ABSTRACT
The consequences of imperfect combustion, which can be undesirably negative, are many, resulting usually in the following:
• Increased emissions contributing to increased acid rain, global warming, smog, materials degradation, and health hazards
• Reduced energy releases, efficiency, and peak temperature • Increased limitations to the types of fuels that can be used • Increased likelihood of corrosion and degradation of materials • Limitation to the type of devices that can be used • Having to resort to the use of costly improving additives to the fuel • Having to use costly remedial measures such as catalytic converters • Increased problems with lubrication and materials compatibility • Increased maintenance and capital costs • Reduced useful life of equipment
Combustion may be defined as any exothermic chemical reaction that is relatively rapid� It covers, in general, rapid reactive processes that release thermal energy� Normally, these reactions take place between a fuel and an oxidant, often air, in a limited variety of proportions� Hence, all combustion processes involve time-dependent chemically reactive processes with increased products temperature� The type and nature of the fuel burned have profound effects on the rate and course of the combustion process and the performance of the associated device used�
A flame is a reaction front that propagates subsonically while releasing thermal energy via chemical reaction and transport processes� The propagation of the flame within the fuel-air mixture comes about through the energy and active species releases by the chemical reaction transported via heat and mass transfer to the fresh mixture yet to burn, thereby raising in turn its temperature and enhancing its reaction activity to release its own energy�
Figure 8�1 shows schematically the typical changes in temperature and corresponding reactants concentration along a propagating one-dimensional laminar flame front within a homogeneous fuel-air mixture� This way, the flame propagation characteristics depend on the chemical nature of the mixture as well as the nature and rate of transport processes of heat and mass from the reaction zone mainly to the reactants� This propagation of the flame can be laminar or turbulent� Laminar flow is encountered when the particles of the fluid move essentially in straight lines parallel to the bulk direction of the flow� Turbulent flow is encountered when the fluid flow changes the velocity of a given fluid particle constantly both in magnitude and in direction�
Because of the associated vigorous transport processes associated with turbulence, turbulent flames are much faster and can release very much greater amount of energy per unit volume than their laminar counterpart� For example, flames within the gas turbine combustor or spark ignition engine are always turbulent so as to achieve large amounts of rapid energy release in a relatively limited compact space� An important parameter in fuel combustion is the maximum amount of energy that can be released by the chemical reaction per unit volume of combustor and time� This maximum is normally associated with stoichiometric mixtures, while lean or fuel-rich mixtures tend to produce less energy with correspondingly lower propagation rates� Figure 8�2 shows typically the variation of the flame propagation rate of hydrogen with changes in the fuel concentrations in the mixture with oxygenated air for various relative concentrations of oxygen in the mixture� It can be seen that enriching the oxygen concentration in the mixture for the same equivalence ratio or fuel concentration increases the flame speed very significantly�
Similarly, the increased presence of a diluent such as nitrogen or carbon dioxide with a fuel reduces the flame speed� The addition of carbon dioxide produces greater reductions than the addition of nitrogen�
Ignition is the process of initiation of combustion through accelerating the exothermic reactive processes beyond the prevailing rate of dissipation of energy out of the reaction zone to the surroundings� The initiation of ignition requires a minimum amount of energy that depends on the fuel used and other operating conditions� The energy is to be supplied from an external source such as a hot spot, a sparking plug, or through mixing with sufficiently hot gases� Stoichiometric mixtures normally require the least minimum amount of energy for ignition� Figure 8�3 shows the variation of the minimum ignition energy required for a number of common fuels in air with changes in the equivalence ratio� It can be seen that slightly richer than stoichiometric mixtures require the lowest amount of energy�
The term “ignition temperature” normally relates to the lowest temperature of the mixture at which sustained combustion can be initiated� Often in practical combustion systems such as furnaces, gas turbine combustors, or automobile engines, much higher ignition energies and temperatures are used to ensure prompt ignition and continued combustion under sometimes adverse local conditions, such as under cold temperature starting or when using excessively lean or rich mixtures�
Table 8�1 lists the values of the minimum ignition temperature at atmospheric pressure for a number of common fuels in mixtures with air� The corresponding temperature values in oxygen are somewhat lower than those in air� Similarly, in atmospheres containing diluents or some products of combustion, higher temperatures will be needed to ensure satisfactory and prompt ignition�
Autoignition or self-ignition takes place as a result of self heating produced by the acceleration of exothermic chemical reactive processes without the need for an external energy source of ignition� The autoignition characteristics are of paramount importance from the point of safety and suitability
of a fuel for spark ignition and compression ignition engine applications� Fuels for spark ignition automobile type engines require very high resistance to autoignition so as to resist the tendency for the undesirable uncontrolled compression ignition of part of the cylinder mixture� However, those for diesel engine type operation need to have very low temperatures because their combustion proceeds only following compression ignition� In general, heavier long-chain hydrocarbons autoignite more readily and quickly than lighter fuels� Stoichiometric mixtures are usually easier to autoignite than lean or rich fuel-air mixtures� Increasing the initial mixture temperature and/or pressure tends to lower the autoignition temperature� Fuels in oxygen also tend to autoignite much more readily than in air� Some typical minimum values of autoignition temperatures for a number of common fuels in air are listed in Table 8�2� For hydrocarbon fuels, as the number of carbon atoms increases, the lower will be the autoignition temperature� Also, isomers have higher ignition temperatures than their corresponding straight chain compounds�
There are basically two major classes of flames� These are of the unmixed fuel and air, diffusion flames, and those of the premixed type (Figure 8�4)� In diffusion flames, the fuel and oxidants are either separated initially or not
premixed thoroughly� The oxidation reaction and consequent flame propagation are governed by the processes of interdiffusion and the resulting extent of mixing between the fuel and the air� Commonly, in diffusion flames, these mixing processes control the rate of fuel burning and associated energy release� This is because these physically controlled processes are much slower than those of the chemical reaction processes� On this basis, the aerodynamics of the system such as turbulence level and air entrainment rates are more important in determining the combustion characteristics, such as those of the size and stability characteristics of the resulting flame, than the reactivity of the fuel and air� Usually, in diffusion flames, the fuel and oxidant come together in the reaction zone through mixing processes of molecular and turbulent convective diffusion� In practice, diffusion flames in comparison to the premixed type have a relatively wide region over which the local composition changes significantly� Diffusion flames are thus considered to be more stable and tolerant to changes in the quality and type of fuel than the premixed type of flames� Examples of diffusion type flames can be seen in the combustion of solid fuel surfaces and particles, liquid fuel pool surfaces and droplets, candles, and gaseous fuel jets� Figure 8�5 shows schematically a gaseous fuel turbulent jet burning in air after discharging from the orifice of a burner�
As shown schematically in Figure 8�5, the concentration of the fuel at regions away from the nozzle gets gradually reduced and becomes diluted through mixing with air entrained/sucked from the surroundings of the jet� After a certain distance from the orifice, the origin of fuel discharge, a stoichiometric mixture envelope is formed� Before this location, fuel-rich mixtures are formed, while beyond it increasingly lean mixture envelopes are developed� Hence, when the fuel jet is ignited, a diffusion flame will be formed and gets anchored more readily around the most reactive fuel-air mixture region that
is available within the stoichiometric mixture envelope� Peak temperature values will be located around this stoichiometric flame envelope�
The axially symmetrical radial variations in the temperatures are shown for three typical horizontal planes in the axial direction of the axially symmetrical issuing fuel jet� Figure 8�6 shows the axial and radial effective equivalence ratio of the local mixture along a jet diffusion flame of methane in air at ambient conditions� It can be seen that as the fuel travels away from the plane of discharge, it gets diluted by the entrained air while extending outwardly�
A similar set of processes basically takes place around a burning liquid fuel droplet in air in the absence of the effects of gravity, as shown schematically in Figure 8�7� The liquid fuel evaporates and diffuses radially outwardly to meet the surrounding air, forming a wide range of transient local fuel vapor-air mixtures� The resulting diffusion flame will be located around the stoichiometric shell with the fuel vapor on the inside and the air outside� The products of combustion diffuse radially outwardly�
However, in practice, the spherical nature of the diffusional processes cannot be retained due to the influence of gravity producing intense distortion to the shape of the flame due to natural convection�
Nevertheless, as shown in Figure 8�8, a vertically elongated diffusion flame is formed around the droplet with maximum values of temperature along the flame surface which corresponds to the stoichiometric mixture envelope�
Another schematic representation of a typical diffusion flame is that of the common candle, shown in Figure 8�9� Its combustion involves physically controlled processes that bring about the characteristic diffusion type of flame� The burning rate of the candle depends primarily on the rates of melting
some wax, its diffusion up the wick through surface tension effects, and its subsequent vaporization so that the resulting fuel vapor will diffuse outwardly, meet, and react with the surrounding air� The flame can be snuffed if any one of these processes is interrupted�
Some other common examples of diffusion un-premixed type combustion are shown in Figure 8�10 for a liquid fuel pool and in Figure 8�11 for a vertical surface of a solid fuel burning, such as that of paper or plastic sheeting� These examples demonstrate the complex interplay between the multitude of physical and chemical processes that bring about the combustion of the fuel� The pool fire shown in Figure 8�11 is a schematic representation of the diffusion type combustion of a liquid fuel surface involving fuel vaporization through heat transfer off the flame and its subsequent combustion within the vapor phase�
In premixed flames, the fuel and oxidant are already mixed together before combustion starts such as in the case of the conventional spark ignition automobile engine or the common Bunsen burner, shown schematically in Figure 8�12� In such premixed flames, the processes of mixing the fuel and air have been completed before approaching the combustion zone� Accordingly, the values of the associated chemical reaction rates become relatively quite significant and controlling� Hence, the type of fuel and its
relative concentration in air are very important in establishing the local flame propagation rate� However, it remains strongly influenced by the local heat and mass transfer from the flame front region to the fresh reactant mixtures, as was shown in Figure 8�1�
Sometimes, the term “partially premixed flames” is used for premixed systems that are only partially premixed with additional fuel or oxidant required to initiate combustion�
In general, the roles of the chemical processes are significant in situations where the physical mixing processes are fast or completed such as at low temperature� At sufficiently high temperatures, the oxidation reactions that are exponentially dependent on temperature proceed very quickly
in comparison to other physical processes that are approximately linearly dependent on temperature� Often the contribution of the latter for the sake of simplicity may become effectively deemphasized�
In a stationary fuel-air gas mixture following ignition, a flame will propagate, consuming the fuel by combustion� Usually in practical applications, the flame needs to be stabilized and anchored at a fixed region of the combustion device so that the energy release would take place continuously where required while the fuel-air mixture is fed continuously� This is achieved in regions where the local flow velocity is equal and opposite to that of the flame propagation rate� A representation of the combustion processes within a simple gaseous fuel furnace is shown schematically in Figure 8�13� Intense mixing processes, with heat transfer to the walls, take place to ensure complete and steady combustion of the fuel-air mixtures�
Flame flashback takes place in the premixed regions of combustors due to upstream flame propagation backward from the main combustion zone� Premixed fuel-air systems are more prone to flame flashback than diffusion type flames� Flashback can occur in the device in the free stream and in the low-velocity flow within the boundary layer along the surfaces on any solid object such as the flame holder supports and walls� Accordingly, flashback in
the mainstream is due mainly to flow reversal� Also, flashback occurs when the turbulent flame speed is greater than the local flow velocity� The combustion of lean mixtures produces low flame velocities and thus is more prone to increased flame instability and flashback� On the other hand, diffusion flame type combustion will be associated with a widely varying mixture composition with the stoichiometric mixture zone featured prominently in the combustion, ensuring at the same time a wide range of burning rates� Hence, diffusion combustion represents much more stable flame characteristics in comparison to the premixed regions� The local wall temperature, turbulent boundary layer thickness, and flow characteristics will affect the flashback of flames along the boundary layer over the various combustor surfaces�
Another important flame stability phenomenon is that of flame blow off, which occurs when the flame cannot be made to remain where it is intended and needed at the burner exit� The flame moves away from that location, which invariably results in its extinguishment� It is then described as blown off and takes place when the local velocity of the fuel-air mixture is in excess of the velocity of flame propagation within the mixture at the same location�
Lifted flames are flames that become undesirably detached from the issuing nozzle burner because the local mixture stream velocity is greater than the local flame velocity� Yet, the velocity is not sufficiently high to blow the flame off� Lifted flames are unstable and often lead to blowing off� In practice, the operation of combustion systems on a wide range of fuels ought to ensure throughout that stable flames are always involved with no blow off, flashback, or lifting allowed to take place�
The length of the diffusion flame is a gross indication of the overall reaction rate for the various combinations of operating conditions (Figure 8�14)� For the same discharge rate, the longer the mixing and reaction times, the longer is the flame� With laminar flames, the length is linearly related to the burning rate, which can be viewed very approximately as the fuel discharge rate without the flame blown off�
The color and intensity of a flame will be governed by the type of luminous emission, which is dependent on the wavelength and temperature� For example, blue flames are more associated with high-temperature vigorous
burning where oxygen-carrying radicals such as OH predominate, whereas long orange-color flames are of lower average temperature and reflect oxygen-deficient processes� The length of a diffusion flame also has practical significance, for example, in combustors, where it can be an indication of the extent of heat transfer capacity� The length of flames needs to be controlled in furnaces and gas turbine applications to ensure that no excessive heat transfer will result in the buildup of unacceptably high-temperature surfaces� As an example, some of the main requirements of combustors for gas turbine applications are the following:
• The flame must remain lighted over all operating conditions including transient conditions, such as during starting, shutting down, and rapid changing of load�
• The combustion efficiency must be very high and must approach as much as possible 100%�
• The engine must be able to tolerate sufficient variations in fuel composition and its supply rate�
• Operational pressure drop within the combustor must be ensured to be as low as possible, because it affects adversely the power output and efficiency of the system�
• The device needs to be capable of very wide fuel supply rates and high associated turndown ratio�
• Sufficiently low exhaust emissions must be ensured throughout� • The extent of heat transfer to the walls and combustor components
must be sufficiently low so as to protect the material of the walls from thermal damage�
• It must be easy and quick to start and stop the device promptly�
The combustion and consequent energy release rates in gas jet flames are dependent on the rates of the physical mixing processes between the fuel, air, and products and only partly dependent on the speed of the chemical reaction� Their relative contribution can vary depending on the situation being considered, but in general, the mixing processes tend to be sufficiently controlling to reduce the importance of the chemical aspects, and hence are less critically dependent on the type of fuel burned� At high speeds, turbulence becomes a very important consideration for speeding up the mixing processes considerably� Also, in the moving surroundings of the jet, the entrainment rate of air into the fuel jet is an important factor as well as the temperature rise due to some fuel burning in the surrounding flow�
In the presence of a crosswind situation, the mixing processes become quite complex, involving the generation of vortices that contribute to incomplete combustion, flame quenching, and increased emissions� Depending on the velocity, steadiness, and direction of the wind, the stability of the
flame may become undermined significantly, such as in the combustion of gas flares in the open atmosphere (Figure 8�15)�
In a burner operating by the discharge into air of a fuel through an orifice, the flow pattern within the primary zone is of much importance to flame stability� Recirculation of some hot combustion products through vortices to mix with incoming fuel and air is always employed to ensure the stabilization of flames within the combustor� Swirlers, especially around fuel injectors, are often used to induce this flow recirculation and mixing� The presence of solid bluff bodies across the bulk flow in the combustion zone also induces vortex shedding and mixing, aiding the stabilization of flames (Figure 8�16)� Through measures such as these, the control of combustion stability and intensity is improved� Flame stabilization can also be achieved through recirculation via the action of opposing jets such as in some furnaces�
The function of the flame holder, which is often used in fuel combustion systems, is to create within the flow a region where the velocity is sufficiently low so as to be lower than the prevailing burning velocity of the local mixture� Behind such a flame stabilizer, the fresh mixture in the shear layer is ignited by the hot combustion products entrained therein from the recirculation zone� The burning mixture then flows downstream within the shear layer and in turn ignites further on neighboring pockets of fresh mixture� When the fully burned gases leave the shear layer, some portions recirculate back into the wake region, thereby providing a continuous source of ignition to the high-speed incoming fresh mixture (Figure 8�17)�
A pilot jet flame employing an auxiliary fuel such as natural gas is an effective source of a continuous supply of energy and ignition for coflowing
streams of lean mixtures of another fuel and air� The lean mixture spread flame limit is the volumetric fuel concentration in the adjoining flow, which produces sufficient thickening of the jet flame to allow a flame to propagate, setting the whole surrounding flow alight� Pilot flames used, though they may be small in size, are usually more than sufficient to provide the energy needed for combusting the surrounding flow�
The reaction directly between a solid fuel and oxygen is very slow normally in comparison to fuel vapor combustion� Usually, the solid must emit some combustible volatilized material from its exposed surfaces, which diffuses outwardly away to meet and mix sufficiently with the surrounding air� The
resulting gaseous fuel-air mixture with the vaporized components can then react in the form of a diffusion flame to produce thermal energy and products of the oxidation processes� Some of the thermal energy released will then get transferred to the solid surface to enhance the production of yet more volatiles, while the products diffusing outwardly are replaced by fresh air, thereby intensifying the combustion, as shown schematically in Figure 8�17� Accordingly, those solid fuels that contain inherently relatively substantial amounts of volatile matter will be more readily combustible than those producing little volatile matter� The volatiles emitted, particularly initially, usually contain a high proportion of water vapor in the form of moisture while the solid fuel is drying as well as resulting from the breakup of fuel molecules releasing water vapor and other combustible vapors� After establishing intense combustion and high temperatures, there will be an increased likelihood for less-or nonvolatile combustibles reacting to release further thermal energy and different products�
Since the composition of most common solid fuels contain noncombustible solid components, these eventually will appear as leftover ash that contains mostly inorganic material with some missed reacting solid carbon that is described as char�
The region within the resulting diffusion flame contains little or no oxygen with a significant fraction of the heat transfer through radiation, which produces the characteristic orange flames associated with diffusion flames�
Figure 8�18 shows a schematic representation of a solid fuel combustor where the fuel is fed at the top and ash removed at the base� Primary air
that is supplied continuously at the base gets heated in its upward travel� It can be seen that the solid fuel at a region near the base where there is plenty of oxygen available gets oxidized fully to carbon dioxide producing high temperatures� However, the hot product gas on its travel upward propelled by thermal natural convection meets more unconverted solid fuel and gets increasingly reduced to carbon monoxide while the temperature gets reduced� Later on, through the appropriate supply of additional secondary air, these gases may get fully oxidized at the stage of the exit of the combustor (Figure 8�19)�
Fluidized bed combustion is a method of burning fuel in which it is continually fed into a bed of reactive or inert solid chipped material such as silica sand or dolomite while a flow of air passes upward through the bed, partially lifting the particles and causing them to suspend and behave as a turbulent fluid� Fluidized beds have been used for the combustion of lowquality, difficult-to-burn fuels and have been receiving increasing attention and wider applications in recent years as a potential means for effective burning of solid fuels� Figures 8�20 and 8�21 show schematic representations of a fluidized bed combustor and the associated changes in the fluidizing velocity as the pressure difference applied to the bed is increased�
Much research and development have gone into the effective utilization of fluidized bed combustion for the generation of thermal heat energy while using pulverized or granular solid fuels such as coal, biomass, or shredded municipal waste� There is a continuing need to make these combustors yet more effective and environmentally acceptable so that they can be sufficiently attractive and more widely usable�
There are a number of advantages for using fluidized beds for the combustion of granulated solid fuels such as coal� These include the following:
• Most types of coal can be burned� • Sulfur removal may be made by including calcium or magnesium
oxides within the bed�
• Oxides of nitrogen problems are eased very significantly by having low combustion temperatures�
• Ash in the coal remains in the bed and is removed periodically� • Good heat transfer is available within the beds for indirect firing� • The beds can be pressurized�
However, there are also limitations yet to be overcome more effectively to the wider employment of fluidized beds� These include the following:
• The exhaust gas needs treatment to remove fuel and bed material fly ash�
• Poor turn-down ratio with slow start-ups and shut-downs� • Must abstract effectively heat from the hot exhaust gases� • Attrition of the granular bed material contributing to emissions and
increased costs�
1� The figure on the next page shows a schematic diagram of the radial variation of a number of key variables across a turbulent jet flame of natural gas in a furnace� Identify the curves shown from the following list of variables:
a� Velocity b� Fuel concentration c� Oxygen concentration d� Temperature e� Carbon dioxide
2� A stoichiometric mixture contains a theoretical amount of air that will permit ideally the combustion of the fuel� However, in practice, there is no assurance that simply by providing the correct fuel to air ratio, combustion will be initiated or completed� Very briefly outline the reasons for this behavior� Indicate some of the measures that may be taken to ensure more complete combustion�
3� The burning rate in the following processes is predominantly controlled by physical factors such as mixing and diffusion except the following:
a� Combustion of charcoal in a stove b� Combustion of natural gas in a laboratory burner c� Combustion of waste heavy oil fractions in open pools near oil
wells d� Combustion of propane in an automotive spark ignition engine e� Combustion of a candle
(Answer: b and d) 4� The combustion efficiency of a furnace in principle is likely to be
enhanced by the following measures except the ones identified: a� Slight preheating of the fuel b� A reduction of the volume of the furnace c� A reduction in wall surface temperature d� A proportionate increase in the flow rates of the fuel and air e� A reduction in the mean diameter of the liquid fuel droplets
Indicate briefly the basis for your choices�
(Answer: b, c, and d)
All aspects of the performance of fuel-consuming devices are critically dependent on the nature, progress, and completion of the combustion processes involved� It is essential to control these appropriately and optimally depending on the fuel employed and local operating conditions so as to ensure complete, reliable, safe, clean, and efficient exploitation of the fuel resource� Combustion processes may be classified into two broad categories� These are a primarily physically controlled diffusion type combustion and a premixed type that is relatively more affected and controlled by the chemical nature of the fuel and to a much lesser extent by the transport type processes (Figure 8�22)�
