Liquid heaters and vaporizers – Flue – Vertical
Reexamination Certificate
2002-01-23
2003-01-28
Wilson, Gregory (Department: 3749)
Liquid heaters and vaporizers
Flue
Vertical
C122S00400R, C110S345000, C422S173000
Reexamination Certificate
active
06510820
ABSTRACT:
FIELD AND BACKGROUND OF INVENTION
The present invention relates generally to the field of industrial and utility boilers and in particular to a new and useful compartmented exhaust gas flue for control of NO
x
emission via a selective catalytic reduction (SCR) system.
FIG. 1
is a schematic view of a pulverized coal (P.C.) fired boiler
10
. The walls of the boiler
10
are lined with tubes
12
. A fluid (usually water) is circulated through the tubes
12
. As the water flows through the tubes
12
, it absorbs the heat generated within the boiler
10
by heat radiation and/or heat convection. Pulverized coal is fed to the burners
14
wherein the fuel burns in the presence of previously introduced combustion air. The resulting flue or exhaust gas, comprised of heated gases and the combustion by-products, pass upwardly through the boiler
10
in heat exchange relationship with the tubes
12
, through horizontal convection pass
16
and finally through vertical gas pass
18
before ultimately exiting from the boiler
10
. A major portion of the flue gas is then routed to an air heater via main exit gas conduit
44
before it is discharged into a stack (not shown). If necessary, the flue gas may be passed through various types of pollution control equipment (not shown) as well.
As shown in
FIG. 1
, many utility boilers have a boiler outlet flue
100
, located below the convection pass, which directs the flowing flue gas through a 90 degree turn as it leaves the boiler. Hoppers at the bottom of the flue collect some, albeit limited, amount of the particulate before it reaches downstream pollution control equipment, such as NOx removal equipment.
A design which has been used for the purpose of controlling boiler outlet steam temperature over a wide range of load conditions is to withdraw, a portion of the flue gas, called recirculated gas, from the exiting flue gas stream and returned to the boiler
10
. The recirculated gas is a small portion of total flue gas flow, typically less than 20%. Recirculation of the flue gas is accomplished by gas recirculation apparatus
28
and the associated recirculation conduit
68
. A fan
70
is employed to induce recirculation flow.
FIGS. 2 and 3
illustrate a known gas recirculation apparatus
28
, which is used to recirculate part of the flue gas stream back to boiler, as taught in U.S. Pat. No. 4,286,548 to Leslie O. Brash and assigned to The Babcock & Wilcox Company. Arrow
81
indicates the direction of the front of the apparatus
28
. The upper face
32
of the apparatus
28
is partially open and in flow communication with the vertical gas pass
18
. A plurality of triangular-shaped hollow flow channels
34
divide the upper portion of the apparatus
28
into a multiplicity of discrete flow passages
36
.
Flow passage
36
is defined by the side walls
53
of adjacent flow channels (see for example
53
a
and
53
b
) and inclined plate member
55
. Member
55
is rectangular in shape and is obliquely situated, occupying plane
56
. Plate member
55
urges most of the flue gas to make a 90 degree turn toward the front of apparatus
28
, thereby exiting through conduit
44
. Much of the particulate entrained in the flue gas precipitates out onto plate
55
during this turn and slides down member
55
into ash hopper
38
. Member
55
is located on both sides of flow channel
34
.
The typical flow channel
34
is a hollow box, shaped like a triangle, disposed so that the triangle peak is pointing downward. The base
52
of channel
34
is a solid rectangular-shaped member extending from the back to the front of pass
18
. The vertical sides
53
of channel
34
are solid, triangular-shaped members, whose peaks point downward and whose oblique edges define planes
54
and
56
. There are no side members occupying planes
54
and
56
, thereby giving channel
34
its hollow nature.
While the flow channels themselves are hollow, having no members in either plane
54
or
56
, planes
54
and
56
are partially occupied by inclined plates
74
.
Inclined plates
74
provide support for the flow channels and, depending upon the length of plate
74
in planes
54
and
56
, provide a degree of control over the flow of gas through the flow channels.
Disposed below flow channels
34
, but not necessarily in alignment therewith, are a series of ash hoppers
38
. Normally closed, means are provided (not shown) to empty the hoppers
38
of collected particulate matter at periodic intervals. The upper faces
58
of the hoppers
38
are open. The oblique faces of hopper
38
, of which face
59
is typical, are closed.
Situated adjacent to and in front of the hopper
38
is recirculation duct
40
. The front side of duct
40
extends from the bottom of conduit
44
downward where it joins conduit
68
. The back side of duct
40
extends from the bottom face
42
of apparatus
28
downward where it joins conduit
68
. Duct
40
is in flow communication with conduit
68
.
Main exit gas conduit
44
, in flow communication with passages
36
, extends outwardly from the front face
46
of the apparatus
28
, providing egress for the bulk of the flue gas, i.e. the gas not being recirculated.
The back face
50
of apparatus
28
is composed of an upper portion and a lower portion. The upper portion is a vertically disposed member which extends downward from and in the same plane as the back of vertical gas pass
18
. The lower portion of back face
50
is a member extending obliquely from the bottom of the upper portion of back face
50
to bottom face
42
. Back face
50
, bottom face
42
, back face
59
and member
55
define flow space
60
.
The only way in which the gas may enter flow space
60
is via flow channel
34
. The sides of plate member
55
are sealably attached to the side walls
53
of adjacent flow channels. The bottom edge of member
55
is sealably attached to the upper edge of back face
59
of the ash hoppers, the upper edges of adjacent hoppers being sealably attached to one another. Therefore, the only available route to flow space
60
is through flow channels
34
.
The apparatus
28
is designed to provide a serpentine flow passage for the recirculated gas, represented by flow line
64
. The recirculated gas, after coming down pass
18
, undergoes an initial turn of approximately 90 degrees as it turns to flow from the area between adjacent channels toward flow channel
34
. This first turn is best illustrated in FIG.
3
. The recirculated gas then makes a second 90 degree turn as the gas turns to flow through channel
34
. This second turn is best illustrated in FIG.
2
. These turns, due to the effects of gravity and the inertia of the particulate matter, cause a large portion of the particulate matter to drop into ash hoppers
38
.
Most of the flue gas coming down pass
18
will follow the path of least resistance which means it will turn to pass through conduit
44
. This flow is indicated by flow line
62
. While this gas is not recirculated, the change in flow direction causes particulate matter from the main flue gas stream to drop into hopper
38
. This gas, after exiting apparatus
28
via conduit
44
, is destined to be discharged to a stack (not shown).
The recirculated gas, after moving through channel
34
toward the rear of apparatus
28
, enters flow space
60
. Rebounding off back face
50
, the recirculated gas turns roughly 180 degrees and moves toward the front of apparatus
28
. Passing along both sides of hoppers
38
, the gas moves forward until it reaches duct
40
. At this point the direction of flow changes 90 degrees as the gas moves vertically down duct
40
. From duct
40
, the recirculated gas enters conduit
68
, passes therethrough to dust collector
72
, and from there will eventually be introduced to boiler
10
, hence completing the circuit.
Nothing in the art described above teaches or suggests the use of a structure for controlling boiler outlet steam temperature as a means to improve a NOx removal system using selective catalytic reduction (SCR).
Serving a purpose entirely different from the gas
Grant Kathryn W.
Marich Eric
The Babcock & Wilcox Company
Wilson Gregory
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