The most important Fuels used to produce heat are coal, oil and gas. These fuels and air react very slowly at atmospheric temperatures, but will react very rapidly producing a flame when raised to high temperatures. A burner is a device designed to ensure that the flame is stabilized by establishing a suitable flow field to produce the initial temperature rise. The flame is used as the source of heat to preheat the fuel/air mixture to the ignition temperature. In the case of a simple laminar flame propagating through mixture, the heating is caused by the thermal conductivity of the mixture transmitting heat upstream of the flame front. Most burners operate with a turbulent flame, and in this case, hot combustion products are recirculated by a reverse flow region in the burner flow field; mixing of these hot gases with unburned mixture raises its temperature to the ignition point. The presence of highly reactive radicals in the combustion products also contributes to the ignition process. A reliable ignition process generally leads to a stable flame.
In addition to ensuring a stable flame, the burner is also required to achieve a high completeness of combustion, which is often referred to as high ‘combustion efficiency.’ This depends on thorough mixing of the fuel and air, and the retention of high enough temperatures until the reaction is complete. A fundamental feature of the mixing process is that it requires power. The source of this power is usually the fan, which supplies pressurized air to the upstream region of the burner. The burner is designed in such a way that this pressure is converted into a high speed jet (or jets) in the downstream region. There is a steep velocity gradient in the edge of the jet (known as the Shear Layer) and provided that the size of the shear layer is significantly larger than the size of the Kolmogorov eddies, whose Reynolds Number is unity, turbulent eddies will be formed. Most of the energy of the jet is transferred to the Turbulence within about twenty jet widths from the burner head.
Turbulence decays very rapidly through the small-scale Kolmogorov dissipation eddies, so there is little turbulent kinetic energy or dissipation beyond this twenty jet width region. At the molecular level, the dissipation of turbulence is caused by molecules carrying their momentum from one eddy to the next. In a qualitative sense, it is apparent that they will also carry their species to the adjacent eddy and thus turbulent dissipation results in mixing at the molecular level.
Chemical reactions depend on mixing at the molecular level rather than on the average of large-scale rich and lean regions, hence the turbulent dissipation region coincides with the region at which molecular mixing and reaction take place. Of course, it is possible that the mixing process consists of ‘like’ mixing with ‘like.’ In order to minimize this possibility, it is important that adjacent Kolmogorov eddies should consist of the different materials to be mixed. Since small-scale eddies are derived from the large-scale eddies by a stretching process, it must be ensured that adjacent energy containing eddies consist of the materials to be mixed. This can be accomplished by designing the fuel injection system so that the maximum separation of the fuel elements is smaller than the size of the energy containing eddies. These energy containing eddies are comparable in size to the thickness of the shear layers; hence, they can also be decided by the burner designer. Thus, these fundamental principles can be used to determine the geometry of the burner, including such features as the number of gas jets required to distribute the gas in a gas burner. (See also Mixing.)
The aerodynamic design of the burner must, therefore, accomplish two major tasks. The first is to ensure recirculation of hot combustion products to stabilize the flame by some feature, such as a bluff body. The second is to thoroughly mix the fuel and the air in the shear layers downstream of the burner to ensure a high combustion efficiency.
The burner designer must take other factors into account, such as the required shape of the flame or the required heat distribution on the load to be heated. Thus, high velocity jet flames (tunnel burners) are used for intensive convective heating, whilst a ‘wall flame’ may be used when we do not want flame impingement to occur.
One important class of industrial flames rely on strong swirl to stabilize the flame by virtue of the fact that swirl causes a low static pressure on the axis compared to the local ambient static pressure. The radial differential pressure may be calculated by integrating the radial pressure gradient expression:
where vt denotes the tangential velocity which is a function of radius. Since the axial static pressure quickly rises to the ambient pressure, it is apparent that there is an adverse pressure gradient along the axis which leads to reverse flow for high levels of swirl. A recirculation region is thus formed on the axis of the burner extending downstream from the burner throat. The swirl burner has the advantage that the hot recirculation region is formed away from walls, whereas a bluff body flame stabilizer, such as a ‘V’ gutter, has flame in contact with the metal walls. (See also Vortices.)
Other features which must be taken into account in the design of the burner is the minimization of pollutants such as Carbon Monoxide, soot and oxides of nitrogen (NOx).
Carbon monoxide and soot are formed when the oxygen available for combustion is inadequate to oxidize all the fuel to carbon dioxide. The solution to this problem is to operate the burner on the lean side of the stoichiometric mixture point, whilst ensuring good mixing so that there are no locally rich regions. The provision of excess air to minimize carbon monoxide and soot formation brings with it the disadvantage that the quantity of flue gases is increased. At the Chimney, these gases have to be maintained at reasonably high temperatures to avoid condensation and to provide a buoyant plume that will disperse high in the atmosphere. The gases carry sensible heat away from the region where it could be used, and thus decrease the efficiency of the boiler or furnace. An optimum quantity of excess air is, therefore, employed at which the efficiency is as high as possible consistent with acceptable levels of pollutant formation.
The minimization of the oxides of nitrogen (NOx) is slightly more complicated and involves special techniques, such as staged combustion, to avoid high local temperatures, since oxides of nitrogen are formed from the nitrogen in the air at high temperatures. (See Nitric Oxide, Nitrogen Dioxide and Nitrous Oxide.)
Burners designed for oil firing are generally similar to gas fired burners, except that the fuel must be sprayed into the shear layers where the drops can be mixed thoroughly with the air. It is also important that an appropriate proportion of the fuel is carried into the recirculation region to maintain flame stability. The fuel atomizer is thus a critical component, and its design is still somewhat empirical. The energy to atomize the fuel is either derived from a pump, which raises the fuel to a high pressure, or a second fluid, such as high pressure steam, may be used. In either case, shear forces at the surface of a thin film of the liquid lead to instability of the surface, which breaks up into small drops. The drop sizes required for rapid combustion is usually less than 50 μm and are difficult to produce with heavy oil fuel. Heating heavy fuel oil dramatically reduces its viscosity and aids the atomization process; thus, heavy oil fuels are heated before they are burned. (See also Atomization.)
Burners designed for pulverized coal firing are based on the same principles as described above; however, the coal particles are of the order of 70 μm in diameter and require about one second to burn. The furnace chamber must therefore allow sufficient residence time for burnout to occur, and sufficient oxygen must be present. The residual ash from the particle will be either liquid or solid, depending on the local temperature and the ash fusion temperature. It is important that liquid ash does not impinge on boiler components, such as the superheater tubes. This is achieved by removing heat from the hot gases by radiation in the early stages of the boiler. Once the gases have cooled to an acceptable level, then convective heat transfer can be employed. (See also Boilers.)
Burners for gas turbines are required to produce hot gases with a near uniform temperature profile at the turbine entry. Since the acceptable turbine entry temperatures are less than the stoichiometric flame temperature, the hot gases from the first stage of combustion must be diluted by mixing with additional air. As explained above, the dilution mixing process again requires mixing power derived from the pressure difference across the combustion chamber wall. Furthermore, the chamber wall must be kept cool by some mechanism, such as the use of a film of cool air injected through slots parallel to the wall, or through the use of some form of effusion cooling.
One of the problems which may be encountered with burners is the phenomenon of oscillatory combustion. The oscillations may by either random or periodic.
Random fluctuations result from the turbulent combustion process itself and noise is produced due to the rate of change of the rate of heat release. Although the efficiency of this process is very small, typically about 10−8, large burner installations may have capacities of hundreds of MW and several watts of combustion noise is very noisy indeed.
Periodic oscillations result from positive feedback from the acoustic field in the furnace. The mechanism of the feedback typically consists of three stages:
The acoustic sound field consists of standing waves in which the velocity fluctuations are 90°C out of phase with the pressure fluctuations.
The velocity fluctuations cause changes in the fuel/air mixing pattern and the shape or location of the flame.
The changes in the flame result in fluctuations in heat release which have a component in phase with the pressure fluctuations. Energy is then transferred into the pressure fluctuations, thus sustaining the acoustic field in the furnace. All the harmonics of the furnace chamber, for which the acoustic energy input overcomes the damping, will be driven.
The solution to this problem, when it occurs, involves identifying the specific coupling mechanism and modifications to the burner or to the chamber acoustic damping.