Information about the Fire Combustion

A Definition of Fire

One generally accepted definition of combustion or fire, is a process involving rapid oxidation at elevated temperatures accompanied by the evolution of heated gaseous products, and the emission of visible and invisible radiation. 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, light, and other exhaust products. Oxidation, in the strict chemical sense, means the loss of electrons. In 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, 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 where the fuel loses 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 if it is a building on fire we refer to this process as an uncontrolled fire.

The Combustion Modes

The combustion process occurs in two modes:

• Flaming
• Non-flaming, smoldering or glowing embers.

For the flaming mode, it is necessary for solid and liquid fuels to be vaporized. The solid fuel vapors are thermally driven off, or distilled and the liquid fuel vapors evaporated. It is this volatile vapor from the solid or liquid fuels that we see actually burning in the flaming mode. This gas or vapor production emitted from the fuel is referred to as paralysis. Once a flame has been established, heat transfer from the flame to the fuel surface continues to drive off more volatile gases and perpetuates the combustion process. Continued burning in the flaming mode requires a high burning rate, and the heat loss associated with the transfer of heat from the flame area by conduction, convection, and radiation must be less than the energy output of the fire. If the heat loss is greater than the energy output of the fire, the fire will extinguish.

Both modes, flaming and non-flaming, can occur separately or in combination. Flammable liquids and gases only burn in the flaming mode. Wood, straw, and coal are examples where both modes may exist simultaneously.

Flaming combustion can occur in the following forms:

1. Premixed flames where the fuel and oxygen are mixed prior to ignition. For example the flame of a Bunsen burner, gas stove, or propane torch.
2. Diffusion flames, more common, where the fuel and oxygen are initially separate but born in the region where they mix, like burning a pool of flammable liquid or the burning of a log.

Stages of a Fire

There are three generally recognized stages of a fire. The incipient stage, smoldering stage, and flame stage.

The incipient stage is a region where preheating, distillation and slow pyrolysis are in progress. Gas and sub-micron particles are generated and transported away from the source by diffusion, air movement, and weak convection movement, produced by the buoyancy of the products of pyrolysis.

The smoldering stage is a region of fully developed paralysis that begins with ignition and includes the initial stage of combustion. Invisible aerosol and visible smoke particles are generated and transported away from the source by moderate convection patterns and background air movement.

The flaming stage is a region of rapid reaction that covers the period of the initial occurrence of flame to a fully developed fire. Heat transfer from the fire occurs predominantly from radiation and convection from the flame.

Classes of fire

Combustible and flammable fuels involved in fires have been broken down into five categories:

• Class A fires – are fires involving organic solids like paper, wood, etc.
• Class B fires – are fires involving flammable liquids
• Class C fires – are fires involving flammable gasses
• Class D fires – are fires involving metals
• Class F fires – are fires involving cooking oils.

Summary

A fire begins with an external ignition source in the form of a flame, spark, or hot ember. This external ignition source heats the fuel in the presence of oxygen. As the fuel and oxygen are heated, molecular activity increases. If sufficiently heated, a self-sustaining chemical chain reaction or molecular activity occurs between the fuel and oxygen. This will continue the heating process and the resulting chain reaction will escalate without the need for an external ignition source. Once ignition has occurred, it will continue until:

1. All the available fuel or oxidant has been consumed, or
2. The fuel and/or oxygen is removed, or
3. The temperature has been reduced sufficiently, or
4. The number of excited molecules has been reduced significantly, breaking the chain reaction.

Explosions

Generally, an explosion is defined as a very rapid release of high-pressure gas into the environment. The energy from this very rapid release of the high-pressure gas is dissipated in the form of a shock wave.

Explosions can be classified as physical (a balloon bursting), as physical and/or chemical (a boiler explosion), or a chemical reaction of a gas/particle mixture. Our discussion will focus on chemical reaction explosions.

The process of a chemical reaction explosion is similar to the combustion process whereby a fuel and oxidizing agent have mixed prior to ignition, such as petroleum vapor or fine particles of grain dust mixed with air. However, in an explosion the oxidation process proceeds at a greatly accelerated rate. The oxidation process is usually, but not always, confined within an enclosure, such as a tank or grain silo, so that a rapid high-pressure rise occurs with an associated flame front. Generally, it is this high-pressure shock wave that causes the damaging effects of an explosion.

Resultant shock waves that propagate from the point of ignition at a velocity less than the speed of sound are termed deflagration. Shock wave velocities in excess of the speed of sound are termed detonations.

A rise in pressure, creating a shock wave of 6894.76 Pascals is sufficient to knock a person down. If the rise in pressure creates a shock wave of 13789.52 Pascals to 20684.28 Pascals, this is sufficient to shatter an 8 to 12-inch thick concrete wall. A Pascal (Pa) is equivalent to 1 N/m2 (one newton per square meter).