The purpose of any combustion control system is to safely and efficiently maintain the desired boiler output without the need for constant operator attention. Therefore, the combustion process inside the furnace must be controlled while the boiler output changes in response to load demands. The basic principle of combustion control is to meet the boiler load requirements regulating the quantities of fuel and air while achieving optimum combustion and maintaining safe conditions for operators and equipment.
Combustion is the rapid oxidation of material (the fuel) to release energy (heat). The fuel can be a solid, liquid or gas, and the amount of heat released is normally expressed in BTUs (British thermal units) or Calories. For industrial applications in the U.S., we generally use the term BTU. A BTU is defined as “the amount of heat necessary to raise one pound of water one degree Fahrenheit from 60° to 61°F.” A Calorie is defined as “the energy required to raise one Kg of water one degree Celsius.”
The oxygen required to release the energy from the fuel normally comes from the air and represents 20.9% of the normal atmosphere. The remainder is primarily nitrogen with traces of other elements. Standard air is defined as “air measured at one atmosphere pressure (14.696 pounds per sq. in.) and 60°F with 0% relative humidity.”
In general, one cubic foot of air will release 100 BTUs of heat, regardless of the type of fuel used. (The exact number of BTUs released will vary somewhat with different fuels, but the variation is typically small, so this relationship holds true for all practical purposes.) As fuels are burned with just enough air to release the total BTUs in the fuel, the reaction is said to be “stoichiometric” or burned “on ratio”—when combustion is complete, no free oxygen or unburned fuel remains.
Combustion is a rapid chemical combination of oxygen and the combustible elements of a fuel with a resulting release of heat. Combustion is an exothermic (heat producing) reaction. Of the three major combustible elements in any fossil fuel, carbon, hydrogen, and sulfur, only the first two have significance as a source of heat.
For complete combustion to occur, the combustible elements should be thoroughly burned, resulting in maximum heat release while using the least possible amount of air. The theoretical amount of air is the amount required to burn all the fuel when the fuel and air are mixed perfectly. This quantity is determined by combustion calculations and is called stoichiometric air or theoretical air.
Perfect mixing of fuel and theoretical air is not possible because of the large amount of fuel burned in the boiler. It is difficult to get the fuel particles or droplets fine enough to burn quickly and to get air all around each particle. Therefore, more than the theoretical amount of air is needed for complete combustion.
The extra air used is called excess air. Excess air ensures that there is enough air for complete combustion. The sum of the theoretical and excess air is called the total air.
Excess air is expressed as a percentage of theoretical air required. Thus, 25% excess air indicates that 125% total air is being supplied. In boiler operation, excess air represents a heat loss. This loss must be balanced against losses from incomplete combustion.
'''The optimum balance is one of the major goals of an ongoing plant thermal performance program.'''
As the combustion process takes place in the furnace, oxygen in the combustion air combines chemically with the carbon and hydrogen in the fuel to produce heat. The amount of air that contains enough oxygen to combine with all the combustible matter in the fuel is called the '''stoichiometric value''' or '''theoretical air'''.
It is improbable for every molecule of fuel that enters the furnace to combine chemically with oxygen. For this reason, it is necessary to provide more air than the stoichiometric requirement. For most boilers, it is customary to provide 5 to 20 percent more air that the stoichiometric requirement to ensure complete combustion. This additional air is called ''excess air''. A boiler firing at 1.2 times the stoichiometric air requirement would be said to be firing at 20 percent excess air. If insufficient oxygen is introduced into the furnace, incomplete combustion of fuel will occur. This wastes fuel, causes air pollution, and results in hazardous conditions in the boiler. The unburned fuel may ignite in the boiler and result in secondary combustion, causing a dangerous explosion.
Providing too much combustion air reduces the explosion danger, but also reduces efficiency. The largest energy loss in a boiler is the heat that escapes as hot flue gas. Increasing the excess air increases this energy loss. High excess air can also result in unstable burner conditions due to the lean fuel/air mixture.
In practice, a large number of items that affect boiler efficiency are related to excess air. The proper value of excess air is a function of boiler load, fuel quantity, and air leakage through idle burners, steam temperature, flame stability, and energy losses.
To understand the combustion process in a boiler, we will first investigate the process of combustion.
Three basic elements are required for combustion to occur:
If any of these conditions were removed, there would no longer be a fire. This is typically demonstrated using a fire triangle ('''Figure 1''').
'''Figure 1: Fire Triangle'''
The most common combustible materials or fuels used in heating boilers are oil and gas. These materials supply a rather large amount of heat. The basic combustible elements in these fuels are hydrogen and carbon. Fuels exist as solids, liquids, and gases. All types of fuels contain carbon, hydrogen, and sulfur. The hydrocarbons readily combine with oxygen to produce a different compound and release heat. Fuels produce different amounts of heat and different byproducts when combusted. Typical heating boiler fuel constituents are shown in '''Table 1'''.
The oxygen needed to support combustion comes from the air that surrounds us. Air is a mixture of gases consisting mainly of about 21 percent oxygen and about 78 percent nitrogen of volume. The remaining 1 percent consists of small amounts of argon, carbon dioxide, and other gases.
Even in a simple wood fire oxygen plays a part in combustion. A chemical reaction occurs between the oxygen of the air and the wood fuel. The nitrogen and other gases in the air do not enter into the reaction, but do carry away the gases of combustion.
If a cover is put over the fire, the oxygen will be used up and the fire will go out. If the cover is removed before the fire is completely extinguished, oxygen becomes available and the fire continues.
The ''ignition temperature'' is the temperature that will start a fuel to rapidly ignite with oxygen causing combustion to take place. A chemist would call this process ''oxidation''. Combustion is a form of oxidation that produces heat and light.
If you have ever tried to light a large log in a fireplace with a match, you notice it is very difficult. The large log does not release heat rapidly enough to maintain the ignition temperature of the log. If you shave off pieces of the log and light them with a match, the results will be much different. The burning shavings produce enough heat to maintain ignition. This same concept applies to liquid fuel. Trying to light a container of oil would be difficult, but atomizing the fuel will allow it to burn easier.
Some things burn more readily than others. As fuel is heated, several different gases are released. These gases are hydrogen, carbon monoxide, and hydrocarbons similar to methane. Finally, all that is left is solid carbon and impurities. If air is added to the solid carbon, oxygen from the air will penetrate the surface and break away atoms of carbon. The carbon atoms combine with the oxygen. The products of combustion are carried away by the moving air. This process continues until the burnable carbon has disappeared and only impurities remain.
Only vapors burn, not liquids or solids. Each type of fuel has a different volatility. '''Volatility''' is a measure of how rapidly the liquid turns into vapors. The vapors still must be raised to at least its flash point before ignition can occur.
Fossil fuels burned in a boiler contain two basic elements: hydrogen and carbon. If these elements are combined, the compound is called ''hydrocarbon''. The fuel gas used for ignition is a hydrocarbon. A chemical analysis of the fuel determines how much air must be mixed with it for complete combustion. The relationship between fuel and air is called the ''fuel/air ratio''.
If you have ever worked on an older car with a carburetor, you probably adjusted the fuel/air "mixture." The mixture is typically adjusted by controlling the amount of fuel entering a carburetor. Supplying too much fuel is called a ''rich'' mixture and causes excess emissions or smoke from the exhaust. Supplying too little fuel is called a ''lean'' mixture and causes poor heat generation and a rough running engine.
Fuels can generally be classified as gaseous, liquid, or solid. In cases where a solid fuel is finely ground, such as pulverized coal, and can be transported in an air stream, its control characteristics approach those of a gaseous fuel. Liquid fuels, as they are atomized and sprayed into a furnace, also have control characteristics similar to those of a gaseous fuel. The control treatment of a solid fuel that is not finely ground is quite different from that of a gaseous or liquid fuel.
Whether a fuel is a gas, a liquid, or a solid is determined by the ratio of its two primary chemical ingredients, carbon and hydrogen. Natural gas has an H/C ratio of in excess of 0.3. Fuel oil has an H/C ratio of above 0.1. Since hydrogen is the lightest element and the molecular weight of carbon is six times that of hydrogen, a decrease in the H/C ratio increases the specific gravity and the density of the fuel.
An ideal fuel burning system would have the following characteristics:
In actual practice, some of these characteristics must be compromised to achieve a reasonable balance between combustion efficiency and cost. For example, firing a fuel with no excess air above the theoretical amount would require an infinite residence time at temperatures above the ignition point at which complete burnout of the combustibles takes place. Thus, every firing system requires a quantity of air in excess of the ideal amount to attain an acceptable level of unburned carbon in the byproducts of combustion leaving the furnace. This amount of excess air is an indicator of the burning efficiency of the firing system.
As we have learned, combustion can occur when the following conditions are met:
All flammable material has a "FLASH POINT" and an "IGNITION POINT."
The ignition point is the temperature at which the ignited material provides enough heat to maintain combustion.
Regardless of the fuel, it must be vaporized in order to burn. Oil, a liquid, and coal, a solid, must be heated to the point where gaseous vapors are rapidly given off. It’s these vapors which burn, NOT the solid or liquid. This is what makes it possible, for example, to put out a match in a bucket of light oil that is below its flash point.
Natural gas consists primarily of methane (CH4). The heat is released as the carbon (C) and hydrogen (H2) combine (react) with oxygen and produce water (H20) and carbon dioxide (CO2).
Efficient combustion of any fuel depends on its chemical and physical characteristics, and how well it is mixed with combustion air. Three important factors - time, temperature, and turbulence - control the completeness of combustion and influence the design of boiler equipment and operating practices.
At oil burning plants, the oils burned must be heated on their way to the burner. This accomplishes two tasks. First, the oil flows more readily when heated; secondly, it atomizes better.
The speed at which the chemical reaction between the carbon, hydrogen, and oxygen occurs is crucial to flame performance. By atomizing the oil into very small droplets, more surface area of the oil is available for the oxygen to come in contact with. The more surface area, the quicker the reaction will occur.
Good combustion is very rapid, has a high flame temperature, and is very turbulent. Turbulence is a key factor in boiler furnace combustion. If the turbulence is high, the mixing of the oxygen and fuel will be good, therefore, combustion will occur very rapidly and the result will be a high flame temperature. If the turbulence is low, mixing will not be as good; therefore, more time is required for complete combustion and the result is lower flame combustion and a lower flame temperature.
The chain of events is as follows:
The precise amount of air required to complete combustion with no excess is called "theoretical air." In real combustion systems, an excess amount of air is required above the theoretical amount to complete combustion. This is because mixing of the fuel and air (turbulence) is not perfect and some of the oxygen does not come in contact with the fuel while in the flame zone where temperatures are sufficient for combustion. This additional amount of air is commonly referred to as "excess air" and is expressed as "percent excess air." Since excess air is supplied to the combustion process, all of the available oxygen in the air will not be used.
The oxygen that is not used is referred to as "excess oxygen" and is expressed as "percent excess oxygen." The quantity of excess air required is dependent on several parameters including boiler type, fuel properties, and burner characteristics. O2 is preferred for monitoring furnace performance for the following reasons:
When measuring O2 or CO2, beware of stratification of gases within the duct and the possible intrusion of outside air through leaks in the breaching, air heater seals, etc. Multiple point measurements as close to the boiler outlet as possible are preferred.
The chemical reactions occurring between the fuels and oxygen within the flame are:
'''C + O2 - CO2 + Heat Energy 2H2 + O2 - H2O + Heat Energy'''
If there is a lack of oxygen, some of the carbon will not be completely oxidized and carbon monoxide will be formed. Another way to look at a lack of oxygen is to recognize that since fuel flow can be adjusted, we can also say that the furnace is "fuel rich." This condition is to be avoided. Excess fuel will carry over to other portions of the furnace, through the air preheaters, and out of the stack as black smoke (THIS IS A "FUEL RICH" CONDITION TO BE AVOIDED). When this happens, only a portion of the carbon’s heat energy is released.
Carbon monoxide is, itself, a fuel, with a heating value of 4,355 BTU/lb. It is, therefore, classified as a ''combustible''. By comparison, pure carbon has a heating value of 14,500 BTU/lb. Oil fuel contains small amounts of sulfur and ash which are transformed into sulfur oxides and particulates. These are important from the standpoint of boiler operation and structural integrity. The oil ash (particulates) sticks to tubes and boiler surfaces and leads to boiler fouling if not cleaned regularly (usually with soot blowing equipment). '''A portion of the sulfur oxides combines with the water vapor (formed in the combustion process) to form sulfuric acid, which is the primary chemical responsible for boiler corrosion.'''
The most used gaseous fuel is natural gas. Natural gases vary in their chemical analysis and, thus, in their heating values. The average heating value is approximately 1,000 Btu per standard cubic foot, but may range from 950 to over 1,100 Btu/scf. Note that, in all cases, the amount of methane is over 80% by volume.
Natural gas is the only major fuel that is delivered by the supplier as it is used. '''Figure 2''' shows a typical supply system for natural gas.
'''Figure 2: Gas Pressure Reducing and Metering Arrangement'''
The most common liquid fuel is fuel oil, a product of the refining process. While crude oil as produced from the well is sometimes used, the most common fuel oils used for boiler fuel are the lightweight No. 2 fuel oil and the No. 6 grade of heavy residual fuel oil. '''Figure 3''' shows a typical supply system for oil fuel.
'''Figure 3: Typical Fuel Oil Pumping and Heating Arrangement'''
If the fuel is No. 2 fuel oil, heating of the fuel is normally unnecessary. If the fuel is a heavy oil such as No. 6, it is usually necessary to heat the oil in the tanks so that it can be easily pumped through the system. If heavy fuel oil in a tank is unused for a period of time, the tank heating may cause the evaporation of some of the lighter constituents, ultimately making the oil too thick to remove from the tank by any normal means.
'''Figure 4: Fuel Oil Temperature vs. Viscosity'''
Most burners are designed for a viscosity of 135 to 150 Saybolt universal seconds (SSU). A very important aspect of oil firing is viscosity. The viscosity of oil varies with temperature: the hotter the oil, the more easily it flows. Indeed, most people are aware that heavy fuel oils need to be heated in order to flow freely. What is not so obvious is that a variation in temperature, and hence viscosity, will have an effect on the size of the oil particle produced at the burner nozzle. 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'' is the rapid oxidation of fuel in a mixture of fuel and air with heat produced and carried by the mass of flue gas generated. Combustion takes place only under the conditions shown in '''Figure 5'''.
'''Figure 5: Combustion Requirements'''
Time, Temperature, and Turbulence are the three '''T’s''' of combustion. 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. Natural gas cannot be ignited if less than 64% of the theoretical air required for combustion is present.
For any fuel, a precise amount of combustion air is needed to furnish the oxygen for complete combustion of that fuel’s carbon and hydrogen (see '''Figure 6'''). The precise amount of air is called the ''theoretical air'' for that particular fuel.
'''Figure 6: Basic Combustion Chemistry and Products of Combustion'''
The amount of carbon and oxygen for complete combustion of carbon is represented by the formula:
'''C + O2 =CO2 + 14,100 Btu/lb °C'''
Twelve pounds of carbon combine with 32 pounds of oxygen to form 44 pounds of carbon dioxide + heat.
The formula for combustion of hydrogen is:
'''2H2 + O2 = 2H2O + 61,000 But/lb H2'''
Four pounds of hydrogen combine with 32 pounds of oxygen to form 36 pounds of water.
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.
'''2C + O2 =2CO + 4,345 Btu/lb C'''
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.
'''2CO + O2 = 2CO2 + 4,345 Btu/lb CO'''
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.
'''Figure 7: Boiler Efficiency Graph'''
Excess air can be determined by the amount of oxygen in the flue gas and calculated by:
Excess air (%) =
where: K = 0.9 for gas and 0.94 for oil.
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. ''Draft'' is the difference between atmospheric pressure and the static pressure of the combustion gases in a furnace. 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. 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 (0.2 to 0.5 inches of water). An airfoil centrifugal fan is typically used.
Natural draft boilers operate with draft formed by the stack alone.
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.
The FD fans supply combustion air. The forced draft fans, in conjunction with O2 trim, maintain the proper fuel/air ratio for maintaining proper combustion and furnace safety. 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.
Burners are the devices responsible for:
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. Experience has shown that oil particles in the range of 20 to 40 µm are the most successful. Particles which are:
Each of the burner types uses a nozzle to provide the spray of liquid fuel. 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. '''Table 5''' is a summary of oil atomization systems.
'''Figure 8''' shows a typical burner assembly.
'''Figure 8: Typical Burner Assembly'''
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.
'''Figure 9: Pressure Jet Burner'''
Varying the pressure of the fuel oil immediately before the orifice (nozzle) controls the flow rate of fuel from the burner.
Advantages of pressure jet burners:
Disadvantages of pressure jet burners:
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.
'''Figure 10: Rotary Cup Burner'''
Because the atomization is produced by the rotating cup, rather than by some function of the fuel oil (e.g., pressure), the turndown ratio is much greater than the pressure jet burner.
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 2.5 and 10 mbar. 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''').
'''Figure 11: Low-Pressure Gas Burner'''
These operate at higher pressures, usually between 12 and 175 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'''). These burners are designed with gas as the main fuel, but have an additional facility for burning fuel oil.
'''Figure 12: Dual Fuel Burner'''
The following procedure is an example of how the changeover from gas to oil is accomplished:
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. Before we start, here are a couple of terms and their meaning that you’ll need to understand.
'''Excess Air'''- The extra amount of air added to the burner above that is required to completely burn the fuel
'''Turndown'''- The ratio of the burner’s maximum BTUH firing capability to the burner’s minimum BTUH firing capability.
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. '''Figure 13 '''shows the process with the amount of air and fuel required for perfect combustion.
'''Figure 13: Combustion Process'''
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 less excess air used results in the least amount of "waste." Let’s examine just why excess air is a waste of fuel.
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. Because of this control, the same BTU’s are absorbed per hour, no matter the amount of excess air that is supplied.
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.
In a boiler, as the excess air is increased, the stack temperature rises and the boiler’s efficiency drops. It takes fewer BTU’s of input to the burner to get the same number of BTU’s out of the boiler if lower excess air can be used. 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.
'''Figure 14''' shows a simplified burner head. 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.
'''Figure 14: Simplified Burner Head'''
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.
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.
'''Figure 15''' graphically displays how excess air affects the efficiency and operating cost of a boiler. The data was compiled on an actual boiler.
'''Figure 15: Effect of Excess Air on Boiler Efficiency'''
'''Figure 16''' shows the savings realized with a 100 horsepower load at various efficiencies caused by different excess air levels.
'''Figure 16: Effect of Excess Air on Fuel Costs'''
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 1.7 to 1 is like having a car that can only go between speeds of 59 MPH and 100 MPH. 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.
'''Figure 17''' shows how the turndown ratio of a burner impacts the fuel cost needed to run a 100 horsepower boiler for heating. When you combine the effects of low excess air and high turndown, the operating cost savings can range from 10% to 15% below a brand new burner that does not have those characteristics.
'''Figure 17: Effect of Turndown on Fuel Costs'''
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 100 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. Gas valves, ignition transformers, etc. are all less prone to fail if they never have to cycle. If the burner stays on, they don’t have to turn on and off and, therefore, last longer.