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COMBUSTION OF FLAMMABLE GASES

THE SIMPLIFIED EXPLANATION

The process commonly known as combustion is a simple chemical reaction which involves two main components.  One component is the fuel, the other is oxygen. Oxygen is found as a significant part of atmospheric air.
Most, although not all, of the compounds which we consider to be fuels are primarily made up of carbon and hydrogen and are known as hydrocarbons.  The carbon and hydrogen atoms, which make up molecules of these fuels, are held together by atomic forces which represent "locked up" energy in the molecule.

Normally, when a fuel mixes with air at ambient atmospheric conditions there is no reaction or activity until something happens to make a small part of the mixture very hot.  Typically this heat could be generated in an electrical spark as in the case of a modern domestic gas stove or an automobile engine, or the heat could be created by the proximity of some other hot source such as a glowing hot coal or another flame.  As soon as a very small part of the fuel becomes hot enough, its molecular structure begins to break apart and some of the "locked-up" energy is released.  This is called ignition and, when it happens, the released energy and the high temperature it creates is sufficient to start the process in any adjacent section of the fuel and the process gradually spreads out from the point of initial ignition. The minimum amount of energy needed to create this ignition is called the activation energy.  Different fuels need different temperatures and energy levels to ignite the combustion but the common ignition methods mentioned above are usually sufficient for all fuels.

When the combustion reaction occurs, the various components break up into their elemental parts which then take on new forms.  When this happens, there is a change in the internal energy structure of the compounds, and some energy which was previously "locked up" in the fuel is released.  This energy primarily takes the form of heat and so the process generates high local temperatures.  This process is "exothermic".

The amount of heat energy available to be unlocked from any fuel is known as its Calorific value.

When the fuel converts some of its material (Hydrogen) to water, this is generated in the form of steam/water vapor.  It is theoretically possible to collect this water and condense it, thereby recovering the latent heat of evaporation.
Those calorific values which are calculated to include the recovery of the latent heat of the water formed during combustion are designated "Higher calorific value" [Higher Heating Value / Gross Calorific Value / HCV / HHV].
Those calorific values which exclude recovery of the latent heat of water formed during combustion are designated "Lower calorific value" [Lower Heating Value / Nett Calorific value / LCV / LHV].
Most combustion problems involve the Lower calorific value of the fuel.

The zone of the chemical reaction contains a variety of transient components, some of which are temporarily solid, like soot, and these glow red-hot because of the high local temperature.  This allows the reaction zone to be visible and it is known as a flame.

The manner in which the flame spreads out through a mixture is known as flame propagation.

The speed at which the flame propagates is known as the flame speed and depends on several different factors but is chiefly conditioned by the nature of the fuel and concentrations of the fuel and oxygen in the mixture.

In order to burn completely, all fuels need a fixed amount of oxygen.  The amount varies from fuel to fuel as a function of the composition but it is always the same for any given fuel.  The name given to the exact amount for any fuel is the stoichiometric quantity. and its numerical value in relation to the fuel quantity is the stoichiometric ratio.  Stoichiometry is determined by performing a combustion calculation based on the chemical composition of the fuel.

As an example of these characteristics, the stoichiometric quantity of air needed for Natural gas is very roughly ten (10) cubic feet of air for each cubic foot of natural gas.  At this mixture, the normal flame speed is approximately 1 foot per second.

If there is not quite enough air to burn the gas completely, combustion will still occur but it will leave some of the gas unburned.  With less than complete conversion, the flame is not as "powerful" and cannot travel as quickly into the mixture so in this case the flame speed is less than the stoichiometric flame speed.
The same thing happens when there is too much air, more than the stoichiometric amount, and again the flame slows down.
At normal atmospheric conditions, when there is less than about 60% or more than 150% of the stoichiometric air , the distribution of the energy release into the overall mixture has changed sufficiently for the flame to slow down so much that it can no longer burn and no mixture outside these approximate limits will support a flame.
(60% and 150% are representative values only and are not true for all gases)

The active flame-speed is a very important factor in the stability of a flame burning at the end of a pipe or at an orifice (jet).

If the gas in the pipe contains no air (or oxygen) then it must find all the oxygen it needs from the atmosphere.  As the gas emerges into the atmosphere it begins to mix (or diffuse) into the surrounding air and the flame will only occur in those areas where there is enough oxygen mixed with the gas to allow the mixture burn.  This causes the flame to take on a characteristic shape, rather like two cones base to base, and this flame is called a diffusion flame.

In a diffusion flame, the visible flame forms an envelope around a core of unburned gas which is constantly mixing with air and feeding into the visible flame.  As the gas spreads out from the pipe in a pseudo conical shape, it slows down and the envelope of flame establishes where the actual sideways velocity matches the flame speed of the mixture.  This type of flame can withstand wide changes in the velocity of gas emerging from the pipe because the flame simply grows larger if the velocity increases and smaller if it decreases.

If the gas in the pipe feeding a flame contains a quantity of air which is approximately stoichiometric it is called a premixed or partially premixed flame.  It needs little or no extra air from the atmosphere and tries to burn immediately at the end of the pipe.
If the velocity from the pipe is equal to the flame speed, the flame will be exactly balanced at the end of the pipe and will be stable.
Because of flame speed dependence on mixture strength the premixed flame is easily destabilized and does not readily withstand wide variations in discharge velocity.