JPH0429926B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION BACKGROUND OF THE INVENTION The present invention relates to a method of operating a pulverized coal-fired furnace.

BACKGROUND ART For a long time, pulverized coal has been successfully burned in a floating state in a furnace using a circular combustion method. According to such a combustion method, pulverized coal and air are introduced into the furnace from the four corners of the furnace, oriented tangentially to an imaginary circle at the center of the furnace.

This spiral combustion method has many advantages, such as good mixing of pulverized coal and air, stable flame conditions, and the length of time the combustion gases stay in the furnace. It has the advantage of being long.

Recently, however, it has become important to minimize air pollution as much as possible. For this reason, several improvements have been made to the standard angular combustion method. An example of this method is as follows. According to this method, pulverized coal and air are introduced into the furnace from a number of burners at a lower level, directed tangentially to an imaginary circle in the center of the furnace and directed in one direction. Also, from a number of upper level burners, pulverized coal and air are directed tangentially to the imaginary circle in the center of the furnace and in a direction opposite to that of the lower level burners described above, Introduced into the furnace.

According to such a method, a better mixing of pulverized coal and air is therefore achieved, which reduces the amount of excess air than when used in ordinary angular combustion furnaces. Generally, pulverized coal is burned with 20-30% excess air.
Reducing the amount of excess air in this way helps minimize the production of NOx, which is the majority of air pollution in pulverized coal furnaces. Also, as a result,
Increases furnace efficiency.

Although the conventional methods described above reduce NOx, they have the following drawbacks. That is, the opposite rotations of the two gas streams formed in the furnace cancel each other out, so that the gas flows generally in a nearly straight line through the upper part of the furnace, and thus the gas flow in the upper part of the furnace The amount of unburned carbon leaving the furnace increases due to reduced turbulence and mixing of the carbon.

Additionally, slag and unburned carbon may adhere to and accumulate on the furnace wall. Therefore, these deposits attached to the furnace wall reduce the efficiency of heat transfer to the water-cooled tubes constituting the furnace wall, increase the rotation at which the soot blower is operated, and further shorten the life of the tubes.

Summary of the Invention The present invention has been made to solve these conventional problems.

According to the present invention, similar to the conventional method described above,
It is possible to provide a method for operating a furnace that can mix pulverized coal and air well and burn pulverized coal in a suspended state. This furnace then has all the above-mentioned advantages of a circular combustion furnace due to the formation of a swirling or rotating fireball within the furnace. The furnace walls are covered and protected by a layer of air, which reduces slugging on the furnace walls.

Such a method involves introducing pulverized coal and primary air into the
auxiliary air is introduced into the furnace from the level tangentially to the imaginary circle at the center of the furnace, and an amount of auxiliary air that is at least twice the amount of the primary air is introduced into the second level directly above the first level. The primary air is introduced into the furnace tangentially to another imaginary circle in the center of the furnace and in the opposite direction to the primary air.
This is accomplished by providing a plurality of two levels, one above the other.

That is, as a result of introducing the auxiliary air into the furnace in a larger amount than the primary air in this way,
The entire fireball that is finally formed in the furnace will rotate in the direction of this auxiliary air introduction. and,
As a result, the pulverized coal introduced in the direction opposite to the rotating direction of the entire fireball inside the furnace, after entering the furnace,
Change direction relative to the direction of the overall combustion gas flow within the furnace. Therefore, due to this effect,
Optimal turbulence is created with good mixing of pulverized coal and air.

A better mixing of pulverized coal and air can therefore reduce the amount of excess air introduced into the furnace from high levels. Also, good mixing of pulverized coal and air can promote carbon conversion, thereby increasing the overall heat release rate of the furnace and reducing slugging and fouling on the furnace walls.

The auxiliary air is directed tangentially to an imaginary circle having a larger diameter than the imaginary circle in which the pulverized coal is directed, thereby forming a layer of air adjacent the furnace wall. Also, overfire air, which essentially constitutes all of the excess air supplied to the furnace, is directed tangentially to yet another imaginary circle from a level a considerable distance above all primary and auxiliary air introduction levels. and is directed into the furnace in a direction opposite to the direction of introduction of the auxiliary air.

DESCRIPTION OF PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail below with reference to the drawings.

In Figure 1, a square combustion type pulverized coal-fired furnace 1
0 has a plurality of burners 12 attached at different heights to each of its four corners. Air is supplied from fan 16 through ducts 18 and 20 to each burner. Air is also supplied through the crusher 22 and duct 24. Pulverized coal is transported to each burner in an air stream through ducts 26 and 28. A plurality of pulverized coal-air ducts branching off from these ducts 26, 28 to respective independent burners are each provided with separate valves and controllers (not shown). Moreover, each burner can also be controlled independently.

Combustion gases swirling upward through the furnace 10 are joined by fluid flowing through a number of tubes 30 forming a four-sided furnace wall before exiting the furnace through a horizontal passage 32 and being directed to a rear gas passage 34. Transfer heat. Both the horizontal passage 32 and the back gas passage 34 of the furnace 10 are provided with other heat exchange surfaces (not shown) for feedwater heating and steam superheating, as is well known in the art. .

According to the invention, therefore, pulverized coal and air are introduced into the furnace in a special way, which will be explained in detail below.

Pulverized coal, which is generally ground to a powder size in a grinder 22, is conveyed from the grinder to each burner 12 in an air stream. The air that carries this pulverized coal is generally called primary air. In addition, as clearly shown in FIG. 2, other air, generally referred to as secondary air, flows through the fuel injection nozzle 38.
It is introduced into the furnace through nozzles 36 from directly above and below. These nozzles 36 are tiltable together with nozzles 38 which introduce pulverized coal and primary air into the furnace. This secondary air is necessary for initial ignition and for maintaining stable combustion conditions. The amount of primary and secondary air is estimated to be approximately 20-30% of the total amount of air required to completely or stoichiometrically burn the coal.

Referring again to FIG. 2, an auxiliary or tertiary air nozzle 40 is located above or below the secondary air nozzle 36. The remaining amount of air necessary for complete or stoichiometric combustion of the coal is introduced into the furnace through these nozzles 40. Typically, approximately 70-80% of the total amount of air required to stoichiometrically burn the coal is introduced into the furnace through these auxiliary nozzles 40.

Next, FIGS. 3 and 4 show the directions in which pulverized coal, primary air, secondary air, and auxiliary air are respectively introduced into the furnace.

First, as shown in Figure 3, the pulverized coal and primary air, together with the secondary air, are
2 into the furnace.

Also, as shown in FIG. 4, auxiliary air is introduced into the furnace in a vacuum and directly below the fireball formed by the imaginary circle 42 and directed tangentially to the imaginary circle 44. This auxiliary air, however, is introduced into the furnace with rotation in a direction opposite to that of the pulverized coal and primary air. As a result,
Better mixing and combustion efficiencies are obtained than are obtained with conventional angled combustion furnaces. Therefore,
This allows excess air in the furnace to be used in less amounts than required by conventional methods.

The fireball that is ultimately formed in the furnace and ascends through the furnace rotates in the same direction as the direction of rotation of the auxiliary air.
This is because the amount of auxiliary air introduced in the direction of rotation is several times greater than the amount of primary and secondary air introduced in the opposite direction. The velocity of the auxiliary air is comparable to the velocity of the primary and secondary air.

This feature is also achieved by introducing the auxiliary air into the furnace tangentially to the imaginary circle 44 in the furnace so that it rotates in a direction opposite to that of the pulverized coal and the primary air. In addition, the fact that the imaginary circle 44 is larger than the imaginary circle 42 allows maintaining a layer of air adjacent the furnace walls, thereby minimizing slugging to the furnace walls.

Again in Figure 1, all excess air is removed from furnace 1.
0 into the furnace in the upper part. This excess air or overfire air is passed through another nozzle (not shown) provided at a considerable distance above the highest nozzle 12, and then tangentially to another virtual circle (not shown). It is also introduced in a direction opposite to the direction of rotation of the fireball that is directed at Ru. Since this amount of overfire air is relatively small (5-20%), the flow of combustion gases leaving the furnace will have some swirl or rotation in the direction of rotation of the auxiliary air. This therefore creates some temperature imbalance in the gas leaving the furnace.

Finally, an example of the specifications of a furnace in which the method of the present invention is implemented will be described.

Pulverized coal and primary air are introduced into the furnace at a 6° angle from the vertical central axis of the furnace to the radial line. The auxiliary air is introduced at an angle of 5 to 15 degrees to the same vertical central axis of the furnace, but its direction is opposite to that of the pulverized coal and the primary air.

As described above, since the amount of the auxiliary air is larger than that of the primary air, the direction of rotation of the entire vortex or fireball that is finally formed in the furnace is the direction in which the auxiliary air is introduced.

Six burners are installed at different heights in the four corners of the furnace, making a total of 24 burners. The burners can then be arranged within a range of furnace heights of about 9 meters (30 feet) starting about 15 meters (50 feet) above the opening in the bottom of the furnace. The top wall of the furnace is approximately 30m above the highest burner.
(100 feet) away. Also,
Excess or overfire air is introduced into the furnace approximately 60 feet above this highest burner location.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of a rectangular combustion type pulverized coal-fired furnace in which the present invention is implemented, and FIG. 2 is a diagram showing the arrangement relationship of a plurality of nozzles provided at one corner of the furnace.
3 is a sectional view taken along the line 3--3 in FIG. 1, and FIG. 4 is a sectional view taken along the line 4--4 in FIG. 10...Furnace, 12...Burner, 16...Fan, 18, 20...Duct, 22...Crusher, 2
4,26,28...Duct, 30...Furnace wall tube, 3
2...Horizontal gas passage, 34...Rear gas passage, 3
6... Secondary air nozzle, 38... Fuel nozzle, 4
0... Auxiliary air nozzle, 42, 44... Virtual circle.

Claims (1)

[Claims] 1. Pulverized coal and primary air are introduced into the furnace from the first level tangentially to the imaginary circle at the center of the furnace, and at least twice the amount of the primary air is introduced into the furnace. auxiliary air is introduced into the furnace from a second level directly above said first level, said auxiliary air being oriented tangentially to another virtual circle and in a direction opposite to said pulverized coal and primary air; , a plurality of said first levels into which said pulverized coal and primary air are introduced, each first level being separated by said second level into which said auxiliary air is introduced, thereby moving upwardly through the furnace; Two fireballs are formed that rotate in opposite directions, but by introducing auxiliary air in a larger amount than the primary air, the entire fireball that is finally formed rotates in the direction of the introduction of the auxiliary air. A method of operating a round combustion type pulverized coal furnace, characterized in that: 2. Claim 1, characterized in that a fluid flows through tubes forming all four walls of the furnace to absorb heat from the pulverized coal burning in the furnace.
The method described in section. 3. Claim 1, characterized in that the auxiliary air is directed tangentially to an imaginary circle having a larger diameter than the imaginary circle to which the pulverized coal and the primary air are directed.
The method described in section. 4 Overfire air is directed from a level a considerable distance above all levels introducing primary and auxiliary air, and further tangentially to another imaginary circle and in a manner opposite to the direction of introduction of auxiliary air. 2. A method as claimed in claim 1, characterized in that it is directed into the furnace. 5. The method of claim 4, wherein the overfire air comprises essentially all of the excess air and is between 5 and 20% excess air.