LOW EXCESS AIR TANGENTIAL FIRING SYSTEM

SYSTEME D'ALLUMAGE A FAIBLE APPORT D'AIR TANGENTIEL

English Abstract

LOW EXCESS TANGENTIAL FIRING SYSTEM
ABSTRACT OF THE INVENTION
A furnace (10) in which pulverized coal is burned in
suspension. The coal is introduced along with primary air,
tangent to an imaginary circle (42). The auxiliary air is
introduced tangent to an imaginary circle (44) directly above
the primary air, in a direction of rotation opposite that of
the primary air. The auxiliary air is directed tangent to a
circle of greater diameter than that of the primary air. There
are a plurality of alternating levels within the furnace where
primary air (38), and then auxiliary air (40), is introduced.

Claims

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THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of operating a tangentially fired pulverized
coal furnace for purposes of achieving a better mixing of the coal
and auxiliary air so that the excess air required for complete
combustion of the coal is kept to a minimum and so that the
quantity of coal, burnt or unburnt, impacting the upper furnace
walls is lowered thus reducing plugging and fouling in the furnace
comprising the steps of:
a.) discharging into the furnace at each of a plurality of
first levels pulverized coal and primary air such that the pulver-
ized coal and primary air being discharged at each of said plural-
ity of first levels is directed in a first direction tangentially
to a corresponding one of a plurality of first imaginary circles
each located in the center of the furnace thereby causing a fire-
ball to be formed within the furnace that rotates in said first
direction while moving within the furnace;
b.) discharging into the furnace at each of said plurality
of first levels secondary air such that the secondary air being
discharged at each of said plurality of first levels is directed
in said first direction tangentially to each of a corresponding
one of said plurality of first imaginary circles each located in
the center of the furnace;
c.) discharging into the furnace at each of a plurality of
second levels that are each located directly above a corresponding
one of each of said plurality of first levels auxiliary air in a
mass more than two times that of the primary air such that the

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auxiliary air being discharged at each of said plurality of second
levels unlike the pulverized coal and the primary air and the
secondary air being discharged at each of said plurality of first
levels is directed in a second direction that is opposite to said
first direction tangentially to a corresponding one of a plurality
of second imaginary circles each located in the center of the
furnace thereby causing a fireball to be formed within the furnace
that rotates in said second direction while moving upwardly within
the furnace; and
d.) causing each fireball formed within the furnace to
rotate in said second direction upon exiting from the furnace
because of the influence exerted thereupon by virtue of the
auxiliary air having a greater mass.
2. The method as set forth in Claim 1 wherein the diameter
of each of said plurality of second imaginary circles is greater
than the diameter of each of said plurality of first imaginary
circles thereby resulting in a protective blanket of air consist-
ing of auxiliary air being provided adjacent to the inner walls of
the furnace.
3. The method as set forth in Claim 2 wherein the pulver-
ized coal and the primary air are each discharged into the furnace
at each of said plurality of first levels at an angle of 6° to the
centerline of the furnace.
4. The method as set forth in Claim 3 wherein the auxiliary

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air is discharged into the furnace at each of said plurality of
second levels at an angle of 5° to 15° to the centerline of the
furnace.
5. The method as set forth in Claim 4 including the further
step of discharging into the furnace at a third level spaced a
considerable distance from both any of said plurality of first
levels and any of said plurality of second levels overfire air so
that the overfire air like the pulverized coal and the primary air
and the secondary air is directed in said first direction
tangentially to a third imaginary circle located in the center of
the furnace.
6. The method as set forth in Claim 5 wherein the overfire
air discharged into the furnace at said third level amounts to 5
to 20% excess air.
7. A method of operating a tangentially fired pulverized
coal furnace for purposes of achieving a better mixing of the coal
and auxiliary air so that the excess air required for complete
combustion of the coal is kept to minimum and so that the quantity
of coal, burnt or unburnt, impacting the upper furnace walls is
lowered thus reducing plugging and fouling in the furnace compris-
ing the steps of:
a.) discharging into the furnace at each of a plurality of
first levels pulverized coal and primary air such that the pulver-
ized coal and primary air being discharged at each of said plural-

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ity of first levels is directed in a first direction tangentially
to a corresponding one of a plurality of first imaginary circles
each located in the center of the furnace thereby causing a fire-
ball to be formed within the furnace that rotates in said first
direction while moving within the furnace;
b.) discharging into the furnace at each of a plurality of
second levels that are each located directly above a corresponding
one of each of said plurality of first levels auxiliary air in a
mass more than two times that of the primary air such that the
auxiliary air being discharged at each of said plurality of second
levels unlike the pulverized coal and the primary air being dis-
charged at each of said plurality of first levels is directed in a
second direction that is opposite to said first direction
tangentially to a corresponding one of a plurality of second
imaginary circles each located in the center of the furnace there-
by causing a fireball to be formed within the furnace that rotates
in said second direction while moving upwardly within the
furnace;
c.) discharging into the furnace at a third level spaced a
considerable distance from both any of said plurality of first
levels and any of said plurality of second levels overfire air so
that the overfire air like the pulverized coal and the primary air
is directed in said first direction tangentially to a third
imaginary circle located in the center of the furnace; and
d.) causing each fireball formed within the furnace to
rotate in said second direction upon exiting from the furnace
because of the influence exerted thereupon by virtue of the

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auxiliary air having a greater mass.
8. The method as set forth in Claim 7 wherein the diameter
of each of said plurality of second imaginary circles is greater
than the diameter of each of said plurality of first imaginary
circles thereby resulting in a protective blanket of air consist-
ing of auxiliary air being provided adjacent to the inner walls of
the furnace.
9. The method as set forth in Claim 8 wherein the pulver-
ized coal and the primary air are each discharged into the furnace
at each of said plurality of first levels at an angle of 6° to the
centerline of the furnace.
10. The method as set forth in Claim 9 wherein the auxiliary
air is discharged into the furnace at each of said plurality of
second levels at an angle of 5° to 15° to the centerline of the
furnace.
11. The method as set forth in Claim 10 wherein the overfire
air discharged into the furnace at said third level amounts to 5
to 20% excess air.

Description

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LOW EXCESS AIR TANGENTIAL FIRING SYSTEM
BACKGROUND OF THE INVENTION
Pulverized coal has been successfully burned in suspension
in furnaces by tangential firing methods for a long time. The
technique invQlves introducing the coal and air into a furnace
from the four corners thereof so that it is directed tangent to an
imaginary circle in the center of the furnace. This type of fir-
ing has many advantages, among them being good mixing of the fuel
and air, stable flame conditions, and long residence time of the
combustion gases in the furnace. In recent times, ~t has become
important to minimize air pollution as much as possible. Thus~
some proposed changes have been made to the standard tangential
firing method. One proposal includes introducing pulverized coal
and air tangentially into the furnace from a number of lower
burner levels in one direction, and introducing coal and air
tangentially into the furnace from a number of upper burner levels
in the opposite direction. By this arrangement, better mixing of
the fuel and air is accomplished, thus permitting the use of less
excess air than with a normal tangentially fired furnace, which
generally is fired with 20-30% excess air. The reduction in
excess air helps minimize the formation of NOx which is a major
air pollutant of coal-fired furnaces. It also results in
increased efficiency of the unit. Although the above firing
technique reduces NOx, it does have some disadvantages. Since
the reverse rotation of the gases in the furnace cancel each other
out, the gases flow in a more or less straight line through the
upper portion of the furnace, increasing the possibility of
unburned carbon particles leaving the furnace due to reduced upper
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furnace turbulence and mixing. In addition, slag and unburned
carbon deposits on the furnace walls can occur. These wall
deposits reduce the efficiency of heat transfer to the water-
cooled tubes lining the walls, increases the need for soot blow-
ing, and reduces the life span of the tubes.
SUMMARY OF THE INVENTION
In accordance with the invention, a furnace is provided
in which pulverized coal is burned in suspension with good mixing
of the coal and air, as previously described. In addition, all of
the advantages previously associated with tangentially fired
furnaces are obtained, by having a swirling, rotating, fire ball
in the furnace. The walls are protected by a blanket of air,
reducing slagging thereof. This is accomplished by introducing
coal and primary air into the furnace tangentially at a first
level, introducing auxiliary air in an amount at least twice that
of the primary air into the furnace tangentially at a second level
directly above the first level,but in a direction opposite to that
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of the primary air, with there being a plurality of such first and
second levels, one above the other. As a result of the greater
2 a mass and velocity of the auxiliary air, the ultimate swirl within
the furnace will be in the direction of the auxiliary air intro-
duction. Because of this, the fuel, which is introduced in a
direction counter to the swirl of the furnace, is forced after
entering the unit, to change direction to that of the overall
fur~ace gases. Tremendous turbulent mixing between the fuel and
air is
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thus created in this process. This increased mixing reduces
the need for high levels of excess air within the furnace.
This increased mixing also results in enhanced carbon
conversion which improves the units over all heat release rate
while at the same time reducing upper furnace slagging and
fouling. The auxiliary air is directed at a circle of larger
diameter than that of the fuel, thus forming a layer of air
adjacent the walls. In addition, overfire air, consisting
essentially of all of the excess air supplied to the furnace,
is introduced into the furnace at a level considerably above
all of the primary and auxiliary air introduction levels, with
the overfire air being directed tangentially to an imaginary
circle, and in a direction opposite to that of the auxiliary
air.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a sectioned perspective of a tangentially
fired pulverized coal furnace incorporating the invention;
Figure 2 is an enlarged sectional view of one corner
of burnersi
Figure 3 is a view taken on line 3-3 of Figure 1; and
Figure 4 is a view taken on line 4-4 of Figure 1.
-~ DESCRIPTION OF THE PREFERRED EMBODIMENT
Looking now to Figure 1, a coal-fired furnace 10 is
shown, having a plurality of levels of burners 12 therein with
each level having a burner mounted in each of the four corners
thereof. Air is supplied to the burners from fan 16 through
ducts 18 and 20. Air is also supplied to pulverizer 22 through
duct 24. Pulverized coal is transported to the burners in an
air stream through ducts 26 and 28. There are separate air and
~-~ 30 fuel ducts leading to each individual burner, with separate
valves and controls (not shown) also, so that each burner can
be independently controlled. The combustion gases swirling
upwardly in the furnace give up heat to the fluid passing
through the tubes 30 lining all four of the furnace walls,
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before exiting the furnace through horizontal pass 32, leading
to rear gas pass 34. Both the furnace and the rear pass
contain other heat exchanger surface (not shown), for
generating and superheating steam, as well known in the art.
The specific manner of introducing the fuel and air
into the furnace will now be described in more detail.
Pulverized coal, senerally ground to a flour-like consistency,
is carried to each burner in a stream of air from the
pulverizer melt 22. This air that carried the coal is
generally referred to as the primary air. As best seen in
Figure 2, more air, generally designated as secondary air, is
introduced directly above and below the fuel-through nozzles
36. These nozzles are tiltable along with the nozzles 38
through the coal and primary air are introduced. This air is
necessary for maintaining initial ignition and stable
combustion conditions. The primary and secondary air
constitutes about 20-30% of the total air ~equired for complete
or stoichiometric combustion of the coal.
Still looking at Figure 2, positioned above and below
each secondary air nozzle 36 are auxiliary, or tertiary air
nozzles 40. The remainder of the air necessary for complete
combustion, or stoichiometric conditions, is introduced through
these nozzles 40. Generally about 70-80% of the stoichiometric
air is introduced through auxiliary nozzles 40.
Looking now to Figures 3 and 4, the manner in which
the coal and primary air, the secondary air, and the auxiliary
- air, is tangentially introduced into the furnace, is shown. As
seen in Figure 3, the coai and primary air along with the
secondary air, are introduced into the furnace tangential to an
imaginary circle 42 in the central portion of the furnace.
Looking now to Figure 4, it can be seen that the auxiliary air
is introduced into the furnace tangential to an imaginary
circle 44, at locations directly above and below the fire ball
42. The auxiliary air is introduced into the furnace rotating
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in a direction reverse~ or opposite to the direction of
rotation of the primary air and fuel. The result of this is a
mixing and combustion efficiency much better than that rea1ized
with the usual tangentially fired furnace. This permits the
use of less excess air in the furnace than previously required.
The ultimate fire ball rising in the furnace rotates in a
direction the same as that of the auxiliary air, since the mass
introduced in this direction ;s several times that introduced
in the opposite direction. The velocity of the auxiliary air
is comparable to that of the primary and secondary air. The
above feature, coupled with the fact that the auxiliary air is
introduced tangential to a circle 44 larger than the circle 42,
keeps a blanket of air adjacent to the furnace walls, thereby
minimizing slagging on these walls.
Looking again at Figure 1, all of the excess air is
introduced into the furnace in the upper portion thereof. This
excess, or overfire, air is introduced through nozzles 50,
which are directed tangential to an imaginary circle 52, in a
direction opposite to that of the rising fire ball; i.e
opposite to the direction of introduction of the auxiliary air
44. Since the amount of excess air is relatively small
(5-20%), the flow leaving the furnace will still be swirling or
rotating somewhat in the direction of rotation of the auxiliary
air introduction. This causes some temperature unbalancing the
gases leaving the furnace. Some statistical data of the
proposed modified furnace will now be given. The primary air
and fuel are introduced into the unit at a 6 angle to the
radial line from the vertical centerline axis of the furnace.
The auxiliary air is introduced at a 5-15 agnle to the same
vertical centerline of the furnace but opposite in direction.
In this manner, the fuel and air are introducing swirl within
the furnace in opposite directions. As stated previously,
however, because of the greater mass and velocity of the
auxiliary air, the ultimate overall swirl within the unit will
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be in the direction of the auxiliary air introduction. There
can be as many as six elevations of burners; i.e. 24 in total,
with six in each corner. These can be spread over a 30-foot
height in the furnace beginning 50 feet above the opening in
the coutant furnace bottom. The top wall of the furnace is
approximately 100 feet about the top burner elevation, and the
excess, or overfire, air is introduced about 60 feet above the
top burner elevations.
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Representative Drawing