally, smoke control and management should also require study and understanding of its behavior if engineers wish to produce an appropriate and efficient design.
Burning is a form of combustion which is a chemical reaction between fuel and an oxidant that resulting in the production of heat and chemical compositions. The heat generating process also produces light in the form of flame or glow. Therefore, presence of a flame is often the result of fuel undergoing combustion or burning. Fuel that can burn includes organic compounds (includes hydrocarbons) in solid state, liquid state or gas phase.
Under normal circumstances, it is very difficult to achieve complete combustion, especially in the case with solid organic materials. The by-product of incomplete combustion will then form smoke particle and gases.
Example of complete combustion
CH4 + 2 O2 → CO2 + 2 H2O + energy
H2S + 6 F2 → CF4 + 2 HF + SF6
A simple example can be seen in the combustion of hydrogen and oxygen, which is a commonly used reaction in rocket engines.
2 H2 + O2 → 2 H2O(g) + heat → water vapour
It is almost impossible to achieve complete combustion, as variety of species-major or minor-such as carbon monoxide and carbon (ash or soot) are always present. Furthermore, several forms of nitrogen oxide will be produced because of the 78% nitrogen permeated in our atmosphere.
The simple word equation for the combustion of a hydrocarbon in oxygen is:
Complete Combustion
To achieve complete combustion, the fuel needs to burn in oxygen, producing limited end products, for eg when a hydrocarbon burns in oxygen, the result is only carbon dioxide and water; carbon will yield Carbon Dioxide; Nitrogen will yield Nitrogen Dioxide; Sulphur will yield Sulphur dioxide, and iron will yield iron oxide.
During fire incidents or industrial combustion process, air instead of oxygen is the source of oxidation. The mixture proportion of Nitrogen to Oxygen in air is approximately 3.76kg to 1kg of Oxygen. Even though Nitrogen does not take part in combustion, some of it will be converted to Nitrogen Oxide at high temperature. Furthermore, when incomplete combustion occurs, some of carbon is converted to carbon monoxide.
A more complete set of equations for combustion of methane in air is therefore:
CH4 + 2 O2 → CO2 + 2 H2O
2 CH4 + 3 O2 → 2 CO + 4 H2O
N2 + O2 → 2 NO
N2 + 2 O2 → 2 NO2
Incomplete Combustion
To be able to achieve a complete combustion is an ideal situation where the energy output is maximum and there is little or no waste generated. However in reality, it can never be achieved due to many factors. In most cases, incomplete combustion occurs because of the insufficient oxygen supply to the fire and the heat sink effect of the solid fuel preventing the fuel to react completely to produce carbon dioxide and water.
In most fire, fuel such as combustible liquid (e.g. petroleum) or organic materials (e.g. coal) or wood, pyrolysis occurs before combustion. When this happens, products of pyrolysis remain unburned due to incomplete oxidation and contaminate the smoke with noxious particles and gases, at times toxic. Partially oxidized compounds could be dangerous, for e.g. carbon can produce toxic Carbon Monoxide.
Smouldering
A flameless form of combustion at low-temperature, smouldering is slow in burning rate sustained by the heat of a condensed fuel when oxygen attacks the surface directly. This is a typical incomplete combustion process.
Solid materials such as wood, cotton, coal, tobacco, peat, synthetic foams, polyurethane foam and dust can sustain a smouldering reaction. A good example of smouldering phenomena will be the initiation of residential fires on upholstered furniture caused by weak heat sources e.g. cigarette.
Rapid Combustion/Explosion
Rapid combustion is another form of fire, in which large amounts of heat and light energy are released, which often results in a flame at the same time. At times, large volume of gas is released during combustion and the sudden release of the gas creates excessive pressure that produces a loud noise. Such combustion is known as an explosion.
During an explosion, there will be a primary wave build up and release, followed by a secondary wave. These high speed pressure waves often cause tremendous damage and destruction. Even building structure may be damaged during the process. An explosion is normally followed by a fire, e.g. 9/11 incident. Combustion needs not involve oxygen; A typical characteristic of combustion would be hydrogen burns in chlorine forms the Hydrogen Chloride that will result in liberating heat and light.
Fuel
Fuel can generally be grouped into two categories:-
(i) Solid fuels
The combustion process of solid fuel is more complex than the liquid fuel as it occurs in three relatively distinct but overlapping stages:
Preheating stage occurs at the beginning when heat is introduced to the surface of the solid fuel. Upon heated up to the flash point, pyrolysis occurs and will release fuel cells into the air to mixture with the oxygen present to form a volatile layer which is gas phase.
Distillation stage or gaseous stage occurs when the pyrolysis phase reaches flash point temperature, ignition will take place. If heat from the initial fire is sufficient to continue and aggravate the pyrolysis process, it will build up to fire point where the entire burning process is self-sustaining. Normally energy from the burning process is presented in the form of form of heat and light.
Charcoal stage or smouldering stage occurs when the entire pyrolysis process slows due to insufficient heat from the fire or insufficient oxygen present to sustain flaming. The surface of the solid fuel will char, and the burning gets to just glow which diminish into smouldering stage.
(ii) Liquid fuels
The combustion of liquid fuel on the other hand is a much simpler phenomenon. When it is subjected to heat or flame, liquid fuel surface will go through the pyrolysis stage which will convert liquid fuel into gas stage before ignition takes place. Liquid fuel will normally ignite once the temperature reaches flash point.
Therefore, under normal circumstances, the combustion of a liquid fuel occurs in the gas stage instead of liquid stage. It is the thin volatile layer or vapour on top of the liquid fuel surface that supports combustion, not the liquid fuel itself.
Flash point of a liquid fuel can be defined as the lowest temperature where it can form an ignitable mixture with the air around it. It is also the lowest temperature at which there is sufficient evaporated fuel in the air for it to start burning. Sometimes, the minimum temperature in which liquid fuel can achieve ignition can be influenced by many other factors such as ambient temperature, the liquid fuel acting as a heat sink and ventilation.
What is a flame?
A flame is a mixture of gases and solids through burning which emits visible and infrared light. The spectrum of light coming out of a flame will depend on the chemical composition of the burning material and its by-products.
Burning of materials, e.g. wood, incomplete combustion will produce the familiar red-orange-yellow glow of ‘fire’ as a result of the glowing solid particles called soot. Complete combustion of gas on the other hand, produces a blue colour dim flame due to the emission of single-wavelength radiation from various electron transitions in the excited molecules formed in the flame. Usually these are the result of oxidation process.
In addition to combustion in the presence of oxygen, there are other types of combustion without oxygen that also produce flame, e.g. hydrogen burning in chlorine producing Hydrogen Chloride (HCL).
The glow and the colour of a flame is complex. Radiation is emitted from soot, gas, and fuel particles. By-products of fire (smoke particles) often carry temperature from the fire. This gives them the buoyancy effect as their temperature is much higher than the ambient temperature. That is the reason smoke particles always float towards the ceiling or the highest point in the room before they move along the ceiling to the other parts of the room.
Radiated heat from the flame is emitted in visible and infrared bandwidth. Colour of the flame will depend on the temperature of the light emitting particles.
The contour of the flame colour changes with the gradient of temperature. This is best demonstrated through a camp fire. The fire is white or bright yellow in colour nearest to the centre of the camp fire where most of the combustion take place. This is the hottest possible colour for organic material in general. Immediately after the yellow region, the colour will change to orange, that is cooler, then red, which is cooler still. Outside the red colour region, combustion no longer occurs, and the un-combustible carbon particles are visible as black smoke.
Heat
Heat is a component of fire. It is also a mechanism of energy transferred from one body to another resulting from thermal contact.
Typical temperatures of fires and flames
Temperatures of flames by appearance
Red
White
Orange
What is Flammability?
Flammability can be termed as how easily something will burn or can be ignited, resulting in a fire or combustion condition. The flammability index of each substance is quantified though tests in the laboratory following test protocol that is stipulated by International or National Standards. The verified flammability index will then be listed in building regulations, fire codes and underwriters’ requirements as guidance for use of building materials.
The flammability rating being an indicator of ease of burning or ignition is another indication of the degree of danger pose by each substance. It is therefore used widely as an indicator for storage and handling of highly flammable substances inside and outside of buildings as well as for transportation (land, sea and air).
Flammability index is normally inversely proportional to the density of organic materials. The higher the density of the substance, the lower will be the flammability index and vice versa.
Category of building occupancies is normally based on fuel loads and fire risks. Type and degree of fuel load is again assessed by referring to the flammability index. Changes in occupancy often bring about changes in fuel load within a building or part of a building.
