Combustion, the process of burning, is defined as a chemical reaction between a combustible reactant the fuel and an oxidizing agent such as air in order to produce heat and in most cases light while new chemical species e. This book covers a gap on the market by providing a concise introduction to combustion. This book provides a brief and clear overview of the combustion basics, suitable for beginners and then focuses on practical aspects, rather than theory, illustrated by a number of industrial applications as examples.
The content is aimed to provide a general understanding of the various concepts, techniques and equipment for students at all level as well as practitioners with little or no prior experience in the field. The authors are all international experts in the field of combustion technology and adopt here a clear didactic style with many practical examples to cover the most common solid, liquid and gaseous fuels.
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The associated environmental impacts are also discussed so that readers can develop an understanding of the major issues and the options available for more sustainable combustion processes. He studied Technical Chemistry at the same university, where he also earned his PhD degree and completed his habilitation. Between and he held several senior positions in the petrochemical industry in Europe and Asia.
Arpad B. At these institutions he continued his research on combustiongenerated aerosols, focusing on soot characterization as well as on the development of fuel additives for the reduction of soot emission from aviation fuel sources. He obtained his academic degrees from the Vienna University of Technology and has specialized in combustion and high temperature reaction kinetics.
For this reason, the temperature needs to be accurately controlled to give consistent conditions at the nozzle. In the previous section, we discussed the requirement for combustion using the fire triangle. The same process holds true in a furnace.
Combustion takes place only under the conditions shown in '''Figure 5'''. A short period of time, high temperature, and very turbulent flame indicates rapid combustion. Turbulence is the key because fuel and air must be thoroughly mixed if the fuel is to be completely burned. When fuel and air are well mixed and all the fuel is burned, the flame temperature will be very high and the combustion time will be shorter.
When the fuel and air are not well mixed, complete combustion may not occur, the flame temperature will be lower, and the fuel will take longer to burn. Less turbulence and longer burning has been known to produce fewer nitrous oxides Nox. In some cases, combustion has been delayed or staged intentionally to obtain fewer nitrous oxides or to obtain desired flame characteristics.
The fuel must be gasified. The oil must be atomized so that the temperature present can turn it into gas. The ignition temperature and flame temperature are different for different fuels if all other conditions are the same. Typical ignition temperatures when mixed with air are shown in '''Table 3'''. Note that the gases have the highest temperature required for ignition. Liquids have the lowest ignition temperatures when properly atomized and mixed with air.
The precise amount of air is called the ''theoretical air'' for that particular fuel. A simple example of the many incomplete combustion reactions resulting in intermediate hydrocarbon compounds is the partial combustion of carbon, resulting in carbon monoxide rather than carbon dioxide. In this case, some of the potential heat from the carbon remains in the carbon monoxide.
Twenty-four pounds of carbon combine with 32 pounds of oxygen to form 56 pounds of carbon monoxide. With the right conditions of time, temperature, and turbulence, and by adding more oxygen to the carbon monoxide, it will further oxidize to carbon dioxide, releasing additional heat energy. As indicated, the combustion process produces heat, but a low percentage of this heat is not useful in transferring heat to the boiler water.
As hydrogen combines with oxygen to form water, the combustion temperature vaporizes the water into superheated steam. This vaporization absorbs latent heat. As the gases pass through the boiler and exit from the system, the gases retain the vaporized water in the form of superheated steam and the heat is lost from the process.
The hydrogen content of the fuel determines this amount of heat loss. It is important to keep in mind that combustion air must be furnished for the total combustion or on the basis of the HHV, while only the LHV has any effect on the heat transfer of the system. The air supply for the combustion process must be adequate for theoretical combustion and also provide "excess air" to ensure complete combustion.
As shown in the graph of '''Figure 7''', as the air is increased the combustion is improved. Once the excessive air becomes too great, the loss of heat reduces boiler efficiency. Excess air can be determined by the amount of oxygen in the flue gas and calculated by:. Typical measurements of oxygen in the flue gas are shown in '''Table 4'''. An adequate flow of air and combustion gases is required for the complete and effective combustion of fuel.
Flow is created and sustained by the stack and fans. The flow of gases can be created by four methods:. Forced draft boilers operate with the air and combustion products maintained above atmospheric pressure. Fans at the inlet to the boiler system, called ''forced draft'' FD fans, provide sufficient pressure to force the air and flue gas through the system. FD fans supply the necessary air for fuel combustion and must be sized to handle the stoichiometric air plus excess air needed for burning the fuel.
They also provide air to make up for air heater leakage and for some sealing air requirements. Radial airfoil centrifugal or variable pitch axial fans are preferred for FD service. FD fans operate in the cleanest environment in the plant associated with a boiler.
Droplets and Sprays
Most FD fans have inlet silencers and screens to protect the fans from entrained particles in the incoming air. Both the air temperature at the power plant and the elevation above sea level affect air density and, therefore, are a direct influence on fan capacity. Induced draft boilers operate with air and combustion pressure below atmospheric. Static pressure is progressively lower as gas travels from the inlet to the induced draft fan.
Induced draft ID fans exhaust combustion products from the boiler. In doing so, they create sufficient negative pressure to establish a slight suction in the furnace 0. An airfoil centrifugal fan is typically used. Balanced draft boilers have a forced draft fan at the boiler inlet and an induced draft fan at the system outlet. This reduces both flue gas pressure and the tendency of combustion gases to escape the furnace. Most modern boilers are balanced draft.
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The FD fans supply combustion air. Flow is controlled by modulating the inlet vanes controls airflow. Air preheaters reclaim some heat from the flue gas and add it to the air required for combustion. Use of preheated air will speed up combustion at all loads, improve combustion at low loads and increase efficiency. Coal, as a boiler fuel, tends to be restricted to specialized applications such as water-tube boilers in power stations. This section reviews the most common fuels for heating boilers.
As previously mentioned, oil must be atomized for optimal combustion. Oil burners are classified according to the method used for atomization:. The ability to burn fuel oil efficiently requires a high fuel surface area-to-volume ratio. Particles which are:. Each of the burner types uses a nozzle to provide the spray of liquid fuel.
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The rate of combustion is limited by vaporization of the liquid fuel. The greater the surface area of the fuel, the greater the combustion capability. Warm up guns normally use air atomization of light oil or steam atomization of heavy oil. Fuel pressure requirements for mechanical atomization are much higher. Excellent atomization, very wide turndown capability; air atomization economical for small boilers or warm-up guns only.
A pressure jet burner '''Figure 9''' is simply an orifice at the end of a pressurized tube. Typically, the fuel oil pressure is in the range of to PSI. In the operating range, the substantial pressure drop created over the orifice when the fuel is discharged into the furnace results in atomization of the fuel. Putting a thumb over the end of a garden hosepipe creates the same effect. Varying the pressure of the fuel oil immediately before the orifice nozzle controls the flow rate of fuel from the burner.
In a rotary cup burner '''Figure 10''' , fuel oil is supplied down a central tube, and discharges onto the inside surface of a rapidly rotating cone. As the fuel oil moves along the cup due to the absence of a centripetal force , the oil film becomes progressively thinner as the circumference of the cap increases. Eventually, the fuel oil is discharged from the lip of the cone as a fine spray.
Because the atomization is produced by the rotating cup, rather than by some function of the fuel oil e. Some advantages of rotary cup burners are that they are robust, have a good turndown ratio, and fuel viscosity is less critical. The major disadvantage of rotary cup burners is they are more expensive to buy and maintain. At present, gas is probably the most common fuel used in the facilities. Atomization is not an issue with a gas, and proper mixing of gas with the appropriate amount of air is all that is required for combustion.
Two types of gas burners in use are low-pressure and high-pressure. These operate at low-pressure, usually between 2. The burner is a simple venturi device with gas introduced in the throat area and combustion air being drawn in from around the outside '''Figure 11'''. These operate at higher pressures, usually between 12 and mbar, and may include a number of nozzles to produce a particular flame shape.
The usual arrangement is to have a fuel oil supply available on site, and to use this to fire the boiler when gas is not available. This led to the development of "dual-fuel" burners '''Figure 12'''.
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These burners are designed with gas as the main fuel, but have an additional facility for burning fuel oil. This operation can be carried out in quite a short period. In some facilities, the changeover may be carried out as part of a periodic drill to ensure that operators are familiar with the procedure, and any necessary equipment is available. However, because fuel oil is only "standby," and probably only used for short periods, the oil firing facility may be basic.
On more sophisticated plants, with a highly rated boiler plant, the gas burner s may be withdrawn and oil burners substituted. There is more to a burner than just blowing fire into a boiler or another heating device. Just what is a burner supposed to do? The following are some basics about how a burner functions. Natural gas will be used as the basic fuel, but fuel oils follow the same rules. Natural gas is primarily composed of methane, or CH4.
When mixed with the proper amount of air and heated to the combustion temperature, it burns. Perfection is absolutely impractical, however. Extra or excess air must be added to assure safe burner operation. Forced draft burners use fans to supply air for combustion. The fan on a burner moves a constant volume of air, not molecules. Any change in temperature or barometric pressure causes a change in the number of air molecules that the fan moves.
The control valves and pressure regulators used to meter the fuel are not perfect devices either so the gas flow cannot be perfectly constant. The gas train is designed to control volume much like the fan, so a change in gas temperature will also change the number of molecules burned. To ensure safe operation at all air and fuel temperatures and at all barometric conditions, the gas burner requires that excess air be supplied. The good news about excess air is that it provides a measure of safety.
The bad news is that it wastes fuel. A prominent manufacturer of burners says that "the heat lost in excess air represents waste heat, and proper burner design will help reduce this to a practical minimum. The boiler is merely a heat exchanger device designed to absorb heat from combustion products and to transfer that heat into water.
When excess air is added to the perfect, or stoichiometric, amount of air, obviously more mass is forced through the boiler. In a boiler, there is a modulating control that meters air and fuel so that the proper amount of heat is added to maintain the proper pressure or temperature. The chart below shows various temperatures leaving a heat exchanger when supplied with different amounts of gas at the same temperature.
As the mass flow is increased through the heat exchanger , the outlet temperature is increased. The mass amount is analogous to the amount of excess air used by a gas burner. Therefore, one of the most important functions of a burner is to burn the fuel at the lowest possible excess air to achieve the greatest overall boiler efficiency. An important function of burners is turndown. This is usually expressed as a ratio and is based on the maximum firing rate divided by the minimum controllable firing rate.
The air is brought into the head by means of a forced draft blower or fan. The gas is metered into the head through a series of valves. In order to get proper combustion, the air molecules must be thoroughly mixed with the gas molecules before they actually burn. The mixing is achieved by burner parts designed to create high turbulence. If insufficient turbulence is produced by the burner, the combustion will be incomplete and samples taken at the stack will reveal carbon monoxide as evidence.
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Since the velocity of air affects the turbulence, it becomes harder and harder to get good fuel and air mixing at higher turndown ratios since the air amount is reduced. Towards the highest turndown ratios of any burner, it becomes necessary to increase the excess air amounts to obtain enough turbulence to get proper mixing.
The better burner design will be one that is able to properly mix the air and fuel at the lowest possible airflow or excess air. The data was compiled on an actual boiler. There are several strong reasons why high turndown and low excess air are important. The first is the operating cost of the burner. You have seen how excess air affects the operating cost, but the turndown ratio of a burner has a big affect as well. Every time the burner starts and stops there is a cost associated. Air is always blown through the boiler to ensure that there is no unburned fuel remaining.
These purges make the boiler work like a chiller because it takes energy out of the system. Two other reasons for having a high turndown relate to lowered maintenance costs and better process or heating control. Do not confuse turndown with "fully modulating" burners. Having a fully modulating burner with only the typical turndown of 1. It is a "fully modulating" car,but try driving it to the grocery store. You would not only look silly, but think of the how the gas mileage would drop. Process control is enhanced with a high turndown. If the load is smaller than the burner can turn down to, it cycles on and off.
When off, the pressure or temperature falls off. On some boilers, we have seen steam pressures drop from psig at burner shutdown to about 40 psig before the burner comes on again. That can cause problems in a manufacturing plant that depends on constant steam pressure. Even on hot water heating systems, control problems occur because of low turndown boilers.
Valves hunt and temperature control becomes erratic. With a high turndown, those fluctuations are eliminated because the burner tracks the load down to the point where it shuts off only when the load is very slight. There is enough stored energy in the system to take up the small fluctuations at that point. Maintenance costs are reduced with a high turndown burner because there is much less thermal cycling taking place in the boiler.
When a burner cycles, the refractory and metal parts expand and contract. Although those materials are built to take it, their life is prolonged if everything stays the same temperature.