Flame involves the Chemical Reaction between one chemical substance called a Fuel, and another chemical which is an oxidizer (or oxidant). In special cases, the fuel and the oxidant may be combined within the same chemical molecule and this is the case in some propellants and explosives. The chemical reaction between the fuel and oxidant is called Combustion; it is accompanied by the release of heat and usually by the emission of light in the visible region of the spectrum. In the case of a premixed hydrocarbon flame burning in air, the light emitted is normally blue if the mixture is fuel lean, and it gives the location of the flame and in particular, because of its greater intensity, the position of the flame front. However, if the mixture entering the flame is fuel-rich a yellow soot-producing flame is produced which is termed a luminous flame.
Most flames result from highly exothermic reactions giving flame temperatures about 2200K although flames may be capable of burning down to about 1300K, depending on the fuel-air ratio. Certain flames can be sustained below this temperature and are termed "cool" flames, but here only partial combustion occurs. Typical flames result from the combustion with air of a gaseous fuel such as natural gas, commercial and industrial liquid fuels, usually termed fuel oils, which are burned as a spray, or by pulverized coal particles suspended in air as is the case in a power station boiler.
Different types of flames can result from the way in which the fuel and oxidant are mixed in a burner and by their flow rates. Premixed gas flames can arise from fuel gas and air being mixed prior to entering the burner, or if they mix after leaving the burner they are called diffusion flames. The gas flow rate may be relatively low, in which case, the incoming gaseous flow of fuel and air is laminar, as is the flame. With high gas flows, they may be turbulent. Thus, flames can be laminar premixed, laminar diffusion, turbulent premixed or turbulent diffusion flames as indicated in Table 1. The transition from laminar to turbulent flame takes place as the flow velocity increases as shown in Figure 1 [Hottel and Hawthorne (1949)]. In addition, they can also be categorized into stationary flames or propagating (travelling) flames, the former being the most widely used in domestic or industrial burners, the latter being involved in explosions.
The type of flame that has been most studied is the laminar premixed flame of a gaseous fuel and oxidant, usually air, because it is the simplest flame and exhibits characteristics common to many other systems. Typical is the Bunsen burner flame, a type of flame widely used in gas fires, gas cookers and central heating units.
The Bunsen burner flame is shown in Figure 2 [Gaydon and Wolfhard (1970)], but for research purposes, a flat flame is used using a special burner which produces uniform flow, as shown in Figure 3(a). The Bunsen burner, however, illustrates both the premixed flame and the principle of the diffusion flame. The inner core is the reaction zone of a premixed flame, but the flame is fuel-rich, so the products of incomplete combustion burn in the outer core as a diffusion flame with the surrounding air. The exact nature of the flame is determined by the fuel to air (mixture) ratio. If there is excess fuel, it would be termed rich and the flame would be a yellow luminous one. If there is excess air (or oxygen), it would be termed lean. If it has exactly the correct amount of fuel and air, it would be termed stoichiometric. Overall the combustion products would be represented by the stoichiometric equation, which in the case of methane (the major constituent of natural gas) is:
where –ΔHc is the heat released by combustion, known as the heat of combustion or calorific value (cv). The term stoichiometric refers to the situation where combustion is complete with no unused fuel or no unused oxidant. The region of incomplete combustion in the flame shown in Figure 2 represents only partial combustion of the fuel resulting in carbon monoxide and hydrogen which subsequently burns with the secondary air to give CO2 and H2O. This two stage combustion can be represented by the reactions, for example for methane, by
In general, all hydrocarbon flames, whether rich or lean, go through such a stage with CO and H2 being formed in the first main reaction zone, and the second stage combustion of the CO and H2 initially formed is characterized by the emission of blue light; this blue emission being a key feature of the combustion of all carbon (and hydrocarbon) containing fuels. This process is more pronounced in slightly fuel-rich flames and is termed afterburning.
Figure 3. (a) Flat flame burner illustrating definition of burning velocity. (b) Flame cone angle and showing definition of burning velocity Su = v sin α.
A premixed flame of a particular fuel-air combination is characterized by three main parameters, the burning velocity, flame temperature and flammability limit, which are also determined by the pressure, temperature and, of course, mixture ratio. Premixed fuel air mixtures have a characteristic burning velocity and this enables flames to be stabilized on a burner as shown in Figures 2, 3(a) and 3(b) if the flow of gas mixture equals the laminar burning velocity. The burning velocity is simply as defined for a flat laminar flame as in Figure 3(a), that is the approach velocity gives the burning velocity (relative to the unburned gas, Su) and usually given as m/s. For a conical flame, the laminar flame is as defined in Figure 3(b). In the case of laminar diffusion flames the fuel and the oxidant only meet at the burner mouth (i.e., they are not premixed) and mix by diffusion processes as the flame burns, as illustrated in Figure 4 In this case, the fuel gas and oxidant gas streams are slots giving a flat flame, but analogous axisymmetric flames can be obtained by the use of concentric tubes with the fuel usually entering via the inner tube.
Flames will only burn if they are within the flammability limit, that is the composition of the fuel-oxidant mixtures that will sustain a stable flame. There are two types of limits associated with the propagation of a laminar flame. The first is associated with the chemical reactive capability of the mixture to support a flame, i.e., the flammability limit. The second is associated with gas flow influences. Typical values are methane where the lower and upper flammability limits are 5 and 14 mol %, the stoichiometric ratio would be 9.47 mol %. In the case of n-heptane the limits would be 1 and 6 mol %, respectively, with a stoichiometric ratio of 1.87 mol %.
The combustion of liquid fuels or pulverized coal (or pulverized fuel, both abbreviated to pf) are both widely used in industrial burners, especially for large scale boilers used to generate steam for power generation. Industrial flames are generally turbulent in nature, and, for convenience and safety reasons, involve diffusion flames where the fuel and air are injected separately for safety reasons and the development of the length of the diffusion flame after the burner exit occurs with increased gas flow velocity as shown in Figure 1. With increasing gas velocity, there is a transition from laminar to turbulent diffusion combustion, although some diffusion mixing takes place, much results from turbulent interaction (turbulent diffusion).
Flames of liquid fuels [Williams (1990)] can range from blue, premixed like flames, through to highly luminous flames akin to coal flames. For liquid fuels to burn, they must be completely vaporized to give a vapor which burns in the same way as a gaseous flame. This is termed "homogenous" spray combustion. For more involatile fuels, the partially volatilized fuel burns as a spherical flame surrounding each droplet as shown in Figure 5, this is called "heterogeneous" spray combustion. An example of the first mode of combustion is in the combustion of aviation kerosine in an aircraft gas turbine where the fuel is largely vaporized after injection as a spray into the combustion chamber, some larger droplets however burn heterogeneously and tend to give smoke. The second mode is termed spray combustion where combustion takes place heterogeneously. This takes place in industrial furnaces and boilers and in diesel engines.
In pulverized fuel combustion, the coal particles, typically 100 mm in diameter, involve the following stages.
The coal particle enters the hot combustion chamber and heats up which results in pyrolysis of some of the more reactive components (typically 50%) of the coal, a process termed devolatilization. The devolatilization products burn in the first part of a pf flame with a yellow gas phase flame. Then the resultant carbonaceous char burns more slowly in a heterogeneous way involving particle surface reactions of the type
leaving residual mineral ash if combustion goes to completion. The flame produced is of a luminous, highly radiating cloud of red hot char and mineral ash particles.
Whilst the most commonly used flames are those of hydrocarbons (natural gas, oil, coal) with air, numerous other combinations should be noted. Of particular interest are:
High temperature flames produced by oxygen-fuel combinations such as acetylene (ethyne) with oxygen for welding and cutting torches, and with natural gas for the rapid melting of metals. The former produces flame temperatures (Tf) of about 3300K and the latter about 2700K. These high temperatures result because of the absence of the diluting nitrogen present in air and the high energy release from energetic (highly exothermic) fuels.
In principle, the combustion of hydrogen with fluorine produces the hottest flame (Tf 4300K). Powdered metals, e.g. aluminum, also produce high flame temperatures which are approximately controlled by the boiling point of the oxide produced which, in the case of aluminum, is 3800K. They can be used for cutting through thick metal sheets such as a safe, or through concrete walls. Metal tubes can also be used and the use of a steel tube with oxygen flowing through it forms the basis of the thermic lance, also used for cutting purposes.
Propellants: Rockets are propelled by the combustion of solid or liquid propellants, that is, by propellants that can be stored. Propellants of liquid fuels may consist typically of liquid hydrogen (LH2) plus liquid oxygen (LOx) (Tf = 3100K) used in the space shuttle, or liquid nitrogen dioxide (NO2) with liquid hydrazine (Tf = 3000K) used in smaller rockets. A solid propellant may be used, e.g., ammonium perchlorate and a rubber polymer binder. The perchlorate sublimes to form a gas (NH3 + HClO4) and the highly oxidizing perchloric acid reacts rapidly with the NH3 and the binder especially at high pressures. The object in all cases is to produce high temperature flames so that thrust can be maximized, although other factors can come into play.
These propellants may contain the fuel and the oxidant in one molecule (monopropellants) or they may be mixed before or during combustion (bipropellants).
Self-decomposition flames: Certain flames can be burned without the addition of an oxidizer because the fuel decomposes exothermically, although from such systems only relatively low flame temperatures (Tf) are achieved. Two well-known examples are that of acetylene (ethyne) and that of hydrazine. In the case of the former, the combustion reaction is:
which gives a flame temperature of 1650K.
For hydrazine, the self decomposition reaction is
giving a flame temperature of 1800K.
Since hydrazine is a storable liquid, it, or its methyl derivatives (CH3)2N2H2, is used in rocketry.
Unusual flames: Flames may be produced by the combustion of air or oxygen with a variety of unusual fuels such as ammonia (NH3, giving products of N2 + H2O), carbon disulphide (CS2, giving CO2 + SO2). Cyanogen (C2N2) and another uncommon fuel carbon subnitride give some of the highest flame temperatures, namely 4800K and 5260K, respectively, because the products, CO and N2, do not dissociate significantly.
Whilst many flames burn in unconfined situations, e.g., in a gas fire or in a gas cooker, most industrial flames, and many flames used in domestic environments such as central heating boilers, are burned in a partially confined situation, namely a combustion chamber. In certain exceptional circumstances, usually during malfunction, explosions can occur. Thus, during ignition, if too much fuel has entered the combustion chamber, or if the rate at which fuel enters the combustion chamber is too great, then the rate of energy generated and volume of combustion products generated is too great for them to escape through the flue. In such circumstances, the expanding flame can result in the build up of pressure and an explosion can occur. In general, such confined combustion processes result in a rapid increase in pressure which can damage the combustion chamber and indeed can result in destruction of the equipment. For this reason, in many industrial combustion processes an explosion (or pressure) relief is built into the combustion chamber (or process vessel) to overcome the problems caused by the high build-up of pressure and the subsequent explosion.
In the extreme case, a confined explosion can result because of the combustion of a flammable mixture in a completely closed system. The most common but safe example is that found within the combustion chamber of a reciprocating piston motor vehicle during its operation. Here, spark ignites the mixture, the flame expands and undergoes transition to a confined explosion, although actually in this case it is only partially confined because of the movement of the piston. Examples of truly confined explosions occur in the case where one has an explosion resulting from a leak of natural gas in a room, or of flammable process mixtures such as hydrocarbon vapor in a chemical plant explosion. Details of this are dealt with in another section. (See Internal Combustion Engines; Explosion Phenomena.)
Gaydon, A. G. and Wolfhard, H. G. (1970) Flames: Their Structure, Radiation and Temperature, Third Edition, Chapman and Hall Ltd., London. DOI: 10.1016/0022-460X(72)90510-X
Glassman, I. (1977) Combustion, Academic Press, New York.
Hottel, H. C. and Hawthorne, W. R. (1949) Third Symposium on Combustion and Flame and Explosion Phenomena, 254, Williams & Wilkins, Baltimore.
Williams, A. (1990) Combustion of Liquid Fuel Sprays, Butterworths, London.