Combustion or fire, is 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.
A process in which substances combine chemically with oxygen from the air and typically give out bright light, heat, and smoke; combustion or burning.
Oxidation occurs all around us in the form of rust on metal surfaces, and in our bodies by metabolizing the food we eat. However, the key word that sets combustion apart from other forms of oxidation is the word “rapid”.
The combustion process is usually associated with the oxidation of a fuel in the presence of oxygen with the emission of heat and light.
Oxidation, in the strict chemical sense, means the loss of electrons. For an oxidation reaction to occur, a reducing agent the fuel, and an oxidizing agent, usually oxygen must be present.
As heat is added, the ignition source, the fuel molecules and oxygen molecules gain energy and become active.
This molecular energy is transferred to other fuel and oxygen molecules which creates a chain reaction.
A reaction takes place where the fuel looses electrons and the oxygen gains electrons.
This exothermic electron transfer emits heat and/or light.
If the fire is in a fire grate/ or furnace we refer to this process as a controlled fire, and it is a building on fire we refer to this process as a uncontrolled fire.
A substance is oxidized when
- Oxygen chemically combine with it
- Any non-metallic element or group of elements combines with it
- Hydrogen is removed from the substance
metallic element or group of elements is removed from the substance
- When positive ions have their positive charge increased
- When negative ions have their negative charge decreased
- However, not all reactions between substance and oxygen are called combustion.
There are many chemical reactions which produce
- heat and light without presence of oxygen
- Burning of powdered metals like Iron, Magnesium, Aluminium or Hydrogen in an atmosphere of Chlorine gas
- Oxidation without heat and light
- Oxidation or rusting of iron
- Light without heat
- Moist yellow phosphorus emits a bluish glow of light without evolution of much heat – Phosphorescence.
According to the nature (rate of combustion), combustion can be classified into
- Slow or Incipient
- Rapid or Active
- Explosion or Deflagration
Slow or Incipient :
- During this phase, the products of combustion may be minimal, the changes in the surrounding atmosphere may be difficult to observe (only some smoke, no detectable flame), and the amount of heat generated will not significantly affect the surrounding area.
The incipient phase can last a few moments (i.e. ignition of a combustible liquid), hours, or even days (i.e., the exothermic reaction seen in spontaneous combustion).
- Rapid or Active – in which a considerable amount of heat & light is emitted within a short time
- Spontaneous : it is a type of combustion which occurs by self-heating (increase in temperature due to exothermic internal reactions), followed by thermal runaway(self heating which rapidly accelerates to high temperatures) and finally, auto ignition.
- In spontaneous combustion, combustion reaction takes place without the application of external heat. Burning of white Phosphorous when brought in to air is an example of this type of combustion.
- Explosion: it is a very rapid combustion with a loud noise within an extremely short time with generation of very high pressure & temperature.
- Deflagration : Which takes place with a considerable rapidity, evolving heat & light
- Detonation: it is a type of combustion involving a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it.
- Detonations occur in both conventional solid and liquid explosives, as well as in reactive gases. The velocity of detonation in solid and liquid explosives is much higher than that in gaseous ones, which allows the wave system to be observed with greater detail (higher resolution).
PRODUCT OF COMBUSTION AND ITS CHARACTERISTICS
The composition of combustion may be flame, heat, fire gas, smoke, etc.
- A flame is a zone in which chemical reaction takes place between gases with evolution of heat and light accompanied by vigorous combustion.
- The characteristics of flame, viz. size, changing shape and colour give an idea of the nature of the combustibles and the temperature produced.
- Gases must be present if flame is tube produced. Substances which do not evolve gaseous flammable products when heated do not exhibit flame while burning.
Example –No flame is produced when Carbon, Iron or Charcoal is burnt.
Flames has grouped in to two classes.
- Luminous flame – Produces heat and light
Example: Flame of a candle burning
- Non-luminous flame – Producing heat and very little light
Example : Burning of Hydrogen or CO
- Most of the flames, in practice, in a fire are open at the top and emit smoke.
- Smoke is a mixture of hot gases, small droplets of liquids ( < 0.001 mm diameter – 1 micron ) and solid carbon particles.
- The gases contain a vide range of pyrolysis products and other elements such as Nitrogen, Oxides of Carbons, Hydrogen, Sulphur, etc. The liquids present are also complex pyrolysis products but of high molecular weight. In case of improperly burned carbonaceous materials and hydrocarbons, the un-reacted carbon molecules agglomerate on leaving the combustion zone form long chain of carbon particles –known as soot. The constituents of smoke are combustible.
- The volume of smoke produced, its density and toxicity depends on the material which is burning and on the way it is burning. The structure of the molecule of fuel affects the amount of smoke which is produced. If molecules of a fuel contain oxygen, then the fully aerated flame will give less smoke than the same flame from a chemically similar fuel which has no oxygen. The chemical composition of smoke is regarded as less important than its density.
- Majority of fire deaths result directly or indirectly from the presence of smoke. These deaths could have resulted from one or more of the following reasons:
- A deficiency of oxygen
- Injury caused by the high temperature
- The presence of toxic and corrosive gases and
- The high concentration of solid matter
Fire Gases :
Fire gases vary greatly with the type of material and the way it is burning.
Fire gases can be grouped into three, viz.
- Asphyxiants are gases that occlude ( stop ) oxygen from getting to the body’s cells. This is accomplished either by displacing available oxygen in the air or through a chemical reaction that damages the hemoglobin’s ability to carry oxygen to the cells.
- Example : Natural gas (Obtained from pits during drilling for oil and contains mainly methane – CH4 ( > 80 % ) ) displaces oxygen. Death occurs not due to the breathing of natural gas, but due to the lack of oxygen.
- Carbon monoxide is chemical asphyxiant – red blood cells have a greater affinity to carry CO molecules than O2 molecules.
Fire Gases :
- Irritants are gases that cause irritation to the respiratory tract, causing difficulty in the exchange of oxygen in the lungs
- Example : HCl gas – when inhaled, chlorine mixes with saliva to create hydrochloric acid. The acid attacks the lining of the trachea (windpipe) and possibly the brachial tubes (brachial- relating to the arm), causing fluid to collect in the lungs – breathing becomes labored.
CLASSIFICATION OF FIRES
Fire has been classified as Class A, Class B, Class C, and Class D by IS: 2190. However, the most widely accepted classification of fire is as follows:
Class A Fire
- Fire in easily combustible solid materials of organic nature such as wood, grass, paper, cloth and products thereof. In such cases, fire is extinguished by reducing the temperature of burning materials by water spraying the residue is always carboneous materials.
Class B Fire
- Fire in volatile and inflammable liquids which provide a homogenous media of fire expansion in materials like all petroleum products (petrol, kerosene, naphtha, etc.), Chemicals, Paints, Oils, Grease, Lubricants, fats, etc..
- Absorbing oxygen by creating smothering atmosphere round the flames is the method used to extinguish the fire under this class.
Class C Fire
- Fire involving flammable gases under pressure including liquefied gases, where it is necessary to inhibit the gas at fast rate with an inert gas powder or vaporizing liquid for extinguishments.
Class D Fire
- Fire involving evolution of heat by chemical reaction in combustible materials such as magnesium, aluminium, zinc, sodium, potassium when the burning metals are reactive to water containing agents; and in certain cases, carbon dioxide, halogenated hydrocarbons and ordinary dry powders.
Class E Fire
- Fires involved in electrical equipment due to short circuit or over loading. Fire fighting medium has to be non-conductive and non-magnetic. (BIS does not include Class E Fire, but makes a mention of this type of fire).
- The fire triangle or combustion triangle is a simple model for understanding the necessary ingredients for most fires.
- The fire triangle illustrates the three elements a fire needs to ignite: heat, fuel, and an oxidizing agent (usually oxygen).
- A fire naturally occurs when the elements are present and combined in the right mixture, meaning that fire is actually an event rather than a thing.
- A fire can be prevented or extinguished by removing any one of the elements in the fire triangle.
- For example, covering a fire with a fire blanket removes the oxygen part of the triangle and can extinguish a fire.
- Fuel (Reducing Agent)
- Chemical Chain Reaction
A tetrahedron can be described as a pyramid which is a solid having four plane faces. Essentially all four elements must be present for fire to occur, fuel, heat, oxygen, and a chemical chain reaction.
Removal of any one of these essential elements will result in the fire being extinguished.
The four elements are oxygen to sustain combustion, sufficient heat to raise the material to its ignition temperature, fuel or combustible material and subsequently an exothermic chemical chain reaction in the material.
Removal of un-burnt material from fire area
Without fuel, a fire will stop. Fuel can be removed naturally, as where the fire has consumed all the burnable fuel, or manually, by mechanically or chemically removing the fuel from the fire. Fuel separation is an important factor in wildland fire suppression, and is the basis for most major tactics, such as controlled burns. The fire stops because a lower concentration of fuel vapor in the flame leads to a decrease in energy release and a lower temperature. Removing the fuel thereby decreases the heat.
Cutting off the supply of oxygen from the area.
Without sufficient oxygen, a fire cannot begin, and it cannot continue. With a decreased oxygen concentration, the combustion process slows. Oxygen can be denied to a fire using a carbon dioxide fire extinguisher, a fire blanket or water.
Removal of heat from the burning material/fire area
Without sufficient heat, a fire cannot begin, and it cannot continue. Heat can be removed by the application of a substance which reduces the amount of heat available to the fire reaction. This is often water, which requires heat for phase change from water to steam. Introducing sufficient quantities and types of powder or gas in the flame reduces the amount of heat available for the fire reaction in the same manner. Scraping embers from a burning structure also removes the heat source. Turning off the electricity in an electrical fire removes the ignition source.
4. Breaking of chain reaction
STAGES OF FIRE:
1. Incipient Stage
2. Smoldering Stage
3. Flame Stage
4. Heat Stage
- Oxygen plentiful
- Temperature has not built up to high peak
- Thermal updraft rises, accumulates at highest point
- Breathing not difficult
- Direct application of water at base of fire.
- Ventilation :not a problem
- Little steam production
- Ionization detector response to these particle.
2. Smoldering Stage;
- Oxygen supply not equal to demands of fire
- Temperature throughout building is very high
- Normal not possible
- Oxygen deficiency may cause back-draft
- Maximum steam production from water fog Photoelectric detector can detect this smoke
3. Flame Stage:
A region of fast reaction which covers the period from initial occurrence of flame to a fully developed fire. i.e. flame starts from the point of ignition and smoke decreases and heat increases.
- Flame radiation: Radiant energy emitted by the
flame is transmitted hemispherical to distant locations independently of the
- Convective heating: it becomes important only in the latter phase of fire development where large quantities of thermal energy has been released.
Infrared detectors can detect this stage.
4. Heat Stage:
At this stage large amount of heat, flame and toxic gases are produced. The transition from flame to heat stage is rapid and takes a few seconds.
Thermal detectors respond to this stage.
MODES OF SPREAD OF FIRE:
1.Conduction– it occurs only in solids i.e. Metallic objects
2.Convection– it occurs in both liquids and gases.
3.Radiation– it is neither conductor nor convection. These are heated rays emanating from the hot objects.
- Heat energy is passed on from one molecule to the next
- The molecule vibrates above its mean position
- Pass on heat energy by colliding with their neighbors
- The ability to conduct heat varies between materials.
- When a liquid or gas heated, it expands.
- It becomes less dense.
- The lighter fluid arises being displaced by colder and therefore denser fluid.
- In turn becomes heated and so a circulation is set up.
- Heat energy is carried throughout the fluid by actual movements of molecules.
- It does not involved any contact between bodies
- It is independent of any material in the intervening space
- These are the heated rays, travels in straight line in all directions with same intensity
FACTORS CONTRIBUTING TO FIRE:
They are many. Some are easily detectable while some are hidden. Easily detectable factors contributing to fire are as under:
1. Easy availability of combustible material like rubbish, solvent, paper, wood etc.
2. Easy availability of air, oxygen or any oxidizing material.
3. Sources of ignition like spark, static discharge, contact of hot surfaces, friction etc.
4. Continuous running machinery without proper lubrication and maintenance.
5. Non flame proof electrical fitting in flammable areas.
6. Habit of smoking in flammable areas.
7. No provision of fire detectors in fire prone areas.
8. No provision of fire extinguishers in fire prone areas.
9. Open handling of flammable substances.
10. No compliance of fire safety rules. Some hidden factors contributing to fires are as under:
(i) Chemical reaction going out of control.
(ii) Sudden stoppage of cooling media protecting flammable reaction or distillation of solvent.
(iii) Trapping of metal parts, nails etc. in rollers or moving machinery giving sudden spark.
(IV) Non availability of inert material on reaction of flammable substances.
(V) Sudden lightning from the sky.
Flash Point : It is the lowest temperature at which an inflammable substance gives off sufficient vapours, so as to form a momentary flash on application of a pilot flame.
Fire Point : It is the lowest temperature at which the heat from the combustion of burning vapours is capable of producing sufficient vapours to enable combustion to continue.
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