Burners and Flames in Industrial Furnaces
Burners and Flames in Industrial Furnaces
A burner is the device used to burn the fuel, with an oxidizer to convert the chemical energy in the fuel into thermal energy. A given combustion system can have a single burner or many burners, depending on the size and type of the application. Burners’ heat (thermal energy) is derived by burning fuel and is required for technological processes. Burners are essential components of a furnace. They describe a series of equipment for burning various types of fuel under suitable conditions for perfect combustion.
Burner operates by sucking in the fuel and the combustion support air (oxidizer), mixes them thoroughly together and safely ignites them inside the furnace. While industrial furnaces have their specific requirements, most burners normally have a certain amount of heat release, a chamber pressure at the burner, and an excess air-fuel ratio. The requirements from a burner include stability, cost effectiveness, flame dimensions, reliable in use (ignition, capacity adjustment, and oxygen tolerances), emissions, fuel versatility, turndown ratio, energy efficiency, ease of maintenance, and compact and elegant design.
The main components of a burner are normally the burner nozzle, mixing tube, downstream section and a cross-connected regulator for air-fuel ratio control. Burner mixes the fuel and the air, and generates an optimum form of flame. It employs forced draft in mixing the fuel and air. Burner is an important component of a combustion system. The combustion system besides burner includes the combustion air supply, comprising of the fan and pipes for taking the air to the burner, fuel supply, comprising components used for regulating the fuel flow and guaranteeing the safety of the combustion system, and the electrical and control components required for firing the flame, the electricity supply to the motors and thermal output regulation developed by the burner.
The combustion head is the part of the burner which mixes the combustion supporting air with the fuel and stabilizes the flame which is generated. The components of the combustion head essentially are (i) the fuel metering device consisting of nozzles for liquid fuels and orifices and distributors for gaseous fuels, (ii) the turbulator diffuser disk, which mixes the fuel and the combustion air, and stabilizes the flame to avoid it blowing back into the burner, (iii) the flame ignition system which uses electric arcs produced by high-voltage powered electrodes directly igniting the flame or coupled with a pilot burner, (iv) a flame sensor for motoring the flame, and (v) the flame tube made of profiled metal cylinder which defines the output speed range.
The flame tube and the diffuser disk essentially determine the geometry of the flame developed by the burner. Especially the latter determines the rotational features of the fuel and combustion supporter mixture flow and, consequently, the flame dimensions. The rotational characteristic of the mixture flow is expressed in mathematical terms by the number of swirls defined as S = Af / (AxR) where S is the number of swirls, Af is the angular momentum of the flow, Ax is the axial force, and R is the radius of the nozzle outlet. As a rule, an increase in the number of swirls causes an increase in the flame diameter and a decrease in the flame length.
The section between the sleeve and the diffuser disk determines the amount of secondary air available for the flame. This amount is zero when the disk is closer and in contact with sleeve. In certain burners, it is possible to adjust the space between the disk and the turbulator diffuser. The changes in the secondary air is done by changing the position of the disk itself.
Combustion heads can be of three types namely (i) non-adjustable fixed head, where the position of the combustion head is fixed by the manufacturer and cannot be changed, (ii) adjustable head, where the position of the combustion head can be adjusted by the burner installer during commissioning, and (iii) variable-geometry head, where the position of the combustion head can be varied during the modulating burner operation.
Non-adjustable fixed head burners are generally burners used for industrial processes and dedicated to the generators they are to be with. For adjustable head burners, regulation is preset in correspondence with the maximum output of the burner for the specific application. For easier start-up and configuration operations, a graph is provided for each burner indicating the position of the regulation mechanisms in relation to the required thermal output. This construction type makes the burner suitable for different requirements, which is why monobloc forced draft burners with a low to medium output are predominantly adjustable head types. Variable-geometry burners are generally high output modulating burners. The geometry of the head is also extremely important in reducing polluting emissions especially NOx.
There are several types of burner designs which are available because of the wide variety of fuels, oxidizers, combustion chamber geometries, environmental regulations, thermal input sizes, and heat transfer requirements, which include things like flame temperature, flame momentum, and heat distribution.
There have many changes in the traditional designs which have been used in burners, mainly because of the recent attention in reducing pollutant emissions. In the past, the burner designer was primarily concerned with efficient burning of the fuel and the transfer of the heat energy to the charge. Presently the increasing environment concerns have added the need to consider the pollutant emissions produced by the burner while designing the burner..
In several cases, reduction in the emissions and maximization of combustion efficiency are at odds with each other. As an example, a well-accepted technique for reducing NOx emissions is known as staging, where the primary flame zone is deficient of either fuel or oxidizer. The balance of the fuel or oxidizer can be injected into the burner in a secondary flame zone or, in a more extreme case, can be injected somewhere else in the combustion chamber. Staging reduces the peak temperatures in the primary flame zone and also alters the chemistry in a way which reduces NOx emissions since fuel-rich or fuel-lean zones are less conducive to NOx formation than near-stoichiometric zones.
Another potential issue with staging is that it can increase CO (carbon mono oxide) emissions, which is an indication of incomplete combustion and reduced combustion efficiency. It is also possible that staged combustion may produce soot in the flame, which can increase flame radiation.
In the past, the challenge for the burner designer has been often to maximize the mixing between the fuel and the oxidizer to ensure complete combustion, especially if the fuel has been difficult to burn, as in the case of low heating value fuels. Now the burner designer is to balance the mixing of the fuel and the oxidizer to maximize combustion efficiency while simultaneously minimizing all types of pollutant emissions. This is no easy task since NOx and CO emissions frequently go in opposite directions. When CO is low, NOx can be high, and vice versa. Modern burners are to be environmentally friendly, while simultaneously efficient in transferring of heat to the charge.
Although most of the burners used in the industrial furnaces have common operating characteristics, they can be classified in several ways. The common ways of classifying burners are (i) by motive force such as natural draft, forced draft, and self-aspirated, (ii) flame type such as round, rectangular or flat, (iii) placement in the furnace such as free standing, in the end wall, in the side wall, or in the roof, (iv) fuel type such as solid fuel, gas, oil, or combination, (v) fuel air mixing such as pre-mixing, diffusion, (vi) NOx emission control such as conventional, low NOx, and ultra-low NOx, and (vii) NOx reduction method such as air staging, and fuel staging. Schematic diagrams of some types of burners are shown in Fig 1.
Fig 1 Schematic diagrams of some types of burners
Burners are also classified according to the type of oxidizer used. The majority of industrial burners use air for combustion. In many of the higher temperature heating and melting applications, the oxidizer is pure oxygen (O2). In other applications, the oxidizer is a combination of air and O2, usually referred to as O2 enriched air combustion. Another way to classify the oxidizer is by its temperature. It is common in many industrial applications to recover heat from the exhaust gases by preheating the incoming combustion air either with a recuperator or a regenerator. Such a burner is often referred to as a preheated air burner.
Fuel choice has an important influence on the heat transfer from a flame. In general, solid fuels like coal and liquid fuels produce very luminous flames which contain soot particles that radiate like blackbodies to heat the charge. Gaseous fuels like natural gas often produce nonluminous flames because they burn so cleanly and completely, without producing soot particles. A fuel like hydrogen is completely nonluminous, as there is no carbon available to produce soot. In cases where highly radiant flames are required, a luminous flame is preferred. In cases where convection heat transfer is preferred, a nonluminous flame can be preferred in order to minimize the possibility of contaminating the heat load with soot particles from a luminous flame.
Further burners can be open flame burner, or radiant heat burner. Open flame burners are those burners where the flame is not confined as is the case in radiant tubes, nor are the flames primarily attached to a surface as in porous refractory burners. Open flame burners are normally visible to the naked eye where the radiant heat from the flame, rather than from a surface heated by the flame, can directly heat the load. Radiant burners operate by combusting a fossil fuel, which heats a solid surface that radiates infrared (IR) energy to a load. These burners are used in a number of lower temperature heating and drying applications. Both gas-fired and electric radiant heaters are commonly used.
Apart from its classification as given above, a burner can also be designed based on factors like the combustion chamber geometry, type of oxidizer and also heat transfer requirements which include flame temperature and heat distribution etc. Typical examples of burners which are developed based on the type of oxidizer are the oxy-fuel burner and air fuel burner.
Many different types of burners have been used. These range from fully premixed to diffusion mixing, downstream of the burner exit. In fully premixed burners, the fuel and oxidizer mix prior to reaching the nozzle exit.
A common method for classifying burners is according to how the fuel and the oxidizer are mixed. In premixed burners, the fuel and the oxidizer are completely mixed before combustion begins. Porous radiant burners are usually of the premixed type Premixed burners generally produce shorter and more intense flames, as compared to diffusion flames. This can produce high-temperature regions in the flame, leading to non-uniform heating of the charge and higher NOx emissions. However, in flame impingement heating, premixed burners are useful because the higher temperatures and shorter flames can enhance the heating rates.
In diffusion-mixed burners, the fuel and the oxidizer are separated and are not mixed prior to combustion, which begins where the oxidizer/fuel mixture is within the flammability range. Oxy-fuel burners are usually diffusion burners, primarily for safety reasons, to prevent flashback and explosion in a potentially dangerous system. Diffusion gas burners are sometimes referred to as ‘raw gas’ burners as the fuel gas exits the burner essentially intact with no air mixed with it. Diffusion burners typically have longer flames than premixed burners. They do not have a high temperature hotspot, and usually have a more uniform temperature and heat flux distribution.
It is also possible to have partially premixed burners, where a portion of the fuel is mixed with the oxidizer. This is generally done for stability and safety reasons. The partial premixing helps anchor the flame, but not fully premixing lessens the chance for flashback. This type of burner normally has a flame length and temperature and heat flux distribution which is between the fully premixed and diffusion flames.
The flame in premixed burners can have either a uniform or a non-uniform velocity profile, depending on the nozzle design. It also depends on the distance between the ignition point and the exit. In partially premixed burners, the fuel and oxidizer mix prior to reaching the nozzle exit. However, only a portion of the stoichiometric amount of oxygen (O2) is supplied through the burner. The rest is provided by mixing with the surrounding ambient air, entrained into the flame. At the nozzle exit, the velocity profile is usually non-uniform. Both uniform and non-uniform outlet temperature profiles and compositions have been reported. In diffusion-mixing burners, the fuel and oxidizer begin to mix at the nozzle exit, where the velocity is often non-uniform. In diffusion burners, the exit temperature field is generally homogeneous and equal to ambient conditions. The gas composition at the exit is pure fuel and pure oxidizer, with no combustion products. If the oxidizer is not supplied through the burner, a pure diffusion flame results. The O2 is provided for combustion by ambient air entrainment into the flame.
Another burner classification based on mixing is known as staging. It can be staged air or staged fuel. In staged burner, secondary and sometimes tertiary injectors in the burner are used to inject a portion of the fuel and/or the oxidizer into the flame, downstream of the root of the flame. Staging is often done to reduce NOx emissions and produce longer flames. These longer flames typically have a lower peak flame temperature and more uniform heat flux distribution than non-staged flames.
In natural-draft burners, the air used for combustion is induced into the burner by the negative draft produced in the combustor. In this type of burner, the pressure drop and combustor stack height are critical in producing enough suction to induce enough combustion air into the burners. The main significance of the natural draft type on heat transfer is that the natural-draft flames are usually longer than the forced-draft flames so that the heat flux from the flame is distributed over a longer distance and the peak temperature in the flame is often lower.
In natural draft burner, air enter through a muffler, which dampens the noise from the burner, Then the air control adjust the amount of air flowing through the burner. The air control is often a set of louvers or damper blades which can be rotated to partially or completely close the entrance into the plenum. The plenum or the windbox distributes air to the burner throat and dampens the noise from the firebox. The burner tile is a refractory piece which shapes and stabilizes the flame. One or more burner tips are used to inject the fuel into the air stream. They are connected to the fuel risers which in turn are connected to the fuel manifold. A small pilot burner provides an ignition source for the main burner.
Most industrial burners are known as forced-draft burners. This means that the oxidizer is supplied to the burner under pressure. As an example, in a forced-draft air burner, the air used for combustion is supplied to the burner by a blower.
Forced draft burners can control the combustion of all gaseous fuels and liquid fuels. There are burners which use only one family of fuel (liquid or gaseous) and while there are also burners which can use both the fuels. Such burners are called ‘dual fuel’ (double fuel) burners. Thus, there are three classes of burner. Forced draft burners can also be classified according to the type of construction, namely (i) monobloc burners, and (ii) separate fired burners. In monobloc burners, the fan and pump are an integral part of the burner forming a single body. In separate fired burners, the fan, pump and/or other fundamental parts of the burner are separate from the main body (head). Monobloc burners are those burners which are normally used in output ranges varying from tens of kWs to several Mw output. For higher outputs, or for special industrial processes, separate burners are used.
Depending on output delivery type, forced draft burners can be classified according to their types which are namely (i) single-stage burners, (ii) multi-stage burners, and (iii) modulating burners. Single-stage burners operate with single-state delivery, fuel delivery is invariable and the burner can be switched on or off. Multi-stage burners, usually two-stage or three-stage, are set for running at one or more reduced output speeds or at maximum output with switchover from one stage to another stage which can be automatic or manual. Two-stage burners also include versions called progressive two-stage, where changeover from one stage to another is through a gradual increase in output and not with sudden step increases. In modulating burners, the delivered output is automatically varied continuously between a minimum and maximum value, for optimum delivery of the thermal output in relation to system requirements.
Burners are often classified as to whether there is direct or indirect heating. In direct heating, there is no intermediate heat exchange surface between the flame and the charge. In indirect heating, such as radiant tube burners, there is an intermediate surface between the flame and the charge. This is usually done because the combustion products cannot come in contact with the charge because of possible contamination.
Radiant burners are designed to produce a uniform surface temperature heat source for heating and melting a variety of materials. The uniform surface temperature produces more homogeneous heating of the materials, which normally improves the product quality compared to conventional burners which can produce hotspots. Other advantages of these burners can include (i) high thermal efficiencies, (ii) low pollutant emissions, (iii) directional heating, (iv) very fast response time to load changes, (v) very fast heating compared to convective heating, (vi) burner shape can be tailored to the shape of the heat load to optimize heat transfer, (vii) ability to segment a burner to produce a non-uniform heat output profile, which can be useful in certain types of heating and drying applications, (viii) certain types of radiant burners have very rapid heat-up and cool-down times, (ix) no open flames which can ignite certain types of materials, (x) more control over the heating process because of the known and measurable surface temperature of the radiant surfaces compared to open flames, where the flame temperature is very difficult to measure, and (xi) burners are very modular and can be configured in a wide variety of geometries to accommodate the process heating requirements.
The primary parameters of interest for radiant burners are the power density (firing rate per unit area), radiant efficiency (fraction of fuel heating value converted to thermal radiation), heat up and cool-down times, and pollutant emissions. Other factors of importance include cost, durability, and longevity.
There are also some important limitations of porous refractory burners, compared to more conventional open-flame burners, which include (i) relatively low temperature limit for the radiant surface due to the limits of the refractory material, (ii) fuel and oxidizer is to be clean to avoid plugging the porous radiant surface, which essentially precludes the use of fuels like coal or heavy fuel oil, (iii) some of the radiant surfaces can be damaged by water or by contact with solid materials, which can be prevalent in certain applications, (iv) holes in the radiant surface can cause flashback since these burners are of premix design, (v) some designs can have high pressure drops, which means more energy is needed for the blower to flow the combustion air through the ceramic burner material, (vi) due to the limits in radiant surface temperatures, the firing rate density is usually limited, and (vii) some types of radiant burners using hard ceramic surfaces can have high heat capacitances which can ignite certain charge materials upon a sudden line stoppage.
In these burners, fuel and air are premixed and combusted either just inside a radiating surface or just above the surface, depending on the operating conditions and specific radiant burner design. If the mixture velocity is too low, flashback or flame extinguishment can occur, depending on the design of the burner. Besides the operational considerations, flashback is an obvious safety concern. If the mixture velocity is too high, the flame can blow off or the radiant performance can be severely reduced because the burner surface is not being directly heated by the hot exhaust products. Depending on the specific design of the burner, optimum performance is achieved when the flame is stabilized just inside or just above the outer burner outlet.
The burner material is ‘porous inert media’ which has been made from a wide variety of ceramics. These ceramics often include alumina, zirconia, or silicon carbide. The heat transfer coefficient between the hot exhaust products and the radiant burner material is difficult to predict and measure due to the uncertainty in the surface area of the ceramic structure. These coefficients have traditionally been presented in terms of a volumetric coefficient (kW/ deg C-cum).
Flames are of two types based on their shape. They are (i) round flame, and (ii) flat or rectangular flame. These are the two most common flame shapes produced by the burners. Also, flames can be luminous or non-luminous.
Free standing burners generally have round flames. These burners are normally placed in the middle of furnace chamber. Round flames are appropriate where horizontal flames are needed or where the burners are firing in the downward direction. Flat or rectangular flames are normally produced by the wall fired burners. These burners heat the furnace wall refractories which radiates heat. A solid refractory wall is capable of efficient radiant heat transfer.
Luminous flames are produced by the continuous radiant emission of particles in the flame, such as soot, which radiate approximately as blackbodies. The soot generated in a flame is highly dependent, among other things, on the fuel composition. Luminous radiation is usually important when liquid and solid fuels, like oil and coal, are used. It is usually not significant for gaseous fuels, like natural gas. Fuels with higher carbon-to-hydrogen weight ratios tend to produce sootier flames.
Luminosity is generated by the cracking of fossil fuels into micron-sized solids and gaseous hydrocarbon compounds. The heaviest of those compounds, perhaps with some solid carbon, is called ‘soot’. When the soot particles become very hot and begin to burn, they radiate like other solids. Since solids radiate in all wavelengths and follow the rules of heat transfer between solids, luminous flames transfer more heat than nonluminous flames. The ‘skin’ of a luminous flame is the locus of points where the soot combines with O2 to self-incinerate to carbon dioxide and water vapour. Luminous flames can transfer around 7 % more heat than nonluminous flames. However, modern nonluminous flame and heat transfer techniques, together, can be more effective overall than luminous flames.
Until recently, all long flames were luminous, but that is not true of several modern burners. Flame lengths are important to deliver heat flux as needed by the charge and fit into the space available. In many cases, space limits the firing rate and the type of flame.
The flame temperature is a critical variable in determining the heat transfer from the flame to the charge. The adiabatic flame temperature is affected by the oxidizer and fuel compositions, the mixture ratio, and the air and fuel preheat temperatures. Real flame temperatures are not as high as the adiabatic flame temperature, but the trends are comparable and representative of actual conditions.
There is an inverse relationship between flame luminosity and flame temperatures. That is, if the flame is very luminous, it radiates its energy more efficiently and hence has a lower temperature. If it is very non-luminous, then the flame temperature is generally much higher because it does not as efficiently release its energy.
Staging the fuel normally means that the inner flame region has excess oxidizer or is fuel-lean. Fuel-lean flames tend to be very non-luminous, depending on the mixing, and hence only generate gaseous radiation, with little or no soot formation. The balance of the oxidizer is added downstream of the main flame region and normally brings the overall combustion process from a very fuel-lean condition to slightly lean conditions. Again, this does not favour luminous flame radiation even in the secondary flame region. Hence, fuel staged flames tend to be non-luminous and the heat transfer from these flames is more dominated by forced convection compared to luminous flames. Depending on the application, this may not only be acceptable, but desirable. If putting too much heat near the beginning of the flame can cause overheating, then fuel staging can be an option for stretching out the heat flux over a longer length, while simultaneously reducing the heat flux near the base of the flame.
There are six types of flames as shown in Fig 2. These six types of flames are described below. Arrows in the figure show furnace gas flows induced by the flames.
Fig 2 Different types of flames
Flame type A – It is a conventional forward, feather shaped flame with no swirl. It has little recirculation and axial jet. The flame is moderate to fast mixing and is used in all-purpose combustion chambers.
Flame type C – It is ball shaped with substantial swirl (swirl number is higher than 0.6). The flame has hot reverse flow into the centre and cold forward flow at sides. There is intense mixing and the secondary jet velocity is more than the primary jet velocity. The flame is used for combustion chambers which are more or less cubicle in shape.
Flame type E – Type E flame is flat. It has coanda effect which is the tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to entrain fluid from the surroundings so that a region of lower pressure develops. The flame has a very high swirl (swirl number is around 2). There is some recirculation and fast mixing. Swirled flow is normally contained with refractory work. The convex type (as shown in the Fig 2) is used to avoid flame impingement on the furnace charge and provides a wider heat spread. A concave type focuses radiation in large ‘hot spots’. Both types increase and direct refractory radiation.
Flame type F – This flame is long luminous and lazy flame. It has no swirl and no recirculation. There is low fuel and air jet velocity. It has laminar jet. The flame has buoyancy controlled. It has delayed, slow, and diffusion mixing. The flame is used for coverage in long chambers, and to add luminous radiation.
Flame type G – This type of flame is a long luminous flame with a ‘fire hose’ appearance. The flame has no swirl. Fuel and/or atomizing medium jet velocity is much greater than the air jet velocity. The flame is thrust controlled. The fuel is directed for the flame and there is stretched out mixing. The flame is used for uniform coverage in long chambers and for the addition of the luminous radiation.
Flame type H – The flame has a high velocity and low swirl. It has a high furnace recirculation. The flow is contained by refractory brick. There is burning inside and outside the refractory brick. The mixing is fast. The flame is used to drive heat into loosely stacked charges. The flame is used to force flow around backs of the furnace charges. The flame increases convention for reaching long distances and for firing between piers.