The objective is to burn coal in a way such that as much of the energy in the coal as possible is converted to electricity with the minimum of environmental impact, and at an economical cost. This is done by burning the coal in a furnace which converts the heat energy to high pressure, high temperature steam.
There are many ways to burn coal. Using a stoker or a fluidized bed... but pulverized coal is the most common method used for utility power generation. Pulverized coal has a number of advantages, including a much smaller particle size which results in higher combustion rates that are close to those of oil and gas. That means pulverized coal is easy to burn, and it allows very large steam generators (furnaces) to be constructed.
As with any fossil fuel, the pulverized coal, with sufficient air for combustion, must be brought together with an ignition source. Thorough and even mixing of the air and coal is necessary so that all of the coal is brought into contact with oxygen for combustion. However, the combustion process must also be controlled so as to limit undesirable emissions and the accumulation of furnace slag.
The overall goals are:
Making improvements in one of the above areas can result in a negative impact on other areas. So efficient, safe, environmentally sound coal firing requires a balance of the above goals.
Coal burns in two stages, devolatilization and char.
Devolatilization – when a pulverized coal particle is exposed to high temperatures devolatilization takes place. This is the release of low boiling point gases, typically long chain hydrocarbons, leaving behind a carbon and ash char. These hydrocarbons immediately burn, providing the heat required for the ignition of the carbon char. Devolatilization typically takes place in about 1/100 of a second.
Char – the particle remaining after devolatilization is composed of carbon, moisture, ash, and other materials. The carbon must burned in the furnace zone, or it will be carried out of the furnace as unburned carbon resulting in reduced efficiency. Typically combustion of the char takes one to two seconds.
With pulverized coal the coal is dried in the pulverizer. This means that all of the moisture from the coal is carried by the Primary Air (PA) and is delivered to the furnace. This moisture can suppress ignition.
Coal contains ash. When heated the ash can become sticky and adhere to furnace walls and other heat absorbing surfaces. The ash absorbs heat, and is an insulator that reduces the heat transfer to the furnace and other heat transfer surfaces. Ash can also cause plugging in the superheater, reheater and convention surfaces (including the air preheater), restricting the flow of gases exiting the furnace.
Coal contains nitrogen. NOx is a major pollutant. When coal is burned the nitrogen in the coal can combine with oxygen to produce NOx. In addition, atmospheric nitrogen can also form NOx in the furnace. Designing the combustion process to reduce NOx formation is a major objective.
There are two types of firing systems, horizontal wall fired systems and tangentially fired systems. They take fundamentally different approaches to burning pulverized coal in a furnace.
In a horizontal wall fired system coal burners are mounted in the front wall of the furnace (and sometimes also in the rear wall). The burner mixes the coal and air, imparting a strong swirling motion to the mixture. The mixture ignites in the burner throat, creating several combustion zones, including a recirculation zone. Each burner creates its own flame.
In tangential firing pulverized coal streams are blown into a square furnace from each of the four corners. They are directed tangential to an imaginary circle in the center of the furnace. As a result they create a swirling fireball in the middle of the furnace. The “burner” in this type of system is considered to be entire lower furnace as that is where the mixing, ignition and combustion takes place.
Both firing systems use similar approaches to getting the coal from the pulverizer to the furnace. The amount of coal delivered to each burner or coal nozzle is determined by the coal feeder that supplies coal to the pulverizer. Leaving the pulverizer the coal is transported by the primary air using coal pipes. A velocity of about 3,000 feet/min. is required to keep the coal in suspension. If needed, riffle distributors may be used to divide the coal/PA flow from a single pipe into two pipes. In addition, restrictions in the coal pipes may be used to balance the flow among coal pipes of various lengths.
Let's take a look at each of the coal firing methods in more detail:
In horizontal wall firing the coal is carried to the burner by primary air (PA). In each individual burner the coal and PA are given a mixing rotation within the nozzle. Secondary air (SA), at a temperature of about 600 degrees and supplied through a windbox, is introduced into the furnace in the throat of the burner.
The goal is to control the resulting flame to establish multiple zones within the flame. This provides complete combustion, controls the flame temperature, and also reduces emissions. In particular the burner needs to be designed to establish a recirculation pattern that extends into the furnace.
A major portion of the combustion takes place in the recirculation zone. In addition, the recirculation brings the flame back to the burner throat where it ignites the coal being introduced into the burner. A flame front that is close to the burner over a range of operating conditions is needed for a stable burning.
Two factors impact the amount of primary air that will be delivered to the burner:
Whichever of these two air flows is greater sets the minimum PA flow. At low loads this can create a problem in the burner because there will be a high PA flow relative to the amount of coal being burned. A higher air flow results in a high coal nozzle velocity. If the nozzle velocity is too high (too much PA air flow), the coal ignition point will move further into the furnace, moving it away from the optimum ignition point in the burner throat. The result is an unstable burner flame.
The way to avoid this problem is to reduce the number of burners in service, running fewer burners at higher firing rates. Heat input to the furnace is controlled by varying the number of burners in service. The rule of thumb is to have fewer burners operating at the higher end of their capacity so as to promote better mixing and flame stability.
A coal burner is designed to maximize coal/air mixing. There is also furnace mixing that results from burner induced turbulence in the secondary air and the design of the furnace, as well as turbulence resulting from the rapid expansion of the coal-air mixture as its temperature quickly increases.
Circular Burner – This is an older design that has provided the basis for most current coal burner designs. It uses a central nozzle through which primary air and pulverized coal are supplied. Swirling secondary air is introduced around the central nozzle. This design provides a physically small burner, and allows for a small furnace size, because it delivers a concentrated, high level of heat input.
S-Type Burner – The next type of burner to be developed was the S-Type Burner. It was similar to the circular burner except that it provided separate control of the amount of secondary air and the swirl imparted to the secondary air. This allowed the amount of secondary air to be more precisely controlled. The result was higher combustion efficiency.
Low Nox Burners – In the 1970's it became apparent that NOx emissions needed to be reduced. This led to a series of improvements in coal burner design. Reducing NOx requires better control of the flame temperature. A technology called “air staging” was developed. It took part of the secondary air that had been introduced through the burner and introduced it through other locations. One of the objectives was to reduce the oxygen in the devolatilization zone where fuel nitrogen was combining with oxygen to form NOx. This reduced mixing, reduced the flame temperature and increased the flame size.
Further improvements in burner design used an increase in the number of zones within the flame to decrease NOx formation. An inner zone was established to stabilize the flame. The majority of the secondary air was supplied to an outer zone where it extended the flame size and induced a more moderate swirl. Current low NOx burner designs typically have six zones.
Tangentially firing is a very different approach to burning pulverized coal. Coal and air nozzles are located in each of the four corners of the furnace. The primary air carries the coal into the furnace through the coal nozzles. Secondary air, from a windbox mounted on each furnace corner, is introduced through air nozzles located directly above and below each coal nozzle.
The air and coal flow is directed tangentially to an imaginary circle in the center of the furnace. This creates a “cyclone effect” that mixes the coal and air in the furnace to produce a single flame envelope. Turbulence in the center of the furnace mixes the coal and air to achieve combustion. Unlike a coal burner in which ignition takes place in the burner throat, in a tangentially fired furnace the coal ignites several feet in front of the coal nozzle.
Tangential firing of coal has the same primary air limits as coal burners. These are the limits imposed by the 3,000 fpm velocity needed in coal pipes and the minimum air flow required by the pulverizes. While small variations in load can be accommodated by changes in the feed rate of the coal, major changes in the firing rate are controlled by changing the number of levels of coal nozzles that are in service. Each pulverizer supplies coal to one level of coal nozzles. As load is increased, additional pulverizers are started and coal is introduced to the furnace through additional levels of coal nozzles.
A tangentially fired system allows the fireball position in the furnace to be controlled. The coal and air nozzles can be tilted up or down to move the fireball up and down in the furnace. This is done, for example, to adjust for ash and slagging conditions in the furnace. As ash builds up on the furnace walls, reducing heat transfer, the fireball can be moved lower in the furnace to increase heat transfer. When the furnace walls are cleaned by soot blowers, removing the ash, the fireball can be moved up in the furnace to reduce heat transfer.
The tilting of the coal and air nozzles, moving fireball up and down, can also be used to control superheat and reheat temperatures as load changes.
In tangentially fired systems NOx is reduced through the use of overfire air and air staging. In the 1970's overfire air nozzles, that introduced secondary air higher in the furnace, stretched out the fireball, lowering flame temperatures and reducing NOx formation. Since then additional advances in low NOx tangential pulverized coal firing technology include:
Proper labeling eliminates confusion and errors. All pipes and tubing around each burner, or tangential corner, should be marked. This includes labeling ignitor fuel lines, air lines, valves, damper actuators and levers, flame sensors, instruments and transmitters. In addition, the burners, or tangential corners and levels, should be clearly identified.
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